Elucidating the Substitution Pattern of a Hydroxyethyl Cellulose Thickening Agent by DNP-Enhanced Solid-State NMR

Hydroxyethyl cellulose (HEC) is a widely used industrial thickening agent added to formulations such as water-based lubricants. The nature and location of the substituents on the cellulose backbone impact its physicochemical properties. In this study, we investigated the functionalization pattern of a commercially available HEC sample using a multifaceted solid-state NMR study. First, leveraging the substantial signal amplification provided by Dynamic Nuclear Polarization (DNP), we performed a ¹³C-¹³C DNP-enhanced refocused INADEQUATE experiment on natural abundance HEC. This enabled the identification of the substitution sites within the cellulose backbone. Then, we acquired a room-temperature quantitative single-pulse ¹³C spectrum to quantify the length and distribution of the functionalized groups. This approach comprehensively describes the HEC substitution scheme, yielding crucial structural information. We unambiguously identified two substitution sites C2 and C6, and determined their degree of substitution (respectively 75 and 60%), as well as the ratio between single and multiple hydroxyethyl groups (33%). The proposed methodology opens the way to the development of new industrial analytical tools to ensure the quality and consistency of hydroxyethyl cellulose in its solid state or in complex formulations.