Determinationof Particle Size Distribution of Oil-in-Water Emulsions by Electronic Counting Thomas R. Lien and Colin R. Phillips’ Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Ont., Canada
The particle size distribution of a hydrocarbon-in-water emulsion was determined by a Coulter counter model T. Dilution of the emulsion prior to the measurement resulted in a large decrease in particle numbers for the size range studied-i.e., 0.75-12.5 p. This decrease in numbers after dilution was time-dependent and was attributed to the dissolution of the oil particles into the diluting electrolyte solution. Prior saturation of the electrolyte solution with hydrocarbon and a method of zero-time extrapolation are suggested to obtain a correct particle size distribution.
Particle size distributions of oil-in-water (O/W) emulsions are useful in studies of general and rheological properties of oil emulsions (Sherman, 1968) in the laboratory and in the environment. Coulter counting (Coulter Electronics, Hialeah, Fla.), by which large numbers of particles can be sized automatically and very rapidly, has usually been preferred over optical methods. The purpose of this paper is to illustrate various problems in using the Coulter counter, in particular the time dependency of the particle count after diluting the O/W emulsion with electrolyte solution. To determine the correct size distribution, prior saturation of the electrolyte solution with oil and zero-time extrapolation are proposed. Problems in the application of the Coulter counter are three. Background Noise. It was found that random background noise in the submicron range occurred, apparently because of electrical and mechanical interferences. Ultimately, low and reproducible background count could be attained by placing the counter in an isolated room and by shielding the sample stand (manometer) of the counter with copper screen. Limited Size Range of Aperture Tube. The problem arose mainly from the emulsion system used in this study. The size of the oil particles ranged from submicron to about 15 p and, depending on the O/W ratio, concentration of surfactant, and the treatment of emulsions, some oil particles larger than 15 p were also observed. With such a broadly dispersed system, a two-tube technique should be employed. However, methods of separating oil particles dispersed in water can hardly give a satisfactory quantitative analysis with such a two-tube technique. Up until now, only one 30-p aperture tube has been used in this study. This tube is recommended for the range 0.512.5 p (manual of Coulter counter model T ) . However, it was found that electronic noise intruded at a size of about 0.65 p . Apparently, a small aperture tube cannot retain the same range of coverage as a larger one-e.g., 100 p . Effect of Dilution on Emulsion Stability. The third problem is the most serious limitation on the application of the Coulter counter to sizing O/W emulsions since it requires that the emulsions be diluted with electrolyte solution prior to the analysis. Depending on the concentration of oil droplets, a dilution factor of over 10,000 is often To whom correspondence should be addressed 558
Environmental Science & Technology
required to make the dispersed oil droplets dilute enough to avoid serious coincidence loss. The dilution can then cause instability of the dispersed emulsion system (Groves, 1968). Although the Coulter counter has been widely used for studying aggregation and flocculation behavior of O/W emulsions, the effect of dilution has been largely overlooked or ignored (for instance, Rowe, 1965). One of the few studies dealing with the dilution effect is that of Singleton and Brown (1965) who found that the number of particles in diluted intravenous fat emulsions was dependent upon the elapsed time of contact of the emulsions with the diluting saline electrolyte. The original particle numbers were then determined by extrcipolation to zero time. The increase in particle numbers observed after dilution was attributed to coalescence or aggregation of particles smaller than the sizing limit. This coalescence can be retarded by adding glycerin or other stabilizer to the electrolyte (Groves, 1968) but the amount of glycerin must first be determined by trial and error for each sample (Sprow, 1967). The rate of aggregation was dependent on the critical micelle concentration of surfactant in the emulsion. A loss in count with elapsed time was observed for emulsions with concentration of surfactant lower than the critical micelle concentration (Shotten and Davis, 1968). For all the oil emulsions prepared in the present investigation a consistently large reduction in count was observed. especially in the first few minutes after dilution (Table Ia). Although the loss in count was noted by Shotten and Davis (1968), and by Loriston-Clarke and Thew (1969), a decrease of this magnitude was not reported before. The rate of loss in count after dilution was less if oilsaturated electrolyte solution was used for the dilution (Table Ib), which indicated that dissolution of the oil phase in the diluent could be the main cause of the observation. When we saturated the electrolyte solution with the oil and employed the method of zero-time extrapolation, a more accurate size distribution was obtained without employing such special techniques as polymer dispersion (Rehfeld, 1967). Results of this analysis method are presented.
Experimental Preparation of Emulsions. Tetralin of chemical reagent grade was chosen as the dispersed oil phase because its density is close to that of water and thereby the sedimentation of oil droplets during the sizing could be eliminated. A wide range of O/W ratios and surfactant concentrations has been studied but, for the purpose of this discussion, only one emulsion is described here. The emulsion consisted of 20 ml tetralin and 180 ml filtered water containing 1.11%by weight of Tween 80 so that the final emulsion contained 1% by weight of the surfactant. The mixture was emulsified with a Virtis homogenizer model 25 at a speed of 10,000 f 1000 rpm for 10 min. Electrolyte Solutions, A saline solution with 3% sodium chloride was filtered three times through 0.3 p Millipore filter membranes. The solution was then shaken with filtered tetralin and left to separate overnight. This solution was then saturated with tetralin. The unsaturated
Tables la and Ib. Particle Size Distribution of 10% Tetralin/Water Emulsion Dilution factor = 1.27 X lo4. Volume of sample sized = 0.05 ml la. The diluent was not saturated with tetralin Ib. The diluent was saturated with tetralin Coulter counter channel no.
14O 13 12 11 10 9 8 7 6 5 4 3 2 1 0 z
Particle diam, p no. of particles counted
0.50-0.75 0.75-1.0 1.0-1.5 1.5-2.0 2.0-2.5 2.5-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 10-12.5 12.5-
Table la. Elapsed time, sec -
30
60
90
120
150
Table Ib. Elapsed time, sec 180
600
30
60
90
120
150
__
Zero time
180
.. . .
.. . .
.. ..
... .
....
.. . .
.. ..
. .. .
. . ..
. .. .
... .
. .. .
....
....
1809 3680 2969 2222 1254 1503 377 108 39 12 8 4 3 1
1926 1518 1428 969 588 796 217 73 21 6 5 1 2 1
2482 1012 675 469 263 355 131 46 15 8 2 2 0 1
2848 732 339 321 182 237 84 37 23 5 4 1 0 0
2614 761 184 132 85 136 51 28 10 4 3 1 5 0
2354 780 163 102 59 94 45 46 19 8 5 7 5 3
1800 430 50 11 1 5 2 0 0 0 1 0 0 0
4012 6917 5739 4021 2134 2647 671 199 72 23 9 6 2 0
2847 4760 3897 2806 1524 1976 487 184 57 19 12 0 2 0
2637 3495 2858 2170 1196 1501 390 147 49 24 4 2 1
2243 2661 2292 1651 917 1168 355 115 49 12 5 3
2294 2314 1709 1280 737 964 314 94 41 22 6 3 0 1
2386 1842 1415 1035 564 801 245 101 33 13 7 1 1 0
6000 10000 7500 6000 3000 3700 900 260 90 28 9 4 2 0
0
1 0
The counts in channel 1 4 are ignored because of t h e large background noise.
and the saturated electrolytes were used as indicated in Tables Ia and Ib. All glassware used was rinsed with the appropriate electrolyte. Correction for background count was not necessary for particle sizes larger than 1.50 p and was not more than 10% for the size range 0.75-1.0 p . The temperature a t which the solution and the emulsion were stored was 70°F. Counter. The particle size analyses were obtained by a Coulter counter model T with an adjustable threshold unit employing a 30-p aperture tube calibrated with standard latex particles of 2.02 and 3.49 p . The advantage in using the model T is that it can produce a complete particle size distribution in 15 increments (channels) in a few seconds whereas the model B requires manual adjustment of channels, one at a time, over many minutes. Procedure for Particle Counting, The adjustable threshold unit was set as shown in Tables l a and Ib to cover the whole aperture range. The emulsion was diluted twice before the analysis. Exactly 1 ml of the emulsion was first diluted with 100 ml of the electrolyte solution; then 1, 2, or 3 ml of this first dilution was again diluted with 230 ml of' the electrolyte solution in a sample beaker for analysis. For convenience, these final solutions are designated as A, B. and C, respectively. The elapsed time after the second dilution was noted. The first count was made at an elapsed time of 30 sec to ensure that the oil droplets were uniformly dispersed. Generally a 30-sec interval was kept between the countings. The time for each counting was set at 14 sec, which represented a volume of 0.05 ml diluted emulsion. At all times, the diluted emulsion was gently stirred.
Results and Discussion Typical results of the counting of particle numbers for the diluted emulsion B as described above a t a series of elapsed times are given in Table Ib. For comparative purposes, results from an identical experiment performed using diluting electrolyte not previously saturated with tetralin are presented in Table Ia. The decrease in numbers in most size ranges in Table Ia is so great that the original size distribution cannot accurately be reconstructed.
When we use Table Ib, the correct count a t zero time can be obtained by extrapolating the plot of particle number against elapsed time. These results indichte that the loss in count for oil droplets less than 3 p in diameter amounted to about 50% in the first minute, and that the percentage loss is inversely related to the particle diameter. The dependence of the particle number on the elapsed time is better illustrated in Figure 1 in which the cumulative particle numbers are plotted against the particle diameter. The solid curve represents the cumulative particle number at zero time. Particles larger than 7 fi in diameter were small in number so that fluctuation in the count was expected; nevertheless, it appears that a slight loss in number with respect to the elapsed time occurred. T o examine dilution changes quantitatively, the emulsion was diluted to various extents. The results are plotted in Figure 2 for the diluted emulsion solutions A. B, and C as described above, and show that the cumulative particle numbers of B or C were about twice or three times that of A at a particular particle diameter as expected. The correct size distribution in the present investigation, then, could only be established if the electrolyte solution was saturated with oil and if the method of zerotime extrapolation was employed. This is apparently due to the dissolution of the oil droplets into the electrolyte solution as indicated before. If we take the diluted emulsion B, for example, the total volume of all oil particles ml. However, using the coramounted to only 1.98 x relation of Leinonen et al. (19711, the estimated solubility of tetralin in water at 25°C and atmospheric pressure is about 100 ppm, which is close to the measured values for many hydrocarbons. Thus the amount of oil required to saturate 250 ml of the electrolyte solution in the final dilution is about 2.5 x ml which is 16 times more than the amount of oil phase present in emulsion B. To examine the dissolution process further, it is of interest to estimate the dissolution rate of the suspended oil particles. The first problem, of course, is to determine the mass transfer coefficient, h, of the oil from the particle surface to the continuous phase. Since the system was constantly under slow stirring during the counting, the Volume 8, Number 6. June 1974
559
value of the Sherwood number (NSh = k d / D , where d is particle diameter and D is the diffusivity) was expected to be greater than the limiting value two for a spherical particle when the Reynolds number approaches zero ( N R e 0) (Grassman. 1971). Since there exists no rigorous correlation for calculating N S h , the following two correlations were used (Grassman, 1971; Treybal, 1963):
-
If the oil particles are assumed to be spherical, and if the relative velocity of oil particles with respect to the continuous phase is 1 cm/sec, then for an oil particle of 2.5 pm in diameter (which was the mean diameter for the original particle distribution as given in Table Ib), the first correlation (Equation 1) gives N S h = 3.3, whereas the second correlation (Equation 2) gives Nbh = 2.4. Accordingly, the mass transfer coefficients were estimated to be 0.051 and 0.03? g-mol/(sec) (cm2) (g-mol/cm3), and the initial dissolution rate to be 1.0 x and 0.75 x 1 O - I 2 g/sec, respectively. The mass of a 2-pm diameter oil pargram. Thus, the particle would disticle is 8.0 x solve into the continuous phase completely in about 10 sec, since the dissolution rate would increase as the particle diameter decreases. However, the data show that about 58% by weight of initial oil particles, or about 63% by number (which was about the percentage of initial oil particles less than 2 pm in diameter) were dissolved into the water after 30 sec (Table Ia).
1x105
A difference of this magnitude between the calculated and the measured values could be due to any of the following four items: The concentration of oil in the aqueous phase is greater than zero and increases during the dissolution. The surface of the oil particles may be somewhat protected by the surfactant which would considerably decrease the dissolution rate. The diluted emulsion itself contains a swarm of particles which would decrease the dissolution rate compared to the calculated value based on a free particle suspended in the aqueous phase. The estimate of particle-liquid relative velocity (1 cm/ sec) may be in error. I t is, therefore, not surprising that the dilution of the oil droplets with the electrolyte solution resulted in a drastic change in number with respect to elapsed time, especially for particles in the submicron range. Hence, presaturation of the electrolyte solution with oil and use of the zero-time extrapolation is of prime importance. It may seem puzzling that a loss in count was always observed in the present investigation even though the electrolyte was saturated with tetralin. A microscopic examination did not reveal any evidence of deaggregation, and the sedimentation effect during the counting was minimized by constant stirring. Evaporation may be a minor factor. The effect of the Kelvin equation (Davies and Rideal, 1963) for submicron particles is insignificant. A possible explanation is that the electrolyte solution was not fully saturated with tetralin because of the method of preparation. If the electrolyte solution were only 10% un-
L
1
lnw
o4
W N
Jci)
1
‘h 1
1
1
0
2
4
6
a
1A0
PARTiCLE DAMETER , y Figure 1. Dependence of measured particle size distributions of 10% tetralin/water emulsion on the elapsed time after dilution with tetraiin-saturated electrolyte. Dilution ratio = 1.27 X I O 4 0 30sec A 120sec 0 60sec 0 150sec A 90sec 180 sec
-zero-time extrapolation
560
Environmental Science & Technology
1
12 1
U
2
4
6
8
12
10
Figure 2. Correct particle size distributions of 10% tetralin/ water emulsion at different dilution ratios. Diluent, tetralin-saturated electrolyte Curve
Dilution ratio
A
2.54x 104
B C
o 85 x
1 27
x
104 104
dersaturated with the oil, the dispersed oil particles would dissolve into the solution. The emulsion after the first dilution was very stable. Repeat measurements using this emulsion for second dilutions over a period of 2 hr showed excellent reproducibility with fluctuations of less than 5% in count. This can be explained from considerations of solubility, since the first dilution contained a total of 0.1 ml of oil and it took only about 0.01 ml of oil to saturate this 100-ml electrolyte solution. A comparison of the total volume of oil particles counted with the original volume of the oil phase should provide a good check on the analysis. If a geometric mean diameter can be assumed for each size range as shown in Table Ib, a total volume of 2.7 x lo-’ ml is obtained for 0.05 ml diluted emulsion B sized. This accounts for about 70% of the original volume of 3.9 x lo-’ ml oil used to make up the emulsion. Higuchi et al. (1962) examined a relatively monodispersed emulsion and obtained not better than 50% of the oil phase. The missing volume of the oil phase may be due to incomplete sizing, evaporation, or the emulsification technique.
Conclusions Coulter counting is an extremely valuable method for determining particle size distributions of O / W emulsions, but care should be taken in its use. The Coulter orifice assembly should be shielded, especially when sizing particles in the submicron range. In addition, the dilution effect should be carefully examined. A correct count of par-
ticle numbers can be deduced if the diluting electrolyte solution is saturated with oil and the method of zero-time extrapolation is employed.
Acknowledgment Thanks are due to J. Schmidt and J. Diodati of Coulter Electronics for their helpful discussions. Literature Cited Davies, J. T., Rideal, K.. “Interfacial Phenomena,” 2nd ed., p p 10-11, Academic Press, New York, N.Y., 1963. Grassman, P . “Physical Principles of Chemical Engineering.” p p 590-2. Pereamon Press. 1971. Groves, M . J:. Proc. Soc. Anal. Chem., 5,166-7 (1968). Higuchi. 0 . 0., (1962). Phillips, C. R., Can. J. Chem. Eng., Leinonen, P . J . , Mackay, D.. 49,288-90 (1971). Loriston-Clarke. A. G.. Thew. M . T.. a uauer presented a t the 7th Coulter Counter Users Conference in Lbndon; England, 1969. Rehfeld, S. J., J. Colloid Interface Sei.. 24, 365-85 (1967). Rowe, E . L., J . Pharm. Sei., 54, 260-4 (1965). Sherman, P.. Ed.. “Emulsion Science,” p 153. Academic Press, Xew York, N.Y., 1968. Shotten, E.. Davis, S. S . , J . Pharm. Pharmac., 20, 780-9 (1968). Sineleton. W . S.. Brown. M. L.. J . Amer. Oil C h e m . Soc.. 42, 312-14 (1965). Sprow. F. B., A I C h E J , 13,995-8 (1967). Treybal. R. E., “Liquid Extraction.” p 157, 2nd ed.. McGrawHill, S e w York. N.Y.. 1963. Receiced f o r recieii, Februac, 14, 1973. Accepted Decem b w 31, 1973. Work supported b ) the Department of the Environment, O t t a u a , the Institute of Environmental Sciencea and Engiwering of the Lniversit? of Toronto, and Imperial Oii Ltd.
Potential Effects of Thermal Discharges on Aquatic Systems Ronald M . Bush,’ Eugene B. Welch, and Brian W . M a r Department of Civil Engineering, University of Washington, Seattle. Wash. 98195
Ecological changes in the aquatic environment caused by temperature increase are estimated in order to aid the decision process in siting cooling water discharges. Principal consideration is given to the fish community for which predictions of ‘change in composition are based on lethal and preferred temperature. Six representative river systems in the country are considered. Insufficient data precluded similar community response estimations for freshwater invertebrates. However, data indicate that adequate protection of fish species results in like protection of the invertebrate fauna and thus the estimated effect of elevated temperature on freshwater fish communities will suffice as a guideline for both the protection of fish and invertebrates. Similar predictions were not possible for the marine communit) since thermal requirements are known for only a few species. However existing data and site studies are summarized for use as guidelines.
In response to the predicted increase in use of natural waters for cooling purposes, several comprehensive literature reviews have been compiled to provide quick reference to effects of changes in water temperature upon aquatic organisms (Brett, 1966; Wurtz and Renn, 1965; Altman and Dittmer. 1966; Welch and Wojtalik, 1968:
Jensen et al.. 1969: Parker and Krenkel. 1969; deSylva. 1969; Hawkes, 1969; Coutant. 1968b, 1969, 1970, 1971; Marble and Mowell, 1971). Despite this concentration of literature there has been a great reluctance by biologists to estimate the response of an aquatic system to increased water temperatures. One reason, and perhaps the basic one, is that many of the multitude of interactions within an aquatic community are unknown and the understanding of the known is hazy at best. Thus, it may seem unrealistic to predict the consequences of altering one basic environmental parameter. Other reasons for reluctance stem from the fact that the great majority of the data available are from research programs investigating the effects of temperature elevation on single test organisms. There is a paucity of studies at the population or community level. Also, much of the research has been performed in laboratories. and there is hesitance to extrapolate laboratory results to field situations. However. in planning the use of a waterway for cooling water discharge, it is essential to determine the amount of heated water that can be safely introduced or make some estimation of the consequences of altering the natural Present address Department of the Army, Seattle District. Corps of Engineers. 1519 Alaskan \Yay So.. Seattle, L$-ash. 98134. T o whom correspondence should be addressed. Volume 8 , Number 6, June 1974
561
