SALT EFFECTS ON INTRAMICELLAR INTERACTIONS AND MICELLIZATION OF NONIONIC SURFACTANTS IN AQUEOUS-SOLUTIONS

The effects of adding LiCl, NaCl, KCl, KBr, and KI to aqueous solutions containing the alkyl poly(ethylene oxide), C(i)E(j), nonionic surfactants, C(12)E(6), C(12)E(8) and C(10)E(6) have been investigated through a combined theoretical and experimental approach. The theoretical studies involve (1) the generalization of a recently developed molecular theory of micellization to characterize and quantify salt effects on intramicellar interactions and the utilization of this theory to predict and rationalize (i) the various contributions to the free energy of micellization, (ii) the critical micellar concentration (cmc), and (iii) the salt constants and (2) the development of a new description of the interactions between the poly(ethylene oxide), PEO, hydrophilic (E(j)) chains which are ''grafted'' at one end to the micellar-core surface. The theoretical description in (2) accounts explicitly for the chainlike character of PEO and utilizes a rotational isomeric state Monte Carlo approach to generate the average conformational characteristics of the ''grafted'' PEO chains. The experimental study involves surface tension measurements, conducted at 25 degrees C, which are utilized to determine cmc's as a function of salt type and concentration, as well as the salt constants of the five salts examined. The theoretically predicted cmc's and salt constants are in very good agreement with those determined experimentally. The theoretical and experimental results are consistent with the observation that the salts examined have a much more pronounced effect on the hydrophobic alkyl (C-i) moieties than on the hydrophilic PEO (E(j)) moieties, primarily by inducing a decrease in the solubility of the alkyl moieties when added to aqueous solutions. This, in turn, results in both a lowering of cmc's and surface tensions of the C(i)E(j) aqueous salt solutions examined.