Monolayers, insoluble states

The terminology of L-B films originates from the names of two scientists who invented the technique of film preparation, which transfers the monolayer or multilayers from the water-air interface onto a solid substrate. The key of the L-B technique is to use the amphiphih molecule insoluble in water, with one end hydrophilic and the other hydrophobic. When a drop of a dilute solution containing the amphiphilic molecules is spread on the water-air interface, the hydrophilic end of the amphiphile is preferentially immersed in the water and the hydrophobic end remains in the air. After the evaporation of solvent, the solution leaves a monolayer of amphiphilic molecules in the form of two-dimensional gas due to relatively large spacing between the molecules (see Fig. 15 (a)). At this stage, a barrier moves and compresses the molecules on the water-air interface, and as a result the intermolecular distance decreases and the surface pressure increases. As the compression from the barrier proceeds, two successive phase transitions of the monolayer can be observed. First a transition from the gas" to the liquid state. 

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 same way with PPV LB films was not applicable to MOPPV, because its polyion complex was unstable in solid state. The elimination reaction of a sulfbnium leaving group in the polyion complex rapidly progressed in solid state even at room temperature and the complex consequently became insoluble in the conventional organic solvents. Then, there is no way to form the polyion complex monolayer at the air/water interface. 

Next let us consider some of the physical properties of the spread monolayer we have described. Equation (1) states that the surface tension of the covered surface will be less than that of pure water. It is quite clear, however, that the magnitude of 7 must depend on both the amount of material adsorbed and the area over which it is distributed. The spreading technique already described enables us to control the quantity of solute added, but so far we have been vague about the area over which it spreads. Fortunately, once the material is deposited on the surface, it stays there —it has been specified as insoluble and nonvolatile for precisely this reason. This means that some sort of barrier resting on the surface of the water may be used to corral the adsorbed molecules. Furthermore, moving such a barrier permits the area accessible to the surface film to be varied systematically. In the laboratory this adjustment of area is quite easy to do in principle. As we see below, the actual experiments must be performed with great care to prevent contamination. 

It is not difficult to propose and develop a model for the gaseous state of insoluble monolayers. The arguments parallel those developed in kinetic molecular theory for three-dimensional gases and lead to equally appealing results. The problem, however, is that many assumptions of the model are far less plausible for monolayers than for bulk gases. To see this, a brief review of the derivation seems necessary. 

If the area of an insoluble monolayer is isothermally reduced still further, the compressibility eventually becomes very low. Because of the low compressibility, the states observed at these low values of a are called condensed states. In general, the isotherm is essentially linear, although it may display a well-defined change in slope as tt is increased, as shown in Figure 7.6. As menlioned above, the (relatively) more expanded of these two linear portions is the liquid-condensed state LC, and the less expanded is the solid state S. It is clear from the low compressibility of these states that both the LC and S states are held together by strong intermolecular forces so as to be relatively independent of the film pressure. 

The principal requirements for an ideal gaseous film are that the constituent molecules must be of negligible size with no lateral adhesion between them. Such a film would obey an ideal two-dimensional gas equation, ttA kT, i.e. the it-A curve would be a rectangular hyperbola. This ideal state of affairs is, of course, unrealisable but is approximated to by a number of insoluble films, especially at high areas and low surface pressures. Monolayers of soluble material are normally gaseous. If a surfactant solution is sufficiently dilute to allow solute-solute interactions at the surface to be neglected, the lowering of surface tension will be approximately linear with concentration - i.e. 

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. 

Ruckenstein, E. Li, B. Surface equation of state for insoluble surfactant monolayers at the air/water interface. J. Phys. Chem. 1998,102, 981. 

Ruckenstein," E. Li, B. A surface equation of state based on clustering of surfactant molecules of insoluble monolayers. Langmuir 1995,11, 3510. 

Fainerman, V.B. Vollhardt, D. Melzer, V. Equation of state for insoluble monolayers of aggregating amphiphilic molecules. J. Phys." Chem. 1996, 100, 15478. 

As far as the spread molecules are concerned, the system is closed in the thermo-d5mamic sense, (even if colloquially these substances are referred to as surfactants ). Upon compression or expansion they cannot leave or enter the monolayer because they are insoluble in the liquid (although we continue calling it solvent or water ). This layer is, via a barrier, separated from a surface that does not contain surfactants. However, water molecules and molecules dissolved in it, including electrolytes, can pass underneath the barrier, so for these components the system is open. The ensuing stationary state is a typical example of a membrane equilibrium, that is an equilibrium between two phases when (at least) one of the components is present in one of the phases only (sec. 1.2.12). 

The strongly amphipathic nature of proteins, resulting from their mixture of polar and nonpolar side chains, causes them to be concentrated at interfaces. As a result of their great stability in the adsorbed state, it is possible to study them at fluid interfaces by the classical techniques of insoluble monolayers. A review of early work along these lines in this series (Bull, 1947) serves as an excellent introduction to the subject. The effect of adsorption on the biological activity of proteins was treated by Rothen (1947) in the same volume. Further... 

The derivative d In c ldT is calculated for each adsorption isotherm, and then the integration in Equation 5.5 is carried out analytically. The obtained expressions for J are listed in Table 5.2. Each surface tension isotherm, oCEi), has the meaning of a two-dimensional equation of state of the adsorption monolayer, which can be applied to both soluble and insoluble surfactants. ... 

Davies (16) applied both equations to a wide collection of data at fairly low A for both soluble and insoluble ionized species and achieved only limited agreement between theory and experiment for some mono-layers. Nevertheless, the Davies term is the basis of nearly every subsequent discussion on the isotherms of ionized monolayers. We discuss elsewhere (25, 26) the validity of Equations 7 and 8 for intermediate and high surface charge densities as well as other proposed equations of state (14, 27, 28, 29, 30, 31, 32). In this paper we establish whether these two equations are suitable limiting forms at high A where many of the assumptions used in their derivation should be more valid. In particular we are interested in the limit of UA at zero n for both interfaces. 

As we have already seen, the state of soluble as well as insoluble monolayers can deviate from a equilibrium state defined at constant temperature, pressure, bulk and surface concentrations. A deviation from the equilibrium state of the corresponding adsorption layer can be triggered by vertical and lateral concentration gradients due to adsorption/desorption processes or by hydrodynamic or aerodynamic shear stresses, as shown in Fig. 3.1. 

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... 

The integration for constant p2 between the limits corresponding to two states of the monolayer (I and II, respectively) with different F values for the soluble or insoluble component, yields the difference of the product (AiFj) in these states [143,146]... 

As the second term in Eq. (2.153) is non-zero, the chemical potential of the insoluble component does not depend on the adsorption of the soluble component provided that both surface pressure and adsorption of the insoluble component are fixed. In turn, as the surface concentration of the insoluble component is fixed, the requirement for constant activity of this component implies the independence of this activity coefficient of adsorption of the soluble component. Clearly, this requirement is satisfied not only for the trivial case of an ideal monolayer, but also for non-ideal monolayers, provided that the activity cross-coefficients of the components (or intermolecular interaction parameters) vanish. For example, if the equation of state Eq. (2.35) is used for a non-ideal (with respect to the enthalpy) mixed two-component monolayer, it follows from Eq. (2.153) that Eqs. (2.151) and (2.152) are applicable when ai2 = 0. Clearly, the condition of Eq. (2.153) imposes certain restrictions to the applicability of Pethica s model. The generalised Pethica equation (2.151) was thermodynamically analysed in [64, 65]. Moreover, an attempt to verify Eq. (2.151) experimentally was undertaken in [65], which also confirms its validity for mixed monolayers comprised of two non-ionic surfactants, or for mixtures of non-ionic and ionic surfactant, or two ionic surfactants. 

The application of Butler s equation has been illustrated in form of the equation of state and adsorption isotherm for non-ideal monolayers (with respect to both enthalpy and entropy) comprised of an insoluble 1 and a soluble 2 component. In this case the equation of state is Eq. (2.31), but the adsorption isotherm (2.32) is true for the soluble component 2 only... 

The generalised Volmer equation (2.159) is the basis for the theoretic analysis of the behaviour of insoluble monolayers. The equation of state for insoluble monolayers with a bimodal distribution (large clusters and monomers or small aggregates) was derived in [25-27]... 

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