THE HUMAN NAIL: SORPTION ISOTHERMS
Bbx Ccx v=---+--
1 +bx 1 -ex
3
(1)
where B1 b1 C and c are disposable constants. The first term on the right hand side of
equation 1 is a Langmuir-like term describing primary adsorption in that sense B
corresponds to the monolayer volume, v
m
(20). The second term describes multilayer
sorption as discussed in (1). We also considered the Guggenheim-Anderson-deBoer
(GAB) model (equation 2):
V cKx
v
m (1 -Kx)(l -Kx +cKx) (2)
Equation 2 is a modified BET isotherm (20) indeed it was called just that in a previous
report from our group (21). The parameters c and vm are the binding constant and the
monolayer volume, respectively. The parameter K [equal to 1/a0 in (21)} is a positive
constant, with a value less than 1, that is associated with weakly bound water in the
second and successive layers surrounding the keratin fibers. The GAB model has found
wide applicability in hydrophilic polymer (22,23) and food (24) systems and has con-
siderable theoretical justification (25). In particular, non-unit values of K arise from a
phenomenon known as "jamming" that is physically more realistic than the unimpeded
sorption process inherent in the BET model (2 5 ).
The above sorption models were fit to individual data sets (nail clippings and intact
cadaver nails) for both the sorption and desorption phase, using nonlinear regression
(SigmaPlot®, Jandel Scientific). Values were compared by one-way ANOVA, and dif-
ferences having p 0.05 were considered significant.
RESULTS AND DISCUSSION
The results of human nail water sorption studies conducted in our laboratory are shown
in Figure 1, along with data from Baden (17). The results show that the human nail has
a maximum water uptake of �0.3 g H
2
O/g dry tissue at 100% RH. The magnitude of
water sorption in nail clippings was comparable to that of intact cadaver nails, but
showed less hysteresis, as discussed below.
The nail water sorption isotherm is a Type II isotherm with a characteristic hysteresis
between uptake and desorption, similar to that observed in wool, hair, and porcupine
quills (8). The hysteresis may be attributed to an unfolding of the keratin bundles on
adsorption that is not immediately reversed on desorption (5 ).The hysteresis was more
pronounced in some nail samples than in others. In particular, it was higher in intact
cadaver nails than in nail clippings. We postulate the observed difference may be related
to the differences in surface-to-volume ratio. The hydrostatic pressure exerted by the
adsorbed water on the samples having a higher surface-to-volume ratio (clippings) would
be less in comparison to those having a lower ratio (intact nails). Other explanations are
possible. El-Shimi and Princen (26) suggested that the hysteresis effect for human
stratum corneum was correlated with the process of aging and the associated change in
stratum corneum elasticity. Watt (8) argued that hysteresis in wool was associated with
relaxation processes taking place within the fibers and also discussel Lhe influence of the
drying process. All of these arguments invoke the concept of stress within the microfibril

4 JOURNAL OF COSMETIC SCIENCE
Q)
0.3 J 2
:::::,
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