The Effect of pH and Ionic Strength on the Bulk and Interfacial Properties of Starches

A combination of UV spectroscopy and turbidity techniques were used in this work to examine the effects of pH and salt (type and concentration) on the bulk and interfacial properties of starch and anthracene labelled-starch. The study of starch interaction with monovalent, divalent and trivalent cations revealed significant differences in turbidity values (Td). The exchangeable cations working in this study are Al3+, Ca2+, and Na+ in order of decreasing valency. An interpretation of the gained results suggests that Td is significantly affected by the valency and size of the interacted cations. A decrease in the valency of the cations is combined with a sharp decrease in the Td values, whereas, for a constant valency an increase in the turbidity of starch solution was recorded in this order I- greater than Br- greater than Cl-, this was credited to that the halogen atom radius becoming bigger. By adding more halide salt (NaX) the Td values were sharply decreased because of the direct interaction between the X- ions and hydrogen in hydroxyl group of starches. On the other hand, an increase in the turbidity values were observed at high alumina content, however increasing the pH was a combined with a decrease in the turbidity. The absorbance data for anthracene labelled starch adsorbed onto alumina agreed with the turbidimetric measurements. Finally, adding calcite also lead to an increase in the turbidity but decreasing the pH caused an in a decrease in the turbidity.