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Particle—fluid interactions with application to solid-stabilized emulsions part I. The effect of asphaltene adsorption
Colloids
and Surfaces,
19 (1986)
89
89-105
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
PARTICLE-FLUID SOLIDSTABILIZED
INTERACTIONS EMULSIONS
WITH APPLICATION
TO
PART I. THE EFFECT OF ASPI-IALTENE ADSORPTION
V.B. MENON and D.T. WASAN Department of Chemical 60616 (U.S.A.)
Engineering,
Illinois
Institute
of Technology,
Chicago,
IL
(Received 9 July 1985 ; accepted in final form 27 January 1986)
ABSTRACT Solid-stabilized water-in-oil emulsions are commonly encountered in the production of crude oil and synthetic fuel liquids. The ability of finely divided solids to stabilize these emulsions is due to the adsorption of asphaltenes and other heavy ends from the oil on the mineral surface which imparts to the solid its hydrophobic nature. This study presents results which quantify the particle-fluid interactions by means of adsorption measurements of asphaltenes on sodium montmorillonite particles. The influence of asphaltene adsorption on the contact angle at the water/oil/solid interface and on the zeta potential of the particles was investigated. The hydrophobic nature of the solids was found to be directly related to the amount of asphaltenes adsorbed on their surface. The effect of asphaltenes on the contact angle is explained in terms of the interfacial tensions at the oil/water and solid/water interfaces. The influence of pH and surfactant concentration in the aqueous phase on the contact angle and the zeta potential is assessed.
INTRODUCTION
The ability of solid particles to stabilize emulsions has been reported in the literature since 1903 when Ramsden [l] first observed emulsions stabilized by the presence of solid or highly viscous matter at the interface of two liquids. Since then the stability of such emulsions has been studied by many investigators [24]. It is generally accepted that water-inoil (W/O) emulsions are stabilized when the solids are hydrophobic, and oilin-water (O/W) emulsions are favored when the solids are hydrophilic. In the petroleum and synthetic fuel industries the separation of particulate matter from oil is commonly accomplished by dispersing water into the oil whereby the solids are transferred to the oil/water interface forming stable W/O emulsions. The water phase is subsequently removed by gravity settling and/or electrostatic coalescence. At this stage of the process the ready coalescence of the emulsions is desirable. The hydrophobic nature of
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0 1986 Elsevier Science Publishers B.V.
90
the mineral particles (owing to adsorbed asphaltic matter from the oil) is responsible for the stability of such emulsions and in order to destabilize these emulsions more readily, the particles must be made more hydrophilic. The wettability of the particles is thus crucial since it determines the nature and stability of the emulsions. The hydrophobicity of mineral matter from crude oil arises from the adsorption of asphaltenes and resins from the oil onto the initially hydrophilic clay [ 51. The asphaltenes comprise a major portion of the surface-active component of oil and have been known to adsorb irreversibly on clay surfaces [ 5,6]. They are large polyaromatic and polycyclic condensed ring compounds which contain heteroatoms such as N, S and 0. Such heteroatoms imply the presence of polar functional groups [ 7-91. Henry et al. [lo] have investigated the effect of particle wettability and pH on the effectiveness of particle separation from oil to a receiving water phase. They found that the effectiveness of separation is improved significantly as the hydrophilicity of the solids increases (contact angle decreases) and as the aqueous phase pH becomes more basic. Gelot et al. [ll] found that the type and stability of such emulsions are strongly dependent on the particle size, density and the effect of surfactants on their wettability. Taubman and Koretskii [12] contend that while certain surfactants are effective in breaking solid-stabilized emulsions others increase the stability by forming chemical complexes on the partices, thereby anchoring them to the oil/water interface. Svitova et al. [13] have found ionic water-soluble polymers to be very effective in breaking water/crude oil emulsions stabilized by asphaltic material while nonionic polymers were not. They also attribute the emulsion stability to be directly related to the nature and extent of surfactant adsorption on the solid surface. In a previous paper [14] we discussed some of our results on the study of the stability of water-in-shale oil emulsions with and without solid fines in the oil. It was found that 0.1% (w/w) of dust particles in the shale oil produced emulsions which were three times more stable than the emulsions without the fines. The concentration of the solids and hence their packing at the interface are therefore very important to the stability of the emulsions. In this study we have made an attempt at understanding the factors that influence the wettability. We therefore investigated the influence of adsorbed asphaltenes on the contact angle at the solid/oil/water interface. The interfacial tension at the oil/water interface and the zeta potential of the solid particles at the solid/water interface were measured to explain the wettability behavior in terms of the surface energies at the solid/water and oil/water interfaces. The influence of surfactants and aqueous phase pH on the contact angle and zeta potential was also investigated. While most of our results are for a model clay, sodium montmorillonite, which was coated with asphaltenes, we have attempted, wherever possible, to compare our observations with that of actual dust filtered from raw shale oil.
91 EXPERIMENTAL
The asphaltenes used in this study were extracted from shale oil obtained from Rundle, Australia, and was supplied to us by Exxon Research and Engineering. The shale oil had a viscosity of 0.032 Poise and a density of 0.83 g cme3. The extraction procedure involves the precipitation of the asphaltenes from the oil by an excess of hexane. Thus the asphaltenes we have extracted comprise the hexane-insoluble portion of the oil. Sodium montmorillonite from Crook County, WY, was used as the model clay. The dry clay had a BET surface area of 29.4 m2 g-‘. The clay fines had an average diameter of 4.0 pm and a density of 2.6 g cmm3. Sodium montmorillonite has been reported to have a cation exchange capacity in the range of 90-100 meq per 100 g [15]. This gives a surface charge density of -0.12 C me2. A 1 :l volume mixture of n-heptane and toluene was used as the organic phase. This liquid, which we shall call ‘heptol’, has a density of 0.76 g cme3 and a surface tension of 24 dyne cm-‘. The surfactant, Aerosol-OT, was supplied by American Cyanamid Co. and was used as supplied. The adsorption isotherm of the asphaltenes on sodium montmorillonite was determined by contacting 0.76 g of clay with 100 ml of a solution of asphaltenes in heptol and stirring for 24 h. The amount of asphaltenes adsorbed on the clay was determined by measuring the absorbance of the initial solution and the equilibrium solution using a Beckman Spectrophotometer at a wavelength of 336 nm. A calibration curve of absorbance versus asphaltene concentration in heptol was constructed at this wavelength. The spectrophotometer measurements were reproducible to within 5%. The zeta potential of the clay dispersions in the aqueous phase was derived from a measurement of the electrophoretic mobility using a commercial model of the Zetameter. Mobilities were measured at room temperature in an electrophoresis cell of 4.4 mm tube diameter and 10 cm length. The polarity of the electrodes was reversed to minimize systematic error and the mean of lo-15 readings is the reported value. The contact angle of a drop of the aqueous phase on the clay, immersed in oil, was measured at 25°C using the goniometric technique. The solid surface was prepared by compressing the particles into a disc in a potassium bromide die at a pressure of 10 000 psi. The disc had a diameter of 0.8 cm and a thickness of 0.2-0.3 cm. This technique for measuring the contact angle was compared with another method where asphaltene-coated glass beads of 10 I.trn diameter were allowed to straddle the oil/water interface. At equilibrium, the position of a bead at the interface was obtained by a photomicroscopic technique. From the equilibrium position of the bead at the interface the contact angle was estimated. For glass beads completely coated with asphaltenes (by equilibration in shale oil for 1 month) we obtained a contact angle of
92
138”, which compares well with the value of 140” obtained for shale dust using the compressed disc of the clay. The interfacial tension of the oil/water interface was determined using the spinning drop tensiometer for low interfacial tensions and the Du Nuoyring tensiometer for tensions in excess of 10 dyne cm-‘. All measurements were made at 25°C. RESULTS
The influence of hydrophobic shale dust particles on the stability of a W/O emulsion becomes obvious when we observe the manner in which these particles arrange themselves at the O/W interface. Figure la is a microphotograph of the distribution of the particles at the O/W interface. These particles have an average diameter of 4.0 pm and have a contact angle of 140” which implies that they are very hydrophobic. The solids are finely dispersed at the interface forming a rigid layer. A water drop that is covered with these hydrophobic particles is shown in Fig. lb. The particles coat the drop so completely that the drop does not coalesce with other drops or with the bulk oil/water interface even after vigorous shaking. Figure lc shows the same drop resting at the bulk oil/water interface which itself
Fig. 1. (a) Distribution in oil; (c) solid-stabilized
of particles at an O/W interface; (b) solidscovered water water drop resting on a particle-covered O/W interface.
drop
93
has a layer of solids. The photograph was taken using transmitted light through an immersion objective below the bulk interface. In the region where the drop touches the interface there appears to be a high density of particles (seen by the dark area) which effectively prevents coalescence. The hydrophobic nature of the particles is ultimately responsible for the stability of these emulsions. Hence, our attention was directed towards studying the influence of asphaltene adsorption on a hydrophilic clay and thereby on the wettability of that clay. The adsorption isotherm of asphaltenes on sodium montmorillonite is shown in Fig. 2. The adsorption density increases rapidly with asphaltene at low concentrations. The isotherm changes slope at an equilibrium concentration of -0.5 g 1-l and then increases at a slower rate at higher concentrations. Figure 2 also shows the interfacial tension of the heptol/water interface where the heptol phase contains the asphaltenes. The critical micelle concentration (CMC) of.asphaltenes in heptol occurs at -0.55 g 1-l which happens to be the concentration at which the adsorption isotherm changes slope. Q
EOUILIBRIUM
Fig. 2. Variation tion in oil.
ASPHALTENE
of adsorption
CONCN.
density
G/LITER
and interfacial
tension
with asphaltene
concentra-
The effect of asphaltene adsorption on the wettability of the clay can be seen in Fig. 3 where 0 is measured through the water phase. In the absence of any adsorbed asphaltenes, the sodium montmorillonite is completely water wet and has a tendency to swell. The contact angle of the clay is plotted with increasing asphaltene concentration in heptol. This asphaltene concentration is in equilibrium with the adsorbed asphaltenes on the model clay. The contact angle increases with increasing amounts of adsorbed asphaltenes and attains a steady value of 150” at an equilibrium asphaltene concentration of 0.4 g 1-l. This value of the contact angle compares reasonably well with the contact angle of 140” that was obtained for dust filtered
94
05
0
Fig. 3. Variation in heptol.
20
15
IO
EQUILIBRIUM
ASPHALTENE
CONCN..
25
G/LITER
of contact angle and interfacial
tension with asphaltene concentration
from raw shale oil. Figure 3 also shows the interfacial tension of the heptol/ water interface with the asphaltenes in the oil. The contact angle shows a one-to-one correspondence with the interfacial tension which decreases with increasing asphaltene concentration and attains a constant value of 12 dyne cm-‘. The effect of asphaltene adsorption on the zeta potential of the clay particles in water-is shown in Fig. 4 along with the interfacial tension. The absolute value of the zeta potential appears to increase rather slowly with increasing amounts of adsorbed asphaltenes. Sodium montmorillonite in the absence of any adsorbed asphaltenes has a zeta potential of -40 mV in deionized water.
,
0 0
I 05
EQUILIBRIUM
I
I IO
I
ASPHALTENE
I 15 CONCN,
I
I 20
I
_” 2.5
G/LITER
Fig. 4. Variation of zeta potential and interfacial in heptol.
tension with asphaltene concentration
95
The influence of aqueous phase pH on the wettability of the clay is depicted in Fig. 5. The contact angle of the shale dust increases slowly at very low and very high pH. The sodium montmorillonite with varying amounts of adsorbed asphakenes does not show any change in contact angle over the entire range of pH studied. The interfacial tension at the oil/water. interface goes through a maximum at about neutral pH for all the systems, measured, as shown in Fig, 6. The systems are: (1) heptol/water; (2) heptol + 0.1 g 1-l asphaltenes/water; and (3) shale oil/water. The effect of pH on the zeta potential of the clays displays a complex trend as can be seen from Fig. 7. In the absence of any adsorbed asphaltenes, the zeta potential for the model clays goes through a maximum at a pH
PH
Fig. 5. Effect of aqueous phase pH on the contact angle of the asphaltenecovered so-
, _
I
0 0
, a
I
SHALE
I 2
I
,
1
,
I
,
,
,
,
I 6
1
I 8
I
I 10
I
,
,
OIL/WATER
I 4
I
I 12
14
PH
Fig. 6. Effect of aqueous phase pH on the O/W interfacial tension.
clay.
96 100
,
,
,
n
Fig.
7.
,
SHALE
,
I
(
,
,
,
,
-I
NA-MONTMORILLIONITE
v
45.8
of
,
DVST
q
Effect
,
MG
ASPHALTENES/G
aqueous
OF CLAY
phase
pH
on
the
zeta
potential
of the asphaltenecovered
particles.
of 3.5-4.5, a minimum at a pH of 6-7 and with further increase in pH the zeta potential increases to a steady value. With increasing amounts of adsorbed asphaltenes, the first maximum appears to be depressed while the steady values attained at highly basic pH values appear to be elevated. We shall discuss later a possible explanation for such complex behavior. Besides studying the influence of pH, the effect of an anionic surfactant (Aerosol-OT) on the contact angle, zeta potential and O/W interfacial tension was also investigated. Figure 8 is a plot of the variation of the contact angle with surfactant concentration in the aqueous phase. The contact angles for the clays with different amounts of adsorbed asphaltenes do not seem to be influenced to any great degree by the addition of Aerosol-
100
0
1
a
SHALE
7.7
458
MG
ASPHALTENES/G
OF CLAY
0
91 3 MG
ASPHALTENES/G
OF CLAY
1
200
1
1
400
0”ST
j
CONCENTRATION
Fig. phase.
8.
Variation
of
1
600 OF
the
1
1
BOO
’
AEROSOL-OT.
contact
angle
1
1000
’
,200
PPM
with
surfactant
concentration
in the aqueous
97
OT. The contact angle for shale dust increases slowly with increasing surfactant concentration and reaches a steady value at high concentrations. On the other hand, the zeta potential passed through maxima with increasing surfactant concentration for all the clays (Fig. 9) except shale dust where it increased to a steady value. The interfacial tension of the heptol/water interface with asphaltenes in the oil and Aerosol-OT in the water decreases with increasing Aerosol-OT concentration and reaches the CMC at -300 ppm (Fig. 10). The presence of asphaltenes in the oil shifts the interfacial tension plots slightly but the CMC does not seem to be affected to any significant degree. The interfacial tension plot for shale oil/water is a little different from the other plots probably due to the presence of other surfaceactive species, in addition to asphaltenes, in the oil. 100
,
0
,
I
1 0
200
,
,
9.
0
Effect
200
SHALE
NkMONTMOR,LLIONITE
0
91 3 MG ASPHALTENESIG
I, 600
400
I
OF CLAY
800
concentration
600
800
OF AEROSOL-OT.
of surfactant
1
,
OF AEROSOL-OT,
of surfactant
I
,
D”ST
/
400
CONCENTRATION
Fig. 10. Effect
/
v
CONCENTRATION Fig.
,
a
I
I
,
,
concentration
1000
_
, 1200
PPM
on the zeta potential.
1000
I200
PPM
on the O/W interfacial
tension.
98
DISCUSSION
Effect
of asphaltene adsorption
The adsorption isotherm of asphaltenes on sodium montmorillonite (as shown in Fig. 2) shows a change of slope at an equilibrium asphaltene concentration of 0.5 g 1-l. This concentration corresponds roughly to the CMC of asphaltenes in oil as seen from the interfacial tension plot in Fig. 2. The decreased slope of the isotherm beyond the CMC appears to be because of the preference of the asphaltene monomers to form micelles in the bulk rather than to adsorb on the solid surface. While some monomer adsorption does occur the competition between micellization in the bulk and adsorption on the surface reduces the rate of adsorption. From the interfacial tension plot it is possible to calculate (using the Gibbs adsorption equation) the average area per molecule occupied by the asphaltene molecule at the oil/ water interface. The value of 33 A2 that we obtained compares well with the base area of 38 A2 obtained from the data reported by Yen et al. [16] for a shale asphaltene molecule. The BET surface area of sodium montmorillonite was known to be 29.4 m2 g-‘. Hence, using a molecular weight of 650 [ 161, we calculated the maximum adsorption density possible on the clay for monolayer adsorption. This value turns out to be 100 mg per gram of clay. The change in the slope of the isotherm occurs when the surface is -50% covered by asphaltenes. The variation of the zeta potential of asphaltene-coated particles in water shows a slow increase with increasing concentration of asphaltenes (Fig. 4). The sodium montmorillonite has a negative charge because of isomorphous substitution of the metal ions in the clay, i.e. during the formation stage of the clay the tetravalent Si is sometimes replaced by trivalent Al and this in turn may be replaced by divalent Mg. This replacement of an atom of higher valence by an atom of lower valence results in an excess of negative charge. This excess of negative charge is compensated by the adsorption on the surface layers of exchangeable cations which are too large to be accommodated in the interior of the crystal [15]. This net negative charge is usually treated as a fixed surface charge. In addition to the fixed surface charge, there exists a smaller pH-dependent charge arising from the ionization of AlOH and SiOH groups which are usually located at the particle edges. Figure 4 shows that in the presence of adsorbed asphaltenes the absolute value of the zeta potential increases slowly. This increase in the zeta potential is due to the ionization of some of the polar groups present in the asphaltenes. The ionizable species of asphaltenes have been reported to be amphoteric in nature [17,18]. While there will be a marked effect of such an amphoteric species on the zeta potential at high and low pH it appears that there is only a small contribution to the charge of the clay at neutral pH. The influence of asphaltene adsorption on the wettability of the clay
99
is dramatic (Fig. 3). The contact angle increases rapidly with increasing asphaltene concentration to attain a highly hydrophobic value of 150”. The variation of the oil/water interfacial tension is also shown in Fig. 3. The relationship between interfacial tension and contact angle is given by Young’s equation, which can be written as cos
e
=
YOS -
-
Tow
YWS -
(1)
Tow
are the oil/solid, water/solid and oil/water interwhere yes, yws and yaw facial tensions, respectively, and 0 is measured through the water phase (Fig. 11). The increase in adsorption of asphaltenes at the soild/oil interface would decrease yes. The adsorption isotherm is a measure of the variation of the solid/oil interfacial tension. From the Gibbs adsorption equation and the adsorption isotherm (Fig. 2) we estimated the initial slope of the solid/oil interfacial tension-concentration plot to be -26 dyne 1 cm-’ g-’ for an asphaltene molecular weight of 650. The corresponding slope for the oil/water interfacial tension plot (Fig. 2) is -46 dyne 1 cm-’ g-l. Thus, yes decreases more slowly than yaw with increasing asphaltene concentration. Hence, the ratio (y,Jy,,) in the Young’s equation will increase with increasing asphaltene concentration.
Fig.
11.
Contact
angle defined
through
the water phase.
The zeta potential at the solid/water interface is an indirect measure Since Fig. 4 shows no significant change of the solid/water interactions. in the zeta potential with increasing asphaltene concentration, it can be is approximately constant. Thus, the ratio (yws/yow) assumed that yws will increase with increasing asphaltene concentration. The increase in (yws/yow) will be much larger than the increase in (yos/yow) since yosdecreases while yws is a constant. Hence, cos 6’ becomes more negative with increasing asphaltene content, i.e. 0 increases. In fact, the variation of the contact angle (Fig. 3) shows a strong correlation with the oil/water interfacial tension, suggesting that the change in (yes-yws) is small in comparison to the change in yaw. Hence, the contact angle attains a plateau at high asphaltene concentrations in a manner similar to the variation of yaw. The alteration of the wettability would require the modification of the solid surface or the two liquid phases. In this study we varied the aqueous phase pH as well as the surfactant in the aqueous phase.
100
Effect
of pH
Sodium montmorillonite possesses acid groups on the surface whose ionization is dependent on the pH of the aqueous solution. The zeta potential exhibits a complicated behavior with increasing pH (Fig. 7). Such complicated behavior is not uncommon for clays. Friend and Hunter [19] observed multiple maxima of zeta potential with increasing electrolyte concentration for vermiculite clay. Hall et al. [20] found that the zeta potential for Athabasca tar suspensions in water goes through a maximum with increasing pH. Baver [21] obtained plots very similar to Fig. 7 for Putnam clay while Williams and Williams [22] observed that the zeta potential for kaolinite particles in water goes through a plateau with increasing PH. The presence of two acid sites (SOH and TOH) on the clay [23], one dissociating at low pH (strong acid) and the other dissociating at high pH (weak acid), would explain the zeta potential plot for sodium montmorillonite. The two acids dissociate as: SOH = SO- + H+
(P&I,
strong acid)
(2)
TOH = TO- + H’
(P&,,
weak acid)
(3)
If SOH is a strong acid dissociating at pH -4.5 and TOH is a weak acid dissociating at pH -9.0, then the zeta potential of sodium montmorillonite is explained. At very low pH, the high ionic strength and the small dissociation of SOH and TOH keep the zeta potential low. As the pH increases beyond 3.0, the acid SOH begins to dissociate increasing the zeta potential rapidly. However, as the pH approaches 7.0 (deionized water) the dissociation of the pH-dependent species decreases and the zeta potential tends towards the fixed charge present due to isomorphous substitution. At highly basic pH values the acid TOH dissociates rapidly leading to an increase in the zeta potential. James and Parks [23] have used the two acid site dissociation model to explain Baver’s data [21] for the zeta potential of Putnam clay. Figure 7 shows a decrease in the first maximum of zeta potential and an increase in the second maximum when asphaltenes are adsorbed on sodium montmorillonite. As mentioned earlier, if we treat the asphaltenes to be amphoteric then, in the acidic pH range, the species AOH dissociates as: AOH + H’ = A+ + Hz0 The positively charged surface ions decrease clay thereby reducing the first maximum. dissociation of AOH occurs as: AOH + OH- = AO- + Hz0
(4) the total negative charge of the At highly basic pH values the (5)
The AO- ions add to the negative charge on the clay leading to a net increase in the zeta potential at high pH over that in the absence of adsorbed
101
asphaltenes. When the clay is completely coated with asphaltenes, the effect of the two acid sites of sodium montmorillonite should be drastically reduced and the zeta potential should reflect only the dissociation of the amphoteric species AOH. Thus, in the acidic region the zeta potential should decrease with decreasing pH and in the basic region it should increase with increasing pH. This is indeed what is observed for the variation of the zeta potential of shale dust with aqueous phase pH (Fig. 7). At high pH the zeta potential decreases again probably owing to the rapid increase in the ionic strength of the aqueous solution. The influence of pH on the interfacial tension of the oil/water interface is depicted in Fig. 6. In the absence of any asphaltenes at the interface, the interfacial tension goes through a maximum at neutral pH. The presence of 0.1 g I-’ asphaltenes in the oil phase shifts the interfacial tension curve to lower values. With shale oil as the oil phase, the curve shifts to even lower values of interfacial tension because of the presence of large amounts of asphaltenes in the oil. When asphaltenes are present in the oil, the maxima occur at pH 7-8. This nature of the interfacial tension versus pH plot is common. The decrease in interfacial tension at the extremes of pH is due to the increase in the number concentration of charges at the interface. The variation of contact angle with pH shows a rather simple behavior as compared to the zeta potential (Fig. 5). For shale dust the contact angle increases at the extremities of pH while for the clays with lower concentrations of adsorbed asphaltenes the effect of pH on the contact angle appears by the changes in insignificant. The in terfacial tension yoS is unaffected aqueous phase pH. Hence, the Young’s equation can be written as follows:
c-yws
cos 8 = -
Yaw
(6)
where C is a constant. The interfacial tension of the shale oil/water interface goes through a maximum at pH -8.0 (Fig. 6). The zeta potential of shale dust particles also goes through a maximum at pH -8.0 (Fig. 7). A maximum in zeta potential indirectly implies a minimum in ywS since the increase of charges at the solid/water interface will lower the surface tension of that interface. It should be noted that such a correlation is qualitative and in actuality the irregularity of the solid surface will cause a nonuniform distribution of the charge. Thus, y ow goes through a maximum while ywS goes through a minimum. Hence, the numerator of Eqn (6) goes through a minimum and the denominator goes through a maximum. Therefore, the contact angle goes through a minimum. For the clays with differing amounts of adsorbed asphaltenes (sodium montmorillonite based clays), yaw again goes through a maximum with increasing pH (Fig. 6). The contact angle can once again be interpreted in terms of the variations of ywS and yaw. However, since ywS shows a very
102
complex trend (as reflected in the zeta potential) with an increase in pH, such interpretations will necessarily be complicated. From the above discussion it is apparent that the knowledge of the variations in the oil/water interfacial tension and the zeta potential of the solids in water makes it possible to predict qualitatively the behaviour of the wettability of a hydrophobic solid. The behavior of the contact angle of shale dust with changing pH shows that there is some improvement in hydrophilicity at pH -8.0. For the other clays the contact angle seems to be essentially unaffected by the changes in pH. For destabilizing W/O emulsions where the solid stabilizer is shale dust, pH -8.0 is therefore recommended. Besides manipulating the aqueous phase pH, another common means of changing the wettability of the solids is by the addition of surfactant to the aqueous phase. Effects of surfactan t The influence of an anionic surfactant on the interfacial tension of the oil/water interface is shown in Fig. 10. The surfactant, sodium dioctyl sulfosuccinate (Aerosol-CT), was added to the water phase while the asphaltenes were added to the oil phase. The asphaltenes generally do not partition into the water but the Aerosol-OT does have some solubility in oil. The interfacial tension decreases with increasing surfactant concentration. The presence of asphaltenes in oil is significant at low surfactant concentration but does not change the CMC of Aerosol-OT in water or the final value of the interfacial tension. It is likely that the superior surface activity of Aerosol-CT (as compared with that of the asphaltenes) causes the preferential adsorption of the former at the oil/water interface. This will result in the interface attaining a final value of the interfacial tension which is unaffected by the asphaltene concentration in the oil. The interfacial tension of shale oil/water shows a slightly different behavior attaining a steady low interfacial tension of 5.4 dyne cm-‘. The CMC of this system is also lower probably because shale oil contains many other surface-active species in addition to asphaltenes. The zeta potential of the clay fines in oil goes through a maximum for all the clays, except shale dust where it attains a steady value at high surfactant concentrations (Fig. 9). The initial increase in the absolute value of the zeta potential is due to the increase in the adsorption of the anionic surfactant from water. At concentrations above 600 ppm the zeta potential decreases, probably because of the compression of the double layer owing to the high concentration of surfactant in the bulk aqueous phase or due to the desorption of asphaltenesurfactant complexes from the clay surface. The contact angle for spent shale dust increases with increasing surfactant concentration and reaches a steady value at high concentrations (Fig. 8). From the interfacial tension plot (Fig. 10) we know that yaw (shale oil/water) decreases with increasing Aerosol-OT concentration in
103
water and attains a constant low value at concentrations in excess of 600 ppm. Figure 9 reveals that the zeta potential for shale dust increases slowly and reaches a steady value at concentrations above 600 ppm. An increase in zeta potential is associated with a decrease in the solid/water tension due to. the increase in surface charges. Hence, both -yWsand yoW decrease with increasing surfactant concentration. Also, yes is unaffected by the addition of surfactant to the aqueous phase. The contact angle would therefore be represented by Eqn (6). Since the initial decrease in the interfacial tension, yoW, is rather steep as compared with the initial decrease in yWs, it can be said that the contact angle would initially rise with increasing surfactant content. At high concentrations, both yoW and yws reach constant values, so the contact angle should also remain constant. The trend in the contact angle observed experimentally is as predicted by the variations in yaw and yws. For the sodium montmorillonite-based clays the experimental results show that the contact angle does not change with addition of surfactant in the aqueous phase. The oil/water interfacial tension decreases rapidly with increasing surfactant concentration and attains a constant low value at high surfactant concentrations. The adsorption of Aerosol-OT on hydrophobic clays such as carbon black has been known to follow the Langmuir adsorption equation [ 241. If we assume that the surfactant adsorption follows the Langmuir equation in our clay system also, then yws would decrease with increasing surfactant concentration and attain a constant value at high surfactant concentrations. Thus, both yaw and yws would show similar trends. Hence, the contact angle in Eqn (6) would remain unchanged. The variation of 0 can also be interpreted in terms of the change in zeta potential (Fig. 9). The increase in zeta potential up to the maximum value is due to the adsorption of surfactant onto the solid surface. Beyond the maximum the decrease in zeta potential is probably due to the compaction of the double layer owing to the increase in electrolyte concentration. Thus, the adsorption isotherm of Aerosol-OT on the clay would level off at -600 ppm. Corresponding to the zeta potential curve, the water/solid surface energy would decrease with increasing Aerosol-OT concentration and level off at -600 ppm. If the variation in (C-r,,) in Eqn (6) is proportional to that of yaw then the contact angle will indeed be a constant. From the above observations it is seen that the addition of the surfactant Aerosol-OT is detrimental to the improvement in wettability of shale dust while for the other clays with smaller amounts of adsorbed asphaltenes, the impact of surfactant on the wettability is insignificant. From a process engineering viewpoint this means that the addition of surfactant may not always aid in the improvement of wettability of the solid fines or help in destabilizing solid-stabilized emulsions. This may be the reason why many industries use electrostatic coalescers in addition to chemical demulsifiers to coalesce these emulsions.
104 CONCLUSIONS
The isothermal adsorption density of asphaltenes in oil onto sodium montmorillonite increased sharply as the CMC point was approached. Beyond the CMC, adsorption density increased, but more slowly. The adsorption of very small amounts of asphaltenes was found to make the initially hydrophilic clay attain a contact angle of 140”, indicating a hydrophobic surface. The zeta potential of the particles in water did not vary significantly with asphaltene concentration. The three-phase contact angle was interpreted in terms of the surface energies of the O/W, O/S and W/S interfaces from measurements of adsorption density, O/W interfacial tension and zeta potential. A pH of 8.0 was found to improve the wettability of shale dust but had little effect on the contact angle of the other clays. The anionic surfactant, Aerosol-OT, made shale dust particles more hydrophobic but had no effect on the other clays. ACKNOWLEDGEMENTS
This study was supported by Exxon Research and Engineering Co. as part of their Frontiers of Separation Program. The help provided by Drs L.A. Kaye, D.L. Smith, E.C. Hsu and R. Gupta, and the valuable discussions with Dr A.D. Nikolov are gratefully acknowledged.
REFERENCES W. Ramsden, Proc. R. Sot. London, 72 (1903) 156. S.U. Pickering, J. Sot. Chem. Ind. London, 29 (1910) 129. P. Finkle, H.D. Draper and J.H. Hilderbrand, J. Amer. Chem. Sot., 45 (1923) 2780. J.H. Schulman and J. Leja, Trans. Faraday Sot., 50 (1954) 598. D.M. Clementz, Clays Clay Miner., 24 (1976) 312. S.H. Collins and J.C. Melrose, Paper presented at the SPE Int. Symp. on Oilfield and Geothermal Chemistry, Denver, CO, 1983. 7 J.D. Dickie and T.F. Yen, Anal. Chem., 39 (1967) 1847. 8 T.F. Yen, Am, Chem. Sot. Div. Pet. Chem. Prepr., 17 (1972) F102. 9 P.A. Witherspoon and R.S. Winniford, in B. Nagy and U. Columbo (Eds), Fundamental Aspects of Petroleum Geochemistry, Elsevier, New York, 1967. 10 J.D. Henry, M.E. Prudich and C. Lau, Colloids Surfaces, 1 (1980) 335. 11 A. Gelot, W. Friesen and H.A. Hamza, Colloids Surfaces, 12 (1984) 271. 12 A.B. Taubman and A.F. Koretskii, Adv. Colloid Chem., Nauka, Moscow, 1973 (in Russian). 13 T.F. Svitova, V.A. Spirodonova and S.N. Tolstaya, Kolloidn. Zh,, 46 (1984) 501. 14 V.B. Menon and D.T. Wasan, Sep. Sci. Technol., 19 (1984) 555. 15 H. Van Olphen, An Introduction to Clay Colloid Chemistry, Wiley, New York, 1977. 16 T.F. Yen, C.S. Wen, J.T. Kwan and E. Chow, in G.P. Strausz and E.M. Lown (Eds), Oil Sand and Oil Shale Chemistry, Verlag Chemie, New York, 1978.
105 17 18 19 20 21 22 23 24
K.C. Tewari and N.C. Li, in J.W. Bunger and N.C. Li (Eds), Chemistry of Asphaltenes, Advances in Chemistry (Series 195), ACS, Washington, DC, 1981, pp. 173-182. J.M. Charlesworth, Structure of Coal Derived Asphaltenes, Fourth Australian Workshop on Coal Hydrogenation, Richmond, Victoria, 1979. J.P. Friend and R.J. Hunter, Clays Clay Miner., 18 (1970) 275. A.C. Hall, S.H. Collins and J.C. Melrose, Sot. Pet. Eng. J., 23 (1983) 249. L.D. Baver, Soil Sci., 29 (1930) 291. D.J.A. Williams and K.P. Williams, J. Colloid Interface Sci., 65 (1978) 79. R.O. James and G.A. Parks, in E. MatijeviE (Ed.), Surface and Colloid Science, Vol. 12, Plenum, New York, 1982. A, Kitahara, T. Tamura and K. Kon-no, Sep. Sci. Technol., 15 (1980) 249.
and Surfaces,
19 (1986)
89
89-105
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
PARTICLE-FLUID SOLIDSTABILIZED
INTERACTIONS EMULSIONS
WITH APPLICATION
TO
PART I. THE EFFECT OF ASPI-IALTENE ADSORPTION
V.B. MENON and D.T. WASAN Department of Chemical 60616 (U.S.A.)
Engineering,
Illinois
Institute
of Technology,
Chicago,
IL
(Received 9 July 1985 ; accepted in final form 27 January 1986)
ABSTRACT Solid-stabilized water-in-oil emulsions are commonly encountered in the production of crude oil and synthetic fuel liquids. The ability of finely divided solids to stabilize these emulsions is due to the adsorption of asphaltenes and other heavy ends from the oil on the mineral surface which imparts to the solid its hydrophobic nature. This study presents results which quantify the particle-fluid interactions by means of adsorption measurements of asphaltenes on sodium montmorillonite particles. The influence of asphaltene adsorption on the contact angle at the water/oil/solid interface and on the zeta potential of the particles was investigated. The hydrophobic nature of the solids was found to be directly related to the amount of asphaltenes adsorbed on their surface. The effect of asphaltenes on the contact angle is explained in terms of the interfacial tensions at the oil/water and solid/water interfaces. The influence of pH and surfactant concentration in the aqueous phase on the contact angle and the zeta potential is assessed.
INTRODUCTION
The ability of solid particles to stabilize emulsions has been reported in the literature since 1903 when Ramsden [l] first observed emulsions stabilized by the presence of solid or highly viscous matter at the interface of two liquids. Since then the stability of such emulsions has been studied by many investigators [24]. It is generally accepted that water-inoil (W/O) emulsions are stabilized when the solids are hydrophobic, and oilin-water (O/W) emulsions are favored when the solids are hydrophilic. In the petroleum and synthetic fuel industries the separation of particulate matter from oil is commonly accomplished by dispersing water into the oil whereby the solids are transferred to the oil/water interface forming stable W/O emulsions. The water phase is subsequently removed by gravity settling and/or electrostatic coalescence. At this stage of the process the ready coalescence of the emulsions is desirable. The hydrophobic nature of
0166-6622/86/$03.50
0 1986 Elsevier Science Publishers B.V.
90
the mineral particles (owing to adsorbed asphaltic matter from the oil) is responsible for the stability of such emulsions and in order to destabilize these emulsions more readily, the particles must be made more hydrophilic. The wettability of the particles is thus crucial since it determines the nature and stability of the emulsions. The hydrophobicity of mineral matter from crude oil arises from the adsorption of asphaltenes and resins from the oil onto the initially hydrophilic clay [ 51. The asphaltenes comprise a major portion of the surface-active component of oil and have been known to adsorb irreversibly on clay surfaces [ 5,6]. They are large polyaromatic and polycyclic condensed ring compounds which contain heteroatoms such as N, S and 0. Such heteroatoms imply the presence of polar functional groups [ 7-91. Henry et al. [lo] have investigated the effect of particle wettability and pH on the effectiveness of particle separation from oil to a receiving water phase. They found that the effectiveness of separation is improved significantly as the hydrophilicity of the solids increases (contact angle decreases) and as the aqueous phase pH becomes more basic. Gelot et al. [ll] found that the type and stability of such emulsions are strongly dependent on the particle size, density and the effect of surfactants on their wettability. Taubman and Koretskii [12] contend that while certain surfactants are effective in breaking solid-stabilized emulsions others increase the stability by forming chemical complexes on the partices, thereby anchoring them to the oil/water interface. Svitova et al. [13] have found ionic water-soluble polymers to be very effective in breaking water/crude oil emulsions stabilized by asphaltic material while nonionic polymers were not. They also attribute the emulsion stability to be directly related to the nature and extent of surfactant adsorption on the solid surface. In a previous paper [14] we discussed some of our results on the study of the stability of water-in-shale oil emulsions with and without solid fines in the oil. It was found that 0.1% (w/w) of dust particles in the shale oil produced emulsions which were three times more stable than the emulsions without the fines. The concentration of the solids and hence their packing at the interface are therefore very important to the stability of the emulsions. In this study we have made an attempt at understanding the factors that influence the wettability. We therefore investigated the influence of adsorbed asphaltenes on the contact angle at the solid/oil/water interface. The interfacial tension at the oil/water interface and the zeta potential of the solid particles at the solid/water interface were measured to explain the wettability behavior in terms of the surface energies at the solid/water and oil/water interfaces. The influence of surfactants and aqueous phase pH on the contact angle and zeta potential was also investigated. While most of our results are for a model clay, sodium montmorillonite, which was coated with asphaltenes, we have attempted, wherever possible, to compare our observations with that of actual dust filtered from raw shale oil.
91 EXPERIMENTAL
The asphaltenes used in this study were extracted from shale oil obtained from Rundle, Australia, and was supplied to us by Exxon Research and Engineering. The shale oil had a viscosity of 0.032 Poise and a density of 0.83 g cme3. The extraction procedure involves the precipitation of the asphaltenes from the oil by an excess of hexane. Thus the asphaltenes we have extracted comprise the hexane-insoluble portion of the oil. Sodium montmorillonite from Crook County, WY, was used as the model clay. The dry clay had a BET surface area of 29.4 m2 g-‘. The clay fines had an average diameter of 4.0 pm and a density of 2.6 g cmm3. Sodium montmorillonite has been reported to have a cation exchange capacity in the range of 90-100 meq per 100 g [15]. This gives a surface charge density of -0.12 C me2. A 1 :l volume mixture of n-heptane and toluene was used as the organic phase. This liquid, which we shall call ‘heptol’, has a density of 0.76 g cme3 and a surface tension of 24 dyne cm-‘. The surfactant, Aerosol-OT, was supplied by American Cyanamid Co. and was used as supplied. The adsorption isotherm of the asphaltenes on sodium montmorillonite was determined by contacting 0.76 g of clay with 100 ml of a solution of asphaltenes in heptol and stirring for 24 h. The amount of asphaltenes adsorbed on the clay was determined by measuring the absorbance of the initial solution and the equilibrium solution using a Beckman Spectrophotometer at a wavelength of 336 nm. A calibration curve of absorbance versus asphaltene concentration in heptol was constructed at this wavelength. The spectrophotometer measurements were reproducible to within 5%. The zeta potential of the clay dispersions in the aqueous phase was derived from a measurement of the electrophoretic mobility using a commercial model of the Zetameter. Mobilities were measured at room temperature in an electrophoresis cell of 4.4 mm tube diameter and 10 cm length. The polarity of the electrodes was reversed to minimize systematic error and the mean of lo-15 readings is the reported value. The contact angle of a drop of the aqueous phase on the clay, immersed in oil, was measured at 25°C using the goniometric technique. The solid surface was prepared by compressing the particles into a disc in a potassium bromide die at a pressure of 10 000 psi. The disc had a diameter of 0.8 cm and a thickness of 0.2-0.3 cm. This technique for measuring the contact angle was compared with another method where asphaltene-coated glass beads of 10 I.trn diameter were allowed to straddle the oil/water interface. At equilibrium, the position of a bead at the interface was obtained by a photomicroscopic technique. From the equilibrium position of the bead at the interface the contact angle was estimated. For glass beads completely coated with asphaltenes (by equilibration in shale oil for 1 month) we obtained a contact angle of
92
138”, which compares well with the value of 140” obtained for shale dust using the compressed disc of the clay. The interfacial tension of the oil/water interface was determined using the spinning drop tensiometer for low interfacial tensions and the Du Nuoyring tensiometer for tensions in excess of 10 dyne cm-‘. All measurements were made at 25°C. RESULTS
The influence of hydrophobic shale dust particles on the stability of a W/O emulsion becomes obvious when we observe the manner in which these particles arrange themselves at the O/W interface. Figure la is a microphotograph of the distribution of the particles at the O/W interface. These particles have an average diameter of 4.0 pm and have a contact angle of 140” which implies that they are very hydrophobic. The solids are finely dispersed at the interface forming a rigid layer. A water drop that is covered with these hydrophobic particles is shown in Fig. lb. The particles coat the drop so completely that the drop does not coalesce with other drops or with the bulk oil/water interface even after vigorous shaking. Figure lc shows the same drop resting at the bulk oil/water interface which itself
Fig. 1. (a) Distribution in oil; (c) solid-stabilized
of particles at an O/W interface; (b) solidscovered water water drop resting on a particle-covered O/W interface.
drop
93
has a layer of solids. The photograph was taken using transmitted light through an immersion objective below the bulk interface. In the region where the drop touches the interface there appears to be a high density of particles (seen by the dark area) which effectively prevents coalescence. The hydrophobic nature of the particles is ultimately responsible for the stability of these emulsions. Hence, our attention was directed towards studying the influence of asphaltene adsorption on a hydrophilic clay and thereby on the wettability of that clay. The adsorption isotherm of asphaltenes on sodium montmorillonite is shown in Fig. 2. The adsorption density increases rapidly with asphaltene at low concentrations. The isotherm changes slope at an equilibrium concentration of -0.5 g 1-l and then increases at a slower rate at higher concentrations. Figure 2 also shows the interfacial tension of the heptol/water interface where the heptol phase contains the asphaltenes. The critical micelle concentration (CMC) of.asphaltenes in heptol occurs at -0.55 g 1-l which happens to be the concentration at which the adsorption isotherm changes slope. Q
EOUILIBRIUM
Fig. 2. Variation tion in oil.
ASPHALTENE
of adsorption
CONCN.
density
G/LITER
and interfacial
tension
with asphaltene
concentra-
The effect of asphaltene adsorption on the wettability of the clay can be seen in Fig. 3 where 0 is measured through the water phase. In the absence of any adsorbed asphaltenes, the sodium montmorillonite is completely water wet and has a tendency to swell. The contact angle of the clay is plotted with increasing asphaltene concentration in heptol. This asphaltene concentration is in equilibrium with the adsorbed asphaltenes on the model clay. The contact angle increases with increasing amounts of adsorbed asphaltenes and attains a steady value of 150” at an equilibrium asphaltene concentration of 0.4 g 1-l. This value of the contact angle compares reasonably well with the contact angle of 140” that was obtained for dust filtered
94
05
0
Fig. 3. Variation in heptol.
20
15
IO
EQUILIBRIUM
ASPHALTENE
CONCN..
25
G/LITER
of contact angle and interfacial
tension with asphaltene concentration
from raw shale oil. Figure 3 also shows the interfacial tension of the heptol/ water interface with the asphaltenes in the oil. The contact angle shows a one-to-one correspondence with the interfacial tension which decreases with increasing asphaltene concentration and attains a constant value of 12 dyne cm-‘. The effect of asphaltene adsorption on the zeta potential of the clay particles in water-is shown in Fig. 4 along with the interfacial tension. The absolute value of the zeta potential appears to increase rather slowly with increasing amounts of adsorbed asphaltenes. Sodium montmorillonite in the absence of any adsorbed asphaltenes has a zeta potential of -40 mV in deionized water.
,
0 0
I 05
EQUILIBRIUM
I
I IO
I
ASPHALTENE
I 15 CONCN,
I
I 20
I
_” 2.5
G/LITER
Fig. 4. Variation of zeta potential and interfacial in heptol.
tension with asphaltene concentration
95
The influence of aqueous phase pH on the wettability of the clay is depicted in Fig. 5. The contact angle of the shale dust increases slowly at very low and very high pH. The sodium montmorillonite with varying amounts of adsorbed asphakenes does not show any change in contact angle over the entire range of pH studied. The interfacial tension at the oil/water. interface goes through a maximum at about neutral pH for all the systems, measured, as shown in Fig, 6. The systems are: (1) heptol/water; (2) heptol + 0.1 g 1-l asphaltenes/water; and (3) shale oil/water. The effect of pH on the zeta potential of the clays displays a complex trend as can be seen from Fig. 7. In the absence of any adsorbed asphaltenes, the zeta potential for the model clays goes through a maximum at a pH
PH
Fig. 5. Effect of aqueous phase pH on the contact angle of the asphaltenecovered so-
, _
I
0 0
, a
I
SHALE
I 2
I
,
1
,
I
,
,
,
,
I 6
1
I 8
I
I 10
I
,
,
OIL/WATER
I 4
I
I 12
14
PH
Fig. 6. Effect of aqueous phase pH on the O/W interfacial tension.
clay.
96 100
,
,
,
n
Fig.
7.
,
SHALE
,
I
(
,
,
,
,
-I
NA-MONTMORILLIONITE
v
45.8
of
,
DVST
q
Effect
,
MG
ASPHALTENES/G
aqueous
OF CLAY
phase
pH
on
the
zeta
potential
of the asphaltenecovered
particles.
of 3.5-4.5, a minimum at a pH of 6-7 and with further increase in pH the zeta potential increases to a steady value. With increasing amounts of adsorbed asphaltenes, the first maximum appears to be depressed while the steady values attained at highly basic pH values appear to be elevated. We shall discuss later a possible explanation for such complex behavior. Besides studying the influence of pH, the effect of an anionic surfactant (Aerosol-OT) on the contact angle, zeta potential and O/W interfacial tension was also investigated. Figure 8 is a plot of the variation of the contact angle with surfactant concentration in the aqueous phase. The contact angles for the clays with different amounts of adsorbed asphaltenes do not seem to be influenced to any great degree by the addition of Aerosol-
100
0
1
a
SHALE
7.7
458
MG
ASPHALTENES/G
OF CLAY
0
91 3 MG
ASPHALTENES/G
OF CLAY
1
200
1
1
400
0”ST
j
CONCENTRATION
Fig. phase.
8.
Variation
of
1
600 OF
the
1
1
BOO
’
AEROSOL-OT.
contact
angle
1
1000
’
,200
PPM
with
surfactant
concentration
in the aqueous
97
OT. The contact angle for shale dust increases slowly with increasing surfactant concentration and reaches a steady value at high concentrations. On the other hand, the zeta potential passed through maxima with increasing surfactant concentration for all the clays (Fig. 9) except shale dust where it increased to a steady value. The interfacial tension of the heptol/water interface with asphaltenes in the oil and Aerosol-OT in the water decreases with increasing Aerosol-OT concentration and reaches the CMC at -300 ppm (Fig. 10). The presence of asphaltenes in the oil shifts the interfacial tension plots slightly but the CMC does not seem to be affected to any significant degree. The interfacial tension plot for shale oil/water is a little different from the other plots probably due to the presence of other surfaceactive species, in addition to asphaltenes, in the oil. 100
,
0
,
I
1 0
200
,
,
9.
0
Effect
200
SHALE
NkMONTMOR,LLIONITE
0
91 3 MG ASPHALTENESIG
I, 600
400
I
OF CLAY
800
concentration
600
800
OF AEROSOL-OT.
of surfactant
1
,
OF AEROSOL-OT,
of surfactant
I
,
D”ST
/
400
CONCENTRATION
Fig. 10. Effect
/
v
CONCENTRATION Fig.
,
a
I
I
,
,
concentration
1000
_
, 1200
PPM
on the zeta potential.
1000
I200
PPM
on the O/W interfacial
tension.
98
DISCUSSION
Effect
of asphaltene adsorption
The adsorption isotherm of asphaltenes on sodium montmorillonite (as shown in Fig. 2) shows a change of slope at an equilibrium asphaltene concentration of 0.5 g 1-l. This concentration corresponds roughly to the CMC of asphaltenes in oil as seen from the interfacial tension plot in Fig. 2. The decreased slope of the isotherm beyond the CMC appears to be because of the preference of the asphaltene monomers to form micelles in the bulk rather than to adsorb on the solid surface. While some monomer adsorption does occur the competition between micellization in the bulk and adsorption on the surface reduces the rate of adsorption. From the interfacial tension plot it is possible to calculate (using the Gibbs adsorption equation) the average area per molecule occupied by the asphaltene molecule at the oil/ water interface. The value of 33 A2 that we obtained compares well with the base area of 38 A2 obtained from the data reported by Yen et al. [16] for a shale asphaltene molecule. The BET surface area of sodium montmorillonite was known to be 29.4 m2 g-‘. Hence, using a molecular weight of 650 [ 161, we calculated the maximum adsorption density possible on the clay for monolayer adsorption. This value turns out to be 100 mg per gram of clay. The change in the slope of the isotherm occurs when the surface is -50% covered by asphaltenes. The variation of the zeta potential of asphaltene-coated particles in water shows a slow increase with increasing concentration of asphaltenes (Fig. 4). The sodium montmorillonite has a negative charge because of isomorphous substitution of the metal ions in the clay, i.e. during the formation stage of the clay the tetravalent Si is sometimes replaced by trivalent Al and this in turn may be replaced by divalent Mg. This replacement of an atom of higher valence by an atom of lower valence results in an excess of negative charge. This excess of negative charge is compensated by the adsorption on the surface layers of exchangeable cations which are too large to be accommodated in the interior of the crystal [15]. This net negative charge is usually treated as a fixed surface charge. In addition to the fixed surface charge, there exists a smaller pH-dependent charge arising from the ionization of AlOH and SiOH groups which are usually located at the particle edges. Figure 4 shows that in the presence of adsorbed asphaltenes the absolute value of the zeta potential increases slowly. This increase in the zeta potential is due to the ionization of some of the polar groups present in the asphaltenes. The ionizable species of asphaltenes have been reported to be amphoteric in nature [17,18]. While there will be a marked effect of such an amphoteric species on the zeta potential at high and low pH it appears that there is only a small contribution to the charge of the clay at neutral pH. The influence of asphaltene adsorption on the wettability of the clay
99
is dramatic (Fig. 3). The contact angle increases rapidly with increasing asphaltene concentration to attain a highly hydrophobic value of 150”. The variation of the oil/water interfacial tension is also shown in Fig. 3. The relationship between interfacial tension and contact angle is given by Young’s equation, which can be written as cos
e
=
YOS -
-
Tow
YWS -
(1)
Tow
are the oil/solid, water/solid and oil/water interwhere yes, yws and yaw facial tensions, respectively, and 0 is measured through the water phase (Fig. 11). The increase in adsorption of asphaltenes at the soild/oil interface would decrease yes. The adsorption isotherm is a measure of the variation of the solid/oil interfacial tension. From the Gibbs adsorption equation and the adsorption isotherm (Fig. 2) we estimated the initial slope of the solid/oil interfacial tension-concentration plot to be -26 dyne 1 cm-’ g-’ for an asphaltene molecular weight of 650. The corresponding slope for the oil/water interfacial tension plot (Fig. 2) is -46 dyne 1 cm-’ g-l. Thus, yes decreases more slowly than yaw with increasing asphaltene concentration. Hence, the ratio (y,Jy,,) in the Young’s equation will increase with increasing asphaltene concentration.
Fig.
11.
Contact
angle defined
through
the water phase.
The zeta potential at the solid/water interface is an indirect measure Since Fig. 4 shows no significant change of the solid/water interactions. in the zeta potential with increasing asphaltene concentration, it can be is approximately constant. Thus, the ratio (yws/yow) assumed that yws will increase with increasing asphaltene concentration. The increase in (yws/yow) will be much larger than the increase in (yos/yow) since yosdecreases while yws is a constant. Hence, cos 6’ becomes more negative with increasing asphaltene content, i.e. 0 increases. In fact, the variation of the contact angle (Fig. 3) shows a strong correlation with the oil/water interfacial tension, suggesting that the change in (yes-yws) is small in comparison to the change in yaw. Hence, the contact angle attains a plateau at high asphaltene concentrations in a manner similar to the variation of yaw. The alteration of the wettability would require the modification of the solid surface or the two liquid phases. In this study we varied the aqueous phase pH as well as the surfactant in the aqueous phase.
100
Effect
of pH
Sodium montmorillonite possesses acid groups on the surface whose ionization is dependent on the pH of the aqueous solution. The zeta potential exhibits a complicated behavior with increasing pH (Fig. 7). Such complicated behavior is not uncommon for clays. Friend and Hunter [19] observed multiple maxima of zeta potential with increasing electrolyte concentration for vermiculite clay. Hall et al. [20] found that the zeta potential for Athabasca tar suspensions in water goes through a maximum with increasing pH. Baver [21] obtained plots very similar to Fig. 7 for Putnam clay while Williams and Williams [22] observed that the zeta potential for kaolinite particles in water goes through a plateau with increasing PH. The presence of two acid sites (SOH and TOH) on the clay [23], one dissociating at low pH (strong acid) and the other dissociating at high pH (weak acid), would explain the zeta potential plot for sodium montmorillonite. The two acids dissociate as: SOH = SO- + H+
(P&I,
strong acid)
(2)
TOH = TO- + H’
(P&,,
weak acid)
(3)
If SOH is a strong acid dissociating at pH -4.5 and TOH is a weak acid dissociating at pH -9.0, then the zeta potential of sodium montmorillonite is explained. At very low pH, the high ionic strength and the small dissociation of SOH and TOH keep the zeta potential low. As the pH increases beyond 3.0, the acid SOH begins to dissociate increasing the zeta potential rapidly. However, as the pH approaches 7.0 (deionized water) the dissociation of the pH-dependent species decreases and the zeta potential tends towards the fixed charge present due to isomorphous substitution. At highly basic pH values the acid TOH dissociates rapidly leading to an increase in the zeta potential. James and Parks [23] have used the two acid site dissociation model to explain Baver’s data [21] for the zeta potential of Putnam clay. Figure 7 shows a decrease in the first maximum of zeta potential and an increase in the second maximum when asphaltenes are adsorbed on sodium montmorillonite. As mentioned earlier, if we treat the asphaltenes to be amphoteric then, in the acidic pH range, the species AOH dissociates as: AOH + H’ = A+ + Hz0 The positively charged surface ions decrease clay thereby reducing the first maximum. dissociation of AOH occurs as: AOH + OH- = AO- + Hz0
(4) the total negative charge of the At highly basic pH values the (5)
The AO- ions add to the negative charge on the clay leading to a net increase in the zeta potential at high pH over that in the absence of adsorbed
101
asphaltenes. When the clay is completely coated with asphaltenes, the effect of the two acid sites of sodium montmorillonite should be drastically reduced and the zeta potential should reflect only the dissociation of the amphoteric species AOH. Thus, in the acidic region the zeta potential should decrease with decreasing pH and in the basic region it should increase with increasing pH. This is indeed what is observed for the variation of the zeta potential of shale dust with aqueous phase pH (Fig. 7). At high pH the zeta potential decreases again probably owing to the rapid increase in the ionic strength of the aqueous solution. The influence of pH on the interfacial tension of the oil/water interface is depicted in Fig. 6. In the absence of any asphaltenes at the interface, the interfacial tension goes through a maximum at neutral pH. The presence of 0.1 g I-’ asphaltenes in the oil phase shifts the interfacial tension curve to lower values. With shale oil as the oil phase, the curve shifts to even lower values of interfacial tension because of the presence of large amounts of asphaltenes in the oil. When asphaltenes are present in the oil, the maxima occur at pH 7-8. This nature of the interfacial tension versus pH plot is common. The decrease in interfacial tension at the extremes of pH is due to the increase in the number concentration of charges at the interface. The variation of contact angle with pH shows a rather simple behavior as compared to the zeta potential (Fig. 5). For shale dust the contact angle increases at the extremities of pH while for the clays with lower concentrations of adsorbed asphaltenes the effect of pH on the contact angle appears by the changes in insignificant. The in terfacial tension yoS is unaffected aqueous phase pH. Hence, the Young’s equation can be written as follows:
c-yws
cos 8 = -
Yaw
(6)
where C is a constant. The interfacial tension of the shale oil/water interface goes through a maximum at pH -8.0 (Fig. 6). The zeta potential of shale dust particles also goes through a maximum at pH -8.0 (Fig. 7). A maximum in zeta potential indirectly implies a minimum in ywS since the increase of charges at the solid/water interface will lower the surface tension of that interface. It should be noted that such a correlation is qualitative and in actuality the irregularity of the solid surface will cause a nonuniform distribution of the charge. Thus, y ow goes through a maximum while ywS goes through a minimum. Hence, the numerator of Eqn (6) goes through a minimum and the denominator goes through a maximum. Therefore, the contact angle goes through a minimum. For the clays with differing amounts of adsorbed asphaltenes (sodium montmorillonite based clays), yaw again goes through a maximum with increasing pH (Fig. 6). The contact angle can once again be interpreted in terms of the variations of ywS and yaw. However, since ywS shows a very
102
complex trend (as reflected in the zeta potential) with an increase in pH, such interpretations will necessarily be complicated. From the above discussion it is apparent that the knowledge of the variations in the oil/water interfacial tension and the zeta potential of the solids in water makes it possible to predict qualitatively the behaviour of the wettability of a hydrophobic solid. The behavior of the contact angle of shale dust with changing pH shows that there is some improvement in hydrophilicity at pH -8.0. For the other clays the contact angle seems to be essentially unaffected by the changes in pH. For destabilizing W/O emulsions where the solid stabilizer is shale dust, pH -8.0 is therefore recommended. Besides manipulating the aqueous phase pH, another common means of changing the wettability of the solids is by the addition of surfactant to the aqueous phase. Effects of surfactan t The influence of an anionic surfactant on the interfacial tension of the oil/water interface is shown in Fig. 10. The surfactant, sodium dioctyl sulfosuccinate (Aerosol-CT), was added to the water phase while the asphaltenes were added to the oil phase. The asphaltenes generally do not partition into the water but the Aerosol-OT does have some solubility in oil. The interfacial tension decreases with increasing surfactant concentration. The presence of asphaltenes in oil is significant at low surfactant concentration but does not change the CMC of Aerosol-OT in water or the final value of the interfacial tension. It is likely that the superior surface activity of Aerosol-CT (as compared with that of the asphaltenes) causes the preferential adsorption of the former at the oil/water interface. This will result in the interface attaining a final value of the interfacial tension which is unaffected by the asphaltene concentration in the oil. The interfacial tension of shale oil/water shows a slightly different behavior attaining a steady low interfacial tension of 5.4 dyne cm-‘. The CMC of this system is also lower probably because shale oil contains many other surface-active species in addition to asphaltenes. The zeta potential of the clay fines in oil goes through a maximum for all the clays, except shale dust where it attains a steady value at high surfactant concentrations (Fig. 9). The initial increase in the absolute value of the zeta potential is due to the increase in the adsorption of the anionic surfactant from water. At concentrations above 600 ppm the zeta potential decreases, probably because of the compression of the double layer owing to the high concentration of surfactant in the bulk aqueous phase or due to the desorption of asphaltenesurfactant complexes from the clay surface. The contact angle for spent shale dust increases with increasing surfactant concentration and reaches a steady value at high concentrations (Fig. 8). From the interfacial tension plot (Fig. 10) we know that yaw (shale oil/water) decreases with increasing Aerosol-OT concentration in
103
water and attains a constant low value at concentrations in excess of 600 ppm. Figure 9 reveals that the zeta potential for shale dust increases slowly and reaches a steady value at concentrations above 600 ppm. An increase in zeta potential is associated with a decrease in the solid/water tension due to. the increase in surface charges. Hence, both -yWsand yoW decrease with increasing surfactant concentration. Also, yes is unaffected by the addition of surfactant to the aqueous phase. The contact angle would therefore be represented by Eqn (6). Since the initial decrease in the interfacial tension, yoW, is rather steep as compared with the initial decrease in yWs, it can be said that the contact angle would initially rise with increasing surfactant content. At high concentrations, both yoW and yws reach constant values, so the contact angle should also remain constant. The trend in the contact angle observed experimentally is as predicted by the variations in yaw and yws. For the sodium montmorillonite-based clays the experimental results show that the contact angle does not change with addition of surfactant in the aqueous phase. The oil/water interfacial tension decreases rapidly with increasing surfactant concentration and attains a constant low value at high surfactant concentrations. The adsorption of Aerosol-OT on hydrophobic clays such as carbon black has been known to follow the Langmuir adsorption equation [ 241. If we assume that the surfactant adsorption follows the Langmuir equation in our clay system also, then yws would decrease with increasing surfactant concentration and attain a constant value at high surfactant concentrations. Thus, both yaw and yws would show similar trends. Hence, the contact angle in Eqn (6) would remain unchanged. The variation of 0 can also be interpreted in terms of the change in zeta potential (Fig. 9). The increase in zeta potential up to the maximum value is due to the adsorption of surfactant onto the solid surface. Beyond the maximum the decrease in zeta potential is probably due to the compaction of the double layer owing to the increase in electrolyte concentration. Thus, the adsorption isotherm of Aerosol-OT on the clay would level off at -600 ppm. Corresponding to the zeta potential curve, the water/solid surface energy would decrease with increasing Aerosol-OT concentration and level off at -600 ppm. If the variation in (C-r,,) in Eqn (6) is proportional to that of yaw then the contact angle will indeed be a constant. From the above observations it is seen that the addition of the surfactant Aerosol-OT is detrimental to the improvement in wettability of shale dust while for the other clays with smaller amounts of adsorbed asphaltenes, the impact of surfactant on the wettability is insignificant. From a process engineering viewpoint this means that the addition of surfactant may not always aid in the improvement of wettability of the solid fines or help in destabilizing solid-stabilized emulsions. This may be the reason why many industries use electrostatic coalescers in addition to chemical demulsifiers to coalesce these emulsions.
104 CONCLUSIONS
The isothermal adsorption density of asphaltenes in oil onto sodium montmorillonite increased sharply as the CMC point was approached. Beyond the CMC, adsorption density increased, but more slowly. The adsorption of very small amounts of asphaltenes was found to make the initially hydrophilic clay attain a contact angle of 140”, indicating a hydrophobic surface. The zeta potential of the particles in water did not vary significantly with asphaltene concentration. The three-phase contact angle was interpreted in terms of the surface energies of the O/W, O/S and W/S interfaces from measurements of adsorption density, O/W interfacial tension and zeta potential. A pH of 8.0 was found to improve the wettability of shale dust but had little effect on the contact angle of the other clays. The anionic surfactant, Aerosol-OT, made shale dust particles more hydrophobic but had no effect on the other clays. ACKNOWLEDGEMENTS
This study was supported by Exxon Research and Engineering Co. as part of their Frontiers of Separation Program. The help provided by Drs L.A. Kaye, D.L. Smith, E.C. Hsu and R. Gupta, and the valuable discussions with Dr A.D. Nikolov are gratefully acknowledged.
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