Effect of Sodium Carbonate on the Cloud Point in Alkyl Ether/Brine Systems: Apparent Relation with Dynamic Interfacial Tension Minimum

The effect of Na₂CO₃ on the cloud point in Na₂CO₃/surfactant/brine was investigated using two series of nonionic surfactants, C₁₃EO_x and C₁₇EO_x. The cloud point, T_cp, was found to decrease linearly with increasing Na₂CO₃ concentration. This was attributed to Na⁺ and particularly to CO₃²⁻salting-out effect. The slope a = dTcp/d[Na₂CO₃] became more and more negative as the degree of ethoxylation is increased, suggesting that the higher the number of ethylene oxide (EO) groups the stronger is the cloud point depression for a given increment in Na⁺and CO₃²⁻ions in solution. This was also illustrated by the linear variation of ΔT_cp = T_cp,0 − T_cp,[Na2CO3] with the surfactant degree of ethoxylation.