Surface vapour pressure

The value of the surface pressure in the two-phase region where it is constant is called the surface-vapour pressure it measures the tendency of the molecules to escape from the liquid film into the vapour film, and is smaller the greater the lateral adhesion between the molecules. The adhesion increases steadily the longer the hydrocarbon chains in tride-cylic acid it first becomes great enough to form a separate liquid phase lauric acid, with twelve carbons, is at, or just above, its critical point. ... 

Table II shows the principal measurements of surface-vapour pressure recorded. While many coherent films show no measurable pressure at areas slightly greater than that at which the molecules are closely packed, owing to the lateral adhesion being very high, at least six different homologous series have been found to show surface-evaporation phenomena with a measurable constant pressure. The highest surface-vapour pressure yet recorded is 0 39 dyne this may seem small, but considering the extreme thinness of the film, it is really fairly large. The molecules are lying flat, and therefore the thickness of the film perpendicular to the surface is about 4 5 A. a surface pressure of 0 39 dyne per cm. is thus roughly 8 7 x 106 dyne per sq. cm., or 8 5 atmospheres. This is smaller than the critical pressure of most liquids in three dimensions, but is not many times smaller.

The variation of surface-vapour pressure with temperature has not yet been measured the difficulties of sufficiently accurate measurement except at room... 

The expanded films tend to a definite area at low compressions, about 48 sq. A., at room temperature they are coherent, and show a surface-vapour pressure of about 0-19 dyne (compare Fig. 13). At areas greater than this the surface is heterogeneous and can be shown by electrical or optical methods to consist of two types of film. As the temperature is increased there is no change of form in the expanded curves, but they move nearly parallel to themselves to somewhat larger areas. The pressure required to initiate the change from expanded to condensed films increases by approximately 1 dyne for every degree rise in temperature. 

This type of expansion very frequently occurs among long-chain aliphatic compounds. All the fatty acids show it, and the expansion is very similar with the a-bromo-acids, the nitriles, alcohols, amides, ureas, oximes, amines, and acetamides. In all these series the expanded film is probably liquid, with a definite surface-vapour pressure, and the area is about 48 sq. A. at the lowest compression. Some other substances form similar films but of different areas the p-alkyl phenols tend to 39 sq. A. (j), and the a-monoglycerides (j) and the a-glyceryl ethers1 of long-chain alcohols,... 

The assumption that the partial surface pressure due to the single molecules is constant throughout the transition is equivalent to saying that the micelles have a constant surface-vapour pressure of dissociation Fj. The manner in which this varies with temperature is most interesting. Langmuir found that the difference Fj—F0 obeys an equation of the Clapeyron type ... 

Flere is the volume of gas required to saturate the monolayer, V the total volume of gas adsorbed, P the sample pressure, P the saturation vapour pressure and C a constant related to the enthalpy of adsorption. The resulting shape of the isothemi is shown plotted in figure Bl.26.6 for C = 500. A plot of P/V(P - Pq) against P/Pq should give a straight line having a slope (C - )/y C and an intercept The BET surface area is... 

The equilibrium vapour pressure, P, over a curved surface is defined by tlie Kelvin equation... 

The vapour pressure of a liquid increases with rising temperature. A few typical vapour pressure curves are collected in Fig. 7,1, 1. When the vapour pressure becomes equal to the total pressure exerted on the surface of a liquid, the liquid boils, i.e., the liquid is vaporised by bubbles formed within the liquid. When the vapour pressure of the liquid is the same as the external pressure to which the liquid is subjected, the temperature does not, as a rale, rise further. If the supply of heat is increased, the rate at which bubbles are formed is increased and the heat of vaporisation is absorbed. The boiling point of a liquid may be defined as the temperature at which the vapour pressure of the liquid is equal to the external pressure dxerted at any point upon the liquid surface. This external pressure may be exerted by atmospheric air, by other gases, by vapour and air, etc. The boiling point at a pressure of 760 mm. of mercury, or one standard atmosphere, may be termed the normal boiling point. 

The model implies that at any pressure below the saturation vapour pressure, the fractions of the surface covered with 1, 2,. .., i molecules will be 6 02,.., 0i respectively, so that the thickness of the adsorbed layer will not be constant throughout. On the specific surface area A, therefore, the total number Z of molecules adsorbed will be... 

As already mentioned, the choice of the supercooled liquid as reference state has been questioned by some workers who use the saturation vapour pressure of the solid, which is measured at the working temperature in the course of the isotherm determination. The effect of this alternative choice of p° on the value of a for argon adsorbed on a number of oxide samples, covering a wide range of surface areas, is clear from Table 2.11 the average value of is seen to be somewhat higher, i.e. 18 OA. ... 

Following the pioneer work of Beebe in 1945, the adsorption of krypton at 77 K has come into widespread use for the determination of relatively small surface areas because its saturation vapour pressure is rather low (p° 2Torr). Consequently the dead space correction for unadsorbed gas is small enough to permit the measurement of quite small adsorption with reasonable precision. Estimates of specific surface as low as 10 cm g" have been reported. Unfortunately, however, there are some complications in the interpretation of the adsorption isotherm. 

D = diffusilivity of the solvent vapour from the sample solution (cm s ) o = surface tension of sample solution (erg-cm ) p = vapour pressure of sample solution (mm Hg X 1-359 = g cm )... 

This equation is derived by considering die transfer of material from a flat surface to a droplet. For the U ansfer of a small mass Sm from the flat surface of vapour pressure p° to the droplet of vapour pressure p, the Gibbs energy of transfer is... 

There is a qualitative distinction between these two types of mass transfer. In the case of vapour phase transport, matter is subtracted from the exposed faces of the particles via dre gas phase at a rate determined by the vapour pressure of the solid, and deposited in the necks. In solid state sintering atoms are removed from the surface and the interior of the particles via the various diffusion vacancy-exchange mechanisms, and the centre-to-cenU e distance of two particles undergoing sintering decreases with time. 

The boiling point of a liquid varies with the atmospheric pressure to which it is exposed. A liquid boils when its vapour pressure is the same as the external pressure on its surface, its normal boiling point being the temperature at which its vapour pressure is equal to that of a standard atmosphere (760mm Hg). Lowering the external pressure lowers the boiling point. For most substances, boiling point and vapour pressure are related by an equation of the form. 

---