Estimation of surface free energy and solubility parameters of solid ionic surfactants

The surface free energy (γ) and solubility (δ) parameters are two important characteristic parameters describing physicochemical properties of substances, but knowledge about the characteristic parameters (γ and δ) of surfactants is still lacking. Possible relationships of the characteristic parameters of surfactants with their head group types and alkyl chain lengths as well as with the surface tension (σ) of their aqueous solutions are worth exploring.

Solid surfactants including 10 anionic and 14 cationic ones were chosen. Their γ and δ parameters were estimated using the contact angle (θ) and dielectric constant (ε) methods. The dependence of the characteristic parameters on their head group type and alkyl chain length was analyzed, and possible relationships of the characteristic parameters with the σ of their aqueous solutions at the critical micelle concentration and nearly saturated concentration (denoted as σ_cmc and σ_sat, respectively) were explored.

The γ and δ values of 24 ionic surfactants were obtained. The γ parameters include total (γ_t), dispersive (γ_d), and polar (γ_p) ones, and the δ parameters include the Hildebrand parameter (δ_t) and the Hansen dispersive (δ_d), polar (δ_p), and hydrogen-bonding (δ_h) ones. With an increase in the carbon number (n) of alkyl chains, all of the γ and δ parameters linearly decrease, independent of the type of head groups. In addition, the γ_t of the surfactants is linearly positively correlated with their σ_cmc and σ_sat. This work deepens our understanding of the nature of surfactants.