Spreading of aqueous trisiloxane surfactant solutions and conventional surfactant solutions over PTFE AF coated silicone wafers

Kinetics of spreading of aqueous trisiloxane solutions Tn (with n = 4, 6 and 8 ethoxy groups) and conventional aqueous surfactant solutions (Tween 20, C12E4, SDS) over silicon wafers coated with PTFE AF is experimentally investigated. It has been found that trisiloxane solutions spread on highly hydrophobic PTFE AF coated silicone wafers; however, they do not show superspreading behavior on these highly hydrophobic substrates. Solutions of conventional non-ionic surfactants investigated show kinetics of spreading similar to trisiloxanes. Three regimes of spreading have been found: (i) at low concentrations complete non-wetting both at an initial and at final stages of spreading, (ii) a transition from initial non-wetting to partial wetting at the end of the spreading process at intermediate concentrations and (iii) partial wetting both at the beginning and the end of the spreading process at higher concentrations. Transition from the first regime (i) to the second regime (ii) takes place at the critical aggregation concentration (CAC) or critical micelles concentration (CMC), transition from regime (ii) to regime (iii) happens at the critical wetting concentration (CWC). In the case of regime (i) the spreading of non-ionic surfactants solutions investigated on PTFE AF coated silicone wafers is slow and follows a theoretically predicted law. In the case of regimes (ii) and (iii) the spreading of the non-ionic surfactant solutions investigated proceeds in two stages: the fast short first stage, which is followed by a much slower second stage. The latter stage is similar to the case of C < CAC/CMC and follows the similar theoretical law. It is shown that the slow stages in both cases develop according to a previously described theoretical model (Starov et al. J. Colloid Interface Sci., (2000), 227(1), 185). According to this theory the surfactant molecules adsorb in front of the moving three phase contact line (autophilic phenomenon), which results in a partial hydrophilisation of an initially hydrophobic substrate and a spreading as a consequence. We assume that the first stage of the spreading is related to the disintegration of surfactant aggregates in the vicinity of the moving three phase contact line.