w

Langmuir 1995,11, 1966-1971

1966

Rheological Behavior of Highly Concentrated Oil-in-Water (o/w) Emulsions R. Pons,*>+,$ C. Solans,? and Th. F. Tadross Departamento de Tensioactiuos, CID 1CSIC, c / Jordi Girona 18-26, 08034 Barcelona, Spain, and Zeneca Agrochemicals (Formerly Part of ICI Group), Jealott’s Hill Research Station, Bracknell, Berkshire RG12 6EY, U.K. Received February 15, 1994. I n Final Form: March 20, 1995@ Oil-in-water(olw)highly-concentratedemulsions have been studied in ternary waterlpolyethyleneoxidepolypropylene oxide-polyethylene oxide (ABA block copo1ymer)hydrocarbonsystems in order to compare their rheological behavior with that of water-in-oil (wlo) highly concentrated emulsions. Viscoelastic measurements have been performed as a function of dispersed phase volume fraction and temperature. The structural parameters of the system, droplet size, interfacial tension and continuous phase viscosity have been determined in order to correlate these parameters with the rheological behavior. The shear modulus has been shown to depend on the volume fraction according to Princen’s equation. The relaxation times were described using a Maxwell liquid model in which the viscosity is related to a reduced continuous phase viscosity. The effect of temperature on the shear modulus and relaxation time is significantly different from that observed for wlo highly concentrated emulsions. These differences in the rheological behavior of the emulsions were discussed in terms of the chemical structure of the surfactants.

Introduction During the last 30 years increasing interest has been focused on highly concentrated e m u l ~ i o n s . l -This ~ ~ is due to their technological applications, e.g. in food and cosmetic emulsions. In addition fundamental studies of the rheology of these systems provide a good insight into the interdroplet interactions. Many of the initial studies were performed on oil-in-water During the last 10 years several investigations considered the properties of water-in-oil e m u l s i o n ~ . ~ -Most l ~ of the investigations on the rheological behavior of highly concentrated emulsions were undertaken by Princen and co-workers, both from theoretical3 and from experimental points of vie^.^,^ These studies were focused on the static parameters, namely the yield stress and the static shear modulus. Princen et al. related the above-mentioned parameters to the structure of the highly concentrated emulsion and interfacial t e n ~ i o n .A~model of a foamlike structure with volume fraction of the dispersed phase greater than 74%

* Author to whom correspondence should be addressed. +

CID/CSIC.

Present address: Zeneca Agrochemicals. 8 Zeneca Agrochemicals. Abstract published in Advance A C S Abstracts, May 15, 1995. (1)Lissant, K. J. J . Colloid Interface Sci. 1966,22,462. (2)Lissant, K. J.;Mayhan, K. G. J . Colloid Interface Sci. 1973,42, f

@

201. (3)Princen, H.M. J . Colloid Interface Sci. 1979,71,55. (4)Princen, H.M.; Kiss, A. D. J . Colloid Interface Sci. 1986,112, 427. ( 5 ) Schwartz, L.W.; Princen, H. M. J . Colloid Interface Sci. 1987, 118, 201. (6)Kunieda, H.;Solans, C.; Shida, N.; Parra, J. L. Colloids Surf. 1987,24,225. (7)Solans, C.; Azemar, N.; Parra, J. L. Prog. Colloid Polym. Sci. 1988,76,224. ( 8 )Solans, C.; Dominguez, J. G.; Parra, J. L.; Heuser, J.;Friberg, S. E. Colloid Polym. Sci. 1988,266,570. (9)Kunieda, H.; Yano, N.; Solans, C. Colloids Surf. 1989,36,313. (10)Kunieda, H.: Evans, D. F.: Solans, C.:Yoshida,M. Colloids Surf. 1990,47,35. (11)Ravey, J. C.; Stebe, M. J. Phys. B 1989,394,156. (12)Ravey, J. C.; Stebe, M. J. Prog. Colloid Polym. Sci. 1990,82, 218. (13)Bampfield, A.;Cooper, J. In Encyclopedia of Emulsion Technology; Becher, Ed.; Marcel Dekker: New York, 1988;Vol. 3, p 281. (14)Kizling, J.; Kronberg, B. Colloids Surf. 1990,50,131. (15)Ruckenstein, E.; Ebert, G.; Platz, G. J . Colloid Interface Sci. 1989,133,432.

and a characteristic size of the unit cell with mean radius

R was considered by these authors. In a recent paper Princen et al. developed a theory for the flow characteristics of these emulsion^.^ Recently, we have carried out a systematic study of the viscoelastic behavior of highly concentrated water-in-oil (w/o) emulsions using oscillatory techniques.16J7 To our knowledge, these were the first studies t h a t were carried out in the linear viscoelasticity region, and hence the analysis of the rheological parameters was relatively more simple. In these investigations, it was possible to use the equation suggested by Princen that relates the static shear modulus to the volume fraction droplet radius and interfacial tension. The static shear modulus was simply replaced by the dynamic shear modulus a t high frequency. The storage and loss moduli ( G and G )were then determined as a function of frequency to obtain the relaxation time z of the system. The relaxation time depended on the volume fraction, mean droplet radius, interfacial tension, and continuous phase viscosity.16J7 The behavior of these emulsions was interpreted on the basis of a Maxwell fluid model with an elastic component related to the structure and a viscous element related to the continuous phase viscosity. In this paper we present results of linear viscoelastic measurements for oil-in-water highly-concentrated emulsions. These emulsions were prepared using decane and an A-B-A block copolymer, polyethylene oxide-polypropylene oxide-polyethylene oxide. These block copolymers are of great industrial interest and are used in cosmetic and pharmaceutical emulsions. 1 8 3 1 9 Various emulsions were prepared by varying the surfactant composition and concentration in order to choose stable systems that are suitable for rheological investigations. The effect of volume fraction and temperature on the rheological behavior were systematically studied. As we will see, both dynamic shear modulus and relaxation time seem to follow the general trends of the wlo emulsions studied before.16J7 (16)Pons, R.; Solans, C.; Stebe, M. J.; Erra, P.; Ravey, J. C. Prog. Colloid Polym. Sci. 1992,89, 110. (17)Pons, R.;Erra, P.; Solans, C.; Ravey, J. C.; Stebe, M. J. J . Phys. Chem. 1993,97,12320. (18)Schmolka, I. R. JAOCS 1977,54,110. (19) Schmolka,I. R. InNonionic Surfactants;Shick, Ed.; Marcel Dekker: New York, 1967;p 300.

0743-746319512411-1966$09.00100 1995 American Chemical Society

Highly Concentrated O /W Emulsions

Langmuir, Vol. 11, No. 6, 1995 1967

Table 1. Properties of the Surfactants and Composition of the Emulsions Used

emulsion CP CP code surf. mw l%a HLB nb mc %surf.d El L92 3650 26°C 16°C 1-7 17 50 10% E2 L64 2900 40°C 58°C 12-18 26 30 20% a Cloud point at 1%or 10%w/w. Mean number of ethylene oxide units per molecule. Mean number of propylene oxide units per molecule. Composition of the aqueous phase of the emulsion. Table 2. Emulsion Surface Weighed Mean Droplet Size, RSZ,and Distribution Standard Deviation C T R ~as ~ ,a Function of Dispersed Phase Volume Fraction, 9,for the Emulsions Studied at 20 "C

0.964 0.936 0.908 0.879 0.849

2.95 3.18 3.23 3.00 2.95

1.6 1.8 1.8 1.8 1.7

3.04 3.03 3.20 3.56 3.67

1.9 1.9 2.3 2.5 2.6

However, some differences were observed and these will be discussed in the present paper. The different behavior can be understood by considering the very different structure of the surfactants used.

Materials and Methods Materials. Decane was obtained from Fluka, P.S. grade, and used as received. Water was doubly distilled in an all-glass apparatus. The polymericsurfactants L92 and L64 were supplied by IC1 (U.K.) and used as received. The chemical composition and some of the properties of these surfactants are shown in Table 1. Emulsion Preparation. The desired amount of surfactant was dissolved in water, and decane was added dropwise. The mixtures were strongly shaken by hand, and further addition of decane was continued until the desired dispersed phase volume ratio was reached. The resulting emulsion was sheared using an Ystral mixer for 5 min. During the process of emulsification the viscosity of the system gradually increased until the emulsion appeared solidlike. Under microscope these emulsions showed the typical structure of highly concentrated emulsions. The oilin-water nature was proven by the fact that they can be diluted in water but not in oil. From the stock emulsions, other systems with lower volume fractions were prepared by dilution with water. A measured amount ofwater was added to the stock emulsions and the system was gently mixed overnight by keeping it on a roller. Dilution with water changes the overall concentration of surfactant; however, the total concentration of surfactant is much higher than the critical micellar concentration and in this conditions the interfacial tension is little affected by surfactant concentration. Droplet size measurements showed no significant change of the mean droplet size nor of the overall distribution when the system was diluted. These results are reported in Table 2. Droplet Size Measurements. A Coulter Counter Multisizer I1 was used in conjunction with 30 and 100 ym apertures. An aliquot of the emulsion was first diluted in 1%aqueous solution of Brij35 to prepare a highly diluted emulsion. Droplets of the diluted emulsion were dispersed in IsotonII in the measuring cell. Interfacial Tension. Interfacial tensions of surfactant solution against decane were measured using a CI Robal microbalance by means of the Wilhelmy Plate method. The plate is roughened platinum with a length of 2 cm. The plate was first wet by the lower phase, and the upper phase was carefully added on the top. The interfacial tension was monitored as a function of time, and a reading was taken when no changes were observed for 2 h. Measurements as a function of temperature were performed on preequilibrated phases. Our setup did not allow us to use temperatures higher than 60 "C. We assumed linear behavior to extrapolate the results at high temperature. This assumption

Table 3. Interfacial Tension for the Systems Studied y (mN m-l)

T ("C) 20 25 30 35 40 45 50 55 60 65 70 75 80 Extrapolated (see text).

El 1.10 1.01b 0.91b 0.7gb 0.72"~~

E2 1.68 1.60 1.60 1.46 1.38 0.60'~~ 1.22 0.47"~~ 1.01 0.91 0.79 0.72 0.60a 0.47O 0.36a Extrapolated from L64 results.

probably provides an overestimation of the interfacial tension as the temperature approaches phase inversion. Phase separation was difficult for the system E l , and only at 20 "C could two clear phases be separated. We extrapolated this interfacial tension with the assumption that the changes with temperature for both systems are the same. These results are shown in Table 3. The Pluronic surfactants are formed by three chains, two of PEO and a central one of PPO. In these surfactants the PPO chain acts as the hydrophobic moiety of the molecule, however, the PPO is insoluble in the oil. Therefore there is not any solubilization of the surfactant in oil. Above the cloud point of the surfactant in water the system is essentially a ternary system formed by oil, water, and surfactant aggregates dispersed in the water. In addition the behavior of systems involving nonionic surfactant of the alkyl alcohol polyoxyethylene type show cloud points of the binary mixtures well below the phase inversion temperature (the difference with these systems is that in this case the surfactant can solubilize in the oil). In the case of the L92 surfactant, the cloud point of the binary solution is close to the starting temperature of the experiments; however, we can assume the same kind of behavior because what is produced is an oil-in-water emulsion and therefore the system is below the phase inversion temperature. Rheological Measurements. Rheological measurements were carried out using a controlled stress Bholin CS and controlled strain Bholin VOR apparatus (BholinRheologie, Lund, Sweden). Concentric cylinder platens C25 were used. The samples were placed in the outer cylinderusing a spatula and the inner cylinder was carefully lowered in the emulsion to avoid appreciable shearing of the sample. The emulsions were kept for 5 min at the measurement temperature to reach equilibrium. Temperature sweeps were carried out using the same emulsion samples. Coalescence was not detected during the measurements at temperatures lower than 40 "C for system El. Measurements performed on a sample undergoing a cycle of temperature from 20 to 40 "C and back to 20 "C showed a decrease in G ofless than a 3%. Stress sweep and strain sweep measurements at a fured frequency (0.1 Hz) were performed in order to determine linear viscoelastic region (where the rheological parameters are independent ofthe amplitude). Frequency sweep measurements were then performed at a given stress or strain in the frequency region 0.01-10 Hz. The resulting spectra, G and G as a function of frequency, were used to fit the Maxwell fluid equations. The storage and loss modulus equations for a Maxwell fluid arezo (la)

(20) Courraze, G.; Grossiord,J. L. Initiation a la Rheologie, Technique et Documentation; Lavoisier: Pans, 1983; pp 19-43.

Pons et al.

1968 Langmuir, Vol. 11, No. 6, 1995 z

--TO

- Go

(IC)

where Go is the dynamic elastic modulus, zo is the relaxation time, w is the frequency in radians s-l and 90 is the zero shear viscosity. The two parameters, Go and TO, were obtained from the least-squares fit of G and G using eq 1. The computation was performed simultaneously on both quantities,using the same procedure as described before.17 In Figure 1 examples of the rheological measurements and the fit of a Maxwell element are shown. The error in the fit of Go is typically in the range of 1-2%. to is deterapined with reasonable accuracy (error of the order of 10%)only for values within the measurement range (Le. 20 > to > 0.02). Extrapolated values are only indicative of the order of magnitude. For liquidlike behavior a single Maxwell element gives a reasonable picture of the behavior. When w is very low, ( o r ) 2