The relationship between the interaction between the same two surfactants in a mixed monolayer at the air/aqueous solution interface (βσ) and their interaction in a mixed micelle in the aqueous phase (βM) was investigated, in 0.1 M NaCl at 25 °C. All the interactions investigated were attractive (having negative β values) and had relatively stronger interaction in the mixed monolayer than in the mixed micelle, except for mixtures of anionic surfactants and some polyoxyethylenated (POE) nonionic surfactants, where relatively stronger interaction occurs in the mixed micelle. It is shown that the relative strengths of the interactions at the interface and in the micelle in the anionic−POE nonionic mixtures are determined by the branching and bulkiness of the hydrophobic and hydrophilic groups of the surfactants in the mixture. Interaction in the mixed micelle is reduced by branching in the hydrophobic group in either surfactant of the mixture. Branching close to the hydrophilic group of the anionic surfactant in the mixture reduces the interactions both in the mixed micelle (βM) and in the mixed monolayer (βσ), with the effect on the interaction in the mixed micelle (βM) greater than that in the mixed monolayer (βσ). When both the anionic and the nonionic surfactants have branched hydrophobic groups and the anionic surfactant, in addition, has branching near the hydrophilic group, interactions in both the mixed monolayer and the mixed micelle are weak. Interaction in the anionic−POE nonionic mixed micelle is sharply enhanced by an increase of the oxyethylene units in the POE group of the nonionic surfactant of the mixture to a number sufficient to complex the Na+ of the anionic surfactant. Consequently, for synergy in surface tension reduction effectiveness to exist in anionic−POE nonionics, the number of oxyethylene units in the nonionic must be small.
The aggregation behavior of four series of bis(quaternary ammonium halide) surfactants (gemini surfactants) having diethyl ether, dihydroxybutyl, monohydroxypropyl, and dimethylene phenylene spacer groups has been studied using steady-state and time-resolved fluorescence spectroscopy. Aggregation numbers were determined using the time-resolved single photon counting method with pyrene as the probe. At certain surfactant concentrations, aggregation numbers of 2, expressed as gemini molecules per micelle, were obtained in all four series when the number of carbon atoms in the alkyl chain length (n) increased beyond a maximum. These long-chain geminis also have a critical micelle concentration (cmc) greater than expected on the basis of plots of log cmc vs n for the shorter chain homologues. This deviation has been attributed to the formation of premicellar aggregates in the surfactant concentration region between the observed and the expected cmc values. The aggregation numbers obtained here indeed point to the existence of dimers in this region. Steady-state fluorescence measurements of I1/I3 values, which are indicative of the polarity of the probe environment, are followed as a function of surfactant concentration. These ratios are used to compare cmc values obtained by surface tension methods as well as to confirm the more hydrophilic nature of systems where dimers are thought to be present.
Interfacial adsorptions of aqueous solutions of trisiloxane surfactant (SILWET L77), N-alkyl pyrrolidinones [N-butyl-(C4P), N-cyclohexyl-(CHP), N-hexyl-(C6P), N-2-ethyhexyl-(C2,6P), N-octyl-(C8P), and N-decyl-(C10P)], and their mixtures have been measured. The adsorptions at the air/aqueous solution interface were obtained from the plots of surface tension versus log(C) by use of the Gibbs equation. The adsorptions onto the powdered polyethylene surface were determined by the use of UV spectroscopy for pyrrolidinones and two-phase titration for SILWET L77, respectively. The adsorptions at the solid/air interface were evaluated by the use of an equation derived from the Gibbs and Young equations. It was found that the addition of the N-alkyl pyrrolidinones to the solution produces little or no enhancement of the total surfactant adsorption at the air/aqueous solution interface. At the solid/aqueous solution interface, the enhancement effectiveness of the pyrrolidinones decreases in the order C2,6P > C8P > C6P > CHP > C4P, and their enhancement efficiency decreases in the order C2,6P > C8P > CHP > C6P > C4P. However, C10P, the most effective surface-active agent among the pyrrolidinones, produces no enhancement in the adsorption of L77 at both the air/aqueous solution and the solid/aqueous solution interfaces. The adsorptions of the individual surfactants and their mixtures at the solid/air interface are shown to be smaller by 1 order of magnitude than those at the air/aqueous solution and solid/aqueous solution interfaces. Consequently, the most significant fact of the addition of N-alkyl pyrrolidinones to an aqueous solution of L77 is the enhancement of the adsorption of L77 at the solid/aqueous solution interface.
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