- Email: [email protected]
Multivariate Screening Analysis of Water-in-Oil Emulsions in High External Electric Fields as Studied by Means of Dielectric Time Domain Spectroscopy
Journal of Colloid and Interface Science 227, 262–271 (2000) doi:10.1006/jcis.2000.6921, available online at http://www.idealibrary.com on
Multivariate Screening Analysis of Water-in-Oil Emulsions in High External Electric Fields as Studied by Means of Dielectric Time Domain Spectroscopy III. Model Emulsions Containing Asphaltenes and Resins Øivind Midttun,∗ Harald Kallevik,† Johan Sj¨oblom,†,1 and Olav M. Kvalheim∗ ∗ Department of Chemistry, University of Bergen, All´egaten 41, N-5007 Bergen, Norway; and †Statoil R&D Centre, Rotvoll, N-7005 Trondheim, Norway E-mail: [email protected] Received May 17, 1999; accepted April 17, 2000
The effect of crude oil resins with various polar characters on the stability of w/o model emulsions containing asphaltenes is investigated using a mixture design. The resins were extracted using an adsorption–desorption technique. One asphaltene fraction and four different resin fractions from one European crude oil were used. The stabilities are measured using time-domain dielectric spectroscopy in high external electric field. It is found that resins with different polar character have different effects on the emulsion stability. At asphaltene/resin ratios of 1 and 5 : 3 the resins in some cases lead to an emulsion stability higher than that of a similar emulsion stabilized by asphaltenes only, while at low asphaltene/resin ratios (∼1 : 3) the emulsion stability is reduced by the resins. The effect on emulsion stability of combining two different resin fractions depended on the resin types combined as well as the relative amount of resins and asphaltenes. Also, an increase in the stability of some of the emulsions containing resins and asphaltenes for a period of 50–300 min after the emulsification was observed. This time-dependence of emulsion stability is attributed to the mobility of resins at the oil–water interface and the slow buildup of a stabilizing interfacial film consisting of resins and asphaltenes. °C 2000 Academic Press Key Words: crude oils; w/o emulsions; crude oil resins; asphaltenes; high electric fields; mixture design; time-dependent emulsion stability.
INTRODUCTION
In crude oil production and transport, asphaltenes are precipitated as a result of mixing of crude oils and temperature and pressure changes, causing asphaltene particle-stabilized water-in-oil emulsions to form. These emulsions are commonly modeled in the laboratory using asphaltenes precipitated from the crude in an excess of an alkane. The precipitated asphaltene fraction is then used to stabilize model emulsions (1–3). Ferworn et al. (4) have shown that the size distribution of asphaltene particles 1
To whom correspondence should be addressed.
0021-9797/00 $35.00
C 2000 by Academic Press Copyright ° All rights of reproduction in any form reserved.
precipitated using n-heptane is a log-normal one and Barr´e et al. (5) found a log-normal distribution of asphaltene particles in toluene. In Refs. (4) and (5) the mean asphaltene particle size varied from 5 to 300 µm, depending on the precipitation conditions and the crude. The molecules included in the asphaltene fraction are polyaromatic molecules with molecular weights in the order of about one thousand to a few thousand (6–9). The asphaltene molecules are found to be polyaromatic sheets surrounded by aliphatic branches (10). Some heteroatoms are also found in the asphaltene fraction (9, 11). The presence of aromatic solvents influences the solubility of the asphaltenes in the oil phase. It is believed that the asphaltenes, themselves highly aromatic compounds, do not form particles in aromatic solvents as they do in aliphatic solvents but rather are present as small aggregates resembling micelles (12). These aggregates do not stabilize emulsions. This was shown by Førdedal et al. (3), who found that the emulsion stability changes with the aromatic character of the oil phase in a w/o model emulsion system. A pure toluene phase gave no stable emulsion, while a pure decane phase gave an emulsion of high stability when asphaltenes were used as stabilizers. The highest emulsion stability was observed for an oil phase consisting of about 80% decane and 20% toluene. McLean and Kilpatrick have reported similar results (13). The resin fraction can be defined by its solubility in, e.g., liquid propane and is per definition not precipitated when a solvent such as n-pentane is added to the crude to precipitate the asphaltenes. Resins are commonly extracted from the crude by adsorption onto a solid surface, such as silica particles (1, 14). The resins have been much less studied than the asphaltenes with respect to their structure, size, and emulsion stabilizing properties. Koots and Speight (9) found the molecular weight of resins to be about 1000. Heteroatoms are also found in resin fractions (9, 11, 15). Barr´e et al. (5) and Bardon et al. (10) studied the effect of resins on the molecular weight of asphaltene aggregates in toluene
262
MULTIVARIATE SCREENING ANALYSIS OF W/O EMULSIONS
solutions and found that an increased concentration of resins led to a reduced molecular weight of the asphaltene aggregates. Koots and Speight (9) also found that it was possible to dissolve the asphaltenes by adding resins. In a work by Førdedal et al. (16) it was shown that resins obtained by adsorption on silica particles and desorbed by a methanol/dichloromethane mixture increased the stability of model emulsions (at an asphaltene/ resin ratio of 1) as compared to model emulsions stabilized by asphaltene particles alone. The explanation given was that the resins used in their experiments are more interfacially active than the asphaltenes, reaching the fresh water/oil interface first. Afterward, the asphaltene particles aggregate at the interfacial resin film, giving a stable emulsion. The effect of resins on asphaltene-stabilized w/o emulsions was found by McLean and Kilpatrick (13) to be destabilizing in most cases, but cases of increased stabilization were also observed (for asphaltene/resin ratios lower than about 1 : 3). They also found that in emulsions with low asphaltene/resin ratios (below ∼1 : 3) the emulsions were always less stable than emulsions stabilized by asphaltene particles alone (at the same asphaltene concentration). Their explanation for this variation in stability is that the resins disperse the asphaltene particles, which must be of “correct” size to have a maximum emulsion-stabilizing ability, and that the asphaltene/resin ratio and the solubilizing effect of the resins affect this size. Acevedo et al. (17) have showed that resins adsorb to asphaltene particles in toluene/heptane mixtures in a complex manner. In a previous work (15) we separated different resin fractions from different crude oils by utilizing their adsorptions onto various particle surfaces and redissolving the adsorbed resins using different solvent combinations. These resins were characterized using infrared spectroscopy. The emulsions stabilized by asphaltene aggregates can be very stable. Several separation methods are in use to study the stability of emulsions, including the bottle test (13) and the use of a high external electric field (3, 18, 19, 20). The use of an external electric field causes the water droplets to polarize due to the presence of electrolyte, and the electric field thus facilitates the coalescence of droplets in the w/o emulsion (21). A suggested mechanism for coalescence is ions rupturing the interfacial films between the droplets. In dilute emulsions the droplets are elongated if an electric field is applied (Fig. 1b), while in a concentrated emulsion the droplets assemble into rows as shown in Fig. 1c. A picture of a typical undisturbed emulsion is shown in Fig. 1a, while a log-normal droplet size distribution of an undisturbed emulsion is illustrated in Fig. 2. The dielectric instrumentation we use to study the emulsions in high electric fields has the additional advantage that it enables us to study the emulsion stability within a time frame of minutes. This can be used to study the stability of emulsions at discrete time intervals, and in this way it is possible to measure changes in emulsion stability as a function of time. Several reports on time-dependent phenomena involving surfactants can be found in the literature (22–27), but none involve surfactants extracted from crude oils. The phenomena reported
263
are surfactant–polymer interactions at interfaces explained by surfactant-induced conformational changes in the polymers (22, 23, 24) and competitive adsorption (25, 27). Also of interest is the reported increase in oil–water interface coverage by proteins with time (25). In (22) surfactants were found to aggregate with polymers, causing a change in their hydrophilic character and thereby affecting their interfacial activity. In addition, reorientation of polymers and surfactants at the water–oil interface was suggested to take place, leading to a time-dependence of the measured surface tension (22, 24). The time scale of adsorption can vary considerably, with equilibrium conditions at the interface being reached in times ranging from milliseconds to hours (24). The longer time scales are observed when reorientation of proteins at an oil–water interface is studied (25). An interesting study of the behavior of interfacial films consisting of asphaltenes and resins, alone and in combination at a water– air interface, was made by Ese et al. (28). They found that films consisting of asphaltenes were very rigid while films consisting of polar resins were less rigid. This interfacial mobility of resins was explained by movement and reorientation of the resins at the interface. When asphaltenes and resins were combined, the compressibility and mobility of the surfactant film were found to be between that of pure asphaltenes and that of pure resins, depending on the asphaltene/resin (A/R) ratio. In this work we use resin fractions extracted according to the same method to study their effect on the stability of asphaltenestabilized w/o model emulsions. Based on their infrared spectra, the relative polar character of the resins is estimated. MATERIALS AND METHODS
Chemicals The crude was supplied by Elf Aquitane and is from a production field in France. n-Pentane, dichloromethane (DCM), methanol (MeOH), toluene, decane, and benzene were all of p.a. quality from Merck (Darmstadt, Germany). Silanol was obtained from Waters. CaO, NaCl, and talc were from Merck (Darmstadt, Germany). The siloxane was obtained from Elkem. Qualitative filter papers used were of type 1 from Whatman, England. De-ionized water was used. Crude Oil The crude oil is an European crude with specific gravity of 0.88 g/ml at 50◦ C, a viscosity of 16.5 mPa s at a shear rate of 1000 s−1 at 50◦ C. The asphaltene content is ∼9 wt% (1 : 5 pentane precipitation as specified in the next paragraph). The crude sample was stored for several years on a jerrycan, thus exposed to air. The amount of resins, aromatics, and contamination of production chemicals is not known. Separation of Asphaltenes and Resins The asphaltenes were precipitated by mixing 10 ml of crude with 50 ml of n-pentane, stirring of the mixture, followed by
264
MIDTTUN ET AL.
FIG. 1. (a) Water droplets in an undisturbed model water-in-oil emulsion. (b) Elongated water droplets when an electrical field is applied to a dilute w/o emulsion. (c) Chains of droplets in a concentrated emulsion when an electrical field is applied. The droplets that are not spherical are the result of coalescence of two droplets; the resulting droplet has not had time to reach its equilibrium shape.
centrifugation for 10 min at 2400 rpm. To adsorb the resins the particles were added to the supernatant from the precipitation, the mixture was stirred and centrifuged, and the supernatant was removed. The particles were then washed with benzene (when using silanole or siloxene particles) or DCM (when using CaO or talc particles) until the filtrate was colorless. The resins were then obtained from the particles by desorption with a mixture of 7% MeOH in DCM (when using CaO, silanol, or siloxane) or benzene (when using talc). The solvent was evaporated under a
nitrogen blanket. The particles and solvents used in each step are given in Table 1, and the extraction method is fully described by Midttun et al. (15). All the fractions are isolated from one single European crude oil. Preparation of Emulsions and Emulsion Stability Measurements The compositions of oil phase of the samples studied are given in Table 2. The surfactant concentration (asphaltenes + resins)
MULTIVARIATE SCREENING ANALYSIS OF W/O EMULSIONS
265
FIG. 2. Droplet size distribution of a typical asphaltene-stabilized model emulsion. The asphaltene concentration is 1.3% (w/w), and the oil phase consists of 30% toluene and 70% decane. The total number of droplets is 545.
is 2% (by weight) of the oil phase in samples 1–24. Samples 25 and 26 contain only asphaltenes at the same concentration as in samples of type b, c, and e (Table 2). According to Ref. (3) water-in-crude oil model emulsions stabilized by resins alone are not stable enough, so we omitted such systems. The lowest asphaltene/resin ratio chosen is 1 : 3, giving 1.5 wt% as the maximum resin concentration and 0.5 wt% as the minimum asphaltene concentration. All the relative compositions of the surfactant mixture chosen for study, except for the centerpoint and samples 25 and 26, are marked in Fig. 3. The oil phase contains 20 wt% toluene and 80 wt% decane before addition of the surfactant. The amount of water in the emulsions is 30% of the total sample weight. The aqueous phase of the samples contain 3.5 wt% NaCl. The emulsions were prepared and analyzed in random order to prevent systematic errors. Toluene was added to the asphaltenes,
and the sample was left to equilibrate for 2 h before the decane was added. This mixture was then left to equilibrate for another 2 h, and the resins (if any) were added. The sample was then left overnight before addition of water and emulsification using a Citenco rotor emulsifier at 3000 rpm for 5 min. The sample was shaken to obtain a homogenous sample, and a small part of it was taken out and inserted into the measuring cell. The thickness of the cell in which the emulsions were kept during stability measurements was 0.23 mm. By increasing the voltage over the emulsion in steps of 0.043 kV/cm every third minute, the electric field was increased until the emulsion separated. The emulsion stabilities are given as kV/cm, and higher numbers indicate higher emulsion stability. The recorded times in Table 2 are the times elapsed from emulsification to separation as observed by conductivity through the samples. This process was repeated for several samples of the emulsion for a period
TABLE 1 Coding of the Resins Used in the Surfactant Mixture Particle Solvent 1 Solvent 2 Resin Acronym in Ref. (14)
Calcium oxide (C) Dichloromethane (D) Methanol (M)/dichloromethane A FCDM
Silanol (S) Benzene (B) Methanol (M)/dichloromethane B FSBM
Siloxane (X) Benzene (B) Methanol (M)/dichloromethane C FXBM
Note. Based on the extraction procedure and IR spectra, the polarity of the fractions is assumed to be A > B > C > D (see text).
Talc (T) Dichloromethane (D) Benzene (B) D FTDB
266
MIDTTUN ET AL.
TABLE 2 Model Emulsions % of each surfactant in model oil Sample no.
Sample type
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
a b c b c b c b c d e d e d e d e d e d e Centerpoint Centerpoint Centerpoint 1.25% asph. 0.5% asph.
A/R ratio
1.67 0.33 1.67 0.33 1.67 0.33 1.67 0.33 1.00 0.33 1.00 0.33 1.00 0.33 1.00 0.33 1.00 0.33 1.00 0.33 0.67 0.67 0.67
AS
B
C
A
D
Stability (kV/cm)
Time (min)
2.00 1.25 0.50 1.25 0.50 1.25 0.50 1.25 0.50 1.00 0.50 1.00 0.50 1.00 0.50 1.00 0.50 1.00 0.50 1.00 0.50 0.80 0.80 0.80 1.25 0.50
0.00 0.75 1.50 0.00 0.00 0.00 0.00 0.00 0.00 0.50 0.75 0.50 0.75 0.50 0.75 0.00 0.00 0.00 0.00 0.00 0.00 0.30 0.30 0.30 — —
0.00 0.00 0.00 0.75 1.50 0.00 0.00 0.00 0.00 0.50 0.75 0.00 0.00 0.00 0.00 0.50 0.75 0.50 0.75 0.00 0.00 0.30 0.30 0.30 — —
0.00 0.00 0.00 0.00 0.00 0.75 1.50 0.00 0.00 0.00 0.00 0.50 0.75 0.00 0.00 0.50 0.75 0.00 0.00 0.50 0.75 0.30 0.30 0.30 — —
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.75 1.50 0.00 0.00 0.00 0.00 0.50 0.75 0.00 0.00 0.50 0.75 0.50 0.75 0.30 0.30 0.30 — —
0.51 0.68 0.23 0.58 0.11 0.51 0.14 0.78 0.00 0.57 0.10 0.59 0.00 0.54 0.37 0.61 0.22 0.74 0.32 0.46 0.35 0.43 0.52 0.43 0.68 0.46
0 180 0 50 0 0 0 0 0 200 0 0 0 300 0 100 0 150 0 50 0 0 0 0 0 0
Note. The composition of the surfactant mixture, the measured stability of the corresponding emulsions and the approximate time necessary for the samples to reach maximum stability is given. For the samples where the time is given as zero, no change in stability was observed. AS is the asphaltenes.
of 2–4 h after mixing for each model emulsion. The electric field necessary to break the emulsion was measured with the instrumentation described by Førdedal et al. (20). Experimental Design A mixture design (29) was chosen to study the effect of varying the composition of the surfactant mixture of the system. A mixture design gave a good distribution of the analyzed compositions in the domain spanned by the components in the surfactant phase of the samples and a three-dimensional simplex pyramid is shown in Fig. 3. Each corner represents a pure compound; the sides represent two-compound mixtures and the faces threecompound mixtures. Using five surfactant components (asphaltene and four different resins), we constructed a five-dimensional simplex, each pure surfactant component represented by one corner in the pyramid.
22 × 275. The similarities/dissimilarities of these fractions are easily found by use of a mathematical decomposition technique called principal component analysis (PCA). In PCA the X matrix is decomposed into principal components called score and loading vectors. The scores and loadings contain information on how the fractions and the wavelengths in X relate to each other,
RESULTS AND DISCUSSION
In a previous work by our group (15) we sampled infrared spectra from 22 fractions of the specific crude used in the present work. Twenty-two infrared spectra with the absorption recorded at 275 wavelengths results in a data matrix X with dimension
FIG. 3. Simplex design in three dimensions. The compositions of the different sample types used in the present work are marked (see Table 1). The centerpoints are not shown.
267
MULTIVARIATE SCREENING ANALYSIS OF W/O EMULSIONS
respectively. Mathematically, the procedure can be expressed as Xn×m = t1 pT1 + t2 pT2 + ti piT + · · · . + tk pTk + En×m .
TABLE 3 Band Assignments for DRIFT Spectra
[1]
In Eq. [1], X is the original data matrix, ti are the score vectors and pi are the loading vectors. E is the residual matrix. The direction of the first principal component (the vectors t1 and p1 ) is defined as the straight line that best describes the variation within the X matrix. The direction of the second principal component is the one that best describes the variation in X that is not explained by the first component. The third and fourth components and so on can be found in the same manner. This procedure results in k orthogonal pairs of score and loading vectors. If these components are plotted against each other, the relation between samples is easily detected. The choice of resins was such that they should span the space represented by the first four principal components in the previous work (15) well (Fig. 4), with the following restrictions: • Resin fractions separated using all of the four particle types should be represented. • The yield of the chosen fractions should not be too low.
Thus, four resin fractions designated A-D (see Table 1 and Fig. 5) were chosen. In the infrared spectra of the resins (Fig. 5) it is seen that resin type A has a very high intensity in the amide
FIG. 4. Distribution of the resin fractions from Ref. (14). A cross marks the origin.
Functional groups Ester Ketones Aldehydes Amide C==C (conjugated and aromatic) C–CH3, C−CH2 (asymmetric bending) C–CH3 (symmetric bending) Ester Aromatic ether Alcohol Aliphatic ether Sulfoxide
Absorption bands (cm−1 ) 1750–1710 1730–1700 1730–1690 1670–1620 1600 1465 1377 1320–1100 1300–1200 1210–1000 1150–1050 1060–1020
band (1670–1620 cm−1 ). Amide groups are highly polar (30), and these resins are therefore considered to be the most polar of the four resin fractions used in the present work. The B resins also have a strong amide band in their spectrum although it is much weaker than that of the A resins. In addition the B resins have a relatively strong absorption band attributed to the ketone/aldehyde functional group (1730–1690 cm−1 ), which is not present in the spectrum of the A resins. The B resin fraction thus also contains a significant amount of polar groups. The C resins have absorption bands of both amide and ketone/aldehyde groups, but they are of considerably lower intensity than in the B resins. A very strong band is also found in the ether/sulfoxide region (∼1030 cm−1 ). Some polar groups are thus present in the C resins. The amide and ketone/aldehyde bands are practically absent from the spectrum of the D resin fraction, and no other strong bands assigned to significantly polar functional groups are found in the spectrum. Resin fraction D is therefore considered to have a very low polarity. Thus the polarity of the four resin fractions is assumed to be
FIG. 5. DRIFT spectra of the four resin fractions. Band assignments are given in Table 3.
268
MIDTTUN ET AL.
A > B > C > D. It should also be noted from the infrared spectrum of the asphaltene fraction (15) that it has a very low content of heteroatom functionalities and is therefore considered to be of low polarity. When the samples were inspected after they had been left overnight with all of the surfactants present (asphaltenes + resins), it was found that all of them contained some precipitate. Effect of Changing the Asphaltene Concentration Only When using only asphaltenes as surfactant (sample nos. 1, 25, and 26), it is seen that the most stable emulsion is obtained when using 1.25% asphaltenes. An electric field of 0.68 kV/cm was needed to break this emulsion. The emulsions with 2% and 0.5% asphaltenes show approximately equal stability, requiring an electric field of about 0.5 kV/cm for separation. There thus seems to exist an optimal asphaltene concentration with regard to maximizing the emulsion-stabilizing ability, located at an asphaltene concentration somewhere between 0.5% and 2% in this system. This can be explained by the varying size of the asphaltene particles (13). An increase in the asphaltene concentration above the “optimal” concentration led to the formation of larger asphaltene particles. These larger particles are less able to stabilize emulsions since they pack less efficiently at the interface. Also, a concentration lower than the “optimal” asphaltene concentration results in fewer asphaltene particles, reducing the ability to stabilize emulsions due to inability to cover the large area of a large number of small droplets. Effect of Aging on the Stability of the Emulsions The stability of several of the samples increased for the first 50–300 min after emulsification, and the times from emulsification until the equilibrium stabilities are reached is given in Table 2. To our knowledge this time-dependency of emulsion stability in emulsions stabilized by surfactants extracted from crude oils has not been reported in the literature. Figure 6 shows the stabilities for a period of 150–300 min after mixing for some representative samples. This increase in stability were observed only in samples with A/R ratios ≥ 1, i.e., samples of types b and d. In all cases the stabilities increased from a level of 0.1–0.3 kV/cm shortly after emulsification to a maximum of 0.5–0.9 kV/cm depending on the type(s) of resins present. Emulsions of types a, c, e, the centerpoint, and sample nos. 25 and 26 did not show any time dependency in the emulsion stability. It is clear from the present data that the time needed for the indigenous surfactants to reach equilibrium at the oil–water interface depends on the type(s) of resins present and the relative amount of resins and asphaltenes. It is assumed that the time (about 16 h) allowed for the surfactants to equilibrate in the oil phase before the introduction of water is sufficient, and the new equilibrium condition is thus a consequence of the introduction of the fresh oil–water interface. For the samples with observed time dependence the increase in stability is from a level comparable to that of the samples with low A/R ratios to stability levels of samples with higher
FIG. 6. Some representative results from the stability measurements. The reproducibility of the method is seen to be acceptable, and the increases in stability of some of the emulsions are illustrated by sample nos. 2 and 14. All the measurements for all three centerpoints (sample nos. 22–24) are given.
A/R ratios and asphaltenes alone. It is therefore reasonable to explain the increasing stability as a process in which the emulsionstabilizing ability of the interfacial surfactant film changes from one dominated by the resins to one dominated by the asphaltenes (but whose strength is influenced by the resins), as discussed by Førdedal et al. (14). Of the samples containing only one resin fraction in combination with asphaltenes, only those of type b with B or C resins show this time-dependent effect. This also applies for all of the samples of type d (which contain two resin fractions and asphaltenes) except the one containing both A and B resins. The B and C resins contain some polar groups and are considered to be of intermediate polarity compared to the A and D resins. The increase in emulsion stability may be explained as follows: if resins are present in the model oil containing both asphaltenes and resins, they disperse some of the asphaltene particles. The amount dispersed depends on the A/R ratio and the character of the resin and asphaltene fraction (5, 9, 10, 13). This will shift the size distribution of the asphaltene particles toward smaller aggregates (13). When water is added to the system, the highly interfacially active resins reach the oil–water interface first, followed by the asphaltenes. Thus a new equilibrium condition is established in which a complex interfacial layer of resins and asphaltenes decides the stability of the emulsion (14). It has been shown by Friberg et al. (31) that the B resins are interfacially active. As noted in the introduction, Ese et al. (28) have shown that the resin fraction called B in this paper forms a film at the water–air interface in which the resins are mobile, showing that movement/reorientation of the resins at the interface is possible. This suggests that the resins will also be mobile at the oil– water interface and opens up the possibility of the asphaltenes
MULTIVARIATE SCREENING ANALYSIS OF W/O EMULSIONS
to combine with the resins in the interfacial film. This may explain the time dependence of the emulsion stability observed in the present work. Since the molecules and aggregates involved are quite large and of complex structure, it is reasonable that this process require time scales on the order of hours (24). For the samples of type b containing A and D resins in addition to asphaltenes no time dependence is observed. This may be explained either by the change in equilibrium conditions (upon the introduction of the fresh oil–water interface) taking place so quickly that it is difficult to observe with the method used, or that no significant change in stability occur. Because the D resins contain very small amounts of polar functional groups (15), they are not expected to have a significant interfacial activity at the oil-water interface. Thus no significant change in the emulsion stability would be expected for the sample of type b containing D resins (sample no. 8). For the A resins it is suggested that the orientation of the resins at the oil–water interface take place quickly due to the large amount of highly polar amide groups present, and/or that the high amounts of polar groups inhibit their mobility at the interface. Both of these effects can reduce the time span of a possible change in equilibrium condition. It may also be that the time-dependent effect only takes place in a small range of A/R ratios, and that this ratio depends on the character of the resins and asphaltenes so that it is not observed in samples containing all the resin types used in this work. In order to investigate this possibility, a more detailed map of the time dependence with regard to A/R ratios must be made. The complexity of the resins and asphaltenes makes it difficult to present a detailed discussion about the results from samples containing two resin fractions. However, the mechanisms that are suggested for the samples containing one resin fraction should be relevant in such systems as well, in addition to interactions between the different resin fractions. As mentioned, the emulsions of types a, c, e, the centerpoints, and sample nos. 25 and 26 did not show any change in stability upon aging. For samples of type c and e and for the center samples, this is explained by the low asphaltene concentration and low A/R ratio: the resins dominate the properties of the surfactant phase. In sample nos. 1, 25, and 26, there are no resins present and the data thus suggest that the emulsion-stabilizing properties of the asphaltene aggregates are not significantly affected by the introduction of a fresh oil–water interface. Also, the asphaltenes were found by Ese et al. (28) to have a very restricted mobility at water–air interfaces, and this may well also be valid for oil–water interfaces. Thus, no significant change in emulsion stability is observed with aging in these emulsions. Final Stability of the Emulsions Containing Asphaltenes and Resins in Combination In this section the emulsion stabilities discussed are those observed after the samples have reached a “steady state,” that is, 0–300 min after mixing depending on sample composition. The values of the measured final stabilities of all the 26 emulsions are given in Table 2.
269
Samples of type b give emulsions that have higher stabilities than samples of type c, and samples of type d give emulsions that are more stable than samples of type e. Thus, for samples containing the same resin type(s), the lower stability is found in the sample with the lower A/R ratio. The most stable emulsions are no. 8 (0.78 kV/cm), which is of type b containing D resins in low concentration (A/R ratio of 5/3), and no. 18 (0.74 kV/cm), which is of type d containing resins of type C and D in low concentrations (A/R ratio of 1). The least stable emulsions are no. 9 (type c containing D resins in high amount, A/R ratio of 1/3) and no. 13 (type e containing A and B resins, A/R ratio of 1/3), both separating within a few minutes after emulsification. Samples with One Resin Type The following results are found when comparing the stabilities of samples of type b with sample 25 (which contain the same amount of asphaltenes but no resins): • •
resins of types A and C reduce the stability; resins of type B have no detectable effect with the resolution of these experiments; and • resins of type D increase the emulsion stability. In samples of type c the resins always reduce the stability of the emulsion compared to the emulsion containing only asphaltenes at the same concentration (sample no. 26). The emulsions of type c have stabilities ranging from 0.00 kV/cm when using D resins to 0.23 kV/cm when using B resins, showing that the type of resins also influences the stability at these A/R ratios. From the effect of the various resins at the A/R ratios explored, it is not possible to draw definite conclusions about their quantitative effect throughout the full range of A/R ratios (0 to 1). McLean and Kilpatrick (13) found that some resin fractions can give an enhanced stability in regions of A/R ratios larger than about 1 : 3, while others were found to reduce the emulsion stability at all A/R ratios. Since the main goal of this work was to do a screening analysis of the effect of various resin fractions extracted by the method presented in (15) on emulsion stability, it was decided not to increase the number of sample compositions. As the number of A/R ratios investigated is only 2, it is not possible to say for certain whether all the studied resin fractions will give an increase in emulsion stability at some A/R ratio. The present stabilities show that resin fractions of very different polarity have a significant effect on the emulsion stabilizing properties of asphaltenes. In previous reports for in the literature, the effect of the resins has been attributed to the content of polar groups, and thus a significant degree of polar interactions has been assumed. The effects of single-resin fractions presented in this paper show that resins of very low polarity also have a strong interaction with the asphaltenes. This leads to the conclusion that it is not just the polar functional groups of resins that interact with the asphaltene aggregates, since the D resins contain virtually no such groups. Thus, the type of intermolecular force is concluded to be important in solubilizing the asphaltenes. This is consistent
270
MIDTTUN ET AL.
with the conclusion by Brandt et al. (32) that dispersion forces are of importance in the aggregation of asphaltenes. Samples with Two Resin Types In samples of type d, which contain asphaltenes and two types of resins with A/R rations of 1, the lowest stability is obtained when using the most and least polar resins together (sample no. 20, A + D resins, E crit = 0.46 kV/cm) and the highest stability when combining the two least polar resins (sample no. 18, C and D resins, E crit = 0.74 kV/cm). All the other samples of this type form emulsions with stabilities of 0.54–0.61 kV/cm, which are considered to be equal with the accuracy of the method. The least stable emulsions of type e (A/R ratio of 1 : 3) is found in the sample containing the two most polar resin types (A and B, sample no. 13) in combination; this emulsion separated within minutes after preparation. The emulsions containing resins of the least polar type (type D) in combination with another resin in samples of type e (nos. 15, 19, and 21) are the most stable emulsions of this type. They all have stabilities of 0.32– 0.37 kV/cm, which must be considered equal with the resolution of the stability measurement method used. All of the emulsions of this type have stabilities lower than the sample with the same asphaltene concentration but no resins (sample no. 26), so in samples of type e all the resin combinations have a destabilizing effect on the emulsion stability. Summarizing the effect of combining two resin types with asphaltenes, it is clear that there are large variations in the effect of the various resin combinations on the emulsion stability. Also, the effect of a given combination of two resin types is not the same at the two A/R ratios investigated in this study. It is found that the emulsion stabilities are in general lower in samples of type e than in samples of type d. This is partly explained by the reduced asphaltene concentration. The observations of the effects of combining two different resin types show that the interaction between the resins is significant and at different A/R ratios the effect of the same resin X/resin Y ratio may be totally different. This makes the interactions very difficult to explain based on the present knowledge about these crude oil fractions. Error Associated with Sample Preparation and Analysis The centerpoint emulsions (sample nos. 22–24) are seen to be good replicates, giving stabilities of 0.43, 0.43, and 0.52 kV/cm and indicating that the error associated with sample preparation and stability measurement is low. CONCLUSIONS
Due to the lack of a good description of the structure and behavior of the resins, it is not possible to present a detailed model of the interactions involving these natural surfactants from the obtained stability data. Some qualitative mechanisms are, however, summarized in the following paragraphs, but it should be pointed out that the present understanding of the oil fractions involved is far from complete.
The effect of some resin fractions of various polar character in combination with asphaltenes on the stability of model emulsions have been investigated using a mixture design. It has been observed that the presence of some resins at asphaltene/resin ratios of 1 and 5 : 3 causes the stability of the prepared emulsions to increase for a period of 50–300 min after emulsification. The stability increases from a level comparable to that of emulsions dominated by the resins to one comparable to emulsions stabilized by asphaltenes. This stability increase is attributed to interactions between the resins and asphaltenes that take place when a fresh oil–water interface is added to the model oil. Mobility of resins at the interface is considered to enable the surfactants to build up a film that can effectively stabilize an emulsion, given that the asphaltene/resin ratio is not too low. These interactions can lead to the formation of a surfactant film that have significant emulsion-stabilizing properties, similar to and in some cases exceeding those of asphaltene aggregates. By measuring the effect of resins on the stability of asphaltenestabilized emulsions, it has been found that both polar and nonpolar resins interact strongly with the asphaltenes. At asphaltene/resin ratios of 1 and 5 : 3, this interaction can lead to an enhanced or reduced stability of model w/o emulsions, while at asphaltene/resin ratios of 1/3 the effect is always destabilizing, in some cases completely removing the emulsion-stabilizing ability of the asphaltenes. The fact that nonpolar resins also interact strongly with the asphaltenes shows that it is not only the polar functional groups of resins that interacts with asphaltenes; the dispersion type of intermolecular interaction is also important. ACKNOWLEDGMENTS Øivind Midttun acknowledges the Norwegian Research Council (NFR) for a Ph.D. grant. The technology program Flucha and the Ph.D. grant for Harald Kallevik was financed by NFR and industry. The dielectric instrumentation was also financed by NFR. Ph.D. student Øystein Sæter is thanked for the analysis of the droplet size distributions.
REFERENCES 1. Mingyuan, L., Christy, A. A., and Sj¨oblom, J., Kluwer, in “Emulsions: A Fundamental and Practical Approach” (J. Sj¨oblom, Ed.), NATO ASI Series C, Vol. 363, p. 157–172. Kluwer, Dordrecht, 1992. 2. Sj¨oblom, J., Mingyang, L., Tiren G., and Christy, A. A., Colloids Surf. 66, 55–62 (1992). 3. Førdedal, H., Midttun, Ø., Sj¨oblom, J., Kvalheim, O. M., Schildberg, Y., and Volle, J.-L., J. Colloid Interface Sci. 182, 117 (1996). 4. Ferworn, K. A., Svrcek, W. Y., and Mehrotra, A. K., Ind. Eng. Chem. Prod. Res. Dev. 32, 955–959 (1993). 5. Barre, L., Espinat, D., Rosenberg, E., and Scarsella, M., Rev. Inst. Fr. Petrole 52(2), 161–175 (1997). 6. Mullins, O. C., and Sheu, E. Y., “Structures and Dynamics of Asphaltenes.” Plenum Press, New York, 1998. 7. Calemma, V., Iwanski, P., Nali, M., Scotti, R., and Montanari, L., Energy Fuels 9, 225–230 (1995). 8. Nali, M., and Manclossi, A., Fuel Sci. Tech. Int. 13(10), 1251–1264 (1995). 9. Koots, J. A., and Speight, J. G., Fuel 54, 179–184 (1975). 10. Bardon, Ch., Barre, L., Espinat, D., Guille, V., Li, M. H., Lambard, J., Ravey, J. C., Rosenberg, E., and Zemb, T., Fuel Sci. Tech. Int. 14(1&2), 203–242 (1996).
MULTIVARIATE SCREENING ANALYSIS OF W/O EMULSIONS 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
Christy, A. A., Dahl, B., and Kvalheim, O. M., Fuel 68, 430–435 (1989). Andersen, S. I., and Birdi, K. S., J. Colloid Interface Sci. 142, 497 (1991). McLean, J. D., and Kilpatrick, P. K., J. Colloid Interface Sci. (in press). Førdedal, H., Schildberg, Y., and Sj¨oblom, J., and Volle, J.-L. Colloids Surf. 106, 33–47 (1996). Midttun, Ø., Sj¨oblom, J., and Kvalheim, O. M., Prog. Colloid Polymer Sci. (submitted). Fødedal, H., Schildberg, Y., Sj¨oblom, J., and Volle, J.-L., Colloids Surf. 106, 33–47 (1996). Acevedo, S., Ranaudo, M. A., Escobar, G., Guiterrez, L., and Ortega, P., Fuel 74(4), 595–598 (1995). Bailes, P. J., Trans. IChemE 73(A), 559–566 (1995). Drelich, J., Bryll, G., Kapczynski, J., Hupka, J., Miller, J. D., and Hanson, F. V., Fuel Proc. Tech. 31, 105–113 (1992). Førdedal, H., Nodland, E., Sj¨oblom, J., and Kvalheim, O. M., J. Colloid Interface Sci. 172, 396–405 (1995). Lee, C.-J., Wang, S-S., and Chan, C-C., J. Chinese Inst. Eng. 26(4), 263–275 (1995).
22. 23. 24. 25. 26. 27. 28. 29.
30. 31. 32.
271
Nahringbauer, I., Prog. Colloid Polym. Sci. 84, 200–205 (1991). Nahringbauer, I., J. Colloid Interface Sci. 176, 318–328 (1995). Nahringbauer, I., Langmuir 13, 2242–2249 (1997). Chen, J., and Dickinson, E. J. Sci. Food Agric. 62, 283–289 (1993). Jocic, D., Julia, M. R., and Erra, P., Colloid Polym. Sci. 274, 375–383 (1996). Courthaudon, J.-L., Dickinson, E., and Dalgleish, G., J. Colloid Interface Sci. 145(2), 390–395 (1991). Ese, M.-H., Yang, X., and Sj¨oblom, J., Colloid Polym. Sci. 276, 800–809 (1998). Myers, R. H., and Montgomery, D. C., “Response Surface Methodology. Process and Product Optimization Using Designed Experiments,” p. 535. John Wiley & Sons, New York, 1995. Weast, R. C., “CRC Handbook of Chemistry and Physics,” 60th ed. CRC Press, Boca Raton, FL, 1980. Friberg, S., Yang, H., Midttun, Ø., Sj¨oblom, J., and Aikens, P. A., Colloids Surf. A: Physicochem. Eng. Aspects 136, 43–49 (1998). Brandt, H. C. A., Hendriks, E. M., Michels, M. A. J., and Visser, F., J. Phys. Chem. 99, 10430–10432 (1995).
Multivariate Screening Analysis of Water-in-Oil Emulsions in High External Electric Fields as Studied by Means of Dielectric Time Domain Spectroscopy III. Model Emulsions Containing Asphaltenes and Resins Øivind Midttun,∗ Harald Kallevik,† Johan Sj¨oblom,†,1 and Olav M. Kvalheim∗ ∗ Department of Chemistry, University of Bergen, All´egaten 41, N-5007 Bergen, Norway; and †Statoil R&D Centre, Rotvoll, N-7005 Trondheim, Norway E-mail: [email protected] Received May 17, 1999; accepted April 17, 2000
The effect of crude oil resins with various polar characters on the stability of w/o model emulsions containing asphaltenes is investigated using a mixture design. The resins were extracted using an adsorption–desorption technique. One asphaltene fraction and four different resin fractions from one European crude oil were used. The stabilities are measured using time-domain dielectric spectroscopy in high external electric field. It is found that resins with different polar character have different effects on the emulsion stability. At asphaltene/resin ratios of 1 and 5 : 3 the resins in some cases lead to an emulsion stability higher than that of a similar emulsion stabilized by asphaltenes only, while at low asphaltene/resin ratios (∼1 : 3) the emulsion stability is reduced by the resins. The effect on emulsion stability of combining two different resin fractions depended on the resin types combined as well as the relative amount of resins and asphaltenes. Also, an increase in the stability of some of the emulsions containing resins and asphaltenes for a period of 50–300 min after the emulsification was observed. This time-dependence of emulsion stability is attributed to the mobility of resins at the oil–water interface and the slow buildup of a stabilizing interfacial film consisting of resins and asphaltenes. °C 2000 Academic Press Key Words: crude oils; w/o emulsions; crude oil resins; asphaltenes; high electric fields; mixture design; time-dependent emulsion stability.
INTRODUCTION
In crude oil production and transport, asphaltenes are precipitated as a result of mixing of crude oils and temperature and pressure changes, causing asphaltene particle-stabilized water-in-oil emulsions to form. These emulsions are commonly modeled in the laboratory using asphaltenes precipitated from the crude in an excess of an alkane. The precipitated asphaltene fraction is then used to stabilize model emulsions (1–3). Ferworn et al. (4) have shown that the size distribution of asphaltene particles 1
To whom correspondence should be addressed.
0021-9797/00 $35.00
C 2000 by Academic Press Copyright ° All rights of reproduction in any form reserved.
precipitated using n-heptane is a log-normal one and Barr´e et al. (5) found a log-normal distribution of asphaltene particles in toluene. In Refs. (4) and (5) the mean asphaltene particle size varied from 5 to 300 µm, depending on the precipitation conditions and the crude. The molecules included in the asphaltene fraction are polyaromatic molecules with molecular weights in the order of about one thousand to a few thousand (6–9). The asphaltene molecules are found to be polyaromatic sheets surrounded by aliphatic branches (10). Some heteroatoms are also found in the asphaltene fraction (9, 11). The presence of aromatic solvents influences the solubility of the asphaltenes in the oil phase. It is believed that the asphaltenes, themselves highly aromatic compounds, do not form particles in aromatic solvents as they do in aliphatic solvents but rather are present as small aggregates resembling micelles (12). These aggregates do not stabilize emulsions. This was shown by Førdedal et al. (3), who found that the emulsion stability changes with the aromatic character of the oil phase in a w/o model emulsion system. A pure toluene phase gave no stable emulsion, while a pure decane phase gave an emulsion of high stability when asphaltenes were used as stabilizers. The highest emulsion stability was observed for an oil phase consisting of about 80% decane and 20% toluene. McLean and Kilpatrick have reported similar results (13). The resin fraction can be defined by its solubility in, e.g., liquid propane and is per definition not precipitated when a solvent such as n-pentane is added to the crude to precipitate the asphaltenes. Resins are commonly extracted from the crude by adsorption onto a solid surface, such as silica particles (1, 14). The resins have been much less studied than the asphaltenes with respect to their structure, size, and emulsion stabilizing properties. Koots and Speight (9) found the molecular weight of resins to be about 1000. Heteroatoms are also found in resin fractions (9, 11, 15). Barr´e et al. (5) and Bardon et al. (10) studied the effect of resins on the molecular weight of asphaltene aggregates in toluene
262
MULTIVARIATE SCREENING ANALYSIS OF W/O EMULSIONS
solutions and found that an increased concentration of resins led to a reduced molecular weight of the asphaltene aggregates. Koots and Speight (9) also found that it was possible to dissolve the asphaltenes by adding resins. In a work by Førdedal et al. (16) it was shown that resins obtained by adsorption on silica particles and desorbed by a methanol/dichloromethane mixture increased the stability of model emulsions (at an asphaltene/ resin ratio of 1) as compared to model emulsions stabilized by asphaltene particles alone. The explanation given was that the resins used in their experiments are more interfacially active than the asphaltenes, reaching the fresh water/oil interface first. Afterward, the asphaltene particles aggregate at the interfacial resin film, giving a stable emulsion. The effect of resins on asphaltene-stabilized w/o emulsions was found by McLean and Kilpatrick (13) to be destabilizing in most cases, but cases of increased stabilization were also observed (for asphaltene/resin ratios lower than about 1 : 3). They also found that in emulsions with low asphaltene/resin ratios (below ∼1 : 3) the emulsions were always less stable than emulsions stabilized by asphaltene particles alone (at the same asphaltene concentration). Their explanation for this variation in stability is that the resins disperse the asphaltene particles, which must be of “correct” size to have a maximum emulsion-stabilizing ability, and that the asphaltene/resin ratio and the solubilizing effect of the resins affect this size. Acevedo et al. (17) have showed that resins adsorb to asphaltene particles in toluene/heptane mixtures in a complex manner. In a previous work (15) we separated different resin fractions from different crude oils by utilizing their adsorptions onto various particle surfaces and redissolving the adsorbed resins using different solvent combinations. These resins were characterized using infrared spectroscopy. The emulsions stabilized by asphaltene aggregates can be very stable. Several separation methods are in use to study the stability of emulsions, including the bottle test (13) and the use of a high external electric field (3, 18, 19, 20). The use of an external electric field causes the water droplets to polarize due to the presence of electrolyte, and the electric field thus facilitates the coalescence of droplets in the w/o emulsion (21). A suggested mechanism for coalescence is ions rupturing the interfacial films between the droplets. In dilute emulsions the droplets are elongated if an electric field is applied (Fig. 1b), while in a concentrated emulsion the droplets assemble into rows as shown in Fig. 1c. A picture of a typical undisturbed emulsion is shown in Fig. 1a, while a log-normal droplet size distribution of an undisturbed emulsion is illustrated in Fig. 2. The dielectric instrumentation we use to study the emulsions in high electric fields has the additional advantage that it enables us to study the emulsion stability within a time frame of minutes. This can be used to study the stability of emulsions at discrete time intervals, and in this way it is possible to measure changes in emulsion stability as a function of time. Several reports on time-dependent phenomena involving surfactants can be found in the literature (22–27), but none involve surfactants extracted from crude oils. The phenomena reported
263
are surfactant–polymer interactions at interfaces explained by surfactant-induced conformational changes in the polymers (22, 23, 24) and competitive adsorption (25, 27). Also of interest is the reported increase in oil–water interface coverage by proteins with time (25). In (22) surfactants were found to aggregate with polymers, causing a change in their hydrophilic character and thereby affecting their interfacial activity. In addition, reorientation of polymers and surfactants at the water–oil interface was suggested to take place, leading to a time-dependence of the measured surface tension (22, 24). The time scale of adsorption can vary considerably, with equilibrium conditions at the interface being reached in times ranging from milliseconds to hours (24). The longer time scales are observed when reorientation of proteins at an oil–water interface is studied (25). An interesting study of the behavior of interfacial films consisting of asphaltenes and resins, alone and in combination at a water– air interface, was made by Ese et al. (28). They found that films consisting of asphaltenes were very rigid while films consisting of polar resins were less rigid. This interfacial mobility of resins was explained by movement and reorientation of the resins at the interface. When asphaltenes and resins were combined, the compressibility and mobility of the surfactant film were found to be between that of pure asphaltenes and that of pure resins, depending on the asphaltene/resin (A/R) ratio. In this work we use resin fractions extracted according to the same method to study their effect on the stability of asphaltenestabilized w/o model emulsions. Based on their infrared spectra, the relative polar character of the resins is estimated. MATERIALS AND METHODS
Chemicals The crude was supplied by Elf Aquitane and is from a production field in France. n-Pentane, dichloromethane (DCM), methanol (MeOH), toluene, decane, and benzene were all of p.a. quality from Merck (Darmstadt, Germany). Silanol was obtained from Waters. CaO, NaCl, and talc were from Merck (Darmstadt, Germany). The siloxane was obtained from Elkem. Qualitative filter papers used were of type 1 from Whatman, England. De-ionized water was used. Crude Oil The crude oil is an European crude with specific gravity of 0.88 g/ml at 50◦ C, a viscosity of 16.5 mPa s at a shear rate of 1000 s−1 at 50◦ C. The asphaltene content is ∼9 wt% (1 : 5 pentane precipitation as specified in the next paragraph). The crude sample was stored for several years on a jerrycan, thus exposed to air. The amount of resins, aromatics, and contamination of production chemicals is not known. Separation of Asphaltenes and Resins The asphaltenes were precipitated by mixing 10 ml of crude with 50 ml of n-pentane, stirring of the mixture, followed by
264
MIDTTUN ET AL.
FIG. 1. (a) Water droplets in an undisturbed model water-in-oil emulsion. (b) Elongated water droplets when an electrical field is applied to a dilute w/o emulsion. (c) Chains of droplets in a concentrated emulsion when an electrical field is applied. The droplets that are not spherical are the result of coalescence of two droplets; the resulting droplet has not had time to reach its equilibrium shape.
centrifugation for 10 min at 2400 rpm. To adsorb the resins the particles were added to the supernatant from the precipitation, the mixture was stirred and centrifuged, and the supernatant was removed. The particles were then washed with benzene (when using silanole or siloxene particles) or DCM (when using CaO or talc particles) until the filtrate was colorless. The resins were then obtained from the particles by desorption with a mixture of 7% MeOH in DCM (when using CaO, silanol, or siloxane) or benzene (when using talc). The solvent was evaporated under a
nitrogen blanket. The particles and solvents used in each step are given in Table 1, and the extraction method is fully described by Midttun et al. (15). All the fractions are isolated from one single European crude oil. Preparation of Emulsions and Emulsion Stability Measurements The compositions of oil phase of the samples studied are given in Table 2. The surfactant concentration (asphaltenes + resins)
MULTIVARIATE SCREENING ANALYSIS OF W/O EMULSIONS
265
FIG. 2. Droplet size distribution of a typical asphaltene-stabilized model emulsion. The asphaltene concentration is 1.3% (w/w), and the oil phase consists of 30% toluene and 70% decane. The total number of droplets is 545.
is 2% (by weight) of the oil phase in samples 1–24. Samples 25 and 26 contain only asphaltenes at the same concentration as in samples of type b, c, and e (Table 2). According to Ref. (3) water-in-crude oil model emulsions stabilized by resins alone are not stable enough, so we omitted such systems. The lowest asphaltene/resin ratio chosen is 1 : 3, giving 1.5 wt% as the maximum resin concentration and 0.5 wt% as the minimum asphaltene concentration. All the relative compositions of the surfactant mixture chosen for study, except for the centerpoint and samples 25 and 26, are marked in Fig. 3. The oil phase contains 20 wt% toluene and 80 wt% decane before addition of the surfactant. The amount of water in the emulsions is 30% of the total sample weight. The aqueous phase of the samples contain 3.5 wt% NaCl. The emulsions were prepared and analyzed in random order to prevent systematic errors. Toluene was added to the asphaltenes,
and the sample was left to equilibrate for 2 h before the decane was added. This mixture was then left to equilibrate for another 2 h, and the resins (if any) were added. The sample was then left overnight before addition of water and emulsification using a Citenco rotor emulsifier at 3000 rpm for 5 min. The sample was shaken to obtain a homogenous sample, and a small part of it was taken out and inserted into the measuring cell. The thickness of the cell in which the emulsions were kept during stability measurements was 0.23 mm. By increasing the voltage over the emulsion in steps of 0.043 kV/cm every third minute, the electric field was increased until the emulsion separated. The emulsion stabilities are given as kV/cm, and higher numbers indicate higher emulsion stability. The recorded times in Table 2 are the times elapsed from emulsification to separation as observed by conductivity through the samples. This process was repeated for several samples of the emulsion for a period
TABLE 1 Coding of the Resins Used in the Surfactant Mixture Particle Solvent 1 Solvent 2 Resin Acronym in Ref. (14)
Calcium oxide (C) Dichloromethane (D) Methanol (M)/dichloromethane A FCDM
Silanol (S) Benzene (B) Methanol (M)/dichloromethane B FSBM
Siloxane (X) Benzene (B) Methanol (M)/dichloromethane C FXBM
Note. Based on the extraction procedure and IR spectra, the polarity of the fractions is assumed to be A > B > C > D (see text).
Talc (T) Dichloromethane (D) Benzene (B) D FTDB
266
MIDTTUN ET AL.
TABLE 2 Model Emulsions % of each surfactant in model oil Sample no.
Sample type
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
a b c b c b c b c d e d e d e d e d e d e Centerpoint Centerpoint Centerpoint 1.25% asph. 0.5% asph.
A/R ratio
1.67 0.33 1.67 0.33 1.67 0.33 1.67 0.33 1.00 0.33 1.00 0.33 1.00 0.33 1.00 0.33 1.00 0.33 1.00 0.33 0.67 0.67 0.67
AS
B
C
A
D
Stability (kV/cm)
Time (min)
2.00 1.25 0.50 1.25 0.50 1.25 0.50 1.25 0.50 1.00 0.50 1.00 0.50 1.00 0.50 1.00 0.50 1.00 0.50 1.00 0.50 0.80 0.80 0.80 1.25 0.50
0.00 0.75 1.50 0.00 0.00 0.00 0.00 0.00 0.00 0.50 0.75 0.50 0.75 0.50 0.75 0.00 0.00 0.00 0.00 0.00 0.00 0.30 0.30 0.30 — —
0.00 0.00 0.00 0.75 1.50 0.00 0.00 0.00 0.00 0.50 0.75 0.00 0.00 0.00 0.00 0.50 0.75 0.50 0.75 0.00 0.00 0.30 0.30 0.30 — —
0.00 0.00 0.00 0.00 0.00 0.75 1.50 0.00 0.00 0.00 0.00 0.50 0.75 0.00 0.00 0.50 0.75 0.00 0.00 0.50 0.75 0.30 0.30 0.30 — —
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.75 1.50 0.00 0.00 0.00 0.00 0.50 0.75 0.00 0.00 0.50 0.75 0.50 0.75 0.30 0.30 0.30 — —
0.51 0.68 0.23 0.58 0.11 0.51 0.14 0.78 0.00 0.57 0.10 0.59 0.00 0.54 0.37 0.61 0.22 0.74 0.32 0.46 0.35 0.43 0.52 0.43 0.68 0.46
0 180 0 50 0 0 0 0 0 200 0 0 0 300 0 100 0 150 0 50 0 0 0 0 0 0
Note. The composition of the surfactant mixture, the measured stability of the corresponding emulsions and the approximate time necessary for the samples to reach maximum stability is given. For the samples where the time is given as zero, no change in stability was observed. AS is the asphaltenes.
of 2–4 h after mixing for each model emulsion. The electric field necessary to break the emulsion was measured with the instrumentation described by Førdedal et al. (20). Experimental Design A mixture design (29) was chosen to study the effect of varying the composition of the surfactant mixture of the system. A mixture design gave a good distribution of the analyzed compositions in the domain spanned by the components in the surfactant phase of the samples and a three-dimensional simplex pyramid is shown in Fig. 3. Each corner represents a pure compound; the sides represent two-compound mixtures and the faces threecompound mixtures. Using five surfactant components (asphaltene and four different resins), we constructed a five-dimensional simplex, each pure surfactant component represented by one corner in the pyramid.
22 × 275. The similarities/dissimilarities of these fractions are easily found by use of a mathematical decomposition technique called principal component analysis (PCA). In PCA the X matrix is decomposed into principal components called score and loading vectors. The scores and loadings contain information on how the fractions and the wavelengths in X relate to each other,
RESULTS AND DISCUSSION
In a previous work by our group (15) we sampled infrared spectra from 22 fractions of the specific crude used in the present work. Twenty-two infrared spectra with the absorption recorded at 275 wavelengths results in a data matrix X with dimension
FIG. 3. Simplex design in three dimensions. The compositions of the different sample types used in the present work are marked (see Table 1). The centerpoints are not shown.
267
MULTIVARIATE SCREENING ANALYSIS OF W/O EMULSIONS
respectively. Mathematically, the procedure can be expressed as Xn×m = t1 pT1 + t2 pT2 + ti piT + · · · . + tk pTk + En×m .
TABLE 3 Band Assignments for DRIFT Spectra
[1]
In Eq. [1], X is the original data matrix, ti are the score vectors and pi are the loading vectors. E is the residual matrix. The direction of the first principal component (the vectors t1 and p1 ) is defined as the straight line that best describes the variation within the X matrix. The direction of the second principal component is the one that best describes the variation in X that is not explained by the first component. The third and fourth components and so on can be found in the same manner. This procedure results in k orthogonal pairs of score and loading vectors. If these components are plotted against each other, the relation between samples is easily detected. The choice of resins was such that they should span the space represented by the first four principal components in the previous work (15) well (Fig. 4), with the following restrictions: • Resin fractions separated using all of the four particle types should be represented. • The yield of the chosen fractions should not be too low.
Thus, four resin fractions designated A-D (see Table 1 and Fig. 5) were chosen. In the infrared spectra of the resins (Fig. 5) it is seen that resin type A has a very high intensity in the amide
FIG. 4. Distribution of the resin fractions from Ref. (14). A cross marks the origin.
Functional groups Ester Ketones Aldehydes Amide C==C (conjugated and aromatic) C–CH3, C−CH2 (asymmetric bending) C–CH3 (symmetric bending) Ester Aromatic ether Alcohol Aliphatic ether Sulfoxide
Absorption bands (cm−1 ) 1750–1710 1730–1700 1730–1690 1670–1620 1600 1465 1377 1320–1100 1300–1200 1210–1000 1150–1050 1060–1020
band (1670–1620 cm−1 ). Amide groups are highly polar (30), and these resins are therefore considered to be the most polar of the four resin fractions used in the present work. The B resins also have a strong amide band in their spectrum although it is much weaker than that of the A resins. In addition the B resins have a relatively strong absorption band attributed to the ketone/aldehyde functional group (1730–1690 cm−1 ), which is not present in the spectrum of the A resins. The B resin fraction thus also contains a significant amount of polar groups. The C resins have absorption bands of both amide and ketone/aldehyde groups, but they are of considerably lower intensity than in the B resins. A very strong band is also found in the ether/sulfoxide region (∼1030 cm−1 ). Some polar groups are thus present in the C resins. The amide and ketone/aldehyde bands are practically absent from the spectrum of the D resin fraction, and no other strong bands assigned to significantly polar functional groups are found in the spectrum. Resin fraction D is therefore considered to have a very low polarity. Thus the polarity of the four resin fractions is assumed to be
FIG. 5. DRIFT spectra of the four resin fractions. Band assignments are given in Table 3.
268
MIDTTUN ET AL.
A > B > C > D. It should also be noted from the infrared spectrum of the asphaltene fraction (15) that it has a very low content of heteroatom functionalities and is therefore considered to be of low polarity. When the samples were inspected after they had been left overnight with all of the surfactants present (asphaltenes + resins), it was found that all of them contained some precipitate. Effect of Changing the Asphaltene Concentration Only When using only asphaltenes as surfactant (sample nos. 1, 25, and 26), it is seen that the most stable emulsion is obtained when using 1.25% asphaltenes. An electric field of 0.68 kV/cm was needed to break this emulsion. The emulsions with 2% and 0.5% asphaltenes show approximately equal stability, requiring an electric field of about 0.5 kV/cm for separation. There thus seems to exist an optimal asphaltene concentration with regard to maximizing the emulsion-stabilizing ability, located at an asphaltene concentration somewhere between 0.5% and 2% in this system. This can be explained by the varying size of the asphaltene particles (13). An increase in the asphaltene concentration above the “optimal” concentration led to the formation of larger asphaltene particles. These larger particles are less able to stabilize emulsions since they pack less efficiently at the interface. Also, a concentration lower than the “optimal” asphaltene concentration results in fewer asphaltene particles, reducing the ability to stabilize emulsions due to inability to cover the large area of a large number of small droplets. Effect of Aging on the Stability of the Emulsions The stability of several of the samples increased for the first 50–300 min after emulsification, and the times from emulsification until the equilibrium stabilities are reached is given in Table 2. To our knowledge this time-dependency of emulsion stability in emulsions stabilized by surfactants extracted from crude oils has not been reported in the literature. Figure 6 shows the stabilities for a period of 150–300 min after mixing for some representative samples. This increase in stability were observed only in samples with A/R ratios ≥ 1, i.e., samples of types b and d. In all cases the stabilities increased from a level of 0.1–0.3 kV/cm shortly after emulsification to a maximum of 0.5–0.9 kV/cm depending on the type(s) of resins present. Emulsions of types a, c, e, the centerpoint, and sample nos. 25 and 26 did not show any time dependency in the emulsion stability. It is clear from the present data that the time needed for the indigenous surfactants to reach equilibrium at the oil–water interface depends on the type(s) of resins present and the relative amount of resins and asphaltenes. It is assumed that the time (about 16 h) allowed for the surfactants to equilibrate in the oil phase before the introduction of water is sufficient, and the new equilibrium condition is thus a consequence of the introduction of the fresh oil–water interface. For the samples with observed time dependence the increase in stability is from a level comparable to that of the samples with low A/R ratios to stability levels of samples with higher
FIG. 6. Some representative results from the stability measurements. The reproducibility of the method is seen to be acceptable, and the increases in stability of some of the emulsions are illustrated by sample nos. 2 and 14. All the measurements for all three centerpoints (sample nos. 22–24) are given.
A/R ratios and asphaltenes alone. It is therefore reasonable to explain the increasing stability as a process in which the emulsionstabilizing ability of the interfacial surfactant film changes from one dominated by the resins to one dominated by the asphaltenes (but whose strength is influenced by the resins), as discussed by Førdedal et al. (14). Of the samples containing only one resin fraction in combination with asphaltenes, only those of type b with B or C resins show this time-dependent effect. This also applies for all of the samples of type d (which contain two resin fractions and asphaltenes) except the one containing both A and B resins. The B and C resins contain some polar groups and are considered to be of intermediate polarity compared to the A and D resins. The increase in emulsion stability may be explained as follows: if resins are present in the model oil containing both asphaltenes and resins, they disperse some of the asphaltene particles. The amount dispersed depends on the A/R ratio and the character of the resin and asphaltene fraction (5, 9, 10, 13). This will shift the size distribution of the asphaltene particles toward smaller aggregates (13). When water is added to the system, the highly interfacially active resins reach the oil–water interface first, followed by the asphaltenes. Thus a new equilibrium condition is established in which a complex interfacial layer of resins and asphaltenes decides the stability of the emulsion (14). It has been shown by Friberg et al. (31) that the B resins are interfacially active. As noted in the introduction, Ese et al. (28) have shown that the resin fraction called B in this paper forms a film at the water–air interface in which the resins are mobile, showing that movement/reorientation of the resins at the interface is possible. This suggests that the resins will also be mobile at the oil– water interface and opens up the possibility of the asphaltenes
MULTIVARIATE SCREENING ANALYSIS OF W/O EMULSIONS
to combine with the resins in the interfacial film. This may explain the time dependence of the emulsion stability observed in the present work. Since the molecules and aggregates involved are quite large and of complex structure, it is reasonable that this process require time scales on the order of hours (24). For the samples of type b containing A and D resins in addition to asphaltenes no time dependence is observed. This may be explained either by the change in equilibrium conditions (upon the introduction of the fresh oil–water interface) taking place so quickly that it is difficult to observe with the method used, or that no significant change in stability occur. Because the D resins contain very small amounts of polar functional groups (15), they are not expected to have a significant interfacial activity at the oil-water interface. Thus no significant change in the emulsion stability would be expected for the sample of type b containing D resins (sample no. 8). For the A resins it is suggested that the orientation of the resins at the oil–water interface take place quickly due to the large amount of highly polar amide groups present, and/or that the high amounts of polar groups inhibit their mobility at the interface. Both of these effects can reduce the time span of a possible change in equilibrium condition. It may also be that the time-dependent effect only takes place in a small range of A/R ratios, and that this ratio depends on the character of the resins and asphaltenes so that it is not observed in samples containing all the resin types used in this work. In order to investigate this possibility, a more detailed map of the time dependence with regard to A/R ratios must be made. The complexity of the resins and asphaltenes makes it difficult to present a detailed discussion about the results from samples containing two resin fractions. However, the mechanisms that are suggested for the samples containing one resin fraction should be relevant in such systems as well, in addition to interactions between the different resin fractions. As mentioned, the emulsions of types a, c, e, the centerpoints, and sample nos. 25 and 26 did not show any change in stability upon aging. For samples of type c and e and for the center samples, this is explained by the low asphaltene concentration and low A/R ratio: the resins dominate the properties of the surfactant phase. In sample nos. 1, 25, and 26, there are no resins present and the data thus suggest that the emulsion-stabilizing properties of the asphaltene aggregates are not significantly affected by the introduction of a fresh oil–water interface. Also, the asphaltenes were found by Ese et al. (28) to have a very restricted mobility at water–air interfaces, and this may well also be valid for oil–water interfaces. Thus, no significant change in emulsion stability is observed with aging in these emulsions. Final Stability of the Emulsions Containing Asphaltenes and Resins in Combination In this section the emulsion stabilities discussed are those observed after the samples have reached a “steady state,” that is, 0–300 min after mixing depending on sample composition. The values of the measured final stabilities of all the 26 emulsions are given in Table 2.
269
Samples of type b give emulsions that have higher stabilities than samples of type c, and samples of type d give emulsions that are more stable than samples of type e. Thus, for samples containing the same resin type(s), the lower stability is found in the sample with the lower A/R ratio. The most stable emulsions are no. 8 (0.78 kV/cm), which is of type b containing D resins in low concentration (A/R ratio of 5/3), and no. 18 (0.74 kV/cm), which is of type d containing resins of type C and D in low concentrations (A/R ratio of 1). The least stable emulsions are no. 9 (type c containing D resins in high amount, A/R ratio of 1/3) and no. 13 (type e containing A and B resins, A/R ratio of 1/3), both separating within a few minutes after emulsification. Samples with One Resin Type The following results are found when comparing the stabilities of samples of type b with sample 25 (which contain the same amount of asphaltenes but no resins): • •
resins of types A and C reduce the stability; resins of type B have no detectable effect with the resolution of these experiments; and • resins of type D increase the emulsion stability. In samples of type c the resins always reduce the stability of the emulsion compared to the emulsion containing only asphaltenes at the same concentration (sample no. 26). The emulsions of type c have stabilities ranging from 0.00 kV/cm when using D resins to 0.23 kV/cm when using B resins, showing that the type of resins also influences the stability at these A/R ratios. From the effect of the various resins at the A/R ratios explored, it is not possible to draw definite conclusions about their quantitative effect throughout the full range of A/R ratios (0 to 1). McLean and Kilpatrick (13) found that some resin fractions can give an enhanced stability in regions of A/R ratios larger than about 1 : 3, while others were found to reduce the emulsion stability at all A/R ratios. Since the main goal of this work was to do a screening analysis of the effect of various resin fractions extracted by the method presented in (15) on emulsion stability, it was decided not to increase the number of sample compositions. As the number of A/R ratios investigated is only 2, it is not possible to say for certain whether all the studied resin fractions will give an increase in emulsion stability at some A/R ratio. The present stabilities show that resin fractions of very different polarity have a significant effect on the emulsion stabilizing properties of asphaltenes. In previous reports for in the literature, the effect of the resins has been attributed to the content of polar groups, and thus a significant degree of polar interactions has been assumed. The effects of single-resin fractions presented in this paper show that resins of very low polarity also have a strong interaction with the asphaltenes. This leads to the conclusion that it is not just the polar functional groups of resins that interact with the asphaltene aggregates, since the D resins contain virtually no such groups. Thus, the type of intermolecular force is concluded to be important in solubilizing the asphaltenes. This is consistent
270
MIDTTUN ET AL.
with the conclusion by Brandt et al. (32) that dispersion forces are of importance in the aggregation of asphaltenes. Samples with Two Resin Types In samples of type d, which contain asphaltenes and two types of resins with A/R rations of 1, the lowest stability is obtained when using the most and least polar resins together (sample no. 20, A + D resins, E crit = 0.46 kV/cm) and the highest stability when combining the two least polar resins (sample no. 18, C and D resins, E crit = 0.74 kV/cm). All the other samples of this type form emulsions with stabilities of 0.54–0.61 kV/cm, which are considered to be equal with the accuracy of the method. The least stable emulsions of type e (A/R ratio of 1 : 3) is found in the sample containing the two most polar resin types (A and B, sample no. 13) in combination; this emulsion separated within minutes after preparation. The emulsions containing resins of the least polar type (type D) in combination with another resin in samples of type e (nos. 15, 19, and 21) are the most stable emulsions of this type. They all have stabilities of 0.32– 0.37 kV/cm, which must be considered equal with the resolution of the stability measurement method used. All of the emulsions of this type have stabilities lower than the sample with the same asphaltene concentration but no resins (sample no. 26), so in samples of type e all the resin combinations have a destabilizing effect on the emulsion stability. Summarizing the effect of combining two resin types with asphaltenes, it is clear that there are large variations in the effect of the various resin combinations on the emulsion stability. Also, the effect of a given combination of two resin types is not the same at the two A/R ratios investigated in this study. It is found that the emulsion stabilities are in general lower in samples of type e than in samples of type d. This is partly explained by the reduced asphaltene concentration. The observations of the effects of combining two different resin types show that the interaction between the resins is significant and at different A/R ratios the effect of the same resin X/resin Y ratio may be totally different. This makes the interactions very difficult to explain based on the present knowledge about these crude oil fractions. Error Associated with Sample Preparation and Analysis The centerpoint emulsions (sample nos. 22–24) are seen to be good replicates, giving stabilities of 0.43, 0.43, and 0.52 kV/cm and indicating that the error associated with sample preparation and stability measurement is low. CONCLUSIONS
Due to the lack of a good description of the structure and behavior of the resins, it is not possible to present a detailed model of the interactions involving these natural surfactants from the obtained stability data. Some qualitative mechanisms are, however, summarized in the following paragraphs, but it should be pointed out that the present understanding of the oil fractions involved is far from complete.
The effect of some resin fractions of various polar character in combination with asphaltenes on the stability of model emulsions have been investigated using a mixture design. It has been observed that the presence of some resins at asphaltene/resin ratios of 1 and 5 : 3 causes the stability of the prepared emulsions to increase for a period of 50–300 min after emulsification. The stability increases from a level comparable to that of emulsions dominated by the resins to one comparable to emulsions stabilized by asphaltenes. This stability increase is attributed to interactions between the resins and asphaltenes that take place when a fresh oil–water interface is added to the model oil. Mobility of resins at the interface is considered to enable the surfactants to build up a film that can effectively stabilize an emulsion, given that the asphaltene/resin ratio is not too low. These interactions can lead to the formation of a surfactant film that have significant emulsion-stabilizing properties, similar to and in some cases exceeding those of asphaltene aggregates. By measuring the effect of resins on the stability of asphaltenestabilized emulsions, it has been found that both polar and nonpolar resins interact strongly with the asphaltenes. At asphaltene/resin ratios of 1 and 5 : 3, this interaction can lead to an enhanced or reduced stability of model w/o emulsions, while at asphaltene/resin ratios of 1/3 the effect is always destabilizing, in some cases completely removing the emulsion-stabilizing ability of the asphaltenes. The fact that nonpolar resins also interact strongly with the asphaltenes shows that it is not only the polar functional groups of resins that interacts with asphaltenes; the dispersion type of intermolecular interaction is also important. ACKNOWLEDGMENTS Øivind Midttun acknowledges the Norwegian Research Council (NFR) for a Ph.D. grant. The technology program Flucha and the Ph.D. grant for Harald Kallevik was financed by NFR and industry. The dielectric instrumentation was also financed by NFR. Ph.D. student Øystein Sæter is thanked for the analysis of the droplet size distributions.
REFERENCES 1. Mingyuan, L., Christy, A. A., and Sj¨oblom, J., Kluwer, in “Emulsions: A Fundamental and Practical Approach” (J. Sj¨oblom, Ed.), NATO ASI Series C, Vol. 363, p. 157–172. Kluwer, Dordrecht, 1992. 2. Sj¨oblom, J., Mingyang, L., Tiren G., and Christy, A. A., Colloids Surf. 66, 55–62 (1992). 3. Førdedal, H., Midttun, Ø., Sj¨oblom, J., Kvalheim, O. M., Schildberg, Y., and Volle, J.-L., J. Colloid Interface Sci. 182, 117 (1996). 4. Ferworn, K. A., Svrcek, W. Y., and Mehrotra, A. K., Ind. Eng. Chem. Prod. Res. Dev. 32, 955–959 (1993). 5. Barre, L., Espinat, D., Rosenberg, E., and Scarsella, M., Rev. Inst. Fr. Petrole 52(2), 161–175 (1997). 6. Mullins, O. C., and Sheu, E. Y., “Structures and Dynamics of Asphaltenes.” Plenum Press, New York, 1998. 7. Calemma, V., Iwanski, P., Nali, M., Scotti, R., and Montanari, L., Energy Fuels 9, 225–230 (1995). 8. Nali, M., and Manclossi, A., Fuel Sci. Tech. Int. 13(10), 1251–1264 (1995). 9. Koots, J. A., and Speight, J. G., Fuel 54, 179–184 (1975). 10. Bardon, Ch., Barre, L., Espinat, D., Guille, V., Li, M. H., Lambard, J., Ravey, J. C., Rosenberg, E., and Zemb, T., Fuel Sci. Tech. Int. 14(1&2), 203–242 (1996).
MULTIVARIATE SCREENING ANALYSIS OF W/O EMULSIONS 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
Christy, A. A., Dahl, B., and Kvalheim, O. M., Fuel 68, 430–435 (1989). Andersen, S. I., and Birdi, K. S., J. Colloid Interface Sci. 142, 497 (1991). McLean, J. D., and Kilpatrick, P. K., J. Colloid Interface Sci. (in press). Førdedal, H., Schildberg, Y., and Sj¨oblom, J., and Volle, J.-L. Colloids Surf. 106, 33–47 (1996). Midttun, Ø., Sj¨oblom, J., and Kvalheim, O. M., Prog. Colloid Polymer Sci. (submitted). Fødedal, H., Schildberg, Y., Sj¨oblom, J., and Volle, J.-L., Colloids Surf. 106, 33–47 (1996). Acevedo, S., Ranaudo, M. A., Escobar, G., Guiterrez, L., and Ortega, P., Fuel 74(4), 595–598 (1995). Bailes, P. J., Trans. IChemE 73(A), 559–566 (1995). Drelich, J., Bryll, G., Kapczynski, J., Hupka, J., Miller, J. D., and Hanson, F. V., Fuel Proc. Tech. 31, 105–113 (1992). Førdedal, H., Nodland, E., Sj¨oblom, J., and Kvalheim, O. M., J. Colloid Interface Sci. 172, 396–405 (1995). Lee, C.-J., Wang, S-S., and Chan, C-C., J. Chinese Inst. Eng. 26(4), 263–275 (1995).
22. 23. 24. 25. 26. 27. 28. 29.
30. 31. 32.
271
Nahringbauer, I., Prog. Colloid Polym. Sci. 84, 200–205 (1991). Nahringbauer, I., J. Colloid Interface Sci. 176, 318–328 (1995). Nahringbauer, I., Langmuir 13, 2242–2249 (1997). Chen, J., and Dickinson, E. J. Sci. Food Agric. 62, 283–289 (1993). Jocic, D., Julia, M. R., and Erra, P., Colloid Polym. Sci. 274, 375–383 (1996). Courthaudon, J.-L., Dickinson, E., and Dalgleish, G., J. Colloid Interface Sci. 145(2), 390–395 (1991). Ese, M.-H., Yang, X., and Sj¨oblom, J., Colloid Polym. Sci. 276, 800–809 (1998). Myers, R. H., and Montgomery, D. C., “Response Surface Methodology. Process and Product Optimization Using Designed Experiments,” p. 535. John Wiley & Sons, New York, 1995. Weast, R. C., “CRC Handbook of Chemistry and Physics,” 60th ed. CRC Press, Boca Raton, FL, 1980. Friberg, S., Yang, H., Midttun, Ø., Sj¨oblom, J., and Aikens, P. A., Colloids Surf. A: Physicochem. Eng. Aspects 136, 43–49 (1998). Brandt, H. C. A., Hendriks, E. M., Michels, M. A. J., and Visser, F., J. Phys. Chem. 99, 10430–10432 (1995).