Insoluble monolayer relaxation

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. 

There may be diverse causes for the dissipative part. In an ideally insoluble monolayer dissipation is exclusively caused by relaxation processes in the mono-layer, such as breaking and reforming bonds between the adsorbed molecules, phase transitions and reconformation of the molecules themselves. See sec. 3.6h. In Gibbs monolayers there is an extra dissipative term due to exchange processes between the bulk and the monolayer. For the case of continuous equilibrium between the monolayer and the bulk dy = 0 and, hence the surface rheological... 

For insoluble monolayers of cholesterol and dipalmitoyl choline the relaxation at pressures below the collapse point were studied by Joos et al. ), using oscillatory and stress relaxation techniques. They found experimental evidence (and presented theory) for a double-exponential decay, representing two consecutive processes. The longer r s are 0(10 s) and 0(10 s) for cholesterol and the lipid, respectively, so these relaxations are relatively slow and may therefore be overlooked, especicJly in automated apparatus. No molecular mechanism was proposed the two r s did not exhibit a clear relationship with the surface pressure at which the experiments were carried out. 

At TT > TTg the relaxation phenomena for insoluble monolayers are caused by the transformation of a homogeneous monolayer phase into a heterogeneous monolayer-collapse phase system. However, some differences exist between saturated-LMWE and unsaturated-LMWE monolayers (Eigure 14.6b). Relaxation phenomena in saturated-LMWE monolayer are controlled predominantly by the collapse mechanism because the surface pressure relaxes to TTg. Eor these systems the monolayer collapses by nucleation and growth of critical nuclei. Unsaturated-LMWE monolayers behave differently to saturated-LMWE monolayers. As the surface pressure relaxes from the collapse value, which is close to TTg, towards values lower than TTg at longer times, the collapse competes with a desorption mechanism (Patino and Nino, 1999). 

Insoluble monolayers on an aqueous substrate have been investigated by means of the capillary wave method for many years. Lucassen and Hansen (1966) in their pioneering work neglected the surface viscosity and considered only pure elastic films. Subsequent studies showed that the surface elasticity of real surface films is a complex quantity, and both the equilibrium surface properties and the kinetic coefficients of relaxation processes in the films influence the characteristics of surface waves. However, it has been discovered recently that the real situation is even more complicated and the macroscopic structure of surface films influences the dependency of the damping coefficient of capillary waves on the area per molecule (Miyano and Tamada 1992, 1993, Noskov and Zubkova 1995, Noskov et al. 1997, Chou and Nelson 1994, Chou et al. 1995, Noskov 1991, 1998, Huhnerfuss et al. this issue). Some peculiarities of the experimental data can be explained, if one takes into account the capillary wave scattering by two-dimensional particles (Noskov et al. 1997). 

Another familiar experiment is the compression or dilation of insoluble monolayers on a Langmuir trough. By this operation the film passes different states, such as mesophases. The transition of the film fi-om one state into another needs time, which is a characteristic parameter for such processes starting from a non-equilibrium state and directed to the reestablishment of equilibrium. The principle of "relaxation" coordinates for any process was first introduced by Maxwell (1868) in his work on relaxations of tensions. After Maxwell, a liquid body under deformation can be described by the shear stress... 

A typical effect related to surface relaxations is obtained in measurements of ti-A isotherms of insoluble monolayers. In most of the measurements with spread amphiphiles there are differences between the curve for compression and expansion of the surface films. Usually this characteristic behaviour is described as hysteresis. One experimental example of a spread dipalmitoyl lecithin is shown in Figs 3.12. This phenomenon corresponds to one or more of these surface relaxations. 

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). 

Figure 3.48 gives an illustration for proteins. Protein monolayers will be mostly deferred to Volume V, but they are virtually insoluble, i.e. behave as Langmuir monolayers. Hence, observed relaxations must be attributed to proeesses inside the layer. The behaviour of the two proteins is similar. The surface concentration is the primary determining variable even pH changes affect the results only in as far as different pH values lead to different F s. In the plateau region, (F> 1.6 mg m" ) the modulus becomes independent of the surface concentration. Here, relaxations... 

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