Tension, surface, pure aqueous solutions

Adipic (aa) and succinic (sa) acid surface tension in 2 wt% aqueous NaCl solutions was studied by Heiming et al. [166]. Three different mixtures (in mass%) of the organics and salt were tested between 273 and 306 K 93%aa/5%sa, 80% aa/18%sa, and 5%aa/93%sa. The concentrations of organics were chosen to correspond to those at the moment of droplet activation for dry particles with d = 50, 100 nm (mixtures 1 and 2), and d = 40, 50, and 100 nm (mixmre 3). All mixtures showed a linear dependence of surface tension on temperature, and pure adipic acid was fotmd to cause more surface tension depression than pure succinic acid. Surface tension depression in this non-reactive system was described very well by a linearly additive model based on the S-L equation. 

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]... 

In the case of liquid/liquid interfaces we have the experiments of W. C. McC. Lewis (1908), who examined the relations at the surface of separation between an aqueous solution and paraffin oil or mercury. If o-, a are the surface tensions between paraffin oil and pure water and the solution, respectively, it was found that cr < [Pg.439]

The CMC of commercial AOS and other surfactants at 40°C has been determined by Gafa and Lattanzi [6] who plotted the surface tension of aqueous surfactant solutions against concentration. The surface tensions were determined with the ring method according to du Nouy. Table 5 gives their CMC values in mmol/L and the surface tension at the CMC in mN/m. Table 5 also contains CMC values of isomerically pure sodium alkyl sulfates, sodium alkylbenzene-sulfonates, sodium hydroxyalkanesulfonate, and sodium alkenesulfonates at 40°C, taken from the literature [39 and references cited therein]. 

All aqueous solutions of anionic or cationic dyes have lower surface tension than pure water. Solutions of the free sulphonic acids give lower values than solutions of the sodium salts of anionic dyes. The decrease in surface tension depends more on the degree of sulphonation of the dye than on its relative molecular mass, although alkyl substituents may... 

In Table 3 are the values of surface tension for the aqueous LAS homolog solutions. Values of molar volume used are those for the pure LAS homolog independent of water. The justification for this comes from the Winsor R model (20, 21) and work by Scriven and Davis (30) who showed that accurate CED values can be obtained from a statistical mechanical treatment of an interface using only 2 or 3 atomic or molecular layers of that interface. For a surfactant solution, the surfactant will predominate in the interface, hence the choice of pure LAS for the solution molar volumes. 

It is a well-known fact that bubble sizes in aqueous electrolyte solutions are much smaller than in pure water with equal values of viscosity, surface tension, and so on. This can be explained by the electrostatic potential of the resultant ions at the liquid surface, which reduces the rate of bubble coalescence. This fact should be remembered when planning experiments on bubble sizes or interfacial... 

In several previous papers, the possible existence of thermal anomalies was suggested on the basis of such properties as the density of water, specific heat, viscosity, dielectric constant, transverse proton spin relaxation time, index of refraction, infrared absorption, and others. Furthermore, based on other published data, we have suggested the existence of kinks in the properties of many aqueous solutions of both electrolytes and nonelectrolytes. Thus, solubility anomalies have been demonstrated repeatedly as have anomalies in such diverse properties as partial molal volumes of the alkali halides, in specific optical rotation for a number of reducing sugars, and in some kinetic data. Anomalies have also been demonstrated in a surface and interfacial properties of aqueous systems ranging from the surface tension of pure water to interfacial tensions (such as between n-hexane or n-decane and water) and in the surface tension and surface potentials of aqueous solutions. Further, anomalies have been observed in solid-water interface properties, such as the zeta potential and other interfacial parameters. 

First let us note that experiment revealed long ago that not all ions prefer the bulk to the interface [8]. Gibbs adsorption equation predicts that the surface tension increases with the electrolyte concentration when the total surface excess of ions is negative. The conventional picture, that the ions prefer the bulk, is probably due to Langmuir, who noted that the increase in the surface tension of aqueous solutions of simple salts with increasing concentration can be explained by assuming a surface layer of pure solvent with a thickness of about 4 A [9]. However, because the aqueous solutions of some simple acids (such as HC1) possess surface tensions smaller than that of pure water [8], Gibbs adsorption equation indicates a positive total... 

Surfactant surface activity is most completely presented in the form of the Gibbs adsorption isotherm, the plot of solution surface tension versus the logarithm of surfactant concentration. For many pure surfactants, the critical micelle concentration (CMC) defines the limit above which surface tension does not change with concentration, because at this stage, the surface is saturated with surfactant molecules. The CMC is a measure of surfactant efficiency, and the surface tension at or above the CMC (the low-surface-tension plateau) is an index of surfactant effectiveness (Table XIII). A surfactant concentration of 1% was chosen where possible from these various dissimilar studies to ensure a surface tension value above the CMC. Surfactants with hydrophobes based on methylsiloxanes can achieve a low surface tension plateau for aqueous solutions of —21-22 mN/m. There is ample confirmation of this fact in the literature (86, 87). 

The surface tension of most concentrated aqueous solutions of inorganic salts, such as those employed in OD as strip solutions, is considerably greater than that of pure water. Intrusion of these solutions into microporous, hydrophobic membranes of the types used in OD is, therefore, unlikely under moderate operating pressures. However, some aqueous feeds contain amphiphilic components that may depress the liquid surface tension, and thereby reduce the critical penetration pressure. In such cases it may be necessary to use a membrane with a pore diameter of less than 0.1 p, to prevent liquid intrusion. For most applications however, membranes with a nominal pore diameter of 0.2 p have been found to be suitable. 

Surface tensions of aqueous solutions of inorganic electrolytes are well known to be larger than the surface tension of pure water at the same temperature. The surface tension of 2 M (10.5 mass%) sodium chloride solution is about 3.3 mN/m larger than the surface tension of... 

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