Adsorption and Conformations of Starch at Solid–liquid Interfaces Using Spectrophotometry and Turbidity Techniques

The adsorption behaviour of starch on silica surface, which is mimic active sites existing on the surface of kaolinite, was investigated by a combination of spectrophotometry and turbidity techniques. It was demonstrated that the conformational behaviour of starch chain in aqueous solution is temperature and pH sensitive. Starch solution showed a rapid in­crease in turbidity, with decreasing temperature. This suggested that collapse is occurring causing the biopolymer chains to aggregate. The decrease in turbidity indicated that an open coil starch was formed at higher temperature. The starch with negatively charged alkoxides (R-O-), due to the deprotonating of hydroxyl groups at mild basic pH values, showed a larger decrease in turbidity than the starch at low pH values, which indicates that chain aggregation is favored by a neutral starch chain and that the formation of an expanded shape is fa­vored by charged starch macromolecules. The point of zero charge results suggested that electrostatic interactions are mainly responsible for the adsorption of starch on SiO2 suspension molecules. The UV-absorption spectra of anthracene labelled-starch proposed that the adsorption density of starch on silica is pH dependent. Starch adsorption on silica is extensively increased by decreasing pH to 7, as the electrostatic attraction between the R-C-OH2+ groups in starch and the negatively charged silica (-Si-O-) was enhanced. The flocculation of silica suspensions by starch chains depends on the adsorption behaviour of starch on the silica particles. The bridging flocculation of silica particles by starch is rather weak at pH 11, since starch shows extremely low affinity towards silica. When starch shows high affinity towards silica at pH 9 and 7, it starts to play a significant role in the flocculation of silica particles.