Effects of sodium sulfonate emulsifiers with different alkyl chain lengths on the structure and surface properties of corn starch

The dispersion and stability of starch have been confined to the high surface hydrophilicity and propensity after retrogradation. The emulsifiers can be expected to modify the surface properties of starch, thereby improving its dispersion within bio-based material systems. Previous studies have focused predominantly on the effects of emulsifier concentration on the starch properties. However, it is still lacking on the influence of the molecular chain length on the surface evolution of starch. This study aims to further investigation into how the molecular chain length of emulsifiers impacted on the starch functionality and performance of composite materials. A systematic investigation was also made to clarify the influence of the sodium sulfonate emulsifiers with the different alkyl chain lengths (sodium hexadecyl sulfonate: SHS), sodium dodecyl sulfate: SDS, and sodium octadecyl sulfate: SOS) on the modification of corn starch. The modified starch was characterized using Fourier transform infrared (FTIR) spectroscopy, X-ray diffractometer (XRD), scanning electron microscope (SEM), and contact angle measurement. The results show that the solubility of starch increased, as the concentration of the emulsifier increased. While the swelling degree first increased and then decreased. The swelling degrees of SHS, SDS, and SOS reached their peak values of 7.1, 9.0 and 8.2 g/g at the concentrations of 3%, 2%, and 1%, respectively. The medium-chain sodium sulfonate emulsifiers were more conducive to the starch swelling. FTIR analysis revealed that both SHS and SOS were enhanced the starch crosslinking through self-bridging, thereby increasing the short-range ordered structure. Nevertheless, the emulsifiers were incorporated to weaken the intermolecular interactions of hydrogen bonding in starch. The XRD analysis demonstrated that there were the distinct effects of short- and long-chain emulsifiers on the starch recrystallization and inclusion complex formation. Specifically, the SDS was facilitated to form the V-type crystalline structures, compared with the SHS and SOS. While the high concentrations of SHS were significantly inhibited the starch retrogradation. DSC analysis indicated that the SDS and SOS were more readily formed the inclusion complexes with starch, indicating the prominent endothermic peaks between 90–110°C. SEM and particle size distribution analysis suggested that the SHS and SOS were indeed promoted the starch crosslinking, whereas the SDS was primarily emulsified starch to improve the dispersion. There was the accordance to that from FTIR analysis. Furthermore, Zeta potential and contact angle analyses reveal that there was no significant impact of the SHS on the zeta potential of starch, thus remaining below -1 eV. While the SDS and SOS markedly increased the surface charge of starch with the increasing concentration. The SOS shared the most significant enhancement, reaching -37 eV. The charged groups of long-chain alkyl sulfonates were more readily exposed after interaction with starch. Furthermore, all three emulsifiers were improved the hydrophobicity of starch at appropriate concentrations. The short- and medium-chain alkyl sulfonates shared the better enhancement, with the contact angles of 60°. The head groups of short- and medium-chain alkyl sulfonates were better connected with the starch, thus exposing hydrophobic groups for the high hydrophobicity. By contrast, the head groups of long-chain alkyl sulfonates were more readily exposed to enhance the electrostatic repulsion between starch granules. This finding can provide a significant theoretical foundation for the application of emulsifiers in the starch composite materials.