Dilational Rheology of Extended Surfactants at the Air/Water and Oil/Water Interfaces: Effect of Propylene Oxide Group Numbers.

In this paper, the interfacial properties of extended surfactants with different oxypropylene (PO) groups were comprehensively investigated by using interfacial dilational rheology. The differences in molecular orientation, spatial configuration, and relaxation process were compared at the gas-water interface and oil-water interface. The influences of the PO groups on the interface viscoelasticity were analyzed, providing important theoretical support for the wide application of extended surfactants. Experimental results show that the lower number of PO groups in extended surfactants does not cause differences in their presence states on the interface; however, once it increases, the longer PO segment will spiral up in the direction perpendicular to the interface, forming a spatial configuration like a thin cylinder. Compared with air, the PO group has better solubility in the oil phase. The chain segment can still maintain a helical extension from the beginning to the end as a result. However, the upper layer of the thin cylinder will collapse to a certain extent at the surface. Moreover, the orientation of the hydrophobic side has a dynamic process of "tilting to upright" with the increase of adsorption amount or in response to interfacial dilation and compression. The increase of PO number or the insertion of oil molecules has little effect on dilational modulus, and the interfacial film strength is generally relatively low. That is to say, the better emulsifying and solubilizing ability of PO-containing extended surfactants may be more attributed to the matching steric effect at interface or better packing action in bulk phase than to higher film strength.

The array and interfacial activity of sodium dodecyl benzene sulfonate and sodium oleate at the oil/water interface.

There is a close correlation between the interfacial activity and the adsorption of the surfactant at the interface, but the detailed molecular standard information was scarce. The interfacial activity of two traditional anionic surfactants sodium dodecyl benzene sulfonate (SDBS) and sodium oleate (OAS) were studied by experimental and computer simulation methods. With the spinning drop method and the suspension drop method, the interfacial tension of oil/aqueous surfactant systems was measured, and the influence of surfactant concentration and salinity on the interfacial tension was investigated. The dissipative particle dynamics (DPD) method was used to simulate the adsorption of SDBS and OAS at the oil/water interface. It was shown that it is beneficial to decrease interfacial tension if the hydrophobic chains of the surfactant and the oil have similar structure. The accession of inorganic salts causes surfactant molecules to form more compact and ordered arrangements and helps to decrease the interfacial tension. There is an osculation relation between interfacial density and interfacial activity. The interfacial density calculated by molecular simulation is an effective parameter to exhibit the interfacial activity.