Clouding Behavior of C ₁₃ EO _x (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, T _cp , in C ₁₃ EO _x (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 dT _cp /dS was given by dT _cp /dS = α _b + β _b S with α _b < 0 and β _b > 0. This trend suggests that dT _cp /dS is the result of two opposing effects, an entirely (negative) surfactant-dependent effect (α _b ) and a combined (positive) surfactant-solvent (β _b S)-dependent effect. In addition, the rate dT _cp /dS was found to become increasingly less negative with increasing salinity (dT _cp /dS) < 0), suggesting an overwhelming contribution of the α _b effect as compared to the β _b S effect. On the other hand, at fixed S, the rate was given by dT _cp /dx = α _x + β _x x 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 dT _cp/ dS, the rate dT _cp /dx was found to become increasingly less positive with increasing x values (dT _cp /dx > 0), suggesting that α _x is the dominant factor in the surfactants’ susceptibility to be salted in. Both constants α _b 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.