Gerald L. Pollack

Why gases dissolve in liquids

The thermodynamics and statistical mechanics of solubility are fairly well understood. It is still very difficult, however, to make quantitative predictions of solubility for real systems from first principles. The purposes of this article are to present the results of solubility experiments in some prototype solute-solvent systems, to show how far they may be understood from molecular first principles, and to discuss some of the things that are still missing. The main systems used as examples have the inert gas xenon as solute and some simple organic liquids as solvents.

Pressure dependence of the solubility of nitrogen, argon, krypton, and xenon in water

Measurements are reported for the pressure dependence of the Ostwald L and mole‐fraction x2 solubilities for nitrogen, argon, krypton, and xenon in water at 25.0 °C. The approximate pressure ranges were: N2, 44–116 atm; Ar, 22–101 atm; Kr, 33–81 atm; and Xe, 5–48 atm. Our experimental technique is a modified Van Slyke method which allows the simultaneous determination of L, x2, and partial molar volume v2 of the solute gas in the solvent. For N2, Ar, and Kr we observe deviations from Henry’s Law, f2=kHx2 which vary linearly with pressure. For example, the measured values of Henry’s constant range from kH (25 °C, 44.6 atm)=90 269 atm to kH (25 °C, 115.8 atm)=100 340 atm for N2 in water. Extrapolation of the data to low pressure yields results for L and x2 which agree with literature values. For Xe we observe a sharp increase in Henry’s constant at pressures above 20 atm. The measured partial molar volume also changes sharply from 47 to ∼125 cm3/mol. This is attributed to the observed onset of ice‐like clat...

Solubility of xenon in liquid n‐alkanes: Temperature dependence and thermodynamic functions

Measurements of the Ostwald solubility as a function of temperature L(T) are reported for 133Xe gas in liquid n‐alkanes. All the alkanes from n‐C5H12 through n‐C20H42 were investigated and the temperatures ranged from 10.0 to 50.0 °C. The experimental method is recently developed and has some unique aspects. From the temperature dependent results the chemical potential Δμ20(T), the enthalpy ΔH20, and the entropy ΔS20 of solution were determined. Values for Δμ20(T) ranged from 2546 cal/mol for n‐C7H16 at 50.0 °C to 1811 cal/mol for n‐C14H30 at 10.0 °C. The range of ΔH20 was from −2818 cal/mol for n‐C5H12 to −2117 cal/mol for n‐C20H42, and the corresponding range in ΔS20 was −16.90 to −13.58 cal/mol K. A Barclay–Butler plot of the thermodynamic functions and a plot of the dependence of solubility on the Hildebrand solubility parameter for this system are displayed. All of the data obtained in this experiment may be given to better than 0.8% by the empirical relation Δμ20=−RT ln x2=−0.0637nT+2.575×10−2T2...

Diffusion of xenon in liquid alkanes: Temperature dependence measurements with a new method. Stokes–Einstein and hard sphere theories

Measurements are reported of the diffusion constant D(T) for xenon gas, in the form of the radioisotope 133Xe, through liquid n‐octane, n‐decane, and n‐tetradecane, in the range 10–40 °C. The values range from D (10.0 °C, Xe→n‐C14H30)=1.32×10−5 cm2/s to D (40.0 °C, Xe→n‐C8H18)=6.02×10−5 cm2/s. A new experimental method is used in which D is obtained by monitoring the decrease in concentration as gas diffuses into the liquid in an effectively one‐dimensional geometry. As expected, the results do not agree with the Stokes–Einstein law. They do follow the usual correlation Dηp=AT, with p=0.708 and A=9.80×10−8, where η is the liquid viscosity in centipoises and T is in K. Application to these results of the rough‐hard‐sphere theory of diffusion is discussed. A quantitative analysis cannot be made until molecular dynamics results for smooth‐hard‐sphere diffusion are available.

Solubility of xenon in 45 organic solvents including cycloalkanes, acids, and alkanals: Experiment and theory

New measurements are reported of the Ostwald solubility L(T), as a function of temperature in the approximate range 10.0–50.0 °C, for 133Xe gas in 13 liquid organic solvents, viz., three cycloalkanes, six carboxylic acids, and four normal alkanals. From our data for each solute–solvent system we determine the mole‐fraction solubility x2(T), and the following thermodynamic functions of solution: chemical potential Δμ0ρ2(T)=−RT ln L, enthalpy ΔH0ρ2, and entropy ΔS0ρ2, where Δμ0ρ2=ΔH0ρ2 −TΔS0ρ2S0ρ2, all based on the number density scale. New results are considered together with previous measurements of xenon solubility in liquid normal alkanes, alkanols, and perfluoroalkanes; in all, data and theory are treated for xenon solubility in 45 organic solvents from six homologous series. The average observed entropy of solvation of Xe is ΔS0ρ2=−4.1± 0.5 cal/mol K, remarkably independent of solvent. The results are analyzed with scaled‐particle theory from which are obtained hard‐core diameters a1, and cavity...

Solubility of xenon in liquid n‐alkanols: Thermodynamic functions in simple polar liquids

Measurements of the Ostwald solubility L(T) as a function of temperature in the range 10.0–50.0 °C are reported for 133Xe gas in 13 liquid normal alcohols: methanol through n‐dodecanol, and n‐tetradecanol. From the data for each solute–solvent system we find the mole fraction solubility x2(T), and calculate the following thermodynamic functions of solution: chemical potential Δμ○2(T) =−RT ln x2, enthalpy ΔH○2, and entropy ΔS○2, all based on the mole fraction scale. We also calculate directly from the Ostwald solubility the corresponding quantities based on the number density scale. These quantities are the chemical potential Δμ○ρ2(T) =−RT ln L, enthalpy ΔH○ρ2, and entropy ΔS○ρ2. The solute was chosen as an inert monatomic gas and the solvents were chosen in a polar homologous series in order to construct prototype solute–solvent systems. Solubility results and the associated thermodynamic functions are analyzed and compared with simple models.

Solubility of xenon in n‐alkanes: n‐pentane through n‐hexadecane

Measurements of the Ostwald solubilities (L) for 133Xe gas in the 12 liquid n‐alkanes from n‐pentane through n‐hexadecane at 20.0 °C are reported. The results range from L(133Xe→n‐C5H12) = 5.39 to L(133Xe→n‐C16H34) = 3.14. The corresponding values of the partial molar Gibbs energy Δμ20 range, respectively, from 2150 to 1920 cal/mol.

Solubility of xenon in perfluoroalkanes: Temperature dependence and thermodynamics

Measurements are reported of Ostwald solubility (L) and mole fraction solubility (x2) for Xe gas, in the form of the radioisotope 133Xe, in three liquid perfluoroalkanes from 5–25 or 5–50 °C. Results at 25.0 °C are L(133Xe in n‐C6F14)=2.11±0.01, L(133Xe in C7F16) =1.95±0.01, and L(133Xe in C8F18)=1.85±0.01. From these data the chemical potentials on the number density scale and mole fraction scale, respectively, Δμ0ρ2 and Δμ02, are calculated as functions of temperature. The corresponding average enthalpy and entropy of solvation, on both scales, are also determined. Results of scaled particle theory and of surface energy theory are compared with experimental thermodynamic quantities. The comparisons are suggestive but better data on solvent properties are needed for a definitive comparison.

Dielectric and Optical Properties of Crystalline Argon

Studies of the crystal optics, dielectric properties from 10 Hz to 10 kHz, and the electro‐optical properties of crystalline argon, grown with a modified Bridgman technique, are reported. The optical character of the solids was examined using polarized light, with varying crystal growth parameters, viz., rate, annealing, and pressure. Using crossed polaroids to examine the transparent crystals, three basic kinds of optical regions were observed: optically isotropic, optically anisotropic (birefringent), and sharp, horizontal, alternate isotropic and anisotropic striations. Low‐order strain birefringence with broad and diffuse patterns is also described. The observations are considered in terms of impurity segregation and strain effects and it is concluded, with support from annealing and electro‐optic studies up to 40 kV/cm, that the birefringent regions have hcp structure and the isotropic regions have fcc structure. The dielectric constant (e) was directly measured for solid argon between two concentric...

Solubility of inert gases in PFC blood substitute, blood plasma, and mixtures.

Measurements are reported of the solubility of nonreactive gases, e.g., hydrogen and xenon, in the following liquids: (a) Oxypherol (FC-43 emulsion) blood substitute, (b) blood plasma, (c) mixtures of Oxypherol and blood plasma, and (d) perfluorotributylamine. Typical results for Ostwald solubility at 25 degrees C for Xe gas in various liquids are 0.118 in H2O, 0.12 in blood plasma, and 1.51 in N(C4F9)3. Observed solubilities for the mixtures can be calculated from the relation: L(mixture) = L(emulsion)xv(emulsion) + L (plasma)xv(plasma), in which the vs are the volume fractions in the mixture. This linear relation implies that the gas dissolves independently in each liquid in the mixture. The effect of the emulsifier (Pluronic F-68, 2.6%), on gas solubility in the mixture, is small. Results for the temperature dependence of Ostwald solubility, L(T), in the range 10-37 degrees C are reported.