Monolayers experimental techniques

Such a development would be impossible without the generation of new experimental techniques extending the possibilities of studies of supramolecular systems. One such new technique is the vibrational (or infrared-visible) spectroscopy sum frequency generation [8] enabling one to lookatjustthe outer monolayer, thus providing more information than more standard methods like contact angle and surface tension measurements [9]. 

The material in this chapter is organized broadly in two segments. The topics on monolayers (e.g., basic definitions, experimental techniques for measurement of surface tension and sur-face-pressure-versus-area isotherms, phase equilibria and morphology of the monolayers, formulation of equation of state, interfacial viscosity, and some standard applications of mono-layers) are presented first in Sections 7.2-7.6. This is followed by the theories and experimental aspects of adsorption (adsorption from solution and Gibbs equation for the relation between... 

The formation of monolayers and their thermodynamic investigation was described in the last chapter. A good introduction to the classic experimental techniques and results is given by Refs. [587] [588], We start by discussing optical techniques. 

Experimental techniques for studying insoluble monolayers Surface pressure... 

There are several experimental techniques suitable for studying e. Some of them are Relaxation after a sudden compression of the monolayer Electrocapillary waves An oscillatory barrier Light Scattering by thermally excited capillary waves. The first two techniques are used in the low - frequency range, below 1 Hz. The last one in the kilohertz range. 

In considering the available experimental techniques and the development of new techniques for the study of surface reactions, the following factors should be considered (1) the method should give a continuous or semicontinuous record of the reaction over a wide range of time, temperature and pressure, (2) sufficient sensitivity should be available so that a monolayer or less of reaction product can be determined on specimens of practical size and thickness, and (3) the experimental information should be directly correlated with a minimum of assumptions. 

One of the most useful and more accessible experimental techniques allowing the behavior of molecules within membranes to be modeled is the study of monolayers. The easiest type of experiments to undertake in this field is the measurement of simple area/pressure isotherms using a Langmuir film balance. 

Unimolecular rectifiers are a reality. They have been made and they have been measured. However, there is still substantial room for improvement in the experimental techniques used to study the molecules and in the molecules used to give improved thermal stability. Current experiments need to be modified so that noble metal electrodes may be used to study a dilute monolayer, ideally at low temperature where molecular orbital signatures will be observable. This already has been achieved with robust molecules such as phthalocyanines, which suggests that the technological step to be made is not too great. 

Over a long period of time experimental results on amphiphilic monolayers were limited to surface pressure-area ( r-A) isotherms only. As described in sections 3.3 and 4, from tc[A) Isotherms, measured under various conditions, it is possible to obtain 2D-compressibilities, dilation moduli, thermal expansivities, and several thermodynamic characteristics, like the Gibbs and Helmholtz energy, the energy cmd entropy per unit area. In addition, from breaks in the r(A) curves phase transitions can in principle be localized. All this information has a phenomenological nature. For Instance, notions as common as liquid-expanded or liquid-condensed cannot be given a molecular Interpretation. To penetrate further into understanding monolayers at the molecular level a variety of additional experimental techniques is now available. We will discuss these in this section. 

G. L. Gaines jr. Insoluble Monolayers at Liquid-Gas Interfaces. Interscience (1966). (Relatively broad and introductory much practical information experimental techniques, examples and applications. Theory is very elementary.)... 

Photophysical processes can be used to probe the local structure of organized systems that are difficult to characterize by conventional analytical protocols. Photophysical probes provide valuable structural information for monolayers assembled on metal supports and for polymeric composites. In the latter case, it has been possible to determine the level of thermal residual strain and to monitor architectural deformations. Fluorescence from single molecules continues to attract great attention while the dynamics of photodissociative processes have been reviewed, with emphasis given to experimental techniques. ... 

Information collected in this table is based on data from Kuntz and Kauzmann (67) and from Richards (16). Numerical values given here represent approximations based on numerous proteins and diverse experimental techniques. The class designated as Monolayer is included for purposes of comparison with Surface water in particular. The latter is, presumably, an actual layer of uneven thickness that is variable from protein to protein. The former is an idealized concept of a uniform layer, one molecule in thickness, covering the protein surface. Freezability denotes whether or not a particular class of water is capable of transition into normal ice. 

Figure 6. Comparison of results of the three experimental techniques. (A) The nou,)/nc(n) (6 80°) ratios obtained from the XPS(o) measurements according to Equation 3 as a function of depth of sulfonation in monolayers...

Before discussing compressed monolayers with their even richer behavior than the submonolayer regime we note that the experimental determination of the coverage gets more difficult due to possible second-layer adsorption [90, 93]. Thus, the coverages marking a phase boundary may differ between various experimental techniques and groups (see the discussion in Section I.C). 

Quantitative information on penetrated layers under dynamic and equilibrium conditions require much attention in respect to the experimental technique. There are a number of penetration experiments with different advantages and drawbacks. The classical experiment is the injection technique where a soluble component is injected into the subphase below a spread monolayer. Experiments can then be performed at constant monolayer coverage [212, 213, 214] or by compression and expansion cycles [215, 216]. Another possibility is to exchange the subphase below a spread monolayer using a laminar pumping system. Other experiments were performed by using the sweeping technique as described in [217, 218]. 

The pendant drop experiments are a very new experimental technique to study penetration systems. The insoluble monolayer is spread onto the drop surface carefully by using a microsyringe [221]. The exchange of the drop bulk phase can be easily performed by using a coaxial double-capillary as shown in Fig. 4.20. 

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