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Properties of Palm Oil-in-Water Emulsions Stabilized by Nonionic Emulsifiers
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
181, 595–604 (1996)
0417
Properties of Palm Oil-in-Water Emulsions Stabilized by Nonionic Emulsifiers K. AHMAD,* C. C. HO,* ,1 W. K. FONG,†
AND
D. TOJI†
*Department of Chemistry, University of Malaya, 59100 Kuala Lumpur, Malaysia; and †Sumirubber Ind. (M) Sdn. Bhd., Bakar Arang Industrial Estate, 08000 Sungai Petani, Kedah, Malaysia Received November 13, 1995; accepted February 26, 1996
Palm oil-in-water emulsions prepared using various ethoxylated nonionic emulsifiers were studied with respect to the effect of concentration and type of the selected emulsifiers, and the emulsification process itself on the properties of the emulsions obtained. The stability of the emulsion during storage was monitored. The physical properties were characterized by particle size analysis, turbidity, interfacial tension, and electrophoretic mobility measurements. Results show that emulsion stability was strongly dependent upon the types of palm oil and emulsifier used and their concentrations. Stable emulsions with minimal creaming at room temperature for up to 6 months can be prepared with the right choice of emulsifier and under the correct conditions of emulsification. The influence of the interfacial layer of emulsifier on emulsion stability is discussed. q 1996 Academic Press, Inc. Key Words: palm oil emulsions; nonionic surfactants; emulsifiers; oil-in-water emulsions; emulsion stability.
INTRODUCTION
Palm oil is widely used as a cooking oil and also as a vegetable shortening. In contrast, its applications in pharmaceutical, engineering, and industry have not been fully explored and therefore remained rather limited. The incorporation of mineral oils into polymers for special applications has been known for some time, for example, in the manufacturing of oil-extended styrene butadiene rubber ( 1 ) . The oil functions as a softener or diluent in most of these applications. However, there does not seem to be any report on the use of palm oil, directly or as emulsions, for this purpose. There is considerable potential of using palm oil in various industrial processes and as additives, judging from recent reports (2, 3) on progress made with other vegetable oils. Recently, Ooi (4) has described the potential use of esters derived from palm oil as plasticizers for polyvinyl chloride. An innovative use of palm oil in such industrial applications would open up a new vista for nonfood applications of palm oil. 1
To whom correspondence should be addressed.
An added advantage is that palm oil is a vegetable oil and therefore nonexhaustive and renewable. In some instances, it may be necessary to use the oil in the form of an emulsion. An advantage of an emulsion is the large surface area that the particulate system offers in enhanced interaction at the oil/water/substrate interface. Further, palm oil emulsion does not require any organic solvent in its preparation. An important consideration when employing palm oil emulsions in any processing is their dispersion stability. The emulsion must be able to remain dispersed, unaggregated, and resistant to creaming within the time frame needed. In this study various palm oil emulsions were prepared under different emulsification conditions, and their physical properties and stability were monitored. Since palm oil consists of mainly triglycerides of different degrees of unsaturation in the fatty acid components, even commercial refined, bleached, and deodorized palm (RBDP) olein would eventually crystallize when the conditions are right. Thus to prevent crystallization (5), the oil was kept at above 257C. This is because the properties of the emulsions, specifically the droplet size, are dependent on the chemical nature and molecular structure of the triglycerides present. MATERIALS AND METHODS
Three types of commercial palm oils were used as received throughout this work, namely unfractionated RBDP oil (PO), RBDP olein (OL), and RBDP stearin (ST). Oil from the same batch was used for each series of emulsification procedures to minimize possible compositional variations in oil properties such as iodine values and impurities present therein which may influence the emulsion properties. The compositions of these oils are given in Table 1. The oil was first heated in a water bath to 557C until it became clear and then cooled to 407C. It was maintained at this temperature and the appropriate amount of emulsifier was added. Distilled water, also preheated to 407C, was then added to the oil and homogenized using a variable-speed homogenizer (6) (Tokushu Kikakogyo, Model 2.5) at 407C.
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0021-9797/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
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TABLE 1 Characteristics of Different Palm Oils Fatty acid (%) Saturated Lauric Myristic Palmitic Stearic Arachidic Monounsaturated Palmitoleic Oleic Polyunsaturated Linoleic Linolenic Iodine value (Wijs) Melting point (7C) Cloud point (7C)
Symbol
PO
OL
ST
12:0 14:0 16:0 18:0 20:0
0.1–0.4 0.9–1.5 41.8–46.8 4.2–5.1 0.2–0.7
0.1–0.6 0.9–1.4 37.9–41.7 4.0–4.8 0.2–0.5
0.1–0.4 1.1–1.9 47.2–73.8 4.4–5.6 0.1–0.6
16:1 18:1
0.1–0.3 37.3–40.8
0.1–0.4 40.7–43.9
0.05–0.2 15.6–37.0
18:2 18:3
9.1–11.0 0.0–0.6 52.3 48.5 21.5
10.4–13.4 0.1–0.6 56.0 16.0 6.0
3.2–9.8 0.1–0.6 37.2 51.0 31.0
Various nonionic emulsifiers of the polyoxyethylene (POE) ether types (Emulgen series, Kao Corp.) were used without further purification. Cloud points of these emulsifiers were determined at 1% w/w solution in the appropriate medium indicated in Table 2. The emulsification conditions for each investigation are given in Table 3. Characterization of Emulsion Phase separation. Immediately after emulsification, 100 ml of the emulsion was poured into a 100-ml measuring cylinder. The volumes of the different phases that separated out were recorded as a function of time. Direct observation of the emulsion under a polarizing microscope to assess the state of aggregation of the droplets was also carried out. Turbidity measurement. One milliliter of emulsion was withdrawn, at regular intervals, from the the middle portion
of the measuring cylinder containing the emulsion (see text later), diluted with distilled water to 500 ml in a volumetric flask. Care was taken that the dispersion in the flask was not subjected to any undue agitation. This is to ensure that minimum disturbance was experienced by the droplets. The transmittance of the dispersion was measured immediately at 480 nm (7). A Shimadzu UV3101PC was used for these measurements. Total oil content determination. About 1.0 g sample of emulsion was withdrawn from the middle portion, with due care taken to exclude any of the top phases while sampling. The sample was dried in an aluminum dish at 1057C for 1.5 h. The weight of oil remaining was determined. Duplicate determinations were carried out. Particle size analysis. The particle size distribution (PSD) of the emulsion was determined using the Malvern Mastersizer E (laser diffraction technique). The sample was analyzed within 1 min of emulsification and also during storage. Viscosity measurement. The viscosity of oil emulsion was determined at 287C using a Brookfield LVF viscometer in a 600-ml beaker at 60 rpm. Reading was taken after 1 min. Determination of zeta potential. The zeta potentials of the oil droplets in the OL emulsions were determined, using a Zetamaster S (Malvern Instruments), as a function of electrolyte concentration and also pH. pH was adjusted using sodium hydroxide or hydrochloric acid solutions. Interfacial tension measurement. The ring method was used to determine the oil/water interfacial tension with a Kruss K8 tensiometer at 307C. RESULTS AND DISCUSSION
After emulsification, an emulsion thus formed would, on standing, establish a concentration gradient: a phase bound-
TABLE 2 Types of Emulsifiers Used and Their HLB Numbers and Cloud Points Cloud point (7C)
Emulsifiers E105 E106 E404 E408 E705 E903 E905 E906 E911 E930
Chemical names Polyoxyethylene Polyoxyethylene Polyoxyethylene Polyoxyethylene Polyoxyethylene Polyoxyethylene Polyoxyethylene Polyoxyethylene Polyoxyethylene Polyoxyethylene
lauryl ether lauryl ether oleyl ether oleyl ether fatty alcohol ether nonylphenyl ether nonylphenyl ether nonylphenyl ether nonylphenyl ether nonylphenyl ether
EO No.
HLB
In water
In 25% acetic acid
In 35% acetic acid
5 6 4 8 5 3 5 6 11 30
9.7 10.5 8.8 10.0 10.5 7.8 9.2 10.8 13.7 15.1
Oil-soluble Oil-soluble Oil-soluble Oil-soluble Oil-soluble Oil-soluble Oil-soluble Oil-soluble 71.3 97.2
42.3 69.5 29.3 50.5 60.2 õ5 õ5 30.1 N.D. N.D.
N.D. N.D. N.D. N.D. N.D. õ5 8.2 N.D. N.D. N.D.
Note. N.D., not determined.
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TABLE 3 Emulsification Conditions Emulsifying parameters Effect Effect Effect Effect Effect Effect
of of of of of of
Types of emulsifiers
Emulsifier (% w/w)
Oil (% w/w)
Type of oil
Homogenizer speed (rpm)
Temperature (7C)
Varied E906 E906 E906 E906 E906
10, 12 Varied 12 12 12 12
20 20 Varied 20 20 20
OL OL OL Varied OL OL
6000 6000 6000 6000 Varied 6000
30, 40 30, 40 40 40 30, 40 Varied
type of emulsifiers concentration of emulsifiers (%) oil content (%) type of oil shear rate (rpm) emulsifying temperature (7C)
ary would appear separating the top from the bottom layer, the rate of formation of which depends on the inherent instability of the emulsion. Due to coalescence and flocculation, an increase in the concentration gradient with time is expected. Thus by sampling the emulsion at the middle portion of the measuring cylinder (the 50-ml mark), the oil content can be taken as a reference to the stability of the emulsion. For emulsion containing 20% w/w oil, the phase boundary between the cream and the bottom aqueous layer would be way above the 50-ml mark if it is unstable and thus would have a low oil content at the middle portion. The stability of the palm oil-in-water emulsions can thus be monitored and characterized during storage. An emulsion is considered stable if it remains as a single phase without phase separation over the entire period under study. It is deemed to be unstable if the emulsion separates into an oil layer on top and an aqueous layer at the bottom. Most of the emulsions prepared exhibited some degree of instability, such as creaming, intermediate between the two extremes. Optimum emulsification occurred at a shear rate of 6000 rpm for 30 min at 407C. A higher shear produced the same droplet size in a shorter period of time but would not reduce the size of the oil droplet further. Thirty minutes of shearing time seemed sufficient for the comminution process. Prolonged shearing can in fact cause instability due to shearinduced flocculation. The optimal emulsification temperature for E906 was 407C, under which the smallest oil droplets were produced. The emulsification process was scaled up and tested with an industrial homogenizer of similar design (with a capacity of up to 150 kg) in a factory. It was found that the droplet size of the emulsions prepared in the factory was comparable to those from the laboratory under similar optimum emulsification conditions. Oil-in-water emulsions were prepared using different: (i) types of commercial emulsifiers; (ii) concentrations of emulsifier; (iii) oil contents in emulsion;
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It is noticed that emulsifiers having about the same HLB values (all except E903, E911, and E930; see Table 2) gave emulsions of varying stability as indicated by the oil retained (Fig. 1). In addition, emulsifiers with different hydrophobes but with the same EO number (E105, E705, E905) differ in their emulsifying capability. This is also true for those with the same hydrophobe but different EO numbers (E903, E905, E906, E911, E930). There does not appear to be any correlation between the cloud points of the emulsifiers and the HLB number either. The stability of these emulsions showing various stages of phase separation is illustrated in Fig. 2. The slightly inferior stability of emulsions prepared from E705 compared to those from E105 may be due to the presence of the alcohol group in the former which renders the polar and nonpolar ends of the emulsifier molecule less distinguishable (8). An O/W emulsion could not be formed when E905 was used as the emulsifier. Tajima et al. (7) found that interaction between the p-electron of the phenyl group and the adjacent EO groups in POE nonylphenyl ether type emulsifiers (E905) altered its molecular configuration at the O/W interface compared to that of the lauryl ether
FIG. 1. Effect of HLB and cloud point on the stability of emulsions prepared using different emulsifiers: 20% w/w OL with 10% w/w emulsifier emulsified at 6000 rpm for 30 min at 307C.
(iv) types of palm oils.
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FIG. 2. Phase separation of emulsions in 100-ml measuring cylinders. Emulsions prepared using different emulsifiers at 20% w/w OL, 10% w/w emulsifier at 6000 rpm for 30 min at 307C.
types (E105), resulting in different emulsion stability of liquid paraffin oil involving these two types of emulsifiers. It was found that for the POE lauryl (E105, E106) and oleyl (E404, E408) ether series, stability of the emulsions decreased as the EO number increased. For the POE nonylphenyl (E903, E905, E906, E911, E930) ether series, the most stable emulsions were those prepared with the oilsoluble E906, with 6 EO numbers. E903 and E905 are more hydrophobic than E906, but E911 and E930 are both watersoluble in comparison with E906 and therefore would be less likely to desorb from the oil–water interface into the oil globule. Since it is known that a nonionic emulsifier with a higher free energy of adsorption gives more stable emulsions (7, 9), both E903 and E905 are predicted to give less stable emulsions compared with those with longer EO chains as was observed here. The less stable emulsions formed by the water-soluble emulsifiers E911 and E930 could also possibly be depletion flocculated at high emulsifer concentration as observed by other workers (10). It is not clear why emulsion stability should peak at E906 and not increase with EO chain length of the emulsifier all the way to E930 as expected. Tajima et al. (7), working on liquid paraffin O/W emulsions stabilized with POE nonylphenyl ethers with EO groups ranging from 2 to 12, have found that emulsions stabilized by emulsifiers with less than
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6 EO numbers were not stable at pH ca. 3. However, there was little difference in the stability of polyethylene latex particles stabilized by POE octylphenyl ethers with 9.5 and 16 EO numbers; instead, at longer chain length, a decrease in stability was actually observed (11). On the other hand, for anisole and chlorobenzene in water emulsions stabilized by hexadecyl POE glycol ethers (9), it was found that the emulsion stability increased with an increase in EO number but the electrophoretic mobility of the emulsion droplets decreased. Shinoda and Sagitani (12) concluded that for cyclohexane-in-water emulsions stabilized by POE nonylphenyl ethers, the most stable O/W emulsion was formed with those having 4.3–5.6 EO numbers. Thus no clearcut trend is apparent for these emulsions. It would appear that the stability of OW emulsions stabilized by ethoxylated type nonionic emulsifiers depends strongly on factors that influence the prevailing conditions at the O/W interface and also on the nature of the interfacial layer formed. Completely different behavior was observed for different oil emulsions. It would appear from this work that E906 is the best emulsifier for palm olein. The particle size distribution of palm olein droplets in Fig. 3 shows that emulsions prepared with E906 gave the smallest average droplet size and the narrowest spread in size. All subsequent experiments were performed using this (E906).
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FIG. 3. Particle size distribution of OL droplets prepared using different emulsifiers: 20% w/w OL with 10% w/w emulsifier emulsified at 6000 rpm for 30 min at 307C.
It should be mentioned that palm oil itself contains trace amount of naturally occurring emulsifiers such as phospholipids which can cause self-emulsification of palm oil. The presence of trace amounts of mono- and diglycerides in the oil can also affect the overall composition of the naturally occurring emulsifiers present at the O/W interface. Thus in considering the effect of emulsifier on the emulsification of palm oil, one should also be aware of the possible contribution from these naturally occurring emulsifiers.
ture of the interfacial film formed and needs a higher positive gi (15). The gi of emulsifiers with very low and very high HLB, namely E903 and E930, were significantly higher than those of the other emulsifiers having intermediate HLB values. However, the stability of the emulsions thus prepared does not reflect the same order as represented by the HLB values. Further, the behavior of E905 was rather puzzling as men-
Effect of Emulsifier on Interfacial Tension at O/W Interface The interfacial tensions, gi , of the different emulsifiers at the O/W interface shown in Fig. 4 indicate that at 1.0% w/ w, the gi of E404 and E408 were lower than that of E906, even though emulsions prepared using E906 were the most stable of the three. This perhaps could be attributed to the presence of the double bond in the oleyl ether group of E404 and E408 preventing close-packing of the hydrocarbon tails at the curvaceous O/W interface of an emulsion drop. Furthermore there is ample evidence (13, 14) that reduction of gi alone does not ensure emulsion droplet stability, especially when a high-shear homogenizer is used in the emulsification process. This is because emulsification is enhanced by low gi whereas emulsion stability is related to the struc-
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FIG. 4. The variation of HLB and interfacial tension of the OL–water interface with different types of emulsifiers. Concentration of emulsifier was 1% w/w.
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FIG. 5. The effect of E906 concentration on the interfacial tension of the OL–water interface measured at 30 and 407C, respectively.
FIG. 6. The effect of E906 concentration on the stability of the emulsions formed at 20% w/w OL, emulsified at 6000 rpm for 10 min at 307C.
tioned above. At 0.1% w/w, its gi was comparable to that of E906 and yet it failed to form any emulsion. It should be noted that E905 has a very low cloud point of 8.07C in 35% w/w acetic acid which indicates its very low solubility in water. The gi of the O/W interface as a function of concentration of E906 at 30 and 407C is shown in Fig. 5. It can be seen that it decreased gradually from about 1.2 mN m01 to just above zero at 1.0% w/w. No abrupt change in slope was discernible. The interface remained clear throughout the measurements. Preliminary study had also shown no time effect of interfacial tension of the OL/water interface; i.e., the interfacial tension remained constant with contact time of the two phases. When emulsifier was added to and present in the oil phase, again no change in interfacial tension with time was noted and equilibrium was reached within ca. 3 min after the OL/water interface was formed in all cases. This tends to imply that for the present system emulsifier molecules were adsorbed rapidly at the OL/water interface. Thus, during the emulsification process, this was able to produce a protective layer around the droplet rapidly. The role played by the kinetics of the process is not so important.
droplets in these emulsions as revealed in the d ( 0.9 ) values, the diameter at which 90% of the droplets fall below these as shown in Fig. 7. It is clear that emulsion prepared with 12% w / w E906 contained the lowest number of big droplets and therefore creamed the least. Thus all subsequent experiments were performed at an optimal emulsifier concentration of 12% w / w. An apparent emulsifier layer thickness can be calculated from the known volume fraction of the emulsifier with respect to that of the oil phase and the area-weighted drop diameter D(3,2) (16, 17). It is assumed that all emulsifiers are located at the surface of the oil droplets but of course in reality the emulsifiers are also found dissolved in the oil or the water phases depending on whether the emulsifier is oil- or water-soluble. Various parameters calculated for OL emulsions emulsified with the oil-soluble E906 are given in Table 4. The number of adsorbed layers on the droplet was calculated based on the assumption that the length of the ˚ , estimated from the molecular E906 molecule is 33.45 A
Effect of Emulsifier Concentration Optical microscopy observation reveals that relatively small oil droplets were obtained in emulsion prepared with as low as 8% w / w E906. The amount of creaming was the lowest at 12% w / w E906 ( Fig. 6 ) , above which the stability of the emulsion decreased, but the mean particle size remained essentially constant at 2.0 mm over an E906 concentration range of 8 – 14% w / w ( Fig. 7 ) . Similarly the polydispersity of droplet size was also the lowest observed over this concentration range. The higher creaming observed for emulsions prepared with less than 12% w / w E906 corresponded to the presence of much bigger
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FIG. 7. The effect of E906 concentration on the droplet size and polydispersity of the emulsions formed. Conditions as in Fig. 6.
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TABLE 4 Droplet Size, Specific Surface Area, and Adsorbed Emulsifier Layer Thickness Calculated for OL-in-Water Emulsions E906 (%)
D(4, 3) (mm)
D(3, 2) (mm)
r a (mm)
SSAb (m2/g)
fc
˚) Dd (A
Number of emulsifier layer, Ne
2 4 6 8 10 12 14 16
7.97 5.40 3.53 2.99 2.92 2.61 2.67 2.68
2.47 1.94 1.68 1.51 1.49 1.42 1.42 1.42
1.25 0.97 0.84 0.76 0.75 0.71 0.71 0.71
2.6735 3.4375 3.9792 4.4280 4.4845 4.6784 4.6900 4.6932
0.0173 0.0346 0.0519 0.0692 0.0865 0.1038 0.1212 0.1385
71 111 143 170 209 238 276 314
2.1 3.3 4.3 5.1 6.2 7.1 8.3 9.4
a
r Å D(3, 2)/2. SSA, specific surface area of emulsion droplet. c f, volume fraction of emulsifier to oil, roil 0.90, rE906 1.04. d D, apparent emulsifier layer thickness Å {fr 3 / r 3}1/3 0 r. e ˚ based on molecular model calculations. N Å D/33.45, where length of E906 is 33.45 A b
model. It can be seen (Fig. 8) that the specific surface area of the emulsions increased rapidly with E906 concentration until ca. 12% w/w, beyond which it remained almost constant. Similarly D(3,2) also remained unchanged above 12% w/w E906. This shows that despite an increase in E906 concentration to more than 12% w/w, smaller droplets were not formed. On the other hand, there was a slight increase in D(3,4), the volume-weighted diameter, when E906 concentration was increased from 12 to 16% w/w. The apparent thickness of the emulsifier layer increased ˚ with increasing concentration of E906. The value of 71 A ˚) at 2% w/w E906 is comparable to that for b-casein (80 A on hexadecane O/W emulsions (18). At 16% w/w E906, ˚ (0.03 mm), equivalent the surface layer thickness was 314 A to about nine layers of emulsifiers. If the smallest droplet formed at 12% w/w E906 was 2.61 mm [D(4,3)], then an additional 0.03 mm contributed by nine layers of adsorbed emulsifiers would result in a droplet size of 2.67 mm, in
close agreement with the D(4,3) of the droplets of 16% w/w E906 emulsion. Thus the slight increase in D(4,3) observed when E906 concentration was increased from 12 to 16% w/ w can be attributed to multilayer adsorption of E906 at the O/W interface. Below 12% w/w E906, the magnitude and the decrease in D(4,3) with E906 concentration was too big for it to be significant. This observation is corroborated by electrokinetic mobility data and creaming results. Increasing the E906 concentration caused a decrease in zeta potential of the emulsion droplets (see Fig. 13). An increase in the layer thickness of adsorbed emulsifiers consequent to increasing E906 concentration would lead to a shift of the plane of shear away from the surface, resulting in a drop in zeta potentials. The increase in creaming observed (Fig. 6) with emulsions containing more than 12% w/w E906 could be explained by an increase in the size of the oil droplets given above. Effect of Oil Concentration
FIG. 8. The droplet size and surface area of the emulsions as a function of E906 concentration. Emulsification conditions as in Fig. 6.
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As the oil content of the emulsion increased at constant emulsifier concentration, the droplet size increased gradually to ca. 2.5 mm at 40% w/w oil, beyond which a sudden drop in particle size to 1.2 mm occurred at 50% w/w oil and the size then increased again on further increase of the oil concentration (Fig. 9). The occurrence of a minimum particle size at a particular oil content is not uncommon for emulsions containing nonionic surfactants where HLB value consideration is important. It has been noted previously (19) that variation in EO number distribution of an emulsifier can affect the size of the emulsion droplets formed. Using E906 from the same batch, this variation was minimized. The viscosities of the emulsions (same as those described in Fig. 9) increased with oil concentration over the whole range studied, from 3.6 cP at 10% w/w oil to the highest viscosity of 2980 cP at 60% w/w oil. High viscosities of stable emulsion are associated with the net repulsive force
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FIG. 9. Effect of oil concentration on the droplet size of OL emulsion: E906 at 12% w/w emulsified at 6000 rpm for 30 min at 407C.
acting among the particles (20). The PSD of droplets in the 50% w/w emulsion remained unchanged even after 6 months storage at room temperature and was stable. Effect of Palm Oil Type ST emulsion was found to consist of large, irregularly shaped, crystallite-like particles (ST crystallites) in contrast to the small oil droplets found in PO and OL emulsions. These large particles were formed probably as a result of partial coalescence, i.e., crystallites from one droplet penetrating into the liquid region of another droplet, causing them to aggregate. Even though PO contains both OL and ST fractions, the emulsion droplets prepared using PO appeared smooth and did not reveal the presence of any free disperse stearin particles and the appearance of the droplets was similar to that prepared with OL. There are two possibilities for this. First is probably because the ST crystallites were soluble in the OL fraction (liquid at room temperature) of the PO droplets and second is because of supercooling effects. If crystallization of ST was inhibited by supercooling, the number of crystallites protruding from the oil droplets would be reduced and the stability of emulsion droplets would improve. The PSD of the droplets in PO emulsion was found to be almost identical to that of the OL emulsion and was not intermediate between those for OL and ST as expected (Fig. 10) if crystallites of ST particles were present. This would seem to concur also with results reported recently (21) on n-hexadecane-in-water emulsions that the rate of crystallization of oil droplets depends on the effects emulsifier molecules have on the interaction between droplets. OL and ST oils were blended to give oils of various iodine values. These were then homogenized and the PSD of the emulsions was determined. It can be seen from Fig. 11 that crystallization of stearin occurred only when the iodine values of the oil fell below 50. Thus PO, with an iodine value
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FIG. 10. Effect of storage time on the stability of emulsions of different types of palm oils: E906 at 12% w/w emulsified at 6000 rpm for 30 min at 407C.
of ca. 52, was in a region where crystallization of stearin was not expected to occur. This would tend to support the reasons given above on why the saturated triglycerides in PO failed to crystallize. Consequently the PSD of PO droplets was similar to that of OL droplets. It is known that the interaction between the oil and the emulsifier at the O/W interface depends strongly on the nature of both of these substances. The types of palm oils used vary both in triglyceride composition and in the degree of unsaturation. E906, being nonionic, is very sensitive to the balance of unsaturation of the oil. A highly saturated oil is comparatively more hydrophobic and would require a more hydrophobic nonionic emulsifier and vice versa. This perhaps explains the diversity in the behavior found for the different oils and the ease with which these oil could be emulsified using different emulsifiers. Stability of Emulsion during Storage Emulsions prepared using 12% E906 were stable during storage at room temperature for up to at least 1 month with-
FIG. 11. The effect of the degree of unsaturation of the oil (as measured by its iodine value) on the droplet size of the emulsion formed.
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out creaming and aggregation, except those prepared from ST oil. Even though substantial creaming was observed for emulsions prepared with E705, E911, and E930, the aggregated oil droplets were able to maintain their individuality and, upon agitation, became redispersed. Thus for the OL emulsions, destabilization of the emulsion by Ostwald ripening can be excluded as can be seen from Fig. 10: the particle size remained unchanged after almost 1 month. For Ostwald ripening to occur the dispersed phase must have some degree of solubility in the aqueous medium. Solubility of palm oil in water was found to be well below 100 mg dm03 at 287C. For dilute O/W microemulsions the rate of ripening decreased as the volume fraction increased (22). The emulsions reported here contained 20% w/w oil. It is observed that droplets of ST emulsion became bigger 1 day after storage at room temperature. This could be explained by growth via aggregation following crystallization of ST during cooling down from 407C to room temperature as mentioned above. Change in droplet size then ceased thereafter. It has been observed that partially crystalline emulsions such as n-hexadecane aggregate on cooling (21). Electrokinetic Behavior of Palm Olein Emulsions The zeta potential–pH curves for the emulsions in the presence of 15 1 10 03 mol dm03 NaCl and 1.5 1 10 04 mol dm03 CaCl2 are shown in Fig. 12. It can seen that the oil droplets stabilized by E906, a nonionic surfactant, were negatively charged and the zeta potential increased significantly as the pH was increased. The same behavior has been reported by Tajima et al. (7) previously and is typical of nonionogenic surfaces where the negative charges resulted from adsorption of hydroxyl ions onto the droplet surface. Figure 13 shows that a gradual drop in zeta potential with increase in E906 concentration was observed except that there was a steeper dip at 12% w/w followed by a sharp
FIG. 13. The variation of zeta potential and droplet size of emulsions as a function of E906 concentration. 20% w/w OL, emulsified at 6000 rpm for 30 min at 307C.
increase. The drop in potential at 12% w/w seems to coincide with the smallest droplet size of the emulsion at this E906 concentration. A decrease in the zeta potential of silver iodide sol particles with an increase in the concentration of POE glycol monoethers was noted previously (23) and was attributed to multilayer adsorption of the emulsifier at the particle surface. The present results are in close agreement with this except at 12% w/w E906 where perhaps the influence of the particle is also important. CONCLUSION
The most stable emulsions of palm olein were obtained using a nonionic emulsifier (E906), ethoxylated nonylphenyl ether with 6 EO numbers. The emulsification conditions for producing these stable emulsions have been identified. Some interesting effects of molecular structure of the emulsifier, finely divided solids (ST) at the O/W interface, and excess emulsifier on the emulsion stability were noted and explained by known interfacial phenomena. ACKNOWLEDGMENT This work was partially supported by Sumitomo Rubber Industries Japan and Sumirubber Industries Malaysia.
REFERENCES
FIG. 12. The zeta potential–pH curves for OL-in-water emulsion droplets in the presence of 15 1 10 03 mol dm03 NaCl and 1.5 1 10 04 mol dm03 CaCl2 . E906 at 12% w/w emulsified at 6000 rpm for 30 min at 407C.
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1. Allens, P. W., and Jones, K. P., in ‘‘Natural Rubber Science and Technology’’ (A. D. Roberts, Ed.), p. 13. Oxford University Press, London, 1988. 2. Vesala, A. M., Rosemholm, J. B., and Laiho, S., J. Am. Oil Chem. Soc. 62, 1379 (1985). 3. Schwab, A. W., Nielsen, H. C., Brooks, D. D., and Pryde, E. H., J. Dispersion Sci. Tech. 4(1), 1 (1983). 4. Ooi, K. S., ‘‘A Study of Some Esters Derived from Palm Oil: Synthesis,
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5. 6. 7. 8. 9. 10. 11. 12. 13.
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Characterization and Potential Applications.’’ M.Sc thesis, University of Malaya, Kuala Lumpur, 1991. Oh, C. H., ‘‘Solvent Fractionation and Crystallization Behaviour of Palm Oil.’’ M.Sc. thesis, University of Malaya, Kuala Lumpur, 1979. Matsumoto, S., in ‘‘Non-ionic Surfactants’’ (M. J. Schick, Ed.), p. 549. Marcel Dekker, New York, 1987. Tajima, K., Koshinuma, M., and Nakamura, A., Colloid Polym. Sci. 270, 759 (1992). Garrett, H. E., ‘‘Surface Active Chemicals,’’ p. 18. Pergamon Press, London, 1972. Elworthy, P. H., and Florence, A. T., J. Pharm. Pharmacol. 21, 70S (1969). McClements, D. J., Colloids Surf. A: Physico Eng. 90, 25 (1994). Stryker, H. K., Helin, A. F., and Mantell, G. J., J. Appl. Polym. Sci. 10, 81 (1966). Shinoda, K., and Sagitani, H., J. Colloid Interface Sci. 64, 68 (1978). Garti, N., and Katz, M., J. Dispersion Sci. Tech. 6, 149 (1984).
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14. Chow, M. C., and Ho, C. C., J. Am. Oil Chem. Soc. 73(1), 47 (1996). 15. Rosano, H. L., Jon, D., and Whittam, J. H., J. Am. Oil Chem. Soc. 59, 360 (1982). 16. Becher, P., ‘‘Emulsions: Theory and Practice,’’ p. 2. Reinhold, New York, 1966. 17. Ostberg, G., Bergenstahl, B., and Hulden, M., Colloids Surf. A: Physico Eng. 94, 161 (1995). 18. Dickinson, E., J. Chem. Soc. Faraday Trans. 88, 2973 (1992). 19. Shinoda, K., and Friberg, S., in ‘‘Emulsion and Solubilization,’’ p. 148. Wiley, New York, 1986. 20. Van den Tempel, M., in ‘‘Rheology of Emulsions’’ (P. Sherman, Ed.), p. 1. Pergamon Press, Elmsford, NY, 1963. 21. McClements, D. J., Dickinson, E., Dungan, S. R., Kinsella, J. E., Ma, J. G., and Povey, M. J. W., J. Colloid Interface Sci. 160, 293 (1993). 22. Taylor, P., and Ottewill, R. H., Colloids Surf. A: Physico Eng. 88, 303 (1994). 23. Hunter, R. J., ‘‘Zeta Potential in Colloid Science,’’ p. 324. Academic Press, London, 1981.
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0417
Properties of Palm Oil-in-Water Emulsions Stabilized by Nonionic Emulsifiers K. AHMAD,* C. C. HO,* ,1 W. K. FONG,†
AND
D. TOJI†
*Department of Chemistry, University of Malaya, 59100 Kuala Lumpur, Malaysia; and †Sumirubber Ind. (M) Sdn. Bhd., Bakar Arang Industrial Estate, 08000 Sungai Petani, Kedah, Malaysia Received November 13, 1995; accepted February 26, 1996
Palm oil-in-water emulsions prepared using various ethoxylated nonionic emulsifiers were studied with respect to the effect of concentration and type of the selected emulsifiers, and the emulsification process itself on the properties of the emulsions obtained. The stability of the emulsion during storage was monitored. The physical properties were characterized by particle size analysis, turbidity, interfacial tension, and electrophoretic mobility measurements. Results show that emulsion stability was strongly dependent upon the types of palm oil and emulsifier used and their concentrations. Stable emulsions with minimal creaming at room temperature for up to 6 months can be prepared with the right choice of emulsifier and under the correct conditions of emulsification. The influence of the interfacial layer of emulsifier on emulsion stability is discussed. q 1996 Academic Press, Inc. Key Words: palm oil emulsions; nonionic surfactants; emulsifiers; oil-in-water emulsions; emulsion stability.
INTRODUCTION
Palm oil is widely used as a cooking oil and also as a vegetable shortening. In contrast, its applications in pharmaceutical, engineering, and industry have not been fully explored and therefore remained rather limited. The incorporation of mineral oils into polymers for special applications has been known for some time, for example, in the manufacturing of oil-extended styrene butadiene rubber ( 1 ) . The oil functions as a softener or diluent in most of these applications. However, there does not seem to be any report on the use of palm oil, directly or as emulsions, for this purpose. There is considerable potential of using palm oil in various industrial processes and as additives, judging from recent reports (2, 3) on progress made with other vegetable oils. Recently, Ooi (4) has described the potential use of esters derived from palm oil as plasticizers for polyvinyl chloride. An innovative use of palm oil in such industrial applications would open up a new vista for nonfood applications of palm oil. 1
To whom correspondence should be addressed.
An added advantage is that palm oil is a vegetable oil and therefore nonexhaustive and renewable. In some instances, it may be necessary to use the oil in the form of an emulsion. An advantage of an emulsion is the large surface area that the particulate system offers in enhanced interaction at the oil/water/substrate interface. Further, palm oil emulsion does not require any organic solvent in its preparation. An important consideration when employing palm oil emulsions in any processing is their dispersion stability. The emulsion must be able to remain dispersed, unaggregated, and resistant to creaming within the time frame needed. In this study various palm oil emulsions were prepared under different emulsification conditions, and their physical properties and stability were monitored. Since palm oil consists of mainly triglycerides of different degrees of unsaturation in the fatty acid components, even commercial refined, bleached, and deodorized palm (RBDP) olein would eventually crystallize when the conditions are right. Thus to prevent crystallization (5), the oil was kept at above 257C. This is because the properties of the emulsions, specifically the droplet size, are dependent on the chemical nature and molecular structure of the triglycerides present. MATERIALS AND METHODS
Three types of commercial palm oils were used as received throughout this work, namely unfractionated RBDP oil (PO), RBDP olein (OL), and RBDP stearin (ST). Oil from the same batch was used for each series of emulsification procedures to minimize possible compositional variations in oil properties such as iodine values and impurities present therein which may influence the emulsion properties. The compositions of these oils are given in Table 1. The oil was first heated in a water bath to 557C until it became clear and then cooled to 407C. It was maintained at this temperature and the appropriate amount of emulsifier was added. Distilled water, also preheated to 407C, was then added to the oil and homogenized using a variable-speed homogenizer (6) (Tokushu Kikakogyo, Model 2.5) at 407C.
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TABLE 1 Characteristics of Different Palm Oils Fatty acid (%) Saturated Lauric Myristic Palmitic Stearic Arachidic Monounsaturated Palmitoleic Oleic Polyunsaturated Linoleic Linolenic Iodine value (Wijs) Melting point (7C) Cloud point (7C)
Symbol
PO
OL
ST
12:0 14:0 16:0 18:0 20:0
0.1–0.4 0.9–1.5 41.8–46.8 4.2–5.1 0.2–0.7
0.1–0.6 0.9–1.4 37.9–41.7 4.0–4.8 0.2–0.5
0.1–0.4 1.1–1.9 47.2–73.8 4.4–5.6 0.1–0.6
16:1 18:1
0.1–0.3 37.3–40.8
0.1–0.4 40.7–43.9
0.05–0.2 15.6–37.0
18:2 18:3
9.1–11.0 0.0–0.6 52.3 48.5 21.5
10.4–13.4 0.1–0.6 56.0 16.0 6.0
3.2–9.8 0.1–0.6 37.2 51.0 31.0
Various nonionic emulsifiers of the polyoxyethylene (POE) ether types (Emulgen series, Kao Corp.) were used without further purification. Cloud points of these emulsifiers were determined at 1% w/w solution in the appropriate medium indicated in Table 2. The emulsification conditions for each investigation are given in Table 3. Characterization of Emulsion Phase separation. Immediately after emulsification, 100 ml of the emulsion was poured into a 100-ml measuring cylinder. The volumes of the different phases that separated out were recorded as a function of time. Direct observation of the emulsion under a polarizing microscope to assess the state of aggregation of the droplets was also carried out. Turbidity measurement. One milliliter of emulsion was withdrawn, at regular intervals, from the the middle portion
of the measuring cylinder containing the emulsion (see text later), diluted with distilled water to 500 ml in a volumetric flask. Care was taken that the dispersion in the flask was not subjected to any undue agitation. This is to ensure that minimum disturbance was experienced by the droplets. The transmittance of the dispersion was measured immediately at 480 nm (7). A Shimadzu UV3101PC was used for these measurements. Total oil content determination. About 1.0 g sample of emulsion was withdrawn from the middle portion, with due care taken to exclude any of the top phases while sampling. The sample was dried in an aluminum dish at 1057C for 1.5 h. The weight of oil remaining was determined. Duplicate determinations were carried out. Particle size analysis. The particle size distribution (PSD) of the emulsion was determined using the Malvern Mastersizer E (laser diffraction technique). The sample was analyzed within 1 min of emulsification and also during storage. Viscosity measurement. The viscosity of oil emulsion was determined at 287C using a Brookfield LVF viscometer in a 600-ml beaker at 60 rpm. Reading was taken after 1 min. Determination of zeta potential. The zeta potentials of the oil droplets in the OL emulsions were determined, using a Zetamaster S (Malvern Instruments), as a function of electrolyte concentration and also pH. pH was adjusted using sodium hydroxide or hydrochloric acid solutions. Interfacial tension measurement. The ring method was used to determine the oil/water interfacial tension with a Kruss K8 tensiometer at 307C. RESULTS AND DISCUSSION
After emulsification, an emulsion thus formed would, on standing, establish a concentration gradient: a phase bound-
TABLE 2 Types of Emulsifiers Used and Their HLB Numbers and Cloud Points Cloud point (7C)
Emulsifiers E105 E106 E404 E408 E705 E903 E905 E906 E911 E930
Chemical names Polyoxyethylene Polyoxyethylene Polyoxyethylene Polyoxyethylene Polyoxyethylene Polyoxyethylene Polyoxyethylene Polyoxyethylene Polyoxyethylene Polyoxyethylene
lauryl ether lauryl ether oleyl ether oleyl ether fatty alcohol ether nonylphenyl ether nonylphenyl ether nonylphenyl ether nonylphenyl ether nonylphenyl ether
EO No.
HLB
In water
In 25% acetic acid
In 35% acetic acid
5 6 4 8 5 3 5 6 11 30
9.7 10.5 8.8 10.0 10.5 7.8 9.2 10.8 13.7 15.1
Oil-soluble Oil-soluble Oil-soluble Oil-soluble Oil-soluble Oil-soluble Oil-soluble Oil-soluble 71.3 97.2
42.3 69.5 29.3 50.5 60.2 õ5 õ5 30.1 N.D. N.D.
N.D. N.D. N.D. N.D. N.D. õ5 8.2 N.D. N.D. N.D.
Note. N.D., not determined.
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TABLE 3 Emulsification Conditions Emulsifying parameters Effect Effect Effect Effect Effect Effect
of of of of of of
Types of emulsifiers
Emulsifier (% w/w)
Oil (% w/w)
Type of oil
Homogenizer speed (rpm)
Temperature (7C)
Varied E906 E906 E906 E906 E906
10, 12 Varied 12 12 12 12
20 20 Varied 20 20 20
OL OL OL Varied OL OL
6000 6000 6000 6000 Varied 6000
30, 40 30, 40 40 40 30, 40 Varied
type of emulsifiers concentration of emulsifiers (%) oil content (%) type of oil shear rate (rpm) emulsifying temperature (7C)
ary would appear separating the top from the bottom layer, the rate of formation of which depends on the inherent instability of the emulsion. Due to coalescence and flocculation, an increase in the concentration gradient with time is expected. Thus by sampling the emulsion at the middle portion of the measuring cylinder (the 50-ml mark), the oil content can be taken as a reference to the stability of the emulsion. For emulsion containing 20% w/w oil, the phase boundary between the cream and the bottom aqueous layer would be way above the 50-ml mark if it is unstable and thus would have a low oil content at the middle portion. The stability of the palm oil-in-water emulsions can thus be monitored and characterized during storage. An emulsion is considered stable if it remains as a single phase without phase separation over the entire period under study. It is deemed to be unstable if the emulsion separates into an oil layer on top and an aqueous layer at the bottom. Most of the emulsions prepared exhibited some degree of instability, such as creaming, intermediate between the two extremes. Optimum emulsification occurred at a shear rate of 6000 rpm for 30 min at 407C. A higher shear produced the same droplet size in a shorter period of time but would not reduce the size of the oil droplet further. Thirty minutes of shearing time seemed sufficient for the comminution process. Prolonged shearing can in fact cause instability due to shearinduced flocculation. The optimal emulsification temperature for E906 was 407C, under which the smallest oil droplets were produced. The emulsification process was scaled up and tested with an industrial homogenizer of similar design (with a capacity of up to 150 kg) in a factory. It was found that the droplet size of the emulsions prepared in the factory was comparable to those from the laboratory under similar optimum emulsification conditions. Oil-in-water emulsions were prepared using different: (i) types of commercial emulsifiers; (ii) concentrations of emulsifier; (iii) oil contents in emulsion;
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It is noticed that emulsifiers having about the same HLB values (all except E903, E911, and E930; see Table 2) gave emulsions of varying stability as indicated by the oil retained (Fig. 1). In addition, emulsifiers with different hydrophobes but with the same EO number (E105, E705, E905) differ in their emulsifying capability. This is also true for those with the same hydrophobe but different EO numbers (E903, E905, E906, E911, E930). There does not appear to be any correlation between the cloud points of the emulsifiers and the HLB number either. The stability of these emulsions showing various stages of phase separation is illustrated in Fig. 2. The slightly inferior stability of emulsions prepared from E705 compared to those from E105 may be due to the presence of the alcohol group in the former which renders the polar and nonpolar ends of the emulsifier molecule less distinguishable (8). An O/W emulsion could not be formed when E905 was used as the emulsifier. Tajima et al. (7) found that interaction between the p-electron of the phenyl group and the adjacent EO groups in POE nonylphenyl ether type emulsifiers (E905) altered its molecular configuration at the O/W interface compared to that of the lauryl ether
FIG. 1. Effect of HLB and cloud point on the stability of emulsions prepared using different emulsifiers: 20% w/w OL with 10% w/w emulsifier emulsified at 6000 rpm for 30 min at 307C.
(iv) types of palm oils.
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FIG. 2. Phase separation of emulsions in 100-ml measuring cylinders. Emulsions prepared using different emulsifiers at 20% w/w OL, 10% w/w emulsifier at 6000 rpm for 30 min at 307C.
types (E105), resulting in different emulsion stability of liquid paraffin oil involving these two types of emulsifiers. It was found that for the POE lauryl (E105, E106) and oleyl (E404, E408) ether series, stability of the emulsions decreased as the EO number increased. For the POE nonylphenyl (E903, E905, E906, E911, E930) ether series, the most stable emulsions were those prepared with the oilsoluble E906, with 6 EO numbers. E903 and E905 are more hydrophobic than E906, but E911 and E930 are both watersoluble in comparison with E906 and therefore would be less likely to desorb from the oil–water interface into the oil globule. Since it is known that a nonionic emulsifier with a higher free energy of adsorption gives more stable emulsions (7, 9), both E903 and E905 are predicted to give less stable emulsions compared with those with longer EO chains as was observed here. The less stable emulsions formed by the water-soluble emulsifiers E911 and E930 could also possibly be depletion flocculated at high emulsifer concentration as observed by other workers (10). It is not clear why emulsion stability should peak at E906 and not increase with EO chain length of the emulsifier all the way to E930 as expected. Tajima et al. (7), working on liquid paraffin O/W emulsions stabilized with POE nonylphenyl ethers with EO groups ranging from 2 to 12, have found that emulsions stabilized by emulsifiers with less than
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6 EO numbers were not stable at pH ca. 3. However, there was little difference in the stability of polyethylene latex particles stabilized by POE octylphenyl ethers with 9.5 and 16 EO numbers; instead, at longer chain length, a decrease in stability was actually observed (11). On the other hand, for anisole and chlorobenzene in water emulsions stabilized by hexadecyl POE glycol ethers (9), it was found that the emulsion stability increased with an increase in EO number but the electrophoretic mobility of the emulsion droplets decreased. Shinoda and Sagitani (12) concluded that for cyclohexane-in-water emulsions stabilized by POE nonylphenyl ethers, the most stable O/W emulsion was formed with those having 4.3–5.6 EO numbers. Thus no clearcut trend is apparent for these emulsions. It would appear that the stability of OW emulsions stabilized by ethoxylated type nonionic emulsifiers depends strongly on factors that influence the prevailing conditions at the O/W interface and also on the nature of the interfacial layer formed. Completely different behavior was observed for different oil emulsions. It would appear from this work that E906 is the best emulsifier for palm olein. The particle size distribution of palm olein droplets in Fig. 3 shows that emulsions prepared with E906 gave the smallest average droplet size and the narrowest spread in size. All subsequent experiments were performed using this (E906).
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FIG. 3. Particle size distribution of OL droplets prepared using different emulsifiers: 20% w/w OL with 10% w/w emulsifier emulsified at 6000 rpm for 30 min at 307C.
It should be mentioned that palm oil itself contains trace amount of naturally occurring emulsifiers such as phospholipids which can cause self-emulsification of palm oil. The presence of trace amounts of mono- and diglycerides in the oil can also affect the overall composition of the naturally occurring emulsifiers present at the O/W interface. Thus in considering the effect of emulsifier on the emulsification of palm oil, one should also be aware of the possible contribution from these naturally occurring emulsifiers.
ture of the interfacial film formed and needs a higher positive gi (15). The gi of emulsifiers with very low and very high HLB, namely E903 and E930, were significantly higher than those of the other emulsifiers having intermediate HLB values. However, the stability of the emulsions thus prepared does not reflect the same order as represented by the HLB values. Further, the behavior of E905 was rather puzzling as men-
Effect of Emulsifier on Interfacial Tension at O/W Interface The interfacial tensions, gi , of the different emulsifiers at the O/W interface shown in Fig. 4 indicate that at 1.0% w/ w, the gi of E404 and E408 were lower than that of E906, even though emulsions prepared using E906 were the most stable of the three. This perhaps could be attributed to the presence of the double bond in the oleyl ether group of E404 and E408 preventing close-packing of the hydrocarbon tails at the curvaceous O/W interface of an emulsion drop. Furthermore there is ample evidence (13, 14) that reduction of gi alone does not ensure emulsion droplet stability, especially when a high-shear homogenizer is used in the emulsification process. This is because emulsification is enhanced by low gi whereas emulsion stability is related to the struc-
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FIG. 4. The variation of HLB and interfacial tension of the OL–water interface with different types of emulsifiers. Concentration of emulsifier was 1% w/w.
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FIG. 5. The effect of E906 concentration on the interfacial tension of the OL–water interface measured at 30 and 407C, respectively.
FIG. 6. The effect of E906 concentration on the stability of the emulsions formed at 20% w/w OL, emulsified at 6000 rpm for 10 min at 307C.
tioned above. At 0.1% w/w, its gi was comparable to that of E906 and yet it failed to form any emulsion. It should be noted that E905 has a very low cloud point of 8.07C in 35% w/w acetic acid which indicates its very low solubility in water. The gi of the O/W interface as a function of concentration of E906 at 30 and 407C is shown in Fig. 5. It can be seen that it decreased gradually from about 1.2 mN m01 to just above zero at 1.0% w/w. No abrupt change in slope was discernible. The interface remained clear throughout the measurements. Preliminary study had also shown no time effect of interfacial tension of the OL/water interface; i.e., the interfacial tension remained constant with contact time of the two phases. When emulsifier was added to and present in the oil phase, again no change in interfacial tension with time was noted and equilibrium was reached within ca. 3 min after the OL/water interface was formed in all cases. This tends to imply that for the present system emulsifier molecules were adsorbed rapidly at the OL/water interface. Thus, during the emulsification process, this was able to produce a protective layer around the droplet rapidly. The role played by the kinetics of the process is not so important.
droplets in these emulsions as revealed in the d ( 0.9 ) values, the diameter at which 90% of the droplets fall below these as shown in Fig. 7. It is clear that emulsion prepared with 12% w / w E906 contained the lowest number of big droplets and therefore creamed the least. Thus all subsequent experiments were performed at an optimal emulsifier concentration of 12% w / w. An apparent emulsifier layer thickness can be calculated from the known volume fraction of the emulsifier with respect to that of the oil phase and the area-weighted drop diameter D(3,2) (16, 17). It is assumed that all emulsifiers are located at the surface of the oil droplets but of course in reality the emulsifiers are also found dissolved in the oil or the water phases depending on whether the emulsifier is oil- or water-soluble. Various parameters calculated for OL emulsions emulsified with the oil-soluble E906 are given in Table 4. The number of adsorbed layers on the droplet was calculated based on the assumption that the length of the ˚ , estimated from the molecular E906 molecule is 33.45 A
Effect of Emulsifier Concentration Optical microscopy observation reveals that relatively small oil droplets were obtained in emulsion prepared with as low as 8% w / w E906. The amount of creaming was the lowest at 12% w / w E906 ( Fig. 6 ) , above which the stability of the emulsion decreased, but the mean particle size remained essentially constant at 2.0 mm over an E906 concentration range of 8 – 14% w / w ( Fig. 7 ) . Similarly the polydispersity of droplet size was also the lowest observed over this concentration range. The higher creaming observed for emulsions prepared with less than 12% w / w E906 corresponded to the presence of much bigger
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FIG. 7. The effect of E906 concentration on the droplet size and polydispersity of the emulsions formed. Conditions as in Fig. 6.
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TABLE 4 Droplet Size, Specific Surface Area, and Adsorbed Emulsifier Layer Thickness Calculated for OL-in-Water Emulsions E906 (%)
D(4, 3) (mm)
D(3, 2) (mm)
r a (mm)
SSAb (m2/g)
fc
˚) Dd (A
Number of emulsifier layer, Ne
2 4 6 8 10 12 14 16
7.97 5.40 3.53 2.99 2.92 2.61 2.67 2.68
2.47 1.94 1.68 1.51 1.49 1.42 1.42 1.42
1.25 0.97 0.84 0.76 0.75 0.71 0.71 0.71
2.6735 3.4375 3.9792 4.4280 4.4845 4.6784 4.6900 4.6932
0.0173 0.0346 0.0519 0.0692 0.0865 0.1038 0.1212 0.1385
71 111 143 170 209 238 276 314
2.1 3.3 4.3 5.1 6.2 7.1 8.3 9.4
a
r Å D(3, 2)/2. SSA, specific surface area of emulsion droplet. c f, volume fraction of emulsifier to oil, roil 0.90, rE906 1.04. d D, apparent emulsifier layer thickness Å {fr 3 / r 3}1/3 0 r. e ˚ based on molecular model calculations. N Å D/33.45, where length of E906 is 33.45 A b
model. It can be seen (Fig. 8) that the specific surface area of the emulsions increased rapidly with E906 concentration until ca. 12% w/w, beyond which it remained almost constant. Similarly D(3,2) also remained unchanged above 12% w/w E906. This shows that despite an increase in E906 concentration to more than 12% w/w, smaller droplets were not formed. On the other hand, there was a slight increase in D(3,4), the volume-weighted diameter, when E906 concentration was increased from 12 to 16% w/w. The apparent thickness of the emulsifier layer increased ˚ with increasing concentration of E906. The value of 71 A ˚) at 2% w/w E906 is comparable to that for b-casein (80 A on hexadecane O/W emulsions (18). At 16% w/w E906, ˚ (0.03 mm), equivalent the surface layer thickness was 314 A to about nine layers of emulsifiers. If the smallest droplet formed at 12% w/w E906 was 2.61 mm [D(4,3)], then an additional 0.03 mm contributed by nine layers of adsorbed emulsifiers would result in a droplet size of 2.67 mm, in
close agreement with the D(4,3) of the droplets of 16% w/w E906 emulsion. Thus the slight increase in D(4,3) observed when E906 concentration was increased from 12 to 16% w/ w can be attributed to multilayer adsorption of E906 at the O/W interface. Below 12% w/w E906, the magnitude and the decrease in D(4,3) with E906 concentration was too big for it to be significant. This observation is corroborated by electrokinetic mobility data and creaming results. Increasing the E906 concentration caused a decrease in zeta potential of the emulsion droplets (see Fig. 13). An increase in the layer thickness of adsorbed emulsifiers consequent to increasing E906 concentration would lead to a shift of the plane of shear away from the surface, resulting in a drop in zeta potentials. The increase in creaming observed (Fig. 6) with emulsions containing more than 12% w/w E906 could be explained by an increase in the size of the oil droplets given above. Effect of Oil Concentration
FIG. 8. The droplet size and surface area of the emulsions as a function of E906 concentration. Emulsification conditions as in Fig. 6.
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As the oil content of the emulsion increased at constant emulsifier concentration, the droplet size increased gradually to ca. 2.5 mm at 40% w/w oil, beyond which a sudden drop in particle size to 1.2 mm occurred at 50% w/w oil and the size then increased again on further increase of the oil concentration (Fig. 9). The occurrence of a minimum particle size at a particular oil content is not uncommon for emulsions containing nonionic surfactants where HLB value consideration is important. It has been noted previously (19) that variation in EO number distribution of an emulsifier can affect the size of the emulsion droplets formed. Using E906 from the same batch, this variation was minimized. The viscosities of the emulsions (same as those described in Fig. 9) increased with oil concentration over the whole range studied, from 3.6 cP at 10% w/w oil to the highest viscosity of 2980 cP at 60% w/w oil. High viscosities of stable emulsion are associated with the net repulsive force
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FIG. 9. Effect of oil concentration on the droplet size of OL emulsion: E906 at 12% w/w emulsified at 6000 rpm for 30 min at 407C.
acting among the particles (20). The PSD of droplets in the 50% w/w emulsion remained unchanged even after 6 months storage at room temperature and was stable. Effect of Palm Oil Type ST emulsion was found to consist of large, irregularly shaped, crystallite-like particles (ST crystallites) in contrast to the small oil droplets found in PO and OL emulsions. These large particles were formed probably as a result of partial coalescence, i.e., crystallites from one droplet penetrating into the liquid region of another droplet, causing them to aggregate. Even though PO contains both OL and ST fractions, the emulsion droplets prepared using PO appeared smooth and did not reveal the presence of any free disperse stearin particles and the appearance of the droplets was similar to that prepared with OL. There are two possibilities for this. First is probably because the ST crystallites were soluble in the OL fraction (liquid at room temperature) of the PO droplets and second is because of supercooling effects. If crystallization of ST was inhibited by supercooling, the number of crystallites protruding from the oil droplets would be reduced and the stability of emulsion droplets would improve. The PSD of the droplets in PO emulsion was found to be almost identical to that of the OL emulsion and was not intermediate between those for OL and ST as expected (Fig. 10) if crystallites of ST particles were present. This would seem to concur also with results reported recently (21) on n-hexadecane-in-water emulsions that the rate of crystallization of oil droplets depends on the effects emulsifier molecules have on the interaction between droplets. OL and ST oils were blended to give oils of various iodine values. These were then homogenized and the PSD of the emulsions was determined. It can be seen from Fig. 11 that crystallization of stearin occurred only when the iodine values of the oil fell below 50. Thus PO, with an iodine value
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FIG. 10. Effect of storage time on the stability of emulsions of different types of palm oils: E906 at 12% w/w emulsified at 6000 rpm for 30 min at 407C.
of ca. 52, was in a region where crystallization of stearin was not expected to occur. This would tend to support the reasons given above on why the saturated triglycerides in PO failed to crystallize. Consequently the PSD of PO droplets was similar to that of OL droplets. It is known that the interaction between the oil and the emulsifier at the O/W interface depends strongly on the nature of both of these substances. The types of palm oils used vary both in triglyceride composition and in the degree of unsaturation. E906, being nonionic, is very sensitive to the balance of unsaturation of the oil. A highly saturated oil is comparatively more hydrophobic and would require a more hydrophobic nonionic emulsifier and vice versa. This perhaps explains the diversity in the behavior found for the different oils and the ease with which these oil could be emulsified using different emulsifiers. Stability of Emulsion during Storage Emulsions prepared using 12% E906 were stable during storage at room temperature for up to at least 1 month with-
FIG. 11. The effect of the degree of unsaturation of the oil (as measured by its iodine value) on the droplet size of the emulsion formed.
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PALM OIL-IN-WATER EMULSIONS
out creaming and aggregation, except those prepared from ST oil. Even though substantial creaming was observed for emulsions prepared with E705, E911, and E930, the aggregated oil droplets were able to maintain their individuality and, upon agitation, became redispersed. Thus for the OL emulsions, destabilization of the emulsion by Ostwald ripening can be excluded as can be seen from Fig. 10: the particle size remained unchanged after almost 1 month. For Ostwald ripening to occur the dispersed phase must have some degree of solubility in the aqueous medium. Solubility of palm oil in water was found to be well below 100 mg dm03 at 287C. For dilute O/W microemulsions the rate of ripening decreased as the volume fraction increased (22). The emulsions reported here contained 20% w/w oil. It is observed that droplets of ST emulsion became bigger 1 day after storage at room temperature. This could be explained by growth via aggregation following crystallization of ST during cooling down from 407C to room temperature as mentioned above. Change in droplet size then ceased thereafter. It has been observed that partially crystalline emulsions such as n-hexadecane aggregate on cooling (21). Electrokinetic Behavior of Palm Olein Emulsions The zeta potential–pH curves for the emulsions in the presence of 15 1 10 03 mol dm03 NaCl and 1.5 1 10 04 mol dm03 CaCl2 are shown in Fig. 12. It can seen that the oil droplets stabilized by E906, a nonionic surfactant, were negatively charged and the zeta potential increased significantly as the pH was increased. The same behavior has been reported by Tajima et al. (7) previously and is typical of nonionogenic surfaces where the negative charges resulted from adsorption of hydroxyl ions onto the droplet surface. Figure 13 shows that a gradual drop in zeta potential with increase in E906 concentration was observed except that there was a steeper dip at 12% w/w followed by a sharp
FIG. 13. The variation of zeta potential and droplet size of emulsions as a function of E906 concentration. 20% w/w OL, emulsified at 6000 rpm for 30 min at 307C.
increase. The drop in potential at 12% w/w seems to coincide with the smallest droplet size of the emulsion at this E906 concentration. A decrease in the zeta potential of silver iodide sol particles with an increase in the concentration of POE glycol monoethers was noted previously (23) and was attributed to multilayer adsorption of the emulsifier at the particle surface. The present results are in close agreement with this except at 12% w/w E906 where perhaps the influence of the particle is also important. CONCLUSION
The most stable emulsions of palm olein were obtained using a nonionic emulsifier (E906), ethoxylated nonylphenyl ether with 6 EO numbers. The emulsification conditions for producing these stable emulsions have been identified. Some interesting effects of molecular structure of the emulsifier, finely divided solids (ST) at the O/W interface, and excess emulsifier on the emulsion stability were noted and explained by known interfacial phenomena. ACKNOWLEDGMENT This work was partially supported by Sumitomo Rubber Industries Japan and Sumirubber Industries Malaysia.
REFERENCES
FIG. 12. The zeta potential–pH curves for OL-in-water emulsion droplets in the presence of 15 1 10 03 mol dm03 NaCl and 1.5 1 10 04 mol dm03 CaCl2 . E906 at 12% w/w emulsified at 6000 rpm for 30 min at 407C.
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Characterization and Potential Applications.’’ M.Sc thesis, University of Malaya, Kuala Lumpur, 1991. Oh, C. H., ‘‘Solvent Fractionation and Crystallization Behaviour of Palm Oil.’’ M.Sc. thesis, University of Malaya, Kuala Lumpur, 1979. Matsumoto, S., in ‘‘Non-ionic Surfactants’’ (M. J. Schick, Ed.), p. 549. Marcel Dekker, New York, 1987. Tajima, K., Koshinuma, M., and Nakamura, A., Colloid Polym. Sci. 270, 759 (1992). Garrett, H. E., ‘‘Surface Active Chemicals,’’ p. 18. Pergamon Press, London, 1972. Elworthy, P. H., and Florence, A. T., J. Pharm. Pharmacol. 21, 70S (1969). McClements, D. J., Colloids Surf. A: Physico Eng. 90, 25 (1994). Stryker, H. K., Helin, A. F., and Mantell, G. J., J. Appl. Polym. Sci. 10, 81 (1966). Shinoda, K., and Sagitani, H., J. Colloid Interface Sci. 64, 68 (1978). Garti, N., and Katz, M., J. Dispersion Sci. Tech. 6, 149 (1984).
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14. Chow, M. C., and Ho, C. C., J. Am. Oil Chem. Soc. 73(1), 47 (1996). 15. Rosano, H. L., Jon, D., and Whittam, J. H., J. Am. Oil Chem. Soc. 59, 360 (1982). 16. Becher, P., ‘‘Emulsions: Theory and Practice,’’ p. 2. Reinhold, New York, 1966. 17. Ostberg, G., Bergenstahl, B., and Hulden, M., Colloids Surf. A: Physico Eng. 94, 161 (1995). 18. Dickinson, E., J. Chem. Soc. Faraday Trans. 88, 2973 (1992). 19. Shinoda, K., and Friberg, S., in ‘‘Emulsion and Solubilization,’’ p. 148. Wiley, New York, 1986. 20. Van den Tempel, M., in ‘‘Rheology of Emulsions’’ (P. Sherman, Ed.), p. 1. Pergamon Press, Elmsford, NY, 1963. 21. McClements, D. J., Dickinson, E., Dungan, S. R., Kinsella, J. E., Ma, J. G., and Povey, M. J. W., J. Colloid Interface Sci. 160, 293 (1993). 22. Taylor, P., and Ottewill, R. H., Colloids Surf. A: Physico Eng. 88, 303 (1994). 23. Hunter, R. J., ‘‘Zeta Potential in Colloid Science,’’ p. 324. Academic Press, London, 1981.
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