The effect of calcium salts on the viscosity and adsorption behavior of xanthan

The effect of CaCl2, Ca(NO3)2, CaSO4, CaCO3 and Ca3(PO4)2 on the flow behavior of xanthan gum solutions was investigated. Regardless the concentration and type of calcium salt used, xanthan solutions presented pseudoplastic behavior. The soluble salts (CaCl2 and Ca(NO3)2) induced the disordered state in the xanthan chains at concentration of 1.0 g/L or 10 g/L, decreasing the flow consistency index (K) values. At 100 g/L soluble salts K values were similar to those found for pure xanthan solutions, whereas at the same concentration of insoluble particles the K values increased 20%. The adsorption of xanthan gum onto Si/SiO2 surfaces in the presence of calcium salts was investigated by ellipsometry and atomic force microscopy (AFM). The adsorbed layer of xanthan onto Si/SiO2 consisted of two regions: (i) a thin acid resistant sublayer, where xanthan chains were like highly entangled fibers and (ii) a thick upperlayer, whose morphology was calcium salt dependent.