Tribology Letters Vol. 10, No. 1-2, 2001 81 The relationships between interfacial friction and the conformational order of organic thin films Scott S. Perry ∗ , Seunghwan Lee, Young-Seok Shon, Ramon Colorado, Jr. and T. Randall Lee ∗ Department of Chemistry, University of Houston, Houston, TX 77204-5641, USA E-mail: perry@uh.edu; trlee@uh.edu This report describes studies of the relationships between the structures of organic monolayers and their molecular-scale frictional properties. Several distinct self-assembled monolayers (SAMs) were formed by the adsorption of a series of spiroalkanedithiols and a single structurally related normal alkanethiol. Measurements of hexadecane wettability, infrared vibrational spectroscopy, and X-ray photoelectron spectroscopy revealed that the films possessed a wide range of interfacial structures and conformational orders. Atomic force microscopy was used to measure the frictional properties of the well-characterized SAMs on the molecular scale. Comparison of the frictional data with structural information derived from complementary analytical techniques revealed a high correlation between the conformational order of the films and the observed frictional response. KEY WORDS: self-assembled monolayers; alkanethiol; spiroalkanedithiols; gold substrates; wettability; infrared spectroscopy; X-ray photoelectron spectroscopy; AFM 1. Introduction Over the past decade, significant progress has been made toward understanding interfacial friction on the molecular and atomic scales [1,2]. These advances have been realized through the development of analytical techniques capable of sensing forces arising from small ensembles of atoms or molecules [3,4] and the development of computational tools capable of modeling these ensembles and their interactions across an interface [5–7]. In addition, the need to understand interfacial friction on the nanometer scale has been driven by the development of technologies and products that possess nanometer-scale interfacial features and require the control of interfacial friction and wear at this level. Progress has been realized through studies of both model systems and ac- tual devices. For our investigations of molecular-level friction, we uti- lize model thin-film systems in which the composition and structure of the films can be controlled through organic synthesis and subsequently characterized by complementary surface analytical techniques. These studies have largely in- volved self-assembled monolayers (SAMs) derived from the adsorption of organosulfur molecules onto gold substrates. Organosulfur-based SAMs on gold have enjoyed increasing popularity both because of the ease with which the mono- layers can be prepared and because of their relative stabil- ity. Our contributions in this area have involved the prepara- tion and characterization of SAMs derived from organothiols possessing unique structures and atomic compositions. The aim of this approach has been to build into the monolayer film specifically tailored nanoscale features and composi- tions and then to correlate these properties with fundamental measurements of interfacial friction. ∗ To whom correspondence should be addressed. Our studies have utilized SAMs composed of linear hy- drocarbon chains [8], selectively fluorinated hydrocarbon chains [8–12], branched hydrocarbon chains [10], terminally functionalized (phenyl, C 60 ) hydrocarbon chains [13,14], and tethered hydrocarbon chains bound to the surface via chelating dithiol head groups [15–20]. Monolayer films generated by the adsorption of these molecules onto gold have been characterized through wettability studies using a wide range of contacting liquids, vibrational studies using polarization modulation infrared reflection absorption spec- troscopy (PM-IRRAS), compositional analysis using X-ray photoelectron spectroscopy (XPS), and interfacial friction measurements using atomic force microscopy (AFM). The information gleaned from these complementary techniques has provided the unique opportunity to interpret the fric- tional properties of the monolayer films on a truly molecular scale. In this paper, we present a summary of the interfacial frictional properties of a series of SAMs that differ with re- gard to their molecular packing on the surface and their re- lated conformational order. A more complete description of these monolayers and their frictional properties can be found in previous publications [8–20]. Most of the molecules used to generate SAMs in this study possess the spiroalka- nedithiol structure illustrated in figure 1. In the spiroalka- nedithiols, two alkyl chains are connected via a quaternary carbon to two mercaptomethyl groups, which can, in turn, bind in a bidentate fashion to metal surfaces. Previous re- ports from our groups have detailed a wide variety of sym- metrical (R = R ′ ) and unsymmetrical (R = R ′ ) spiroalka- nedithiol derivatives that can be used to prepare highly sta- ble and structurally defined SAMs on gold [15–20]. Here, we will discuss SAMs derived from only the molecules shown in figure 1. These molecules include the biden- 1023-8883/01/0300-0081$19.50/0  2001 Plenum Publishing Corporation