Langmuir-Wilhelmy balance

In our model studies (42,56) we investigated the surface tension properties of the surfactant film at the air-liquid interface. In the studies conducted in vitro, nsing a modified Langmuir-Wilhelmy balance, we have not considered the structure of the subphase and its viscoelastic properties the subphase was taken as a newtonian fluid. 

Figure 6 shows polystyrene (PS) particles of an average diameter of 11.9 xm retained on a freshly excised piece of sheep trachea and on a surfactant film of dipalmitoylphosphatidylcholine (DPPC) in a modified Langmuir-Wilhelmy balance. The surface tension in both cases is approximately 30 mN/m, measured by a drop-spreading technique on the trachea and by a Wilhelmy plate in the surface balance. The aqueous substrate for the film in the balance was 0.9% NaCl with 55% sucrose. The density of the substrate was 1.26 g/mL, considerably higher than that of the particle (1.05 g/cm ). Most of the particles retained on the tracheal surface appear to be submerged in the substrate, and only a small segment of the spheres is exposed to air above the three-phase line. This is equivalent to the situation in the surface balance. 

Devise a method to accurately use two Wilhelmy plates (see Fig. 6) to measure the surface tension of a Langmuir film. Do not forget about the gravitational and buoyancy forces. Such a device is called a Wilhelmy balance. 

Serrien et al. (1992) measured the damping of planar longitudinal waves, generated with a barrier on a Langmuir trough and detected with a Wilhelmy balance. The longitudinal waves correspond to slow periodic compressions/dilations of the protein adsorption layer (BSA, casein). The results for BSA are shown in Fig. 6.18. Further experiments using a stress... 

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. 

All surface-active molecules, such as soaps and lipids, can be prepared as monomolecular layers at the air-water interface. Driven by the reduction of the surface free energy of water, these molecules spread when applied to the surface, for example, from a volatile solvent (Fig. 10). The physical properties of these monolayers were first investigated in the 1940s by Langmuir after his first work on molecules in insoluble monolayers [41]. A so-called Langmuir trough filled with water defines an exactly known area for the spread molecules (Fig. 10). At low lateral density, these molecules behave like a quasi-two-dimensional gas. If the area for these molecules is reduced by a movable barrier, this lateral compression will eventually lead to a measurable lateral pressure n (force F per unit length of barrier). Which can be measured by a so-called Wilhelmy balance (see Kuhn et al. [42]), It is the difference between the surface tension of the free, y, and the layer-covered, y, water surface ... 

A commonly used alternative to the Langmuir film balance method of determining surface pressure is to measure surface tension using a Wilhelmy plate (Section 4.3.1), dipped into the monolayer at different stages of compression. 

Martin DA, Vogler EA (1991) Immersion depth independent computer analysis of Wilhelmy balance hysteresis curves. Langmuir 7 422 29... 

The film pressure is defined as the difference between the surface tension of the pure fluid and that of the film-covered surface. While any method of surface tension measurement can be used, most of the methods of capillarity are, for one reason or another, ill-suited for work with film-covered surfaces with the principal exceptions of the Wilhelmy slide method (Section II-6) and the pendant drop experiment (Section II-7). Both approaches work very well with fluid films and are capable of measuring low values of pressure with similar precision of 0.01 dyn/cm. In addition, the film balance, considerably updated since Langmuir s design (see Section III-7) is a popular approach to measurement of V. 

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. 

The thermodynamics of spread films at the air-water interface can be expressed by the tt-A isotherm, obtained in a Langmuir- or Wilhelmy-type film balance (Figure 14.4a). From the tt-A isotherm, different structures can be deduced for emulsifier monolayers as a function of emulsifier, temperature, and surface density or surface pressure. A phase diagram... 

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.

For many years, the main experimental tool for these studies was the film balance (see Figure 4.19). Various workers, including Pockels, Langmuir, and Adam, made major contributions to its development (see Gaines, 1966). A small, known quantity of the surfactant to be studied is dissolved in a volatile solvent and deposited carefully by pipette on the surface of a pool of water. The solvent is chosen so that it spreads rapidly over the water and then evaporates, leaving the surfactant uniformly distributed as a monomolecular layer (or monolayer) in the region between the two barriers. One of the barriers is movable, so that the area occupied by the surfactant film can be varied. A torsion balance is provided to measure the surface pressure (i.e., the difference between the surface tension of pure water and that of the film-covered surface). More commonly in modem instruments, a Wilhelmy plate is used to measure surface tension in the film region. 

We employed a Langmuir type oil/water interfacial film balance (10). The relative film pressure during compression and expansion of the oil/water interface was determined. This was measured using a hydrophobic Wilhelmy plate, which remained oil-wet even in the presence of added demulsifiers. During film compression, solvent molecules may be squeezed out of the film, surfactant molecules not tightly held in the interface may redissolve into the bulk phases and molecular rearrangements of the adsorbed species may occur. 

The amplitude and frequency of barrier oscillation and hence wave propagation are variable. Frequencies between 0.005 - 1 Hz can be obtained and amplitudes up to 0.5 cm can be used. The other barrier can be smoothly adjusted to alter the extent of the interfacial area, as in a conventional Langmuir type film balance. The change of interfacial tension produced by the area variation is monitored continuously. Use is made of Wilhelmy plate suspended from one arm of a microforce balance (Beckman microforce balance). The output of this feeds one arm of an X-Y recorder. A position transducer monitors the movement of the oscillating barrier and this feeds the Y axis of the recorder. Lissajou figures are therefore produced on the X-Y recorder. Here 0 is defined as being... 

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. 

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