Monolayer analysis

Analysis of Heterogeneity. The monolayer analysis consists of three elements an adsorption isotherm equation, a model for heterogeneous surfaces, and an algorithm such as CAEDMON, which uses the first two elements to extract the adsorptive energy distribution and the specific surface from isotherm data. Morrison and Ross developed a virial isotherm equation for a mobile film of adsorbed gas at submonolayer coverage (6) ... 

It will be going beyond the scope of the present review to give a detailed description of the techniques available, or to provide a comprehensive list of all methods used in surface analysis. Such information could be found in a number of monographs and review papers414,15. Analytical Chemistry publishes reviews every two years on the latest developments in surface characterization methods16. We will attempt to discuss briefly the most powerful and most readily available methods applicable to routine gold-thiol monolayer analysis with particular emphasis on the kind of information which can be obtained from these systems. 

Static or low-damage secondary ion mass spectrometry (SIMS) can be used to characterize polymers and also detect very small amounts of metals transferred to polymers or in polymer wear debris. While SIMS uses an ion beam to sputter remove surface species and is therefore inherently destructive, damage can be minimized by using low ion beam fluxes (3x10 A/cm ). This allows for monolayer analysis. 

The emission of secondary ions of both polarities from instrument construction materials produced as a by-product of ion source operation was first observed by Amot [86,87] and was developed into an analytical instrument by Herzog and Viebock [88]. Inspired by these experiments, an early SIMS instrument was constructed in the 1960s to analyze lunar rocks from NASA s Apollo missions. Much of the breakthrough of SIMS for i) bulk analysis of solids, ii) depth profiling of thin layers, Hi) imaging, and iv) monolayer analysis is also the merit of the Ben-ninghoven group [82,84,89,90]. 

Neumann has adapted the pendant drop experiment (see Section II-7) to measure the surface pressure of insoluble monolayers [70]. By varying the droplet volume with a motor-driven syringe, they measure the surface pressure as a function of area in both expansion and compression. In tests with octadecanol monolayers, they found excellent agreement between axisymmetric drop shape analysis and a conventional film balance. Unlike the Wilhelmy plate and film balance, the pendant drop experiment can be readily adapted to studies in a pressure cell [70]. In studies of the rate dependence of the molecular area at collapse, Neumann and co-workers found more consistent and reproducible results with the actual area at collapse rather than that determined by conventional extrapolation to zero surface pressure [71]. The collapse pressure and shape of the pressure-area isotherm change with the compression rate [72]. 

The external reflection of infrared radiation can be used to characterize the thickness and orientation of adsorbates on metal surfaces. Buontempo and Rice [153-155] have recently extended this technique to molecules at dielectric surfaces, including Langmuir monolayers at the air-water interface. Analysis of the dichroic ratio, the ratio of reflectivity parallel to the plane of incidence (p-polarization) to that perpendicular to it (.r-polarization) allows evaluation of the molecular orientation in terms of a tilt angle and rotation around the backbone [153]. An example of the p-polarized reflection spectrum for stearyl alcohol is shown in Fig. IV-13. Unfortunately, quantitative analysis of the experimental measurements of the antisymmetric CH2 stretch for heneicosanol [153,155] stearly alcohol [154] and tetracosanoic [156] monolayers is made difflcult by the scatter in the IR peak heights. 

While both models find some experimental support, the monolayer has been much more amenable to simple analysis. As a consequence, most of the discussion in this chapter is in terms of die monolayer model, although occasional... 

Stem layer adsorption was involved in the discussion of the effect of ions on f potentials (Section V-6), electrocapillary behavior (Section V-7), and electrode potentials (Section V-8) and enters into the effect of electrolytes on charged monolayers (Section XV-6). More speciflcally, this type of behavior occurs in the adsorption of electrolytes by ionic crystals. A large amount of wotk of this type has been done, partly because of the importance of such effects on the purity of precipitates of analytical interest and partly because of the role of such adsorption in coagulation and other colloid chemical processes. Early studies include those by Weiser [157], by Paneth, Hahn, and Fajans [158], and by Kolthoff and co-workers [159], A recent calorimetric study of proton adsorption by Lyklema and co-workers [160] supports a new thermodynamic analysis of double-layer formation. A recent example of this is found in a study... 

There are three advantages to study molecular recognition on surfaces and interfaces (monolayers, films, membranes or soHds) (175) (/) rigid receptor sites can be designed (2) the synthetic chemistry may be simplified (J) the surface can be attached to transducers which makes analysis easier and may transform the molecular recognition interface to a chemical sensor. And, which is also a typical fact, this kind of molecular recognition involves outside directed interaction sites, ie, exo-receptor function (9) (see Fig. 5b). 

Few-monolayer thin-film analysis, e.g., adsorbate and very thin-film reactions submicron detection of metal hydrides... 

Static SIMS is labeled a trace analytical technique because of the very small volume of material (top monolayer) on which the analysis is performed. Static SIMS can also be used to perform chemical mapping by measuring characteristic molecules and fiagment ions in imaging mode. Unlike dynamic SIMS, static SIMS is not used to depth profile or to measure elemental impurities at trace levels. 

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