SURFACE ACTIVITY OF SOLID EMULSIFIERS J. MITCHELL FAIN AND FOSTER DEE SNELL Foster D. Snell, Inc., Brooklyn, N. Y.
70 per cent by volume of water could be dispersed in 30 per cent by volume of kerosene when one gram of dry carbon (best American carbon black) was used; a very viscous emulsion, stable for several weeks, was formed. Bhatnagar ( 2 ) used zinc hydroxide, aluminum hydroxide, and lead oxide for making emulsions of water in oil. Bechhold, Dede, and Reiner (1) carried out an extensive investigation on the use of finely divided solids as emulsifiers. They found that the formation of emulsions depended on: ( a ) the grain size of the powder-the smaller the grain the greater was the emulsifying power until an optimum was reached, after which small grains had inferior emulsifying properties; ( b ) the quantity of powder-the more powder there was available the more globules that could be covered, provided the powder was sufficiently fine. The solids investigated were zinc dust, iron powder, clay, and kieselguhr; they were found to be as efficient as yeast, hemoglobin, or egg albumin solutions for emulsifying benzene, paraffin, nitrobenzene, isobutyl aldehyde, carbon disulfide, and several other organic liquids in water. Thomas (18) pointed out that, in order for a powder to serve as an emulsifying agent, it must be wetted to a certain extent by both liquids. When a liquid does not spread over a solid, the surface of the liquid joins the solid a t an angle, which within certain limits is definite. The angle between the surface of the liquid and the solid-liquid interface is the contact angle for the solid and liquid (5). When a liquid completely wets a solid, the contact angle is zero. The distribution of finely divided solids and colloidal suspensions between two immiscible liquids was discussed by Reinders (14.)’ Let 8 1 2 = interfacial tension between solid 1 and liquid 2 Sls = interfacial tension between solid 1 and liquid 3 8 2 8 = interfacial tension between the two liquids, 2 and 3
Finely divided solids, acting as emulsifiers, appear in the interface between the two emulsion liquids, and their distribution m a y be determined from a consideration of their contact angles. The aqueous dispersion of asphalt with the aid of bentonite is a large-scale industrial application of finely divided solids as emulsifiers. Its study throws light on their fundamental properties. The increase in hydrogen-ion concentration of a bentonite slip by addition of sulfuric acid results in increase in viscosity. Partial flocculation or coagulation of the emulsifier is found necessary for optimum emulsification. The effect of a weak flocculating agent is to counteract the peptization of the solid suspension and to force the solid into the interface. The juxtaposition of the bentonite granules, situated as they are in a thin layer around the already emulsified asphalt particles, causes them to serve as a cutting edge for the subdivision of new asphalt fed into the emulsion. The addition of citric acid in small amounts to an emulsion of asphalt in water, with bentonite as emulsifier, substantially decreases the time required for plastic flow. The addition of citric acid to the bentonite slip causes a reduction in viscosity up to a certain point, after which the viscosity increases with increase in the amount of citric acid added. The addition of oxalic acid produces a similar effect on the viscosity of the bentonite slip. Two hypotheses to account for this action are discussed. One is based on the preferential adsorption of the solid emulsifier for the organic compounds by virtue of their hydroxyl or carboxyl groups; the other is based on the action of the organic acid in bringing into solution the iron oxide contained in the bentonite.
When a powder of the solid is shaken with liquids 2 and 3, the following cases must be considered: 1.
812
> 813 + 8 2 3
+
11. 818 > 8 1 2 8 2 3 111. Sea > 8 1 2 f f l l 3 IV. None of the three interfacial tensions is greater than the sum of the other two.
T
HE first extensive survey of insoluble emulsifiers was
made by Pickering (If?) who found that the basic sulfate of iron was the best, followed by those of copper and nickel. The basic sulfates of zinc and aluminum generally gave good emulsions a t first; but aggregation of the particles seemed to occur, causing partial de-emulsification. Other useful emulsifying agents were freshly precipitated calcium carbonate and arsenate, lead arsenate, and various unheated fine clays. Pickering (13) believed that emulsification depended on the size of the particles constituting the emulsifier, and that when the particles became too big, they ceased to emulsify. Likewise, the size of the emulsion globules varied directly with the size of the particles of the emulsifying agent. He also pointed out that for emulsions of oil in water the finely divided solid emulsifier must be more readily wetted by water than by oil. Briggs (3) demonstrated that hydrous ferric oxide, arsenious sulfide, and finely powdered silica promote emulsions of benzene and kerosene in water. Schlaepfer (17) laid down the rule that “insoluble particles, which are more easily moistened by oil than by water, will have a tendency to facilitate the emulsification of water in oil.” He found that
I n the first case the solid will remain entirely in liquid 3, in the second case, entirely in liquid 2. In the third and fourth cases the powder goes to the interface, tending to separate the two liquids. When none of the three tensions exceeds the sum of the other two, the three phases meet a t a certain contact angle. All finely divided solids which act as emulsifiers appear in the interface between the two emulsion liquids. Their distribution may be determined from a consideration of the contact angles. If 1 in Figure 1 is a solid sphere a t the interface between liquids 2 and 3, the condition for equilibrium is 812
where 8
=
818
f
823
6
angle of contact
If Xlz > 513,then cos 0 is positive and 0 is less than 90”. The solid will tend to be drawn into liquid 3; i. e., its equator is in liquid 2. If Slz< 813,then cos 0 is negative and 0 is greater 48
JANUARY, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
than 90". The solid sphere will tend to be drawn into liquid 2. If there are enough solid particles to fill the interface, the tendency of the interface t o contract will cause it t o bend (6), as shown in Figure 2, in the direction of the more poorly wetting liquid, which makes it easy for the latter to become the enclosed phase. I n order to behave in this way, a solid must be easily dispersed in the outer liquid; its particles should not tend t o agglomerate in the liquid nor to stick together when serving as a protective armor for the emulsified drops. It
2 \
/ F I Q U R1~
should be possible, therefore, to predict whether or not a given solid powder can stabilize an emulsion and also which liquid will become the dispersed phase, if the angle of contact of the interface and the solid is known.
Dispersion of Asphalt with Bentonite The aqueous dispersion of asphalt with the aid of bentonite is a large-scale industrial application of finely divided solids as emulsifiers. Ccrtain features of the industry merit study for the light they throw on the fundamental properties of solid emulsifiers. Bentonite, usually considered the result of devitrification and partial decomposition of glassy volcanic ash, has been found by Ross and Shannon (15) to be commonly composed of montmorillonite and less often of beidellite, minerals resembling micas. Montmorillonite is designated as (Mg,Ca)O.Al2O3.5SiO2.nH20,with n varying from 5 to 8. Larsen and Wherry (10) named beidellite and assigned to i t the formula Al2Os.3SiO2.xH20,in which x is frequently equal to 4 and the alumina is replaceable by other oxides. Ross and Shannon (16) concluded that this mineral consists of isomorphous series of Al2O3.3SiOz.nHz0and Fe20a.3SiO*.nH20with n equal to approximately 4 and with the water replaceable by alkali or alkaline-earth oxides. The colloidal properties of bentonite were attributed by Wherry (19) to a felted texture and a micaceous structure, in which the crystals have appreciable size in two dimensions, but a thickness of colloidal magnitude and the property of splitting up into still thinner plates. The statement of Davis and Vacher (4) that a consideration of the behavior of the gel of Wyoming bentonite leads to its classification as an inorganic, hydrophilic colloid appears to be justified. The action of bentonite as a solid emulsifier does not depend on surface tension lowering. When bentonite was shaken with water, no permanent froth formed, which indicated that bentonite was not adsorbed a t the water-air interface. Surface tension measurements ( 4 ) by the capillary tube and the drop weight and du Nouy apparatus methods were the same for distilled water as for water to which bentonite had been added. These results indicate that bentonite does not appreciably change the surface tension of pure water. The action of bentonite and of solids generally as emulsifiers depends, according to Pickering (la), on the covering of the droplet with small insoluble solid particles. The mechanism of the action whereby the asphalt is subdivided into particles has not been described. Kirschbraun (7)stated that there is apparently some relation between the various
49
emulsifying agents-for kxample, clays-and between the bitumen and the clays, or combinations of clays, which widely influences the nature of the emulsifying operation. The difference in behavior results not only in variations in degree of dispersion and amount of bitumen capable of being emulsified, but also in the shape of the particles and the color of the finished emulsion. For example, in using a clay combination of five parts of bentonite t o two parts of New Jersey clay, the resulting emulsion is brown; on the other hand, by substituting a different clay, such as Ohio fire clay, for the Kew Jersey pottery clay or by omitting the New Jersey clay entirely, the emulsion is black and in the latter case the particles are rohnd. The emulsifying operation carried out in the presence of the New Jersey clay produces particles more or less ovoid and flat. On microscopic examination they appear quite transparent. The brown color of any emulsion is due to the finer particle size of the bitumen. The proportioning of the clays affects the pH of the clay system, inasmuch as the bentonite is alkaline and the New Jersey pottery clay is acidic. Subsequent work (9) proved that similar variations in emulsion char a c t er is t ics could be obtained OUTER L I Q U I D merely by adjusting the pH of the bentonite with buffer salts without addition of other clays. K i r s c h b r a u n and Levin (9) found i NTER FA CE that the optimum pH of bentonite for emulINNER LIQUID sifying steam-refined FIGURE2 Mexican asphalt with a m e l t i n g p o i n t of 120" F. is 6.3. The acid nature of some binders and the alkaline nature of others make it necessary to adjust the pH of the emulsion system as dispersion proceeds, in order to maintain optimum emulsification.
m
Effect of pH on Viscosity I n order to understand more clearly the nature of the emulsifying operation, the effect of pH adjustment on the viscosity of a bentonite slip was studied. Dried crushed bentonite was made up into a 10 per cent paste. Eighty-gram portions of the paste were weighed out, and varying amounts of 0.690 N sulfuric acid added. Water was added and stirred in to give a smooth slip of 5 per cent dry clay content. The slip was screened through a 200-mesh copper sieve. The pH was determined electrometrically with a quinhydrone electrode, and the viscosity with a Dudley pipet. The results are as follows: Cc. of 0.690 N
Sulfuric Scid
Added
cc. of Water Added
PH
Viscosity. Seconds
Figure 3A illustrates the relation between pH and viscosity for the bentonite slip whose hydrogen-ion concentration has been adjusted with sulfuric acid. At the optimum pH for emulsification of Mexican asphalt, an increase in viscosity is noted over that of the unadjusted bentonite. This increase in viscosity is probably the result of flocculation of the bentonite particles. Interfacial-tension lowering plays no part in emulsification with bentonite. The amount of work required to produce an asphalt emulsion is determined by the increase in surface area of the asphalt particles and the asphalt-water interfacial ten-
INDUSTRIAL AND ENGINEERING CHEMISTRY
50
sion. Yet emulsification with a bentonite slip whose pH has been adjusted to 6.3 gives an emulsion of finer particle size t h a n one whose pH has not been a d j u s t e d . Increasing the 70 amount of work done on the‘ emul60 sion will not affect the size of the asphalt particles. 5. PH ADJUSTMEh 60Additional mixing s WITH CITRIC AC a beyond the point z 0 where the smallest 0 W particle size for 0 any definite set of 55>conditions is obc tained will result 0 only in coalescence 0 v, of the asphalt and “breaking” of the emulsion. A consideration of the factors involved in the emulsification process gives rise to the hypothesis that flocculation of the bentonite is responsible for the nature of the emulHYDROGEN-ION CONCENTRATION ( p W ) sifying operation. FIUURE3. EFFECTOF HYDROGEN- At a pHbf 6.3 the ION CONCENTRATION ON VISCOSITY bentonite particles OF 5 PERCENTBENTONITE SLIP are flocculated to the proper degree for optimum emulsification of steam-refined Mekcan asphalt. The effect of a weak flocculating agent is to counteract the peptization of the solid suspension and to force the solid into the interface. The juxtaposition of the bentonite granules, situated as they are in a thin layer around the already emulsified asphalt particles, causes them to serve as a cutting edge for the subdivision of new asphalt fed into the emulsion. The appearance of the asphalt particles seems to confirm this view. They are flat, as would be expected after comminution by a material having a micaceous structure in which the crystals have appreciable size in two dimensions but a thickness of colloidal magnitude. The edges are not smooth but scaly, which indicates that mechanical tearing is involved. The ovoid rather than the round shape is also probably due to this cause. Increased flocculation, as may be accomplished by the further addition of acids to the bentonite slip, does not result in increase of asphalt particle size when the ratios of bitumen to clay are low. When more bitumen is added, the particle size tends to increase. The bentonite loses efficiency as an emulsifying agent. Apparently the increased flocculation of the bentonite granules causes them to agglomerate in relatively large masses. The number of separate cutting edges is reduced. In the absence of flocculation, subdivision of the additional asphalt is not accomplished with the same violent tearing action. The asphalt is subdivided into particles of larger size. The edges are smooth.
A. PH ADJUSTMEN[ WITH SULFURIC ACID
v)
’
i f
/-
’
VOL. 31, NO. 1
The necessity for adjusting the pH of bentonite to effect optimum emulsification of asphalt is paralleled by the action of colloidal ferric oxide which, as reported by van der Minne ( I I ) , does not emulsify oil in water untilit has been coagulated to a certain extent. Briggs (3) also reported that moderate flocculation of hydrous ferric oxide and of arsenious sulfide were necessary for successful emulsification. Finely divided solids, which have the ability to maintain water-immiscible liquids in emulsified form in water, form coatings over the dispersed particles and prevent them from coalescing. Such properties of the emulsion as viscosity, plasticity, mobility, and suspendability of the dispersed particles are affected importantly by the emulsifier. Reaction characteristics of the solid emulsifier are reflected in the physical properties of the emulsion. Kirschbraun (8) discovered that the addition of citric acid in very small amounts to an emulsion of asphalt in water, with bentonite as emulsifier, substantially decreases the time required for plastic flow. The apparent‘viscosity as measured with a Gardner mobilometer decreased from 28 seconds when no citric acid was present to a minimum of 9.5 seconds when 0.007 per cent citric acid was added. Further additions of citric acid resulted in an increase in the viscosity. He found that other acids, notably tannic, tartaric, pyrogallic, maleic, phthalic, and pyromucic acids, as well as hydroquinone and resorcinol, exhibit the same effect. From this he concluded that the effect is due to the presence of hydroxyl and/or carboxyl groups in the molecule. I n order to ascertain the role played by the solid emulsifier in this phenomenon, additions of varying amounts of a 10 per cent citric acid solution were made to 80-gram portions of a 10 per cent bentonite paste. Water was then added to bring the total up to 160 grams, which made the solid clay content of each portion 5 per cent. The pH of each portion was determined electrometrically with a quinhydrone electrode and the viscosity with a Dudley pipet. The results are shown in Figure 3B and the following table: cc. of 10% Citric Acid Added
cc. of Water Added
0.4 1.0 5.0 20.0
80.0 79.8 79.6 79.0 75.0 60.0
... 0.2
PH 7.3 6.9
6.35
5.65 3.70 2.95
Viscosity, Seconds 55.0 64.4 53.2 52.2 56.6 59.2
As in the case of the bentonite-dispersed asphalt, the addition of citric acid to the bentonite slip caused a reduction in viscosity up to a certain point, after which the viscosity increased with increase in the amount of citric acid added. The addition of oxalic acid to a bentonite slip is similar in its effect to citric acid. Varying amounts of a 0.1151 N oxalic acid solution were added to separate 80-gram portions of a 10 per cent bentonite slip, except in one case where 30 cc. of a 10 per cent oxalic acid solution was added t o effect a more marked lowering of pH than was possible with the more dilute solution. Water was added to bring the total weight in each case up to 160 grams; the solid clay content of each portion was thus exactly 5 per cent. The pH and viscosity of each portion were determined with results shown in Figure 3C and the following table:
cc. of Oxalio Acid S o h . Added
cc. of Water Added
Viscosity, Seconds
The explanation (8) offered for the action of citric acid in lowering the viscosity of the emulsion is that the adsorption of
JANUARY, 1939
INDUSTRIAL AND ENGINEERING C H E M I S T R Y
water by clay, bentonite, colloidal oxides, and similar emulsifying agents is due to the attraction between the colloidal particles of these emulsifying agents and the hydroxyl group of water. In the presence of citric acid and in the absence of flocculating concentrations of flocculating ions, the emulsifying agents selectively adsorb the citric acid in preference to the water by virtue of the hydroxyl group possessed by the citric acid, and consequently the water adsorption of the emulsifying agent is reduced. This enables water that would otherwise be adsorbed by the emulsifying agent to remain in the intermicellar space of the emulsion system. In lieu of the above or perhaps in combination with it, the strictly chemical action of acids such as citric or oxalic as opposed to that of acids such as sulfuric may be a factor in the viscosity-pH relations. Small amounts of iron oxide present in the bentonite are precipitated by sulfuric acid as insoluble sulfates. The action of citric and oxalic acids is to dissolve the iron oxide. As a result, the reduction in viscosity of the bentonite slip is more than sufficient, with small concentrations of the organic acids, to offset the increase in viscosity due to the flocculating action of the hydrogen ions. With increase in acid content, the latter effect becomes increasingly important and the viscosity becomes greater.
51
Literature Cited (1) Bechhold, H., Dede, L., and Reiner, L., Kolloid-Z., 28, 6 (1921). (2) Bhatnagar, 5. S., J . Chem. SOC.,119, 1760 (1921). (3) Briggs, T. R., J. IND. ENG.CHEM.,13, 1008 (1921).
(4) Davis, C. W., and Vacher, H. C., U.9. Bur. Mines, Tech. Paper 438 (1928). (5) Edser, E., Brit. Assoc. Advancement Sci. 4th Rept., 1922,289. (6) Finkle, P., Draper, H. D., and Hildebrand, J. H., J . Am. Chem. Soc , 45,2780 (1923). (7) Kirschbraun, L., U. 9. Patent 1,691,768 (Nov. 13, 1928). (8) Ibid., 1,918,759 (July 18, 1933). (9) Kirsohbraun, L., and Levin, H. L., Ibid., 1,691,767 (Nov. 13, 1928). (10) Larsen, E. S.,and Wherry, E. T., J . Wash. Acad. Sci., 15,465 (1925). (11) Minne, J. L. van der, Chem. Weekblad,35, 125 (1938). (12) Pickering, S.U., J. Chem. SOC.,91,2001 (1907). (13) Pickering, S.U., J . SOC.Chem. Ind., 29, 129 (1910). (14) Reinders, W., Kolloid-Z., 13, 235 (1913). (15) Ross, C. S.,and Shannon, E. V., J . Am. Ceram. SOC., 9,77 (1926). (16) Ross, C. S.,and Shannon, E. V., J . Wash. Acad. Sci., 15,467 (1925). (17) Schlaepfer, A. U. M., J. Chem. SOC.,113,522 (1918). (18) Thomas, A. W., J . Am. Leather Chem. Assoc., 22,171 (1927). (19) Wherry, E. T., Am. Mineral., 10,120 (1925). RECEIVED September 12, 1938.
SURFACE-ACTIVE PROPERTIES OF HEXAMETAPHOSPHATE G. B. HATCH AND OWEN RICE Hall Laboratories, Inc., Pittsburgh, Pa.
Besides possessing definite surface-active properties of its own, hexametaphosphate has the ability of forming soluble complexes with many multivalent cations; their concentration is thereby reduced to such an extent as practically to eliminate their agglomerating action on numerous colloid systems. Examples of these properties are found in its action as a peptizing agent, as a depressor in selective flotation, and in its effect upon monolayers. Recently amounts of hexametaphosphate very much below those required for complete calcium sequestration have been found effective in preventing calcium carbonate deposition upon moderate treatment of bicarbonate waters with heat or alkali.
G
LA8SY sodium metaphosphate, (iTaPOB),, commonly termed Graham’s salt or sodium hexametaphosphate, was discovered in 1833 by Graham ( 3 ) . For almost a century it remained a scientific curiosity, and not until Hall’s use of this glassy form as a water-conditioning agent (4, 6) did it become commercially available. Though it is still commonly known chiefly as a water-treating chemical, in the past’few years numerous other uses for sodium hexametaphosphate have been developed, ranging from the treatment of occupational dermatoses (8) to the tanning of leather (14). This paper will be limited to the discussion of the behavior of hexametaphosphate as a surface-active agent, with
Thus the addition of 2 p. p. m. of hexametaphosphate to a water containing 200 p. p. m. of calcium bicarbonate will obviate precipitation when the water is treated with 500 p. p. m. of sodium carbonate or is heated to 80” C. for one hour. This “threshold treatment” with amounts of hexametaphosphate of the order of 1 to 5 p. p. m. has proved very useful in the prevention and removal of carbonate scale in many industrial processes. Data are presented showing the effect of temperature and metaphosphate concentration on the efficacy of this treatment, and demonstrating hysteresis effects on calcium carbonate and metal surfaces. Indications of the colloidal nature of threshold treatment are discussed.
particular emphasis upon the peculiar ability of as little as 1 or 2 parts per million to inhibit the precipitation of calcium carbonate.
Colloidal Properties Hexametaphosphate has two properties which are of considerable interest with respect to colloidal phenomena. It possesses definite surface-active properties of its own a t solidaqueous solution interfaces, and it has the ability of forming soluble complexes with numerous multivalent cations, thereby reducing their concentration to such low values as practically to eliminate their agglomerating action towards various col-
