Rheological and sedimentation studies on Instant Clearjel and Primojel suspensions

Abstract

Abstract The suitability of Instant Clearjel and Primojel for use as extemporaneous suspending agents has been investigated by continuous shear and creep analysis using a specially constructed air turbine viscometer. Aqueous dispersions of both of the modified starches exhibited non-Newtonian, pseudoplastic flow with little thixotropy and some irreversible shear breakdown. Apparent viscosities, determined from the apices of flow curves obtained at constant maximum shear rate, decreased progressively with increase in temperature over the range 10–50 °C. An Arrhenius plot gave a low apparent activation energy of flow for Primojel. The two modified starches showed a marked rise in apparent viscosity with rise in pH from 3 to 11 of the dispersion medium. Only Instant Clearjel dispersions exhibited a linear viscoelastic region in creep testing, studies which confirmed that this system behaved as a viscoelastic liquid. Sedimentation studies on 10% w/v sulphadimidine suspensions containing varying concentrations of either suspending agent indicated that 3% w/v Primojel could form permanent suspensions even after 6 months storage at room temperature. The significance of these results in relation to the usefulness of these starch derivatives as suspending agents is discussed.