Asymmetric thin liquid films

Similarity of Foam Films with Emulsion and Asymmetric Thin Liquid Films... 

Maldarelli, C., Jain, R. K., Ivanov, I. B., and Ruckenstein, E., Stability of symmetric and asymmetric thin liquid films to short and long wavelength perturbations, J. Colloid Interface Sci., Vol. 78, pp. 118-143, 1980. 

The microinterferometric method has widely been used by many authors to investigate both symmetric thin liquid films (foam and emulsion films) and asymmetric ones (wetting films). This method has been described in several papers, for example. Refs. [2, 3, 32, 33, 37] as well as in a book [1]. Here we give only a brief outline. 

Initially devised to measure interactions in single soap films (air/water/air) [8], the TFB technique has been progressively improved and its application has been broadened to emulsion films (oil/water/oil) [ 12] and asymmetric films (air/water/oil or air/water/solid) [13,14]. In a classical setup, a thin porous glass disk is fused on the side to a capillary tube and a small hole is drilled in the center of the disk. The liquid solution fills the disk, part of the capillary, and a thin horizontal film is formed across the hole. The disk is enclosed in a hermetically sealed box, with the capillary tube exposed to a constant reference pressure Pr. Under the effect of the pressure difference AP between the box and the reference, the... 

More complex with respect to molecular interaction is the case of formation of non-aqueous films on the surface of aqueous solutions from non-ionic surfactants [528], Films from octane were obtained by adsorption from drops of octane/non-adsorbing diluent (squalane) mixture. Occasionally the spreading of alkanes on aqueous surfactant solution gives stable thin oil films (e.g. on solutions of the anionic surfactants Aerosol OT) [529,530], Some evidence about the stability of asymmetric films can be derived from the data about the surface pressure and spreading coefficients of liquids on water surface. These data are known for many organic liquids [531,532], It should be also noted that the techniques for determination of the spreading coefficients have improved considerably [533,534]. Most precise values were obtained by measuring the surface pressure of a monolayer with a special substance introduced as an indicator [533]. 

Summarizing, two conditions must be fulfilled in order to obtain from the simulations a confinement-induced and thickness-dependent distribution of the end-to-end distance for terminal subchains. First, a chain should be in contact with both interfaces. This happens only when the film thickness becomes comparable to the size of the chains and, obviously, explains why the confinement-induced mode does not exist in the bulk. Second, the interactions at the interfaces should be asymmetric One interface should immobilize the polymer chains, while the second one should only reflect them. This asymmetry could be induced by the nonequivalent preparation of the electrodes in the experiment While one interface is prepared by spin-coating, the other one is prepared by evaporation of aluminium on top of the polymer film (see Section II for details). A similar picture of asymmetry was found in studies on thin PS films, with a preparation procedure identical with ours. For thin PS films capped between two aluminum electrodes a three-layer model was proposed, in which, in addition to a middle-layer having bulk properties, a dead (immobilized) layer and a liquid-like layer were assumed to be present at the interfaces. 

Most commercially available RO membranes fall into one of two categories asymmetric membranes containing one polymer, or thin-film composite membranes consisting of two or more polymer layers. Asymmetric RO membranes have a thin ( 100 nm) permselective skin layer supported on a more porous sublayer of the same polymer. The dense skin layer determines the fluxes and selectivities of these membranes whereas the porous sublayer serves only as a mechanical support for the skin layer and has little effect on the membrane separation properties. Asymmetric membranes are most commonly formed by a phase inversion (polymer precipitation) process (16). In this process, a polymer solution is precipitated into a polymer-rich solid phase that forms the membrane and a polymer-poor liquid phase that forms the membrane pores or void spaces. 

A stratification phenomenon in foam films from sodium oleate aqueous solutions has been described by Johonnott [314] and Perrin [318] in the beginning of the century. The stepwise thinning has been studied by many authors [e.g. 235,345], It became evident that this phenomenon is universal and has been observed also in emulsion films [346,347], asymmetric films of the air-water/oil type [348], films from latex suspensions [349] and liquid crystalline films [350],... 

In isolation, the BHT ion is tetrahedral, and consequently only two fundamentals, the asymmetric BH stretch (V3) and asymmetric BH4 deformation (V4) are IR active for the isolated ion, whereas all four fundamentals are Raman active. The Raman active fundamentals were characterized in liquid ammonia solutions, whereas IR spectra of thin films of NaBH4 on alkali halide crystals or diluted in an alkali halide host crystal have been reported ". Raman and IR studies focused specifically on NaBH4 and LiBH4 have also been reported. The vibrational modes in borohydrides are of three distinct types librational (below 1000 cm ), B-H bending (1127 cm ) and B-H stretching (2200-2400 cm ). The overtone of the deformation mode (2V4) occurs around 2228 cm ... 

In another set of experiments we used the asymmetric block copolymer PBh-PEO(21.1-4.3), where PEO presents the minority phase of about 17 volume percent and forms spherical cells with a diameter of about 12 nm. A typical AFM image of the surface of a thin film prepared from this polymer is shown in Fig. 13. The first point to realize is the abihty of the AFM phase mode to distinguish even between two liquid polymers, the hquid PEO cells embedded within the liquid PBh matrix. 

The membrane can be a solid, a liquid, or a gel, and the bulk phases can be liquid, gas, or vapor. Membranes can be classified according to their structures. Homogeneous or symmetric membranes each have a structure that is the same across the thickness of the membrane. These membranes can be porous or have a rather dense uniform structure. Heterogeneous or asymmetric membranes can be categorized into three basic structures (1) integrally skinned asymmetric membrane with a porous skin layer, (2) integrally skinned asymmetric membrane with a dense skin layer, and (3) thin film composite membranes [13]. Porous asymmetric membranes are made by the phase inversion process [14,15] and are applied in dialysis, ultrafiltration, and microfiltration, whereas integrally skinned asymmetric membranes with a dense skin layer are applied in reverse osmosis and gas separation applications. 

Passivation of the surface of n-GaAs is possible with thin films of plasma-polymerized thiophene [837]. A composition containing an electrically nonconductive polymer matrix and POT can also be used in paraboloid antennas, reflectors for radar, heating systems, photoelectric devices, and electric circuits and apparatus [779]. PTs are used for manufacturing a nonlinear two-terminal device. This deviee is not asymmetrical, gives stable electrical characteristics, and is useful as a display device [838, 839], Liquid crystal display devices contain an electrically conductive polymer, e.g., PT or POT as oriented film [840-842]. PTs are also used for the production of color filters for liquid crystal displays [843]. 

---