Adsorption layer structures and spreading behavior of aqueous non-ionic surfactants on graphite

The adsorbed self-assembly structures of a series of trisiloxane M(D′En)M, n=6, 8, and 12 and Tris (ethylene oxide) dodecyl ether C12E3 non-ionic surfactants on graphite have been studied by AFM using tapping mode for imaging and contact mode for force measurements. In the concentration range of 2–5 times of the CMC, C12E3 aggregates are arranged in parallel stripes, oriented along the three graphite crystal axes. At higher C12E3 concentrations, AFM reveals featureless structures, repeating the morphology of the underlying graphite surface. Trisiloxane surfactants basically show the same self-assembly behavior on graphite, but the rate of self-organization into elongated aggregates is substantially slower. C12E3 aggregates can be imaged as soon as the cantilever, solid substrate, and solution are equilibrated, whereas for trisiloxane surfactants it takes from several hours for M(D′E6)M to a couple of days for M(D′E12)M arrangement into stripe-like aggregates. The self-aggregation behavior of surfactants on graphite is compared with their wetting behavior on this substrate. Critical wetting concentrations (CWC) found for M(D′En)M solutions on graphite are in agreement with ones found for other hydrophobic substrates in our previous studies [Langmuir 14 (1998) 5023]. At C≥CWC, a transition occurs from partial wetting to complete spreading. At these concentrations, M(D′En)M and C12E3 form featureless multilayer adsorption structures, as revealed by AFM. We find for pure wetting liquids that the location of the main-drop three-phase contact line propagates as a power law in time, R∼ktn, with n varying from 0.12 to 0.2. Drop radius histories for aqueous, non-ionic surfactant solutions spreading on graphite at concentrations above the CWC also obey a power-law functionality. However, now spreading occurs in three regimes. At a time of several seconds, n is approximately 0.2. Next, an approximate square-root-exponent time regime emerges. Finally, catastrophic irregular spreading occurs with the formation of preceding dendrites and fingers, most likely caused by local roughness heterogeneities and/or local interfacial tension gradients. Spreading in precursor channel feet ahead of the main drop is important on graphite surfaces, which are rough on the macroscopic scale.