Phase and Phase Transition in Agarose
Masayuki Tokita
Department of Physics, Graduate School of Science, Kyushu University
6-10-1 Hakozaki, Fukuoka, Fukuoka 812-8581, Japan
We present evidence for the existence of phase separation in the gel state of
agarose having the mixture of water and methanol as the gel solvent. Firstly, the sol-gel
transition line and the cloud point line are determined independently as a function of the
concentration of agarose as well as the concentration of methanol in the mixed solvent
by the quasi-equilibrium cooling of the solutions. Then the spinodal line is determined
by quenching the solutions below the sol-gel transition line. We find that the spinodal
line appears below the cloud point line and both lines are entirely buried below the
sol-gel transition line in the aqueous agarose system. The concentration fluctuations are,
therefore, frozen into the polymer network of agarose gel that promotes the opacity of
the resultant gel. The structure of agarose gel is observed by the confocal laser scanning
microscope (CLSM) imaging technique that reveals that the density fluctuations are
grown up to micrometer scale in space. The phase separation boundary is found to shift
to the higher temperature region than the sol-gel transition line when the concentration
of methanol in the mixed solvent is increased. The results indicate that the position of
the phase separation boundary in relative to the sol-gel transition line varies with the
quality of solvent. These results are in agreement with the theory of the sol-gel
transition in which both the divergence of the connectivity and the thermodynamic
instability are taken into account.