Clouding Behavior of C₁₃EO_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 C₁₃EO_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_bS 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_bS)-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_bS effect. On the other hand, at fixed S, the rate was given by dT_cp/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 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.