Cellulose model surfaces studied by femtosecond sum frequency spectroscopy

Sum frequency spectroscopy (SFS) is a surface specific non-linear pulsed laser technique capable of providing detailed molecular-level orientation and conformational information of interfacial species. Application of this technique to thin film structures results in complex spectra which require theoretical deconvolution. In the present work, a new co-propagating model for SF emission from model cellulose surfaces has been developed, thereby enabling the study of cellulose surface characteristics and various aspects of wood utilization. Immobilized model cellulose films have been prepared on gold coated silicon wafers for characterization by SFS.;Before quantitatively analyzing SF spectra derived from cellulose, the thickness dependent interference effect between multiple SF sources in the cellulose/gold system was investigated theoretically. Comparisons between experimental and simulated SF spectra enable an accurate understanding of thickness/phase and thickness/intensity interference effects, which are essential to the application of SFS to cellulose and other thin films on gold.;In addition, SFS was employed to study the alkyl ketene dimer (AKD) sizing mechanism employed in the papermaking industry for hydrophobization of cellulose. The AKD was spun coat onto model cellulose films, which resulted in ≈2.6 nm thick AKD layers. The chain orientation of AKD molecules during the sizing process was measured at different temperatures. It was demonstrated that the chain orientation and conformation do not correlate with sizing. The distribution of AKD molecules on glass coverslips as a function of time and temperature was imaged via Fluorescence Microscopy to complement SFS measurements. It was concluded that the distribution of AKD plays a major role in the sizing effect.;A preliminary investigation of SFS application to the study of cellulose enzymatic degradation is presented. However, resultant SF spectra of enzymatically degraded cellulose films contained methyl resonances indicating the likelihood of incomplete removal of adsorbates from cellulose surfaces, which prevented the drawing of conclusion regarding cellulose surface changes. Complementary AFM and ellipsometry measurements quantified the degradation process of model cellulose surfaces, and demonstrated that cellulose film thickness was statistically invariant with pH and enzyme concentration. This work demonstrates the potential use of SFS for studying enzymatic degradation of cellulose films, which is a promising biofuel source.