Department of Chemical and Biochemical Engineering, Rutgers, The State University of New Jersey, 98 Brett Road, Piscataway, NJ 08854, USA.
DuPont Central Research & Development, Corporate Center for Analytical Sciences, Macromolecular Characterization, Wilmington, DE 19803, USA.
J Colloid Interface Sci. 2016 Nov 1;481:181-93. doi: 10.1016/j.jcis.2016.07.019. Epub 2016 Jul 15.
Polymer adsorption is a ubiquitous phenomenon with numerous technological and healthcare applications. The mechanisms of polymer adsorption on surfaces and in pores are complex owing to a competition between various entropic and enthalpic factors. Due to adsorption of monomers to the surface, the chain gains in enthalpy yet loses in entropy because of confining effects. This competition leads to the existence of critical conditions of adsorption when enthalpy gain and entropy loss are in balance. The critical conditions are controlled by the confining geometry and effective adsorption energy, which depends on the solvent composition and temperature. This phenomenon has important implications in polymer chromatography, since the retention at the critical point of adsorption (CPA) is chain length independent. However, the mechanisms of polymer adsorption in pores are poorly understood and there is an ongoing discussion in the theoretical literature about the very existence of CPA for polymer adsorption on porous substrates. In this work, we examine the mechanisms of chain adsorption on a model porous substrate using Monte Carlo (MC) simulations. We distinguish three adsorption mechanisms depending on the chain location: on external surface, completely confined in pores, and also partially confined in pores in so-called "flower" conformations. The free energies of different conformations of adsorbed chains are calculated by the incremental gauge cell MC method that allows one to determine the partition coefficient as a function of the adsorption potential, pore size, and chain length. We confirm the existence of the CPA for chain length independent separation on porous substrates, which is explained by the dominant contributions of the chain adsorption at the external surface, in particular in flower conformations. Moreover, we show that the critical conditions for porous and nonporous substrates are identical and depend only on the surface chemistry. The theoretical results are confirmed by comparison with experimental data on chromatographic separation of a series of linear polystyrenes.
聚合物吸附是一种普遍存在的现象，具有许多技术和医疗保健应用。由于各种熵和焓因素的竞争，聚合物在表面和孔中的吸附机制非常复杂。由于单体吸附到表面，链获得焓，但由于限制效应而失去熵。这种竞争导致吸附的临界条件存在，即焓增益和熵损失达到平衡。临界条件由限制几何形状和有效吸附能控制，这取决于溶剂组成和温度。这种现象在聚合物色谱学中具有重要意义，因为在吸附的临界点（CPA）处的保留与链长无关。然而，聚合物在孔中的吸附机制理解得很差，在理论文献中存在关于聚合物在多孔基质上吸附的 CPA 是否存在的持续讨论。在这项工作中，我们使用蒙特卡罗（MC）模拟研究了模型多孔基质上链吸附的机制。我们根据链的位置区分三种吸附机制：在外部表面上、完全限制在孔中以及在所谓的“花”构象中部分限制在孔中。通过增量规胞 MC 方法计算不同吸附链构象的自由能，该方法允许确定作为吸附势、孔径和链长函数的分配系数。我们证实了在多孔基质上存在链长独立分离的 CPA，这可以通过外部表面上的链吸附的主要贡献来解释，特别是在花构象中。此外，我们表明，多孔和非多孔基质的临界条件相同，仅取决于表面化学。理论结果通过与一系列线性聚苯乙烯的色谱分离实验数据进行比较得到了证实。
