Particle—fluid interactions with application to solid-stabilized emulsions part II. The effect of adsorbed water

Colloids Elsevier

and Surfaces, 19 (1986) Science Publishers B.V..

107-122 Amsterdam

PARTICLE--FLUID INTERACTIONS STABILIZED EMULSIONS

107 -

Printed

WITH APPLICATION

PART II. THE EFFECT OF ADSORBED

V.B.

MENON

Department (U.S.A.) (Received

and D.T. of Chemical

9 July

1985;

in The Netherlands

TO SOLID-

WATER

WASAN Engineering, accepted

Illinois

in final

form

Institute

of Technology,

27 January

Chicago,

IL 60616

1986)

ABSTRACT The adsorption of asphaltic material from crude oil and other synthetic oils onto fine solid particles enables them to stabilize water-in-oil emulsions. In Part I of this series of papers we presented results of our study on the modification of solid wettability through the variation of surfactant and pH in the aqueous phase. In this paper we study the change in asphaltene adsorption behavior and wettability with modification of the solid surface through the use of preadsorbed and coadsorbed water. The adsorption of asphaltenes was reduced significantly when preadsorbed water of a suitable pH or surfactant concentration was used. The coadsorption of water along with the asphaltenes was not very effective in decreasing asphaltene adsorption. The wettability of the solid improved with a decrease in the surface concentration of asphaltenes.

INTRODUCTION

The presence of adsorbed layers of asphaltenes from crude oil or other synthetic oils on finely divided solids can dramatically alter the wettability and characteristics of these solids, thus enabling them to stabilize water-in-oil (W/O) emulsions. The asphaltene-particle and particle-fluid interactions in such systems were discussed in a previous paper [ 11. In this study we attempted to decrease the stability of these emulsions by modifying the surface properties of the solids. Since the adsorbed asphaltenes are responsible for the hydrophobicity of the solid particles, it seems obvious that the stability of the W/O emulsions can be decreased either by decreasing the asphaltene adsorption or by changing the fluid-particle interaction through the use of surfactants or pH-adjusted water. The ability of layers of surface water to decrease asphaltene adsorption on clays has been known for a long time. Lyutin and Burdyn [Z] and Clementz [3] observed that heavyend adsorption from petroleum onto mineral surfaces in reservoir cores was strongly dependent on the presence of residual water. When the residual water was absent, the heavy-end adsorption was larger, while an increase in the water content lowered the adsorption density. 0166-6622/86/$03.59

0 1986

Blsevier

Science

Publishers

B.V.

108

Hall et al. [4] estimated the thickness of water layers stabilized between clay surfaces and oil in a pore. Their analysis shows that stable water films can exist on the clay only if the surface potentials at the water/clay and water/oil interfaces have the same sign. Collins and Melrose [5] found that the presence of water, either preadsorbed on the mineral surface or coadsorbed with the asphaltenes, decreased the amount of asphaltene adsorption on the clay. As a general rule, the presence of water decreases but does not eliminate the adsorption of asphaltenes. We have systematically studied the adsorption of asphaltenes derived from shale oil onto a model clay, sodium montmorillonite, in the presence of preadsorbed or coadsorbed water. The influence of the pH of the aqueous phase and the surfactant concentration on the adsorption was investigated. The modification of the surface of the clay due to asphaltene adsorption must alter the wettability of the clay, therefore the contact angle of the clay was measured and correlated with the adsorption density. EXPERIMENTAL

The asphaltenes were extracted from shale oil obtained from Rundle, Australia, and supplied to us by Exxon Research and Engineering. The extraction procedure is described elsewhere [l]. The clay, sodium montmorillonite, from Crook County, WY, had a BET surface area of 29.4 m2 g-’ and a cation exchange capacity of 90 meq per 100 g of clay. The surfactant, Aerosol-OT, was supplied by American Cyanimid Co., and was used as supplied. For the experiments with preadsorbed water, a known amount of doubledistilled water was dispersed by intense agitation in 100 ml of a solution of “heptol” which is a 1:l volume mixture of heptane and toluene. A 0.76 g sample of the clay was added to this solution with vigorous stirring. The oil was stirred overnight after which the clay was filtered and added to another heptol solution containing a known amount of asphahenes. This solution was stirred for 24 h. The adsorption process was monitored by measuring the absorbance of the equilibrium heptol solution at a wavelength of 336 m using a Beckman Spectrophotometer. For experiments with coadsorbed water, both the asphaltenes and the water were simultaneously dispersed in the heptol solution and the clay was contacted with this solution for 24 h. The contact angles and interfacial tensions were measured at 25” C using the methods described elsewhere [l] . RESULTS

Figure 1 depicts the adsorption isotherms of asphaltenes on sodium montmorillonite in the presence of various concentrations of preadsorbed water. For all concentrations of preadsorbed water, the isotherms go through

109

I

I

I

I

100 t

I

1.

0

I CM3

WATER/G

OF

CLAY

b

0 8 CM3

WATER/G

OF

CLAY

WATER/G

OF

CLAY

CONCN

I

I

0

WATER

3

Cd

OF ASPHALTENES,

of preadsorbed

Effect

I

NO

V

EQUILIBRIUM Fig.

I

I

0

water

G/LITER

on the adsorption

isotherms

of asphaltenes

on sodium

montmorillonite.

maxima with increasing asphaltene concentration. The isotherm in the absence of preadsorbed water increases with increasing asphaltene concentration (with a change in slope at 0.5 g 1-l) in the range of concentrations studied. The nature of the isotherms shows that the presence of water can have a significant influence on the adsorption behavior of the asphaltenes. The maximum adsorption density for each concentration of preadsorbed water decreases with increasing preadsorbed water content. This is seen from Fig. 2 where the maximum adsorption density drops from 80 mg

loo2

b------i 0

I

I

I

I

I

I

IO

15

20

2.5

30

3.5

I 05

CONCENTRATION

Fig. 2. Variation sorbed water.

OF PAEADSORBEO

of the maximum

WATER,

CM?‘G

in adsorption

4.0

CLAY

density

with

concentration

of pread-

110

g-’ for 0.1 cm3 g-’ preadsorbed water to a steady value of 55 mg g-’ for water contents in excess of 1.0 cm3 g-l of clay. The influence of coadsorbed water on the adsorption density of asphaltenes is depicted in Fig. 3. The initial asphaltene concentration that was used for the adsorption experiments was held at 0.76 g 1-l. The adsorption density stays essentially constant with increasing concentration of coadsorbed water until 0.8 cm3 g-* of clay, beyond which it decreases to a final value of 40 mg ’ at high coadsorbed water concentrations.

I

0 0 INITIAL

Fig. 3. Effect

I 10 CONCN

I OF WATER

of coadsorbed

I 20 IN

I OIL.

water

CM3/G

I 30

I 4.0

OF CLAY

on the adsorption

density.

The adsorption of asphaltenes on clay surfaces changes the surface characteristics of the clay dramatically. In a previous paper [l] we reported that the hydrophilicity of montmorillonite decreased rapidly with increasing amounts of adsorbed asphaltenes. Contact angle measurements were made for all the clays used in this study to determine the influence of preadsorbed water, coadsorbed water, pH and surfactant concentration on the ultimate wettability of the clay. Figure 4 is a plot of the three-phase contact angle, measured through the water phase, for the clays containing asphaltenes adsorbed in the presence of preadsorbed water. Sodium montmorillonite is strongly water-wet, but the addition of small quantities of asphaltenes decreases the wettability drastically. After the initial rapid increase in the contact angle, the curve levels off to a constant value. This final value varies from 153” for a preadsorbed water concentration of 0.1 cm3 g-l to -142” at 3.8 cm3 g-l preadsorbed water. An increase in the preadsorbed water concentration appears to show an improvement in the hydrophilicity. The dotted portions of the plots

111

in Fig. 4 are from extrapolated values of the contact angle. Coadsorption of water does not seem to affect the wettability to any significant degree (Fig. 5).

0

I 60 .p II

NO WATER

q 0 I CM3 WATER/G

OF CLAY

V

OF CLAY

3 8 CM3 WATER/G

II ‘%O II

0; 0

02

EQUILIBRIUM

04

06

ASPHALTENE

08 CONCN,

Fig. 4. Variation of the contact sorbed water concentrations.

164

-

116

-

I

100

0

1 IO

CONCENTRATION

Fig.

5. Variation

I

I 20

OF COADSORBED

of contact

J

IO

12

G/LITER

angle

I

with

asphaltene

I

I 4.0

30 WATER,

concentration

CMJ/G

angle with coadsorbed

CLAY

water

content.

at various

pread-

112

eoc y

GO-

”

H

PH

Fig. 6. Effect

OF PREADSORBED

WATER

of the pH of preadsorbed

water on the adsorption

behavior.

A preadsorbed water concentration of 1.3 cm3 g-’ of clay was selected to study the effects of the pH of the preadsorbed water on the adsorption behavior of asphaltenes on sodium montmorillonite. This concentration corresponds to that at which the maximum adsorption density attains its lowest value (Fig. 2) and was selected with our ultimate objective in mind, i.e. the reduction of the adsorption of asphaltenes on the clay. The initial asphaltene concentration was held at 0.76 g 1-l. Figure 6 shows the influence of pH on the adsorption density. The adsorption density decreases with increasing pH at very low pH, goes through a minimum, then increases to a maximum at about neutral pH. Beyond this maximum the adsorption density drops again to a minimum and then rises at high pH values. The effect of pH on

4 I-

116 -

II

100 0

,I 2 PH

Fig. 7. Effect

I 4 OF

III 6

PREADSORBED

8

IIll IO

I 12

_ 14

WATER

of the pH of preadsorbed

water

on the contact

angle.

113

the adsorption behavior appears to be very complex. The effects of the pH of coadsorbed and preadsorbed water on the wettability are shown in Figs 7and8. 180 , , , , , I 1 I 1 I 1 , 164

116

-

-

100

I

0

I

I

I

I

I

4

2 PH

Fig. 8. Effect

I

6

OF

I

I

I

8

GOADSORBED

I

I

10

I

12

, 14

WATER

of the pH of coadsorbed

water

on the contact

angle

The presence of an anionic surfactant in the preadsorbed water appears to play an important role in modifying the adsorption behavior of asphaltenes. The adsorption density (Fig. 9) decreases rapidly with increasing concentration of Aerosol-OT in the preadsorbed water, reaches a low value / loo-

0

0

,

I,

I

,

0

I 3 CM3/G

PREADSORBED

D

2 0 d/G

COADSORBEO

I

I 200

I

I 400

CONCENTRATION

I

II

I,

I

WATER

WATER

Ill 600

Fig. 9. Variation of adsorption and coadsorbed water.

II

I

800

OF AEROSOL-OT.

density

1000

1200

PPM

with

Aerosol-OT

concentration

in preadsorbed

114

ppm, then increases at high surfactant concentraof -10 mg g-l at -400 tions. The influence of surfactant concentration in the coadsorbed water is also shown in Fig. 9. In these experiments, the initial asphaltene concentration was maintained at 0.76 g 1-l. The adsorption of asphaltenes in the presence of coadsorbed water containing surfactant is completely different from that in the presence of preadsorbed water. With coadsorbed water, the adsorption density increases with increasing surfactant concentration in water and goes through a maximum between 400 and 600 ppm. The effect of the surfactant, Aerosol-OT, on the contact angle is shown in Fig. 10.

164

-

116

-

I

I

100 0

CONCN

200

I

I

OF AEROSOL-OT

Fig. 10. Effect

I

I

400

600

I

I,

IN PREADSOREED

of Aerosol-OT

I

800

I

1000 WATER,

on the contact

1200 PPM

angle.

DISCUSSION

Adsorption

behavior

Sodium montmorillonite is a layer silicate with a strong propensity to swell in the presence of water. It has been reported to absorb as much as 10 g water per gram of clay and increase in volume by a factor of 20 [6,7]. The water is first accommodated in the interlayers of the clay where it can modify the adsorption characteristics of organic molecules onto the clay. The literature in the area of clay mineralogy is replete with references about the effect of water and organic molecules on the basal spacing of montmorillonite. When the amount of water is low, this water is accommodated completely in the interlayer. The amount of water that can be so accommodated depends on the clay in question. In the Appendix we have calculated the limiting amount of the interlayer water that can be accommodated in sodium montmorillonite, using literature reported values of the d(OO1) spacing [8]. This limiting value of interlayer water is approximately equal to 0.25 cm3 g-’

115

for the clay in the collapsed state (no swelling) and 0.5 cm3 g-’ for the most expanded state. We selected the concentrations of preadsorbed water for our experiments in such a way that 0.1 cm3 g-’ corresponds to a water content lower than that required to fill the interlayer without swelling. All the other concentrations were in excess of that required to fill the interlayer, even in the most swollen state. This excess water usually exists as external layers around the clay particles, often helping to form liquid bridges between particles [ 91. The adsorption isotherms in the absence and presence of preadsorbed water are depicted in Fig. 1. The abscissa represents the equilibrium asphaltene concentration in heptol. There appears to exist three mechanisms of adsorption in such systems: (1) adsorption in the absence of preadsorbed water; (2) adsorption in the presence of trace amounts of preadsorbed water, and (3) adsorption in the presence of larger amounts of water. In the absence of water the isotherm for asphaltene adsorption increases with increasing asphaltene concentration with a change in slope around the CMC of asphaltenes in oil (0.55 g I-‘) [l] . The decreased slope of the isotherm beyond the CMC was explained as being due to the process of micellization in the bulk which decreases the number of free monomers available for adsorption. The isotherm for 0.1 cm3g-’ preadsorbed water increases rapidly with increasing asphaltene concentration and then attains a saturation adsorption density of 80 mg g-’ of clay at large asphaltene concentrations. At water concentrations in excess of 0.1 cm3 g-l the isotherms go through maxima, near the CMC of asphaltenes in oil. When trace amounts of water, such as 0.1 cm3 g-‘, are adsorbed on the clay, the water penetrates into the interlayer and inevitably causes a small amount of swelling. X-Ray diffraction measurements of asphaltene adsorption of montmorillonite [lo] have revealed that the asphaltenes do not form stable complexes in the interlayer and that all the adsorption occurs on the external surfaces of the clay. Since the trace amounts of water reside mainly in the interlayer and since the asphaltenes do not adsorb inside the interlayer, the adsorption of the latter should not be hindered by the presence of the interlayer water. In fact, the small swelling that inevitably occurs may increase the surface area of the clay and contribute to an enhanced adsorption density. This may be the reason for the different nature of the adsorption isotherm with 0.1 cm3g-’ preadsorbed water. The thin aqueous interlayers between the platelets of montmorillonite have also been known to possess structural properties different from that of bulk water [ll, 121. Such structural forces may also be partly responsible for the steeper slope of the adsorption isotherm in the presence of 0.1 cm3 g-’ preadsorbed water. The concentration of preadsorbed water for all the other isotherms is well in excess of the limiting amount required to swell the clay. The excess water would therefore form layers around the clay. The adsorption of asphaltenes occurs through the water layers probably by hydrogen bonding [ 131. Be-

116

yond the CMC of asphaltenes in oil (0.55 g l-l), the adsorption densities decrease with increasing concentrations of asphaltenes unlike the case where water was absent. The decrease in the adsorption density beyond the CMC may be due to the desorption of the surface layers of the preadsorbed water to be solubilized in the asphaltene micelles. If the solubilization process in the bulk is very much favored, the asphaltene monomer concentration in oil will drop, thereby leading to a decrease in the adsorption density. The maximum adsorption density attained in the presence of preadsorbed water increases sharply with increasing water content at very low water concentrations and then decreases with increasing concentration of water as shown in Fig. 2. The initial increase in the maximum adsorption density is due to the presence of trace amounts of water in the clay, while the decrease at high water concentrations is due to the formation of surface layers of water around the clay. The maximum adsorption density of asphaltenes decreases with an increase in the amount of water present in the surface layers. Figure 3 is a plot of the adsorption density with increasing concentration of coadsorbed water. For all the experiments with coadsorbed water the initial asphaltene concentration in oil was fixed at 0.76 g 1-l. This asphaltene concentration is higher than its CMC in oil. The value of the initial asphaltene concentration reported here is for coadsorbed water, while the equilibrium asphaltene concentration mentioned is for adsorption in the absence of water. This is because the equilibrium asphaltene concentration for the former case changes with a change in coadsorbed water content. The water droplets and the asphaltene molecules have to compete for the adsorption sites on the clay. At low concentrations of coadsorbed water, the adsorption density of asphaltenes does not seem to be affected, probably because of the solubilization of water droplets in the asphaltene micelles. This solubilization process prevents the water molecules from affecting the adsorption of asphaltenes at the solid surface. However, as the water concentration in the oil increases, the solubilization of water in asphaltene micelles may result in a depletion of the asphaltene monomers that are available for adsorption. Also, some of the water may reach the solid surface and adsorb creating an impediment to asphaltene adsorption. Both these factors will lead to a decrease in the adsorption density at high coadsorbed water concentrations. The magnitudes of the adsorption density observed with coadsorbed water and preadsorbed water (for the same initial asphaltene concentration) are about the same. Under optimum conditions of pH and surfactant, however, the adsorption density of asphaltenes with preadsorbed water can be reduced to a value much lower than that with coadsorbed water. We shall discuss this in more detail at a later point. Figures 4 and 5 show the influence of preadsorbed and coadsorbed water on the contact angle of the clay. For preadsorbed water, the presence of increasing amounts of water causes a small improvement in the final contact angle; the most hydrophobic clay being that for which the preadsorbed water concentration is the lowest. In all cases, the increase in the amount of

117

adsorbed asphaltenes causes a rapid decrease in the hydrophilicity of the clay. An equilibrium asphaltene concentration of 0.15 g 1-l is sufficient to make the initially hydrophilic sodium montmorillonite extremely hydrophobic. A comparison of the contact angles with the maxima in the adsorption densities of Fig. 2 uncovers the fact that the lowest value of the maximum adsorption density corresponds to the lowest value of the contact angle. Thus, there is a direct correlation between the asphaltene adsorption and the wettability. The coadsorption of water along with the asphaltenes does not appear to influence the contact angle to any significant degree (Fig. 5) and the contact angle stays the same for all concentrations of coadsorbed water. Effect

of aqueous phase pH

The results of the previous paper [l] revealed that the pH of the aqueous phase plays an important role in modifying the surface characteristics of the solid. The pH of the preadsorbed water was varied while maintaining a constant water and initial asphaltene concentration. Figure 6 shows the results of our experiments. The observed complex behavior of the asphaltene adsorption density with pH has also been reported by other investigators. Helmy et al. [14] observed a maximum in the uptake of quinoline from an aqueous solution at pH -7.0. They attribute this maximum to the favored adsorption of neutral molecules over ionic species. Siffert and Espinasse [15] also observed uptake-pH plots very similar to ours for the adsorption of polyacrylate from water. The adsorption of asphaltenes on a solid surface coated with water layers is very similar to the adsorption of asphaltene molecules at the oil/water interface. Since the asphaltene molecules are known to possess amphoteric sites, the adsorption on the solid could either be that of neutral molecules or of species that form ionic bonds at the surface. The maximum in the adsorption density at neutral pH implies that adsorption of neutral molecules appears to be the most favored. The lesser the extent of ionization the greater the adsorption on the surface. The minima in the adsorption density observed at pH 5 and 8 should therefore correspond to the pH at which the dissociation of the amphoteric species is most favored. The pK, values of the amphoteric sites would therefore be at pH 5 and 8. As the pH acquired values away from the pK,, the extent of dissociation would decrease and hence the adsorption density would increase. The variation of the adsorption density with pH, shown in Fig. 6, follows the expected behavior. Since the preadsorbed water layers on the solid create an interface that is akin to the oil/water interface, the adsorption of asphaltene molecules at the oil/water interface should reflect the changes that were observed on the solid surface. The drop in interfacial tension (which is a measure of the adsorption) of the clean oil/water interface as a consequence of asphaltene adsorption is shown as a function of pH in Fig. 11. This drop in interfacial tension

118

PH

Fig. 11. Effect of pH on the drop to asphaltene adsorption.

by subtracting

tension

of a clean

oil/water

interface

due

the tension of the asphaltene-covered interinterface. Indeed, the interfacial tension difference goes through a maximum at pH -7.0 and shows two minimaone at pH -5.0 and the other at pH -8.5. Thus the nature and mechanism of asphaltene adsorption on clays containing preadsorbed water independently corroborates the adsorption measurements at the oil/water interface. Figure 6 provides an important result - the adsorption density of asphaltenes drops to almost zero in the presence of 1.3 cm3 g-’ preadsorbed water of pH 8.0. The decrease in the adsorption density at pH 5.0 is also noteworthy. The use of aqueous solutions of the above-mentioned pH could therefore help in decreasing asphaltene adsorption such that the solids are ultimately prevented from forming stable W/O emulsions. The effect of the pH of coadsorbed water on the adsorption density is shown in Fig. 12. The concentration of coadsorbed water in oil was 2.0 cm3 g-’ of clay and the initial asphaltene concentration was 0.76 g 1-l. This concentration of coadsorbed water was chosen because it resulted in the lowest value of the adsorption density at neutral pH (Fig. 3). The effect of pH on asphaltene adsorption with coadsorbed water is insignificant in contrast to that of preadsorbed water. The adsorption density does not change appreciably except at very high pH. As the water content on the clay increases, the expected influence of pH also increases. With coadsorbed water, part of the water is solubilized in asphaltene micelles and the rest has to compete with the asphaltenes for adsorption sites, hence the amount of water on the surface would decrease. Commensurately, the influence of pH would decrease. The effect of the pH of the preadsorbed water on the contact angle is seen in Fig. 7. The contact angle shows minima at pH -4.5 and -8.0. The minima in the contact angle are due to the minima in the adsorption density observed at the same pH (Fig. 6). The higher value of the contact angle obcan

be obtained

in interfacial

face from that of the pure oil/water

119

0

2

4

PH

Fig.

12. Effect

6

OF

COADSOREED

10

8

WATER

of the pH of coadsorbed

water

on the adsorption

behavior.

served at neutral pH is because the adsorption density of asphaltenes on the solid surface is highest at this pH. The contact angle thus appears to depend on the amount of adsorbed asphaltenes with an improvement in hydrophilicity being observed at low adsorption densities. The contact angle for the clays involving coadsorbed water increases slowly with increasing pH as seen in Fig. 8. The correlation with the adsorption density behavior is once again reasonable, with the contact angle increasing slowly with increasing pH, the effect being most prominent at high pH. An increase in the adsorption density would call for a corresponding increase in the contact angle, and this is indeed what is observed. The use of preadsorbed water of pH 8.0 appears to be extremely effective at decreasing the asphaltene adsorption and therefore the hydrophobicity of the clay. This set of conditions would therefore be effective in destabilizing the solid-stabilized W/O emulsion. Effect

of surfactant

Most of the studies on adsorption of a species in the presence of surfactants involve adsorption from aqueous solutions, Polymer adsorption from aqueous solution onto clays in the presence of surfactants has been studied by some researchers [16,17]. The effect of surfactants present in the preadsorbed water or coadsorbed in small water droplets along with the adsorbate from nonaqueous solutions does not appear to have been investigated. The surfactant that was used in our study was Aerosol-OT which is an anionic surfactant with a molecular weight of 444. Figure 9 shows the variation of the adsorption density of asphaltenes from oil with the surfactant concentration present in the preadsorbed and coadsorbed water. The preadsorbed water content was fixed at 1.3 cm3 g-’ of clay and the initial as-

120 phaltene concentration selected was 0.76 g 1-l. For the preadsorbed water systems the adsorption density of asphaltenes decreases with increasing surfactant concentration and passes through a minimum of -400 ppm. The surface activity of Aerosol-OT was found to be greater than that of asphaltenes on water layers present around the solid particles. The presence of Aerosol-OT in the surface water layers prevent the adsorption of asphaltenes owing to their superior surface activity. Hence, the adsorption density in Fig. 9 decreases with increasing surfactant concentration. The CMC for Aerosol-OT in water is -500 ppm. An explanation may be that the asphaltenes begin to form complexes with the Aerosol-OT which is present on the solid surface. Such asphaltene-surfactant complexes may then increase the adsorption density. Figure 9 also shows the influence of Aerosol-OT in the coadsorbed water on the adsorption of asphaltenes onto the clay. In contrast to the behavior seen previously with preadsorbed water, the adsorption density increases with increasing surfactant concentration and goes through a maximum at -600 ppm. When Aerosol-OT is present in the water droplets that are COadsorbed with the asphaltenes from the oil, the hydrophobic tails of the surfactant molecules will be oriented towards the external oil phase. Hence, these water droplets will be preferentially retained in the bulk oil thereby allowing the adsorption of asphaltenes. This would explain the rise in the asphaltene adsorption density with increasing surfactant concentration. in J the coadsorbed water. Figure 10 delineates the infuence of Aerosol-OT in the water on the wettability of the clay. For the preadsorbed water systems, the contact angle decreases with increasing surfactant concentration and goes through a minimum at -300 ppm. This behavior is once again in accordance with the change in the adsorption density due to the presence of surfactant in the preadsorbed water (Fig. 9). The adsorption density goes through a minimum of -400 ppm while the contact angle goes through a minimum at -600 ppm and the minimum in contact angle is due to the decrease in the surface concentration of asphaltenes. With coadsorbed water, the contact angle again goes through a minimum, this time at a concentration of 200 ppm. CONCLUSIONS

The presence of preadsorbed water decreased the adsorption density of the asphaltenes, especially at high asphaltene concentrations. The isotherms went through maxima for all preadsorbed water concentrations in excess of 0.1 cm3 g-l. Preadsorbed water containing 200-600 ppm of Aerosol-OT or at pH 8.0, at a water content of 1.3 cm3 g-l, decreased asphaltene adsorption considerably. While coadsorbed water caused a small decrease in asphaltene adsorption,

121

no improvements in wettability were observed when surfactant was added or when the pH was varied. A decrease in asphaltene adsorption improved wettability in all cases examined. APPENDIX

A typical montmorillonite clay in the sodium form has a unit cell formula [71 (Sib

(AL.3

Mg,.,,

1 OZO(OH),

The unit-cell weight for this formula is 734. Therefore, 734 g of this clay contains 6.02 X 1O23unit cells. The interlayer [d(OOl)] spacing in montmorillonit is reported to vary from 9.5 a in the collapsed state to 19 8, in the most swollen state [8]. Since the water molecule has a diameter of 3.71 A, a minimum of 2.6 and a maximum of 5.1 water layers can be accommodated between two platelets of the clay. Four molecules of water are usually accommodated in one water layer of a unit cell [7]. Hence the number of water molecules per unit cell is 10.4 in the collapsed state and 20.4 in the swollen state. The amount of interlayer water in the collapsed state, therefore, is 0.25 g g-’ of clay and in the swollen state it is 0.50 g g-’ of clay. ACKNOWLEDGEMENTS

This work was supported by Exxon Research and Engineering Company as part of their Frontiers of Separation Programs. 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 V.B. Menon and D.T. Wasan, Colloids Surfaces, 19 (1986) 89. L.V. Lyutin and T.A. Burdyn, Tr. Vses. Neftegazov. Nauchno-Issled. Inst., 53 (1970) 117. D.M. Clementz, Clays Clay Miner., 24 (1976) 312. A.C. Hall, S.H. Collins and J.C. Melrose, Sot. Pet. Eng. J., 23 (1983) 249. S.H. Collins and J.C. Melrose, Paper presented at the SPE International Symposium on Oilfield and Geothermal Chemistry, Denver, CO, 1983. K. Norrish, Discuss. Faraday Sot., 18 (1954) 120. H. Van Olphen, An Introduction to Clay colloid Chemistry, Wiley, New York, NY, 1977. B.K.G. Theng, The Chemistry of Clay-Organic Reactions, Wiley, New York, NY, 1974. Y. Kawashima and C.E. Capes, Ind. Eng. Chem. Fundam., 19 (1980) 312.

122 10 11 12 13 14 15 16 17

E. Czarnecka and J.E. Gillott, Clays Clay Miner., 28 (1980) 197. D. Green-Kelly and B.V. Derjaguin, Trans. Faraday Sot., 60 (1964) 449. P.F. Low, Soil Sci. Sot. Amer. Proc., 40 (1976) 500. R.E. Grimm, Clay Mineralogy, McGraw-Hill, New York, NY, 1968. A.K. Helmy, S.G. De Bussetti and E.A. Ferreiro, Clays Clay Miner., 31 (1983) B. Siffert and P. Espinasse, Clays Clay Miner., 28 (1980) 381. Y. Bocquenet and B. Siffert, Colloids Surfaces, 9 (1984) 147. Th.F. Tadros, J. Colloid Interface Sci., 46 (1974) 528.

29.