surface phenomena resulting at lowering of temperature
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Many surface phenomena come under the heading of adsorption, which consists in the adhesion of substances on the surfaces of solids and liquids (the latter being then called adsorbents). Adsorption can take place from gases or liquids, and a solute may be adsorbed from solution. For example, many gases are adsorbed on the surface of carbon, silica gel, or the majority of metals; carbon adsorbs various organic compounds from solution. The degree of adsorption is described by the surface concentration, which is the quantity of the substance per unit area of the surface of the adsorbent.
Adsorption phenomena are widely found in Nature, and play an important part in technology. In order to adsorb a large quantity of a substance, we must evidently use substances which have the maximum area for a given mass, such as porous or finely powdered materials. To describe this property of adsorbents, we use the specific area, which is the area per unit mass of the substance. In good adsorbents, such as specially prepared porous carbons, it reaches hundreds of square metres per gram. Such large values of the specific area are not surprising if we consider how rapidly the surface area increases when a body is permeated by pores or is finely crushed. For example, 1 cm3 of material in spheres of radius r cm will have a total area of 3/r cm2, and when r ∼ 10−6 this amounts to hundreds of square metres.
The concentration of adsorbed gas depends (at a given temperature) on the gas pressure over the surface of the adsorbent. This dependence is shown by a curve, called an adsorption isotherm, of the form shown in Fig. 114. The surface concentration at first increases rapidly with pressure. As the pressure continues to rise, the concentration increases more slowly, and finally reaches a limit or saturation value. Experiment shows that the saturation of adsorption corresponds to a more or less dense occupation of the adsorbent surface by a single layer of adsorbed molecules (called a monomolecular layer).
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FIG. 114.
A very important property of adsorption is the change which it causes in the surface tension at the interface between media. Usually the surface of a liquid is concerned. Adsorption always reduces the surface tension, since otherwise adsorption would not occur. Here again there is a tendency to reduce the surface energy: this reduction can be achieved not only by decreasing the surface area but also by changing the physical properties of the surface. Because of their effect on the surface tension, substances which can be absorbed (on the surface of a given liquid) are said to be surface-active. On water, for example, various soaps are surface-active.
The total quantity of a substance which can be absorbed on the surface of a liquid is very small. Thus even small quantities of surface-active substances accumulating on the surface of a liquid may considerably affect its surface tension. The surface tension of a liquid is very sensitive to impurities: for example, even very small quantities of soap can reduce the surface tension of water by a factor of more than three.
Adsorbed monomolecular films on the surfaces of liquids are a very curious physical phenomenon, forming as it were a two-dimensional state of matter, in which the molecules are distributed over a surface in two dimensions and not over a volume in three dimensions. In this state there can exist various phases, “gas”, “liquid” and “solid”, exactly analogous to three-dimensional phases.
In a “gaseous” film the adsorbed molecules have a comparatively rarefied distribution on the surface of the liquid and can move freely on it. In “liquid” and “solid” films the molecules are close together, either retaining some freedom of relative motion (so that a liquid film can “flow”), or so firmly held together that the film behaves as a solid. Liquid and solid films may be anisotropic, forming two-dimensional analogues of liquid and solid crystals; in the liquid film we have a regular orientation of molecules on the surface of the adsorbent, and in the solid film a type of two-dimensional crystal lattice with a regular configuration of molecules. It is noteworthy that such anisotropic films may occur at an interface between two isotropic media, a liquid and a gas.
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The river is like the transient human life. The sea reminds the poet of eternity.