Structure and Frictional Properties of Self-Assembled Surfactant Monolayers Yihan Liu and D. Fennell Evans* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 Qun Song and David W. Grainger Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523 Received June 23, 1995. In Final Form: October 26, 1995 X Frictional properties of monolayers formed from 11 double-chain quaternary ammonium surfactants self-assembled onto mica were measured using lateral force microscopy. Frictional forces differ by orders of magnitude and can increase or decrease with increasing humidity, and frictional force vs velocity curves in some cases display maximum. These differences correlate at a molecular level with variations in the surfactant’s chemical composition, degree of unsaturation, chain length, and ω functional group. These differences can also be directly related to the monolayer’s structure, phase transition temperature, compressibility and surface hydrophobicity as determined by X-ray diffraction, scanning calorimetry, contact angle, and simulation data. Comparison between lateral force measurements using a standard AFM tip and measurements modified by replacing the tip with a sphere provide a relationship between nanoscale and microscale frictional properties. These observations establish a relation between frictional properties and molecular properties of thin films which are important in many applications including lubrication and tribology. Introduction Ultrathin molecular films show considerable techno- logical importance in areas involving electronic and optical devices, sensors and transducers, protective and lubricate layers, and patternable materials, etc. 1,2 In recent years, considerable effort has been directed toward establishing relationships between structure, forces, and electrical and mechanical properties of organic thin films on surfaces. Self-assembled monolayers of surfactant molecules con- stitute model systems which permit incorporation of diverse chemical and physical properties and ease of preparation. Insights gained from these model studies can be codified and applied to more complex technological and biological systems. Our present understanding of friction and boundary lubrication is mainly based on empirical correlation. However, the development of modern instrumentation, particularly the surface force apparatus 3-7 and the scan- ning force microscopy, 8-12 provides a way to characterize frictional properties of systems at a molecular level. In this paper, we present a systematic study of frictional properties of self-assembled surfactant monolayers using lateral force microscopy. 13,14 The surfactants are double- chain quaternary ammonium salts, differing in composi- tion, chain length, degree of unsaturation, and functional groups. Our goal is to develop a structure-property relationship. We establish that frictional force can be related to the structure of constituent surfactant molecules in the monolayer film, which in turn provides unique insights into the physics and chemistry of the surfactants as well as into the thermodynamics of two-dimensional systems. Experimental Details Surfactants. Ditetradecyldimethylammonium acetate (2C14- N2C 1OAc), dihexadecyldimethylammonium acetate (2C16N2C1- OAc), and dioctadecyldimethylammonium acetate (2C 18N2C1- OAc) were the same compounds as those used in ref 15 (2C14) and ref 16 (2C16 and 2C18). The surfactants were prepared by ion exchanging from the corresponding bromide salts (Sogo Pharmaceuticals, Japan) and recrystallized. Dieicosyldimethyl- ammonium bromide (2C 20N2C1Br) and didocosyldimethylam- monium bromide (2C 22N2C1Br) were a gift from Professor Robert Moss and were used as received. Dodecyloctadecyldimethylam- monium bromide (C 12C18N2C1Br) was synthesized by Dr. J. E. Brady, using the method described in ref 17. Two ω-hydroxy- substituted surfactants, (16,16′-dihydroxydihexadecyl)dimethyl- ammonium bromide (2(HOC 16)N2C1Br) and (16-hydroxydihexa- decyl)dimethylammonium bromide (HOC 16C16N2C1Br), were a gift from Dr. John Trend and were used as received. 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