vibrations of carbonyl group (C O)

vibrations of carbonyl group (C O) of carboxylate anions. Among
which, the characteristic absorption peaks at 1648 cm
−1
of raw
agar was obviously enhanced after carboxymethylation. The characteristic absorption peaks at 1370 cm
−1
could be attributed
to scissoring vibration of methylene ( CH2 ) in carboxymethyl
group. All these differences between the FT-IR spectra of raw agar
and CMA could verify that carboxymethyl group had been successfully introduced into the agar molecule.
3.2. Determination of physicochemical properties
The dissolving temperature, the gelling temperature, the gel
melting temperature, the gel strength, the content of 3,6-anhydrol-galactose and the sulfate content of raw agar and CMAs were
tested and are shown in Table 1 and Fig. 2. It could be seen that
the CMA could be dissolved in water at room temperature when
the DS was equal or above 0.559. This indicated that the introduction of carboxymethyl groups into agar molecules could confer
the raw agar better aqueous solubility and make CMA lose its
gelation property at room temperature. Both the gel melting temperature and the gelling temperature of CMA decreased in good
linear correlation with the increase of DS value from 0 to 0.495,
and their linear equations were Ya= 78.4 − 18.0X (correlation coefficient Ra= −0.99004) and Yc= 38.4 − 30.3X (correlation coefficient
Rc= −0.99841), respectively. But it was quite different for the dissolving temperature and the gel strength of CMA. With the increase
of DS, the dissolving temperature and the gel strength of CMA also
decreased in good linear correlation at first. The linear equation
of DS value dependence of dissolving temperature of CMA was
Yb= 77.8 − 95.5X (correlation coefficient Rb= −0.99962) when the
DS value was lower than 0.251, and the linear equation of DS value
dependence of gel strength of CMA was Yd= 1919.8 − 9413.7X (correlation coefficient Rd= −0.99951) when the DS value of CMA was
lower than 0.155. However, when the DS value was larger than
0.155 for the gel strength, or larger than 0.251 for the dissolving
temperature, the decline of the gel strength and the dissolving
Fig. 3. Temperature dependence of the optical rotation for solutions of raw agar and
CMA on cooling.
temperature of CMA became slowly with the increase of its DS,
respectively, as shown in Fig. 2.
3.3. Analysis of optical rotation
Studies showed that the mechanism of sol–gel transition for
agar solution was due to the double helix structure formed between
agar molecules (Arnott et al., 1974; Nordqvist & Vilgis, 2011). When
agar aqueous solution was cooled, agar molecules turned into double helix, and when further cooled, the double helix was gathered
and generated hard gel. Temperature dependence of the optical
rotation for solutions of raw agar and CMA (DS = 0.155) on cooling
is shown in Fig. 3. It could be seen that the optical rotation values
of both raw agar and CMA were negative, and almost constant at
high temperature (more than 70
◦
C) and low temperature (less than
30
◦
C), and their variation trend on cooling were approximately the
same. The variation process of agar molecules in solution from coil
to helix could be observed not only in a higher concentration solution (0.25 wt%) at which the solution could form a gel, but also in a
lower concentration solution (0.05 wt%) at which even the solution
could not form a gel. The temperature of the transition beginning
point (from coil to helix) of the agar molecules was higher in a
higher concentration solution (0.25 wt%) than that in a lower concentration solution (0.05 wt%), but its temperature of the transition
end point (from coil to helix) was almost the same in both higher
and lower concentration solution. As for this, it might be attributed
to that the probability of collision between agar molecules was
greater in the higher concentration solution than that in the lower
concentration solution. Meanwhile, it could be also found that the
temperature of the transition end point of CMA was obviously lower
than that of raw agar. This meant that the introduction of carboxymethyl groups into the agar molecule could hinder the agar
molecules to form double helix structure in water solution.