Using Surface Tension Data to Predict Differences in Surface and Bulk Concentrations of Nonelectrolytes in Water

Recently, we developed a quantitative interpretation of surface tension increments (STI) of salts, acids, and bases in terms of the solute (or salt ion) partitioning model (SPM). Here, we obtain an analogous SPM-based interpretation of surface tension increments of nonelectrolytes,. which yields local-bulk partition coefficients (K-p) quantifying the accumulation or exclusion of these solutes in the local region near the air-water surface, and the amount of water per unit area of that region (b(1)(0)). Sucrose exhibits the largest positive STI (approximately 1.4 ergs cm(-2) Osm(-1)). Assuming that K-p = 0 for sucrose (i.e., that it is completely excluded from the surface of water), these STI provide a minimum estimate of b(1)(0) of 0.20 H-2 O/angstrom(2), or a minimum thickness of the surface region of approximately two layers of water at bulk density. This is the same value as obtained previously from analysis of surface tension and hydrocarbon solubility increments of Na2SO4 and also for the interaction of glycine betaine with anionic carboxylate surface, indicating that this quantity is not a function of the type of solute or surface investigated and therefore that it may represent the molecular thickness of the region. Partition coefficients of other nonelectrolytes investigated range from moderately excluded (e.g., urea) to moderately accumulated (e.g., glycerol, ethylene glycol); strongly accumulated surface active solutes (e.g., monosubstituted alcohols) were not included in this analysis. Partition coefficients for many salt ions obtained from STI and hydrocarbon solubility increments fall in a rank order which corresponds to the Hofmeister series for protein folding and protein solubility, indicating a common pattern of accumulation or exclusion of salt ions at the air-water surface and nonpolar surfaces of dissolved hydrocarbons and proteins; no such patterns are observed for nonelectrolytes.