Abstract:
Hydrogels obtained by chemical crosslinking of Acacia melliferagum with citric
acid were prepared and characterized in this study. Different amounts of citric
acid, varying heating temperatures and heating times were carried out. The
samples were characterized by the gel content, FT-IR, swelling behaviour and
the variation of swelling behaviour with time and pH.
The results of gel content have shown that the amount of citric acid, temperature
and time, have all influenced the crosslinking efficiency. Sample S7 (0.1g,
160o
C and 5 minutes) was found to have the best swelling characteristics
(993.5%). The results of the variation of swelling behaviour with timehave
shown an increase in water absorption with increasing time up to 36 hours;
while the results of the variation of swelling characteristics with pH showed that
the swelling ratio decreases in acidic medium and vice versa in alkaline
medium. This is because COO- - COO– ion repulsionin acidic medium is
screened by H+ ions which did not allowed the network to expand, but in
alkaline medium the dominant species in the hydrogels are carboxylate anions
and the hydrogels were swollen due to intra-ionic repulsion between carboxylate
anions. The highest swelling ratio was found at pH 10 (11.52).
Finally, the FTIR results have displayed almost identical absorption bands for
the crude gum and the hydrogel samples. However, the intensities of the
stretching and the bending vibrations of –OH group were noticeably decreased
for the hydrogel sample as result of crosslinking.
