April, 1963
STABILIZATION OF WATER-IN-OILEMULSIOKS BY SOLID PAI~TICLES S[Cl] = Ksp
+ K2Ksp2P2
(12)
where once more (6. section A3) the assumption Ka = K 4 .. . . . Kn is invoked. Although eq. 12 rests on somewhat dubious approximations, it leads to the entirely reasonable conclusion that the solubility of the dye evidently is enhanced as the extent of polymerization increases and that this effect is the cause of the very high dye solubility a t low [Cl-1. However, it is not possible to determine by these methods how many C1 are included in the polymer. Judging by the appearance of the curve S[Cl-] us. [Cl-] in Fig. 9, however, polymerization is encouraged a t fairly low [Cl-]. Evidently, as anions become more plentiful, the tendency to form the insoluble uncharged species containing relatively more C1- overtakes the polymerization process and the dye salts out. If each polymer unit is singly charged entity, the total concentration of all such units in a solution of pure THCl must be equal to the concentration of anions, [Cl-1. The solubility, Le., the total number of dye
731
ions (associated and otherwise) in a saturated solution of pure dye, then, is the average degree of polymerization multiplied by the number of polymers, or P[C1-] = S. This relation may be used with eq. 12 and the values of K2, Ksp, and S for pure dye to estimate a minimum value, P = 6, under these circumstances. If, as the case seems to be, fewer anions are included in the polymers, P must be correspondingly larger. 2. Sulfate Systems.-We are unable to give quailtitative data on the dependence of S[SO4-2l112 on [S04-2]1/2 in analogy to the chloride data since we were not able to prepare acceptable crystals of (TH),S04. The solubility of the chloride salt in sulfate systems, of course, has no direct bearing on the question since the composition of the solid phase is not determined in such systems. We report the solubility of crude (TH)$304 in Table I. Acknowledgments.-We gratefully acknowledge the full support of this research by the Air Force Office of Scientific Research under Contract Af 19(604)-6643.
STABILIZATION OF \;\’ATER-IS-OIL EMULSIONS BY SOLID PARTICLES’ BY E. H. LCCASSES-REYXDERS ASD M.
VAN DEN
TEMPEL
Unilever Research Laboratory, Vlaardingen, Netherlands Received M a y 1.3, 1969 Emulsions of water in paraffin oil can be stabilized by cryetals of glycerol tristearate, provided that small amounts of surface-active agents are also present. Both oil- and water-soluble surfactants can be used, in concentrations which have no appreciable stabilizing power in the absence of tristearate crystals. The contact angle of water and paraffin oil against tristearate crystals was found to be unaffected by the addition of surfactants. The increased stabilizing power in the presence of surfactants is due to a decreased interaction of the tristearate crystals, which allows the crystals to reach the surface of the water droplets before an appreciable amount of coalescence has occurred. The effect of surfactants on the interaction energy of the crystals is investigated by means of steady-state viscosity measurements a t very low shear rates. The interaction energy is decreased by about 1 kl’ unit at shear rates of the order of 0.001 sec.-l, by adding a surfactant to the dispersion of crystals in oil. The estimated effect of the decreased interaction energy on the emulsion stability is in agreement with experimental results.
affected by the addition of surface active materials16 Jntroduction even though this results in a considerably increased Emulsions may be stabilized by solid particles if emulsion stability. The value of the contact angle (i) the contact angle between the two liquids and the (measured in the water phase with Bartell’s method) particle surface has a value which favors adsorption of is l l O o , and this accounts for the high stability of the the particle at the liquid-liquid interface2 and (ii) the emulsions compared with the stability of particles are in a state of incipient f l o ~ c u l a t i o n . ~ ~water-in-oil ~ inverse type emulsions.2b This behavior usually is explained by considering that a I n the present paper it is shown that the effect of dense layer of solid particles a t the oil-water interface surfactants in this system is due to a decreased interis required for stability. action between the triglyceride crystals, as found by The relation between emulsion stability and contact rheological measurements on their suspensions in angle has been investigatcd in emulsions containing paraffin oil. water, benzene, and barium sulfate as solid particle^.^ It was shown that the contact angle in this system Experimental can be varied by adding surface-active materials, Materials.-Glyceryl tristearate was produced by recrystsllizing fully hydrogenated linseed oil from acetone until the meltand this affects the stability of the emulsions formed. ing point exceeded 71”. Crystals with an average diameter On the other hand, in the system containing triglyceride of about 10-6 cm. were obtained by rapid crystallization of a crystals, paraffin oil and water the contact angle is not hot, 25% solution in paraffin oil. After recrystallization had (1) The contents of this paper have been presented b y M. vdT. a t the 142nd National Meeting of the American Chemical Society in Atlantic City, September 9-14, 1962; they are part of a thesis of E. L.. prepared under the supervision of Prof. Dr. J. Th. G. Overbeek of Utrecht University. (2) (a) S. U. Pickering, J . Chem. Soc.. 91, 2001 (1907); (b) J. L. van der Minne, “Over Emulsies,” Thesis, Amsterdam, 1928. (3) T. R. Briggs. Ind. Eng. Chem.. 13, 1008 (1921). (4) M. van der Waarden, Kollotd-2.. 166, 116 (1958). (5) J. H. Schulman and J. Leja. Trans. Pawday SOC.,60, 598 (1954).
been allowed to proceed for a t least several days, the plastic mass was diluted with paraffin oil t o a concentration of lY0by weight. The following surfactants were added in the oil used for diluting: glyreryl a-monodeate (purity 98y! by weight as determined by oxidation with periodic acid; iodine value 71.5; melting point (6) E. H. Lucnssen-Reynders, paper submitted for publication in J . Phya. Chem.
E. H. LUCASSE~\--REYXDERS AND $1. VAN
732
DEN TEMPEL
1701
TABLE I SURFACTAXTS O N THE STABILIZATION EMULSIONS OF WATERIN PARAFFIN OIL BY GLYCERYL TRISTEARATE CRYSTALS
INFLUENCE OF
Surfactant NO SURFACTANT AEROSOL UT CETYLALCOHOL MONO OLEAI'E
I;
Kone Glyceryl monooleate Cetyl alcohol Aerosol OT
Y
0
20
10
SILAR
Fig. 1.-Effect of surfactants on the stress-shear curve of a 1y0 glyceryl tristearate suspension in paraffin oil, a t a shear rate of 0.00767 sera-'. Surfactant concentrations are given in the text. 35"); sodium diethyl hexyl sulfosuccinate (Aerosol OT; from American Cyanamid Co.); cctyl alcohol (purity 92y0 by weight as determined by gas chromatography, the residue consisting of higher and lower homologs). In one single case (the stability measurement with Aerosol OT) the surfactant was added t o the water phase. The paraffin oil was a water-white, medical grade having a density of 0.864%.~ m . - ~ a naviscosity d of 0.76 poise, both at 20". Its surface tension against water was not affected by purification with alumina; this shows the absence of surface-active impurities. Emulsion stability was fomd from the change in reflertance of the coagulating emulsions, containing a water-soluble dye in the interior of the droplets. The amoant of light of a suitable wave length reflected from the upper sirface of such emulsions is a simple function of the specific interfacial area, as has been shown empirically by Lloyd7
log R
=
b log A
+c
R being the percentage of normally incident light reflected from the emulsion under an angle of about 10'; A is the area of interface per unit volume of the emulsion, and b and c are constants not depending on A . This equation can be transformed easily into a relation between reflectance and time in cases where the coagulation follows a first-order reaction mechanism,8 with probability of coalescence K (in see.+)
log R
=
bK t 3.2,30 ~
+ constant
Thus the probability of coalescence K can be found from the slope of the log reflectance-time plot. Knowledze of the additional constant is not necessary. The constant b, however, has to be known; its value was evaluated from the reflectances of a coarse and a fine emulsion with identical composition and a mixture of these two, 119 suggested by Lloyd. The accuracy and reproducibility of the values of b obtained in this way were not better than by a factor of about 2 , and hence the uncertainty in the value of K is of the same m a g d u d e . Since the addition of surfactant results in changes of K with several orders of magnitude, the measurement is considered sufficiently accurate for the purpose of the present investigation. Table I shows the results of stability measurements on emulsions of water (20% by volume) in paraffin oil which contained 1% by weight of glyceryl tristearate crystals, upon addition of various amounts of surfactants. Surfactant concentrations refer t o the phase in which the surfactant had been dissolved: oil for the first two materials, and water for Aerosol OT. A Coalescence rate of about 0.01 see.-', as found in the emulsion containing only tristearate and no surfactant, indicates that the emulsion is completely broken in less than 10 min. In (7) N. E. Lloyd, J . Colloid Sei., 19, 441 (1959). (8) M. van den Tempel, Proc. 2nd Intern. Congr. Surface Activity, London, I, 1957. P. 439.
Conon., 10-6 moles cm.-a
... 1
2 5 5 25 10
67
OF
Probability of coalescence of emulsion, sec. -1
-io x 2x 0 01 x 0 3 0 08 o 04
x x x
10-3 10-3 10-3
10-3 10-3
emulsions containing only surfactants but no tristearate, breaking occurred a t the same high rate. Flocculation of fat crystals dispersed in oil was investigated by means of steady-state viscosity measurements a t very low shear rates. A concentric cylinder instrument was used in which the outer cylinder (internal diameter 3.15 cm.) is slowly rotated a t a constant, low speed. The inner cylinder (diameter 2.80 cm., hei.ght 8.58 em.) is freely suspended from a torsion wire. The surfaces of the cylinders are provided with vertical grooves of triangular cross-section and a depth of 0.1 mm. Rotation of the inner cylinder is recorded by means of an optical lever. The cylinders are carefully aligned and centered before a measurement starts; the error due to imperfect adjustment could be made smaller than about lo%, which is small compared with the effects to be measured. The instrument is permanently held at 20.0'. The range of shear rates that can be covered with this instrument extends from about 1 see.-' to about 10-5 see.-'. At these low speeds, transient effects can be measured in the beginning of an experiment. -4plot of the shear stress in the sample as a function of the shear generally will show a maximum stress a t shear values of the order of unity (cf. Fig. 1). At higher shear values, the stress drops to a stationary value, which measures the steadystate viscosity. Stress-shear curves of this type zenerally have been found for disperse systems at sufficiently low shear rates.g The discussion of the present paper is restricted to the behavior in the steady state. For measurements at shear rates in excess of 50 see.-', a cone-and-plate instrument (Ferranti-Shirley) was used. I n the dilute suspensions (17 0 by weight of glyceryl tristearate), no sedimentation could be observed even after storage periods of several weeks. At shear rates exceedins 100 see.-', the suspensions showed newtonian behavior with a viscosity coefficient of 0.95 poise, both in the absence and presence of surfactants. In performing a measurement a t low shear rate, the suspension was thoroughly stirred before being poured into the viscometer. The measurement was started about 1 hr. after assembling and adjustment of the instrument had been completed. Typical stress-shear curves are shown in Fig. 1. Steady-state viscosities obtained from these curves have been assembled in Fig. 2 . Altogether, four different dispersions were made without surfactant, using two different batches of tristearate crystals. The steady-state viscosities of these dispersions varied over a range indicated by the height of the vertical lines in Fiz. 2. Surfactants investigated were glycerol monooleate, Aerosol OT, and cetyl alcohol, in concentrations of, respectively, 2.5, 10, and 10 pmoles/cm.a of oil. The results collected in Fig. 2 show that, a t shear rates less than about 0.1 see.-', the steady-state viscosities of the dispersions containing surfactant are significantly lower than those of the dispersions without surfactant. Results obtained with dispersions that had been carefully dried or to which 1% of water had been added, did not differ significantly from those shown in Fig. 2.
Discussion (a) Flocculation of Solid Particles.-Dispersed triglyceride crystals in oil are always flocculated to a high degree, because attractive forces prevail a t all distances between the particles. The absence of sedimentation in the quiescent, dilute dispersions shows that the tri(9) A. A Trapeenikov and V. A. Fedotova, Proc. Alcad. Sci. U.8.S.R.. 81, 1101 (1951).
April, 1963
STABILIZATIOS OF WATER-IN-OIL EMIULSIONS BY SOLID PARTICLES
glyceride crystals are flocculated to form a threedimensional network sufficiently strong to withstand the action of gravihy. In more concentrated suspensions the strength of this network has been measuredlo; the average energy content of the bonds between consecutive particles in the network was estimated a t about 40 LT units. Addition of surfactant causes a reduced interaction energy between the particles in the network and this results in a decreased viscosity as shown in Fig. 2 . The presence of the crystal network in the dilute suspensions, and the effect of surfactants on the strength of the network, can easily be demonstrated by observing the movements of water droplets through a layer of the suspension. I n a suspension containing surfactant, a water droplet of 0.3 cm. diameter falls regularly a t a velocity of the order of 1 cm.,/min. In the absence of surfactant, a droplet is trapped in the upper layer of the suspension where it may remain stationary for very long periods. A more quantitative estimate of the effect of surfactants on the strength of the crystal network can be made as follows. The network is considered to consist of an assembly of branched and interlinked chains, each chain consisting of a linear arrangement of solid particles. The distance between consecutive particles in a chain is small compared with the average particle diameter. The properties of the network are described by the free energy (AG) of the interparticle bonds and by the number (n) of bonds per em. chain length, or alternatively, by the number (iV) of chains cutting through 1 cm.2 cross-section. I n a steady-state viscosity measurement at a shear rate D, the stress supported by the network is S = qD. The average load upon each interparticle bond is SIN and the rate of breaking of bonds is given by reaction rate theory as
-1 dN N dt
=
.A’ exp
(-g)sinh ($)
(3)
A’ is a proportionality constant which need not be specified and X measures the distance over which the particles must be moved apart to break the bond. The shear rate is proportional to the rate of breaking of bonds
.O
=
(
cg)
(=) (4)
nXA‘ exp - - sinh NLT
I n comparing the behavior of two dispersions a t the same shear rate, it will nom be assumed that any difference in viscosity (and hence in X) is due to different values of the bond energy AG, whereas variations in the values of X and n (or of A T ) are negligible. Under these coiiditions eq. 4 may be considered as a relation giving the stress S as a function of the bond energy AG, and in which all the other parameters are constant. It is then easily derived that X
)
where
x=-
AS NkT
(10) M. Yan den Tempei, J . Colloid Sei.. 16, 284 (1901).
(5)
733
LOG VISCOSITY (POISE),
t
-4
+2
0
-2
LOG (SHEAR RATE).
Fig. 2.-Effect
of shear rate on the steady-state viscosity of
1%glyceryl tristearate suspensions in paraffin oil, with and with-
out surfactants (concentrations given in the text).
Under the conditions used in the present investigation, the value of x
