DODECYLTRIMETHYLAMMONIUM BROMIDE IN WATER UREA MIXTURES - STRUCTURE AND ENERGETICS

Enthalpies of dilution and osmotic coefficients of dodecyltrimethylammonium bromide (DTAB)-water-urea mixtures as functions of urea (m(u)) and surfactant (m(s)) concentrations were measured at 298 and 310 K. Small-angle neutron scattering (SANS) measurements were also performed on some DTAB-D2O-urea mixtures at two surfactant concentrations and different urea concentrations at 298 K. From the thermodynamic data, the apparent (L(PHI)) and partial molar relative enthalpies (L2) and the nonideal free energies (G2ni) and entropies (TS2ni) of DTAB were derived as functions of m(u) and m(s) at 298 K. At constant m(u), the trends of G2ni and TS2ni as functions of m(s) behave regularly while those of L2 show anomalies at about 0.30-0.35 mol kg-1. This peculiarity has been attributed to a micelle structural transition, in agreement with other literature data. At constant m(s), the addition of urea causes a decrease of G2ni due to the effect on L2 since no dependence on m(u) is observed for TS2ni. In the range of m(u) investigated, the partial molar relative enthalpy of monomeric surfactant at the cmc does not depend on m(u) while that of the micellized surfactant decreases up to 3m(u) and, then, it is constant up to 7m(u). Consequently, the enthalpies of micellization, evaluated on the basis of the pseudophase transition model, do not change linearly with mu. From SANS data analysis the shape of the micelles and the aggregation number (N), the charge (Z), and, hence, the degree of counterion dissociation (beta) were computed. The increase of urea concentration determines a decrease in the size of the micelles and an increase of beta. At fixed m(u) a small difference in the aggregation number and a strong variation in beta were observed between the two DTAB concentrations. Thc effect of added urea on the degree of counterion dissociation of DTAB micelles derived from conductivity and SANS measurements is discussed in terms of the change of the aggregation number with m(u). These conclusions were supported by a comparison of the pair correlation functions obtained from the fitted structure functions.