Clouding Behavior of C13EOx (x = 10–19) Micelles in a Wide Range of Salinity and Hardness: Concomitant Effects of Salinity and Degree of Ethoxylation

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 = α b + β bS with α b < 0 and β b > 0. This trend suggests that dTcp/dS is the result of two opposing effects, an entirely (negative) surfactant‐dependent effect (α b) and a combined (positive) surfactant‐solvent (β bS)‐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 α b effect as compared to the β bS effect. On the other hand, at fixed S, the rate was given by dTcp/dx = α x + β xx with α x > 0 and β x < 0. This trend also suggests the existence of two opposing effects, a strictly (positive) solvent‐dependent effect (α x) and a concomitant solvent (β x)‐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 α x is the dominant factor in the surfactants’ susceptibility to be salted in. Both constants αb and α x were attributed to a dual temperature‐salinity effect on the structure of water molecules not involved in surfactant's hydration. On the other hand, β b and β x were associated with the change in the structure of water due to the dehydration of surfactants’ oxyethylene moieties with increasing temperature.