Hydration of Microcrystalline Cellulose and Milled Cellulose Studied by Sorption Calorimetry

Abstract

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.

Thermal and mechanical properties of chitosan nanocomposites with cellulose modified in ionic liquids

“…The reason for that was the presence of cellulosic filler and cellulose is a hydrophilic material which is well known for being able to absorb high amounts of water. In general, it is believed that, because of the contrasting sorption mechanisms (bulk for C II and surface for C I), less crystalline C II can adsorb more water than C I [47,48]. This relation can be seen for samples of CHT/5 C I and CHT/5 CII, but in our case it is no longer noticeable for composites with nanometric filler.…”

Factors affecting the hygroexpansion of paper

“…Kocherbitov et al evaluated the hydration of microcrystalline cellulose (cellulose I and II) and amorphous cellulose. They found that the water sorption increases from cellulose I to cellulose II to amorphous cellulose [73]. During the process of water absorption by amorphous cellulose, water molecules are first of all bound to the O6 and O2 hydroxyl groups of cellulose.…”

Dynamic vapor sorption and thermoporometry to probe water in celluloses

“…For celluloses that retain their native cellulose I crystal structures (which are the focus of this work), cellulose forms aggregates of microfibrils containing highly-ordered (crystalline) and less-ordered structures (Henriksson and Lennholm 2009). Moreover, cellulose ultrastructure affects hydration, as monolayer water mass depends on cellulose crystallinity indexes; and hydration affects ultrastructure, as water-filled pores collapse upon drying (Mihranyan et al 2004;Kocherbitov et al 2008). Nevertheless, in addition to such general properties, one also needs analytical techniques able to accurately and precisely measure material-specific ultrastructural parameters.…”