Osmotic dehydration phenomena in gel systems

Swelling or contraction plays an important role in affecting the mass trans fer during osmotic dehydration (OD) of hydrophilic and hydrophobic gels. Th e initial stage of the OD process is characterized by mass loss until the g el volumes reach a minimal value. Two types of behaviours are observed afte r this point. The first one (Type I) shows a turning point and re-swelling of the gel. The main reason for such behaviour is the relatively high swell ing pressure of the gel in a sugar or salt solution. In certain solutions, such as of sugars and sodium chloride, hydrophilic polyacrylamide gels swel l even to a larger extent than in water. The second type of behaviour (Type II) is characterized by total lack of re-swelling capability, which is mos t likely the result of a phase separation process. In hydrophobic gels, pha se transition may take place under critical conditions that are determined by the combined effect of temperature, type and concentration of low molecu lar solutes. In hydrophilic gels, on the other hand, "salting-out" is the m ost likely reason for phase separation. The driving forces of all these phe nomena are attributed to the effects of the preferential interaction betwee n the low molecular solutes and the gel polymeric matrix. (C) 2001 Elsevier Science Ltd. All rights reserved.