Films stabilisers

A PP film stabilised with AOl and PI (structure shown in Fig. 6) in the ratio 1 2 that has been initiated using UV light according to a previously described procedure [79] can be seen in Fig. 7. Only the area between the dotted lines has been irradiated, the rest of the film was covered with aluminium foil during initiation. 

Initiation of a polypropylene sample was also done by placing a piece of copper in contact with a PP film stabilised with the antioxidant system in... 

The FRAP method has been applied to the measurements of molecular lateral diffusion of molecules adsorbed at the interface of equilibrium common thin foam films and of black foam films [39-43], Initially Clark et al. reported FRAP measurement of surface diffusion of the fluorescence probe 5-N(octadecanoyl)aminofluorescein incorporated into foam films stabilised with NaDoS [39]. Then followed the measurements of protein-stabilised foam films where the protein was covalently labelled with fluorescein [40,41], Studies of FRAP measurements of surface lateral diffusion in equilibrium phospholipid common thin foam films and black foam films were also reported [42,43]. 

Thin liquid films (especially foam films) stabilised with phospholipids, proteins, etc., prove to be very suitable in the study of surface forces, since they could model the interacting biological membranes in aqueous medium. 

The pH at which the film always ruptured is 3.6. Therefore, this pH value can be denoted as the pH for the investigated microscopic films stabilised with lyso PC. At this point the surface charge and potential practically disappear and no further screening effect occurs. 

The result of the homogeneous layer treatment seems physically sound under these experimental conditions electrostatic repulsion is negligible and the contribution of van-der-Waals attraction is rather small [127]. Therefore, film stabilisation may be attributed to steric interactions between the hydrophilic PEO chains protruding into the solution. 

The values of the Hamaker s constant A are of a reasonable order of magnitude (Table 3.7). In Fig. 3.38 are shown plots of Eq. (3.86) calculated with the value for water (3.7-10 13 erg [246]) and for polymers (8-10" 3 erg [246]). In view of the experimental scatter the general agreement is acceptable. Note that such behaviour has not been detected in SFA experiments [242]. Electrostatic repulsion cannot be responsible for film stabilisation and by inference steric forces are operative [127], i.e. the conclusion reached within the single layer treatment remains unaltered. Though seemingly firm, this qualitative hypothesis evades quantitative treatment. 

In Section 3.3.1 it was shown that the state of thin foam films is described by the Fl(/ ) isotherm of disjoining pressure. For relatively thick films, stabilised by surfactants, this isotherm is consistent with the DLVO-theory. However, black foam films exhibit a diversion from the DLVO-theory which is expressed in the specific course of the disjoining pressure isotherm. 

DMPC films. Direct measurement of interaction forces in films stabilised by neutral phospholipids has been first done with microscopic foam films from suspensions of small unilamellar DMPC vesicles [286]. Foam films formation from phospholipids is a difficult task since they are insoluble in water. Sonicated dispersions of insoluble phospholipids is an option, employed by Yamanaka et al. [287],... 

The course of h(Cci) dependence indicating the decrease in equilibrium thickness up to the transition to NBF as well as the course of n(Ii) isotherm with a distinct barrier transition, reveal the electrostatic character of the forces acting in the film. Thus, double electric layer can be estimated, knowing that n / = pc+T vw The capillary pressure pa was measured experimentally while Tlvw was calculated from Eq. (3.89). The potential was determined within the electrolyte concentration range of 5-10 4 to 10 3 mol dm 3 (Fig. 3.48) in which the films were relatively thick, yielding a value of (po = 36 3 mV. In this respect films stabilised with the zwitterionic lipid DMPC are very similar to those stabilised with non-ionic surfactants [e.g. 100,186,189] (see also Section 3.4.1.1). The low ( -potential leads to the low barrier in the FI(Ii) isotherm which can easily be overcome at relatively low electrolyte concentrations and low pressure values. 

Lyso PC and Lyso PE films. The knowledge in the field of interaction forces in foam films stabilised with soluble zwitterionic phospholipids lyso PC (lysophosphatidylcholine) and lyso PE (lysophosphatidylethanolamine) has improved due to the studies of microscopic foam films [e.g. 191,192,292], The main dependences studied were of film thickness vs. electrolyte concentration and disjoining pressure vs. thickness, under specially chosen conditions in the presence of Na+ and Ca2+. The /i(pH) dependence proved to be very informative for understanding the charge origin in films from the neutral phosopholipids lyso PC and lyso PE (see Section 3.3.2). 

The mechanism of Ca2+ binding is not clear yet. However, increase in repulsive double layer forces between neutral diacylphosphatidylcholine bilayer in aqueous media in the presence of divalent ions has been identified by other methods as well [293-296]. These systems differ from the foam- film model by virtue of their interface ordered lipid phase/water in place of the air/water interface of foam films. Nevertheless, the CaCb concentration where the transition from NBF to silver films is observed in experiments with foam films is very close to the concentrations where increase in the distance between the bilayers was found [293,294,296]. Results with microscopic films are also in good agreement with the established increase in the free energy of formation of macroscopic films stabilised with lysolecithin in the presence of CaCl2 [287]. 

The observed change at 2-1 O 2 mol dm 3 CaCl2 is accompanied by the appearance of black spots, leading to the formation of black films that decrease in thickness with the increase in Cei. This shows that CBF are obtained. Thus, a transition from silver to CBF is established. This process is usually observed in films stabilised with ionic surfactants [171]. Here it is possible to interpret the results as additional evidence for the increase in the diffuse electric layer potential as a consequence of Ca2+ ion binding. The next established transition is evidently from CBF to NBF. It occurs at a Cei close to the critical electrolyte concentration of transition to NBF observed for a typical ionic surfactant (NaDoS 1-1 valent electrolyte) [251] (see Section 3.4.1.3). The fact that NBF thickness at high Cei equals that of the NBF obtained... 

These considerations indicate that above a definite CaCl2 concentration in the solution, the lyso PC films become very similar to films stabilised with ionic surfactants. In this context it is of interest to examine to n(/t) isotherms shown in Fig. 3.51. It can be seen that they follow a course with a barrier transition to NBF. 

Fig. 3.53. Aqueous core thickness d2 of thin liquid films stabilised with lyso PC as a function of CaCl2...

Fig. 3.54. Diffuse electric layer potential (po and surface charge density Oo of thin liquid films stabilised...

The experiments indicated that foam films rupture at pressures lower than nmax is not due to occasional reasons. Critical pressure pcr was observed with different types of films (common foam, CBF and NBF) stabilised with various kinds of surfactants [171,303]. Similar effect has been observed by Black and Herrington [261] who studied films stabilised with three anion-active surfactants. However, details on the critical pressure of film rupture will not be discussed here, since a satisfactory theoretical explanation of this effect has not been proposed so far. There are some hypothesis on the matter. Nevertheless, this parameter has been successfully employed in clarifying the role of foam films in foam stability (see Chapter 7). No doubt, this parameter provides information about the stability of the different types of foam films and is awaiting its qualitative interpretation. 

One of the parameters characterising quantitatively the process of film rupture is the lifetime ta of a black film under a-particle irradiation. Fig. 3.65 shows the ra(Cei) dependences for films stabilised with various kinds of surfactants (non-ionic, anionic and cationic) subjected to a-particle irradiation (Am241) [97,324,325]. At fist the lifetime decreases, reaching a minimum value for all films studied. Then, with the increase in electrolyte concentration in the initial solution, the lifetime starts to increase (for non-ionic and anionic surfactants) or remains constant (for films from the cationic surfactant). There is either a flexion or a plateau in the curves which correspond to Ceicr for the CBF/NBF transition. The sharp downward slope of the curve for all studied films reflects the decrease in thickness of CBF with the increase in electrolyte concentration. However, the right-hand side... 

In the case of pHcr, the barrier in the FU/i) isotherm is lowered, due to the reduced OH concentration which provokes the transition to a NBF at very low electrolyte concentrations. The independence of the thickness of the lyso PC film in the presence of CaCl2 on pH at the same Cs conforms with the above mechanism of surface charge formation and long-range interactions in films stabilised with neutral phospholipids. 

The data given above definitely prove also the metastable character of the CBF/NBF transition and provide the experimental base for further quantitative explanation of this process. The transition to NBF can be regarded as a transition to bi-dimensional state. For instance, the process of grey film/NBF transition in films stabilised by non-ionic surfactants and CBF/NBF transition in films stabilised by ionic surfactants can be presented as a nucleation process of a new phase. So far this approach is applied only to analyse the stability of NBF (see Section 3.4.4). 

Several studies show that the values of the lateral diffusion coefficient (D, cm2 s 1) in foam films stabilised by phospholipid(s) depend on two main groups of factors. The first is related to the type of the film, its thickness and radius, and the lipid composition of film monolayers. The second is related to the dependence of the surface diffusion within the limits of given film type and composition on the molecular characteristics of the lipid(s) building the film (molecular charge, length, lipid phase, etc.). 

Fig. 3.112. Temperature dependence of diffusion coefficient D of surface-adsorbed 5-N-(octadecanoyl)aminofluorescein in black foam films stabilised by DLPE ( ) DMPE (A) DPPE ( ) and DOPE (+) [492],...

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. 

Study of microscopic O/W films has been performed by Velev et. al. [514-516] and a new phenomenon spontaneous cyclic formation of a dimple (thicker lens-like formations) in O/W emulsion films stabilised by a non-ionic surfactant (Tween 20) was observed. This phenomenon was described as a diffusion dimple formation in contrast to the dimple created as a result of hydrodynamic resistance to thinning in liquid films [55,56,63,237,517], The dimple shifted from the centre to the periphery and periodically regenerated. Photos of the different periods of a dimple growth are shown in Fig. 3.115 and the process is schematically presented in Fig. 3.116. 

An important characteristics of this type of films (in contrast to the symmetric films foam and emulsion) is their ability to form thermodynamically stable films in the absence of a surfactant on the account of the molecular attraction forces [e.g. 522,523]. In the presence of a surfactant, however, it is possible to obtain stable asymmetric films stabilised with... 

For aqueous films on organic substrate the condition s < 0 is most often realised. Hence, these films would be unstable over a large range of thicknesses (except diluted adsorption layers) but a possibility to form thicker metastable (II >0) films stabilised by surfactants cannot be neglected (analogous to foam and emulsion films). 

Several authors [25,29-32] believe that film elasticity is an important factor for film and foam stability. However, an immediate relation between foam stability and film elasticity has not been established but if it exists, it should not be directly proportional. Even at large values of elasticity modulus the insoluble surfactant monolayer are poor film stabilisers. The modulus of elasticity of such films does not depend on film thickness and, therefore, it can be... 

As shown in Section 3.4. the rate of drainage of liquid films depends on surface rheological properties. This is valid for foam and emulsion films. However, the drainage of the films between fluid particles is only the first common step in stability, the DLVO theory controls films and films stabilised in the second minimum of the it/A- isotherm. 

Figure 12.9 (a), 1 film stabilised by a non-ionic surfactant (b) free energy of interaction as a function of film thickness for a stern ally stabilised film. 

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