A sedimentation study of gel systems

Abstract

When an agar gel was subjected to a high centrifugal field, a well-defined gel/solvent in terface formed, and moved in the direction of the field, sometimes with a self-sharpening effect. Little non-gelling solute occurred. At high centrifugal fields, there was no observable time lag between application of the field and motion of the boundary. A plot of log x against t ( x being distance of the interface from the centre of rotation and t the time) was almost linear initially but the slope decreased progressively and the curve approached a well-defined limiting value log x ∞ appropriate to the field and initial concentration. This behaviour is probably due to the continuously increasing gel concentration which is caused as the gelling solute is confined to a progressively smaller volume between the gel interface and the bottom of the cell. When no further motion occurs, equilibrium has been attained, a tall levels of the cell, between the partial hydrostatic pressure involved and the swelling pressure of the gels. At low fields, a considerable time lag (induction period), observed between application of the field and motion of the boundary, probably indicates the need for a considerable break ­ down of structure before flow can occur. A plot of initial slope (d log x /d t ) against the square of the rotational speed is linear within experimental error and passes through the origin. The yield point for gels may be associated with the occurrence of the induction period. On the basis of a crude lattice model, the mean pore size for a 1 % agar gel was calculated to be 70 to 100 Å, in fair agreement with previous estimates. Gelatin gels behave similarly to agar in the general features of the curves of log x against t but a much higher proportion of gelatin is observable in true solution (30 to 40% ), sedimenting more slowly than the gel interface.