Vapor Pressure-Volatilization Relationship

If a chemical is placed in an empty vessel that is greater in volume than the chemical itself, a portion of the chemical volatilizes to fill the remaining free space of the vessel with vapors. The pressure in the vessel at eqnilibrium is affected only by the temperature and is independent of the vessel volnme. The pressure that develops, called vapor pressure, characterizes any chemical in the liquid or solid state. 

When the temperatnre increases, the proportion of molecules with energy in excess of the cohesive energy also increases, and an excess vapor pressnre is observed. The Clansins-Clapeyron eqnation describes the variation of vapor pressure with temperature as follows  

Becanse the vapor pressnre of chemicals is a key factor in controlling their dissipation within the snbsnrface, and from the snbsnrface to the atmosphere, accurate estimation of this valne is reqnired. Comprehensive reviews on this subject are given by Plimmer (1976) and Glotfelty and Schombnrg (1989). For contaminants with low vapor pressnre that reach the snbsnrface as a result of a nonpoint disposal (e.g., pesticides nsed in agricnltnral practices), their vapor pressure is sufficiently low to be below detection limits, which may explain some discrepancies in the reported results. 

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Gas-liquid relationships, in the geochemical sense, should be considered liquid-solid-gas interactions in the subsurface. The subsurface gas phase is composed of a mixture of gases with various properties, usually found in the free pore spaces of the solid phase. Processes involved in the gas-liquid and gas-solid interface interactions are controlled by factors such as vapor pressure-volatilization, adsorption, solubility, pressure, and temperature. The solubility of a pure gas in a closed system containing water reaches an equilibrium concentration at a constant pressure and temperature. A gas-liquid equilibrium may be described by a partition coefficient, relative volatilization and Henry s law. 

Vapor Pressure The volatility of a given component is expressed by its vapor pressure, which increases with increasing temperature (Figure 2). The lower the vapor pressure, the lower the volatility and, thus, the more difficult to remove the component from the oil. For each specific component, the vapor pressure-temperature relationship can be expressed by the Equation of Antoine ... 

The vapor pressure of a compoimd is a measure of the ease with which its molecules escape the surface of a liquid. When the liquid is composed of two volatile components, in this case X and Y, the number of molecules of X and of Y in a given volume of the vapor above the mixture will be proportional to their resjjective partial vapor pressures. This relationship is expressed mathematically by Ecjuation 4.5, where N /Ny is the ratio of the mole fractions of X and Y in the vapor phase. The mole fraction of each component may be calculated from the equations N = Px/(Px + y)... 

Let us first consider binary mixtures which we shall term ordinary by this is meant that the liquid components dissolve in all proportions to form homogeneous solutions which are not necessarily ideal and that no complications of maximum or minimum boiling points occur. We shall consider component A of the binary mixture A-B as the more volatile, i.e., the vapor pressure of pure A at any temperature is higher than the vapor pressure of pure B. The vapor-liquid equilibrium for each pure substance of the mixture is of course its vapor-pressure-temperature relationship, as indicated in Fig. 7.1. For binary mixtures an additional variable, concentration, must likewise be considered. Mole fractions are the most convenient concentration terms to use, and throughout this discus-... 

Relative volatility is the volatility separation factor in a vapor-liquid system, i.e., the volatility of one component divided by the volatility of the other. It is the tendency for one component in a liquid mixture to separate upon distillation from the other. The term is expressed as fhe ratio of vapor pressure of the more volatile to the less volatile in the liquid mixture, and therefore g is always equal to 1.0 or greater, g means the relationship of the more volatile or low boiler to the less volatile or high boiler at a constant specific temperature. The greater the value of a, the easier will be the desired separation. Relative volatility can be calculated between any two components in a mixture, binary or multicomponent. One of the substances is chosen as the reference to which the other component is compared. 

All the refractory metals of Group IV and Group V form volatile suboxides at high temperatures. Just as the stability of carbon monoxide increases with an increase in temperature, these oxides also become more stable at higher temperatures. The vapor pressures of these suboxides can be calculated from the relationship ... 

Such a relationship describes how a chemical will partition between water and the atmosphere under equilibrium conditions and is appropriate only for dilute solutions which are typically observed in the environment. Certain hydrocarbons despite possessing relatively low vapor pressures, may tend to partition significantly toward the air. This is largely a result of their correspondingly low water solubilities which result in low values for Kw. Therefore, chemicals which have low values for Kw have a greater tendency to partition towards the air and volatilize from solution. 

Example 2.7. To show what form the energy equation takes for a two-phase system, consider the CSTR process shown in Fig. 2.6. Both a liquid product stream f and a vapor product stream F (volumetric flow) are withdrawn from the vessel. The pressure in the reactor is P. Vapor and liquid volumes are and V. The density and temperature of the vapor phase are and L. The mole fraction of A in the vapor is y. If the phases are in thermal equilibrium, the vapor and liquid temperatures are equal (T = T ). If the phases are in phase equilihrium, the liquid and vapor compositions are related by Raoult s law, a relative volatility relationship or some other vapor-liquid equilibrium relationship (see Sec. 2.2.6). The enthalpy of the vapor phase H (Btu/lb or cal/g) is a function of composition y, temperature T , and pressure P. Neglecting kinetic-energy and potential-energy terms and the work term,... 

Void formation and growth in addition curing composite laminates is primarily due to entrapped volatiles. Higher temperatures result in higher volatile pressures. Void growth will potentially occur if the void pressure (i.e., the volatile vapor pressure) exceeds the actual pressure on the resin (i.e., the hydrostatic resin pressure) while the resin is a liquid (Fig. 10.9). The prevailing relationship, therefore, is ... 

Equilibrium of headspace volatile lipids is influenced by a number of factors including the solubility of the component in the food matrix and the vapor pressure over the food system. This relationship may be very complex and must be determined experimentally by spiking the sample with known quantities of the compound of interest and subtracting background levels (Reineccius, 1993). 

Raoult s Taw gives the quantitative relationship between vapor pressure and solute concentration. Raoult s law states the vapor pressure of a volatile component of a solution (P) is equal to the vapor pressure of the pure substance (F°) times the mole fraction (x) of that substance. 

Gtxckel, W., Kastel, R., Lewerenz, J., Synnatschke, G. (1982) A method for determining the volatility of active ingredients used in plant protection. Part HI The temperature relationship between vapor pressure and evaporation rate. Pest. Sci. 13,161-168. 

Figure 3.10 shows the vapor pressure/composition curve at a given temperature for an ideal solution. The three dotted straight lines represent the partial pressures of each constituent volatile liquid and the total vapor pressure. This linear relationship is derived from the mixture of two similar liquids (e.g., propane and isobutane). However, a dissimilar binary mixture will deviate from ideal behavior because the vaporization of the molecules A from the mixture is highly dependent on the interaction between the molecules A with the molecules B. If the attraction between the molecules A and B is much less than the attraction among the molecules A with each other, the A molecules will readily escape from the mixture of A and B. This results in a higher partial vapor pressure of A than expected from Raoult s law, and such a system is known to exhibit positive deviation from ideal behavior, as shown in Figure 3.10. When one constituent (i.e., A) of a binary mixture shows positive deviation from the ideal law, the other constituent must exhibit the same behavior and the whole system exhibits positive deviation from Raoult s law. If the two components of a binary mixture are extremely different [i.e., A is a polar compound (ethanol) and B is a nonpolar compound (n-hexane)], the positive deviations from ideal behavior are great. On the other hand, if the two liquids are both nonpolar (carbon tetrachloride/n-hexane), a smaller positive deviation is expected.

The volatility of agent VX is relatively low (vapor pressure 0.0007 mm HG (DA, 1974 MacNaughton and Brewer, 1994). A vapor concentration of 10.5 mg/m has been reported for a temperature of 25°C (DA, 1974) (although not adequately described in the reference, this is presumably the saturation concentration above a pure liquid). Because VX does not absorb UV radiation above 290 mn (Rewick et al., 1986), photodegradation is not a significant environmental fate process. Based on structure-activity relationships, VX is predicted to react in the troposphere with photochemically produced hydroxyl radicals, with a half-hfe estimated to be 0.24 days (Atkinson, 1987). 

If a gas phase is present, chemical species may volatilize from the liquid or solid phase, which is an important partitioning process in a variety of circumstances (e.g., transport in the unsaturated zone, or for treatment processes). The equilibrium vapor pressure can be used with the ideal gas law to estimate the mass in a given volume and temperature under equilibrium conditions. For solutions with more than one component, Raoult s law can be used to quantify the vapor pressure of each component. For dilute aqueous solutions, Henry s law describes the equilibrium relationship between dissolved chemicals and their vapor pressure ... 

In order to determine the activity of a component in solution, one must measure its vapor pressure. In the case of volatile liquids such as those discussed in most of this chapter, vapor pressure measurement is not a problem so that very accurate determination of activity is possible over the whole composition range for which a solution is formed. However, many solutes, for example, most solids, have negligible vapor pressures. Under these circumstances, one makes use of the Gibbs-Duhem relationship between the activities of the two-components in solution. Since the vapor pressure of the solvent can be measured, its activity can be determined, and then used to estimate the activity of the solute. 

The material in this chapter explains the relationship between the concentration of a solution component and its activity. The activity is monitored through the vapor pressure of the components, which are volatile for most of the examples considered. Thus, it is very easy to understand why the activity of a given component can also be defined as its escaping tendency. It is obvious from the fact that most solutions are non-ideal that the relationship between activity and concentration is not simple. When the solution is very dilute, Henry s law holds for the solute and Raoult s law for the solvent. Then the activity is proportional to the concentration over a finite concentration range which must be determined for each system. 

By factoring out solute volatility, the enhancement factor allows comparison of solvent and secondary solute effects. Empirically, there is a linear relationship between the log of the enhancement factor and solvent density. For nonpolar and polar solutes in supercritical carbon dioxide, plots of enhancement factor coincide, indicating that differences in solubility are primarily due to vapor-pressure differences. Nonlinear behavior is noted in the case of high solubilities. The enhancement m pure fluids is relatively independent of solute structure but is sensitive to solvent polarity and density. 

Vapor pressure and volatility Less than nitroglycerine. Nitro isobutyl glycerine trinitrate is little volatile at room temperature without odor. It is slightly volatile at 30 °C with odor of tar and acridity. The volatility is increased at elevated temperature. It is obviously volatile at 50 °C. Its volatility at 25 °C is 0.127 x 10 mg/ cm/day. The relationship of the volatility of nitro isobutyl glycerine trinitrate and temperature is shown in Fig. 5.19 and its vapor pressures are listed in Table 5.49. 

The vapor pressure of a volatile solvent above a solution containing a nonvolatile solute is proportional to the solvent s concentration in the solution. This relationship is expressed quantitatively by Raoult s law, which states that the partial pressure exerted... 

This linear relationship between the total pressure, P, and the mole fraction, x, of the most volatile species is a characteristic of Raoult s law, as shown in Figure 7.18a for the benzene-toluene mixture at 90°C. Note that the bubble-point curve (P-x) is linear between the vapor pressures of the pure species (at x, = 0, 1), and the dew-point curve (P-yJ lies below it. When the (x, yi) points are graphed at different pressures, the familiar vapor-liquid equilibrium curve is obtained, as shown in Figure 7.18b. Using McCabe-Thiele analysis, it is shown readily that for any feed composition, there are no limitations to the values of the mole fractions of the distillate and bottoms products from a distillation tower. 

All of the 23 plasticizers in Table 18.1 occur as viscous or oily liquids that range from colorless to an amber color. If these liquids were spilled on soil or sediments, a portion of the liquid could volatilize into the air, depending on the specific compound, but most of the 23 plasticizers have vapor pressures that are less than 10 mm Hg at 25°C (Table 18.13). The vapor pressures of nine of the compounds have not been measured. For these plasticizers, vapor pressures were estimated using the Fragment Constant Method. As noted earlier, most of these chemicals will also be adsorbed by soil and sediments which would reduce the extent of volatiUzation. The rate of volatilization of plasticizers from soil has not been measured. For the purpose of illustration, the Dow Method was applied to estimate the half-life of each plasticizer if it was spilled on the surface of a dry soil. The Dow Method is a simple relationship that was derived for the evaporation of pesticides from bare soil ... 

This relationship is important in measurements of gases confined over volatile liquids. The evaporation of the liquid adds molecules of a new kind to the gas and makes up a part of its total pressure in accordance with Dalton s law. Fortunately for measurement purposes, the pressure contribution from such evaporation at constant temperature reaches a constant value called the VAPOR PRESSURE OF THE LIQUID. In laboratory practice, the confining liquid is often water. 

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