Water model

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https://en.wikipedia.org/wiki/Water_model 

 

A water model is defined by its geometry, together with other parameters such as the atomic charges and Lennard-Jones parameters.

In computational chemistry, a water model is used to simulate and thermodynamically calculate water clusters, liquid water, and aqueous solutions with explicit solvent. The models are determined from quantum mechanics, molecular mechanics, experimental results, and these combinations. To imitate a specific nature of molecules, many types of models have been developed. In general, these can be classified by the following three points; (i) the number of interaction points called site, (ii) whether the model is rigid or flexible, (iii) whether the model includes polarization effects.

An alternative to the explicit water models is to use an implicit solvation model, also termed a continuum model, an example of which would be the COSMO solvation model or the polarizable continuum model (PCM) or a hybrid solvation model.

Simple water models

The rigid models are considered the simplest water models and rely on non-bonded interactions. In these models, bonding interactions are implicitly treated by holonomic constraints. The electrostatic interaction is modeled using Coulomb's law, and the dispersion and repulsion forces using the Lennard-Jones potential. The potential for models such as TIP3P (transferable intermolecular potential with 3 points) and TIP4P is represented by

${\displaystyle E_{ab}=\sum _{i}^{{\text{on }}a}\sum _{j}^{{\text{on }}b}{\frac {k_{C}q_{i}q_{j}}{r_{ij}}}+{\frac {A}{r_{\text{OO}}^{12}}}-{\frac {B}{r_{\text{OO}}^{6}}},}$

where k_C, the electrostatic constant, has a value of 332.1 Å·kcal/(mol·e²) in the units commonly used in molecular modeling; q_i and q_j are the partial charges relative to the charge of the electron; r_ij is the distance between two atoms or charged sites; and A and B are the Lennard-Jones parameters. The charged sites may be on the atoms or on dummy sites (such as lone pairs). In most water models, the Lennard-Jones term applies only to the interaction between the oxygen atoms.

The figure below shows the general shape of the 3- to 6-site water models. The exact geometric parameters (the OH distance and the HOH angle) vary depending on the model.

2-site

A 2-site model of water based on the familiar three-site SPC model (see below) has been shown to predict the dielectric properties of water using site-renormalized molecular fluid theory.

3-site

Three-site models have three interaction points corresponding to the three atoms of the water molecule. Each site has a point charge, and the site corresponding to the oxygen atom also has the Lennard-Jones parameters. Since 3-site models achieve a high computational efficiency, these are widely used for many applications of molecular dynamics simulations. Most of the models use a rigid geometry matching that of actual water molecules. An exception is the SPC model, which assumes an ideal tetrahedral shape (HOH angle of 109.47°) instead of the observed angle of 104.5°.

The table below lists the parameters for some 3-site models.

	TIPS	SPC	TIP3P	SPC/E
r(OH), Å	0.9572	1.0	0.9572	1.0
HOH, deg	104.52	109.47	104.52	109.47
A, 103 kcal Å12/mol	580.0	629.4	582.0	629.4
B, kcal Å6/mol	525.0	625.5	595.0	625.5
q(O)	−0.80	−0.82	−0.834	−0.8476
q(H)	+0.40	+0.41	+0.417	+0.4238

The SPC/E model adds an average polarization correction to the potential energy function:

${\displaystyle E_{\text{pol}}={\frac {1}{2}}\sum _{i}{\frac {(\mu -\mu ^{0})^{2}}{\alpha _{i}}},}$

where μ is the electric dipole moment of the effectively polarized water molecule (2.35 D for the SPC/E model), μ⁰ is the dipole moment of an isolated water molecule (1.85 D from experiment), and α_i is an isotropic polarizability constant, with a value of 1.608×10⁻⁴⁰ F·m². Since the charges in the model are constant, this correction just results in adding 1.25 kcal/mol (5.22 kJ/mol) to the total energy. The SPC/E model results in a better density and diffusion constant than the SPC model.

The TIP3P model implemented in the CHARMM force field is a slightly modified version of the original. The difference lies in the Lennard-Jones parameters: unlike TIP3P, the CHARMM version of the model places Lennard-Jones parameters on the hydrogen atoms too, in addition to the one on oxygen. The charges are not modified. Three-site model (TIP3P) has better performance in calculating specific heats.

Flexible SPC water model

Flexible SPC water model

The flexible simple point-charge water model (or flexible SPC water model) is a re-parametrization of the three-site SPC water model. The SPC model is rigid, whilst the flexible SPC model is flexible. In the model of Toukan and Rahman, the O–H stretching is made anharmonic, and thus the dynamical behavior is well described. This is one of the most accurate three-center water models without taking into account the polarization. In molecular dynamics simulations it gives the correct density and dielectric permittivity of water.

Flexible SPC is implemented in the programs MDynaMix and Abalone.

Other models

• Ferguson (flexible SPC)
• CVFF (flexible)
• MG (flexible and dissociative)
• KKY potential (flexible model).
• BLXL (smear charged potential).

4-site

The four-site models have four interaction points by adding one dummy atom near of the oxygen along the bisector of the HOH angle of the three-site models (labeled M in the figure). The dummy atom only has a negative charge. This model improves the electrostatic distribution around the water molecule. The first model to use this approach was the Bernal–Fowler model published in 1933, which may also be the earliest water model. However, the BF model doesn't reproduce well the bulk properties of water, such as density and heat of vaporization, and is thus of historical interest only. This is a consequence of the parameterization method; newer models, developed after modern computers became available, were parameterized by running Metropolis Monte Carlo or molecular dynamics simulations and adjusting the parameters until the bulk properties are reproduced well enough.

The TIP4P model, first published in 1983, is widely implemented in computational chemistry software packages and often used for the simulation of biomolecular systems. There have been subsequent reparameterizations of the TIP4P model for specific uses: the TIP4P-Ew model, for use with Ewald summation methods; the TIP4P/Ice, for simulation of solid water ice; and TIP4P/2005, a general parameterization for simulating the entire phase diagram of condensed water.

Most of four-site water models use OH distance and HOH angle matching that of the free water molecule. An exception is OPC model, on which no geometry constraints are imposed other than the fundamental C_2v molecular symmetry of the water molecule. Instead, the point charges and their positions are optimized to best describe the electrostatics of the water molecule. OPC reproduces a comprehensive set of bulk properties more accurately than commonly used rigid n-site water models. OPC model is implemented in AMBER force field.

	BF	TIPS2	TIP4P	TIP4P-Ew	TIP4P/Ice	TIP4P/2005	OPC	TIP4P-D
r(OH), Å	0.96	0.9572	0.9572	0.9572	0.9572	0.9572	0.8724	0.9572
HOH, deg	105.7	104.52	104.52	104.52	104.52	104.52	103.6	104.52
r(OM), Å	0.15	0.15	0.15	0.125	0.1577	0.1546	0.1594	0.1546
A, 103 kcal Å12/mol	560.4	695.0	600.0	656.1	857.9	731.3	865.1	904.7
B, kcal Å6/mol	837.0	600.0	610.0	653.5	850.5	736.0	858.1	900.0
q(M)	−0.98	−1.07	−1.04	−1.04844	−1.1794	−1.1128	−1.3582	−1.16
q(H)	+0.49	+0.535	+0.52	+0.52422	+0.5897	+0.5564	+0.6791	+0.58

Others:

• q-TIP4P/F (flexible)

5-site

The 5-site models place the negative charge on dummy atoms (labeled L) representing the lone pairs of the oxygen atom, with a tetrahedral-like geometry. An early model of these types was the BNS model of Ben-Naim and Stillinger, proposed in 1971, soon succeeded by the ST2 model of Stillinger and Rahman in 1974. Mainly due to their higher computational cost, five-site models were not developed much until 2000, when the TIP5P model of Mahoney and Jorgensen was published. When compared with earlier models, the TIP5P model results in improvements in the geometry for the water dimer, a more "tetrahedral" water structure that better reproduces the experimental radial distribution functions from neutron diffraction, and the temperature of maximal density of water. The TIP5P-E model is a reparameterization of TIP5P for use with Ewald sums.

	BNS	ST2	TIP5P	TIP5P-E
r(OH), Å	1.0	1.0	0.9572	0.9572
HOH, deg	109.47	109.47	104.52	104.52
r(OL), Å	1.0	0.8	0.70	0.70
LOL, deg	109.47	109.47	109.47	109.47
A, 103 kcal Å12/mol	77.4	238.7	544.5	554.3
B, kcal Å6/mol	153.8	268.9	590.3	628.2
q(L)	−0.19562	−0.2357	−0.241	−0.241
q(H)	+0.19562	+0.2357	+0.241	+0.241
RL, Å	2.0379	2.0160
RU, Å	3.1877	3.1287

Note, however, that the BNS and ST2 models do not use Coulomb's law directly for the electrostatic terms, but a modified version that is scaled down at short distances by multiplying it by the switching function S(r):

${\displaystyle S(r_{ij})={\begin{cases}0&{\text{if }}r_{ij}\leq R_{\text{L}},\\{\frac {(r_{ij}-R_{L})^{2}(3R_{\text{U}}-R_{\text{L}}-2r_{ij})}{(R_{\text{U}}-R_{\text{L}})^{2}}}&{\text{if }}R_{\text{L}}\leq r_{ij}\leq R_{\text{U}},\\1&{\text{if }}R_{\text{U}}\leq r_{ij}.\end{cases}}}$

Thus, the R_L and R_U parameters only apply to BNS and ST2.

6-site

Originally designed to study water/ice systems, a 6-site model that combines all the sites of the 4- and 5-site models was developed by Nada and van der Eerden. Since it had a very high melting temperature when employed under periodic electrostatic conditions (Ewald summation), a modified version was published later optimized by using the Ewald method for estimating the Coulomb interaction.

Other

• The effect of explicit solute model on solute behavior in biomolecular simulations has been also extensively studied. It was shown that explicit water models affected the specific solvation and dynamics of unfolded peptides, while the conformational behavior and flexibility of folded peptides remained intact.
• MB model. A more abstract model resembling the Mercedes-Benz logo that reproduces some features of water in two-dimensional systems. It is not used as such for simulations of "real" (i.e., three-dimensional) systems, but it is useful for qualitative studies and for educational purposes.
• Coarse-grained models. One- and two-site models of water have also been developed. In coarse-grain models, each site can represent several water molecules.
• Many-body models. Water models built using training-set configurations solved quantum mechanically, which then use machine learning protocols to extract potential-energy surfaces. These potential-energy surfaces are fed into MD simulations for an unprecedented degree of accuracy in computing physical properties of condensed phase systems.
  • Another classification of many body models is on the basis of the expansion of the underlying electrostatics, e.g., the SCME (Single Center Multipole Expansion) model 

Computational cost

The computational cost of a water simulation increases with the number of interaction sites in the water model. The CPU time is approximately proportional to the number of interatomic distances that need to be computed. For the 3-site model, 9 distances are required for each pair of water molecules (every atom of one molecule against every atom of the other molecule, or 3 × 3). For the 4-site model, 10 distances are required (every charged site with every charged site, plus the O–O interaction, or 3 × 3 + 1). For the 5-site model, 17 distances are required (4 × 4 + 1). Finally, for the 6-site model, 26 distances are required (5 × 5 + 1).

When using rigid water models in molecular dynamics, there is an additional cost associated with keeping the structure constrained, using constraint algorithms (although with bond lengths constrained it is often possible to increase the time step).

Diet and obesity

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https://en.wikipedia.org/wiki/Diet_and_obesity 

 

Map of dietary energy availability per person per day in 1961 (kcal/person/day).

 

Map of dietary energy availability per person per day in 1979-1981 (kcal/person/day).

 

Map of dietary energy availability per person per day in 2001–2003 

 

(kcal/person/day).

  no data

  <1600

  1600-1800

  1800-2000

  2000-2200

  2200-2400

  2400-2600

  2600-2800

  2800-3000

  3000-3200

  3200-3400

  3400-3600

  >3600

Diet plays an important role in the genesis of obesity. Personal choices, food advertising, social customs and cultural influences, as well as food availability and pricing all play a role in determining what and how much an individual eats.

Dietary energy supply

Average per capita energy consumption of the world from 1961 to 2002

The dietary energy supply is the food available for human consumption, usually expressed in kilocalories per person per day. It gives an overestimate of the total amount of food consumed as it reflects both food consumed and food wasted. The per capita dietary energy supply varies markedly between different regions and countries. It has also changed significantly over time. From the early 1970s to the late 1990s, the average calories available per person per day (the amount of food bought) has increased in all part of the world except Eastern Europe and parts of Africa. The United States had the highest availability with 3654 kilo calories per person in 1996. This increased further in 2002 to 3770. During the late 1990s, Europeans had 3394 kilo calories per person, in the developing areas of Asia there were 2648 kilo calories per person, and in sub-Sahara Africa people had 2176 kilo calories per person.

Average calorie consumption

Change over time of the macronutrient composition of the US male diet.

 

Change over time of the macronutrient composition of the US female diet.

 

USDA chart showing the increase in soda consumption and the decrease in milk consumption from 1947 to 2001.

From 1971 – 2000, the average daily number of calories which women consumed in the United States increased by 335 calories per day (1542 calories in 1971 and 1877 calories in 2000). For men, the average increase was 168 calories per day (2450 calories in 1971 and 2618 calories in 2000). Most of these extra calories came from an increase in carbohydrate consumption, though there was also an increase in fat consumption over the same time period. The increase in caloric consumption is attributed primarily to the "consumption of food away from home; increased energy consumption from salty snacks, soft drinks, and pizza; and increased portion sizes". Other sources note that the consumption of soft drinks and other sweetened beverages now accounts for almost 25 percent of daily calories in young adults in America. As these estimates are based on a person's recall, they may underestimate the amount of calories actually consumed.

Fast food

As societies become increasingly reliant on energy-dense fast-food meals, the association between fast food consumption and obesity becomes more concerning. In the United States, consumption of fast food meals has tripled and calorie intake from fast food has quadrupled between 1977 and 1995. Consumption of sweetened drinks is also believed to be a major contributor to the rising rates of obesity.

Portion size

A comparison of a typical cheeseburger 20 years ago (left) which had 333 calories with a modern cheeseburger (right) which contains 590 calories as per the National Heart, Lung, and Blood Institute

The portion size of many prepackage and restaurant foods has increased in both the United States and Denmark since the 1970s. Fast food servings, for example, are 2 to 5 times larger than they were in the 1980s. Evidence has shown that larger portions of energy-dense foods lead to greater energy intake and thus to greater rates of obesity.

Meat consumption

A 2010 study published in the American Journal of Clinical Nutrition closely tracked 373,803 people over a period of 8 years across 10 countries. At its conclusion, the study reported that meat consumption (processed meat, red meat, & poultry) is positively associated with weight gain and increased abdominal obesity in men and women. In response, the National Cattlemen's Beef Association countered that increased meat consumption may not be associated with fat gain. However, a subsequent response controlled for just abdominal fat across a sample of 91,214 people found that even when controlling for calories and lifestyle factors, meat consumption is linked with obesity. Further population studies, reviews, and meta-analysis studies have corroborated the claim that greater meat consumption is linked to greater rates of obesity, especially in regards to red meat and processed meat.

Sugar consumption

Drinking more sugary beverages (including fruit juices, soft drinks, fruit drinks, sports drinks, energy and enhanced water drinks, sweetened iced tea, and lemonade) increases overall energy intake, and thus increases the risk of metabolic syndrome, obesity and type 2 diabetes (see Obesity#Pathophysiology). Children who consume more added sugar in foods and beverages have a higher risk of becoming overweight. By itself, sugar is not a factor causing obesity and metabolic syndrome, but rather – when over-consumed – is a component of unhealthy dietary behavior. Adults who increase or decrease their free sugar intake increase or decrease their weight.

Reviews indicate that governmental health policies should be implemented to discourage intake of sugar-sweetened beverages, and reduce the obesity in children and adults. Obesity has been rising in the 21st century. Other than adding excessive calories, the mechanisms by which high sugar consumption causes obesity are unclear because of limitations in clinical research involving uncontrolled factors, such as overall diet, physical activity, and sedentary lifestyle.

Ultraprocessed food consumption

Numerous large studies have demonstrated that eating ultraprocessed food has a positive dose-dependent relationship with both abdominal obesity and general obesity in both men and women. Consuming a diet rich in unprocessed and minimally processed foods is linked with lower obesity risk and less chronic disease. These results are consistent among American, Canadian, Latin American, British, Australian, French, and Spaniard populations.

Particular processing ingredients used in ultraprocessed foods have been linked with increasing the risk of obesity further. Intake of trans fat from industrial oils has been associated with increased abdominal obesity in men and increased weight and waist circumference in women. These associations were not attenuated when fat intake and calorie intake was accounted for.

Similarly, heavy consumption of fried food is linked to greater obesity risk on a population level. On a more individual level, the relative risk of fried food consumption and increased weight gain seems to depend on genetic predisposition.

Social policy and change

New agricultural technologies have led to an overall reduction in the cost of food relative to household income, especially in high-income countries. In his popular book, "The Omnivore's Dilemma," the journalist Michael Pollan linked the subsidies offered to farmers of corn, soy, wheat, and rice through the U.S. farm bill to over-consumption of calories derived from these crops and to rising obesity rates. While increased consumption of foods derived from these commodities is correlated with an increase in BMI (at the population level), no current research supports a causal relationship between farm subsidies and obesity. From a policy perspective, the cost of sugar would actually decrease in the US if the commodity support programs in the farm bill were removed, largely due to the tariffs in the farm bill that restrict the importation of lower-cost sugar available on the global market.

Participation by adults in the United States Department of Agriculture Supplemental Nutrition Assistance Program (i.e. Food Stamps) is positively associated with obesity, waist circumference, elevated fasting glucose, and metabolic syndrome.

Metabolism

Evidence does not support the commonly expressed view that some obese people eat little yet gain weight due to a slow metabolism. On average obese people have a greater energy expenditure than normal weight or thin people and actually have higher BMRs. This is because it takes more energy to maintain an increased body mass. Obese people also underreport how much food they consume compared to those of normal weight. Tests of human subjects carried out in a calorimeter support this conclusion.

Electromagnetic absorption by water

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https://en.wikipedia.org/wiki/Electromagnetic_absorption_by_water 

 

Absorption spectrum (attenuation coefficient vs. wavelength) of liquid water (red), atmospheric water vapor (green) and ice (blue line) between 667 nm and 200 μm. The plot for vapor is a transformation of data Synthetic spectrum for gas mixture 'Pure H₂O' (296K, 1 atm) retrieved from Hitran on the Web Information System.

 

Liquid water absorption spectrum across a wide wavelength range

The absorption of electromagnetic radiation by water depends on the state of the water.

The absorption in the gas phase occurs in three regions of the spectrum. Rotational transitions are responsible for absorption in the microwave and far-infrared, vibrational transitions in the mid-infrared and near-infrared. Vibrational bands have rotational fine structure. Electronic transitions occur in the vacuum ultraviolet regions.

Liquid water has no rotational spectrum but does absorb in the microwave region. Its weak absorption in the visible spectrum results in the pale blue color of water.

Overview

The water molecule, in the gaseous state, has three types of transition that can give rise to absorption of electromagnetic radiation:

• Rotational transitions, in which the molecule gains a quantum of rotational energy. Atmospheric water vapour at ambient temperature and pressure gives rise to absorption in the far-infrared region of the spectrum, from about 200 cm⁻¹ (50 μm) to longer wavelengths towards the microwave region.
• Vibrational transitions in which a molecule gains a quantum of vibrational energy. The fundamental transitions give rise to absorption in the mid-infrared in the regions around 1650 cm⁻¹ (μ band, 6 μm) and 3500 cm⁻¹ (so-called X band, 2.9 μm)
• Electronic transitions in which a molecule is promoted to an excited electronic state. The lowest energy transition of this type is in the vacuum ultraviolet region.

In reality, vibrations of molecules in the gaseous state are accompanied by rotational transitions, giving rise to a vibration-rotation spectrum. Furthermore, vibrational overtones and combination bands occur in the near-infrared region. The HITRAN spectroscopy database lists more than 37,000 spectral lines for gaseous H₂¹⁶O, ranging from the microwave region to the visible spectrum.

In liquid water the rotational transitions are effectively quenched, but absorption bands are affected by hydrogen bonding. In crystalline ice the vibrational spectrum is also affected by hydrogen bonding and there are lattice vibrations causing absorption in the far-infrared. Electronic transitions of gaseous molecules will show both vibrational and rotational fine structure.

Units

Infrared absorption band positions may be given either in wavelength (usually in micrometers, μm) or wavenumber (usually in reciprocal centimeters, cm⁻¹) scale.

Rotational spectrum

Part of the pure rotation absorption spectrum of water vapour

 

Rotating water molecule

The water molecule is an asymmetric top, that is, it has three independent moments of inertia. Rotation about the 2-fold symmetry axis is illustrated at the left. Because of the low symmetry of the molecule, a large number of transitions can be observed in the far infrared region of the spectrum. Measurements of microwave spectra have provided a very precise value for the O−H bond length, 95.84 ± 0.05 pm and H−O−H bond angle, 104.5 ± 0.3°.

Vibrational spectrum

The three fundamental vibrations of the water molecule

ν₁,O-H symmetric stretching
3657 cm⁻¹ (2.734 μm)

 

ν₂, H-O-H bending
1595 cm⁻¹ (6.269 μm)

 

ν₃, O-H asymmetric stretching
3756 cm⁻¹ (2.662 μm)

The water molecule has three fundamental molecular vibrations. The O-H stretching vibrations give rise to absorption bands with band origins at 3657 cm⁻¹ (ν₁, 2.734 μm) and 3756 cm⁻¹ (ν₃, 2.662 μm) in the gas phase. The asymmetric stretching vibration, of B₂ symmetry in the point group C_2v is a normal vibration. The H-O-H bending mode origin is at 1595 cm⁻¹ (ν₂, 6.269 μm). Both symmetric stretching and bending vibrations have A₁ symmetry, but the frequency difference between them is so large that mixing is effectively zero. In the gas phase all three bands show extensive rotational fine structure. In the Near-infrared spectrum ν₃ has a series of overtones at wavenumbers somewhat less than n·ν₃, n=2,3,4,5... Combination bands, such as ν₂ + ν₃ are also easily observed in the near-infrared region. The presence of water vapor in the atmosphere is important for atmospheric chemistry especially as the infrared and near infrared spectra are easy to observe. Standard (atmospheric optical) codes are assigned to absorption bands as follows. 0.718 μm (visible): α, 0.810 μm: μ, 0.935 μm: ρστ, 1.13 μm: φ, 1.38 μm: ψ, 1.88 μm: Ω, 2.68 μm: X. The gaps between the bands define the infrared window in the Earth's atmosphere.

The infrared spectrum of liquid water is dominated by the intense absorption due to the fundamental O-H stretching vibrations. Because of the high intensity, very short path lengths, usually less than 50 μm, are needed to record the spectra of aqueous solutions. There is no rotational fine structure, but the absorption bands are broader than might be expected, because of hydrogen bonding. Peak maxima for liquid water are observed at 3450 cm⁻¹ (2.898 μm), 3615 cm⁻¹ (2.766 μm) and 1640 cm ⁻¹ (6.097 μm). Direct measurement of the infrared spectra of aqueous solutions requires that the cuvette windows be made of substances such as calcium fluoride which are water-insoluble. This difficulty can alternatively be overcome by using an attenuated total reflectance (ATR) device rather than transmission.

In the near-infrared range liquid water has absorption bands around 1950 nm (5128 cm⁻¹), 1450 nm (6896 cm⁻¹), 1200 nm (8333 cm⁻¹) and 970 nm, (10300 cm⁻¹). The regions between these bands can be used in near-infrared spectroscopy to measure the spectra of aqueous solutions, with the advantage that glass is transparent in this region, so glass cuvettes can be used. The absorption intensity is weaker than for the fundamental vibrations, but this is not important as longer path-length cuvettes can be used. The absorption band at 698 nm (14300 cm⁻¹) is a 3rd overtone (n=4). It tails off onto the visible region and is responsible for the intrinsic blue color of water. This can be observed with a standard UV/vis spectrophotometer, using a 10 cm path-length. The colour can be seen by eye by looking through a column of water about 10 m in length; the water must be passed through an ultrafilter to eliminate color due to Rayleigh scattering which also can make water appear blue.

The spectrum of ice is similar to that of liquid water, with peak maxima at 3400 cm⁻¹ (2.941 μm), 3220 cm⁻¹ (3.105 μm) and 1620 cm⁻¹ (6.17 μm)

In both liquid water and ice clusters, low-frequency vibrations occur, which involve the stretching (TS) or bending (TB) of intermolecular hydrogen bonds (O–H•••O). Bands at wavelengths λ = 50-55 μm or 182-200 cm⁻¹ (44 μm, 227 cm⁻¹ in ice) have been attributed to TS, intermolecular stretch, and 200 μm or 50 cm⁻¹ (166 μm, 60 cm⁻¹ in ice), to TB, intermolecular bend

Visible region

Predicted wavelengths of overtones and combination bands of liquid water in the visible region

ν1, ν3	ν2	wavelength /nm
4	0	742
4	1	662
5	0	605
5	1	550
6	0	514
6	1	474
7	0	449
7	1	418
8	0	401
8	1	376

Absorption coefficients for 200 nm and 900 nm are almost equal at 6.9 m⁻¹ (attenuation length of 14.5 cm). Very weak light absorption, in the visible region, by liquid water has been measured using an integrating cavity absorption meter (ICAM).^[16] The absorption was attributed to a sequence of overtone and combination bands whose intensity decreases at each step, giving rise to an absolute minimum at 418 nm, at which wavelength the attenuation coefficient is about 0.0044 m⁻¹, which is an attenuation length of about 227 meters. These values correspond to pure absorption without scattering effects. The attenuation of, e.g., a laser beam would be slightly stronger.

Visible light absorption spectrum of pure water (absorption coefficient vs. wavelength)

Electronic spectrum

The electronic transitions of the water molecule lie in the vacuum ultraviolet region. For water vapor the bands have been assigned as follows.

• 65 nm band — many different electronic transitions, photoionization, photodissociation
• discrete features between 115 and 180 nm
  • set of narrow bands between 115 and 125 nm
    Rydberg series: 1b₁ (n₂) → many different Rydberg states and 3a₁ (n₁) → 3sa₁ Rydberg state
  • 128 nm band
    Rydberg series: 3a₁ (n₁) → 3sa₁ Rydberg state and 1b₁ (n₂) → 3sa₁ Rydberg state
  • 166.5 nm band
    1b₁ (n₂) → 4a₁ (σ₁*-like orbital)

At least some of these transitions result in photodissociation of water into H+OH. Among them the best known is that at 166.5 nm.

Microwaves and radio waves

Dielectric permittivity and dielectric loss of water between 0°C and 100°C, the arrows showing the effect of increasing temperature

 

See also: Radio window

The pure rotation spectrum of water vapor extends into the microwave region.

Liquid water has a broad absorption spectrum in the microwave region, which has been explained in terms of changes in the hydrogen bond network giving rise to a broad, featureless, microwave spectrum. The absorption (equivalent to dielectric loss) is used in microwave ovens to heat food that contains water molecules. A frequency of 2.45 GHz, wavelength 122 mm, is commonly used.

Radiocommunication at GHz frequencies is very difficult in fresh waters and even more so in salt waters.

Atmospheric effects

See also: Infrared window

 

Synthetic stick absorption spectrum of a simple gas mixture corresponding to the Earth's atmosphere composition based on HITRAN data created using Hitran on the Web system. Green color - water vapor, WN – wavenumber (caution: lower wavelengths on the right, higher on the left). Water vapor concentration for this gas mixture is 0.4%.

Water vapor is a greenhouse gas in the Earth's atmosphere, responsible for 70% of the known absorption of incoming sunlight, particularly in the infrared region, and about 60% of the atmospheric absorption of thermal radiation by the Earth known as the greenhouse effect. It is also an important factor in multispectral imaging and hyperspectral imaging used in remote sensing because water vapor absorbs radiation differently in different spectral bands. Its effects are also an important consideration in infrared astronomy and radio astronomy in the microwave or millimeter wave bands. The South Pole Telescope was constructed in Antarctica in part because the elevation and low temperatures there mean there is very little water vapor in the atmosphere.

Similarly, carbon dioxide absorption bands occur around 1400, 1600 and 2000 nm, but its presence in the Earth's atmosphere accounts for just 26% of the greenhouse effect. Carbon dioxide gas absorbs energy in some small segments of the thermal infrared spectrum that water vapor misses. This extra absorption within the atmosphere causes the air to warm just a bit more and the warmer the atmosphere the greater its capacity to hold more water vapor. This extra water vapor absorption further enhances the Earth's greenhouse effect.

In the atmospheric window between approximately 8000 and 14000 nm, in the far-infrared spectrum, carbon dioxide and water absorption is weak. This window allows most of the thermal radiation in this band to be radiated out to space directly from the Earth's surface. This band is also used for remote sensing of the Earth from space, for example with thermal Infrared imaging.

As well as absorbing radiation, water vapour occasionally emits radiation in all directions, according to the Black Body Emission curve for its current temperature overlaid on the water absorption spectrum. Much of this energy will be recaptured by other water molecules, but at higher altitudes, radiation sent towards space is less likely to be recaptured, as there is less water available to recapture radiation of water-specific absorbing wavelengths. By the top of the troposphere, about 12 km above sea level, most water vapor condenses to liquid water or ice as it releases its heat of vapourization. Once changed state, liquid water and ice fall away to lower altitudes. This will be balanced by incoming water vapour rising via convection currents.

Liquid water and ice emit radiation at a higher rate than water vapour (see graph above). Water at the top of the troposphere, particularly in liquid and solid states, cools as it emits net photons to space. Neighboring gas molecules other than water (e.g. Nitrogen) are cooled by passing their heat kinetically to the water. This is why temperatures at the top of the troposphere (known as the tropopause) are about -50 degrees Celsius.

Criticism of fast food

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Criticism_of_fast_food 

 

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Human body weight

The fall 2013 issue of Ms. magazine promotes the need for higher fast food worker wages

Criticism of fast food includes claims of negative health effects, animal cruelty, cases of worker exploitation, children targeted marketing and claims of cultural degradation via shifts in people's eating patterns away from traditional foods. Fast food chains have come under fire from consumer groups, such as the Center for Science in the Public Interest, a longtime fast food critic over issues such as caloric content, trans fats and portion sizes. Social scientists have highlighted how the prominence of fast food narratives in popular urban legends suggests that modern consumers have an ambivalent relationship (characterized by guilt) with fast food, particularly in relation to children.

Some of these concerns have helped give rise to the slow food and local food movements. These movements seek to promote local cuisines and ingredients, and directly oppose laws and habits that encourage fast food choices. Proponents of the slow food movement try to educate consumers about what its members consider the environmental, nutritional, and taste benefits of fresh, local foods.

Health based criticisms

A sign advertising inclusion of highly processed meat and sugar in a sandwich.

Many fast foods are rich in calories as they include considerable amounts of mayonnaise, cheese, salt, fried meat, and oil, thus containing high fat content (Schlosser). Excessive consumption of fatty ingredients such as these results in unbalanced diet. Proteins and vitamins are generally recommended for daily consumption rather than large quantities of carbohydrates or fat. Due to their fat content, fast foods are implicated in poor health and various serious health issues such as obesity and cardiovascular diseases. Additionally, there is strong empirical evidence showing that fast foods are also detrimental to appetite, respiratory system function, and central nervous system function (Schlosser). In a cross-sectional data study from more than 100-thousand adolescents in 32 countries, which included low-income, middle-income, to high-income countries, it has been found that fast food is associated with an increase in suicide attempts.

McDonald's has received criticism for serving food high in saturated fat and calories.

According to the Massachusetts Medical Society Committee Jeff Nutrition, fast foods are commonly high in fat content, and studies have found associations between fast food intake and increased body mass index (BMI) and weight gain. In particular many fast foods are high in saturated fats which are widely held to be a risk factor in heart disease. In 2010, heart disease was the number 1 ranking cause of death. A 2006 study fed monkeys a diet consisting of a similar level of trans fats as what a person who ate fast food regularly would consume. Both diets contained the same overall number of calories. It was found that the monkeys who consumed higher levels of trans fat developed more abdominal fat than those fed a diet rich in unsaturated fats. They also developed signs of insulin resistance, an early indicator of diabetes. After six years on the diet, the trans fat fed monkeys had gained 7.2% of their body weight, compared to just 1.8% in the unsaturated fat group. A five-year study conducted in Singapore showed that frequent fast food consumers (more than 2 time per week) had a significant increased risk of developing type 2 diabetes and an increased risk of death rate from coronary heart disease, when compared to non-consumers. The American Heart Association recommends consumption of about 16 grams of saturated fats a day.

The director of the obesity program for the Children's Hospital Boston, David Ludwig, says that "fast food consumption has been shown to increase caloric intake, promote weight gain, and elevate risk for diabetes". Excessive calories are another issue with fast food. According to B. Lin and E. Frazao, from the US Department of Agriculture(USDA), the percentage of calories which can be attributed to fast-food consumption has increased from 3% to 12% of the total calories consumed in the United States. A regular meal at McDonald's consists of a Big Mac, large fries, and a large Coca-Cola drink amounting to 1,430 calories. The USDA recommends a daily caloric intake of 2,700 and 2,100 kcal (11,300 and 8,800 kJ) for men and women (respectively) between 31 and 50, at a physical activity level equivalent to walking about 1.5 to 3 miles per day at 3 to 4 miles per hour in addition to the light physical activity associated with typical day-to-day life, with the French Agency for Food, Environmental and Occupational Health & Safety guidance suggesting roughly the same levels.

Students buying fast food.

However, besides fast food consumption, there are many other the other reasons for over weighting among children while they are growing, including sex development, "hormonal changes," and social interactions. At those moments kids can feel depressed, which may lead to increase or decrease in appetite. In fact, increased hunger may lead to obesity in some cases. “…seasonal effective disorder affects 1.7 – 5.5% of youths ages 9-19 years old based on a community study of over 2,000 youth."

The fast food chain D'Lites, founded in 1978, specialized in lower-calorie dishes and healthier alternatives such as salads. It filed for bankruptcy in 1987 as other fast food chains began offering healthier options. McDonald's has been attempting to offer healthier options besides salads. They have incorporated fruit and milk as options of happy meals and have promoted healthier ads and packaging for kids. The Alliance for a Healthier Generation has set a standard in hopes of pressuring fast food companies to make recommended healthier adjustments.

Food poisoning risk

Besides the risks posed by trans fats, high caloric intake, and low fiber intake, another cited health risk is food poisoning. In his book Fast Food Nation: The Dark Side of the All-American Meal, Eric Schlosser argues  that meatpacking factories concentrate livestock into feedlots and herd them through processing assembly lines operated by employees of various levels of expertise, some of which may be poorly trained, increasing the risk of large-scale food poisoning.

Manure on occasion gets mixed with meat, possibly contaminating it with salmonella and pathogenic E. coli. Usually spread through undercooked hamburgers, raw vegetables, and contaminated water, it is difficult to treat. In 2008, the Health Protection Agency in England showed that in a Salmonella Typhimurium infection of 179 cases, consumption of pre-packaged egg sandwiches was associated with illness. Although supportive treatment can substantially aid inflicted individuals, since endotoxin is released from gram-negative bacteria such as E. coli upon death, antibiotic use to treat E. coli infections is not recommended. About 4% of people infected with E. coli 0157:H7 develop hemolytic uremic syndrome, and about 5% of children who develop the syndrome die. The rate of developing HUS is 3 in 100,000 or 0.003%. E. coli 0157:H7 has become the leading cause of renal failure among American children. These numbers include rates from all sources of poisoning, including lettuce; radish sprouts; alfalfa sprouts; unpasteurized apple juice/cider; cold cooked or undercooked meat; and unpasteurized animal milk. Additional environmental sources include fecal-contaminated lakes, nonchlorinated municipal water supply, petting farm animals and unhygienic person-to-person contact. An average of sources leads to the number of 0.00000214% for undercooked beef.

Food-contact paper packaging

Fast food often comes in wrappers coated with polyfluoroalkyl phosphate esters (PAPs) to prevent grease from leaking through them. These compounds are able to migrate from the wrappers into the packaged food. Upon ingestion, PAPs are subsequently biotransformed into perfluorinated carboxylic acids (PFCAs), compounds which have long attracted attention due to their detrimental health effects in rodents and their unusually long half-lives in humans. While epidemiological evidence has not demonstrated causal links between PFCAs and these health problems in humans, the compounds are consistently correlated with high levels of cholesterol and uric acid, and PAPs as found on fast food packaging may be a significant source of PFCA contamination in humans.

Fast food and diet

Percent of obese adults and number of fast food restaurants in each state 2011.

On average, nearly one-third of U.S. children aged 4 to 19 eat fast food on a daily basis. Over the course of a year this is likely to result in a child gaining 6 extra pounds every year. In a research experiment published in Pediatrics, 6,212 children and adolescents ages 4 to 19 years old were examined to extrapolate some information about fast food. Upon interviewing the participants in the experiment, it was reported that on any given day 30.3% of the total sample had eaten fast food. Fast-food consumption was prevalent in both males and females, in all racial/ethnic groups, and in all regions of the country.

Additionally, in the study children who ate fast food, compared to those who did not, tended to consume more total fat, carbohydrates, and sugar-sweetened beverages. Children who ate fast food also tended to eat less fiber, milk, fruits, and non-starchy vegetables. After reviewing these test results, the researchers concluded that consumption of fast food by children seems to have a negative effect on an individual's diet, in ways that could significantly increase the risk for obesity. Due to having reduced cognitive defenses against marketing, children may be more susceptible to fast food advertisements, and consequently have a higher risk of becoming obese. Fast food is only a minuscule factor that contributes to childhood obesity. A study conducted by researchers at The University of North Carolina at Chapel Hill's Gillings School of Global Public Health showed that poor diet and obesity as an overall factor are the leading causes of rising obesity rates in children. "While reducing fast-food intake is important, the rest of a child's diet should not be overlooked," Jennifer Poti, co author and doctoral candidate in the university's Department of Nutrition. 

Contrary evidence has been documented that questions the correlation of a fast food diet and obesity. A 2014 People Magazine article recounts the experience of John Cisna, a science teacher at Colo-NESCO High School, who ate a fast food diet for 90 days. At the end of 90 days he had lost 37 pounds and his cholesterol level went from 249 to 170. Cisna kept to a strict 2,000 calorie limit a day and walked 45 minutes a day. Harley Pasternak, a celebrity trainer and nutrition expert, supports Cisna's experiment by saying, "While I don’t think it’s a great idea to eat too much fast food...I do think he is right. Fast food, while far from healthy, doesn’t make people gain weight. Eating too much fast food too often is what can make you gain weight—the same way eating too much of anything can pack on the pounds." A cross-sectional study in China shows that the relationship between BMI and times per week fast food consumption was not significant.

Jared Fogle’s drastic weight loss on a Subway diet, which is considered a fast food chain, is another case that poses a fair debate against the idea that the culprit of obesity is fast food. Fogle dropped 235 pounds by consuming Subway sandwiches for lunch and dinner daily. With no cheese or mayonnaise, the calories of both sandwiches totaled less than 1,000 calories in a day.

Fast food labels without the calorie amount increase the risks of obesity. In the article of M. Mclnerney et al. is examined the impact of fast food labeling on college students’ overweighting. In the study the students required to label the calories of fast foods in the items’ lists. The results showed positive effects on the decreased significance of weighting among college students. Thus, fast food restaurants need to write down the exact calories of the products to inform the consumers about their food choices in order to prevent obesity.

Fast food commercials

A 2012, estimated report by the US Federal Trade Commission revealed a $7.9 billion marketing expenditure difference between expenditure on marketing to all audiences and expenditure on marketing strictly to children and adolescents. According to this report, Fast food industries spent approximately $9.7 billion on marketing food and beverages to the general audience while they spent only $1.8 billion on marketing to children and adolescents.

Consumer responsibility

Spokespeople for the fast food industry claim that there are no good or bad foods, but instead there are good or bad diets. The industry has defended itself by placing the burden of healthy eating on the consumer, who freely chooses to consume their product outside of what nutritional recommendations allow.

Many fast food restaurants added labels to their menus by listing the nutritional information below each item. The intent was to inform consumers of the caloric and nutritional content of the food being served there and result in directing consumers to the healthier options available. However, reports do not display any significant drop in sales at sandwich or burger locations which highlights no change in consumer behavior even after food was labeled.

Fast food is also affordable on people's incomes and expenses relating to the regions they live. "Healthy foods including whole-grain products, low-fat dairy foods, and fresh fruits and vegetables may be less available, and relatively costlier, in poor and minority neighborhoods." So, fast food stores are located in the areas where the demand by the population is high.

Some other studies show that eating fast food is not dependent on a person's income. Researchers found that an amount of fast food consumed does not correlate with a person's income level. The article "Wealth doesn't equal health Wealth: Fast food consequences not just for poor," discusses the issue: not all rich people are healthy food consumers, nor do they consume fast food less frequently than poor people. Additionally, fast food customers work harder and longer than those who do not eat fast food daily. So, it is dependent on a person to choose their meal based on their lifestyle.

A study conducted in 20 Fast Food restaurants in Australia showed that despite the availability of healthy meal options on the menu less than 3% of the consumers observed opted for a healthy meal which emulated results of other recent Australian research on consumption of healthy meals at Fast Food locations. In this 12 hour observational study, about 34% of meals purchased were take-away, meals that were excluded from the study, and 65% represented the unhealthy eat-in meals while the remaining 1% represented the healthy meals purchased.

Restrained eating, or excessive consumption of fast food and other unhealthy foods high in sugar and sodium, is a category of different eating habits derived from results of a cross-sectional study in 2014. This study depicted a prominent association between restrained eating and nurses working overnight shifts and those who are under high stress. Fruits and vegetables were reported as the least likely to be consumed under stress. About 395 nurses participated in this study. All these nurses were employees of two major hospitals in the capital city, Riyadh, of the Kingdom of Saudi Arabia.

The research gathered from a nationwide study in China strictly concentrated on the link between fast food consumption and the growing obesity epidemic in children, ranging from ages 6–18. Although end results weren't completely inconclusive, there was no significant relationship found between the said two parties. The variables taken into consideration to support and narrow the study, displayed that with the presence of any of the following variables low-income households, peer influence, geographical location, pocket money and independence, fast food consumption rates increased. Fast food consumption rates escalated when older children were surveyed whilst consumption rates for younger children appeared normal. Also, western fast food was preferred by children of all ages because they associated western fast food with high quality food.

"The McLawsuit" was a group of overweight children that filed a class action lawsuit against McDonald's seeking compensation for obesity related reasons.

The CSR Halo Effect

The Corporate Social Responsibility (CSR) Effect is a phrase used to judge a category based on judgments from other similar categories or is in relation to them. To put it in terms of the fast food industry, a customer who had a bad experience at a McDonald's would associate that experience with other McDonald's, casting a per-conspired image in their mind of how all other Mcdonald's are. Ioannis Assiouras states that "positive prior CSR leads to higher sympathy and lower anger and schadenfreude toward the company, than negative prior CSR or lack of CSR information."

Worker discrepancies and strikes

See also: Fast food worker strikes

Many fast food employees are adults who earn minimum wage, which in the United States is around $7.25 for every hour. Around 60% of fast food workers are twenty-five years and older.

Many employees have protested to raise the minimum wage. On December 5, 2013, protesters from 100 cities in the United States held demonstrations for a $15 hourly wage. This protest was one of a series of strikes that began 2012, in New York City, protesting against low wages.

In the largest increase in decades, fast food workers have seen a 10% increase in pay since 2020.

There has been a study over employee wages at the fast food companies, the study suggests the fast food industry needs to increase an hourly payment from "7.25 to 10.25" for the beginners of the job. Besides, they recommend to rise that to 5 dollars after few years of experience. From that it is clear to understand that how increased minimum wages has its effects on employees services, life style, and well-being. Because workers start to work better when their life style changes as well as their payments.

Fast food and the pandemic

The 2019 global pandemic, COVID-19, has created many challenges for businesses to stay afloat. The pandemic lead to many business cutting hours for minimum wage workers, which lead them to seek new employment opportunities.

Fast food workers continue to face hostile work environments for those who choose to stay. They are already in a low paying job and need to go to work so they continue show up even if they show any symptoms due to lack of coverage and legislation that only support COVID-19 related illnesses. With two thirds of front line workers being women with a minor child at home, the only option is to show up for work.

Social distancing is not always an option in fast food restaurants due to limited space. Fast food workers were not always provided with proper PPE, one worker even cited utilizing the same mask for up to a week at a time. Both of these variables contributed to fast food workers being at a higher risk for contracting the virus.

The pandemic intensified the mental health issues that fast-food workers had already faced. On top of the known mental health issues, they now had a fear of losing their jobs, contracting COVID-19 and spreading the virus to others.

The fast food workers have said they have dealt with being sexually harassed and mentally abused. More than 60% of workers have experienced some form of abuse since the pandemic.

Packaging waste

Further information: packaging waste

A 2011 study of litter in the Bay area by Clean Water Action found that nearly half of the litter present on the streets was fast food packaging. The Natural Resources Defense Council's paper “Waste and Opportunity 2015: Environmental Progress and Challenges in Food, Beverage, and Consumer Goods Packaging” reported that no fast food brands were meeting best practices for use of recycled materials or promotion of recycling of the used packaging. The EPA states that only a tiny proportion of the plastic waste generated by the fast food industry is recycled.

Disposable Tableware as a Business Model

The use of disposable tableware shifts costs from in-house employment to the municipal waste stream. By convincing consumers to bus disposable tableware, mostly in the 1960-1975 time period, formula fast food restaurants were able gain competitive advantage over full-service lunch counter operations, despite the additional cost of the disposable items. Some attempts at discouraging this have been made, but the custom of busing disposable items is still widespread. Other measures include "Carryout Bag" laws and restrictions on formula restaurants.

Fast food industry's response to criticism

John Merritt, senior vice president of public affairs for Hardee's says their "strategy is not necessarily to move towards healthier items" but "to move towards more choice."

In 2013, McDonald's and Dunkin’ Brands publicly pledged to transition out of their use of foam hot beverage cups. McDonald's has replaced foam with paper cups, but Dunkin’ has not initiated transition. The use of foam cups can still be seen at Chick-fil-A, Burger King, and KFC. Chipotle uses aluminum meal lids that are made from 95% recycled material, but they do not have postconsumer recycling, so the lids that are left on-site are landfilled.

Animal cruelty

In 2015, a gruesome video clip of a T&S farm in Dukedom, Tennessee was released by animal rights activists, where workers were caught abusing chickens. Tyson Foods, the company which delivers chicken nuggets to the fast food giant McDonald's, cancelled their contract with the farm stating "animal well-being" is their utmost priority. McDonald's supported Tyson Foods' decision and described the workers actions as unacceptable.

In the fall of 2007, an investigator working for the Humane Society of the United States documented inhumane treatment of downed dairy cows, those too weak to walk, at a slaughterhouse in Chino, California. Plant workers at the Hallmark/Westland facility were filmed using a forklift to forcibly move cows who could not rise to their feet, dragging them with chains, kicking them, spraying high-pressure water hoses into their nostrils and shocking them with electric prods, all in an effort to get them to stand long enough for the U.S. Department of Agriculture (USDA) veterinary inspector to pass them for slaughter.

Nutrition and health

In 2013, McDonald's announced that they would include fruits and vegetables in their menu combinations. Don Thompson, McDonald's chief executive stated, "We’ve been trying to optimize our menu with more fruits and vegetables and giving customers additional choices when they come to McDonald’s."

In 2016 the company replaced the high-fructose corn syrup in its hamburger buns with sugar and removed antibiotics that are "important to human medicine" from its chicken. They also removed artificial preservatives from their cooking oil, pork sausage patties, eggs served on the breakfast menu, and Chicken McNuggets. The skin, safflower oil and citric acid from the McNuggets was also replaced with pea starch, rice starch and powdered lemon juice. These changes were made in an effort to target "health-conscious consumers."

Source reduction

Many fast food chains have reduced their material usage by “lightweighting”, or reducing material in a package by weight. McDonald's made over 10 reduction in packaging weight in 2012, such as a 48% reduction in the chicken sandwich paperboard carton, and an 18-28% reduction in its plastic cold cups. Starbucks has reduced their water bottle weight by 20% and cold cups by 15%.

Cage-free hens

Over 160 companies in the food sector have announced that they are planning to shift to eggs from only cage-free hens, most by the year 2025. The list includes McDonald's, Dunkin’ Donuts, Carl's Jr., Burger King, Denny's, Jack in the Box, Quiznos, Shake Shack, Starbucks, Sonic, Taco Bell, Wendy's, White Castle, and Subway, among others. The full list can be seen at: https://web.archive.org/web/20170306143016/http://cagefreefuture.com/docs/Cage%20Free%20Corporate%20Policies.pdf 

Proximity of fast food locations

A study of students who live within a half mile from fast food locations have been reported to consume fewer amounts of fruit and vegetables, consume more soda, and are more likely to be overweight. More other studies show that the exposure to poor-quality food environments has important effects on adolescent eating patterns and obesity. Therefore, it seems that policy interventions limiting the proximity of fast-food restaurants to schools could help reduce adolescent obesity.