稀薄膠體溶液之有效電荷; Effective charge of a dilute colloidal solution

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题名:	稀薄膠體溶液之有效電荷;Effective charge of a dilute colloidal solution
作者:	王子瑜;Tzu-yu Wang
贡献者:	化學工程與材料工程研究所
关键词:	唐南平衡;沉降平衡;聚電解質;有效電荷;膠體;donnan equilibrium;sedimentation equilibrium;polyelectrolytes;effective charge;colloids
日期:	2008-12-02
上传时间:	2009-09-21 12:25:26 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	在稀薄的膠體粒子溶液中，由於膠體粒子會解離出反離子(counterion)，使粒子帶有電荷Z。散佈在溶液中的反離子受到膠體粒子電荷的影響，而靠近粒子表面，即稱之為反離子凝聚(condensation)現象。因此遠處所感受到膠體粒子的電荷不再是Z價電荷，而是有效電荷Z*。此現象可藉由蒙地卡羅模擬其反離子的離子化程度( )來進行研究。離子化程度定義為實際測得之反離子濃度與其本質上應有之反離子濃度的比值。在實驗上，利用擁有相同反離子化學勢能的電解質溶液濃度可直接測得反離子濃度，例如使用離子選擇性電極。本實驗之膠體粒子溶液系統，分別就未添加鹽類(salt free)、添加一價鹽類(monovalent salt)、添加多價鹽類(multivalent salt)等三個不同方向進行研究。在未添加鹽類系統中，解離度會受到膠體粒子的濃度、本質價數、溶劑種類以及反離子價數的影響；在添加鹽類系統中，解離度隨著鹽類濃度增加而變大，但是膠體的有效電荷卻隨鹽類濃度增加而變小。此外，不同價數的鹽類對解離度之影響亦不同。利用上述之研究方法亦成功的模擬反離子凝聚現象對聚電解質(polyelectrolyte)溶液性質之影響。在稀薄的膠體溶液中，膠體粒子受到重力和熱擾動互相競爭而最終達到沉降平衡，形成一個非均勻濃度分佈，稱之為大氣分佈(barometric distribution)。然而，即使在低體積分率下(例如：10-4)，帶電粒子之間的靜電作用亦會造成其濃度分佈與大氣分佈有著重大的偏差。在稀薄、未加鹽的膠體溶液系統中，發現在沉降平衡系統中因其濃度的分佈，可以粗略的區分為五個部份。分別從這五個部份的膠體粒子和反離子的濃度分佈中，進而觀察反離子凝聚效應的程度。由模擬結果中得知造成Non-barometric distribution的原因主要來自於膠體粒子彼此之間的斥力作用，而和膠體粒子與反離子之間的吸引力無關。再進一步觀察反離子的濃度分佈發現，當膠體粒子與反離子之間為弱吸引力時，反離子在系統溶液中均勻分佈，並隨著彼此之間吸引力增加的同時，反離子會逐漸吸附在膠體粒子週遭，使得膠體粒子彼此之間斥力作用減弱，膠體粒子的濃度分佈則遵守大氣分佈。整體而言，在Non-barometric distribution情況下，溶液的局部並不會達到電中性，而內部電場的存在並不是造成Non-barometric distribution的主要原因。膠體溶液於沉降平衡中，通常都會伴隨著唐南平衡(Donnan equilibrium)的發生。當系統中兩區域達到唐南平衡時，此兩區域之間存有唐南位能(Donnan potential)，此位能會受膠體粒子的大小、本質電荷、濃度與反離子電荷的影響。唐南位能會隨著膠體濃度與有效電荷增加而增加，並且隨著介電常數增加而降低。而唐南位能來至於兩區域內的淨電荷密度，當靜電荷越高則唐南位能越大。 The effective charge is often invoked to account for the accumulation of counterions near the colloid with intrinsic charge . Although the ion concentrations are not uniform in the solution due to the presence of the charged particle, their chemical potentials are uniform everywhere. Thus, on the basis of ion chemical potential, effective ion concentrations, which can be experimentally measured by potentiometry, are defined with the pure salt solution as the reference state. The effective charge associated with the charged particle can then be determined by the global electroneutrality condition. Monte Carlo simulations are performed in a spherical Wigner-Seitz cell to obtain the effective charge of the colloid. In terms of the charge ratio , the effects of salt free concentration, added salt concentration, counterion valency, and particle charge are examined. The effective charge declines with increasing salt concentration and the multivalent salt is much more efficient in reducing the effective charge of the colloidal solution. Moreover, the extent of effective charge reduction is decreased with increasing intrinsic charge for a given concentration of added salt. Besides the colloid system, the effective charge of polyelectrolytes is also investigated in a salt free solution. The sedimentation profile of a dilute colloidal solution follows the barometric distribution owing to the balance between gravitational force and thermal fluctuation. However, the electrostatic interactions may lead to significant deviation even in the low volume fraction limit (e.g. 10−5). For a dilute, salt-free colloidal dispersion, five regimes can be identified through the resulting colloidal sedimentation profile and the counterion distribution. The electrostatic interactions depends on the Coulomb strength, , defined as the ratio of the Bjerrum length to the colloid size. At weak colloid-ion attractions (small ), counterions tend to distribute uniformly in the container. However, both barometric and inflated profiles of colloids can be observed. On the contrary, at strong colloid-ion attraction (large ), counterions accumulate in the vicinity of the colloids. Significant counterion condensation effectively decreases the strength of colloid-colloid repulsion and barometric profile of colloids can be obtained as well. As a result, the sedimentation profile and counterion distribution are indicative of the strength of effective colloid-colloid and colloid-ion interactions. It is also found that local electroneutrality condition is generally not satisfied and charge separation (or internal electric field) is neither a sufficient nor necessary condition for non-barometric distributions. Donnan equilibrium of a salt-free colloidal dispersion has been investigated by Monte Carlo simulations. The influences of colloidal characteristics, including particle size R, intrinsic particle charge Z, couterion valency , and concentration , on Donnan potential and effective charge are directly calculated by considering two chambers separated by a semipermeable, fictitious membrane. Donnan potential is increased by increasing and and by decreasing dielectric constant . In principle, is determined by the ratio of net charge density to dielectric constant in a chamber and charge density distribution. The former reveals that the existence of net charge is responsible for Donnan potential, while the latter illustrates the influence of colloid-ion interaction, which is associated with colloidal characteristics.
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