Direct measurement from solid surfaces

Niederwieser and G. Pataki (Editors), Progress in Thin-layer Chromatography and Related Methods, Vols. I and II, Ann Arbor Sci. Publ., Ann Arbor, Mich., 1970 and 1971. 

Shellard (Editor), Quantitative Paper and Thin-Layer Chromatography, Academic Press, New York, London, 1968. 

Frei and J.D. McNeil, Diffuse Reflectance Spectroscopy in Environmental Problem Solving, Chemical Rubber Co. Press, Cleveland, Ohio, 1973. 

Sedgley and H.F. Walton, Anal. Chim. Acta, 33(1967) 102. 

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DIRECT MEASUREMENTS FROM SOLID SURFACES 2.3.1 Densitometry (transmittance)... 

Equation (5.7.5], and variants thereof, have been widely invoked to assess the surface tension of solid surfaces by working with organic liquids (such as hydrocarbons) in which dispersion forces prevail. For those one may set = y " so that the equation can be solved for y , which, in turn, may be equated to y for a Van der Waals solid. However, we see from [5.7.7] that the situation is more complicated. First, neglection of the TAS term is not allowed, it may account for 20-30% of the interaction Helmholtz energy. Second, even if an assumption is made on, or if it is directly measured from the heat of adhesion, only the... 

Neutral species represent the majority of particles desorbed during electron stimulated desorption (ESD) experiments from solid surfaces or other desorption experiments with other types of ionizing radiation (Eeulner and Menzel 1995). However, the study of emission of these species has received little interest compared to charged particles. The comparative scarcity of information on this subject reflects both the difficulties in measuring the neutral species and the number and complexity of desorption mechanisms (Bazin et al. 2010). In fact, the origins of the neutral species are multiple DEA, electron-hole pair recombination, dipolar excitations, and multihole final states with or without recombination of different particles (Kimmel et al. 1994). Neutral species can also be produced by direct electronic excitation of a molecule to a repulsive state leading to electronic excitation dissociation (EED) (Eigure 16.3). 

At low surfactant concentrations it is observed that an attraction dominates at short separations. The attraction becomes important at separations below about 12 nm when the surfactant concentration is 0.01 mM, and below about 6 nm when the concentration is increased to 0.1 mM. Once the force barrier has been overcome the surfaces are pulled into direct contact between the hydrophobic surfaces at D = 0, demonstrating that the surfactants leave the gap between the surfaces. The solid surfaces have been flocculated. However, at higher surfactant concentrations (1 mM) the surfactants remain on the surfaces even when the separation between the surfaces is small. The force is now purely repulsive and the surfaces are prevented from flocculating. Emulsion droplets interacting in the same way would coalesce at low surfactant concentrations once they have come close enough to overcome the repulsive barrier, but remain stable at higher surfactant concentrations. Note, however, that the surfactant concentration needed to prevent coalescence of emulsion droplets cannot be accurately determined from surface-force measurements using solid surfaces. 

Interfacial tension between two fluid phases is a definite and accurately measurable property depending on the properties of both phases. Also, the contact angle, depending now on the properties of the three phases, is an accurately measurable property. Experimental approaehes are described, e.g., in Refs. 8,60, and 63 and in Ref. 62, where especially detailed discussion of the Wilhelmy technique is presented. Theories sueh as harmonie mean theory, geometric mean theory, and acid base theory (reviewed, e.g., in Refs. 8, 20, and 64) allow caleulation of the solid surfaee energy (beeause it is difficult to directly measure) from the contaet angle measurements with selected test liquids with known surface tension values. These theories require introduction of polar and dispersive components of the surface free energy. 

The integral heat of adsorption Qi may be measured calorimetrically by determining directly the heat evolution when the desired amount of adsorbate is admitted to the clean solid surface. Alternatively, it may be more convenient to measure the heat of immersion of the solid in pure liquid adsorbate. Immersion of clean solid gives the integral heat of adsorption at P = Pq, that is, Qi(Po) or qi(Po), whereas immersion of solid previously equilibrated with adsorbate at pressure P gives the difference [qi(Po) differential heat of adsorption q may be obtained from the slope of the Qi-n plot, or by measuring the heat evolved as small increments of adsorbate are added [123]. 

Now, in principle, the angle of contact between a liquid and a solid surface can have a value anywhere between 0° and 180°, the actual value depending on the particular system. In practice 6 is very difficult to determine with accuracy even for a macroscopic system such as a liquid droplet resting on a plate, and for a liquid present in a pore having dimensions in the mesopore range is virtually impossible of direct measurement. In applications of the Kelvin equation, therefore, it is almost invariably assumed, mainly on grounds of simplicity, that 0 = 0 (cos 6 = 1). In view of the arbitrary nature of this assumption it is not surprising that the subject has attracted attention from theoreticians. 

It is important to distinguish clearly between the surface area of a decomposing solid [i.e. aggregate external boundaries of both reactant and product(s)] measured by adsorption methods and the effective area of the active reaction interface which, in most systems, is an internal structure. The area of the contact zone is of fundamental significance in kinetic studies since its determination would allow the Arrhenius pre-exponential term to be expressed in dimensions of area"1 (as in catalysis). This parameter is, however, inaccessible to direct measurement. Estimates from microscopy cannot identify all those regions which participate in reaction or ascertain the effective roughness factor of observed interfaces. Preferential dissolution of either reactant or product in a suitable solvent prior to area measurement may result in sintering [286]. The problems of identify-... 

In terms of hydrodynamics, the boundary layer thickness is measured from the solid surface (in the direction perpendicular to a particle s surface, for instance) to an arbitrarily chosen point, e.g., where the velocity is 90-99% of the stream velocity or the bulk flow ((590 or (599, respectively). Thus, the breadth of the boundary layer depends ad definitionem on the selection of the reference point and includes the laminar boundary layer as well as possibly a portion of a turbulent boundary layer. 

This chapter comprises two sections. The first describes the most usual techniques to directly measure force versus distance profiles between solid or liquid surfaces. We then describe different long-range forces (range >5 nm) accessible to evaluation via these techniques for different types of surface active species. The second section is devoted to attractive interactions whose strong amplitude and short range are difficult to determine. In the presence of such interactions, emulsion droplets exhibit flat facets at each contact. The free energy of interaction can be evaluated from droplet deformation and reveals interesting issues. 

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