Foam bilayers experimental results

The foam bilayer is the main model system used to obtain experimental results for the stability of bilayers. The proof that the studied foam films are bilayers is based on the experimentally measured h(Cei) dependences and I"I(/i) isotherms. In both cases films with the same thickness are obtained, which corresponds to that of bilayers and does not change with further increase in Cei or IT (e.g. Figs. 3.44, 3.57, 3.62). This leads to the conclusion that the NB foam films do not contain a free aqueous core between its two monolayer of surfactant molecules. A similar conclusion is drawn from the investigatigations of NB foam films by infrared spectra [320,417] and by measuring longitudinal electric conductivity of CB and NB foam films [328,333,418]. 

To verify whether the rupture of a-particle-irradiated foam bilayers can also occur by the hole mechanism the experimental TjC) dependence for NB foam films of NaDoS has been analysed using Eq. (3.132) [415]. The circles in Fig. 3.88 show the experimental data, and the solid line is drawn according to Eq. (3.132) as a result of the best fit in the range C = 3 to 6T0 4 mol dm 3. The % value of 2-10 11 J m 1 obtained is in good agreement with the % value for spontaneous rupture. The size of the smallest irradiation-active hole is i = 3, and nucleus hole consists of = 12 to 83 NaDoS vacancies. The abrupt rise of the slope in the... 

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. 

The good agreement between theoretical and experimental results of hole-mediated permeability of foam bilayers to air allows the determination of the permeability coefficient of bilayers of both ionics and nonionics. Though the mechanism of hole-mediated permeation of foam bilayers is not entirely clarified, its efficiency for lower surfactants concentrations in a wide range of temperatures is firmly established. This finding is in strong support of the basic idea of the existence of randomly nucleated microholes in the amphiphile bilayer. 

The temperature dependence of the DMPC critical concentration for formation of foam bilayer is shown in Fig. 3.95 (co-ordinates In Cc vs. 1 IT) [384,386,439]. Experimental results are denoted by circles and the solid lines are the least-squares fits. A clearly pronounced break in the In Cc vs. 1/7 dependence corresponds to the temperature of the thickness transition at 23°C. This should be contrasted with the invariable slope of the dependence in the range of the low-temperature thickness transition in foam bilayers. It is clear that the temperature dependence of Cc is very sensitive to the occurrence of some phase transitions in foam bilayers and may be used for their detection. 

The determination of the binding energy of DMPC molecule in the foam bilayer was carried out using the experimental results for the temperature dependence of the critical concentration for formation of foam bilayer (Fig. 3.95) and the theory of Kashchiev-Exerowa (see Section 3.4.4.2). The concentrations Cc and Ce (Eq. (3.129)) are specific constants of each system which determine the ability of a foam bilayer to exist in a metastable state within the concentration range Cc< C < Ce. When C >Ce the foam bilayer is thermodynamically stable (there is no driving force for the whole nucleation process in the foam bilayer). It follows from the theory that the critical concentration of amphiphile molecules in the solution equals the equilibrium one (Cc = Ce) in the case of a missing metastable region when only thermodynamically stable foam bilayers are formed. As mentioned above, the DMPC foam... 

The values of Q obtained from the best fit of Eq. (3.115) (the solid lines in Fig. 3.95) to the experimental data (the circles in Fig. 3.95) assuming Cc = Ce are (1.93 0.04)-10 9 J for temperatures lower than 23°C and (8.03 0.19)-10 2° J for temperatures higher than 23°C. The possible error arising from the assumption that Cc = Ce is analysed elsewhere [384] it can raise the Q value by up to 20%. The good fit of the experimental results to the theoretical dependence and the high stability of the foam bilayers with respect to their rupture even under a-particle irradiation, show that in the case of DMPC bilayers the assumption Cc = Ce is probably accurate. 

Phase diagrams of DMPC foam bilayers. The analysis of the experimental results for the foam bilayer thickness and the critical concentration for formation of the foam bilayer... 

These results are a reliable basis for further improvement of the theoretical model here presented with regard to new experimental research in order to relate better the macroscopic and microscopic characteristics of foam bilayers. 

Fig. 3.85 depicts the x(C) dependence for bilayer foam films of NaDoS [419,420], each point being the result of about 300 measurements. It can be seen that in a rather narrow C range, x changes from fractions of a second to practically infinite times. Since after ca. 15 s the process of rupture reaches a steady-state, it is appropriate to analyse the experimentally obtained x(C) dependence with the aid of Eq. (3.124). The solid line in Fig. 3.85 shows the best-fit theoretical curve drawn according to Eq. (3.124) with three independent parameters A, B and Ce the most probable values of which are give in Table 3.12. 

Muller et. al. [421] have studied the behaviour of emulsion Newton bilayer films and compared it to that of foam films. They determined the dependence of the lifetime on surfactant concentration of emulsion films stabilised with 22-oxythylated dodecyl alcohol (see Section 3.4.1). Experimental data for both kinds of films proved to be in conformity with the theory of bilayer stability (see Section 3.4). The values of the equilibrium concentrations Ce calculated for emulsion films were higher (Ce 10 3 mol dm 3) than those for foam films (Ce 3 1 O 5 mol dm 3). It is worth noting that Ce value of foam films from certain surfactants is lower than CMC (C < CMC) while for emulsion films - Ce > CMC. That is why it is impossible to obtain thermodynamically stable films in the latter case. This result is of particular importance for the estimation of stability of aqueous emulsions with bilayer films between the drops of the organic liquid. 

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