Critical wetting concentrations of trisiloxane surfactants

Trisiloxane surfactants also known as “superspreaders” are characterized by a critical wetting concentration (CWC) which has been determined by Svitova et al (Langmuir. 1998. 14, 50235031). CWC is a concentration above which a transition from partial wetting to complete wetting occurs at spreading over moderately hydrophobic surfaces, hence, the CWC is associated with the beginning of the superspreading. The latter shows that the knowledge of the CWCs is important for many wetting/spreading applications. We suggest a new method, which allows determining CWCs for a range of trisiloxanes using wetting behaviour of aqueous trisiloxane solutions on highly hydrophobic smooth Teflon AF coated silicon wafers. Using the new method the CWCs for trisiloxane surfactants, Tn with a wide range in the number (n = 4 − 9) of ethylene oxide units have been measured. The critical aggregation concentration (CAC) for trisiloxane surfactants has been measured using the surface tension methods. Additionally, spreading kinetics of all trisiloxane surfactants studied over moderately hydrophobic substrates has been investigated. It has been found that for all trisiloxanes studied the CWCs are several times higher than their corresponding CAC values. The CWCs for T6 and T8 trisiloxanes obtained according to our approach are in agreement 2 with those measured by Svitova et al. (Langmuir. 1998. 14, 5023-5031) for the case of complete wetting. It has been verified that indeed at CWCs estimated according to our approach trisiloxane surfactant solutions demonstrate superspreading behaviour on moderately hydrophobic surfaces with the exception of trisiloxane T4, which still shows only partial wetting. Thus, the CWC does not depend on the degree of hydrophobicity of substrates and could be considered as a distinct property of surfactants that characterize the highest limit of their spreading capability.