| Summary: | 碩士 === 國立中央大學 === 化學工程與材料工程研究所 === 97 === A colloidal dispersion, consisting of many charged particles and small ions, is a very complicated system owing to the long-range nature of the electrostatic interactions. Counterions are not distributed evenly in the solution but prefer to stay in the vicinity of the oppositely charged colloid. The extent of counterions residing near the particles is generally depicted by the concept of counterion condensation, which mediates how colloids interact among themselves and thus influence the physical properties of colloidal dispersion.
The concept of counterion condensation basically characterizes a battle fought between energy and entropy in minimizing the free energy of a solution of mobile charges in the neighborhood of charged particles. The counterion-condensation transition (CCT) at infinitely long charged cylinders in the infinite-dilution limit and infinitely low added salt concentration has been extensively studied. For example, Manning''s theory of counterion condensation predicts a certain quantity of counterions condenses onto a stiff polymer whose charge density exceeds a critical value.
In reality, the colloidal dispersion is at finite concentration. In this work the CCT at charged spheres is investigated using Monte Carlo simulations for an ideal model (neglecting effects of inhomogeneous surface charges) based on Wigner-Seitz cell. The crossover temperature T* is identified by the behavior of energy and heat capacity. In addition to establish the relation between effective charge and CCT, the effects of particle charge, particle concentration and salt addition on the crossover temperature are explored as well.
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