[acs symposium series] colloidal dispersions and micellar behavior volume 9 || electric...

6 Electric Emulsification

AKIRA WATANABE and KEN HIGASHITSUJI Faculty of Textile Science, Kyoto University of Industrial Arts and Textile Fibers, Matsugasaki, Kyoto 606, Japan

KAZUO NISHIZAWA Nippon Filcon Co. Ltd., Nagaokakyo, Kyoto 617, Japan

Introduction

Although the d ispers ion of l i q u i d s by electric method has been known for a long time (1, 2, 3, 4, 5), it has rare ly been used i n p rac t ica l emuls i f icat ion processes. This is because of the fact that , except for the emuls i f icat ion of mercury by I lkovic (3), the liquid is dispersed in air at first and then introduced into another liquid phase containing stabilizing agent. In add i t ion , the voltage used is very h igh , ranging from several to several ten thousand volts.

In connection with the studies on e l e c t r o c a p i l l a r y phenomena at o i l /water inter faces (6, 7, 8, 9), the present authors have succeeded i n producing w/o or o/w type emulsions directly by using much lower voltages as compared with the above method. The emulsions thus formed are i n general h ighly monodisperse. Since phys ica l propert ies of emulsions are strongly dependent on the s ize d i s t r i b u t i o n , deta i led studies on the present method appear to provide us with a useful means to c l a r i f y the mechanism of emuls i f icat ion as wel l as of propert ies of emulsions i n general .

Experimental

Mater ia ls . Except for the cases of app l ica t ions , the mater i a l s used are as follows :

The inorganic (KCl) and organic (tetrabutylammonium c h l o r i d e , TBAC) e lec t ro ly tes were of Ana ly t i ca l Grade, which were used without further p u r i f i c a t i o n . The surfactants , sodium dodecylsulfate (SDS) and cetylpyr idinium chlor ide (CPC), were of Extra Pure Grade, and Span 80 was of Commercial Grade. The organic solvent , methylisobutylketone (MIBK) , of Extra Pure Grade, washed by sodium hydroxide aqueous solut ion and pure water, was d i s t i l l e d and then saturated with water before use. Ion exchange water was r e d i s t i l l e d from an a l l Pyrex apparatus, which gave water of s p e c i f i c conduct iv i ty ca . 1 micromho/cm. This was used for a l l experiments.

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In Colloidal Dispersions and Micellar Behavior; Mittal, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

COLLOIDAL DISPERSIONS A N D M I C E L L A R BEHAVIOR

I

Figure 1. Schematic of the apparatus for electric emulsification. A: Teflon tube, B: Syringe, C: Driving motor, D: Three-way cock, E: Water phase reservoir, F: D.C. power supply, G, H: Ft electrodes, O: Oil phase, V: Voltmeter, W:

Water phase.

Figure 2. Photographs of electric emulsification of water in MIBK system. A: 0 volt, B: 100 volts, C: 250

volts, D: 500 volts.

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6. W A T A N A B E E T A L . Electric Emulsification 99

A l l glassware was thoroughly cleaned and steamed before use.

Apparatus. The out l ine of the apparatus for e l e c t r i c emulsif i c a t i o n i s shown in Figure 1. The water phase in the syringe Β i s introduced into the o i l phase through the Tef lon tube A ( inner d i a . , c a . 0.5 mm) at the t i p of the glass syr inge, by d r i v ing the motor G . The platinum electrodes G and H i n water and o i l phases, respect ive ly , are connected to the var iable D.C. supply F. When s u f f i c i e n t l y high voltage i s applied to the o/w in ter face , the dispersion of water phase takes place at the t i p of the c a p i l l a r y and w/o type emulsion i s produced.

In the case of formation of o/w type emulsions', the o i l phase i s introduced from the syringe into water phase.

In the present paper the sign of applied voltage Ε i s always that of water phase with reference to that of o i l phase.

Hycam 16 mm High Speed Camera i s used to observe d e t a i l s of the phenomena which occur during the emuls i f ica t ion .

Results

Emuls i f ica t ion of 0.01 mol/dm 3 KCl i n MIBK. When the a p p l i ed voltage Ε i s zero, large drops are successively formed at the t i p as shown by the photograph in Figure 2, A. The s ize of drops decreases with increasing Ε (Figure 2, Β ) , and at 250 vo l ts a shower of f ine drops i s formed, although r e l a t i v e l y large drops are a lso formed simultaneously (Figure 2, C ) . This i s the minimum voltage necessary to give r i s e to continuous emuls i f i ca t ion and i s ca l led "the c r i t i c a l voltage of emuls i f i ca t ion" , E ( c r i t . ) , a measure of the d i f f i c u l t y of emuls i f icat ion (4). At 500 vo l ts the emuls i f icat ion takes place v i o l e n t l y , presenting a f o g - l i k e appearance (Figure 2, D).

Photographs taken by the high speed camera are shown in Figure 3. I t i s noticed that the o/w inter face i s strongly deformed at high vol tages, with a very sharp protuberance. At the c r i t i c a l voltage the dispersion takes place at th is po in t . The protuberance moves around rap id ly and i r r e g u l a r l y at th is stage.

The E f fec t of Composition on the C r i t i c a l Voltage of Emulsi f i c a t i o n . The c r i t i c a l voltage of emuls i f icat ion Ε ( e x i t . ) i s strongly influenced by the composition of o i l and water phases. As an example the ef fect of SDS concentration of o i l phase on Ε ( e x i t . ) for w/o type emuls i f icat ion i s shown i n Figure 4 (9), where the water phase contains 0.0001 mol/dm 3 or 0.01 mol/dm 3

K C l . The o i l phase i s MIBK containing various concentrations of anionic surfactant SDS. I t i s noticed that , when the SDS concentrat ion in MIBK i s increased, E ( c r i t . ) decreases at f i r s t . This i s ascribed to the adsorption of SDS at the w/o inter face and also to the increase in e l e c t r i c conduct iv i ty , that i s the decrease in ohmie drop in o i l phase.

However, for SDS concentrations higher than 0.0001 mol/dm 3

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C O L L O I D A L DISPERSIONS A N D M I C E L L A R BEHAVIOR

Figure 3. Photographs of electric emulsification taken by the high speed camera for water in MIBK system. A: 0 volt, 200 exposures/sec. B: 150 wits, 200 exposures/sec. C: 250 volts, 8,000 exposures/sec.

D: 1,000 volts, 8,000 exposures/sec.

log [SDS], (M)

Figure 4. The effect of SDS concentration in oil phase on E(crit.). Water phase: KCl concentration, Δ: 0.0001 mol/dm3,

Q: 0.01 mol/dm3. Oil phase: SDS in MIBK.

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6. W A T A N A B E E T A L . Electric Emulsification 101

in the case of 0.0001 mol/dm 3 K C l , E ( c r i t . ) increases again, the emuls i f icat ion becoming more d i f f i c u l t .

More or less the same behavior i s found in the curve for 0.01 mol/dm 3 K C l .

It i s in terest ing to f ind that th is behavior i s re la ted to the change in the conduct iv i ty of o i l phase. Figure 5 shows the r e l a t i o n between E ( c r i t . ) and the s p e c i f i c conduct iv i ty of o i l phase κ ( ο ) for w/o type emuls i f i ca t ion , the water phase being 0.0001 mol/dm 3 KCl aq. and the o i l phase MIBK containing various concentrations of SDS. I t i s noticed that E ( c r i t . ) decreases with increasing κ ( ο ) , the emuls i f icat ion becoming eas ier . However, when κ ( ο ) approaches that of water phase K ( W ) , i . e . 18.4 micro-mho/cm, no emuls i f icat ion takes place under present experimental condi t ions, E ( c r i t . ) increasing i n d e f i n i t e l y .

Figure 6 shows the r e l a t i o n between E ( c r i t . ) and K(W) for w/o type emuls i f i ca t ion , the o i l phase being 0.00005 mol/dm 3

CPC in MIBK and the water phase containing various concentrations of KCl (9). I t i s noticed that E ( c r i t . ) increases with decreasing κ ( w ) , and no emuls i f icat ion takes place when the conduct i v i t y of water phase approaches that of o i l phase κ ( ο ) , i . e . 1.04 micromho/cm.

Opposite re la t ions are found for o/w type emuls i f icat ion as shown i n Figure 7, where E ( c r i t . ) i s p lo t ted against κ (w) . The water phase contains various concentrations of KCl and the o i l phase i s 0.01 mol/dm 3 TBAC in MIBK. I t i s noticed that E ( c r i t . ) increases i n d e f i n i t e l y as κ ( w ) approaches κ ( ο ) , i . e . 140 micromho/cm (Figure 7) .

I t i s worth mentioning here that TBAC i s e l e c t r o c a p i l l a r y inact ive (JL) , and hence the e l e c t r i c emuls i f icat ion takes place even i n the absence of surface act ive mater ia ls .

These observations lead to the conclusion that the s p e c i f i c conduct iv i ty of the discontinuous phase must always be higher than that of continuous phase i n order for the e l e c t r i c e m u l s i f i cat ion to take p lace .

P a r t i c l e Size D is t r ibu t ions . The s ize d is t r ibu t ions of emulsions thus formed e l e c t r i c a l l y are much narrower than those of emulsions prepared mechanically (8). This i s c lear from Figure 8, i n which the p a r t i c l e s ize d i s t r i b u t i o n s , measured microscopica l ly , are shown for w/o type emulsions of the same composition, prepared by e l e c t r i c (A) and mechanical (B) methods. The water phase contains 0.01 mol /dm 3 SDS and the o i l phase i s 0.5 % Span 80 i n toluene, the appl ied voltage being -300 v o l t s i n the case of e l e c t r i c emuls i f i ca t ion .

Emulsions of much higher d i s p e r s i t y , with the average d i a meter lower than 0.1 μ m, can be formed by choosing proper sur factants at proper concentrat ions. This can be proved by using the l i g h t - s c a t t e r i n g technique (10).

S t a b i l i t y . The emulsions formed e l e c t r i c a l l y are very

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log κ(ο), (mho/cm)

Figure 5. The influence of the specific conductivity of oil phase on E(crit.). Water phase: 0.0001 mol/dm3. Oil

phase: SDS in MIBK.

Figure 6. The influence of the specific conductivity of water phase on E(crit). Water phase: KCl. Oil phase: 0.00005

mol/dm3 CFC in MIBK.

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W A T A N A B E E T A L . Electric Emulsification

-600

log κ(w), (mho/cm)

Figure 7. The influence of the specific electric conductivity of water phase on E(crit.). Water phase: KCl. Oil phase:

0.01 mol/dm3 TBAC in MIBK.

~ 50

•2 40

ê.

( A )

-

( Β )

Γ 1. * ι m 0 2 4 6 8 0 2 4 6 8 10

Diameter of p a r t i c l e s (ym)

Figure 8. Particle size distributions measured by optical microscopy. A: Electric emulsification. B: Mechanical

emulsification.

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104 COLLOIDAL DISPERSIONS A N D M I C E L L A R BEHAVIOR

stable as compared with those prepared mechanically. This i s due to the small drop s i z e , high degree of monodispersity and the repulsive force between p a r t i c l e charges. The photographs of emulsions, prepared by introducing fountainpen ink into MIBK, are shown in Figure 9, both of which are taken 60 days af ter preparat i o n . The l e f t hand side i s the emulsion formed by using the homogenizer and the r ight hand side i s formed e l e c t r i c a l l y . Since these emulsions contain no s t a b i l i z e r s other than those i n the o r i g i n a l ink , drops begin to s e t t l e i n a few min. a f ter preparat ion and separate out completely a f ter 3 days, in the case of mechanical emuls i f i ca t ion . However, the emulsion prepared e l e c t r i c a l l y i s very s tab le , as shown by the photograph.

Discussion

Since the process of emuls i f icat ion i s in p r i n c i p l e a non-equi l ibr ium phenomenon, as i s c lear from the fact that the o/w inter face i s strongly deformed and i n ceaseless v io len t motion, there are many factors which govern th is phenomenon.

In the f i r s t p lace , we must consider the decrease i n (macroscopic) i n t e r f a c i a l tension by the appl ied potent ia l d i f fe rence , the e l e c t r o c a p i l l a r y phenomena at o/w inter faces ((>, 8) . In Figure 10 the i n t e r f a c i a l tension γ i s p lo t ted against E , where the water phase i s 1 mol /dm 3 KCl and the o i l phase 0.0001 mol / dm 3 CPC or SDS i n MIBK. I t i s noticed that γ decreases over the negative po la r i za t ion range in the case of ca t ion ic surfactant CPC, becoming almost zero at Ε = - 30 v o l t s . For the anionic si irfactant SDS, on the other hand, γ decreases over the pos i t i ve po la r i za t ion range, becoming almost zero at Ε = + 50 v o l t s . These values of Ε for zero γ almost coincide with those of E ( c r i t . ) , the c r i t i c a l voltages of emuls i f ica t ion .

I t has been found, however, that the emuls i f icat ion a lso takes place over the po la r i za t ion range i n which γ does not decrease i n Figure 10 or even in the absence of surfactants , i f a s u f f i c i e n t l y high voltage i s appl ied . Since γ i s higher than zero at the c r i t i c a l voltage of emuls i f icat ion i n these cases, we can conclude that the decrease i n (macroscopic) i n t e r f a c i a l tension i s not the necessary condit ion of e l e c t r i c emuls i f ica t ion .

From microscopic point of view, the f luctuat ion of i n t e r -f a c i a l tension always takes place by l o c a l changes in curvature and concentration and a lso by the uneven d i s t r i b u t i o n of current density (11, 12, 13, 14). Hence, there are l o c a l regions in which γ i s lower than the average macroscopic value. Hence, i t i s expected that a sharp protuberance can grow at such a point on the surface of l i q u i d drop and the l o c a l curvature increases. Hence, the f luctuat ion i s ampl i f ied there, thus increasing the tendency to emuls i f i ca t ion .

In the second p lace , the strong e l e c t r i c f i e l d at the i n t e r face must be taken into account, which helps to break the mechani c a l equi l ibr ium at the inter face l o c a l l y .

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6. W A T A N A B E ET A L . Electric Emulsification 105

Figure 9. The stability tests of w/o type emulsions. A: Electric emulsification. B: Mechanical emulsification by

using the homogenizer.

-υ

200 -200

Figure 10. Electrocapillary curves at o/w interfaces. Water phase: 1 mol/dm3 KCl. Oil phase: 0.0001

mol/dm3 CPC (Q) or SDS ( · ) .

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106 COLLOIDAL DISPERSIONS A N D M I C E L L A R BEHAVIOR

The mechanical force due to the e l e c t r i c f i e l d i s equal to the di f ference in the Maxwell stresses at both sides of the i n ter face . When the e l e c t r i c f i e l d i s perpendicular to the i n t e r face , the normal force f act ing from phase 1 to 2 i s given by

f = (1/2) [ e 'Cd φ ' / ά χ ) ^ = 0 " ε ( d f / d x ) ^ 1

where ε and ε 1 are d i e l e c t r i c constants, ψ and ψ 1 e l e c t r i c pot e n t i a l s , of phases 1 and 2, respect ive ly , and χ i s the distance from the inter face i n the d i rec t ion from phase 1 to 2. In the absence of ion ic adsorpt ion, we have for χ = 0

φ = ψ ·

and

K(di|//dx) = κ · ( ά ψ ' / ( 1 χ )

where κ and κ ' are s p e c i f i c conduct iv i t ies of phases 1 and 2, respect ive ly . From these three equations we obta in:

f = 0 for κ //ε = κ V / ε »

Thus, d i rec t ion of the force f i s governed by the values of κ / / ε at both sides of the in ter face . The e f fect of ion ic adsorption on the surface potent ia l can be neglected, when a s u f f i c i e n t l y high voltage i s appl ied .

The above discussion i s i n accord with the experimental facts mentioned e a r l i e r . Since the change i n s p e c i f i c conduct i v i t y dominates under the present experimental condi t ions, the square roots of d i e l e c t r i c constants of water and o i l phases being almost equal to each other, the mechanical force i s d i rected from the phase of higher to that of lower s p e c i f i c conduct iv i t ies . This explains the re la t ion between the type of emulsion formed and the s p e c i f i c conduct iv i t ies of water and o i l phases.

I t i s therefore concluded that the mechanical force due to the d iscont inu i ty of e l e c t r i c f i e l d at the in te r face , together with the l o c a l f luctuat ion of Υ , acts to break the mechanical equi l ibr ium at the inter face and drops are thrown into the other phase with complicated motion of the l i q u i d . Then, the deformat ion and the ve loc i ty of drop formation increase, and the continuous emuls i f icat ion i s accelerated.

Appl icat ions

In the case of ordinary emuls i f icat ion processes, propert ies of emulsions, e . g . p a r t i c l e s ize d i s t r i b u t i o n , s t a b i l i t y , are usual ly contro l led by se lect ing proper surfactants at high concentrat ions. This causes i n many cases a considerable disadvantage, and i n addit ion the use of synthetic surface act ive mater i -

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als often gives r i s e to various d i f f i c u l t i e s . The present technique has i n th is sense a great advantage, since s tab le , nearly monodisperse, f ine emulsions are formed i n the absence o f , or i n the presence of very small amounts o f , surface act ive mater ia ls .

This method has a vast f i e l d of app l ica t ions , for example the dyeing, cosmetics, food and pharmaceutical industr ies (16). A few examples w i l l be presented here b r i e f l y .

The emulsion for solvent dyeing can be prepared e l e c t r i c a l l y by introducing the aqueous solut ion of dye into te t rach loroethy l -ene in the presence of sorbitan mono-oleate. The leve l ing e f fec t i s good. Moreover, i t i s a lso possible in th is method to contro l the p a r t i c l e charge according to the nature of the f ibe r to be dyed.

A cosmetic cream of high qual i ty i s formed e l e c t r i c a l l y by introducing water into l i q u i d paraf f ine containing 0.3 % sorbitan mono-oleate. The amount of surfactant i s about one tenth of that in ordinary commercial cosmetic creams.

The e lec t rocap i l l a ry spinning i s a modif icat ion of the present procedure (15). For instance, we can spin threads out of ge la t in by introducing 20 % aqueous solut ion of ge la t in into absolute ethanol through a Tef lon tube of c a . 0.5 mm in inner diameter (15). When the appl ied voltage i s zero, the ge la t in solut ion passes into the ethanol phase slowly and coagulates read i ly . At s u f f i f i e n t l y high vol tages, however, the ge la t in solu t ion forms a thread, which moves around v i o l e n t l y at the front of the t i p and i s dehydrated rap id ly . In Figure 11 photographs taken by the high speed camera at the speed of 200 exposures per sec. are shown. The ge la t in powder thus formed has the s p e c i f i c area much larger than that of untreated one, and hence d issolves in water qu ick ly . The i n i t i a l d isso lu t ion ve loc i ty of the ge la t i n sample treated at 1,000 vo l ts i s about ten times larger than that of untreated one. This technique of e lec t rocap i l l a ry spinning can be applied to any other soluble high polymers, e . g . polyv iny la lcohol and gum arabic .

Acknowledgements

The authors wish to express the i r grati tude to Messrs. K. Kamada, M. Takayama and K. Ish izak i and to Misses K. Takubo and Y. Ebe for the i r assistance in the experimental work.

L i terature Ci ted

1. Zeleny, J., Phys. Rev. (1914) 3, 69. 2. Vonnegut, B. and Neubauer, R., J . Colloid Sci. (1952) 7,

616. 3. I l kov ic , D. , Coll. Trav. Chim. Tchécosl. (1932) 4, 480. 4. Nawab, M. and Mason, S., J. C o l l o i d Sci. (1958) 13, 179. 5. Gopal, E . S . R . , in "Emulsion Science", P. Sherman, Ed., p. 55,

Academic Press, London, 1968.

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Figure 11. Photographs of electrocapillary spinning of gelatin taken by the high speed camera at 200 exposures/sec. A: 0 volt,

B: 250 volts, C: 800 volts, D: 1,000 volts. Dow

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6. Watanabe, A., Matsumoto, M. and Gotoh, R., Nippon Kagaku Kaish i (1966) 87, 941.

7. Watanabe, Α., Matsumoto, M. , Tamai, H. and Gotoh, R., Kolloid-Z. Z . Polym. (1967) 220, 152.

8. Watanabe, Α., Nippon Kagaku Kaish i (1971) 92, 575. 9. H i g a s h i t s u j i , K., Nishizawa, K . , Kamada, K. and Watanabe, Α.,

Nippon Kagaku Kaishi (1974) 1974 (6),995. 10. H i g a s h i t s u j i , Κ., Takayama, M., Nishizawa, K. and Watanabe,

Α., (1974), unpublished data. 11. Tolman, R . C . , J. Chem. Phys. (1949) 17, 333. 12. Kirkwood, J.C. and Buff , F.P., ibid. (1949) 17, 338. 13. Buff , F.P., ibid. (1955) 23, 419. 14. H i g a s h i t s u j i , Κ., Nishizawa, K. and Watanabe, Α., unpubl ish

ed data. 15. Ebe, Y., Takayama, M., Kamada, Κ., Nishizawa, K. and Watana

be, Α., (1974), unpublished data. 16. Watanabe, Α., H i g a s h i t s u j i , K. and Nishizawa, Κ., (1974), un

published data.

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