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Surface free energy components of monodisperse zinc sulfide
MATERIALS CHE~VMTR~~~ND Materials
ELSEVIER
Chemistry
and Physics
Surface free energy components
38 (1994) 42-49
of monodisperse
zinc sulfide
J.D.G. Durban, A.V. Delgado and F. Gontilez-Caballero* Departamento de Fisica Aplicada, Facultad de Ciencias, Universidad de Granada, 18071 Granada (Spain)
E. Chibowski Department of Physical Chemistry, Faculty of Chemistry, Maria Curie-Sklodowska University, Lublin, 20-031 (Poland)
(Received July 12,1993; accepted December 10,1993)
Abstract Apolar, Lifshitz-van der Waals (y Lw) and polar, acid-base (Lewis) (y AB) surface free energy components, of monosized, spherical ZnS colloidal particles were experimentally determined using the thin-layer wicking technique. The results were compared with those obtained on a pure, commercial sample of zinc sulfide. The fact that the synthetized sample showed a very high electron-donor ( TV-)component suggested that some species, related in principle to surface oxidation of ZnS during the synthesis reaction, could be formed on the surface of the particles. To check for this possibility, the surface free energy components of pure and oxidized ZnS, and of zinc oxide and sulfate, were determined by both the contact angle and thin-layer wicking methods. The similarity between x components obtained with both methods for pure ZnS lead to the conclusion that both techniques give reliable results. The high y,value determined for synthetic ZnS particles cannot be explained simply by surface oxidation of the colloidal particles, since partially oxidized ZnS, or even zinc oxide or sulfate, gave very similar ys-,always lower than that found for synthetic spherical ZnS. The fact that the Lifshitz-van der Waals ( */sLw)component of the surface free energy is somewhat higher for the latter material is believed to be related to elemental sulfur present on the surface of the particles. The general compatibility of the results obtained by both methods suggests that the thin-layer wicking technique is useful to further characterize these or other systems, particularly if the examined material is sufficiently monodisperse.
Introduction Zinc sulfide is a widely studied inorganic compound mainly because of its numerous technological applications, that include pigment preparation, water purification or electroluminescent phosphors preparation. In fact, according to Schlam [l], it is the material that has been used both in cathodoluminescent and electroluminescent applications. Most of the special properties of ZnS are related to the surface properties of the bulk phase of this material or of its thin films [2]. For this reason, examination of the surface properties of ZnS is interesting not only by itself but also with practical applications in mind. It is well known that many phenomena occurring at the solid-liquid interface, such as wetting, printing, stability of colloidal suspensions, par*Author
to whom correspondence should be addressed.
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1994
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titles adhesion to solid substrates, etc., depend to a large extent on the kind and magnitude of the surface and interfacial free energies involved. However, the study of these properties, when dealing with the surface of sulfide, is difficulted by the complicated changes in the structure and composition of the surface layer of the solid. The changes can take place as a consequence of chemical interactions of water and oxygen with the particle surface [3,4]. Thus, different compounds, ranging from hydroxides, through sulfates, thiosulfates, to elemental sulphur, can form onto the sulfide surface [3-61. These compounds should influence forces interacting on the sulfide surface, and lead to changes in the surface free energy components of the sulfide. In an earlier work [7], we have analyzed some surface electric properties of ZnS by determining the electrophoretic mobility of its aqueous suspensions in different conditions, particularly for various states of surface
J.D.G.
Durdn
et al. I Materials
oxidation. Use was made of the recent development of highly pure, spherical, zinc sulfide colloidal particles, exceedingly homogeneous in size and shape [8]. Particles thus prepared are most adequate to be used as a model system for adhesion studies or for testing stability conditions of dispersed systems. Although there is a number of studies in the literature dealing with the surface free energy of sulfide (minerals) [5,9-lo], they do not, however, treat directly the components of the surface free energy of zinc sulfide and their changes under the influence of oxidation processes. Therefore, in this paper, interest will be focussed on the determination of Lifshitz-van der Waals and acid-base (in the sense of Lewis) contributions to the total surface free energy of this solid. It is our aim to contribute new data on these quantities that, together with the electric characterization previously carried out [7, 111, may give a rather complete picture of the surface properties of zinc sulfide.
Theory
Only a brief review of some equations employed in the calculation of the surface free energy components of ZnS will be given in this paragraph. The reader is referred to refs. [12-141 where a detailed description of the van Oss, Good and coworkers’ approach to the surface free energy formulation of solids can be found. According to these authors, the total surface free energy xToTof a solid (or the surface tension of a liquid) can be expressed as the sum of two components:
xTOT=
xjr.W+
XAH,
where xLw1sthe apolar or Lifshitz-van der Waals component, and xA”is the acid-base contribution, which in many cases is due to hydrogen bonding. It has been proposed that xAHdepends on the electron donor (Lewis base) and electron aceptor (Lewis acid) properties of the phase i (x- and x’, respectively) as follows:
In the case of two phases i and j the total interaction can be expressed as [12-141:
When a phase 1 (solid or immiscible liquid) is dispersed in a polar liquid 2, according to these authors [1214], the free energy of interaction of two identical entities 1 (particles, molecules, etc), immersed in the polar liquid 2 can be calculated from the equation:
Chemistry
4249
43
-4(,!77~+~-Jg-3-Jj3F)
(4)
and Physics
38 (1994)
where the symbols are as defined above. From eq.(3), it is seen that three liquids of known surface tension components are needed and thus three equations of the type (3) must be solved simultaneously to determine the three unknown components of the surface free energy of a given solid, i.e., yb+,ySand ysLw. For this purpose, at least two techniques can be used: contact angle measurements [15], and the thin-layer wicking method, based on Washburn’s equation [15]. The later technique was very recently proposed by van Oss et al. [ 161and further elaborated upon by Chibowski etnl. [17-191. In the case of contact angle method, the starting point is Young’s equation written as:
where yL is the surface tension of a liquid forming a contact angle 0 on the solid. It should be stressed here that no film pressure behind the drop of the liquid used is taken into account in this equation. If 8 is measured for three liquids (two polar and one nonpolar) the three can be obtained. We have unknowns, ysLu’,ys+ and ysys’ used the following triads of liquids: water-formamidediiodomethane and water-formamide-a-bromonaphtalene. The thin-layer wicking method [16] is based on the measurement of the time (t) that a liquid takes to penetrate a distance (x) through a thin porous layer of a solid. The most general description of the process is based on a modified version of Washburn’s equation [17-19,20-211:
where AG is the change in surface free energy characteristic of the substitution of the solid- air interface by the solid-liquid interface in the wicking process. According to equation (6), at a constant temperature, for a liquid having viscosity n the square of penetrated distance should be a linear function of the time. In eq.(6), R is an effective radius of the capillary network forming the thin-layer of the solid. Chibowski et al. [17-191 have proposed an experimental methodology for the determination of the solid surface free energy components. The authors distinguished four wicking systems in which different values of AG appear. If the used liquid wets the solid completely, and the solid has been saturated with its vapour, in such a way that its duplex film is formed onto the surface (precontacted plate), eq.(6) reads [19]: 1 Kt .y- = 2 i
yL’
44
J.D.G. Durcin et al. I Maters
and this form of Washburn’s equation can be used to estimate R, if the surface tension ye and the viscosity of the liquid are known. Usually, the liquids best preferred for this purpose are n-alkanes, like n-octane or n-decane. When the same liquid is used on a bare plate, of the same solid, equation (6) is valid [19] with AaGgiven by: AG=2,/sw-2y,
(8)
If a non-polar liquid (y~q~~‘“) is used, eqs.(6) and (8) can be used to determine ysLw.We have employed n-decane in our experiments. In order to estimate y,’ and y;, two bipolar [12-141 liquids partially wetting the solid (i.e., forming anon-zero static contact angle with it) must be used. The experiments were carried out on both bare and precontacted surfaces and the following equation applies 1191: AGba, -AC,,
Where AGbare and AC,,, were obtained in separate wicking experiments (under the two above experimental conditions) from an equation of type (6). We have used water and formamide, and solved the resulting system of two equations to obtain ys+and ys-,provided ye, nLW, yL+and y,_-were known.
The synthesis of homogeneous in size, spherical ZnS particles (diameter 320t20 nm) was carried out as described in [8] and details of the preparation of its suspensions are given in [7]. Ail chemicals used were of anlytical quality, including ZnS (Merck), Hz02 (Panreac), ZnO (Probus), ZnS04.H20 (Panreac), diiodomethane (Merck), a-bromonaphtalene (Carlo Erba), formamide (Carlo Erba) and n-decane (Sigma). Two different techniques were used to estimate the different contributions to the surface free energy of zinc sulfide, namely contact angle and thin-layer wicking. Contact angles were measured using a Ram&Hart lOO07-00 goniometer, on pellets obtained by compressing dry ZnS powder under lo4 kplcm’ for 10 min. Prior to contact angle determinations, the pellets were either dried at 105°C and kept in a desiccator (“dry” pellets in what follows), or saturated with water vapor at room temperature (“wet” pellets hereafter). Thin-layer wicking technique [16] was used as described by Chibowski and Gonzalez-Caballero 1191. Microscope glass slides covered by a thin-layer of ZnS were prepared by spreading 2 ml bf 50 g/l suspensions and drying then for 24 hours at room temperature. Then
Chem~t~ and
Physics38 (1994) 42-49
they were either oven-dried at 60°C for 6 hours and stored in a desiccator (“bare” plates), or contacted for several hours with the vapor of the liquid to be studied (“precontacted” plates). In order to oxidize the commercial sulfide particles, 2 g samples were suspended in 3.5 ml of 3% or 15% aqueous H202 solutions for 60 min, in polyethylene bottles. The suspensions thus prepared were then cleaned by repeated cycles of centrifugation and redispersion in doubly distilled, deionized, filtered water.
Results and Discussion Surface free energy from thin-layer wicking According to eq. (6), if the time t that the liquid boundary takes to travel a distance x is plotted as a function of x2, a straight line should be obtained. Fig. 1 shows the results obtained when n-decane was used as the penetrating liquid in both bare and precontacted plates prepared from spherical ZnS particles. Different numbers in this and the following figures correspond to different plates prepared following the method described above. It is worth to note how the linearity predicted by Washburn’s equation is in fact obtained, although a change in the slope of the line was found in some cases. At present [19] there is no definite explanation for the existence of such inflection, although it can be suggested that it has its origin in the experimental
1
0 0
20
40
60 x2
80
100
(cm*)
Fig. 1. Penetration rate of n-decane in thin-layers of spherical colloidal Z&S, both bare and precontacted with the liquid vapor.
J.D.G. Durdn et al. / ~a~e~a~ Chemistry and Physics 38 (1994) 42-49
methodology, for example, in inhomogeneity of the layer porosity. The advantage of the use of homogeneous original material is clear concerning the reproducibility of the results; the lines corresponding to different bare plates almost coincide, and the same can be said as to the hehaviour of precontacted plates (note in table 1 the reproducibility of the values of the parameter R). Furthermore, this is true not only in the case of n-decane (Fig. l), but also of water (Fig. 2) and formamide (Fig. 3), for instance. In our earlier work [7], we considered the possibility that some surface compounds, other than pure ZnS, could be present in our synthetic sample due to oxidation, as a consequence of the fact that the synthesis is carried out in a strongly oxidizing environment (high HN03 concentration). To check for that possibility, experiments were also carried out in this work on pure and oxidized ZnS with the aim of estimating how the surface oxidation affects the free energy components, For the sake of brevity, we will not show all the t-x’ plots obtained but will rather focus on surface free energy components values. Figs. 4 and 5 are just two examples obtained with the commercial ZnS sample, and corresponding to several plates precontacted with n-decane (Fig. 4) and formamide (Fig. 5). From the plots in Fig. 1, the R parameter (using precontacted plates) and the value of the Lifshitz-van der Waals component, ysLw(data from bare plates), of synthetic ZnS can be obtained. The same types of plots (not shown) were used for commercial ZnS. The
5
6
z “0 c c-’
4
2
_I
0
0
20
40
60 x2
80
100
(cm’)
Fig. 3. Penetration of formamide in thin-layers (dry and precontacted with formamide vapor) of spherical ZnS.
2.5
8-
-z 6 ‘+J
6 I .o
0.5
0.0 0
10
20
30
40
50
60
70
x2 (cm*) x2 (cm’) Fig. ?.
Same as Fig. I. for water.
Fig. 4. Time against squared distance of penetration thin layers of commercial ZnS precontacted atmosphere of n-decane vapor.
of n-decane on with a saturated
J.D.G. D&n
46
et al. I Materials Chemistry and Physics 38 (1994) 4249 TABLE 1. Effective pore radius, R, of synthetic and commercial ZnS as estimated from thin-layer wicking in different plates.
r
plate
Plate No.
Material
5 prec
Temperature(C)
Penetrated distance
Rx105 (cm)
(cm)
12 -
1
24
o-3 4-9
1.73 1.49
2
23
o-3 4-9
1.67 1.50
1
22
o-3 3-8
2.133 1.818
2
23
o-4 5-8
3.074 3.703
13
23
Synthetic ZnS 10 -
Commercial ZnS
a-
3
x2 (cm’) Fig. 5.
Same as Fig. 4, for formamide.
results are shown in Tables 1 and 2 for both samples: the average values of ysLw for synthetic and commercial samples are 52+4 and 4.5*4 mJ/m*, respectively, so that it can be stated that, concerning Lifshitz-van der Waals interactions, both samples are similar. Note also how, as expected, R is larger for the plates where commercial material was used, since the average particle size is also larger in this case. As explained above, in order to completely characterize the contribution to ysToT,data of AGbare and AG,,, are needed with two bipolar liquids wetting the solid (eq.(9)). The experimental results for synthetic ZnS are shown in Figs. 2, 3, and the estimation of AGbare and AG,,, as well of ys+ and h‘ for both, synthetic and commercial ZnS are shown in Table 3. The values of yLLw,ye+ and rid-of the liquids used can be found in ref.
[W A significant difference between ys-for the synthetic and commercial material is worth to be noted. Our spherical ZnS particles are thus characterized by a higher electron donor (Lewis base) nature than commercial zinc sulfide particles. At least two reasons could be invoked to explain such a difference: i) the spherical particle surface is oxidized, at least partially; ii) the results obtained by the thin-layer wicking technique are not too reliable for the sulfide. In order to confirm or reject the first hypothesis, we estimated the ys component for differently oxidized samples of commercial ZnS, as well as for ZnO and ZnS04.H20. The second
23
o-3
2.219
4-7
2.396
o-5 6-8
3.091 2.592
TABLE 2. Lifshitz-van der Waals surface free energy component, ysLw, and effective pore radius of synthetic and commercial ZnS, as estimated from thin-layer wicking experiments.
Plate No.
Material
Synthetic
ZnS
Temp. (“C)
Penet. dist. (cm)
RxlO” (cm)
AG (mJ/m2)
XLW (mJ/m2)
4
23.5
4-9
1.49a
20.8
49.2
5
24
o-3 4-9
1 .67b 1.50b
21.3 25.6
49.9 56.3
3
23.5
o-4 5-9
2.63c
19.3 21.8
47.1 50.7
6
23
o-4 4-5
2.63c
16.9 12.1
43.8 37.5
7
23
o-4 5-8
2.63c
19.6 17.9
47.5 45.2
Average Commercial ZnS
52+4
Average
45*4
a R parameter from plate 1. b R parameter from plate 2. c Average R value for commercial
TABLE 3. free energy deduced
ZnS.
Average acid-base components (mJ/m*) of the surface of synthetic and commercial ZnS. AG (mJ/m2) values
from thin-layer
penetration
plots are included.
Material
AG;z
AG;r!ie’ AG&;
AGFe6” h.
Synthetic
22.7
10.5
45.2
48.3
88.3
0.2
2.8
4.0
7.5
8.6
54.7
0.5
Commercial
YF+
possibility was checked by determination of h components using the contact angle technique. The results will be discussed in the following paragraph.
J.D.G. Durdn et al. I ~aie~ls
Chem&y
and Physics 38 (1994) 42-49
47
TABLE 4. Contact angles (deg) of the liquid indicated on pure and oxidized commercial ZnS. and zinc oxide and sulfate pellets. Data corresponding to wet pellets are given between parentheses. Liquid
SOLID ZnS (Commercial)
Water
40 (24)
ZnS (3% H?O?) 37 (44)
ZnS (15% I+O:)
ZnO
45 MS)
32 (25)
ZnSO+H,O
15”
Formam
32 (32)
29 (3’))
38 (111)
25 (22)
13
Diiodo
16 (32)
20 (26)
20 (.?Jj
17 (281
20
a-BrNph
11 (17)
15 (lhj
12 (181
13 (14j
10
a ZnSO+H20 soluble in water.
Surface free energy components from contact angle measurements
Four liquids, two polar (water and formamide) and two non polar (diiodomethane and cx-bromonaphtalene) were used. Thus, we can use two polar-polar-non polar triads to solve eq.(5). Contact angles 8 were measured on dry and wet pellets, as shown in Table 4. The surface free energy components of the dry and wet powders are shown in Table 5 and 6, respectively. First of all, it can be noted that the ys components obtained with wet non-oxidized commercial ZnS are very similar to those obtained by the thin-layer wicking technique (compare data in Tables 2,3 and 4). This can be considered as a demonstration that the method works for ZnS too, and that hypothesis (ii) above can be rejected. Let us now consider the values of the ys components corresponding to the studied materials, i.e., synthetic ZnS, commercial ZnS, 3% and 15% oxidized ZnS, ZnO, and finally, ZnS04.H20 (tables 2-3, 5-6). Concerning the electron acceptor component, ysys’. it can be seen that for all materials studied it is close to zero, including ZnO and ZnS04.H20. No significant differences were found between dry and wet samples either. That is, all the species studied are strong electron donors. Moreover, the Lifshitz-van der Waals component, Ysl,w,is again similar for the solids, except that synthetic ZnS showed a somewhat higher value (52 mJm_*, see table 2). It should be kept in mind that elemental sulfur, if present, undergoes essentially Lifshitz-van der Waals interactions only, ha~~ingy~L~~lZ5mJ/m’ 122,231. Hence, it may be suggested that elemental sulfur, S”, is present in some amount on the surface of synthetic zinc sulfide, at the pHz5.5 of the original suspensions used to make the thin-layer. Unlike these particles, commercial ZnS probably does not possess any measurable surface S”, in agreement with our previous findings concerning the electrophoretic mobility of the particles [7]. The difference between y; of the synthetic ZnS (- 88 mJ/m2) and of any of the other systems (< 53 mJ/m’) is remarkable: the surface species existing on the ZnS
particles must be different than those produced byoxidation (even ZnO or ZnS04.H20). At present, we do not TABLE 5. Surface free energy components of the dry solids computed from contact angles of two triads of the liquids. SOLID/triads of liquids ZnS (Commercial) W-F-D” ~~-F-BrNphb ZnS (3% HzOz) W-F-D W-F-BrNph
48.9 42.8
38.0 37.5
0.0 0.3
47.7 42.1
39.5 39.0
0.0 0.5
ZnS (lS% HZOz) W-F-D W-F-BrNph
47.6 42.6
35.3 34.9
0.0 0.1
ZnO W-F-D W-F-BrNph
48.6 42.5
43.4 42.9
0.8 0.6
ZnSO,t.H20 W-F-D W-F-BrNph
47.7 42.9
52.8 52.3
0.3 0.7
* Water-~ormamide-Diiodomethane. h Water-Formamide-a-Bromonaphtalene. TABLE 6. Surface free energy components of the wet solidscomputed from contact angles of two triads of liquids. SOLID/triads of liquids
Ysrw
Ys-
Ys’
ZnS (Commercial) W-F-D” W-F-BrN~hb
43.2 41.7
56.7 56.5
0.0 0.1
ZnS (3% H,Oz) W-F-D W-F-BrNph
45.6 41.8
38.0 37.6
0.0 0.1
ZnS (IS% H202) W-F-D W-F-BrNph
46.5 41.5
37.2 36.8
0.0 0.1
ZnO W-F-D W-F-BrNph
45.0 42.3
49.1 48.X
0.3 0.5
a Water-Formamide-diiodomethane. b Water-~ormamide-a-Bromonaphtalene.
48
J.D.G. L)urrin et al. f Mate~Is
have a clear picture of what those species could be, although it can be suggested that there are still adsorbed on the surface some entities originating from the precipitating procedure [8]. Finally, some comments on the behaviour of y9*in dry and wet pellets may be given. An increase in that quantity is observed between dry and wet (commercial) ZnS; this may be attributed to the appearance in the latter of hydrated species, like Zn(SH) (OH) or Z&.x H20, although they are probably not present in either 3% or 15% H202 treated samples, since y; does not significantly change upon wetting these samples (see table 5 and 6). The partially oxidized ZnS particles are probably covered by Zn(OH), or Zn2(0H)$04, the behaviour of which must be very similar to ZnO or ZnS04 when contacted with water vapor. This is confirmed by the fact that ZnO undergoes a very small ys-change between dry and wet conditions. Free energy of interaction between~artic~es of zi~cs~~de in water
The thermodynamic theory of interfacial interactions proposed by van Oss et al. [E] allows calculation of the free energy of interaction between particles suspended in a liquid medium, provided that interfacial solid/liquid free energies are known. For this purpose, eq.(4) is used, after determining the surface free energy components of both the solid and the liquid. Knowledge of the free energy of interaction between particles is of interest in order to predict the stability behaviour of the suspensions. Of course, other interactions, mainly those of electrostatic nature are also needed. Using the Lifshitz-van der Waais and acid-base components of the surface free energy for the various zinc compounds (phase 1) above studied, as well as water (phase 2) surface tension, we have calculated from eq.(4) the free energy change, AGm. The results are shown in Table 7 for both synthetic and commercial zinc TABLE 7. Free energy of interaction, AGIZIToT(mJ/m$ between particles immersed in water. AG,,,“” and AG,,,- correspond to the Lifshitz-van der Waals and acid-base component, respectively.
Materiai
AC;;
AC&?
AG;yT
ZnS (synthetic) (wicking)
-9.34
87.81
78.47
ZnS (commercial) (wicking)
-5.50
47.39
41.89
ZnS (commercial) (0) ZnS (3% H202)
-4.30
49.96
45.66
-4.87 -5.01 -4.85 -3.36
22.19 20.86 39.32
17.32 15.85 34.47
44.42
41.07
ZnS (1.5% H202) ZnO ZnS04.H20
Chemdy
and Physics 38 (1994) 42-49
sulfide, as well as for oxidized zinc sulfide, zinc oxide and zinc sulphate. It can be seen in Table 7 that AGIzILw is similar for all the compounds, the differences being related to acid-base contribution, AGIZIAB.As expected for a strong electron-donor monopolar substance [12-141, zinc sulfide particles in water have an important repulsion due to the high acid-base contribution to AGIZIToT. Comparatively to the small attraction due to the apolar contribution to the above energy of interaction, the high repulsion (previously termed “hydration forces”) should ensure stability of ZnS dispersions {electrostatic repulsion will contribute in the same way, see ref.[7]). It can be seen in Table 7 that AG121ToT for synthetic ZnS equals 78.5 mJ/m:, whereas for commercial ZnS is around 44 mJ/m2. Thus, surface characteristics of the specially synthetized zinc sulfide particles show, among others, better properties concerning stability of the monodisperse system. The effect of oxidation of zinc sulfide particles is somewhat relevant from the point of view of a significant modification of the (repulsive) free energy of interaction between the particles. A pretreatment of the particles with only 3% Hz02 solution is enough to decrease the acid-base contribution to AGm by 25 mJ/m’. Higher oxidation does not have significant influence on the interfacial repulsion. Concerning the free energy of interaction between particles of zinc oxide and zinc sulphate, they are 34 and 41 mJ/m2, respectively. These values are not apparently related to those obtained for the various samples of ZnS used in this work. So, the problem of interpretation of the reduction in the energy of repulsion between ZnS particles as a consequence of their oxidation still remains open.
Conclusions Although more experimental work is needed, mainly using monodisperse systems, our results show that the thin-layer wicking technique is suitable to characterize the polar components of the surface free energy of inorganic materials. Results thus obtained are complementary to others dealing with electrostatic interactions, obtained through the common ele~trok~netic techniques, in order to obtain a better picture of the interactions involved in colloidal suspensions. The thin-layer technique compares well with contact angle determinations, although it can be considered more adequate when dealing with powdered samples. We have also found that spherical, monodisperse ZnS colloidal particles have an extremely strong electron donor (basic) nature. This intense acid-base activity could be useful in applications such as catalysis, adhesion to different substrates, and so on. However,
J.D.G.
Durdn et al. / ~ute~als
Chemirtry and Physics 38 (I!%%] 42-49
some features of the surface composition of the synthetized material must still be clarified.
Acknowledgments This work was financed Areces. Spain.
by Fundacicin
Ram6n
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563.
9. E. Chibowski, L. Holysz, J. Adhesion Sci. Technot., 3 (1989) 571. 10. B. Bilinski, W. Wojcik. Colloids Surfaces 36 (1989) 77.
11. J’.D.G. Duran, M.C. Guindo. A.V. Delgado, Prog. Colloid Polym. Sci. 93 (1993) 215. 12. C.J. Van Oss. M.K. Chaudhury, R.J. Good. C/rem. Rev. tr8 (1988) 927 13. R.J. Good, M.K. Chaudhury, in L-H. Lee (ed.), Fundamentals of Adhesion, Plenum Press, New York, 1991 p. 137. 14. R.J. Good, C.J. Van Oss, in M.E. Schrader and G.I. Loeb (eds.), i%iodernA~~rouches to ~etta~~~ity.Plenum Press, New York, 1992, p. 1. 15. A.W. Adamson. Physical Chern~str?l qf Surfaces , 4th ed. Wiley New York. 1982. p.436. 16. R.F. Giese, P.M. C’onstanzo. CJ. Van Oss, J. Phps.Chem. Miner 17 (1991) 611. 17. E. Chibowski, L. Holysz, Langmuir, 8 (1992) 710. 18. E. Chibowski. J. Adhesion Sci. Technol. 6 (1992)1069. 19. E. Chibowski, F. Gonzalez-Caballero, Langmuir, 9 (1993) 330. 20. R.J Good. J. Colloid Interface Sci., 42 (1973) 473. 21. R.J Good. N.Y. Lin, J.Colloid Interface Sci., 54 (1976) 52 22. B. .Janczuk. A. Waksmundzki, W. W6jcik. Roc:niki Chem. 51 (1977) 985. 23. B. Janczuk, W. Wojcik. A. Zdziennicka. J. ~iater~~l~sSci. 27 (1992) 6447.
F. Gonzalez-Caballero.
ELSEVIER
Chemistry
and Physics
Surface free energy components
38 (1994) 42-49
of monodisperse
zinc sulfide
J.D.G. Durban, A.V. Delgado and F. Gontilez-Caballero* Departamento de Fisica Aplicada, Facultad de Ciencias, Universidad de Granada, 18071 Granada (Spain)
E. Chibowski Department of Physical Chemistry, Faculty of Chemistry, Maria Curie-Sklodowska University, Lublin, 20-031 (Poland)
(Received July 12,1993; accepted December 10,1993)
Abstract Apolar, Lifshitz-van der Waals (y Lw) and polar, acid-base (Lewis) (y AB) surface free energy components, of monosized, spherical ZnS colloidal particles were experimentally determined using the thin-layer wicking technique. The results were compared with those obtained on a pure, commercial sample of zinc sulfide. The fact that the synthetized sample showed a very high electron-donor ( TV-)component suggested that some species, related in principle to surface oxidation of ZnS during the synthesis reaction, could be formed on the surface of the particles. To check for this possibility, the surface free energy components of pure and oxidized ZnS, and of zinc oxide and sulfate, were determined by both the contact angle and thin-layer wicking methods. The similarity between x components obtained with both methods for pure ZnS lead to the conclusion that both techniques give reliable results. The high y,value determined for synthetic ZnS particles cannot be explained simply by surface oxidation of the colloidal particles, since partially oxidized ZnS, or even zinc oxide or sulfate, gave very similar ys-,always lower than that found for synthetic spherical ZnS. The fact that the Lifshitz-van der Waals ( */sLw)component of the surface free energy is somewhat higher for the latter material is believed to be related to elemental sulfur present on the surface of the particles. The general compatibility of the results obtained by both methods suggests that the thin-layer wicking technique is useful to further characterize these or other systems, particularly if the examined material is sufficiently monodisperse.
Introduction Zinc sulfide is a widely studied inorganic compound mainly because of its numerous technological applications, that include pigment preparation, water purification or electroluminescent phosphors preparation. In fact, according to Schlam [l], it is the material that has been used both in cathodoluminescent and electroluminescent applications. Most of the special properties of ZnS are related to the surface properties of the bulk phase of this material or of its thin films [2]. For this reason, examination of the surface properties of ZnS is interesting not only by itself but also with practical applications in mind. It is well known that many phenomena occurring at the solid-liquid interface, such as wetting, printing, stability of colloidal suspensions, par*Author
to whom correspondence should be addressed.
0254-0584/94/$07.00
8
1994
SSDI 0254-0584(94)01349-L
Elsevier Science S.A. All rights reserved
titles adhesion to solid substrates, etc., depend to a large extent on the kind and magnitude of the surface and interfacial free energies involved. However, the study of these properties, when dealing with the surface of sulfide, is difficulted by the complicated changes in the structure and composition of the surface layer of the solid. The changes can take place as a consequence of chemical interactions of water and oxygen with the particle surface [3,4]. Thus, different compounds, ranging from hydroxides, through sulfates, thiosulfates, to elemental sulphur, can form onto the sulfide surface [3-61. These compounds should influence forces interacting on the sulfide surface, and lead to changes in the surface free energy components of the sulfide. In an earlier work [7], we have analyzed some surface electric properties of ZnS by determining the electrophoretic mobility of its aqueous suspensions in different conditions, particularly for various states of surface
J.D.G.
Durdn
et al. I Materials
oxidation. Use was made of the recent development of highly pure, spherical, zinc sulfide colloidal particles, exceedingly homogeneous in size and shape [8]. Particles thus prepared are most adequate to be used as a model system for adhesion studies or for testing stability conditions of dispersed systems. Although there is a number of studies in the literature dealing with the surface free energy of sulfide (minerals) [5,9-lo], they do not, however, treat directly the components of the surface free energy of zinc sulfide and their changes under the influence of oxidation processes. Therefore, in this paper, interest will be focussed on the determination of Lifshitz-van der Waals and acid-base (in the sense of Lewis) contributions to the total surface free energy of this solid. It is our aim to contribute new data on these quantities that, together with the electric characterization previously carried out [7, 111, may give a rather complete picture of the surface properties of zinc sulfide.
Theory
Only a brief review of some equations employed in the calculation of the surface free energy components of ZnS will be given in this paragraph. The reader is referred to refs. [12-141 where a detailed description of the van Oss, Good and coworkers’ approach to the surface free energy formulation of solids can be found. According to these authors, the total surface free energy xToTof a solid (or the surface tension of a liquid) can be expressed as the sum of two components:
xTOT=
xjr.W+
XAH,
where xLw1sthe apolar or Lifshitz-van der Waals component, and xA”is the acid-base contribution, which in many cases is due to hydrogen bonding. It has been proposed that xAHdepends on the electron donor (Lewis base) and electron aceptor (Lewis acid) properties of the phase i (x- and x’, respectively) as follows:
In the case of two phases i and j the total interaction can be expressed as [12-141:
When a phase 1 (solid or immiscible liquid) is dispersed in a polar liquid 2, according to these authors [1214], the free energy of interaction of two identical entities 1 (particles, molecules, etc), immersed in the polar liquid 2 can be calculated from the equation:
Chemistry
4249
43
-4(,!77~+~-Jg-3-Jj3F)
(4)
and Physics
38 (1994)
where the symbols are as defined above. From eq.(3), it is seen that three liquids of known surface tension components are needed and thus three equations of the type (3) must be solved simultaneously to determine the three unknown components of the surface free energy of a given solid, i.e., yb+,ySand ysLw. For this purpose, at least two techniques can be used: contact angle measurements [15], and the thin-layer wicking method, based on Washburn’s equation [15]. The later technique was very recently proposed by van Oss et al. [ 161and further elaborated upon by Chibowski etnl. [17-191. In the case of contact angle method, the starting point is Young’s equation written as:
where yL is the surface tension of a liquid forming a contact angle 0 on the solid. It should be stressed here that no film pressure behind the drop of the liquid used is taken into account in this equation. If 8 is measured for three liquids (two polar and one nonpolar) the three can be obtained. We have unknowns, ysLu’,ys+ and ysys’ used the following triads of liquids: water-formamidediiodomethane and water-formamide-a-bromonaphtalene. The thin-layer wicking method [16] is based on the measurement of the time (t) that a liquid takes to penetrate a distance (x) through a thin porous layer of a solid. The most general description of the process is based on a modified version of Washburn’s equation [17-19,20-211:
where AG is the change in surface free energy characteristic of the substitution of the solid- air interface by the solid-liquid interface in the wicking process. According to equation (6), at a constant temperature, for a liquid having viscosity n the square of penetrated distance should be a linear function of the time. In eq.(6), R is an effective radius of the capillary network forming the thin-layer of the solid. Chibowski et al. [17-191 have proposed an experimental methodology for the determination of the solid surface free energy components. The authors distinguished four wicking systems in which different values of AG appear. If the used liquid wets the solid completely, and the solid has been saturated with its vapour, in such a way that its duplex film is formed onto the surface (precontacted plate), eq.(6) reads [19]: 1 Kt .y- = 2 i
yL’
44
J.D.G. Durcin et al. I Maters
and this form of Washburn’s equation can be used to estimate R, if the surface tension ye and the viscosity of the liquid are known. Usually, the liquids best preferred for this purpose are n-alkanes, like n-octane or n-decane. When the same liquid is used on a bare plate, of the same solid, equation (6) is valid [19] with AaGgiven by: AG=2,/sw-2y,
(8)
If a non-polar liquid (y~q~~‘“) is used, eqs.(6) and (8) can be used to determine ysLw.We have employed n-decane in our experiments. In order to estimate y,’ and y;, two bipolar [12-141 liquids partially wetting the solid (i.e., forming anon-zero static contact angle with it) must be used. The experiments were carried out on both bare and precontacted surfaces and the following equation applies 1191: AGba, -AC,,
Where AGbare and AC,,, were obtained in separate wicking experiments (under the two above experimental conditions) from an equation of type (6). We have used water and formamide, and solved the resulting system of two equations to obtain ys+and ys-,provided ye, nLW, yL+and y,_-were known.
The synthesis of homogeneous in size, spherical ZnS particles (diameter 320t20 nm) was carried out as described in [8] and details of the preparation of its suspensions are given in [7]. Ail chemicals used were of anlytical quality, including ZnS (Merck), Hz02 (Panreac), ZnO (Probus), ZnS04.H20 (Panreac), diiodomethane (Merck), a-bromonaphtalene (Carlo Erba), formamide (Carlo Erba) and n-decane (Sigma). Two different techniques were used to estimate the different contributions to the surface free energy of zinc sulfide, namely contact angle and thin-layer wicking. Contact angles were measured using a Ram&Hart lOO07-00 goniometer, on pellets obtained by compressing dry ZnS powder under lo4 kplcm’ for 10 min. Prior to contact angle determinations, the pellets were either dried at 105°C and kept in a desiccator (“dry” pellets in what follows), or saturated with water vapor at room temperature (“wet” pellets hereafter). Thin-layer wicking technique [16] was used as described by Chibowski and Gonzalez-Caballero 1191. Microscope glass slides covered by a thin-layer of ZnS were prepared by spreading 2 ml bf 50 g/l suspensions and drying then for 24 hours at room temperature. Then
Chem~t~ and
Physics38 (1994) 42-49
they were either oven-dried at 60°C for 6 hours and stored in a desiccator (“bare” plates), or contacted for several hours with the vapor of the liquid to be studied (“precontacted” plates). In order to oxidize the commercial sulfide particles, 2 g samples were suspended in 3.5 ml of 3% or 15% aqueous H202 solutions for 60 min, in polyethylene bottles. The suspensions thus prepared were then cleaned by repeated cycles of centrifugation and redispersion in doubly distilled, deionized, filtered water.
Results and Discussion Surface free energy from thin-layer wicking According to eq. (6), if the time t that the liquid boundary takes to travel a distance x is plotted as a function of x2, a straight line should be obtained. Fig. 1 shows the results obtained when n-decane was used as the penetrating liquid in both bare and precontacted plates prepared from spherical ZnS particles. Different numbers in this and the following figures correspond to different plates prepared following the method described above. It is worth to note how the linearity predicted by Washburn’s equation is in fact obtained, although a change in the slope of the line was found in some cases. At present [19] there is no definite explanation for the existence of such inflection, although it can be suggested that it has its origin in the experimental
1
0 0
20
40
60 x2
80
100
(cm*)
Fig. 1. Penetration rate of n-decane in thin-layers of spherical colloidal Z&S, both bare and precontacted with the liquid vapor.
J.D.G. Durdn et al. / ~a~e~a~ Chemistry and Physics 38 (1994) 42-49
methodology, for example, in inhomogeneity of the layer porosity. The advantage of the use of homogeneous original material is clear concerning the reproducibility of the results; the lines corresponding to different bare plates almost coincide, and the same can be said as to the hehaviour of precontacted plates (note in table 1 the reproducibility of the values of the parameter R). Furthermore, this is true not only in the case of n-decane (Fig. l), but also of water (Fig. 2) and formamide (Fig. 3), for instance. In our earlier work [7], we considered the possibility that some surface compounds, other than pure ZnS, could be present in our synthetic sample due to oxidation, as a consequence of the fact that the synthesis is carried out in a strongly oxidizing environment (high HN03 concentration). To check for that possibility, experiments were also carried out in this work on pure and oxidized ZnS with the aim of estimating how the surface oxidation affects the free energy components, For the sake of brevity, we will not show all the t-x’ plots obtained but will rather focus on surface free energy components values. Figs. 4 and 5 are just two examples obtained with the commercial ZnS sample, and corresponding to several plates precontacted with n-decane (Fig. 4) and formamide (Fig. 5). From the plots in Fig. 1, the R parameter (using precontacted plates) and the value of the Lifshitz-van der Waals component, ysLw(data from bare plates), of synthetic ZnS can be obtained. The same types of plots (not shown) were used for commercial ZnS. The
5
6
z “0 c c-’
4
2
_I
0
0
20
40
60 x2
80
100
(cm’)
Fig. 3. Penetration of formamide in thin-layers (dry and precontacted with formamide vapor) of spherical ZnS.
2.5
8-
-z 6 ‘+J
6 I .o
0.5
0.0 0
10
20
30
40
50
60
70
x2 (cm*) x2 (cm’) Fig. ?.
Same as Fig. I. for water.
Fig. 4. Time against squared distance of penetration thin layers of commercial ZnS precontacted atmosphere of n-decane vapor.
of n-decane on with a saturated
J.D.G. D&n
46
et al. I Materials Chemistry and Physics 38 (1994) 4249 TABLE 1. Effective pore radius, R, of synthetic and commercial ZnS as estimated from thin-layer wicking in different plates.
r
plate
Plate No.
Material
5 prec
Temperature(C)
Penetrated distance
Rx105 (cm)
(cm)
12 -
1
24
o-3 4-9
1.73 1.49
2
23
o-3 4-9
1.67 1.50
1
22
o-3 3-8
2.133 1.818
2
23
o-4 5-8
3.074 3.703
13
23
Synthetic ZnS 10 -
Commercial ZnS
a-
3
x2 (cm’) Fig. 5.
Same as Fig. 4, for formamide.
results are shown in Tables 1 and 2 for both samples: the average values of ysLw for synthetic and commercial samples are 52+4 and 4.5*4 mJ/m*, respectively, so that it can be stated that, concerning Lifshitz-van der Waals interactions, both samples are similar. Note also how, as expected, R is larger for the plates where commercial material was used, since the average particle size is also larger in this case. As explained above, in order to completely characterize the contribution to ysToT,data of AGbare and AG,,, are needed with two bipolar liquids wetting the solid (eq.(9)). The experimental results for synthetic ZnS are shown in Figs. 2, 3, and the estimation of AGbare and AG,,, as well of ys+ and h‘ for both, synthetic and commercial ZnS are shown in Table 3. The values of yLLw,ye+ and rid-of the liquids used can be found in ref.
[W A significant difference between ys-for the synthetic and commercial material is worth to be noted. Our spherical ZnS particles are thus characterized by a higher electron donor (Lewis base) nature than commercial zinc sulfide particles. At least two reasons could be invoked to explain such a difference: i) the spherical particle surface is oxidized, at least partially; ii) the results obtained by the thin-layer wicking technique are not too reliable for the sulfide. In order to confirm or reject the first hypothesis, we estimated the ys component for differently oxidized samples of commercial ZnS, as well as for ZnO and ZnS04.H20. The second
23
o-3
2.219
4-7
2.396
o-5 6-8
3.091 2.592
TABLE 2. Lifshitz-van der Waals surface free energy component, ysLw, and effective pore radius of synthetic and commercial ZnS, as estimated from thin-layer wicking experiments.
Plate No.
Material
Synthetic
ZnS
Temp. (“C)
Penet. dist. (cm)
RxlO” (cm)
AG (mJ/m2)
XLW (mJ/m2)
4
23.5
4-9
1.49a
20.8
49.2
5
24
o-3 4-9
1 .67b 1.50b
21.3 25.6
49.9 56.3
3
23.5
o-4 5-9
2.63c
19.3 21.8
47.1 50.7
6
23
o-4 4-5
2.63c
16.9 12.1
43.8 37.5
7
23
o-4 5-8
2.63c
19.6 17.9
47.5 45.2
Average Commercial ZnS
52+4
Average
45*4
a R parameter from plate 1. b R parameter from plate 2. c Average R value for commercial
TABLE 3. free energy deduced
ZnS.
Average acid-base components (mJ/m*) of the surface of synthetic and commercial ZnS. AG (mJ/m2) values
from thin-layer
penetration
plots are included.
Material
AG;z
AG;r!ie’ AG&;
AGFe6” h.
Synthetic
22.7
10.5
45.2
48.3
88.3
0.2
2.8
4.0
7.5
8.6
54.7
0.5
Commercial
YF+
possibility was checked by determination of h components using the contact angle technique. The results will be discussed in the following paragraph.
J.D.G. Durdn et al. I ~aie~ls
Chem&y
and Physics 38 (1994) 42-49
47
TABLE 4. Contact angles (deg) of the liquid indicated on pure and oxidized commercial ZnS. and zinc oxide and sulfate pellets. Data corresponding to wet pellets are given between parentheses. Liquid
SOLID ZnS (Commercial)
Water
40 (24)
ZnS (3% H?O?) 37 (44)
ZnS (15% I+O:)
ZnO
45 MS)
32 (25)
ZnSO+H,O
15”
Formam
32 (32)
29 (3’))
38 (111)
25 (22)
13
Diiodo
16 (32)
20 (26)
20 (.?Jj
17 (281
20
a-BrNph
11 (17)
15 (lhj
12 (181
13 (14j
10
a ZnSO+H20 soluble in water.
Surface free energy components from contact angle measurements
Four liquids, two polar (water and formamide) and two non polar (diiodomethane and cx-bromonaphtalene) were used. Thus, we can use two polar-polar-non polar triads to solve eq.(5). Contact angles 8 were measured on dry and wet pellets, as shown in Table 4. The surface free energy components of the dry and wet powders are shown in Table 5 and 6, respectively. First of all, it can be noted that the ys components obtained with wet non-oxidized commercial ZnS are very similar to those obtained by the thin-layer wicking technique (compare data in Tables 2,3 and 4). This can be considered as a demonstration that the method works for ZnS too, and that hypothesis (ii) above can be rejected. Let us now consider the values of the ys components corresponding to the studied materials, i.e., synthetic ZnS, commercial ZnS, 3% and 15% oxidized ZnS, ZnO, and finally, ZnS04.H20 (tables 2-3, 5-6). Concerning the electron acceptor component, ysys’. it can be seen that for all materials studied it is close to zero, including ZnO and ZnS04.H20. No significant differences were found between dry and wet samples either. That is, all the species studied are strong electron donors. Moreover, the Lifshitz-van der Waals component, Ysl,w,is again similar for the solids, except that synthetic ZnS showed a somewhat higher value (52 mJm_*, see table 2). It should be kept in mind that elemental sulfur, if present, undergoes essentially Lifshitz-van der Waals interactions only, ha~~ingy~L~~lZ5mJ/m’ 122,231. Hence, it may be suggested that elemental sulfur, S”, is present in some amount on the surface of synthetic zinc sulfide, at the pHz5.5 of the original suspensions used to make the thin-layer. Unlike these particles, commercial ZnS probably does not possess any measurable surface S”, in agreement with our previous findings concerning the electrophoretic mobility of the particles [7]. The difference between y; of the synthetic ZnS (- 88 mJ/m2) and of any of the other systems (< 53 mJ/m’) is remarkable: the surface species existing on the ZnS
particles must be different than those produced byoxidation (even ZnO or ZnS04.H20). At present, we do not TABLE 5. Surface free energy components of the dry solids computed from contact angles of two triads of the liquids. SOLID/triads of liquids ZnS (Commercial) W-F-D” ~~-F-BrNphb ZnS (3% HzOz) W-F-D W-F-BrNph
48.9 42.8
38.0 37.5
0.0 0.3
47.7 42.1
39.5 39.0
0.0 0.5
ZnS (lS% HZOz) W-F-D W-F-BrNph
47.6 42.6
35.3 34.9
0.0 0.1
ZnO W-F-D W-F-BrNph
48.6 42.5
43.4 42.9
0.8 0.6
ZnSO,t.H20 W-F-D W-F-BrNph
47.7 42.9
52.8 52.3
0.3 0.7
* Water-~ormamide-Diiodomethane. h Water-Formamide-a-Bromonaphtalene. TABLE 6. Surface free energy components of the wet solidscomputed from contact angles of two triads of liquids. SOLID/triads of liquids
Ysrw
Ys-
Ys’
ZnS (Commercial) W-F-D” W-F-BrN~hb
43.2 41.7
56.7 56.5
0.0 0.1
ZnS (3% H,Oz) W-F-D W-F-BrNph
45.6 41.8
38.0 37.6
0.0 0.1
ZnS (IS% H202) W-F-D W-F-BrNph
46.5 41.5
37.2 36.8
0.0 0.1
ZnO W-F-D W-F-BrNph
45.0 42.3
49.1 48.X
0.3 0.5
a Water-Formamide-diiodomethane. b Water-~ormamide-a-Bromonaphtalene.
48
J.D.G. L)urrin et al. f Mate~Is
have a clear picture of what those species could be, although it can be suggested that there are still adsorbed on the surface some entities originating from the precipitating procedure [8]. Finally, some comments on the behaviour of y9*in dry and wet pellets may be given. An increase in that quantity is observed between dry and wet (commercial) ZnS; this may be attributed to the appearance in the latter of hydrated species, like Zn(SH) (OH) or Z&.x H20, although they are probably not present in either 3% or 15% H202 treated samples, since y; does not significantly change upon wetting these samples (see table 5 and 6). The partially oxidized ZnS particles are probably covered by Zn(OH), or Zn2(0H)$04, the behaviour of which must be very similar to ZnO or ZnS04 when contacted with water vapor. This is confirmed by the fact that ZnO undergoes a very small ys-change between dry and wet conditions. Free energy of interaction between~artic~es of zi~cs~~de in water
The thermodynamic theory of interfacial interactions proposed by van Oss et al. [E] allows calculation of the free energy of interaction between particles suspended in a liquid medium, provided that interfacial solid/liquid free energies are known. For this purpose, eq.(4) is used, after determining the surface free energy components of both the solid and the liquid. Knowledge of the free energy of interaction between particles is of interest in order to predict the stability behaviour of the suspensions. Of course, other interactions, mainly those of electrostatic nature are also needed. Using the Lifshitz-van der Waais and acid-base components of the surface free energy for the various zinc compounds (phase 1) above studied, as well as water (phase 2) surface tension, we have calculated from eq.(4) the free energy change, AGm. The results are shown in Table 7 for both synthetic and commercial zinc TABLE 7. Free energy of interaction, AGIZIToT(mJ/m$ between particles immersed in water. AG,,,“” and AG,,,- correspond to the Lifshitz-van der Waals and acid-base component, respectively.
Materiai
AC;;
AC&?
AG;yT
ZnS (synthetic) (wicking)
-9.34
87.81
78.47
ZnS (commercial) (wicking)
-5.50
47.39
41.89
ZnS (commercial) (0) ZnS (3% H202)
-4.30
49.96
45.66
-4.87 -5.01 -4.85 -3.36
22.19 20.86 39.32
17.32 15.85 34.47
44.42
41.07
ZnS (1.5% H202) ZnO ZnS04.H20
Chemdy
and Physics 38 (1994) 42-49
sulfide, as well as for oxidized zinc sulfide, zinc oxide and zinc sulphate. It can be seen in Table 7 that AGIzILw is similar for all the compounds, the differences being related to acid-base contribution, AGIZIAB.As expected for a strong electron-donor monopolar substance [12-141, zinc sulfide particles in water have an important repulsion due to the high acid-base contribution to AGIZIToT. Comparatively to the small attraction due to the apolar contribution to the above energy of interaction, the high repulsion (previously termed “hydration forces”) should ensure stability of ZnS dispersions {electrostatic repulsion will contribute in the same way, see ref.[7]). It can be seen in Table 7 that AG121ToT for synthetic ZnS equals 78.5 mJ/m:, whereas for commercial ZnS is around 44 mJ/m2. Thus, surface characteristics of the specially synthetized zinc sulfide particles show, among others, better properties concerning stability of the monodisperse system. The effect of oxidation of zinc sulfide particles is somewhat relevant from the point of view of a significant modification of the (repulsive) free energy of interaction between the particles. A pretreatment of the particles with only 3% Hz02 solution is enough to decrease the acid-base contribution to AGm by 25 mJ/m’. Higher oxidation does not have significant influence on the interfacial repulsion. Concerning the free energy of interaction between particles of zinc oxide and zinc sulphate, they are 34 and 41 mJ/m2, respectively. These values are not apparently related to those obtained for the various samples of ZnS used in this work. So, the problem of interpretation of the reduction in the energy of repulsion between ZnS particles as a consequence of their oxidation still remains open.
Conclusions Although more experimental work is needed, mainly using monodisperse systems, our results show that the thin-layer wicking technique is suitable to characterize the polar components of the surface free energy of inorganic materials. Results thus obtained are complementary to others dealing with electrostatic interactions, obtained through the common ele~trok~netic techniques, in order to obtain a better picture of the interactions involved in colloidal suspensions. The thin-layer technique compares well with contact angle determinations, although it can be considered more adequate when dealing with powdered samples. We have also found that spherical, monodisperse ZnS colloidal particles have an extremely strong electron donor (basic) nature. This intense acid-base activity could be useful in applications such as catalysis, adhesion to different substrates, and so on. However,
J.D.G.
Durdn et al. / ~ute~als
Chemirtry and Physics 38 (I!%%] 42-49
some features of the surface composition of the synthetized material must still be clarified.
Acknowledgments This work was financed Areces. Spain.
by Fundacicin
Ram6n
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submitted.
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