Monomolecular Layer

MONOMOLECULAR LAYER (abbreviated "monolayer"), a layer of any substance consisting of only a single row of molecules and forming under certain conditions on the surface of a foreign liquid under the influence of capillary forces. Thus, if a clean water surface is touched with an oil-contaminated needle, the latter instantly spreads into a thinnest film, forming a continuous spot with sharply defined edges. As Rayleigh already noted, such films have a characteristic thickness for each substance, which for example is 10Å for olive oil and 13Å for castor oil. In other cases, the existence of a monomolecular layer can be established indirectly by means of its characteristic properties; these properties were studied mainly by Langmuir and Harkins. If the surface of a cuvette with a solution of a capillary-active substance is partitioned across with a paper strip, then when moving it along the cuvette, it can push aside the molecules adsorbed in the surface film. As long as the latter lie isolated, this movement occurs with great ease. However, it suddenly encounters sharp resistance only when the approaching molecules come into contact with each other; this serves as an excellent signal for the formation of a monomolecular layer, thereby opening up a convenient method for the precise measurement of the latter. The relationships between the molecules adsorbed in the surface film and the monomolecular layer present an interesting analogy with the properties of vapor and liquid. Adsorbed molecules move in all directions over the entire free surface of the liquid, just as vapor molecules strive to occupy the entire available volume to them. Upon compression, the molecules form in the first case a monomolecular layer, in the second a liquid, and as a result of such condensation, compressibility in both cases sharply decreases. With repeated increase of the free surface, the molecules that make up the monomolecular layer again "evaporate" over the entire surface film. In short, completely similar phenomena occur—only not in three-dimensional, but in two-dimensional space. According to later experiments by Volmer, this correspondence, which was already established by Langmuir, has not only a qualitative but even a quantitative character. The condition for the spreading of a substance on a water surface is a certain structure of its molecule, the presence in it, along with hydrophobic radicals, of an active hydrophilic group. A typical example of a hydrophobic group can be methyl (CH₃); hydrophilic groups are those containing oxygen, e.g., hydroxyl (OH) and carboxyl (COOH). Substances having such hydrophilic groups are adsorbed and spread on the water surface. In this case, as Langmuir showed, in the monomolecular layer the molecules take up a characteristic arrangement. They are oriented in it perpendicular to the surface in such a way that their hydrophilic groups are turned inward (toward the water), and hydrophobic groups outward (toward the air). When spreading adsorbed molecules over a larger boundary surface, beyond the monomolecular layer, the molecules can "float" on it, lying on the surface with their longitudinal axis. Conversely, when compressing the monomolecular layer, the molecules pile up in several layers on top of each other; It is interesting to note that substances that do not have hydrophilic groups (such as liquid paraffin hydrocarbons) do not possess capillary activity and do not spread on the water surface, but remain on it in the form of droplets. Such a structure of the monomolecular layer makes it possible to determine the size of the molecules contained in it by simple calculations. First of all, for the precise measurement of extremely small amounts of substance going to form the monomolecular layer, use is made of the method proposed by Devaux: the substance under investigation is first dissolved in some volatile solvent (e.g., gasoline) and then a measured amount of such a solution is placed on the water surface; the film forms after the solvent evaporates. Knowing the amount of substance taken and its specific gravity, its volume is found. Dividing the latter by the area of the monomolecular layer gives its thickness, and consequently the length of the molecule. Knowing the number of molecules in a given amount of substance [for a gram-mole it is expressed by the so-called "Avogadro number" (see Avogadro's law)], it is not difficult in the same way to calculate the area occupied by the cross-section of each molecule from the total area of the monomolecular layer. The results obtained are in excellent agreement with the data of structural chemistry, thereby confirming the correctness of the theoretical considerations regarding the structure of the monomolecular layer. Another very important property of the monomolecular layer is its effect on the surface tension of the liquid. As long as the adsorbed molecules lie isolated, they have a very weak effect on the surface tension of the solution. The latter sharply drops at the moment when the approaching molecules form a monomolecular layer. Further piling up of adsorbed molecules, disrupting the regular structure of the monomolecular layer, in some cases can even initially lead to a slight increase in surface tension. The minimum clearly emerges if concentration of the capillary-active substance is plotted on the abscissa axis, and surface tension of the solution on the ordinate axis. As du Noüy showed, it can be used as a very convenient criterion for the precise determination of the moment of formation of the monomolecular layer, and consequently for establishing its dimensions. Du Noüy established that the dissolved capillary-active substance forms a monolayer of oriented molecules over its entire boundary surface—not only on the free surface in contact with air, but also on the inner boundary with glass. It must be assumed that such a layer of strictly oriented (directed by like groups in the same direction) molecules is formed in many cases in the body on various boundary surfaces, in particular on the blood-washed surface of the endothelium, as well as on the surface of other cells.