Mv. Alexeeva et al., INFLUENCE OF THE 3RD COMPONENT ON THE TEMPERATURE OF THE CRYSTAL SURFACTANT-MICELLAR SOLUTION EQUILIBRIUM, Langmuir, 12(22), 1996, pp. 5263-5270
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.