Gelation of cholesterol-bearing pullulan by surfactant and its rheology

Gelation of cholesterol-bearing pullulan (CHP) with SDS in water was studied by rheological measurements. The apparent viscosity of the CHP (2% (w/w))/SDS mixture increased with an increase in the SDS concentration up to approximate to 1% and then decreased after a maximum. In the presence of large amounts of SDS, the CHP self-aggregate certainly dissociated. With 3% (w/w) CHP, a macroscopic gel was formed by the addition of SDS above 0.5% (w/w). At higher concentrations of SDS (above 4.5% (w/w)), the gel changed to a sol. The mechanism of the gelation and the transition to the sol is related to the formation of mixed aggregates between the cholesteryl groups of CHP and SDS. Due to the strong association of the cholesteryl groups, large amounts of SDS were required to achieve the complete solubilization of cholesteryl groups. Oscillatory shear measurements were carried out for the gel of the CHP/SDS mixture. In the low frequency region (omega < 0.1 Hz), G " (loss modulus) showed a maximum, while G' (storage modulus) reached a plateau with an intersection of the G " curve. This is a trend typical of a Maxwellian fluid. An extremely long relaxation time (approximate to 20 s) was observed for the CHP/SDS gel at relatively low SDS concentrations. Such a long relaxation time would be ascribed to the strong association of the cholesteryl groups of CHP. (C) 1999 Elsevier Science B.V. All rights reserved.