Molecular Orientation and Conformation

Zhuang X, Miranda P B, Kim D and Shen Y R 1999 Mapping molecular orientation and conformation at interfaces by surface nonlinear optics Phys. Rev. B 59 12 632-40... 

Multilamellar bilayers in the fluid phase are also ordered in the sense that they are smectic liquid crystals. Of great interest is the range of molecular order this is long-range in the sense that the molecules are confined to two dimensions there is also some kind of short-range order in molecular orientations and conformations, but the range of this latter ordering is not known at present. 

Fourier-Transform Infrared (FTIR) spectroscopy as well as Raman spectroscopy are well established as methods for structural analysis of compounds in solution or when adsorbed to surfaces or in any other state. Analysis of the spectra provides information of qualitative as well as of quantitative nature. Very recent developments, FTIR imaging spectroscopy as well as Raman mapping spectroscopy, provide important information leading to the development of novel materials. If applied under optical near-field conditions, these new technologies combine lateral resolution down to the size of nanoparticles with the high chemical selectivity of a FTIR or Raman spectrum. These techniques now help us obtain information on molecular order and molecular orientation and conformation [1],... 

The observation that these solute molecules rearrange on the surface agrees with the report of a rearrangement of AFP molecules on the ice surface." An early stage of adsorption can be pictured as isolated adsorbed molecules, conformationally disordered and randomly distributed on the surface of the substrate. The final stage involves close-packed adsorbents with relatively uniform molecular orientation and conformation. We speculate that initially, not all wfAFP molecules would contact the silica surface with their... 

Information about molecular orientation and conformation at a surface, and sometimes even surface structure, is gained from the dependence of the SFG signal on... 

Reprinted with permission of Elsevier Science from Cunningham, A., Ward, I. M., Willis, H. A. and Zichy, V., An infra-red spectroscopic study of molecular orientation and conformational changes in poly(ethylene terephthalate) . Polymer 15, 749-756. Copyright 1974. 

The current large interest in this method results from the fact that vibrational spectra of adsorbed species which provide information complementary to IR can be recorded under non-vacuum conditions. As an example, very recently the technique has been used to investigate molecular orientations and conformations in confined systems in the context of tribological phenomena [OOEIS]. In addition it should be noted that the method can be used to study very fast processes with a time resolution down to the pico/femto second regime. In the past the complexity of the equipment necessary to carry out SFG experiments has limited the number of studies but recent advances in laser technology have strongly reduced the effort (and the expenses) needed to set up an experiment. 

Mesophase materials of linear, flexible macromolecules have gained attention only more recently, when it was found that parallel molecular orientation is easily achieved in some of these mesophases. This orientation can lead to high modulus and tensile strength 6). Presently there exists a certain amount of confusion in the literature about the description, properties, and nomenclature of these macromolecular mesophases and their place in the arrangement of all matter. Even for the better understood small-molecule mesophases there are some problems in the description of glasses and in the separation of orientational and conformational disorder. Also, the distinction between mesophases based on molecular structure and on super-molecular structure is not always made. We will try in this review to clarify some of these points. 

This chapter examines the orientation and conformation of adsorbate and solvent molecules at liquid/liquid interfaces, and from this information, it further examines the interactions between the molecules present at these interfaces. The summary begins with a description of the theoretical underpinnings of the technique and describes the experimental apparatus employed in our studies of these interfaces. In each case, one of the liquids is water. The remainder of the chapter describes studies of molecular adsorbates at liquid/liquid interfaces and studies of neat liquid/liquid interfaces. 

The studies presented here illuminate just a few of the exciting possibilities for the use of VSFS to study chemistry at liquid/liquid surfaces. Solvents and adsorbates can be probed and orientations and conformations obtained. Molecular dynamics has recently been employed to gain additional information using the constraints provided by the spectroscopy. The future of this technique lies in expanding the spectrum to longer wavelengths so that more vibrations can be probed in each molecule and more complicated molecules can be studied. The study of interfacial dynamics will also offer exciting opportunities for the future. 

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