Temperature dependence of enthalpic properties of aqueous micellar solution of some N-alkylnicotinamide chloride surfactants

Abstract

The dependence on the temperature of the enthalpic properties of some nicotinamide derivatives has been studied by measuring the heats of dilution, using an LKB flow microcalorimeter. The measurements were performed at 10, 18 and 40 °C in the case of N-nonylnicotinamide (C₉NAC) and N-decylnicotinamide chloride (C₁₀-NAC) and at 40 °C for N-heptylnicotinamide chloride (C₇-NAC). An increase in the temperature produces a lowering of enthalpic curves vs. m that is directly proportional to the temperature change. Assuming a constant heat capacity in the temperature range examined, the trends of heat capacities vs. m through the finite ratios ΔΦ_L/ΔT have been derived. These trends confirm the presence of a rearrangement in the micellar structure at ca. 0.35 mol kg^–1 for C₉-NAC and 0.25 mol kg^–1 for C₁₀-NAC, in agreement with the behaviour of the terms with eight and 12 carbon atoms, previously studied.

A pseudo-phase-transition model has been assumed, in order to extract the enthalpy of micellization at each temperature. From the linear dependence of the enthalpy changes for the micellization process vs. temperature, the standard heat capacities of micellization have been evaluated as –299 J K^–1 mol^–1 for C₉-NAC and of –385 J K^–1 mol^–1 for C₁₀-NAC.

For the class of compounds under investigation, a group contribution to micellization heat capacity for each CH₂ group, of –62 J K^–1 mol^–1 has been evaluated.