  Thermal phase transition temperatures of high and low melting point agarose
gels were investigated by using the UV-vis spectroscopy techniques. Transmitted
light intensities from the gel samples with different agarose concentrations
were monitored during the heating (gel-sol) and cooling (sol-gel) processes. It
was observed that the transition temperatures Tm, defined as the location of
the maximum of the first derivative of the sigmoidal transition paths obtained
from UV-vis technique, slightly increased with the agarose type and
concentration. Here, we express the phase transitions of agar-water system, as
a representative of reversible physical gels, in terms of a modified
Susceptible-Infected-Susceptible epidemic model whose solutions are the
well-known 5-point sigmoidal curves. The gel point is hard to determine
experimentally and various computational techniques are used for its
characterization. Based on previous work, we locate the gel point T0 of sol-gel
and gel-sol transitions in terms of the horizontal shift in the sigmoidal
transition curve. For the gel-sol transition (heating), T0 is greater than Tm,
i.e. later in time, and the difference between T0 and Tm is reduced as agarose
content increases. For the sol-gel transition (cooling), T0 is again greater
than Tm, but it is earlier in time for all agarose contents and moves forward
in time and gets closer to Tm as the agarose content increases.
