The physical properties of colloids and suspensions are strongly dependent on the nature and extent of the particle-liquid interface; the behavior of aqueous dispersions being especially sensitive to the electrical and ionic structure of the interface. The production and stability of colloids and suspensions are both intimately related to the so-called electrical double layer that characterizes the interface.
Information relating to stability is therefore of considerable importance. It should be noted that the term stability, when applied to colloidal dispersions, is generally relative in meaning and intended to express the resistance to change of the dispersion with time. Zeta potential measurements are directly related to the nature and structure of the electric double layer at the particle-liquid interface. Zeta potential measurements are key factors in the following processes:
1. THE PREPARATION OF COLLOIDAL DISPERSIONS for useful purposes as in paints, inks, pharmaceutical and cosmetic preparations, food products, drilling muds, dyestuffs, foams and agricultural chemicals.
2. THE USE OF COLLOIDAL DISPERSIONS AS A STEP IN THE MANUFACTURIING PROCESSES, for example in ceramic casting, cements and plaster, brickmaking and pottery, paper coatings and catalyst supports.
3. THE UTILIZATION OF COLLOIDAL PHENOMENA including: detergency, capillary phenomena which are important in the wetting of powders, the expulsion of oil from reservoir rock, the retention of moisture and nutrients in soil, and in the coating of surfaces, the flotation of minerals, the absorption of impurities as in sugar refining, solvents recovery and electrophoretic deposition of paints.
4. THE DESTRUCTION OF UNWANTED COLLOIDAL DISPERSIONS in water purification, the fining of wines and beers, sewage disposal, the breaking of oil emulsions, the dewatering of sludges and the dispersal of aerosols and fogs.
When stability is an important factor in your colloidal process, then consider zeta potential measurements.
Some of the areas of applications are:
Microelectrophoresis is a most useful technique for characterizing the surface of organisms such as bacteria, blood cells, viruses etc. In contrast to chemical methods of analysis which can disrupt the organism, zeta potential measurement has the particular merit of providing information which refers specifically to the outermost regions of an organism. This is of particular value since the surface of these organisms are the sites of many phenomena of biological importance. The main constituents of biological material (protein, lipid, polysaccharide, nucleic acid) show characteristic charge behavior, the magnitude, sign and distribution of which profoundly affects its interaction with any other surface or molecule. Changes in the zeta potential of blood elements, tissue cells, vessel walls or components of body fluids can produce metabolic changes or alterations in blood flow properties, cellular or vessel wall determinability, cellular aggregation and adsorption/desorption equilibrium of body fluid components.
Clays consist mainly of plate-like particles, which when in contact with water, usually have negatively charged faces and positively charged edges. The physical properties of clay-water systems such as sedimentation, filtration, swelling, viscosity, yield stress and structural strength are extremely sensitive to the nature of the electric double layer around the particles and the tendency of the particles to aggregate. Zeta potential measurements provide particularly relevant information where colloid stability and/or ion adsorption is involved. Results for a concentrated suspension can be related to results of shear and consolidation tests. Control of the mechanical behavior of clay suspensions, using additives in the liquid phase, is also an important feature of soil treatment, oil well drilling, ceramics and other processes involving clay-like materials.
(a) SOIL MECHANICS
To achieve good aeration and drainage, it is necessary to maintain agricultural soil in the reasonably flocculated state and treatment with flocculants such as calcium salts or organic polyelectrolytes is common practice. The movement of nutrients through soils is affected by the degree of flocculation of the soil. In road and building engineering, soils are sometimes stabilized against swelling by precipitating soaps onto the particles to reduce their hydrophilic nature and response to water. Conversely, canals are sometimes lined with clays which swell on contact with water to reduce seepage losses.
(b) OIL WELL TECHNOLOGY
In the drilling of oil wells a drilling mud is used (1) as a coolant, (2) for removing the cutting from the bore-hole and (3) to seal the sides of the bore-hole with an impermeable filter cake. The pumping and sealing features of this operation are most effective if the drilling mud is deflocculated. However, a certain amount of mud rigidity is required to prevent cuttings from falling to the bottom of the bore-hole during an interruption of circulation. These opposite requirements are reconciled by maintaining the drilling mud in a partially flocculated, thixotropic, state. If the drilling mud stiffens, partial deflocculation can be effected by addition of a small amount of a peptizing agent such as a polyphosphate, the main function of which is to reverse the positive charge on the edge of the clay particles. The relatively small edge area of the clay particles makes this process economically attractive.
Mobility information is useful in the manufacture of felted ceramics which are produced by a paper making technique. Finely divided material and fibers are first made into an aqueous suspension. The charges on the surface of the fibers and particulate matter are measured and adjusted so that the particulate material is attracted and holds on the surface of the fiber. As the flocculated material settles, water is removed, and the flocked fibers form the felt, or compact.
WATER PURIFICATION AND INDUSTRIAL WASTE
Municipal water supplies are usually obtained from sources which are contaminated with materials (both mineral and organic in nature) derived from soil, domestic sewage and industrial waste.
To facilitate the filtration of these contaminants, small quantites of flocculating agents are used. Electrophoretic measurements play an important part in the study of this flocculation process. Before being discharged into streams industrial and mining wastes are also treated by the addition of coagulating additives to accelerate settling of suspended matter. Zeta potential measurements have established the optimum coagulation condition for removal of particulate matter and organic dyestuffs from such waste waters.
MINERALS AND ORE FLOTATION
Many valuable minerals including those containing copper, lead, zinc, tungsten and others are separated from the gangue by grinding the ore and mixing it with a collector in a water medium.
The subsequent enrichment of metal ores by flotation is enhanced by the addition of collector oils which adsorb on the metal-rich particles, (but not on siliceous material), making them more hydrophobic. As might be expected, the efficiency adsorption of collector oil and flotation recovery processes depends on the sign and magnitude or the charge on the one particle. Anionic collector oils adsorb effectively on positive metal ore surfaces and vice-versa. At low zeta potential little collector is adsorbed (anionic or cationic) and flotation recovery is minimal.
Interparticle repulsion due to the overlap of similarly charged electric double layers is an important stabilizing mechanism in O/W emulsions. When ionic emulsifying agents are used, lateral electric double layer repulsion may prevent the formation of a close-packed film. This film expanding effect can be minimized by using a mixed ionic plus non-ionic film and/or by increasing the electrolyte concentration in the aqueous phase.
Since stability of an emulsion is, in part, related to the electrokinetic conditions of the interface, electrophoretic mobility measurements on droplets can thus provide information regarding both shelf storage stability and freeze-thaw stability.
The important factor in detergency is the prevention of dirt redeposition after its removal. Redeposition is governed by the influence of detergent adsorption, not only on the zeta potential of the dirt particles, but also on the zeta potential of the fabric surface.
The degree of dispersion of finely divided material is nowhere of greater importance than in the paint industry. When dispersion of pigment particles is incomplete, the paint will behave as though it contains larger particles. The degree of dispersion can markedly affect the color quality, the gloss, the texture and flow/dispersion relationships. Increased dispersion increases brightness and gloss (specular reflection) and gives a marked decrease in haze (diffuse reflection). Bronzing occurs only at coarse levels of dispersion, but disappears at finer levels. Electrophoretic measurements under various conditions of composition, particle size and additives are useful in establishing optimum dispersion characteristics, and hence can assist in selection of mills for specific products and their maximum efficiency in operation and the development of maximum product value from raw materials, e.g. realizing the full intrinsic color value from pigments. Zeta potential measurements also are useful in the study and control of storage stability and behavior during application and drying.
The application of paint to metal objects such as automobile bodies, by electrodeposition is a major advance in corrosion protection. The metal article to be painted is made the anode and the container the cathode.
When a suitable voltage is applied, negatively charged paint particles are deposited, from a moderately dilute (ca. 10 per cent) aqueous suspension, as a coherent film. The process is, in essence, an electrophoretic phenomenon in which the colloidal paint particles move through the aqueous electrolyte to the part being painted due to charge on the electric double layer. Electrophoretic mobility measurements are valuable in establishing the optimum pigment dispersion with various additives. Alkyds, epoxies, melamines, acrylics and other functional groups have been employed in formulation paint for different electrocoating purposes.
The dispersion of coating colors in paper manufacture changes as various ingredients are added, and flow properties shift accordingly. Electrophoretic motility measurements are informative in investigating the effect of various coating ingredients, singly or in combination, on the rheology of the system. There is evidence that the charge on the particles controls the rheology more strongly than any other single factor.
Zeta potential measurements are also valuable in understanding the retention mechanism of filler pigments as well as the degree of retention of these materials, and in optimizing paper-mill wet-end chemistry for improved drainage, retention and chemical efficiency.
One of the most difficult problems in the pharmaceutical industry is the preparation of physically stable dispersions of a drug in a suitable suspension vehicle. A stable dispersion is one in which the particles are completely deflocculated. An effective method for producing a colloidally stable dispersed system can best be devised after the conditions of maximum dispersion are established by means of electrophoretic mobility measurements. The stability of parenteral and oral drugs in aqueous suspensions affects both dosage requirements and shelf life. Caking tendencies of pharmaceutical suspensions may also be studied with the aid of electrophoretic measurements.
Zeta potential measurements are useful in establishing optimum conditions for purification in a number of chemical processes. Impurities in raw sugar cane juice, for example, may be removed by precipitating an inorganic material which serves to collect the impurities. To achieve maximum removal of these impurities, the proper zeta potential is determined, and optimum conditions are maintained for rapid sedimentation.