Foam bilayers rupture

The molecular model of amphiphile bilayers with holes in them is a good basis also for the description of the rupture of NB foam films by a-particle irradiation [331,415,416]. The mean lifetime ra of the foam bilayer shortens dramatically under irradiation, and probability considerations [416] show that only a small area Sh S of the bilayer is active for the passage of the a-particles. Assuming that 5 is the overall area of those holes in the foam bilayer which are large enough to be irradiation-active makes it possible to represent Sh as... 

Thus, the foam bilayer can indeed be regarded as a system of two amphiphile monolayers adsorbed onto each other. In view of the strong effect of the concentration C of surfactant in the solution on the bilayer lifetime T, it is very convenient to use the t(C) dependence for experimental verification of the theory [399,402,403] of hole-mediated rupture of bilayers. 

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. 

The z(C) dependence has been investigated with the help of microscopic foam bilayers of both ionics and nonionics [419,420]. Due to the fluctuation character of the film rupture, the film lifetime is a random parameter. Experimentally, the film mean lifetime r has been determined by averaging from a great number of measurements. Because of the assumption that the monomer and the total surfactant concentrations are practically equal, in all t(C) dependences given below, C refers to the total concentration. Using Eq. (3.120) to analyse the experimentally obtained time dependence of the probability P(t) of film rupture it was found... 

Because of the strong t(C) dependence at and below a given surfactant concentration Cc the foam bilayer cannot be observed experimentally, as it ruptures instantaneously. Hence, Cc is the experimental limit of bilayer metastability and is determined by the resolution of the measuring equipment. For NaDoS foam bilayers Cc - 1.2-10 4 mol dm 3, a value which coincides with that of the lowest bulk surfactant concentration at which maximum packing of the adsorption monolayer at the solution/air interface is attained [332,366]. 

Rupture of foam bilayers by a-particle irradiation. By means of a-particle irradiation a controlled external influence can be exerted on the rupture of black foam films [331,415,416]. The measuring cell in which the studied microscopic foam bilayer is formed is shown in Fig. 2.10. The a-source is placed at a distance of 3.5 cm away from the bilayer the Bragg distance at which the particle energy is almost constant. The statistical character of bilayer rupture is evidenced in experiments at different irradiation rates [416]. The bilayer mean lifetime ra is therefore an appropriate parameter for assessing the destructive action of the a-particles. 

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 conclusion is, therefore, that both spontaneous and forced rupture of foam bilayer by a-particles are mediated by microscopic holes of surfactant vacancies and can be described from a unified point of view with the aid of the nucleation theory of bilayer rupture [399,402,403]. However, studying the effect of a-particle irradiation of the bilayer lifetime is an independent way of proving the applicability of the hole mechanism of bilayer rupture. 

Effect of temperature on the stability of foam bilayers. The effect of temperature on the rupture of foam bilayers has also been studied [414] with the help of microscopic NaDoS NB films with a radius of 250 pm. The dependence of the bilayer mean lifetime ton the surfactant concentration C in the presence of 0.5 mol dm 3 electrolyte (NaCI) at 10, 22 and 30°C has been obtained, the temperature being kept constant within 0.05°C. As in the above mentioned case, the NB foam films formed via black spots and the measurements were carried out after a sufficiently long time in order to allow equilibration of the system. At each of the NaDoS concentrations used and at the corresponding temperature, x was determined statistically and the comparison of the experimental with the theoretical x Q dependences was done by means of non-linear optimisation of the constants A, B and Ce. 

The analysis of the effect of temperature on the mean lifetime of foam bilayers provides further evidence for the applicability of the theory of bilayer rupture by hole nucleation [399,402,403]. The experiments show that the foam bilayers become less stable with increasing temperature, due both to the Boltzmann-type thermal activation of the hole nucleation and to the decreasing work of a nucleus hole formation. 

The adsorption isotherms of NaDoS vacancies calculated in [424], from Eq. (3.13) with the aid of the values of Q and Co given previously are shown in Fig. 3.90 for 10°C (curve 1), 22°C (curve 2) and 30°C (curve 3). The equilibrium NaDoS concentrations Ce above which NaDoS foam bilayers of infinitely long lifetime are to be obtained are marked by arrows. In practice, this cannot be achieved because CMC < Ce. The hatched area shows the investigated concentration interval in which the gas of NaDoS vacancies in the foam bilayer undergoes a 2D first-order phase transition from a dilute phase (existing metastable bilayer) into a condensed phase (ruptured bilayer). 

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. 

Temperature dependence of the critical concentration Ce of a foam bilayer formation. The Cc concentration (see Eq. (3.129)) of formation of DMPC foam bilayer was determined on the basis of observations of the final state which the foam film reached during its drainage (see Section 3.2), i.e. either rupture at a definite critical thickness without formation of black spots occurs, or formation of foam bilayer via black spots is observed. Rupture at critical thickness occurred at lower DMPC concentrations in the solution (C < Cc) and black spots were formed at higher concentrations (C > Cc). These black spots encountered the film turning it into a foam bilayer of constant radius. At each temperature a series of observations were carried out at various DMPC concentrations for the determination of Cc (the minimum DMPC concentration at which a foam bilayer is formed). This concentration is... 

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. 

Hole-nucleation rupture of foam bilayers was considered on the basis of formation of nucleus-holes from molecular vacancies existing in the film in Section 3.4.4. The experimentally determined parameters of film rupture along with the hole-nucleation theory of rupture of amphiphile bilayers of Kashchiev-Exerowa [300,301,354,402] made it possible to evaluate the coefficient of lateral diffusion of vacancies in foam bilayer. 

It is shown in Section 3.4.4. that microscopic foam bilayers (NBF) can be used to measure different parameters characterising their rupture. A time dependence J(t) expressed as a ratio of the number of films ruptured within the interval t + (/ + At) to the total number of films with lifetime longer than t, was derived to evaluate Dv. It is clearly seen in Fig. 3.114 that for all NaDoS films studied the J(t) dependence has a non-steady-state character. 

Using 6, = 21 O 4 (see Section 3.4.4) the lateral diffusion of NaDoS molecules in a bilayer foam film can be estimated Dv = 10 6 cm2 s 1. However, this value should be regarded only as an approximate one. Unfortunately, a comparison cannot be done since the conditions of NaDoS film formation of the FRAP technique (CBF) and by their rupture J(t) dependence (NBF) are different. The value of D for NaDoS molecules in the foam bilayer is by one to two... 

Fig. 11.4 shows separately curve 1 from Fig. 11.3 which is the dependence of W on the DPPC concentrations in the AF. The W(C) curves allow to determine the threshold concentration C i.e. the minimum phospholipid concentration at which there is a 100% probability of observation of black films (see Eq. (3.130)). At concentrations lower than C, NBFs are no more observed, since W sharply decreases to zero (films rupture). At concentrations higher than C, (W = 1), NBFs always form. Special studies with phospholipid analysis of amniotic fluid indicate that of all phospholipids in the AF, it is the DPPC that stabilises the foam bilayers. This analysis gives grounds to conclude that the concentration of each phospholipid (except DPPC) in the native AF is of an order lower than the corresponding... 

The systematic study of foam bilayers from phospholipids [28,38-40] reveals that they do not rupture spontaneously at any concentration allowing their formation. That is why in the case of phospholipid foam bilayer the dependence of their mean lifetime on the bulk amphiphile concentration cannot be measured in contrast to foam bilayer from common surfactants [41,42], This infinite stability of phospholipid foam bilayers is the cause for the steep W(d) and W(C) dependences. In the case of AF foam bilayers this high stability was confirmed by a very sensitive method [19,43] consisting of a-particle irradiation of foam bilayers. As discussed in Sections 2.1.6 and 3.4.2.2, the a-particle irradiation substantially shortens the mean lifetime of foam bilayers. The experiments showed that at all temperatures and dilutions studied (even at d,), the foam bilayers from AF did not rupture even at the highest intensity of irradiation applied, 700 (iCi. 

Study of processes leading to rupture of foam films can serve to establish the reasons for their stability. The nature of the unstable state of thin liquid films is a theoretical problem of major importance (it has been under discussion for the past half a century), since film instability causes the instability of some disperse systems. On the other hand, the rupture of unstable films can be used as a model in the study of various flotation processes. The unstable state of thin liquid films is a topic of contemporary interest and is often considered along with the processes of spreading of thin liquid films on a solid substrate (wetting films). Thermodynamic and kinetic mechanisms of instability should be clearly distinguished so that the reasons for instability of thin liquid films could be found. Instability of bilayer films requires a special treatment, presented in Section 3.4.4. 

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. 

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. 

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. 

Dependence of the probability of observing a bilayer in a foam film on the concentration of dissolved surfactant. Experimental verification of the theory [399,402,403] of hole-mediated rupture of bilayers has also been conducted [382] by analysing data for the W(C) dependence with the help of Eq. (3.128). Studying this dependence is possible and particularly convenient at lower C values when the bilayer mean lifetime t is comparable with tr (see Eq. (3.122)). A characteristic feature of W, according to Eq. (3.128), is its sensitivity to changes of C only in a very narrow range. 

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-... 

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). 

Fig. 3.114. Time dependence of rupture frequency rupture J(t) of bilayer foam films from NaDoS solution...

In order to determine the statistical distribution of amniotic fluid samples taken at different gestation weeks, two relations are studied rupture of foam films (VP = 0) and development of RDS, and formation of a bilayer foam film (W = 1) and normal respiratory status of neonates. These correlations allowed to develop a new diagnostic method for estimation of lung maturity [20]. The function of the threshold dilution of various amniotic fluid samples (corresponding to C,) on the gestation age and the clinical results (i.e. yes/no RDS in neonates) is given in Fig. 11.5. The respiratory status of the neonates is studied with the screening system of Masson et al. [26], modified by Hobel et al. [29]. 

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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