ADSORPTIONOF ORGANICCOMPOUNDS FROM AQUEOUS SOLUTION BY MERCURY
July, 1957
953
FUNCTIONAL GROUP EFFECTS I N THE ADSORPTION OF ORGANIC COhlPOUNDS FROM AQUEOUS SOLUTION BY MERCURY' BY ROBERT8. HANSEN, ROBERT E. MINTTJRNAND DONALD A. HICKSON Contribution No. 611 f r o m the Institute for Atomic Research and Department of Chemistry, Iowa State College, Ames, Zowa. (Work was performed in the Ames Laboratory of the U . S. Alomic Energy Commisssion) Received March 6,1867
Isotherms for the adsorption of pentanol-1, pentanone-3, pentanenitrile, 2,4-pentanedione, phenol and octanoic acid from aqueous perchloric acid solution by mercury are inferred from differential double layer capacitance measurements in these systems. Standard free energies of adsorption are calculated using standard states based on unit activity coefficient in solution for infinitely dilute solute and unit activity coefficient in monolayer for infinitely dilute monolayer. The standard free energy of adsorption is approximately proportional to the difference in optical polarizabilities per unit volume between adsorbate and solvent and to the adsorbate molar volume raised to the two-thirds power. The interaction free energy in the monolayer (difference in free energy between adsorbate standard states based on unit activity coefficients in infinitely dilute and complete monolayers) is approximately one-half the corresponding difference in bulk solution. In the compounds investigated, functional groups appear to influence adsorption only to the extent that they influence molecular polarizability and character of adsorbate-solvent interactions.
Introduction We have published recently a method for inferring isotherms for adsorption from solution from differential double layer capacitance measurements2 based on the following considerations. If the isotherm for adsorption at an unpolarized surface is of the fairly general regular localized monolayer form
in which 0 is the fractional surface coverage, a the solute activity, and BOand a are constants, then Frumkin4 has shown that adsorption at the polarized surface is governed by
in which s is the molar area of the adsorbate and Q =
so"
[&w -
c' (v - v ~ ) ] d v
(3)
In eq. 3, Qw is the boundary charge density a t polarization V relative to the electrocapillary maximum, C' is the capacitance per unit area of a complete monolayer, and VN is the potential (referred t o the potential at the electrocapillary maximum in the absence of adsorbate) a t which there is no charge on the double layer when this contains adsorbate a t 0 = 1. Pursuing this model further, we showed that the dependence of differential capacitance on polarization and adsorbate activity was given by
%1-
cC'(V w - -C' v N, - 12/
(4)
in which CW is the differential capacitance per unit area in the absence of adsorbate and 0 is given by eq. 2. Since the molar area appears as a param(1) Based in part upon a dissertation submitted b y Robert E. Minturn to the Graduate School, Iowa State College, in partial fulfillment of the requirements for the degree of Doctor of Philosophy. 1955. ( 2 ) R . S. Hansen, R. E. Mintum and D. A. Hickson, THISJOURNAL, 60,1185 (195G). (3) The signs of the exponential terms in eq. 4 and 5 in our previous paper were, due t o our proofing error, shown as negative. They are correctly given in the corresponding eq. 1 and 2 in the present paper, (4) A. Frumkin, 2. Physilc, 35, 792 (1926).
eter in this treatment it is possible in principle t o establish not only fractional surface coverages, but also the number of moles adsorbed per unit area by this technique. I n our first paper2 we documented applicability of the theoretical treatment using the system mercury-0.1 M aqueous perchloric acid solutionpentanoic acid. The present work applies the method t o the investigation of functional group effects in the adsorption of organic compounds from aqueous solution. New results are given for the adsorbates pentanol-1, pentanenitrile, phenol, pentanone-3, 2,4-pentanedione and octanoic acid, and these results together with those previously obtained for pentanoic acid are compared to obtain an evaluation of the structures of the adsorbed layers and of the factors responsib:e for their adsorption. Theoretical The parameters ad justable for the representation of the experimentally observed dependence of differential double layer capacitance on solute activity and electrode polarization are Bo, C', VN, a and s. The parameter s gives the molecular area occupied by the adsorbed species, and suggests its orientation. The parameter VN is related t o the quotient of the normal component of the dipole moment of the adsorbed species and the dielectric constant of the adsorbed layer, thus (5)
The value of E is uncertain, but should be approximately the optical dielectric constant, ie., approximately 2 for the substances considered in this work. The sign and magnitude of p~ are also suggestive as to molecular orientation in the adsorbed layer. The parameter a is related to the change in interaction free energy per mole when the adsorbed species is transferred from an in-. finitely dilute monolayer to a monolayer of pure adsorbate; in our earlier paper2 we noted that a should be in order of magnitude '/4 Info, where fo is the activity coefficient of the adsorbate a t infinite dilution in solvent, referred to pure liquid adsorbate as standard state. The adsorption isotherm equation, eq. 1, implies the following chemical potential balance be-
RORERT S.HANREN, R.OBERTE. MINTURN AND DONALD A. HICKSON
954
PHENOL
.0.IN
Vol. 61
PERCHLOR6 ACID-O.OOl IN FUTPSSIUM CHLORIDE
..l.O
I
PENTANOLACTIVITY X- ,694 0- ,222
0- , 1 3 9 0- ,083
A- ,042 V-,028 .-.O14
APPLIED FOTENTlPL RELATIVE '10 ECM
Fig. 1.-Dependence of apparent fractional surface coverage on polarization and adsorbate activity. Adsorbate is pentanol-I, curves are calculated from eq. 4, points are experimental.
tween adsorbate in the adsorbed layer and in bulk solution . pA =
fit
e - 2aRTe = pb = pg + R T In a + R T In 1-e (6)
in which p a and PI, are the chemical potentials of adsorbate in the adsorbed layer and in bulk solution, respectively, and pi and p1Lg are the corresponding standard state chemical potentials. The standard state for the adsorbed species in the monolayer is implied by eq. 6 to be such that its activity coefficient (e-*Or0) approaches unity as 0 + 0 and is therefore one based on infinite dilution; the standard state for adsorbate in solution depends, of course, on the choice of activity basis, and we shall base this on pure liquid solute. Comparing eq. 6 and eq. 1, we obtain R T In Bo =
-(pfl
- fi:)
(7)
Equation 7 implies two useful standard free energies of adsorption, namely
-
AFg = -RT In Bo R T Info AFp = -RT In BO - 2aRT
(8) (9)
The first of these, AF& is the standard free energy of adsorption referred to standard states based on infinite dilution of adsorbate in both solution and monolayer, and the second of these, AF?, is the standard free energy of adsorption referred to
Fig. 2.-Dependence of apparent fractional surface coverage on polarization and adsorbate activity. Adsorbate is phenol, curves are calculated from eq. 4, points are experimental.
standard states based on pure adsorbate in both solution and monolayer.
Experimental Except as otherwise stated, experimental techniques were those used in our earlier work.2 The pentanol-1, octanoic acid, pentanone-3 and 2,4-pentanedione were Eastman best grade chemicals; the acetic acid (used as diluent in determination of the octanoic acid adsorption isotherm) was Baker and Adamson glacial acetic acid; the phenol was "Baker Analyzed" Reagent Phenol; and the pentanenitrile was prepared by an unknown student and furnished by the laboratory in organic chemistry a t Iowa State College. The pentanol-1, pentanenitrile, octanoic acid and 3-pentanone were purified by distillation through a 30-plate Oldershaw column a t reflux ratio 10: 1; central fractions used had boiling ranges (corrected to 760 mm. pressure) of 138.4-138.G0, 141.55-141.57", 238.9", and 101.2-102.2", respectively. The acetic acid was purified by two fractional freezing operations. The 2,4-pentanedione was purified by distillation under reduced pressure a t about 30"; and was further purified by four fractional freezing operations; the freezing point of the fraction used was -23.8'. The phenol was twice fractionally frozen and distilled in a single stage glass still. The corrected boiling point was 178.7'. In all cases adsorbate solutions were 0.1 M in HClOa and 0.001 M in KCl. Modifications of the previously reported experimental technique were omission of pre-electrolysis (but not the helium degassing operation) in the case of pentanol-1 and pentanenitrile, and use of acetic acid as diluent for the octanoic acid to permit accurate volumetric addition of octanoic acid to the aqueous perchloric acid solution. The amount of acetic acid used was such that an aqueous solution saturated with respect to octanoic acid was 0.1 M with respect to acetic acid; a blank run using
ADSORPTIONOF ORGANICCOMPOUNDS FROM AQUEOUS SOLUTION BY MERCURY
July, 1957
9 55
A a=OO2 I 0.004 0 SODIS
w‘ 4
98 Y
?iCL
5:
c
PENTANENITRILE ACTIVITY
0-.64 -.426 x -.236 0-,142
U
2 a a
4 --.5
A-.094 ..-.6
~-.047
.--7
--.a
APPLIED POTENTIAL RELATIVE E.C.M. IN O.IN HCIO4.
Fig. 3.-Dependence of apparent fractional surface coverage on polarization and adsorbate activity. Adsorbate is pentanenitrile, curves are calculated from eq. 4,.points are experimental. Solid curves correspond to a choice S = 29 h.2,dotted curves to a choice S = 32 aqueous perchloric acid solution 0.1 M in acetic acid showed that acetic acid in this concentration had very little effect on the capacitance-polarization curve.
Fig. 4.-Dependence of apparent fractional surface coverage on polarization and adsorbate activity. Adsorbate is octanoic acid, curves are calculated from eq. 4, points are experimental.
both cases there appears to be a dependence of C’ on polarization qualitatively similar to that observed for CWand too large to be safely ignored. Parameters chosen for the best representation of data are shown in Table I. Data for the pentanoic acid system previously reported are included; in the cases of pentanone-3 and 2,4-pentanedione the parameters Bo and a were estimated from apparent adsorption isotherms a t a number of polarieations in the neighborhoods of the maxima in the eapp-vplots.
Results Results are presented graphically in Figs. 1-6. In Figs. 1-4, experimental Oapp-V-a points are compared with theoretical curves, calculated from eq. 4 using the best choices of the parameters Bo, C’, VN, a and 8, for the adsorbates pentanol-1, phenol, pentanenitrile and octanoic acid. RepreTABLE I sentation of experimental results by theory is as ADSORPTION PARAMETERS IMPLIED BY DEPENDENCE OF good in these systems as that previously reported DOUBLE LAYER CAPACITANCE ON POLARIZATION A N D for the adsorbate pentanoic acid. In Figs. 5 and 6 SOLUTEACTIVITIES (See eq. 1-4) experimental C-V-a curves are given for the adC’, CO sorbates pentanone-3 and 2,4-pentanedione. We Substance .Bo //cm.2 V N , v. a S, R . 9 moles/l. were unable to represent these data adequately with Pentanoic acid 10.5 6 . 2 1 0.24 1.00 33 0.365 6.0 5.38 ,345 1 . 3 8 32 ,222 constant choices of the parameters. I n the case Pentanol-1 Pentanenitrile 2.45 9 . 5 ,391 1 . 6 4 32 ,138 of pentanone-3, the cathodic shift of the central Octanoic acid 16.3 4.50 .25 1.05 62 ,0051 maximum in the eap,-v curve with increasing Pentanone-3 ... .36 1 . 6 i= .. . GI0 6.3 adsorbate activity, moderately apparent in the 0.2 2,349 cases of pentanoic acid,2pentanol-1, pentanenitrile 2,4-Pentanedione 23.7 , . . . . 0 . 9 =t 0.1 and octanoic acid, becomes so pronounced as to Phenol 5 . 5 11.05 0.23 1.22 40.0 0.898 render impossible adequate representation of the data with a single choice of the parameter VN. The variations in standard free energies of adI n the case of 2,4-pentanedione, the desorption sorption AFX and AFP among the different adsorbminima in the Oapp-V curves are far too small ates should be indicative of the types of forces for the theory to represent them adequately. I n responsible for the adsorption of these compounds, I
,
, ,
950
ROBERT 8. HANSEN, ROBERTE. MINTURN AND
DoN.4LD
A. HICKSON
Vol. 61
if this difference be denoted by AP and the molar volume of the adsorbate be denoted by 'v, then the standard free energy of adsorption AF; is represented fairly well by KV.'/s A P
-AF,O
(10)
and nearly as well by -AF,O = K'V. AP
(11)
AF: for the adsorption of phenol is poorly represented by both equations; omitting phenol, eq. 10 is satisfied within an average deviation of G%, and eq. 11 within an average deviation of 10%. Models for which these equation can be justified theoretically involve spherical molecules and a non-metallic solid of such a character that 1 v,/vm constant where v, and vm are natural frequencies of solid and molecule, respectively; with such models eq. 10 corresponds to a situation in which the adsorbate molecule displaces an equivalent area of solvent molecules from the surface and the effective distance of each molecule from the surface is its radius, and eq. 11 corresponds to a situation in which the adsorbate molecule displaces an equivalent volume of solvent molecules from the surface and the effective distances of solvent and adsorbate molecules from the surface are the same. 0'' .2 .4 .6 .8 1.0 1.2 1.4 Both models are far too simple to be correct for APPLIED POTENTIAL RELATIVE TO Ag -AgCI. the present systems, in that the adsorbate moleFig. 5.-Dependence of differential double-layer capaci- cules are not spherical, their polarizabilities, from a tance on polarization and adsorbate activity. Adsorbate is molecular point of view, are anisotropic, and the 3-pentanone. adsorbent is a metal whose van der Waals interaction with the adsorbate should depend significantly on the dispersion frequency of the latter.6 Equations 10 and 11 must hence be regarded as semi-empirical for the time being, but their applicability strongly suggests that dispersion forces are much more important than dipole image forces or specific functional group effects (e.g., acidity, basicity, ability to donate or accept hydrogen bonds) in determining the standard free energy of adsorption from aqueous solutions. Standard free energies of adsorption and quotients of normal component of dipole moment and dielectric constant in the compact double layer are given in Table 11. The approximate validity of eq. 10 is demonstrated, and quotient of 4a and In j o t expected to be approximately unity, is also tabulated.
-
+
TABLEI1 SUMMARY OF STANDARD FREE ENERGIES OF ADSORPTION A N D DERIVED QUANTITIES AF: -
-
0806
0
Substance
4
~
1
'
1I
I
I
'
I
1
'
1
1
1
1
Fig. &-Dependence of differential double-layer capacitance on polarization and adsorbate activity. Adsorbate is 2,4pentanedione.
since initial and final standard states reflect similar environments except for the adsorbent surface. The simplest representation of our data appears to be in terms of the difference in optical polarizability per cc. between adsorbate and solvent;
Pentanoic acid Pentanol-1 Pentanenitrile Pentanone-3 2,4-Pentanedione Phenol Octanoic acid
-AF;~
-AF&
kcal.
kcal.
2.58 2.70 2.48 2.89 0.14 2.91 0.15 5.21 2.90
* *
4rr debyes
0.21 4.37 .29 4.34 .33 4.10 3 . 5 7 =t 0.14 2.91 zt 0.15 0.24 6.21 .41 7.16
+
+
Info 0.83 1.00 1.09 1.5 0.2 1.2 0.2 1.18 0.45
* *
VVr AP kcal./ om.'
4.36 4.34 5.14 5.12 4.75 2.87 4.62
I n computing the last column, AP was taken as - l)/(nz 2) (ni l)/(n:+2), in which
(nt
+ -
( 5 ) H. Margenau and W. G .
-
Pollard, Phye. Rev., 60, 128 (1941).
July, 1957
THEHEATOF SUBLIMATION OF BORON
na and nw are the indices of refraction for the sodium D line of adsorbate and water. I n the case of 2,4-pentanedione, an index of refraction was calculated assuming additive molar refractions for the keto form. This was done because pure 2,4pentanedione is largely (78y0)in the enol form, whereas a t infinite dilution in water it is largely (85y0)in the keto form6 and we are concerned with its behavior at infinite dilution. It may be noted that in every case the adsorbate molecule is adsorbed in such a manner that the positive end of its dipo1.e is oriented toward the surface: this orientation tends to place carbons toward the surface and functional groups away from the surface. .. Marked deviations of 4a/ln f o from unity, in the case of octanoic acid, and of -AF:/V’/lAP from 4.7 kcal./cm.2 in the case of phenol invite speculation as t o their cause. We suggest that the first deviation is due to a marked change in orientation of the octanoic acid between infinite dilution and complete monolayer states. If the adsorption forces were significantly less in the complete monolayer than a t infinite dilution this would be reflected in a n abnormally small value of a,and this would be expected if the octanoic acid molecules tended to “stand up” a t high concentrations and to “lie down” a t low concentrations in the monolayer. (6) G. S. Hammond, “Steric Effects in Organic Chemistry,” Chap. 9, M. 8. Newman, Ed., John Wiley and Son, New York, N. Y., 1956, esp. Tables XIV, XVI, discussion page 452.
957
Such behavior is quite in accord with observed curvatures of capacitance-potential curves which suggest markedly lower molecular areas a t high concentration. The deviation in the case of phenol may be due t o its anisotropic polarizability or to a greater than normal distance between phenol and surface. The desorption peaks in the 2,4-pentanedione system are anomalously small. This may be due to electrolytic oxidation and reduction of the dione on anodic and cathodic sides, or to stabilization of an oxidation product of mercury, presumably Hg(I1), through formation of an acetylacetone complex. Such phenomena would destroy the (essentially) ideally polarizable character of the electrode. An alternate explanation is interesting in its possible application to field effects on equilibrium. The enol form is more polarizable, but the keto form is stabilized by the polar solvent. Polarization of the electrode causes displacement of adsorbate (low dielectric constant) by water (high dielectric constant) but if the adsorbate has two forms the high electric field should stabilize the more polarizable form. This form is also more strongly held to the surface (eq. ll), and if this stabilization occurs it will cause the desorption peaks to be reduced in height and to be spread out, in accord with experiment. Acknowledgment.-We are indebted to Dr. L. S. Bartell for a helpful discussion of the ideas embodied in eq. 11 and 12.
THE HEAT OF SUBLIMATION OF BORON AND THE GASEOUS SPECIES OF THE BORON-BORIC OXIDE SYSTEM’ BY ALANW. SEARCY AND CLIFFORD E. MYERS Contribution f r o m the Ceramics Laboratories, Division of Mineral Technology, University of California, Berkeley, Calif,, and the Department of Chemistry, Purdue University, Lafayette, Indiana Received March 9, 1967
The sublimation pressure of boron has been measured in three kinds of crucibles. The heat of sublimation a t 2RS01
