Valle-Delgado J J, Molina-Bolívar J A, Galisteo-González F, Gálvez-Ruiz M J, Feiler A, Rutland M W
Grupo de Física de Fluidos y Biocoloides, Departamento de Física Aplicada, Facultad de Ciencias, Universidad de Granada, Spain.
J Chem Phys. 2005 Jul 15;123(3):34708. doi: 10.1063/1.1954747.
Silica is a very interesting system that has been thoroughly studied in the last decades. One of the most outstanding characteristics of silica suspensions is their stability in solutions at high salt concentrations. In addition to that, measurements of direct-interaction forces between silica surfaces, obtained by different authors by means of surface force apparatus or atomic force microscope (AFM), reveal the existence of a strong repulsive interaction at short distances (below 2 nm) that decays exponentially. These results cannot be explained in terms of the classical Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory, which only considers two types of forces: the electrical double-layer repulsion and the London-van der Waals attraction. Although there is a controversy about the origin of the short-range repulsive force, the existence of a structured layer of water molecules at the silica surface is the most accepted explanation for it. The overlap of structured water layers of different surfaces leads to repulsive forces, which are known as hydration forces. This assumption is based on the very hydrophilic nature of silica. Different theories have been developed in order to reproduce the exponentially decaying behavior (as a function of the separation distance) of the hydration forces. Different mechanisms for the formation of the structured water layer around the silica surfaces are considered by each theory. By the aid of an AFM and the colloid probe technique, the interaction forces between silica surfaces have been measured directly at different pH values and salt concentrations. The results confirm the presence of the short-range repulsion at any experimental condition (even at high salt concentration). A comparison between the experimental data and theoretical fits obtained from different theories has been performed in order to elucidate the nature of this non-DLVO repulsive force.
二氧化硅是一个非常有趣的体系，在过去几十年里得到了深入研究。二氧化硅悬浮液最显著的特性之一是它们在高盐浓度溶液中的稳定性。除此之外，不同作者通过表面力仪或原子力显微镜（AFM）获得的二氧化硅表面之间直接相互作用力的测量结果表明，在短距离（低于2纳米）存在强烈的排斥相互作用，且这种相互作用呈指数衰减。这些结果无法用经典的德亚金、朗道、维韦和奥弗贝克（DLVO）理论来解释，该理论只考虑两种力：电双层排斥力和伦敦-范德华吸引力。尽管关于短程排斥力的起源存在争议，但二氧化硅表面存在结构化水分子层是对此最被认可的解释。不同表面的结构化水层重叠会导致排斥力，即所谓的水化力。这一假设基于二氧化硅极强的亲水性。为了重现水化力（作为分离距离的函数）的指数衰减行为，人们发展了不同的理论。每种理论都考虑了二氧化硅表面周围结构化水层形成的不同机制。借助AFM和胶体探针技术，已在不同pH值和盐浓度下直接测量了二氧化硅表面之间的相互作用力。结果证实，在任何实验条件下（即使在高盐浓度下）都存在短程排斥力。为了阐明这种非DLVO排斥力的本质，已对实验数据与从不同理论获得的理论拟合结果进行了比较。
