Surface Orientational Order

G.P. Crawford, A. Scharkowski, Y.K. Fung, J.W. Doane, and S. Zumer, Internal surface, orientational order, and distribution of a pol mier network in a liquid crystal matrix, Phys. Rev. E 52, R1273-76 (1995). 

An experimental technique has recently been developed to investigate the surface orientational order of liquid crystals optical second-harmonic generation [48]. Because of its high sensitivity to surface polar ordering and its surface specificity, this method is able to probe the orientational distribution of polar monolayers of liquid crystal molecules located at the surface of a substrate, whether they are covered by a bulk liquid crystal or not. 

Fig. 19 shows an example of the orientational density profile evaluated for the system 10. Most of the particles of the first adlayer assume two limiting configurations parallel to the surface and vertical to the surface. The second adlayer exhibits also well pronounced orientational ordering ... 

Other forces can arise as a result of elastic strain on the growing film, which can be due to a surface-induced ordering in the first few layers that reverts to the bulk liquid structure at larger distances. This elastic energy is stored in intermolecular distances and orientations that are stretched or compressed from the bulk values by the influence of the substrate at short distances [7]. Similar phenomena are well known to occur in the growth of epitaxial layers in metals and semiconductors. 

FIG. 7 Schematics of the SHG process at the surface of a sphere of a centrosymmetrical medium with a radius much smaller and of the order of the wavelength of light. The cancellation or the addition of the nonlinear polarization contribution is given explicitly and underlines the effect of the electromagnetic field and the surface orientation. 

Thin polymer films have been prepared by surface catalysis in ultrahigh vacuum and electrochemical deposition from solution. These two routes of synthesis result in poly(thiophene), poly(aniline) and poly(3,5-lutidine) films that have similar infrared spectra. These polymer films are highly orientationally ordered the rings are perpendicular to the surface in poly(thiophene) and poly(3,5-lutidine) films, and the phenyl rings are parallel to the surface in poly(aniline). 

In this paper we describe the preparation of thin polymer films by surface catalysis and anodic deposition. The results indicate that both synthesis routes produce orientationally ordered films that have similar infrared spectra. It is also shown that thin ordered films of poly(thiophene) have different electrochemical behavior than the fibrous films that are electrically conducting. 

Aniline. Aniline black is a well known polymer of aniline formed by electrophilic additionf3.41. Numerous investigators have formed poly(aniline) films by anodic deposition of Pt and other electrode materials. We have examined the interaction of aniline with clean Ni(lll) and Ni(100) surfaces in ultrahigh vacuum and found aniline to form an orientationally ordered, thermally stable polymer film. Electrochemically prepared poly(aniline) films also show the high degree of orientational ordering. 

Here the angular brackets mean that the ensemble average is taken, and the angle 3 can be defined and computed for each bond in the molecule. When a bond is perfectly aligned normal to the bilayer surface, the order parameter assumes a value of unity. In a random orientation, the order parameter is zero. Bonds that are perfectly aligned parallel to the surface gives a negative order parameter of S = —0.5. 

The polarization properties of the evanescent wave(93) can be used to excite selected orientations of fluorophores, for example, fluorescent-labeled phosphatidylethanolamine embedded in lecithin monolayers on hydrophobic glass. When interpreted according to an approximate theory, the total fluorescence gathered by a high-aperture objective for different evanescent polarizations gives a measure of the probe s orientational order. The polarization properties of the emission field itself, expressed in a properly normalized theory,(94) can also be used to determine features of the orientational distribution of fluorophores near a surface. 

Natural biological membranes consist of lipid bilayers, which typically comprise a complex mixture of phospholipids and sterol, along with embedded or surface associated proteins. The sterol cholesterol is an important component of animal cell membranes, which may consist of up to 50 mol% cholesterol. As cholesterol can significantly modify the bilayer physical properties, such as acyl-chain orientational order, model membranes containing cholesterol have been studied extensively. Spectroscopic and diffraction experiments reveal that cholesterol in a lipid-crystalline bilayer increases the orientational order of the lipid acyl-chains without substantially restricting the mobility of the lipid molecules. Cholesterol thickens a liquid-crystalline bilayer and increases the packing density of lipid acyl-chains in the plane of the bilayer in a way that has been referred to as a condensing effect. 

First, we consider the thermodynamics of a stepped surface. The step density, n = HI, is related to the surface orientation 9 by tan 9 = na, where a is the step height. To third order in n, the surface free energy E( ) is given by [17]... 

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