TRANSIENT ELECTRIC BIREFRINGENCE STUDY OF HIGHLY DILUTE AGAROSE SOLUTIONS

In order to characterize the first step of agarose gelation, highly dilute solutions (2.10(-3) to 0.5 g/L) have been studied by means of the transient electric birefringence technique. The field-free decay curves of the birefringence are well described by a stretched-exponential B(t) almost-equal-to exp(-t/tau)-beta., the value of the exponent beta is close to 0.5 whatever the agarose concentration. The suspended particles observed by electron microscopy present a rod-like shape with a constant diameter (approximately 50 angstrom), without any branching; they are polydisperse with a distribution of lengths approximately exponential. The mean length of these fibers, deduced from their mean rotational diffusion coefficient, is proportional to the 0.37 power of the agarose concentration in the solution. Furthermore, these particles possess a strong permanent electrical dipole confirming the side-to-side arrangement of helices into bundles; this dipole is roughly proportional to the particle length, indicating a self-similarity in the unidirectional growth of the agarose fibers, even when approaching the gelling concentration.