Proceedings of SerbiaTrib '13

According to the AFM results in figure 2.a, theadsorbed film can be the homogeneous SAM on themica. Hysteresis in water CA indicates ahydrophobic but not homogeneous film. Similarcontact angles have been reported in the literature.The decay in CA values with time shows instabilityof the formed layer, due to desorption of theadsorbed molecules into the water drop. A similarobservation has been also made 24 and interpretedas water penetration into the monolayer, aphenomenon already known to Langmuir in 1938.The degree of water penetration can also beinfluenced by the local environment, e.g. relativehumidity or temperature 24.In all experiments hydrophobic surfaces withadvancing contact angles between 75° and 90° havebeen obtained. The significant hysteresis suggests achemical heterogeneity or roughness in thesurfactant film. Measurements of hexadecanecontact angle represent one of the most sensitivetools to determine the conformational order ofhydrocarbon thin films. High hexadecane contactangles are observed on hydrocarbon SAMs onlywhen the monolayers are densely packed. Usinghexadecane we have observed lower values than theexpected 40° for ideally ordered films.In order to relate our SAM preparation protocolsto previous work, we have compared our resultswith those described in the literature. Much workhas been devoted to the study of the adsorption ofCTAB on mica using a variety of adsorptionprotocols and measurement techniques. For bothsolution concentrations, below and above the cmc,the picture drown in the literature demonstrates ahigh degree of variability.The protocol proposed by Zhao 25 wasreported to produce cylinder aggregates on the micasurface at a solution concentration of 2cmc (figure3.a). Due to the measured height of ~6nm of thefeatures, the layer has been described as consistingof two bilayers.At the CTAB concentration 10 -5 M monolayersand bilayers have been observed 22, using asimilar adsorption protocol at room temperature(around 25°C), as shown in figure 3.b. The AFMwas operated in the surfactant solution.We can conclude that the literature confirms thateven small differences in the SAM adsorptionprotocol can significantly affect the surfactant filmmorphology on mica, especially above the cmc andthe Krafft temperature. These observations areconfirmed by our experimental results and there isthe need to explain these variations by structuralchanges of the surfactant solution 26. There aresignificant structural changes around the Kraffttemperature. Below the bulk cmc and below theKrafft temperature, an equilibrated solution isexpected to be free of micelles. At the Kraffttemperature, the solubility becomes equal to thecmc and micelles will form in the solution and thistemperature is very often described in the literature.But, reported values of T k for CTAB in water varyconsiderably ( from 20°C to 25°C), which is veryclose to a room temperature and complicates theexplanation of experimental results.Figure 3. a) AFM topographic image of a mica surfaceprepared unspecified temperature conditions, using“CTAB in/CTAB out” protocol (immersion time-1min),at 2cmc, without rinsing after removal from the solution,dried with nitrogen before AFM imaging 25; b) AFMimage of a CTAB adsorbed layer on mica in a 10 -5 Msolution at 25°C for 25min immersion time, observed inthe surfactant solution 22.It is important to note that most of theexperiments described in the literature do notmention the problem of temperature control, and itis not possible to reconstruct this importantparameter from the information provided. We havea clear evidence that temperature is the key factordetermining SAM morphology. Therefore, we havesystematically studied temperature effects byrigorously controlling the temperature during allprocedures.6. CONCLUSIONThe concept of self-assembly on surfaces hasbeen treated in this research and the experimentshave revisited the adsorption of CTAB onto amuscovite mica. These results demonstrate theinfluence of a large number of parameters on theadsorption process, as well as the morphology and13 th International Conference on Tribology – Serbiatrib’13 181