Trisiloxane surfactants: surface interfacial tension dynamics and spreading on hydrophobic surfaces

Dynamics of surface (at the solution/air interface) and interfacial (at the solution/n-dodecane interface) tension of nonionic siloxane surfactants, some of which are known as ''superwetter'', and ethoxylated isododecyl alcohols was studied by the drop volume method. The influence of surfactant concentration and hydrophilicity (length of the ethoxy chain) on surface/interfacial tension dynamics and spreading of aqueous solutions on the liquid hydrocarbon surface was investigated. Surface and interfacial tension fall rates were estimated on the basis of the Hua and Rosen approach (Hua, X. Y.; Rosen, M. J. J. Colloid Interface Sci. 1988, 124 (2), 652). It was found that concentrated solutions of surfactants with intermediate ethoxy chain length show unusually high surface/interfacial tension fall rates. These solutions spread very fast on a liquid hydrocarbon surface: a drop of aqueous solution with a volume of about 3 mu L forms a thin spreading film with an area of several square centimeters in 5-10 s. The rate of spreading and the resulting film thickness were found to depend on the surfactant concentration and the hydrophilicity and hydrocarbon subphase chain length. A good correlation between surface/interfacial tension fall rate, rate of spreading, and the dynamic spreading coefficient was found. Diffusion coefficient values were calculated according to the method of Fainerman et al. (Fainerman, V.; Makievski, A.; Miller, R. Colloids Surf. A 1994, 87, 61), and it was found that for the siloxane surfactant with eight ethoxy groups the diffusion coefficient values are 1 order of magnitude higher than that of the hydrocarbon analogue with 5 ethoxy-groups. An increase of the ethoxy chain length for siloxane as well as for hydrocarbon surfactants causes a decrease of the diffusion coefficient and the surface/interfacial tension fall rate and leads to a suppression of surfactant spreading ability.