30 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS NIPA-ESTER COMBINATIONS AS PRESERVATIVES AND ANTISEPTICS By ERICH BOEHM, Ph.D., F.R.I.C., •,•D ELIZABETH JONES, B.Sc. I•TRO•)UCTtO• AS FAR back as thirty years ago, when one of us, in conjunction with Th. Sabalitschka, discovered the inhibitory action of the aliphatic esters of •-hydroxybenzoic acid upon fungi and investigated in great detail their suitability for the preservation of foods, medicaments, cosmetics and a number of technical products, it occurred to us that the simultaneous use of two such esters had a favourable effect. This effect could be explained when different types of micro-organisms are present in the material to be preserved (as is usually the case), by the differing ability of different classes of organisms to resist a particular ester. In other words, there is a difference of degree in the capacity of the individual esters to harm the different types of organisms. The behaviour of the individual esters against the different kinds of bacteria in the maximum concentrations obtainable in water does, in fact, vary considerably, although in general the bactericidal effect of the esters increases with the size of the ester alkyl group. An example of the above generalisations is afforded by the study of the preservation of sugar solutions. The lowest member of the ester series, the methyl ester--Nipagin M--successfully prevents mould growth in concentrated sugar solutions, but fails to suppress fermentation by yeasts even in weak sugar solutions. For the latter, the propyl ester--Nipasol M-- proves considerably more effective. The combined use of the two esters will accordingly afford to a substance protection against attack and spoilage by both moulds and yeasts. But a combination of two esters frequently shows a stronger harmful effect upon a pure culture of one type of micro-organisms than one ester alone in the corresponding concentration. How can this be explained ? The esters of •b-hydroxybenzoic acid, indeed, differ from each other only in the size of the alkyl group introduced into their carboxyl group, and consequently the mechanism of their action on bacteria should be qualita- tively the same they should become concentrated in the same position in the bacterial cell and react in the same way with the cell contents. A genuine synergism of two esters is therefore hardly conceivable, i.e., that the two esters of the mixture should attack different regions of the cell, or the same region by a different mechanism. Nevertheless, the different activity of individual esters, in the same concentration on the same species of bacterium, can be due to their differing physico-chemical properties. * From the Research Department of Nipa Laboratories Ltd., Treforest Industrial Estate, Pontypridd, Glam.

NIPA-ESTER COMBINATIONS AS PRESERVATIVES AND ANTISEPTICS 31 Thus the speed and intensity of the interaction between esters and bacterial cell contents are influenced by the surface activity, adsorption capacity, lipoid/water distribution, and rapidity of saponification of the ester concerned. These facts have already been referred to by Th. Sabalitschka and one of us •. Accordingly, as already mentioned, the antibacterial effect of the esters increases with the size of the alkyl group. The hypothesis of a simple addition of the action of the two ester concentrations under consideration may sometimes be sufficient to explain the strengthening of the antibacterial action when two esters are used at the same time. But this simultaneous use of two esters can frequently give the impression of an increase in action. It is possible that one ester promotes the concen- tration of the other on the water surface, or bacterial cell surface, or that it increases its penetration through the cell wall or adsorption in definite regions of the cell. This gives a total action with a combination greater than simple addition of that of each separate ester. The failure of one single ester to kill or inhibit bacteria in an aqueous medium is usually because the necessary concentration is not soluble. If a second ester is added to the .highest obtainable concentration of the first one this failure can be obviated because the total ester concentration has been increased. If this second ester is at all effective on its own against bacteria in its highest concentration, the amount needed to be added to the original ester to get good results may be much below this maximum concentration. This use of a combination of two esters, therefore, affords the possibility of getting satisfactory antibacterial results with smaller total amounts of preservative than if one is employed alone, which has, of course, great practical advantages. In fact, two ester concentrations which when used separately are ineffective against a specific type of bacterium, when combined may show, all other conditions being constant, a strong action. This increase of bacteriostatic effect by the use of a suitable ester combination is very useful when the material to be preserved is of a difficult composition or the organisms likely to be encountered are particularly resistant, and it is also useful when attempts are being made at obtaining a bactericidal action. PRACTICAL CONSIDERATIONS It is obvious that ester combinations are as important for the various cosmetic preparations as they are for pharmaceutical and medicinal products. More than 20 years ago one of us •'*'4'• had already stated that pre- servatives for use in cosmetics should have the following properties: 1. The inhibiting effect should be lasting and should not depend upon volatile materials whose effects decrease because of continual evaporation.