Aqueous films, stability

Concerning the structure of dispersed CLAs, the model originally proposed by Sebba [57] of a spherical oil-core droplet surrounded by a thin aqueous film stabilized by the presence of three surfactant layers is, in our opinion, essentially correct. However, there is still little direct evidence for the microstructure of the surfactant interfaces. From an engineering point of view, however, there is now quantitative data on the stability of CLAs which, together with solute mass transfer kinetics, should enable the successful design and operation of a CLA extraction process. 

The aqueous films stabilized with fluorine containing surfactants on organic substrates are an exception since they have very low surface tension (- 10 mM m 1). However, there are no literature data about them. 

Aqueous film stability is dependent on the adhesive force or negative interfacial tension at the two-phase (i.e., solid/liquid) boundary. The force balance at the two-phase boundary may change independently from the three-phase force balance due to surface configuration change of interfacing surface state moieties, which occurs in order to minimize interfacial tension with water as described in previous chapters. 

Three different cases of aqueous film stability indicated by FHT are depicted in Figure 26.23. Corresponding force loops and diagrams of the continuous water film... 

Figure 26.31 depicts the same measuring procedure applied to a contact lens material sheet (A) untreated, (B) (CH4 + air) plasma treated, and (C) (CH4 + air) plasma treated and then O2 plasma treated. Characterizing aqueous film stability on untreated and plasma-modified contact lens materials using artificial tear fluid by... 

The FHT measured by the Wilhelmy balance method can be effectively used to compare the liquid holding capabilities of different surfaces. The value of FHT depends on the experimental parameters and cannot be used in an absolute sense. In general, the aqueous film stability is obtained when spontaneous wetting occurs on imperturbable surfaces. However, moderately hydrophilic and possibly even some hydrophobic surfaces that are perturbable by water were found to be capable of holding continuous films of water. Multicomponent fluid, such as a dilute solution of protein used as a simulated tear fluid, may yield misleading liquid holding characteristics of surfaces due to preferential adsorption of components on a surface. 

In reality, aqueous films stabilized with ionic surfactant, without electrolyte, also rupture, especially at surfactant concentrations below the cmc. The latter fact cannot be explained in the framework of the quasistatic approximation (117—119) this is still an open problem in the theory of li(juid-film stability. 

Colloidal liquid aphrons (CLAs), obtained by diluting a polyaphron phase, are postulated to consist of a solvent droplet encapsulated in a thin aqueous film ( soapy-shell ), a structure that is stabilized by the presence of a mixture of nonionic and ionic surfactants [57]. Since Sebba s original reports on biliquid foams [58] and subsequently minute oil droplets encapsulated in a water film [59], these structures have been investigated for use in predispersed solvent extraction (PDSE) processes. Because of a favorable partition coefficient for nonpolar solutes between the oil core of the CLA and a dilute aqueous solution, aphrons have been successfully applied to the extraction of antibiotics [60] and organic pollutants such as dichlorobenzene [61] and 3,4-dichloroaniline [62]. 

Measurements have been carried out on the excess tensions, equilibrium thicknesses, and compositions of aqueous foam films stabilized by either n-decyl methyl sulfoxide or n-decyl trimethyl ammonium-decyl sulfate, and containing inorganic electrolytes. 

Figure 5 shows dynamic film tension of soybean oil films stabilized by 0.5 wt % SPAN 80 emulsifier between aqueous phases under expansion by various flow rates. The increase in film tension from equilibrium is higher at higher rates of interface expansion because the flux of surfactant that can adsorb during expansion is lower at higher rates. 

Figure 1. Schematic diagram showing the possible mechanisms of thin film stabilization, (a) The Marangoni mechanism in surfactant films (b) The viscoelastic mechanism in protein-stabilized films (c) Instability in mixed component films. The thin films are shown in cross section and the aqueous interlamellar phase is shaded.

Correa and Saramago [282] describe the calculation of disjoining pressures for non-aqueous films. In this case the dispersion forces were found to be the most important in determining thin-film stability. 

Wasan, D.T. Koczo, K. Nikolov, A.D. Mechanisms of Aqueous Foam Stability and Antifoaming Action With and Without Oil A Thin Film Approach in Foams, Fundamentals and Applications in the Petroleum Industry, Schramm, L.L. (Ed.), American Chemical Society Washington, DC, 1994, pp. 47-114. 

Data on emulsion film formation from insoluble surfactant monolayer are rather poor. It is known, however, that such films can be obtained when a bubble is blown at the surface of insoluble monolayers on an aqueous substrate [391,392]. Richter, Platikanov and Kretzschmar [393] have developed a technique for formation of black foam films which involves blowing a bubble at the interface of controlled monolayer (see Chapter 2). Experiments performed with monolayers from DL-Py-dipalmitoyl-lecithin on 510 3 mol dm 3 NaCl aqueous solution at 22°C gave two important results. Firstly, it was established that foam films, including black films, with a sufficiently long lifetime, formed only when the monolayer of lecithin molecules had penetrated into the bubble surface as well, i.e. there are monolayers at both film surfaces on the contrary a monolayer, however dense, formed only at one of the film surfaces could not stabilize it alone and the film ruptured at the instant of its formation. Secondly, relatively stable black films formed at rather high surface pressures of the monolayer at area less than 53A2 per molecule, i.e. the monolayer should be close-packed, which corresponds to the situation in black films stabilized with soluble surfactants. 

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]. 

Stability of both foam and asymmetric aqueous films at the surface of organic liquids of different polarity has been studied as function of the surfactant concentration [551], Microscopic foam films were obtained in a glass cuvette (Tig. 3.120,c) by blowing an air bubble at the tip of a vertical capillary immersed into the surfactant solution. With a micrometric screw (not shown in the figure) the bubble was pressed carefully onto the solution/air interface, thus forming the film. 

The dependence of the surfactant concentration at which equilibrium asymmetric aqueous films are formed on the nature of the organic phase is presented in Table 3.18 [551]. It is seen that the stabilising ability of the surfactants strongly reduces with the increase in organic phase polarity. The experiments performed in [552] have shown that at high capillary pressures in the meniscus the stability of films on organic substrate substantially depends on the surfactant concentration. 

The effect of disjoining pressure on thickness and stability of aqueous films on dodecane and tetradecane substrates has been reported in [542], A comparison with the stability of foam films from the same surfactants was also presented. In all systems studied... 

The concentration of black spot formation in microscopic films Cm characterises not only the threshold concentration of the surfactant at which stable foams and emulsions can be obtained but it can also be used as an indirect measure of film stability. The relations between film stability and Cm of the emulsifier depend on the polarity of the organic phase of the emulsion films (aqueous and hydrocarbon) [58], on the hydrophilic-lipophilic balance of the surfactant mixture [59] as well as on other properties. 

The term moisture, usually defined as wetness conferred by an unidentified liquid, is assumed here to be due to water. Thus, the scope of this article is the characterization of and consequences due to relatively small amounts of water associated with solids of pharmaceutical interest. Chemical stability, crystal structure, powder flow, compaction, lubricity, dissolution rate, and polymer film permeability are some properties of pharmaceutical interest that have been demonstrated to be influenced by the presence of moisture. Wet granulation, extrusion, spheronization, tray drying, freeze drying, spray drying, fluid-bed drying, tableting, and aqueous film coating are some unit operations that obviously depend on the amount and state of water present. 

A more rigorous procedure to study emulsion stability using the ultracentrifuge is to observe the system at various speeds of rotation. At relatively low centrifuge speeds, the expected opaque cream layer may be observed, but at sufficiently high speeds a coalesced oil layer and a cream layer may be observed that are separated by an extra layer of deformed oil droplets. This deformed layer resembles a foam that is, it consists of oil droplets separated by thin aqueous films. 

Only a few studies about aqueous films of amphiphilic random polyelectrolytes are reported in the literature. Millet et al. [239-241] have investigated by x-ray reflectivity the behavior of vertical free-standing films (Figure 29) of a series of hydrophobically modified poly(acrylic acid) sodium salt (HMPAANa) and poly(acrylic acid) (HMPAAH). The chemical structure of the polymer was presented in Sec. II.C (Eq. 2a). One of the aims of this work was to determine the microscopic structure of the films to explain the (macroscopic) stability behavior of the dodecane-in-water emulsions studied by Perrin et al. [188,189], who used the same series of amphiphilic polyelectrolytes as primary emulsifiers. The aqueous polyelectrolyte films have been used as model systems for the interstitial films separating two neighboring oil droplets of an emulsion creamed layer. The authors have assumed that the oil/water interface encountered in emulsions was suitably described by the air/water interface of the films. The HMPAANa and HMPAAH co-... 

The behavior of the pseudoemulsion film is controlled by mechanisms similar to foam films the interfadal rheological properties of the surfactant molecules at low surfactant concentrations and micellar ordering at high surfactant concentrations (i.e., much above the CMC). Not much above the CMC, both of these mechanisms can play a role in film thinning and stability. The stability of a thin pseudoemulsion film also depends on the van der Waals interactions between the phases at the two sides of the film, that is between air and oil, acting across the aqueous film. In a water pseudoemulsion film the Hamaker constant is generally negative, the van der Waals interactions are repulsive and stabilize the film. 

The effect of oil on aqueous foam stability is controlled by the behavior of the pseudoemulsion films. In the previous sections, two extreme cases of the foam-oil interactions were shown foam stabilizing when the pseudoemulsion films are stable and antifoaming that is, fast foam rupture, when these films are very unstable. 

Figure 2 illustrates what is coined a discontinuous-gas foam (2, 9), in that the entire gas phase is made discontinuous by lamellae, and no gas channels are continuous over sample-spanning dimensions. Gas is encapsulated in small packets or bubbles by surfactant-stabilized aqueous films. These packets transport in a time-averaged sense through the porous medium (20). 

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