Influence of Spacer on Association Behavior and Thermodynamic Parameters of Dimeric Cationic Surfactants

Abstract

AbstractNew dimeric surfactants [16‐s‐16, s: (CH2)4; CH2–COO–(CH2)2–OCO–CH2; CH2–COO–ISO–OCO–CH2 (ISO = d‐isorbide) and CH2–CONH–(CH2)2–NHCO–CH2] were synthesized and characterized (I–IV, respectively). The micellization behavior of dimeric surfactants with various spacers was studied by conductance/steady‐state fluorescence quenching (SSFQ) measurement and compared with the conventional surfactant (cetyltrimethylammoniumbromide, CTAB). The critical micelle concentration (CMC) values of several surfactants were found to be nearly the same by both techniques. Conductance measurements were used exclusively to obtain CMC and degree of dissociation (α) values at various temperatures, while SSFQ measurements were used to obtain a few CMC values (for comparison) and an aggregation number (Nagg). A decrease in CMC and Nagg values and an increase in α were observed for dimeric surfactants when compared with CTAB. CMC decreases and then increases with a continuous increase in temperature (U‐shaped behavior). The temperature of minimum CMC, Tm, was distinctly different for CTAB and the dimeric surfactants. Tm values were dependent on the nature of the spacer. The temperature dependence of CMC and α value was used to calculate the thermodynamic parameters of micelle formation for two classes of surfactant (conventional and dimeric). Enthalpy‐entropy compensation plots exhibited fair linearity except for IV. Compensation lines for spacers containing ester groups are parallel to one another. Micro‐polarity and apparent dielectric constant (εa) data suggest that the micellar environment felt by pyrene is similar to that of an alkanol with a 3–4 carbon chain.