Santo Kolattukudy P, Vishnyakov Aleksey, Brun Yefim, Neimark Alexander V
Department of Chemical and Biochemical Engineering, Rutgers University , Piscataway, New Jersey 08854, United States.
DuPont Central Research & Development , Wilmington, Delaware 19803, United States.
Langmuir. 2018 Jan 30;34(4):1481-1496. doi: 10.1021/acs.langmuir.7b02914. Epub 2017 Oct 3.
This work explores interactions of functionalized nanoparticles (NP) with polymer brushes (PB) in a binary mixture of good and poor solvents. NP-PB systems are used in multiple applications, and we are particularly interested in the problem of chromatographic separation of NPs on polymer-grafted porous columns. This process involves NP flow through the pore channels with walls covered by PBs. NP-PB adhesion is governed by adsorption of polymer chains to NP surface and entropic repulsion caused by the polymer chain confinement between NP and the channel wall. Both factors depend on the solvent composition, variation of which causes contraction or expansion of PB. Using dissipative particle dynamics simulations in conjunction with the ghost tweezers free energy calculation technique, we examine the free energy landscapes of functionalized NPs within PB-grafted channels depending on the solvent composition at different PB grafting densities and polymer-solvent affinities. The free energy landscape determines the probability of NP location at a given distance to the surface, positions of equilibrium adhesion states, and the Henry constant that characterizes adsorption equilibrium and NP partitioning between the stationary phase of PB and mobile phase of flowing solvent. We analyze NP transport through a polymer-grafted channel and calculate the mean velocity and retention time of NP depending on the NP size and solvent composition. We find that, with the increase of the bad (poor) solvent fraction and respective PB contraction, NP separation exhibits a transition from the hydrodynamic size exclusion regime with larger NPs having shorter retention time to the adsorption regime with smaller NPs having shorter retention time. The observed reversal of the sequence of elution is reminiscent of the critical condition in polymer chromatography at which the retention time is molecular weight independent. This finding suggests the possibility of the existence of an analogous special regime in nanoparticle chromatography at which NPs with like surface properties elute together regardless of their size. The latter has important practical implications: NPs can be separated by surface chemistry rather than by their size employing the gradient mode of elution with controlled variation of solvent composition.
本工作探索了功能化纳米颗粒（NP）与聚合物刷（PB）在良溶剂和不良溶剂二元混合物中的相互作用。NP-PB体系有多种应用，我们特别关注聚合物接枝多孔柱上NP的色谱分离问题。此过程涉及NP流经孔道，孔道壁上覆盖着PB。NP与PB的粘附受聚合物链在NP表面的吸附以及NP与孔道壁之间聚合物链受限引起的熵排斥作用控制。这两个因素都取决于溶剂组成，溶剂组成的变化会导致PB收缩或膨胀。结合耗散粒子动力学模拟和虚镊子自由能计算技术，我们研究了接枝PB的通道内功能化NP的自由能景观，该景观取决于不同PB接枝密度和聚合物-溶剂亲和力下的溶剂组成。自由能景观决定了NP在距表面给定距离处的位置概率、平衡粘附状态的位置以及表征吸附平衡和NP在PB固定相和流动溶剂流动相之间分配的亨利常数。我们分析了NP通过聚合物接枝通道的传输，并根据NP尺寸和溶剂组成计算了NP的平均速度和保留时间。我们发现，随着不良（差）溶剂分数的增加以及相应PB的收缩，NP分离表现出从较大NP保留时间较短的流体动力学尺寸排阻模式到较小NP保留时间较短的吸附模式的转变。观察到的洗脱顺序逆转让人联想到聚合物色谱中的临界条件，即保留时间与分子量无关。这一发现表明在纳米颗粒色谱中可能存在类似的特殊模式，在该模式下具有相似表面性质的NP无论其尺寸如何都会一起洗脱。后者具有重要的实际意义：可以通过表面化学而非尺寸来分离NP，采用溶剂组成可控变化的梯度洗脱模式。
