Use of cleavable surfactants for alkyl ketene dimer (AKD) dispersions

Alkyl ketene dimer (AKD) dispersions were made with combinations of nonionic and cationic cleavable surfactants. The dispersion stability and the rate of surfactant hydrolysis were studied at different pH values both at room temperature and at elevated temperature. 1H NMR was used for studying surfactant degradation and measurements were made both above and below the critical micelle concentration (CMC) of the surfactant. The particle size was monitored as a function of time in either pH-adjusted water or in aqueous buffer solutions with the view to elucidate whether the concept of micellar catalysis can be correlated to the rate of degradation of the surfactant monolayer that stabilizes the dispersion. A series of poly(ethylene glycol) esters of oleic acid with varying number of oxyethylene units were employed as nonionic surfactants and two ester quats and one betaine ester were used as cationic surfactant. The relative rate of surfactant degradation was nonionic surfactant < ester quat < betaine ester. The betaine ester-containing systems exhibited micellar catalysis, but for the systems based on ester quat the hydrolysis rate was the same above as below the CMC. The fatty alcohol resulting from hydrolysis of the betaine ester was found to contribute to the emulsification of AKD, as shown by its effect on the particle size obtained. This result is in accordance with results from previous work based on stable surfactants [K. Mohlin, H. Leijon, K. Holmberg, J. Disper. Sci. Technol. 22 (2001) 569; K. Mohlin, K. Holmberg, J. Esquena, C. Solans, Colloids Surf. A 218 (2003) 189].