Polymer and Biophysics Laboratory, School of Physical Sciences, Jawaharlal Nehru University, New Delhi 110 067, India.
J Phys Chem B. 2012 Jun 21;116(24):7113-21. doi: 10.1021/jp3022024. Epub 2012 Jun 13.
A comprehensive study of hydration of polyanionic agar molecules in its solution and gel phase in glycerol-water binary solvent is reported. Raman spectroscopy results predict differential water structure arrangement for glycerol-water binary solvent, 0.02% (w/v) agar in glycerol solution and 0.3% (w/v) agar organogel. The 3200 cm(-1) Raman band pertaining to ice-like structure of water was found to increase in gel phase alike in glycerol-water solvent while it decreased in agar solutions with increase in glycerol concentration. In contrast, the partially structured water corresponding to the component 3310 cm(-1) of Raman spectra increased in agar solution, and decreased in gel phase similar to glycerol-water solvent case. We have explained these observations based on a simple model where the available oxygen to hydrogen atom ratio in a given solvent-polymer system uniquely defines hydration in solution and gel phases. The gelation concentration was found to increase from 0.18 (for water) to 0.22% (w/v) (50% v/v glycerol solution) as the glycerol concentration was raised. Correspondingly, the gelation temperature, T(g), showed a decline from 40 to 20 °C, and the gel melting temperature, T(m), revealed a reduction from 81 to 65 °C in the same glycerol concentration regime. Two distinctive features are evident here: (i) presence of glycerol as a cosolvent does not favor the gelation of agar as compared to water and (ii) agar organogels are softer than their hydrogels. A unique 3D phase diagram for the agar organogel is proposed. Circular dichroism data confirmed that the agar molecules retained their biological activity in these solvents. Thus, it is shown that thermo-mechanical properties of these organogels could be systematically tuned and adapted as per application requirement.
本文全面研究了多阴离子琼脂分子在甘油-水二元溶剂中的溶液和凝胶相中的水合作用。拉曼光谱结果表明，甘油-水二元溶剂、0.02%（w/v）琼脂在甘油溶液中和 0.3%（w/v）琼脂有机凝胶中存在不同的水分子结构排列。发现 3200cm-1处与冰状结构水有关的拉曼带在凝胶相中随着甘油浓度的增加而增加，类似于甘油-水溶剂中的情况。相比之下，与拉曼光谱的组分 3310cm-1相对应的部分有序水在琼脂溶液中增加，而在凝胶相中类似于甘油-水溶剂的情况减少。我们基于一个简单的模型解释了这些观察结果，该模型认为在给定的溶剂-聚合物系统中，可用的氧-氢原子比唯一地定义了溶液和凝胶相中的水合作用。发现凝胶化浓度从 0.18%（水）增加到 0.22%（w/v）（50%v/v 甘油溶液），随着甘油浓度的升高。相应地，凝胶化温度 T(g)从 40°C 下降到 20°C，凝胶熔化温度 T(m)在相同的甘油浓度范围内从 81°C 降低到 65°C。这里有两个明显的特点：（i）与水相比，甘油作为共溶剂的存在不利于琼脂的凝胶化；（ii）琼脂有机凝胶比其水凝胶更软。提出了琼脂有机凝胶的独特 3D 相图。圆二色性数据证实，琼脂分子在这些溶剂中保留了其生物活性。因此，表明这些有机凝胶的热机械性能可以根据应用要求进行系统地调整和适应。
