Autophilic Effect: Wetting of Hydrophobic Surfaces by Surfactant Solutions

Abstract

This paper resolves questions in the literature regarding the autophilic effect (i.e., movement of surfactant past the advancing contact line-leading to an increase in drop radius beyond that due to the advance) and its importance to quasi-static sessile drop wetting. Various systems (SDS, HTAB, and MEGA 10 surfactant solutions at three concentrations each and pure water and ethylene glycol on hydrophobic Teflon and OTS-coated silicon) are probed to determine the existence, time constant, and magnitude of the autophilic effect, using quasi-static advancing and receding sessile drops. From spreading results and advancing contact angle measurements, it is inferred that the autophilic effect does not occur for our systems (in contradiction of some literature) for the following reasons. First, no relation exists between the time constant for spreading and surfactant concentration, meaning the spreading seen is likely inertial in cause and not due to surfactants. Second, advancing contact angle decreases between tests on clean surfaces and those pre-exposed to surfactant, ruling out the possibility that the autophilic effect is faster than the advance. Third, spreading is seen after the end of the advance over both clean and pre-exposed surfaces, ruling out the possibility that the autophilic effect is slower than the advance. Finally, the pure liquids spread in a similar fashion to surfactant solutions on Teflon and similar contact angle measurements are seen for surfactant solutions and pure liquids of similar surface tension.

Diffusion-controlled evaporation of sodium dodecyl sulfate solution drops placed on a hydrophobic substrate

“…Ivanova et al [38] examined the spreading of SDS solution on the PTFE-AF coated wafer surface and reported that the change of droplet volume caused by evaporation during the whole spreading process was around 5% [38]. Milne and Amirfazli studied the system of spreading SDS solution drop on TEFLON-AF spin-coated smooth silicon wafer, using the quasi-static sessile drop technique and reported that there was no SDS surfactant adsorption on the solid-vapor interface ahead of the contact line on the first advance over the clean hydrophobic surface, and also there was no autophilic effect (i.e., movement of surfactant past the advancing contact line, leading to an increase in drop radius beyond that due to the advance) although drop spreading occurs after the volume increase of the SDS solution drops stopped during the advancing contact angle measurements within 20 s and the time for spreading is independent of SDS concentration [39].…”

“…There is no previous publication of the SDS adsorption on TEF-LON-FEP surface, however conflicting reports were published on the SDS adsorption and spreading on fluorocarbon polymer surfaces in the literature [36][37][38][39]. Starov et al found a 3-5% increase of the contact radius, r b of the SDS solution drops on polytetrafluoroethylene (PTFE) surface within 10 s for SDS concentrations below cmc [36].…”

Potentiodynamic study of Al–Mg alloy with superhydrophobic coating in photobiologically active/not active natural seawater

“…Since in investigated conditions pollution from CTAB or similar artificial surfactants is negligible, an indirect mechanism driven by naturally present organic matter is more likely involved. In particular, it is referred to phenomena like the autophilic effect reported by some authors, accounting for alternative changes in composition by adsorption from water-air to solid interface [51,52]. In addition, it is pointed out that CA measurement represents the status of the surface after exposure to the natural seawater environment, undergoing the effects of DOM sedimentation and accumulation implying possible macroscopic changes in the interface composition.…”

Time-dependent Dynamic Receding Contact Angles Studied during the Flow of Dilute Aqueous Surfactant Solutions through Fluorinated Microtubes

“…6 Surfactants adsorb onto the liquid vapor (LV) and solidliquid (SL) interface; adsorption onto the solidvapor (SV) interface is still under debate. 7,8 The dynamic wetting of surfactant solutions on hydrophobic solids has been mostly studied through drop-spreading 9,10 and spontaneous capillary-filling experiments. 11,12 In the latter category of experiments, penetration of surfactant solutions into hydrophobic capillaries showed different kinetic regimes.…”