Spreading film surface

While we have, in accord with established convention, used the term "surface pressure, it must be realized that what we really mean is the difference in tension between that of the clean surface and that of the surface in the presence of the spread film. "Surface pressure has the dimensions of a tension and not of a pressure. 

Before proceeding to the main subject of this chapter—namely, the behavior and properties of spread films on liquid substrates—it is of interest to consider the somewhat wider topic of the spreading of a substance on a liquid surface. Certain general statements can be made as to whether spreading will occur, and the phenomenon itself is of some interest. 

A surfactant for evaporation control has an equilibrium film pressure of 15 dyn/cm. Assume a water surface and 25°C and calculate the distance traveled by the spreading film in 8 sec. 

Replacement of gas by the nonpolar (e.g., hydrocarbon) phase (oil phase) has been sometimes used to modify the interactions among molecules in a spread film of long-chain substances. The nonpolar solvent/water interface possesses an advantage over that between gas and water in that cohesion (i.e., interactions between adsorbed molecules) due to dipole and van der Waals s forces is negligible. Thus, at the oil/water interfaces, the behavior of adsorbates is much more ideal, but quantitative interpretation may be uncertain, in particular for the higher chains, which are predominantly dissolved in the oil phase to an unknown extent. The oil phase is poured on the surface of an aqueous solution. Thus, the hydrocarbon, such as heptane or decane, forms a membrane a few millimeters thick. It is thicker than the adsorbed monolayer. Owing to the small difference in dielectric constant between the air and a hydrocarbon oil, the... 

Harkins et al., 1940), where ne is the ESP, and Ae is the average area per molecule at the ESP as obtained from the 11/ 4 isotherm of the spread film. The temperature dependence of the ESP may then be used to calculate the excess surface entropies from (5) and enthalpies of spreading from (6). 

These results for spread film and equilibrium spreading suggest that films of racemic N-(a-methylbenzy 1) stearamide may be resolved by seeding the racemic film with crystals of either pure enantiomer. Indeed, when a monolayer of racemic jV- (a-methylbenzyl) stearamide is compressed to 45 A2/molecule (27 dyn cm-1), deposition of a crystal of either R( +)- or S( — )-enantiomer results in a decay of surface pressure from the initial 28 dyn cm-1 film pressure to 3.0 dyn cm-1, the ESP of the enantiomeric systems on a pure 10n sulfuric acid subphase (Table 1). When the experiment is repeated with racemic crystals, the system reaches an equilibrium surface pressure of 11 dyn cm-1, nearly the ESP of the racemic crystal on the clean acidic interface. In either case, equilibrium pressure is reached within a two hour time period. 

When spread from a benzene/hexane solution on to a slightly acidic water subphase, spread films of racemic and enantiomeric STy exhibit nearly the same IT/A isotherms (Fig. 22) and surface shear viscosities (Harvey et al., 1990). The shapes of these isotherms and the apparently small differences between the compression/expansion characteristics of these fluid homochiral and heterochiral monolayers is conserved throughout the... 

In order to test the mechanism of recognition, equilibrium spreading pressures of both racemic and enantiomeric forms of SSME were obtained in pre-spread films of palmitic acid/SSME mixtures. The films were spread from solution and then compressed to their lift-off areas. A crystal of the racemic SSME was placed on surface film mixtures of the fatty acid with racemic SSME, and the enantiomeric crystals were placed on surface film mixtures of the fatty acid and enantiomeric SSME. The results of the equilibrations are given in Fig. 27. 

There are several parameters that affect on what type of LB film is produced. These are the nature of the spread film, subphase composition and temperature, surface pressure during deposition and deposition speed, the type and nature of the solid substrate, and the time the solid substrate is stored in air or in the subphase between deposition cycles. 

Fig. 12. Eddy brings fresh liquid surface into the interface, but this is opposed by the back pressure All of the spread film.

When a thin film of fatty acid is spread on the surface of water the surface tension of the water is reduced from yo to 7- A barrier placed between pure water and water with a surface film will experience a pressure difference resulting from the tendency of the film to spread. This surface pressure , n, is given by... 

Rodriguez Nino, M.R., Sanchez, C.C., Rodriguez Patino, J.M. (1999). Interfacial characteristics of (3-casein spread films at the air-water interface. Colloids and Surfaces B Biointerfaces, 12, 161-173. 

A two-dimensional analog of PV work can be recognized in fluid films that exhibit surface tension (tendency of the film surface to contract against an opposing spreading force). The surface tension work wsurf (of, for example, a soap film) can be measured by a rectangular wire-frame device with moveable edge, as shown in Fig. 3.5. 

Fig. 6 [79]. Metals such as copper are known to decompose hydroperoxides and thereby initiate oxidation [143]. The location of the copper wire is clearly seen at the centre of the PP film. The copper wire was placed in direct contact with the PP film surface. The images in Fig. 11 show the initiation and spreading of oxidation from the location where the PP film is in contact with the copper wire to the rest of the film the experiments were run at 150 °C in air. The initiation by copper was not caused by an increase in heat transfer since initiation with aluminium, which is not able to decompose hydroperoxides, failed.

Keywords Interface Langmuir monolayer Adsorption process Spreading solvent Surface tension Surface isotherm Langmuir-Blodgett film Collapse pressure Amphiphilic polymer... 

Fig. 15 and Table III give the results of measurements on numerous other series of compounds. Curve I is identical with curve I of the preceding figure, and is obtained on over a dozen different homologous series at some surface pressure. With all these compounds the area is the same, within about 1 sq. A., i.e. 20-5 sq. A. at no compression. Curves to the left of curve I, i.e. with smaller areas, have never been obtained with fully spread films, which show no unspread patches when examined by Zocher and Stiebers dark-ground illumination. Moreover, when areas as small as this are found, the films are always very incompressible, and the curves very steep. 

Rideal and his collaborators found that solid particles of protein spread very rapidly on a clean surface their method of spreading is to coat a quartz fibre, first covered with paraffin wax, with a little protein this is dipped into the water surface, and the amount of protein passing into the surface ascertained by weighing the fibre on a simple micro-balance.1 By this method somewhat larger areas can sometimes be obtained, and films formed from proteins which cannot be made to form homogeneous films when spread by Gorter s technique and in certain cases the average thickness of the fully spread film is only about 3 A. 

The equilibrium between the crystal, or material in bulk form, and the surface film is also determined by the relative intensities of the tendencies of the chains to lie side by side, and of the heads to reach the water. If the chains become very long, a mass on the surface becomes more stable than the spread film the crystal will not spread spontaneously, and the film, if spread by the aid of a solvent, may become unstable and collapse readily. These facts will appear in 35. 

With the aliphatic compounds the contribution (as measured by /x) of each molecule to the surface potential is similar to that of insoluble molecules with the same end groups in spread films. In some cases there may be some change in /x as the film becomes more crowded, indicating a change in tilt of the dipoles to the surface in others, /x apparently remains constant the data are scarcely full enough, however, to afford much information on this point. 

Energy requirements for stable spreading. When a drop of a liquid is placed on the surface either of another immiscible liquid, or of a solid, it may spread to a film, or may remain as a drop without spreading. The surface tensions of the two liquids, and the interfacial tension between them, determine whether or no the liquid spreads and the same holds if the lower phase is a solid. 

In the case of adsorbed layers, by analogy with the two-dimensional pressure of spread films, a surface pressure (n) is introduced defined as the change of - interfacial tension (y) caused by the addition of a given species to a base solution. At constant cell potential (E) this is... 

---