Emulsification Rheokinetics of Nonionic Surfactant-Stabilized Oil-in

Aug 4, 2001 - A further enhancement of the emulsion droplet network takes place in a few hours after emulsification. This rearrangement may be explain...
0 downloads 7 Views 88KB Size
Download PDF
5410

Langmuir 2001, 17, 5410-5416

Articles Emulsification Rheokinetics of Nonionic Surfactant-Stabilized Oil-in-Water Emulsions M. C. Sa´nchez,† M. Berjano,‡ A. Guerrero,*,‡ and C. Gallegos† Dpto. de Ingenierı´a Quı´mica, Universidad de Huelva, Campus de La Ra´ bida 21819 Palos de la Frontera (Huelva), Spain; and Dpto. de Ingenierı´a Quı´mica, Universidad de Sevilla, c/ Prof. Garcı´a Glez, s/n. 41012 Sevilla, Spain Received May 26, 2000. In Final Form: June 6, 2001 The emulsification process of o/w emulsions containing poly(ethylene glycol) nonylphenyl ether has been investigated by following the evolution of torque, droplet size distribution (DSD), and linear viscoelastic properties with emulsification time. The emulsification process was carried out in a controlled-rotational speed mixing rheometer using an anchor or a helical ribbon impeller under different processing conditions. The kinetics of emulsification has been discussed in terms of two stages: breakup of droplets (counterbalanced by coalescence) and transport (and adsorption) of surfactant molecules to the o/w interface to prevent coalescence. An increase in the emulsification time yields lower values of the mean droplet size and, subsequently, an increase in the linear viscoelastic properties of the emulsion at emulsification temperatures up to 25 °C. Above this temperature, the evolution with emulsification time is different, since a higher degree of coalescence takes place. As a result, emulsion DSD tends to be bimodal at long emulsification time. A further enhancement of the emulsion droplet network takes place in a few hours after emulsification. This rearrangement may be explained in terms of the development of a depletion-flocculation process, so that the surfactant molecules act both as emulsifying and depleting agents.

Introduction Emulsion stability is considered a primary requirement for a wide variety of the many industrial applications of emulsions (i.e., foodstuffs, cosmetics, pharmaceuticals, agrochemicals, and petrochemicals). An emulsion is stable, from a kinetic point of view, when the number, droplet size distribution, and arrangement of droplets do not undergo any discernible change over the storage time scale.1 Emulsion rheology and stability are closely related both depending on several structural parameters, as reviewed by different authors (i.e., refs 2-4). The emulsification process usually requires a considerable amount of mechanical energy, to disperse one of the liquids in the form of small droplets in the continuous phase. Janssen and Meijer5 considered a subdivision of the emulsification process in three stages based on the value of the local capillary number:

Ca )

ηcγ˘ R σ

(1)

where ηc is the viscosity of the continuous phase, γ˘ is the * To whom correspondence should be addressed. E-mail address: [email protected]. † Universidad de Huelva. ‡ Universidad de Sevilla. (1) Dickinson, E. Proceedings of the 2nd World Congress on Emulsions; EDS: Bourdeaux, France, 1997. (2) Darby, R. Emulsions and Emulsion Technology III; Marcel Dekker: New York, 1984. (3) Rahalkar, R. R. Viscoelastic Properties of Foods. Elsevier Applied Science: London, 1992. (4) MacClements, D. J. Food Emulsions: Principles, Practice and Techniques; CRC Press LLC: New York, 1999. (5) Janssen, J. M. H.; Meijer, H. E. H. Polym. Eng. Sci. 1995, 35, 1766-1780.

shear rate, and σ/R is the interfacial stress (σ being the interfacial tension and R the radius of the disperse phase droplets). During the emulsification process both the capillary (Ca) and Reynolds numbers (Re) decrease as a result of the length scale reduction and the increase in viscosity, respectively. The stages considered by these authors are as follows: (a) The first is stretching of dispersed drops when Ca is much higher than a critical value, Cacrit, above which the local stress overrules the interfacial stress. Such stresses can be generated either in laminar or turbulent flow.6 This mechanism is typically produced at the beginning of the process and leads to formation of large liquid threads. (b) The second is consecutive breakup of threads and drops which takes place when Ca > Cacrit. The value of Cacrit depends on the ratio between disperse and continuous phase viscosities.7 (c) The last is coalescence of the disperse droplets when Ca , Cacrit. As stated by Walstra,8 the two critical steps in emulsification are the consecutive disruption of droplets and their coalescence, both of which are favored by an intense agitation. The evolution of this process, as well as the microstructure of the final emulsion, depends on many variables related to the processing conditions (i.e., type and geometry of the emulsification equipment, residence time, agitation speed, temperature of emulsification, etc.), or to the nature of the phases involved (i.e., rheological properties of the continuous and disperse phases, interfacial tension, etc.). Mason and Bibette9 have studied the (6) Walstra, P.; Smulders, P. Proceedings of the 2nd World Congress on Emulsions; EDS: Bourdeaux, France, 1997. (7) Grace, H. P. Chem. Eng. Commun. 1982, 14, 225-277. (8) Walstra, P. Encyclopedia of Emulsion Technology; Marcel Dekker: New York, 1983. (9) Mason, T. G.; Bibette, J. Langmuir 1997, 13, 4600-4613.

10.1021/la000723w CCC: $20.00 © 2001 American Chemical Society Published on Web 08/04/2001

Emulsification Rheokinetics

shear induced rupturing of droplets in viscoelastic complex fluids. They found that the viscoelasticity of the complex fluid used in emulsification can lead to a dramatic alteration of the rupturing phenomena conditioned by the partial elasticity of the complex fluid. Moreover, processing variables influence the structural parameters yielding dramatic differences in the emulsification kinetics and rheological properties of the emulsions.10,11 Knowledge of structural parameters and rheological properties is important for both the control of processing and the stability of the emulsion. An emulsifier is typically required to improve both the emulsification process and emulsion stability. The role of the emulsifier molecules during the emulsification process is 2-fold: 1. They adsorb at the o/w interface, which reduces the interfacial tension, yielding a decrease in the amount of mechanical energy required to deform and disrupt the droplets and thereby favoring emulsification. For example, a reduction from 50 to 5 mNm-1 in the interfacial tension should decrease the droplet size by an order of magnitude under laminar flow conditions.12 Moreover, if surfactant concentration is high and interfacial tension very low, “spontaneous emulsification” may be produced by the strong interfacial tension gradients induced. However, this fact is only important at the earlier stages of emulsion formation.13 2. They prevent recoalescence as stated by Walstra12 who explained this effect in terms of the Gibbs-Marangoni effect, caused by formation of interfacial tension gradients during emulsification. Other authors attributed the effect to formation of a protective barrier around the oil droplets, which must provide a repulsive force strong enough to prevent droplet from aggregating with each other.4 Nevertheless, a number of other factors, such as the ability to enhance the interfacial rheological properties of emulsion droplets14 or the kinetics of adsorption to the interface,13 also determine the efficiency of emulsifier molecules. Thus, under the same homogenization conditions, it has been shown that emulsifiers which adsorb rapidly produce smaller droplet sizes than those which adsorb slowly. In fact, most food emulsifiers do not adsorb rapidly enough to completely prevent droplet coalescence.15 The influence of surfactant concentration for different lowmolecular-weight emulsifiers was studied in a previous paper.11 A correlation between the plateau modulus, a linear viscoelastic parameter related to the strength of interdroplet interactions, and the volume mean diameter of droplets was obtained in this paper. This correlation was derived from the equation of Princen for small strain obtained from a two-dimensional analysis of the stress vs strain relationship of highly concentrated, monodisperse fluid-fluid dispersions.16,17 The objective of this work was to study the evolution of some structural and rheological parameters, namely droplet size distribution, torque, and linear viscoelastic properties, during the emulsification process of o/w emulsions, under different processing conditions. (10) Franco, J. M.; Guerrero, A.; Gallegos, C. Rheol. Acta 1995, 34, 513-524. (11) Sa´nchez, M. C.; Berjano, M.; Guerrero, A.; Brito, E.; Gallegos, C. Can. J. Chem. Eng. 1998, 76, 479-485. (12) Walstra, P. Chem. Eng. Sci. 1993, 48, 333-349. (13) Schubert, H.; Armbruster, H. Int. Chem. Eng. 1992, 32, 14-21. (14) Williams, A.; Janssen, J. J. M.; Prins, A. Colloid Surf. A 1997, 125, 189-195. (15) Stang, M.; Karbstein, H.; Schubert, H. Chem. Eng. Proc. 1994, 33, 307-313. (16) Princen, H. M. J. Colloid Interface Sci. 1985, 105, 150-171. (17) Pons, R.; Erra, P.; Solans, C.; Ravey, J. C.; Ste´be´, M. J. J. Phys. Chem. 1993, 97, 12320-12324.

Langmuir, Vol. 17, No. 18, 2001 5411

Figure 1. Evolution of torque with emulsification time for emulsions prepared with two different impellers, at 25 °C (solid line, eq 4; dashed line, eq 5).

A low-molecular-weight emulsifier able to complete the emulsification process at low energetic conditions, which fulfils the above-mentioned properties related to the reduction of the interfacial tension and the adsorption kinetics, has been selected in order to carry out this study. Experimental Section Emulsions were prepared using 75wt % sunflower oil (Hijos de Ybarra, S.A., Seville, Spain) and 7wt % poly(ethylene glycol), EO ) 10, nonylphenyl ether, PEG-NPE (Sigma, St. Louis, MO) (HLB: 13) as emulsifier. The emulsification was carried out at different temperatures (5, 15, 25, 35, and 50 °C), with a controlledrotational speed mixing rheometer, Ika-Visc (Ika, Steufen, Germany) using either an anchor or a helical ribbon impeller in a batch tank. The emulsions were prepared at different agitation speeds comprised between 100 and 300 rpm. Sampling was accomplished by using an automatic pipet, Multipette Plus, from Eppendorf (Hamburg, Germany). A tip suitable for highly viscous fluids was used. Dynamic shear tests were done in a RS-100 controlled-stress rheometer from Haake (Karlsruhe, Germany), using a cone-plate geometry (4°, 60 mm) at 25 °C. All the measurements were performed in the linear viscoelasticity range. Droplet size distributions (DSD) were determined with a Mastersizer X analyzer from Malvern Instruments Ltd. (Malvern, U.K.). A Coulter Counter model ZB analyzer from Coulter Electronics Ltd. (Luton, U.K.) was used to contrast Mastersizer measurements, showing a good concordance with the results. Two types of mean diameters (volume diameter, d43, and numeric diameter, d10) as well as the uniformity parameter, U, which is related to the deviation of droplet size from the median of the distribution, were used in the present paper. These parameters may be expressed as follows:

dxy )

U)

∑n d ∑n d

x

i

i

i

i

(2)

y

∑ V |d(v,0,5) - d | d(v,0,5) ∑ V i

i

(3)

i

where ni is the number of droplets with a diameter di, d(v, 0,5) is the median of the distribution, and Vi is the volumen of droplets with a diameter di. In all cases, at least three replicates of each test were performed.

Results and Discussion Evolution of in Situ Emulsion Torque. The emulsification process has been always followed by studying the evolution of torque with emulsification time. The results obtained with two different impellers, at two different agitation speeds, are shown in Figure 1. Initially,

5412

Langmuir, Vol. 17, No. 18, 2001

Sa´ nchez et al.

Table 1. Kinetic Parameters for the Emulsification Processes Carried out at 25 °C type of impeller anchor anchor anchor anchor anchor helical ribbon helical ribbon helical ribbon helical ribbon helical ribbon

agitation speed k Me tin pre(rpm) (1/min) (mN‚m) (s) emulsification 100 150 200 250 300 100 150 200 250 300

0.26 0.70 2.37 3.70 6.15 0.31 0.56 1.84 2.02 3.72

139 173 248 289 367 81 118 144 152 245

38 48 25 28

no no no no no yes yes yes yes yes

the system at rest was completely separated into two phases, the oil phase and the aqueous micellar solution of the surfactant used, which was placed at the bottom of the tank. It is worth pointing out that the contribution of the two separated phases to the torque is negligible. Immediately after the beginning of the mixing process a very rapid increase in torque takes always place. However, in some cases (i.e., at the lowest energetic conditions), this initial increase in torque is rapidly dampened, giving rise to a period of time at which the value of torque becomes nearly constant. This period of time is considered as an induction time, previous to a further exponential increase in torque. Finally, an equilibrium torque value is obtained. A first-order kinetic equation reproduces the abovementioned evolution of torque fairly well:

M ) Me[1 - exp(-k(t - tin))]

Figure 2. Evolution of torque with emulsification time for emulsions prepared, at 250 rpm, with the anchor impeller.

(4)

where Me is the torque for the final emulsion, k is the kinetic constant, and tin is the induction time. The only exception found corresponds to those emulsification processes carried out with the helical ribbon at the highest agitation speeds. In those cases, a second kinetic constant is needed to reproduce the evolution of torque properly:

Figure 3. Evolution of kinetic parameters with emulsification temperature for emulsions prepared, at 250 rpm, with the anchor impeller.

The values of the kinetic parameters (eq 4) are shown in Table 1. As may be observed, parameters k and Me undergo a continuous increase with the agitation speed, irrespective of the impeller used, being more moderate for the helical ribbon. On the other hand, the induction time tends to vanish with increasing agitation speed. If the induction time is too long (i.e., when a helical ribbon is used), formation of a stable emulsion is not possible. In this case, a preemulsification method, which consists of three stepss premixing, resting time, and emulsificationsis needed to produce the emulsion. The evolution of torque during the emulsification process does not show any significant differences with or without the preemulsification stage (once the induction time is over). Similarly, a preemulsification stage was required, at high concentration of surfactant, when in the aqueous phase used for the emulsification process a highly viscous hexagonal liquidcrystalline phase was formed.11 The preemulsification procedure also eliminates the appearance of the induction time, as has been previously reported.18 Figure 2 shows the evolution of torque with the emulsification time for emulsification processes carried out, at different temperatures, with the anchor impeller.

An increase in the temperature of emulsification gives rise to a relevant decrease in torque. An interesting parameter related to the emulsification kinetics is the characteristic emulsification time, which may be defined as the elapsed time of processing necessary to reach 95% of the final torque. This value continuously decreases as emulsification temperature increases up to 35 °C and remains constant (around 0.8 min) at the highest temperatures. The values of the kinetic parameters, eq 4, for emulsification processes carried out using an anchor impeller, as a function of emulsification temperature, are shown in Figure 3. An increase in temperature yields a decrease in the equilibrium torque, as well as an increase in the kinetic constant. This increase is linear up to a temperature close to 35 °C. However, above this critical temperature, a significant reduction in the slope of variation of k is then observed. The kinetics of the emulsification process may be explained taking into account the following stages: I. The consecutive breakup of droplets, which corresponds to the two first steps described by Janssen and Meijer.5 II. The transport of surfactant molecules, and adsorption, to the o/w interface, this stage being important to prevent coalescence, as mentioned before.8 Both stages largely depend on temperature. Thus, Saito and Shinoda19 found a decrease in the interfacial tension upon raising temperature below the phase inversion

(18) Sa´nchez, M. C.; Berjano, M.; Guerrero, A.; Gallegos, C. Grasas y Aceites 2000, 51, 230-236.

(19) Saito, H.; Shinoda, K. J. Colloid Interface Sci. 1970, 32, 647657.

M ) Me[1 - A exp(-k1(t - tin)) - (1 - A) exp(-k2(t - tin))] (5)

Emulsification Rheokinetics

Langmuir, Vol. 17, No. 18, 2001 5413

Table 2. Values of DSD Parameters for the Emulsification Processes Carried out at 250 rpm, Using the Anchor Impeller temp of emulsiDSD fication (°C) params 5 15 25 35 50

1

2

time (min) 3 4 5

d4,3 (µm) 3.42 2.73 3.16 d1,0 (µm) 1.15 1.20 1.23 U 0.53 0.51 0.51 d4,3 (µm) 3.79 2.78 2.86 d1,0 (µm) 1.21 1.25 1.21 U 0.57 0.44 0.50 d4,3 (µm) 3.40 3.20 3.20 d1,0 (µm) 1.23 1.29 1.30 U 0.56 0.51 0.44 d4,3 (µm) 3.74 3.72 2.81 d1,0 (µm) 1.32 1.22 1.27 U 0.57 0.54 0.46 d4,3 (µm) 10.02 6.12 3.50 d1,0 (µm) 1.34 1.21 0.77 U 0.65 0.61 0.84

3.09 1.22 0.45 2.73 2.93 1.27 1.31 0.43 0.40 2.77 1.36 0.39 3.97 1.22 0.46 6.34 0.83 0.61

7

final

3.05 1.28 0.40 2.59 1.26 0.40

2.80 1.34 0.38 2.48 1.23 0.39 2.77 1.37 0.38 3.78 1.18 0.46 5.00 0.90 0.61

temperature, PIT,20 which favors stage I. This is the effect that Saito and Shinoda19 found for a system containing water/PEG (EO ) 8,6)-NPE/cyclohexane. Moreover, the viscosity of both the aqueous and oil phases decrease with temperature,21 which favors the transport/adsorption stage. The emulsification process studied always starts under turbulent conditions. At the beginning of the process the increase in torque is very rapid, because the Ca number is high and the local stress is predominant, leading to droplet breakup. (i) If the second stage is slower than the first one (i.e., at low agitation speed), an induction time may take place inmediately after the initial torque growth, due to a pseudoequilibrium between breakup and coalescence of droplets. The induction time finishes when the velocity of the stage I decreases in such a way that the surfactant can stabilize the new interface generated. After the induction period, the normal evolution of the process takes place (first-order kinetics). (ii) If the second stage is much slower (i.e., at low temperature), no stable emulsion can be formed unless a preemulsification procedure is used. The first two steps of the preemulsification process allow surfactant transport to the interface, which makes emulsification possible. (iii) If the process is controlled by the generation of interface (i.e., at high temperature) the emulsification is rapid, according to a first-order kinetic equation without induction time. The final emulsion results from the dynamic equilibrium between breakup and coalescence phenomena.22,23 A high value in the emulsification temperature promotes droplet bursting but also coalescence, leading to bimodal distributions, as reported in a previous paper.11 Evolution of Emulsion Droplet Size Distribution. The evolution of droplet burst during the emulsification process was monitored by taking samples from the evolving emulsion at different processing times, and analyzing their DSD. The values of both mean diameters (d43 and d10) and uniformity are shown in Table 2 as a function of time and emulsification temperature. As can be observed, the evolution of these structural parameters with the emul(20) Shinoda, K. J. Colloid Interface Sci. 1967, 24, 4-15. (21) Sa´nchez, M. C. Ph.D. Thesis, University of Seville, Seville, Spain, 1998. (22) Janssen, J. M. H.; Meijer, H. E. M. J. Rheol. 1993, 37, 597-608. (23) Salager, J. L.; Pe´rez-Sa´nchez, M.; Ramı´rez-Gouveia, M.; Bricen˜oRivas, M. I.; Garcı´a, Y. Proceedings of the 2nd World Congress on Emulsions; EDS: Bourdeaux, France, 1997.

Figure 4. Droplet size distribution curves, for emulsions prepared at 15 °C and 250 rpm, as a function of the emulsification time.

Figure 5. Droplet size distribution curves for emulsions prepared at 250 rpm and 50 °C, as a function of the emulsification time.

sification time depends on the emulsification temperature. In the low-temperature region studied (from 5 to 25 °C), a significant decrease in d43 and an increase in d10, although moderate, takes place as emulsification proceeds. Consequently, a continuous reduction in the size of the largest droplets and a decrease in the number of the smallest droplets are produced. These results suggest that both breakup and coalescence (of small droplets) occur. This evolution results in a reduction of polydispersity as deduced from the values of U. Moreover, as may be observed in Figure 4, all the emulsions studied show unimodal log-normal DSD within this temperature range. The evolution of the DSD curves with the emulsification time at the highest temperatures of emulsification studied (35 and 50 °C) is quite different, as may be deduced from the values shown in Table 2. A clear example of this evolution can be observed in Figure 5. At the beginning of the emulsification process the DSD is unimodal and a continuous reduction in size and polydispersity occurs. However, at longer times an increase in size and polydispersity, as well as a tendency to a bimodal distribution, is noticed. Thus, the droplet size distribution curves of emulsions processed at 50 °C, for emulsification times longer than 3 min, show a well-pronounced maximum for droplet sizes larger than 4 µm and a very smooth maximum at a lower droplet size. This behavior can be explained assuming that the morphology of the final emulsion is the result of an equilibrium between breakup and coalescence of droplets, both of which are favored by a high temper-

5414

Langmuir, Vol. 17, No. 18, 2001

ature and an intense agitation.8,23 Thus, the second peak, obtained at high temperature, suggests that the mechanism of coalescence plays an important role under such conditions. It must be emphasized that this mechanism is evident once the torque reaches it maximum value. For example, an emulsion processed at 250 rpm with the anchor impeller, and 50 °C, shows a characteristic emulsification time lower than 1 min, although the predominant effect of coalescence, which modifies DSD, is apparent for a processing time of 4 min or longer. It is also interesting to compare the results of the DSD of final emulsions processed at different temperatures of emulsification. Thus, an increase in temperature above 25 °C produces an increase in droplet size and polydispersity, as may be deduced from the values of the volume mean diameter, d43, and the uniformity parameter, U, shown in Table 2. However, a high temperature of emulsification also produces a higher number of small droplets since the numeric mean diameter, d10, decreases. These results are consistent with those found in the literature. Thus, Marszall24 found that the o/w emulsions are stable at temperature by 25-60 °C below PIT.20 The emulsions studied fit this requirement (Shinoda and Sagitani25 and Shinoda and Kunieda26 reported a value of 72-75 °C for the PIT value for the PEG-NPE); however, as stated by Shinoda and Saito,27 the rate of coalescence increases with increasing temperature, particularly near the PIT. Kabalnov and Wennerstro¨m28 and Kabalnov and Weers29 provided a model for the correlation between emulsion stability against coalescence and the monolayer spontaneous curvature for a wide variety of surfactants systems. According to this concept, increases in droplet coalescence result from decreases in the monolayer spontaneous curvature with increasing temperature, which in turn depends on temperature, on HLB value of surfactant used and on the degree of penetration of the dispersed oil phase in the surfactant monolayer. Consequently, a high temperature favors disruption of droplets, which explains the increase in the kinetic constant shown in Figure 3, but also induces coalescence, leading to bimodal distributions above 25 °C, which is responsible for the change in the tendency of the evolution of the kinetic constant. The disruption of droplets is more difficult at 5 °C, probably due to a higher viscosity of the continuous phase, yielding a minimum in droplet size (and polydispersity) at 15 °C. Evolution of Emulsion Linear Viscoelastic Properties. The analysis of the evolution of emulsion microstructure during the emulsification process was completed by means of the characterization of its linear viscoelastic properties. Strain sweep tests were performed on samples taken after different emulsification times, to establish the linear viscoelastic range. As can be observed in Figure 6, the above-mentioned range generally extends up to strains around 2%, irrespective of the emulsification time and temperature of processing used. The frequency dependence of the dynamic viscoelastic properties (storage modulussshowing the elastic component of the materialsand loss modulussindicating the viscous component of the samplesG′ and G′′, respectively), at 25 °C, of samples processed at 15 °C, and during different (24) Marszall, L. Nonionic Surfactants: Physical Chemistry; Marcel Dekker: New York, 1987. (25) Shinoda, K.; Sagitani, H. J. Colloid Interface Sci. 1978, 64, 6877. (26) Shinoda, K.; Kunieda, H. Encyclopedia of Emulsion Technology; Marcel Dekker: New York, 1983. (27) Shinoda, K.; Saito, H. J. Colloid Interface Sci. 1969, 30, 258270. (28) Kabalnov, A.; Wennerstro¨m, H. Langmuir 1996, 12, 276-292. (29) Kabalnov, A.; Weers, J. Langmuir 1996, 12, 1931-1936.

Sa´ nchez et al.

Figure 6. Evolution of complex viscosity with shear strain for emulsions prepared at 250 rpm and 15 °C.

Figure 7. Evolution of the storage and loss moduli with frequency (T ) 25 °C): influence of the emulsification time (agitation speed: 250 rpm; temperature of emulsification: 15 °C) (solid symbols, G′, and open symbols, G′′).

emulsification times, is shown in Figure 7. The behavior of all the emulsions studied corresponds to a mechanical spectrum showing only the plateau and transition region. This behavior is typical of concentrated oil-in-water emulsions, with unimodal DSD, stabilized either by lowmolecular weight11,30 or by macromolecular emulsifiers.10,31,32 Occurrence of a well-developed plateau region, where approximately constant values of G′ can be observed, in surfactant-stabilized emulsions has been previously related to the formation of an elastic structural network which confers a high stability to the emulsion.30,33 Figure 7 shows a continuous development of the plateau region with emulsification time that may be attributed to an increase in the interactions among droplets, as a consequence of the continuous decrease in droplet size and polydispersity. Therefore, for temperatures of emulsification lower than 35 °C, both an enhancement of the elastic network and a stability improvement of the emulsion are continuously being produced as emulsification proceeds. The evolution of the linear viscoelasticity properties with the emulsification time, for emulsions processed at high temperature, may be observed in Figure 8, where (30) Guerrero, A.; Partal, P.; Gallegos, C. J. Rheol. 1998, 42, 13751388. (31) Gallegos, C.; Berjano, M.; Choplin, L. J. Rheol. 1992, 36, 465478. (32) Guerrero, A.; Ball, H. R. J. Texture Studies 1994, 25, 363-381. (33) Franco, J. M.; Sa´nchez, M. C.; Guerrero, A.; Gallegos, C. Rheol. Fluid Mech. Nonlinear Mater. ASME 1996, 217, 177-183.

Emulsification Rheokinetics

Langmuir, Vol. 17, No. 18, 2001 5415

Figure 8. Evolution of loss tangent with frequency (T ) 25 °C) for emulsions processed at 250 rpm and 35 °C. Table 3. Values of the Plateau Modulus for Final Emulsions Processed at 250 Rpm, with the Anchor Impeller temp of emulsification (°C) GON (Pa)

5 213

15 239

25 188

35 134

50 47

the frequency dependence of the loss tangent, tan δ (ratio of the loss and storage moduli), is shown. The emulsion obtained after 1 min of processing shows similar values of the storage and loss moduli in the low-frequency region (tan δ ≈1). An increase in the emulsification time yields an increase in the elastic characteristics of the emulsion (tan δ , 1), which is accompanied by a decrease in emulsion droplet size and polydispersity. A further increase in emulsification time favors the appearance of a terminal region in the mechanical spectrum, in this low-frequency region (tan δ > 1). This behavior is related to the occurrence of a bimodal DSD, as was previously discussed, which leads to a decrease in the interactions among droplets, being a consequence of the more efficient packing of droplets. Table 3 includes the values of GoN, plateau modulus, obtained as a function of the emulsification temperature. The values shown in Table 3 clearly reflect the different behavior found at low and high temperature in the kinetic constant, DSD and the mechanical spectrum. An approximate value of the plateau modulus can be obtained from the frequency at which a minimum in the loss tangent appears.34 These results confirm the importance of the temperature of emulsification on the emulsification process and, hence, on the stability, microstructure and rheological properties of the final emulsions.33,35 Aging Effect. Once the emulsification process was finished, both the DSD and viscoelastic properties of the emulsions were measured, as a function of aging, to get information about emulsion stability. As may be observed in Figure 9, the lack of any change in DSD indicates the absence of droplet coalescence after the emulsification process, provided that the emulsions prepared at high temperature were cooled inmediately after emulsification. Creaming was not detected either over the storage time scale (at least 1 month). However, a significant evolution of the mechanical spectrum, as a function of aging, takes place since the plateau region expands after emulsification, with a slight increase in the plateau modulus. This evolution, shown in Figure 10, seems to be restricted to the first few hours, since no further modification in G′ and G′′ takes place (34) Wu, S. J. Polym. Sci. 1989, 27, 723-741. (35) Barnes, H. A. Colloid Surf. A 1994, 91, 89-95.

Figure 9. Droplet size distribution curves, as a function of aging, for emulsions prepared at 250 rpm and 25 °C.

Figure 10. Evolution of the storage and loss moduli with frequency (T ) 25 °C) for emulsions prepared at 250 rpm, as a function of aging, at 25 °C (G′, solid symbols; G′′, open symbols).

after 8 h. These results are consistent with those reported by Kiosseoglou and Sherman36 which describe the development of three-dimensional network structures as a consequence of droplets flocculation. The above-mentioned effect responds to a mechanism of structural development, which does not modify the DSD obtained and is much (36) Kiosseoglou, V. D.; Sherman, P. J. Texture Studies 1983, 14, 397-417.

5416

Langmuir, Vol. 17, No. 18, 2001

slower than the emulsification process. This enhancement of the emulsion droplet network may be attributed to a reversible flocculation process. The addition of nonadsorbing macromolecules to protein-stabilized emulsions has been reported to cause reversible flocculation by a depletion mechanism.37 A critical concentration of the depleting agent is required in order to generate an osmotic driving force strong enough to induce flocculation. In fact, a mechanism of depletion-flocculation has been reported to occur when an excess of surfactant, in the form of micelles, is available in the continuous phase. The surfactant acts as both emulsifying agent and depleting agent.1 The emulsions studied in this paper seem to be in this case, since the concentration of surfactant is high enough to be in excess after stabilization of the oil-inwater. Similar results were recently reported for emulsions stabilized by sucrose palmitate.30,38 Depletion flocculation can lead to emulsion destabilization by creaming,39 However, as reported by Dickinson and Hong,40 occurrence of this mechanism in a concentrated emulsion leads to the formation of a weak gellike structure, which may be disrupted by mechanical agitation. On the other hand, the development of the plateau region has been also related to an increase in the stability of oil-in-water emulsions.41 Finally, it must be pointed out that the measurements of all the viscoelastic properties were carried out 24 h after sampling. As may be inferred from Figure 10, this aging time is long enough to reach a completely developed microstructure in the bulk emulsion. (37) Cao, Y.; Dickinson, E.; Wedlock, D. J. Food Hydrocolloids 1990, 4, 185-195. (38) Partal, P.; Guerrero, A.; Berjano, M.; Gallegos, C. JAOCS 1997, 74, 1203-1212. (39) Jenkins, P.; Snowden, M. Adv. Colloid Interface Sci. 1996, 68, 57-96. (40) Dickinson, E.; Hong, S. T. J. Agric. Food Chem. 1995, 43, 25602566. (41) Franco, J. M. 1995 Ph.D. Thesis. University of Seville, Seville.

Sa´ nchez et al.

Concluding Remarks All the phenomena noticed during the emulsification process studied (namely, first-order kinetic evolution of torque, induction time and preemulsification) may be explained in terms of two stages: breakup of droplets (counterbalanced by coalescence) and transport (and adsorption) of surfactant molecules to the oil/water interface to prevent coalescence. The evolution of the structural parameters (i.e., droplet size) and the linear viscoelastic properties largely depends on the emulsification temperature. In the low-temperature region, an increase in emulsification time yields a reduction in droplet size and polydispersity, showing unimodal DSD, as well as an extended plateau region, due to increasing interactions among droplets. However, a change from unimodal to bimodal DSD tends to be produced at high emulsification temperature because the coalescence of oil droplets is then favored. As a result, a minimum in droplet size and polydispersity is found at an intermediate temperature, which coincides with the widest plateau region. Consequently, the morphology of the final emulsion, immediately after processing, results from the dynamic equilibrium between droplet breakup and coalescence. A further enhancement of the emulsion droplet network takes place in a few hours after emulsification. An interpretation for this arrangement may be based on the development of a reversible depletion-flocculation process, which is much slower than the emulsification process itself. The surfactant molecules act both as emulsifying and depleting agents. Acknowledgment. This work is part of a research project sponsored by the CICYT, Spain (reference PB920664). The authors gratefully acknowledge its financial support. LA000723W