The purpose of soaking is to bring the hide to the same condition in which it was immediately after separation from the carcass. Recovered softness makes it easier to introduce small-molecule substances into the hide. During soaking the mechanical impurities: scud, blood, salt of other preservations used, a part of nonstructural proteins and remains of fat and meat are removed. The hide becomes swollen in the process. The collagen and glycosaminoglycans remain probably intact through the tanning process.

Mature crosslinked collagen is water insoluble but it swells. Extent of swelling is, in such a system, inversely dependent on the number of crosslinks. In a fiber network the solvent may occupy the inter or intra fibrillar spaces, the general regularity however remains. Swelling of collagen depends on two factors. Osmotic and Lyotropic ones. 

Osmotic swelling (Donnan swelling) occurs due to a high concentration of bound, nondiffusing ions located inside the structure. It takes place when pH of the solution is off the isoelectric point, the ionic strength of the solution is small and the temperature is low. Changes of pH in the range 4 to 8  do not affect markedly the length and diameter of fibrils. Outside these pH values almost 10 fold increase of fiber volume may be observed. Greatest swelling effects may be observed at pH 2 and 12. If pH drops below 2, then the volume decreases. The increase of ionic strength suppresses collagen swelling. It is reversible by straining of the fibers, changed pH or increase of ionic strength of the solution by increase of salt concentration.
The Donnan effect comes from the increase of charge bound at protein surface, as the pH is drifting away from the isoelectric point. According to Donnan's theory occurrence of localized charges causes formation of excess ions having opposite charges inside the gel, which in turn initiates action of osmotic forces. Donnan effect does not elucidate satisfactorily the mechanism of attachment of solvent molecules to the biopolymer, although from the thermodynamic point of view it describes very well the influence of pH on the degree of swelling. 

Lyotropic swelling which is due to neutral salts at considerable ionic strength, decreases the cohesion of the fibers and is not completely reversible.
Both swelling components may act together, and their contributions are condition dependent.
Lyotropic swelling may be observed in solutions of the salts, in which the forces, causing Donnan phenomenon are insignificant. One may be observe it at every pH if only salt concentration is high enough (over 0.5 molar) or in solutions having lower salt conc., and a neutral pH. Increasing salt concentration causes at first swelling increase (salting in) and then decrease (salting out) of swelling. Gelatin behaves like collagen. Comparing swelling effect of various salts have been ordered in a Lyotropic or Hofmeister series. 

F- < Cl- < CH3COO- < NO3- < Br- < SCN- < SO4-2
K+ < Mg+2 < Na+ < Cs+ < Li+ < Ca+2 

HCl and H2SO4 have strongest swelling power. Maximum swelling occur in their 1.5% solutions.

Swelling in nonaqeous solutions and stabilization of structure of raw hide :
The purpose of investigating swelling of collagen in nonaqueous solutions has been to find out the length of carbon chains(in alcohols) which may fit between the collagen side chains, stiffening them and making the structure immobile.
This may be observed if
i.  Surface of tension of water in which leather is  immersed  is decreased by surfactants 
ii.  The hide is treated with neutral salt soln.
iii.  The hide is dehydrated with organic solvents (ethanol, ether, acetone)
iv.  The hide is lyophilised.

The result of all operations or their combination is the removal of the substance which closes the pores during drying and considerable amounts of water, whereas the pore size do not change. The influence of dehydrating agents on hides is expressed by two factors: Volume decrease during drying and apparent specific gravity.

The characteristic property of a dehydrated hide is the loss of its properties after re-soaking in water. This process is reversible unlike the tanning. If however the dehydrated hide is impregnated i.e. by silicons it remains in this state even after multiple wetting and drying. 

It is concluded that in a denaturation process besides intra molecular H-bonds, the bonds joining together the water molecules bound to protein become split. Thus the cooperative process of rebuilding the hydration layers is one of the peculiarities of "intra molecular" melting of the collagen macromolecule.

Hide behaves like an ionite resin, may behave like a molecular sieve or an inactive bulk polymer (depending on reference system).

Hide in the soaking process imbibes and binds more water, the lower the process temperature is. Simplest explanation is the increase of mobility of water molecules i.e. its entropy decrease with temperature. A substance or action increasing the distances among molecules has such an effect on an unordered system. Increase of ordering, however, is connected with entropy decrease. There is a temperature point (TO or TS ) at which the long range order structure sharply decreases. Problems of water structure are related to hide-water system, in which the structure making function belongs to the functional groups of hide proteins and to structure making ions contained in the system. X-ray show long range interactions (7A0) disappear at higher temperature.

In heating the hide one observes a shrinkage of over 50% of the sample length. This is best observed if the sample is immersed in water. The temperature of shrinkage (Ts) depends on degree of crosslinking; it is lower for the raw hide, higher for the leather.
The non-swollen collagen is, a highly ordered polymer, which is synonymous with its crystallinity.It has high elasticity and a limited liability to deformation.