Conditioning Agents for Hair and Skin

124 Carson and Gallagher
oilier-feeling ester or, if the molecular weight is high enough, an ester with a
waxier feel.
In our example of the simple ester IPM, we treated it like a single chemical
entity. It must be stressed that this is typically not the case for cosmetic ingredients.
Usually, an ester like IPM consists of isopropyl esters of a variety of
fatty acids. IPM consists mainly, but not exclusively, of isopropyl esters myristic
acid. Also present, depending on the fatty acid distribution of the starting
material, will be quantities of isopropyl laurate, palmitate, and stearate. This
fact is often overlooked in the comparisons of emollient esters since these
comparisons are most often performed on the commercially available materials.
Such comparisons are practically useful, but they often are filled with ambiguity,
or at the least imprecision, when they are used for the scientific purpose
of relating the structure of the ester to performance. We need to keep
these limitations in mind when reviewing such information. These limitations,
however, do not prevent us from making useful generalizations or interpretations
based upon our understanding of the individual components.
The straight-chain, saturated fatty acids typically used in simple emollient
esters range from lauric (Cizio) acid to stearic (Ci8:o) acid. Shorter hydrocarbon
chains are not capable of delivering the nongreasy, lubricating emollient feel
associated with cosmetic esters. Higher hydrocarbon chains are likely to produce
esters with melting points well above skin temperature. It is difficult to
consider these esters as emollients; they are probably better categorized as wax
esters. Of course, the simple esters can also be based on unsaturated fatty acids
such as oleic (Cis:i) and linoleic (Cisa) acids, as well as the less common palmitoleic
(Ci6:i) acid.
Although specific comparative data for the isopropyl esters are not complete,
based on the information available we can make some generalizations
concerning these straight-chain fatty chains. As the molecular weight of the
isopropyl esters increases, so do the solidification or melt point, viscosity, surface
tension, and amount of ester that persists on the skin after 1 or 2 hours
(Table 6).
With regard to the correlation of the skin feel of these esters in formulations
to their chemical structure, the "Rosetta Stone" of such information is still the
Goldemberg Article. In this paper, various esters are compared for their "initial
slip" and "end feel" in a simple oil-in-water emulsion. Goldemberg notes
that the isopropyl esters were often ranked the best in each series of esters
studied. The data suggest that although the feel properties of the myristate
and palmitate are very similar, the myristate scored slightly higher in initial
slip but slightly lower in end feel. Perhaps the most interesting observation is
that the isopropyl linoleate (Cisa) ester is significantly poorer for both initial
slip and end feel. Perhaps this is why the saturated-chain and branched-chain
isopropyl esters have been preferred in actual use.