Distinguishing Surface versus Bulk Hydroxyl Groups of Cellulose Nanocrystals Using Vibrational Sum Frequency Generation Spectroscopy

In plant cell walls and cellulose-containing composites, nanocrystalline cellulose interacts with water molecules or matrix polymers through hydrogen bonding of the hydroxyl groups at the cellulose surface. These interactions play key roles in cellulose assembly in plant cell walls and mechanical properties of cellulose composites; however, they could not be studied properly due to the spectroscopic difficulty of selectively detecting the surface hydroxyl groups of nanocrystalline domains. This study employed the sum frequency scattering principle to distinguish the hydroxyl groups inside of the crystalline nanodomain of cellulose and those exposed at the surface of crystalline domains. The comparison of the spectra at various scattering angles revealed that the OH peak near ∼3450 cm^-1 comes from the weakly hydrogen-bonded OH groups at the surface of crystalline cellulose. Also, a time delay measurement found that the sharp vibrational features observed near 3700 cm^-1 can be attributed to isolated OH groups not accessible by ambient water molecules. These findings allow the distinction of surface versus bulk OH groups in sum frequency generation vibrational spectroscopy.