EXTREMELY STRONG INTERACTION OF SODIUM DECYL SULFATE AND DECYLTRIMETHYLAMMONIUM BROMIDE IN MOLECULAR AGGREGATES

The thermodynamic behavior of a mixture of sodium decyl sulfate (SDeS) and decyltrimethylammonium bromide (DeTAB) in aqueous solution and in molecular aggregates such as surface adsorbed films and micelles was investigated by measuring the electric conductivity and surface tensio n of the aqueous solutions. The thermodynamic quantities in solution a nd those in the molecular aggregates were evaluated from the experimen tal conductivity and surface tension data. The results for molar condu ctivity showed that dimerization or ion-pair formation of the SDeS and DeTAB molecules does not occur in aqueous solution and the mixture be haves as uni-univalent strong electrolyte below the critical micelle c oncentration (CMC). Contrary to the results in the aqueous solution, w e found significant nonideal behavior in the phase diagrams of surface adsorption; that is, equimolar mixture of SDeS and DeTAB exists in th e adsorbed film at almost all compositions irrespective of the bulk co mposition in the solution. The same result was also observed in the ph ase diagram of micelle formation. There was no difference in phase dia grams between surface adsorption and micelle formation at the CMC. The great nonideal mixing of SDeS and DeTAB in the molecular aggregates i s undoubtedly attributable to the extreme attractive interaction betwe en oppositely charged polar head groups of surfactants as well as to c ohesion between hydrophobic groups. Further, in a low concentration re gion, it turned out that equimolar composition is preserved in the fil m during phase transition of the mixed adsorbed film of SDeS and DeTAB from a gaseous state to an expanded state. (C) 1997 Academic Press.