Proceedings of SerbiaTrib '13

electrostatics [17]. Due to the flexibility inchoosing the molecular architecture, organicmolecules have many interesting applications, suchas biosensors, for lubrication or in controlling wateradsorption. Therefore, in recent years, muchattention has been directed to the study of SAMs.However, a discrepancy still exists between thetheoretical understanding and the practicalimportance involved in the formation of suchlayers.Cationic surfactants are a small subgroup withsome unique properties that are not present in othersurfactants. In order to help understanding ofadsorption of cationic surfactants, adsorption ofquaternary ammonium surfactants onto inorganicsubstrates, such as mica, has been widely studied[18, 19, 20]. The process of adsorption has beeninvestigated by different techniques [21], such as x-ray photoelectron spectroscopy (XPS), the surfaceforces apparatus, (SFA) [15] or contact angle (CA)measurements [22]. This paper focuses onquaternary ammonium surfactants with a cationichead group, single-tailedhexadecyltrimethylammonium bromide, CTAB,with the molecular structureCH 3 (CH 2 ) 15 N + (CH 3 ) 3 Br - (figure 2b).4. SAMS PREPARATION ANDCHARACTERIZATIONA standard protocol, which can produce a welldefinedand reproducible hydrophobic CTAB filmon mica, does not exist. Namely, previous studiesof CTAB adsorption on various substrates,suggested that the behavior of CTAB is morecomplex than the behavior of other cationicsurfactants [22, 23, 24], but the reason for thissingularity has not been clearly determined [17].The original goal of these experiments was toproduce self-assembled monolayers and use themas model systems to study boundary lubrication.But, there was a problem concerning the resultsbeing repeated, as well as the characterization ofthe adsorbed CTAB layers on muscovite mica indetail. The various SAM morphologies, found onmica by the use of different adsorption protocols,demonstrate the influence of a large number ofexperimental parameters on the adsorption process,such as concentration, pH, temperature andhumidity. They are rarely described in theliterature.Figure 3. CTAB layer on muscovite micaIn order to characterize and determine theproperties of SAMs, two techniques have beenextensively used in this work, contact anglemeasurements and the atomic force microscopy(AFM). For example, freshly cleaved mica hascontact angle less than 10° and contact angle on theSAM produced by CTAB adsorption on mica canbe 140° [22]. A contact angle greater than 90° isdetermined on hydrophobic surfaces.In our experiments the contact anglemeasurements have been used in order to define thedegree of hydrophobicity of the modified micasurface and molecular order after surfactantadsorption, performed by using ultra pure water.All contact angle measurements were averagedover several samples.5. EXPERIMENTAL PROCEDUREWe made self-assembled monolayers ofquaternary ammonium surfactants on mica. Singletailedhexadecyltrimethylammonium bromide,CTAB, with the molecular structureCH 3 (CH 2 ) 15 N + (CH 3 ) 3 Br - , was purchased fromFluka. For further purification, CTAB wasrecrystallized from an ethanol/acetone mixture. Asa solvent, ultra pure water of resistivity 18.3MΩcmwas prepared using a Barnstead EASYpurebatch-fed water purification system. The samewater quality was also used for the sample rinsing,before drying with a clean nitrogen stream.The glassware and bottles used in theexperiments were consistently cleaned by piranhasolution and then rinsed with purified water toavoid any organic contamination. All the employedtools were previously cleaned in order to minimizethe occurrence of molecular contamination,particularly on the high-energy mica surface.Muscovite mica purchased from Spruce PineMica Company Inc. (USA) was used for theadsorption experiments. Small mica samples of 1-1.5cm 2 size, were cut by scissors. Then, they werefreshly cleaved on both sides before immersion intothe surfactant solution. The adsorption was13 th International Conference on Tribology – Serbiatrib’13 155