Peculiarity of Phase Transition CI into CII for Nanocrystallites of Cellulose

In this paper, using a method of wide-angle X-ray diffraction, sizes of cellulose nanoscale crystallites were determined and phase transition of nanosized crystallites CI into CII was studied after treatment of cellulose samples with solutions of sodium hydroxide with various concentrations, 5 to 20% (1.3 to 6.1 M). It was found that the phase transition proceeds in a certain interval of hydroxide concentrations; moreover, a correlation between average concentration (C) of hydroxide and average lateral sizes (D) of nanocrystallites was observed. Methods of chemical thermodynamics of nanophases allowed to derive an equation, which describes the relationship between C and D: lnC = lnC_o–KD^–1, where C_o is maximum concentration of hydroxide, which is required for the phase transition of large crystals of CI. Thus, the decrease in hydroxide concentration at the phase transition CI into CII, is explained by decreasing of lateral size of CI nanocrystallites. By means of the derived equation, minimum, average and maximum lateral sizes of CI nanocrystallites were determined, as well as polydispersity in lateral sizes of crystallites was studied. It has been shown that crystallites of organo-solvent celluloses were the most uniform, whereas aggregated crystallites of Kraft celluloses were the most heterogeneous.