Langmuir Balance

Ai-Stearoylamino acids and their methyl esters were synthesized from enantiomeric and racemic forms of tyrosine, serine, alanine, and tryptophan (Fig. 16). Analogs of these molecules were investigated initially over 30 years ago by Zeelen and Havinga, who found stereochemical differentiation in the monolayer HjA isotherms of these materials (Zeelen, 1956 Zeelen and Havinga, 1958). We have extended this study using more sensitive Langmuir balances, a wider array of dynamic and equilibrium techniques, and the A-stearoyl methyl esters of the amino acids (Harvey et al., 1989 Harvey and Arnett, 1989). 

Several crystals, such as vaterite and calcite forms of CaC03, or a-glycine, have been nucleated (induced oriented crystallization) at the water surface covered with a monolayer film of carboxylic acids or aliphatic alcohols (compressed to "suitable" distances of the hydrophilic groups with a Langmuir balance) (Mann et al., 1988). 

Figure 10. Development of equilibrium spreading pressure of film spread from pure crystals. Film pressure is measured on a Langmuir balance with barriers stationary. From Thompson (101).

By means of this apparatus, it is possible to vary the area of a spread monolayer and measure the corresponding film pressure directly. Many different variations of the film balance are available, and a number of instrumentational techniques can be combined with the Langmuir balance to obtain information on the microstructure of the films and the properties of the films. Figure 7.4b illustrates, for example, a laser optics arrangement to monitor the molecular orientation of the hydrocarbon tails of the surfactant molecules. Below in this... 

A convenient way to discuss the Langmuir balance is to examine the difficulties involved in measuring each of the two-dimensional state variables tt, A, T, and, of course, the number of moles n of material in the insoluble layer. 

Until now we have concentrated on those difficulties in using the Langmuir balance that... 

An extensive discussion of the Langmuir balance technique and a comparison with the Wilhelmy plate method are given by MacRitchie (1990). This book also discusses modifications that are possible to the techniques and other experimental details. 

As we saw in Chapter 3, the heart of any osmotic pressure experiment is a semipermeable membrane that allows the solvent but not the solute to pass. The float of a Langmuir balance accomplishes this. That portion of surface with the monolayer is considered to be the two-dimensional solution the clean surface is the two-dimensional solvent. The solvent can certainly pass from one region to another (remember that the mechanism of the partitioning has... 

The dependence of the surface tension of a Langmuir layer containing a given amount of surfactant, as a function of its area, reveals many of the properties of the surfactant. (At constant temperature, such measurements are analogous to determining the P-V isotherm of a gas.) One device for making such measurements is the Langmuir balance, the essential principles of which are shown in Fig. 10. 

The first theoretical equation relating the quantity of gas adsorbed to the equilibrium pressure of the gas was proposed by Langmuir [16]. Using the kinetic theory of gases, Langmuir balanced the rates of condensation and evaporation of the gas molecules at the solid surface, giving the volume adsorbed, V, as a fimetion of the gas pressure, P ... 

Experimental determination of spreading pressure in monolayers Langmuir balance... 

If the molecular weight, M, of the studied substance is unknown, it is not possible to estimate adsorption in moles/m2, nor can therefore, the area per molecule, sM, be established from a known sample weight m. In this case, instead of nsM, the product of the two-dimensional pressure and the macroscopic area S between the barriers in the Langmuir balance (see Fig II-10) is plotted against n, and eq. (11.23) can be rewritten as... 

The solubility of lauric (Cl2) acid is sufficiently high, so that one is able to carry out the surface tension measurements. Yet at the same time, its solubility and dissolution rate are low enough for one to be able to study the properties of the adsorption layers formed with the Langmuir balance, provided that one works fast. The latter feature allowed Frumkin to compare trends in the 7rsM(7r) curves obtained for the same substance using methods established for both soluble and insoluble surfactants. The experimental... 

The Langmuir-balance technique was adapted for the study of Indian crudes by Singh and Pandey (69). They separated the crudes into anionic, cationic, and nonionic fractions and studied the effects of electrolytes and pH on their film properties. Maximum film pressure was observed for films made with the anionic fraction and minimum film pressure for the cationic fractions. Increased electrolyte concentrations caused increased viscosity and less resolution of emulsions. Film pressures were maximum at pH 12 and more stable emulsions resulted. The electrolyte also had an adverse effect on the demulsifier. 

Mohammed et al. (144, 145), in a series of studies, examined several aspects of emulsion films with and without demulsifiers as well as their chemistries. Using the Langmuir balance for studying the air/ crude/water interface, they examined the sinface pressme n-area isotherm for monolayers of Buchan crude s asphaltenes and resins and their mixtures, spread on distilled water at pH 6.2 and 25°C. They found that the asphaltenes upon compression formed solid films, that could withstand pressures up to 45 mN m" in contrast to the resin films at 7 mN m which thereafter collapsed. The asphaltenes formed highly stable emulsions in contrast to the resins alone, which formed the least stable emulsions. They found that film compressibility and emulsion stability decreased as resin content increased. Temperature increases caused no significant effects on asphaltene monolayer compressibility as was observed earlier by Reis-berg and Doscher (126) for natural crude oil films. 

Nordli et al. (146), using the Langmuir-balance technique, studied the monolayer properties of the interfacially active fractions extracted from six North Sea crude oils over a subphase of distilled water and simulated formation water. The pH and salinity were varied. They compared additives such as butanol, benzyl alcohol, and octylamine, added to the subphase, to note changes in film compressibilities. A typical phase-change pattern for Langmuir ciuves of siuface pressure versus area was observed for all cases. The smallest specific area attributed to the liquid ex-... 

Fig. 4.7. Typical appearance of surface pressure curves obtained using the Langmuir balance. 7T is surface pressure and a molecular area. Curve (2) was obtained at higher temperature than curve (1) and reveals a transition for the surface film at a = ac...

To form a Langmuir monolayer, the molecule of interest is dissolved in a volatile organic solvent (frequently chloroform or hexane) that will not react with or dissolve in the subphase (1,2,4). A quantity of this solution is placed on the surface of the subphase, and as the solvent evaporates, the siuTactant molecules spread and alter the surface pressure of the water surface. A barrier designed to measure this surface pressure (D), relative to that of the pin-e subphase, is the principle behind the Langmuir balance. Alternatively, the siuTace pressure is measured as the difference between the surface tension (y) of the monolayer and that of the pure subphase iyo), n = yo — K- A common method for measining surface tension involves using a Wilhelmy plate, usually a piece of platimun or paper that is wetted by the subphase, suspended from a balance. As the monolayer is compressed by using the moveable barrier to reduce the sinface area, the surface pressure increases. A plot of the siuTace pressin-e versus surface area is called a pressure versus area isotherm (or Il-A isotherm). Isotherms are normally plotted in terms of area/molecule, and the imits of surface pressure are mN/m. 

The experiments were performed by a computer controlled KSV MiniMicro Langmuir balance (KSV Instruments, Finland). The trough (51 x 169 x4 mm) was made of Teflon while the material of the barrier was polyoxymethylene recommended for work with lipid layers [16]. Trough was cleaned with dichloromethane and double distilled water, while the barriers with methanol and double distilled water. Lipid dissolved in chloroform (Ig/L) was spread on the water surface. 15pL of solution was applied dropwise by a Hamilton syringe to form the monolayer. Prior recording the isotherms the solvent was let to evaporate for 10 minutes. 

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