Biodegradable Surfactants Containing Hydrolyzable Bonds
Surfactants with built-in weak bonds, so-called cleavable surfactants, are interesting candidates in the search for more environmentally friendly alternatives to traditional surfactants. The objective of this thesis was to perform a systematic study in which cleavable surfactants with different hydrolyzable bonds were compared with regard to physicochemical properties, and chemical and biological stability. Nonionic surfactants where the head group and the tail are linked by an ester, an amide or a carbonate bond were prepared and characterized. The stability was investigated by subjecting the surfactants to base-catalyzed hydrolysis, enzyme-catalyzed hydrolysis and biodegradation. The effect of steric hindrance near the hydrolyzable bond on the surfactant stability was also investigated. A series of surfactants with different degree of substitution at the α-carbon of the hydrophobic chain were prepared and compared.
Physicochemical properties, such as critical micelle concentration (CMC) and cloud point, showed that the carbonate group and the ester group are more hydrophobic than the amide group. It was found that the ester bond was more labile than a carbonate with respect to base-catalyzed hydrolysis, while the amide was virtually stable. The surfactants showed a pronounced difference in stability with respect to the type of substitution in the vicinity of the hydrolyzable bond. Hydrolysis studies above the CMC revealed that the ester bond of aggregated surfactant is protected from attack by hydroxide ions. Biodegradation tests showed that all three types of hydrolyzable bonds give surfactants that fulfill the main criterion for being classified as readily biodegradable.
Furthermore, mixed surfactant systems containing cleavable surfactants were investigated. It was shown that it is possible to govern the hydrolysis rate of a hydrolyzable surfactant by the addition of a second surfactant species