Surface tension mixtures

Surface tensions for aqueous solutions are more difficult to predict than those for nonaqueous mixtures because of the nonlinear dependence on mole fraction. Small concentrations of the organic material may significantly affect the mixture surface tension value. For many binary organic-water mixtures, the method of Tamura, Kurata, and Odanfi maybe used ... 

The surface tension is important for the calculation of mass transfer coefficients and the specific contact area (see Section 9.4.4). Depending on the availability of necessary parameters, the surface tension for a molecular species can be determined either with the simplest method of Hakim-Steinberg-Stiel or with a more complex DIPPR-method (see Ref. [52]). The mixture surface tension can be obtained via a mixing rule. A further extension to cover electrolyte mixtures is realized by the method of Onsager and Samaras (see Ref. [44]). The latter uses an additive term which can be estimated using the dielectric constant of the mixture and molar volumes of electrolytes. 

Because surface-active compounds are generally organic solutes, we can modify (17.A.33) to include the effects of surface-active, slightly soluble material (Shulman et al. 1996). Note that this correction affects only the Kelvin term. First, we assume that the mixture surface tension can be approximated as the mole fraction weighted average of the surface tensions of pure water and pure organic solute ... 

We can then use this mixture surface tension crmix in place of aw in the Am term. The resulting expression can be written in shorthand as... 

Cm = mixture surface tension, mN/m Xij = mole fraction of component i orj in the liquid mixture pLi.j = pure component liquid density of component t orj, kmol/m ... 

The first test was an assessment of surface activity of SML/ESMIS mixtures. Surface tension (o) measurements of their aqueous solutions at concentrations of 0.001%, 0.01%, 0.1%, 1%, 2%, and 4 wt% were made. A TDl Lauda tensiometer was used. The test was carried out at 20°C. The dependences of surface tension of aqueous SML/ESMIS solutions vs. concentration are presented in fig. 18.2. 

Pretest predictions were performed based on Equation (3.16). To do so, knowledge of mixture surface tension, mixture contact angle, and effective pore diameters is required. Here, pore diameters were based on Method 1 in Table 4.2, pure reference fluid bubble point tests. Mixture surface tension for all mass fractions was estimated from an equation of state and Langmuir isotherm (1916) fit to data available in the literature. Mixture contact angles were measured as a function of methanol mass fraction. 

Liquid/Vapor Surface Tension of the Methanol/Water Mixture Surface tension values were previously measured at different mass fractions of methanol in water at constant temperature from 293 to 323 K, and are available in the literature (Vazquez et al., 1995). Bubble point tests here were conducted over a colder range of temperatures ( 275-295 K), so measurements from Vazquez et al. (1995) at 293 K are used to... 

FIGURE 4.9 Binary Mixture Bubble Point Predictions as a Function of (a) Mixture Surface Tension and (b) Methanol Mass Fraction. 

The liquid densities may be assumed to be benzene -0.879 g/cm toluene -0.866 g/cm. The mixture surface tension may be assumed to be -20 dyne/cm in both cases. The tray spacing is 12 inches. 

We have considered the surface tension behavior of several types of systems, and now it is desirable to discuss in slightly more detail the very important case of aqueous mixtures. If the surface tensions of the separate pure liquids differ appreciably, as in the case of alcohol-water mixtures, then the addition of small amounts of the second component generally results in a marked decrease in surface tension from that of the pure water. The case of ethanol and water is shown in Fig. III-9c. As seen in Section III-5, this effect may be accounted for in terms of selective adsorption of the alcohol at the interface. Dilute aqueous solutions of organic substances can be treated with a semiempirical equation attributed to von Szyszkowski [89,90]... 

It was noted in connection with Eq. III-56 that molecular dynamics calculations can be made for a liquid mixture of rare gas-like atoms to obtain surface tension versus composition. The same calculation also gives the variation of density for each species across the interface [88], as illustrated in Fig. Ill-13b. The density profiles allow a calculation, of course, of the surface excess quantities. 

It has been pointed out [138] that algebraically equivalent expressions can be derived without invoking a surface solution model. Instead, surface excess as defined by the procedure of Gibbs is used, the dividing surface always being located so that the sum of the surface excess quantities equals a given constant value. This last is conveniently taken to be the maximum value of F. A somewhat related treatment was made by Handa and Mukeijee for the surface tension of mixtures of fluorocarbons and hydrocarbons [139]. 

Calculate, using the data of Fig. III-9a and Eq. III-53, the surface tension versus mole fraction plot for mixtures of cyclohexane and benzene. 

What is the critical surface tension for human skin Look up any necessary data and make a Zisman plot of contact angle on skin versus surface tension of water-alcohol mixtures. (Note Ref. 136.)... 

This database provides thermophysical property data (phase equilibrium data, critical data, transport properties, surface tensions, electrolyte data) for about 21 000 pure compounds and 101 000 mixtures. DETHERM, with its 4.2 million data sets, is produced by Dechema, FIZ Chcmic (Berlin, Germany) and DDBST GmhH (Oldenburg. Germany). Definitions of the more than SOO properties available in the database can be found in NUMERIGUIDE (sec Section 5.18). 

Surface tension is usually predicted using group additivity methods for neat liquids. It is much more difficult to predict the surface tension of a mixture, especially when surfactants are involved. Very large molecular dynamics or Monte Carlo simulations can also be used. Often, it is easier to measure surface tension in the laboratory than to compute it. 

Revised material in Section 5 includes an extensive tabulation of binary and ternary azeotropes comprising approximately 850 entries. Over 975 compounds have values listed for viscosity, dielectric constant, dipole moment, and surface tension. Whenever possible, data for viscosity and dielectric constant are provided at two temperatures to permit interpolation for intermediate temperatures and also to permit limited extrapolation of the data. The dipole moments are often listed for different physical states. Values for surface tension can be calculated over a range of temperatures from two constants that can be fitted into a linear equation. Also extensively revised and expanded are the properties of combustible mixtures in air. A table of triple points has been added. 

Larch gum is readily soluble in water. The viscosity of these solutions is lower than that of most other natural gums and solutions of over 40% soHds are easily prepared. These highly concentrated solutions are also unusual because of their Newtonian flow properties. Larch gum reduces the surface tension of water solutions and the interfacial tension existing in water and oil mixtures, and thus is an effective emulsifying agent. As a result of these properties, larch gum has been used in foods and can serve as a gum arabic substitute. 

Vinyl acetate is a colorless, flammable Hquid having an initially pleasant odor which quickly becomes sharp and irritating. Table 1 Hsts the physical properties of the monomer. Information on properties, safety, and handling of vinyl acetate has been pubUshed (5—9). The vapor pressure, heat of vaporization, vapor heat capacity, Hquid heat capacity, Hquid density, vapor viscosity, Hquid viscosity, surface tension, vapor thermal conductivity, and Hquid thermal conductivity profile over temperature ranges have also been pubHshed (10). Table 2 (11) Hsts the solubiHty information for vinyl acetate. Unlike monomers such as styrene, vinyl acetate has a significant level of solubiHty in water which contributes to unique polymerization behavior. Vinyl acetate forms azeotropic mixtures (Table 3) (12). 

Isoxazole dissolves in approximately six volumes of water at ordinary temperature and gives an azeotropic mixture, b.p. 88.5 °C. From surface tension and density measurements of isoxazole and its methyl derivatives, isoxazoles with an unsubstituted 3-position behave differently from their isomers. The solubility curves in water for the same compounds also show characteristic differences in connection with the presence of a substituent in the 3-position (62HC(17)1, p. 178). These results have been interpreted in terms of an enhanced capacity for intermolecular association with 3-unsubstituted isoxazoles as represented by (9). Cryoscopic measurements in benzene support this hypothesis and establish the following order for the associative capacity of isoxazoles isoxazole, 5-Me, 4-Me, 4,5-(Me)2 3-Me> 3,4-(Me)2 3,5-(Me)2 and 3,4,5-(Me)3 isoxazole are practically devoid of associative capacity. 

In general, the surface tension of a Hquid mixture is not a simple function of the pure component surface tensions because the composition of the mixture surface is not the same as the bulk. For nonaqueous solutions of n components, the method of Winterfeld, Scriven, and Davi is apphcable ... 

Example 42 Estimate surface tension of a mixture. At 298.15 K, Daiibert et al. " report the hqiiid density of n-pentane to be 8.617 kmol/nd and its surface tension to be 15.47 mN/m. From the same source, the corresponding values for dichloromethane are 15.52 kmol/m and 27.22 mN/m. Using Eqs. (2-170) and (2-169) for a mixture of 0.1606 mole fraction n-pentane and 0.8394 mole fraction dichloromethane ... 

The bulk properties of mixed solvents, especially of binary solvent mixtures of water and organic solvents, are often needed. Many dielectric constant measurements have been made on such binary mixtures. The surface tension of aqueous binary mixtures can be quantitatively related to composition. ... 

---