Yuan Fang, Wang Shihu, Larson Ronald G
Department of Chemical Engineering, University of Michigan , Ann Arbor, Michigan 48109-2136, United States.
Langmuir. 2015 Feb 3;31(4):1336-43. doi: 10.1021/la5044393. Epub 2015 Jan 21.
We calculate potentials of mean force (PMFs) and mean first passage times for a surfactant to escape a micelle, for both ionic sodium dodecyl sulfate (SDS) and nonionic ethoxylated alcohol (C12E5) micelles using both atomistic and coarse-grained molecular dynamics (MD) simulations. The PMFs are obtained by umbrella sampling and used in a Smoluchowski first-passage-time theory to obtain the times for a surfactant to escape a micelle. The calculated mean first passage time for an SDS molecule to break away from a micelle (with an aggregation number of 60) is around 2 μs, which is consistent with previous experimental measurements of the "fast relaxation time" for exchange of surfactants between the micellar phase and the bulk solvent. The corresponding escape time calculated for a nonionic ethoxylated alcohol C12E5, with the same tail length as SDS, is 60 μs, which is significantly longer than for SDS primarily because the PMF for surfactant desorption is about 3kT smaller than for C12E5. We also show that two coarse-grained (CG) force fields, MARTINI and SDK, give predictions similar to the atomistic CHARMM force field for the nonionic C12E5 surfactant, but for the ionic SDS surfactant, the CG simulations give a PMF similar to that obtained with CHARMM only if long-range electrostatic interactions are included in the CG simulations, rather than using a shifted truncated electrostatic interaction. We also calculate that the mean first passage time for an SDS and a C12E5 to escape from a latex binder surface is of the order of milliseconds, which is more than 100 times longer than the time for escape from the micelle, indicating that in latex waterborne coatings, SDS and C12E5 surfactants likely bind preferentially to the latex polymer interface rather than form micelles, at least at low surfactant concentrations.
我们使用原子istic和粗粒度分子动力学（MD）模拟，计算了离子型十二烷基硫酸钠（SDS）和非离子型乙氧基化醇（C12E5）胶束中表面活性剂逃离胶束的平均力势（PMF）和平均首次通过时间。通过伞形采样获得PMF，并将其用于Smoluchowski首次通过时间理论中，以获得表面活性剂逃离胶束的时间。计算得出，SDS分子从胶束（聚集数为60）脱离的平均首次通过时间约为2微秒，这与先前关于表面活性剂在胶束相和本体溶剂之间交换的“快速弛豫时间”的实验测量结果一致。对于具有与SDS相同尾长的非离子型乙氧基化醇C12E5，计算出的相应逃逸时间为60微秒，这明显长于SDS，主要是因为表面活性剂解吸的PMF比C12E5小约3kT。我们还表明，对于非离子型C12E5表面活性剂，两种粗粒度（CG）力场MARTINI和SDK给出的预测与原子istic CHARMM力场相似，但对于离子型SDS表面活性剂，只有在CG模拟中包含长程静电相互作用，而不是使用移位截断静电相互作用时，CG模拟才能给出与CHARMM获得的PMF相似的结果。我们还计算得出，SDS和C12E5从乳胶粘合剂表面逃逸的平均首次通过时间为毫秒级，比从胶束逃逸的时间长100多倍，这表明在乳胶水性涂料中，至少在低表面活性剂浓度下，SDS和C12E5表面活性剂可能优先结合到乳胶聚合物界面而不是形成胶束。
