Thixotropy:
Only exhibited by plastic & pseudoplastic flow not dilatent. It is an isothermal but comparatively slow recovery on understanding of a material whose consistency last through shearing. It depends upon:
1). Rate of shear increased/decreased
2). Length of time (time of shear)
3). Degree of structure in a sample
Antithixotropy: e.g. Magnesia magma, clay suspension, low content of solid i.e. 1-10% flocculated. The equilibrium state is sol. There is increased collision frequency of dispersed particles or polymer molecules in suspension resulting in increased interparticle bonding with time. This changes the original state i.e. initially small flocule to relatively large floccules at equilibrium state. At rest, larger floccules break up and gradually return to original state of small floccules & individual particles.
Colloidal Ag → Germicidal e.g. Silver Suphadiazine
Colloidal Cu → Anticancer
Colloidal Au → Anti Gout
Lyophobic colloids are prepared by special method like (i) dispersion by –
a. Ultrasonic
b. Electric arc
c. Milling & Grinding (Colloid mill)(ii) Condensation method/aggregation method that involves high degree of initial supersaturation followed by the formation & growth of nuclei.Hardy-Schule rule: for lyophobic sols precipitating power of anion on a disperse phase of opposite charge increases with increase in valence charge of the ion/capacity of ions to coagulate lyophobic/hydrophobic sols.Cations: Al3+ > Ba2+ > Li2+Hofmeister rule for lyophilic colloid/salting out: Coagulation power of cation/anion is related to removal of solvent sheath from lyophilic colloid. So its an ability of ion to separate water molecules from colloidal particles.Cation: Mg2+ > Ca2+ > Na+ > K+
Anion: Citrates > tartarates > Sulfates > AcetatesFlocculation Number: Coagulating power of an electrolyte is expressed in flocculation number. Minimum concentration of electrolyte required to precipitate the sol in 2 hrs. lower the flocculation value, higher the precipitation power of an ion.Coacervate: When –ve & +ve charged hydrophilic colloid mixed then the particles separate from the dispersion to form a layer of colloidal aggregates called coacervate.
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