Proceedings of SerbiaTrib '13

Mica samples preparationFor the adsorption experiments, we usedmuscovite mica purchased from Spruce Pine MicaCompany Inc. (USA). Small mica samples of 1-1.5cm 2 size were cut by scissors and after thatfreshly cleaved on both sides and immersed into thesurfactant solution. The adsorption was performedfrom the surfactant solution in a volume of 20ml.of 18°C or 30°C. It is important to emphasize thatall the used chemicals, tools and substrates wereequilibrated at the defined temperature before theadsorption experiments.3. EXPERIMENTAL PARTSeveral experiments, described in the literature,suggested that the transition through the threephaseboundary is an important step, with asignificant influence on the surfactant filmmorphology. To distinguish effects due toadsorption at the solid-liquid interface and thedeposition at the three-phase boundary (TPB), wehave systematically varied theimmersion/extraction protocol and defined fourdifferent experiments. All four adsorptionprotocols, separately described below, have beenused with/without temperature control. The resultsobserved under the temperature controlledconditions have been described.Every experiment consists of four steps:adsorption, rinsing, drying and analysis (AFM andCA). Varying the immersion and extraction of micasample into and out of the surfactant solution, wehave defined four different adsorption protocols(figure 1).The first option, called “CTAB in/CTAB out“,involves immersion and extraction from thesurfactant solution at the nominal concentration.The second adsorption type (“CTAB in/diluteout”) uses immersion into the nominal surfactantsolution, a rest time and subsequent rapid(10seconds) dilution with pure water prior toextraction, in order to eliminate surfactantdeposition during the transition through the TPB.The third option (“dilute in /dilute out”) preventssurfactant deposition at the TPB in both steps,immersion into solution and extraction from it. Inthis case, CTAB is added to the solution after thesample is submerged.The fourth option (“dilute in/CTAB out”) allowsdeposition at the TPB during extraction only.In the water dipping step, the mica samples weredipped into 20ml of ultra pure water to remove theexcess surfactant molecules.The same set of adsorption experiments shownin figure 1 has been repeated at controlledtemperature in the laboratory, at 18C (below theKrafft temperature) and 30°C (above the Kraffttemperature). A stock solution, 250ml of 10 -3 MCTAB, has been prepared at controlled temperatureFigure 1. Schematic representation of four differentadsorption protocols used to discriminate differentdeposition mechanisms at the three-phase boundary. Allsamples were dipped into ultra pure water to strip offpossible excess CTAB (e.g. incomplete second layer)After the post-rinsing step, the modified micasurface was gently blown dry with nitrogen beforethe AFM imaging or contact angle measurements.Each type of protocol (at different concentrations)was repeated several times to also assess thereproducibility.The samples were imaged with an Atomic ForceMicroscope (Digital Instruments, Nanoscope IIIa),which was operated under ambient conditions. Theimages were systematically collected for differentscan sizes (i.e. 10µmx10µm, 5µmx5µm and1µmx1µm, and again 10µmx10µm) and they wererepeated by scanning several different areas on agiven sample. At least two samples of eachpreparation protocol were analyzed.The contact angle is the angle conventionallymeasured through the liquid, where aliquid/vaporinterface meets a solid surface. It quantifies thewettability of a solid surface by a liquid. On everysample advancing (maximal) contact angle and thereceding (minimal) contact angle are measured. Awater contact angle greater than 90° is determinedon hydrophobic surfaces. For example, freshlycleaved mica has a contact angle less than 10, anda contact angle on the SAM produced by CTABadsorption on mica can be 140.13 th International Conference on Tribology – Serbiatrib’13 179