ARTICLE pubs.acs.org/JPCC
Adsorption of Unsaturated Amides on a Graphite Surface: trans-Unsaturated Amides Tej Bhinde,† Adam Y. Brewer,† Stuart M. Clarke,*,† and Tamsin K. Phillips† †
BP Institute and Department of Chemistry, University of Cambridge, Madingley Road, Cambridge CB3 0EZ, United Kingdom
Thomas Arnold‡ and Julia E. Parker‡ ‡
Diamond Light Source Limited, Diamond House, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0DE, United Kingdom
bS Supporting Information ABSTRACT: The adsorption of three unsaturated amides on graphite is investigated using a combination of synchrotron X-ray diffraction and differential scanning calorimetry (DSC). In this work unsaturated amides with chain lengths of six and nine carbons have been studied with one trans double bond in the alkyl chain at different positions. The structure of each unsaturated amide has been separately determined using synchrotron X-ray diffraction at submonolayer coverages. These monolayer structures are compared with the structure of the saturated amide of the same chain length. The hydrogen bond geometries in these structures are discussed, and these indicate that the position of the double bond relative to the amide headgroup has an important influence on the stability of the monolayer, a finding also supported by the DSC results. Binary mixtures of saturated and trans-unsaturated amides have also been studied using DSC and the behavior discussed using the monolayer structures of the individual species.
’ INTRODUCTION Understanding the adsorption behavior of molecules at the solid/liquid interface has been a subject of considerable study owing to the wide variety of commercial and academic applications of such systems. Lamellar solids such as graphite are advantageous to study adsorption phenomena owing to their chemical inertness and near-ideal uniform surface.1 Reports on the adsorption of various organic species on graphite can be found in literature using a wide range of experimental and theoretical approaches. These include X-ray, neutron, and electron scattering,2-4 calorimetry,5-7 scanning probe microscopy,8-10 NMR,11 and simulation12 techniques. A summary of some of these studies can be found in reviews 13-15. Alkylamides are an important class of organic compounds which have a number of commercial and academic interests. Long chain (e.g., C18 and C22) saturated/unsaturated amides are used as friction modifiers in polymer systems,16 among other applications. The amide headgroup imparts these molecules with interesting properties such as the ability to form very stable hydrogen-bonded networks. Indeed there is significant interest in amides across a variety of areas particularly in biology where polypeptides are held together by amide linkages and the hydrogen bonds they provide. They provide an interesting extension of previous studies of related species, such as alcohols and carboxylic acids, which can also hydrogen bond to form lines or dimers. These contrast to the amides where a similar chemical r 2011 American Chemical Society
group can make hydrogen bonds that can make both dimers and lines of bonded molecules. Many commercial materials derive from natural products and have one or more double bonds; hence, the focus of this work on unsaturated species is of particular interest here. Adsorbed layers of amides have been the focus of several studies recently. We have investigated the adsorption of amides on a graphite surface using a variety of techniques such as X-ray and neutron diffraction and 17,18 differential scanning calorimetry (DSC)19 and on polymer surfaces by environmental scanning electron microscopy (ESEM).20 An initial study on the adsorption of amides on graphite by DSC showed the formation of stable solid ordered layers of amides.19 Binary mixtures of alkylamides investigated were reported to mix surprisingly well on the surface. Structural investigations into the 2D crystal structures of individual saturated amide molecules have revealed the symmetry and hydrogen bonding in the amide monolayers with some detail.17 Other workers have employed techniques such as scanning tunneling microscopy (STM),21,22 atomic force microscopy,23 and spectroscopy techniques24,25 to observe and characterize amide layers at various surfaces, with broadly similar findings. Received: December 21, 2010 Revised: February 11, 2011 Published: March 07, 2011 6682
dx.doi.org/10.1021/jp1121257 | J. Phys. Chem. C 2011, 115, 6682–6689
The Journal of Physical Chemistry C
Figure 1. Saturated and unsaturated alkylamides used in this work.
Many commercial materials are often derived from natural sources and consist of mixtures of alkyl species of different chain lengths and saturation; hence, it is interesting to investigate their mixing behavior. An understanding of the monolayer structures of the individual mixing components can be helpful in rationalizing the mixing behavior observed. This work aims to address the formation of solid ordered layers of trans-unsaturated amides on graphite, characterize their monolayer structures, and study 2D mixtures of these compounds with their saturated analogues.
’ EXPERIMENTAL SECTION A convenient system for naming alkylamides has been adopted here19 where the total alkyl chain length and the number of unsaturated bonds are given, with the cis/trans nature of the bonds also indicated. For example, the samples of interest here are denoted as: trans-2-hexenoic acid amide, C6:1 t-2; trans-3hexenoic acid amide, C6:1 t-3; trans-2-nonenoic acid amide, C9:1 t-2; hexanamide, C6:0; and nonanamide, C9:0. It is rather difficult to obtain appropriate unsaturated fatty acids with the short alkyl chains of interest here (which are converted to the amide) limiting the choices of species we can investigate. Samples. Alkylamides used in this work (Figure 1) were either purchased directly from Sigma-Aldrich or prepared from the corresponding carboxylic acid (purchased from Sigma-Aldrich) using the method outlined in ref 26, i.e., by conversion to the acid chloride and then reaction with ammonia. The purities of the prepared/purchased amides were analyzed using elemental analysis, 13C and 1H NMR, and liquid chromatography-mass spectrometry, and most samples were found to have a reasonable purity of >98.5%. The only exceptions were the unsaturated amides C6:1 t-2 and C9:1 t-2, which show evidence for a small amount of impurity (
