Langmuir 1995,11, 3648-3655
3648
Stability of Monodisperse Zinc Sulfide Colloidal Dispersions J. D. G. Duran, M. C. Guindo, A. V. Delgado, and F. Gonzalez-Caballero” Departamento de Fisica Aplicada, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain Received August 10, 1994. I n Final Form: June 12, 1995@ The stability of monodisperse, spherical colloidal particles of zinc sulfide, in the presence of NaCl and CaCl2 solutions, has been studied in this work. The so-called extended DLVO theory of stability is used to explain the data. In this model, it is proposed that Lewis acid-base (AB) interactions have to be considered for better explainingthe stability of ZnS colloidal dispersions. Theoretical interaction energydistance curves are computed and compared t o experimental determinations of the stability of the suspensions,obtained from time evolutionofboth their optical absorbance and particle diameter. Previously, the ( potential of the particles and their surface free-energy components were determined as a function of electrolyte concentration,using, respectively,electrophoreticmobility measurements and the thin-layer wicking method. The effect of NaCl concentration on the 5; potential of the particles is typical of indifferent electrolytes, whereas Ca2+cations appear to specifically interact with the ZnS surface. The stability of M, whereas higher concentrations seem t o stabilize the suspensions is lowest for concentrations around the suspensions. After calculation of the surface free-energy components of the particles, potential energy ofinteraction curves are computedfor different interparticle distances. A comparisonis carried out between the predictions of both classical and extended DLVO models and experimental stability data. A good qualitative agreement between theoretical and experimental results is found when the latter model is used. The inclusion of (Lewis)acid-base interactionsbetween the particles is thus a useful tool t o adequately describe the stability of ZnS suspensions. The results support the previous findings (van Oss, C. J.;et al. Clays Clay Min. 1990,38, 151) on the suitability of adding acid-base (Lewis)forces t o electrostatic and Lifshitz-van der Waals forces to have a powerful theory capable of predicting many aspects ofthe behavior of colloidal suspensions.
Introduction Zinc sulfide is a n inorganic compound, widely used because of its numerous technological applications, including water purification, pigment fabrication,’ and manufacturing of electroluminescent and cathodoluminescent panels. Furthermore, its semiconductor properties make zinc sulfide a typical component of solar cells. Some of those uses require the adhesion ofZnS on different substrates (usually glass or other semiconductors like Si, Ge, or GaAs). This explains the recent interest2 in the investigation of chemical bath deposition procedures to attain the adhesion of zinc sulfide on the desired substrate. Whatever the deposition method used, it is the physicochemical interactions between zinc sulfide particles or between the particles and the substrate that determine the extent and stability of the adhesion. In previous works, we have undertaken the study of the surface properties of ZnS in connection with the determination of the characteristic interactions of the zinc sulfide-solution interface, both of an electrostatic3 and a nonelectrostatic n a t ~ r e The . ~ whole analysis was carried out using monodisperse, spherical, colloidal particles of zinc ~ u l f i d e . ~ In this work, we intend to analyze the colloidal stability of such particles in the presence of NaCl and CaC12 solutions, as well as t o explain the observed behavior, taking into account the different contributions to the Abstract published in Advance A C S Abstracts, September 1, 1995. (1)Parker, D. H. Principles of Surface Coating Technology; Interscience: New York, 1965; p 79. (2) Nicolau, Y. F.; Dupuy, M. J . Electrochem. SOC.1991,137, 2915. (3)Duran, J. D. G.; Guindo, M. C.;Delgado, A. V.Prog. Colloid Polym. Sei. 1993,93, 215. (4) Duran,J . D. G.; Delgado,A.V.; Chibowski, E.; Gonzalez-Caballero, F.Mater. Chem. Phys. 1994,38, 49. (5) Wilhelmy, D. M.; Matijevic, E. J. Chem. Soc., Faraday Trans. 1 1984,80, 563. @
energy of interaction between them. The physical basis for this study is that of the so-called “extended DLVO theory”, where not only Lifshitz-van der Waals (LW) attraction and electrostatic repulsion (EL) are taken into account but also the structural o r solvation forces.6 In this paper, we will follow the treatment proposed by van Oss, Good et al.7-9for the estimation of the latter type of interactions; i.e., we will assume that they are a manifestation of the existence of Lewis acid-base (or electron donor-electron acceptor) contributions to the surface free energy of the particles. Thus, following van Oss et al.’s proposal,* we will consider that the total energy of interaction (V) between the particles consists of three components: electrostatic (EL), Lifshitz-van der Waals (LW), and acid-base (AB).
v = VEL+ VLW+ vAB
(1) It is the variation of each term of eq 1with the distance H between the particle surfaces that determines the stability of the system. The electrostatic term has been well established by the classical DLVO theory;1° assuming constant, moderate surface potentials, a reasonable expression is
VEL = ~
X E , E ~ IU n [~l +J e~ x~
p ( - ~ ~ ) ~ (2) where a is the radius of the spherical particles, K is the reciprocal of the Debye length, cr is the dielectric constant (6) Pashley, R. M. In Colloid Chemistry in Mineral Processing; Laskowski, J . S., Ralston, J.,Eds.; Elsevier: Amsterdam, 1992;pp 97114. ( 7 ) van Oss, C. J.; Chaudhury, M. K.; Good, R. J. Chem. Reu. 1988,
88. - - , 927. (8) van Oss, C. J.; Giese, R. F.; Constanzo, P. M. Clays Clay Min.
1990,38, 151. (9) Good, R. J. In Contact Angle, Wettability and Adhesion; Mittal, K. L., Ed.; VSP: Utrecht, 1993; pp 3-36. Good, R. J. J . Adhesion Sci. Technol. 1992,6 , 1269. (10)Hunter, R. J.FoundationsofColZoidScience;Clarendon: Oxford,
1987
0743-746319512411-3648$09.00/0 0 1995 American Chemical Society
Stability of Monodisperse ZS Colloidal Dispersions
Langmuir, Vol. 11, No. 10, 1995 3649
of the medium, EO is the permittivity of a vacuum, and vo is a surface electric potential that many authors assume to be equal to the electrokinetic or 5 potentials. The Lifshitz-van der Waals attractive energy of interaction between the same spheres can be computed fromll
+ “1
‘a2 (2a
+ W 2+ I n H(4a (2a + W 2 (3)
the constant A being the well-known Hamaker constant, dependent on the characteristics of both the particles and the liquid medium. Typical values of A range between and J for a large number of colloidal systems. However, its determination is not free from uncertainty. It has been demonstrated, even experimentally,12J4that interfacial interactions ofthe Lifshitz-van der Waals type are responsible for the existence of the LW component of the total energy of interaction. Hence, there must be some relationship between the value of A for a given disperse system and the LW contribution to the total interfacial energy of the particles. In fact, it has been shown8 that (4) where the parameter HO is the so-called equilibrium separation distance between interfaces. According to van Oss et al.,7the most reasonable value estimated for Ho is 1.58 f 0.08 A, as deduced from the application of eq 4 to a large number of systems, including gases like helium, metals, and many different colloidal suspensions, y;:, the LW contribution to the interfacial tension of the zinc sulfide (material l)/solution (material 2) system, can be written15J6in terms of the LW contributions to the surface free energy of the individual materials, both experimentally accessible:
positive in the case of structural repulsion (hydrophilic surfaces) and negative in the case of structural attraction (hydrophobicsurfaces). The parameter A is the correlation length of the water molecules. If water molecules were not hydrogen bonded to each other, the theoretical value for ilwould be close to 2 A.23 But it is admitted that about 10% of water molecules are engaged in hydrogen bonds, so il 1nm would be a more reasonable figure.8 Other authors have estimated similar values for ilin a number of hydrophilic surfaces,22 and the range 0.5-1.1 nm includes all estimations. In this work, we have chosen il = 1nm, although the exact knowledge of ilas a measure of structural forces is still a n open problem. The fact that such forces are manifested mainly a t short distances to the particle-solution interface suggests that they must have a n interfacial origin. According to van Oss et al.’s surface thermodynamic a p p r ~ a c hthey , ~ must be related to the polar (or Lewis acid-base) component of the interfacial tension between materials 1 and 2:
ZT-
YlY2)
(7)
where y: ( y l ) stands for the electron-acceptor (electrondonor) component of the surface free energy of material i. These quantities can also be experimentally determined, as will be described below. The expression for the AB term in eq 1 is, finally,
vAB = -2y:nail The factor
iHon7
exp -
(9) measures the strength of the acid-base interaction between particles of material 1immersed in material 2, a t the distance of equilibrium Ho. The higher its value, the stronger the repulsion (for AGEl (Ho) > 0) between the particles. In this work, we will use eqs 1-5, 8, and 9 to analyze the stability properties of ZnS suspensions for different electrolyte concentrations, by comparing the theoretical predictions (as energy-distance diagrams) with experimental determinations of the stability of the systems.
As mentioned above, there is a considerable body of experimental data that demonstrate that some colloidal systems do not fit into the classical DLVO scheme of colloidal ~ t a b i l i t y . ~ s ~The J ~most - ~ ~ plausible explanation for this fact lies on the reasonable supposition that the presence of a surface may alter the nature of a fluid in contact with it, at least in the regions close to the surface. As a consequence, forces (solvation forces) may appear between colloidal particles, not considered in the DLVO Experimental Section approach. Such forces have been experimentally measured with the surface force apparatus (SFA) developed Materials. The zinc sulfide spherical particles were prepared following the method described by Wilhelmy and M a t i j e ~ i cTEM .~ by Israelachvili and his g r o ~ p . Practically ~!~~ all authors micrographs showed them to be spherical and considerably have used a n empirical exponential function of distance for the structural forces between p a r t i c l e ~ : ~ , ~ J ~ , ~ ~ monodisperse (average diameter 320 i20 nm). TheX-ray powder
F?H, = Fos exp(-H/A)
(6)
where FoS reflects the wettability of the surface; i.e., it is ~~
~~
~
~~
~
(11)Gregory, J. J . Colloid Interface Sci. 1981,83,138. (12)Christenson, H. K.;Claesson, P. M.; Berg, J.; Herder, P. C. J . Phys. Chem. 1989,93,1472. (13)Yaminsky, V. W.; Yushchenko, V. S.; Amelina, E. A.; Shchukin, E. D. J . Colloid Interface Sci. 1983,96,301. (14)Yushchenko, V. S.;Yaminsky, V. W.; Shchukin, E. D. J . Colloid Interface Sci. 1983,96, 307. (15)Fowkes, F. M. J . Phys. Chem. 1963,67,2538. (16)Good, R.J.;Girifalco, L. A. J. Phys. Chem. 1960,64,561. (17)Allen, L.H.;Matijevic, E. J . Colloid Interface Sci. 1969,31,31. (18)Skvarla, J.;b e t , S. Int. J . Min. Process. 1991,32, 111. (19)Yotsumoto, H.; Yoon, R. H. J . Colloid Interface Sci. 1993,157, 426. (20)During the time of refereeing of this work, an interesting paper
was published (Wu, W.; Giese, R. F.; van Oss, C. J. Colloids Surf. A: dealing with the interpretation Physicochem. Eng. Aspects 1994,89,241) of the stability of charged polar surfaces (clays, glass, and calcite) in aqueous suspensions, in terms of EL, LW, and AB interactions. (21)Israelachvili, J.Acc. Chem. Res. 1987,20, 415.
diffractogram of the particles corresponded to pure sphalerite. The specific surface area of the solids was 43.7 m2/g, as deduced from the BET method using a Quantasorb J r . apparatus (Quantachrome). The chemicals used in the preparation of the particles and the suspensions, and in the measurement of the surface free energy of ZnS, were supplied by either Merck or Carlo Erba with analytical quality. They were used as received, without further purification, except in the case of the thioacetamide used in the particle synthesis, which was recrystallized from spectroscopic quality benzene. The water used in the preparation of the suspensions was twice distilled, deionized, and filtered (Milli-Q reagent water system, Millipore). The mother dispersions were kept in the dark in refrigerated polyethylene bottles. Their pH was 5-5.5. (22)Israelachvili, J. Intermolecular Surface Forces, 2nd ed.; Academic: New York, 1992;p 277. (23)Chan, D: Y. C.; Mitchell, D. J.; Ninham, B. W.; Pailthorpe, B. A. In Water; Franks, F., Ed.; Plenum: New York, 1979;Vol. 6,pp 239279.
D u r a n et al.
3650 Langmuir, Vol. 11, No. 10, 1995
0. In this work, eq 11allows the determination ofthe solid-surface free-energy components if it is correctly applied and interpreted for the various experimental conditions considered in the wicking processes. In this approach, four wicking systems can be distinguished in which different values of AG in eq 11 appear: (i) If the liquid used wets the solid completely and the solid h a s been saturated w i t h its vapor in such a way that a duplex film is formed onto the solid (precontacted plate), then AG in the wicking process equals y2 and eq 11 reads (12)
and this form of Washburn's equation can be used to estimate R if the surface tension, y 2 , and the viscosity of the liquid are known. Usually, the liquids best preferred for this purpose are n-alkanes. (ii) When the same liquid as above has penetrated through a porous thin layer of a bare (not precontacted) plate, of the solid, eq 11 is valid, with AG given by32
with W, and W, being the work of adhesion of the liquid to the solid surface and the work of liquid cohesion, respectively. If a nonpolar liquid ( y z = y ; = 0; this is true for n-decane, for instance) is used, eqs 12 and 13 allow us to obtain ytw. In order to obtain y: and y;, two bipolar liquids7 partially wetting the solid (i.e., forming a non-zero static contact angle with it) must be used (water or aqueous electrolyte solutions and formamide). Thus, in systems iii and iv, the wicking experiments were carried out with a bipolar liquid on both bare and precontacted plates, formed by porous thin layers of the same solid under study. Thus, AGbareand AG,,, were obtained in the separate wicking experiments (under the two above experimental conditions) from a n equation of type (111, and the following equation applies:32
describes the process correctly only in the case of a liquid completely wetting the solid surface (contact angle 6 = 01, and the duplex film of the liquid has been formed on the surface or AGbare - AGpre = + + - 2Y2 is formed well ahead of the penetrating front of the l i q ~ i d . ~ * , ~ ~ (14) However, in such cases, the solid surface free energy has no The experimental procedure was as follows. Microscope glass influence on the wicking rate of the liquid, as seen from eq 10. slides covered by a thin layer ofZnS were prepared by spreading However, if no duplex film is present ahead of the penetrating front, or if the wicking liquid forms a defined contact angle, then 2 mL of 50 g/L suspensions and drying them for 24 h a t room eq 10 does not describe the x 2 =fit) function, because additional temperature. Then they were either oven-dried at 60 "C for 2 h a n d stored in a desiccator (bare plates) or contacted for 6 h with changes of the free energy accompanying the penetration process the vapor of the liquid to be usedin the wicking experiments. For can take the determination of the surface free-energy components of ZnS van Oss et al.25326proposed a method for the determination of with adsorbed ions from solutions ofNaCl or CaC12, such solutions the contact angle ofliquids on porous solids based on Washburn's were used instead of pure water in the estimation of &'bare and equation.27 The method, called thin-layer wicking, allows the AGPre. The same plates, after being dried, were employed for determination of solid-surface free-energy components. Later, penetration with formamide. The maximum electrolyte conChibowski et al.30-32interpreted the wicking processes in terms centration that could be used in these experiments was 10-' M, of the surface and interface free-energy changes, and no contact since for higher concentrations penetration offormamide on plates angles have to be determined from the penetration rates of a pretreated with solution was impossible. The viscosity and liquid into a porous layer of a solid. In what follows, we will use surface tension of the solutions employed were taken from the the methodology for the thin-layer wicking technique as proposed literat~re.~~ by Chibowski and G~nzalez-Caballero.~~ Briefly, the most general description of the process is given by a generalized form of Washburn's equation: Results and Discussion
24m'&
(11)
where AG is the change in surface free energy associated to the process of replacement of a soliaair interface by a solidfliquid (24) O'Brien, R. W.; White, L. R. J. Chem. SOC.,Faraday Trans. 2 1978, 74, 1607. (25)van Oss, C. J.; Giese, R. F.; Li, Z.; Murphy, K.; Norris, J.; Chaudhury, M. K.; Good, R. J. Adhesion Sci. Technol. 1992, 6, 413. (26) Giese, R. F.; Constanzo, P. M.; van Oss, C. J. J.Phys. Chem. Min. 1991, 17, 611. (27) Washburn, E. W. Phys. Rev. 1921, 17, 273. (28) Good, R. J. J.Colloid Interface Sci. 1973, 42, 473. (29) Good, R. J.;Lin, N. Y. J. Colloid Interface Sci. 1976, 54, 52. (30) Chibowski, E.; Holysz, L. Langmuir 1992, 8, 710. (31) Chibowski, E. J.Adhesion Sci. Technol. 1992, 6, 1069. (32) Chibowski, E.; Gonzdlez-Caballero,F. Langmuir 1993,9, 330.
2m
6 Potential. As m e n t i o n e d above, a t h o r o u g h electrokinetic c h a r a c t e r i z a t i o n of the s y n t h e t i c ZnS spheres has b e e n c a r r i e d o u t e l ~ e w h e r e .The ~ isoelectric p o i n t of the particles w a s f o u n d t o be a r o u n d pH = 5.5, a value s o m e w h a t higher than that r e p o r t e d b y o t h e r a u t h o r s , probably due t o s u r f a c e oxidation and h y d r a t i o n reactions provoked b y the strongly acid and oxidizing solutions used in the synthesis.
The p o t e n t i a l of the p a r t i c l e s is s h o w n in Figure 1 as a function of the concentration of b o t h NaCl and CaClz at pH 4. Note, first of all, that 5 is practically zero for concentrations z 10-1 M of either electrolyte; strictly (33)Handbook ofchemistry and Physics, 61st ed.; CRC: Boca Raton, FL, 1980-1981; pp F-43, D-233.
Langmuir, Vol. 11, No. 10, 1995 3651
Stability of Monodisperse ZS Colloidal Dispersions 40
120 A
I
.
5
20
c
\
-E Len
._
-e
-5
-4
-3
-2
-1
0
2ot I
1
log C (MI
Figure 1.
