2326
P. URONE,Y. TAKAHASHI, AND G. H. KENNEDY
conceivable that at the extremely high temperatures possible within the cavitation bubblell that the amount of energy available so far exceeds that necessary to
bring about the chemical reaction that any change in the energy available brought about by the addition of surfactantsis negligible.
Sorption Isotherms of Polar-Nonpolar Systems on Liquid-Coated Adsorbents by Paul Urone, Yoshihiro Takahashi, Department of Chemistry, University of Colorado, Boulder, Colorado 80602
and George H. Kennedy Department of Chemistry, Colorado School of Mines, Golden, Colorado 80401
(Received August 5 , 1960)
Experimental studies of polar-nonpolar liquid-coated adsorbent systemsat low liquid coatingsreveal basic sorption relationshipsnot previously understood. Sorption isothermswere obtained for the bulk liquids, the uncoated adsorbents,and adsorbents coated with 0.1-20% liquid phase. Relative pressures ( P / P o )were varied from 0,001 to 0.30, and temperature studies were made for thermodynamic evaluation. Bulk liquid partition coefficients and Henry's law constants at 30' are given. Sorption of polar solutes on squalane-coated adsorbents equal the sum of the contributions of the squalane and the adsorbent, and nonpolar solutes show slight sorption from solution equilibrium effects. Tri-o-tolyl phosphate (TOTP) and tris(cyanoethoxypr0pane) (TCEP) interact with the adsorbent to form a modified surface which gives reduced, and sometimes linear, isotherms. Polar solutes show isotherms that are a sum of the contributions of the liquid-modified surface and the bulk liquid. The isotherms of hexane, as a nonpolar solute, on TOTP-coated adsorbents show the possibility of a small contribution from adsorption on the liquid surface in addition to the contributions of the bulk liquid and the liquid-modified surface. The demonstrated modified surface and the additivity of the sorption terms have long-range potentials for the measurements of thermodynamic properties and the development of specialized surfaces. An earlier study of sorption isotherms of acetone on liquid-coated adsorbentsl showed that the amount of solute sorbed at a given partial pressure (Qtota1) was equal to the sum of: (1) the amount adsorbed by the surface of the adsorbent modified by a thin layer of coating liquid (Qmod) and ( 2 ) the amount sorbed by the remainder of the coating liquid (&liquid). &total
=
&mod
+
(1)
&liquid
The amounts sorbed in each case depended upon the partial pressure of the solute and its isotherm on each of the respective phases; i e . , a t constant temperature Qt
= W,
lp
dP
+ WL
sop
dP dP
(2)
ivhere Qp is the amount of solute sorbed a t a given partial pressure, P; dQ,,d/dP and d&L/dP describe the isotherms of the modified surface of the adsorbent and of the bulk liquid, respectively. W , is the weight of the modified adsorbent and WL is the weight of liquid coating above that needed to modify the surface. When, as is common at low conThe Journal of Physical Chemistry, Vola74,No. 11, 1070
centrations, the isotherm for the solute in the liquid is linear, d&L/dP is constant and eq 2 becomes Qt =
W,
sop
d&mod d P d P + WLKLP
(3)
where K L , the partition coefficient, is the ratio of the concentration of the solute in the liquid to that in the gas phase. When squalane was used as the coating liquid, the adsorbent surface was only slightly modified, and the amount sorbed was essentially the same as that sorbed by the uncoated adsorbent. This report extends the studies to cover in more detail examples of polarnonpolar solutes, solvents, and modified adsorbent surfaces.
Experimental Section The sorption isotherms were determined gravimetrically using the apparatus and techniques described previously1 except that an electrotorsion Bourdon gauge was added to the vacuum system to measure (1) P. Urone, Y. Takahashi, and G. H. Kennedy, Anal. Chem., 40, 1130 (1968).
SORPTION ISOTHERMS OF POLAR-NONPOLAR SYSTEMS
2327
solute pressures with greater sensitivity and accuracy. The gauge (Ruska Instrument Corp., Houston, Texas) was capable of measuring pressures from 0 to 10 Torr Torr. with a sensitivity of f Materials. Acid-washed firebrick (AWFB) (60-80 mesh) (Matheson Coleman and Bell), 60-80 mesh Chromosorb W (John Mansville), and 60-80 mesh acid-washed dimethyldichlorosilane-treated firebrick (DMCS) (Varian Aerograph) were used as adsorbents. Before coating with liquid phase, the adsorbents were dried at 110’ for more than a day. Tri-0-tolyl phosphate (TOTP) (Matheson Coleman and Bell) was used without further purification. 1,2,3-Tris(cyanoethoxy)propane (TCEP) (Wilkens Instrument and Research) was degassed in a vacuum oven at 50” for 6 hr, and squalane, CaoHe2 (Eastman Organic Chemicals), was purified by passing through a silver nitrate modified silica gel column.2 When unpurified squalane was used, experimental results were influenced by the impurities present and inconsistent effects were observed.’ Acetone, methanol, and benzene vi-ere all Nanograde (llallinckrodt Chemical Works) ; n-hexane was reagent grade, boiling range of 0.4” (J. T. Baker Chemical Go.) ; and 2,2,4-trimethylpentane (isooctane) was also reagent grade (Fisher Scientific). Dissolved oxygen and carbon dioxide were degassed from the water before using. Coating of the Supports. To obtain a uniform liquid coating on the supports, the method developed by Parcher and Urone was used.3 About 20 g of the solid support was placed in a sintered glass filter funnel which was connected to a suction flask. Approximately 100 ml of the solution of the liquid phase having the desired concentration was poured into the funnel. The solid and the solution were very gently mixed with a glass rod before suction was applied. Care was taken not to break any support particles. The solution was then filtered out with suction until the particles were free flowing, after which dried N2 was blown through the bottom end of the funnel while heating tape was used to warm the funnel walls. The actual amount of liquid coated on the adsorbent was determined by washing 1 g of the sample several times with a total of 250 ml of the same solvent used for making the solution, and obtaining the loss in weight after drying. Measurement of Sorption Isothermsand Solubility Curves. A sample was hung on one side of the microbalance, carefully counterbalanced with glass weights, and degassed. When the sample was an uncoated solid it was degassed for hr at to Torr’ When the sample was a liquid or a liquid-coated solid, it was degassed a t room temperature for 3 hr a t to Torr, depending upon the vapor pressure of the liquid phase used. Most of the isotherms were taken a t 30 f 0.1”. A Small amount of Solute Vapor Was introduced,
and after equilibrium was reached, the pressure of the vapor and the weight change were recorded. I n general it took no more than 2 to 5 min to reach equilibrium for all samples except the pure liquid samples, which took as much as 12 hr. For the coated adsorbents, Q was calculated as micromoles sorbed per gram of coated adsorbent. The above steps were repeated until the relative pressure, PIPo, reached approximately 0.1. Po is the saturation pressure of the solute at the temperature of the experiment. In order to calculate the surface area of a sample, PIPo readings had to be taken to relative pressures of 0.3 or higher. Surface areas were calculated using the well-known BET m e t h ~ d . ~ The solubilities of the solutes in the nonvolatile solvents were measured in the same manner as described for measuring the sorption isotherms. The evaporation rates of the solvents in the presence of comparable helium pressures were determined and applied when necessary. Buoyancy corrections were obtained by using a nonadsorptive gas (Nz) at room temperature and were found to be negligibly small. For surface area measurements at liquid nitrogen temperatures, buoyancy corrections were obtained with helium. They were found to be significant, and the proper corrections were made.
Discussion Bulk Liquids. The isotherms, or solubilities, of the four solutes in the various nonvolatile coating liquids were obtained as described above. In each of the cases studied plots of mole fraction sorbed vs. the relative pressure gave straight lines showing adherence to Henry’s law for the pressure ranges studied. Table I gives the slopes of the lines expressed as the partition coefficient K L , where K L = dx/d(P/Po), and x is the mole fraction of the solute in the solvent. The amounts of n-hexane and methanol which may have dissolved in TCEP or squalane, respectively, were so low they could not be observed directly. The Table I: Partition Coefficients at 30.0’:
Solvent----
----,
Solute
a
K L = dx/d(P/Po)
Squalane
TOTP
-
TCEP
%-Hexane 1.55 2.95 a Isooctane 1.32 ... ... Acetone 0.30 0.94 0.97 Methanol a 0.60b . * , Below experimental error. See text. * Measured a t 45.0’.
(2) B. deVries, J . Amer. Oil Chem. Soc., 41, 403 (1964). (3) J. F. Parcher and P. Urone, J. Gas Chromatog., 2 , 184 (1964). (4) S. J. Gregg and K. S. W. Sing, “Adsorption, Surface Area and Porosity,” Academic Press, New York, N. Y., 1967.
The Journal of Physical Chemistry, Vol. 74, No. 11, 1970
2328
P. URONE,Y. TAKAHASHI, AND G. H. KENNEDY
evaporation of the TCEP or squalane over the many hours required for attaining equilibrium with the bulk liquids a t each datum point was of the same order or greater than the amounts sorbed in these two cases. Corrections for evaporation, consequently, gave indeterminant results.
Solute
Isooctane %-Hexane Methanol Acetone
47.3 120.6 b 932.3
k
45.6 121.6 b 946.4
-9
16.0
-D
12.0
3.
80
0
___ ---TOTP--
47
20.0
-2
Table 11: Henry's Law Constantsa a t 30.0" ____-___-Solvent -Squalane--k k'
24 0
4.0 7
-TCEP-k'
k
...
...
...
634.2 544c 300.5
639.9 587~ 319.2
b
k'
b
...
290.9
322.1 Un-
4.0
P/p, x
...
...
a Where k = P/x and k' = f/x; = r"Po. P in Torr. able to detect amount dissolved. c Taken at 45.0".
2 .o
0
8.0
6.0
.
102
Figure 1. Sorption isotherms for acetone on TCEP-coated AWFB at 30". 24
Table I1 gives the Henry's law constants for the same systems. These were calculated using the observed solute pressures for IC and fugacities for k' where P was measured in Torr and
P f
=
kx IC'X
=
-!"Pox
=
(4)
(5) (54
hence
IC' = -!"Po
-P> - (Bz2 - v2°)(P20 RT
(7)
where Bzz,V20and PZO are the second virial coefficient, the molar volume, and the saturation pressure of the solute, respectively. Polar Solute-Polar Coating. In the earlier paper,l the isotherms of acetone on TOTP coated columns were used to develop and confirm eq 2 . In extending the studies, several systems representing a wide range of relative polarities were selected. For polar solutepolar coating systems the isotherms of acetone in TCEP and methanol in TOTP coated adsorbents were obtained. Acetone and TOTP are intermediate in polarity while methanol and TCEP are of relatively high polarity. Figure 1shows a typical series of isotherms of acetone on the adsorbent (AWFB) coated with increasing amounts of TCEP. As the amount of coating was increased, the adsorbent surface became modified and a The Journal of Physical Chemistry, Vol. 74, No. 21, 1970
I
I .o
I 20
I
30
PERCENT LIQUID COATING
I 40
3
Figure 2. Sorption of acetone on TCEP-coated AWFB at 30 ' (A) and of methanol on TOTP-coated AWFB at 46' (B) at PIP0 = 0.04 as a function of per cent liquid coating.
(6)
The activity coefficients a t infinite dilution were obtained by extrapolation from plots of the natural log of the activity coefficient us. the mole fraction using6 In y = In PIPOX
4tI
0
drop in the isotherms was observed. A minimum isotherm developed at an amount of coating roughly equivalent to that necessary for a monomolecular layer covering approximately 0.5% wt/wt for an adsorbent with area of 4 m2/g. Beyond the minimum, the sorption isotherms increased in proportion to the amount of TCEP used. Figure 2 shows the variation of the amount sorbed at a given pressure, PIP0 = 0.04, as a function of the per cent of liquid coating for both acetone in TCEP (curve A) and methanol in TOTP (curve B). The data were taken from isotherms similar to those shown in Figure 1. The amount of solute sorbed decreased sharply as the per cent of liquid coating was increased from 0 to approximately 0.5%. After a minimum developed, the amount sorbed increased in proportion to the per cent of liquid coating used (Figure 1). The difference in the slopes beyond the minimum for the two solutes (Figure 2) was caused by the difference in solubilities of the solutes in the respective liquids. Figure 3 graphically portrays the calculated contributions of the modified surface (Qmod) and the bulk (6) Y . Takahashi, P. Urone, and G. H. Kennedy, J . Phys. Chem., 74, 2333 (1970).
2329
SORPTION ISOTHERMS OF POLAR-NONPOLAR SYSTEMS
I
24.01
wpo
'"I
x 102
Figure 3. Calculated sorption isotherms and experimental data points at 30' for acetone on 4.23% TCEP-coated AWFB. Dashed lines give the contributions of the modified surface, Qm, and the mating liquid in excess of that needed to modify the surface, Q ~ i ~ ~ i The d . solid line gives the sum of the dashed lines, and the circles are the observed data points. 0
I
I
I
2.0
4.0
6.0
I 8.0
I 10.0
I: 3
PERCENT SQUALANE 24
20
1
1
I 0 :OBSERVED DATA POINTS
,
I
I
20
4.0
6D
_--_ _ _ I - -
0
I 80
"Po x IO2
Figure 4. Calculated sorption isotherms and experimental data points for methanol on-4.93% TOTP-coated AWFB at 45'. Curve identities as noted in Figure 3.
liquid (&liquid) to the observed amount sorbed (Qtotal) for acetone on the AWFB adsorbent coated with 4.23% TCEP. The individual contributions (dashed lines, Figure 3) were calculated according to eq 2 using the isotherms obtained from the 0.55% TCEP-coated AWFB and the bulk TCEP, respectively. The solid line represents the sum of the calculated contributions while the small circles indicate the observed data points. There is close agreement between the calculated solid line and the observed data points. Figure 4 represents the same type of data calculated for methanol on 4.93Oj, TOTP-coated AWFB. In this case, the contribution of the modified adsorbent surface (&mod) was greater than that seen in Figure 3, principally because of the differences in the relative polarities and solubilities of the solutes and coating liquids involved. Nevertheless, the calculated isotherm and the observed data points again are in good agreement. Polar Solute-Nonpolar Coating. Although acetone and methanol were only slightly soluble in squalane,
Figure 5. Sorption a t 30' and PIPo = 0.04 as a function of per cent squalane: A, acetone on AWFB; B, n-hexane on AWFB; C, isooctane on AWFB; D, acetone on DMCS-treated AWFB.
the isotherms for these compounds on squalane-coated AWFB showed that both acetone and methanol easily penetrated the nonpolar squalane to interact with the adsorbent surface as if the squalane were not there (Figure 5, curve A). In these cases the total amount sorbed by the liquid-coated adsorbent is more appropriately expressed by &total
=
&solid
+
&liquid
(8)
where &solid is the amount sorbed by the adsorbent whose surface is not modified with respect to the solute by the coating liquid. Figure 5 shows the amounts sorbed a t constant relative pressure, PIP0 = 0.04, for several solutes on squalane coated adsorbents. Curves A and D are for acetone on two different types of adsorbents: surface active AWFB and deactivated DMCS, respectively. Although the squalane contribution in both cases was identical, curve A is much higher than curve D because the surface activity of the AWFB adsorbent used for curve A was much greater than that of the DMCS adsorbent used for curve D. The DMCS treatment converts the active hydrogen sites on the surface to trimethylsilyl groups imparting a paraffinic character to the surface. Other chemical treatments, a strongly adsorbed compound, or a thin layer of a polar coating liquid can easily modify the adsorbent surface when a nonpolar coating liquid is used. The use of water or steam, for example, to modify the properties of an adsorbent is well known.6 (6) H. 8. Knight, Anal. Chem., 30, 2030 (1958). The Journal of Physical Chemistry, Vol. 74, No. 11, 1970
2330
P . URONE,Y . TAKAHASHI, AND G. H. KENNEDY
Both curves A and D (Figure 5 ) obeyed eq 8 strictly. A curve for methanol on AWFB (not shown) was out of the vertical range of Figure 5 , but it also obeyed eq 8. Curve D emphasizes what has been previously observed, that DMCS treament does not completely deactivate the support surface.' Curves B and C are for n-hexane and isooctane and will be discussed in the following section on nonpolar solutes. Figure 6 shows the effect on the isotherm of acetone on squalane-coated AWFB by the addition of 0.6% TCEP to the approximately 3% squalane used. The adsorbent without the TCEP has a high, nonlinear sorption isotherm. The one with the TCEP has a much reduced and essentially linear isotherm. The small amount of TCEP (which was added with the squalane during the coating process) adhered to and modified the surface to give it a more homogeneous and less adsorptive character. Nonpolar Solute-Nonpolar Coating. For a nonpolar system, the isotherms of n-hexane and isooctane on squalane (a saturated hydrocarbon) were studied. Figure 7 shows representative isotherms for isooctane on squalane coated AWFB. As the per cent of squalane was increased, a slight minimum in the sorption isotherms was observed, following which the isotherms continually increased with increasing amounts of squalane. In this case, the squalane competed with the isooctane for the active adsorption sites probably setting up a steady-state adsorption equilibrium such as that proposed by Everetta8 aL8
+ bSL = aLL + bSa
r-----
L IY
3.23% SOUALAN
2.9% SOUALANE t 0.6% TCEP
I
/
Figure 6. Effect of small amount of TCEP on sorption isotherm of acetone on squalane-coated AWFB.
12.0
10.0
8.0
e -8
6.0
0
E
3
4.0
0
2.0
2.0
0
4.0 P/po x 102
6.0
8.0
Figure 7. Sorption isotherms of isooctane on squalane-coated AWFB at 30".
(9)
where L denotes the coating liquid, S the solute, and the superscripts s and L indicate the sorbed and liquid phases, respectively. The coefficients u and b are introduced because the size difference in the solute and coating liquid molecules may not allow a 1:1 solute-liquid interchange. The amount of isooctane sorbed in the modified surface is small compared to its solubility in squalane. Hence, the isotherms of Figure 7 appear to be linear although the modified surface continued to adsorb isooctane irrespective of the amount of squalane present. Such pseudolinear isotherms were observed whenever adsorption by the support was small relative to the amount of solute dissolved by the coating liquid. Curves B and C in Figure 5 clearly show the variation of the amounts of n-hexane and isooctane sorbed with increasing amounts of squalane. A slight minimum is developed after which the curves rise rapidly because of a high solubility in squalane. The difference in the slopes indicates the difference in the solubilities of the two solutes. Nonpolar Solute-Polar Coating. The solubility of a nonpolar solute in a polar solvent will vary considerably depending upon the type and relative polarity of the solvent. Two systems were studied in this respect: The Journal of Physical Chemistry, Vol. 74, No. 11, 1970
20 0
hexane on TOTP and hexane on TCEP-coated adsorbents. Hexane was measureably soluble in TOTP, but it was practically insoluble in TCEP. The isotherms for n-hexane on TOTP-coated AWFB adsorbents showed the typical development of a minimum isotherm at approximately 0.3% TOTP with an increase in the isotherms as the percentage of TOTP coating was increased. When the data were studied for conformance to eq 2, it was found that the calculated total sorbance values matched the experimental data better with a three-component equation, Recognizing the possibility of contributions by all phases p r e ~ e n t ,eq ~ 1 was then expanded to include contributions from the solid-liquid and the liquid-gas interfaces in addition to the modified surface and the bulk coating liquid. &total
= &solid
+
&mod
+
&liquid
QAL
(IO)
where Qmod is the amount sorbed by the modified ~ the amount sorbed by adsorbent surface and Q A is (7) J. Bohemen, S. H. Langer, R. H. Perrett, and J. H. Purnell, J . Chem. SOC.,2444 (1960). (8) D. H. Everett, Trans. Faraday SOC.,6 0 , 1803 (1964); 61, 2478 (1965). (9) R. L. Pecsok and B. H. Gump, J . Phys. Chem., 71, 2209 (1967).
2331
SORPTION ISOTHERMS OF POLAR-XONPOLAR SYSTEMS the coating liquid surface. At constant temperature Qt
=
W,
lp WL
dP
+ Wm
1'$$
lp%
dP
+
0 * OBSERVED
+A L L
0 8.0
% d
I
'*,so/
dP (11)
where W , is the weight of the unmodified adsorbent present, AL is the surface area of the coating liquid, and dQ,/dP and d&A,/dP describe the respective isotherms. I n general, the term disappears when the adsorbent surface is modified, and the Q A term ~ is negligibly small except for the special case of a nonpolar solute on a highly polar coating liquid. At a given pressure and temperature and for liquid coatings above 1%, it can be assumed that the distribution coefficients for the solutes on each of the respective phases remain constant and reasonably independent of the amount of liquid or liquid surface present. Equation 11 simplifies to &($,PI=
POINTS
W
P
dP
DATA
WmKmod4- WLKL-k ALKA
Figure 8. Calculated sorption isotherms and experimental data points for n-hexane on TOTP-coated AWFB at 30" and PIPo = 0.04. Curve identities as noted in Figure 3.
=d"---
2 .O
(12)
where Kmod,KL, and K A are distribution coefficients for the modified surface, the bulk liquid, and the liquid surface area, respectively, at the given solute pressure and temperature.'O Figure 8 shows the observed sorption as well as the hypothesized contributions of the modified adsorbent surface and the liquid surface as calculated by eq 12 for n-hexane on TOTP-coated AWFB. The bulk liquid term was obtained from the bulk liquid isotherm, and the other two terms were calculated from eq 12 using simple two-equation, two-unknown techniques. The calculated contribution of the liquid surface was found to be small and possibly within experimental error (Figure 8)) but it did help fit the experimental data better, particularly a t low per cent liquid coatings. Figure 9 shows the sorption isotherms of n-hexane on TCEP-coated AWFB. I n this instance n-hexane is nearly insoluble in the TCEP. A minimum is developed, but there is a slight increase in the isotherms as the amount of TCEP coating is increased. If the surface area of the liquid TCEP were a major contributor to the sorption isotherms, there would have been a relatively large increase in the isotherms as the liquid surface developed beyond the monolayer covering followed by a drop in the isotherms as the per cent of TCEP became large. This is because the surface area of a liquid on an adsorbent decreases when the micropores and capillaries are filled." Figure 10 compares the amount of sorption of nhexane and the surface area as a function of the per cent TCEP liquid coating. The surface area curve shows the expected initial rapid drop followed by a more gradual drop as the amount of liquid coating was increased. The amount sorbed followed the same general pattern except that there was a slight increase
0 0.0%
Q 4.23%
090%
6 989%
0.51 0
0
I
I
I
2.0
4.0
6.0
PI$
0 205%
0 0.55%
I 8.0
x 102
Figure 9. Sorption isotherms for n-hexane on TCEP-coated AWFB a t 30".
a t the higher per cent liquid coating. If adsorption on the liquid surface had been a dominating factor there should have been a noticeable decrease in the amount of n-hexane sorbed. Of particular interest is the fact that the amount sorbed at 1-2% TCEP is two to three times the amount of increased sorption that occurs when the liquid coating is increased from 2 to 10%. Because acetone and the highly polar methanol were easily able to penetrate the nonpolar squalane to interact completely with the sorbent surface and because the chemical potential of a vapor or solute is the same in all phases of a system at equilibrium, it must be assumed that n-hexane tends to equilibrate in all phases of a TCEP coated adsorbent-including the modified surface, the bulk liquid, and the liquid surface. Figures 9 and 10 consequently show that sorption by the modified adsorbent surface, as shown by the minimum a t about 1%, predominates over the amount of increased sorption beyond the minimum that might be attributed to the sum of the bulk liquid and liquid surface contributions. The possibility of (10) J. R. Conder, D. C. Locke, and J. H. Purnell, J. Phys. Chem., 73, 700 (1969). (11) S. Ross and E. D. Tolles in "The Solid-Gas Interface," Vol. 2, E. A . Flood, Ed., Marcel Dekker, New York, N. Y., 1967, p 652.
The Journal
of
Physical Chemistry, Vol. 74, No. 11, 1 ~ 7 0
2332
P. URONE,Y. TAKAHASHI, AND G. H. KENNEDY
continued adsorption by the modified adsorbent surface was not taken into consideration by previous workers. 11-14
Summary Equation 10 is expected to hold for situations involving volatile solutes and adsorbents coated with relatively nonvolatile liquids. Under most conditions one or more of the right-hand terms of eq 10 are negligibly small or nonexistent. The &solid contribution disappears rapidly with per cent coating for highly polar coating liquids on adsorbents of low specific surface areas and persists up to high per cent liquid coatings for adsorbents having high specific areas. The &mod contribution develops a t the expense of the &solid term, while the &liquid term develops with the formation of liquid pools. Adsorption by the liquid surface, & A ~ , cannot become a factor until a liquid surface is developed, and except for nonpolar solutes on polar coating liquids, its magnitude is generally too small to be observed. Figure 11 helps summarize the contributions of the various phases t o the total amount of sorption for the systems studied. Schematically, it shows the expected total amount of sorption with a solid line and the principal contributing sources as areas under dashed lines. A constant temperature and solute pressure is assumed in all cases. It must be emphasized that the relative magnitudes of the respective contributions can change with solute pressure and per cent coating liquid depending upon the linearity of the sorption isotherms and the relative intensities of the sorption forces. Curve A (Figure 11) shows that a nonpolar coating liquid is not able to compete effectively with a polar solute as the per cent of coating liquid is increased. The contribution of the solid is large and remains constant a t constant pressure and temperature. The contribution of the coating liquid rises slowly because the solubility of polar solutes in nonpolar solvents is generally low. For surfaces treated with a silanizing agent or a highly polar compound, the contribution of the surface is significantly reduced but not eliminated (curve D, Figure 5). Curve B indicates that there is some competition between the nonpolar coating and nonpolar solute for the active sites of an adsorbent. A modified surface is developed at low per cent coatings, but its contribution is small because its interaction with the solute is small. Beyond the minimum, the liquid phase contribution increases rapidly because of the greater solubility of the solute in the liquid. Polar coatings rapidly develop a modified surface giving a minimum and constant contribution to the total amount of polar solutes sorbed as the per cent coating is increased (curve C, Figure 11). The relative polarities of polar substances cover a wide range, and The Journal of Physical Chemistry, Val. 74, No. 11, 1970
I
0
I
I
IO
20
I
I
I
PERCENT 30 4 0 TCEP 50
"
>
/
90
1 100
Figure 10. Comparison of amount of n-hexane sorbed at 30" and PIP0 = 0.04 os. surface area as a function of per cent TCEP coating on AWFB.
A
B
NONPOLAR COATING NONPOLAR COATING NONPOLARSOLUTE POLAR SOLUTE
C
D
POLAR COATING POLAR SOLUTE
POLAR COATING NONPOLAR SOLUTE (EXPANDED SCALE)
PERCENT LIQUID COATING
Figure 11. Representative sorption profiles at constant pressure and temperature as a function of per cent coating for various pola r-nonpolar systems.
the magnitude of the contributions could vary considerably. A small amount of contribution by the liquid surface is shown to develop after the modified surface is formed. This is not easily seen experimentally, and, if present, may be negligible compared to the contribution of the coating liquid. Curve D represents those types of systems which have been reported to exhibit liquid surface adsorption.l1-l4 Because the interaction of a nonpolar solute with either the adsorbent or the polar coating liquid is small, an expanded vertical scale is used. At low per cent coating a minimum in the sorption isotherm is developed. The increase in sorption beyond the minimum may be attributed t o sorption by the bulk liquid or the liquid surface or both. Previous studies have not considered the contribution of the modified surface to the total amount sorbed.14 Conder, Locke, and Purnell'o recently discussed on a qualitative basis the type of chromatographic retention behavior to be expected when the solid-liquid as well as the gas-liquid interfaces contribute to the retention (12) R. L. Martin, Anal. Chem., 33, 347 (1961). (13) D.E.Martire, R. L. Pecsok, and J . H. Purnell, Trans. Faraday Sac., 61, 2496 (1965). (14) D.E. Martire, Anal. Chem., 38, 244 (1966).
2333
SOMETHERMODYNAMIC PROPERTIES OF LIQUID-COATED ADSORBENTS of the coating liquid. Their hypothetical curves based on a three-component retention equation and constant solute concentration in the gas phase may be compared to those observed in this study. Although close comparisons cannot be made because of differing emphasis on the relative strengths of the independent retention factors, there is reasonably good agreement. A possible major area of difference is the recognition in this study of the development of a modified adsorbent surface which becomes and remains active as the original solid surface becomes covered; i.e., the total surface area of the adsorbent is at all times active either in its original form ( Q 8 ) or a modified form (Qmod). The modified adsorbent surface can be represented by a complicated adsorption-from-solution equilibrium expression, the solution of which would give the Qmod curves obtained from data similar to those shown in Figures 1, 7, and 9 and used in Figures 3 and 4. For those who desire to make adjustments for the
combined modified adsorbent and liquid surface contributions, a correction factor, &’adsorbent, may be obtained from two or three batches of adsorbent coated with amounts of liquid increasing beyond the amount needed for developing a minimum. A plot of the amount sorbed (or in chromatography, the retention time or volume) vs. the per cent liquid would give a straight line for a constant solute pressure or gas-phase concentration. Extrapolation to zero per cent coating would give an apparent sorption correction factor for the adsorbent, &’adsorbent, and the slope of the line would give the partition coefficient for the liquid. The apparent &‘adsorbent would apply for all coatings beyond the minimum and would make possible study of and allowances for the adsorbent and surface area effects.
Acknowledgment. This study was made possible by National Science Foundation Grants No. GP9456, GP5400, and GY4093.
Some Thermodynamic Properties of Liquid-Coated Adsorbents by Yoshihiro Takahashi, Paul Urone, Department of Chemistry, University of Colorado, Boulder, Colorado 80302
and George H. Kennedy Department of Chemistry, Colorado School of Mines, Golden, Colorado Sod01
(Received August 6 , 1069)
The thermodynamic properties of n-hexane, isooctane, acetone, and methanol on liquid-coated diatomaceous earth adsorbents show contributions from both the coating liquid and the liquid-modified adsorbent surface. Static equilibrium sorption isotherms were obtained for the solutes in squalane, tri-o-tolyl phosphate, tris(cyanoethoxypropane),and on the adsorbents having 0.1-20% coatings of the liquids. Activity coefficients at infinite dilution of the solutes on the liquid-coated adsorbents are calculated from the respective isotherms after subtraction of the contribution of the isotherm of the liquid-modified support surface. The values agree with those obtained from the bulk liquids and those found in the literature. Observed isosteric heats of sorption are shown to equal the sum of the contributions from adsorption on the liquid-modified surface and solution in the bulk coating liquid. Entropy effects are calculated using the isosteric heats of adsorption and partial molar free energies of adsorption. These show that acetone adsorption on the modified surface conform closely to localized adsorption layer theory. The thermodynamic properties of liquid-coated adsorbents are of potential interest to workers in a number of fields of science. I n the field of aas-liauid chromatorrraphy, it has long been realized -that partition chromatography is not only a powerful analytical tool but it is also capable of easily and rapidly measuring partition coefficients, activity coefficients, and heats and entropies of I n the fields of catalysis and surface chemistry, liquid-coated adsorbents give a new dimenY
sion for the development of surfaces with specific chemical and physical proper tie^.^ (1) J. R. Coder in “Progress in Gas Chromatography,” vel. 6, J. H. Purnell, Ed., Interscience Publishers, New York, N. Y., 1967, p 209.
(2) Y. Kobayashi, P. S. Chappelar, and H. A. Deans, Ind. Eng. 591 63 (1967)* (3) A. B. Littlewood, C. S. G . Phillips, and D . T. Price, J . Chem. sot,, 1480 (1955). (4) J. H. Purnell, Endeavor, 23,142 (1964).
The J O U T ~ ofU ~Physical Chemistry, Vol. 74, No. 11, 1970
