Secondary minimum flocculation

The presence of the large repulsive potential barrier between the secondary minimum and contact prevents flocculation. One can thus see why increasing ionic strength of a solution promotes flocculation. The net potential per unit area between two planar surfaces is given approximately by the combination of Eqs. V-31 and VI-22 ... 

The charge on a droplet surface produces a repulsive barrier to coalescence into the London-van der Waals primary attractive minimum (see Section VI-4). If the droplet size is appropriate, a secondary minimum exists outside the repulsive barrier as illustrated by DLVO calculations shown in Fig. XIV-6 (see also Refs. 36-38). Here the influence of pH on the repulsive barrier between n-hexadecane drops is shown in Fig. XIV-6a, while the secondary minimum is enlarged in Fig. XIV-6b [39]. The inset to the figures contains t,. the coalescence time. Emulsion particles may flocculate into the secondary minimum without further coalescence. 

At larger particle separation, a second minimum may occur in tire potential energy. In many cases, tliis minimum is too shallow to be of much significance. For larger particles, however, tire minimum may become of order kT. Aggregation in tliis minimum is referred to as secondary minimum flocculation. 

Flocculation a relatively reversible aggregation often associated with the secondary minimum of a potential energy diagram. Particles are held together loosely with considerable surface separations. 

PVA and TaM -for the 88%-hydrolyzed PVA. The same dependence was found for the adsorbed layer thickness measured by viscosity and photon correlation spectroscopy. Extension of the adsorption isotherms to higher concentrations gave a second rise in surface concentration, which was attributed to multilayer adsorption and incipient phase separation at the interface. The latex particle size had no effect on the adsorption density however, the thickness of the adsorbed layer increased with increasing particle size, which was attributed to changes in the configuration of the adsorbed polymer molecules. The electrolyte stability of the bare and PVA-covered particles showed that the bare particles coagulated in the primary minimum and the PVA-covered particles flocculated in the secondary minimum and the larger particles were less stable than the smaller particles. 

It is of interest to note that both VA and Vr increase as particle radius a becomes larger and thus VM in Figure 5.9 would be expected to increase with the sol becoming more stable also if a increases then VSM increases and may become large enough to produce secondary minimum flocculation. 

Secondary minimum flocculation is considered to play an important role in the stability of certain emulsions and foams. 

For situations i) and ii) the coagulated state, i.e. with the particles in intimate contact, is desirable. For other purposes, the flocculated state is required, i.e. with the particles still essentially individual and separated by a thin layer of liquid, thus giving control of the rheological properties of the sytem. Frequently, the secondary minimum plays a significant role in flocculated systems. 

With this size of latex particle it becomes possibfe to make direct observations on particles over a perind of time and record them with a high-speed camera. Using this technique Cornell et al (1975)) discovered that particles in an associated unit could be quite mobile. It was observed that as well as some particles leaving the aggregated unit as single particles and returning to the disperse phase there was a continued rearrangement of the particles. This was also observed with floccules at salt concentrations well above the ccc. These observations clearly support the contention that association can occur in a secondary minimum and that in this situation a liquid film is maintained between the particles. 

Occurrence of flocculation may be explained if secondary minima aggregation is assumed. This is confirmed by the V/KT vs (H (A)) curves, which indicate that at higher values of H, repulsion becomes negligible and attraction predominates and emulsion flocculates. It is also observed that the depth of secondary minimum is more at higher concentration, 5-20 KT, which is deep enough for reversible aggregation, while at low concentrations, the depth of the secondary minima is too shallow to trap the particles. 

Under some conditions (depending on electrolyte concentration and particle size), flocculation into the secondary minimum may occur, although such flocculation is weak and reversible. On increasing the electrolyte concentration decreases until, at a given concentration, it vanishes and particle coagulation occurs. This is illustrated in Figure 7.10, which shows the variation of with h at various electrolyte concentrations. 

Another mechanism of flocculation is that involving the secondary minimum (Gmin) which is few kT units. In this case, the flocculation is weak and reversible and hence both the rate of flocculation (forward rate k ) and deflocculation (backward rate k ) must be considered. In this case, the rate or decrease of particle number with time is given by the expression ... 

State (g) represents the case of weak and reversible flocculation. This occurs when the secondary minimum in the energy distance curve is deep enough to cause flocculation. This situation can occur at moderate electrolyte concentrations, in particular with larger particles the same occurs with sterically and electrosterically stabihsed suspensions. It also occurs when the adsorbed layer thickness is not very large, particularly with large particles. The minimum depth required to cause... 

As discussed above, the total energy-distance of separation curve for electrostatically stabilised shows a shallow minimum (secondary minimum) at a relatively long distance of separation between the droplets. However, by adding small amounts of electrolyte, such minima can be made sufficiently deep for weak flocculation to occur. The same applies to stericaUy stabihsed emulsions, which show only one minimum, but whose depth can be controlled by reducing the thickness of the adsorbed layer. This can be achieved by reducing the molecular weight of the stabiliser and/or the addition of a nonsolvent for the chains (e.g., an electrolyte). 

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