TALKING OF PERFUMES AGAIN Xn• I MOLE FRACTION X^=O X. =0 X. ,• I Fig. I represents the vapour pressure effect when two substances, similar in character, are mixed. P• TOTAL '" -,.. x N ' X^• I MOLE FRACTION X•,•O Xe •0 Xe FiG. II P7 Fig. II indicates how the total and partial vapour pressures of a system may increase above the normal due to the presence of dissimilar substances in solution. X,• I MOLE FRACTION Xa•O X.=O X. --I Fig. III shows how two substances whose molecules attract one another will tend to possess relatively low total and partial vapour pressures when in common solution. I X^• I MOLE FRACTION X^•0 X.=0 'X.--•I FIG. IV In Fig. IV the effect of mixing two sub- stances possessing intermolecular attrac- tion for one another is again shown, but in this instance one ingredient is non- volatile. p,O _--_ Vapour pressure of pure Liquid A. B = Vapour pressure of pure Liquid B. Xx = Mole Fraction A in mixture. X• -- Mole Fraction B in mixture. 197

JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS by a careful choice of ingredients, a severe limit on odour effects would be imposed by too narrow a selection. The balance of odour effects and the stability of these effects is decided by the compounder and therein lies the skill of the perfumer. Let us therefore consider the matter from the view of a perfumer who, from whatever the reason, desires to retard the rate of evaporation of one or more of the ingredients in his perfume he desires to add to his pre-made mixture a substance which we call our fixative. From previous indications this must have a low vapour pressure and a low molecular weight, two properties not often to be found in one chemical substance, especially when oil-soluble. In most cases we are forced to use relatively non-volatile sub- stances of comparatively high molecular weight, although the molecular weight may not be high in comparison with other ingredients of the perfume. Our third requirement is, perhaps, the most important, and here we must select a product with physical attraction for the molecules of the more volatile members of our perfume. This attraction of molecule for molecule could be explained on the grounds of dipole moments that is, an electrostatic attraction between polar molecules or on the grounds of hydrogen bonding, or possibly both. Hydro- gen bonding accounts for the fact that water boils at 100 ø C., whereas other considerations would indicate a considerably lower figure. If we consider the structural formube of two isomeric compounds, dimethyl ether and ethyl alcohol, we gather some idea of the way in which bonding effects their physi- cal properties. Thus dimethyl ether CH•OCH3 boils at --24 ø C., whilst ethyl alcohol CHsCH•OH boils at q-78.4 ø C. Turning closer to our per- fumery materials we can expect an association of molecules possessing hydroxyl radicals in the following manner :-- H R--O .... H 0 .... H 0 .... R R This particular example refers to alcohols or phenols with carboxylic acids the effect would be to produce a double molecule: OH- -0 / % R C C R % o- -- .HO One more example of a bonding effect accompanied by a lowering of vapour pressure is given by a study of the properties of two isomeric com- pounds, ortho and para hydroxy benzaldehyde. The former is very volatile in steam, whilst the para isomer is not steam distillable. This effect is shown below: 198