Abstract: Surfactant molecules will form an ordered structure at the oil-water interface to promote film formation at the interface. To clarify the effect of this molecular adsorption behavior on film formation at the oil-water interface and its micromechanisms is of great significance for establishing the analysis methods of stability and instability of oil-water emulsions in crude oil production and transportation. Using the sodium dodecyl benzene sulfonate (SDBS), a common anionic surfactant, as an example, and the moving plate wall surface to assign a shear flow field to the "crude oil-SDBS-mineral water" interface simulation system, in combination with the non-equilibrium molecular dynamics (NEMD) method, the paper reveals the effect of adsorption behavior of surfactant molecules on film formation at the oil-water interface in the shear flow field, and also explains the mechanisms by which shear velocity, temperature, pressure, and surfactant concentration affect the stability of film formation at the interface. The results show that the stability of film formation at the interface can be affected by the adsorption configuration of SDBS molecules, the hydrogen bonds formed between SDBS molecules and water molecules, and the diffusivity of oil oleic and aqueous phase molecules; applying shear stress is a process of instability to stability for the oil-water interface system, and the stability of film formation at the surface decreases with the increase of shear velocity; in the range of SDBS concentration of 0.15-0.70 mol/L, the thickness of the interfacial film increases from 1.43-2.13 nm under static working conditions to 2.74-2.93 nm under dynamic shear working conditions; the increase of temperature will destroy the hydrogen bonds formed between molecules, leading to interface instability, and the increase of pressure will shorten the length of hydrogen bonds formed between molecules, enhancing the strength of intermolecular interactions and being conductive to the stability of the interface.

Key words: shear flow field, surfactant, oil-water interfacial film, molecular simulation, non-equilibrium dynamics

摘要： 表面活性剂分子在油-水界面会形成排列有序的结构从而促进界面成膜,厘清这种分子吸附行为对油-水界面成膜的影响及其微观机制,对于构建原油采输工艺油-水乳状液稳定与失稳的方法具有重要意义。以常见阴离子型表面活性剂十二烷基苯磺酸钠(SDBS)为例,利用移动平板壁面为原油-SDBS-矿物水界面模拟体系赋予剪切流场,采用非平衡动力学模拟(NEMD)方法,揭示了剪切流场中表面活性剂分子吸附行为对油-水界面成膜的影响,解释了剪切速度、温度、压力及表面活性剂浓度影响界面成膜稳定性的作用机制。研究结果表明,SDBS分子的吸附构型、SDBS分子与水分子形成的氢键,以及油、水相分子的扩散能力均会影响界面成膜稳定性;施加剪切对于油-水界面体系是一个先失稳而又重构稳定的过程,且界面成膜稳定性会随剪切速度的增大而降低;在SDBS浓度同为0.15~0.70 mol/L的区间内,界面膜厚度由静态工况时的1.43~2.13 nm增加至动态剪切工况下的2.74~2.93 nm;温度升高会破坏分子间形成的氢键导致界面失稳,压力升高则缩短了分子间形成氢键的键长,增强了分子间相互作用强度,益于界面保持稳定。

关键词: 剪切流场, 表面活性剂, 油-水界面膜, 分子模拟, 非平衡动力学