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Coalescence during Emulsification
Journal of Colloid and Interface Science 254, 165–174 (2002) doi:10.1006/jcis.2002.8561
Coalescence during Emulsification 3. Effect of Gelatin on Rupture and Coalescence Lloyd Lobo1 Eastman Kodak Company, Imaging Materials and Media, Research and Development, 1999 Lake Avenue, Rochester, New York 14650-2108 Received October 17, 2001; accepted June 27, 2002
An oil-soluble fluorescent probe, undecyl pyrene (UDP), is used to measure the amount of coalescence that occurs during the emulsification of tri-2-ethylhexyl phosphate using a high-pressure homogenizer. From these measurements, the roles of anionic surfactant (SDS) and gelatin in stabilizing drops against coalescence and promoting drop rupture during emulsification are deduced. It is found that gelatin aids in reducing coalescence, whereas SDS aids in rupture of drops. The effect of variables such as gelatin MW, surfactant type, and pH on coalescence and final drop size is investigated. C 2002 Elsevier Science (USA) Key Words: emulsification; gelatin; SDS; photographic dispersions; molecular weight; polymers.
INTRODUCTION
Gelatin is a well-known stabilizer of colloidal materials used in a variety of industrial applications. In the photographic industry, gelatin is used in the manufacture of many film components such as solid particle pigment dispersions, oil-in-water coupler dispersions, and silver halide emulsions. Gelatin not only provides long-term stability of the colloidal dispersion but also determines the morphology and dimensions of the colloidal particles during formation. Several studies address the adsorption behavior of gelatin and its conformation at interfaces. Howe (1) has provided a comprehensive summary of the research done in this area. Small angle neutron scattering and fluorescence labeling have been used to deduce the adsorption and conformation of gelatin adsorbed to organic surfaces (2, 3). The gelatin layer thickness obtained in these studies is closely matched to the values obtained from models using rheological data (4, 5). Because most photographic dispersions and coating solutions contain gelatin and anionic surfactants, the behavior of such systems is of interest. Studies describing the solution behavior of these systems are listed in Ref. (1). At the air–water interface, Cooke et al. have carried out adsorption measurements (6), while Pitt (7) 1 Present address: NexPress Solutions LLC, 2600 Manitou Rd., Rochester, NY 14624. Fax: (585) 613-2105. E-mail: lloyd [email protected].
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has studied the dynamic surface tension. At the oil–water interface, which is of relevance to this study, Muller et al. (8, 9) have studied the adsorption of surfactant and gelatin. Howe et al. (4) characterized oil-in-water emulsions containing gelatin and anionic surfactants by measuring the adsorbed amounts of the individual species and their role in modifying the rheological properties of the emulsions. Although emulsification with gelatin as a stabilizer is widely practiced, there are not many studies published on the role of gelatin in emulsification. Dickinson and co-workers (10, 11) studied the emulsifying behavior of gelatin using a jet homogenizer. They found that gelatin yielded large oil drops compared with other proteins such as sodium caseinate and that the drop size went through a minimum with the gelatin concentration. Muller et al. (9) have studied the emulsification of mixtures of gelatin and sodium dodecyl benzene sulfonate (SDBS) solutions with polar versus nonpolar oils. They observed a general result that polar oils that are typically used in the photographic industry show a smaller drop size than the nonpolar oils, when a high-pressure homogenizer is used. They attributed this dependence to the lower interfacial tension of polar oils. Toledano and Magdassi (12) have studied the emulsifying and foaming properties of hydrophobically modified gelatin. They found that the hydrophobic groups enhance adsorption at the oil/water interface and decrease in the drop size, for stirred systems. There are many interesting facets to the properties of gelatin, which can affect its efficacy as an emulsifier: Interfacial tension: Although gelatin adsorbs strongly to an interface, it shows modest reduction in surface tension (68 dyn/cm to about 62 dyn/cm at 35◦ C), when compared to anionic surfactants (25 to 30 dyn/cm) (13, 14). However, because of its relatively high molecular weight it is not obvious if it can adsorb to surfaces on the short time scales that surfaces are formed in a homogenizer. Turbulence suppression: Polymers are known to alter the flow fields at the dimensions of the smaller eddies (15), and thus gelatin has the potential to alter the rupture mechanism. Sensitivity to pH: Gelatin is a polyampholyte whose charge depends on pH, and the pH affects the gelatin binding as well as its conformation at an interface (2, 5), particularly in the presence of anionic surfactant (9). Interaction with surfactants: While anionic 0021-9797/02 $35.00
C 2002 Elsevier Science (USA)
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surfactants are reported to increase the gelatin adsorption below their cmc’s (9), nonionic surfactants can displace gelatin from the surface (16, 17). In solutions, Greener has shown that the viscosity and gelatin conformation will change because of the interaction with surfactant (18), which can alter the fluid dynamics during homogenization. Molecular weight: Gelatin obtained from a typical manufacturing process is a mixture of collagenlike molecules with MW ranging from 10,000 to 1,000,000. Depending on the point of extraction in the process the weight average molecular weight can range from 100,000 to 200,000. The MWD can affect the flow field at the small scale, the viscous dissipation in the bulk solution, and the thickness of the adsorbed gelatin shell. The interaction with surfactants can also be affected. Olijve (19) has studied the effects of gelatin MW on its interaction with SDBS and its role on emulsion stability properties. He concludes that the low-MW gelatin forms more surface-active complexes with surfactants and is therefore able to stabilize emulsions more effectively. In this study, we relate some of the above-mentioned properties of gelatin to its role in the emulsification process. In Part 1 of this series (20) we have described a method that uses an excimer-forming oil soluble fluorescent probe that monitors the amount of coalescence induced mixing. The probe, at a high concentration, is introduced in a fraction of the drops before emulsification. The ratio of the excimer peak intensity to the monomer peak intensity (E/M) is monitored. Because this ratio is proportional to the probe concentration in the oil phase, random mixing of the contents of the oil drops, caused by coalescence, results in an effective dilution of the probe and a decrease in the ratio. The change in the value of E/M, during the emulsification process, is an indication of the amount of coalescence that has occurred. The quantitative relation between the signal change and the degree of coalescence induced mixing has been obtained via Monte Carlo simulation as discussed in Part 1. The simulation shows that this signal is not affected by the drop size or the polydispersity of the distribution. We have used the method in this report to measure the amount of coalescence and to infer the relative importance of the role of gelatin in the rupture and stabilization (to coalescence) during emulsification. The effect of gelatin concentration is first studied for pure gelatin solutions and then for gelatin/SDS mixtures. The effect of anionic vs nonionic surfactants in the presence of gelatin is discussed next. The role of the MW of gelatin on its emulsifying properties is also investigated. Finally, the role of pH for two gelatins having different isoelectric points (iep) is studied in the absence and presence of surfactant. EXPERIMENTAL
Materials The oil phase in these experiments was tri-2-ethylhexyl phosphate (TEP), obtained from Eastman Chemicals. It is one of the materials that are used to prepare photographic coupler dispersions. The anionic surfactants used were sodium dodecyl sulfate
and Alkanol-XC a commercial surfactant made by DuPont—a mixture of di- and triisopropylnaphthalene sulfonate. The nonionic surfactant used in this study was the ethoxylated alcohol 10 lauryl ether obtained from Sigma. Type IV gelatin was used in sample preparation. Because of the relatively high viscosity of the oil phase, the more efficient undecyl pyrene (UDP) was used as the oil soluble fluorescent probe. The reasons for the choice and the synthesis procedure of the probe are described in part I of the series (20). Gelatin Several varieties of gelatin were used in this study. The basic Type IV Lime Processed Ossein (LPO), with a mean MW of 170,000 and iep of about 4.9, was obtained from Eastman Gelatine. The acid-processed ossein (APO) gelatin, with an iep of 7.0 and a MW of about 130,000, was obtained from Croda. Three other gelatins with varying MW were prepared by enzymatic hydrolysis of a high-MW type IV gelatin obtained from Eastman Gelatine. The proteolytic enzyme Neutrase was made by Novo Nordisk. A 20% gelatin solution with an initial viscosity of 320 cp (at 50◦ C) was hydrolyzed at 50◦ C by adding the enzyme broth to the solution at a concentration of 100 ppm. When the temporally monitored viscosity reached a predetermined value, the solution temperature was raised to 80◦ C and kept at that temperature for 10 min, to irreversibly denature the enzyme and stop the reaction. The final viscosities of the three hydrolyzed gelatin solutions were 92 cp, 29.5 cp, and 8 cp, measured at 50◦ C. The gelatin content of the final solutions was measured by drying samples of the gels in an oven. Molecular Weight Measurement Absolute molecular weight determinations of gelatin were made using size-exclusion chromatography (SEC) with twoangle laser light scattering (TALLS) detection using a mobile phase of aqueous 0.25 M calcium chloride and 5% by volume ethylene glycol. Two SUP8103R-LMB PSS Suprema mixed bed columns were used for the separation. The absolute weight average molecular weight Mw was determined by integration of the light-scattering chromatogram from the 15◦ forward angle signal without using the differential refractometer (DRI) signal (21). Emulsification Experiments A Microfluidics microfluidizer was used as a high-pressure homogenizer. All the experiments were carried out at 45◦ C by immersing the microfluidizer into a water bath. In a microfluidizer, a coarse emulsion, which is the feed, is separated into two fluid streams that are accelerated using high pressures. The two fluid streams are impinged upon each other in an interaction zone, where emulsification occurs. Two coarse emulsions (called premixes) were made with a Silverson rotor-stator device at a stirring speed of 7000 rpm. Each premix was identical, except that one of them had the UDP at a level of 2% in the oil phase. Samples were taken of each premix. The premixes
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Samples of the emulsions, taken during the emulsification experiment, were stabilized by mixing them with a warm SDS/gelatin mixture, and stirred in an ice bath so that the gelatin sets into a gel. The samples were stored in a refrigerator until measurements were made (within 24 h). The final composition of all samples was 1.25% oil, 2.2% gelatin, and 1% SDS. In Part 1, we have shown that this treatment of the sample is able to arrest the coalescence process and in addition, does not result in change in the fluorescence signal because of diffusion transport of the probe via the aqueous phase. Fluorescence Measurements Fluorescence measurements were made on a SPEX Fluorolog spectrofluorometer. Because we are measuring fluorescence of dispersed drops, front face acquisition2 is used to obtain the emission. Excitation was at 340 nm. Emission acquisition was between 370 and 600 nm. The two monomer peaks monitored were the M1 at 376 nm and M2 at 397 nm. The excimer peak was measured around 477 nm. All the data are reported using the ratio of the excimer (E) and monomer (M2) as the E/M2 signal. Because all emulsion samples contained 2% gelatin and 1.25% oil phase the contribution of the fluorescence signal from gelatin was the same across samples. For the probe concentration (2%) and mix ratio (1 : 9) chosen, the maximum E/M2 signal is close to 0.95, while complete mixing due to coalescence will reduce the signal to about 0.1. Based on the Monte Carlo simulations discussed in Part 1 of this series (20), E/M2 reaches its lowest value when each drop experiences approximately two coalescence events. Data showing the changes in E/M2, will be referred to as the coalescence data.
RESULTS AND DISCUSSIONS
Gelatin with Anionic Surfactants Emulsions were prepared in which the gelatin concentrations in the aqueous phase ranged from 0.05 to 8 wt%. Type IV gelatin, with a weight average MW of 170,000, was used in all the experiments. Three series of experiments were done, each at different levels of SDS—0, 8 × 10−4 , and 1.6 × 10−2 M in the aqueous phase. Figure 1 shows the change in the fluorescence signal as a function of the gelatin concentration at the three
1 0.9 0.8 0.7
E M2
Sample Stabilization
FL-70 (obtained from Fisher Scientific) with 0.01% sodium azide to minimize bacterial growth. The average drop size data in this report uses the third moment of the distribution—the volume average mean.
0.6 0.5 0.4 0.3
pass 1 pass 2 pass 4 pass 6 pass 8 pass 10
a) no surfactant
0.2 0.1 0 0.01
0.9 0.8
0.1
1
10
gelatin conc. (wt%)
b) 0.8 mM SDS
0.7 0.6
E M2
were subsequently mixed in a ratio of 1 : 9 (1 part of the emulsion with the probe). The mix was sampled prior to passing it through the microfluidizer at a pressure of 7000 psi. The emulsion was passed through sequentially, 10 times, with samples being taken after each pass and stabilized with SDS/gelatin solution. Selected samples were measured for fluorescence. The drop sizes of the premixes were measured by either the Coulter counter or the CFFFF. In all the emulsification experiments, the oil fraction was maintained at 8 wt% (8.6 vol%).
0.5 0.4 0.3 0.2 0.1 0 0.01
0.1
1
10
gelatin conc. (wt%) 0.9 0.8
Drop Size Measurements
c) 16 mM SDS
0.7
2
Front face acquisition is where the excitation beam incidence and emission collection are at the same face of the spectrophotometric cell. The acquired signal is only from the part of the sample adjacent to the cell wall (2-dimensional collection). While this method of measurement presents some optical artifacts, it does not affect the type of signal (E/M) used in this study.
E M2
0.6
Coarse emulsions with drop sizes greater than a micrometer were measured using a Coulter counter—Coulter Multisizer II. The smaller emulsions were sized using a cross flow field flow fractionation (CFFFF) instrument, Model F-1000, made by FFFractionation, LLC. The eluent used was 0.1% surfactant
0.5 0.4 0.3 0.2 0.1 0 0.01
0.1
1
10
gelatin conc. (wt%)
FIG. 1. Coalescence in a microfluidizer as a function of gelatin concentration. SDS concentration in (a) is 0; in (b) it is 8 × 10−4 M, and in (c) it is 1.6 × 10−2 M.
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a) 0.8 mM SDS drop size ( m)
0.45 0.4 0.35 0.3 0.25
1st pass 2n pass 4th pass 6th pass 8th pass 10th pass
0.2 0.01
0.1
1
10
1
10
gel conc. (wt%) 0.5
drop size ( m)
b) 16 mM SDS 0.45 0.4 0.35 0.3 0.25 0.2 0.01
0.1
gel conc. (wt%)
FIG. 2. Drop size as a function of gelatin concentration: (a) 8 × 10−4 M SDS, (b) 1.6 × 10−2 M SDS.
different SDS levels. In each of the plots, data are presented for the 1st, 2nd, 4th, 6th, and 10th passes through the homogenizer. Figure 2 shows the drop size data obtained in these experiments. The drop size data for emulsions made in the absence of SDS is not shown, because the drop sizes varied by over an order of magnitude and it was not possible to capture the data accurately. As mentioned earlier, the degree of coalescence is proportional to the drop in the fluorescence signal (0.95 in the absence of coalescence). In the absence of surfactant (Fig. 1a), we see a steep dependence of coalescence on the gelatin level. At 8 wt%, gelatin stabilizes the emulsion adequately—based on our simulations in Part 1, the fluorescence signal reaches its lowest value when the average number of coalescence events per drop is 2. At the low SDS concentration (Fig. 1b), because of the stability afforded by the SDS itself, gelatin concentration has a smaller impact on coalescence. At the high surfactant concentration, addition of low levels of gelatin shows a slight increase in stability, relative to the emulsion made with SDS alone (0% gelatin is shown on the log–log graph as 0.01 wt% gelatin). Above 0.05%, gelatin concentration has relatively little influence on the coalescence. The decrease in coalescence with increasing gelatin concentration is similar to the effect seen with changing SDS concentration, discussed in Part 2 of this series (22). However, the reason for this dependence is different in this case. In the case of SDS alone, the surfactant surface coverage increases, up to the cmc. However, in the case of gelatin, the saturation coverage of gelatin is reached at very low levels of gelatin (3, 8). By using
the drop size data (Fig. 2) and a value for saturation coverage of 4.0 mg/m2 of gelatin (4, 3), the amount of gelatin binding and the depletion from solution can be calculated (a drop diameter of 0.75 µm was estimated for the no surfactant case). We estimate that, in the absence of SDS, gelatin depletion was occurring when the total gelatin concentration was below 0.1 wt%. At the two levels of SDS studied here, because of the smaller drop sizes, the gelatin depletion occurred up to 0.5% gelatin. At concentrations below which gelatin saturation is achieved, flocculation is known to occur. Thus, the increased coalescence at the low gelatin concentration can be explained as due to gelatin depletion. At the higher gelatin concentrations, depletion cannot explain the stability dependence on gelatin concentration. The increased stability probably has to do with the dynamics in the homogenizer. At the short time scales, related to the time scale of forming new surface, the amount of gelatin absorbed will be dependant on the gelatin concentration in the aqueous phase (based on diffusion/adsorption kinetics). The results obtained with gelatin/SDS systems should be viewed in light of the interactions between the two species. Based on the investigations of Whitesides and Miller (13), as gelatin is added to a solution of SDS, which is above the cmc, the micelles bind on to gelatin at a specific stoichiometry with a maximum of 5 micelles per average gelatin molecule (molecular weight of 100,000). Thus, at high SDS-to-gelatin ratios, free (unbound) micelles exist and the gelatin/micelle complex has a low surface activity relative to that of free gelatin and free SDS. However, for a fixed SDS level, when the gelatin level is increased beyond the stoichiometric maximum for bound micelles (when there are no free micelles), there is an increased amount of gelatin binding at the interfaces (see Ref. (8)). Based on the data of Whitesides and Miller, it is calculated that at the high SDS concentration (16 mM), free micelles exist below gelatin concentrations of 1 wt%. However, we are always above the cmc of SDS in gelatin (1mM). The stability obtained because of the free SDS is high enough that the addition of gelatin does not affect it. At the low SDS concentration (0.8 mM), we are below the cmc of free SDS (8 mM) as well as below the cmc of SDS in the presence of gelatin. Therefore, the addition of gelatin results in the formation complexes of single SDS molecules and submicellar species onto gelatin, which are surface active. Thus, at SDS levels below the cmc, the addition of gelatin, even at low levels, results in increased gelatin being adsorbed at the interface. At the lower level SDS is not adequate at providing stability and, thus, the adsorbed gelatin contributes to the stability against coalescence. Because of large drop size and difficulty in measurement, drop size data is not shown for the surfactant-free system. However, the qualitative result that can be stated is that the drop sizes, in the absence of any surfactant, were larger than that obtained at both levels of SDS, when the data was compared for the same gelatin concentration. At the low SDS level (Fig. 2a), the drop size dependency with gelatin concentration changes, depending on the number of passes through the homogenizer.
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1 0.9
a
0.8
E M2
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0
2
4
6
8
10
12
number of passes
0.4
a) 0.05 wt% gelatin
0.8
no surf. 0.5% SDS 2.0% SDS 0.5% AXC 2.0% AXC
1 0.9
drop size ( m)
For the first and second pass, where there is relatively less coalescence, there are relatively small changes in drop size. For the later passes the drop sizes decrease with gelatin concentration, which can be attributed to attenuation in the amount of coalescence by the gelatin concentration (seen in Fig. 1). At the high SDS level (Fig. 2b), smaller drops are obtained. In this case, gelatin concentration does not affect the coalescence behavior, but drop sizes decrease on increasing the gelatin concentration beyond 1 wt%, suggesting that increasing gelatin concentration enhances rupture. In studies with a similar homogenizer Muller et al. (9) obtained a similar result as reported here—the drop size decreases with increased surfactant concentration. Figure 3 shows the above coalescence data plotted to observe the effect of surfactant concentration. Data are selectively presented for three gelatin levels. At each gelatin level, the effect of changing the surfactant level is shown. At the low gelatin
b
0.35 0.3 0.25 0.2
E M2
0.7
0.15
0.6
0
0.5 0.4
2
4
6
8
10
12
number of passes
0.3 0.2
FIG. 4. Coalescence (a) and drop size (b) as functions of anionic surfactant concentration with 8 wt% gelatin in the aqueous phase.
0.1 0 0
2
4
6
8
10
12
E M2
number of passes 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
b) 1.0 wt% gelatin
0
2
4
6
8
10
12
10
12
E M2
number of passes 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
c) 8.0 wt% gelatin
0
2
4
6
8
number of passes
FIG. 3. Coalescence as a function of number of passes through the homogenizer: (a) 0.05%, (b) 1%, and (c) 8% gelatin. SDS concentration is 0 (diamonds), 0.8 mM (squares), and 16 mM (triangles).
level (0.05 wt%), gelatin by itself is inadequate at protecting against coalescence. Thus, increasing levels of surfactant results in increased stability. At the intermediate gelatin level (1 wt%), the emulsions made without SDS and the ones made at a low SDS level, behave similarly, while the high SDS level shows increased stability. At the high gelatin level (8%), the emulsion without any surfactant is the most stable, while the high SDS level shows definitely lower stability. In order to verify the generality of this finding, another experiment was carried out using two different anionic surfactants—SDS and AlkanolXC. The cmc of Alkanol-XC, in the absence of gelatin, is about 0.38 wt% versus 0.23 wt% for SDS. In the presence of gelatin the cmc is about 0.04% for Alkanol-XC and 0.03% for SDS.3 Figure 4 shows the amount of coalescence that occurs and the drop size change, with the number of passes through the homogenizer, with the varying levels of anionic surfactant and with 8% gelatin. It is clearly seen that increasing the amount of anionic surfactant decreases the fluorescent signal, suggesting increased coalescence. Using a value of max of 3.16 µmol/m2 for both surfactants (based on the data for SDS obtained in Part 2 (22)), the value of unbound surfactant was estimated. The resulting concentrations of surfactant in solution obtained are well above the cmc of the respective surfactant, in the presence of gelatin. Muller et al. (8) have shown that the amount 3
T. Whitesides, unpublished results.
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Gelatin with Nonionic Surfactants The ethoxylated alcohol C12E10 was used for this study. The interfacial tension for a 1 wt% surfactant, 2 wt% gelatin TEP interface was 2.96 dyn/cm versus 13.2 dyn/cm for a 2% gelatin solution. The cmc for this surfactant is about 1 × 10−4 M or 0.007 wt%. The emulsification experiment was carried out with the same oil phase composition and homogenization procedure described above. The gelatin concentration in the aqueous phase was 8 wt% and the two levels of nonionic surfactant was used 0.01 wt% (slightly above the cmc) and 0.4 wt%. As a comparison the data with 8% gelatin and 0.4% SDS was also generated. Figures 5a and 5b show the coalescence data and the drop size data, respectively, for these emulsions. At the low level of nonionic surfactant, the coalescence behavior is similar to that of the emulsion made with gelatin alone. At 0.4 wt% nonionic surfactant, there is substantially more coalescence and the fluorescence signal is lost after the first pass. In comparison, the SDS/gelatin emulsion shows much less coalescence, although, as before, we see that the coalescence is greater than in the pure gelatin case. On comparing the drop size data, we see that the emulsion with
1.2
no surfactant
a
0.4wt% SDS
1
0.01wt% C12E10 E M2
0.8
0.4wt% C12E10
0.6 0.4 0.2 0 0
2
4
6
8
10
number of passes 0.55
drop size ( m)
of adsorbed gelatin and the thickness of the adsorbed gelatin layer, increases and then decreases as the amount of anionic surfactant is increased. The position of the maximum occurs at or around the cmc of the surfactant, in the presence of gelatin. Similar results have been obtained at the air–water interface (6, 23). The decrease in the adsorbed gelatin above the cmc can be used to explain the increase in coalescence with surfactant concentration. The drop size data (Fig. 4b) shows that even though the amount of coalescence increases with increasing surfactant, the drop sizes also goes down, suggesting that increasing surfactant concentration enhances the rupture. One possible explanation is that the dynamic interfacial tension decreases with increased surfactant concentration, which can facilitate rupture, as is posited by Muller et al. (9). However, in Part 2 (20) we have shown, by inference, that when the coalescence of two emulsions is comparable, the variation in interfacial tension does not affect the drop size. That is, the rupture does not depend too strongly on the interfacial tension. In this case, the aqueous phase also has gelatin. Because polymers affect the modes of viscous dissipation in a turbulent field, the mode of rupture can be different. Therefore, surfactant level (above cmc) can have different possible impacts-(a) Surfactant affects the gelatin conformation and, consequently, the viscosity of the solution (2, 18). Therefore, they can alter the momentum transfer to the drops themselves (via changes in the eddy formation). (b) If the fluid flow is substantially different in the presence of polymer, the interfacial tension may now become important to the rupture process (as we have shown in Part 2, there are different dependencies for a stirred tank versus a high-pressure homogenizer). However, it is hard to point out the exact mechanism that can explain why the drops get smaller as the surfactant concentration increases.
b
0.5 0.45 0.4 0.35 0.3 0
1
2
3
4
5
6
7
8
9
10
number of passes FIG. 5. Coalescence (a) and drop size (b) with 8% gelatin and varying levels of nonionic surfactant C12E10.
the high level of C12E10 has the largest drop size. The low level of C12E10 has the same drop size as the pure gelatin system— coinciding with the coalescence data. As before, the emulsion with the SDS has the smallest drop size. The effect of the nonionic surfactant on coalescence can be explained based on the findings of Chen and Dickinson (17). They found that addition of the nonionic surfactant Tween20 to a protein-stabilized emulsion (gelatin or β-lactoglubulin) resulted in displacement of the proteins from the oil–water interface. For a gelatin-stabilized emulsion, it took about 0.4 wt% of Tween 20 to displace the gelatin completely. This behavior is unlike that observed with gelatin and anionic surfactants (8) discussed earlier. The reason for this difference is that nonionic surfactants have shown no interactions with gelatin, as inferred from surface tension and viscosity measurements (18, 24). Bagchi et al. (16) have also shown that addition of nonionic surfactant to a gelatin-containing dispersion can substantially reduce its viscosity by displacing the thick adsorbed gelatin shell. However, it is not clear from either work why the surfactant concentration needs to be several times above the cmc in order to cause gelatin desorption. There are two possible explanations: (1) The interaction between gelatin and nonionic surfactant is not zero, but is much weaker than in the anionic surfactant case. (2) At the low surfactant concentration, depletion of the nonionic surfactant, caused by adsorption at the drop surfaces, reduces its actual concentration to below the cmc. At this point, we are not sure which of the two factors is operating. However, it is clear
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TABLE 1 Mean Molecular Weight (Mw ) of the Gelatins Used for Emulsification
1.4
gel 4
1.2
gel 3
1
gel 2
Gel i.d.
Mw
1 2 3 4
488,000 126,000 83,500 22,700
0.8
gel 1 0.6 0.4 0.2
4
4.5
5
5.5
6
6.5
7
log M
FIG. 6.
MWD of gelatins used for emulsification.
that in the presence of gelatin, the nonionic surfactant that is able to lower the interfacial tension substantially is not capable of stabilizing the drops and, therefore, not able to produce small drop sized emulsions.
comes important and the influence of the gelatin MW on stability will be diminished. There is not much difference between the coalescence of emulsions containing 8% gelatin versus 2% gelatin in the presence of SDS. It is useful to compare the coalescence data obtained by Olijve (19) with tricresyl phosphate emulsions made with varying gelatin MW. The emulsions were prepared with gelatins whose MW varied in a range similar to that in the
4
A. Howe, unpublished results.
0.5
a
0.4 0.3 0.2 0.1 0 0
2
4
6
8
10
12
E M2
number of passes 1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
0.5
b
0.4 0.3 0.2 0.1 0 0
2
4
6
8
10
12
number of passes
E M2
Four gelatins having different MW were prepared as described in the experimental section. The molecular weight distributions of the gelatins are shown in Fig. 6. The mean MW of each gelatin is given in Table 1. The oil phase fraction and the homogenization conditions were similar to the experiments described above. Three emulsions were made with each gelatin type—(a) 8% gelatin, (b) 8% gelatin + 16 mM SDS, and (c) 2% + 16 mM SDS gelatin. Figure 7 shows the coalescence and the drop sizes obtained for the emulsions made with the four different gelatins. In the absence of surfactant (a), the stability decreases (coalescence increases) in proportion to the decrease in the MW of the gelatin. It has been found that the thickness of the adsorbed gelatin shell decreases with the MW of the gelatin used4 , and the stability against coalescence decreases as a direct consequence of this. As was postulated in Part 2 of this series (22), the collisions between drops in a homogenizer results in very short times of contact, and the mechanism that is responsible for stability is more likely due to the resistance to the drainage of the intervening thin film. The two factors that affect the thin film drainage for gelatin are the continuous phase viscosity and the interfacial viscosity; the latter is related to the adsorbed layer thickness (25). Both the viscosities will increase with increasing MW of the gelatin. With the emulsions made with 8% and 2% gelatin with 0.46% SDS, a similar trend is observed—the coalescence increases with decreasing MW of the gelatin. However, the differences between the gelatins are smaller than in the absence of surfactant. This is because at these high levels of SDS, some of the gelatin has been displaced. The role of SDS as a stabilizer be-
E M2
Gelatin Molecular Weight
1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
drop size ( m)
3.5
drop size ( m)
3
1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
gel 1 Gel 2 gel 3 gel 4
c
0
2
4
6
8
10
0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0
drop size ( m)
0
12
number of passes
FIG. 7. Coalescence (closed symbols) and drop sizes (open symbols) obtained with varying MW gelatins: (a) 8% gelatin, (b) 8% gelatin + 16 mM SDS, (c) 2% gelatin + 16 mM SDS. The data are represented by diamonds for gel1, squares for gel2, triangles for gel3, and circles for gel4.
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present study, at a concentration of 0.5 wt%, and the change in the drop size was measured when the emulsions were stored at 40◦ C over 4 h. He found that the drop size change (allegedly caused by coalescence) decreased with decreasing gelatin MW, implying the low-MW gelatins were better at stabilizing against coalescence. This is exactly opposite to the data reported here, the difference being in the higher collision forces occurring during an emulsification process, compared to the static condition. Another difference is in the gelatin concentrations used in the two studies, although both use concentrations above the overlap concentration of gelatin. Furthermore, tricresyl phosphate has some solubility in water, and its emulsions are prone to Ostwald ripening, which can confound the results in long term studies. The drop size data shows that the drop sizes of the emulsions are very dependent on the MW of the gelatin. In all the three emulsion formulations, the drop sizes decrease with decreasing MW, except for the lowest MW. Whereas coalescence also increases with decreasing gelatin MW, it can be confidently inferred that the decrease in drop size can be attributed to increased efficiency in drop rupture as the MW of the gelatin decreases. Moreover, the attenuation of the drop size with the MW is the maximum for the emulsions made with 8% gelatin, in the absence of surfactant. To get a better picture of the relative effects of the MW and the level of SDS, the coalescence and drop size data are plotted in Figs. 8a and 8b after the second pass through the homogenizer. For the three emulsion formulations, the fluorescence signal decreases monotonically with the MW of the
0.8 0.7
a
8% gelatin + 16mM SDS
0.6
E M2
8% gelatin
0.5
2% gelatin + 16mM SDS
0.4 0.3 0.2 0.1 0 10000
100000
1000000
MW 0.48
drop size( m)
0.46
8% gelatin
b
0.44 0.42 0.4
8% gelatin + 16mM SDS 2%gel + 16mM SDS
0.38 0.36 0.34 0.32 0.3
10000
100000
1000000
MW
FIG. 8. (a) Coalescence and (b) drop sizes of emulsions as a function of the gelatin MW, after the second pass through the homogenizer.
gelatin, whereas the drop size shows a minimum around a MW of 80,000. The size increase at the lowest MW is probably not a reflection of the coalescence, because the fluorescence data is similar at the two lower values of MW. Olijve (19) found that the surface tension of SDBS/gelatin mixtures decreases with decreasing gelatin MW (the SDBS complexes with lower MW gelatin are more surface active than the complexes with higher MW gelatin) at low surfactant concentrations. At the higher surfactant concentrations, he found the surface tension isotherms for the different MW gelatins coincide. In this study, the surfactant concentration used is high and it was verified that the gelatin MW had no impact on the static interfacial tensions, in the presence or absence of surfactant. Thus, interfacial tension cannot be used to explain the drop size results. This data suggests that there is a maximum in the efficiency of drop rupture at a certain MW of gelatin, which in these experiments is between 20,000 and 80,000. We can explain these results in light of the studies by Walstra (15) on emulsification with polymers with varying MW, where he has shown that addition of polymers (PVA), with MW greater than 35,000, to the continuous phase, results in the elimination of the smaller drops in a distribution— the mean drop size goes up and the polydispersity goes down. He attributed this result to the suppression of eddies whose length scales were of the order of the end-to-end length of the polymer molecule. Thus, drops whose sizes are smaller than the smallest eddy are not easily formed. The MW of a single α chain of gelatin is around 100,000 and the end–end length is around ˚ which is close to the size of the smaller drops present in 3000 A, the distributions. As the MW decreases, we see that the number of small drops in the distribution also increases, which may be attributed to the presence of smaller eddies involved in the drop rupture. For the lowest MW gelatin, the molecular size may not limit the formation on the drop size, but rather Walstra found that the polymer simply acts as a viscosifier and a higher viscosity produces smaller drops. Thus, at the lowest gelatin MW, the reduction in viscosity may cause the increase in the drop size. For the emulsions made with gelatin and SDS, we see similar relationship between drop size and MW. However, the dependency on the MW is less than in the absence of surfactant, and the drops are smaller. This difference may be a reflection on the differences in the mode of rupture in the presence or absence of surfactants. It is postulated that when surfactants are present, a large number of small drops are formed by tip streaming— breakup of small drops from the tips of deformed large drops, rather than the conventional breakup of drops (26, 27). This effect is not due to lowering of interfacial tension, but is attributed to the change in the mobility of the interface due to interfacial tension gradients. The relationship between the gelatin MW and the drop size also depends on the method of emulsification and the fluid flow characteristics during emulsification. The above discussion was based on results obtained in a high-pressure homogenizer. Table 2 shows the drop sizes obtained for three emulsion formulations, when the rotor-stator device (Silverson) was used to
COALESCENCE DURING EMULSIFICATION, 3
TABLE 2 Drop Sizes (in µm) of Premixes Made with a Rotor–Stator Device for the Emulsions Having the Different Gelatins 2% Gel + surf
8% Gel
8% Gel + surf
5.12 7.54 7.62 8.55
6.07 7.34 8.55 12.36
3.16 1.88 3.46 5.87
Gel 1 Gel 2 Gel 3 Gel 4
prepare the premixes. In this device, the calculated Kolmogorov length scale of the smallest eddies is larger than the drop sizes, implying that the rupture is occurring in the viscous dissipation regime. In this case, it is clear that the drop size decreases when the continuous phase viscosity is increased, by either increasing the gelatin level, increasing the MW or adding SDS. Effect of Gelatin pH Gelatin is a polyampholyte with titratable positive and negative charges. The isoelectric pH of gelatin produced from ossein depends on the mode of hydrolysis. Lime-hydrolyzed ossein has a pH of around 5.0, whereas acid-processed ossein has distribution of isoelectric point varying between 5 and 9, and has a mean from 7 to 9. The pH of the environment in which the gelatin is present has a strong influence on its behavior. In solution, the pH influences the viscosity of the solution caused by
1.2
0.5
a
0.45
0.8
0.4
0.6
0.35
0.4
0.3
0.2
0.25
0
drop size ( m)
E M2
1
0.2 0
2
4
6
8
10
12
1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
b
typeIV pH5.6
0.55
type IV pH5.0
0.5
type IV pH4.5 APO pH 5.6
0.45
APO pH 7.0
0.4 0.35
drop size ( m)
E M2
number of passes
0.3 0.25 0
2
4
6
8
10
12
number of passes
FIG. 9. Coalescence and drop sizes of emulsions containing 2 wt% gelatin at varying pH: (a) no SDS, (b) 4 mM SDS. Solid lines represent coalescence data and dashed lines represent drop size data.
173
the conformational changes of the molecule (28). pH modulates the interactions of ionic species with the gelatin. The viscosity of a gelatin solution can be increased or decreased on adding anionic surfactant, depending on whether the pH is above or below the iep (18). The stoichiometry of anionic surfactant binding to the gelatin molecules is also strongly dependent on pH (13). The effect of pH on the adsorption characteristics of gelatin on latex particles were studied by SANS (2) and light scattering (5). At the air–water surface it was found that the pH affected the concentration and the thickness of the adsorbed gelatin layer (23) in the presence of SDS. Adsorption studies on silica surfaces showed that the manner and stoichiometry of surfactant modulated gelatin adsorption, was a strong function of pH— desorption of gelatin from a surface occurred at lower surfactant concentrations if the pH was below the iep of gelatin (8). Emulsions were prepared with a lime-processed ossein (LPO) gelatin with a mean iep of 5.0 and an acid-processed ossein gelatin (APO) with a mean iep of 7.0. The oil composition and homogenization conditions were the same as the previous experiments. The gelatin concentration was 2 wt% and emulsions were made without and with SDS, at a level of 4 mM in the solution. The pH of the aqueous phase for the LPO gelatin was varied at three values—4.5, 5.0, and 5.6—and for the APO it was 5.6 and 7.0. Figures 9a and 9b show the fluorescence and the drop size data for the two emulsion formulations. We see that the pH of the gelatin has no effect on the stability against coalescence. In the absence or presence of surfactant, the thickness of the adsorbed gelatin layer is expected to increase with the change in pH away from the isoelectric point.5 However, this change does not affect stability. Similarly, the pH does not have a significant impact on the drop size, when SDS is not present. However, in the presence of surfactant, we see that for a given gelatin type the drop size decreases as the pH is raised. That is, the pH affects the rupture processes when SDS is present. In view of the fact that addition of SDS to a gelatin containing aqueous phase reduces the drop sizes during emulsification (see Fig. 2 and Fig. 7), we can explain the results in Fig. 9b. Whitesides (13) has shown that the amount of surfactant that is bound to gelatin, in solution, goes up as the pH is lowered. Consequently, the amount of free SDS is lowered with the lower pH. Thus, lowering the pH is tantamount to lowering the free SDS concentration. If the mode of rupture described by Milliken and Leal (26) is operational, then the amount of surfactant that would promote tip breakage of drops and yield smaller drops decreases with decreasing pH. In fact, 1 for the LPO gelatin at the iep (5.0) and below the iep (4.5), the drop sizes are not changed by adding surfactant. This observation also holds for the APO gelatin below (pH 5.6) the iep. That is, for these emulsions, the addition of surfactant at a low level does not impact the drop size. For the APO gelatin at the iep (pH 7.0) and the LPO gelatin above the iep (pH 5.6), the emulsions with surfactant have a significantly smaller size, which is probably due to the higher amount 5
Bagchi, P., unpublished results.
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of free SDS in the latter two emulsions. Thus, when designing formulations for emulsions made in the presence of gelatin, the amount of surfactant required to reduce the drop size will depend on the pH of the formulation relative to the iep of the gelatin. Muller et al. (9) also found a similar pH dependency with the SDBS/gelatin system. The gelatin used in their study is the LPO gelatin. CONCLUSIONS
This paper presents an extensive, if not exhaustive, study of the emulsification of oil into water, in the presence of gelatin. Some of the key areas addressed are gelatin level and MW, gelatin interaction with surfactant, and finally the gelatin pH. Gelatin by itself is shown to be an adequate stabilizer of emulsions, even in high-pressure homogenizers, when the rates of surface generation are high. However, gelatin is not as effective at drop rupture and formation of small drops, compared to systems where surfactants are present. The explanation we are offering is that surfactants are needed to be able to change the mobility of surfaces, via interfacial tension gradients, and promote a different mode of rupture as described by Milliken and Leal (26). At substantially high levels of anionic surfactant, coalescence is actually higher than that obtained with gelatin in the absence of surfactant, while the drop sizes are smaller. Nonionic surfactants, on the other hand, are very inefficient at stabilizing drops in a gelatin containing system. This is because they work to completely displace gelatin from drop surfaces, which in turn can promote flocculation. The high level of coalescence results in the production of large drop size emulsions. The gelatin MW has a significant impact on the rupture of drops as evidenced by the decrease in drop size as the gelatin MW is reduced, even as the amount of coalescence increases. It is believed that the gelatin molecules are attenuating the turbulent flow characteristics, particularly the eddy size, and the size of the gelatin molecules determines the size of the eddies that are allowed to exist. There is an optimal MW at which the rupture is most efficient. Below this MW we believe that the decrease in the viscosity results in less efficient transfer of momentum across drop interface. If drop rupture occurs in the shear-controlled regime, as in some rotor-stator devices, then the viscosity of the continuous phase becomes important, and the smaller drops are formed with the higher MW gelatins. Although the pH of the gelatin impacts the density and thickness of the adsorbed layer, it was not found to impact the coalescence in a homogenizer. In the absence of added surfactant, the pH did not have an impact on the drop size. In the
presence of surfactant, while the coalescence was not affected by pH, the drop sizes were. At the low pH, because of high surfactant binding to the free gelatin, there is insufficient free surfactant to promote rupture. ACKNOWLEDGMENTS I thank Kurt Schroeder for help with the drop size measurements. Thanks to Aileen Svereika and Eileen Beverly for help with the experiments with gelatin and SDS. The help of Xavier Lallemand and Christophe Amyot with the preparation of the low-MW gelatins and the homogenization experiments with those gelatins is gratefully acknowledged.
REFERENCES 1. Howe, A., Curr. Opin. Colloid Interface Sci. 5, 288 (2000). 2. Cosgrove, T., Hone, J. H. E., Howe, A. M., and Heenan, R. K., Langmuir 14, 5376 (1998). 3. Vaynberg, K. A., Wagner, N. J., Sharma, R., and Martic, P. J., Colloid Interface Sci. 205, 131 (1998). 4. Howe, A. M., Clarke, A., and Whitesides, T. H., Langmuir 13, 2617 (1997). 5. Hone, J., Howe, A., and Whitesides, T., Colloids Surf. 161, 283 (2000). 6. Cooke, D. J., Dong, C., Thomas, R. K., Howe, A., Simister, E., and Penfold, J., Langmuir 16, 6546 (2000). 7. Pitt, A., Prog. Colloid Polym. Sci. 103, 307 (1997). 8. Muller, D., Malmstem, M., Bergentahl, B., Hessing, J., Olijve, J., and Mori, F., J. Imaging Sci. 45, 229 (1997). 9. Muller, D., Malmstem, M., Bergentahl, B., Hessing, J., Olijve, J., and Mori, F., Langmuir 14, 3107 (1998). 10. Dickinson, E., and Lopez, G., J. Food Sci. 66, 118 (2001). 11. Chesworth, S. M., Dickinson, E., Searle, A., and Stainsby, G., Lebens. Wiss. U. Technol. 18, 230 (1985). 12. Toledano, O., and Magdassi, S., J. Colloid Interface Sci. 200, 235 (1998). 13. Whitesides, T., and Miller, D., Langmuir 19, 2899 (1994). 14. Knox, W. J., and Parshall, T. O., J. Colloid Interface Sci. 40, 290 (1972). 15. Walstra, P., Chem. Eng. Sci. 29, 885 (1974). 16. Bagchi, P., Mcsweeny, G., and Sargent, S., U.S. Patent 5,013,640. 1991. 17. Chen, J., and Dickinson, E., Food Hydrocolloids 9, 35 (1992). 18. Greener, J., Contestable, B., and Bale, M., Macromolecules 20, 2490 (1987). 19. Olijve, J., European Patent Application 971262A. 2000. 20. Lobo, L., Nair, M., and Svereika, A., J. Colloid Interface Sci. 253, 409 (2002). 21. Mourey, T. H., and Coll, H., in “Advances in Chemistry” (T. Provder, Ed.), Vol. 247. Am. Chem. Soc., Washington, D.C., 1995. 22. Lobo, L. A., and Svereika, A., submitted for publication. 23. Wustneck, R., and Muller, H. J., Coll. Polym. Sci. 264, 97 (1986). 24. Howe, A. M., Wilkins, A. G., and Goodwin, J. W., J. Photogr. Sci. 40, 234 (1992). 25. Lobo, L., and Wasan, D. T., AIChE Symp. Ser. 86, 25 (1990). 26. Milliken, W. J., and Leal, L. G., J. Colloid Interface Sci. 166, 275 (1994). 27. Groeneweg, F., van Dieren, F., and Agterof, W. G., Colloids Surf. A 91, 207 (1994). 28. Rose, P. I., in “The Theory of the Photographic Process” (T. H. James, Ed.), Macmillan, New York, 1997.
Coalescence during Emulsification 3. Effect of Gelatin on Rupture and Coalescence Lloyd Lobo1 Eastman Kodak Company, Imaging Materials and Media, Research and Development, 1999 Lake Avenue, Rochester, New York 14650-2108 Received October 17, 2001; accepted June 27, 2002
An oil-soluble fluorescent probe, undecyl pyrene (UDP), is used to measure the amount of coalescence that occurs during the emulsification of tri-2-ethylhexyl phosphate using a high-pressure homogenizer. From these measurements, the roles of anionic surfactant (SDS) and gelatin in stabilizing drops against coalescence and promoting drop rupture during emulsification are deduced. It is found that gelatin aids in reducing coalescence, whereas SDS aids in rupture of drops. The effect of variables such as gelatin MW, surfactant type, and pH on coalescence and final drop size is investigated. C 2002 Elsevier Science (USA) Key Words: emulsification; gelatin; SDS; photographic dispersions; molecular weight; polymers.
INTRODUCTION
Gelatin is a well-known stabilizer of colloidal materials used in a variety of industrial applications. In the photographic industry, gelatin is used in the manufacture of many film components such as solid particle pigment dispersions, oil-in-water coupler dispersions, and silver halide emulsions. Gelatin not only provides long-term stability of the colloidal dispersion but also determines the morphology and dimensions of the colloidal particles during formation. Several studies address the adsorption behavior of gelatin and its conformation at interfaces. Howe (1) has provided a comprehensive summary of the research done in this area. Small angle neutron scattering and fluorescence labeling have been used to deduce the adsorption and conformation of gelatin adsorbed to organic surfaces (2, 3). The gelatin layer thickness obtained in these studies is closely matched to the values obtained from models using rheological data (4, 5). Because most photographic dispersions and coating solutions contain gelatin and anionic surfactants, the behavior of such systems is of interest. Studies describing the solution behavior of these systems are listed in Ref. (1). At the air–water interface, Cooke et al. have carried out adsorption measurements (6), while Pitt (7) 1 Present address: NexPress Solutions LLC, 2600 Manitou Rd., Rochester, NY 14624. Fax: (585) 613-2105. E-mail: lloyd [email protected].
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has studied the dynamic surface tension. At the oil–water interface, which is of relevance to this study, Muller et al. (8, 9) have studied the adsorption of surfactant and gelatin. Howe et al. (4) characterized oil-in-water emulsions containing gelatin and anionic surfactants by measuring the adsorbed amounts of the individual species and their role in modifying the rheological properties of the emulsions. Although emulsification with gelatin as a stabilizer is widely practiced, there are not many studies published on the role of gelatin in emulsification. Dickinson and co-workers (10, 11) studied the emulsifying behavior of gelatin using a jet homogenizer. They found that gelatin yielded large oil drops compared with other proteins such as sodium caseinate and that the drop size went through a minimum with the gelatin concentration. Muller et al. (9) have studied the emulsification of mixtures of gelatin and sodium dodecyl benzene sulfonate (SDBS) solutions with polar versus nonpolar oils. They observed a general result that polar oils that are typically used in the photographic industry show a smaller drop size than the nonpolar oils, when a high-pressure homogenizer is used. They attributed this dependence to the lower interfacial tension of polar oils. Toledano and Magdassi (12) have studied the emulsifying and foaming properties of hydrophobically modified gelatin. They found that the hydrophobic groups enhance adsorption at the oil/water interface and decrease in the drop size, for stirred systems. There are many interesting facets to the properties of gelatin, which can affect its efficacy as an emulsifier: Interfacial tension: Although gelatin adsorbs strongly to an interface, it shows modest reduction in surface tension (68 dyn/cm to about 62 dyn/cm at 35◦ C), when compared to anionic surfactants (25 to 30 dyn/cm) (13, 14). However, because of its relatively high molecular weight it is not obvious if it can adsorb to surfaces on the short time scales that surfaces are formed in a homogenizer. Turbulence suppression: Polymers are known to alter the flow fields at the dimensions of the smaller eddies (15), and thus gelatin has the potential to alter the rupture mechanism. Sensitivity to pH: Gelatin is a polyampholyte whose charge depends on pH, and the pH affects the gelatin binding as well as its conformation at an interface (2, 5), particularly in the presence of anionic surfactant (9). Interaction with surfactants: While anionic 0021-9797/02 $35.00
C 2002 Elsevier Science (USA)
All rights reserved.
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surfactants are reported to increase the gelatin adsorption below their cmc’s (9), nonionic surfactants can displace gelatin from the surface (16, 17). In solutions, Greener has shown that the viscosity and gelatin conformation will change because of the interaction with surfactant (18), which can alter the fluid dynamics during homogenization. Molecular weight: Gelatin obtained from a typical manufacturing process is a mixture of collagenlike molecules with MW ranging from 10,000 to 1,000,000. Depending on the point of extraction in the process the weight average molecular weight can range from 100,000 to 200,000. The MWD can affect the flow field at the small scale, the viscous dissipation in the bulk solution, and the thickness of the adsorbed gelatin shell. The interaction with surfactants can also be affected. Olijve (19) has studied the effects of gelatin MW on its interaction with SDBS and its role on emulsion stability properties. He concludes that the low-MW gelatin forms more surface-active complexes with surfactants and is therefore able to stabilize emulsions more effectively. In this study, we relate some of the above-mentioned properties of gelatin to its role in the emulsification process. In Part 1 of this series (20) we have described a method that uses an excimer-forming oil soluble fluorescent probe that monitors the amount of coalescence induced mixing. The probe, at a high concentration, is introduced in a fraction of the drops before emulsification. The ratio of the excimer peak intensity to the monomer peak intensity (E/M) is monitored. Because this ratio is proportional to the probe concentration in the oil phase, random mixing of the contents of the oil drops, caused by coalescence, results in an effective dilution of the probe and a decrease in the ratio. The change in the value of E/M, during the emulsification process, is an indication of the amount of coalescence that has occurred. The quantitative relation between the signal change and the degree of coalescence induced mixing has been obtained via Monte Carlo simulation as discussed in Part 1. The simulation shows that this signal is not affected by the drop size or the polydispersity of the distribution. We have used the method in this report to measure the amount of coalescence and to infer the relative importance of the role of gelatin in the rupture and stabilization (to coalescence) during emulsification. The effect of gelatin concentration is first studied for pure gelatin solutions and then for gelatin/SDS mixtures. The effect of anionic vs nonionic surfactants in the presence of gelatin is discussed next. The role of the MW of gelatin on its emulsifying properties is also investigated. Finally, the role of pH for two gelatins having different isoelectric points (iep) is studied in the absence and presence of surfactant. EXPERIMENTAL
Materials The oil phase in these experiments was tri-2-ethylhexyl phosphate (TEP), obtained from Eastman Chemicals. It is one of the materials that are used to prepare photographic coupler dispersions. The anionic surfactants used were sodium dodecyl sulfate
and Alkanol-XC a commercial surfactant made by DuPont—a mixture of di- and triisopropylnaphthalene sulfonate. The nonionic surfactant used in this study was the ethoxylated alcohol 10 lauryl ether obtained from Sigma. Type IV gelatin was used in sample preparation. Because of the relatively high viscosity of the oil phase, the more efficient undecyl pyrene (UDP) was used as the oil soluble fluorescent probe. The reasons for the choice and the synthesis procedure of the probe are described in part I of the series (20). Gelatin Several varieties of gelatin were used in this study. The basic Type IV Lime Processed Ossein (LPO), with a mean MW of 170,000 and iep of about 4.9, was obtained from Eastman Gelatine. The acid-processed ossein (APO) gelatin, with an iep of 7.0 and a MW of about 130,000, was obtained from Croda. Three other gelatins with varying MW were prepared by enzymatic hydrolysis of a high-MW type IV gelatin obtained from Eastman Gelatine. The proteolytic enzyme Neutrase was made by Novo Nordisk. A 20% gelatin solution with an initial viscosity of 320 cp (at 50◦ C) was hydrolyzed at 50◦ C by adding the enzyme broth to the solution at a concentration of 100 ppm. When the temporally monitored viscosity reached a predetermined value, the solution temperature was raised to 80◦ C and kept at that temperature for 10 min, to irreversibly denature the enzyme and stop the reaction. The final viscosities of the three hydrolyzed gelatin solutions were 92 cp, 29.5 cp, and 8 cp, measured at 50◦ C. The gelatin content of the final solutions was measured by drying samples of the gels in an oven. Molecular Weight Measurement Absolute molecular weight determinations of gelatin were made using size-exclusion chromatography (SEC) with twoangle laser light scattering (TALLS) detection using a mobile phase of aqueous 0.25 M calcium chloride and 5% by volume ethylene glycol. Two SUP8103R-LMB PSS Suprema mixed bed columns were used for the separation. The absolute weight average molecular weight Mw was determined by integration of the light-scattering chromatogram from the 15◦ forward angle signal without using the differential refractometer (DRI) signal (21). Emulsification Experiments A Microfluidics microfluidizer was used as a high-pressure homogenizer. All the experiments were carried out at 45◦ C by immersing the microfluidizer into a water bath. In a microfluidizer, a coarse emulsion, which is the feed, is separated into two fluid streams that are accelerated using high pressures. The two fluid streams are impinged upon each other in an interaction zone, where emulsification occurs. Two coarse emulsions (called premixes) were made with a Silverson rotor-stator device at a stirring speed of 7000 rpm. Each premix was identical, except that one of them had the UDP at a level of 2% in the oil phase. Samples were taken of each premix. The premixes
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Samples of the emulsions, taken during the emulsification experiment, were stabilized by mixing them with a warm SDS/gelatin mixture, and stirred in an ice bath so that the gelatin sets into a gel. The samples were stored in a refrigerator until measurements were made (within 24 h). The final composition of all samples was 1.25% oil, 2.2% gelatin, and 1% SDS. In Part 1, we have shown that this treatment of the sample is able to arrest the coalescence process and in addition, does not result in change in the fluorescence signal because of diffusion transport of the probe via the aqueous phase. Fluorescence Measurements Fluorescence measurements were made on a SPEX Fluorolog spectrofluorometer. Because we are measuring fluorescence of dispersed drops, front face acquisition2 is used to obtain the emission. Excitation was at 340 nm. Emission acquisition was between 370 and 600 nm. The two monomer peaks monitored were the M1 at 376 nm and M2 at 397 nm. The excimer peak was measured around 477 nm. All the data are reported using the ratio of the excimer (E) and monomer (M2) as the E/M2 signal. Because all emulsion samples contained 2% gelatin and 1.25% oil phase the contribution of the fluorescence signal from gelatin was the same across samples. For the probe concentration (2%) and mix ratio (1 : 9) chosen, the maximum E/M2 signal is close to 0.95, while complete mixing due to coalescence will reduce the signal to about 0.1. Based on the Monte Carlo simulations discussed in Part 1 of this series (20), E/M2 reaches its lowest value when each drop experiences approximately two coalescence events. Data showing the changes in E/M2, will be referred to as the coalescence data.
RESULTS AND DISCUSSIONS
Gelatin with Anionic Surfactants Emulsions were prepared in which the gelatin concentrations in the aqueous phase ranged from 0.05 to 8 wt%. Type IV gelatin, with a weight average MW of 170,000, was used in all the experiments. Three series of experiments were done, each at different levels of SDS—0, 8 × 10−4 , and 1.6 × 10−2 M in the aqueous phase. Figure 1 shows the change in the fluorescence signal as a function of the gelatin concentration at the three
1 0.9 0.8 0.7
E M2
Sample Stabilization
FL-70 (obtained from Fisher Scientific) with 0.01% sodium azide to minimize bacterial growth. The average drop size data in this report uses the third moment of the distribution—the volume average mean.
0.6 0.5 0.4 0.3
pass 1 pass 2 pass 4 pass 6 pass 8 pass 10
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gelatin conc. (wt%)
b) 0.8 mM SDS
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were subsequently mixed in a ratio of 1 : 9 (1 part of the emulsion with the probe). The mix was sampled prior to passing it through the microfluidizer at a pressure of 7000 psi. The emulsion was passed through sequentially, 10 times, with samples being taken after each pass and stabilized with SDS/gelatin solution. Selected samples were measured for fluorescence. The drop sizes of the premixes were measured by either the Coulter counter or the CFFFF. In all the emulsification experiments, the oil fraction was maintained at 8 wt% (8.6 vol%).
0.5 0.4 0.3 0.2 0.1 0 0.01
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Drop Size Measurements
c) 16 mM SDS
0.7
2
Front face acquisition is where the excitation beam incidence and emission collection are at the same face of the spectrophotometric cell. The acquired signal is only from the part of the sample adjacent to the cell wall (2-dimensional collection). While this method of measurement presents some optical artifacts, it does not affect the type of signal (E/M) used in this study.
E M2
0.6
Coarse emulsions with drop sizes greater than a micrometer were measured using a Coulter counter—Coulter Multisizer II. The smaller emulsions were sized using a cross flow field flow fractionation (CFFFF) instrument, Model F-1000, made by FFFractionation, LLC. The eluent used was 0.1% surfactant
0.5 0.4 0.3 0.2 0.1 0 0.01
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gelatin conc. (wt%)
FIG. 1. Coalescence in a microfluidizer as a function of gelatin concentration. SDS concentration in (a) is 0; in (b) it is 8 × 10−4 M, and in (c) it is 1.6 × 10−2 M.
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a) 0.8 mM SDS drop size ( m)
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gel conc. (wt%)
FIG. 2. Drop size as a function of gelatin concentration: (a) 8 × 10−4 M SDS, (b) 1.6 × 10−2 M SDS.
different SDS levels. In each of the plots, data are presented for the 1st, 2nd, 4th, 6th, and 10th passes through the homogenizer. Figure 2 shows the drop size data obtained in these experiments. The drop size data for emulsions made in the absence of SDS is not shown, because the drop sizes varied by over an order of magnitude and it was not possible to capture the data accurately. As mentioned earlier, the degree of coalescence is proportional to the drop in the fluorescence signal (0.95 in the absence of coalescence). In the absence of surfactant (Fig. 1a), we see a steep dependence of coalescence on the gelatin level. At 8 wt%, gelatin stabilizes the emulsion adequately—based on our simulations in Part 1, the fluorescence signal reaches its lowest value when the average number of coalescence events per drop is 2. At the low SDS concentration (Fig. 1b), because of the stability afforded by the SDS itself, gelatin concentration has a smaller impact on coalescence. At the high surfactant concentration, addition of low levels of gelatin shows a slight increase in stability, relative to the emulsion made with SDS alone (0% gelatin is shown on the log–log graph as 0.01 wt% gelatin). Above 0.05%, gelatin concentration has relatively little influence on the coalescence. The decrease in coalescence with increasing gelatin concentration is similar to the effect seen with changing SDS concentration, discussed in Part 2 of this series (22). However, the reason for this dependence is different in this case. In the case of SDS alone, the surfactant surface coverage increases, up to the cmc. However, in the case of gelatin, the saturation coverage of gelatin is reached at very low levels of gelatin (3, 8). By using
the drop size data (Fig. 2) and a value for saturation coverage of 4.0 mg/m2 of gelatin (4, 3), the amount of gelatin binding and the depletion from solution can be calculated (a drop diameter of 0.75 µm was estimated for the no surfactant case). We estimate that, in the absence of SDS, gelatin depletion was occurring when the total gelatin concentration was below 0.1 wt%. At the two levels of SDS studied here, because of the smaller drop sizes, the gelatin depletion occurred up to 0.5% gelatin. At concentrations below which gelatin saturation is achieved, flocculation is known to occur. Thus, the increased coalescence at the low gelatin concentration can be explained as due to gelatin depletion. At the higher gelatin concentrations, depletion cannot explain the stability dependence on gelatin concentration. The increased stability probably has to do with the dynamics in the homogenizer. At the short time scales, related to the time scale of forming new surface, the amount of gelatin absorbed will be dependant on the gelatin concentration in the aqueous phase (based on diffusion/adsorption kinetics). The results obtained with gelatin/SDS systems should be viewed in light of the interactions between the two species. Based on the investigations of Whitesides and Miller (13), as gelatin is added to a solution of SDS, which is above the cmc, the micelles bind on to gelatin at a specific stoichiometry with a maximum of 5 micelles per average gelatin molecule (molecular weight of 100,000). Thus, at high SDS-to-gelatin ratios, free (unbound) micelles exist and the gelatin/micelle complex has a low surface activity relative to that of free gelatin and free SDS. However, for a fixed SDS level, when the gelatin level is increased beyond the stoichiometric maximum for bound micelles (when there are no free micelles), there is an increased amount of gelatin binding at the interfaces (see Ref. (8)). Based on the data of Whitesides and Miller, it is calculated that at the high SDS concentration (16 mM), free micelles exist below gelatin concentrations of 1 wt%. However, we are always above the cmc of SDS in gelatin (1mM). The stability obtained because of the free SDS is high enough that the addition of gelatin does not affect it. At the low SDS concentration (0.8 mM), we are below the cmc of free SDS (8 mM) as well as below the cmc of SDS in the presence of gelatin. Therefore, the addition of gelatin results in the formation complexes of single SDS molecules and submicellar species onto gelatin, which are surface active. Thus, at SDS levels below the cmc, the addition of gelatin, even at low levels, results in increased gelatin being adsorbed at the interface. At the lower level SDS is not adequate at providing stability and, thus, the adsorbed gelatin contributes to the stability against coalescence. Because of large drop size and difficulty in measurement, drop size data is not shown for the surfactant-free system. However, the qualitative result that can be stated is that the drop sizes, in the absence of any surfactant, were larger than that obtained at both levels of SDS, when the data was compared for the same gelatin concentration. At the low SDS level (Fig. 2a), the drop size dependency with gelatin concentration changes, depending on the number of passes through the homogenizer.
169
COALESCENCE DURING EMULSIFICATION, 3
1 0.9
a
0.8
E M2
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0
2
4
6
8
10
12
number of passes
0.4
a) 0.05 wt% gelatin
0.8
no surf. 0.5% SDS 2.0% SDS 0.5% AXC 2.0% AXC
1 0.9
drop size ( m)
For the first and second pass, where there is relatively less coalescence, there are relatively small changes in drop size. For the later passes the drop sizes decrease with gelatin concentration, which can be attributed to attenuation in the amount of coalescence by the gelatin concentration (seen in Fig. 1). At the high SDS level (Fig. 2b), smaller drops are obtained. In this case, gelatin concentration does not affect the coalescence behavior, but drop sizes decrease on increasing the gelatin concentration beyond 1 wt%, suggesting that increasing gelatin concentration enhances rupture. In studies with a similar homogenizer Muller et al. (9) obtained a similar result as reported here—the drop size decreases with increased surfactant concentration. Figure 3 shows the above coalescence data plotted to observe the effect of surfactant concentration. Data are selectively presented for three gelatin levels. At each gelatin level, the effect of changing the surfactant level is shown. At the low gelatin
b
0.35 0.3 0.25 0.2
E M2
0.7
0.15
0.6
0
0.5 0.4
2
4
6
8
10
12
number of passes
0.3 0.2
FIG. 4. Coalescence (a) and drop size (b) as functions of anionic surfactant concentration with 8 wt% gelatin in the aqueous phase.
0.1 0 0
2
4
6
8
10
12
E M2
number of passes 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
b) 1.0 wt% gelatin
0
2
4
6
8
10
12
10
12
E M2
number of passes 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
c) 8.0 wt% gelatin
0
2
4
6
8
number of passes
FIG. 3. Coalescence as a function of number of passes through the homogenizer: (a) 0.05%, (b) 1%, and (c) 8% gelatin. SDS concentration is 0 (diamonds), 0.8 mM (squares), and 16 mM (triangles).
level (0.05 wt%), gelatin by itself is inadequate at protecting against coalescence. Thus, increasing levels of surfactant results in increased stability. At the intermediate gelatin level (1 wt%), the emulsions made without SDS and the ones made at a low SDS level, behave similarly, while the high SDS level shows increased stability. At the high gelatin level (8%), the emulsion without any surfactant is the most stable, while the high SDS level shows definitely lower stability. In order to verify the generality of this finding, another experiment was carried out using two different anionic surfactants—SDS and AlkanolXC. The cmc of Alkanol-XC, in the absence of gelatin, is about 0.38 wt% versus 0.23 wt% for SDS. In the presence of gelatin the cmc is about 0.04% for Alkanol-XC and 0.03% for SDS.3 Figure 4 shows the amount of coalescence that occurs and the drop size change, with the number of passes through the homogenizer, with the varying levels of anionic surfactant and with 8% gelatin. It is clearly seen that increasing the amount of anionic surfactant decreases the fluorescent signal, suggesting increased coalescence. Using a value of max of 3.16 µmol/m2 for both surfactants (based on the data for SDS obtained in Part 2 (22)), the value of unbound surfactant was estimated. The resulting concentrations of surfactant in solution obtained are well above the cmc of the respective surfactant, in the presence of gelatin. Muller et al. (8) have shown that the amount 3
T. Whitesides, unpublished results.
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LLOYD LOBO
Gelatin with Nonionic Surfactants The ethoxylated alcohol C12E10 was used for this study. The interfacial tension for a 1 wt% surfactant, 2 wt% gelatin TEP interface was 2.96 dyn/cm versus 13.2 dyn/cm for a 2% gelatin solution. The cmc for this surfactant is about 1 × 10−4 M or 0.007 wt%. The emulsification experiment was carried out with the same oil phase composition and homogenization procedure described above. The gelatin concentration in the aqueous phase was 8 wt% and the two levels of nonionic surfactant was used 0.01 wt% (slightly above the cmc) and 0.4 wt%. As a comparison the data with 8% gelatin and 0.4% SDS was also generated. Figures 5a and 5b show the coalescence data and the drop size data, respectively, for these emulsions. At the low level of nonionic surfactant, the coalescence behavior is similar to that of the emulsion made with gelatin alone. At 0.4 wt% nonionic surfactant, there is substantially more coalescence and the fluorescence signal is lost after the first pass. In comparison, the SDS/gelatin emulsion shows much less coalescence, although, as before, we see that the coalescence is greater than in the pure gelatin case. On comparing the drop size data, we see that the emulsion with
1.2
no surfactant
a
0.4wt% SDS
1
0.01wt% C12E10 E M2
0.8
0.4wt% C12E10
0.6 0.4 0.2 0 0
2
4
6
8
10
number of passes 0.55
drop size ( m)
of adsorbed gelatin and the thickness of the adsorbed gelatin layer, increases and then decreases as the amount of anionic surfactant is increased. The position of the maximum occurs at or around the cmc of the surfactant, in the presence of gelatin. Similar results have been obtained at the air–water interface (6, 23). The decrease in the adsorbed gelatin above the cmc can be used to explain the increase in coalescence with surfactant concentration. The drop size data (Fig. 4b) shows that even though the amount of coalescence increases with increasing surfactant, the drop sizes also goes down, suggesting that increasing surfactant concentration enhances the rupture. One possible explanation is that the dynamic interfacial tension decreases with increased surfactant concentration, which can facilitate rupture, as is posited by Muller et al. (9). However, in Part 2 (20) we have shown, by inference, that when the coalescence of two emulsions is comparable, the variation in interfacial tension does not affect the drop size. That is, the rupture does not depend too strongly on the interfacial tension. In this case, the aqueous phase also has gelatin. Because polymers affect the modes of viscous dissipation in a turbulent field, the mode of rupture can be different. Therefore, surfactant level (above cmc) can have different possible impacts-(a) Surfactant affects the gelatin conformation and, consequently, the viscosity of the solution (2, 18). Therefore, they can alter the momentum transfer to the drops themselves (via changes in the eddy formation). (b) If the fluid flow is substantially different in the presence of polymer, the interfacial tension may now become important to the rupture process (as we have shown in Part 2, there are different dependencies for a stirred tank versus a high-pressure homogenizer). However, it is hard to point out the exact mechanism that can explain why the drops get smaller as the surfactant concentration increases.
b
0.5 0.45 0.4 0.35 0.3 0
1
2
3
4
5
6
7
8
9
10
number of passes FIG. 5. Coalescence (a) and drop size (b) with 8% gelatin and varying levels of nonionic surfactant C12E10.
the high level of C12E10 has the largest drop size. The low level of C12E10 has the same drop size as the pure gelatin system— coinciding with the coalescence data. As before, the emulsion with the SDS has the smallest drop size. The effect of the nonionic surfactant on coalescence can be explained based on the findings of Chen and Dickinson (17). They found that addition of the nonionic surfactant Tween20 to a protein-stabilized emulsion (gelatin or β-lactoglubulin) resulted in displacement of the proteins from the oil–water interface. For a gelatin-stabilized emulsion, it took about 0.4 wt% of Tween 20 to displace the gelatin completely. This behavior is unlike that observed with gelatin and anionic surfactants (8) discussed earlier. The reason for this difference is that nonionic surfactants have shown no interactions with gelatin, as inferred from surface tension and viscosity measurements (18, 24). Bagchi et al. (16) have also shown that addition of nonionic surfactant to a gelatin-containing dispersion can substantially reduce its viscosity by displacing the thick adsorbed gelatin shell. However, it is not clear from either work why the surfactant concentration needs to be several times above the cmc in order to cause gelatin desorption. There are two possible explanations: (1) The interaction between gelatin and nonionic surfactant is not zero, but is much weaker than in the anionic surfactant case. (2) At the low surfactant concentration, depletion of the nonionic surfactant, caused by adsorption at the drop surfaces, reduces its actual concentration to below the cmc. At this point, we are not sure which of the two factors is operating. However, it is clear
171
COALESCENCE DURING EMULSIFICATION, 3
TABLE 1 Mean Molecular Weight (Mw ) of the Gelatins Used for Emulsification
1.4
gel 4
1.2
gel 3
1
gel 2
Gel i.d.
Mw
1 2 3 4
488,000 126,000 83,500 22,700
0.8
gel 1 0.6 0.4 0.2
4
4.5
5
5.5
6
6.5
7
log M
FIG. 6.
MWD of gelatins used for emulsification.
that in the presence of gelatin, the nonionic surfactant that is able to lower the interfacial tension substantially is not capable of stabilizing the drops and, therefore, not able to produce small drop sized emulsions.
comes important and the influence of the gelatin MW on stability will be diminished. There is not much difference between the coalescence of emulsions containing 8% gelatin versus 2% gelatin in the presence of SDS. It is useful to compare the coalescence data obtained by Olijve (19) with tricresyl phosphate emulsions made with varying gelatin MW. The emulsions were prepared with gelatins whose MW varied in a range similar to that in the
4
A. Howe, unpublished results.
0.5
a
0.4 0.3 0.2 0.1 0 0
2
4
6
8
10
12
E M2
number of passes 1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
0.5
b
0.4 0.3 0.2 0.1 0 0
2
4
6
8
10
12
number of passes
E M2
Four gelatins having different MW were prepared as described in the experimental section. The molecular weight distributions of the gelatins are shown in Fig. 6. The mean MW of each gelatin is given in Table 1. The oil phase fraction and the homogenization conditions were similar to the experiments described above. Three emulsions were made with each gelatin type—(a) 8% gelatin, (b) 8% gelatin + 16 mM SDS, and (c) 2% + 16 mM SDS gelatin. Figure 7 shows the coalescence and the drop sizes obtained for the emulsions made with the four different gelatins. In the absence of surfactant (a), the stability decreases (coalescence increases) in proportion to the decrease in the MW of the gelatin. It has been found that the thickness of the adsorbed gelatin shell decreases with the MW of the gelatin used4 , and the stability against coalescence decreases as a direct consequence of this. As was postulated in Part 2 of this series (22), the collisions between drops in a homogenizer results in very short times of contact, and the mechanism that is responsible for stability is more likely due to the resistance to the drainage of the intervening thin film. The two factors that affect the thin film drainage for gelatin are the continuous phase viscosity and the interfacial viscosity; the latter is related to the adsorbed layer thickness (25). Both the viscosities will increase with increasing MW of the gelatin. With the emulsions made with 8% and 2% gelatin with 0.46% SDS, a similar trend is observed—the coalescence increases with decreasing MW of the gelatin. However, the differences between the gelatins are smaller than in the absence of surfactant. This is because at these high levels of SDS, some of the gelatin has been displaced. The role of SDS as a stabilizer be-
E M2
Gelatin Molecular Weight
1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
drop size ( m)
3.5
drop size ( m)
3
1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
gel 1 Gel 2 gel 3 gel 4
c
0
2
4
6
8
10
0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0
drop size ( m)
0
12
number of passes
FIG. 7. Coalescence (closed symbols) and drop sizes (open symbols) obtained with varying MW gelatins: (a) 8% gelatin, (b) 8% gelatin + 16 mM SDS, (c) 2% gelatin + 16 mM SDS. The data are represented by diamonds for gel1, squares for gel2, triangles for gel3, and circles for gel4.
172
LLOYD LOBO
present study, at a concentration of 0.5 wt%, and the change in the drop size was measured when the emulsions were stored at 40◦ C over 4 h. He found that the drop size change (allegedly caused by coalescence) decreased with decreasing gelatin MW, implying the low-MW gelatins were better at stabilizing against coalescence. This is exactly opposite to the data reported here, the difference being in the higher collision forces occurring during an emulsification process, compared to the static condition. Another difference is in the gelatin concentrations used in the two studies, although both use concentrations above the overlap concentration of gelatin. Furthermore, tricresyl phosphate has some solubility in water, and its emulsions are prone to Ostwald ripening, which can confound the results in long term studies. The drop size data shows that the drop sizes of the emulsions are very dependent on the MW of the gelatin. In all the three emulsion formulations, the drop sizes decrease with decreasing MW, except for the lowest MW. Whereas coalescence also increases with decreasing gelatin MW, it can be confidently inferred that the decrease in drop size can be attributed to increased efficiency in drop rupture as the MW of the gelatin decreases. Moreover, the attenuation of the drop size with the MW is the maximum for the emulsions made with 8% gelatin, in the absence of surfactant. To get a better picture of the relative effects of the MW and the level of SDS, the coalescence and drop size data are plotted in Figs. 8a and 8b after the second pass through the homogenizer. For the three emulsion formulations, the fluorescence signal decreases monotonically with the MW of the
0.8 0.7
a
8% gelatin + 16mM SDS
0.6
E M2
8% gelatin
0.5
2% gelatin + 16mM SDS
0.4 0.3 0.2 0.1 0 10000
100000
1000000
MW 0.48
drop size( m)
0.46
8% gelatin
b
0.44 0.42 0.4
8% gelatin + 16mM SDS 2%gel + 16mM SDS
0.38 0.36 0.34 0.32 0.3
10000
100000
1000000
MW
FIG. 8. (a) Coalescence and (b) drop sizes of emulsions as a function of the gelatin MW, after the second pass through the homogenizer.
gelatin, whereas the drop size shows a minimum around a MW of 80,000. The size increase at the lowest MW is probably not a reflection of the coalescence, because the fluorescence data is similar at the two lower values of MW. Olijve (19) found that the surface tension of SDBS/gelatin mixtures decreases with decreasing gelatin MW (the SDBS complexes with lower MW gelatin are more surface active than the complexes with higher MW gelatin) at low surfactant concentrations. At the higher surfactant concentrations, he found the surface tension isotherms for the different MW gelatins coincide. In this study, the surfactant concentration used is high and it was verified that the gelatin MW had no impact on the static interfacial tensions, in the presence or absence of surfactant. Thus, interfacial tension cannot be used to explain the drop size results. This data suggests that there is a maximum in the efficiency of drop rupture at a certain MW of gelatin, which in these experiments is between 20,000 and 80,000. We can explain these results in light of the studies by Walstra (15) on emulsification with polymers with varying MW, where he has shown that addition of polymers (PVA), with MW greater than 35,000, to the continuous phase, results in the elimination of the smaller drops in a distribution— the mean drop size goes up and the polydispersity goes down. He attributed this result to the suppression of eddies whose length scales were of the order of the end-to-end length of the polymer molecule. Thus, drops whose sizes are smaller than the smallest eddy are not easily formed. The MW of a single α chain of gelatin is around 100,000 and the end–end length is around ˚ which is close to the size of the smaller drops present in 3000 A, the distributions. As the MW decreases, we see that the number of small drops in the distribution also increases, which may be attributed to the presence of smaller eddies involved in the drop rupture. For the lowest MW gelatin, the molecular size may not limit the formation on the drop size, but rather Walstra found that the polymer simply acts as a viscosifier and a higher viscosity produces smaller drops. Thus, at the lowest gelatin MW, the reduction in viscosity may cause the increase in the drop size. For the emulsions made with gelatin and SDS, we see similar relationship between drop size and MW. However, the dependency on the MW is less than in the absence of surfactant, and the drops are smaller. This difference may be a reflection on the differences in the mode of rupture in the presence or absence of surfactants. It is postulated that when surfactants are present, a large number of small drops are formed by tip streaming— breakup of small drops from the tips of deformed large drops, rather than the conventional breakup of drops (26, 27). This effect is not due to lowering of interfacial tension, but is attributed to the change in the mobility of the interface due to interfacial tension gradients. The relationship between the gelatin MW and the drop size also depends on the method of emulsification and the fluid flow characteristics during emulsification. The above discussion was based on results obtained in a high-pressure homogenizer. Table 2 shows the drop sizes obtained for three emulsion formulations, when the rotor-stator device (Silverson) was used to
COALESCENCE DURING EMULSIFICATION, 3
TABLE 2 Drop Sizes (in µm) of Premixes Made with a Rotor–Stator Device for the Emulsions Having the Different Gelatins 2% Gel + surf
8% Gel
8% Gel + surf
5.12 7.54 7.62 8.55
6.07 7.34 8.55 12.36
3.16 1.88 3.46 5.87
Gel 1 Gel 2 Gel 3 Gel 4
prepare the premixes. In this device, the calculated Kolmogorov length scale of the smallest eddies is larger than the drop sizes, implying that the rupture is occurring in the viscous dissipation regime. In this case, it is clear that the drop size decreases when the continuous phase viscosity is increased, by either increasing the gelatin level, increasing the MW or adding SDS. Effect of Gelatin pH Gelatin is a polyampholyte with titratable positive and negative charges. The isoelectric pH of gelatin produced from ossein depends on the mode of hydrolysis. Lime-hydrolyzed ossein has a pH of around 5.0, whereas acid-processed ossein has distribution of isoelectric point varying between 5 and 9, and has a mean from 7 to 9. The pH of the environment in which the gelatin is present has a strong influence on its behavior. In solution, the pH influences the viscosity of the solution caused by
1.2
0.5
a
0.45
0.8
0.4
0.6
0.35
0.4
0.3
0.2
0.25
0
drop size ( m)
E M2
1
0.2 0
2
4
6
8
10
12
1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
b
typeIV pH5.6
0.55
type IV pH5.0
0.5
type IV pH4.5 APO pH 5.6
0.45
APO pH 7.0
0.4 0.35
drop size ( m)
E M2
number of passes
0.3 0.25 0
2
4
6
8
10
12
number of passes
FIG. 9. Coalescence and drop sizes of emulsions containing 2 wt% gelatin at varying pH: (a) no SDS, (b) 4 mM SDS. Solid lines represent coalescence data and dashed lines represent drop size data.
173
the conformational changes of the molecule (28). pH modulates the interactions of ionic species with the gelatin. The viscosity of a gelatin solution can be increased or decreased on adding anionic surfactant, depending on whether the pH is above or below the iep (18). The stoichiometry of anionic surfactant binding to the gelatin molecules is also strongly dependent on pH (13). The effect of pH on the adsorption characteristics of gelatin on latex particles were studied by SANS (2) and light scattering (5). At the air–water surface it was found that the pH affected the concentration and the thickness of the adsorbed gelatin layer (23) in the presence of SDS. Adsorption studies on silica surfaces showed that the manner and stoichiometry of surfactant modulated gelatin adsorption, was a strong function of pH— desorption of gelatin from a surface occurred at lower surfactant concentrations if the pH was below the iep of gelatin (8). Emulsions were prepared with a lime-processed ossein (LPO) gelatin with a mean iep of 5.0 and an acid-processed ossein gelatin (APO) with a mean iep of 7.0. The oil composition and homogenization conditions were the same as the previous experiments. The gelatin concentration was 2 wt% and emulsions were made without and with SDS, at a level of 4 mM in the solution. The pH of the aqueous phase for the LPO gelatin was varied at three values—4.5, 5.0, and 5.6—and for the APO it was 5.6 and 7.0. Figures 9a and 9b show the fluorescence and the drop size data for the two emulsion formulations. We see that the pH of the gelatin has no effect on the stability against coalescence. In the absence or presence of surfactant, the thickness of the adsorbed gelatin layer is expected to increase with the change in pH away from the isoelectric point.5 However, this change does not affect stability. Similarly, the pH does not have a significant impact on the drop size, when SDS is not present. However, in the presence of surfactant, we see that for a given gelatin type the drop size decreases as the pH is raised. That is, the pH affects the rupture processes when SDS is present. In view of the fact that addition of SDS to a gelatin containing aqueous phase reduces the drop sizes during emulsification (see Fig. 2 and Fig. 7), we can explain the results in Fig. 9b. Whitesides (13) has shown that the amount of surfactant that is bound to gelatin, in solution, goes up as the pH is lowered. Consequently, the amount of free SDS is lowered with the lower pH. Thus, lowering the pH is tantamount to lowering the free SDS concentration. If the mode of rupture described by Milliken and Leal (26) is operational, then the amount of surfactant that would promote tip breakage of drops and yield smaller drops decreases with decreasing pH. In fact, 1 for the LPO gelatin at the iep (5.0) and below the iep (4.5), the drop sizes are not changed by adding surfactant. This observation also holds for the APO gelatin below (pH 5.6) the iep. That is, for these emulsions, the addition of surfactant at a low level does not impact the drop size. For the APO gelatin at the iep (pH 7.0) and the LPO gelatin above the iep (pH 5.6), the emulsions with surfactant have a significantly smaller size, which is probably due to the higher amount 5
Bagchi, P., unpublished results.
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LLOYD LOBO
of free SDS in the latter two emulsions. Thus, when designing formulations for emulsions made in the presence of gelatin, the amount of surfactant required to reduce the drop size will depend on the pH of the formulation relative to the iep of the gelatin. Muller et al. (9) also found a similar pH dependency with the SDBS/gelatin system. The gelatin used in their study is the LPO gelatin. CONCLUSIONS
This paper presents an extensive, if not exhaustive, study of the emulsification of oil into water, in the presence of gelatin. Some of the key areas addressed are gelatin level and MW, gelatin interaction with surfactant, and finally the gelatin pH. Gelatin by itself is shown to be an adequate stabilizer of emulsions, even in high-pressure homogenizers, when the rates of surface generation are high. However, gelatin is not as effective at drop rupture and formation of small drops, compared to systems where surfactants are present. The explanation we are offering is that surfactants are needed to be able to change the mobility of surfaces, via interfacial tension gradients, and promote a different mode of rupture as described by Milliken and Leal (26). At substantially high levels of anionic surfactant, coalescence is actually higher than that obtained with gelatin in the absence of surfactant, while the drop sizes are smaller. Nonionic surfactants, on the other hand, are very inefficient at stabilizing drops in a gelatin containing system. This is because they work to completely displace gelatin from drop surfaces, which in turn can promote flocculation. The high level of coalescence results in the production of large drop size emulsions. The gelatin MW has a significant impact on the rupture of drops as evidenced by the decrease in drop size as the gelatin MW is reduced, even as the amount of coalescence increases. It is believed that the gelatin molecules are attenuating the turbulent flow characteristics, particularly the eddy size, and the size of the gelatin molecules determines the size of the eddies that are allowed to exist. There is an optimal MW at which the rupture is most efficient. Below this MW we believe that the decrease in the viscosity results in less efficient transfer of momentum across drop interface. If drop rupture occurs in the shear-controlled regime, as in some rotor-stator devices, then the viscosity of the continuous phase becomes important, and the smaller drops are formed with the higher MW gelatins. Although the pH of the gelatin impacts the density and thickness of the adsorbed layer, it was not found to impact the coalescence in a homogenizer. In the absence of added surfactant, the pH did not have an impact on the drop size. In the
presence of surfactant, while the coalescence was not affected by pH, the drop sizes were. At the low pH, because of high surfactant binding to the free gelatin, there is insufficient free surfactant to promote rupture. ACKNOWLEDGMENTS I thank Kurt Schroeder for help with the drop size measurements. Thanks to Aileen Svereika and Eileen Beverly for help with the experiments with gelatin and SDS. The help of Xavier Lallemand and Christophe Amyot with the preparation of the low-MW gelatins and the homogenization experiments with those gelatins is gratefully acknowledged.
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