Monolayer Study by VSFS: In Situ Response to Compression and Shear in a Contact Ahmed Ghalgaoui, ⊥,† Ryosuke Shimizu, ⊥,‡ Saman Hosseinpour, ⊥ Rube ́ nA ́ lvarez-Asencio, ⊥ Clayton McKee, ⊥,∥ C. Magnus Johnson, ⊥ and Mark W. Rutland* ,⊥,§ ⊥ KTH Royal Institute of Technology, Division of Surface and Corrosion Science, Drottning Kristinas Vä g 51, SE-100 44 Stockholm, Sweden § SP Chemistry, Materials and Surfaces, SP Technical Research Institute of Sweden, Box 5607, SE-114 86 Stockholm, Sweden ABSTRACT: Self-assembled octadecyltrichlorosilane ((OTS), CH 3 (CH 2 ) 17 SiCl 3 ) layers on hydroxyl-terminated silicon oxide (SiO 2 ) were prepared. The monolayers were characterized with atomic force microscopy (AFM) and contact angle measurements; their conformation was studied before, during, and after contact with a polymer (either PDMS or PTFE) surface using the vibrational sum frequency spectroscopy (VSFS) technique. During contact, the effect of pressure was studied for both polymer surfaces, but in the case of PTFE, the effect of shear rate on the contact was simultaneously studied. The VSFS response of the monolayers with pressure was almost entirely due to changes in the real area of contact with the polymer and therefore the Fresnel factors, whereas sliding caused disorder in the previously all-trans monolayer, as evidenced by a significant increase in the population of gauche defects. ■ INTRODUCTION Boundary lubricants form a thin barrier layer, typically a monolayer, preventing direct contact between two surfaces. Exactly how such a layer behaves under pressure and shear remains a topic of debate. A self-assembled silane layer 1,2 provides a good model system for an in situ spectroscopic study. Octadecyltrichlorosilane (OTS) as a self-assembled monolayer (SAM) on silicon oxide has been widely studied by many groups in recent years. 3−5 Such SAMs offer a facile means to alter and control the chemical nature of a surface. Some examples of current industrial uses of SAMs are lubricants, coupling agents, coatings, and templates. 6 OTS is a particularly interesting type of SAM because it can bind to oxide surfaces and is thus of use in Si-based micro- electromechanical systems, which have a native amorphous oxide layer, 7 and where tribological and “stiction” issues are very important. The characteristics of these SAMs have been investigated extensively by various methods, such as FTIR, AFM, and ellipsometry. 8,9 In this study, we are interested in the change in the molecular conformation of an OTS SAM in a tribological contact. In the literature, the molecular conformation of OTS SAM at the solid−solid interface as a function of pressure and sliding is limited to a theoretical study. 10−12 However, Shen 13 has experimentally studied the case of a monolayer of octadecyl alcohol confined between two equivalent solids (a quartz flat and a quartz lens). Vibrational sum frequency spectroscopy (VSFS) was employed, and the signal disappeared when the surfaces were placed in contact. This was interpreted as a conformational change resulting in the molecules lying flat on the substrate at a pressure of 50 MPa. Bain 14 also used VSFS to study a monolayer of zinc arachidate at the sapphire−silica interface and in contrast reported that the monolayer is resistant to pressure- and shear-induced conformational disorder. The authors claimed that there was intimate contact of the sapphire and silica over the entire contact region based on root-mean-square (rms) roughness measurements and contact mechanical theory. Both frequency shifts and changes in peak intensity ratios were observed and were ascribed to monolayer transfer between the surfaces. These observations are consistent with observations by Du et al. 13 and Fraenkel et al., 15 who explained that the transfer of the monolayer between solid surfaces during contact could explain part of the observed drop in the VSFS signal intensity. The Dhinojwala 16 group has studied the PDMS−PPMA interface for fresh and aged lenses. They observed an unusual increase in adhesion hysteresis and frictional forces for poly(dimethylsiloxane) (PDMS) lenses sliding on smooth, glassy surfaces after a period of aging. VSFS studies were carried out on the PDMS surfaces immediately after they slid out of contact. The increase in the VSFS signals from PDMS suggested significant ordering of the PDMS chains induced by sliding., The same group 17 studied the interface of an oxidized (PDMS ox ) elastomer and a methyl-terminated self- assembled monolayer of OTS on sapphire substrates. A strong Received: November 13, 2013 Revised: February 13, 2014 Published: February 18, 2014 Article pubs.acs.org/Langmuir © 2014 American Chemical Society 3075 dx.doi.org/10.1021/la4042474 | Langmuir 2014, 30, 3075−3085