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10"2 10"1 Concentration (mol kg"1)

Figure 6.5 Surface tension, y, as a function of log molal concentration at 30°C showing the increase of hydrophobicity associated with a Br substituent on the phenyl ring of an antihistamine.

Reproduced from D. Attwood and O. K. Udeala, J. Pharm. Pharmacol., 27, 754 (1975) with permission.

that is associated with the Br substituent on the phenyl ring of an antihistaminic drug. Similarly, substitution on the phenothiazine ring systems increases surface activity in the order CF, » C1 >H.

6.2.6 Insoluble monolayers

In section 6.2.4 we examined the case in which the surface of a solution containing an amphiphile became covered with a monomol-ecular film as a result of spontaneous adsorption from solution. The molecules in such films are in equilibrium with those in the bulk of the solution, i.e. there is a continuous movement of molecules between the surface and the solution below it. If, however, a surfactant has a very long hydrocarbon chain it will be insufficiently water-soluble for a film to be formed in this way. In such cases we can spread a film on the surface of the solution by dissolving the surfactant in a suitable volatile solvent and carefully injecting the solution on to the surface. The insoluble monolayer formed by this process contains all of the molecules injected on the surface; there is no equilibrium with the bulk solution because of the low water solubility of the surfactant. Conse quently, the number of molecules per unit area of surface is generally known directly.

Although the films are called insoluble films, this is not meant to imply that any insoluble substance will form a stable mono-layer; in fact, only two classes of materials will do so. The simpler and larger of the two classes includes the water-insoluble amphiphiles discussed above. Such structures orientate themselves at the water surface in the manner of typical surfactants, with the polar group acting as an anchor and the hydrocarbon chain protruding into the vapour phase. The other class of film-forming compounds includes a range of polymeric materials such as proteins and synthetic polymers. With these compounds a high degree of water insolubility is not so essential and stable films will form, providing there is a favourable free energy of adsorption from the bulk solution.

Experimental study of insoluble films

One of the earliest studies of insoluble films was conducted by Benjamin Franklin in 1765 on a pond in Clapham Common in London. Surprisingly Franklin's experiment was sufficiently controlled to establish that olive oil formed a film of monolayer thickness (quoted as one ten millionth of an inch: approximately 2.5 nm). Figure 6.6 illustrates a commonly used apparatus - the Langmuir trough - for study of monolayers on a laboratory scale.

In its simplest form the apparatus consists of a shallow trough with waxed or Teflon sides (nonwetting), along which a nonwetting barrier may be mechanically moved. In use, the trough is filled completely so as to build up a meniscus above the level of the sides. The surface is swept clean with the movable barrier and any surface impurities are sucked away using a water pump. The film-forming material is dissolved in a suitable volatile solvent and an accurately measured amount, usually about 0.01 cm3, of this solution is carefully distributed onto the surface. The solvent evaporates and leaves a uniformly spread film which can now be compressed using the movable barrier. For each setting of the barrier, a force is applied to a torsion wire attached to the float to maintain the float at a fixed position. This force is a direct measure of the surface pressure, n, of the film, that is, the difference between the surface tension of the clean surface y0 and that of the film-covered surface, y m:

enclosed between the float and the barrier as shown by the following example.

EXAMPLE 6.3 Calculation of the area per molecule in an insoluble monolayer

When 1 cm3 of a solution containing 8.5 mg per 100 cm3 of stearic acid (mol. wt. = 284.3) dissolved in a volatile organic solvent is placed on the surface of water in a Langmuir trough, the solvent evaporates off, leaving the stearic acid spread over the surface as an insoluble monomolecular film. If the surface area occupied by the film is 400 cm2, calculate the area occupied by each molecule of stearic acid in the film.

Answer

1 cm3 of the solution contains 8.5 x 10 5 g of stearic acid = 2.99 x 10 7 mol.

Since 1 mol contains 6 x 1023 (Avogadro constant) molecules, the solution contains

1.79 x 1017 molecules

Therefore, the area per molecule of stearic acid in the film is

The results are generally presented as graphs of n against the surface area per molecule, A, which is readily calculated from the number of molecules added to the surface and the area

1.79 x 1017

Film

Movable bar

Movable bar

Side view

Figure 6.6 Langmuir trough for monolayer studies (not to scale).

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