The structure and interactions of coated silica nanoparticles have beenstudied in water using molecular dynamics simulations. For 5 nm diameteramorphous silica nanoparticles we studied the effects of varying the chainlength and grafting density of polyethylene oxide (PEO) on the nanoparticlecoating's shape and on nanoparticle-nanoparticle effective forces. For shortligands of length $n=6$ and $n=20$ repeat units, the coatings are radiallysymmetric while for longer chains ($n=100$) the coatings are highlyanisotropic. This anisotropy appears to be governed primarily by chain length,with coverage playing a secondary role. For the largest chain lengthsconsidered, the strongly anisotropic shape makes fitting to a simple radialforce model impossible. For shorter ligands, where the coatings are isotropic,we found that the force between pairs of nanoparticles is purely repulsive andcan be fit to the form $(R/2r_\text{core}-1)^{-b}$ where $R$ is the separationbetween the center of the nanoparticles, $r_\text{core}$ is the radius of thesilica core, and $b$ is measured to be between 2.3 and 4.1.
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机译：已使用分子动力学模拟在水中研究了包覆的二氧化硅纳米粒子的结构和相互作用。对于直径为5 nm的非晶态二氧化硅纳米粒子，我们研究了改变聚环氧乙烷（PEO）的链长和接枝密度对纳米粒子涂层形状和纳米粒子-纳米粒子有效力的影响。对于长度为$ n = 6 $和$ n = 20 $重复单元的短配体，涂层是径向对称的，而对于较长链（$ n = 100 $），涂层是高度各向异性的。这种各向异性似乎主要由链长决定，而覆盖率起次要作用。对于考虑的最大链长，强烈的各向异性形状使得无法拟合简单的径向力模型。对于较短的配体（涂层是各向同性的），我们发现成对的纳米粒子之间的力是纯排斥力，可以拟合为$（R / 2r_ \ text {core} -1）^ {-b} $的形式，其中$ R $是纳米粒子中心之间的距离，$ r_text {core} $是二氧化硅核心的半径，而$ b $的测量值介于2.3和4.1之间。
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