INFLUENCE OF THE 3RD COMPONENT ON THE TEMPERATURE OF THE CRYSTAL SURFACTANT-MICELLAR SOLUTION EQUILIBRIUM

The effect of additives on the temperature of precipitation of a nonio nic surfactant from micellar solutions is studied experimentally and o n the basis of thermodynamic relationships for the pseudophase separat ion model. The derived formulas connect the decrease in the precipitat ion temperature (Delta T effect) produced by amphiphilic additives wit h the composition of mixed micelles ''basic surfactant-additive'' and also with the temperature and the heat of the dissolution of the indiv idual surfactant. Experimental data on the dissolution temperatures of crystalline N-dodecanoyl-N-methylglucamine (MEGA-12) in aqueous micel lar solutions containing additives of various chemical nature are pres ented; the data for systems with alkanol C-2-C-4 additives relate to a wide range of the surfactant and alkanol concentrations. The enthalpi es of dissolution of the crystalline MEGA-12 in water and water-alkano l mixtures determined by the Calvet calorimetry are given. The depende ncies of the Delta T effect on the concentrations of MEGA-12 and alkan ol (ethanol, propanol, or butanol) calculated from the model are in go od agreement with the experimental data. The composition of mixed mice lles formed by MEGA-12 and an alkanol C-2-C-4 is found from the Delta T data with the help of the model; the distribution of the alkanols be tween the micellar and aqueous pseudophases is evaluated. Nonideality of the aqueous alkanol solutions is taken into account in the model ca lculations. The results of measurements for aqueous and aqueous-alkano l mixtures of MEGA-12 containing salt additives evidence that the addi tion of salts (potassium, sodium, and ammonium chlorides, nitrates, ca rbonates, and sulfates) to aqueous solutions of MEGA-12 does not chang e significantly the temperature of the solid surfactant-micellar solut ion equilibrium, but a pronounced decrease of the temperature is obser ved when the salts are added to aqueous MEGA-12 mixtures containing al kanols, which is in correspondence with the model predictions.