Development of a method for measurement of relative solubility of nonionic surfactants

Relative solubility number (RSN) provides a practical alternative to the HLB method of assessing hydrophilic–lipophilic balance of surfactants. Traditionally, RSN has been determined by titration of a surfactant in benzene/dioxane solution with water. Owing to the toxicity of these solvents, a new procedure for determination of RSN, using less-toxic substances, toluene and ethylene glycol dimethyl ether (EGDE), was developed. The relationship between RSN and classic HLB values for some well-characterized classes of surfactants was investigated. Some factors that might affect RSN value were also studied. sults showed that RSN values determined using toluene/EGDE were linearly correlated with those determined using benzene/dioxane, indicating that toluene/EGDE solvent can be used to replace benzene/dioxane. It was found that, within the same surfactant family, RSN values determined at certain molar concentration showed a good linear relationship with classic HLB values. The surfactant concentration in RSN titration solvent affected the RSN values; the effect of salt concentration in the titration water was negligible. A generalized regression model was used to correlate RSN values with the structure parameters of surfactants such as carbon number in hydrophobic groups, CO number and free OH number.