Effect of trisodium citrate on swelling property and structure of cationic starch thin film

Abstract

Effects of trisodium citrate (citrate), as an ionic crosslinker of cationic starch (CS), on swelling property of citrate-CS thin film immersing into various pH solutions were studied. The maximal turbidity of film forming solutions (FFS) indicates the most extensive ionic crosslinking of citrate to CS occurring at charge ratio (R) = 3 instead of the charge neutrality point (R = 1) due to the co-existence of electrostatic force and hydrogen bonding. The CS FFS is composed of leached amylose and unbursted swollen granules mainly occupied by amylopectin. Proved by the rheological properties of citrate-CS FFS and the microscopic images of citrate-CS thin films during swelling studies, citrate governs the pH-dependent swelling behavior within the unbursted swollen granule via ionic crosslinking either in solution or in thin films. Besides, the ionic crosslinking from citrate is able to decrease the surface hydrophilicity of CS thin film by hindering the exposure of cationic group. The small angle X-ray scattering (SAXS) patterns of swollen citrate-CS thin films are well fitted by employing the correlation length model. The rise of the exponent in low q region (n) is found to be correlated with the presence of deprotonated citrate induced aggregates, constructing a strong network against osmotic pressure, thus leading to a low water retention capacity of the thin film. Proposed nano-structure inside swollen granule from SAXS can explain the results of swelling test and microscopic images. The citrate-CS thin film can potentially serve as an active coating for pH-triggered releasing of active compounds.