Artie McFerrin Department of Chemical Engineering, Texas A&M University, 3122 TAMU, College Station, Texas 77843-3122, United States.
Langmuir. 2010 Dec 7;26(23):18459-67. doi: 10.1021/la1035129. Epub 2010 Nov 4.
Tetramethylammonium (TMA)- and tetrapropylammonium (TPA)-silica mixtures containing monovalent salts were studied to determine how salt impacts nanoparticle stability and organocation-silica interactions. Dynamic light scattering (DLS) results show that salt concentrations as low as 5 mM can induce nanoparticle aggregation. The extent of aggregation increases with the ionic size of the alkali-metal cations, consistent with the Hoffmeister series. Thus specific ion effects are observed in these mixtures. Pulsed-field gradient (PFG) NMR shows a more obvious increase in the self-diffusion coefficient of TPA than TMA in the presence of salt, indicating TPA is more easily displaced from the nanoparticle surface due to the background electrolyte. A two-site model is used to describe the exchange between tetraalkylammonuim (TAA) adsorbed on the nanoparticles and TAA in solution, from which the binding isotherms of the organocations at low electrolyte concentration was obtained and analyzed using the Langmuir formalism. This analysis also shows specific-ion effects, with the amount of TPA adsorbed to be much smaller than TMA and also much more sensitive to the presence of salt. In the context of the oriented aggregation mechanism proposed previously in the literature, the current work suggests one route for tuning the organocation-particle interaction and thus a route to controlling the rates of some steps in the mechanism.
研究了含有单价盐的四甲基铵（TMA）和四丙基铵（TPA）-硅胶混合物，以确定盐如何影响纳米颗粒的稳定性和有机阳离子-硅胶相互作用。动态光散射（DLS）结果表明，盐浓度低至 5mM 即可诱导纳米颗粒聚集。聚集的程度随碱金属阳离子的离子尺寸增加而增加，与霍夫曼系列一致。因此，在这些混合物中观察到特定离子效应。脉冲梯度（PFG）NMR 表明，在存在盐的情况下，TPA 的自扩散系数比 TMA 增加得更明显，这表明由于背景电解质，TPA 更容易从纳米颗粒表面置换。使用双位点模型来描述吸附在纳米颗粒上的四烷基铵（TAA）与溶液中 TAA 之间的交换，从该模型获得了低电解质浓度下有机阳离子的结合等温线，并使用朗缪尔形式对其进行了分析。该分析还表明存在特定离子效应，吸附到 TPA 的量比 TMA 小得多，并且对盐的存在也更加敏感。在文献中先前提出的定向聚集机制的背景下，目前的工作表明了一种调节有机阳离子-颗粒相互作用的途径，从而控制该机制中某些步骤的速率。
