On the Structure of the Surface Layer of Liquids

(Irving Langmuir. The Shapes of group molecules forming the surfaces of liquids. Proceed. of the National Academy of Sciences. Washington. Vol. 3. Num 4 p. 251—1917).

In Langmuir’s works \[Journal Amer. Chem. Soc. 38. 1916 (2221)\] views were developed regarding the forces acting in solid and liquid bodies as forces analogous to the forces responsible for chemical compounds. Condensation, evaporation, crystallization, wetting, adsorption, and surface tension are determined by these same forces. According to this conception, every atom of a substance is chemically bonded in a solid body with its neighbors. And, depending on the magnitude of the bond, which may be large or small, Langmuir, just like Werner, distinguishes primary and secondary valence, of which the first corresponds to considerable bonding forces, the second to small ones. The significance of primary or secondary valence is especially clear in organic solids or liquids. Every complex molecule of an organic body is formed from atoms held together by considerable forces of primary valence, whereas ...

individual molecules unite into a solid or liquid body, being held close to one another by forces of secondary valence. From this point of view, the phenomena of adsorption and surface tension are due to individual atoms lying at the surface. Langmuir proceeds from the fact, established by Pockels in 1891, that extremely small quantities of oil have no effect on the surface tension of water, which, when the quantity of oil poured on is increased, begins suddenly to decrease, if a certain limit is reached.

Rayleigh, Devaux, and Marcelin found that the thickness of the oil layer causing the sudden decrease of surface tension corresponds to molecular dimensions. Thus, for example, Devaux found that triolein begins to diminish the magnitude of the surface tension at a thickness of \(11 \times 10^{-8}\) cm. Knowing the density of the oil, its molecular weight, and the value of Avogadro’s constant, one can compute the diameter of the molecule, which turns out to be equal to \(11.3 \times 10^{-8}\) cm. (the molecule is assumed spherical).

According to Langmuir’s conceptions, if we have an organic acid containing the radical \(COOH\), or an alcohol with the group \(OH\), then this group must have a greater affinity for water than the hydrocarbon chain, for example, \(CH_3—CH_2—CH_2—CH_2—\), since alcohols and acids are soluble, whereas hydrocarbons themselves are insoluble; transferring these considerations to molecules at the interface, we arrive at Langmuir’s conclusion. Hence it is clear that if we take a complex acid with hydrocarbon radical \(R\) and this acid is poured onto water, then in the water there is arranged the group \(COOH\), which gives chemical combination with water, while \(R\) turns away to the side. According to this theory, pure hydrocarbons should not form films, and this is in fact observed.

A molecule of a complex acid is considered to be, in this manner, standing vertically to the surface of the liquid; moreover, if the molecule has a complex composition, for example \(C_{15}H_{31}COOH\), then the length of the molecule depends on the number of \(CH_2\) groups entering into the chain. The \(CH_3\) group, the side chains, determine its width. Therefore, for different liquids of one homologous series, for example the series of fatty acids, the influence of replacing \(CH_3\) by \(CH_2\), \(CH_2—CH_2\), is manifested in an increase in the length of the molecule. This is found also in changes in the limiting thickness of the oil layer on water at which a change in surface tension is obtained.

Langmuir calculates the magnitude of the cross section of a molecule \(a\) as follows: let \(u\) be the mass condensed on water, and \(A\) the area occupied by it; if \(M\) is the molecular weight and \(N\) the number of gram-molecules per gram, then

\[ a=\frac{AM}{uN}. \]

The length of the molecule \(\tau\) is calculated as follows: the volume of the molecule is equal to

\[ \frac{M}{\rho N}, \]

where \(\rho\) is the density of the oil, and

\[ \tau=\frac{M}{a\rho N}. \]

As the measurements of the thickness of the oil layer show, the length of the molecule \(\tau\) increases in proportion to the number of carbon atoms in the hydrocarbon chain.

Calculations show that the areas occupied on water by molecules of different acids are identical, and this indicates that attachment occurs by the group \(COOH\) (or, in the case of alcohols, by the group \(OH\)). Unsaturated acids or esters occupy a larger area than saturated ones and have a shorter length, which is due to the double bond. A number of experiments even proves that the hydrocarbon chain can bend, and thus fully justifies the name “chain.”

The same method can be extended to substances soluble in water, by causing them to spread over mercury.

Further, proceeding from the same views, Langmuir develops a theory of the changes in the concentration of dissolved substances in water at its surface. The theory is not mathematically substantiated, but it allows the author to draw a number of interesting conclusions. Finally, Langmuir points to a method for measuring the cross-section of vapor molecules adsorbed by the surface of water.

P. Lazarev.