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

170 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS due to some kind of attraction or association between odorant and fixa- tive. Pickthall (2) has shown that the hypothesis that fixation is due to dilution only cannot account for some facts familiar to every perfumer, and that it is reasonable to assume some degree of molecular association. If this is so, it follows that the chemical structure of the substances in- volved is important and that the action of a given fixative should be more or less selective. In general, molecular association and hence fixative action would tend to be more pronounced when the odorants and fixatives are closely related chemically than in cases where the two have widely different structures. This would mean that in the case of a complex per- fume mixture we can, by the proper selection of fixatives, control to some extent which components will be most pronounced in the end note (i.e., the odor of the perfume in its final stages of evaporation). Let us add to this the observation that a fixative, by lowering the rate of evaporation, not only strengthens the odor of the fixed odorants in the final evaporation stages, but that it also weakens it in the earlier stages. We are then forced to conclude that if molecular association and electro- static attraction really play an important role, the choice of a fixative affects the odor balance of the perfume throughout its evaporation. Since all this has not only theoretical, but also far-reaching practical importance, it certainly seems worthwhile to study the action of fixatives more closely by means of experiments. EXPERIMENTS ON FIXATION The most straight-forward way of studying the action of a fixative would be to make a dilution of some perfume material, add a fixative to a portion of this, let the solutions both with and without fixative evaporate, and ob- serve in which one the odor of the perfume material remains noticeable for the longest time. Anyone who has ever taken a perfume blotter and tried to indicate the moment at which the odor of the material on it ceases to be perceptible realizes that this is a difficult and poorly reproducible determination. Two perfume blotters dipped simultaneously into the same solution usually do not lose their odor at exactly the same rate. It is hard for any in- vestigator to reproduce his end point, since much depends on the degree of freshness or fatigue of the nose. If several persons participate in this test, you find that it is nearly impossible to reach any close agreement. For this reason we tried to find an experimental approach that is less sub- jective and more reproducible and reliable. The general plan of our experiments was as follows: We took two odorants that differed in one or more of the structural features thought to play a role in electrostatic attraction (e.g., an aliphatic and an aromatic alcohol, or an aliphatic ester and an aliphatic alcohol).