Isothermal titration calorimetry study of pectin-ionic surfactant interactions

Isothermal titration calorimetry (ITC) was used to measure enthalpy changes resulting from injection of anionic (sodium dodecyl sulfate, SDS) or catio nic (dodecyl trimethylammonium bromide, DTAB) surfactants into aqueous 1 wt % pectin solutions (30, 60, or 90% methoxylated). In the absence of pectin , the critical micelle concentrations (cmc) determined by ITC were 14.7 mM for DTAB and 7.7 mM for SDS. Binding of DTAB to pectin was endothermic and was attributed to electrostatic attraction between the cationic surfactant and anionic biopolymer. Binding of SDS to pectin was exothermic and was att ributed to hydrophobic interactions. Pectin reduced the cmc of SDS, probabl y because of long-range electrostatic repulsion between the molecules. Abov e a particular concentration, which depended on pectin and surfactant type, both ionic surfactants promoted pectin aggregation (monitored by turbidity increase). This study demonstrates the potential of ITC for providing valu able information about interactions between polysaccharides and amphiphiles .