Spreading of Aqueous Solutions of Trisiloxanes and Conventional Surfactants over PTFE AF Coated Silicone Wafers

Abstract

Kinetics of spreading of aqueous trisiloxane surfactant T(n) (with n = 4, 6, and 8 ethoxy groups) solutions 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 nonionic surfactants investigated show kinetics of spreading similar to trisiloxanes. Three regimes of spreading have been identified (i) complete non-wetting during the spreading process at low concentrations, (ii) a transition from initial nonwetting 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 micelle 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 nonionic surfactants solutions investigated on PTFE AF coated silicone wafers is slow and follows a theoretically predicted law (Starov; et al. J. Colloid Interface Sci. 2000, 227 (1), 185). In the case of regimes (ii) and (iii), the spreading of the nonionic surfactant solutions investigated proceeds in two stages: the fast short first stage, which is followed by a much slower second stage. It is shown that the slow stage develops according to a previously described theoretical model. 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.

Why do aqueous surfactant solutions spread over hydrophobic substrates?

“…To summarize experimental findings presented in 15 : the slow stage of spreading of aqueous trisiloxane surfactant solutions over highly hydrophobic PTFE AF coated silicon wafers is described in the frame of the proposed theory 23 , and a good agreement between experimental and predicted data has been found. Figure 5 shows the comparisons between the experimental data on the time evolution of the diameter of the droplet base and theoretical predictions according to Eqs.…”

“…It was shown earlier 15 that at low concentrations below CAC the kinetics of spreading is described by the theory presented in 23 .…”

“…According to 15,23 the dynamic of contact angle of a spreading droplet of surfactant solution over hydrophobic substrate at concentrations below CAC can be described as…”

“…[9][10][11][12][13][14][15][16] Obviously the spreading of surfactant solutions is affected by adsorption of surfactant molecules at interfaces, which could drastically changes interfacial tensions and energy balance at a three-phase contact line, promote the spreading of aqueous surfactant solutions: the rate of adsorption of surfactant molecules and depletion of surfactant at the expanding interface during the spreading reflects on the changing of spreading rate of droplet. 17 The mechanisms proposed by a number of researchers [4][5][6][7]10 for spreading of surfactant solution over hydrophobic surfaces relayed only on the adsorption processes on (i) the liquid-vapor and/or (ii) the solid-liquid interfaces, and completely excluded the possibility of adsorption of surfactant on (iii) the solid-vapour interface in front of the moving three phase contact line.…”

Kinetics of spreading of synergetic surfactant mixtures in the case of partial wetting

“…The theoretical basis of the slow spreading of surfactant solutions due to surfactant adsorption on the solid/air interface in the front of contact line was proposed in [20,21]. Experimental evidence of such adsorption was provided in [22].The slow spreading of long duration was found also in [23], for several surfactant solutions, both ionic and non-ionic. Kinetics of spreading in this case was in good agreement with the theory developed in [20,21].…”