A METHOD FOR MEASURING AVERAGE PARTICLE SIZE OF EMULSIONS
BY WHEELER P. DAVEY
Experiment shows that if a drop of an oil-in-water emulsion is allowed to fall into clean water with such force as to break through the water surface the original drop of emulsion will retain its identity for some time, diffusing only slowly into the body of the water. If, however, it is deposited gently on the surface of the water without making a splash, it will spread on the water like an oi1.l The spreading takes place with extreme rapidity and may be considered to be a two-dimensional explosion. If we assume that the layer on the surface is one particle thick and that the droplets of the disperse phase are small enough in diameter to hold a spherical shape, then the average diameter of the droplets may be measured by the apparatus which Langmuir used for measuring the length of oil molecules. Trial shows that, if all other conditions of the experiment are kept constant, successive measurements of the same emulsion a t different concentrations which are still of the same order of magnitude are consistent with each other and are independent of the quantity of emulsion used. The technique of the measurement is obviously as follows. An enamelled iron tray about eight inches wide, thirty inches long and about one-fourth inch deep is filled with water. A piece of paraffined aluminum foil is floated on the surface at one end of the tray and attached to a balance such as was used by Langmuir, or to a torsion wire such as is used in the Du Nouy apparatus. The spaces between the walls of the tray and the ends of the aluminum float are sealed by constant pressure air jets. The rest of the water surface is then swept free of monomolecular layers of grease, oil, etc. by means of glass sweepers, and a drop of the emulsion is spread on this clean surface from a micrometer pipette. The area of the film is determined exactly as in Langmuir's experiments with oil films. The calculation of particle size requires a knowledge of the total volume of the droplets of the disperse phase as they ezist in the emulsion. Although in many cases this volume is practically the same as that of the same mass of undispersed material, it cannot be assumed that this must always be the case. The volume may be obtained by curdling a known volume of emulsion with a known volume of a solution of a suitable electrolyte, removing any included water from the curd and adding it to the rest of the water phase. The total volume of the water phase is then measured and the volume of the disperse phase is found by difference. This gives a t once the concentration C of the 1
Davey: Science, 64,252 (1926).
* Langmuir: Proc. Nat. Acad. Sci., 3, 251 (1917).
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WHEELER P. DAVEY
emulsion. If V is the volume of the drop of emulsion which was spread on the water, then CV is the volume of the disperse phase in that drop. The mean diameter of the particles of disperse phase is therefore
where A is the area which was covered. The method is subject to the following limitations: ( I ) The emulsion must not be so concentrated that it cannot be diluted by the simple addition of water a t room temperature without vigorous stirring. (2) The emulsion must not be so dilute that it cannot always present to the water on which it is spread a substantially continuous surface of disperse phase. (3) The water on which the emulsion is spread must be neutral3 ( p H = 7). This condition should ordinarily be met even if it be necessary to add a buffer to the water and later make the necessary corrections as outlined below. (4) The restoring force on the aluminum float must be very small if uncertainties in the calculated particle size are to be avoided. These four restrictions will be discussed in order. This restriction makes it impossible to use the method for deter(I) mining the size of aggregates of disperse phase in concentrated emulsions which are almost ready to gel. I t is obvious that the method requires the emulsion to be dilute enough so that each droplet of disperse phase can float on the water surface without being tied up with any other droplet. The method is, therefore, best adapted to measuing the size of the ultimate d r o p lets of disperse phase. This limitation on concentration is not only due to the properties of the emulsion itself, but is due also to the practical consideration that only a small volume of disperse phase can be spread on a water surface of convenient size. The accurate measurement of the volume of disperse phase requires, of course, considerable dilution. The probable mechanism of the water-spreading of emulsions is of (2) considerable interest. We are a t first sight tempted to consider a hanging drop of an emulsion as being surrounded by a sort of bag of oriented molecules of free emulsifying agent. Such a picture may be true in some cases, but in the cases with which the writer has worked, it is hard to see why, on the basis of such a picture, the drop retains its identity when it hits the water with such force that it goes below the surface. It seems simpler to assume that at least a large part of the surface of the drop is covered with a monoparticle layer of disperse phase, This layer would correspond to the membrane around a living cell. Its existence on the surface of the drop would be consistent with the water-spreading phenomenon of emulsions which forms the basis of the method described in this paper. If such a picture is adopted, it follows that the concentration of the emulsion must be such that, as the drop flattens out on the water surface, there will be a constantly available supply Weeks: Phys. Rev., 35,668, (1930);See also Weeks thesas for M.S. degree, The Pennsylvania State College.
MEASURING AVERAGE PARTICLE SIZE OF EVULSION
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of disperse phase to keep the surface completely coated. Otherwise, droplets of disperse phase would find their way into the body of the water in the tray and the measurements of particle size would not be substantially independent of concentration. It should be noted that we have dealt with extremes of concentration in ( I ) and ( 2 ) . It is not intended to imply that our measurements of average particle size are independent of the tendency for particles of disperse phase to become aggregated as the concentration is increased. ( 3 ) When particle-size measurements are made on water whose pH is kept a t 7 , the area covered by the emulsion is constant with time. If the pH of the water is greater than 7, the area covered by the emulsion (after subtracting the area covered by buffer in the same time) approaches its final value slowly, and the final value is larger than when it is measured a t p H = 7 . The effect is what might be expected if we assumed the extra alkali to saponify some free acid in the disperse phase, thus causing the average particle size of the disperse phase to become smaller. Besides, the soap molecules thus formed being smaller than the original particles of disperse phase, would tend to lower the average particle size. When the p H is less than 7 , the area covered by the emulsion increases with time without reaching an equilibrium value, so that the disperse phase acts like a two-dimensional gas just above its critical temperature. (4) The monoparticle layer of disperse phase floating on the surface of the water in the tray can hardly be expected to be as rigid as a corresponding monomolecular layer of a fatty acid. Excessive horizontal pressure on the layer may be expected to crumple the layer rather easily, thus giving a layer which, in some portions a t least, would be more than one particle thick. For this reason, the force-area curves for emulsions do not show as sharp a point on inflection (Langmuir’s S point) as is shown by fatty acids and soaps. This introduces some error in the h a 1 calculated size. Since (3) implies that the restoring force on the aluminum float lies in the optimum range for pH = 7 , it is recommended that this pH be used even if it is necessary to add a buffer to the water in the tray and then correct for whatever buffer material may be adsorbed on the water surface. School of Chemastry and Physzcs, T h e Pennsylvanza State College, State College, Pennsyloanza.
