Aggregation in the Mixture of Branched Carboxylate Salts and Sulfonate Surfactants with Different Oligomeric Degrees

Understanding the effects of molecules with branched structures on surface activities and micellization of star-shaped oligomeric surfactants will promote the applications of oligomeric surfactants. The present work has studied the interactions and aggregation of branched carboxylate 2-hexyldecanoic acid (HDA) and 2,2,4,8,10,10-hexamethylundecane-5-carboxylic acid (HMLCA) with single chain sodium dodecyl sulfonate (SDoS) and star-shaped tetrameric sulfonate surfactant (EDA-(C12SO3Na)(4)) in aqueous solution of pH 11 by surface tension, zeta-Potential and small angle neutron scattering (SANS). Surface tension measurements have shown that the addition of HDA or HMLCA can significantly reduce the surface tension at critical micellar concentration (CMC), meanwhile, the CMC values increase slightly as the mole fraction of HDA or HMLCA increases. The interaction parameter (beta), calculated according to the non-ideal mixed solution model, indicate that different interaction degrees exist between branched carboxylate and sulfonate surfactant on surface activities and micellization. The four mixtures all exhibit synergism in surface tension reduction efficiency. The mixtures of single chain sulfonate surfactant and branched carboxylate also exhibit synergism in micelle formation, whereas the mixtures of tetrameric sulfonate surfactant and branched carboxylate do not, although the attractive interaction of HDA/EDA-(C12SO3Na)(4) and HMLCA/EDA-(C12SO3Na)(4) is stronger than that of HDA/SDoS and HMLCA/SDoS in mixed micelles. Taking HDA as a representative, SANS and zeta-Potential results reveal that the addition of HDA into these two sulfonate surfactants leads to different aggregate transitions in the solution. For HDA/SDoS, when the molar fraction of HDA (X-HDA) is constant and the total surfactant concentration increases, spherical micelles transfer into rod-like micelles. For the HDA/EDA-(C12SO3Na)(4) mixture, the rod-like micelles become shorter as X-HDA increases at a fixed total surfactant concentration, while the rod-like micelles are growing longer with increasing X-HDA at a fixed total surfactant concentration. This observation suggests that the branched structure of carboxylates can improve the aggregation ability of the mixed system. In addition these mixtures show excellent performance at emulsifying dodecane, and HDA or HMLCA can greatly reduce the dosage of sulfonate surfactants.