The temperature dependence of the critical micelle concentrations of cationic surface‐active agents

The temperature dependence of the critical micelle concentrations, CMC values, of dodecyl pyridinium bromide (Adderson & Taylor, 1964), tetradecyl pyridinium, trimethyl ammonium and benzyl dimethyl ammonium bromides (Adderson & Taylor, 1967) and alkyl a-picolinium bromides (Adderson & Taylor, 1970) has been previously reported and an explanation of the micellization process advanced. The present data are complementary and reinforce this explanation. Dodecyl benzyl dimethyl ammonium bromide, decyl and dodecyl trimethyl ammonium bromides were prepared by methods previously reported. CMC values were determined by conductance technique and are recorded in Table 1. The fraction of theoretical charge, p/N has been calculated both for these salts and from the conductance-concentration data for the tetradecyl homologues (Adderson & Taylor, 1967), using the method of Evans, (1956). Aggregation numbers, N, of Clo = 36, C,, = 50 & C,, = 75 (Debye 1949) have been used throughout (Table 2). As observed for alkyl picolinium bromides, (Adderson & Taylor, 1970) the degree of dissociation increases with temperature and decreases with increase in length of the principal alkyl chain. The dissociation is less in the alkyl trimethyl salts than in the corresponding compounds containing a cyclic structure, suggesting that the benzyl screens the charged nitrogen to a greater extent than a methyl group. Thermodynamic parameters, Fig. 1 a and b, have been calculated as previously, using the uncharged phase-change model and standard states of a mol fraction of unity for the monomeric species and the micelle itself for its species, and assuming that the heats of dilution from standard state to CMC are negligible compared to the heat of micellization. Hence, AGOm = 2RT In CMC, and AHm = AHOm = -2RT2 ( a h CMC/