Pressure Cell Assisted Solution Characterization of Polysaccharides. 1. Guar Gum

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Abstract
To reduce time-dependent aggregation phenomena and achieve true “molecular” solution, the “pressure cell” solubilization method of Vorwerg and co-workers was applied to solutions of guar galactomannans (three samples of different molecular weights), using various heating, time, and pressure profiles. Physicochemical characterization of the guar samples before and after pressure cell treatment included measurements of intrinsic viscosity [η] by capillary viscometry and Mw and radius of gyration from size exclusion chromatography coupled to multiangle laser light scattering (SEC/MALLS). Heating the guar solutions (100−160 °C) without pressurization produced chain degradation with [η] and Mw values being reduced significantly, whereas this effect was reduced substantially for samples subject to initial pressurization (∼5−10 bar). The constants in the Mark−Houwink−Sakurada equation, relating [η] and Mw were established and the characteristic ratio C∞ and chain persistence length Lp were calculated using both the Burchard−Stockmayer−Fixman (BSF) method for flexible and semiflexible chains and the Hearst method more appropriate for stiffened chains. Definitive conclusions can now be drawn on the flexibility of the guar chain backbone, with Lp ≈ 4 nm from the BSF plot, in good agreement with previously published work using such geometric methods. This contrasts with the higher values obtained from extrapolation of data for polyelectrolytes with a similar backbone geometry, such as sodium carboxymethyl cellulose, to “infinite” ionic strength.