Acid soap formation of oleic acid and catanionic complex formation in the alkyldimethylamine oxide/sodium oleate equimolar mixtures

Abstract

The influence of adding alkyldimethylamine oxide (CnDMAO) with varying alkyl chain lengths (nc) on the acid soap formation of oleic acid was investigated. The solutions of equimolar mixtures of CnDMAO and sodium oleate (Na+Ol−), each 25 mmol kg−1, became turbid at a certain critical pH (pHc) on decreasing pH. Values of the pHc depended on nc and showed the minimum at C10DMAO/NaOl mixture. The presence of the minimum was interpreted in terms of two different kinds of the complex formed in the micelles depending on nc: the catanionic complex (CnDMAOH+/Ol−) in the mixed micelles of nc=16, 14, 12 and 10, and the acid soap of oleic acid for C6DMAO/NaOl and C8DMAO/NaOl mixtures. At pHc where the amounts of these complexes of double-chain nature reached certain critical values in the mixed micelles, a phase separation (most probably lamella formation) took place. It was expected that the critical amount of the catanionic complex was smaller for the mixtures of higher nc values and hence pHc increased with nc for the mixtures nc≥10. For the mixtures of nc<10, it was expected that the amount of the acid soap in the mixed micelles increased with decreasing nc at a given pH and the pHc increased with decreasing nc. Micelle compositions at cmc were evaluated on the basis of the regular solution theory coupled with the pseudo phase approximation. The micelle compositions at 100 mmol kg−1 were examined with 13C-NMR. The results showed the mixed micelle formation for nc=16–10, while the micelles mostly consisting of oleic acid for the mixtures of nc=8 and 6. The assumption of two different complexes for the two groups of the mixture was thus supported. The cmc range of mixed micelles was evaluated and it was well correlated with the observed concentration range of pyrene fluorescence change.