| Abstract
| - The self-associating structures at the solid−liquid interface of three nonionic trisiloxane surfactants((CH3)3SiO)2Si(CH3)(CH2)3(OCH2CH2)nOH (n = 6, 8, and 12), or BEn, are studied as a function of substrateproperties by atomic force microscopy (AFM) imaging and force measurement. These trisiloxane surfactantsare known as superwetters, which promote rapid spreading of dilute aqueous solutions on low-energysurfaces. This study also attempts to relate the BEn surface aggregate structures at the solid−liquidinterface to their superwetting behavior. Four substrates are used in the study: muscovite mica, highlyoriented pyrolytic graphite, and oxidized silicon wafer with and without a full monolayer of self-assembledn-octadecyltrichlorosilane (OTS). The concentration of BEn is fixed at 2 times the critical aggregationconcentration (CAC). The BEn surfactants are only weakly attracted to hydrophilic surfaces, more onoxidized silicon than on mica. All three form ordinary planar monolayers on HOPG and OTS-coveredoxidized silicon. The significance of surfactant adsorption on the AFM tip is investigated by comparingthe force curves obtained by tips with and without thiol modification. The surface aggregate structuresof the BEn surfactants correlate with their bulk structures and do not exhibit anomalous adsorptionbehavior. The adsorption behavior of the BEn superwetters is similar to that of the CmEn surfactants. Thus,our results confirm previous work showing that superwetting shares its main features with other classesof surfactants.
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