Counterion adsorption on flexible polyelectrolytes: comparison of theories.
Counterion adsorption on flexible polyelectrolytes: comparison of theories.
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DOI：
10.1021/ma801799e
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发表时间：
2009-02-24
期刊：
影响因子：
5.5
通讯作者：
Muthukumar, M.
中科院分区：
文献类型：
--
作者：
Kumar, Rajeev;Kundagrami, Arindam;Muthukumar, M.
在球形空腔中，考虑了反离子在柔性主链上的吸附，即在链上取“排列”电荷分布，使“吸附的”反离子沿着主链移动。我们用自洽场理论（SCFT）计算电离度，并与以前发展的变分理论进行比较。在这两种理论中的自由能的各种贡献的分析表明，达到电离的平衡度主要是作为一个相互作用的抗衡离子的吸附能的骨干，小离子的平移熵，和它们相关的密度波动。SCFT计算的电离度明显低于变分法计算的电离度。这种差异完全是由于系统中小离子的密度波动造成的，这在变分过程中得到了解释。当这些波动被故意抑制在截断变分过程中，出现了显着的定量协议的各种因素的平衡电离度，尽管在这两个计划中使用的近似和计算程序的根本差异。此外，它被发现，从截断变分程序和SCFT的总自由能是在定量协议在低单体密度和彼此不同，在较高的单体密度。在更高的单体密度的分歧是由于变分计算，以准确地计算在更高的浓度下的溶剂熵的能力。静电能（相对较小）的比较表明，与泊松-玻尔兹曼估计相比，变分理论中使用的德拜-休克尔估计对静电能的估计过高。然而，由于小离子的密度波动的显着影响不被捕获的SCFT，由于SCFT和其他贡献因素之间的更透明的变分过程中的密切协议，后者是一个更好的计算工具，用于获得电离度。
Counterion adsorption on a flexible polyelectrolyte chain in a spherical cavity is considered by taking a “permuted” charge distribution on the chain so that the “adsorbed” counterions are allowed to move along the backbone. We compute the degree of ionization by using self-consistent field theory (SCFT) and compare with the previously developed variational theory. Analysis of various contributions to the free energy in both theories reveals that the equilibrium degree of ionization is attained mainly as an interplay of the adsorption energy of counterions on the backbone, the translational entropy of the small ions, and their correlated density fluctuations. Degree of ionization computed from SCFT is significantly lower than that from the variational formalism. The difference is entirely due to the density fluctuations of the small ions in the system, which are accounted for in the variational procedure. When these fluctuations are deliberately suppressed in the truncated variational procedure, there emerges a remarkable quantitative agreement in the various contributing factors to the equilibrium degree of ionization, in spite of the fundamental differences in the approximations and computational procedures used in these two schemes. Furthermore, it is found that the total free energies from the truncated variational procedure and the SCFT are in quantitative agreement at low monomer densities and differ from each other at higher monomer densities. The disagreement at higher monomer densities is due to the inability of the variational calculation to accurately compute the solvent entropy at higher concentrations. A comparison of electrostatic energies (which are relatively small) reveals that the Debye-Hückel estimate used in the variational theory is an overestimation of electrostatic energy as compared to the Poisson-Boltzmann estimate. Nevertheless, since the significant effects from density fluctuations of small ions are not captured by the SCFT, and due to the close agreement between SCFT and the other contributing factors in the more transparent variational procedure, the latter is a better computational tool for obtaining the degree of ionization.
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影响因子：
4.4
作者：
Muthukumar, M
通讯作者：
Muthukumar, M
影响因子：
--
作者：
EDWARDS, SF
通讯作者：
EDWARDS, SF
影响因子：
4.4
作者：
MANNING, GS
通讯作者：
MANNING, GS
影响因子：
8.6
作者：
Hsiao, Pai-Yi;Luijten, Erik
通讯作者：
Luijten, Erik
影响因子：
4.4
作者：
Liu, S;Muthukumar, M
通讯作者：
Muthukumar, M