Molecular dynamics simulation of binary betaine and anionic surfactant mixtures at decane - Water interface

The interfacial properties of binary betaine and anionic surfactant mixtures at decane-water interface have been studied via molecular dynamics simulations. Two betaines chosen are alkyl sulfobetaine (ASB) and alkylbenzene sulfobetaine (BSB), while three anionic surfactants chosen are sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS) and sodium octadecyl sulfonate (SOS), respectively. The synergistic influences between betaines and anionic surfactants are explored using mass density profile, interfacial thickness, atomic spacing and oblique angle. Hydrophilic size of betaine has been calculated through atomic spacing and oblique angle. The calculated value of hydrophilic size is about 81 Å2, which is approximately equal to experimental saturated adsorption area and simulated critical micelle area. Two anionic surfactant layers exist, a small amount of molecules are paralleled with ASB to generate competitive adsorption. More anionic surfactants are located in the cracks of ASB hydrophobic chains to form mixed adsorption. Mixed adsorption dominates surfactant adsorbent process for betaine/anionic surfactant mixtures. SDBS distribution range is exactly same as ASB range at the oil phase, which implies the consistent of hydrophobic chain lengths between above two surfactants. Interfacial compactness can be quantitative calculated by comparing betaine number and anionic surfactant number at mixed adsorbent layer. Simulated interfacial compactness perfectly corresponds with experimental interfacial tension, which may stand for the discovery about reliable evaluation criteria of comparing different betaine mixtures. Both surfactant hydrophilic-lipophilic balance and consistency of hydrophilic-hydrophobic structure can affect interfacial properties. The mechanism about surfactant mixed adsorption is also perfected at a molecular level, which is important for the enhanced oil recovery processes.