Institute of Solution Chemistry, Russian Academy of Sciences, Akademicheskaya St. 1, 153045 Ivanovo, Russia.
J Chem Phys. 2013 Nov 21;139(19):194901. doi: 10.1063/1.4828435.
We report a novel phenomenon of a surface-induced phase transition in salt-free solutions of charged colloids. We develop a theory of this effect and confirm it by Molecular Dynamics simulations. To describe the colloidal solution we apply a primitive model of electrolyte with a strong asymmetry of charge and size of the constituent particles - macroions and counterions. To quantify interactions of the colloidal particles with the neutral substrate we use a short-range potential which models dispersion van der Waals forces. These forces cause the attraction of colloids to the surface. We show that for high temperatures and weak attraction, only gradual increase of the macroion concentration in the near-surface layer is observed with increase of interaction strength. If however temperature drops below some threshold value, a new dense (liquid) phase is formed in the near-surface layer. It can be interpreted as a surface-induced first-order phase transition with a critical point. Using an appropriately adopted Maxwell construction, we find the binodal. Interestingly, the observed near-surface phase transition can occur at the absence of the bulk phase transition and may be seemingly classified as prewetting transition. The reported effect could be important for various technological applications where formation of colloidal particle layers with the desired properties is needed.
我们报告了在无盐带电胶体溶液中一种新的表面诱导相转变现象。我们发展了这一效应的理论，并通过分子动力学模拟进行了验证。为了描述胶体溶液，我们应用了一种具有强烈不对称电荷和尺寸的电解质的原始模型，其中包括大分子离子和抗衡离子。为了量化胶体颗粒与中性基底的相互作用，我们使用了一个短程势，该势模型了色散范德华力。这些力导致胶体对表面的吸引力。我们表明，对于高温和弱吸引力，仅在增加相互作用强度时，才会观察到近表面层中大分子离子浓度的逐渐增加。然而，如果温度降至某个阈值以下，则会在近表面层中形成新的密集（液态）相。可以将其解释为具有临界点的表面诱导一级相变。使用适当采用的麦克斯韦构造，我们找到了双节点。有趣的是，观察到的近表面相变可能发生在没有体相相变的情况下，并且可能被归类为预湿转变。所报道的效应可能对各种技术应用很重要，这些应用需要形成具有所需性质的胶体颗粒层。
