Hydration/Dehydration Behavior of Cellulose Ethers in Aqueous Solution
Hydration/Dehydration Behavior of Cellulose Ethers in Aqueous Solution
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DOI：
10.1021/acs.macromol.7b00848
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发表时间：
2017-08-08
期刊：
影响因子：
5.5
通讯作者：
Shikata, Toshiyuki
中科院分区：
文献类型：
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作者：
Arai, Kengo;Shikata, Toshiyuki
水合数：使用高达50 GHz的极高频介电光谱方法测定溶解在水溶液中的纤维素醚的含量，其中自由水分子主要介电分散的频率范围清晰可见。甲基纤维素、羟丙基甲基纤维素和羟乙基甲基纤维素样品在相对高的甲基取代度范围为1.4至1.9，这是水溶性的基本原因，和由羟丙基或羟乙基进行的0至0.25的额外低摩尔取代，其具有20 × 10（3）至300 × 10（3）的摩尔质量（M-w），在这项研究中进行了检查。所有样品在低于室温的冷水中溶解良好，约为200 ℃。25摄氏度;然而，它们的水溶液在高温如38-53 ℃下显示出明显的浊点，这与M-w值无关，而基本上取决于取代条件。测定的水溶液中纤维素醚的每吡喃葡萄糖单元的水合数（n（H））在10 ℃时大于12，并受取代条件的轻微影响。随着温度的升高，纤维素醚表现出明显的脱水行为，然后n（H）值显著降低至约100 ℃。5几乎与取代条件和M-w无关，在由取代条件控制的下临界溶解温度（LCST）附近。这些观察结果有力地表明，在每个LCST附近的n（H）= 5的值是纤维素醚溶解到水中的最小临界量，并且当n（H）的值小于在高于浊点的温度下的值时，纤维素醚的水溶液变得不透明。
The hydration numbers: of cellulose ethers dissolved into aqueous solution were determined using extremely high-frequency dielectric spectroscopic methods, up to 50 GHz, in which the frequency range of the major dielectric dispersion of free water molecules is clearly observable. Methyl cellulose, hydroxypropylmethyl cellulose, and hydroxyethylmethyl cellulose samples at relatively high degrees of substitution by methyl groups ranged from 1.4 to 1.9, which are an essential reason for water solubility, and additional low molar substitution by hydroxypropyl or hydroxyethyl groups from 0 to 0.25 bearing molar masses (M-w) ranging from 20 X 10(3) to 300 X 10(3) was examined in this study. All the samples dissolve well into cold water below room temperature, ca. 25 degrees C; however, their aqueous solutions show distinct cloud points at high temperatures, such as 38-53 degrees C, depending not so much on the M-w values but on the substitution condition substantially. The determined hydration numbers per glucopyranose unit (n(H)) for the cellulose ethers in aqueous solution were more than 12 at 10 degrees C and were slightly influenced by the substitution condition. The cellulose ethers showed remarkable dehydration behavior with increasing temperature; then, the n(H) values significantly decreased to the value of ca. 5 almost irrespective of the substitution condition and M-w in the vicinities of then lower critical solution temperatures (LCSTs) controlled by the substitution condition. These observations strongly suggest that the value of n(H) = 5 near each LCST is the minimum critical quantity for the cellulose ethers to be dissolved into water, and when the value of n(H) is less than the value at higher temperatures than cloud points, aqueous solutions of cellulose ethers become opaque.