Stability of Allophane, Allophanic Clay, and Allophane-Halloysite Floc in Aqueous Solutions of an Anionic Exocellular Heteropolysaccharide (Gum Xanthan) from Xanthomonas campestris

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Stability of allophane with positive and/or negative charges in the presence of gum xanthan (GX) was mainly investigated at three pH levels. At pH 4.5, allophane showed polymeric flocculation (PFL) at low GX concentrations, and polymeric stabilization (PST) at medium GX concentrations. Flocculated allophane (i.e.p.) at pH6.5 showed polymeric weak-stabilization (PWST) at extremely low GX concentrations, besides PFL and PST described above. At pH 8.5, allophane did not exhibit any change in stability with GX addition. The PFL and PWST can be regarded as charge neutralization between allophane and GX, Allophanic clay with i.e.p. at pH 10 exhibited both PFL and PST but no PWST, even at pH8.5. The PWST can be explained by the disintegration of the floc when interparticle attraction was weakened by the adsorption of GX. The lower PWST and PST of allophane-halloysite mutual floc strongly suggested the presence of a more stable arrangement among these dissimilar particles. Stability of crystalline clays was also studied for comparison. The above behavior was analyzed on the basis of (i) the suspended microaggregate of allophane, and (ii) the double-stranded wormlike structure of GX, which was taken in this study as a model substance of anionic microbial polysaccharide.
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