EVAPORATION AND THE ODOR OUALITY OF PERFUMES By J. STEPHAN JELLINEK, PH.D.* Presented October 5, 1960, New York Chapter THE FUNCTION and the reason for existence of any perfume is to be smelled. A perfume is a thing of no importance or interest until and unless someone perceives it. Perception of a perfume is possible only after some evaporation has taken place: molecules of the perfume material must leave their point of origin and travel to the olfactory membrane of the nose in order to be perceived as an odor. Although in the past perfumers have not given it much active attention, evaporation is a key process in perfumery. Since perfumes are usually complex mixtures containing materials of widely different vapor pressures, it is also a complicated process. The object of our research is and has been to study the process of evapora- tion of perfumes in all its ramifications, in the hope and certainty that our findings will have not only theoretical, but also distinctly practical, in- terest to the perruiner. EVAPORATION OF PERFUME MATERIALS Let us first take a general look at evaporation. If you have a perfume in an open bottle and you warm the bottle, the odor will be intensified. This is perfectly logical and we are all familiar with it. There is, however, another change which is more significant and less obvious. The odor character also changes somewhat as the perfume is heated. A perfume has many different ingredients, and it would be ex- tremely unlikely that the rate of evaporation of the various odorants would be increased in the same proportion by a given rise in temperature. This means that the vapor concentration over the perfume is different at the new temperature than it was at room temperature. Since what we per- ceive is not the liquid perfume in the bottle, but the vapor over it, a change in vapor composition means a qualitative change in the perceived odor. Apart from temperature there are other factors that influence the rate of evaporation of odorants and hence both the intensity and the quality of the * Polak's Frutal Works, Inc., Middletown, N.Y. 168

EVAPORATION AND THE ODOR QUALITY OF PERFUMES 169 odor of perfumes: surface area, air currents, attractive forces between odorant molecules and surrounding molecules, etc. In our work we have concentrated on the attractive forces, since they seem to play an important role in many ways. Take the phenomena of "blending" and "fixation," or the fact that the effective odor of a perfume depends on the medium in which it is incor- porated, or that a perfume smells very differently on the skin than in the bottle, and that it may smell differently on different persons. We believe that these phenomena, which are just the ones which have traditionally been regarded as proofs of the esoteric and intangible nature of perfumes, are readily understandable in terms of physical forces between molecules. T•E CoNcEPT ov FtXATtON The experiments discussed in this paper fall into two groups: one related to fixation and blending, the other to the evaporation of odorants from different media. Let us begin with fixation. To the perfumer, fixation is the phenomenon whereby a perfume material or a group of perfume materials is made to evaporate more slowly by the addition of another material. The added material, the "fixative," may have a distinct odor of its own, or it may be odorless. The terms fixation and fixative have often been used very loosely in the past. The distinction between a material which in itself has a long-lasting odor (this is not necessarily a fixative) and a true fixative, which causes the odor of other materials to last longer, has sometimes been neglected. A very important point, brought up by Wells (1), has generally been over- looked namely, that we cannot blithely assume that a substance which works as a fixative when the perfume mixture is allowed to evaporate from a paper blotter is also a fixative when the perfume is used on the skin, incorporated in a cream, or sprayed as an aerosol. Since the conditions of evaporation are different in all of these cases, the effect of a given material on the evaporation of the others is not necessarily the same in each case. Hence, we should not say that a substance is a "fixative," but only that it has fixative action in whatever medium this has actually been demon- strated. So ill-defined is our knowledge of the action of fixatives that a number of authors who have recently discussed the problem of fixation found it necessary to carry out simple experiments to demonstrate that this phenomenon exists at all (2, 3). We know even less about the mechanism of fixation. Various possibili- ties have been suggested, but of these only two are worthy of consideration. If an odorant is mixed with a fixative and its evaporation thereby re- tarded, it may be that this is due to a simple dilution effect (lowering of partial vapor pressure due to the addition of the fixative), or that it is