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

• Published in: Colloids and surfaces. A, Physicochemical and engineering aspects Vol. 183; pp. 607 - 620
• Main Authors: Svitova, T, Hill, R.M, Radke, C.J
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.07.2001
• Subjects: AFM imaging; Graphite; Non-ionic surfactants; Spreading dynamics; Surface roughness; Spreading dynamics; Surface roughness; Non-ionic surfactants; Graphite; AFM imaging
• ISSN: 0927-7757; 1873-4359
• DOI: 10.1016/S0927-7757(01)00517-9

The adsorbed self-assembly structures of a series of trisiloxane M(D′E n )M, n=6, 8, and 12 and Tris (ethylene oxide) dodecyl ether C 12E 3 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, C 12E 3 aggregates are arranged in parallel stripes, oriented along the three graphite crystal axes. At higher C 12E 3 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. C 12E 3 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′E 6)M to a couple of days for M(D′E 12)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′E n )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′E n )M and C 12E 3 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∼ kt n , 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.