Department of Physics, North Dakota State University, Fargo, North Dakota 58108-6050, USA.
J Chem Phys. 2021 Dec 7;155(21):214904. doi: 10.1063/5.0064282.
Ionic microcapsules are hollow shells of hydrogel, typically 10-1000 nm in radius, composed of cross-linked polymer networks that become charged and swollen in a good solvent. The ability of microcapsules to swell/deswell in response to changes in external stimuli (e.g., temperature, pH, and ionic strength) suits them to applications, such as drug delivery, biosensing, and catalysis. The equilibrium swelling behavior of ionic microcapsules is determined by a balance of electrostatic and elastic forces. The electrostatic component of the osmotic pressure of a microcapsule-the difference in the pressure between the inside and outside of the particle-plays a vital role in determining the swelling behavior. Within the spherical cell model, we derive exact expressions for the radial pressure profile and for the electrostatic and gel components of the osmotic pressure of a microcapsule, which we compute via Poisson-Boltzmann theory and molecular dynamics simulation. For the gel component, we use the Flory-Rehner theory of polymer networks. By combining the electrostatic and gel components of the osmotic pressure, we compute the equilibrium size of ionic microcapsules as a function of particle concentration, shell thickness, and valence. We predict concentration-driven deswelling at relatively low concentrations at which steric interactions between particles are weak and demonstrate that this response can be attributed to crowding-induced redistribution of counterions. Our approach may help to guide the design and applications of smart stimuli-responsive colloidal particles.
离子微胶囊是水凝胶的中空壳，通常半径为 10-1000nm，由交联聚合物网络组成，在良溶剂中带电荷并溶胀。微胶囊能够响应外部刺激（例如温度、pH 值和离子强度）而溶胀/收缩的能力使其适用于药物输送、生物传感和催化等应用。离子微胶囊的平衡溶胀行为取决于静电和弹性力的平衡。微胶囊渗透压的静电分量——粒子内外的压力差——在确定溶胀行为方面起着至关重要的作用。在球形细胞模型中，我们推导出了微胶囊的径向压力分布以及渗透压的静电和凝胶分量的精确表达式，我们通过泊松-玻尔兹曼理论和分子动力学模拟来计算这些表达式。对于凝胶分量，我们使用聚合物网络的 Flory-Rehner 理论。通过组合渗透压的静电和凝胶分量，我们计算了离子微胶囊的平衡尺寸作为粒子浓度、壳层厚度和价态的函数。我们预测了在相对较低的浓度下，由于粒子间的空间相互作用较弱而导致的浓度驱动的去溶胀，并且证明这种响应可以归因于拥挤诱导的抗衡离子重新分布。我们的方法可能有助于指导智能响应性胶体粒子的设计和应用。
