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Particle transfer from a continuous oil to a dispersed water phase: Model particle study
Particle Transfer from a Continuous Oil to a Dispersed Water Phase: Model Particle Study 1 C H E - A N K U , J O S E P H D. H E N R Y , JR., 2 R A N J A N I S I R I W A R D A N E , AND L L O Y D R O B E R T S Department of Chemical Engineering, West Virginia University, Morgantown, West Virginia 26506-6101
Received September 7, 1984; accepted December 6, 1984 The surface free energy of glass microbeads was controlled by varying the extent of reaction between t-butyldimethylchlorosilane with the surface silanols. This system was chosen so that the surface free energy of the particles could be varied without introducing an extraneous wetting agent. The wicking method was used to obtain the liquid-particle-air contact angles. Water-air contact angles on glass beads increased with increasing silanizing reaction. The dispersive and the nondispersive contributions of the surface free energy were calculated from the contact-angle data. Nondispersive contribution decreased with increasing silanizing reaction while dispersive component remained unchanged. These silanized glass beads were dispersed in the continuous oil phase and the extent of particle retention at the water-oil interface and distribution to the water phase were determined. The experimental data indicate that the surface free energy of particles is the controlling parameter determining the transfer of particles from the oil phase to the water phase. Particle size in the range 5-20 #m had no significant effect, either on the particles retained or distributed to the water droplets. © 1985AcademicPre*~inc. INTRODUCTION T h e ash content o f coal fed to coal liquefaction process is ultimately contained in the coal-derived liquid product. It is desirable that ash levels be reduced to decrease erosion a n d obtain better efficiency (1). T h u s the products o f coal liquefaction processes are either filtered or centrifuged to remove the ash (2). Because o f the fine sizes o f mineral particles and the high viscosities o f coalderived liquids, elevated temperatures and pressures are utilized for these separation processes (2). It has been shown that currently available separation processes are costly (4). A n added disadvantage is the loss o f liquid h y d r o c a r b o n products in the cake or slurry. Therefore, the need for an alternative, m o r e cost-effective separation process is well justified. ~This paper was presented at the 58th Colloid and Surface Science Symposium, ACS, Pittsburgh; 1984. 2 To whom all correspondence should be addressed.
H e n r y (3) proposed a new separation process which involved removal o f mineral matter by transfer f r o m the oil to an aqueous phase. T h e process mechanics is shown in Fig. 1. Coal-derived liquid containing ash is contacted with water by making a water/oil emulsion. Ash transfer from the oil to the water phase is p r o m o t e d by surfactant. Subsequently the emulsion is b r o k e n and separated into two phases giving an ash-free oil. The h y d r o c a r b o n losses will be m i n i m a l because the solubility o f oil in the aqueous phase is very low. Hence, this new separation process can potentially o v e r c o m e the operational problems and e c o n o m i c disadvantages o f conventional ash separation processes. This process has been studied by Prudich (4) using mineral particles extracted f r o m coal-derived liquid. It was reported that the distribution o f particles into the aqueous phase which was described by the distribution c o e f ~ c i e n t BE decreased with increasing mixing time. T h e result is shown in Fig. 2. A 377 0021-9797/85 $3.00 Copyright © 1985 by AcademicPress, Inc.
Journal of Colloid and Interface Science, Vol. 106, No. 2, August 1985
All rights of reproduction in any form reserved.
378
K U ET AL.
surface characteristics were fixed and did not change during the particle transfer process. Thus the anomalous artifacts of the real, and more complicated, system could be avoided. Silica in high percentages is found in coal mineral mineral ash and mineral particles (6); in addition, it matter@ matter has highly reactive hydroxyl group. The modification of the surface free energy of mineral glass plates by chemisorption of different matter organochlorosilanes has been studied by Menawat (7). The conventional sessile drop min~alQ matter method was used to measure both water-air and xylene-water contact angles on model ~ G . 1. P r ~ e s s m e c h a n i c s o f l i q u i d - l i q u i d - s o l i d s e ~ plate systems produced by reagents of differarationpr~s. ent concentrations. It was reported that tbutyldimethylchlorosilane could produce stapartial detergency model (1) based on the ble, reproducible, homogeneous surfaces fate of the physically adsorbed surfactant was which did not change their surface free enerproposed. The experimental result was ex- gies during particle transfer in a liquid-liquid plained as due to the partial removal of (xylene-water) system. This work extended the technique of procarbonaceous coating by surfactant. This is ducing stable model surface to particle sysshown in Fig. 3. However, there are some artifacts asso- tems. Model colloids which had a range of surface free energies and were stable in the ciated with his experimental system (4). water-oil system were developed. These 1. The physically adsorbed surfactant was model particle systems were then characterbeing transferred to the aqueous phase and ized by measuring contact angle using wicking adsorbed at the water-oil interface during method and used in the distribution experithe particle transfer process. ment to study the mechanism of the particle 2. Experimental error occurred due to dif- transfer process. ficulty in splitting the aqueous phase from the interface and the oil phase, and to heterogeneities in the mineral matter samples. 3. Some material balance error was attribo uted to organic material being dissolved from ~ 6o0 rpcn the particles into the oil phase. 4. Asphaltenes which coat the mineral particles were also shown (5) to form rigid films at the water-oil interface and to have different surface characteristics under different ~2 pH conditions. ~DROCARBON P~SE
In order to elucidate the mechanism of this particle removal process it is necessary to understand the fundamental effects of particle surface characteristics on the particle transfer process. This involved the use of a well-defined model particle system whose Journal of Colloid and lmerface Science, Vol. 106, No. 2, August 1985
~-E]. ~; ~ A ~ A ~ . A ' 10
_ _
80p~m 4opt
' ¢o 20 TffF, EE PHASE MIXING TI?~E,MINUTES
D -A 4;
FIG. 2. Distribution coefficient, 82, versus three-phase mixing time, parametric in initial suffactant concentration.
PARTICLE
• .'. -
.'. -.'..'.-..'-. ~.-....-. :.-.." .'. - ". -...
inteatrface ~t~O~ ~
MINERAL
I I~
379
TRANSFER
SURFACE
:... ' . . . ' . . . . . - ... - .-...... - - -....'.-....-.
on ~surfacelI~II ~
= [ ~
-
intea:face
®
of surfactant in bulk
aggregatein bulk OILPHASE
WATERPHASE
FIG. 3. Phenomenologicalmodel of solubilizationdetergency.
EXPERIMENTAL
Preparation of Model Particle System The substrate used was sodalime glass microbeads (Potters Industries Co.). The particle size is within 5-20 #m. The beads were first washed with methylethylketone (Fisher) and rinsed with distilled water several times. Then they were washed in detergent and again rinsed with distilled water. Then the beads were placed in concentrated HNO3 and rinsed several times with distilled water. The beads were separated from the reagent by filtration at each step. An ultrasonic bath was used to enhance the cleaning effect. This substrate preparation step has been used by Menawat et al. (7) in cleaning the glass plates and has been performed in the removal of impurities from the surface using auger electron spectroscopy (7). All glassware used was cleaned by the same procedure. After cleaning, the beads and a cleaned magnetic stirrer were put into the reaction vessel and outgassed at 115°C and 10 - 3 m m Hg for 24 h to remove the physically adsorbed water. After outgassing, 50 ml of n-hexane
(Fisher) was transferred into the reaction vessel via a funnel without breaking the vacuum and a liquid seal was maintained. The magnetic stirrer was used to break up any particle lumps and keep the particles in suspension. The vessel was then placed in the ultrasonic bath to ensure the proper wetting in hexane. Solutions of t-butyldimethylchlorosilane were prepared in 100 ml of n-hexane. The solution was then transferred to the reaction vessel without breaking the vacuum. The reaction was carried out for 24 h and the contents were stirred by the magnetic stirrer during the reaction. After the reaction the beads were rinsed 3 times with n-hexane and outgassed at 115°C and 10 - 3 m m Hg for 4 h.
Characterization of Model Particle System The method based on the wicking of liquids into a packed column of particles was used to characterize the surface of the model particle system. This method has been used to measure contact angles of powder and fiber (15, 16) by previous workers. Journal of Colloid and Interface Science, Vol. 106, No. 2, August 1985
380
KU ET AL.
The packed column of particles can be considered to consist of a bundle of capillaries of mean effective radius R. The mean effective radius is constant for a given packing of particles. Thus applying the Washburn equation (8) which describes the penetration of liquid into a small capillary to this system yields h2
3"R cos 0 - - - t 2#
[1]
where h is the wicking height, t is the wicking time, 0 is the advancing contact angle, 3' is the surface tension, and ~ is the viscosity of the liquid. The Washburn equation is a simplification of the more general WashburnRideal equation which includes the hydrostatic pressure term. The hydrostatic pressure term is negligible compared to the capillary pressure term for the wicking of liquid into a bed of 5- to 20-tzm particles which was the case for our experiments. A magnitude analysis indicated that the maximum gravitational force for this study was only 0.18% of the capillary force. Also this can be verified by wicking data which would only produce a linear h2-vs-t curve if gravitational effects are negligible. Crow and Woodridge (20), Chwastiak (16), and others (15, 21, 22) have also found it appropriate to neglect the hydrostatic pressure term in the Washburn-Rideal equation. The value of R can be calculated if a liquid is chosen for which 0 = 0 °. Since the particle size is not changed by the surface reaction, once R is known, the contact angles of subsequent batches of modified particles, for other liquids, can be determined from wicking results. A known weight of particles was placed in a glass tube of 0.7-cm i.d. The lower end of the tube was closed with a glass flit. The tube was filled to the same height with the same amount of particles to ensure a constant packing. Gradual addition of small amounts of particles and tapping with a glass rod in the tube maintained a constant packing. ReJournal of Colloid and Interface Science, Vol. 106, No. 2, August 1985
producibility of the experimental data was found to be dependent mainly on proper packing. The experimental set up is shown in Fig. 4. The packed column was placed vertically in the liquid and the wicking height (h) was measured as a function of time. For the calibration of R, particles in the packed column were equilibrated with the saturated vapor of each wicking liquid to avoid the additional driving force due to the reduction in free energy of the solid on becoming covered by the adsorbed films as recommended by Good (9). All experiments were carried out at room temperature (20°C).
The Distribution Experiment of Model Particle System Model particle systems which had welldefined surface characteristics were used in the distribution experiments to study the effect of the surface characteristics of particles on the particle transfer process in a liquidliquid system. Xylene (Fisher, certified ACS grade) was used as the model hydrocarbon phase and deionized triple distilled water was used as the aqueous phase. Model particles (0.2 g) were dispersed in 100 ml of xylene phase using an ultrasonic bath. A 300-ml cylindrical Teflon autoclave liner (Parr No. 762 HC) with i.d. = 2.5 in was used as the mixing vessel. The vessel containing the particle-oil dispersion was then moved to a mixer (Parr No. 4561). A 3.85-cm diameter turbine impeller with 4 fiat pitched blades provided vigorous mixing and
GLASS
SCALE
WICKING FRONT
--
TUBE
PARTICLES
GLASS
FILTER
LIQOID
FIG. 4. Experimental setup of wicking method.
381
PARTICLE TRANSFER
rapid liquid-liquid dispersion in the mixing vessel. The impeller was coated with a 0.4m m Teflon film. Teflon equipment was used because the particles tended to stick to glass. The impeller height from vessel bottom was equal to one impeller diameter according to the Standard Tank Configuration (19). A 50-ml water phase was introduced into this particle-oil dispersion while the system was being mixed. The time of mixing with the aqueous phase was called the three-phase mixing time. Upon completion of mixing, the oil/water/particle mixture was transferred into a 250-ml Teflon separatory funnel and allowed to separate into phases. The resolution times ranged from 2 to 10 s. Three fractions, the aqueous phase, the oil phase, and the interface, were then drawn from the funnel separately. These fractions were filtered through a Millipore filter (Millipore H A W P 047, 0.45 #m in pore size), dried, and weighed to determine the distribution of particles. The size distributions of particles before and after the distribution experiments were determined u s i n g a particle size analyzer (Microtrac 7991-3) which utilizes low-angle, forward scattering of light from a laser beam. The entire apparatus was washed with methylethylketone, detergent, nitric acid, and triple-distilled water and dried before each experiment. RESULTS AND DISCUSSION
Characterization of Model Particle System The mean effective radius of the packed column, R, was determined by conducting wicking experiments on cleaned glass beads with hydrocarbons which were known to spread on clean glass plates (contact angle - 0°). A plot of h 2 vs t is shown in Fig. 5. The wicking experiment was very reproducible under the same condition and the experimental error was within 3%. In all cases linear plots of h 2 vs t were obtained. This justified the assumption that the packing was
,0 ACETONE 0/0 O/
/~7 N-HEXANE
~z/~
o/
/ ~/" /0 V / ~It i0 z" ~II o- 7 / ~'~" o I ,.~ ~ I ~ ,~ o ' ~ / / . . < > o" ,,.~
~= o
o>V..,---o
CHLOROBENZENE N-OCTANE XYLENE ,-~EC~E
,,-
1~,3~:..o ._zx.:
I- tJ', ~
o .~S.[~
i
.
2
.
3
.
.
4
.
5
.
6
7
WICKING TIME,MIN. FIG. 5. Wicking results of liquids on clean substrate.
uniform. The values of R cos 0 were calculated from the slopes of the h2-vs-t plot and with the knowledge o f surface tensions and viscosities of the liquids. The physical properties of these liquids and the R cos 0 values are shown in Table I. Since these liquids have either zero or very low contact angles on clean glass surface, the values of cos 0 should be nearly 1. This was verified by the closeness of the values of the calculated R cos 0 for different liquids as shown in Table I. The maximum value, 3.73 X 10 -5 cm, was used as the mean effective radius of the packed column. The plot of h 2 vs t for water on model particle systems prepared using different concentrations of t-butyldimethylchlorosilane is shown in Fig. 6. The particle-water-air contact angles were determined from the slope of this plot. Increasing hydrophobicity indicated by the increasing water-air contact angle as shown in Fig. 6 was observed with increasing reagent concentration. This may be due to the replacement of more hydroxyl groups with increasing alkyl silanizing agents. Similar observation was made by Menawat et al. (7) on glass plates after reacting with silanizing agents. The hydroxyl groups on Journal o f Colloid and Interface Science,
Vol. 106, No. 2, August1985
382
KU ET AL. TABLE 1 The Results of the Wicking Experiments of Liquids on Clean Glass Beads at 20°C Liquid
3' (dyn/cm)
~u (g/cm-s)
Correlation coetficienff
Slope (cmX/s)
R cos 0 (cm)
n-Hexane n-Octane n-Decane Acetone Xylene Chlorobenzene
18.4 24.4 22.2 23.7 30.1 33.6
0.326 0.542 0.920 0.327 0.648 0.799
0.9995 0.9998 0.9980 0.9996 0.9990 0.9992
0.1004 0.0754 0.0461 0.1308 0.0803 0.0729
3.73 X 10-5 3.51 X 10-5 3.67 × 10-5 3.67 X 10-5 3.46 × 10-5 3.59 X 10-5
a Least-squares linear plot of h 2 vS I. the glass surface participate in the reaction with t-butyldimethylchlorosilane as shown in
dispersion force and other intermolecular forces are additive 7 = yd + 7p
CH3CH3
I
I
I
I
[4]
and the approximation solid-liquid interfacial tension from either Wu's equation (13)
- - S i s - - O H + C 1 - - S i - - C - - CH3 CH3CH3
~+~
~,2=~,+~2
~+~
[5]
CH3CH3
I
I
- - S i s - - O - - S i - - C - - CH3 + HC1.
I
[21
I
~l~=Vl+V2--2~--2~
CH3CH3
The surface free energy of solid surface can be calculated from contact-angle data using Young's equation (10) 7re
(6]
it can be shown by derivation (13, 14) that % is related to contact-angle data through either one of the following equations:
Surface Free Energy Calculation
"Ys = "YLvCOS 0 + "YSL +
or the Modified Fowkes equation (14)
[3]
1 + cos 0 4
~-y~ TL
1 + cos 0 YL
--
-y~_~g
y~_ + 3'~----~+ 3'~_+ ~
[7]
/~L -- ~ "
where 7re is the film pressure of the solid surface which is usually negligible on smooth, homogeneous solid surface with contact angle greater than zero (1 1), which is the case in this work. The surface tension of liquid, 3'LV, and the contact angle, 0, can be measured. Hence, if the solid-liquid interfacial tension, 3'SL is known, the surface free energy of solid surface, %, can be determined. However, since YSL cannot be measured experimentally, an estimation is needed in order to obtain the surface free energy of solid surface. With the Fowkes' assumption (12) that the Journal of Colloid and Interface Science, Vol. 106, No. 2, August 1985
2
[8] It should be noted that Eq. [5] uses the harmonic mean, (2~,~,z°)/(y~ + 3'~), to approximate the dispersive interracial interaction while Eq. [6] uses the geometric mean, (7~,2d) 1/2, to estimate the dispersive interfacial interaction. Theoretically, the former is applicable when the polarizabilities of the volu m e elements in the two phases are nearly equal while the latter is applicable when the electronic vibrational frequencies are nearly
PARTICLE TRANSFER SYMBOL
[]
CONCENTRATION (mg/ml)
CORRELATION COEFFICIENT
SLOPE (cm2/sec)
CONTACT ANGLE (degree)
0.001
0.9997
0.0903
49.7
0.004
0.9994
0.0466
69.1
0.04
0.9981
0.0153
82.2
WICKING LIQUID = WATER
~'
383
15].[Z]-[]
[7],.['7"IZ]" ]
..[].~9 o.D-a .v-v []---.v-v .
..~..V ~V'V ^
^
_A-A-A
o N/--~'~--'~-A-A-A-A - / ' x - / ' x - ' -
It
I 2
13
I 4
I 5
I 6
WICKING TIME, MIN.
FIG. 6. Wicking results of water on model particle systems.
equal (13). The nondispersive interracial interaction may include dipole-dipole, dipoleinduced dipole, and hydrogen bonding. Both the harmonic mean and the geometric mean for the nondispersive interfacial interaction should be regarded as empirical in nature, since they include all the interaction not accounted for by the dispersion term (18). The term (3,~3,~)1/2 is applicable to dipoledipole interaction. However, it has been shown that the dipole-induced dipole interaction predominates in liquids or liquid/solid systems. The dipole-induced dipole interaction should lead to the arithmetic mean (l 8). In order to use either Eq. [7] or Eq. [8], two liquids of known dispersive and nondispersive surface tensions and their contact angles on the solid surface need to be known. The two liquids used in this study were water and methylene iodide. The surface tension values of dispersive and nondispersive com-
ponents for the two liquids used in this study are listed on Table II (13, 14). The contact-angle data of these liquids on particle systems were determined using the wicking method. The results are shown in Table III. The surface free energies of model particle systems calculated using Eqs. [7] and [8] are listed in Table IV. Results calculated from TABLE II Dispersive ( ~ ) and Nondispersive ('r~) Components of the Surface Tensions of Liquids
Liquid
(1) a
(2) b
(1)
(2)
Water 22.1 21.8 50.7 51.0 Methylene iodide 44.1 49.5 6.7 1.3
"rt = 3'~. + "/~.
72.8 50.8
a (1) Determined from Wu's equation. b(2) Determined from modifiedFowkes's equation. Journal of Colloid and Interface Science, Vol. 106, No. 2, August 1985
384
KU ET AL. TABLE II1 The Contact Anglesof Model Particle Systems Model particle system
Correlation coefficient
Wicking slope (cm2/s)
Contact angle (deg)
Water
A B C D E
0.9992 0.9997 0.9993 0.9994 0.9800
0.1079 0.0903 0.0806 0.0466 0.0056
34.3 49.7 55.0 69.1 87.7
Methylene iodide
A B C D E
0.9997 0.9992 0.9996 0.9990 0.9991
0.0167 0.01234 0.0115 0.0085 0.0037
85.0 86.3 86.6 87.4 88.9
Liquid
both equations are of the same order of magnitude and show a similar trend. It is known (18) that both the harmonic mean and geometric mean equations give similar surface tension and components when contact angles are used for the calculation. However, very large differences are obtained when Eqs. [5] and [6] are used to calculate interfacial tensions. The harmonic mean equation has been shown to be more accurate (18). A plot of "y~ and 3,p calculated using Wu's equation vs water-air contact angle is shown in Fig. 7. It is interesting to note that the nondispersive surface free energy, %P, decreased with increasing contact angle (hydrophobicity) while the dispersive surface free energy, -y~,
remained almost constant. Thus, the London dispersion force contribution of the model surfaces did not change when the hydroxyl groups were gradually replaced by the alkyl groups. This is quite reasonable in view of the similar polarizability of water molecules and methane molecules (17). The decrease of the nondispersive surface free energy with increasing hydrophobicity can be explained as being due to the loss of the number of the polar hydroxyl groups which have strong hydrogen-bonding ability.
The Distribution Experiment Particles were either retained at the wateroil interface or distributed to the aqueous phase after the resolution of the emulsion. The a m o u n t of particles in the oil phase was negligible. However, during resolution, a small a m o u n t of particles was observed settling onto the water-oil interface; hence, some of the particles at the interface after resolution were originally in the oil phase before the resolution of the emulsion. No particles were observed settling from the interface into the aqueous phase after the resolution of the emulsion. To prevent the particles at the water-oil interface from being drawn out with the aqueous phase, only 90% of the aqueous phase was drawn from the separation funnel for analysis. The water-oil interface and the
TABLE IV Dispersive and Nondispersive Components of the Surface Free Energy Values for Silanized Glass Beads Model particle system
Ow (deg)
(I)a
(2~
(I)
(2)
(1)
(2)
A B C D E
34.3 49.7 55.0 69.1 87.7
9.5 9.3 9.4 9.6 10.1
7.3 7.7 8.0 8.8 9.9
55.9 43.5 39.0 27.9 15.7
56.9 43.3 38.1 24.7 10.6
65.4 52.8 48.4 37.5 25.8
64.2 51.0 46.1 33.5 20.5
a(1) Calculated using Wu's equation. b(2) Calculated using modified Fowkes'sequation. Journal of Colloid and Interface Science, Vol. 106, No. 2, August 1985
PARTICLE TRANSFER
385
remaining aqueous phase were then drawn from the separation funnel together for analA ysis. The mass of particles in each phase and Ys x[ ] . V YP the interface was then back-calculated from yPr:'-7 ~ [ ] Ys = yd + yp material balance. It has been experimentally verified that the particle concentration in the aqueous phase is constant during the period o f analysis (less than 10 s). The results of the distribution experiments using model particles of different surface free energies are shown in Fig. 8. The a m o u n t of particles retained at the oil-water interface at steady state decreased while the amount 2 YdA A-& & A o f particles distributed to the aqueous phase increased with increasing surface free energy i 30 ~o ~o 6~ ¢o ~o of the model particle systems. Operating variCONTACT ANGLE 0w , degree ables such as mixing speed, water/oil ratio, FIG. 7. Effect of water-air contact angle on surface and initial particle concentration were kept free energy ofmodel particle systems. constant for the present work. d
[]"D
YS' TOTAL
SURFACE
FREE ENERGY,
20
30
40
50
i
!
i
!
dyne/cm 60 l
70 !
WATER/OIL RATIO = 0 . 5 INITIAL P A R T I C L E C O N C E N T R A T I O N M I X I N G SPEED = 600 rpm
= O. 23 %
P A R T I C L E S IN W A T E R PHASE
| "
RETAINED PARTICLES
y
z
I
I P y , NON-DISPERSIVE SURFACE FREE F.NER~, d y n e / c m
FIG. 8. Effectof surface free energy on particle distribution in xylene-waterphases for silanized glass beads. Journal of Colloid and Interface Science, Vol. 106, No. 2, August 1985
386
KU ET AL.
An explanation for these results is that the particles with higher surface free energy have more hydroxyl groups (less alkyl groups) on the surface; hence, they have more affinity for the water phase. It is also interesting to note that the polar contribution to the surface free energy is the major parameter controlling the distribution of particles to the aqueous phase. The behavior of particles at the interface of two immiscible liquid phases is known to be related to the forces acting at the interface,
such as surface force, gravitational force, and buoyancy force. In order to determine the contribution of this mechanism on the distribution process, the size distribution of particles before and after the distribution experiment were measured using the Microtrac 7991-3 particle size analyzer. The results are shown on Fig. 9. No significant change of particle size distribution was observed. Therefore, the particle transfer process in the water/oil system does not depend on particle size in the range of 5-20 ~tm.
ORIGINAL
--
--s
- ~
WATER
--
PHASE
RETAINED
m . _ _
It.
u
E
g
A 5
! I0 PARTICLE
!
I
l
15
20
25
SIZE,
~m
FIG. 9. Particle size distribution data before and after particle transfer experiments. Journal of Colloid and Interface Science, Vol. 106, No. 2, August 1985
387
PARTICLE TRANSFER CONCLUSIONS
REFERENCES
The surface characteristics of model particle 1. Prudich, M. E., and Henry, J. D., Jr., AIChE J. 24, systems were controlled by reacting the sur788 (1978). face with t-butyldimethylchlorosilane. The 2. Henry, J. D., Jr., and Patel, B. S., Interim Report, prepared for the Office of Coal Liquefaction liquid-air contact angles on these model Study on August 14, 1974. particles were obtained from the wicking 3. Henry, J. D., Jr., Research proposal submitted to method. Increasing hydrophobicity was obU.S. Bureau of Mines on April 30, 1974. served with increasing reagent concentration 4. Prudich, M. E., Ph.D. dissertation, West Virginia as indicated by increasing contact angles. University, Morgantown, 1979. 5. Bartell, F. E., and Neiderhauser, D. O., in "API The surface free energies of model particle Drilling and Production Practices," The University systems were calculated from the liquidof Michigan, Ann Arbor, 1949. particle-air contact-angle data obtained from 6. Menawat, A., M.S. thesis, West Virginia University, the wicking method. The dispersive surface Morgantown, 1982. free energy did not change while the nondis- 7. Menawat, A., Henry, J. D., Jr., and Siriwardane, R., J. Colloid Interface Sci. 101, 110 (1984). persive surface free energy decreased signifi8. Washburn, E. W., Phys. Rev. 1, 273 (1921). cantly with increasing replacement of hy9. Good, R. J., £ Colloid Interface Sci. 42, 473 (1973). droxyl groups by alkyl groups. 10. Young, T., "Miscellaneous Works" ((3. Peacock, The distribution experiments of particles Ed.), Vol. 1. Murray, London, 1855. in two liquid phases indicated that the particle I 1. Good, R. J., J. Colloid Interface Sci. 52, 308 (1975). transfer process was not due to body force 12. Fowkes, F. M., Ind. Eng. Chem. 56, 40 (1964). difference for particles in the size range of 13. Wu, S., J. Polym. Sci. C34, 19 (1971). 14. Owens, D. K., and Wendt, R. C., J. Appl. Polym.Sci. 5-20 txm. The distribution of particles to 13, 1741 (1969). water phase decreased with decreasing surface 15. Garbsva, S., Contreras, S., and Goldfarb, J., Colloid free energy of particles. The nondispersive Polym. Sci. 256, 241 (1978). surface free energy, 3'~, was mainly respon- 16. Chwastiak, S., J. Colloid Interface Sci. 42, 298 (1973). sible. The effect of 3'~ on particle transfer 17. Barrow, G. M., "Physical Chemistry," 3rd ed. could not be studied because 3'~ remained McGraw-Hill, New York, 1973. essentially the same before and after surface 18. Wu, S., "Polymer Interface and Adhesion." Dekker, modification. New York, 1982. ACKNOWLEDGMENTS We thank Dr. Tomas W. Healy for many suggestions for developing model particles for liquid and liquidparticle transfer experiments. This work was supported by the National Science Foundation through Grant ENG-7918386.
19. Holland, H. H., Chem. Eng. 69 (19), 179 (1962). 20. Crowl, V. T., and Wooldrige, W. D. S., SCI Monograph, 25, 200 (1967). 21. Bruil, H. G., and van Aartsen, J. J., Colloid Polym. Sci. 252, 32 (1974). 22. Fukuoka, E., Kimura, S., and Yamazaki, M., Chem. Pharm. Bull. 29, 205 (1981).
Journal of Colloid and Interface Science, Vol. 106,No. 2, August1985
Received September 7, 1984; accepted December 6, 1984 The surface free energy of glass microbeads was controlled by varying the extent of reaction between t-butyldimethylchlorosilane with the surface silanols. This system was chosen so that the surface free energy of the particles could be varied without introducing an extraneous wetting agent. The wicking method was used to obtain the liquid-particle-air contact angles. Water-air contact angles on glass beads increased with increasing silanizing reaction. The dispersive and the nondispersive contributions of the surface free energy were calculated from the contact-angle data. Nondispersive contribution decreased with increasing silanizing reaction while dispersive component remained unchanged. These silanized glass beads were dispersed in the continuous oil phase and the extent of particle retention at the water-oil interface and distribution to the water phase were determined. The experimental data indicate that the surface free energy of particles is the controlling parameter determining the transfer of particles from the oil phase to the water phase. Particle size in the range 5-20 #m had no significant effect, either on the particles retained or distributed to the water droplets. © 1985AcademicPre*~inc. INTRODUCTION T h e ash content o f coal fed to coal liquefaction process is ultimately contained in the coal-derived liquid product. It is desirable that ash levels be reduced to decrease erosion a n d obtain better efficiency (1). T h u s the products o f coal liquefaction processes are either filtered or centrifuged to remove the ash (2). Because o f the fine sizes o f mineral particles and the high viscosities o f coalderived liquids, elevated temperatures and pressures are utilized for these separation processes (2). It has been shown that currently available separation processes are costly (4). A n added disadvantage is the loss o f liquid h y d r o c a r b o n products in the cake or slurry. Therefore, the need for an alternative, m o r e cost-effective separation process is well justified. ~This paper was presented at the 58th Colloid and Surface Science Symposium, ACS, Pittsburgh; 1984. 2 To whom all correspondence should be addressed.
H e n r y (3) proposed a new separation process which involved removal o f mineral matter by transfer f r o m the oil to an aqueous phase. T h e process mechanics is shown in Fig. 1. Coal-derived liquid containing ash is contacted with water by making a water/oil emulsion. Ash transfer from the oil to the water phase is p r o m o t e d by surfactant. Subsequently the emulsion is b r o k e n and separated into two phases giving an ash-free oil. The h y d r o c a r b o n losses will be m i n i m a l because the solubility o f oil in the aqueous phase is very low. Hence, this new separation process can potentially o v e r c o m e the operational problems and e c o n o m i c disadvantages o f conventional ash separation processes. This process has been studied by Prudich (4) using mineral particles extracted f r o m coal-derived liquid. It was reported that the distribution o f particles into the aqueous phase which was described by the distribution c o e f ~ c i e n t BE decreased with increasing mixing time. T h e result is shown in Fig. 2. A 377 0021-9797/85 $3.00 Copyright © 1985 by AcademicPress, Inc.
Journal of Colloid and Interface Science, Vol. 106, No. 2, August 1985
All rights of reproduction in any form reserved.
378
K U ET AL.
surface characteristics were fixed and did not change during the particle transfer process. Thus the anomalous artifacts of the real, and more complicated, system could be avoided. Silica in high percentages is found in coal mineral mineral ash and mineral particles (6); in addition, it matter@ matter has highly reactive hydroxyl group. The modification of the surface free energy of mineral glass plates by chemisorption of different matter organochlorosilanes has been studied by Menawat (7). The conventional sessile drop min~alQ matter method was used to measure both water-air and xylene-water contact angles on model ~ G . 1. P r ~ e s s m e c h a n i c s o f l i q u i d - l i q u i d - s o l i d s e ~ plate systems produced by reagents of differarationpr~s. ent concentrations. It was reported that tbutyldimethylchlorosilane could produce stapartial detergency model (1) based on the ble, reproducible, homogeneous surfaces fate of the physically adsorbed surfactant was which did not change their surface free enerproposed. The experimental result was ex- gies during particle transfer in a liquid-liquid plained as due to the partial removal of (xylene-water) system. This work extended the technique of procarbonaceous coating by surfactant. This is ducing stable model surface to particle sysshown in Fig. 3. However, there are some artifacts asso- tems. Model colloids which had a range of surface free energies and were stable in the ciated with his experimental system (4). water-oil system were developed. These 1. The physically adsorbed surfactant was model particle systems were then characterbeing transferred to the aqueous phase and ized by measuring contact angle using wicking adsorbed at the water-oil interface during method and used in the distribution experithe particle transfer process. ment to study the mechanism of the particle 2. Experimental error occurred due to dif- transfer process. ficulty in splitting the aqueous phase from the interface and the oil phase, and to heterogeneities in the mineral matter samples. 3. Some material balance error was attribo uted to organic material being dissolved from ~ 6o0 rpcn the particles into the oil phase. 4. Asphaltenes which coat the mineral particles were also shown (5) to form rigid films at the water-oil interface and to have different surface characteristics under different ~2 pH conditions. ~DROCARBON P~SE
In order to elucidate the mechanism of this particle removal process it is necessary to understand the fundamental effects of particle surface characteristics on the particle transfer process. This involved the use of a well-defined model particle system whose Journal of Colloid and lmerface Science, Vol. 106, No. 2, August 1985
~-E]. ~; ~ A ~ A ~ . A ' 10
_ _
80p~m 4opt
' ¢o 20 TffF, EE PHASE MIXING TI?~E,MINUTES
D -A 4;
FIG. 2. Distribution coefficient, 82, versus three-phase mixing time, parametric in initial suffactant concentration.
PARTICLE
• .'. -
.'. -.'..'.-..'-. ~.-....-. :.-.." .'. - ". -...
inteatrface ~t~O~ ~
MINERAL
I I~
379
TRANSFER
SURFACE
:... ' . . . ' . . . . . - ... - .-...... - - -....'.-....-.
on ~surfacelI~II ~
= [ ~
-
intea:face
®
of surfactant in bulk
aggregatein bulk OILPHASE
WATERPHASE
FIG. 3. Phenomenologicalmodel of solubilizationdetergency.
EXPERIMENTAL
Preparation of Model Particle System The substrate used was sodalime glass microbeads (Potters Industries Co.). The particle size is within 5-20 #m. The beads were first washed with methylethylketone (Fisher) and rinsed with distilled water several times. Then they were washed in detergent and again rinsed with distilled water. Then the beads were placed in concentrated HNO3 and rinsed several times with distilled water. The beads were separated from the reagent by filtration at each step. An ultrasonic bath was used to enhance the cleaning effect. This substrate preparation step has been used by Menawat et al. (7) in cleaning the glass plates and has been performed in the removal of impurities from the surface using auger electron spectroscopy (7). All glassware used was cleaned by the same procedure. After cleaning, the beads and a cleaned magnetic stirrer were put into the reaction vessel and outgassed at 115°C and 10 - 3 m m Hg for 24 h to remove the physically adsorbed water. After outgassing, 50 ml of n-hexane
(Fisher) was transferred into the reaction vessel via a funnel without breaking the vacuum and a liquid seal was maintained. The magnetic stirrer was used to break up any particle lumps and keep the particles in suspension. The vessel was then placed in the ultrasonic bath to ensure the proper wetting in hexane. Solutions of t-butyldimethylchlorosilane were prepared in 100 ml of n-hexane. The solution was then transferred to the reaction vessel without breaking the vacuum. The reaction was carried out for 24 h and the contents were stirred by the magnetic stirrer during the reaction. After the reaction the beads were rinsed 3 times with n-hexane and outgassed at 115°C and 10 - 3 m m Hg for 4 h.
Characterization of Model Particle System The method based on the wicking of liquids into a packed column of particles was used to characterize the surface of the model particle system. This method has been used to measure contact angles of powder and fiber (15, 16) by previous workers. Journal of Colloid and Interface Science, Vol. 106, No. 2, August 1985
380
KU ET AL.
The packed column of particles can be considered to consist of a bundle of capillaries of mean effective radius R. The mean effective radius is constant for a given packing of particles. Thus applying the Washburn equation (8) which describes the penetration of liquid into a small capillary to this system yields h2
3"R cos 0 - - - t 2#
[1]
where h is the wicking height, t is the wicking time, 0 is the advancing contact angle, 3' is the surface tension, and ~ is the viscosity of the liquid. The Washburn equation is a simplification of the more general WashburnRideal equation which includes the hydrostatic pressure term. The hydrostatic pressure term is negligible compared to the capillary pressure term for the wicking of liquid into a bed of 5- to 20-tzm particles which was the case for our experiments. A magnitude analysis indicated that the maximum gravitational force for this study was only 0.18% of the capillary force. Also this can be verified by wicking data which would only produce a linear h2-vs-t curve if gravitational effects are negligible. Crow and Woodridge (20), Chwastiak (16), and others (15, 21, 22) have also found it appropriate to neglect the hydrostatic pressure term in the Washburn-Rideal equation. The value of R can be calculated if a liquid is chosen for which 0 = 0 °. Since the particle size is not changed by the surface reaction, once R is known, the contact angles of subsequent batches of modified particles, for other liquids, can be determined from wicking results. A known weight of particles was placed in a glass tube of 0.7-cm i.d. The lower end of the tube was closed with a glass flit. The tube was filled to the same height with the same amount of particles to ensure a constant packing. Gradual addition of small amounts of particles and tapping with a glass rod in the tube maintained a constant packing. ReJournal of Colloid and Interface Science, Vol. 106, No. 2, August 1985
producibility of the experimental data was found to be dependent mainly on proper packing. The experimental set up is shown in Fig. 4. The packed column was placed vertically in the liquid and the wicking height (h) was measured as a function of time. For the calibration of R, particles in the packed column were equilibrated with the saturated vapor of each wicking liquid to avoid the additional driving force due to the reduction in free energy of the solid on becoming covered by the adsorbed films as recommended by Good (9). All experiments were carried out at room temperature (20°C).
The Distribution Experiment of Model Particle System Model particle systems which had welldefined surface characteristics were used in the distribution experiments to study the effect of the surface characteristics of particles on the particle transfer process in a liquidliquid system. Xylene (Fisher, certified ACS grade) was used as the model hydrocarbon phase and deionized triple distilled water was used as the aqueous phase. Model particles (0.2 g) were dispersed in 100 ml of xylene phase using an ultrasonic bath. A 300-ml cylindrical Teflon autoclave liner (Parr No. 762 HC) with i.d. = 2.5 in was used as the mixing vessel. The vessel containing the particle-oil dispersion was then moved to a mixer (Parr No. 4561). A 3.85-cm diameter turbine impeller with 4 fiat pitched blades provided vigorous mixing and
GLASS
SCALE
WICKING FRONT
--
TUBE
PARTICLES
GLASS
FILTER
LIQOID
FIG. 4. Experimental setup of wicking method.
381
PARTICLE TRANSFER
rapid liquid-liquid dispersion in the mixing vessel. The impeller was coated with a 0.4m m Teflon film. Teflon equipment was used because the particles tended to stick to glass. The impeller height from vessel bottom was equal to one impeller diameter according to the Standard Tank Configuration (19). A 50-ml water phase was introduced into this particle-oil dispersion while the system was being mixed. The time of mixing with the aqueous phase was called the three-phase mixing time. Upon completion of mixing, the oil/water/particle mixture was transferred into a 250-ml Teflon separatory funnel and allowed to separate into phases. The resolution times ranged from 2 to 10 s. Three fractions, the aqueous phase, the oil phase, and the interface, were then drawn from the funnel separately. These fractions were filtered through a Millipore filter (Millipore H A W P 047, 0.45 #m in pore size), dried, and weighed to determine the distribution of particles. The size distributions of particles before and after the distribution experiments were determined u s i n g a particle size analyzer (Microtrac 7991-3) which utilizes low-angle, forward scattering of light from a laser beam. The entire apparatus was washed with methylethylketone, detergent, nitric acid, and triple-distilled water and dried before each experiment. RESULTS AND DISCUSSION
Characterization of Model Particle System The mean effective radius of the packed column, R, was determined by conducting wicking experiments on cleaned glass beads with hydrocarbons which were known to spread on clean glass plates (contact angle - 0°). A plot of h 2 vs t is shown in Fig. 5. The wicking experiment was very reproducible under the same condition and the experimental error was within 3%. In all cases linear plots of h 2 vs t were obtained. This justified the assumption that the packing was
,0 ACETONE 0/0 O/
/~7 N-HEXANE
~z/~
o/
/ ~/" /0 V / ~It i0 z" ~II o- 7 / ~'~" o I ,.~ ~ I ~ ,~ o ' ~ / / . . < > o" ,,.~
~= o
o>V..,---o
CHLOROBENZENE N-OCTANE XYLENE ,-~EC~E
,,-
1~,3~:..o ._zx.:
I- tJ', ~
o .~S.[~
i
.
2
.
3
.
.
4
.
5
.
6
7
WICKING TIME,MIN. FIG. 5. Wicking results of liquids on clean substrate.
uniform. The values of R cos 0 were calculated from the slopes of the h2-vs-t plot and with the knowledge o f surface tensions and viscosities of the liquids. The physical properties of these liquids and the R cos 0 values are shown in Table I. Since these liquids have either zero or very low contact angles on clean glass surface, the values of cos 0 should be nearly 1. This was verified by the closeness of the values of the calculated R cos 0 for different liquids as shown in Table I. The maximum value, 3.73 X 10 -5 cm, was used as the mean effective radius of the packed column. The plot of h 2 vs t for water on model particle systems prepared using different concentrations of t-butyldimethylchlorosilane is shown in Fig. 6. The particle-water-air contact angles were determined from the slope of this plot. Increasing hydrophobicity indicated by the increasing water-air contact angle as shown in Fig. 6 was observed with increasing reagent concentration. This may be due to the replacement of more hydroxyl groups with increasing alkyl silanizing agents. Similar observation was made by Menawat et al. (7) on glass plates after reacting with silanizing agents. The hydroxyl groups on Journal o f Colloid and Interface Science,
Vol. 106, No. 2, August1985
382
KU ET AL. TABLE 1 The Results of the Wicking Experiments of Liquids on Clean Glass Beads at 20°C Liquid
3' (dyn/cm)
~u (g/cm-s)
Correlation coetficienff
Slope (cmX/s)
R cos 0 (cm)
n-Hexane n-Octane n-Decane Acetone Xylene Chlorobenzene
18.4 24.4 22.2 23.7 30.1 33.6
0.326 0.542 0.920 0.327 0.648 0.799
0.9995 0.9998 0.9980 0.9996 0.9990 0.9992
0.1004 0.0754 0.0461 0.1308 0.0803 0.0729
3.73 X 10-5 3.51 X 10-5 3.67 × 10-5 3.67 X 10-5 3.46 × 10-5 3.59 X 10-5
a Least-squares linear plot of h 2 vS I. the glass surface participate in the reaction with t-butyldimethylchlorosilane as shown in
dispersion force and other intermolecular forces are additive 7 = yd + 7p
CH3CH3
I
I
I
I
[4]
and the approximation solid-liquid interfacial tension from either Wu's equation (13)
- - S i s - - O H + C 1 - - S i - - C - - CH3 CH3CH3
~+~
~,2=~,+~2
~+~
[5]
CH3CH3
I
I
- - S i s - - O - - S i - - C - - CH3 + HC1.
I
[21
I
~l~=Vl+V2--2~--2~
CH3CH3
The surface free energy of solid surface can be calculated from contact-angle data using Young's equation (10) 7re
(6]
it can be shown by derivation (13, 14) that % is related to contact-angle data through either one of the following equations:
Surface Free Energy Calculation
"Ys = "YLvCOS 0 + "YSL +
or the Modified Fowkes equation (14)
[3]
1 + cos 0 4
~-y~ TL
1 + cos 0 YL
--
-y~_~g
y~_ + 3'~----~+ 3'~_+ ~
[7]
/~L -- ~ "
where 7re is the film pressure of the solid surface which is usually negligible on smooth, homogeneous solid surface with contact angle greater than zero (1 1), which is the case in this work. The surface tension of liquid, 3'LV, and the contact angle, 0, can be measured. Hence, if the solid-liquid interfacial tension, 3'SL is known, the surface free energy of solid surface, %, can be determined. However, since YSL cannot be measured experimentally, an estimation is needed in order to obtain the surface free energy of solid surface. With the Fowkes' assumption (12) that the Journal of Colloid and Interface Science, Vol. 106, No. 2, August 1985
2
[8] It should be noted that Eq. [5] uses the harmonic mean, (2~,~,z°)/(y~ + 3'~), to approximate the dispersive interracial interaction while Eq. [6] uses the geometric mean, (7~,2d) 1/2, to estimate the dispersive interfacial interaction. Theoretically, the former is applicable when the polarizabilities of the volu m e elements in the two phases are nearly equal while the latter is applicable when the electronic vibrational frequencies are nearly
PARTICLE TRANSFER SYMBOL
[]
CONCENTRATION (mg/ml)
CORRELATION COEFFICIENT
SLOPE (cm2/sec)
CONTACT ANGLE (degree)
0.001
0.9997
0.0903
49.7
0.004
0.9994
0.0466
69.1
0.04
0.9981
0.0153
82.2
WICKING LIQUID = WATER
~'
383
15].[Z]-[]
[7],.['7"IZ]" ]
..[].~9 o.D-a .v-v []---.v-v .
..~..V ~V'V ^
^
_A-A-A
o N/--~'~--'~-A-A-A-A - / ' x - / ' x - ' -
It
I 2
13
I 4
I 5
I 6
WICKING TIME, MIN.
FIG. 6. Wicking results of water on model particle systems.
equal (13). The nondispersive interracial interaction may include dipole-dipole, dipoleinduced dipole, and hydrogen bonding. Both the harmonic mean and the geometric mean for the nondispersive interfacial interaction should be regarded as empirical in nature, since they include all the interaction not accounted for by the dispersion term (18). The term (3,~3,~)1/2 is applicable to dipoledipole interaction. However, it has been shown that the dipole-induced dipole interaction predominates in liquids or liquid/solid systems. The dipole-induced dipole interaction should lead to the arithmetic mean (l 8). In order to use either Eq. [7] or Eq. [8], two liquids of known dispersive and nondispersive surface tensions and their contact angles on the solid surface need to be known. The two liquids used in this study were water and methylene iodide. The surface tension values of dispersive and nondispersive com-
ponents for the two liquids used in this study are listed on Table II (13, 14). The contact-angle data of these liquids on particle systems were determined using the wicking method. The results are shown in Table III. The surface free energies of model particle systems calculated using Eqs. [7] and [8] are listed in Table IV. Results calculated from TABLE II Dispersive ( ~ ) and Nondispersive ('r~) Components of the Surface Tensions of Liquids
Liquid
(1) a
(2) b
(1)
(2)
Water 22.1 21.8 50.7 51.0 Methylene iodide 44.1 49.5 6.7 1.3
"rt = 3'~. + "/~.
72.8 50.8
a (1) Determined from Wu's equation. b(2) Determined from modifiedFowkes's equation. Journal of Colloid and Interface Science, Vol. 106, No. 2, August 1985
384
KU ET AL. TABLE II1 The Contact Anglesof Model Particle Systems Model particle system
Correlation coefficient
Wicking slope (cm2/s)
Contact angle (deg)
Water
A B C D E
0.9992 0.9997 0.9993 0.9994 0.9800
0.1079 0.0903 0.0806 0.0466 0.0056
34.3 49.7 55.0 69.1 87.7
Methylene iodide
A B C D E
0.9997 0.9992 0.9996 0.9990 0.9991
0.0167 0.01234 0.0115 0.0085 0.0037
85.0 86.3 86.6 87.4 88.9
Liquid
both equations are of the same order of magnitude and show a similar trend. It is known (18) that both the harmonic mean and geometric mean equations give similar surface tension and components when contact angles are used for the calculation. However, very large differences are obtained when Eqs. [5] and [6] are used to calculate interfacial tensions. The harmonic mean equation has been shown to be more accurate (18). A plot of "y~ and 3,p calculated using Wu's equation vs water-air contact angle is shown in Fig. 7. It is interesting to note that the nondispersive surface free energy, %P, decreased with increasing contact angle (hydrophobicity) while the dispersive surface free energy, -y~,
remained almost constant. Thus, the London dispersion force contribution of the model surfaces did not change when the hydroxyl groups were gradually replaced by the alkyl groups. This is quite reasonable in view of the similar polarizability of water molecules and methane molecules (17). The decrease of the nondispersive surface free energy with increasing hydrophobicity can be explained as being due to the loss of the number of the polar hydroxyl groups which have strong hydrogen-bonding ability.
The Distribution Experiment Particles were either retained at the wateroil interface or distributed to the aqueous phase after the resolution of the emulsion. The a m o u n t of particles in the oil phase was negligible. However, during resolution, a small a m o u n t of particles was observed settling onto the water-oil interface; hence, some of the particles at the interface after resolution were originally in the oil phase before the resolution of the emulsion. No particles were observed settling from the interface into the aqueous phase after the resolution of the emulsion. To prevent the particles at the water-oil interface from being drawn out with the aqueous phase, only 90% of the aqueous phase was drawn from the separation funnel for analysis. The water-oil interface and the
TABLE IV Dispersive and Nondispersive Components of the Surface Free Energy Values for Silanized Glass Beads Model particle system
Ow (deg)
(I)a
(2~
(I)
(2)
(1)
(2)
A B C D E
34.3 49.7 55.0 69.1 87.7
9.5 9.3 9.4 9.6 10.1
7.3 7.7 8.0 8.8 9.9
55.9 43.5 39.0 27.9 15.7
56.9 43.3 38.1 24.7 10.6
65.4 52.8 48.4 37.5 25.8
64.2 51.0 46.1 33.5 20.5
a(1) Calculated using Wu's equation. b(2) Calculated using modified Fowkes'sequation. Journal of Colloid and Interface Science, Vol. 106, No. 2, August 1985
PARTICLE TRANSFER
385
remaining aqueous phase were then drawn from the separation funnel together for analA ysis. The mass of particles in each phase and Ys x[ ] . V YP the interface was then back-calculated from yPr:'-7 ~ [ ] Ys = yd + yp material balance. It has been experimentally verified that the particle concentration in the aqueous phase is constant during the period o f analysis (less than 10 s). The results of the distribution experiments using model particles of different surface free energies are shown in Fig. 8. The a m o u n t of particles retained at the oil-water interface at steady state decreased while the amount 2 YdA A-& & A o f particles distributed to the aqueous phase increased with increasing surface free energy i 30 ~o ~o 6~ ¢o ~o of the model particle systems. Operating variCONTACT ANGLE 0w , degree ables such as mixing speed, water/oil ratio, FIG. 7. Effect of water-air contact angle on surface and initial particle concentration were kept free energy ofmodel particle systems. constant for the present work. d
[]"D
YS' TOTAL
SURFACE
FREE ENERGY,
20
30
40
50
i
!
i
!
dyne/cm 60 l
70 !
WATER/OIL RATIO = 0 . 5 INITIAL P A R T I C L E C O N C E N T R A T I O N M I X I N G SPEED = 600 rpm
= O. 23 %
P A R T I C L E S IN W A T E R PHASE
| "
RETAINED PARTICLES
y
z
I
I P y , NON-DISPERSIVE SURFACE FREE F.NER~, d y n e / c m
FIG. 8. Effectof surface free energy on particle distribution in xylene-waterphases for silanized glass beads. Journal of Colloid and Interface Science, Vol. 106, No. 2, August 1985
386
KU ET AL.
An explanation for these results is that the particles with higher surface free energy have more hydroxyl groups (less alkyl groups) on the surface; hence, they have more affinity for the water phase. It is also interesting to note that the polar contribution to the surface free energy is the major parameter controlling the distribution of particles to the aqueous phase. The behavior of particles at the interface of two immiscible liquid phases is known to be related to the forces acting at the interface,
such as surface force, gravitational force, and buoyancy force. In order to determine the contribution of this mechanism on the distribution process, the size distribution of particles before and after the distribution experiment were measured using the Microtrac 7991-3 particle size analyzer. The results are shown on Fig. 9. No significant change of particle size distribution was observed. Therefore, the particle transfer process in the water/oil system does not depend on particle size in the range of 5-20 ~tm.
ORIGINAL
--
--s
- ~
WATER
--
PHASE
RETAINED
m . _ _
It.
u
E
g
A 5
! I0 PARTICLE
!
I
l
15
20
25
SIZE,
~m
FIG. 9. Particle size distribution data before and after particle transfer experiments. Journal of Colloid and Interface Science, Vol. 106, No. 2, August 1985
387
PARTICLE TRANSFER CONCLUSIONS
REFERENCES
The surface characteristics of model particle 1. Prudich, M. E., and Henry, J. D., Jr., AIChE J. 24, systems were controlled by reacting the sur788 (1978). face with t-butyldimethylchlorosilane. The 2. Henry, J. D., Jr., and Patel, B. S., Interim Report, prepared for the Office of Coal Liquefaction liquid-air contact angles on these model Study on August 14, 1974. particles were obtained from the wicking 3. Henry, J. D., Jr., Research proposal submitted to method. Increasing hydrophobicity was obU.S. Bureau of Mines on April 30, 1974. served with increasing reagent concentration 4. Prudich, M. E., Ph.D. dissertation, West Virginia as indicated by increasing contact angles. University, Morgantown, 1979. 5. Bartell, F. E., and Neiderhauser, D. O., in "API The surface free energies of model particle Drilling and Production Practices," The University systems were calculated from the liquidof Michigan, Ann Arbor, 1949. particle-air contact-angle data obtained from 6. Menawat, A., M.S. thesis, West Virginia University, the wicking method. The dispersive surface Morgantown, 1982. free energy did not change while the nondis- 7. Menawat, A., Henry, J. D., Jr., and Siriwardane, R., J. Colloid Interface Sci. 101, 110 (1984). persive surface free energy decreased signifi8. Washburn, E. W., Phys. Rev. 1, 273 (1921). cantly with increasing replacement of hy9. Good, R. J., £ Colloid Interface Sci. 42, 473 (1973). droxyl groups by alkyl groups. 10. Young, T., "Miscellaneous Works" ((3. Peacock, The distribution experiments of particles Ed.), Vol. 1. Murray, London, 1855. in two liquid phases indicated that the particle I 1. Good, R. J., J. Colloid Interface Sci. 52, 308 (1975). transfer process was not due to body force 12. Fowkes, F. M., Ind. Eng. Chem. 56, 40 (1964). difference for particles in the size range of 13. Wu, S., J. Polym. Sci. C34, 19 (1971). 14. Owens, D. K., and Wendt, R. C., J. Appl. Polym.Sci. 5-20 txm. The distribution of particles to 13, 1741 (1969). water phase decreased with decreasing surface 15. Garbsva, S., Contreras, S., and Goldfarb, J., Colloid free energy of particles. The nondispersive Polym. Sci. 256, 241 (1978). surface free energy, 3'~, was mainly respon- 16. Chwastiak, S., J. Colloid Interface Sci. 42, 298 (1973). sible. The effect of 3'~ on particle transfer 17. Barrow, G. M., "Physical Chemistry," 3rd ed. could not be studied because 3'~ remained McGraw-Hill, New York, 1973. essentially the same before and after surface 18. Wu, S., "Polymer Interface and Adhesion." Dekker, modification. New York, 1982. ACKNOWLEDGMENTS We thank Dr. Tomas W. Healy for many suggestions for developing model particles for liquid and liquidparticle transfer experiments. This work was supported by the National Science Foundation through Grant ENG-7918386.
19. Holland, H. H., Chem. Eng. 69 (19), 179 (1962). 20. Crowl, V. T., and Wooldrige, W. D. S., SCI Monograph, 25, 200 (1967). 21. Bruil, H. G., and van Aartsen, J. J., Colloid Polym. Sci. 252, 32 (1974). 22. Fukuoka, E., Kimura, S., and Yamazaki, M., Chem. Pharm. Bull. 29, 205 (1981).
Journal of Colloid and Interface Science, Vol. 106,No. 2, August1985