Energy barrier flocculation

The stabilization of an emulsion iavolves slowiag the destabilization, primarily the flocculation process. This may be achieved ia two principal manners by reduciag the mobiHty of droplets through enhanced viscosity or by inserting an energy barrier between them (see also Dispersants Flocculating agents). 

Flocculation kinetics. We have droplets in an emulsion with a density of one droplet per (10 /uu)3. Calculate the initial decrease of the concentration, assuming no energy barrier. 

Each particle is considered as a center to which other particles diffuse by Browian motion. Thus the rate of flocculation is proportional to the square of the number of particles. The original treatment assumed that all interparticle collisions were effective in causing flocculation. Later modifications (6) assumed that the potential energy barrier between particles resists flocculation and that only those collisions with sufficient energy to overcome this barrier will cause flocculation. The agitation-induced flocculation has also been analyzed theoretically (7). 

Figure 9.6. Schematic of repulsion or attraction forces (which vary with distance from the particle surface) between particles in suspension. Curves 1 and 6 are examples of repulsion and attraction curves, respectively, which vary with the colloid and the kinds and amounts of electrolytes. A summation of curves 1 and 6 for different conditions produces curves 2-5. In curve 2, the energy of repulsion predominates and a stable suspension is formed. Increasing electrolyte produces curves 3, 4, or 5 owing to suppression of the electric double layer. Curve 3 shows there is still an energy barrier to be overcome prior to flocculation. When the colloids surmount this energy barrier and approach closer than point C, flocculation occurs because the forces of attraction predominate. Curve 5 suggests spontaneous flocculation without redispersion unless there is a shift toward curve 2 by reexpanding the double layer through changing kinds and/or amounts of electrolytes (adapted from Kruyt, 1952).

Dilute suspensions may be studied using the Coulter principle. It is sometimes found that the count level at the lower sizes decreases with time and this may be attributed to flocculation or dissolution. A problem arises in that the suspending liquid needs to be electrically conducting and one would expect this to reduce the potential energy barrier and decrease suspension stability [108]. 

In a completely deflocculated system the particles are not associated pressure on the individual particles can lead in this layer to close packing of the particles to such an extent that the secondary energy barriers are overcome and the particles become irreversibly bound together. In flocculated systems (where the repulsive barriers have been reduced) particles settle as floes and not as individual particles. The supernatant clears but, because of the random arrangement of the particles in the floes, the sediment is not closely packed and caking does not readily occur. 

The formation of a surfactant film around droplets facilitates the emulsification process and also tends to minimize the coalescence of droplets. Macroemulsion stability in terms of short and long range interactions has been discussed. For surfactant stabilized macroemulsions, the energy barrier obtained experimentally is very high, which prevents the occurrence of flocculation in primary minimum. Several mechanisms of microemulsion formation have been described. Based on thermodynamic approach to these systems, it has been shown that interfacial tension between oil and water of the order of 10- dynes/cm is needed for spontaneous formation of microemulsions. The distinction between the cosolubilized and microemulsion systems has been emphasized. 

The resulting nanosuspensions must be maintained colloidally stable by using surfactants and/or polymers that provide an effective energy barrier against flocculation. 

The surfactants used for the preparation of disperse systems are seldom effective in maintaining the long-term physical stabihty (absence of flocculation and/or coalescence) of the formulation. This is due to their weak and reversible adsorption and lack of the presence of a high-energy barrier that prevents flocculation as a result of van der Waals attractions. For this reason, dispersants and emulsifiers of polymeric nature that are strongly and irreversibly adsorbed at the interface are required. In addition, these polymeric dispersant provide effective repulsive forces (referred to as steric repulsion) that overcomes the van der Waals attractions. The criteria for an effective dispersant are [1, 2] ... 

Gjnax < 5 IcT then flocculation will occur. Two types of flocculation kinetics may be distinguished (i) fast flocculation with no energy barrier and (ii) slow flocculation, where an energy barrier exists. 

Figure 10.5 Schematic representation of free energy path for breakdown (flocculation and coalescence) for systems containing an energy barrier.

This can occur if the energy barrier is small or absent (for electrostatically stabilised emulsions) or when the stabilising chains reach poor solvency (for sterically stabilised emulsions, that is if />0.5). For convenience, the flocculation of electrostatically and sterically stabilised emulsions will be discussed separately. 

Stability in colloidal dispersions is defined as resistance to molecular or chemical disturbance, and the distance the system is removed from a reference condition may be used as a measure of stability. The stability can be analyzed from both energetic and kinetic standpoints. The kinetic approach uses the stability ratio, as a measure of the stability. W is defined as fhe ratio of the rate of flocculation in the absence of any energy barrier to that when there is an energy barrier due to adsorbed surfactant or polymer. These processes are referred to as rapid and slow flocculation with rate constants kj and kg, respectively, such that W = kjlk. The stability of colloidal suspensions can be evaluated using various techniques. In practice, two methods are mainly used sedimentation and rheology measurements. 

The magnitude of the electrostatic contribution to stabilization of the dispersion against flocculation can be determined from the electrostatic energy barrier shown in Figure 2, a plot vs. interparticle distance H of the electrostatic repulsion plus the dispersion force attraction term, U 0, (in units of kT at 20°C). 

At 1% NaCl, there is a monotonic increase in attractive potential energy, therefore, oil drop flocculation is irreversible and oil droplets can be brought together at contact distances. For the system with no sodium chloride, there is a repulsive energy barrier between oil droplets at interparticle distances below 600A, therefore, oil-drop flocculation is reversible, and hence conditions are extremely unfavorable for coalescence. Thus the emulsion is extremely stable. 

---