Henry constant, surface activity

Many simple systems that could be expected to form ideal Hquid mixtures are reasonably predicted by extending pure-species adsorption equiUbrium data to a multicomponent equation. The potential theory has been extended to binary mixtures of several hydrocarbons on activated carbon by assuming an ideal mixture (99) and to hydrocarbons on activated carbon and carbon molecular sieves, and to O2 and N2 on 5A and lOX zeoHtes (100). Mixture isotherms predicted by lAST agree with experimental data for methane + ethane and for ethylene + CO2 on activated carbon, and for CO + O2 and for propane + propylene on siUca gel (36). A statistical thermodynamic model has been successfully appHed to equiUbrium isotherms of several nonpolar species on 5A zeoHte, to predict multicomponent sorption equiUbria from the Henry constants for the pure components (26). A set of equations that incorporate surface heterogeneity into the lAST model provides a means for predicting multicomponent equiUbria, but the agreement is only good up to 50% surface saturation (9). 

TWO types of physical loss processes should be considered In the external removal of a chemical species. First, the species of Interest may be lost to the atmosphere through water-air exchange. This process depends on the Henry s Law constant for the chemical species, as well as the atmospheric concentration and the structure of the surface microlayers (. Wind stress and turbulence of the water body surface have a pronounced effect, especially for surface-active materials for which bubble scavenging and surface film ejection as aerosol takes place. Transfer rates at the air-water interface are complex problems In themselves and are not dealt with In this Chapter. 

Yun and coworkers studied the adsorption equilibrium of dichloromethane, trichlomethane, and trichloroethylene on activated carbons using static volumetric technique. The adsorption isotherms were measured for the pure vapors in the temperature range 283 to 683 K and at different pressures. The Dubinin-Redushkevich equation was applied to experimental adsorption isotherms to determine thermodynamic properties such as heat of adsorption and the Henry constant. The value of isosteric heat of adsorption was found to vary with surface loading. Li et al. ... 

An overview of some basic mathematical techniques for data correlation is to be found herein together with background on several types of physical property correlating techniques and a road map for the use of selected methods. Methods are presented for the correlation of observed experimental data to physical properties such as critical properties, normal boiling point, molar volume, vapor pressure, heats of vaporization and fusion, heat capacity, surface tension, viscosity, thermal conductivity, acentric factor, flammability limits, enthalpy of formation, Gibbs energy, entropy, activity coefficients, Henry s constant, octanol—water partition coefficients, diffusion coefficients, virial coefficients, chemical reactivity, and toxicological parameters. 

It is a well-known fact that upon covalent immobilization at the surface, the dissociation constant of the acid-base indicator changes by as much as 3 pK units. This shift clearly illustrates the dramatic effect that the interphase has on the ionization equilibria. It is perhaps the most serious problem with optical sensors that the surface concentration of any species is related to its corresponding bulk activity value through an adsorption isotherm which, with the exception of Henry s law, is a highly nonlinear and variable relationship. It is also known (Davies and Rideal, 1963) that the surface pH is different from the bulk value due to the electrostatic repulsion. 

Air-saturated water has a dissolved O2 concentration dependent on temperature, the Henry s law constant and ionic strength. In pure water at 0 °C, O2 saturation is 450 p,M at 25 °C, saturation falls to 270 p,M. Other solutes reduce O2 solubility, such that at normal seawater salinities, O2 saturation is reduced by —25%. Seawater is, of course, rarely at perfect O2 saturation. Active photosynthesis may locally increase O2 production rates, resulting in supersaturation of O2 and degassing to the atmosphere. Alternately, aerobic respiration below the sea surface can consume dissolved O2 and lead to severe 02-depletion or even anoxia. 

The picture in Fig. 2 may be considered as the distribution of adsorption activity over the surface of an amorphous oxide. It shows that the contribution to the Henry s Law constant (which is a measure of adsorption activity for the entire surface) comes not from all parts of the surface as would be the case for a homogeneous surface such as the graphite basal plane but from special places corresponding to the sharp peaks in Fig. 2 which well may be called adsorption sites . The concept of adsorption sites... 

Releases of carbon disulfide to the environment as a result of industrial activity are expected to be primarily to the atmosphere. Any carbon disulfide released to surface waters in effluent streams is expected to partition rapidly to the atmosphere as a result of the high ratio of vapor pressure to the solubility (Henry s law constant = 1.01 x 10"2 atm m3/mol) of the compound. Hydrolysis is not a significant removal mechanism since the evaporation half-life from a saturated solution is estimated to be 11 minutes (EPA 1978a). 

Fischer et al, [122] proposed a model to predict the adsorption isotherm of krypton in porous material at supercritical temperature. In their study, a model pore of infinite length is formed by concentric cylindrical surfaces on which the centers of solid atoms are located. The interaction between an adsorbate and an individual center on the pore wall is described by the LJ 12-6 theory, and the overall potential is the integral of this interaction over the entire pore surface. With thermodynamic relations, Fischer et al. obtained the functional dependence of the saturation adsorption excess and the Henry s law constant on the pore structure. The isotherm was then produced by the interpolation between Henry s law range and saturation range. They tested their theory with the adsorption of krypton on activated carbon. It was shown that, with information on the surface area of the adsorbent and thermodynamic properties of the adso bate, their model gives more than quantitative agreement with experimental data. If a few experimental data such as the Henry s law constant at one temperature are available, the isotherms for all temperatures and pressures can be predicted with good quality. 

It is more convenient, however, to compare reaction rates at a constant concentration in the solution. The activation energy determined under such conditions is known as the apparent value and represents the energy of activation of the elementary act itself as well as heat change of the preceding equilibrium. At low surface concentration C of a reagent (in the Henry s isotherm region), C varies with volume concentration C and exponentially depends on the heat of adsorption... 

The diffusion and permeabihty are closely interconnected with the solubility of a polymer. The permeation of the permeants through polymeric membrane film occurs in three stages (1) Sorption includes the initial adsorption, absorption, penetration, and dispersal of penetrant into the voids of the polymer membrane surface and cluster formation. The distribution of permeant in the membrane may depend on penetrant size, concentration, temperature, and swelling of the matrix as well as on time. The extent to which permeant molecules are sorbed and their mode of sorption in the polymer depends upon the enthalpy and entropy of permeant-polymer mixing, i.e., upon the activity of the permeant within the polymer at equilibrium. When both polymer-permeant and permeant-permeant interactions are weak relative to polymer-polymer interactions, i.e., dilute solution occurs, Henry s law is obeyed. The solubihty coefficient S is a constant independent of sorbed concentration at a given temperature. (2) Diffusion includes the transfer of the penetrant through the polymer membrane which depends on penetrant concentration that leads to a plasticization effect, penetrant size and shape, polymer Tg, time, and temperature. The diffusion coefficient is determined by Pick s first law of diffusion. (3) Desorption includes release of the penetrant from the opposite side of the membrane face. 

The film surface, x = 0, is assumed to be at equilibrium with the gas phase, expressed through a dimensionless partition or distribution coefficient m (g/m )/(g/m ). m can be extracted from Henry s constants (see Table 6.1) or from activity coefficients (see Illustration 6.14). Some typical values appear in Table 4.7. 

These QSPR methods can be used to predict any chemical property that is dependent upon molecular structure. Publications are available that provide examples of uses in a wide variety of applications for chemical or physical property estimation or biological activity estimation. Our group has published papers on property estimation dealing with normal boiling points, critical temperatures, surface tension, Henry s law constants, aqueous solubility, supercritical CO2 solubility, autoignition temperature, gas chromatographic retention times, and ion mobility constants. Several specific examples are given as illustrations of the capabilities of the method. 

---