Complex formation in alkyldimethylamine oxide/sodium palmitate/water mixtures

Abstract

The complex formation between nonionic alkyldimethylamine oxide (C nDMAO, n = 14 , 16, and 18) and sodium palmitate (NaPa) in the solid phase of C nDMAO/NaPa mixtures and the dependence of the interaction parameter β of the regular solution theory (RST) on the mixed micelle composition of C16DMAO/NaPa mixtures were investigated. The dissolution temperature showed a maximum at a NaPa mole fraction X Pa * of 0.3–0.4 for C16DMAO/NaPa and 0.2 for C18DMAO/NaPa. The compositions of the complexes suggested by X Pa * are C16DMAO: NaPa = 3:2 or 2:1 and C18DMAO:NaPa = 4:1. The composition X Pa * depended on the chain length of the amine oxides. The maximum was not observed in the case of the C14DMAO/NaPa/water system. In the range 0.7 ⩽ X Pa ⩽ 1.0 , dissolution temperature depression was observed with decreasing X Pa . The dissolution temperature depression was analyzed by taking into account the nonideal behavior in the mixed micelles and the counterion binding on the mixed micelle surface. The negative β values were obtained for all three mixed systems. It was shown that the counterion activity remained practically constant in the range of 0.7 ⩽ X Pa ⩽ 1.0 . The cmc values of C16DMAO/NaPa mixtures were determined by pyrene fluorescence measurement. For C16DMAO/NaPa mixtures, the dependence of the RST interaction parameter β on the mixed micelle composition X Pa was determined for a wide range ( 0.2 ⩽ X Pa ⩽ 0.9 ). In the range 0.2 ⩽ X Pa ⩽ 0.5 , the β values were obtained from an analysis of cmc based on the RST. In the range 0.7 ⩽ X Pa ⩽ 0.9 , the β values were obtained from an analysis of the dissolution temperature depression. From the analysis of the micelle composition dependence of the β values, a short-range attractive interaction between the headgroup of C16DMAO and palmitate anion is suggested.