Dipartimento CMIC, Politecnico di Milano, via Ponzio 34/3, 20133 Milano, Italy.
J Chem Phys. 2010 Mar 28;132(12):124902. doi: 10.1063/1.3366690.
A large variety of engaging phenomena stems from the occurrence of short-ranged attractive depletion forces. Yet, so far, most experimental studies have been interpreted on the basis of the simple Asakura-Oosawa model, where the depletion agent can be regarded as ideal. Here, conversely, we focus on a system where strong electrostatic coupling is present in the suspension. Specifically, from measurements of equilibrium sedimentation profiles, we obtain an equation of states for a colloidal system where depletion forces are tuned by the addition of a surfactant. At fixed colloid volume fraction, colloidal aggregation takes place when the surfactant concentration reaches a critical value which rises for increasing ionic strength. Screening repulsive electrostatic interactions inhibits the depletion mechanism and weakens the effective colloid-colloid attraction. The metastable coexistence curve displays the universal scaling behavior predicted for short-ranged potentials. The experimental data are compared with the theoretical predictions of a simple model which includes only electrostatic interactions. The effective depletion force on the colloids is evaluated by using the hypernetted-chain equation of liquid state theory at different salt concentrations. This model provides a convincing interpretation of the observed enhancement of the depletion mechanism by Coulomb repulsion.
大量吸引人的现象源于短程吸引力耗散力的发生。然而，到目前为止，大多数实验研究都是基于简单的浅野-大泽模型进行解释的，在该模型中，耗散剂可以被视为理想的。相比之下，我们关注的是悬浮液中存在强静电耦合的系统。具体来说，通过测量平衡沉降曲线，我们获得了胶体系统的状态方程，其中通过添加表面活性剂来调节耗散力。在固定胶体体积分数的情况下，当表面活性剂浓度达到临界值时，胶体聚集会发生，而临界值会随着离子强度的增加而升高。屏蔽排斥静电相互作用会抑制耗散机制并削弱有效胶体-胶体吸引力。亚稳共存曲线显示了短程势预测的普遍标度行为。实验数据与仅包括静电相互作用的简单模型的理论预测进行了比较。通过在不同盐浓度下使用液体状态理论的超网链方程，评估了胶体上的有效耗散力。该模型为观察到的库仑排斥增强耗散机制提供了令人信服的解释。
