Surface-enhanced order

Although surface enhanced order is commonly found for various liquid crystal transitions, the layer-by-layer transitions have only been characterized in the following five cases the SmA-SmI transition of 90.4 [47], the SmA-SmB ex transition of nmOBC [45], the SmA-SmB ex transition of 54COOBC [48], the SmA-B transition of 40.8 [49] and the SmA-isotropic transition [50, 51]. Thus far the sequence of the transition temperatures can be described by the following simple power law [52] ... 

As discussed previously the extra hydrodynamic mode due to the molecular tilt in the SmC phase can be described by the variable 2. As demonstrated by Meyer et al. [14], free-standing films offer a unique geometry to single out this variable. For wavelengths that are large compared to the film thickness (approx. 2.5 nm/layer) and at a temperature far away from other transitions, without the complications of the surface-enhanced order, SmC free-standing films behave essentially like two-dimensional nematics. 

MOLE, however, is more sensitive than ETIR (<1 samples compared to about 100 p.m ). With surface-enhanced Raman spectroscopy the Raman signal is enhanced by several orders of magnitude. This requires that the sample be absorbed on a metal surface (eg, Ag, Cu, or Au). It also yields sophisticated characterization data for the polytypes of siUcon carbide, graphite, etc. 

Surface-enhancement Electromagnetic and chemical effects can enhance the Raman intensities from substances in close proximity to appropriate metal surfaces by several orders of magnitude. 

Enhanced thermal stability enlarges the areas of application of protein films. In particular it might be possible to improve the yield of reactors in biotechnological processes based on enzymatic catalysis, by increasing the temperature of the reaction and using enzymes deposited by the LB technique. Nevertheless, a major technical difficulty is that enzyme films must be deposited on suitable supports, such as small spheres, in order to increase the number of enzyme molecules involved in the process, thus providing a better performance of the reactor. An increased surface-to-volume ratio in the case of spheres will increase the number of enzyme molecules in a fixed reactor volume. Moreover, since the major part of known enzymatic reactions is carried out in liquid phase, protein molecules must be attached chemically to the sphere surface in order to prevent their detachment during operation. 

This has been verified for polydimethylsiloxanes added to crude oils. The effect of the dilatational elasticities and viscosities on crude oil by the addition of polydimethylsiloxanes is shown in Table 21-1. Under nonequilibrium conditions, both a high bulk viscosity and a surface viscosity can delay the film thinning and the stretching deformation, which precedes the destruction of a foam. There is another issue that concerns the formation of ordered structures. The development of ordered structures in the surface film may also stabilize the foams. Liquid crystalline phases in surfaces enhance the stability of the foam. 

LC-IR using surface-enhanced IR absorption spectroscopy (SEIRAS) was recently designed in order to develop a highly sensitive and rapid analysis method for polymer additives [506]. The method, which consists of spraying the LC eluents on to a metal film of Ag on a BaF2 substrate, allows an enhancement factor of about 90. 

A related effect has been described for IR spectroscopy - Surface Enhanced Infrared Absorption spectroscopy (SEIRA). However, as the enhancement factors are significantly lower than for SERS and both the required metal particle size and the activation distance between the target molecule and the particle are more than one order of magnitude smaller, no practically applicable SEIRA sensors have been demonstrated up to now. 

Tian Z.Q., Ren B., Wu D.Y., Surface-enhanced Raman scattering From noble to transition metals and from rough surfaces to ordered nanostructures, J. Phys. Chem. B 2002 106 9463-9483. 

In contrast to Raman scattering, the absorption of infrared (IR) radiation is a first-order process, and in principle a surface or an interface can generate a sufficiently strong signal to yield good IR spectra [6]. However, most solvents, in particular water, absorb strongly in the infrared. There is no special surface enhancement effect, and the signal from the interface must be separated from that of the bulk of the solution. 

There are different paths to achieving surface specificity. One can exploit optical susceptibilities and resonances that are nonzero only at the surface or only for the molecular species of interest adsorbed on the surface. Examples include the use of second-order nonlinear mixing processes such as second harmonic generation7-9 for which the nonlinear susceptibility tensor is nonzero only where inversion symmetry is broken. Spectroscopic techniques with very high selectivity for molecular resonances such as surface-enhanced infrared or Raman spectroscopy10-12 may also be used. 

Much of this research effort has been directed to the study of the fundamental basis of surface-enhanced Raman scattering (SERS), in order to understand the underlying principles. There have also been many applications of SERS to situations in which in situ vibrational... 

Ever since Albrecht and Creighton [85], and Jenmarie and van Duyne [86], observed that the Raman cross-section for pyridine absorbed on a roughened silver electrode was larger than that in solution by six orders of magnitude, surface enhanced Raman spectroscopy (SERS) has steadily gained ground in analytical instrumentation. The sensitivity of this technique... 

---