Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, USA.
J Phys Chem B. 2013 Jun 27;117(25):7653-74. doi: 10.1021/jp302589y. Epub 2013 Jun 18.
Interfacial regions between certain gels and their surrounding solutions were observed by Pollack and co-workers to exhibit several unexpected phenomena: (1) long-range exclusion of charged microspheres out to typical distances of ~100-200 μm from the gel surface; (2) significant electrostatic potentials extending over comparable distances; (3) a reduced intensity of upward spontaneous thermal IR emission over a region 300-500 μm wide at or near the gel-solution interface; and (4) a significantly lower proton T2 and an apparently reduced H2O self-diffusion coefficient over a zone ~60 μm wide at or near the gel-solution interface in high resolution NMR imaging experiments. To account for such observations, they proposed that a region of long-range ordered water bearing a net negative charge, but lacking mobile charge carriers, extended ~100-200 μm outward from the gel surface. In this paper, various problems associated with the ordered water hypothesis, including contradictions by experiments from many other laboratories, are briefly discussed, and testable alternative explanations for the observed phenomena are proposed. Exclusion zones are suggested to arise from chemotaxis of the microspheres in long-range diffusion gradients of OH(-) (or H(+)) and salt, the theory of which was developed and compared with the observations on non-ionic gels in a companion paper. The same theory together with the expected directions of ion transfers between gel and solution are now used to predict qualitatively the exclusion/attraction behavior of microspheres in the presence of ionic gels and ionomers. The electrostatic potentials are interpreted as long-range liquid-junction potentials arising from the same long-range diffusion gradients of OH(-) (or H(+)) and salt in the unstirred solutions of Pollack and co-workers. Alternative explanations in terms of plausible experimental artifacts are suggested for both the reduced intensity of IR thermal emission and the lower proton T2 and apparent H2O diffusion coefficient in the NMR imaging experiments.
波拉克及其同事观察到，某些凝胶与其周围溶液的界面区域表现出几种出人意料的现象：（1）带电荷的微球被长程排斥到距凝胶表面典型距离100-200μm 之外；（2）延伸相当距离的显著静电势；（3）在靠近凝胶-溶液界面的300-500μm 宽的区域，向上自发热红外发射强度降低；（4）在高分辨率 NMR 成像实验中，在靠近凝胶-溶液界面的60μm 宽的区域，质子 T2 显著降低，H2O 自扩散系数明显降低。为了解释这些观察结果，他们提出，从凝胶表面向外延伸100-200μm 的区域存在带净负电荷但缺乏可移动电荷载体的长程有序水。本文简要讨论了有序水假说所涉及的各种问题，包括来自其他许多实验室的实验矛盾，并提出了观察现象的可检验替代解释。排斥区被认为是由微球在 OH(-)（或 H(+)）和盐的长程扩散梯度中的趋化作用引起的，该理论在本文的姊妹篇中得到了发展并与非离子凝胶的实验进行了比较。同样的理论以及凝胶和溶液之间离子转移的预期方向现在用于定性预测离子凝胶和离聚物存在时微球的排斥/吸引行为。静电势被解释为长程液体结电位，源自波拉克及其同事的未搅拌溶液中相同的 OH(-)（或 H(+)）和盐的长程扩散梯度。对于在 NMR 成像实验中观察到的红外热发射强度降低以及质子 T2 和 H2O 扩散系数降低，提出了基于合理实验假象的替代解释。
