Adsorption of corn starch molecules at hydrophobic mineral surfaces

Abstract

The adsorption of corn starch at high purity hydrophobic minerals, HOPG and talc, was found to be due to hydrophobic interaction. These results suggest that the previous thinking, in which acid-base interactions between hydroxylated metallic ions such as Mg and Fe and the hydroxyl groups of polysaccharides were thought to explain adsorption, needs to be reconsidered. Crystal and powder sample surfaces were investigated using traditional surface analysis techniques, including bubble attachment time measurements, and micro-flotation experiments. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) analyses on the crystal samples showed little or no impurity highlighting the point that metallic sites/ions do not play a significant role in the adsorption process. The captive bubble contact angle of both HOPG and talc decreased and the bubble attachment time increased significantly after conditioning in starch solution. In the micro-flotation experiments, the recovery of the powdered samples decreased drastically after starch adsorption, which results complement the bubble attachment time results and show that even in the absence of impurities, adsorption of starch takes place at hydrophobic minerals. Tapping mode atomic force microscopy (TMAFM) images reveal the adsorption morphology at the crystal surfaces. Unique structures of starch were found at both HOPG and talc surfaces, confirming the adsorption. These results provide substantial evidence that the starch adsorption at the surface of hydrophobic minerals is governed by hydrophobic interactions between the nonpolar mineral surfaces and the hydrophobic portion of the starch molecule.