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Self-aggregation of tetradecyltrimethylammonium bromide (TTAB, [CH3(CH2)13N+(CH3)3Br-]) and polyoxyethylene 23 lauryl ether (Brij-35, [CH3 (CH2)11(OCH2CH2)23OH]) binary surfactant mixture in aqueous medium was studied using tensiometric, conductometric, density, quasielastic light scattering, potentiometric, and fluorometric measurements. The binary surfactant mixture was studied well above the Krafft temperature, which was evaluated by conductance measurements. Rubingh's nonideal solution theory predicted nonideal mixing and attractive interaction between the constituent surfactants in the mixed micelle. Moreover, attractive interaction between the two surfactants in the mixed micelle is explained by assuming that water acts as a bridge between the hydrophilic polar groups of the surfactant molecules. The chain-chain interaction among the surfactant does not seem to be high in this case. The partial specific volume of pure as well as binary surfactant mixtures was also evaluated, and it was inferred that the mixed micelles are more hydrated compared to individual components. The excess Gibbs free energy of mixing was evaluated, and it indicated relatively more stable mixed micelles for this binary combination. Surface tension measurements indicate an existence of a second state of aggregation for the mixed surfactant system, which is supported by the break in conductance−concentration of surfactant profile. The Krafft temperature of TTAB decreases as the nonionic surfactant content increases in the mixed system. Quasielastic light scattering studies suggest an increase in the hydrodynamic radius of the micelle in the mixed surfactant system.

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The interfacial and thermodynamic properties of nonionic surfactants and their mixtures are of both theoretical and practical interest. The nonionic surfactants used in this study are polyoxyethylene (10) alkyl ether [CnE10; m ) 10 and n ) 12, 16] and N-decanoyl-N-methylglucamine (MEGA-10). The critical micelle concentrations of pure surfactants and their mixtures were determined by surface tension measurements at different mixed ratios and temperatures. Interfacial parameters such as the maximum surface excess (Γmax) and the minimum area per molecule (Amin) at the air/water interface were also determined from surface tension data. Standard thermodynamic parameters of micellization and adsorption were also computed and discussed. Steady-state fluorescence studies were also carried out to determine the micellar aggregation number (Nagg) and the microenvironment/polarity in the mixed micelle from the I1/I3 ratio. The interaction parameters that measure the interaction between the surfactant molecules in the mixed micelle were computed by Rubingh's approach. 1 H NMR was also used to investigate the interaction between the surfactants.

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A short account of the developments and perspectives of IKR (iso-kinetic relation) and EEC (enthalpy (H) - entropy (S) compensation) has been presented. The IKR and EEC are known to be extra thermodynamic or empirical correlations though linear H-S correlation can be thermodynamically deduced. Attempt has also been made to explain the phenomena in terms of statistical thermodynamics. In this study, we have briefly revisited the fundamentals of both IKR and EEC from kinetic and thermodynamic grounds. A detailed revisit of the EEC phenomenon on varied kinetic and equilibrium processes has been also presented. Possible correlations among the free energy (ΔG), enthalpy (ΔH), and entropy (ΔS) changes of different similar and nonsimilar chemical processes under varied conditions have been discussed with possible future projections.

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