Лако-красочные материалы — производство

We have touched briefly on how reduced vapour pressures can lead to reduced rates of evaporation. This means that physicochemical proper­ties not only influence perfume volatility per se but also affect other aspects of fragrance behaviour, such as substantivity and retention. These terms are used within the industry to denote perfume longevity in use, usually with respect to a particular substrate and/or surface (e. g. skin, hair, cloth, etc.).

To understand perfume behaviour on these surfaces and/or matrices, we must consider the range of attractive or repulsive forces between the perfume components and the surface itself. The situation is compli­cated by the way in which perfume is delivered to the surface. For example, for a perfume ingredient in a soap bar to be substantive it must first be efficiently delivered to the skin during washing, it must then survive rinsing and, finally, it must be retained for some time on the skin. Definitions of‘substantivity’ and of‘retention’ vary, but here ‘retention’ is used to indicate the affinity a perfume has for a substrate when delivered to it, whilst ‘substantivity’ also includes delivery barriers.

As implied above, the delivery of at least some perfume to a surface is, except perhaps for air fresheners, a key requirement for perfume longevity in use. For certain products, such as cologne or deodorant, perfume delivery is very effective since the perfume is applied directly to the target substrate (the skin). Delivery from other products, such as soap or laundry powder, involves perfume transferring from an aqueous detergent solution or dispersion to a substrate or surface. This process has analogies with partitioning of materials between different phases, and, perhaps not surprisingly, we find that the octanol-water partition parameter (log P) may often provide insights into physical behaviour. For example, in dilute wash systems the deposition of ingredients onto substrates is often moderately or strongly correlated with log P. It is likely that the log P values reflect the solubilities of the perfume ingredients. By definition, materials with high log P such as the macrocyclic musks (log P values typically of 4.5 to 6.5) tend to be hydrophobic (‘water hating’) and partition where possible into more lipophilic (‘oil loving’) phases. Conversely, materials with low log P such as 2-phenylethanol (log P of 1.5), are hydrophilic and either soluble or sparingly soluble in water.

In fact, several well known models in the literature allow solubilities in water to be estimated reasonably well from a knowledge of log P (and according to the functional groups present). For example, Hansch et al. (1968) has published several linear free energy relationships (LFERs) between molal solubility and log P for various classes of monofunctional molecules. The correlation coefficients for the LFERs (a measure of ‘goodness of fit’) were in the range 0.93-0.99, indicating that solubility estimations, at least for some classes of material, are likely to be relatively accurate.

We may conclude from the above that values of log P appear to give some guidance to the tendency of perfume ingredients to move from aqueous systems to (presumably) less polar surfaces (skin, hair, etc.).

Analogous partition data are available for solvents other than octanol, e. g. olive oil, and these may be more pertinent in certain situations. However, log P values based on octanol/water partition are easily accessible for thousands of substances and, for most of the compounds found in perfumery, may be estimated reasonably well using one of the mathematical prediction models described in the literature [the two most well known are those due to Rekker (1977), and to Leo et al. (1971)]. Additionally, it is often true that for many materials partition coefficients determined in different solvent-water systems often corre­late strongly with one another.

Unfortunately, the log P of an ingredient does not always suffice to describe behaviour adequately. For example, in concentrated aqueous detergent systems it is probable that perfume partitioning into complex surfactant phases becomes dominant, and knowledge of log P provides only a partial understanding. It then becomes necessary to search for other parameters which may be of more use. As mentioned above, a large number of parameters may be considered, but here we look more closely at just one, the solubility parameter (introduced earlier).

The Hildebrand solubility parameter has its origins in the develop­ment of what is known as ‘regular solution theory’. As can be seen from equation (5), it is essentially a measure of how much energy is needed to disrupt intermolecular cohesion: the higher the sp value the more cohesive the material (‘sticky’ at the molecular level), and the harder it is to separate into individual molecules in the gas phase. Originally, sp was exploited primarily in the paint and polymer industry, but has since been found useful across a number of applications (Barton, 1985), too numerous to discuss here, but dealing with properties such as viscosity, surface adhesion, miscibility and, of course, volatility. As a working rule, different molecules with similar values of sp are likely to have significant interaction. This is similar to some of the conclusions made above when discussing polarity, and it also suffers from the same drawback, viz. the overall interaction may be complex, deriving from a superposition of mechanisms. It is possible to resolve sp into different components reflecting different interactions (e. g. hydrogen bonding, dispersion, etc.), and these may sometimes be more useful than the overall sp value.

The sp values of most perfumery ingredients fall between ca. 16 MPa0,5 (non-polar materials such as terpene hydrocarbons) and ca. 25 MPa0’5 (polar materials such as alcohols). In general, we expect materials to have lower activity coefficients in microenvironments characterized by similar values of sp. For instance, limonene (sp value of 16.5) is expected to be compatible with plastics such as polyethylene and polypropylene (sp range typically 16-18), and to exhibit good solubility and retention in these polymers. We anticipate that other ingredients, such as phenylethanol (sp value of 23.7), would have less desirable interactions with these polymers, for example promoting phase separation, crazing, stress cracking, etc. On the other hand, in partially hydrolysed polyvinyl acetate (sp range typically 22-24), the situation is reversed. The solubility parameter thus finds good practical use for understanding perfume interactions with plastic packaging, as well as for providing a basis for understanding affinities in general.

It is only rarely that we have explicit values of sp for a surface or substrate of interest, but this does not impede study and model building from the perfumery perspective. The approaches outlined earlier, together with appropriate parameters capable of ‘capturing’ the major types of interaction present in a system, may all be used to help build up a picture of the key features in the delivery of perfume to skin, fabric, etc. Once on the target site, the affinity between various perfume ingredients and the site may be quantified analytically, and investigated theoretically. It is important to recall, however, that the single most important property for prediction of substantivity remains, except where partitioning is extremely discriminating, the ingredient vapour pressure.

CONCLUSIONS

In summary, the volatility and headspace behaviour of perfume components is broadly comprehensible in terms of molecular interac­tions, both within products such as shampoos and colognes, and on or within substrates such as cloth or hair. However, the extreme complex­ity of the interactions, and the number of components invariably present, renders it difficult to predict a priori the headspace composi­tions in any given situation. Similar comments also apply to the related phenomena of ingredient or perfume fixation and substantivity. Never­theless, it is possible to:

—quantify perfume behaviour analytically for any given product and in-use combination;

—analyse the data obtained from the first stage to identify ingredi­ents that perform well, and to seek (empirical) mathematical models that explain the behaviour for a particular system.

The knowledge and understanding gained in this manner is part of the cycle of learning, perfume creation and performance evaluation that is a fundamental element of modern perfumery.

REFERENCES

A. F. M. Barton, Handbook of Solubility Parameters and other Cohesion Parameters, CRC Press, Boca Raton, FL, 1985.

J. M. Behan and K. D. Perring, Perfume Interactions with Sodium Dodecyl Sulphate Solutions, Int. J. Cosmet. Sci., 1987, 9, 261-268.

C. Hansch, J. Quinlan and G. Lawrence, The Linear Free-Energy Relationship between Partition Coefficients and the Aqueous Solubilities of Organic Liquids, J. Org. Chem., 1968, 33, 347-350.

A. Leo, C. Hansch and D. Elkins, Partition Coefficients and their Uses, Chem. Rev., 1971,71, 525-616.

R. F. Rekker, The Hydrophobic Fragmental Constant, Elsevier, Oxford, 1977.