Ideal Solid Substrates

FIGURE 7.5. (a) Photographs of the growth of a dry zone in a PDMS film of molecular mass 28,000 deposited on fluorinated silicon. The total height of the image is 4 cm and the film thickness is 30 pm. (b) Time dependence of the radius /2(t) of the expanding dry hole. (From C. Redon, F. Brochard, and F. Rondelez, In Physical Review Letters, 66 p. 715 (1991), Americal Physical Society. Reproduced by permission.) 

FIGURE 7.6. Schematic diagram of a film s profile during de wet ting. A circular ridge is bounded by the contact line A, which recedes on the dry solid, and by point B, which advances on the wet solid. 

The cross section of the ridge is circular. The pressure comes to equilibrium rapidly in the central portion. Hence, in accordance with Laplace s law, the curvature is constant. Accordingly, the profile of the rim is a circular arc that intersects the horizontal with the same dynamical angle 6d Sit points A and B. 

The ridge collects the liquid that initially filled the hole. Because of conservation of volume, the size I of the ridge is related to the radius R of the hole  

The viscous dissipation is dominated by the fluid s flow within the ridge (hydrodynamic model). The motion of the ridge results from an equilibrium between two forces  

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The quantity and quality of the deposited monolayer on a solid support is measured by a so-called transfer ratio, tr. This is defined as the ratio between the decrease in monolayer area during a deposition stroke, Al, and the area of the substrate, As. For ideal transfer, the magnitude of tr is equal to 1. Depending on the behavior of the molecule, the solid substrate can be dipped through the film until the desired thickness of the film is achieved. Different kinds of LB multilayers can be produced and/or obtained by successive deposition of monolayers on the same substrate (see Figure 4.11). The most common one is the Y-type multilayer, which is produced when the monolayer deposits on the solid substrate in both up and down directions. When the monolayer deposits only in the up or down direction, the multilayer structure is called either Z-type or X-type. Intermediate structures are sometimes observed for some LB multilayers, and they are often referred to be XY-type multilayers. 

Because of the presence of a well-defined energy gap between the conduction and the valence band, semiconductors are ideally suited for investigation of the interfacial interactions between immobilized molecular components and solid substrates. In this chapter, interfacial assemblies based on nanocrystalline TiOz modified with metal polypyridyl complexes will be specifically considered. It will be shown that efficient interaction can be obtained between a molecular component and the semiconductor substrate by a matching of their electronic and electrochemical properties. The nature of the interfacial interaction between the two components will be discussed in detail. The application of such assemblies as solar cells will also be considered. The photophysical processes observed for interfacial triads, consisting of nanocrystalline TiO 2 surfaces modified with molecular dyads, will be discussed. Of particular interest in this discussion is how the interaction between the semiconductor surface and the immobilized molecular components modifies the photophysical pathways normally observed for these compounds in solution. 

The ACCA and APCA on an ideal solid surface are identical by definition, and are referred to as the "ideal contact angle (ICA)". As will be explained below, it is the value of the ICA that is required for the characterization of the wettability of a solid substrate in terms of its surface tension. Also, all predictions of wetting behavior start with the ICAas their basis. Therefore, even though ideal surfaces are rarely encountered in practice, the concept is of fundamental importance. 

Generally, monolayer formation can be caused either by non-localized or localized adsorption. TTie first case can be described by 2D ideal or real gas models [3.253, 3.254] which apply to systems with solid substrates at relatively high temperatures or... 

Naturally and ideally, the most prevalent LB films should be Y-type films. When a monolayer is deposited during immersion, the hydrophobic tails attach to the solid substrate and the hydrophilic ends of the amphiphilic molecules remain on the outside in contact with the liquid subphase creating a hydrophilic surface and the necessary condition for deposition during removal. On the other hand, when a monolayer is deposited during removal, the hydrophilic ends attach to the substrate and the hydrophobic tails are in contact with the air creating a hydrophobic surface, which will in turn determine the necessary condition for deposition of the next monolayer during immersion. In practice, under experimental conditions, forces generated by electrical double layers between the carboxylic acid end and the cations of the metal salts of the liquid subphase deteriorate Y-type films and allow the formation of X- or Z-type films. 

Standards are prepared as solids or in solution from pure elements or compounds. Standard solutions sold for inductively coupled plasma (ICP) spectrometric techniques are ideal. These are prepared as multielement standards for use as liquids or dried on a suitable solid substrate. The standard should have a similar geometry to that of the sample. The composition is not so critical, however, and standards can be used over several orders of magnitude. If irradiation and counting conditions are kept constant during analysis, the standard data may be stored in a database for use on future occasions, with inclusion of a monitor to check the irradiation and counting conditions. In variable conditions of irradiation, decay and y-ray spectrometry standards are included with the samples during analysis, and this is the common... 

In general, any material that binds nucleic acids can be used as the solid phase. The ideal solid phase is one with high selectivity for nucleic acids over proteins. The solid phase must not react with the enzyme substrate itself. A final criterion is that the solid phase must bind sufficiently tightly so that nucleic acids will not be lost during blocking, washing, or detection steps. Solid phases that allow covalent immobilization of DNA are preferable but are not required. 

If correctly applied, Secondary Ion Mass Spectrometry (SIMS) has the abihty to provide a wealth of information on the elemental and/or molecular distributions on or within the near snrface region of any solid substrate of interest. There are, however, a myriad of conditions that can be applied before, during, and after data collection. As a result (and as stated in Section 4.1), understanding the ideal conditions required for a specific type of analysis is often considered an art. Gaining this understanding requires, at the very least, knowledge of ... 

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