Films, condensed solid

Equation (6.25) not only allows us to calculate the Hamaker constant, it also allows us to easily predict whether we can expect attraction or repulsion. An attractive van der Waals force corresponds to a positive sign of the Hamaker constant, repulsion corresponds to a negative Hamaker constant. Van der Waals forces between similar materials are always attractive. This can easily be deduced from the last equation for 1 = e2 and n = n2 the Hamaker constant is positive, which corresponds to an attractive force. If two different media interact across vacuum ( 3 = n3 = 1), or practically a gas, the van der Waals force is also attractive. Van der Waals forces between different materials across a condensed phase can be repulsive. Repulsive van der Waals forces occur, when medium 3 is more strongly attracted to medium 1 than medium 2. Repulsive forces were, for instance, measured for the interaction of silicon nitride with silicon oxide in diiodomethane [121]. Repulsive van der Waals forces can also occur across thin films on solid surfaces. In the case of thin liquid films on solid surfaces there is often a repulsive van der Waals force between the solid-liquid and the liquid-gas interface [122],... 

Gas Solid S/G Adsorption, catalysis, corrosion, oxidation, diffusion, surface states, thin films, condensation and nucleation, permeation, energy transfer. 

Condensed (solid) films, in which the molecules are closely packed and steeply orientated towards the surface. 

Another example of phase change during reaction is chemical vapor deposition (CVD), a process used to manufacture microelectronic materials. Here, gas-phase reactants are deposited (analogous to condensation) as thin films on solid surfaces (see Problem P3-25). One such reaction is the production of gallium arsenide, which is used in computer chips. 

The technique of filling the sample holder is to fill it 1/3 to 1/2 full of liquid. Place the lower end into the dry ice-acetone or isopropanol container, and tilt the holder to about a 60 angle or as flat as possible without spilling the liquid. Slowly turn the holder so a film of solid sample freezes around the inside of the sample holder. Freeze the sample for a few minutes longer, connect the top section and then connect it to the condenser before it begins to melt. This rotation technique increases the surface area, and the water in this thin film, about 1 cm thick at most, can be sublimed rapidly. 

Zinsmeister, G., A contribution to Frenkel s theory of condensation. Vacuum, 16, 529, 1966 Zinsmeister, G., Theory of thin film condensation. Part B Solution of the simplified condensation equation. Thin Solid Films, 2, 497, 1968. 

M. Kaviany, Boundary-Layer Treatment of Film Condensation in the Presence of a Solid Matrix, Int. J. Heat Mass Transfer, (29) 951-954,1986. 

A second aspect of the film properties of typical known surfactants, which is of relevance to the in situ film pressure measurements, concerns the usefulness of the film pressure as a measure of the amount of oi anic material present in a surface film. For solid and liquid-condensed films, the film pressure is almost independent of area/molecule in the region of interest, i.e., those pressures above 1 X 10" N m Fora liquid-expanded film. Fig. 1 suggests that the area/molecule might change by a factor of 2 or 3 if the film pressure was increased 30-fold from this lower limit to 30 X 10 N Finally for gaseous films, which may not in fact exist on the sea in this film pressure range, the film pressure should be approximately linear with the amount of organic material present per unit of surface. Only in this extreme case, therefore, is the film pressure data a reliable indicator of the amount of natural surfactant molecules at the water surface. For all of the other... 

Single-layer PE films containing solid VCI (G-2 and NDA) additions outdo conventional PE films by a factor of 10-15 in terms of their protective properties [72]. The corrosion rate of the steel samples insulated by PE films modified by PI and Cl and placed in a I N solution of Na2S04 (simulating a harsh condensed medium of atmospheric corrosion) is lowered by three to five times [104,118]. 

Microporous membranes are thought to function less effectively when the surface is covered with a layer of condensation. Solid film hydrophilic films, on the other hand, transport the water effectively without the need for re-evaporation. The high water content of the polymer caused by condensation plasticises and swells the polymer, improving the water vapour transport at relatively high temperatures. However, low temperatures offset the benefit of plasticisation and so the lining material should be chosen to optimise the overall performance of the garment. 

Condensed (solid) films, which are coherent, rigid (essentially incompressible), and densely packed, with high surface viscosity. The molecules have little mobility and are oriented perpendicular (or almost so) to the surface (Fig. 8.13a). 

Phenomena of condensation and aggregation of continuous films on solid surfaces. 

In the vacuum deposition of the precursors PMDA and ODA at temperature of 120-145 °C reaction of polycondensation to P AA with opening of the anhydride ring of PMDA takes place (Ac-Sn2 -reaction). These processes are to great extent accelerated and controlled in the thermal treatment of the condensed solid phase which represents PAA, with regard to their transformation to PI by means of reaction of polycyclodehydration in solid state to linear PI [7,10,11]. The FTIR spectra of individual films of PMDA, ODA and PAA are shown in figure 2. 

Figure 9.13. Typical forms of monolayer films at L/L and LN interfaces (a) moderately close-packed with significant chain mobility (liquid expanded) (b) close-packed with tilted orientation and reduced chain mobility (liquid condensed) (c) close-packed with essentially vertical orientation and very limited chain mobility (condensed solid).

It is known that even condensed films must have surface diffusional mobility Rideal and Tadayon [64] found that stearic acid films transferred from one surface to another by a process that seemed to involve surface diffusion to the occasional points of contact between the solids. Such transfer, of course, is observed in actual friction experiments in that an uncoated rider quickly acquires a layer of boundary lubricant from the surface over which it is passed [46]. However, there is little quantitative information available about actual surface diffusion coefficients. One value that may be relevant is that of Ross and Good [65] for butane on Spheron 6, which, for a monolayer, was about 5 x 10 cm /sec. If the average junction is about 10 cm in size, this would also be about the average distance that a film molecule would have to migrate, and the time required would be about 10 sec. This rate of Junctions passing each other corresponds to a sliding speed of 100 cm/sec so that the usual speeds of 0.01 cm/sec should not be too fast for pressurized film formation. See Ref. 62 for a study of another mechanism for surface mobility, that of evaporative hopping. 

This description is traditional, and some further comment is in order. The flat region of the type I isotherm has never been observed up to pressures approaching this type typically is observed in chemisorption, at pressures far below P. Types II and III approach the line asymptotically experimentally, such behavior is observed for adsorption on powdered samples, and the approach toward infinite film thickness is actually due to interparticle condensation [36] (see Section X-6B), although such behavior is expected even for adsorption on a flat surface if bulk liquid adsorbate wets the adsorbent. Types FV and V specifically refer to porous solids. There is a need to recognize at least the two additional isotherm types shown in Fig. XVII-8. These are two simple types possible for adsorption on a flat surface for the case where bulk liquid adsorbate rests on the adsorbent with a finite contact angle [37, 38]. 

In general there are two factors capable of bringing about the reduction in chemical potential of the adsorbate, which is responsible for capillary condensation the proximity of the solid surface on the one hand (adsorption effect) and the curvature of the liquid meniscus on the other (Kelvin effect). From considerations advanced in Chapter 1 the adsorption effect should be limited to a distance of a few molecular diameters from the surface of the solid. Only at distances in excess of this would the film acquire the completely liquid-like properties which would enable its angle of contact with the bulk liquid to become zero thinner films would differ in structure from the bulk liquid and should therefore display a finite angle of contact with it. 

The formation of a liquid phase from the vapour at any pressure below saturation cannot occur in the absence of a solid surface which serves to nucleate the process. Within a pore, the adsorbed film acts as a nucleus upon which condensation can take place when the relative pressure reaches the figure given by the Kelvin equation. In the converse process of evaporation, the problem of nucleation does not arise the liquid phase is already present and evaporation can occur spontaneously from the meniscus as soon as the pressure is low enough. It is because the processes of condensation and evaporation do not necessarily take place as exact reverses of each other that hysteresis can arise. 

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