Ellipsometry adsorption layers

Quite similar results have been reported by van Vliet [226]. He found that the equilibrium thickness of foam films from aqueous solutions of polyvinyl alcohol at low pressure is much larger than twice the adsorption layer thickness measured by ellipsometry. The core thickness is again several times the radius of gyration of the polymer molecules in... 

As mentioned above, the study of the dynamics of adsorption layers at liquid interfaces is mainly restricted to surface and interfacial tension measurements. Only for slow adsorption processes, methods such as radiotracer technique [163, 164], the significantly improved surface ellipsometry [165, 166], or the very recently developed technique of neutron reflectivity [167, 168, 169, 170] can be used to directly follow the change of surface concentration with time. Neutron reflectivity allows even distinguishing between different species adsorbed at a fluid interface [171, 172, 173]. These techniques are reviewed in more detail in the preceding chapter 3 as they yield data most of all for the equilibrium state of adsorption layers. 

The following chapter presents the basics of ellipsometry and discusses some recent advances. The article covers the formalism and theory used for data analysis as well as instrumentation. The treatment is also designed to familiarize newcomers to this field. The experimental focus is on adsorption layers at the air-water and oil-water interface. Selected examples are discussed to illustrate the potential as well as the limits of this technique. The authors hope, that this article contributes to a wider use of this technique in the colloidal physics and chemistry community. Many problems in our field of science can be tackled with this technique. 

Figure 9 Characterization of the equilibrium properties of the adsorption layer by ellipsometry and Surface second harmonic generation, SHG. The SHG-signal I (P = 45, d = 90) (circles) is proportional to the surface coverage and increases monotonously with the bulk concentration. The ellipsometric quantity dA = A - Ao (triangles) shows an extremum at an intermediate concentration far below the cmc. The inset clearly shows the nonmonotonic dependency of dA on the adsorbed amount, (redrawn from (20 )...

In short, ellipsometry applied to adsorption layers of ionic soluble surfactants does not measure the surface excess. The ellipsometric signal may show a non-monotonic behaviour which is caused by a redistribution of the ions between compact and diffuse layer. The data analysis within the classical model of a charged double layer yields an estimate of the prevailing ion distribution. 

Ellipsometry applied to adsorption layers of nonionic surfactant directly yields the surface... 

Adsorption of biologically important materials, such as fibrinogen from plasma proteins and barbiturates, " and organic molecules including corrosion inhibitors have been studied, often as functions of potential of the substrate, by ellipsometry in situ. Jovancicevic, Yang, and Bockris used time-resolved ellipsometry to measure the thickness, refractive index, and extinction coefficient of the adsorption layer of l-octyne-3-ol on iron as functions of time. They showed that the layer thickness increased stepwise, due to change of the adsorption configuration from flat... 

From these relations they estimated the electromodulation effect on the ellipsometry measurements, the magnitude of which turned out to be of the same order of magnitude as the effect of adsorption layer. 

It is also useful to define the total amount of monomers per unit area which take part in the adsorption layer. This is the so-called surface excess F it is measured experimentally using, e.g., ellipsometry, and is defined as... 

Measurements of the adsorption of inhibitors on corroding metals are best carried out using the direct methods of radio-tracer detection and solution depletion measurements . These methods provide unambiguous information on uptake, whereas the corrosion reactions may interfere with the indirect methods of adsorption determination, such as double layer capacity measurements", coulometry", ellipsometry and reflectivity Nevertheless, double layer capacity measurements have been widely used for the determination of inhibitor adsorption on corroding metals, with apparently consistent results, though the interpretation may not be straightforward in some cases. 

The aim of this paper is to offer experimental results for the molecular weight dependence of adsorption of polystyrene-sulfonate) onto a platinum plate from aqueous NaCl solution at 25 °C. Measurements of poly(styrenesulfonate) adsorption were carried out by ellipsometry. The dependences of molecular weight and added salt concentration on the thickness of the adsorbed layer and also the adsorbances of polymer and salt are examined. 

Adsorption of putidaredoxin on gold electrodes has been studied using dynamic spectroscopic ellipsometry and differential capacitance measurements [307]. In Ref. 307, a method for the measurement of metal surface optical perturbation during protein adsorption at a constant potential has been described. The method is based on the concept that the charged transition layer develops between the electrode substrate and the adsorbate. 

Morrissey 53) used transmission infrared spectroscopy to study protein adsorption onto silica particles in a heavy water (DzO) buffer. By observing the shift in the amide I absorption band, he could deduce the fraction of protein carbonyl groups involved in bonding to the silica surface. He found that bovine IgG had a bound fraction of 0.20 at low bulk solution concentrations, but only about 0.02 at high solution concentrations. However, neither prothrombin nor bovine serum albumin exhibited a change in bound fraction with concentration. Parallel experiments with flat silica plates using ellipsometry showed that the IgG-adsorbed layers had an optical thickness of 140 A and a surface concentration of 1.7 mg/m2 at low bulk solution concentration — in concentrated solutions the surface amount was 3.4 mg/m2 with a thickness of 320 A (Fig. 17). 

In 1985 Allara and Nuzzo [354, 355] published the results of an extensive investigation in which adsorption took place on to an aluminium oxide layer formed on a film of aluminium deposited in vacuo on to a silicon wafer. Various carboxylic acids were dissolved in high purity hexadecane and allowed to adsorb from this solution on to the prepared aluminium oxide surface. The monolayers so formed were examined by ellipsometry and infrared spectroscopy. Contact angle measurements were made on the monolayer surfaces and radioactive labelled (tritiated) compounds were employed to study the interchange of adsorbed molecules with those in solution. Various other techniques of less immediate relevance to our present interests were also employed and reference to these two papers should be made for further particulars. Aluminium... 

Concerning the experimental side of polymer adsorption studies the quantity A was only measurable at the early stage of the study, but in 19SS the thickness of the adsorbed layer became accessible to measurement by a hydrodynamic method and in 1961 the quantity p was first determined by infrared spectroscopy. Ellipsometry came up in 1963, which enabled both the adsorbance and the thickness of the adsorbed layer to be measured simultaneously. 

Takahashi et al.67) prepared ionene-tetrahydrofuran-ionene (ITI) triblock copolymers and investigated their surface activities. Surface tension-concentration curves for salt-free aqueous solutions of ITI showed that the critical micelle concentration (CMC) decreased with increasing mole fraction of tetrahydrofuran units in the copolymer. This behavior is due to an increase in hydrophobicity. The adsorbance and the thickness of the adsorbed layer for various ITI at the air-water interface were measured by ellipsometry. The adsorbance was also estimated from the Gibbs adsorption equation extended to aqueous polyelectrolyte solutions. The measured and calculated adsorbances were of the same order of magnitude. The thickness of the adsorbed layer was almost equal to the contour length of the ionene blocks. The intramolecular electrostatic repulsion between charged groups in the ionene blocks is probably responsible for the full extension of the... 

Figure 9.13 Adsorption of nitrogen to a single basal plane of graphite at a temperature of 46.2 K as determined by ellipsometry. Plotted is the change in the ellipsometric angle A versus pressure. The subsequent adsorption of at least four layers at defined pressures can clearly be distinguished. Redrawn from Ref. [389].

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