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Experimental area/molecule data (Aexpt) at the air/aqueous solution interface after mixing, ideal mixing area/molecule data (Aideal), based upon area/molecule data at that interface before mixing, and regular solution theory have been used to explain the values of surfactant molecular interaction (β) parameters observed in mixed monolayers and mixed micelles. The value of the β parameter reflects the difference in surfactant-surfactant interactions before and after mixing. In ionic-nonionic surfactant mixtures, when surfactant-surfactant interactions are weak, reduction in electrostatic self-repulsion interaction energy of the ionic surfactant, due to the dilution effect upon mixing, is suggested as a major contributor to the negative β values observed for mixed monolayers and mixed micelles. Steric effects appear when surfactant molecular structure varies in the size of the headgroups and in the branching of the hydrophobic groups. It is suggested that the effect of an increase in the size of the hydrophilic group of the ionic surfactant in anionic-nonionic surfactant mixtures is to decrease electrostatic self-repulsion of the ionic surfactant before mixing, producing less negative β values. In contrast, the observation of more negative β values with increased length of the polyoxyethylene chain in POE nonionic-anionic surfactant mixtures, which increases the size of the hydrophilic group of the nonionic surfactant, may be due to the acquisition of a positive charge by the polyoxyethylene group in the nonionic surfactant upon being mixed with an anionic surfactant, which would cause attractive electrostatic interaction with the anionic surfactant in both the mixed monolayer and mixed micelle after mixing. Bulkiness due to branching in the hydrophobic group of the ionic surfactant decreases electrostatic self-repulsion before mixing with a nonionic surfactant having a linear hydrophobic group. Branching in the hydrophobic group of a polyoxyethylenated nonionic surfactant causes some contraction upon mixing with an anionic surfactant with a linear hydrophobic group, due to the greater steric repulsion in the bulky molecule of the nonionic surfactant before mixing and its reduction after mixing, resulting in slightly more negative β σ values than with the corresponding linear hydrophobic chain nonionic surfactant.

Micellization and premicellar behavior of the two series of cationic surfactants, each with two hydrophilic and two hydrophobic groups in the molecule ("gemini" surfactants), one series with a rigid, hydrophobic spacer, and second with a flexible, hydrophilic one, have been studied by use of surface tension measurements. The data show the expected regular increase in surface activity with an increase in alkyl chain length for the shorter chain homologs but show increased deviation from the regularity with an increase in chain length when the number of carbon atoms in the alkyl chain exceeds a certain number. This deviation in surface activity appears to be due to the formation of small, non-surface active aggregates. Equilibrium constants calculated for the aggregation reaction show that the conditions facilitating micelle formation also favor formation of these premicelles, such as lower temperature, stronger ionic strength of the solution, and increased alkyl chain length. Geminis with a flexible, hydrophilic spacer appear to aggregate more readily than geminis with a rigid, hydrophobic spacer. Their shorter homologs are also more surface active than those having a rigid, hydrophobic spacer.

Bis(quaternary ammonium halide) surfactants (gemini surfactants) having, variously, diethyl ether, monohydroxypropyl, and dihydroxybutyl spacer groups have been investigated by surface tension, interfacial tension, and steady-state fluorescence techniques. The critical micelle concentration (cmc) and area per molecule (A min) are shown to deviate from the expected patterns of behavior as the number of carbon atoms in the alkyl chain (n) increases beyond a certain maximum. This aberrant behavior is observed at the hydrocarbon/water as well as the aqueous/air interface. The unexpected values of the physicochemical parameters at long alkyl chain length have been interpreted on the basis of a concentration region in which submicellar or multilayer structures are forming. Fluorescence measurements provide confirmation of cmc values by an alternative technique. Comparison of the fluorescence emission maxima profiles of the gemini surfactants with those of their monoquaternary analogues demonstrates that there is a continuously changing shape with change in n for the geminis, whereas the profiles for nongeminis are invariant. Long-chain geminis exhibit a gradually sloping sigmoidal profile, indicating a variety of environments experienced by pyrene-3-carboxaldehyde between the totally aqueous environment at low surfactant concentration and the hydrophobic (entirely micellar) environment at high surfactant concentration. The large variation in the polarity of the probe environment between these two extremes may be attributed to the formation of submicellar structures.

The literature, including patents, describing the emerging area of gemini surfactants is reviewed. The differences in structure/property relationships between gemini and comparable conventional surfactants are described and discussed in terms of their predicted performance properties. Supportive performance data are enumerated. JSD 1, 547-554 (1998).