Thermodynamics of Micellization of Nonionic Saccharide-Based N-Acyl-N-alkylaldosylamine and N-Acyl-N-alkylamino-1-deoxyalditol Surfactants

Eight homologous series of nonionic carbohydrate-derived surfactants in which the alkyl chains are linked through N-acylated amine bonds were synthesized, and their critical micelle concentrations (cmc's) and standard enthalpies of micellization were determined using titration microcalorimetry. Gibbs energies of micellization (Δmic G°) were calculated from the critical micelle concentrations. N-Acyl-N-alkylaldosylamines (acyl = acetyl/propionyl, aldosyl = glucosyl/lactosyl) and N-acyl-N-alkylamino-1-deoxyalditols (acyl = acetyl/propionyl, alditol = glucitol/lactitol) with alkyl chain lengths of 8, 10, and 12 carbon atoms show a 10-fold decrease in cmc when the length of the chain is increased by two methylene groups. The enthalpograms for the C8 analogs are more complicated than those for the C10 and C12 analogs. Therefore, the enthalpograms were modeled using a computer-based program which takes account of the nonideal properties of the solutions, yielding enthalpies of micelle formation. Increments in the thermodynamic parameters show satisfactory self-consistency. Each CH2 group contributes −2.4 kJ mol-1 to Δmic H°, Δmic G°(CH2) is −3.0 kJ mol-1 for each series, and TΔmic S°(CH2) is 0.7 kJ mol-1 at 40 °C. Although the change in entropy is the main driving force for micellization, the enthalpy of micellization may also contribute significantly to the Gibbs energy of micellization, particularly for longer alkyl chains.