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Investigations of the properties of water films on quartz surfaces
Dbj;ortment
of
Physical
Chemistry.
(Received September $7,.1983;
Institute
ofchemistq,
Maria
Curie-Sklodo~ska.Uniuersity.
Lupiii
(Poland)
in revised form December 20,1983)
SUMMARY Isotherms of water adsorption on and desorption from quartz surfaces have been determined_ Thermal analysis curves of water thermodesorption from quark surfaces have also been determined_ From the experimental data, the energetic changes accompanying the water adsorption and desorption processes were -calculated. The mechanism of water adsorption has been proposed, and conclusions concerning the structure and other properties of water films on quartz surfaces have been drawn.
models of the surface water structure i-L, 9; 14; 15]_ It should be ‘noted that the hypctneses and conclusions formulated on the bzis of the obtained results concerning the structure and other properties of vicinal water do not completely ex&in this problem. For this reason, experiments have been carried out in order to study more closely the structure and thickness of water films on quartz surfaces as well as to determine the energy changes accompanying the adsorption and desorption processes.
EXPERIMENTAL INTRODUCTION The presence of adsorbed water on a solid surface plays a very important role in various processes such as coating the surface with various organic films [ 1, 21, wetting and stabilization of dispersed systems [3, 41, protection and stabilization of macroelectronic elements [5], etc. As yet, these processes have not been completely investigated in many instances, although they possess a practical value besides their cognitive aspect [S] _ Accurate studies of these phenomena are difficult, because water adsorption fiequently exerts an influence on the course of many surface phenomena and a precise regulation of the humidity of systems investigated is often impossible_ The present investigations have shown that the properties of thin water films adsorbed on quartz surfaces are different from those of bulk water [7 - 13]_ There are also many *Mailing address: Dr. Piotr Staszczuk, Department of Physical Chemistry, Institute of Chemistm UhKS, pl. Marii Curie-Sklodowskiej 3, 20-031 Lublin (Poland).
Measurements of the kinetics of water vapor adsorption and desorption on quartz surfaces were made with a previously described [lS, 173 apparatus. For determination of water vapor adsorption and desorption isotherms, a dynamic chromatographic step profile method (Glueckauf method) [lS] was adopted. The details of the method and the apparatus are given in the papers [lS, 173 _ In this method, t.he carrier gas nitrogen saturated with water vapor at the measurement temperature is passed through the sample placed in the detector, until maximum wetting (maximum capacity; plateau on the recorder signal - adsorption equilibrium) is achieved. In this way, a characteristic adsorption curve was obtained. The maximum constant signal obtained on the recorder showed that the vapor adsorption process under the given measurement conditions was complete. Then water desorption from the mineral surface was started, a dry carrier gas (N1) was passed through the sample, and a desorption curve was obtained. Adsorption and desorption isotherms, as a function of the relative vapor were obtained from the shape pressure PIP,, @ Elsevier Sequoia/Printed in Tbe Netherlands
34
of the broadening echo and the back echo of the recorder signal 1181 _ Measurements of water thermodesorption were conducted using a Q-1500 D derivatograph (MOM Hungary). Before the measurements, quartz samples (1.4 g) were wetted with double-distilled water until the saturation state (15%) was obta&ed, and then water was thermodesorbed in the crucible of the apparatus_ The rate of temperature increase in the furnace of the apparatus was 0.6, 1.25 and 10 “C/min. Samples (1 g) of natural Brazilian quartz powder of high purity for optical purposes (80 ppm total impurity level) were used for the investigations. The impurities, determined by the atomic adsorption method, were (in ppm): Al,O, - 30, KP - 10, Na,O - 15, Li,O - 2, Fe,O, - 5, TiO, - 2, CaO - 8, MgO - 5, and the hydroxyl groups, cu. 1 ppm, as determined by intied spectroscopy. Size dictions of 0.15 - 0.25 mm were dried at 423 K for 12 h and stored in a desiccator_ The initial water content, determined by Fisher’s method [19], is 9.3 X lo-’ mmol/g.
RESULTS
AND
a
q.Gld/g 0.8
0.6
0.4
0.2
, /
L 0.2
P/P,
(aI
a
IXU01/g x10
DISCUSSION
The surface properties of quartz depend strongly up on the preparation of the samples, especially on thermal pretreatment, which can affect the number of generated and dissociated silanol groups. Prolonged heating at 473 - 673 K caused the number of OH groups on the surface to decrease [20] _ Since the prepared quartz samples were heated for 12 h at 423 K only, the hydroxyl groups were not completely removed from the surface. Figure 1 presents the isotherms of water vapor adsorption and desorption from the quartz surface at 293 K and 303 K. The adsorption and desorption isotherms form hysteresis loops and are a combination of several shapes. De Boer [21] has presented the sequence of adsorption-desorption isotherm hysteresis loops as a function of the shapes of the pores of the mineral surface. Adsorption hysteresis takes place during adsorption of polar liquids on a heterogenic mineral surface_ The micropores and intergranular spaces are not filled simultaneously. It is well known that adsorption hysteresis is due to capillary condensation effects, which
2
I10
2 0.6
0.4
0.2
0.2
0.6
’
P/P,
(b)
Fig.
1. Water vapor adsorption and desorption quartz (a), at 293 K; (b) at 303 K.
therms on
iso-
are described by the Cohan equation [ 221. The hysteresis loops obtained from the experimental data also .indicate regeneration of a part .of the silanol groups lost during thermal treatment of the quartz. The isotherms obtained have the shape of type Ii of the BET isotherm classification, which can be related to the formation of.a polymolecular adsorption layer. The-isotherms presented here are similar to those of water vapor
:
:.
. ...::
._
:
:.
-. TABLE-l._.
:
at 293/303
.’
: l&ree df . . --hydratioti K
-_ ..
:
:_
Time of
’
..- ’ adsorption
(mmol/m2)
(m~ollg). 1 x 10-Z
.. .35:.” _.:
.. at 2931303 5
“max
:
:
Ex+i&&+r&ult.% .. . Adsorption capacity
._
:
.. 0.-25
pr&zss at.293/3d3.K.
. (min)
Surface:
T%n~ of’
.desorption process . . at 2931303 (rnin)
area
K
(m’/g)
Amount
Statistical-
number of adsorbed water layers at 293/303 K
x 10-Z
0.18
Statist&ii
.number of bonded water
water at 293/303 (mmollg)
K
226
2.8
183
1.4
layers at 293/303
K
c-1
(-1 10-7
0.066
x 1O-2
0.92
7-9
0.069
x lO-2
0.74
0.039 0.74
of
the bouded
m
mmoL/g
x10
2
0-B
0.6
0-k
0.2
1 1
Fig.
2. Kinetics
of adserption
(A) and desorption
2
t.;In
3
(B) of water on quartz at 293 and 303 K.
adsorption and desorption on quartz [ 23j The results presented in Fig. 1 and Table 1 indicate that the adsorption value on a quartz surface at 293 K is 1 X 10e2 mmol/g, i.e. 10.7 statistical water monolayers, and at 303 K, 0.7 X lo-’ mmol/g (7.9 statistical monolayers)_ The statistical number of water monolayers adsorbed on quartz was calculated on the basis of the surface area of quartz (0.039 m*/g), determined by the Nelsen and Eggersten method [24] with an appropriate apparatus [25]. The number of water monolayers formed on a quartz surface shows good agreement with Fowkes’s calculations [l], according to which 10 statistical water monolayers may be zdarbed on a quartz surface from the gaseous phase. From the shape of the isotherms presented in Fig. 1, it can be seen that the desorption curves lie above the adsorptioti curves and form hysteresis loops which do not coincide over the whole range of relative pressures_
This is due to irreversible adsorption of water vapor. Some of this water vapor is irreversibly bonded with the solid surface under the experimental conditions_ This quantity of water is stably bonded with the quartz surface in the form of a hydration layer, the thickness of which corresponds to 0.92 monolayer at 293 K and 0.74 monolayer at 303 K. Figure 2 presents the kinetics of water vapor adsorption (Fig. 2A) and desorption (Fig. 2B) on the quartz surface at 293 K and 303 K. It can be seen that adsorption equilibrium is obtained in a shorter time at a higher temperature, because the diffusion rate of the water molecules from the gaseous phase to the surface layer is then higher. Water desorption from the quartz surface is also more rapid at a higher temperature_ Many different parameters, in particular the porous structure of the surface, the character of mineral-water interactions and the change of diffusion rate, which takes place when the concentration
36
-qzt kJf
PO1
70
60
50
I
I
0.4
0.2
Ll
I
0.6
a.
nzn31/~10
Fig. 3. Dependence of isosteric heat of adsorption adsorption of water on quartz.
2
on
gradient changes, also influence the kinetics of adsorption and desorption [ 20,26]_ From the adsorption data obtained at two temperatures, i.e. 293 and 303 K, we have calculated the isosteric heat of adsorption qsrX_ If it is assumed that the heat is constant for a given concentration of water on the surface over the selected temperature range, use may be made the integrated form of the Clausius-Clapeyron equation [20, 27,281: hp,=-
R$
+c
(1)
where q,t= is the isosteric heat of adsorption at amount adsorbed x, px is the equilibrium pressure at this coverage, and C is the constant of integration. A plot of In p, against reciprocal temperature for a constant amount of adsorbed water x gives a siose of -qstx/R. The slopes of the resulting straight lines were calculated to the highest possible degree of accuracy by use of a least-squares method. Figure 3 presents the relationship between isosteric heat of adsorption and the quantity of water adsorbed on the quartz surface. As seen, the heat of adsorption decreases on adsorption. A high initial value of heat of adsorption (80.26 kJ/mol) at low coverages may be due to water adsorption on the centers of sufficiently high activity. The centers are the coordinately unsaturated silicon atoms on the quartz surface. And
relatively high energy is released during the process of water molecules binding to these centers [9]. Results of qstx calculations presented above are in good agr_eement’with those obtained by Whalen [12] (44.73:Q6.6 kJ/mol) from the isotherms determined gravimetically for water vapor adsorption on quartz at 298 K and 308 K. Similar q,+= values as a function of surface coverage by water were obtained for silica [S, 23], organosilane-treated silica [ 291, kaolinite [ 301, sulfur [ 311, and copper ores [ 32]_ From the chromatographic data, similar slopes were obtained for porous glass [ 281, graphite and carbon black [27] and graphitized carbon black [ 333. This problem was also discussed by Kloubek ef al. 1341, who suggest that isosteric heat of adsorption can be a variable function of the temperature and degree of coverage in the case of heterogeneous adsorbent% The activation energy Eact was calculated using the experimental data obtained by the chromatographic step profile method and using the modified Arrhenius equation [31] (eqn. (2)) and the Kissinger equation [35] (eqn- (3)): E act = -
E act -= R
R Wt,/tz) UT,
-
(2)
117-z
d Wb/Tm2) d(I/T,)
(3)
where ti and t2 are the times of the desorption process at temperatures T, and T,, respectively (T, < T,), b is the sample heating rate and T, is the temperature of the extreme effect point on the DTA curve_ The activation enthalpy AH of the water molecules was calculated from derivatographic measurements using eqn. (4) [ 351: (4) where TP is the sample temperature. The activation energy and activation enthalpy of the water molecules adsorbed on the quartz surface calculated from these equations are greater than the energy necessary to disrupt the hydrogen bond, and are 26.75 kJ/mol, 26.48 kJ/mol and 25.83 kJ/ mol, respectively (Table 2). These values suggest the existence of stionger atomic
.
. .’
: .TABiE 2. Vah.u%of variou¶meteti d.etermined by the step profile’and derjvatogxaphic memo+. .. Par&eter
Activation energy E act
Step profile method
DerivatofP=PGC method
26.75
26.48
kJ/mol
25.83
kJ/mol
kJlmo1
Activation enthalpy fu? Maximum Losteric heat of adsorption (--4stxnax
82.35
MeXimum change in molar adsorption entropy
-152.57
t.ers for the’ next w&x.
mokc&es when the most active centers are saturated~ (secondary adsorption cen~ters). Further water molecules can be adsorbed around them according to scheme II or form aggIomerations of water bonded by hydrogen bonds [9] according to scheme III (see overleaf).
kJ/mol
Scheme II J/(mol
K)
These agglomerations may be formed before the occupation of all free hydroxyl groups, because of protonisation of the water molecules adsorbed in the first stage of the process. The hydrogen bond energy is then higher (heat of condensation L = 43.89 kJ/ mol) than the bonding energy between the hydroxyl groups and water molecules (25.08 kJ/mol) [ 91. A decrease in isosteric heat of adsorption is observed until the water adsorption value becomes equal to 0.4 X lCl_* mmol/g (Fig. 3). A minimum heat occurs on the curve of isosteric heat of adsorption_ A slight heat increase corresponding to about 0.5 X lo-* mmol/g adsorption can be caused by migration of water molecules from less active energetic centers to more energetic ones and by adsorption around them (moving water films)_ The increase in heat of adsorption may also be due to horizontal interactions between water molecules [ 203. The entropy chalges calculated from the known values of the isosteric heat of adsorption and free enthalpy (eqn. (5)) [20] as well as the derivatographic measurements (eqn. (6)) 1353 confcm this hypothesis (eqns. (5)
dAS H do
rrax -101.7 J/(mol
Activation entropy As Maximum change in free enthalpy -AG,,
4.18
K)
kJ/mol
bonds formed between water molecules and quartz surface independently of the existing hydrogen bonds. Such bonds are formed especially during the first stage of the adsorption process, between oxygen atoms of water molecules and unsaturated silicon atoms on the quartz surface ES]. This process may be illustrated schematically as is shown in scheme I. The water molecules adsorbed in the first stage of the process are strongly bonded to the surface. They possess low freedom of orientation and become the adsorption cen-
A -L&O-ii+-&Scheme I
37
and (6)). H
c, + 2H20
__f
H
..
38
Scheme III
d AS -= da AS=R-
-(s,t=
--L)
-
AG
(5)
T
(”
+ln-
RT,
-ln-
R
-1np
mobility and the entropy changes initially increase and then decrease in the adsorption range 0.5 X 10m2 to 0.6 X 10m2 mmol/g. This fact corresponds to the mobility drop caused by the adsorption of water molecules on the secondary adsorption centers. Water molecule agglomerations show smaller angular and translation movement& than water molecules weakly bonded with the surface and thus they possess a smaller entiopy. Therefore, at low coverages, water adsorption on quartz is determined mainly by the entropy changes, while at higher coverages, the formation of agglomerations is determined by molecular stabilization energy [ 93. Water molecules obtain a greater freedom of orientation when the quantity of water adsorbed on the surface increases_ Adsorption of the next water molecules leads to water film formation and filling up of capillaries (capilary condensation). The properties of the adsorption layers are then similar to those of the liquid state (Figs. 3 and 4). The changes in free enthalpy of the water molecules adsorbed on the quartz surface are presented in Fig. 5 in the form of the relationship AG = f(a). It results from this relation-
Tm2 dT,ldt
k Trn h
where d AS/da is the change in molar adsorption entropy, AG = RT In@@,) is the change in free enthalpy, L is the heat of condensation, p. is the saturated vapor pressure of water at temperature T, AS is the activation entropy, k is the Boltzmann constant and h is the Planck constant_ Figure 4 presents the changes in molar adsorption entropy versus the adsorption values of water on the quartz surface. From this relationship and from the data listed in Table 2, it results that water molecules adsorbed on the mineral surface in the first stage of the adsorption process are associated with the lowest values of entropy change (-152.57 J/(mol K)) caused by the loss of linear movement. When the number of adsorbed water molecules increases, their 1G
2
--dG kJ/rad
3
2
d 0.1 Fig. 4. Dependence of entropy tion of water on quartz.
of water OP adsorp-
0.3
0.5
a.
m!mlfgxlO
Fig. 5_ Changes in free entbalpy as a function quantity of water adsorbed on quartz.
of the
2
39:.
L
I
313
Fig. 6. Thermal
I
1
333
I
I
353
I
I
373
analysis curves of water thermodesorption
ship that the AG value is highest in the initial stage of the adsorption process and gradually decreases during the coverage of the surface, reaching a minimal value in the equilibrium state. The direction of the changes indicates that the water adsorption process is spontaneous. From the investigations presented, it can be concluded that the structure and properties of the water adsorbed on a quartz surface are different Tom those of bulk water Surface water films are strongly bonded to a quartz surface and this fact can be conCrmed by the derivatographic measurements presented in Fig. 6. Peak I at 331 K on the DTA curve corresponds to the socalled ‘paradoxal effect’ related to abnormal water properties at this temperature. These abnormalities are related to changes in water structure_ Water possesses the properties of the liquid rather than the solid state, and these properties are
A
T /W
from the quartz surface.
independent of the nature of the solid [14, 361. From the DTA and DTG curves presented in Fig. 6, it results that water desorption from the quartz surface proceeds in two stages. In the first stage, bulk water (peak II at 360 K on the DTA curve) is mainly desorbed, and then water more strongly bonded with the surface (peak III at 375 K). In our previous paper [lS], water film pressure was determined on the basis of the adsorption isotherms obtained at 293 K (Fig. 1). This value is K,,, = 380 mJ/m’. Interpretation of the changes in n values related to film thickness and wetting work was proposed. It has been concluded that the measured values of the film pressure correspond to the work of spreading, immersional and adhesional wetting and to the layer thickness of about 2, 3 and 4 statistical water monovalue in turn corresponds to layers. The 7r,, some work of adhesion of quartz-water and
., .. 40
that of water cohesion_ On the basis of the spreading rs, the immersional x1, the adhewater film sional rrA, and the maximum TV, pressure values, the polar component of the quartz free energy -yeP was determined_ The determined -yQp values were in good agreement, giving the average value rep = 115 mJ/ m2_ In a previous paper [ 373, thermal analysis curves of water thermodesorption from a silica gel surface were presented, which showed peaks analogous to those in Fig. 6. On the basis of the film pressure rr values, it was shown that individual peaks correspond to spreading, immersion and adhesion wetting work, which corresponds to a thickness of 2, 4 and 5 statistical monolayers, respectively_ Because quartz and silica gel surfaces are similar [ 53, the results obtained suggest that points of inflection on the curves plotted on Figs. 3,4 and 6 are related to the thickness of the water film and to the nature of the wetting process of the quartz surface.
CONCLUSION From water
the
data
adsorption
irreversible
under
presented, on a quartz given
it appears surface
experimental
that
is partly condi-
and 7.9 statistical water monolayers are adsorbed at 293 and 303 K, respectively_ The water films remaining on the surface after desorption are more strongly bonded to the quartz surface and therefore possess properties different from those of bulk water. Desorption of this layer requires a dramatic change in conditions, for example, sharp increase in temperature. Such a desorption proceeds in two steps.
tions_
10.7
REFERENCES F. M_ Fowkes, Hydrophobic Surface. Academic Press, New York-London, 1969, p_ 51_ T. D. Blake, Far. Trans. I. 71 (1975) 192. D. W. Fuerstenau and S. Raghavan, in M_ C. Fuerstenau. (Ed.), FZotafion. AIME, New York, 1976, p_ 29. -J. Laskowski, Minerals Sci_ Engng_, 6 (4) (1974) 223. W. F. Khielev, Surface Phenomena on Semiconductors and Dielectrics. Nauka, Moscow, 1970.
6 D. W. Harkins, The Physical Chemistry of S&ace’ Films, Reinhold, New York, 1957. 7 F. M. Fowkes, Ind_ Eng. Chem.. 56 (1664) 1240. Problemy 8 P_ Staszczuk and A. Waksmundzki, Agrofizyki, Ossolineum. (No. 37.) Wroclaw, 1982. J_ CoZZoid Inter9 K. Klier and A_ C. Zettlemoyer, face Sci.. 58 (1977) 216. 10 J. Laskowski and J. A. Kitchener, J. CoZZoid Interface Sci.. 29 (1969) 670. 11 B_ W_ Deryagin (Ed.). Research in Surface Forces, Consultants Bureau, New York-London, vol. 1, 1963; vol. 2, 1966; vol. 3, 1971; vol. 4, 1975. 12 J_ W. Whalen, J. Phys. Chem.. 65 (1961) 1676_ 13 FL. L. Every, W. H. Wade and N. Hackerman, J. Phys. Chem., 65 (1961) 25. 14 W. Drost-Hansen, Znd. Engng. Chem., 61 (1969) 10. 15 J. Lyklema, J. CoZZoid Interface Sci.. 58 (1977) 242_ 16 P. Staszczuk, B. Jaiiczuk and E. Chibowski, Calloids Surfaces. submitted for publication. and P. Stasznuk. Prace Komisji 17 A. Waksmundzki Naukowych PTG. II/12, Warsaw 1679, p_ 12318 J. F. K. Huber and R. G_ Gerritse. J. Chromntom.. 58 (1971) 137_ Methoden. 19 R. Gerhardt_ Neue Massanolvtische Ferdinand Enke Verlag. Stuttgart, 1956. PWN, Warsaw-Ellis 20 J. Ogcik, Adsorption, Harwood, Chichester. 1982. 21 J. H. De Boer, The Structure and Properties of Porous Materials, in Proc. X Symp. CoZston Research Sot. Univ. BtitoZ. Butterworths, London, 1958. 22 L. H. Cohan, J. Am. Chem. Sot.. 60 (1938) 433. 23 P. BarracIough and P. F. Hall, J. Chem. Sot. Faraday I. 74 (1978) 1360. 24 F. M. Nelsen and F. T_ Eggersten. Anal. Chem.. 30 (1958) 1387. 25 Patent PRL. 173 237 T (1974). to A. Waksmundzki, 2. Suprynowicz, J. Gawdzik. A. Gorgol and J. Wojcik_ UMCS, Lublin, 1979, 26 M. Jaroniec, “Thesis”, p_ 25. 27 D. DoBimore, G. R. Heal and D. R. Martin, J. Chem. Sot. Faraday Trans. I. 5 (1972) 832. 28 J. Gawdzik and M. Jaroniec, J. Chromatogr-. 131 (1977)l. and H. H_ Hsing, J. CoZZoid 29 A. C. ZettIemoyer Interface Sci_. 58 (1977) 263. 30 J. Jurinak and D. H. Volman, J. Phys. Chem., 65 (1961) 1853. 31 P. Staszczuk, Powder Technol.. 32 (1982) 211. 32 P. Staszczuk, Powder Technol, 35 (1983) 97. Surface 33 S. J. Gregg and K. S. W. Sing, Adsorption, Area and Porosity, Academic Press, London and New York, 1967, p_ 299. 34 J_ KIoubek, J. Pasek and J. Volf, J_ ColZoid Interface Sci., 51 (1975) 491. 1702. 35 H_ E. Kissinger, Anal. Chem.. 29 j1957) 36 W. Drost-Hansen, J. CoZZoid Interface Sci_. 58 (1977) 251_ 37 P. Staszczuk, J. Thermal Anal., 29 (1984) 217.
of
Physical
Chemistry.
(Received September $7,.1983;
Institute
ofchemistq,
Maria
Curie-Sklodo~ska.Uniuersity.
Lupiii
(Poland)
in revised form December 20,1983)
SUMMARY Isotherms of water adsorption on and desorption from quartz surfaces have been determined_ Thermal analysis curves of water thermodesorption from quark surfaces have also been determined_ From the experimental data, the energetic changes accompanying the water adsorption and desorption processes were -calculated. The mechanism of water adsorption has been proposed, and conclusions concerning the structure and other properties of water films on quartz surfaces have been drawn.
models of the surface water structure i-L, 9; 14; 15]_ It should be ‘noted that the hypctneses and conclusions formulated on the bzis of the obtained results concerning the structure and other properties of vicinal water do not completely ex&in this problem. For this reason, experiments have been carried out in order to study more closely the structure and thickness of water films on quartz surfaces as well as to determine the energy changes accompanying the adsorption and desorption processes.
EXPERIMENTAL INTRODUCTION The presence of adsorbed water on a solid surface plays a very important role in various processes such as coating the surface with various organic films [ 1, 21, wetting and stabilization of dispersed systems [3, 41, protection and stabilization of macroelectronic elements [5], etc. As yet, these processes have not been completely investigated in many instances, although they possess a practical value besides their cognitive aspect [S] _ Accurate studies of these phenomena are difficult, because water adsorption fiequently exerts an influence on the course of many surface phenomena and a precise regulation of the humidity of systems investigated is often impossible_ The present investigations have shown that the properties of thin water films adsorbed on quartz surfaces are different from those of bulk water [7 - 13]_ There are also many *Mailing address: Dr. Piotr Staszczuk, Department of Physical Chemistry, Institute of Chemistm UhKS, pl. Marii Curie-Sklodowskiej 3, 20-031 Lublin (Poland).
Measurements of the kinetics of water vapor adsorption and desorption on quartz surfaces were made with a previously described [lS, 173 apparatus. For determination of water vapor adsorption and desorption isotherms, a dynamic chromatographic step profile method (Glueckauf method) [lS] was adopted. The details of the method and the apparatus are given in the papers [lS, 173 _ In this method, t.he carrier gas nitrogen saturated with water vapor at the measurement temperature is passed through the sample placed in the detector, until maximum wetting (maximum capacity; plateau on the recorder signal - adsorption equilibrium) is achieved. In this way, a characteristic adsorption curve was obtained. The maximum constant signal obtained on the recorder showed that the vapor adsorption process under the given measurement conditions was complete. Then water desorption from the mineral surface was started, a dry carrier gas (N1) was passed through the sample, and a desorption curve was obtained. Adsorption and desorption isotherms, as a function of the relative vapor were obtained from the shape pressure PIP,, @ Elsevier Sequoia/Printed in Tbe Netherlands
34
of the broadening echo and the back echo of the recorder signal 1181 _ Measurements of water thermodesorption were conducted using a Q-1500 D derivatograph (MOM Hungary). Before the measurements, quartz samples (1.4 g) were wetted with double-distilled water until the saturation state (15%) was obta&ed, and then water was thermodesorbed in the crucible of the apparatus_ The rate of temperature increase in the furnace of the apparatus was 0.6, 1.25 and 10 “C/min. Samples (1 g) of natural Brazilian quartz powder of high purity for optical purposes (80 ppm total impurity level) were used for the investigations. The impurities, determined by the atomic adsorption method, were (in ppm): Al,O, - 30, KP - 10, Na,O - 15, Li,O - 2, Fe,O, - 5, TiO, - 2, CaO - 8, MgO - 5, and the hydroxyl groups, cu. 1 ppm, as determined by intied spectroscopy. Size dictions of 0.15 - 0.25 mm were dried at 423 K for 12 h and stored in a desiccator_ The initial water content, determined by Fisher’s method [19], is 9.3 X lo-’ mmol/g.
RESULTS
AND
a
q.Gld/g 0.8
0.6
0.4
0.2
, /
L 0.2
P/P,
(aI
a
IXU01/g x10
DISCUSSION
The surface properties of quartz depend strongly up on the preparation of the samples, especially on thermal pretreatment, which can affect the number of generated and dissociated silanol groups. Prolonged heating at 473 - 673 K caused the number of OH groups on the surface to decrease [20] _ Since the prepared quartz samples were heated for 12 h at 423 K only, the hydroxyl groups were not completely removed from the surface. Figure 1 presents the isotherms of water vapor adsorption and desorption from the quartz surface at 293 K and 303 K. The adsorption and desorption isotherms form hysteresis loops and are a combination of several shapes. De Boer [21] has presented the sequence of adsorption-desorption isotherm hysteresis loops as a function of the shapes of the pores of the mineral surface. Adsorption hysteresis takes place during adsorption of polar liquids on a heterogenic mineral surface_ The micropores and intergranular spaces are not filled simultaneously. It is well known that adsorption hysteresis is due to capillary condensation effects, which
2
I10
2 0.6
0.4
0.2
0.2
0.6
’
P/P,
(b)
Fig.
1. Water vapor adsorption and desorption quartz (a), at 293 K; (b) at 303 K.
therms on
iso-
are described by the Cohan equation [ 221. The hysteresis loops obtained from the experimental data also .indicate regeneration of a part .of the silanol groups lost during thermal treatment of the quartz. The isotherms obtained have the shape of type Ii of the BET isotherm classification, which can be related to the formation of.a polymolecular adsorption layer. The-isotherms presented here are similar to those of water vapor
:
:.
. ...::
._
:
:.
-. TABLE-l._.
:
at 293/303
.’
: l&ree df . . --hydratioti K
-_ ..
:
:_
Time of
’
..- ’ adsorption
(mmol/m2)
(m~ollg). 1 x 10-Z
.. .35:.” _.:
.. at 2931303 5
“max
:
:
Ex+i&&+r&ult.% .. . Adsorption capacity
._
:
.. 0.-25
pr&zss at.293/3d3.K.
. (min)
Surface:
T%n~ of’
.desorption process . . at 2931303 (rnin)
area
K
(m’/g)
Amount
Statistical-
number of adsorbed water layers at 293/303 K
x 10-Z
0.18
Statist&ii
.number of bonded water
water at 293/303 (mmollg)
K
226
2.8
183
1.4
layers at 293/303
K
c-1
(-1 10-7
0.066
x 1O-2
0.92
7-9
0.069
x lO-2
0.74
0.039 0.74
of
the bouded
m
mmoL/g
x10
2
0-B
0.6
0-k
0.2
1 1
Fig.
2. Kinetics
of adserption
(A) and desorption
2
t.;In
3
(B) of water on quartz at 293 and 303 K.
adsorption and desorption on quartz [ 23j The results presented in Fig. 1 and Table 1 indicate that the adsorption value on a quartz surface at 293 K is 1 X 10e2 mmol/g, i.e. 10.7 statistical water monolayers, and at 303 K, 0.7 X lo-’ mmol/g (7.9 statistical monolayers)_ The statistical number of water monolayers adsorbed on quartz was calculated on the basis of the surface area of quartz (0.039 m*/g), determined by the Nelsen and Eggersten method [24] with an appropriate apparatus [25]. The number of water monolayers formed on a quartz surface shows good agreement with Fowkes’s calculations [l], according to which 10 statistical water monolayers may be zdarbed on a quartz surface from the gaseous phase. From the shape of the isotherms presented in Fig. 1, it can be seen that the desorption curves lie above the adsorptioti curves and form hysteresis loops which do not coincide over the whole range of relative pressures_
This is due to irreversible adsorption of water vapor. Some of this water vapor is irreversibly bonded with the solid surface under the experimental conditions_ This quantity of water is stably bonded with the quartz surface in the form of a hydration layer, the thickness of which corresponds to 0.92 monolayer at 293 K and 0.74 monolayer at 303 K. Figure 2 presents the kinetics of water vapor adsorption (Fig. 2A) and desorption (Fig. 2B) on the quartz surface at 293 K and 303 K. It can be seen that adsorption equilibrium is obtained in a shorter time at a higher temperature, because the diffusion rate of the water molecules from the gaseous phase to the surface layer is then higher. Water desorption from the quartz surface is also more rapid at a higher temperature_ Many different parameters, in particular the porous structure of the surface, the character of mineral-water interactions and the change of diffusion rate, which takes place when the concentration
36
-qzt kJf
PO1
70
60
50
I
I
0.4
0.2
Ll
I
0.6
a.
nzn31/~10
Fig. 3. Dependence of isosteric heat of adsorption adsorption of water on quartz.
2
on
gradient changes, also influence the kinetics of adsorption and desorption [ 20,26]_ From the adsorption data obtained at two temperatures, i.e. 293 and 303 K, we have calculated the isosteric heat of adsorption qsrX_ If it is assumed that the heat is constant for a given concentration of water on the surface over the selected temperature range, use may be made the integrated form of the Clausius-Clapeyron equation [20, 27,281: hp,=-
R$
+c
(1)
where q,t= is the isosteric heat of adsorption at amount adsorbed x, px is the equilibrium pressure at this coverage, and C is the constant of integration. A plot of In p, against reciprocal temperature for a constant amount of adsorbed water x gives a siose of -qstx/R. The slopes of the resulting straight lines were calculated to the highest possible degree of accuracy by use of a least-squares method. Figure 3 presents the relationship between isosteric heat of adsorption and the quantity of water adsorbed on the quartz surface. As seen, the heat of adsorption decreases on adsorption. A high initial value of heat of adsorption (80.26 kJ/mol) at low coverages may be due to water adsorption on the centers of sufficiently high activity. The centers are the coordinately unsaturated silicon atoms on the quartz surface. And
relatively high energy is released during the process of water molecules binding to these centers [9]. Results of qstx calculations presented above are in good agr_eement’with those obtained by Whalen [12] (44.73:Q6.6 kJ/mol) from the isotherms determined gravimetically for water vapor adsorption on quartz at 298 K and 308 K. Similar q,+= values as a function of surface coverage by water were obtained for silica [S, 23], organosilane-treated silica [ 291, kaolinite [ 301, sulfur [ 311, and copper ores [ 32]_ From the chromatographic data, similar slopes were obtained for porous glass [ 281, graphite and carbon black [27] and graphitized carbon black [ 333. This problem was also discussed by Kloubek ef al. 1341, who suggest that isosteric heat of adsorption can be a variable function of the temperature and degree of coverage in the case of heterogeneous adsorbent% The activation energy Eact was calculated using the experimental data obtained by the chromatographic step profile method and using the modified Arrhenius equation [31] (eqn. (2)) and the Kissinger equation [35] (eqn- (3)): E act = -
E act -= R
R Wt,/tz) UT,
-
(2)
117-z
d Wb/Tm2) d(I/T,)
(3)
where ti and t2 are the times of the desorption process at temperatures T, and T,, respectively (T, < T,), b is the sample heating rate and T, is the temperature of the extreme effect point on the DTA curve_ The activation enthalpy AH of the water molecules was calculated from derivatographic measurements using eqn. (4) [ 351: (4) where TP is the sample temperature. The activation energy and activation enthalpy of the water molecules adsorbed on the quartz surface calculated from these equations are greater than the energy necessary to disrupt the hydrogen bond, and are 26.75 kJ/mol, 26.48 kJ/mol and 25.83 kJ/ mol, respectively (Table 2). These values suggest the existence of stionger atomic
.
. .’
: .TABiE 2. Vah.u%of variou¶meteti d.etermined by the step profile’and derjvatogxaphic memo+. .. Par&eter
Activation energy E act
Step profile method
DerivatofP=PGC method
26.75
26.48
kJ/mol
25.83
kJ/mol
kJlmo1
Activation enthalpy fu? Maximum Losteric heat of adsorption (--4stxnax
82.35
MeXimum change in molar adsorption entropy
-152.57
t.ers for the’ next w&x.
mokc&es when the most active centers are saturated~ (secondary adsorption cen~ters). Further water molecules can be adsorbed around them according to scheme II or form aggIomerations of water bonded by hydrogen bonds [9] according to scheme III (see overleaf).
kJ/mol
Scheme II J/(mol
K)
These agglomerations may be formed before the occupation of all free hydroxyl groups, because of protonisation of the water molecules adsorbed in the first stage of the process. The hydrogen bond energy is then higher (heat of condensation L = 43.89 kJ/ mol) than the bonding energy between the hydroxyl groups and water molecules (25.08 kJ/mol) [ 91. A decrease in isosteric heat of adsorption is observed until the water adsorption value becomes equal to 0.4 X lCl_* mmol/g (Fig. 3). A minimum heat occurs on the curve of isosteric heat of adsorption_ A slight heat increase corresponding to about 0.5 X lo-* mmol/g adsorption can be caused by migration of water molecules from less active energetic centers to more energetic ones and by adsorption around them (moving water films)_ The increase in heat of adsorption may also be due to horizontal interactions between water molecules [ 203. The entropy chalges calculated from the known values of the isosteric heat of adsorption and free enthalpy (eqn. (5)) [20] as well as the derivatographic measurements (eqn. (6)) 1353 confcm this hypothesis (eqns. (5)
dAS H do
rrax -101.7 J/(mol
Activation entropy As Maximum change in free enthalpy -AG,,
4.18
K)
kJ/mol
bonds formed between water molecules and quartz surface independently of the existing hydrogen bonds. Such bonds are formed especially during the first stage of the adsorption process, between oxygen atoms of water molecules and unsaturated silicon atoms on the quartz surface ES]. This process may be illustrated schematically as is shown in scheme I. The water molecules adsorbed in the first stage of the process are strongly bonded to the surface. They possess low freedom of orientation and become the adsorption cen-
A -L&O-ii+-&Scheme I
37
and (6)). H
c, + 2H20
__f
H
..
38
Scheme III
d AS -= da AS=R-
-(s,t=
--L)
-
AG
(5)
T
(”
+ln-
RT,
-ln-
R
-1np
mobility and the entropy changes initially increase and then decrease in the adsorption range 0.5 X 10m2 to 0.6 X 10m2 mmol/g. This fact corresponds to the mobility drop caused by the adsorption of water molecules on the secondary adsorption centers. Water molecule agglomerations show smaller angular and translation movement& than water molecules weakly bonded with the surface and thus they possess a smaller entiopy. Therefore, at low coverages, water adsorption on quartz is determined mainly by the entropy changes, while at higher coverages, the formation of agglomerations is determined by molecular stabilization energy [ 93. Water molecules obtain a greater freedom of orientation when the quantity of water adsorbed on the surface increases_ Adsorption of the next water molecules leads to water film formation and filling up of capillaries (capilary condensation). The properties of the adsorption layers are then similar to those of the liquid state (Figs. 3 and 4). The changes in free enthalpy of the water molecules adsorbed on the quartz surface are presented in Fig. 5 in the form of the relationship AG = f(a). It results from this relation-
Tm2 dT,ldt
k Trn h
where d AS/da is the change in molar adsorption entropy, AG = RT In@@,) is the change in free enthalpy, L is the heat of condensation, p. is the saturated vapor pressure of water at temperature T, AS is the activation entropy, k is the Boltzmann constant and h is the Planck constant_ Figure 4 presents the changes in molar adsorption entropy versus the adsorption values of water on the quartz surface. From this relationship and from the data listed in Table 2, it results that water molecules adsorbed on the mineral surface in the first stage of the adsorption process are associated with the lowest values of entropy change (-152.57 J/(mol K)) caused by the loss of linear movement. When the number of adsorbed water molecules increases, their 1G
2
--dG kJ/rad
3
2
d 0.1 Fig. 4. Dependence of entropy tion of water on quartz.
of water OP adsorp-
0.3
0.5
a.
m!mlfgxlO
Fig. 5_ Changes in free entbalpy as a function quantity of water adsorbed on quartz.
of the
2
39:.
L
I
313
Fig. 6. Thermal
I
1
333
I
I
353
I
I
373
analysis curves of water thermodesorption
ship that the AG value is highest in the initial stage of the adsorption process and gradually decreases during the coverage of the surface, reaching a minimal value in the equilibrium state. The direction of the changes indicates that the water adsorption process is spontaneous. From the investigations presented, it can be concluded that the structure and properties of the water adsorbed on a quartz surface are different Tom those of bulk water Surface water films are strongly bonded to a quartz surface and this fact can be conCrmed by the derivatographic measurements presented in Fig. 6. Peak I at 331 K on the DTA curve corresponds to the socalled ‘paradoxal effect’ related to abnormal water properties at this temperature. These abnormalities are related to changes in water structure_ Water possesses the properties of the liquid rather than the solid state, and these properties are
A
T /W
from the quartz surface.
independent of the nature of the solid [14, 361. From the DTA and DTG curves presented in Fig. 6, it results that water desorption from the quartz surface proceeds in two stages. In the first stage, bulk water (peak II at 360 K on the DTA curve) is mainly desorbed, and then water more strongly bonded with the surface (peak III at 375 K). In our previous paper [lS], water film pressure was determined on the basis of the adsorption isotherms obtained at 293 K (Fig. 1). This value is K,,, = 380 mJ/m’. Interpretation of the changes in n values related to film thickness and wetting work was proposed. It has been concluded that the measured values of the film pressure correspond to the work of spreading, immersional and adhesional wetting and to the layer thickness of about 2, 3 and 4 statistical water monovalue in turn corresponds to layers. The 7r,, some work of adhesion of quartz-water and
., .. 40
that of water cohesion_ On the basis of the spreading rs, the immersional x1, the adhewater film sional rrA, and the maximum TV, pressure values, the polar component of the quartz free energy -yeP was determined_ The determined -yQp values were in good agreement, giving the average value rep = 115 mJ/ m2_ In a previous paper [ 373, thermal analysis curves of water thermodesorption from a silica gel surface were presented, which showed peaks analogous to those in Fig. 6. On the basis of the film pressure rr values, it was shown that individual peaks correspond to spreading, immersion and adhesion wetting work, which corresponds to a thickness of 2, 4 and 5 statistical monolayers, respectively_ Because quartz and silica gel surfaces are similar [ 53, the results obtained suggest that points of inflection on the curves plotted on Figs. 3,4 and 6 are related to the thickness of the water film and to the nature of the wetting process of the quartz surface.
CONCLUSION From water
the
data
adsorption
irreversible
under
presented, on a quartz given
it appears surface
experimental
that
is partly condi-
and 7.9 statistical water monolayers are adsorbed at 293 and 303 K, respectively_ The water films remaining on the surface after desorption are more strongly bonded to the quartz surface and therefore possess properties different from those of bulk water. Desorption of this layer requires a dramatic change in conditions, for example, sharp increase in temperature. Such a desorption proceeds in two steps.
tions_
10.7
REFERENCES F. M_ Fowkes, Hydrophobic Surface. Academic Press, New York-London, 1969, p_ 51_ T. D. Blake, Far. Trans. I. 71 (1975) 192. D. W. Fuerstenau and S. Raghavan, in M_ C. Fuerstenau. (Ed.), FZotafion. AIME, New York, 1976, p_ 29. -J. Laskowski, Minerals Sci_ Engng_, 6 (4) (1974) 223. W. F. Khielev, Surface Phenomena on Semiconductors and Dielectrics. Nauka, Moscow, 1970.
6 D. W. Harkins, The Physical Chemistry of S&ace’ Films, Reinhold, New York, 1957. 7 F. M. Fowkes, Ind_ Eng. Chem.. 56 (1664) 1240. Problemy 8 P_ Staszczuk and A. Waksmundzki, Agrofizyki, Ossolineum. (No. 37.) Wroclaw, 1982. J_ CoZZoid Inter9 K. Klier and A_ C. Zettlemoyer, face Sci.. 58 (1977) 216. 10 J. Laskowski and J. A. Kitchener, J. CoZZoid Interface Sci.. 29 (1969) 670. 11 B_ W_ Deryagin (Ed.). Research in Surface Forces, Consultants Bureau, New York-London, vol. 1, 1963; vol. 2, 1966; vol. 3, 1971; vol. 4, 1975. 12 J_ W. Whalen, J. Phys. Chem.. 65 (1961) 1676_ 13 FL. L. Every, W. H. Wade and N. Hackerman, J. Phys. Chem., 65 (1961) 25. 14 W. Drost-Hansen, Znd. Engng. Chem., 61 (1969) 10. 15 J. Lyklema, J. CoZZoid Interface Sci.. 58 (1977) 242_ 16 P. Staszczuk, B. Jaiiczuk and E. Chibowski, Calloids Surfaces. submitted for publication. and P. Stasznuk. Prace Komisji 17 A. Waksmundzki Naukowych PTG. II/12, Warsaw 1679, p_ 12318 J. F. K. Huber and R. G_ Gerritse. J. Chromntom.. 58 (1971) 137_ Methoden. 19 R. Gerhardt_ Neue Massanolvtische Ferdinand Enke Verlag. Stuttgart, 1956. PWN, Warsaw-Ellis 20 J. Ogcik, Adsorption, Harwood, Chichester. 1982. 21 J. H. De Boer, The Structure and Properties of Porous Materials, in Proc. X Symp. CoZston Research Sot. Univ. BtitoZ. Butterworths, London, 1958. 22 L. H. Cohan, J. Am. Chem. Sot.. 60 (1938) 433. 23 P. BarracIough and P. F. Hall, J. Chem. Sot. Faraday I. 74 (1978) 1360. 24 F. M. Nelsen and F. T_ Eggersten. Anal. Chem.. 30 (1958) 1387. 25 Patent PRL. 173 237 T (1974). to A. Waksmundzki, 2. Suprynowicz, J. Gawdzik. A. Gorgol and J. Wojcik_ UMCS, Lublin, 1979, 26 M. Jaroniec, “Thesis”, p_ 25. 27 D. DoBimore, G. R. Heal and D. R. Martin, J. Chem. Sot. Faraday Trans. I. 5 (1972) 832. 28 J. Gawdzik and M. Jaroniec, J. Chromatogr-. 131 (1977)l. and H. H_ Hsing, J. CoZZoid 29 A. C. ZettIemoyer Interface Sci_. 58 (1977) 263. 30 J. Jurinak and D. H. Volman, J. Phys. Chem., 65 (1961) 1853. 31 P. Staszczuk, Powder Technol.. 32 (1982) 211. 32 P. Staszczuk, Powder Technol, 35 (1983) 97. Surface 33 S. J. Gregg and K. S. W. Sing, Adsorption, Area and Porosity, Academic Press, London and New York, 1967, p_ 299. 34 J_ KIoubek, J. Pasek and J. Volf, J_ ColZoid Interface Sci., 51 (1975) 491. 1702. 35 H_ E. Kissinger, Anal. Chem.. 29 j1957) 36 W. Drost-Hansen, J. CoZZoid Interface Sci_. 58 (1977) 251_ 37 P. Staszczuk, J. Thermal Anal., 29 (1984) 217.