Emulsion bilayers

Fig. 3.82. Molecular model of (a) foam or emulsion and (b) membrane or emulsion bilayer hollow...

Rupture of emulsion bilayers. Experimental verification of the theory [399,402,403] of hole nucleation rupture of bilayer has also been conducted with emulsion bilayers [421]. A comparative investigation of the rupture of microscopic foam and emulsion bilayers obtained from solutions of the same Do(EO)22 nonionic surfactant has been carried out. The experiments were done with a measuring cell, variant B, Fig. 2.3, a large enough reservoir situated in the studied film proximity was necessary to ensure the establishment of the film/solution equilibrium. The emulsion bilayer was formed between two oil phases of nonane at electrolyte concentration higher than Cei,cr-... 

Since t(Q data for the emulsion bilayers are rather scattered, only Ce and % could be estimated Ce = 0.5 - 3-103 mol dm 3, % = 610"12 J m1. From the comparison of the t(C) dependences for the foam and emulsion bilayers in Fig. 3.92 it is seen that the stability of the foam bilayers is greater than that of the emulsion bilayers and that Ce is much lower for the... 

The critical concentration Cc for formation of foam and emulsion bilayers of Do(EO)22 are 4-10 6 mol dm 3 and 1.6 10 5 mol dm 3, respectively, and are in good correlation with the lowest concentrations, 2-31 O 6 mol dm 3 and 10 5 mol dm 3 [421] at which maximum filling of the surfactant adsorption monolayer is attained. It should also be noted that in the case of the emulsion bilayers, CMC < Ce which implies that it is not possible to obtain infinitely stable (i.e. with r = °°) bilayers of Do(EO)22 between two droplets of nonane under the described conditions. For this reason, it may be thought that thermodynamically stable nonane-in-water emulsions stabilised with Do(EO)22 do not exist. 

The experimental results discussed pertain to foam and emulsion bilayers formed of surfactants of different kinds and provide information about quantities and effects measurable in different ways. It is worth noting that analysing the observed effect of temperature on the rupture of foam bilayers enables the adsorption isotherm of the surfactant vacancies in them to be calculated. This isotherm shows a first-order phase transition of the vacancy gas into a condensed phase of vacancies, which substantiates the basic prerequisites of the theory of bilayer rupture by hole nucleation. 

Some experimentally derived values of yy for foam and emulsion bilayers are listed in Table 3.16. Values of yy for BLMs are also given for comparison. These data are obtained on the basis of an experiment in which the rupture of BLM is caused by an external electric field of intensity U [456,463]. Using the i(U) dependence the value of yy for bilayers from lyso PC and lyso PE is found to be 0.5 to 1.510"11 J m 1 (Table 3.16). For egg lecithin BLM in n-decane yy is also evaluated [459,464], Depending on the adopted model, packing model [465] or liquid-crystalline model [464] two values of yy are obtained yy = 0.75-10" J m 1 and % = 2.M011 J m1. The latter value is also determined in [466] by studying microscopic holes in tube liposomes in electric field (Table 3.16). 

Abstract The effect of lecithin (natural surfactant) addition to gelatin on the surface rheological properties of the water/ heptane interfacial layer and emulsion films formed by these liquids was studied. It was found that the gelatin/ledthin mixtures form complexes in the aqueous phase. Self-assembly of these complexes leads to the formation of viscoelastic interfacial adsorption layers characterized by a yield stress and elastic modules that provide stability of the emulsion films and emulsion systems. The above mentioned parameters evolve in time, though the formation of equilibrium interfacial layers proceeds during several hours emulsion bilayer films require only several minutes. 

Fig. 6 illustrates the diagram of emulsion bilayer film stability. It has been shown that in the range of concentrations... 

Table 1 Elastic modules G, yield stress xys su d viscosity of emulsion bilayer films and infiafacial adsorption layers formed at the interface with heptane from aqueous gelatin/lecithin mixed solutions...

The development of monoalkyl phosphate as a low skin irritating anionic surfactant is accented in a review with 30 references on monoalkyl phosphate salts, including surface-active properties, cutaneous effects, and applications to paste and liquid-type skin cleansers, and also phosphorylation reactions from the viewpoint of industrial production [26]. Amine salts of acrylate ester polymers, which are physiologically acceptable and useful as surfactants, are prepared by transesterification of alkyl acrylate polymers with 4-morpholinethanol or the alkanolamines and fatty alcohols or alkoxylated alkylphenols, and neutralizing with carboxylic or phosphoric acid. The polymer salt was used as an emulsifying agent for oils and waxes [70]. Preparation of pharmaceutical liposomes with surfactants derived from phosphoric acid is described in [279]. Lipid bilayer vesicles comprise an anionic or zwitterionic surfactant which when dispersed in H20 at a temperature above the phase transition temperature is in a micellar phase and a second lipid which is a single-chain fatty acid, fatty acid ester, or fatty alcohol which is in an emulsion phase, and cholesterol or a derivative. 

OS 41a] [R 19] ]P 30] A study was undertaken to compare extended (1 h) processing in small vials (2 cm ) with short-time (100 s) continuous micro reactor and mini-batch (10 cm ) operation for 10 different substrates (C4-C8 alcohols) which were reacted with rhodium(I)-tris(m-sulfophenyl)phosphane [111]. The vials were either directly filled with the two phases yielding a bilayered fluid system with small specific interfaces or by interdigital micro mixer action yielding an emulsion with large specific interfaces. 

The method utilizing ID NMR is simple and eonvenient. Henee the NMR method diseussed here ean be applied to the systematie investigation of the membrane irug inter-aetions, elosely related to the vital function in biomembranes. It is expected that the application can be extended to the lipid-peptide interaction and protein uptake. We are now applying the method to apolipoprotein binding with lipid bilayers and emulsions. Preferential protein binding sites in membranes can be specified by NMR on the molecular level. 

Azuma K, Ippoushi K, Ito H, Higashio H and Terao J. 1999. Evaluation of antioxidative activity of vegetable extracts in linoleic acid emulsion and phospholipid bilayers. J Sci Food Agric 79(14) 2010-2016. 

FIG. 14. Transmission electron micrograph of Voltaren Emulgel the interface hetween the continuous hydrogel and the dispersed emulsion droplets consists of multiple bilayers of hydrated surfactant molecules, bar 500 nm. From Miiller-Goymann, C., and Schutze, W., Mehrschichtige Phasengrenzen in Emulsionen, Dtsch. Apoth. Ztg., 130 561-562 (1990). 

Rgure 2.30. Two adhesive emulsion droplets. A flat hquid fllm stabilized by the surfactant layers is located between the droplets. This fllm being very thin, it can be usually considered as a surfactant bilayer. Yf is the tension of the fllm and y nt the tension of single isolated interface. 

Vesicles are capsules in which the shells are composed of amphiphilic small molecules or polymers. Generally, the shell is an amphiphilic bilayer with an aqueous interior. These differ fundamentally from capsules generated in a water-in-oil emulsion because the oil phase in the vesicle system is only in the shells, which are surrounded by an outer aqueous phase. 

In order to conveniently distinguish between these two cases, they are simply referred to here as (i) bilayer emulsions and (ii) mixed emulsions (Jourdain et al., 2008, 2009 Dickinson, 2008a). 

Figure 7.20 Influence of salt content on properties of bilayer emulsions based on p-lactoglobulin + i-carrageenan at pH = 6.0 (a) zeta potential and (b) mean particle diameter The primary emulsion (open symbols) contained 5 wt% oil and 0.5 wt% protein die secondary emulsions (ftlled symbols) contained an additional 0.1 wt% polysaccharide. Reproduced from Gu el al. (2005b) with permission.

Especially troublesome is bridging flocculation. It is therefore much more convenient to prepare emulsions with protein and polysaccharide components both present together in the aqueous medium before homogenization (Dickinson et al., 1998 Garti et al., 1999 Dickinson, 2008a). Moreover, in a direct comparison between the two techniques (Jourdain et al., 2008), it has been demonstrated that the experimentally more straightforward mixed emulsion approach can actually produce a better level of stability than the bilayer approach. 

The larger thermodynamic affinity for the aqueous medium for the case of normal complexes as compared to interface complexes (Figure 7.16a) correlates well with ( -potential values of oil droplets in mixed and bilayer emulsions (Table 7.3). 

It seems that there is probably greater availability of positively charged residues on the adsorbed protein for electrostatic interaction with sulfate groups of the anionic polysaccharide. This could lead to a greater extent of neutralization of dextran sulfate as a result of complex formation, and consequently to a lower thermodynamic affinity of the complexes for the aqueous medium and a lower value of the ( -potential for emulsion droplets in bilayer emulsions. 

Therefore, two contributory factors may provide an explanation for more effective electrostatic / steric stabilization of the so-called mixed emulsions in comparison with the sequentially assembled biopolymer interfaces of the bilayer emulsions firstly, a greater hydrophilicity of the adsorbed protein-polysaccharide complexes, caused by the larger net negative charge, and, secondly, a more bulky architecture of the normal complexes as compared to the interface complexes. 

Table 7.3 Relationship between molecular parameters (A2, p) of sodium caseinate (0.5 wt%) + dextran sulfate complexes at pH = 6.0 formed in the bulk and at the interface of a protein foam, and the corresponding properties (J43, Q of the bilayer and mixed emulsions (20 vol% oil, 0.5 wt% sodium caseinate) containing 0.1 or 1.0 wt% dextran sulfate (Jourdain et aL, 2008 Semenova et al., 2009).

It was reported by Jourdain et al. (2008) that it was impossible to make a proper emulsion with sodium caseinate alone at pH = 4 because of the very low protein solubility close to pI. Nevertheless, a fine stable emulsion (c/43 1.0 pm) could easily be prepared when dextran sulfate was present in a modest amount. In contrast, the monodisperse bilayer emulsion could not be prepared at low pH because the primary caseinate-stabilized emulsion was already either partially aggregated (pH = 2) or completely insoluble (pH = 4) under these conditions. 

As w ell as lateral heterogeneity in mixed protein layers, there is also the possibility of segregation of biopolymer components perpendicular to the interface, z.e., bilayer formation (Dickinson, 1995, 2009). Let us consider the case of an interface containing casein and whey protein in an emulsion system. The images in Figure 8.4 are confocal micrographs... 

---