Langmuir 1991, 7, 2243-2247
2243
Inverse Gas Chromatographic Studies on Silica: Infinite Dilution and Finite Concentration Measurements Ivo Tijburg,? Jacek Jagiello,t Alain Vidal, and Eugene Papirer' Centre de Recherches sur la Physico-Chimie des Surfaces Solides, CNRS, 24 Avenue du Prbsident Kennedy, 68200 Mulhouse, France Received January 14, 1991. I n Final Form: April 22, 1991 The surface properties of a precipitated silica have been determined by inverse gas chromatography, either at infinite dilution or at finite concentration conditions. The dispersive component of the surface energy is readily assessed, whereas a new approach is proposed for the appraisal of the specific interaction capacity of the silica, starting from alkene adsorption thermodynamic data. Further, the adsorption energy distributions of alkanes and alkenes are calculated, showing the very heterogeneous energetic nature of the silica surface, which interacts strongly with the alkene probes. Introduction Silicas are widely used as reinforcing fillers in In order to improve reinforcing capabilities, organic or silane compounds may be grafted on the silica surface. The effectiveness of grafting strongly depends upon the presence of surface hydroxyl groups on the silica surface. Inverse gas chromatography (IGC) has shown to be a very suitable technique to investigate silica surfaces before and after graftinga4* IGC at infinite dilution can be used to study the thermodynamic parameters of adsorption sites with the highest energy, while IGC a t finite concentration provides information about all sites. If probe molecules are used which interact specifically with surface hydroxyl groups, it is possible to study the presence of these groups before and after grafting. In this paper it will be demonstrated that the energy distribution functions obtained from finite concentration experiments provide complementary information to the thermodynamic parameters obtained from infinite dilution experiments. Further, for the calculation of thermodynamic parameters a new method will be presented.
'characteristic elution point" method was used. Prior to this, superpositionof peak tails was checked for differentamounts of probe injected. All calculations were performed on an Apple Macintoeh computer using software developed at the CNRS. Materials. The probes (n-alkanes, 1-alkenes,and cycloalkanes) were purchased from Aldrich as puriss grade producta and were used without further purification. The silica was a precipitated Akzo-PQsilica with a BET surface area of 150 ma/ g. The silica was grafted with Ca alkyl chains at 150 O C during 2 h at atmospheric pressure, followed by a washing procedure with toluene for 20 h. The grafted silica was subsequentlydried at 80 O C under vacuum (1OaPa) for 16 h. The amount of grafted alkyl chains was determined by analysis of the carbon content. The carbon content of grafted samples was 3 f 0.2%.
t Visiting Scientist,Instituteof Energochemistryof Coal and Physicochemistry of Sorbents, University of Miningand Metallurgy, Krakow., Poland. - ~ -- ~ (1) Ferch, H. B o g . Org. Coat. 1982, 10, 91. (2) Pal, P. K.; De, S. K. Rubber Chem. Technol. 1982,55 (51, 1370. (3) Wagner, M. P. Rubber Chem. Technol. 1976,49 (31,703. (4) Sidai, M.: Lianer, G.: Janiello, J.: Balard. H.: PaDirer, E. Chromatographia 1987,b, 588.. . . (51 Vidal. A,:PaDirer. J. B. Chromatoaradtia _ . E.: .Wann. -.M. J.:Donnet. . - -
Results and Discussion Infinite Dilution Experiments. Inverse gas chromatography (IGC) can readily provide thermodynamic parameters to characterize solid surfaces. Gaseous or liquid molecular probes are injected to study the surface properties of the stationary phase. Two different methods can be distinguished IGC a t infinite dilution and a t finite concentration. Results obtained a t infinite dilution are related to adsorption of molecules on high energy surface sites, while finite concentration chromatography is concerned with all the surface. From IGC a t infinite dilution the enthalpy of adsorption, ASo, and AGO can be obtained. The nonspecific component of surface energy, yad,can be obtained by using the method proposed by Dorris and Gray.' yld can be calculated by using the linear relationship between AGO, the standard variation of free enthalpy of adsorption a t zero coverage of n-alkanes, and the number of carbon atoms of a homologue alkane series. This method was extended by Sidqi et al.4 who used alkene probes instead of alkane probes. When the free enthalpy of adsorption of hydrocarbon probes versus their number of carbon atoms was reported, a 1-alkene line was obtained parallel to, but above, the n-alkane line. The characteristic increase of energy, due to the presence of the double bond in the probe molecules, was called e,. At infinite dilution it is assumed that adsorption is described by Henry's law. In this case the free enthalpy of adsorption, AGO, is given by the following fundamental equation: BVn AGO = -RT In Sm where R is the gas constant, T is the temperature, Vn is
(6) Balard, H.; Sidqi, M.; Papirer, E.; Donnet, J. B.; Tuel, A.;Hommel, H.; Leqand, A. P. Chromatographio 1988,25 (a), 712.
(7) Dorrie, G. M.; Gray, D. G. J. Colloid Interface Sci. 1979, 72, 93.
Experimental Section Inverse Gas Chromatography. Infinite dilution as well as finite concentration experiments were carried out with an Intersmat IGC 121 FL chromatograph. Initial and treated silicas were pressed at 3 x 108Pa, and tabletswere subsequently crushed. Stainlesssteel columns of l / g in. diameter were filled with a sieve fraction of 250-400 Hm. The experimentalprocedure for infinite dilution and finite concentration experiments are described in detail elsewhere." Infinite dilution experiments were carried out at 70-90 O C using homologue series of n-alkanes, 1-alkenes, and cycloalkanes. In finiteconcentrationexperiments,adsorption isotherms were measured for n-pentane, n-hexane, 1-pentene,and 1-hexene.In all experiments helium was used as carrier gas at a flow rate of 15 mL/min. For calculation of the adsorption isotherms the
* Author to whom correspondence should be sent.
+ Visiting Scientist,Akzo-PQ Silica,Amsterdam, The Netherlands.
~
1987; 23 (2); 121.
0743-7463/91/2407-2243$02.50/0
0 1991 American Chemical Society
Tijburg et al.
2244 Langmuir, Vol. 7, No. 10,1991 25
E 3
n Q)
c
20 15
-'
10
&
5
C
0
alkane alkene cycloal kane
o !
I
I
I
I
I
3
4
5
6
7
8
I
V
9
number of C Figure 1. Variation of RT In Va versus the number of carbon atoms of the injected probes used to test nontreated silica (temperature of measurement, 80 OC).
the net retention volume, S and m are the specific surface area and mass of the solid, and B is a constant related to the standard states of gas and adsorbed phases. Equation 1 can be written in a more simple form In V, = RT where C is a constant. Taking into account the well-known linear relationship between AGO and the number of carbon atoms in a family of homologue hydrocarbons we obtain -AGO
-(a
In V , =
+
+ t~ AGc%)
+C (3) RT where n is the number of carbon atoms, AGcH, is the linear increment of AGO, and a is aconstant. Remembering that AG=AH-TAS we arrive at the final general expression
(4)
where AHcH~and A S C H are ~ linear increments of AH and AS, respectively. Because AHcH,,ASCH,, and the constants of eq 5 can be obtained by two-dimensionallinear fitting of experimental data, the true temperature dependence of ysdcan be found with eqs 6 and 7
(7)
Until now this temperature dependence ww calculated by measuring A G O at different temperatures. The method described above makes it possible to predict accurately the values of thermodynamic parameters at different temperatures and for different hydrocarbons of a homologous series. In Figure 1 the relation between R T In V , and the number of carbon atoms of the injected hydrocarbons is shown for initial silica at 80 "C. As expected the experimental points of the different n-alkanes are situated on a straight line. The alkene line runs parallel to, but above, the alkane line. The difference, E,, is equal to 3.1 kJ/mol. This is due to interaction of the ?r bond in alkene molecules with surface hydroxyl groups. The line for cycloalkanes superimposes the n-alkane line. From this
I
I
1
I
3
4
5
6
I
a
7
9
number of C Figure 2. Variation of RT In Va versus the number of carbon atoms of the injected probes used to teat treated silica (temperature of measurement, 80 OC). Table I. Thermodynamical Parametere for Adsorption on Initial and Treated Silica -MCH
kJ/mof
s
-Men J/(mol k) I
kJ/mol
U
44.8 52.2 43.8
0.003 0.014 0.045
35.2 40.5 35.0
0.017 0.007 0.036
Initial Silica n-alkane 1-alkene cycloalkane
6.96 8.15 5.06
n-alkane 1-alkene cycloalkane
5.68 6.25 5.95
-0.011 -0.014 -0.005
Treated Silica -0.009 -0.010 -0.009
phenomenon it can be concluded that adsorption of n-alkanes and cycloalkanes at the silica surface proceeds in a similar way. Structural differencesbetween aliphatic and ring structures do not give rise to differences in adsorption. Probably, cycloalkanes are adsorbed as flat structures, in which each CH2 group interacts with the silica surface. By use of the method of Dorris and Gray,' 7 : was calculated. For initial silica a 7: value of 61.3 mJ/m2 is obtained at 80 "C. Figure 2 show the results obtained for infinite dilution experiments on treated silica. The value of tr has decreased to 1.0 kJ/mol indicating a weaker interaction between 1-alkeneprobes and the solid surface. For 71d,avalue of 41.7 mJ/m2 is obtained at 80 O C . It can also be observed from Figure 2 that the cycloalkane line does not superimpose the n-alkane line. The different adsorption behavior on trehted silica can be due to a different enthalpy of adsorption or a different entropy. In order to investigate the different contributions of enthalpy and entropy to the free enthalpy of adsorption, these parameters were calculated by using eq 5. Table I summarizes the thermodynamical parameters. As an example, the enthalpy of adsorption of hexane has been calculated. The last column shows the standard deviation (a) of the two-dimensional linear fitting procedure. The values for AHcH,and ASCH, differ considerably on initial silica for all probes. However, on grafted silica, values vary in a much narrower range, suggesting similar adsorption processes for all probes. Finite Concentration Experiments. From chromatographic data, the relationship between the amount of adsorbed solute and the equilibrium pressure, the adsorption isotherm is obtained. Different types of analysis have been developed by Conder and Young.8 In this paper, only the principle of analysis by the so-called "characteristic elution points" will be briefly presented. ~
~~
(8)Conder, J. R.;Young,C. L. Physicochemical Measurement by Gas Chromatography; John Wiley: New York, 1979.
Langnuir, Vol. 7, No.10, 1991 2246
ZCC Studies on Silica
This method only requires a simple standard chromatograph. A given amount of solute is injected in a very short time, inducing a peak. Adsorption or desorption can be followed by mathematical examination of the front or tail of the observed peak. It is assumed that the temperature is constant in the whole column, that the volumetric rate of the carrier gas and solute are equal and constant, and that the system is in equilibrium in each section of the column. The use of this method leads readily to the adsorption isotherm. The study of adsorption isotherms will allow a quantitative description of the energetic properties of the solid surface, especially an evaluation of the surface energetic heterogeneity. For such a description, the socalled adsorption energy distribution function, x,is usually accepted. The relation between x and the experimentally measured adsorption isotherm Vis given by the following integral equation
v(~,r)= $‘=e( tI
e,p,r ) x ( 4 de
(8)
where B(e,p,T) is the local isotherm of adsorption on sites having adsorption energy e; p is the pressure and €1 and tm are the minimum and maximum values of the adsorption energy of the adsorption system. The calculated distribution function is dependent on the adsorbed probe molecule. To solve eq 8 with respect to the energy distribution function, we must assume some theoretical local isotherm, 0. We accept here the model of monolayer localized ‘adsorption, with nearest neighbor interactions between adsorbed molecules. Furthermore, we accept the BraggWilliams approximation9 taking these interactions into account. When the interactions between adsorbed molecules are not neglected, then the surface topography of the sites has to be taken into account.l*12 We apply here the random model of surface topographyl3assuming that the adsorption sites are randomly distributed over the surface. This assumption means that the adsorption system is considered as one thermodynamic entity. This model seems to be more realistic for silica surfaces than the “patchwise” model which assumes that sites of the same adsorption energy are grouped into large patches.14 With all these assumptionsthe local adsorption isotherm O(e,p,T) can be written in the following form (9)
where
e,
is the function called condensation energy e,
v(P,r) = -RT In E - zu -
K Vm In this equation, K is the Langmuir constant, z is the number of nearest neighbor adsorption sites, u is the interaction energy between molecules adsorbed on two neighboring sites, and V , is the monolayer capacity. The simplest approximate solution of eq 8 is given by the condensation approximation ( C A P (9) Fowler, R. H.; Guggenheim, E. A. Statistical Thermodynamics; Cambridge University Press: London, 1939. (lO)Adamson, A. W.; Ling, I.; Dormant, L.; Orem, M. J. Colloid Interface Sci. 1966,21,445. (11) Rudzinski, W.; Lajtar, L.; Patrykiejew, A. Surf. Sci. 1977,67,196. (12) Rudeinski, W.;Jagiello, J. J. Low Temp. Phys. 1982,48,307. (13) Hill, T.L. J. Chem. Phys. 1949, 17, 762. (14) Roes, S.; Olivier, J. P. On Physical Adsorption; Interscience: London, 1964. (15) Harrie, L. B. Surf. Sci. 1968,10, 128.
This approximation, however, is only accurate for low temperature conditions. Amore exact approximation was proposed by Rudzinski and Jagiellole X&,)
a”+
= --
a%
; (R‘I? 2
2,sv
at;
It was proved by Jagiello et al.“ that these approximations are special cases of an exact solution given by the following equation:
For practical calculations, however, the two-term formula (12) is recommended. This formula, like the condensation approximation, can be treated as the local solution of the integral equation which means that it does not depend on the limits of integration and, therefore, can be obtained from a limited part, or “windowW,l8of the experimental isotherm. In our calculations we perform a correction in isotherms for multilayer adsorption using the method described in ref 19. For the interaction parameter, we took the estimation given by House et al.,20 who proposed the formula zu = qv/4, where qv is the heat of evaporation. Concerning the Langmuir constant K, we calculated its value by using the method previously described in ref 17. Generally, we found values close to those obtained from the equation of Jaroniec21
To perform the calculation of the energy distribution function, we need to know the functional dependence of the amount adsorbed, V, upon pressure. This dependence can be found from experimental isotherms. The appropriate equation based on a virial expansion was recently discussed.22 It has the following form
where aji are empirical parameters. It was shown that this equation is very flexible, even with a limited number of parameters, and can describe several adsorption systems.n Introducing In p from the above equation into the defintion of ec (eq lo), we obtain for e, and x(ec) functions of two variables, Vand T. Thus, with the aid of this equation, all differentiations in eq 12 can be performed analytically. To study the effect of grafting, isotherms were measured for treated and untreated silicas. Figure 3 shows the initial part (up t o p / p o = 0.1) of n-hexaneand l-hexene isotherms on untreated and treated silica at 60 OC. In both cases, the l-hexene isotherm is situated above that of n-hexane. It can also be observed that the difference in adsorption (16) Rudzhki, W.;Jagiello, J.; Grillet, Y. J. Colloid Interface Sci. 1982, 87,478. (17) Jagiello, J.; Ligner, G.; Papirer, E. J. Colloid Interface Sci. 1990, 137,486. (18) Nederlof, M. M.;Riemsdijk, W. H.; Koopal, L. K. J. Colloid Interface Sci. ISSO, 136 (2), 410. (19) H o w , W. A.: Jaroniec,. M.:. BrHuer.. P.:. Fink, P. Thin SolidFilms lW.1, 85,87. . (20) House, W.A.; Jaycook, M. J. J. Colloid Polym. Sci. 1978,266,62. (21) Jmniec, M.Surf. Scr. 1975,50, 653. (22) Czepirski, L.; Jagiello, J. Chem. Eng. Sci. 1989, 44, 797.
Tijburg et al.
2246 Langmuir, Vol. 7, No.10,1991
0'31
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(Aexane on initial silica hexene on treated silica hexane on treated silica
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70
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P (torr)
Energy (kJlmole)
Figure 3. Comparison of adsorption ieotherms of hexane and hexene on initial and treated silicas (temperatureof measurement, 60 "C).
1
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.
+pentane
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Figure 4. n-Hexane adsorption energy distributions on initial and treated silica. between 1-hexene and n-hexane on initial silica is larger than in the case of treated silica. This can be explained knowing that the double'bond in 1-hexene molecules strongly interacts with surface hydroxyl groups. The amount of hydroxyl groups on initial silica was measured by grafting the silica with labeled methanol. This method is extensively described by Papirer et aLZ8and Mertens et a1.24 It appeared that about 50% of the hydroxyl groups were esterified. A more quantitative description of the studied adsorption systems can be given by adsorption energy distribution functions. Figure 4 shows the adsorption energy distributions of n-hexane on initial and treated silica. The energy distributions were calculated from the respective isotherms by using eq 12 with the aid of eq 15 used to fit the isotherms. In both cases a bimodal distribution is obtained. However, the treatment with alkylchainscauses both peaks to be flattened. Moreover, the high-energy peak is transformed to a broad shoulder. To study the presence of the OH groups on the silica surface, energy distributions of 1-hexene and 1-pentene were calculated. The double bond in the alkene molecules interacts strongly with surface hydroxyl groups. Energy distributions of n-hexane and 1-hexeneare shown in Figure 5a. The hexene distribution is broadened compared to the hexane distribution. The positions of the high-energy peaks are less affected than those of the low-energy peaks. The high-energy side of the hexene distribution is shifted to the right, what is probably caused by the specific interaction between the ?r bond and surface OH groups. (23)Papirer, E.;Keeeaiasia, 2.;Balard, H.Bull. SOC.Chim. Fr. 1980, 441. (24) Mertene, G.; Fripiat, J. J. J. Colloid Interface Sci. 1973,42 (I), 169.
Figures. Comparisonofn-hexaneandl-hexene(a)andofn-pentane and 1-pentene (b)adsorption energy distributions on initial silica. 0.06
0.034 0
0.01
0
4 .io '.O.o.?o.,
,O
The same results were obtained for n-pentane and l-pentene (Figure 5b). In this case, however, broadening of the distribution was less pronounced. It should be remarked that the energy distributions of pentane and pentene are situated at a lower energy compared with hexane and hexene, respectively. This is due to the different numbers of carbon atoms in the hydrocarbon molecules. The difference between the peak maxima of hexane and hexene and pentane and pentene, respectively, is 4.8 k J in all cases. The contribution to AGO of one extra CH2 group as calculated from the infinite dilution experiments is 3.5 kJ. Taking into account the experimental error, these values are in cloae agreement; however, they represent different physical quantities. The presence of surface hydroxyl groups after grafting was studied with the same method as described above.
ZGC Studies on Silica Figure 6 shows the energy distributions of n-hexane and 1-hexeneon treated silica. The shape and position of the n-hexane distribution are not affected compared to initial silica (Figure 5a), but for 1-hexene the changes are dramatic. The distribution now closely resembles that of n-hexane, indicating that the total number of (accessible) OH groups is substantially smaller on treated silica. Conclusion Adsorption energy distributions of initial and grafted silica show a bimodal distribution for CS and Ca probes. Energy distributions for alkenes were broadened compared to alkane distributions on initial silica. On treated silica,
Langmuir, Vol. 7, No.10, 1991 2247 these distributions closely resembled each other, indicating a decrease in (accessible) surface hydroxyl groups. Calculations of thermodynamic parameters obtained from IGC at infinite dilution are in good agreement with results obtained by finite concentration experiments. As IGC at infinite dilution only provides information about adsorption sites with highest energy, the shift of the high energy peak is reflected in the thermodynamicparameters obtained at infinite dilution. Registry No. Si02,7631-86-9; HsC(CH~)SCHS, 109-66-0; HaC(CHz)rCHa, 110-54-3;H2C=CH(CH2)2CHs, 109-67-1; H2C
