Kocherbitov Vitaly, Ulvenlund Stefan, Kober Maria, Jarring Kjell, Arnebrant Thomas
Biomedical Laboratory Science and Technology, Faculty of Health and Society, Malmö University, SE-205 06 Malmö, Sweden.
J Phys Chem B. 2008 Mar 27;112(12):3728-34. doi: 10.1021/jp711554c. Epub 2008 Feb 29.
The hydration of two different polymorphs of microcrystalline cellulose (cellulose I and II), as well as the hydration of amorphous cellulose was studied using water sorption calorimetry, gravimetric water vapor sorption, nitrogen sorption, and X-ray powder diffraction. Amorphous cellulose was prepared by means of ball-milling of microcrystalline cellulose (MCC). Whereas X-ray data showed the untreated MCC to consist of cellulose I, the amorphous cellulose was found to recrystallize into cellulose II after contact with water or water vapor at relative humidities (RHs) above 90%. Sorption isotherms show an increase of water sorption in the sequence cellulose I<cellulose II<amorphous cellulose. The enthalpy of water sorption becomes more exothermic in the same sequence. The specific area of cellulose is dramatically higher when calculated from the water adsorption than when calculated from nitrogen adsorption. A proposed mechanism of water sorption by MCC implies the adsorption of water molecules at solid-solid interfaces, i.e., between neighboring microfibrils, which explains the observed difference between water and nitrogen. The Brunauer-Emmett-Teller (BET) model is therefore not appropriate for the description of the hydration of cellulose. Rather, the Langmuir model represents a more accurate description of water sorption by MCC at low RH. At higher RH, the water adsorption competes with capillary condensation. The thickness of microfibrils, as calculated using the fitting of the sorption isotherm of MCC with the Langmuir equation, is about 4 nm. This value compares favorably with literature data.
采用水吸附量热法、重量法水蒸气吸附、氮气吸附和X射线粉末衍射等方法，研究了微晶纤维素的两种不同多晶型物（纤维素I和纤维素II）的水合作用以及无定形纤维素的水合作用。无定形纤维素通过微晶纤维素（MCC）球磨制备。X射线数据显示未处理的MCC由纤维素I组成，而无定形纤维素在相对湿度（RH）高于90%的情况下与水或水蒸气接触后会重结晶为纤维素II。吸附等温线表明水吸附量按纤维素I<纤维素II<无定形纤维素的顺序增加。水吸附焓也按相同顺序变得更放热。由水吸附计算得到的纤维素比表面积显著高于由氮气吸附计算得到的比表面积。提出的MCC水吸附机制意味着水分子在固-固界面（即相邻微纤丝之间）吸附，这解释了观察到的水和氮气之间的差异。因此，Brunauer-Emmett-Teller（BET）模型不适用于描述纤维素的水合作用。相反，Langmuir模型更准确地描述了低RH下MCC对水的吸附。在较高RH下，水吸附与毛细管凝聚相互竞争。使用MCC吸附等温线与Langmuir方程拟合计算得到的微纤丝厚度约为4nm。该值与文献数据相当。
