Weiand Erik, Rodriguez-Ropero Francisco, Roiter Yuri, Koenig Peter H, Angioletti-Uberti Stefano, Dini Daniele, Ewen James P
Department of Mechanical Engineering, Imperial College London, South Kensington Campus, SW7 2AZ London, UK.
Institute of Molecular Science and Engineering, Imperial College London, South Kensington Campus, SW7 2AZ London, UK.
Phys Chem Chem Phys. 2023 Aug 23;25(33):21916-21934. doi: 10.1039/d3cp02546b.
The properties of solid-liquid interfaces can be markedly altered by surfactant adsorption. Here, we use molecular dynamics (MD) simulations to study the adsorption of ionic surfactants at the interface between water and heterogeneous solid surfaces with randomly arranged hydrophilic and hydrophobic regions, which mimic the surface properties of human hair. We use the coarse-grained MARTINI model to describe both the hair surfaces and surfactant solutions. We consider negatively-charged virgin and bleached hair surface models with different grafting densities of neutral octadecyl and anionic sulfonate groups. The adsorption of cationic cetrimonium bromide (CTAB) and anionic sodium dodecyl sulfate (SDS) surfactants from water are studied above the critical micelle concentration. The simulated adsorption isotherms suggest that cationic surfactants adsorb to the surfaces a two-stage process, initially forming monolayers and then bilayers at high concentrations, which is consistent with previous experiments. Anionic surfactants weakly adsorb hydrophobic interactions, forming only monolayers on both virgin and medium bleached hair surfaces. We also conduct non-equilibrium molecular dynamics simulations, which show that applying cationic surfactant solutions to bleached hair successfully restores the low friction seen with virgin hair. Friction is controlled by the combined surface coverage of the grafted lipids and the adsorbed CTAB molecules. Treated surfaces containing monolayers and bilayers both show similar friction, since the latter are easily removed by compression and shear. Further wetting MD simulations show that bleached hair treated with CTAB increases the hydrophobicity to similar levels seen for virgin hair. Treated surfaces containing CTAB monolayers with the tailgroups pointing predominantly away from the surface are more hydrophobic than bilayers due to the electrostatic interactions between water molecules and the exposed cationic headgroups.
表面活性剂的吸附可显著改变固液界面的性质。在此，我们使用分子动力学（MD）模拟来研究离子型表面活性剂在水与具有随机排列的亲水和疏水区域的异质固体表面之间的界面上的吸附情况，该异质固体表面模拟了人类头发的表面性质。我们使用粗粒度的MARTINI模型来描述头发表面和表面活性剂溶液。我们考虑了具有不同中性十八烷基和阴离子磺酸盐基团接枝密度的带负电荷的原生头发和漂白头发表面模型。研究了在临界胶束浓度以上，阳离子十六烷基三甲基溴化铵（CTAB）和阴离子十二烷基硫酸钠（SDS）表面活性剂从水中的吸附情况。模拟的吸附等温线表明，阳离子表面活性剂通过两阶段过程吸附到表面，最初形成单层，然后在高浓度下形成双层，这与先前的实验一致。阴离子表面活性剂通过疏水相互作用弱吸附，在原生头发和中度漂白头发表面上仅形成单层。我们还进行了非平衡分子动力学模拟，结果表明将阳离子表面活性剂溶液应用于漂白头发可成功恢复原生头发所具有的低摩擦。摩擦力由接枝脂质和吸附的CTAB分子的综合表面覆盖率控制。含有单层和双层的处理过的表面显示出相似的摩擦力，因为后者很容易通过压缩和剪切去除。进一步的润湿MD模拟表明，用CTAB处理的漂白头发将疏水性提高到与原生头发相似的水平。由于水分子与暴露的阳离子头基之间的静电相互作用，含有CTAB单层且尾基主要指向远离表面的处理过的表面比双层更疏水。
