Welan

 

• Introduction

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Introduction

Welan belongs to the gellan family, and it is the simplest of the branched polysaccharides.

Fig. 1 Chemical repeat unit of Welan

 

 It dispays very high viscosity in aqueous solution up to 130° C.  These functional properties make it an ideal candidate for such uses as cement addition in underwater pillars and as viscosifying agent in oil drilling.  

Under the influence of calcium, Welan can be oriented in the form of polycrystalline fibers ic which give X-ray diffractogram of sufficient quality to establish 3-dimensional features . As with gellan, welan occurs in the form of a double helical structure, and crystallises in a trigonal space group.

Fig. 2 Double helical structure of Welan. The pitch (i.e. 2c)  of the welan chain is 57.38 Å The chain conformation is stabilized by six intrachain hydrogen  per pentasaccharide repeat. Three occur within the main chain whereas the three other ones involve the branched components. 

The packing features of the three chains that cross the trigonal unit cell are characterized by both parallel and anti-parallel arrangements of the double helices. The structure is highly hydrated as up to 75 water molecules have been located in the unit cell that complement the occurrence of six calcium ions. Six ions are involved in chains of carboxylate….calcium….water….carboxylate interactions.

Fig. 3 Schematic representation of the packing of welan chains in the trigonal unit cell.