TALKING OF PERFUMES AGAIN in a solvent is to lower the vapour pressure of the latter. In simple form, Raoult's Law states "the relative lowering of vapour pressure is equal to the mole fraction of solute in the solution." The relative lowering of vapour pø_p pressure is given by • where Pø is the vapour pressure of the solvent po and P is the vapour pressure of the solution (solvent plus solute). The mole fraction is the number of molecules of solute divided by the total number of molecules in the solution. This effect is well illustrated by the way the vapour pressure of ether may be considerably lowered by the addition of organic chemicals. Viewed from a perfumery angle this is not as helpful as it seems at first glance. The addition of a second substance has certainly lowered the desire of the molecules of the solvent to leave the surface, but it is virtually only a question of dilution and we are faced with the thought that the more volatile the solute the more effectively we lower the vapour pressure of the solvent, this on consideration of the fact that normally, low molecular- weight bodies are more volatile than those of high molecular weight. If we take two substances A and B of equal molecular weight, then the lowering of the vapour pressure of A will be proportional to the amount of B added. If, however, the molecular weight of B is much higher than that of A, the lowering of vapour pressure will be correspondingly less. An example would be the case of ethyl acetate (molecular weight 88, vapour pressure 72-8 mm. at 20 ø C.) as solvent. If we add to the ethyl acetate one-tenth of its weight of a second substance "B," also with a molecular weight of 88, then we have the following equation according to Raoult's Law. Pø -- P Molecules of substance B __ -- Pø Molecules of ethyl acetate + molecules of substance B 72.8- x 0'6 x 10 TM 72'8 6 x 10 TM + 0'6 x 10 •a x ---- 66.18mm. 6 x 10 •a refers to the number of molecules present in one gram molecule, i.e., 88 gms. of ethyl acetate will contain 6 x 10 TM molecules. In the case of a solute C having a high molecular weight, say, 880, and where an amount equal to one-tenth of the ethyl acetate is added, we have ß Pø -- P Molecules of C Pø Molecules of ethyl acetate 5- molecules of C 72-8- x 0.06 x 10 •a 72'8 6 x 10 •a 5- 0'06 x 10 TM x = 72.08 mm. 195

JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS In other words, an addition of 10 per cent of B (molecular weight 88) has lowered the vapour pressure of ethyl acetate by 6.62 mm., whilst the addition of 10 per cent of C (molecular weight 880) has lowered the vapour pressure by only 0.72 mm. Departing from ideal solutions, that is solutions which obey Raoult's Law, we enter a realm more closely connected with perfumes. In an ideal solution the molecules of the solute are presumed to have no effect upon the intramolecular forces of the solvent, and vice versa. This might be the case if both ingredients in a two-part system were similar in character, e.g., both related polar substances or both non-polar. If, however, the intramolecular attraction of one constituent was much greater than that existing in the second constituent, then the effect would be to increase the vapour pressure of the latter and to force those molecules out of the solution as vapour. Such a case would be exemplified by a mixture of a polar and non-polar compound. On the other hand, the two, constituents of a solution may have a strong attraction for one another then the vapour pressure of both constituents will be lowered not only by virtue of molecular dilution, but also on account of this intermolecular attraction. Figs. I, II, III and IV show how partial and total vapour pressures are affected according to the nature of the ingredients of the solution. Reviewing the four figures we imagine that our effective fixative will therefore have three distinct and important requirements: (a) Low molecular weight. (b) Low vapour pressure. (c) Molecular attraction for the other constituent or constituents. When dealing with a perfume or mixture of perfumery ingredients, it is to be realised that the vapour pressure, or the desire of the molecules to escape, has, for each individual member, already been reduced if only by a dilution phenomenon. Further, if the individual ingredients are so selected that their molecules attract one another, then the fixation of the mixed molecules is enhanced. This is, of course, our physical blending effect. Additional blending effects are obtained by chemical means, e.g., the forma- tion of hemi-acetals between aldehydes and alcohols, Schiff-base formation from aldehydes and amines, etc., but for the time being we are concerned with physical effects. Admitting, therefore, that we are, if our blending technique is good, automatically providing fixation in our perfume, is it possible or even neces- sary to add an additional fixative ? In many cases the answer must be yes. There are so many individual ingredients to be used in one mixture and their chemical and physical characteristics may vary enormously. Whilst one may aim at minimum total and partial vapour pressures in the mixture 196