4 JOURNAL OF COSMETIC SCIENCE
Q)
0.3 J 2
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u,
u,
� 0.2 alt
"C )(-jJ
Cl 6 6 0 X !•
---....+••
0 0 )()(!0.1
X • ♦
Cl
•
-:::.-i 6
�
0.0 • •
0 20 40 60 80 100
Relative Humidity, %
Figure 1. Equilibrium water sorption for human nail (mean ± SD of four donors, n =4-6/donor). Uptake
(•), desorption (0). Data from Baden (17) (x) and from Turek et al. (28) (+).
network as the tissue swells and deswells. The freezing of the cadaver nails during
storage may have been a factor however, it should be noted that the water in nail is
highly bound to keratin. Based on an analogy with stratum corneum at comparable
water contents, it is likely there is no freezable water in cadaver nails (27).
Compared to other reports of nail water sorption (17 ,28), the water uptake values
obtained in our laboratory are slightly lower throughout the entire range of relative
humidity. They are also somewhat more scattered. The lower values may be related to
temperature the literature values were obtained at 25 ° C, whereas our studies were
performed at 32°C. Similar results have been seen in wool-water vapor isotherms where
the amount of water adsorbed at any specified humidity decreases as the temperature is
increased (8). Such a dependence is expected for an exothermic sorption process (29). The
scatter may be related to the use of nonsaturated salt solutions for most of the equili-
brations. While the RH of these solutions can be accurately calculated (18), it can drift
as the solution exchanges water with the environment. Saturated solutions provide a
more reliable RH-see Yabuza (30) for an excellent discussion.
Figure 2 shows a comparison of water uptake in nail to that for other keratinized tissues,
i.e., the hard keratins horn (9), wool (31), and hair (31), and the soft keratin found in
stratum corneum (21). There is a major difference in the total water uptake between soft
and hard keratins. This may be attributed to the presence of high levels of cystine

0.8
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0.0
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THE HUMAN NAIL: SORPTION ISOTHERMS
� □ ◊. ·�---·· ...
◊ � .....................
······•··········
·· · · ··
20 40 60
Relative Humidity, %,
80
5
100
Figure 2. Mean equilibrium water sorption for human nail (e), horn (9) (6), hair (31) (◊ ),wool (31) (□),
and human stratum corneum (21) (dotted line). The nail data are the mean of the uptake and desorption
values in Figure 1.
in hard keratin, which restricts excessive swelling due to the disulphide linkages. It is
also evident that the shape and the magnitude of the nail isotherm are closest to that of
horn. In the range of 30-60% RH, nail, horn, wool, and hair sorbed approximately the
same amount of water. Phylogenetically, these hard keratin structures arise in tissues
that provide a static function and are exposed to more pressure and friction than other
keratinous membranes hence they require a fair amount of rigidity. Excessive water
uptake would soften these structures, detracting from their function. Their low water-
holding capacity corresponds to this requirement.
Fits of the D'Arcy-Watt and GAB models to the uptake and desorption data are shown
in Figure 3, and the regression parameters are shown in Tables I and II. Both models fit
the entire data set with r2 0.99. The GAB model fit the data in the region of higher
water activity very closely but missed the initial bend of the isotherm at water activity
in the range of 0.3-0.5. At lower RH (25%), water molecules are principally bonded
to hydrophilic sites [primarily amino and carbonyl sites (8)} by hydrogen bonds this
region may be described by a Langmuir isotherm. The values of the monolayer volume,
vmJ estimated from the GAB model, were 0.06 and 0.11 g H
2 O/g dry tissue for the
sorption and desorption phases, respectively. The desorption value was higher than that
for uptake, possibly due to the larger number of water-binding sites available on de-
sorption. At higher humidity, additional water is adsorbed, leading ro multilayer sorp-
tion, with the tissue being saturated with 0.3 g H
2
O/g dry tissue at 100% RH.