Surface tension, binary mixtures

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. 

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 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. ... 

SOL.2. I. Prigogine, La tension superficieUe des melanges binaires (Surface tension of binary mixtures), Bull Soc. Chim. Beiges 54, 286-302 (1945). 

SOL.39. 1. Prigogine and A. Englert-Chwoles, On the statistical theory of the surface tension of binary mixtures, II Nuovo Cimento 9, Suppl. 1, 347—355 (1958). 

It will be observed that the F, N curves for such binary mixtures follow the same course—a rapid followed by a more gentle rise of F as iV increases to a well defined maximum followed by a drop and an asymptotic fall in the F value. In the case of alcohol water mixtures F ax. is obtained at about 0 3A. To find an adequate explanation for the complete F, N curve is by no means an easy matter. It is clear that the first portion of the curve may be taken to represent an increasing surface concentration of alcohol and this proceeds to a limiting value—an observation first made by Milner (Phil. Mag. xill. 96, 1907), who showed that for relatively strong solutions of acetic acid the surface tension of the solutions could be expressed as a function of the concentration of the acetic acid in the following form ... 

Results for the various binary mixed surfactant systems are shown in figures 1-7. Here, experimental results for the surface tension at the cmc (points) for the mixtures are compared with calculated results from the nonideal mixed monolayer model (solid line) and results for the ideal model (dashed line). Calculations of the surface tension are based on equation 17 with unit activity coefficients for the ideal case and activity coefficients determined using the net interaction 3 (from the mixed micelle model) and (equations 12 and 13) in the nonideal case. In these calculations the area per mole at the surface for each pure component, tOj, is obtained directly from the slope of the linear region in experimental surface tension data below the cmc (via equation 5) and the maximum surface pressure, from the linear best fit of... 

Good agreement with the model is seen in figure 4 for "the binary mixture of. 10 4 and SDS, with predicted nonideal surface tension behavior similar to that seen for C gPO and SDS in figure 1. 

In a previous publication ( ), results were presented on the micellar properties of binary mixtures of surfactant solutions consisting of alkyldimethylamine oxide (C12 to Cig alkyl chains) and sodium dodecyl sulfate. It was reported that upon mixing, striking alteration in physical properties was observed, most notably in the viscosity, surface tension, and bulk pH values. These changes were attributed to 1) formation of elongated structures, 2) protonation of amine oxide molecules, and 3) adsorption of hydronium ions on the mixed micelle surface. In addition, possible solubilisation of a less soluble 1 1 complex, form between the protonated amine oxide and the long chain sulfate was also considered. 

During the past few years, the determination of the interfacial properties of binary mixtures of surfactants has been an area in which there has been considerable activity on the part of a number of investigators, both in industry and in academia. The Interest in this area stems from the fact that mixtures of two different types of surfactants often have interfacial properties that are better than those of the individual surfactants by themselves. For example, mixtures of two different surface-active components sometimes reduce the interfacial tension at the hydrocarbon/water interface to values far lower than that obtained with the individual surfactants, and certain mixtures of surfactants are better foaming agents than the individual components. For the purpose of this discussion we define synergism as existing in a system when a given property of the mixture can reach a more desirable value than that attainable by either surface-active component of the mixture by itself. 

Synergism in surface tension reduction efficiency. The efficiency of surface tension reduction by a surfactant is defined (9) as the solution phase concentration required to produce a given surface tension (reduction). Synergism in this respect is present in a binary mixture of surfactants when a given surface tension (reduction) can be attained at a total mixed surfactant concentration lower than that required of either surfactant by itself. This is illustrated in Figure 2. 

Figure 3. Synergism in surface tension reduction efficiency for some binary surfactant mixtures.

In order to elucidate the effect of alkyl alcohol on the surface adsorption of CyFNa and C,oSNa, it is useful to calculate the surface molecular interaction parameters of the binary surface active mixtures (y8,) according to the equation at constant surface tension and constant ionic strength (13,14,10) ... 

In the case of surface adsorption, at constant surface tension ofsolutions, an equation of the same form as equation 12 is obtained for the binary surfactant mixture system (15,16) ... 

Several other empirical relations for diffusion coefficients have been suggested Olson and Walton (01) have devised a means for estimating diffusion coefficients of organic liquids in water solution from surface-tension measurements. Hill (H5) has proposed a method based on Andrade s theory of liquids which allows for the concentration dependence of the diffusion coefficient in a binary liquid mixture. The formula of Arnold (A2, T6, p. 102) does not seem generally useful inasmuch as it contains two constants ( abnormality factors ) characteristic of the solute and of the solvent. 

The salt effects of potassium bromide and a series office symmetrical tetraalkylammonium bromides on vapor-liquid equilibrium at constant pressure in various ethanol-water mixtures were determined. For these systems, the composition of the binary solvent was held constant while the dependence of the equilibrium vapor composition on salt concentration was investigated these studies were done at various fixed compositions of the mixed solvent. Good agreement with the equation of Furter and Johnson was observed for the salts exhibiting either mainly electrostrictive or mainly hydrophobic behavior however, the correlation was unsatisfactory in the case of the one salt (tetraethylammonium bromide) where these two types of solute-solvent interactions were in close competition. The transition from salting out of the ethanol to salting in, observed as the tetraalkylammonium salt series is ascended, was interpreted in terms of the solute-solvent interactions as related to physical properties of the system components, particularly solubilities and surface tensions. 

Tables IV-XVI show that the tetraalkylammonium salts have a large effect on both solvents in the binary solvent mixture, especially the larger tetraalkylammonium bromides, i.e., (n-C3H7)4NBr and (n-C4Hg)4NBr. This can be seen from consideration of the boiling temperature alone. This observation is also borne out by the surface tensions and solubilities at 25°C of the individual salts studied, the results of which are tabulated in Table XVII in water, in ethanol, and in an ethanol-water mixture at x = 0.206. For the higher homologs of the R4NBr series, these salts exert a large effect on the surface tensions of the solvent systems studied and show a marked increase in their solubility in ethanol.

In practice, sedimentation volume experiments are performed with binary liquid mixtures as the suspending liquids, in order to have a sufficiently large range of surface tension and to be able to adjust the liquid surface tension to any specific value. 

The -maxima and minima on viscosity-composition curves are reminiscent of those on vapour pressure-composition curves of binary, mixtures. 5 The vapour pressures and viscosities are equal at some temperatures, say T and To, and T and To respectively. Then To/T—To7T =C(T —T), where C is a constant. A plot of TojT—To IT against T—T gives a straight line in many cases, both for vapour pressure and viscosity in other cases, the vapour pressure shows a minimum and the viscosity a maximum, and the vapour pressure a maximum and the viscosity a minimum. Prasad, 6 from the relation with vapour pressure deduced the equation rj =rjjrio= +ac, where c=conc. of non-electrolyte. The theoretical value of a is 0 00652 the observed values were glucose 0 44, fructose 0 44, sucrose 0 78, independent of temperature. According to Errera, the curves depend on the electric dipolarity of the liquids if both are nonpolar, the curve is concave to the composition axis whilst if both are polar, it is convex. Wolkowa found that the viscosity of a solution is approximately proportional to its heat of dilution. There seems to be no relation between the viscosity and surface tension of a mixture of acetic acid and water (cf. salt solutions, 13.VIII E). Mixtures of isomorphous substances obey an approximately linear relation. 

A correlation of surface tension and vapour pressure of binary liquid mixtures was attempted by Worley,2 who thought they changed in opposite directions, but this relation was said not to hold for chloroform and methyl alcohol. De Carvalho found dplp=—kdajRT(fc=const.), hence ... 

Values closer to 2-12 are found if allowance is made for dissociation in calculating M. This would indicate that no change in association is produced by the presence of the electrolyte in the water. The Eotvos constants of binary mixtures seem to depend on the concentration and temperature. The effect of temperature is either (i) normal, when d[a MvY>mdt is about 2-1, or (ii) abnormal, when this coefficient is less than 2-1 ljut increases with temperature from these results, conclusions have been drawn as to the molecular weights of dissolved substances. Light (including ultraviolet) has no influence on the surface tension of solutions. ... 

In order to study molecular associations we may consider binary liquid mixtures whose constituents do not react chemically with one another and investigate the connexion between the polarity of the molecules of the components and other physico-chemical properties such as vapour pressure, viscosity, surface tension, c. 

The surface tension of miscible binary mixtures 4.2a General features and thermodynamics... 

Surfaces of binary liquid mixtures are the simplest Gibbs monolayers that exist. We are considering the surface tension y as a function of the mole fraction X = x. It runs from the value y for pure component 1 to y for pure 2. that is from X = 0 to X = 1. Often such curves cae convex with respect to the x-cuds (as curves 2 and 3 in fig. 4.1), Implying the tendency of the Interface to be richer in the component with the lower y. At the same time this is the most volatile component. Such convex behaviour is the rule for mixtures of simple molecules, like liquid Ar, CH4, N2, CO, etc. but has also been observed for binary mixtures of the Kr-ethene-ethane triod ) and for molten salt mixtures ). Concave curves (2 in the figure) require the surface to be enriched by the component with the higher y, but such... 

Figure 4.1. Surface tension of binary miscible mixtures. Examples of trends (1), near-ideal (2) and (2 ) many organic mixtures (3) ethanol +water.

Figure 4.4. Surface tensions of binary mixtures according to the exchange principle. Monolayer approximation (eq. [4.2.17]) with fixed areas per molecule. Capacity factor (F ) corresponds to 0.4 nm /molecule. Exchange constant K is a measure of the relative preference of the two components for the surface according to [4.2.10]. Top curves plotted with respect to y s 0 bottom absolute value Ay of the deviation from linearity.

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