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

The dependence on the temp. of the enthalpic properties of some nicotinamide derivs. was studied by measuring the heats of diln., using an LKB flow microcalorimeter. The measurements were performed at 10, 18 and 40° in the case of N-nonylnicotinamide (C9NAC) and N-decylnicotinamide chloride (C10-NAC) and at 40° for N-heptylnicotinamide chloride (C7-NAC). An increase in the temp. produces a lowering of enthalpic curves vs. m that is directly proportional to the temp. change. By assuming a const. heat capacity in the temp. range examd., the trends of heat capacities vs. m through the finite ratios Δ.vphi.L/ΔT were derived. These trends confirm the presence of a rearrangement in the micellar structure at ca. 0.35 mol/kg for C9-NAC and 0.25 mol/kg for C10-NAC, in agreement with the behavior of the terms with eight and 12 carbon atoms, previously studied. A pseudo-phase-transition model was assumed, in order to ext. the enthalpy of micellization at each temp. From the linear dependence of the enthalpy changes for the micellization process vs. temp., the std. heat capacities of micellization were evaluated as -299 J/K.mol for C9-NAC and of -385 J/K mol for C10-NAC. For the class of compds. under investigation, a group contribution to micellization heat capacity for each CH2 group, of -62 J/K mol was derived.