Proceedings of SerbiaTrib '13

formed during sliding, becomes more complex dueto change of some experimental parameters, such astemperature [3]. The computer simulation ofprocesses during sliding contact, when severalhundreds of atoms are involved, indicates thatatomic processes cannot be neglected, when wedescribe nanotribology experiments [1, 4]. For thatpurpose, several available methods can be includedfor research at a molecular level [5].2. BOUNDARY LUBRICATION BY SAMSAttractive model systems for boundarylubrication are organic self-assembled monolayers(SAMs). Preparing self-assembled monolayers isone of the most elegant ways to make ultrathinorganic films of controlled thickness. The processof self-assembly is considered as a very importantexample of equilibrium structural organization onthe molecular scale. Organic thin films are anemerging area of materials chemistry and areutilized in many application areas, such aselectronic components, as well as in biomedicalapplication [6]. There is also special interest in thepossibility of manufacturing molecular layers withparticular properties. Molecular self-assembly isrecognized as a powerful strategy for thefabrication of nanoscale structures [7].The interest in these systems has been furtherintensified in order to understand and solve friction,lubrication and related problems [8]. More recently,lubrication in a small-size system, such as themicroelectromechanical system (MEMS) ornanoelectromechanical system (NEMS), is a bigchallenge in scientific work, especially in the studyof new kind of lubricants. Different type ofmonolayers attached to sliding surfaces appears as agood candidate in MEMS lubrication. Therefore,the understanding of behavior between monolayersfilms is of great importance in tribological andnanotribological experiments.Due to very small thickness of monolayers(range of few nanometers), new tools are requiredfor this nanotribological studies. Widely used arethe following: surface-force apparatus (SFA), thescanning tunneling miscorscope (STM), the atomicforce and friction-force microscopes (AFM andFFM). Developed more than 40 years ago, the SFAis usually applied to study properties of molecularlythin films, confined between two molecularlysmooth macroscopic surfaces, with surfaceseparations at the angstrom level and forcesbetween them. A scanning tunneling microscope(STM) is an instrument for imaging surfaces at theatomic level [1]. With the development of a numberof powerful techniques in surface analysis, asmentioned above, academic interest in SAMs hasregained, because of the possibilities to investigatethe growth and the structure of such layers on thenanometer scale [9, 10].3. SURFACTANTS SELF-ASSEMBLYThe word “surfactant”, does not always appearin dictionaries, because it is a contracted form ofthe phrase SURFace ACTive AgeNT. Surfactantsare molecules essential to the chemical industry andin many products such as soaps, detergents,shampoos, softeners, pharmaceutical products, etc.Surfactant molecules have amhiphilic propertiesbecause they consist of two distinct parts - one thathas an affinity for the solvent, and the another onethat does not. This dual structure is responsible fora number of remarkable phenomena, such asmicelle formation in solution at a certainconcentration, the so-called critical micelleconcentration (cmc), and oriented adsorption ofsurfactants at surfaces and interfaces. Micelleformation has attracted a notable part of thesurfactant research, in order to investigate theformation of micelles [11, 12], their shape [13] ortheir interactions [14]. Systems below the cmc havenot been widely studied [15].The self-assembled monolayers can be preparedusing different types of molecules and differentsubstrates. A very often studied SAM model systemcomprises thiol molecules, adsorbed onto gold,silanes on an oxide surfaces, or alkanephosphatemonolayers, which was in detail reviewed byUlman [16]. The choice of the substrates, used inthe self-assembling process, is dictated by themolecules and their interactions, as well as the finalapplication.Self-assembled monolayers form spontaneously,when certain classes of molecules adsorb onto asolid surface from solution. When a surfactantsolution is in contact with a solid surface, thesurfactant molecules adsorb onto the surface,ideally forming an adsorbed layer of high order,termed as self-assembled monolayer (fig.2).Figure 2. a) An organized monolayer on a substrate,b) CTAB chainMany surface properties are influenced by sucha film, e.g. the hydrophobicity or the wetting or154 13 th International Conference on Tribology – Serbiatrib’13