43 HyaluronicALURONIC acid water binding
which is 22,389. Experimental measurements of the number of water molecules bound
per disaccharide repeating unit discussed above gave ca. 14 (±5) waters per disaccharide
repeat unit. The only obvious way water would bind to a carbohydrate polymer is through
hydrogen bonding. However, each hyaluronic acid disaccharide repeating unit has only 12
atoms capable of hydrogen bonding, and each of those can only bind to a maximum of 3
water molecules, for a theoretical maximum of 36 hydrogen-bonded water molecules per
disaccharide. Because the molecular weight of water is 18 and that of the repeating unit
in the HA polymer (N-acetylglucosamine-D-glucuronic acid) is 403, the theoretical mass
ratio for a water-HA hydrogen-bonded complex would be 36 × 18 =648 per 403 amu.
That ratio works out to be 1.61 times the weight of the HA polymer.
While the foregoing seriously compromises the idea that HA binds 1,000 times its weight
in water, there are other synthetic polymers that do bind large amounts of water per unit
Figure 3. DSC data for 10% solution of glycerol scanned from 20 to −40 to 20°C in triplicate.
Figure 4. DSC data for pure water scanned from 20 to −40 to 150°C (control).

44 JOURNAL OF COSMETIC SCIENCE
weight. For example, poly(acrylate) sodium salts are well-known, found in various personal
care products, and may absorb 300 times their weight in water (from 30–60 times their
own volume). These polymers are typically used in a dry state, are envisioned to convert into
their hydrated forms that sequester water, and would not be considered useful ingredients
for cosmetic moisturizers. Given past claims about hyaluronic acid, one might imagine
that the special composition of HA and 1,000 times its weight in water would give a
hydrated form with special properties, such as a stable gel or solid form. However, the
0.1% hyaluronic acid solution prepared in this study is a free-flowing liquid that looks
and behaves like water. Thus, that amount of water is dissolving HA as it would any other
solute to form a dilute solution.
It is appropriate to contemplate the water-binding properties of HA in relation to glycerol.
While the theoretical maximum hydrogen-bonding capacity of glycerol is nine water
Figure 5. DSC data for 0.1% (w/w) solution of hyaluronic acid scanned from 20 to −40 to 150°C.
Figure 6. DSC data for a 10% solution of glycerol scanned from 20 to −40 to 150°C.