The effect of salinity and degree of ethoxylation on the cloud point, Tcp, in C13EOx (x = 10–19) micelles was investigated in distilled water and in the salinity range S = 1.96–196 g L−1. At a fixed x value, the rate dTcp/dS was given by dTcp/dS = αb + βbS with αb < 0 and βb > 0. This trend suggests that dTcp/dS is the result of two opposing effects, an entirely (negative) surfactant-dependent effect (αb) and a combined (positive) surfactant-solvent (βbS)-dependent effect. In addition, the rate dTcp/dS was found to become increasingly less negative with increasing salinity (dTcp/dS) < 0), suggesting an overwhelming contribution of the αb effect as compared to the βbS effect. On the other hand, at fixed S, the rate was given by dTcp/dx = αx + βxx with αx > 0 and βx < 0. This trend also suggests the existence of two opposing effects, a strictly (positive) solvent-dependent effect (αx) and a concomitant solvent (βx)-surfactant (x)-dependent effect. Contrary to dTcp/dS, the rate dTcp/dx was found to become increasingly less positive with increasing x values (dTcp/dx > 0), suggesting that αx is the dominant factor in the surfactants’ susceptibility to be salted in. Both constants αb and αx were attributed to a dual temperature-salinity effect on the structure of water molecules not involved in surfactant's hydration. On the other hand, βb and βx were associated with the change in the structure of water due to the dehydration of surfactants’ oxyethylene moieties with increasing temperature.
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