Hydrolysis of hydroxyethyl methylcellulose with perchloric acid and determination of chemical structure via 13C NMR spectroscopy

Abstract

Hydroxyethyl methylcellulose (HEMC) is widely used in the pharmaceutical, food and construction industries to impart desirable qualities, such as rheology, thixotropy, anti-sedimentation and water retention, to water-dispersible systems. The main application area of the mentioned cellulose ether is in the creation of different construction compositions, in particular, dry mixtures. We studied the hydrolysis of three samples of HEMC, with different degrees of polymerization, under the action of aqueous solutions of perchloric acid (HClO4) at a concentration of 1%, 2%, 3% and 4% and temperature of 80, 90, 100 and 110 °C in a microwave reactor. The completeness of hydrolysis was estimated by the disappearance of the acetal carbon atom signal in the 3C NMR spectrum. It has been shown that the increase in temperature affects less, while the concentration of the acid has the greatest influence on the rate of hydrolysis, and the reaction time is even shorter. Using comparative analysis of the completeness of hydrolysis of various HEMC samples, it was possible to assess the degrees of polymerization of the cellulose ethers qualitatively. The structure of the hydrolysis products was studied by 3C NMR spectroscopy, as a result, the substitution parameters at the 2, 3 and 6 positions (DSC-2, DSC-3and DSC-6) and the number of methyl (DSMe) and hydroxyethyl groups (DSHE) were determined, and the total degree of substitution (DStotal) was calculated. Thus, the optimum conditions for the hydrolysis of hydroxyethyl methylcellulose were selected and defined by the polymer structure, including both distribution of substituents at different positions and the number of each type of substituents in the anhydroglucose unit.