Soluble Monolayers

if we conduct experiments under pressure, part of the monolayer can redissolve in the underlying water. We can, however, monitor E (and II) to some extent by varying the concentration c of the surfactant in the water. 

FIGURE 8.4. Equilibrium curve relating the surface area E per polar head to the applied surface pressure IT. 

What is the relation between E and c We can find the answer by considering the surface/volume equilibrium in terms of chemical potentials. In the bulk liquid (and as long as the concentration is below the cmc), the chemical potential of the surfactant was described by equation (8.10) if the surfactant is of the ionic type. Here we will consider a surfactant that is neutral, giving us the opportunity to illustrate a different case. In this instance, the chemical potential of the surfactant dissolved in the volume of the liquid is 

This relation was derived early on by Gibbs. Gibbs was one of the founders of statistical physics he was also a great devotee of soap films From equation (8.19) we can obtain the chemical potential of the surfactant with the help of the curves describing H(E). 

FIGURE 8.5. Surface tension of a mono-layer as a function of the concentration c of the surfactant in solution. 

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Until now we have discussed only insoluble monolayers. Although their behavior is complex, they have the conceptual simplicity of being localized in the interface. It has been noted, however, that even in the case of insoluble monolayers, the substrate should not be overlooked. The importance of the adjoining bulk phases is thrust into even more prominent view when soluble monolayers are discussed. In this case the adsorbed material has appreciable solubility in one or both of the bulk phases that define the interface. 

Kinetics of Adsorption. Discussion. The interpretation of the results reproduced on the Figure 1 is attempted on the same lines as the desorption of slightly soluble monolayers (20). 

In the method developed by Exerowa, Cohen and Nikolova [144] the insoluble (or slightly soluble) monolayers are obtained by adsorption from the gas phase. A special device (Fig. 2.28) was constructed for the purpose a ring a in the measuring cell of Scheludko and Exerowa for formation of microscopic foam films at constant capillary pressure (see Section 2.1.2.). The insoluble (or slightly soluble) substance from reversoir b is placed in this ring. Conditions for the adsorption of the surfactant on either surface of the bi-concave drop are created in the closed space of the measuring cell. The surfactant used was n-decanol which at temperatures lower than 10°C forms a condensed monolayer. Thus, it is possible to obtain common thin as well as black foam films. The results from these studies can be seen in Section 3.4.3.3. 

The experimental points on curve 1 indicate that some of the thick films transform into NBF, since the potential of the double diffuse electric layer at the decyl alcohol monolayer/air interface is low (energy barrier ITmax in the IT(/i) isotherm. This transition was observed only at temperature below 10°C. Under such conditions a condensed monolayer from n-alcohol forms at the solution/air interface [396]. Metastable films were also observed in the temperature range of 8-10°C, i.e. thick films were produced which transformed into NBF very easily (within a short time) [394]. Therefore, there is a complete similarity with NBF formation from soluble monolayers of various surfactants, i.e. the presence of a close packing in this monolayer is a necessary condition for formation of NBF. 

Reaction at liquid surfaces covers a wide field. However, special experimental methods have been developed mainly for the study of insoluble or partly soluble monolayers at the liquid-air interface. Adsorption and reaction of monolayers at the liquid-air interface and liquid-liquid interfaces can be studied by similar techniques. It is therefore convenient to treat these together. Comprehensive information about liquid interfaces is contained in publications by Adamson, Davies , Alexander , Davies and Rideal and Gaines . ... 

T. Huang R. W. Murray, Visible Luminescence of Water-Soluble Monolayer-Protected Gold Clusters. f Phys. Chem. B 2001, 105, 12498-12502. 

Dynamic Surface Measurements as a Tool to Obtain Equation-of-State Data for Soluble Monolayers... 

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. 

In this paper we propose a dynamic method for tackling the problem of interactions in soluble monolayers. The surface dilational modulus, e, is a dynamic surface quantity very sensitive to interactions. It is defined as the decrease in local surface pressure per unit relative increase in surface area A in an oscillatory experiment ... 

The accurate information about the surface tension-surface coverage relationship which for soluble monolayers is contained in the e0—tt curves, can be used, for example, to interpret rate of adsorption measurements. For fluid-fluid interfaces, adsorption onto an initially clean surface can be assessed only indirectly by measuring the changing interfacial tension. An example is shown in Figure 9, giving the change in surface pressure... 

Figure 5.3 Film balances a. Langmuir trough having a movable barrier and a Wilhelmy tensiometer to measure the spreading pressure, n, for water insoluble monolayers, b. PLAWM (Pockels, Langmuir, Adam, Wilson and McBain) trough used for partially water-soluble monolayers, where a flexible membrane, which is fixed to the barrier, separates the surfactant solution and pure water departments to prevent the passage of dissolved surfactant molecules into the pure water department beneath the barrier.

F. Manea, C. Bindoli, S. Polizzi, L. Lay, and P. Scrimin, Expeditious synthesis of water-soluble, monolayer-proteeted gold nanoparticles of controlled size and monolayer composition, Langmuir, 24 (2008) 4120-4124. 

At low frequency they found that the demulsifier behaved as soluble monolayers and the tension was governed by the bulk concentration and did not change with change in area. At high frequency the demulsifiers behaved as insoluble monolayers and the change in interfacial tension resulting from area change was instantaneous ... 

O. Kohlmann, W.E. Steimnetz, X.A. Mao, W.P. Wuelfing, A.C. Templeton, R.W. Murray, C.S. Johnson, NMR diffiision, relaxation, and spectroscopic studies of water soluble, monolayer-protected gold nanoclusters. J. Phys. Chem. B 105, 8801-8809 (2001)... 

The study of the surface properties and characteristics of insoluble and soluble monolayers gives information concerning molecular dimensions, intermolecular interaction, and interaction with substrate. 

Huang T, Murray RW (2001) Visible luminescence of water-soluble monolayer-protected gold clusters. J Phys Chem B 105 12498-12502... 

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. 

A renewed interest in the structure and properties of amphiphilic molecules which reside at the air-water interface has occurred in the last decade due to the fact that the monolayers comprise an idealized two dimensional system which can be probed in terms of structure, composition and phase transitions, and due to the fact that the monolayers are used to form highly ordered coatings in Langmuir-Blodgett applications. Monolayers form at an air-water interface upon dissolution of amphiphiles in a water immiscible solvent, spreading of Ae solution on a water surface, and lastly, solvent removal. Such monolayers are insoluble in the aqueous phase. Alternately, monolayers which are soluble in the aqueous phase are formed by adsorption of amphiphiles from the bulk aqueous solution to the air-water interface. A primary difference between insoluble and soluble monolayers... 

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