Frantz S, Hübner G A, Wendland O, Roduner E, Mariani C, Ottaviani M F, Batchelor S N
Institute of Physical Chemistry, University of Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany.
J Phys Chem B. 2005 Jun 16;109(23):11572-9. doi: 10.1021/jp050791x.
The effect of water content on the physicochemical properties of the amorphous regions in cotton were investigated by measuring the electron paramagnetic resonance (EPR) of TEMPOL nitroxide radicals, deposited in cotton at different loadings, as a function of the relative humidity (RH) and temperature. Three different components contribute differently to the experimental EPR spectra, corresponding to (a) mobile radicals absorbed in the bulk amorphous region, (b) slow moving radicals adsorbed on the crystallite surfaces in cotton, and (c) aggregated radicals. These components were analyzed by means of computer-aided simulations of the line shapes and simplified line width methods. Polarity and mobility parameters were extracted from the analysis of the spectra. For all loadings and temperatures, the polarity suddenly dropped when the water content fell below approximately 3 wt %, i.e., when water was removed from the bulk amorphous regions. At the lowest loading (2 x 10(-5) mol kg(-1)), the spectra were independent of the RH, and only mobile radicals were observed. At intermediate loading (10(-4)-10(-3) mol kg(-1)) both mobile (fast) and adsorbed (slow) moving radicals were present, the fraction of which depended on the RH. The mobility of the adsorbed and mobile radical signals was smaller at higher loadings, indicating microdomains of different character. The temperature dependence of the rotational correlation times provided the activation energies, which were much lower than in liquids. An equilibrium exists between the mobile and the adsorbed radicals. The temperature dependence of the equilibrium constant, K, gave the enthalpy and the entropy of the adsorption process. At low RH, the enthalpy and the entropy values indicated a simple adsorption process. At 10(-3) mol kg(-1), the values were independent of the RH, but at low loadings the values increased with the increase in the RH, which suggested a displacement of adsorbed water by the radicals at high water content. At loadings above 10(-3) mol kg(-1), signals from radicals strongly interacting via spin exchange were observed, which are assigned to aggregated radicals; simulation of the spectra gave an activation energy of 13 kJ mol(-1) for the spin exchange process. These effects are rationalized on the basis of microdomains of different character within cotton, reflecting the variation in pore sizes (0.5-8 nm) and the relaxation behavior of the cellulose chains.
通过测量不同负载量下沉积在棉花中的TEMPOL氮氧自由基的电子顺磁共振（EPR），研究了含水量对棉花非晶区物理化学性质的影响，该影响是相对湿度（RH）和温度的函数。三种不同的成分对实验EPR谱的贡献不同，分别对应于：（a）在块状非晶区吸收的移动自由基；（b）吸附在棉花微晶表面的慢速移动自由基；（c）聚集自由基。通过线形的计算机辅助模拟和简化线宽方法对这些成分进行了分析。从光谱分析中提取了极性和迁移率参数。对于所有负载量和温度，当含水量降至约3 wt%以下，即当水从块状非晶区去除时，极性突然下降。在最低负载量（2×10⁻⁵ mol kg⁻¹）下，光谱与RH无关，仅观察到移动自由基。在中等负载量（10⁻⁴ - 10⁻³ mol kg⁻¹）下，移动（快速）和吸附（慢速）移动自由基均存在，其比例取决于RH。在较高负载量下，吸附和移动自由基信号的迁移率较小，表明存在不同性质的微区。旋转相关时间的温度依赖性提供了活化能，该活化能远低于液体中的活化能。移动自由基和吸附自由基之间存在平衡。平衡常数K的温度依赖性给出了吸附过程的焓和熵。在低RH下，焓和熵值表明是一个简单的吸附过程。在10⁻³ mol kg⁻¹时，这些值与RH无关，但在低负载量下，这些值随RH的增加而增加，这表明在高含水量下自由基取代了吸附水。在负载量高于10⁻³ mol kg⁻¹时，观察到通过自旋交换强烈相互作用的自由基信号，这些信号归因于聚集自由基；光谱模拟给出自旋交换过程的活化能为13 kJ mol⁻¹。基于棉花内部不同性质的微区，这些效应得到了合理的解释，反映了孔径（0.5 - 8 nm）的变化和纤维素链的弛豫行为。
