Insoluble monolayer dynamics

Viscosity, defined as the resistance of a liquid to flow under an applied stress, is not only a property of bulk liquids but of interfacial systems as well. The viscosity of an insoluble monolayer in a fluid-like state may be measured quantitatively by the viscous traction method (Manheimer and Schechter, 1970), wave-damping (Langmuir and Schaefer, 1937), dynamic light scattering (Sauer et al, 1988) or surface canal viscometry (Harkins and Kirkwood, 1938 Washburn and Wakeham, 1938). Of these, the last is the most sensitive and experimentally feasible, and allows for the determination of Newtonian versus non-Newtonian shear flow. 

The dynamic surface tension of a monolayer may be defined as the response of a film in an initial state of static quasi-equilibrium to a sudden change in surface area. If the area of the film-covered interface is altered at a rapid rate, the monolayer may not readjust to its original conformation quickly enough to maintain the quasi-equilibrium surface pressure. It is for this reason that properly reported II/A isotherms for most monolayers are repeated at several compression/expansion rates. The reasons for this lag in equilibration time are complex combinations of shear and dilational viscosities, elasticity, and isothermal compressibility (Manheimer and Schechter, 1970 Margoni, 1871 Lucassen-Reynders et al., 1974). Furthermore, consideration of dynamic surface tension in insoluble monolayers assumes that the monolayer is indeed insoluble and stable throughout the perturbation if not, a myriad of contributions from monolayer collapse to monomer dissolution may complicate the situation further. Although theoretical models of dynamic surface tension effects have been presented, there have been very few attempts at experimental investigation of these time-dependent phenomena in spread monolayer films. 

The difference between the static or equilibrium and dynamic surface tension is often observed in the compression/expansion hysteresis present in most monolayer Yl/A isotherms (Fig. 8). In such cases, the compression isotherm is not coincident with the expansion one. For an insoluble monolayer, hysteresis may result from very rapid compression, collapse of the film to a surfactant bulk phase during compression, or compression of the film through a first or second order monolayer phase transition. In addition, any combination of these effects may be responsible for the observed hysteresis. Perhaps understandably, there has been no firm quantitative model for time-dependent relaxation effects in monolayers. However, if the basic monolayer properties such as ESP, stability limit, and composition are known, a qualitative description of the dynamic surface tension, or hysteresis, may be obtained. 

As discussed in the following sections, even soluble surfactants can form inhomogeneous adsorption layers. The origin of such inhomogeneities is not fully understood. It could be possible that it is a feature of mixed adsorption layers, formed by commercial surfactants of low purity or by mixtures of particular composition. Such domain-like structures could be formed due to remarkable differences in the interaction strength within the adsorption layer. The formation of 2- and 3-dimensionaI aggregates at interfaces is known from studies of insoluble monolayers (Mohwald et al. 1986 and Barraud et al. 1988, Vollhardt 1993). First dynamic models of aggregate formation in insoluble monolayers were theoretically described by Retter Vollhardt (1993) and Vollhardt et al. (1993). These simple and new, more... 

The formation of interfacial aggregates is known from studies of insoluble monolayers. For example, Mohwald et al. (1986) and Barraud et al. (1988) studied the formation of 30-aggregates in monolayers. The dynamics of aggregate formation in insoluble monolayers was studied by Vollhardt (1993) and 2- and 3-dimensional nucleations are discussed and described... 

As pointed out in the preceding chapter phase transitions in adsorption layers as observed in insoluble monolayers (Mbhwald 1993), can also be obtained for soluble surfactants too. Structures in mixed monolayers in particular were investigated by Henon Meunier (1993). These overlapping processes are now starting to be considered in new studies of equilibrium properties (cf Lin et al. 1991, Lunkenheimer Hirte 1992) or in dynamics of insoluble monolayers in terms of nucleation processes (Vollhardt et al. 1993). So far no attempts have been made to consider such transition or structure formation processes are considered in the dynamics of soluble adsorption layers. 

Very recently this technique was also applied for studies of monolayer isotherms of insoluble surfactants (Kwok et al. 1994) and the dynamic behaviour of insoluble monolayers. In Appendix 6C these investigations are discussed in more detail. 

Adsorbed gelatine molecules alone do not show a frequency dependence of surface elasticity (Fig. 6.19), which corresponds to a behaviour of an insoluble monolayers. The presence of surfactants changes the elastic and relaxation behaviour dramatically. With increasing SDS concentration the elasticity modulus (frequency independent plateau value of the elasticity) first increases and then decreases. The dynamic behaviour of the mixed adsorption layer changes from one completely formed by gelatine molecules to an adsorption layer completely controlled by surfactant molecules (Fig. 6.20). A similar behaviour can be observed for CTAB and a perfluorinated surfactant (Hempt et al. 1985). 

Dynamics in insoluble monolayers were studied for example by Dimitrov et al. (1978). They observed a Marangoni effect during a continuous compression and described it by a mechanism... 

In a very recent development the pendent drop technique was used to study the static and dynamic behaviour of insoluble monolayers. Kwok et al. (1994a) were the first who performed measurements of surface pressure - area isotherms of octadecanol monolayer by using a pendent water drop and ADSA (cf Section 5.4) as a film balance. Consistent results with classical Langmuir-Wilhelmy film balance measurements reveal ADSA as a powerful tool also for monolayers. 

The pendent drop technique has also been extended to studies of insoluble monolayers of phospholipids at the water/n-dodecane interface (Li et al. 1994b). In these experiments first a monolayer is produced on a water drop surface as described above. Then, the water drop is gently immersed into the second liquid, for example n-dodecane. Then the change of the drop size enables one to compress and expand the interfacial film. Again, the isotherm obtained with ADSA shows the same type of dynamic behaviour as measured with the classic Langmuir-Blodgett trough technique (Thoma Mohwald 1994). 

Capillary waves can be successfully applied to investigate dynamic properties and the structure of insoluble monolayers or adsorption layers on liquid subphases. The design of the experimental set-up and the experimental procedure were described in detail elsewhere [198, 199, 200]. The dispersion equation for capillary waves... 

The measurements of the propagation characteristics of the capillary wave, e.g., the propagation velocity and the damping coefficient, are effective for the study of the dynamic properties of materials existing on the gas-liquid interface. The theoretical studies for the insoluble monolayers have been performed by Dorrestein, Mayer and Eliassen", and Mann and Du, while those for the soluble monolayer have been performed by van den Tempel and van de Riet, Hansen and Mann, and Lucassen and Hansen. The former has developed their theories taking account of the surface rheologies, and the latter with the assumption that the rate-determining step of surfactant transfer between the surface and the bulk phase is the diffusion process. 

Another field of application of the drop and bubble shape techniques is in studying the penetration of soluble surface-active molecules into spread insoluble monolayers. In general, the obtaining of quantitative information on penetrated layers under dynamic and equilibrium conditions requires much attention... 

The interface is considered to be a macroscopically planer, dynamic fluid interface. Thus, the interface is regarded as a two-dimensional entity independent of the surrounding three-dimensional fluid. The interface is considered to correspond to a highly viscous insoluble monolayer and the interfacial stress acting within such a monolayer is sufficiently large compared to the bulk-fluid stress acting across the interface and in this way one can define an interfacial shear viscosity qj. 

It should be pointed out at this juncture that strict thermodynamics treatment of the film-covered surfaces is not possible [18]. The reason is difficulty in delineation of the system. The interface, typically of the order of a 1 -2 nm thick monolayer, contains a certain amount of bound water, which is in dynamic equilibrium with the bulk water in the subphase. In a strict thermodynamic treatment, such an interface must be accounted as an open system in equilibrium with the subphase components, principally water. On the other hand, a useful conceptual framework is to regard the interface as a 2-dimensional (2D) object such as a 2D gas or 2D solution [ 19,20]. Thus, the surface pressure 77 is treated as either a 2D gas pressure or a 2D osmotic pressure. With such a perspective, an analog of either p- V isotherm of a gas or the osmotic pressure-concentration isotherm, 77-c, of a solution is adopted. It is commonly referred to as the surface pressure-area isotherm, 77-A, where A is defined as an average area per molecule on the interface, under the provision that all molecules reside in the interface without desorption into the subphase or vaporization into the air. A more direct analog of 77- c of a bulk solution is 77 - r where r is the mass per unit area, hence is the reciprocal of A, the area per unit mass. The nature of the collapsed state depends on the solubility of the surfactant. For truly insoluble films, the film collapses by forming multilayers in the upper phase. A broad illustrative sketch of a 77-r plot is given in Fig. 1. 

Another dynamic factor affecting the rate of diffusion transfer, mentioned long ago by Gibbs [9], is the elasticity of the surfactant monolayers which decreases the capillary pressure in small bubbles during their compression and increases it in large bubbles during their expansion. This effect is most pronounced in bubbles whose adsorption layers contain insoluble surfactants. Analysis of the influence of this factor on diffusion transfer has been reported in [486], However, no experimental verification has been performed so far. 

A dynamic technique is described for obtaining surface elasticity (e0) vs. surface pressure (tt) curves which can be transformed into accurate tt—A curves for soluble monolayers. Small amplitude periodic area variations are used with a sufficiently high frequency to make monolayers effectively insoluble in the time of the experiment even though they behave as soluble in equilibrium measurements. plots are given for some nonionic surfactants. Straight line portions in these plots illustrate that surface interactions are too complex to be described by a Frumkin isotherm. In the limit of very low surface pressures there is no trace of an ideal gaseous region. Some examples show the implications of particular e0—rr curves for equilibrium and dynamic surface behavior. 

The equilibrium and dynamic behaviour of mixed monolayers of soluble and insoluble amphiphiles at fluid/liquid interfaces plays an important role in various technological and biological processes, which was studied in numerous publications [139-157], However, even for very simple systems, say, gaseous mixed monolayers, the thermodynamic analysis is not trivial. For more complicated systems (the formation of two-dimensional domains) such an analysis is very cumbersome due to mathematical difficulties. 

As mentioned in the Sec. 1, an important thermo-dynamic parameter of a surfactant adsorption monolayer is its Gibbs (surface) elasticity. The physical concept of surface elasticity is the most transparent for monolayers of insoluble surfactants, for which it was initially introduced by Gibbs (18, 19). The increments A a and AT in the definition of Gibbs elasticity ... 

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