Allison Stuart
Department of Chemistry, Georgia State University, Atlanta, GA 30303, USA.
J Colloid Interface Sci. 2004 Sep 1;277(1):248-54. doi: 10.1016/j.jcis.2004.04.050.
A spherical gel layer model of colloidal particles is used to analyze the electrophoretic mobility and viscosity of a dilute suspension of the silica sol, Ludox, reported previously by Laven and Stein (J. Laven, H.N. Stein, J. Colloid Interface Sci. 238 (2001) 8-15). The colloid is modeled as a sphere with a solid inner core surrounded by a diffuse gel layer of uniform thickness and comprising a specific fraction, f, of the colloidal particle's mass. The gel layer is accessible to solvent and ions, but the gel layer retards the flow of solvent, which is assumed to obey the Brinkman equation. The colloidal charge is assumed to be spherically symmetric, but its disposition on the surface of the core particle and gel layer (alpha = fraction of charge in the gel layer) is left as an adjustable parameter. Experiments at pH 5.7 and 8.7 over a KCl concentration range of 0.3 to 80 mM are examined. At high salt and/or low pH, the thickness of the gel layer is estimated to be 1.0 to 1.6 nm depending on the assumed fraction of silica present in the gel layer. At low salt (0.3 mM) and high pH, where the net absolute charge of Ludox is large, the thickness of the gel layer is estimated to be 3.7 to 4.1 nm. Thus, the thickness of the gel layer appears to increase with decreasing salt at high pH. The net charge required to simultaneously match experimental and model mobilities and viscosities is sensitive to the choice of f and alpha. Nonetheless, for reasonable choices of these parameters (f = 0.13 and alpha approximately equal to 1.0), the estimated net absolute charges of Ludox from present modeling are in good agreement with the titration charges of Bolt (G.H. Bolt, J. Phys. Chem. 61 (1957) 1166-1169), and Milonjic (S.K. Milonjic, Colloids Surf. 23 (1987) 301-311) over the entire salt concentration range at pH 8.7. At pH 5.7, however, the estimated net absolute charge from current modeling exceeds the Bolt values by about 50%.
采用胶体颗粒的球形凝胶层模型，对先前Laven和Stein（J. Laven, H.N. Stein, J. Colloid Interface Sci. 238 (2001) 8 - 15）报道的硅溶胶Ludox稀悬浮液的电泳迁移率和粘度进行分析。该胶体被建模为一个球体，其内部有一个实心内核，周围是一层厚度均匀的扩散凝胶层，凝胶层占胶体颗粒质量的特定比例f。溶剂和离子可以进入凝胶层，但凝胶层会阻碍溶剂流动，假定溶剂流动服从布林克曼方程。假定胶体电荷呈球对称，但电荷在核心颗粒表面和凝胶层上的分布（α = 凝胶层中电荷的比例）作为一个可调参数。研究了在pH值为5.7和8.7、KCl浓度范围为0.3至80 mM条件下的实验。在高盐和/或低pH值下，根据假定的凝胶层中二氧化硅的比例，凝胶层厚度估计为1.0至1.6 nm。在低盐（0.3 mM）和高pH值下，Ludox的净绝对电荷较大，凝胶层厚度估计为3.7至4.1 nm。因此，在高pH值下，凝胶层厚度似乎随盐浓度降低而增加。同时匹配实验和模型迁移率及粘度所需的净电荷对f和α的选择很敏感。尽管如此，对于这些参数的合理选择（f = 0.13且α约等于1.0），当前模型估计的Ludox净绝对电荷在pH值为8.7的整个盐浓度范围内与Bolt（G.H. Bolt, J. Phys. Chem. 61 (1957) 1166 - 1169）和Milonjic（S.K. Milonjic, Colloids Surf. 23 (1987) 301 - 311）的滴定电荷吻合良好。然而，在pH值为5.7时，当前模型估计的净绝对电荷比Bolt值高出约50%。
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