Characterization of particle wettability by the measurement of floatability

Characterization of Particle Wettability by the Measurement of Floatability ABRAHAM MARMUR,*

WEILIAM CHEN,t

AND G E O R G E

Z O G R A F I t '1

*Department of Chemical Engineering, Technion-IsraelInstitute of Technology, Haifa 32000, Israel; and ~School of Pharmacy, University of Wisconsin-Madison, Madison, Wisconsin 53706 Received August 26, 1985; accepted November 6, 1985 The wettability of particles has been characterized by floatability experiments with model systems of monosized spheres of various materials and surface coatings. Equilibrium floatability experiments have been employed, where each individual particle was tested for flotation on the interface of various aqueous ethanol or methanol solutions. Results are expressed in terms of the percentage of floating particles (out of 40 tested for each experiment) vs the concentration of the solution. The highest concentration of ethanol (methanol) at which all the particles float is termed the "total floating concentration" (TFC), and the lowest ethanol (methanol) concentration at which all the particles sink is called the "total sinking concentration" (TSC). The reasons for the difference between the TFC and TSC are discussed, and the general trends followedby these values are compared with the theoretical condition for floatability equilibrium. The values of the surface tension of the various solid surfaces, as calculated from the present data, closely resemble values of critical surface tension for flat surfaces. The TFC and TSC are shown to be a sensitive measure of variations in the surface properties of the particles. © 1986AcademicPress,Inc. INTRODUCTION Characterization o f solid surfaces in terms o f their wettability and surface energy is important for a variety o f products and processes. Most o f the surfaces hitherto studied have been sufficiently large to allow the direct measurem e n t o f contact angles. H o w e v e r the surfaces o f most particles are too small to a c c o m m o date this type o f measurement. Therefore, other approaches need to be taken toward the characterization o f surfaces o f particles. This problem has not been given sufficient attention, and only recently has some effort been m a d e toward the achievement o f this goal (1-6). A m e t h o d frequently used for the characterization o f solid surfaces is the evaluation o f the critical surface tension, 3'c, developed by Zisman and co-workers (7). In this method, the contact angle, 0, o f each o f a series o f liquids is measured on the solid surface to be studied; the curve o f cos 0 vs YL, the surface

LTo whom correspondence should be addressed.

tension o f the liquid, is then extrapolated to cos 0 = 1 (0 = 0) and the corresponding "~L defined as Yc- It is n o t always convenient to use a series o f different liquids, therefore the use o f solutions has been proposed a n d demonstrated (e.g. (8, 9)). T h e use o f solutions, however, introduces an additional aspect to the problem o f determining the surface energy: the relative adsorption o f solutes to the solidliquid and l i q u i d - v a p o r interfaces (9). This u n k n o w n parameter adds to the complexity o f the problem since the value o f 3'c depends on it. Therefore, reported 3'c values should be supplemented by some estimate o f the relative adsorption o f the solute to the various interfaces. If interpretation in terms o f ~'c is not sought, however, the wettability can be empirically characterized by the lowest concentration o f the less polar c o m p o n e n t in a binary solution, which leads to complete spreading (0 = 0) (10, 1 1). This concentration has been termed the "critical spreading c o n c e n t r a t i o n " (CSC). It has been shown (10, 11) that the CSC is m u c h m o r e sensitive than contact angle measurements in characterizing the wettability

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0021-9797/86 $3.00 Copyright © 1986 by Academic Press, Inc. All rights of reproduction in any form reserved.

Journal of Colloid and Interface Science, Vol. 113, No. 1, September 1986

PARTICLE W E T T A B I L I T Y C H A R A C T E R I Z A T I O N

115

TABLE I of flat solid surfaces and identifying differences due to various surface treatments. Types and Sizes of Beads Used in Floatability Studies Similar ideas have been tested with respect Nominal diameter, Measured to the characterization of surfaces of particles. Type #m radius, #in The few studies which have been reported can be divided into two categories. One approach Glass 75 36.3 + 2.0 100 51.6 + 2.9 uses "lc values, which are known for plane Glass 150 72.0 _ 3.3 surfaces of the same material, to determine a Glass Glass 250 125.0 + 6.5 suitable liquid for separation of particles by Glass 500 258.0 + 4.7 flotation (1-5). The other approach attempts PMMA-coated 150 74.1 + 3.6 to evaluate directly ~/c for particles (6), by ex- PMMA-coated 250 127.0 +_ 5.7 150 72.4 _+ 4.1 trapolating the curve of percentage recovery Silicone-coated 250 124.0 + 5.3 by flotation vs ~L to zero recovery, where it is Silicone-coated claimed that "YL '~C. AS will be shown below, Polystyrenedivinylbenzene 500 236.0 _+ 9.0 this procedure can be improved by taking into account the condition of equilibrium for floatability, which depends not only on the ble I. Particle densities were measured pycinterfacial tensions, but also on the sizes and nometrically at 25 °C. densities of the particles. Liquids. The liquids used were anhydrous Thus, the purposes of this paper are: (a) to methanol, 99.9% (Mallinkrodt), absolute present floatability data for well-defined beads ethanol (Midwest Solvent Co.), and triple-diswith various surface coatings on various tilled water. Surface tension was measured at aqueous solutions and (b) to use these data for 25°C using the Wilhelmy plate method. Liqthe surface characterization of the beads, tak- uid densities were determined pycnometrically ing into account the requirements for floating at 25°C. equilibrium. =

Methods EXPERIMENTAL

Materials Solids. The spherical glass beads and siliconized beads were obtained from the Cataphote Division, Ferro Corporation. The polymethylmethacrylate (PMMA)-coated beads were prepared by coating the glass beads in a Wurster Huidized Bed Coater (Coating Place, Inc., Verona, Wisc.) using a methylene chloride solution of PMMA (Elvacite 2041, DuPont) (12). The polystyrene-divinylbenzene spheres were used as received. The glass beads were cleaned in boiling concentrated nitric acid, followed by thorough rinsing with triple-distilled water and absolute alcohol, followed by drying. All beads were sieved to produce as narrow a size distribution as possible. Particle size distribution was determined microscopically for all solids. Values of average radius and standard deviation are given in Ta-

Floatability of beads. Individual spheres, each examined microscopically, were placed on the surface of a selected liquid maintained at 25°C in a 10-ml beaker. Whether the sphere sank or floated was noted. Consistency in delivering individual spheres to the surface was assured by placing a watchglass, with a hole drilled in its center, over the beaker at a small distance above the liquid surface. For each liquid mixture used, 40 individual beads were tested and the results reported as percentage floatability. A separate experiment with 40 more beads of the solid sample and the same liquid revealed that percentage floatability reported was reproducible within about +2%. RESULTS A N D DISCUSSION

Figures 1-5 summarize floatability data for the various surfaces, sphere sizes, and aqueous solutions in terms of the percentage of foating Journal of Colloid and Interface Science, Vol. 113, No. 1, September 1986

116

MARMUR, CHEN, AND ZOGRAFI I00 I00 i

90 90 80 o

o

80

•

70 TO

6O

@ o o

eo

o ~ LL 50

•

*E

LL

o

4O

P 40 #_

o

3O 20

50 o

o

20

o

o o

o

o

I

I

I

I

I

O

20

50

40

50

60

70

Volume

"- O

0 20

80

I 30

Percent E t h a n o l

I 4-0

I 50

I 60

I 70

o 80

90

t(30

Volume Percent E t h a n o l

FIG. 1. Floatability of glass beads on ethanol-water mixtures: (e) 36.3 #m, (O) 51.6/~m.

FIG. 3. Floatability of siliconized glass beads on ethanolwater mixtures: (@) 72.4 ~m, ( 0 ) 124/zm.

beads vs the volumetric percentage of ethanol (or methanol) in this solution. The volumetric percentages refer to the liquid volumes prior to mixing. All the figures show similar characteristic curves: below a certain ethanol (methanol) concentration, all the beads float; a transition region follows, where the percentage of sinking beads is increasing with the

ethanol (methanol) concentration; finally, above a certain concentration all the beads sink. The lowest ethanol (methanol) concentration, at which all the beads sink, is termed the "total sinking concentration" (TSC). The TSC ranges between 35 and 95% for ethanol solutions with the beads studied. Only two

I00

o

O

I00 '

O Q

90

90

80

80 7O

7O 0 •~

2

60

6o

~ o

o

50

b_

o

50

E

b_

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~ 40

13- 30

30 o

20

20

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i

I0

20 ' Volume

30 ' Percent

&

50 °

60

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I

70

A

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80

Ethanol

FIG. 2. Floatability of PMMA-coated glass beads on ethanol-water mixtures: (O) 74.1/Lm, (O) 127 #m. Journal of Colloid and Interface Science, Vol. t 13, N o . 1, S e p t e m b e r 1986

o

o

,5

'o

~o

"o

5o

~o

~o

Volume Percent Ethanol

FIG. 4. Floatability of 236-tzm polystyrene-divinylbenzene beads on ethanol-water mixtures.

PARTICLE WETTABILITY CHARACTERIZATION I00

•

0

©

•

90

sin(0/2) =

[(PS/PL

-

117

1)(R/V2"YL/aLg)l'925] 1/2"2 [1]

©

80

where g is the gravitational acceleration. The second equation, describing the relationship between the contact angle and the surface tensions of the liquid and solid, is (14)

7O 6o

u_ 5C

Ys = (1 + cos 0)2'yL/4

O o

[2]

40 © 50 20 I0

020

, 30

t 4O

50

60

70

80

9O

iO0

Volume Percent Methonol

FIG. 5. Floatabilityof beads on methanol-watermixtures: (O) 127-1zmPMMA,(O) 124-#m siliconized.

types of beads were studied with methanol, and for them the TSC values were 50 and 70%. The highest ethanol (methanol) concentration, at which all the beads float, can be correspondingly termed "total floating concentration" (TFC). Values of the TFC for ethanol solutions were in the range of 10-75%, while for methanol solutions they were 35 and 50%. The first issue which needs to be discussed concerns the reasons underlying the existence of the transition region, or in other words, why there is a difference between the TSC and the TFC. The reasons for variability in the floating behavior of the beads can be due to one of three factors, or their combination: the size distribution of the beads, variations in their surface energies, or contact angle hysteresis. The latter is associated with possible roughness and chemical heterogeneity of the particle. To determine which factor is most responsible for the existence of the transition region, the following analysis is carried out, varying one factor at a time, using the following two equations. The first equation (13) yields the equilibrium contact angle, which supports a sphere of radius R and density as floating on a liquid of surface tension ")/Land density aL:

where "rs is the surface tension of the solid. This equation is clearly oversimplified; however, its simplicity is useful for the present purpose, since only estimates will be made. Tables II and III summarize the experimental data for the TFC and TSC, respectively, and the corresponding 0 a n d "rs calculated from Eqs. [1] and [2]. The independent variables are the material and coating of the particle, its radius, and the type of solute in the aqueous solution. The surface tension of the liquid and its density refer to the TFC or the TSC. To analyze the possible effect of the size distribution of the particles on the difference between the TFC and TSC, the glass beads of 51.6 #m in radius will be taken as an example. The TSC for this system is 70% ethanol. This corresponds to 3'L = 26.7 dyn/ cm and PL = 0.875 g/cm 3. The contact angle calculated from Eq. [1] is 5.06 °, and 3's calculated from Eq. [2] is 26.6 dyn/cm. If it is assumed that all the glass beads have the same 7s, then it is possible to calculate the contact angle which corresponds to the TFC, using the above value for 7s. At the TFC, which is 20% ethanol ('rL = 41.7 dyn/cm), the contact angle calculated from Eq. [2] is 53.3°. Introducing this value into Eq. [1] yields a corresponding radius of 945 am, which is obviously much beyond any reasonable deviation of the radius from the average which was measured. Therefore, the difference between TSC and TFC, i.e., the existence of the transition region, cannot be attributed to small variations in the radius of the particle. It is interesting to note that previously reported curves of percentage recovery (4-6) are similar in appearance to the curves shown in this study, although the former apply to mixtures of particles ofwide Journal of Colloid and Interface Science, Vol.

113, No. 1, S e p t e m b e r 1986

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MARMUR, CHEN, AND ZOGRAFI TABLE II Analyses of Results at the Total Floating Concentration, TFC

Beads

Radius O~m)

Bead density, as (gem -3)

TFC (%)

3'r~ (dyn cm -l )

Liquid demity, th. (gem -:s)

O" (degrees)

3,sb (dyn em-I )

0.964 0.971 0.862 0.964 0.942 0.983 0.971

3.1 4.0 6.9 5.7 10.1 8.0 4.4

37.7 41.6 25.9 37.6 29.8 51.1 41.6

0.909 0.945

9.4 8.8

36.1 41.8

Ethanol-water Glass Glass Silieonized PMMA Silieonized PMMA PS

36.3 51.6 72.4 74.1 124.0 127.0 236.0

2.47 2.47 2.47 2.43 2.47 2.43 1.07

25 20 75 25 40 10 20

Siliconized PMMA

124.0 127.0

2.47 2.43

50 35

37.8 41.7 26.1 37.8 30.3 51.6 41.7

Methanol-water 36.6 42.3

a Calculated from Eq. [1] in text. b Calculated from Eq. [2] in text.

size distribution. This observation appears to be consistent with the above reasoning. To examine the effect of variations in the surface tension of the solid, assuming the radius to be the same for all beads, Eqs. [ 1] and [2] can be used to calculate the 3's at the TFC for the above-mentioned system. The contact angle corresponding to the TFC is calculated from Eq. [1] to be 4.0 °. Using Eq. [2], the resulting surface tension of the solid is 41.6

dyn/cm. Since such a deviation from the value of 26.6 dyn/em, which was calculated for the TSC, is not too large, variability in the surface energy of the solid appears to be a possible cause for the existence of the transition region. Contact angle hysteresis can also be responsible for the difference between the TFC and the TSC. Equation [ 1] shows that as the concentration of ethanol (methanol) increases, i.e., as 7L and P L decrease, a higher contact angle

TABLE III Analyses of Results at the Total Sinking Concentration, TSC Beads

Radius (Jan)

Bead density, Ps (gcm -3)

TSC (%)

'~L (dy- era-~)

Liquid density, PL (gcm -~)

0" (degrtes)

7sb (dyn cm-')

0.862 0.875 0.804 0.875 0.848 0.950 0.910

3.8 5.0 7.4 6.8 11.0 9.9 6.5

26.0 26.6 23.0 26.5 25.0 32.9 28.6

0.865 0.909

10.0 9.5

30.5 36.1

Ethanol-water Glass Glass Siliconized PMMA Silicoulzed PMMA PS

36.3 51.6 72.4 74.1 124.0 127.0 236.0

2.47 2.47 2.47 2.43 2.47 2.43 1.07

75 70 95 70 80 35 55

Siliconized PMMA

124.0 127.0

2.47 2.43

70 50

26.1 26.7 23.2 26.7 25.5 33.4 28.8

Methanol-water

a Calculated from Eq. [ 1 ] in text. b C a l ~ a t e d from Eq, [21 in text. Journal of Colloid and Interface~ience, VoL 113, No. 1, September 1986

31.0 36.6

PARTICLE

WETTABILITY

is required to support the floatability of a given sphere. For the example which is discussed above, the change from 20% ethanol to 70% corresponds to an increase of only about 1o in the contact angle. The range of stable contact angles associated wtih contact angle hysteresis is usually much larger than 1°. Thus, contact angle hysteresis is very likely to also be responsible for the difference between the TFC and the TSC. The above conclusions, which were based on data for glass beads, are consistent with the results shown in Tables II and III for other solids. The effects of the independent variables can now be analyzed using the data in Tables II and III. With reference to the size of the particles, it is worth noting that all data follow the trends which are expected according to Eq. [1]. Thus, for a given surface coating, both TFC and TSC decrease when the particle radius is increased. Since it is extremely difficult to independently measure the contact angle on the particle, further quantitative testing of the data by comparison to Eq. [ 1] is impossible. Therefore, the following discussion will assume that the data are compatible with

Eq. [1]. Another general observation is that all contact angles calculated for the TFC and TSC are relatively low, so that cos 0 is close to unity. Therefore, all calculated "/s values are close to the corresponding 3'L values. Since under these circumstances the solid-liquid interfacial tension should be very low (14), the extent of approximation introduced by the use of Eq. [2] should be small. However, it is very important to notice that the contact angles are not zero even for the TSC. Therefore, the determination of 3'c from flotation data by extrapolation to 0% recovery (6) is only approximate. By definition, ~c is determined at 0 = 0, and this condition can be achieved at the TSC (or zero recovery) only when the particle size approaches zero. It has been noted above for one example, and Tables II and III show it for all present data, that the differences between the contact angles which correspond to the TFC and TSC,

CHARACTERIZATION

1 19

respectively, are small. However, the corresponding differences in the values of 3~s, as calculated from Eq. [2], may be significant. For all the systems reported in Tables II and III, the calculated 3's values at the TFC are higher than at the TSC. The smallest difference occurs with the siliconized beads, and the largest difference with the PMMA-coated beads on ethanol solutions. Based on our previous conclusions regarding the effect of contact angle hysteresis on the differences between the TFC and TSC, it is possible that the larger differences in 3's with the PMMA-coated beads reflect a rougher surface or less uniform coating. Comparing the calculated 3's values with previously reported "Yc values (12, 15), it can be concluded that 3's which is calculated for the TSC is very close to the value obtained on fiat surfaces of the same composition. It is difficult to judge the values of 3's calculated for the uncoated glass beads, since the nature of this particular glass is unknown. In general, higher values of "Ys are expected for a clean glass surface, and, indeed, larger glass beads which were used in this study (not reported in the tables) all sank even in water. Therefore, the uncoated glass beads which are reported in the tables most probably had an adsorbed layer on their surface, which could not be removed by the cleaning procedure. The use of methanol solutions instead of ethanol solutions does not lead to any significantly different conclusions. In the case of the siliconized beads, the difference between the TFC and TSC is smaller for methanol solutions, but the opposite is true for the PMMAcoated beads. Thus, the present data cannot indicate any specific trend or preference that should be taken. A final comment, which is of practical importance, relates to the sensitivity of the measurements. As can be concluded from Tables II and III, the small differences between the contact angles and surface energies for the various surface coatings are associated with larger differences in the TFC or TSC. Thus, this method of characterization of the wettability of particles is more sensitive to the surJournal of Colloidand Interface Science, Vol. 113, No. 1, September 1986

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MARMUR, CHEN, AND ZOGRAFI

face energy than contact angle measurements, had they been possible for small particles. This observation agrees with similar observations on fiat surfaces (10, 1 1).

SUMMARY AND CONCLUSIONS 1. T h e feasibility o f characterization of wettability and surface energy of particles by measuring floatability has been d e m o n s t r a t e d for well-defined particles o f various sizes and surface properties. D a t a has been presented in terms of the highest concentration o f ethanol (methanol) at which all the particles float (TFC) and the lowest concentration of ethanol (methanol) at which all the particles sink (TSC). 2. Interpretation of such data must take into account the equilibrium condition for floating. In particular, it cannot be assumed that the contact angle is zero at zero recovery or at the TSC. 3. T h e T F C and T S C have been shown t o be sensitive to variations in the surface energy of the particles used in this study. 4. Values of'ys calculated at the TSC closely resemble values of~'c obtained for flat surfaces, using aqueous ethanol solutions.

Journal of Colloid and Interface Science, Vol. 113, No. 1, September 1986

REFERENCES 1. Parekh, B. K., and Appian, F. F., in "Recent Developments in Separation Science" (N. N. L~ Ed.), Vol. IV, pp. 107-113. CRC Press, West Palm Beach, Fla., 1978. 2. Hornsby, D. T., and Leja, J., Colloids Surf !, 425 (1980). 3. Hornsby, D. T., and Leja, J., Colloids Surf 7, 339 (1983). 4. Finch, J. A., and Smith, G. W., Canad. Metall. Q. 14, 47 (1975). 5. Kelebek, S., and Smith, G. W., Int. J. Miner. Process. 14, 275 (1985). 6. Yarar, B., and Kaoma, J., Colloids Surf. 11, 429 (1984). 7. Zisman, W. A., "Contact Angle, Wettability and Adhesion," pp. I-51. Adv. Chem. Set. 43. Amer. Chem. Soc., Washington, D.C., 1964. 8. Dann, J. R., J. Colloidlnterface Sci. 32, 321 (1970). 9. Pyter, R. A., Zografi, G., and Mukerjee, P., J. Colloid Interface Sci. 89, 144 (1982). 10. Marmur, A., Narkis, M., and Woogen, W., J. AppL Pol. Sci. 25, 11253 (1980). 11. Marmur, A., Pesach, D., Dodiuk, H., and Drori, L., in "Proceedings, 2nd Int. Conf. on Fouling and Cleaning in Food Processing." Madison, Wisc., 1985. 12. Yang, Y.-W., "Capillary Flow Phenomena as Related to Wettability in Porous Media," Ph.D. thesis, p. 70. University of Wisconsin-Madison, 1985. 13. Boucher, E. A., and Jones, T. G. J., J. Colloidlnterface Sci. 83, 645 (1981). 14. Adamson, A. W., "Physical Chemistry of Surfaces," 4th ed., p. 357. Wiley, New York, 1982. 15. Dann, J. R., J. Colloid Interface Sci. 32, 302 (1970).