Emulsion bilayers rupture

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-... 

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). 

Generalized emulsion charaeterization, i.e., measurement of droplet size distribution, eleetrokinetie potentials, Hamaker constant, etc., is not always sufficient. Thermodynamic stability with respect to bilayer rupture eannot be quantified with such a characterization proeedure alone. Consequently, measurement of the eoaleseenee time is... 

As already noted, the NB foam films, the bilayer emulsion films and the BLMs, are amphiphile bilayers, and their stability in respect to rupture and their permeability can be considered from a unified point of view. 

Dependence of the lifetime of foam bilayers on the concentration of dissolved surfactant. The stability of foam, emulsion and membrane bilayers can be characterised by their mean lifetime r which is the time elapsing form the moment of formation of a bilayer with a given radius until the moment of its rupture. Obviously, this is a kinetic characteristic of the bilayer stability and can only be applied to thermodynamically metastable bilayers. 

Similar steep x(C) dependences, which are an immediate indication for nucleation-mediated rupture, have been obtained for all investigated cases of foam, emulsion and membrane bilayers [300, 421], In some cases, for example, for bilayers of phospholipids, the rtO dependence is so steep that it is hard to obtain short-lived bilayers. 

In some cases the interdrop film becomes very thin, i.e., only a few micellar or molecular sizes across. It decreases not continuously but stepwise, layer by layer, and could end up in a surfactant bilayer with no solvent content (sometimes referred to as black film because of its color). Such extremely thin films could exhibit a high resistance to rupture as it occurs in so-called foam emulsions (2.3.24). 

The mechanism oifilm rupture by nucleation of pores has been proposed by Deqaguin and Gutop (99) to explain the breaking of very fliin films, built up from two attached monolayers of amphiphilic molecules. Such are the secondary foam and emulsion films and the bilayer lipid membranes. This mechanism was further developed by Deijaguin and Prokhorov (3, 100, 101), Kashchiev and Exerowa(102—104), Chizmadzhev and coworkers (105— 107), and Kabalnov and Wennerstrom (108). The formation... 

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