POTENTIAL UTILITY OF ION-EXCHANGE RESINS 265 the gel structure of the polymer and make contact with the exchange sites. As a general rule, the smaller the particle size of the resin, the higher the reaction rate, although for practical purposes there is a limiting particle size that can be easily produced and beyond which the reaction rate of the resin is not materially affected. This particle size is in the 400 mesh range. "Porosity" or degree of crosslinking of the resin polymer is also important since this factor controls the dimensions of the paths in the resin gel struc- ture through which the adsorbable ions must travel. Fortunately, the porosity of the present-day ion exchange resins can be controlled w•thin reasonable limits. Therefore, it is possible to modify the reaction rate of these polymers to some degree bv varying the concentration of crosslinker in the exchanger. It should be explained, however, that there is a point at which the degree of crosslinking or "porosity" of the resin ceases to be important in determining the adsorption acitvity of the polymer. At this stage, particle size becomes the controlling characteristic in determining reaction rate. To illustrate, various resins of the IRA-411 (XE-98) type varying in crosslinkage or "porosity" can be prepared and the particle size of these polymers can be varied. The effect of each of these resin proper- ties on the adsorption of a typical constituent of axillary perspiration such as lactic, butyric or caprylic acid, can be determined. By plotting the reaction rate values from these two series of resins, one varying in porosity and the other in particle size, on the same graph, the rate curves will cross. Such studies should be of value in determining the optimum combination of resin properties required to achieve maximum effectiveness in the ad- sorption of the known odoriferous constituents of perspiration. Other factors to be considered in such investigations are the size of the ions to be adsorbed and their relative affinities for the exchange group at- tached to the resin polymer. Usually, as the molecular weight of the ad- sorbable ion increases, its affinity for the exchange sites also increases. Of course, concentration of the reacting ions in contact with the resin also affects the activity of the exchanger, particularly if the total quantity of ionizable material to be adsorbed is greater than the total available ex- change capacity of the resin combination. In summarizing this phase of the discussion, it should be kept in mind that an equilibrium reaction governs the function of an ion exchange resin or combination of resins when applied to the skin. It is not a static equilib- rium, however, because while the concentration of the resin phase essen- tially remains constant, the concentration of the reaction phase, i.e., perspir- ation, varies appreciably. The driving force of the adsorption arises from the almost continuous contact of the exchanger with fresh prespiration. In turn, perspiration flow will be governed to some extent by the effective- ness of the astringent present in the formulation. All of these phenomena must be considered in preparing a suitable product.

266 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS SELECTION OF SUITABLE COMPOUI•DING VEHICLES The last point to be considered in gauging the utility of ion exchange resins in antiperspirant-deodorant formulations is the nature of the com- pounding vehicle. Since the resins are solid, insoluble particles, and be- cause their reactivity depends upon the migration of the ionizable con- stituents of perspiration to the exchanger sites of the resins, it is necessary to choose a reagent or combination of reagents as the compounding base which will maintain the ion exchange materials completely dispersed and which will also allow diffusion or transfer of the materials to be adsorbed through the medium to the exchanger sites of the resin particles at a reason- able rate. Moreover, the vehicle must have certain adhesive properties which will maintain the resin in contact with skin at the site of application for reasonable periods of time. While the optimum rate of ion diffusion through the compounding vehicle can only be determined by experimenta- tion, some rough yardsticks can be established by which to select suitable media for this purpose. First, the rate of diffusion, and in turn, ion adsorption, cannot be too rapid because this may result in localized concentrations of hydrogen or hydroxyl ions which could cause skin irritation. On the other hand, the achievement of equilibrium cannot be too slow or the compound will give ineffective results. Second, the choice of resins will determine to some extent the nature of the compounding medium. For examp!e, if strongly acidic or strongly basic resins are employed, the more hydrophobic type vehicles such as white petrolatum, should first be tried. Such materials prevent rapid diffusion of ionic constituents and therefore cut down the over-all reaction rate. Conversely, when weakly basic and weakly acidic exchangers are used, a hydrophylic or polar vehicle should first be investigated, since the speed of adsorption in this case will probably be governed by the reaction rate of the resins themselves, rather than by the rate of diffusion of the perspira- tion through the supporting vehicle. In conclusion, it can be said that the availability of a wide variety of synthetic ion exchange resins, exhibiting the properties of solid acids and bases, has opened a new avenue of investigation for cosmetic chemists. Preliminary reports from dermatologists studying these materials have shown that the novel properties of these polymeric adsorbents should be useful in developing antiperspirant-deodorant preparations which have effective deodorizing action and high buffering power in the optirnum pH range of 5.0-6.0. Through greater knowledge of the physical and chemical properties of ion exchange materials, cosmetic formulatots will be in a better position to assess the utility of these interesting reagents in the preparation of new and improved products.