SKIN COMPONENTS and WATER
Leather production is carried out largely in water. Skin does not dry between the production operations. The discussion of the character and strength of the bonds between water and skin components therefore gains importance.
Energy of ion hydration depends on charge and kind. For H+ approx. 176 kcal / mole (1156 kJ/mole). Interaction of ions and water molecules consist of water-cation and water-anion interactions. Absolute free energies and enthalpies of ion hydration are greater for anions.
Ions of small radii and multivalent ones Li+, Na+, H3O+, Ca+, Al+3, OH-, F- increase the viscosity of water -they show a structure making ability. They produce, apart from polarization, the immobilization and electrostriction (a dielectric deformation of molecules in the external electric field proportional to the square field intensity) of water molecules as well as the decrease of entropy ( due to additional ordering) in the second hydration layer.
Large monovalent ions generally give a structure-breaking effect (entalpy increase).
K+ , NH4+, Cl-, Br-, I-, NO3-, IO3, ClO4- ion increase mobility of water.
Nonpolar substances have a very strong structure forming influence on water (only observed in the first layer of water molecules). The water coordination number is increased to 5 and happens spontaneously. Water-water interaction does not change but hydrocarbon-hydrocarbon interaction decreases as hydrocarbon-water interaction is established.
Collagen – water system:
Water bound to collagen forms a kind a of chain, parallel to the collagen molecule chain. There are two water molecules per tripeptide unit firmly bound by H-bonds to the helical part of the collagen molecule (with binding energy close to 11-12 kcal/mol = 46-50 KJ/mol). Their residence time in their sites is about 0.1-1.0ms. This water accounts for 35 % of collagen weight (“first hydration shell”, and a second one between 35 to 50%). The remaining part of water, in a not strictly limited amount, which is in weak interactions with a number of different sites, forms a multilayer with liquid-like properties.
The first kind of water does not freeze at o C0. The strength of H bonds between “swelling” water of collagen is about 1-2 kcal /mol. There are no sharp limits between strongly and weakly bound water, nor between weakly bound completely free water.
Glycosaminoglycans and water:
It has been suggested that all polar groups of gycosaminoglycans are involved in water binding.
Swelling of hyaluronic acid is greatest.
The swelling degree decreases with the increase of ionic strength of the NaCl soln. This may contribute to swelling on soaking.
It can be expected that the effect of gycosaminoglycans on the behaviour of water in the skin is greater than that of collagen, since the former are more spread out and thus more accesible. Hydration of polysaccharides is directly dependent on the amount of -OH groups occurring in a unit.
Water permeability of skin:
The external surface of skin of land animals is permeable to liquids to a very limited extent. There is no substantial change after the death of the animal.
Penetration of water through skin is time proportional except for the initial period (period necessary to reach dynamic equilibrium).
The ability of ions and non ionic substances to penetrate skin is of great importance for the curing and soaking processes.
Molecules penetrate the skin in a passive way -by diffusion. Three arguments speak in favour:
1) specific permeability remains unchanged even for a long time after skin is removed from animal.
2) diffusion obeys Fick’s law , an exception is Na and K ions which are actively absorbed by the skin
Fick’s law: p = r / c
p = permeability constant
r = penetration rate
c = concentration
3) Stratum corneum of epidermis is resistant to penetration and various compounds (ie. arrest alkyl phosphoric compounds).
Permeability of skin decreases with age; also decreases to half its value as the temperature decrease by 10 K.
