Thermodynamic properties of new gluconamide-based cationic surfactants in aqueous solution: Experimental and modeling approaches

Highlights: • Micellization of new soft gluconamide-type cationic surfactants were studied. • Isothermal titration calorimetry and conductometry measurements were performed. • Thermodynamic functions of micellization and micelle ionization were determined. • Heat capacity and contributions of CH2 to thermodynamic functions were calculated. • Molecular properties of the surfactants affecting aggregation were studied by molecular modeling. -- Abstract: The micellization processes of 2-(alkyldimethylammonio)ethylgluconamide bromides (CnGAB), dodecyltrimethylammonium bromide (DTAB) and dodecylethyldimethylammonium bromide (DEDAB) were studied using solution conductivity and isothermal titration calorimetry (ITC). The critical micelle concentrations (CMC), the degree of counterion binding (β), the enthalpies (ΔHm) and entropies (ΔSm) of micellization as well as the contributions of the headgroups to Gibbs free energies (ΔGm0(hy)) were calculated. Approximate relative partial enthalpies (L2) were evaluated as functions of concentration from the calorimetric curves. In comparison with DTAB, the hydrophobicity of C12GAB is reduced by an extent corresponding to the shortening of DTAB's alkyl chain by one methylene group. The micelles of C12GAB are hydrated by one methylene group deeper than those of DTAB. The free energy of the transfer of the headgroup from the bulk solution to the micelle is more favorable for DTAB than C12GAB. This effect is approximately equal to the ΔGm0(CH2) contribution. In contrast to the usual ionic surfactant behavior, the degree of counterions binding for CnGABs decreases with increasing alkyl chain length. This is supposed to be a consequence of screening of the charged layer of the micelle by the large, hydrated and hydrogen-bonded headgroups from the bromide anions in bulk solution. The computational modeling made it possible to relate the surfactants' molecular properties to the interactions the surfactants enter into, suggesting that, due to the strongly polar character of their headgroup, CnGABs form small premicellar aggregates