Mechanistic studies of particulate soil detergency

Abstract

The ultimate objective of this reasearch was to to investigate the mechanisms of particulate soil removal in an aqueous single surfactant system. In this work, three particulate soils-hydrophobic particulate soil (carbon black) and two hydrophilic particulate soils (ferric oxide and kaolinite) and five surfactants-anionic surfactants; sodium dodecyl sulfate (SDS), methyl ester sulfonate (MES) and linear alkyl benzene sulfonate (LAS), nonionic surfactant; octyl phenol ethoxylate with an average of 10 ethylene oxides per molecule (OP(EO)10-tradename Triton X-100), and cationic surfactant; cetyl trimethyl ammonium bromide (CTAB) were used. The correlations between zeta potential, surfactant adsorption, contact angle, and solid/liquid spreading pressure to detergency performance and antiredeposition over the range of surfactant concentrations at different pH levels were used to reveal the mechanisms of particulate soil detergency. In all cases, detergency was found to improve with increasing solution pH of washing solutions and the maximum detergency performance was found at pH 11. The results of both hydrophobic soil (carbon black) and hydrophilic soils (ferric oxide and kaolinite) showed that the electrostatic repulsion between fabric and soil particles was the primary mechanism responsible for the particulate soil removal. In comparisons among three studied surfactants (SDS, OP(EO)10 and CTAB), SDS provided the best detergency performance followed by OP(EO)10 and CTAB, in which the adsorption of SDS to the negatively charged fabrics and particulate soils, yielding higher negative electrical potentials. For the nonionic surfactant (OP(EO10), the adsorption of OP(EO)10 caused the surface of fabrics and soils to be more negatively charged, and the steric repulsion was found to aid detergency. For the cationic surfactant (CTAB), CTAB strongly adsorbed to the negatively charged fabrics and soils, resulting in charge reversal, poor rinseability and lowest detergency. Additionally, the IFT reduction due to CTAB adsorption was found to aid particulate soil detergency. From the SEM images, all of the studied particulate soils were found to stay on the studied fabrics and no entrapment between the fabric yarns. The performance of methyl ester sulfonate (MES) on both hydrophobic soil (carbon black) and hydrophilic soils (ferric oxide and kaolinite) removal was also studied and compared to SDS (the isomerically pure surfactant) and LAS (the workhorse surfactant used in most detergent products). The results showed that MES exhibited the best detergency performance, corresponding to the highest adsorption, resulting in the highest electrostatic repulsion. The highest MES adsorption results from its long hydrophobic tail length with the smallest head group as compared to the other two anionic surfactants.