Langmuir 1987,3,951-957
951
Articles Adsorption of Aromatic Compounds from Solutions on Titanium Dioxide and Silica? Nina A. Eltekova and Yuri A. Eltekov* Institute of Physical Chemistry of the USSR Academy of Sciences, Leninsky Prospect 31, 117071 Moscow, USSR Received September 15, 1986. In Final Form: February 27, 1987 The adsorption of anisole, phenol, benzaldehyde, and benzoic acid from solutions in n-heptane on hydroxylated and polyoxyethylenated surfaces of titanium dioxide and silica has been studied. The distribution factors, adsorption equilibrium constants, and activity coefficients of the components of binary adsorption solutions have been calculated for a model of monolayer adsorption with consideration of the orientation of adsorbed molecules to the surface. The structure and properties of adsorption layers of poly(oxyethy1ene) on the surfacesof adsorbents have been studied by IR spectroscopy, gas chromatography, and adsorption from solutions. The modification of the surfaces of oxide adsorbents results in a lowering of the isotherms of adsorption of aromatic compounds from solutions in n-heptane and a decrease in adsorption equilibrium constants. Thermodynamic calculations have demonstrated that on the hydroxylated surfaces of oxides (where the adsorbate-adsorbent interaction is strong) the properties of the adsorption solution are close to ideal. In the case of weaker adsorption of aromatic compounds on modified surfaces of oxides, the properties of the adsorption solution show a marked deviation from ideal.
Introduction The surface of many oxide adsorbents is covered with hydroxyl groups that can enter into specific intermolecular interactions with molecules of organic compounds containing functional groups. In some cases, particularly when preparing adsorbents suitable for gas and liquid chromatography, the surface OH groups of porous silica are screened by large molecules of a polymer, which are physically adsorbed.’J The structure of the adsorbed polymer layer depends on the chemical nature and rigidity of the macromolecular chains, the chemical state of the adsorbent surface, and the polarity of the solvent. These factors determine the contribution of the energies of polymer-polymer, polymersolvent, polymer-adsorbent, and solvent-adsorbent intermolecular interactions to the displacement energy. The structure of the adsorbed polymer layers depends on the contribution of each component to the total energy of intermolecular i n t e r a ~ t i o n . ~ - ~ This paper presents the results of a study of the influence of the surface chemistry of adsorbents and the structure of the poly(oxyethylene)(POE) layer attached to the surface of TiOz and silica on the adsorption from solutions of anisole, benzaldehyde, benzoic acid, and phenol in n-heptane and on the character of intermolecular interactions in the adsorbate-adsorbent system. Experimental Section Chemically pure anisole, benzaldehyde, phenol, and benzoic acid were supplied by Khimreaktiv (USSR). The structure of the modifying POE layer was determined by gas chromatography using n-hexane, benzene, diethyl ether, and ethanol. POE (Schuchardt,FRG) with an average molecular weight of 400 was used as the modifying reagent. n-Heptane, predried with zeolite Presented at the “KiselevMemorial Symposium”,60th Colloid
and Surface Science Symposium, Atlanta, GA, June 15-18,1986;K. S. W. Sing and R. A. Pierotti, Chairmen.
0743-7463/87/2403-0951$01.50/0
NaA, and freshly distilled benzene were used as solvents. TiOzof rutile modificationP-1 (S = 5.4 m2/g)with an OH group concentration of 10.8 nm-2,macroporous silica (Silochrom (2-80, S = 100 m2/g) with a pore diameter of 50 nm and OH group concentrationof 4.0 nm-2,and carbon black (S = 85 m2/g) graphitized at 3300 K were used as adsorbents. Specific surface area, S, was determined by argon thermal desorption. The POE adsorbent surfaces were modified by adsorption from solution in benzene. Before the adsorption and modification experiments, the adsorbents were heated at 393 K for 10 h under vacuum. The adsorption measuring technique is described in ref 2 and 5. All of the adsorption data were obtained at 296 K. The adsorption equilibrium was reached in 6 h. The excew adsorption value rJn)was calculated as
rl(n)= Ax,n/mS
(1)
where Ax is a change in the mole fraction of the aromatic compound in solution due to adsorption,n is the totalnumber of moles of the solution components, and m is the adsorbent mass. The IR spectra were recorded by a Hilger H-800 double-beam spectrophotometer with an NaCl prism. The samples of TiOz and silica were pressed into 18- X 4-mm plates. Before the IR spectra were recordea, the modified samples of TiOz and silica were heated at 373 K in air. A gas chromatograph (Tswett 102, Khimavtomatika, USSR) was used with a thermal conductivity detector. Helium of extra-highpurity was used as a carrier gas with a flow rate of 4O cm3/min. The column was 5O cm long, 3-mm i.d. The column was fiied with modified or unmodified adsorbent with a particle size of 0.25-0.50 mm. The volume of sample was (1)Kiselev, A. V.; Yashin, Ya. I. Adsorbrionnaya Gasouaya i Zhidkostnaya Chromatografiya;Khimiya: Moscow, 1979 (Adsorption Gas and Liquid Chromatography, in Russian). (2) Uvarov, A. V.; Chuduk, N. A.; Eltekov, Yu. A. Kolloidn. Zh.1978, 40, 386. (3) Chuduk, N.A.; Eltekov, Yu. A. J.Polym. Sci., Polym. Symp. 1980, 68,135. (4)Howard, G. J.; Ma, C. C.; Yip, C. W. Polymn. Commun. 1983,24, 183. (5)Chuduk, N. A.; Eltekov, Yu. A. Zh. Fiz. Khim. 1981, 55, 1010.
0 1987 American Chemical Society
952 Langmuir, Vol. 3, No. 6,1987
Eltekova and Eltekov
a
a.
i
* nOLE FIMCT:CW, X , Figure 1. Adsorption isotherms of phenol (l),benzoic acid (2), benzaldehyde (31, and anisole (4) from solutions in heptane on hydroxylated surfaces of rutile (a), silica (b), and carbon black (4. 10 pL. The column temperature varied from 343 to 393 K. The absolute retention volume was calculated as VA = (tR - t,)uj/mS (2)
where tR is the sorbate retention time, tois the unsorbed substance outlet time, u is the carrier gas flow rate, and j is the correction factor for pressure drop at an average velocity u.
Results and Discussion Effect of Molecular Structure. The adsorption isotherms were compared for aromatic compounds with different functional groups such as hydroxyl, carbonyl, carboxyl, and ether groups adsorbed from n-heptane solution on the surfaces of oxide adsorbents and carbon black.6*6 Figure 1 shows the adsorption isotherms of anisole, benzaldehyde, benzoic acid, and phenol from nheptane solutions on the surface of Ti02, silica, and carbon black. For a flat orientation of the molecules of these aromatic compounds, w is the value of the surface area occupied by one molecule in a dense monolayer: 0.55 nm2 for anisole, 0.47 nm2for benzaldehyde, 0.50run2for benzoic acid, and 0.45 nm2 for phenol. There is only a slight difference between these values; therefore, the slope and general position of adsorption isotherms at low concentrations are determined not by the molecular size but by the energy of intermolecular interaction with the surface of the adsorbent. When passing from anisole to benzaldehyde, benzoic acid, and phenol, an increase is observed in the slope of the isotherm and a gradual increase in the adsorption values. The arrangement of these adsorption isotherms for Ti02, silica, and carbon black is the same, but the values for the adsorption of aromatic compounds ~
(6)Eltekov, Yu. A.; Chuduk, N. A. Zh. Fiz. Khim. 1982, 56, 425.
Figure 2. (a) IR spectra of benzoic acid (l),of benzoic acid adsorbed by rutile (2), and of rutile initially (3) and after desorption of benzoic acid from the surface (4). (b) IR spectra of benzoic acid (l),of benzoic acid adsorbed by silica (2), and of silica initially (4) and after desorption of benzoic acid from the surface (3). on Ti02 are greater than on silica. High concentrations of hydroxyl groups on the surface of Ti02 that can enter into specific interactions with adsorbate molecules contribute to their increased adsorption on its surface. The benzoic acid adsorption values obtained for silica agree with published values.' Figure 2 shows the IR spectra of benzoic acid adsorbed on the surface of TiOz and silica. These spectra indicate that the surface chemical compounds are formed on the surface of Ti02whereas on silica their formation is not observed. Figure 1 shows that at high equilibrium concentrations the values of phenol adsorption are I'l(n) = 8.5 pmol/m2 on Ti02 and I'l(N) = 4.8 pmol/m2 on silica. These values are apparently due to vertical orientation of the phenol molecules to the surface of the adsorbent. The phenol adsorption data agree with those obtained earlier.819 Structure and Properties of Adsorbed Layer. The surface of Ti02 and silica was modified by adsorbing POE from solution in ben~ene.~JOFor this purpose the adsorption of POE from solution in benzene on the surface of TiOz, silica, and carbon black was studied under static conditions. The adsorption isotherms of POE from the benzene solution per unit surface area of Ti02,silica, and carbon black are shown in Figure 3. The adsorption values (7) Wright, E. H. M.; Pratt, N. C. J. Chem. SOC.,Faraday Trans. 1 1974, 70,1461.
(8) Krasilnikov, K. G.;Kiselev, A. V. Dokl. Akad. Nauk SSSR 1951, 77, 1047. (9) Davis, K. M.; Deuchar, J. A.; Ibbitson, D. A. day Trans. 1 1973,68, 1117.
J. Chem. SOC.,Fara-
(IO) Chuduk, N. A.; Eltekov, Yu. A. Zh. Fiz. Khim. 1982, 56, 1778.
Langmuir, Vol. 3, No. 6, 1987 953
Adsorption of Aromatic Compounds on Adsorbent Surface
Table I. Thickness of Adsorbed POE Layers solvent rm at 20 mg/g, mg/m2
a
if
benzene benzene benzene water
1.1 0.65 -0.15 0.50
7,nm
0.95 0.56 0.45
on the carbon black surface becomes positive." In this cme the POE macromolecular positive adsorption is due to hydrophobic (dispersion) interactions with the carbon black surface. The thickness, T (nm), of the POE layer adsorbed on the surface of different adsorbents was calculated from the ultimate adsorption value, rm,assuming that rmcorresponds to the value of the dense adsorption monolayer and that the density, pa, of the adsorbed polymer is equal to p", the density of the polymer in the bulk specimen (pa p J :
=
t
CONCENTRATION, m j 9 - i
B
c
adsorbent rutile silica carbon black carbon black
I
Pi00
3200
T
C
/
,
3600
3 , rm-i Figure 3. Adsorption isotherms (a) of POE-400from solutions in benzene on the surfaces of rutile (I), silica (2), and carbon black (3) and from water solutions on the carbon black surface (4). IR spectra of initial (1)and modified (2) rutile (b) and silica (c) and the spectra of pure POE (3). of POE on the surface of Ti02and silica are positive over the range of equilibrium concentrations studied. Positive adsorption of POE from benzene solutions by oxides is, apart from nonspecific (dispersion) intermolecular interaction, due to additional specific intermolecular interaction between POE functional groups and the oxide surface OH groups. Thus, in the IR spectrum of silica the POE adsorption is followed by the disappearance of the absorbance band at 3750 cm-', which characterizes stretching vibrations of the silica surface OH groups, increased absorption in the 3200-3400-~m-~ region, and a shift of 40 cm-' to the low-frequency region. For Ti02, the IR spectrum also shows an increased absorbance in the region of vibrations of the surface hydroxyl groups and of the terminal POE groups with its simultaneous band shift of 20 cm-' to the low-frequency region. The IR spectra show the formation of hydrogen bonds between the POE ether oxygen and the surface OH groups, which leads to the orientation of the adsorbed POE macromolecules extended along the surface. In this case, the ether oxygen of the chain is directed toward the surface and the -CHz-CH2-bonds are directed away from the surface toward the bulk solution. The POE adsorption from the benzene solutions on the carbon black surface is negative. Such a trend of the adsorption isotherm is apparently due to stronger dispersion interaction between the benzene molecules and the graphite surface. If water is substituted for benzene, the POE adsorption
= rm/Pa
(3)
The T values calculated from eq 3 are listed in Table I. Table I shows that, in contrast to silica, thicker layers of adsorbed POE molecules are formed on the Ti02 surface, indicating that 1 m2 of the TiO2 surface area contains a greater number of POE molecules, which may be due to a poorer developed surface of TiOz compared with that of the silica, on the one hand, and, on the other hand, to the different chemical nature of the surfaces of these oxide adsorbents. The thickness uf the POE layer adsorbed on a flat surface calculated from the models of close-packed macromolecules orientated parallel to the surface, with a regard for van der Waals radii, is 0.5 nm. When POE is adsorbed from aqueous solutions on the carbon black surface the layers of adsorbed polymers are close to those calculated from the models. To assess the structure and properties of the POE adsorbed layers, gas chromatography was used to examine the adsorbents modified with different quantities of POE. Figure 4 shows the dependence of the retention volumes, V,, of n-hexane, benzene, ether, and ethanol on the coverage (e) of the surface of Ti02and silica with POE macromolecules. The values of 8 were calculated as the ratio ri/rm, where riwas preseleded on the basis of the adsorption isotherms. The values of 6' > 1were obtained for adsorbents on which POE was deposited by evaporation of a solvent. The adsorbent surfaces were covered with POE macromolecules; therefore, the retention volumes of all adsorbates decreased due to a decline in the energy of interaction between the adsorbed molecules and the adsorbent as a result of screening the most active surface adsorption centers after modification. For adsorbates susceptible to specific intermolecular interactions with a hydroxylated surface, a decrease of VA is also due to a change in the specific character of adsorption because of the replacement of the surface active groups with the POE molecules. Similar relationships were obtained for silica,12 carbon black,13 and Ti0214J5 To assess the properties of the modified surfaces, the values of q (adsorption heat) of some adsorbates on the POE monolayer were determined from the temperature relationship between log VA and 1/T. The values of Aq (the energy contribution of a specific interaction of molecules (11)Howard,G. J.; Connell, P. J. J. Phys. Chem. 1967, 71, 2979. (12) Kiselev, A. V.; Kovaleva, N. V.; Nikitin, Yu. S.J. Chromatogr. 1971,58, 19.
(13) Belyakova, L. D.; Kiselev, A. V.; Kovaleva, N. V.; Rosanova, L. N.; Khopina, V. V. Zh. Fiz. Khim. 1968,42, 179. (14) Kiselev, A. V.; Khopina, V. V.; Kovaleva, N. V.;Eltekov, Yu.A. Vysokomol. Soedin. Ser. A 1974,16,1142. (15) Eltekov, Yu.A.; Khopina, V. V.; Kovaleva, N. V.; Kiselev, A. V. J. Colloid Interface Sci. 1974, 47, 792.
954 Langmuir, Vol. 3, No. 6,1987
Eltekoua and Eltekov
a- 1
A-2 0 -3
0-4
1
1
0
n
L
V
0
1 A -Y
1
28
,
2
3
8
Figure 4. Dependence of retention volumes of diethyl ether (l),n-hexane (21, benzene (31, and ethanol (4)on the coverage of the surfaces of silica (a) and rutile (b) with POE at 373 K.
I
a
C
9 'I
RMS
I
POI rpf
mms,
24
Figure 5. Adsorption isotherms of phenol (a, b), benzoic acid (e, d), benzaldehyde (e, f), and anisole (9, h) from solutions in n-heptane on hydroxylated (1)and POE-modified (2, e = 0.5; 3, 0 = 1) surfaces of rutile (a, c, e, g) and silica (b, d, f, h).
with the oxide surface to the total adsorption energy) were determined as the difference between the value of the adsorption heat, q, of a given substance and the interpolated value of adsorption heat, q*, on the plot of the linear dependence of adsorption heat of n-alkanes on polarizability, cy, of molecules.' The values of the contributions of specific intermolecular interaction energy, Aq, to the adsorption heat and the values of adsorption heat, q, of different adsorbates on the unmodified and modified adsorbents are listed in Table 11. As shown in Table 11, the adsorption heat for all adsorbates is lower on the POE monolayers than it is on the unmodified samples of TiOz and silica, which is due to weakening of the dispersion and specific interaction of the molecules with a modified sur-
Table 11. Adsorption Heat q (kJ/mol) of n -Hexane and Benzene and Contribution of Specific Interaction adsorbent
H-CnH,a
TiOz TiOl + POE silica silica + POE carbon black carbon black + POE
30.6 28.8 29.2 25.9 43.5 33.4
C6H6 45.5 31.5 35.5 29.8 41.0 35.0
17.1 6.0 7.5 5.9 2.9
face of oxides because of the replacement of strong specific interactions with oxide surface hydroxyl groups by weaker specific interactions with both ether and hydroxyl POE groups. For carbon black a decrease in q values is due to
Langmuir, Vol. 3, No. 6, 1987 955
Adsorption of Aromatic Compounds on Adsorbent Surface a weaker dispersion interaction of both n-hexane and benzene molecules with the POE monolayer as compared with the interaction of the molecules with the unmodified graphite surface. Effect of Surface Modification. Figure 5 shows a comparison of adsorption isotherms of phenol, benzoic acid, benzaldehyde, and anisole from solutions in n-heptane on hydroxylated surfaces of Ti02 and silica and on the surfaces modified with POE. In all cases there is a decrease in adsorption of the aromatic compounds due to modification of the oxide surfaces. For quantitative assessment of the influence of modification on the parameters of adsorption systems, calculations and correlations were made of the distribution factor, f , and of the adsorption equilibrium constant, K, for adsorption of aromatic compounds on the surfaces of Ti02and silica, taking into account the intermolecular interaction in both the bulk and surface solutions. For binary solutions, the molecules whose componenfs have similar dimensions (displacement factor is l),the adsorption equilibrium constant can be written as
K = Xh3X,Y2/~lYlX!Y!
(4)
where y2/y1and $1 y; are ratios of activity coefficients in the bulk and surface solutions, respectively, and xl, x2, xS,, and x: are mole fractions of components 1and 2 in the bulk and surface (s) solutions. The activity coefficient isotherms for bulk solutions were calculated from the equation applied to strictly regular solutions:16 In y1 =
(6,1/2
Vl - 621/2)2 RT -a2
(5)
where a1 and 62 are the Hildebrand solubility parameters equal to the ratios AUl/ Vl and AUz/ V 2 , AUl and AUZare internal energies of evaporation, Vl and V2 are molar volumes of components 1and 2, respectively, and rpz is the volume fraction of component 2 in the bulk solution. The isotherms of the activity coefficients in the surface solution were calculated by the Everett equation:l7JS
The adsorption equlibrium constant calculated without considering yi and yt is f , a distribution factor reflecting the interactions in the adsorbamlvent-adsorbent system and depending on the concentration of the components of the bulk and surface solutions: f = x;x2/x,xs,
(7)
Values of x ; were calculated by eq 8, where Am,l is the monolayer capacity component and B = wl/wz is the displacement factor. x! =
+ Am,lB- ( B - l)rlcn)
When K and f are calculated, the following assumptions were made: the adsorption is limited by the formation of a monolayer, the orientation of adsorbed molecules of the components of solution in the process of adsorption is (16)Hildebrand, J. H.; Prausnitz, J. M.; Scott, R. L. Regular and Related Solutions, Van Nostrahd Reinhold New York, 1970. (17)Everett, D.H. Trans. Faraday SOC.1964,60, 1803. (18)Everett, D. H. In Adsorption from Solution; Ottewill, R. H., Rochester,C. H., Smith, A. L., Eds.; Academic: New York, 1983;pp 1-50.
I
PIOLE
FRACTIONS
Figure 6. Isotherms of activity coefficients of anisole (component 1)and n-heptane (component 2) in bulk solution (a) and surface solution (b-e) on hydroxylated silica (b) and rutile (d) and on silica (c, 0 = 1) and rutile (e, 0 = 1) modified with POE. Table 111. Effect of Surface Modification on fo and K K adsorbate adsorbent 9 f" f"rzlr1 anisole TiOp 0 270 f 40 122 f 30 110 f 20 1 10f2 40 f 1 8 f 22 silica 0 30 f 5 17 f 4 25 f 5 1 5fl 3f1 5fl benzaldehyde TiOz 0 86Of 130 280 f 70 300 f 60 1 30f5 10 f 3 25 f 5 silica 0 190 f 30 65 f 15 70 f 14 1 18f3 20 f 4 6f2
permanent, and the values of the surface areas occupied by molecules of the components of solution on the adsorbent surface are close fB = 1). Figures 6 and 7 show the activity coefficient isotherms of anisole, benzaldehyde, and n-heptane in the bulk and surface solutions. A comparison of these isotherms reveals a trend toward more ideal surface solution properties as compared with the bulk solution. In the case of anisole and benzaldehyde, the value of yi on the oxide hydroxylated surface is close to unity over almost the whole range of xS,, and for silica this range is -0.5-1. In the case of the adsorption of anisole and benzaldehyde on a modified surface of Ti02and silica, the adsorption solutions deviate from the ideal state. Moreover, in the case of the weak adsorption of anisole and benzaldehyde on a modified surface of oxides, deviations from the ideal state strongly
956 Langmuir, Vol. 3, No. 6, 1987
adsorbate anisole benzaldehyde benzoic acid phenol
Eltekova and Eltekov
Table IV. Relationship between log f* and Physical Characteristics log f* at XI = 0.001 TiOz SiOz a. nm3 u. D 6. J/cm3 8=0 0=1 o=o 0=1 2.6 1.8 1.5 1.0 0.394 1.2 400 2.9 2.1 0.400 2.8 510 2.4 1.8 0.41 1.8 560 3.0 2.1 2.8 1.9
carbon black 0 = 0 0.5
~
0.407
1.5 a
U
540
3.0
2.6
2.5
1.33 2.5 1.7
2.2
/
/
1
;
s,
500 rcm-3 Figure 8. Dependence of logfc for anisole (l),benzaldehyde (2), benzoic acid (3), and phenol (4) on Hildebrand's parameter for rutile (a) and silica (b). 40 0
+
I
1 MOLE F R R C T I O N S
Figure 7. Isotherms of activity coefficients of benzaldehyde (component 1)in n-heptane (component 2) in bulk solution (a) and surface solution (b-g) on hydroxylated silica (b) and rutile (d) and on silica (c, 0 = 1) and rutile (e, 0 = 0.5; g, 6 = 1)modified with POE.
influence the behavior of components in the adsorption solution. In the case of strong adsorption on the hydroxylated surface of oxides, the effect of deviations is insignificant and the properties of the surface solution are close to ideal. Thus,the change in chemistry of the surface of adsorbent by modification substantially influences the intermolecular interaction in the adsorption solution and, consequently, the value of K. Table I11 lists the values of distribution factors, f " , at x: = 0.5 and of the equilibrium adsorption constants, K, calculated by taking into account the values of y1 and y:. A comparison of the values off" and K demonstrated that they are substantially different in the case of the adsorption of anisole and benzaldehyde on hydroxylated oxides. In this case, in calculating the value of K it is important to take into account the influence
of y1 and y2 since the value of 7: is close to unity. If molecules of aromatic compounds are adsorbed on modified surfaces, the influence of intermolecular interactions in a bulk solution on the K values is reduced due to an increase in intermolecular interaction in the adsorption solution. In this case the value of 7: is different from unity. In the w e of adsorption on the modified surface of oxides, the K values calculated with regard for the values of y1 and y2 are close to the values off. As is seen from Table 111, modification of the surface of Ti02and silica with POE reduces the value of K by a factor of 14 for Ti02 and by a factor of 5 for silica due to weakening of the specific interaction between the molecules of anisole and the POE adsorption layer deposited on the surface of TiOz and silica. The adsorption values and the values of log f* for all aromatic compounds investigated in this and previous works were compared with some physical characteristics of their molecular structure (Table IV). Table IV shows that in this series of adsorbates from anisole to benzaldehyde, benzoic acid, and phenol there is a rise in the value of log f*, and polarizability, a,of molecules due to the stronger interaction with the adsorbent surface in this series. No consistency in the values of dipole moment, p , and of polarizability, a,was observed in this series of aromatic compounds. The values of log f* were compared with Hildebrand's parameters of s~lubility,'~ 6, since the character of adsorption of aromatic compounds of this series by Ti02 and silica mainly depends on whether the adsorbate molecules can interact specifically with oxide hydroxylated surfaces due to the formation of hydrogen bonds between functional groups in an adsorbate molecule and OH groups of the oxide surfaces. Figure 8 shows the dependence of log p on Hildebrands solubility paramter, 6, whose general trend is intensification of interaction of aromatic compounds with an increase in the parameter 6 upon adsorption by Ti02 or silica. This indicates the general character of the interaction between these aromatic compounds and hydroxylated surface of oxides. In this case, the adsorption interaction is stronger (19)Timmermans, J. Physico-Chemical Constants of Pure Organic Compounds; Elsevier: New York, 1950.
Langmuir 1987, 3, 957-959 for TiOz due to a higher concentration of OH groups on the TiOz surface.
Conclusions The slopes of the isotherms and the values of adsorption for TiOz, silica, and carbon black increase in the order anisole < benzaldehyde < phenol < benzoic acid. The order of position of these adsorption isotherms for TiOz, silica, and carbon black is the same, but the values of adsorption of aromatic compounds on TiOz are greater than those on silica and carbon black because of the different surface chemistry of the three adsorbents. The thickest layers of adsorbed POE molecules are formed on the TiOz surface, in contrast to silica and carbon black where the POE layers are about 0.5 nm thick. The adsorption heats for n-hexane, benzene, ether, and ethanol are lower on the POE monolayers than on unmodified samples of TiOz, silica, and carbon black. The thermodynamic calculations have demonstrated that in the case of adsorption of aromatic compounds on modified surfaces of oxides the values of K and f decrease and the adsorpticm solution shows a deviation of properties from ideal. A good correlation has been found between Hildebrand‘s parameters of solubility and the values of log f* for adsorption of aromatic compounds on TiOz and silica.
957
Acknowledgment. In November 1948 Andrey Vladimirovich Kiselev, then a professor in the Chemical Department of Lomonosov University, suggested to one of us (E.Yu.A.) that we undertake a theoretical investigation “on the nature of adsorption forces” as a student project. In 1959 he advised us to develop a fundamental study of the thermodynamics of adsorption from solutions and its relationship with surface chemistry. A group of scientists from the Institute of Physical Chemistry assisted with the project. The constant leadership of A. V. Kiselev made it possible to determine the important laws of behavior of molecules at the solid-liquid interface. We are indebted to the late Professor A. V. Kiselev for his useful advice and discussions. We will miss him not only as an outstanding scientist but as an inexhaustible source of scientific ideas and a famous collector of ancient Russian china and furniture. We wish to thank Professor K. s. W. Sing (Brunel University, U.K.) and Professor Robert A. Pierrotti (Georgia Institute of Technology, U.S.A.) for organizing the A. V. Kiselev Memorial Symposium and for their kind invitation to take part in this outstanding meeting. Registry No. TiOz, 13463-67-7; S a z , 7631-86-9; POE, 25322-68-3; phenol, 108-95-2;benzaldehyde, 100-52-7;benzoic acid, 65-85-0; anisole, 100-66-3; n-heptane, 142-82-5;benzene, 71-43-2;
n-hexane, 110-54-3.
Adsorption of Aromatic Hydrocarbons on Hydroxylated and Dehydroxylated Silica Gel? Yuri A. Eltekov* and Valentina V. Khopina Institute of Physical Chemistry of the USSR Academy of Sciences, 117071 Moscow, USSR Received September 15, 1986. I n Final Form: February 27, 1987 The high-performance liquid chromatography (HPLC) of aromatic hydrocarbons on macroporous silica gel with hydroxylated and dehydroxylated surfaces has been studied at 20 O C . The adsorption isotherms of toluene, naphthalene, biphenyl, phenanthrene, anthracene, m-terphenyl, and p-terphenyl in the Henry region were calculated from the chromatograms. The effects of dehydroxylationof the silica gel surface and the chemical structure of aromatic hydrocarbon molecules on Henry’s constants were determined. Henry’s constants were compared with the distribution coefficients of adsorption equilibrium calculated from €he model of an adsorption l a y e r d h a constant thickness and adsorbed molecules oriented parallel to the surface.
Introduction Traditional static methods for studying adsorption at a solid-solution interface cannot be applied to very dilute solutions. By use of high-performance liquid chromatography (HPLC) with highly sensitive detectors, it is possible to study adsorption from dilute solutions in the Henry’s law region. Various procedures for calculating adsorption isotherms on the basis of data obtained by chromatography are discussed in ref.l In this paper, the results of studying the effect of silica gel dehydroxylation on the adsorption of aromatic hydrocarbons from very dilute solutions by an HPLC method are presented. These investigations are being carried out ,~ by the surface chemistry group in o w l a b ~ r a t o r y . ~The t Presented at the “Kiselev Memorial Symposium”, 60th Colloid and Surface Science Symposium, Atlanta, GA,June 15-18,1986; K. S. W.Sing and R. A. Pierotti, Chairmen.
present paper is another in a series of investigations on the effect of surface chemistry on adsorption from solution^."^
Experimental Section Toluene freshly distilled (chromatographically pure), naphthalene, biphenyl, phenanthrene, anthracene, m-terphenyl,and p-terphenyl, all recrystallized from benzene, were used as adSorPtives. (1)Huber, J. F. K.; Gerritae, R. G. J. Chromatogr. 1971, 58, 137. (2) Chuduk, N. A.; Eltekov, Yu. A. Zh.Fir. Khim. 1979, 53, 1032. (3) Eltekov, Yu. A.; Kazakevich, Yu. V.; Kiselev, A. V.; Zhuchkov, A. A. ChromatograPhia 19859 2% 525. (4) Kiselev, A. V.; Khopina, V. V.; Eltekov, Yu. A. Izu. Akad. Nauk sssR,Otd. Khim. Nauk 1958, No, 6,664. ( 5 ) Kiselev, A. V.; Khopina, V. V.; Eltekov, Yu. A. J. Chem. SOC., Faraday Trans. 1 1972,68, 889. (6)Kiselev, A. V.;Khopina, V. V.; Eltekov, Yu. A. Kolloidn. Zh. 1974, 36, 963. (7)Khopina, V. V.;Eltekov, Yu. A. Zh. Fiz. Khim. 1979, 53, 1806.
0743-746318712403-0957$01.50/0 0 1987 American Chemical Society
