Discovery of Cellulose Surface Layer Conformation by Nonlinear Vibrational Spectroscopy.

Significant questions remain in respect to cellulose's structure and polymorphs, particularly the cellulose surface layers and the bulk crystalline core as well as the conformational differences. Total Internal Reflection Sum Frequency Generation Vibrational Spectroscopy (TIR-SFG-VS) combined with conventional SFG-VS (non-TIR) enables selectively characterizing the molecular structures of surface layers and the crystalline core of cellulose, revealing their differences for the first time. From the SFG spectra in the C-H and O-H regions, we found that the surface layers of Avicel are essentially amorphous while the surface layers of Iβ cellulose are crystalline but with different structural and spectroscopic signatures compared with its crystalline core. The differences between hydrogen bonding networks of cellulose surface and crystalline core were also shown by the SFG signal. The discovery here represents yet another instance of the importance of spectroscopic observations in transformative advances to understand the structure of the cellulosic biomass.