Film pressure from adsorption

The second mechanism can be explained by the wall liquid film flow from one meniscus to another. Thin adsorptive liquid layer exists on the surface of capillary channel. The larger is a curvature of a film, the smaller is a pressure in a liquid under the corresponding part of its film. A curvature is increasing in top s direction. Therefore a pressure drop and flow s velocity are directed to the top. 

As discussed in Chapter III, the progression in adsoiptivities along a homologous series can be understood in terms of a constant increment of work of adsorption with each additional CH2 group. This is seen in self-assembling monolayers discussed in Section XI-IB. The film pressure r may be calculated from the adsorption isotherm by means of Eq. XI-7 as modified for adsorption from dilute solution ... 

A. Film Pressure-Area Diagrams from Adsorption Isotherms... 

Interactions with neighboring adsorbed molecules will influence the conformation which the critical complex will adopt. This phenomena is demonstrated in the change in the mode of adsorption of toluene on a liquid mercury surface from a flat to a vertical arrangement as the film pressure is increased (73). In the present context, the attraction of the surface for the substrate, whether chemical or physical, will cause neighboring molecules to crowd one another so that an adsorbed molecule may adopt a conformation which is different from the conformation of lowest energy in the isolated molecule. 

In the previous sections we have noted that the hypothesis of a unimolecular Gibbs layer for solutions of liquids of markedly different internal pressures together with the equation of Gibbs leads to values for molecular areas and thicknesses which are not at all unreasonably different from those determined by means of X-ray measurements, or from a study of insoluble substances on the surface of water, but cannot be said to be identical within the limits of experiment. In one respect, however, such soluble films differ from the insoluble films which we shall have occasion to examine in the next chapter the surface tension of solutions which according to the Gibbs adsorption equation... 

Azzaroni etal. [163] have used STM to study electrochemical reactivity of thiourea toward Au(lll). Sequential STM imaging has shown that thiourea adsorbs as striped arrays that evolve to the hexagonal close-packed structure when surface charge density is decreased. The transient hep structure undergoes electrooxidation to formamidine disulfide, which slowly yields adsorbed sulfur. Adsorption of thiourea on the pc-Au electrode from KCIO4 solutions has also been studied [164]. The film pressure and the Gibbs surface... 

In the case of adsorption from solution, the surfactant layers are in equilibrium with the solution and will de-sorb on dilution. However, it would be very useful to produce adsorbed layers in both air and water, which will remain adsorbed. This can be achieved using the Langmuir-Blodgett deposition technique. The technique is based on the observation that if a surfactant, which is insoluble in water, is dissolved in a volatile, non-aqueous solvent and then spread on water, an insoluble monolayer of orientated surfactant molecules will remain at the air/solution interface. The effect of the spreading surfactant and its surface film pressure can be dramatically demonstrated by spreading hydrophobic talc powder on a clean water surface and then placing a... 

In the Chapter 7, formation of monolayers in air-liquid interfaces and the resulting film pressure and phase transitions are discussed. This chapter also includes a brief discussion of adsorption on solid surfaces from solutions. 

The apparatus developed for yb measurements of BLM deserves brief comment since it can be used not only to examine the effects of various substances on BLM but is readily adaptable for studying other types of interfacial films and related adsorption phenomena at either air-water or oil-water interfaces (and bifaces). Unlike both the Wil-helmy plate and film balance methods, the present technique measures 7i directly. From the description of the apparatus and procedure that the present method relies on the ability to measure the very small pressure difference across an interface (or biface). For certain BLM s, the pressure heads measured are only fractions of a millimeter of water. Therefore, the method described here has been possible only as a result of developing pressure transducers of high sensitivity. 

By using the Gibbs adsorption theorem and the relation between the three-dimensional pressure p and the film pressure de Boer and Hill derived from Eq. 36 and equation for a two-dimensional adsorption with two separate types of interactions4 ... 

Cassel s results indicate gaseous films, the hydrocarbons giving the same direction of curvature as above, i.e. a preponderance of the 6 correction in the van der Waals type of equation but the lower alcohols and nitromethane showed opposite curvature in the F-p2 curves, except at very high pressures this indicates strong lateral adhesion the F-A relations for the adsorbed films, deducible from his results, indicate association into double molecules even at great dilutions. Butyl and iso-amyl alcohols formed adsorbed films intermediate in properties between the lower alcohols and the hydrocarbons. No lowering of tension at all was observed with water. Cassel s results differ considerably from those of other workers since the surface tension of his supposedly clean mercury was some 25 dynes lower than the best values of several other workers, and since no adsorption of water was found, there may have been a considerable amount of contamination initially on the mercury. 

The special properties of thin liquid films, in particular of foam films, involve studying various colloid-chemical aspects, such as kinetics of thinning and rupture of films, transition from CBF to NBF, isotherms of disjoining pressure, thermodynamic (equilibrium) properties, determination of the electrical parameters of surfactant adsorption layer at the liquid/gas... 

Figure 2. Dependencies of the adsorption potential on the pore width calculated according to the equations (3) (SF curve), (4) (curve KJSe) and (5) (curve KJSc). The KJSc curve was calculated via equation (5) using the Harkins-Jura-type expression for the statistical film thickness t, which gives a good representation of the experimental t-curve only in the range of relative pressures from 0.1 to 0.95. Therefore, this curve deviates from points at high values of A, which correspond to low values of p/po. Data for the ZLZ curve are from Zhu et al. [29].

The equilibrium properties of foam films formed from aqueous solutions of decylmethyl sulfoxide have been studied in the presence of sodium chloride and potassium thiocyanate. Stable films were formed whose thicknesses depended on the electrolyte concentration. As the electrolyte concentration was increased, a sudden increase in film thickness occurred but gradually decreased with further electrolyte addition. Examination of the electrophoretic mobility of dodecane droplets stabilized by decylmethyl sulfoxide showed an increase in mobility at about the same concentration. These data indicated that the thicker foam films were charge stabilized owing to the adsorption of the anions. The surface pressures and surface potentials of monolayers of octadecyl sulfoxide were also investigated. 

Gibbs equation. The adsorption process involves the transport of molecules from the bulk solution to the interface, where they form a specially oriented monomolecular layer according to the nature of the two phases. When a Gibbs monolayer forms, it does not necessarily mean that the molecules are touching each other in this monolayer. Instead, if the anchoring from the sub-phase molecules is weak, the molecules may move freely in the two-dimensional interfacial area. Thus, the physically measurable monolayer area is sometimes much larger than the close-packed area where all the molecules touch each other. When any monolayer is fairly well populated with adsorbed molecules, it exerts a lateral spreading (film) pressure, it, which is equal to the depression of the surface tension (see Section 5.5.2). 

Narrowly defined, the main contributions to film pressure or interfacial tension decrease come from the osmotic term and the repulsion of the electrical double layers of ionic surfactants including the effects of counterions. Interactions in mixed adsorption layers are of broad interest for the description of the state of surfactant adsorption layers. For the clarification of the adsorption mechanism at liquid interfaces the replacement of solvent molecules, mainly water, has been intensively studied by Lucassen-Reynders(1981). 

Quite a number of years ago we carried out some experiments on films evaporated from Alloy 99 wire. We observed that films evaporated from this wire exhibited quite different properties than those evaporated from Nickel A or Hoskins 651. The films prepared from Alloy 99 showed (211) preferential orientation and were highly resistant to sintering as measured by the hydrogen adsorption. The activity of these films for carbon monoxide oxidation was 15 times greater than the activity of films prepared from the other nickel wires. For the disproportionation of carbon monoxide, 50-60% more CO2 was found than was theoretically possible. When the wire was heated in hydrogen a large decrease in hydrogen pressure with subsequent water formation was observed. 

Jarvis (1967) placed samples of fresh seawater in a film balance tray, swept the surface free of existing films and, using film pressure and surface potential measurements, followed the build-up (i.e. adsorption) at the air/ water interface of organic films from the bulk seawater. He found that seawater normally contains natural surface active organic compounds which will adsorb at the air/water interface, and that bubbling the seawater as opposed... 

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