Smectites as colloidal stabilizers of emulsions

Applied Clay Science 14 Ž1999. 83–103

Smectites as colloidal stabilizers of emulsions I. Preparation and properties of emulsions with smectites and nonionic surfactants G. Lagaly ) , M. Reese, S. Abend Institute of Inorganic Chemistry, UniÕersity of Kiel, D-24098 Kiel, Germany Received 22 June 1998; revised 9 November 1998; accepted 9 November 1998

Abstract Bentonites, montmorillonites, and hectorites were used as colloidal stabilizers of oil-in-water ŽOrW. emulsions. The enrichment of the solid particles on the oil–water interface was attained by the addition of nonionic coemulsifiers Žglycerol monostearate ŽGMS., decaŽethylene glycol. hexadecyl ether, alkyl polyglucoside, and lecithin.. The clay mineral content of the aqueous dispersion was 2% Žwrw.. Stable emulsions required amounts of 0.5–1.5 g coemulsifier per 100 ml aqueous dispersion. Oil volume fraction was varied between f s 0.17 and f s 0.50. At f ) 0.50 the OrW emulsions changed into water-in-oil ŽWrO. emulsions. The number average diameter of the droplets was about 25 nm. The volume average diameter Ž50–100 nm. more strongly depended on the clay mineralrcoemulsifier combinations. Wyoming bentonite and the corresponding delaminated sodium montmorillonite were useful stabilizers; technical, sodaactivated bentonites yielded unstable emulsions, or emulsification was not successful. A synthetic hectorite which caused pronounced thickening of the coherent phase was an effective stabilizer. Creaming was often observed because of the buoyancy of the large droplets. Most of the creamed emulsions were stable over long periods and did not separate an oil phase. The resistance against creaming increased with the oil volume fraction. An increase of the solid content had to be accompanied by an increase of the coemulsifier concentration to reduce the rate of creaming. q 1999 Elsevier Science B.V. All rights reserved. Keywords: alkyl ethers; alkyl polyŽethylene glycols.; alkyl polyŽethylene oxides.; alkyl polyglucoside; bentonite; creaming; emulsifier; emulsions; glycerol monostearate; hectorite; lecithin; montmorillonite; polyŽethylene oxide.; pickering emulsions

)

Corresponding author. Tel.: q49-431-880-3261; Fax: q49-431-880-1608; E-mail: [email protected] 0169-1317r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 1 3 1 7 Ž 9 8 . 0 0 0 5 1 - 9

84

G. Lagaly et al.r Applied Clay Science 14 (1999) 83–103

1. Introduction Dispersion of an oil in the aqueous phase, yielding an oil-in-water emulsion ŽOrW emulsion. , or of water in an oil ŽWrO emulsion, Fig. 1. usually requires the addition of surfactants as emulsifier and stabilizer which dissolve in the liquid phases. In general, the phase with the higher solubility of the surfactant becomes the coherent phase. Aggregation of the droplets Ž Fig. 2. is called flocculation. This process is reversible; shaking restores the emulsion with more or less equally distributed droplets. As the density of the oil phase is generally lower than that of the coherent aqueous phase, the droplets aggregate to a creaming layer sitting on top of the aqueous phase. This process is also reversible. The emulsion breaks when the droplets flow together forming a coherent oil phase Ž coalescence.. Ramsden Ž1904. and Pickering Ž1907, 1910. stabilized emulsions by fine solid particles of different materials. This principle was known in ancient times when mustard powder was used to prepare mayonnaise by dispersing oil in water. A wide variety of solid materials was reported as colloidal stabilizers Ž Finkle et al., 1923; Schulman and Leja, 1954; Mukerjee and Srivastava, 1956a,b, 1957a,b; Lucassen-Reynders and van den Tempel, 1963; Menon and Wasan, 1986; Levine et al., 1989a; Tambe and Sharma, 1993, 1995; Yan and Masliyah, 1993, 1994, 1995a,b; Puskas ´ ´ et al., 1996; Zhai and Efrima, 1996.. Examples are iron oxides and other oxides, hydroxides and oxyhydroxides, basic salts of metals, silica, barium sulfate, carbons, colloidal silver or solid organic materials such as glycerol stearate. Stabilization by solid materials requires the colloidal particles to form a dense film around the dispersed droplets which impedes coalescence when two droplets approach ŽFig. 3a. ŽFinkle et al., 1923; Tadros and Vincent, 1983; Levine and Sanford, 1985; Menon and Wasan, 1988. . Enrichment of the fine

Fig. 1. Type of emulsions. ŽA. oil-in-water emulsion ŽOrW emulsion.. ŽB. water-in-oil emulsion ŽWrO emulsion..

G. Lagaly et al.r Applied Clay Science 14 (1999) 83–103

85

Fig. 2. Stability and instability of emulsions. Flocculation and creaming are reversible.

particles at the oil–water interface is related to the contact angle u at the water–oil–solid line Ž Rapacchietta and Neumann, 1977; Tadros and Vincent, 1983; Levine and Sanford, 1985; Levine et al., 1989b; Denkov et al., 1992. . In general, stabilization is optimal when u is somewhat below or above 908. When u is too small Ž< 908. or too large Ž4 908., the particles leave the interface and move into the bulk of the water or oil. A general rule is that OrW emulsions are formed at u - 908 and WrO emulsions at u ) 908 ŽFinkle et al., 1923; Schulman and Leja, 1954. but exceptions are known Ž Yan and Masliyah,

Fig. 3. Stabilization of emulsions by solid particles. Ža. stabilization by envelopes of particles around the oil droplets; Žb. stabilization by encapsulation of oil droplets in a three-dimensional network of particles.

86

G. Lagaly et al.r Applied Clay Science 14 (1999) 83–103

1993.. The type of emulsion Ž OrW or WrO. can also be dependent on the way the emulsion is prepared. The shape of the particles is often not important but contact angle hysteresis increases emulsion stability; i.e., rough particles stabilize better than smooth ones Ž Tadros and Vincent, 1983; Denkov et al., 1992. . In many cases surface active agents are added to change the surface properties of the solid materials, and the type of emulsion is determined by the combination of solids and surfactants ŽSchulman and Leja, 1954; Tambe and Sharma, 1993. . One can imagine a second stabilization mechanism Ž Fig. 3b. . When the particles aggregate and build up a three-dimensional network in the coherent phase, the oil droplets can be trapped in the array of particles. The mechanical stability of the network structure reduces the rate of coalescence. A high elasticity of the network seems to be favorable Ž Abend et al., 1998. . Also, the network reduces the mobility of the particles ŽAbend et al., 1998. and, in this way, enhances emulsion stability. Stabilization by a three-dimensional particle– particle structure is comparable to the role of liquid–crystalline phases in stabilizing emulsions ŽFriberg et al., 1988. . Pickering emulsions reveal several favorable properties. They are often insensitive to changes of chemical parameters like pH, type and concentration of salts, composition of oil, etc., and are difficult to break—in most cases an advantage but sometimes, e.g., in crude oil–water systems, a serious disadvantage. The amount of surface active agents can often be reduced. The emulsifying power of several modest emulsifiers can be enhanced in combination with solid materials. This provides the possibility to replace hazardous surfactants by less harmful compounds, in particular in cosmetics and pharmaceutical applications. Addition of a solid to an emulsion can change the type Ž OrW in WrO or reverse. and the flow-behavior Žviscosity, yield values, thixotropy, antithixotropy; cf. Part II of this paper. . It is surprising that clay minerals as colloidal stabilizers were considered in a limited number of papers only though the particles fulfil the required conditions: They are of small sizes; if needed, particle size fractions are obtained by sedimentation. They can strongly increase the viscosity of the bulk phase. They form three-dimensional networks Ž band-type or cardhouse structures. . Sodium smectites delaminate in water. The surface of the particles is easily modified by adsorption, ion exchange, and grafting. Menon and Wasan Ž1986. studied asphaltene adsorption on sodium montmorillonite and its effect on stabilization of oil–water emulsions. Later, Yan and Masliyah Ž1993, 1994, 1995a,b. modified kaolinite with asphaltenes and studied the influence of the contact angle on emulsification and demulsification of mineral oil in water. They prepared OrW emulsions with solids having contact angles greater than 908 ŽYan and Masliyah, 1993. . Jordan and Williams Ž1954. described emulsification of water in oil with dimethyl dioctadecylammonium

G. Lagaly et al.r Applied Clay Science 14 (1999) 83–103

87

Table 1 Preparation of emulsions by dispersing 25 ml of aqueous dispersions of the clay mineral Ž2% solids content. in the oil phase containing 100–400 mg coemulsifier at 808C Aqueous dispersion Žml.

Oil Žml.

Phase ratio ŽOrW.

Oil volume fraction f

25 25 25 25 25 25 25

5.0 7.5 10.0 12.5 15.0 20.0 25.0

0.20 0.30 0.40 0.50 0.60 0.80 1.00

0.17 0.23 0.29 0.33 0.38 0.44 0.50

bentonite as colloidal stabilizer. Tsugita et al. Ž1983. reported stabilization of OrW emulsions by montmorillonite in the presence of monoglycerides Žglycerol monolaurate, glycerol monopalmitate, glycerol monostereate. and a few related compounds. Emulsions stabilized by lipophilic substances like fatty alcohols and monoglycerides are applied in cosmetic and pharmaceutical fields. High-purity monoglycerides are used in food chemistry Ž Dickinson, 1992. .

Fig. 4. Conductivity cell for measuring the rate of creaming.

88

G. Lagaly et al.r Applied Clay Science 14 (1999) 83–103

G. Lagaly et al.r Applied Clay Science 14 (1999) 83–103

89

2. Materials and methods 2.1. Bentonites, montmorillonites, and hectorites Sodium montmorillonite of Wyoming was used in most experiments. It was prepared from Wyoming bentonite Ž ‘Greenbond’, obtained from Sud-Chemie, ¨ Germany, our sample No. M40A. by the usual procedure: removing iron oxides with dithionite in the presence of citrate ions as chelating agent, oxidation of organic material by H 2 O 2 and fractionation Ž- 2 mm fraction. by sedimentation ŽStul and van Leemput, 1982; Tributh and Lagaly, 1986, a,b. . The layer charge of this montmorillonite was j s 0.28 eqrmol Žby alkylammonium exchange; Lagaly, 1994. . The cation exchange capacity calculated from the carbon content of the alkylammonium derivatives was 1.03 meqrg sodium montmorillonite. Besides this montmorillonite we used several industrial test products to see the influence of the processing route. Calcium bentonite from Milos ŽGreece, obtained from Sud-Chemie, Ger¨ many. was activated in the usual way by knealing the material with 3% Ž wrw. soda. This material was air-dried and pulverized. Calcium magnesium bentonite from Kimolos ŽGreece, obtained from Sud¨ Chemie, Germany. was activated with 4.5% Ž wrw. soda. Sodium bentonite of Kimolos was obtained by exchanging the calcium and magnesium ions of the raw bentonite with an ion-exchange resin Ž details were not available.. Hectorite was synthesized from aqueous solutions of MgSO4 , Li 2 SO4 , water glass and soda at 1708C and 8 bar ŽSud-Chemie, Germany.. Sample A was ¨ washed more intensely than sample B. Aqueous dispersions of sample B showed a higher electrical conductivity and were much more viscous than dispersions of sample A. Both dispersions thickened considerably with time. Sample C was impregnated with sodium polyphosphate Ždetails were not available. . 2.2. Viscosity The flow behavior of the dispersions and emulsions was measured in rotational viscosimeters ŽLow Shear viscosimeter type 30, Contraves, Switzerland; rheometer CV 100, Haake, Germany. . The relative apparent viscosity ŽGuven, 1992. of the clay mineral dispersions was obtained from the shear stress ¨ t ) of the dispersion divided by the shear stress of the dispersion medium, both Fig. 5. Creaming behavior of a emulsion with 2% Žwrw. sodium montmorillonite ŽWyoming. in the aqueous dispersion and 300 mg Žv ., 400 mg ŽB., and 500 mg Ž'. GMS in the oil phase Ž25 ml., oil volume fraction f s 0.5. a. changes of conductivity s with time, normalized to s s 0 at t s 0. Open and full symbols: s at the bottom and top of the cell; Žb. difference of conductivity D s s s bottom y stop ; Žc. the same as Žb. but with 2.5% sodium montmorillonite.

90

G. Lagaly et al.r Applied Clay Science 14 (1999) 83–103

measured at the rate of shear g˙ s 94.5 sy1 Žfor details see part II of this paper ŽLagaly and Reese. which is now under review and will appear in 1999. . 2.3. Coemulsifiers The organic emulsifiers, here called coemulsifiers, were technical products: glycerol monostearate, GMS Ž Rewo Chemische Werke, Steinau. ; lecithin, LEC Žfrom eggs, Merck, Darmstadt.; tetraŽ ethylene glycol. hexadecyl ether, TEHE, and decaŽethylene glycol. hexadecyl ether, DEHE Ž both from Condea Chemie, Brunsbuttel ¨ .; alkyl‘poly’glucoside, APG derived from a mixture of C 12rC 14 fatty alcohols and with a degree of ‘polymerization’ of 1.3–1.6 Žfrom Henkel, . ŽBusch et al., 1993; Biermann et al., 1993. . Dusseldorf ¨ 2.4. Emulsification The coemulsifier was dissolved in the oil, the solid stabilizer was dispersed in the aqueous phase. Dissolving the polyŽ ethylene glycol. alkyl ethers or alkyl polyglucoside in the aqueous dispersion yielded emulsions of coarser droplets. Emulsions with glycerol monostearate and lecithin were only obtained by the coemulsifier-in-oil method, at least at the condition of emulsifying described in the following. A dispersion of the clay mineral in water was shaken for 24 h, then heated to 808C. The coemulsifier was also dissolved in the oil Ž paraffin oil, density 0.88 g cmy3, viscosity 150 mPa s; Merck, Germany. at 808C. Within 2 min the aqueous clay mineral dispersion was added to the oil phase under stirring Ž Ultra Turrax T25, IKA, 24,000 rotrmin.. The emulsion was then allowed to cool down to room temperature. Table 2 Average droplet diameter of emulsions prepared from 100 ml of 2% dispersions of bentonite, montmorillonite or hectorite and 100 ml paraffin oil containing 400 mg coemulsifier Clay

Coemulsifier

dN

dV

dV r dN

Montmorillonite Wyoming

GMS GMS a APG APG a TEHE DEHE GMS GMS GMS

22.8 22.9 24.8 25.0 23.5 23.2 26.5 24.3 24.1

51.0 48.3 58.7 75.7 53.9 54.0 73.3 132.9 91.5

2.24 2.11 2.37 3.03 2.29 2.33 2.77 5.47 3.80

Bentonite Wyoming Hectorite, sample A Hectorite, sample C b

d N : number average diameter, d V : volume average diameter, d V r d N : polydispersity index. After stirring for 30 min Ž200 ml of emulsion, propeller agitator, 500 rotrmin.. b Impregnated with sodium polyphosphate. a

G. Lagaly et al.r Applied Clay Science 14 (1999) 83–103

91

The volume of the clay mineral dispersion was 25 ml with a solid content of 2%. The amount of oil was varied from 5 ml to 25 ml, corresponding to oil volume fractions between 0.17 and 0.50 ŽTable 1.. Amounts of 100 to 400 mg coemulsifier were added to the oil phase. 2.5. Characterization of the emulsions The size and size distribution of the oil droplets were measured in a scanning laser microscope ŽSLM, PAR-TEC 1000. . Reliable results were obtained with at least 200 ml emulsion which were prepared immediately before the measurements Žas described above but starting from 100 ml of the smectite dispersion..

Fig. 6. Droplet size distribution of emulsions. Ža. 2% aqueous dispersion of Wyoming bentonite Žv . and the corresponding sodium montmorillonite of Wyoming Ž`., 400 mg GMS; oil volume fraction f s 0.5; Žb. 2% aqueous dispersion of sodium montmorillonite ŽWyoming., 400 mg coemulsifier, f s 0.5; Ž'. GMS ŽB. TEHE Žv . APG.

92

G. Lagaly et al.r Applied Clay Science 14 (1999) 83–103

During the measurements the emulsion was stirred with a propeller agitator Ž 500 rotrmin.. About 5 P 10 4 chord lengths were registered during measuring time periods of 2 s. The values reported are averages of 10 cycles. Stability of the emulsion was controlled by visual inspection and centrifugation tests. Samples of about 1 ml were tested at 11,000 rotrmin ŽBiofuge T, Heraeus. for 10 min. After centrifugation we measured the volume occupied by Ži. the aqueous phase, Žii. the emulsion and creaming layer, and Žiii. the oil phase at the top. The data reported were obtained from five parallel tests. Creaming of the emulsions was followed by measuring the conductivity at the bottom and the top of the emulsion as a function of time at 208C ŽSchambil et al., 1987.. The cell Ž Fig. 4. contained two pairs of platinum electrodes for conductivity measurements Ž LF 530, WTW, Weilheim, Germany.. At the beginning of the experiment, i.e., some minutes after preparation of the emulsion,

Fig. 7. Separation of the emulsion in an aqueous dispersion Žw., a creaming layer Žcr. and a coherent oil phase Žo. after centrifugation. f s oil volume fraction, 2% dispersion of sodium montmorillonite ŽWyoming., 100 mg emulsifier. Ža. glycerol monostearate; Žb. tetraŽethylene glycol. hexadecyl ether; Žc. alkyl polyglucoside; Žd. lecithin.

G. Lagaly et al.r Applied Clay Science 14 (1999) 83–103

93

Fig. 7 Žcontinued..

both pairs of electrodes indicated the same conductivity Ž Fig. 5a. . When the emulsion was stable Žfor 400 and 500 mg GMS in Fig. 5a. , the conductivities remained constant. With 300 mg GMS the emulsion separated into the aqueous clay mineral dispersion and a creaming layer. The conductivity measured by the upper pair of electrodes decreased because oil droplets enriched in the creaming layer. In the lower part of the cell the loss of oil raised the conductivity which approximated the value of the pure clay mineral dispersion. The increase of the difference D s s s bottom y stop with time was considered as a measure of the rate of creaming ŽFig. 5b, c, 9–11. 3. Results 3.1. Preparation Stable emulsions were prepared with 25 ml of the clay mineral dispersion Žsolids contents usually 2% wrw. and 100–400 mg coemulsifier in the oil

94

G. Lagaly et al.r Applied Clay Science 14 (1999) 83–103

Fig. 8. Separation of the emulsion in an aqueous dispersion Žw., a creaming layer Žcr. and a coherent oil phase Žo. after centrifugation. f s oil volume fraction, 100 mg glycerol monostearate, 2% dispersion of hectorite B Ža. and hectorite A Žb..

phase. The oil volume fraction could be varied from f s 0.17 to f s 0.50 ŽTable 1. . Emulsification was not successful without the colloidal stabilizer. As an aside it is relevant to note that WrO emulsions were prepared with glycerol tristearate as colloidal stabilizer provided that small amounts of surface-active agents were present Ž Lucassen-Reynders and van den Tempel, 1963. . In some cases, emulsions could be prepared without a coemulsifier but the conditions varied under which emulsification was successful. A higher input of mechanical energy per volume was often required in the absence of the organic

Fig. 9. Creaming of emulsions at different oil volume fractions Ž D s see Figs. 4 and 5.. Žv . f s 0.29 ŽB. f s 0.38 Ž'. f s 0.50; 2 percent dispersion of sodium montmorillonite ŽWyoming.. Ža. 200 mg lecithin; Žb. 400 mg lecithin; Žc. 200 mg tetraŽethylene glycol. hexadecyl ether.

G. Lagaly et al.r Applied Clay Science 14 (1999) 83–103

95

96

G. Lagaly et al.r Applied Clay Science 14 (1999) 83–103

compounds, and emulsification became much more dependent on the type of bentonite or smectite. 3.2. Droplet size The number average diameter d N of the droplets was about 25 mm and did not change very much with type of coemulsifier and solid but the volume average diameter d V was influenced by the composition ŽTable 2.. Polydispersity expressed by d Vrd N was largest with the synthetic hectorite. The droplet size varied between 10 mm and about 200 mm ŽFig. 6.. In some cases, e.g., with hectorite, a small amount of even larger droplets was observed. The raw Wyoming bentonite and the sodium montmorillonite yielded emulsions with similar size distribution curves Ž Fig. 6a. . Also, the type of coemulsifier did not change the order of magnitude of the droplet sizes Ž Fig. 6b.. 3.3. Stability Bentonites, montmorillonites and synthetic hectorites yielded stable emulsions. In many cases creaming was observed but was reversible, a coherent oil phase was not separated during storage over longer periods Ž up to 1 year at least.. The influence of coemulsifierrclay mineral combinations on emulsion stability was revealed by centrifugation tests when smaller amounts Ž100 mg. of coemulsifier were used ŽFigs. 7 and 8.. The phase diagram of emulsions with sodium montmorillonite of Wyoming and GMS is typical Ž Fig. 7a. . The emulsion formed a creaming layer but resisted centrifugation as long as the oil volume fraction was below f s 0.38. At higher phase ratios a coherent oil phase separated during centrifugation. A similar diagram was observed with DEHE. In the presence of TEHE a coherent oil phase formed at f G 0.28 ŽFig. 7b.. Emulsions containing APG Ž Fig. 7c. and lecithin Ž Fig. 7d. were less stable; the emulsion with lecithin was broken into the aqueous dispersion and the oil phase. 3.4. Creaming Emulsions stabilized by combinations of clay minerals and coemulsifiers differed in their resistance against creaming. The onset of creaming was indicated by the conductivity difference rising with time Ž Figs. 4 and 5.. Increasing amounts of emulsifier at constant montmorillonite content stabilized the uniform distribution of the particles and reduced creaming. GMS was the most effective agent for stabilizing the emulsions against creaming. The synergism between the amounts of solid and emulsifier is revealed by Fig. 5b, c: An emulsion with 2% montmorillonite and 300 mg GMS formed a creaming layer after 450 h Ž Fig. 5b.. When the montmorillonite content was increased to 2.5%,

G. Lagaly et al.r Applied Clay Science 14 (1999) 83–103

97

the emulsion creamed immediately Ž Fig. 5c.. To impede creaming the amount of coemulsifier must be enhanced. Resistance against creaming generally increased with the oil volume fraction f ŽFig. 9.. The relation between stability against creaming and f depended on the amounts of solids and coemulsifier. Emulsions with 200 mg lecithin creamed rapidly at f s 0.29 and 0.38 but were stable for f s 0.50 Ž Fig. 9a.. An emulsion containing 400 mg lecithin ŽFig. 9b. showed rapid creaming for f s 0.29. At higher oil volume fractions agglomeration of the oil droplets was retarded but not completely impeded. This again indicates the synergism between the colloidal stabilizer and the coemulsifier. When the increased amount of lecithin was accompanied by a slight increase of the montmorillonite content Žto about 2.3%. , the emulsion became very viscous and creaming was no longer observed. The strong increase of stability against creaming when f approached 0.5 was also observed for other coemulsifiers ŽFig. 9c.. When the amount of TEHE and DEHE rose above 200 mg, the emulsions thickened strongly and creaming was impeded at all oil volume fractions. Among the coemulsifiers studied the alkyl polyŽ ethylene glycols. had the strongest influence on the rheological behavior. With increasing f the flow became non-newtonian with distinct yield values and antithixotropy changed into thixotropy Ž see part II of this paper. . 3.5. Influence of the clay mineral The suitability of the bentonites, montmorillonites, and hectorites is not easy to understand. Our actual experience is that many raw bentonites, and also their sodium forms are good stabilizers, but not the technical, soda-activated samples. Soda-activated bentonite from Kimolos and Milos were not suited for emulsifi-

Fig. 10. Influence of the clay mineral on creaming Ž D s see Figs. 4 and 5.: 200 mg GMS, 2% montmorillonite or hectorite dispersion, f s 0.5. Žv . hectorite, sample A ŽB. sodium bentonite ŽKimolos. Ž'. sodium montmorillonite ŽWyoming. Žl. hectorite, sample B.

98

G. Lagaly et al.r Applied Clay Science 14 (1999) 83–103

Fig. 11. Creaming of emulsions Ž D s see Figs. 4 and 5. in the presence of salts. 400 mg GMS, 2% sodium montmorillonite ŽWyoming., f s 0.5. Concentration of salts related to the aqueous dispersion. Ž'. 5P10y3 M Na 4 P2 O 7 ; ŽB. 10y2 M NaCl Žv . 10y3 M CaCl 2 .

cation at the conditions of our experiments. Emulsions could be prepared with the sodium bentonite from Kimolos but they creamed rapidly Ž Fig. 10. . Synthetic hectorites can be excellent stabilizers but their suitability not only depends on the conditions during synthesis but also on the processing of the synthesized products. The hectorite with the high relative apparent viscosity yielded very stable emulsions whereas the other sample caused rapid creaming ŽFigs. 8 and 10.. Sodium chloride and calcium chloride stabilized the emulsions against coalescence. The stepwise increase of the conductivity difference D s after sodium chloride addition ŽFig. 11. indicated different states of creaming. Also, visual inspection revealed a stepwise contraction of the creaming layer with time. A stepwise increase of the conductivity was also observed in the presence of small amounts of calcium chloride. The liquefying diphosphate ions Ž Packter, 1956. caused rapid creaming and initiated coalescence. 4. Discussion Solid particles stabilize paraffin-in-water emulsions by forming envelopes around the oil droplets. Modification of the montmorillonite or hectorite surfaces by adsorption of the coemulsifier causes the particles to accumulate at the oil–water interface and to create a mechanical barrier protecting the oil droplets from coalescing with each other. As bentonite of Wyoming ŽM40A. and sodium montmorillonite separated from it are both excellent colloidal stabilizers, the degree of delamination seems to be less important. Thicker particles enriched in a monolayer at the oil–water interface produce a thick mechanical barrier. On the other hand, the thin, flexible silicate layers of delaminated particles encapsulate the droplets by dense

G. Lagaly et al.r Applied Clay Science 14 (1999) 83–103

99

envelopes when the conditions are such that parallel stacking of the particles is ensured. When the electrostatic repulsion is too strong, the particles cannot form a dense film around the oil droplets. In this case, small amounts of calcium ions which cause aggregation of the particles Ž Brandenburg and Lagaly, 1988, Lagaly, 1989; Permien and Lagaly, 1994b. enhance the stability of the emulsion. Clay mineral particles remaining in the bulk of the coherent phase increase the viscosity, which reduces the mobility of the particles and improves the stability of the emulsion. This effect is strong when the clay minerals aggregate to three-dimensional networks. Formation of networks in dispersions of 2% sodium bentonite or sodium montmorillonite is weak in the absence of salt and at pH f 6 ŽLagaly, 1989, see also part II.. A considerable stiffening occurs by addition of layered double hydroxides. Network formation then becomes so strong that coemulsifiers are no longer required to stabilize the emulsion ŽAbend et al., 1998.. An increase of the bentonite or montmorillonite content increases the viscosity but emulsion stability is not much improved as long as the solid content is too low for formation of stable networks. As revealed by Fig. 5b, c an increase in the amount of montmorillonite requires a corresponding increase of the coemulsifier concentration to cover enough particles with surfactant molecules. This illustrates that network formation in the coherent phase is not the decisive mechanism of stabilization, rather it is the encapsulation of the droplets by the solid particles. The process of network formation is more important when salts in appropriate concentrations are added Ž Fig. 11.. Particle–particle aggregation then becomes weakly attractive and band-type networks form Ž Lagaly, 1989; Permien and Lagaly, 1994a.. However, salts which increase the negative charge density of the particles, in particular at the edges, are counterproductive. Typical examples are phosphate, citrate or carbonate anions. Phosphate adsorption strongly increases the negative edge charge density and, therefore, the repulsion between the particles. A typical consequence is the strong increase of the critical coagulation concentration Ž Permien and Lagaly, 1994c; see also Packter, 1956. . The enhanced repulsion impedes close contacts of the particles around the oil droplets, the mechanical stability of the barrier is no longer high enough to protect two approaching particles from coalescing. Higher concentrations of carbonate react in a similar way. This makes the technical, soda-activated products less suitable as colloidal stabilizers. After decomposition of the carbonate emulsification tests proceeded successfully. Synthetic hectorite, sample B, was very effective in stabilizing the emulsion. The presence of small amounts of magnesium ions stiffens the dispersion strongly Žcf. the effect of calcium ions in Fig. 2 in Permien and Lagaly, 1994a. . ) The high relative apparent viscosity of the 2% dispersion Žhrel s 30 after 24 h, Table 3. contributes largely to the emulsion stability Ž for comparison: sodium ) ) montmorillonite of Wyoming, hrel s 7. . Hectorite, sample A, with hrel s 2.5

100

G. Lagaly et al.r Applied Clay Science 14 (1999) 83–103

Table 3 ) Ž Relative shear stress trel relative apparent viscosity. at g˙ s94.4 sy1 of 2% aqueous dispersions of bentonite, montmorillonite and hectorite Sample

) trel after 24 h

) trel after 14 days

Montmorillonite Wyoming Bentonite Kimolos, soda-activated Bentonite Kimolos, sodium form Bentonite Milos, soda-activated Hectorite, sample A Hectorite, sample B Hectorite, sample C a

7.0 36.0 2.5 3.8 2.3 30.0 6.0

7.0 26.0 3.0 4.5 10.0 100.0 65.0

a

Impregnated with sodium polyphosphate.

ŽTable 3. was distinctly less suited as stabilizer. The different behavior of both hectorites reveals that enrichment of the particles around the oil droplets plays a minor role than for montmorillonites, or that the envelopes of hectorite particles are mechanically less stable during collisions of the droplets. This behavior requires further studies. It should be mentioned that dispersions of both hectorites thickened with time ŽTable 3. , a behavior typical of synthetic hectorites ŽWillenbacher, 1996, see part II.. The main cause may be a very slow delamination of the particles. Kroon et al. Ž 1998. assume that the thickening is related to the reorientation of the charged particles. The role of the coemulsifier primarily is surface modification of the clay mineral particles which promotes their enrichment at the oil–water interface. The nonionic surfactants act as a compatibilizing agent at the surface. The organic compounds may slightly reduce weak electrostatic repulsions between the particles and, therefore, strengthen the mechanical stability of the envelopes. The results reported in this paper indicate that this influence is modest at stronger repulsive forces. The situation at the particle edges seems to play an important role in stabilizing the envelopes and, therefore, the colloid-laden emulsions. As the buoyancy of the large droplets ŽTable 2. is high, the emulsions tend to cream. Stability against creaming increases with the oil volume fraction. When

Fig. 12. Retardation of creaming ŽA, B. at higher oil volume fractions when the droplets agglomerate in a three-dimensional structure ŽC..

G. Lagaly et al.r Applied Clay Science 14 (1999) 83–103

101

small amounts of oil are dispersed in the aqueous phase, the oil droplets rise independently. However, the mobility is reduced at higher densities of the oil droplets ŽFig. 12.. Differences in the rate of creaming Ž at constant f . are related to the effect of the particles on the viscosity of the coherent phase. Stiffening by sufficiently high concentrations of repulsive particles Ž cf. Kroon et al., 1998. or network formation of weakly attractive particles decreases the tendency of creaming. When network formation combines with mechanically stable envelopes around the droplets, emulsions are obtained which do not cream and are stable over longer periods. Dispersions of delaminated clay mineral particles exert a high viscosity which reduces the upward movement of the oil droplets. However, when the interaction between the solid particles is repulsive as in case of soda-activated bentonite or certain hectorite samples, the envelope of particles around the droplets cannot protect the droplets from coalescing and the creaming emulsion becomes unstable. 5. Conclusion Bentonites, montmorillonites or hectorites in combination with glycerol monostearate, alkyl polyŽ ethylene oxide. ethers, alkyl polyglycosides and lecithins are useful colloidal stabilizers for paraffin water emulsions. An interesting aspect of these emulsions is the strong influence of the clay mineral and coemulsifier on the rheological properties of the emulsions which will be reported in part II of this paper. Acknowledgements The droplet size distributions were measured in the institute of pharmacy, Professor Dr. Herbert Rupprecht, University of Regensburg, Germany. We are very thankful to Professor Rupprecht for his encouraging interest. We thank the Fonds der Chemischen Industrie for financial support. References Abend, S., Bonnke, N., Gutschner, U., Lagaly, G., 1998. Stabilization of emulsions by heterocoagulation of clay minerals and layered double hydroxides. Colloid Polymer Sci. 276, 730–737. Biermann, M., Schmid, K., Schulz, P., 1993. Alkylpolyglucoside—Technologie und Eigenschaften. StarchrStarke ¨ 45, 281–288. Brandenburg, U., Lagaly, G., 1988. Rheological properties of sodium montmorillonite dispersions. Appl. Clay Sci. 3, 263–279. Busch, P., Hensen, H., Tesmann, H., 1993. Alkylpolyglycoside-eine neue Tensidgeneration fur ¨ die Kosmetik. Tenside Surf. Det. 30, 116–121.

102

G. Lagaly et al.r Applied Clay Science 14 (1999) 83–103

Denkov, N.D., Ivanov, I.B., Kralchevsky, P.A., Wasan, D.T., 1992. A possible mechanism of stabilization of emulsions by solid particles. J. Colloid Interface Sci. 150, 589–593. Dickinson, E., 1992. An Introduction to Food Colloids. Oxford Sci. Publ., Oxford. Finkle, P., Draper, H.D., Hildebrand, J.H., 1923. The theory of emulsification. J. Am. Chem. Soc. 45, 2780–2788. Friberg, S.E., Goldsmith, L.B., Hilton, M.L., 1988. Theory of emulsions. In: Lieberman, H.A., Rieger, M.M., Banker, G.S. ŽEds.., Pharmaceutical Dosage Forms. Marcel Dekker, New York, pp. 49–91. Guven, N., 1992. Rheological aspects of aqueous smectite suspensions. In: Guven, N., Pollastro, ¨ ¨ R.M. ŽEds.., Clay–Water Interface and its Rheological Implications. CMS workshop lectures. Vol. 4. The Clay Mineral Society, Boulder CO, USA, pp. 81–126. Jordan, J.W., Williams, F.J., 1954. Organophilic bentonites: II. Inherante properties. Kolloid Z. 137, 40–48. Kroon, M., Vos, W.L., Wegdam, G.H., 1998. Structure and formation of a gel of colloidal disks. Phys. Rev. E 57, 1962–1970. Lagaly, G., 1989. Principles of flow of kaolin and bentonite dispersions. Appl. Clay Sci. 4, 105–123. Lagaly, G., 1994. Layer charge determination by alkylammonium ions. In: Mermut, A. ŽEd.., Charge Characteristics of 2:1 Clay Minerals. CMS workshop 6, The Clay Mineral Society, Boulder CO, USA, pp. 1–46. Levine, S., Sanford, E., 1985. Stabilization of emulsion droplets by fine powders. Can. J. Chem. Eng. 62, 256–268. Levine, S., Bowen, B.D., Partridge, S.J., 1989a. Stabilization of emulsions by fine particles: I. Partitioning of particles between continuous phase and oilrwater interface. Colloids Surfaces 38, 325–343. Levine, S., Bowen, B.D., Partridge, S.J., 1989b. Stabilization of emulsions by fine particles: II. Capillary and van der Waals forces between particles. Colloids Surfaces 38, 345–364. Lucassen-Reynders, E.H., van den Tempel, M., 1963. Stabilization of water in oil emulsions by solid particles. J. Phys. Chem. 67, 731–734. Menon, V.B., Wasan, D.T., 1986. Particle–fluid interaction with application to solid-stabilized emulsions. Colloids Surfaces 19, 89–105. Menon, V.B., Wasan, D.T., 1988. A review of the factors affecting the stability of solids-stabilized emulsions. Sep. Sci. Technol. 23, 2131–2142. Mukerjee, L.N., Srivastava, S.N., 1956a. Finely divided solids as emulsifiers: Part I. Emulsions stabilized by hydroxides or hydrous oxides of metals. Kolloid Z. 147, 146–152. Mukerjee, L.N., Srivastava, S.N., 1956b. Finely divided solids as emulsifiers: Part II. Emulsions stabilized by basic salts of metals. Kolloid Z. 149, 35–38. Mukerjee, L.N., Srivastava, S.N., 1957a. Finely divided solids as emulsifiers: Part III. Emulsions stabilized by miscellaneous solids. Kolloid Z. 150, 144–148. Mukerjee, L.N., Srivastava, S.N., 1957b. Bordeaux mixture and related compounds as emulsifiers. Kolloid Z. 150, 148–151. Packter, A., 1956. Studies in the rheology of clay water systems: Part I. The viscosity of sodium montmorillonite sols. Kolloid Z. 149, 109–115. Permien, T., Lagaly, G., 1994a. The rheological and colloidal properties of bentonite dispersions in the presence of organic compounds: I. Flow behaviour of sodium montmorillonite in water–alcohol. Clay Min. 29, 751–760. Permien, T., Lagaly, G., 1994b. The rheological and colloidal properties of bentonite dispersions in the presence of organic compounds: II. Flow behavior of Wyoming bentonite in water–alcohol. Clay Min. 29, 761–766. Permien, T., Lagaly, G., 1994c. The rheological and colloidal properties of bentonite dispersions

G. Lagaly et al.r Applied Clay Science 14 (1999) 83–103

103

in the presence of organic compounds: III. The effect of alcohols on the coagulation of sodium montmorillonite. Colloid Polym. Sci. 272, 1306–1312. Pickering, S.U., 1907. Emulsions. J. Chem. Soc. 91, 2001–2021. ¨ Pickering, S.U., 1910. Uber Emulsionen. Z. Chem. Ind. Koll. ŽKolloid Z.. 7, 11–16. Puskas, ´ ´ S., Balazs, ´ J., Farkas, A., Regdon, I., Barkesi, O., Dekany, ´ ´ I., 1996. The significance of colloidal hydrocarbons in crude oil production. I. Colloids Surfaces A 113, 279–293. Ramsden, W., 1904. Separation of solids in the surface-layers of solutions and suspensions ŽObservations on surface-membranes, bubbles, emulsions, and mechanical coagulation.. Proc. R. Soc. London 72, 156–164. Rapacchietta, A.V., Neumann, A.W., 1977. Force and free-energy analysis of small particles at fluid interfaces: II. Spheres. J. Colloid Interface Sci. 59, 555–567. Schambil, F., Paasch, S., Sax, B., 1987. Interfacial and colloidal properties of cosmetic emulsions containing fatty alcohol and fatty alcohol polyglycol ethers. Prog. Colloid Polym. Sci. 73, 37–47. Schulman, J.H., Leja, J., 1954. Control of contact angles at the oil–water–solid interfaces. I, II. Trans. Faraday Soc. 50, 589–605. Stul, M.S., van Leemput, L., 1982. Particle-size distribution, cation exchange capacity and charge density of deferrated montmorillonites. Clay Min. 17, 209–215. Tadros, T.F., Vincent, B., 1983. Emulsion stability. In: Becher, P. ŽEd.., Encyclopedia of Emulsion Technology. Vol. 1, Marcel Dekker, pp. 129–287. Tambe, D.E., Sharma, M.M., 1993. Factors controlling the stability of colloid-stabilized emulsions: I. An experimental study. J. Colloid Interface Sci. 157, 244–253. Tambe, D.E., Sharma, M.M., 1995. Factors controlling the stability of colloid-stabilized emulsions: III. Measurement of the rheological properties of colloid-laden interfaces. J. Colloid Interface Sci. 171, 456–462. Tributh, H., Lagaly, G., 1986. Aufbereitung und Identifizierung von Boden-und Lagerstattentonen: ¨ I. Aufbereitung der Proben im Labor. GIT-Fachzeitschrift fur ¨ das Laboratorium 30, 524–529. Tsugita, A., Takemoto, S., Mori, K., Yoneya, T., Otani, Y., 1983. Studies on OrW emulsions stabilized with insoluble montmorillonite-organic complexes. J. Colloid Interface Sci. 95, 551–560. Willenbacher, N., 1996. Unusual thixotropic properties of aqueous dispersions of laponite RD. J. Colloid Interface. Sci. 182, 501–510. Yan, N., Masliyah, J.H., 1993. Solids stabilized oil-in-water emulsions: scavenging of emulsion droplets by fresh oil addition. Colloids Surfaces 75, 123–132. Yan, N., Masliyah, J.H., 1994. Adsorption and desorption of clay particles at the oil–water interface. J. Colloid Interface Sci. 168, 386–392. Yan, N., Masliyah, J.H., 1995a. Characterization and demulsification of solids-stabilized oil-inwater emulsions: Part 1. Partitioning of clay particles and preparation of emulsions. Colloids Surfaces A 91, 229–242. Yan, N., Masliyah, J.H., 1995b. Characterization and demulsification of solids-stabilized oil-inwater emulsions: Part 2. Demulsification by the addition of fresh oil. Colloids Surfaces A 96, 243–252. Zhai, X., Efrima, S., 1996. Chemical physical aspects of macroemulsions stabilized by interfacial colloids. J. Phys. Chem. 100, 11019–11028.