Solid state 13C CP MAS NMR study of molecular motions and interactions of urea adsorbed on cotton cellulose

Abstract

The surface interaction between cotton cellulose and urea was studied by measuring solid state ¹³C CP MAS NMR spectra and relaxation times. The investigation of cotton cellulose surface interactions is inherently difficult as the surface regions constitute only a small proportion of cotton fibers and an important part of these investigations has been to identify and develop useful experimental methods. The results have provided detailed insight into the interactions and motions of urea at the cotton cellulose surface. It is found that the interactions have no discernible effects on the crystallinity and polymorphic form of cellulose. This suggests that urea does not penetrate the cellulose microfibrils and distort the internal hydrogen bonding structure but interact with the cotton cellulose surface. The relaxation techniques used in this paper make it possible to identify several cellulose components including the crystalline and amorphous regions that are not directly resolved in the spectra. Another advantage of the relaxation methods is that it becomes possible to study details of molecular motion in both the crystalline and amorphous regions that may be related to the mechanical properties of the cellulose microfibrils. It has been possible to describe the surface interaction between urea and cotton cellulose in terms of the motion of the urea molecules. The interaction at the cotton cellulose surface makes the urea molecules more mobile as revealed by the lineshapes and relaxation characteristics. The study of the motion makes it possible to obtain a description of the orientation of the urea molecules at the cotton cellulose surface. This orientation is determined by numerous intermolecular interactions and provides a constraint on the intermolecular potential. The results suggest that the urea molecules interact preferentially at hydrophilic defects on the surface such as depressions and grooves by hydrogen bonding with surface hydroxyl groups. The shape of these surface defects is directly reflected in the orientation of the urea molecules at the binding sites. The experiments have been shown to be selective, monitoring only urea molecules interacting with the cotton cellulose surface. This makes urea an excellent probe molecule to study cotton cellulose surfaces.