Micellar Solution Behavior of Cetylpyridinium Surfactants in 2-Propanol–Water Mixed Media at Different Temperatures

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Self-assembly of surfactant systems in organic solvent–water mixed media is relevant in several industrial and pharmaceutical applications. This article emphasizes the role of 2-propanol (PrOH) as a cosolvent in the micellization of cetylpyridinium chloride (CPCl) and cetylpyridinium bromide (CPBr) surfactants at different temperatures (293.15–308.15 K) studied through the conductometric technique. The results show an enhancement in CMC values for both the surfactants with an increase in volume percentage (vol %) of PrOH (from 0 up to 20 vol %) as well as temperature. The CPBr shows distinct behavior in the lower composition of PrOH–WR (0 and 2.5%) at a higher temperature (308.15 K). The rise in the CMC values depends on lowering of permittivity of the mixed media caused by change in compositions of PrOH–WR mixed media. A similar effect is also seen on the degree of counterion dissociation (α) values with an exception to a few compositions. The increase in α values is found to be relatively less for CPBr as compared to CPCl. Temperature dependency of CMC values has been used to calculate thermodynamic parameters such as the change in Gibbs energy (Δ G m 0 ), enthalpy (Δ H m 0 ), and entropy (Δ S m 0 ) of micellization, and their values were used to analyze the effect of various compositions of PrOH–WR mixed media. The Δ G m 0 values indicate that micellization is spontaneous and becomes less favorable with the inclusion of PrOH. The values of Δ H m 0 (<0) and Δ S m 0 (>0) affirm that the process of micellization is entropically driven within the scope of the studied temperature. Overall results have been analyzed based on the interactions between the solute and solvent, counterions and aggregates, and solvation/desolvation of counterions.

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