Influence of Acetyl Content and Degree of Substitution on the Structural Properties of Cellulose Acetate for Pressure Retarded Osmosis

Abstract

Cellulose acetate (CA) has been extensively utilized as a membrane material in pressure retarded osmosis (PRO) owing to its adjustable hydrophilicity, mechanical robustness, and chemical stability. Although earlier research has highlighted the role of acetylation in modifying the properties of CA, the specific impact of acetyl content and degree of substitution (DS) on structural features pertinent to PRO applications is still not well understood. The present study aims to fill this gap by systematically investigating CA derived from different lignocellulosic feedstocks. The purpose is to assess the relationship between acetyl content, DS, and the resulting structural properties of CA, and to determine the most appropriate biomass precursor for the development of PRO membranes. Cellulose was isolated from oil palm empty fruit bunches (EFB), rice straw, and waste paper, followed by acetylation. The obtained CA was characterized using Fourier-transform infrared spectroscopy (FTIR) to confirm esterification, while acetyl content and DS were quantitatively determined. Commercial CA served as the reference standard. The results demonstrated marked differences among the feedstocks. Waste paper yielded cellulose monoacetate (DS = 1.37; acetyl 26.9%), exhibiting incomplete acetylation with weak ester absorption. EFB produced cellulose diacetate (DS = 2.80; acetyl 43.0%), displaying spectral features comparable to commercial CA. Rice straw resulted in cellulose triacetate (DS = 3.91; acetyl 51.6%), indicating the most extensive substitution and superior structural quality. Higher DS was positively associated with enhanced thermal stability, reduced hydrophilicity, and improved mechanical performance, all of which are critical for PRO operation.