Dispersion steric stabilization

Vincent, B., Luckham, P.F. and Waite, F.A. (1980) The effect of free polymer on the stability of sterically stabilized dispersions. Journal of Colloid and Interface Science, 73 (2), 508-521. 

Influence of Addition of Electrolyte and Increase of Temperature Addition of electrolyte or increase of temperature at a given electrolyte concentration to a sterically stabilized dispersion may result in its flocculation at a critical concentration or temperature, which in many cases coincides with the theta point for the stabilizing chain. At the theta point the mixing term in the steric interaction is zero and any yield value measured should correspond to the residual van der Waals attraction. The energy arising from van der Waals attraction may be calculated from the following approximate relationship,... 

This model was introduced by Neville and Hunter (13,14) for the case of sterically stabilized dispersions which have undergone reversible flocculation. It is assumed that the major contribution to the excess energy dissipation in such pseudoplastic systems comes from the need to provide energy from the shear field to separate contacting particles. Under these conditions, the extrapolated yield value is given by the expression (13,32,33),... 

E. Ruckenstein and LV. Rao Effect of solvent on the stability of mixtures of sterically stabilized dispersions and free polymers, COLLOIDS AND SURFACES 17 (1986) 185-205. 

EFFECT OF SOLVENT ON THE STABILITY OF MIXTURES OF STERICALLY STABILIZED DISPERSIONS AND FREE POLYMERS... 

The phase behavior of mixtures of sterically stabilized dispersions and free polymer molecules is determined by the interparticle potential. In such systems, the interplay between the repulsive steric forces and the attractive forces due to the presence of free polymer molecules and van der Waals interaction between the particles determines the onset of instability. 

When the concentration of the free polymer is set equal to zero, the situation corresponds to pure steric stabilization. The free energy of interaction due to the interpenetration of the adsorbed polymer chains has a range of 26, where 6 is the thickness of the adsorbed layer. This free energy is proportional to the quantity (0.5 - x), where x is the Flory interaction parameter for the polymer-solvent system. Thus, a repulsive potential is expected between two particles when x < 0.5 and this repulsion is absent when x = 0.5. For this reason, it was suggested [25] that instabilities in sterically stabilized dispersions occur for x > 0.5, hence for theta or worse-than-theta conditions. However, the correlation with the theta point only holds when the molecular weight of the added polymer is sufficiently high... 

Fig. 6. Critical particle concentration, p Ip0, above which a dilute disordered phase and a concentrated ordered phase coexist in equilibrium for a sterically stabilized dispersion, as a function of the particle radius, a. System polyisobutene-stabilized silica particles in cyclohexane, 6=5 nm, T = 308 K, xi = 0.47, x2 0.10, A 4.54kT and v = 0.10.

Dependence of the critical particle concentration p fp0, above which disorder—order transition occurs for sterically stabilized dispersions, on the thickness of the adsorbed layer and on the nature of the solvent. System polyisobutene-stabilized silica particles in cyclohexane at 308 K and in ethylbenzene at 298 K. a = 48 nm, A = 4.54kTr v = 0.10, xt = 0.47, x5 = 0.10 for polyisobutene—cyclohexane, and xt 0.477, x2 = 0.32 for polyisobutene—ethylbenzene... 

In addition to the molecular weight of the free polymer, there axe other variables, such as the nature of the solvent, particle size, temperature, and thickness of adsorbed layer which have a major influence on the amount of polymer required to cause destabilization in mixtures of sterically stabilized dispersions and free polymer in solution. Using the second-order perturbation theory and a simple model for the pair potential, phase diagrams relat mg the compositions of the disordered (dilute) and ordered (concentrated) phases to the concentration of the free polymer in solution have been presented which can be used for dilute as well as concentrated dispersions. Qualitative arguments show that, if the adsorbed and free polymer are chemically different, it is advisable to have a solvent which is good for the adsorbed polymer but is poor for the free polymer, for increased stability of such dispersions. Larger particles, higher temperatures, thinner steric layers and better solvents for the free polymer are shown to lead to decreased stability, i.e. require smaller amounts of free polymer for the onset of phase separation. These trends are in accordance with the experimental observations. 

In a sterically stabilized dispersion of adsorbed polymer layers, the additional energy term, Vs, called the steric stabilization, is included in Equation (4.73) ... 

In aqueous dispersion media, the preparation of high solids dispersions that are stabilized electrostatically is often fui d by the gel-hke nature of the product. This is a consequence of the interactions between the double layers surrounding each particle. No comparable increase is evident in the viscosity of sterically stabilized dispersions at high solids content. Of course, in certain applications, such as in paints, the rheological properties exhibited by electrostatically stabilized dispersions at high solids may be a decided advantage rather than a drawback. 

If a polystyrene latex that is stabilized solely by an electrostatic mechanism is coagulated by the addition of electrolyte, that coagulation is usually irreversible to subsequent dilution. In contrast, sterically stabilized dispersions can usually be flocculated by the addition to the dispersion medium of a nonsolvent for the stabilizing moieties mere dilution of the concentration of the nonsolvent to a suitably low value is often sufficient to induce the particles to redisperse spontaneously. 

This difference in behaviour between electrostatically and sterically stabilized dispersions probably springs from the fact that whereas sterically stabilized particles may be thermodynamically stable, dispersions that are electrostatically stabilized are only thermodynamically metastable. In the latter case, the coagulated state is the lowest energy state and, once achieved, it is only reversed if there is an input of work into the system. It must be remembered, however, that there exist many electrostatically stabilized dispersions that undergo reversible coagulation (e.g. clays, Carey Lea silver... 

One important consequence of the thermodynamic stability of sterically stabilized dispersions is that they redisperse spontaneously after drying. Mention has already been made of the instant ink of the Ancient Egyptians. More recently, Everett and Stageman (1978a) have demonstrated the spontaneous redispersion in n-alkanes of freeze dried poly(methyl methacrylate) particles, coated by poly(dimethylsiloxane). 

Sterically stabilized dispersions also often display good freeze-thaw stability, which can be a desirable attribute in some practical applications (e.g. paint systems). 

Typical stabilizing moieties and anchor polymers for sterically stabilized dispersions... 

Many sterically stabilized dispersions can be induced to flocculate simply by decreasing the solvency of the dispersion medium for the stabilizing moieties. This may be achieved for some systems by changing the temperature and/or pressure. Other dispersions, however, resolutely defy all such efforts to induce flocculation merely by adjusting the ambient conditions. This is especially evident with certain aqueous sterically stabilized dispersions. For example poly(vinyl acetate) latices stabilized by poly(oxyethylene) in pure water are stable at 100 C. In these instances, the addition of a substance that reduces the solvency of the dispersion medium for the stabilizing moieties usually permits flocculation to be observed. In aqueous systems, for example, the addition of electrolytes will commonly reduce the solvent power that water displays for the polymeric stabilizing moieties. In nonaqueous media, all that is usually required is the addition of a nonsolvent for the stabilizing chains that is miscible with the dispersion medium. This method is also applicable to the flocculation of aqueous sterically stabilized dispersions. 

As shown in Fig. S.l, sterically stabilized dispersions often display an abrupt transition from long-term (indeed thermodynamic) stability to the onset catastrophic flocculation. Changing the temperature by only a few degrees kelvin is sufficient to transform a very stable dispersion into a flocculated coagulum. This dramatic temperature response contrasts sharply with the more sluggish response to temperature changes exhibited by electrostatically stabilized dispersions. Their stability is normally decreased by heating, as noted by Faraday (1857). 

In order to understand the stability of sterically stabilized dispersions, it is necessary to identify the CFPT. This identification can be accomplished for many systems once the influence of a wide range of system parameters on the CFFT has been established experimentally. Such an experimental study also generates guidelines for optimizing the stability of sterically stabili dispersions. 

Considerably less work has been reported on CFPs than on CFTs. There are two main reasons for this first, the stability of sterically stabilized dispersions is less sensitive to changes in pressure than to changes in temperature second, it is experimentally easier to vary the temperature than to vary the pressure. Of course, in many experiments where the temperature is varied, the pressure also changes signi antly. In such cases, it is advisable to allow for the change in pressure by, for example, extrapolating to zero pressure (Evans and Napper, 1975). 

For most sterically stabilized dispersions whose particles are fully-coated by well-anchored high polymeric stabili ng moieties, the practical thermodynamic limit to stability is the 0-point for the stabilizing moieties in free solution. 

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