The Characteristic Curvature (Cc) of Ionic Surfactants Assessed via Small-Angle X-ray Scattering

The characteristic curvature (Cc), within the hydrophilic-lipophilic difference + net (H_n) - average (H_a) curvature (HLD-NAC) framework, is the dimensionless net curvature, -H_n·L (L is the surfactant's tail length parameter), that a surfactant acquires at the characteristic condition (T = 25 °C, no added cosurfactants, oil with an equivalent alkane carbon number (EACN) of zero and for ionic surfactants, a total salinity (S) of 1 g NaCl/100 mL). A recent article demonstrated the validity of the Cc concept, where H_n was assessed via oil and water solubilization radii. Here, we assess H_n from the characteristic length (ξ) obtained from the analysis of SAXS profiles of microemulsions produced at semicharacteristic conditions (characteristic condition but varying S). The predicted relationship, -L·H_{n,semicharacteristic} = Cc_bi + bi·ln(S), was confirmed with the five ionic surfactants explored. The SAXS-assessed Cc (Cc = Cc_bi/bi) values are consistent with those obtained from solubilization studies and phase inversion scans. The Cc-SAXS method provides a way to assess the hydrophobicity of ionic surfactants directly, avoiding the bias that could be introduced by cosurfactants in phase inversion studies and minimizing the impact of potential uncertainties in the surfactant volume to area ratio (v_s/a_s) required to calculate the solubilization radii in the solubilization method.