ABSTRACT
Surface tension is the oldest method used for the abovementioned purpose. It determines the cmc*, from the decrease on the surface tension, γ, as a function of [S]tot, measured with different tensiometric techniques, i.e., plate, ring, and/or sensible drop methods.[37] It is one of
Fig. 1 Values of the surface tension, γ, as a function of In C, at several constant molar fractions XMEGA−10, for the system MEGA−10+sodium deoxycholate. (From Ref. [38].)
Fig. 2 Experimental values of cmc as a function of molar fraction, αCTAB, for the system TX-100+CTAB. Dot line: ideal behavior. Solid line: Rubingh’s model predictions. (From Ref. [55].)
the most used methodologies in the mixed surfactant studies, given that it is valid both for ionic or nonionic systems. Fig. 1 shows, as an example, the experimental data of γ vs. In C, where C is the total surfactant concentration at several constant molar fractions for the mixed system MEGA-10+sodium deoxycholate in water. Breaks on the plot permit to determine the cmc* of the mixed aggregates.[38]
Surface tension has been used to obtain the cmc* for the following: 1) nonionic-nonionic surfactant systems, as MEGA10/CnE10 (n=12 or 16),[39] alquilylsulfinylethanol,[19] or alkylglycosides[40] with several nonionic surfactants; 2) nonionic-ionic
surfactant systems, as MEGA10 with sulfonate myristic acids,[41,42] bile salts,[38] Tween-80 with bile salts,[43] Tween-40 or Brij56 with C16PyC,[44] various CmEn with alkyl sulfates or sulfonates,[45] C12DMAO with SC12S,[46] several alkylglycosides with anionic, cationic, zwitterionic, and/or cationic gemini surfactants,[40,46-49] CmEn or nonionic saccharides with lithium octyl or dodecylsulfates, hydrocarbonfluorocarbon nonionic-ionic systems,[50,51] and some others;[52-55] and 3) ionic-ionic surfactant systems, such as anionic-anionic systems, i.e., SC12S with SC12S derivatives[56] or bile salts,[57] hydrocarbon-fluorocarbon systems,[58] or cationic-cationic systems, i.e., CnTAB with CnPyC, CnAC,[59,60] or several mixed CnAC,[61] and anioniczwitterionic systems, i.e., SC12S with C12-dimethylbetaine.[62,63]
Most of these experimental studies have been reported together with a theoretical analysis. By applying Rubingh’s model [Regular Solution Theory (RST)], [38-44,46-49,52-54,57,59-61,63-69] the β12 interaction parameter has been reported together with the micellar composition. This β12 parameter has been found to be negative in all the cases, with the only exception of the hydrocarbon/hydrocarbon mixtures.[50,51] Using Motomura’s model,[38,41,53,54,56,68] the micellar composition has been determined. In all the cases, the results have been interpreted in terms of the deviation from the ideal behavior (Clint’s model). Fig. 2 shows a plot of the cmc* vs. molar fractions, αCTAB, for the system Triton X-100+C16TAB.[55] It is worth mentioning the nonideal behavior of the system, whose experimental data are in agreement with the theoretical predictions of Rubingh’s model.
