(317e) Molecular Thermodynamics Modeling of Water Equilibrium and Heat of Sorption In Human Stratum Corneum

Understanding the thermodynamic water interactions in biological materials
plays an important role for many scientific and engineering applications. 
In particular, preservation in artificial environments or skin care using
moisturizers and other drugs. The stratum corneum (SC) is part of the
human skin, which serves as an important barrier to maintain the physical
and chemical equilibrium with the environment for many compounds, in particular water.
Due to SC's rate-limiting diffusion characteristics, it is a good model
to study water-skin interactions, in general. The description of water
in SC is commonly performed using extensions of the Langmuir and
the Brunauer-Emmett-Teller (BET) adsorption models developed
mainly to describe the adsorption of gases in hard surfaces such as metals.
A common extension of this approach is the Guggenheim-Anderson-deBoer (GAB)
isotherm model. These models do not take into account other important
water chemical activity effects in soft-biological materials such as mixing and
elastic contributions. In this work, we use a molecular thermodynamics
framework to describe the equilibrium moisture content (EMC) in SC. The parameters
of the model are obtained using experimental data reported in the literature.
The proposed modeling approach, in addition to describe EMC, is easily expanded to
obtain the water heat of sorption. This molecular-based model provides a
a more fundamental view on the water interactions in biological materials from
a classical thermodynamics perspective. This is achieved by building water
free energy expressions with physical parameters that capture better the contributions
of adsorption, energies of mixing, and elastic or stretching of the material.  
The estimation of water heat of sorption is done naturally by calculating the
difference in enthalpy between the SC and vapor phases -- extracted from the
free energy expressions. The results of the model show very good
agreement for EMC and heat of sorption of previously reported data on  
this system, with model parameters describing water molecular thermodynamic
interactions more naturally.