Langmuir 1997, 13, 3621-3626
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Concentration Profiles in Creaming Oil-in-Water Emulsion Layers Determined with Stray Field Magnetic Resonance Imaging B. Newling, P. M. Glover, J. L. Keddie,* D. M. Lane, and P. J. McDonald Department of Physics, University of Surrey, Guildford, Surrey GU2 5XH, United Kingdom Received October 17, 1996. In Final Form: April 2, 1997X We report the first use of stray field magnetic resonance imaging in the determination of concentration profiles in layers (with submillimeter thickness) of coarse oil-in-water emulsions that are undergoing creaming. We compare our results for emulsions having various oil contents to the predictions of a numerical model. In emulsions having low oil content (12 and 23 vol %), we find that the model adequately predicts the profile shape and time-dependent change in the lower region of the emulsion. In the cream layer at the top of each of the samples, however, the predictions of the model deviate substantially from the observed profiles. Whereas the model assumes that the concentration of oil in the cream layer is constant as the layer increases in thickness, we observe that there is a concentration gradient in the cream layer and that the concentration of oil in the layer increases with time. In explaining our findings, we consider the effects of polydispersity and the presence of gum xanthan in the continuous phase and also the possibility of gradual compaction of oil droplets in the cream layer (a phenomenon not considered in the model).
1. Introduction Creaming is the gravimetric separation of a dispersed phase from the continuous phase of an emulsion.1 Creaming of emulsions can be either problematic or useful. On the one hand, it is a destabilizing process that can reduce the shelf life of foods, pharmaceuticals, and cosmetics. But on the other hand, creaming can be exploited in industrial separation processes. In either case, full control of the process of creaming is not possible without an understanding of its time dependence. The simplest way of studying creaming is through visual observation of the movement of the interface between the cream layer and the continuous phase. This type of experiment can only determine the so-called creaming velocity, however, and does not provide any information about the concentration profiles that develop in the creaming emulsion. Intrusive techniques do provide information on concentration profiles, but they are time consuming and do not have high spatial resolution. For instance, samples from an emulsion can be removed from various heights, frozen, and examined with microscopy.2 Over the past 10 years, the propagation of ultrasound has been employed to determine concentration profiles in creaming oil-in-water (O/W) emulsions. The velocity of ultrasound through an emulsion will depend on the volume fraction of the dispersed phase if the droplet size is much smaller than the wavelength of the ultrasound. The technique has been applied to alkanes-in-water3-5 and to soybean oil-in-water emulsions,6 among others. Concentration profiles and creaming velocities were obtained. X
Theoretical work by Pinfield et al.7 has shown that scattering effects can be significant in cream layers in which the concentration of oil is high. The Urick equation,8 upon which the ultrasound analysis is based, is inadequate in interpreting ultrasound velocity measurements when scattering is not negligible, but there are frequencies above which the equation is valid for some systems. Additional experimental difficulties are encountered in obtaining reliability if air bubbles are not removed from the emulsion.9 Ultrasound measurements are typically obtained at a spacing of 1 mm,10 thereby limiting the number of readings attainable across a sample. Because of these difficulties and limitations with ultrasound experiments, there is clearly a need for other noninvasive techniques to provide detailed concentration profiles in creaming O/W emulsions. One such technique is magnetic resonance imaging (MRI), the potential of which has been demonstrated in previous creaming studies.11,12 MRI relies upon the application of a magnetic field gradient to impart a spatial dependence to the characteristic radiofrequency (rf) spectra of suitable nuclei in a magnetic field. Information about the distribution of MRI-active nuclei may be obtained from any material which is transparent to rf radiation, simply by rf irradiation and detection using a single coil (or probe), which is close to the sample. MRI is truly noninvasive (because rf radiation is nonionizing), and it may be applied to optically-opaque systems (including emulsions). A variety of physicochemical parameters may be quantified by the use of different forms of MRI interrogation; information may be deduced about molecular surroundings,13 reaction rates,14 temperature,15 diffusion, restricted diffusion, and coherent motion.16 Very
Abstract published in Advance ACS Abstracts, June 15, 1997.
(1) Dickinson, E. In Food StructuresIts Creation and Evaluation Blanshard, J. M. V., Mitchell, J. R., Eds.; Butterworths: London, 1988; p 41. (2) Reddy, S. R.; Fogler, H. S. J. Colloid Interface Sci. 1981, 82, 128. (3) Gouldby, S. J.; Gunning, P. A.; Hibberd D. J.; Robins, M. M. In Food Polymers, Gels and Colloids; Dickinson, E., Ed.; Royal Society of Chemistry: Cambridge, U.K., 1991; p 244. (4) Fillery-Travis, A. J.; Gunning, P. A.; Hibberd, D. J.; Robins, M. M. J. Colloid Interface Sci. 1993, 159, 189. (5) Dickinson, E.; Euston, S. R. In Food Polymers, Gels and Colloids; Dickinson, E., Ed.; Royal Society of Chemistry: Cambridge, U.K.; 1991; p 132. (6) Hibberd, D. J.; Howe, A. M.; Mackie, A. R.; Purdy, P. W.; Robins, M. M. In Food Emulsions and Foams; Dickinson, E., Ed.; Royal Society of Chemistry: London, 1987; p 219.
S0743-7463(96)01005-0 CCC: $14.00
(7) Pinfield, V. J.; Dickinson, E; Povey, M. J. J. Coll. Interf. Sci. 1994, 166, 363. (8) Urick, R. J. J. Appl. Phys. 1947, 18, 983. (9) Dickinson, E.; Stainsby, G. In Advances in Food Emulsions and Foams; Dickinson, E., Stainsby, G., Eds.; Elsevier Applied Science: London, 1988; p 1. (10) Cao, Y.; Dickinson, E.; Wedlock, D. J. Food Hydrocolloids 1991, 5, 443. (11) Kauten, R. J.; Maneval, J. E.; McCarthy, M. J. J. Food Sci. 1991, 56(3), 799. (12) Duce, S. L.; Amin, M. H. G.; Horsfield, M. A.; Tyszka, M.; Hall, L. D. Int. Dairy J. 1995, 5(4), 311. (13) Bloembergen, N.; Purcell, E. M.; Pound, R. V. Phys. Rev. 1946, 73, 679. (14) McConnell, H. J. Chem. Phys. 1958, 28, 430.
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large magnetic field gradients occur in the fringe field of a superconducting, MRI magnet and are employed in stray-field MRI (StraFI)17 to give high spatial resolution, particularly in or near solids. StraFI provides spatial information on both the concentration and the dynamics of materials studied. It has already been applied, for instance, to the study of solvent ingress into homopolymers and the accompanying polymer relaxation18 and to polymer penetration into porous materials.19 In this paper we describe the use of StraFI to determine concentration profiles during the creaming of a coarse O/W emulsion. Unlike ultrasound measurements, the StraFI technique is not subject to errors caused by scattering in higher concentrations of droplets (as the emulsion remains transparent to rf radiation). Thus, StraFI is suitable for the study of cream layers. Other forms of magnetic resonance imaging (including conventional MR microscopy16) are equally, if not better, suited to the study of the creaming process. StraFI, however, is a broad line method able to visualize both solids and liquids; it can, therefore, visualize cross-linking and solidification in appropriate emulsions. This capability influenced our choice of StraFI over the other alternatives in MRI for the present study. O/W emulsions are sometimes cast into layers or coatings. Examples include emulsions used in paints, photographic films, lubricants, foods, and wax coatings on floors. The study of films and layers presents a particular challenge, because most noninvasive techniques do not have sufficient resolution for this purpose. We report here the first use of a specialized StraFI planar film probe to study layers of O/W emulsions. The development of the film probe has been reported elsewhere.20 The probe’s resolution and limits depend on the material studied; it can potentially provide resolution on the order of 5 µm. In this work we have analyzed layers less than 600 µm thick. Although this current work is limited to the study of creaming in emulsions, creaming is only one of several processes that occur during the film formation of paints from alkyd emulsions. Other relevant processes are evaporation of water, absorption of water into the substrate, phase inversion, and the oxidative drying (or cross-linking) of the alkyd.21,22 The probe and technique described in this paper will therefore be very useful in future studies of such film formation. 2. Model of Creaming A survey of the approaches to predicting concentration profiles during the creaming of emulsions has been given elsewhere.7 Recently Pinfield et al.7 have presented a numerical model that predicts the concentration profiles in a polydisperse system undergoing creaming. This model ignores the effects of particle acceleration and inertia but instead uses the terminal velocity of the particles (the terminal Stokes’ velocity modified by the effect of steric hindrance due to the presence of surround(15) Doran, S. J.; Carpenter, T. A.; Hall, L. D. Rev. Sci. Instrum. 1994, 65(7), 2231. (16) Callaghan, P. T. Principles of Nuclear Magnetic Resonance Microscopy; Oxford University Press: Oxford, U.K., 1991. (17) Samoilenko, A. A.; Yu, D.; Artemov, L. A. JETP Lett. 1988, 47, 348. (18) Lane, D. M.; McDonald, P. J. Polymer 1997, 38, 2329. (19) Black, S. J.; Lane, D. M., McDonald, P. J.; Hannant, D. J.; Mulheron, M. J.; Hunter, G.; Jones, M. R. J. Mat. Sci. Lett. 1995, 14, 1175. (20) Glover, P. M.; McDonald, P. J. In Abstracts 28th Congress Ampere, 1-6 September 1996; Smith, M. E., Strange, J. H., Eds., University of Kent: Canterbury, U.K., 1996; p 141. (21) Ostberg, G.; Bergenstahl, B.; Hulden, M. J. Colloid Surf. A: Physiochem. Eng. Aspects 1995, 94, 161. (22) Ostberg, G.; Bergenstahl, B.; Sorenssen, K. J. Coatings Technol. 1992, 64, 33.
Newling et al.
ing particles) to describe their movement. The velocity of droplets as a function of concentration is described by an equation based on empirical and theoretical results,23 which gives the velocity of a particle relative to the fluid as
U)
(1 - φ) U (1 + φ)exp[5φ/3(1 - φ)] S
(1)
where US is the Stokes velocity and φ is the oil volume fraction. The effects of thermal diffusion are also considered through the inclusion of an effective diffusion coefficient for particles:
D ) S(φ)B(φ)(kT/6πηr)
(2)
where S(φ) is a thermodynamic particle interaction term and B(φ) is a hydrodynamic term equal to U/US. The final term in the expression is the Stokes-Einstein coefficient (including temperature, T, droplet radius, r, and viscosity of the continuous phase, η). As described elsewhere,7 the emulsion is divided into horizontal layers in the model. The size distribution of droplets is taken into account by division into discrete size fractions or bins. Movement of droplets of different sizes over a short time interval from each layer to the others is then calculated. The procedure is then repeated as appropriate. This model assumes that the cream layer contains a fixed concentration of one phase dispersed in another and that the cream layer thickness increases with time. As the model currently stands, the effects of coalescence, flocculation, electrostatic interactions, and droplet packing and deformation are not considered. In this current work, we compare the predictions of our implementation of the model to concentration profiles determined experimentally using StraFI. Our simulation program is based upon the work of Pinfield et al.,7 although it does not include their correction for the effect of thermal diffusion upon the position of the upper and lower limits for a given droplet-size range. 3. Experimental Procedure Coarse O/W emulsions were prepared from sunflower oil (obtained from J. Sainsbury plc) dispersed in deuterium oxide (obtained from Aldrich Chemical Co. and used as received). The use of deuterium oxide, instead of water, minimized the signal from the continuous phase during the StraFI investigation. A nonionic surfactant, Tween 80 (Atlas Chemical Industries) was used as the emulsifier. Because of the large droplets sizes and the large difference in density between the oil (0.9196 g/cm3) and deuterium oxide (1.107 g/cm3), creaming of the emulsion is quite rapid. In order to slow down the process to a time scale that is more practical experimentally, gum xanthan (practical grade, Aldrich) was used as a thickener. A stock solution of 0.149 wt % gum xanthan in deuterium oxide was first prepared. Surfactant was added to some of the solution and dissolved. Oil was then added to make an overall oil concentration of 39.5 wt % and an overall surfactant concentration of 0.60 wt %. The mixture was shaken by hand for 10 min to give a crude emulsion and shaken again thoroughly before analysis. The 39.5 wt % oil emulsion was diluted with stock solution to make samples at lower oil concentrations. Emulsions were analyzed soon after their preparation. Zero shear-rate viscosity of the continuous phase (0.149 wt % gum xanthan in deuterium oxide) at 20 °C was determined with a CSL250 Autogap Set instrument (TA Instruments, Leatherhead, UK) in controlled stress experiments. Additionally, creep tests were performed to determine the zero shear-rate viscosity. In this second type of measurement, compliance was measured as a function of time after imposing a shear stress of 0.05 Pa. Viscosity was determined by fitting to the Voigt model. (23) Barnea, E.; Mizrahi, J. Chem. Eng. J. 1973, 5, 171.
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We observed the droplet size distributions in each of the “parent” crude emulsions using optical microscopy. For this analysis, the emulsions were diluted to concentrations of about 10 vol % so that they were less opaque. Microscopy combined with image analysis has already been successfully employed by other workers to determine droplet size distributions in emulsions with fairly narrow distributions.24 In the emulsions studied here, there is a very broad distribution, and this distribution led to some inherent problems in microscopy measurements. At higher magnifications, where it is possible to count particles that are in the range of 1 µm, the observation is focused on small particles and larger ones are neglected. Furthermore, creaming during the measurement resulted in the droplet size distribution varying with the depth of focus of the microscope. In our initial observations, the number of larger particles was underestimated as a result of these experimental limitations. We subsequently used microscopy in a semiquantitative way to determine the range and mode of oil droplet sizes obtained in the emulsions. When using the StraFI film probe, emulsions were sealed between parallel glass cover slips, spaced at a distance of approximately 600 µm from each other. The cover slips were sealed around three of their edges with epoxy, and silicone grease (insoluble in the emulsion) was used to seal the fourth side after adding the emulsion. The sample holder was filled completely with the emulsion, so as to avoid the introduction of air bubbles. In our spectrometer, the z-axis (the vertical direction that is also the direction of the magnetic field gradient) is normal to the sample surface and, therefore, allows spatial resolution in the direction of changing oil concentration during the creaming process. The spectrometer operating frequency is stepped between each interrogation, which corresponds to excitation/ detection at different positions in the magnetic field gradient, allowing a 1H single acquisition z-direction profile to be built up in about 300 ms. Interrogation takes the form PX -τ-[PY-τecho detection-τ]n, where Pi is an excitatory rf pulse applied in one of two, orthogonal transmission channels (X or Y) and τ is a delay of 50 µs. Each pulse is 40 µs long and there are 8 (n) signal detection periods at each of 30 z-positions, centered around 225.6 MHz. That frequency is suitable for the detection of signal arising from 1H nuclei, which are principally contained in the sunflower oil (with a small fraction in the thickener and surfactant). Signal is not detected from the deuterium oxide continuous phase. The gradient strength is 58 T/m. Each frequency sweep is repeated 1400 times to improve the ratio of signal to noise, causing profile acquisition to take about 7 min overall. The rf probe itself is a tiny solenoid, which excites only a small area of sample adjacent to (and outside) its windings. Such restricted excitation reduces the problems associated with profile construction of layer samples, for which meniscus formation and inaccurate leveling can limit spatial resolution. The excited region of sample does not experience a uniform rf field, but rather one which falls away with increasing distance from the coil windings. A calibration profile of a rubber section is therefore collected and used to normalize each emulsion profile. Bulk analysis of creaming was carried out in larger samples (1.4 mm × 6 mm i.d. glass tubes) using a contrasting approach, in which the sample is moved relative to a (different) rf probe, and the spectrometer operating frequency is kept constant. The sample is moved through the probe using a stepper motor and is homogeneously excited at each position (τ is 14 µs, the pulses are 6 µs long, n is 16). This technique, therefore, becomes increasingly unsuitable for use in the study of thinner layers. Excitation and detection was in a much larger region of emulsion than with the alternative probe, however, and the consequent increase in signal-to-noise ratio allowed profile acquisition in 1 min (two measurement repeats). In both methods, multiple rf excitations and detections are carried out at each position in the sample to yield a train of n signal maxima. The decay of these maxima can yield information about the molecular mobility within a sample and, in the StraFI technique, are unusually insensitive to additional decay cause by diffusional motion (because τ is very short). Herein, however, the signal maxima are simply summed to increase signal-tonoise ratio in the reconstruction of profiles such as shown in Figures 2-5.
The flow of the gum xanthan solutions is clearly pseudoplastic. Both the controlled stress and the creep measurements on the solutions determined a value of zero shear-rate viscosity on the order of 1 Pa‚s, but the latter method was shown to be more reliable. We found a value of 1.74 Pa‚s with the creep measurements, and we applied this value in subsequent computer simulations. Our observations with optical microscopy revealed that oil droplet sizes varied from ∼1 to ∼100 µm. Figure 1 shows one of the log-normal distributions7 (with a mode of 60 µm) used to describe droplet size in our computer simulations. This distribution, which agrees qualitatively with our microscopic observations, is not unreasonable for an emulsion prepared by hand-shaking.25 Figures 2-5 show StraFI results for layers of emulsions containing 12.0, 23.4, 31.4, and 44.0 vol % oil, respectively. The vertical axes plot the concentration of oil and are calibrated according to the known oil volume fraction at the first time point (or an estimate of the same in circumstances when some creaming has already occurred at the first measurement, as in Figure 5). The horizontal axes show distance from the bottom of the emulsion. Note that the concentrations determined in these experiments
(24) Jokela, P.; Fletcher, P. D. I.; Aveyard, R.; Lu, J. R. J. Coll. Interf. Sci. 1990, 134, 417.
(25) Dickinson, E. An Introduction to Food Colloids; Oxford University Press: Oxford, U.K., 1992; p 117.
Figure 1. Typical oil droplet size distribution in O/W emulsion prepared by hand-shaking. The histogram shows the log-normal distribution used in the computer simulations to describe the distribution.
Figure 2. Concentration profiles in an O/W emulsion containing gum xanthan and 12 vol % oil as a function of time, determined with the StraFI film probe. Data, acquired over 7 min intervals, are shown with filled symbols. Solid lines show the corresponding simulated (instantaneous) profiles time for t ) 0 (horizontal line), 7, 14, and 21 min. The open circle shows the last data set at t ) 63 min.
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Figure 3. Concentration profiles in an O/W emulsion containing gum xanthan and 23.4 vol % oil as a function of time, determined with the StraFI film probe. Data, acquired over 7 min, are shown with filled symbols at 21 min intervals. Solid lines show the corresponding simulated (instantaneous) profiles at t ) 0, 21, 42, 63, and 84 min. The open circles show the last data set at t ) 119 min.
Figure 5. Concentration profiles in an O/W emulsion containing gum xanthan and 44.0 vol % oil as a function of time, determined with the StraFI film probe. Data, acquired over 7 min, are shown with symbols at 21 min intervals. Solid lines show the corresponding simulated (instantaneous) profiles in 21 min intervals, starting at t ) 0. The open circles show the last data set at t ) 119 min.
Figure 4. Concentration profiles in an O/W emulsion containing gum xanthan and 31.6 vol % oil as a function of time, determined with the StraFI film probe. Data, acquired over 7 min, are shown with filled symbols at 21 min intervals. Solid lines show the corresponding simulated (instantaneous) profiles in 21 min intervals, starting at t ) 0. The open circles show the last data set at t ) 119 min.
Figure 6. Concentration profiles in an O/W emulsion containing gum xanthan and 44.0 vol % oil as a function of time, determined in a bulk sample. Data are shown with filled symbols at 10 h intervals. The last two data sets (48 and 71 h) are shown with open circles and squares, respectively.
are time averages over the time interval of the measurement. Simulations of instantaneous profiles, predicted with the model using log-normal droplet size distributions and measured zero shear-rate viscosity, are shown on the figures as solid lines. In general, for emulsions of narrow droplet size distribution, the model predicts an upper and a lower meniscus. The upper meniscus separates the cream layer from the emulsion below it. The lower meniscus separates a region depleted of oil droplets from the emulsion above it. If the droplet size distribution is broad, then the meniscus becomes more diffuse. Furthermore, smaller droplets are subject to Brownian diffusion and likewise create a more diffuse meniscus. Experimentally, the concentration of oil tends to increase over 200 µm or more from the bottom of a layer that is 600 µm thick, as seen in Figures 2-5. A sharp meniscus is not observed in any of the emulsions studied here. For the emulsion with lowest oil content (Figure 2), the predictions of the model adequately agree with the experimentally-observed concentration profiles of oil in the dispersed phase in the lower region of the emulsion. In the cream layer at the top of the emulsion, however, there is a vast disagreement between simulation and experiment. The simulation assumes that the cream layer will maintain a constant oil concentration and that the
thickness of the cream layer will increase with time. Our data show instead that the experimentally-determined concentration of oil in the cream layer increases with time. The concentration of oil is not uniform in the cream layer; instead, the oil concentration is highest at the upper surface and decreases with increasing distance from that surface. At higher oil contents, particularly at 44 vol % oil (Figure 6), the fits of the data to the theoretical predictions are poor, both above and below the lower meniscus of the cream layer. Whereas at lower concentrations the general shape of the profile in the serum layer coincides with the simulation (Figures 2 and 3) even though there are differences in the kinetics of creaming, at higher concentrations there are significant differences between the predicted and experimental shapes of the profile (Figures 4 and 5). In simulations of creaming in the emulsion with the highest oil concentration (Figure 5), we obtain a better fit to the experimental data when a distribution with a mode of 100 µm is applied. A distribution with this mode, however, is not consistent with our observations. The experimental data from the emulsions with lower oil concentrations (Figures 2 and 3), in contrast, showed better agreement with the simulations when a more physically realistic distribution was used. Our StraFI analysis, under the conditions of these measurements, provides measurements of absolute con-
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Langmuir, Vol. 13, No. 14, 1997 3625
centration of oil in the cream layer with an accuracy of about 10%. Consequently, although it is apparent that the oil concentration in the cream increases with time, it is not possible to comment on the small differences between oil concentrations seen in the cream layers in Figures 2-4. The maximum oil concentration in the cream layer is between 45 and 55 vol % for these three samples. This value is lower than that expected for a random packing of monodisperse spheres (about 64 vol %).1 According to these concentration measurements, the oil droplets cannot be densely-packed in the cream. Of course, our emulsions are far from being monodisperse, and so the concentration of oil alone is not adequate to determine the arrangement of droplets in the cream layer. In the most concentrated emulsion (shown in Figure 5) the cream layer contains about 75 vol % oil. 5. Discussion Figures 2-5 show that the model used here was not able to adequately predict the observed concentration profiles, particularly in the cream layer and in emulsions with a high oil concentration. We now seek to explain this discrepancy between simulation and experiment. One possible cause is the presence of gum xanthan in the continuous phase. There have been several other studies on the effects of gum xanthan and other water-soluble polymers on creaming, as outlined below. The addition of water-soluble polymers to O/W emulsions can have one of two effects, depending on the polymer concentration, on the rate of creaming. At low levels of polymer, the creaming rate can be enhanced, as a result of depletion flocculation. The enhancement occurs because the effective droplet size increases. At higher concentrations, however, creaming is inhibited by the increased viscosity of the continuous phase.26 In fact, if there is a Bingham-like yield stress in a pseudoplastic continuous phase, creaming is prevented altogether.9 There is evidence5 that both of these effects can be observed when the anionic polysaccharide, gum xanthan, is present in the continuous phase. At low concentrations (0.05 wt %), gum xanthan was found to lead to depletion flocculation that resulted in enhanced creaming rates. At higher xanthan concentrations (greater than 0.25 wt %), creaming was essentially halted because of the thickening effect, i.e. increased viscosity of the continuous phase. At the intermediate concentration of 0.15 wt %, there was no observable effect on the creaming rate. There is other evidence,6 however, that gum xanthan leads to only temporary stability in emulsions and that flocculation eventually occurs. In the case of emulsions with higher oil concentrations (31.6 and 44.0 vol %), we observed creaming rates that are faster than predicted by our simulations but that correspond to an unrealistically large particle size distribution. This finding could be the result of flocculation of the droplets at higher concentrations. Alternatively, the concentration dependence of the particle velocity23 and effective diffusion coefficient used in the simulations could be less applicable in the relatively high oil concentrations studied. Apart from these effects on the rate of creaming, there is evidence that nonadsorbing, water-soluble polymers can alter the mechanism of creaming. For instance, the addition of (hydroxyethyl)cellulose was found to cause depletion flocculation that subsequently affected the density of the cream layer.4 Flocs consisting of clusters of droplets were believed to initially resist close-packing in the cream layer, leading to a lower density of oil in the (26) Dickinson, E.; Goller, M. I.; Wedlock, D. J. J. Coll. Interf. Sci. 1995, 172, 192.
Figure 7. Concentration profiles in an O/W emulsion containing no gum xanthan 44.0 vol % oil as a function of time, determined in a bulk sample. Data are shown with filled symbols at 1 min intervals. The last data set at 9 min is shown with open circles.
cream. In similar work,3 the extent of flocculation was found to be inversely related to the density of oil droplets in the cream layer. With passing time, however, the packing density of oil droplets increased, probably as a result of compaction and compression of the droplets under the influence of gravity. Thus, flocs do not initially pack densely together but droplets can rearrange gradually to create a closer packed structure. In the results presented here, we observe an increase in oil concentration in the cream layer with increasing time. We propose that this increasing cream density results primarily from the rearrangement of oil droplets into a closer-packed structure under the forces imposed by other droplets. We cannot entirely rule out the effects of coalescence, droplet deformation, and deemulsification. We do not observe, however, either with StraFI or visually, the formation of an oil layer, indicating that deemulsification, if it occurs, is not extensive. In order to explore the effects of the addition of gum xanthan on creaming in our polydisperse system, further StraFI experiments were conducted. Owing to the large difference in density between the oil and deuterium oxide and to the relatively large droplet size, creaming of the emulsions is very fast without the addition of gum xanthan. Consequently, we could not conduct experiments on xanthan-free emulsions using the film probe. Instead, we conducted bulk analysis, as already described. We observed the creaming process in emulsions with 0.149 wt % gum xanthan in the continuous phase, and we compared this to creaming in emulsions without any gum xanthan. Figures 6 and 7 show concentration profiles in two creaming emulsions, both of which contain 44 vol % oil. The emulsion containing xanthan, shown in Figure 6, creams in a manner similar to those observed with the film probe. There is a lower meniscus that separates a depleted region from the rest of the emulsion. This meniscus moves upward with time. The concentration of oil in the cream layer also increases gradually with time. There is a gradient in oil concentration across the cream layer, as was also observed in the films. After 71 h no changes were observed in the oil concentration profile. The maximum oil concentration attained in the cream layer is initially about 65 vol %, slightly more than found in the film samples. This concentration, in light of the broad droplet size distribution, indicates that the oil droplets are not densely packed. After about three days, however, the concentration of oil in the cream layer increases to over 70 vol %, which is roughly consistent
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with the concentration observed in the thin layer (Figure 5) near the completion of creaming. In the emulsion without added gum xanthan, shown in Figure 7, the creaming rate is much faster, as a result of lower viscosity in the continuous phase. After approximately 5 min, separation is complete and no further creaming is observed. Our microscopic analysis indicates that the distribution of droplet sizes is broader than in the emulsions containing gum xanthan. This distribution leads to the broader width of the meniscus. In stark comparison to what is observed in Figure 6, the maximum concentration of oil obtained in the cream layer is remarkably 89 vol %. There is no evidence for deemulsification, and so we hypothesize that the packing of droplets in this cream layer is extremely dense. In the absence of gum xanthan, it appears that the droplets pack much more densely. Volume fractions of oil above what is possible for the cubic closest packing of monodisperse spheres (0.74) can be obtained when there is high polydispersity, as in this emulsion, because smaller droplets can fill the interstices between larger droplets.1 These observations of the creaming in bulk emulsions shed some light on the creaming in the submillimeter emulsion layers. The presence of gum xanthan prevents the packing of oil droplets to maximum density, probably because it causes droplet flocculation, and the flocs create a more open, gellike structure. This low density of packing, however, allows for some rearrangement of the oil droplets, gradually leading to an increase in oil concentration in the cream. Because the model used here does not consider the effects of flocculation and droplet compaction, it is inadequate in predicting the changes that occur in the cream layer. As noted earlier, the faster than expected creaming rates in concentrated emulsions might likewise be the result of flocculation. We also note that the concentration of oil in the cream layers of bulk samples (containing 0.149 wt % gum xanthan) is higher than in the film samples. This difference might result from the compressive force generated in the thick cream layers being greater than in the submillimeter layers, as a result of the cumulative transfer of gravitational forces from droplets near the bottom of the cream layers to those near the top. The emulsions studied here have a broad droplet size distribution. This distribution could also contribute to discrepancies between experiment and model. Larger droplets, which cream at a faster rate, will initially constitute the cream layer. There will be large interstitial void space between the droplets. With time, smaller droplets might be able to move into this void space and thereby increase the overall packing density in the cream layer.1 Such a mechanism, which is not considered in the model, could explain some of our results, as follows. There is a distinct gradient in the oil concentrations within the cream layers. The oil concentration is highest at the top surface of the cream (the first part of the layer to form) and decreases in concentration lower in the sample. These gradients were observed in all of the emulsion samples (with and without gum xanthan, and in both bulk and film form, as already shown). The overall concentration of oil in the cream layer increases with time. The concentration gradients might be related to the droplet size distributions. At the top of the emulsions, smaller droplets might gradually fill the interstices between larger droplets, so that with passing time, a denser cream forms. (27) Ansell, G. C.; Dickinson, E. J. Chem. Phys. 1986, 85, 4079.
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At the lower side of the cream layer, a higher proportion of small droplets exist. Packing is not as efficient with these smaller, more monodisperse droplets, and so the cream in the lower region would be less dense. Droplet size polydispersity therefore might explain both the concentration gradients in the cream layer and the gradual increase in oil concentration with time. Future experiments using the pulse field gradient spin-echo measurements combined with MRI microscopy should be able to provide quantitative information about the spatial variation of droplet sizes within the cream layer. Future theoretical work might be able to refine predictions of creaming velocity and concentration profiles at high oil concentrations (as studied here) and to consider the effects of compaction of droplets. This work has isolated several advantages that StraFI has over other analytical techniques in the study of creaming (or sedimentation). It can be applied to coarse emulsions with a broad droplet size distribution, such as emulsions used in applications such as foods.25 There are no errors introduced by studying higher concentrations. Perhaps most importantly, it has a spatial resolution (∼5 µm) that is comparable to the droplet size in some emulsions. This feature opens up opportunities for comparing experimental systems with simulations of discrete particles undergoing such processes as flocculation, sedimentation, or creaming. For instance, Ansell and Dickinson27 have performed Brownian dynamics simulations of sediment profiles in systems consisting of up to 2500 particles. The differences in sediment profile occurring over distances of tens of particle diameters, which were predicted by their simulations, could be resolved using StraFI. 6. Summary We have used stray field magnetic resonance imaging to study composition profiles in thin layers of creaming oil-in-water emulsions with various oil concentrations. This noninvasive technique yields composition profiles of the oil with resolution up to about 5 µm. We have compared the experimentally-determined profiles to those predicted by simulations developed by Pinfield et al.,7 using experimentally-determined values for the viscosity of the continuous phase. The observed creaming can be adequately described by the model only at low concentrations of oil (12 and 23.4 vol %). In the cream layer and in emulsions with higher oil concentrations, there is a large discrepancy between simulation and experiment. The presence of gum xanthan in the continuous phase leads to lower concentrations of oil in the cream layer, probably because it causes flocculation, and a more open structure is formed from the flocs. The broad distribution of droplet sizes might contribute to the observed gradient in oil concentration within the cream layers. StraFI is capable of analysing concentrated and polydisperse systems at resolutions on the order of droplet sizes. Acknowledgment. We are grateful for funding for the StraFI facility from the Engineering and Physical Sciences Research Council. We gratefully acknowledge the assistance of P. Stevens in determining droplet size distributions. We thank P. Hodder of TA Instruments Ltd (Leatherhead, UK) for performing the rheological evaluation. We also appreciate the very helpful comments of one of the anonymous reviewers. LA9610056
