MELTING & SHRINKAGE TEMPERATURE 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, are removed, a part of nonstructural proteins and remains of fat and meat. The hide becomes swollen in the process. The collagen of glycosaminoglycons remain through the tanning process probably intact. Mature crosslinked collagen is water insoluble but it swells. Extent of swelling is , in such a system, inversely dependent on the crosslinks number. In a fibre 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 and the ionic strength of the solution is small. Greatest swelling effects may be observed at pH 2 and 12. It is reversible by straining of the fibres, changed pH or increase of ionic strength of the solution by increase of salt concentration. Lytotropic swelling which is due to neutral salts at considerable ionic strength, decreases the cohesion of the fibres and is not completely reversible. In heating the hide one observes the 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 highky ordered polymer, which is synonymous with its crystallinity. Osmotic swelling is due to pH change, when the ionic strength is small and temperature 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 fibre volume may be observed. If pH drops below 2, when the volume decreases. The increase of ionic strength suppresses collagen swelling. 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 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 pH neutral. 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. So to make a long story short, acid swell, aint always acid swell.