Structure of poly(acrylic acid) in electrolyte solutions determined from simulations and viscosity measurements
Structure of poly(acrylic acid) in electrolyte solutions determined from simulations and viscosity measurements
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DOI：
10.1021/jp063981w
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发表时间：
2006-11-16
影响因子：
3.3
通讯作者：
Warszynski, P.
中科院分区：
文献类型：
--
作者：
Adamczyk, Z.;Bratek, A.;Warszynski, P.
在这项工作中，通过分子动力学模拟得到的电解质溶液中的聚丙烯酸（PAA）分子结构与动态光散射（PCS）、动态粘度和电泳测量得到的实验数据进行了比较。对分子量为12 kD的聚合物在不同离子强度的支撑电解质（NaCl）下进行了模拟和测量。系统地研究了溶液pH值在4 ~ 9单位范围内变化对聚合物电离度的影响。通过理论模拟预测，在低电解质浓度（10-3 M）和pH = 9 （PAA的名义完全电离）的情况下，分子呈现出有效长度为L-ef = 21 nm的柔性棒的形状，而完全延伸的聚合物链的预测轮廓长度为L-ext = 41 nm。当电解质浓度为0.15 M时，L-ef = 10.5 nm。对于较低的电离度，预测了分子的显著折叠，其形状为半径为2nm的球体。这些理论预测与PCS对分子扩散系数的实验测量结果进行了比较，从而可以计算出其流体动力半径RH。在低离子强度（pH = 9）和高离子强度（pH = 4）条件下，相对湿度在6.6 nm和5.8 nm之间变化。pH = 9时的相对湿度值与粒子形状的理论预测非常吻合，近似为长形球体，弯曲成各种形式。另一方面，在pH = 4时发生的理论形状预测的显著偏差被解释为模拟中忽略的链水化效应。为了获得更多的形状信息，用毛细管粘度计测量了聚电解质溶液的动态粘度。研究发现，在考虑水化校正后，实验结果与布伦纳黏度理论对长形球状悬浮液的影响是一致的。用该理论计算得到的有效长度与分子动力学模拟的预测值一致。
In this work, the structure of poly(acrylic acid) (PAA) molecules in electrolyte solutions obtained from molecular dynamic simulations was compared with experimental data derived from dynamic light scattering (PCS), dynamic viscosity, and electrophoretic measurements. Simulations and measurements were carried out for polymer having a molecular weight of 12 kD for various ionic strengths of the supporting electrolyte (NaCl). The effect of the ionization degree of the polymer, regulated by the change in the pH of the solution in the range 4-9 units, was also studied systematically. It was predicted from theoretical simulations that, for low electrolyte concentration (10-3 M) and pH = 9 (full nominal ionization of PAA), the molecule assumed the shape of a flexible rod having the effective length L-ef = 21 nm, compared to the contour length L-ext = 41 nm predicted for a fully extended polymer chain. For an electrolyte concentration of 0.15 M, it was predicted that L-ef = 10.5 nm. For a lower ionization degree, a significant folding of the molecule was predicted, which assumed the shape of a sphere having the radius of 2 nm. These theoretical predictions were compared with PCS experimental measurements of the diffusion coefficient of the molecule, which allowed one to calculate its hydrodynamic radius RH. It was found that RH varied between 6.6 nm for low ionic strength (pH = 9) and 5.8 nm for higher ionic strength (pH = 4). The RH values for pH = 9 were in a good agreement with theoretical predictions of particle shape, approximated by prolate spheroids, bent to various forms. On the other hand, a significant deviation from the theoretical shape predictions occurring at pH = 4 was interpreted in terms of the chain hydration effect neglected in simulations. To obtain additional shape information, the dynamic viscosity of polyelectrolyte solutions was measured using a capillary viscometer. It was found that, after considering the correction for hydration, the experimental results were in a good agreement with the Brenner's viscosity theory for prolate spheroid suspensions. The effective lengths derived from viscosity measurements using this theory were in good agreement with values predicted from the molecular dynamic simulations.