Colloidal dispersions thixotropic suspensions

Colloidal dispersions often display non-Newtonian behaviour, where the proportionality in equation (02.6.2) does not hold. This is particularly important for concentrated dispersions, which tend to be used in practice. Equation (02.6.2) can be used to define an apparent viscosity, happ, at a given shear rate. If q pp decreases witli increasing shear rate, tire dispersion is called shear tliinning (pseudoplastic) if it increases, tliis is known as shear tliickening (dilatant). The latter behaviour is typical of concentrated suspensions. If a finite shear stress has to be applied before tire suspension begins to flow, tliis is known as tire yield stress. The apparent viscosity may also change as a function of time, upon application of a fixed shear rate, related to tire fonnation or breakup of particle networks. Thixotropic dispersions show a decrease in q, pp with time, whereas an increase witli time is called rheopexy. 

Clays of the montmorillonite family are lamellar aluminosificates [46] used in many industrial processes and in products such as paints, softeners, and composite materials [47]. They swell when brought into contact with water, which is due to the insertion of water molecules between the sheets. Complete exfoliation can be induced leading to dispersions of disk-like particles of 10 A thickness and 300-3000 A in diameter, depending on the variety of clay used. These clay platelets bear a rather large surface electrical charge so that electrostatic interactions between them must be considered and are actually responsible for the colloidal stability of these dispersions. These suspensions have been widely studied as model colloids and also because they form physical thixotropic gels. 

A study was made of the comparative stabilities at various exposures of an upset stomach remedy suspension. This product consisted of a dispersion of bismuth subsalicylate and phenylsalicylate in an aqueous system. Methyl-cellulose and magnesium aluminum silicate were selected as the suspending agents, because the presence of polyvalent metallic ions precluded the use of hydrocolloids affected by these ions. In addition, it was found that methyl-cellulose contributed a demulcent effect. The viscosity, as well as the suspension characteristics of the combination of protective colloids used, was of a synergistic nature. These colloids formed a thixotropic system. The thixotropy undoubtedly aids in stabilizing this system. In order to make this product palatable and impart elegance, color and flavor were added. Sample 1 (with protective colloids) showed no separation, while sample 2 (without... 

The life of an Avicel suspension can be extended by coprecipitating the rodlike structures with a protective colloid after trituration. Avicel-RC19 is limit cellulose that has been physically modified by coprecipitation with CMC to facilite dispersibility. Avicel-RC water suspensions simulate the properties of a hydrosol. At low aqueous concentrations, the apparendy hydrated crystallites assemble into a thixotropic, heat- and acid-stable structure whose viscosity depends direcdy on pH to about pH 10, whereupon it declines precipitously. The suspension coalesces at low pH. The addition of salt after mixing increases viscosity above what it would be if the salt were added at the time of mixing or shearing. 

Viscosity (dynamic) dispersions in water at the 1-2% w/v level are thin colloidal suspensions. At 3% w/v and above, dispersions are opaque. As the concentration is increased above 3% w/v, the viscosity of aqueous dispersions increases rapidly at 4-5% w/v, dispersions are thick, white colloidal sols, while at 10% w/v firm gels are formed. Dispersions are thixotropic at concentrations greater than 3% w/v. The viscosity of the suspension increases with heating or addition of electrolytes, and at higher concentrations with aging. 

Function in Formula Suspending, thickening agent, colloidal thixotrope Use Concentration 1.2% in cosmetic, personal care suspensions 2% in reconstitutable suspensions 1-2% in pharmaceutical suspensions 1.5-2.5% in cosmetic creams and lotions Solubility Characteristics Water-dispersible... 

The suspension should meet the following demands it must be soluble in the dispersion medium or be able to swell the necessary concentration must be as low as possible the micropores of the adsorbent must not be clogged when the adsorbent is activated (200-500 C), decomposition products must not affect the surface, or there must be no decomposition at all and it should not have 8iny tendency to foam during evaporation. Suitable substances are colloids, which in addition should be thixotropic. They make the filling of very long capillaries feasible. 

The aluminum oxide layer was applied to the inner walls of the glass capillary from an aqueous dispersion in the form of aluminum hydroxide and converted In situ into aluminum oxide by heat treatment. By varying heat treatment and by blocking unwanted activities with potassium chloride, adjustment to the desired separation characteristics can be achieved [55]. To prepare the coating suspension, aluminum oxide (particles <2 urn) obtained by calcination of hydroxide is heated for 24 h at 300 °C. 20 g of the alumina is mixed with 70 ml of 5% (w/w) Baymal solution (colloidal aluminum hydroxide) and with 0.3 ml of acetic acid (>96%) and stirred for about 10 min in an ultrasonic bath. Subsequently the mixture is filtered through a wire sieve of 300 mesh and allowed to stand for 24 h for aging. The suspension thus prepared shows thixotropic behavior. 

---