Binding of hexadecyltrimethylammonium bromide to starch polysaccharides. Part I. Surface tension measurements

Surface tension measurements were used to characterise the binding of the cationic surfactant hexadecyltrimethylammonium bromide, CTAB, to three starch polysaccharides, amylose from potato, amylopectin from potato and amylopectin from barley. Surface tension measurements were used according to the axisymmetric drop shape analysis, ADSA, to determine the concentration of free CTAB monomers during consecutive addition of CTAB to the polysaccharide solutions. The interaction between the starch polysaccharides and CTAB was studied at three starch concentrations, 0.1, 0.25 and 0.5% (w/w). Binding isotherms were derived from the concentration of free CTAB and the type of binding was determined with Scatchard plots. The binding capacities of CTAB correlated linearly with the polysaccharide concentration but were independent of the type of starch polysaccharide. All three starch polysaccharides were able to bind 33 mmol CTAB per mole glucose units. The binding of CTAB to amylose was cooperative while the binding of CTAB to amylopectin was of Langmuir type.