Sulfobetaine-Siloxanes: A Class of Self-Destructive Surfactants

Abstract

The hydrolytic susceptibility of sulfobetaine-siloxane surfactants is investigated by comparison of a homologous series in this subclass of surfactants (R-(CH 2 ) 3 N + (Me) 2 (CH 2 ) 3 SO 3 − ; R = (Me 3 SiO) 3 Si-, (Me 3 SiO) 2 Si(Me)-, (Me 2 SiO) 3 -Si(Me)-) with an analogue series of oxyethylene-siloxane surfactants (R-(CH 2 ) 3 (OCH 2 CH 2 ) 10.2 OH; R = (Me 3 SiO) 3 Si-, (Me 3 SiO) 2 Si-(Me)-, (Me 2 SiO) 3 -Si(Me)-). Nuclear magnetic resonance (NMR) monitoring of these surfactants in an aqueous solution shows that the presence of the sulfobetaine head structure greatly enhances the hydrolysis rate of the siloxane tail as compared with oxyethylene-siloxane analogue control experiments. This sulfobetaine effect is confirmed by adding a model compound, (Me) 3 N + (CH 2 ) 3 SO 3 − , to the oxyethylene-siloxane surfactants and observing the large hydrolysis enhancement. Measurements of pH indicate the sulfobetaine presence greatly enhances acidity, but rigorous analysis could discover no source of acid other than the presence of the sulfobetaine structure. Titration measurements confirmed the presence of a tightly bound hydration layer of 4−7 water molecules per sulfobetaine group. It is speculated that the source of acidity may originate from an aqueous exclusion zone nucleated by the hydrated sulfobetaine at the interface of a sulfobetaine-siloxane surfactant bilayer aggregate. Hydrolysis prevention is investigated by addition of a pH 7 phosphate buffer, of an alkyl polyglycoside cosurfactant, and of a combination of both, with a finding of very significant but not complete suppression of the hydrolysis.