Selective Sorption of Phenol and Related Compounds from Aqueous

Nov 1, 1995 - The adsorption of phenol, cyclohexanol, and n-hexanol from their dilute binary aqueous solutions onto graphitized carbon black (gcb) was...
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Langmuir 1996, 12, 423-430

423

Selective Sorption of Phenol and Related Compounds from Aqueous Solutions onto Graphitized Carbon Black. Adsorption and Flow Microcalorimetric Studies Zolta´n Kira´ly* and Imre De´ka´ny Department of Colloid Chemistry, Attila Jo´ zsef University, H-6720 Szeged, Hungary

Erwin Klumpp, Hans Lewandowski, Hans D. Narres, and Milan J. Schwuger Institute of Applied Physical Chemistry, Ju¨ lich Research Centre, D-52425 Ju¨ lich, Germany Received May 19, 1995. In Final Form: September 5, 1995X The adsorption of phenol, cyclohexanol, and n-hexanol from their dilute binary aqueous solutions onto graphitized carbon black (gcb) was investigated at 308.15 K. The adsorption excess isotherms were determined by flow frontal analysis solid/liquid (S/L) chromatography and also by the classical immersion method. The enthalpies of displacement of water by the C6-ols were measured by flow sorption microcalorimetry. A general thermodynamic treatment was developed for the adsorption at the S/L interface from dilute solutions, with further specifications to make the proposed equations operational for the analysis of experimental data. A combination of thermal data with adsorption data gave access to the monomolecular adsorption capacities and the molar surface areas of the C6-ols adsorbed on gcb. The adsorption becomes polymolecular as the consolute phase boundary is approached. The net enthalpies of adsorption, in both monomolecular and multilayer regimes, were determined by subtracting a bulk correction term, related to the heats of solution at infinite dilution, from the total enthalpies of displacement.

Introduction In a series of fundamental investigations, Hansen, Bartell, and co-workers reported on the adsorption of sparingly water-soluble organic compounds from their binary aqueous solutions onto several carbon blacks and graphites.1-5 The adsorption was found to be monomolecular in the whole solubility range (Langmuir-type isotherms on porous carbons), or alternatively multilayer formation occurred in the neighborhood of the saturating concentration (sigmoid-shaped isotherms on low-porosity carbons). It was pointed out that even a completed monolayer may contain an appreciable amount of water besides the organic compound, provided the solid surface is energetically heterogeneous. The solubility may have either a positive or a negative temperature coefficient; hence, the normal temperature effect of the adsorption may be either enhanced or reduced. For instance, the extent of adsorption of n-butanol from water onto nonporous carbon decreases with increasing temperature up to monomolecular surface coverage of the alcohol, but the opposite holds true in the polymolecular adsorption regime.3 The implication of this observation is that the sign and magnitude of the isosteric heat of adsorption are dependent on the sign and relative magnitude of the solubility and normal temperature effects. It has been argued that in the polymolecular adsorption region, where there is no direct interaction between adsorbent and adsorbate molecules, an endothermic “bulk dilution effect” may well overcompensate the expected exothermic character of the “net adsorption”.6-8 In fact, the enthalpy isotherms of displacement X Abstract published in Advance ACS Abstracts, November 1, 1995.

(1) Hansen, R. S.; Fu, Y.; Bartell, F. E. J. Phys. Chem. 1949, 53, 769. (2) Fu, Y.; Hansen, R. S.; Bartell, F. E. J. Phys. Chem. 1949, 53, 1141. (3) Bartell, F. E.; Thomas, T. L.; Fu, Y. J. Phys. Chem. 1951, 55, 1456. (4) Hansen, R. S.; Fackler, W. V., Jr. J. Phys. Chem. 1953, 57, 634. (5) Hansen, R. S.; Craig, R. P. J. Phys. Chem. 1954, 58, 211.

0743-7463/96/2412-0423$12.00/0

of water by nC4-nC6 alcohols in their binary aqueous solutions onto nonporous carbon surfaces were found to be exothermic and endothermic for monolayer and multilayer formation, respectively.6-8 Carbon blacks show a wide variation in structure and surface composition, so that differences are often found between results from various laboratories. It is of interest, therefore, to employ adsorbents which provide reproducible results. Graphitized carbon black (gcb) possesses a well-defined, planar and energetically homogeneous solid surface.9 Adsorption and calorimetric data relating to the adsorption of n-butanol from water onto gcb’s of different origins revealed excellent agreement between the different sets of results.7,8,10 In the present work, we pursue our calorimetric investigations of the adsorption of partially water-miscible materials from binary aqueous solutions onto gcb.6-8,11 Three closely related compounds (phenol, cyclohexanol, and n-hexanol) were selected for study, to get a better understanding of the thermodynamics and mechanism of the adsorption of organics from dilute binary aqueous solutions onto solid surfaces. Of particular interest was how the hydrophobic effect12,13 contributes to the adsorption of single solutes in an aqueous environment. Adsorption data for these organics in the aqueous phase onto artificial graphites, sugar charcoal, furnace black, and channel black were reported over 40 years ago, with minor emphasis paid to thermodynamic considerations.1,2,5 Here, a general thermody(6) Kira´ly, Z.; De´ka´ny, I. In 3rd International Conference on Fundamentals of Adsorption (Sonthofen, May 1989); Mersmann, A. B.; Scholl, S. E., Eds.; Engineering Foundation: New York, 1991; p 425. (7) Kira´ly, Z.; De´ka´ny, I. J. Chem. Soc., Faraday Trans. 1 1989, 85, 3373. (8) Kira´ly, Z.; De´ka´ny, I. Colloids Surf. 1990, 49, 95. (9) Kinoshita, K. Carbon. Electrochemical and Physicochemical Properties; John Wiley & Sons: New York, 1987; Chapter 3. (10) Groszek, A. J.; Partyka, S. Langmuir 1993, 9, 2721. (11) Kira´ly, Z.; De´ka´ny, I. Prog. Colloid Polym. Sci. 1990, 83, 68. (12) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes; Wiley: New York, 1980. (13) Franks, F.; Reid, D. S. In Water: A Comprehensive Treatise; Franks, F., Ed.; Plenum: New York, 1973; Vol. 2, Chapter 5.

© 1996 American Chemical Society

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namic formulation is presented and applied to adsorption and calorimetric data relating to the C6-ol(1)-water(2)/ gcb systems. Theoretical Relations In terms of the surface phase model, adsorption from solution can be described by a displacement reaction of solvent (component 2) by solute (component 1) at the solid/ solution interface:6,7,14

(1)l + r (2)s ) (1)s + r (2)l

∆21h1 )

[ ] ∂Γ1s

s

l

s

l

) (h1 - h1 ) - r (h2 - h2 ) (2)

T,p,A

where ∆21hˆ is the integral enthalpy of displacement of 2 by 1, and Γ1s is the number of moles of component 1 actually present in the adsorption layer; both quantities are interpreted here as relating to unit surface area, A, of the solid. According to Kira´ly and De´ka´ny8 or McGuire and Suffet,16 the displacement process (1) may be subdivided into four distinctive steps. These authors used somewhat different terminologies but essentially the same mechanistic picture: (i) solute molecules leave the bulk solution (hole formation in the liquid phase); (ii) solvent molecules leave the adsorption layer (hole formation in the surface phase); (iii) solute molecules enter the adsorption layer (solute transfer into the surface phase); (iv) solvent molecules enter the bulk solution (solvent transfer into the liquid phase). Clearly, steps i and iv are regarded as bulk phenomena and steps ii and iii as surface phenomena. The net differential molar enthalpy of adsorption, which is a measure of the affinity of component 1 for the surface relative to component 2, may be defined6,7,15,16 as s

∆21h1 )

[ ] ∂(∆21hˆ s) ∂Γ1s

) h1s - r h2s

∆21h1 )

[ ] ∂Γ1s

s

s 2 s, 2

1 s 1

where Γ2s,° is the adsorption capacity of the pure 2nd component and dΓ2s ) -r dΓ1s, according to eq 1. The integral enthalpies of adsorption and bulk dilution can be expressed by analogous formulas:

∆21hˆ s )

∫ΓΓ )0(h1s - r h2s) dΓ1s

(6)

∆21hˆ l )

∫ΓΓ )0(r h2l - h1l) dΓ1s

(7)

s

1 s 1

s

1 s 1

Equation 6 represents the change of state of the adsorption layer, while eq 7 represents the change of state of the bulk liquid phase during the displacement process as a whole. Simply, the enthalpy of displacement is the sum of the net enthalpy of adsorption and the bulk dilution enthalpy:

∆21hˆ ) ∆21hˆ s + ∆21hˆ l

(8)

∆21h1 ) ∆21h1s + ∆21h1l

(9)

The enthalpy of displacement is measured by flow sorption calorimetry. The partial molar enthalpies hil can be derived from the molar enthalpies of mixing, available in solution thermodynamic tables. Furthermore, the adsorption excess isotherm permits calculation of Γis, the real amount adsorbed. Hence, the net enthalpy of adsorption can be obtained via eqs 8 and 9. We may arbitrarily set the energy scale of the system so that the molar enthalpies of the two pure components are equal to zero at the temperature and pressure of interest. The calculations are simplified further if the above considerations are applied for preferential solute adsorption from ideally dilute solutions:7,8,15

where xi is the mole fraction of the ith component in the equilibrium bulk solution, Γ1σ is the surface excess concentration of the solute, and ∆solh1∞ is the heat of solution of solute 1 at infinite dilution in solvent 2. If eq10 holds, eq 5 reduces to

∆21hˆ ) ∆21hˆ s - Γ1σ∆solh1∞ l

(5)

hil,° ) 0; x1 , x2 = 1; Γ1σ = Γ1s; h1l ) ∆solh1∞ (10)

where ∆21hˆ s is the (net) integral enthalpy of adsorption, i.e., the enthalpy change of the adsorption layer in contact with the bulk solution when the concentration in the bulk liquid phase varies from pure 2 to a particular concentration of 1, e.g., in mole fraction terms from x1 ) 0 to x1. According to eqs 2 and 3, the differential molar enthalpy of bulk dilution is given by l

∫ΓΓ )0(h1s - h1l) dΓ1s + ∫ΓΓ °(h2s - h2l) dΓ2s

(3)

T,p,A

∂(∆21hˆ l)

∆21hˆ )

(1)

where superscripts s and l refer to the adsorbed layer and bulk solution, respectively, and r is the number of solvent molecules displaced by one molecule of solute. The differential molar enthalpy of displacement represents the difference between the partial molar enthalpies of the two components in the adsorbed layer and in the bulk solution:6,7,14,15

∂(∆21hˆ )

determined by flow sorption microcalorimetry.6-8,10,11,14,17-22 In practice, a solid sample in a column is equilibrated with pure component 2 (solvent) and replaced by a set of solutions involving a small concentration increment. The step-by-step enthalpies of displacement of 2 by 1 are measured, and the cumulative data are compiled. The mathematical expression for ∆21hˆ may be given as

) r h2 - h1

l

(4)

T,p,A

where ∆21hˆ l is the integral enthalpy of bulk dilution, which denotes the change in enthalpy due to the change in solution composition.6-8,15 The integral enthalpy of displacement is an experimental quantity, which can be (14) Findenegg, G. H. Calorim. Anal. Therm. 1985, 16, 1. (15) Kira´ly, Z.; De´ka´ny, I.; Nagy, L. G. Colloids Surf. 1993, 71, 287. (16) McGuire, M. J.; Suffet, I. H. In Activated Carbon Adsorption of Organics from the Aqueous Phase; Suffet, I. H., McGuire, M. J., Eds.; Ann Arbor Science Publishers: Ann Arbor, Michigan, 1980; Vol. 1, Chapter 4.

(11)

so that the last term, the change in enthalpy resulting from the change in solution composition, can be calculated without difficulty. Although several surface excess quantities are used in the literature (Γ1(2) relative, Γ1(n) molereduced, and Γ1(V) volume-reduced surface excess concen(17) Koch, C. S.; Ko¨ster, F.; Findenegg, G. H. J. Chromatogr. 1987, 406, 257. (18) Woodbury, G. W.; Noll, L. A. Colloids Surf. 1983, 8, 1. (19) Woodbury, G. W.; Noll, L. A. Colloids Surf. 1987, 28, 233. (20) Johnson, I.; Denoyel, R.; Rouquerol, J.; Everett, D. H. Colloids Surfaces 1990, 49, 133. (21) van Os, N. M.; Haandrikman, G. Langmuir 1987, 3, 1051. (22) Groszek, A. J. Proc. R. Soc. London, A. 1970, 314, 473.

Sorption onto Graphitized Carbon Black

Langmuir, Vol. 12, No. 2, 1996 425

trations23 ), each of them becomes equal to Γ1s, the real amount adsorbed, as the adsorption of the solute is strongly preferential over that of the solvent. In such cases, the differential molar enthalpies of the displacement and adsorption are obtained if the corresponding integral enthalpy data are plotted against, and differentiated with respect to Γ1σ, according to eqs 2 and 3. It has been established that the relative enthalpy of immersion ∆21hˆ w (the enthalpy of immersion in a binary liquid mixture minus the enthalpy of immersion in pure component 2) and the relative mole-reduced surface excess enthalpy ∆21hˆ σ(n) are linked by the following relationship: 24,25

∆21hˆ w ) ∆21hˆ σ(n) - Γ1(n) (h1l - h2l)

(12)

For ideally dilute solutions and for preferential solute adsorption, the relative enthalpy of immersion becomes equal to the integral enthalpy of displacement, and the relative mole-reduced surface excess enthalpy becomes equal to the integral enthalpy of adsorption.6-8,15,18-20,26 Accordingly, as the conditions described by eq 10 hold, eq 12 becomes equivalent with eq 11. The aforementioned considerations show that a knowledge of the heat of solution at infinite dilution is of crucial importance when the change of state within the adsorption layer is to be investigated. An excellent data collection of ∆solh1∞ for a large number of organic compounds in binary aqueous solutions at 298.15 K has been published by Abraham.27 If a different temperature is of interest, but direct calorimetric (or heat capacity) data are not available, ∆solh1∞ can still be satisfactorily estimated from the temperature dependence of the solubility. In particular, for dilute solutions of sparingly soluble materials

[

∆solh1∞ ) RT2

]

∂ ln x1,sat ∂T

(13)

p

where x1,sat is the saturating mole fraction of the solute in the solvent. ∆solh1∞ can also be deduced (either numerically or graphically) from the molar excess enthalpy of mixing, HE:28

lim x1f0

[ ] ∂HE ∂x1

T,p

) ∆solh1∞ = lim

x1f0

HE x1x2

(14)

If the heat capacity of a solution of solute 1 at infinite dilution in solvent 2 is available, the enthalpy of solution data can be transformed to different temperatures. For extrapolation from higher temperatures to a lower temperature T, at which the pure solute would undergo freezing, the molar enthalpy of fusion is to be taken into account:

[∆solh1∞ (solidfaq)]T ) [∆solh1∞(liqfaq)]T + [∆fh1]T (15) where ∆fh1 is the temperature-dependent enthalpy of fusion,29 which may be approximated by the enthalpy of melting at temperatures close to the melting temperature. Equation 15 is equivalent to taking the sum of the (23) Kira´ly, Z.; De´ka´ny, I. Colloid Polym. Sci. 1988, 266, 663. (24) Everett, D. H. Pure Appl. Chem. 1981, 53, 2181. (25) Everett, D. H. Colloids Surf. 1993, 71, 205. (26) Denoyel, R.; Rouquerol, F.; Rouquerol, J. J. Colloid Interface Sci. 1990, 136, 375. (27) Abraham, M. H. J. Chem. Soc., Faraday Trans. 1 1984, 80, 153. (28) King, M. B. Phase Equilibrium in Mixtures; Pergamon: Oxford, 1969; p 22. (29) Hildebrand, J. H.; Scott, R. L. The Solubility of Nonelectrolites; Reinhold: New York 1950, pp 26, 270.

enthalpies of the hypothetically separate processes of melting the solid to give the supercooled liquid and the mixing of the liquid with the solvent (with water in the present notation) to produce an infinitely dilute (aqueous) solution. The application of the above thermodynamic equations will be demonstrated for the displacement of water by C6-ols from their dilute binary aqueous solutions onto gcb. Materials Solutions. n-Hexanol (>99%) and phenol (>99.5%) were Fluka chemicals, while cyclohexanol (>99%) was purchased from Merck. The water was doubly distilled. Aqueous solutions of C6-ols were prepared by weighing. The solubilities of n-hexanol, phenol, and cyclohexanol in water at 308.15 K are 54.8, 1053 and 343 mmol dm-3, respectively.30,31 Adsorbents. Printex 90, a product of Degussa, was graphitized at 2973 K in an argon atmosphere for 3 days by Sigri Electrographit GmbH, Meitingen. The graphitized Printex (abbreviated as GP) was purified by Soxhlet extraction in n-heptane for 50 h, dried, and stored at 393 K in a vacuum desiccator overnight before use. The BET specific surface area of the GP was determined in a Gemini 2375 (Micromeritics) automated gas sorption apparatus to be as ) 114 m2 g-1. Hypercarb, a graphitic carbon of Shandon Scientific Ltd., was supplied in a HPLC column. The mass of the column packing was 0.5322 g, and it had a BET surface area of as ) 103 m2 g-1. The column was conditioned with a mixture of 95% methanol/ water.

Methods Static Adsorption Measurements. The adsorption excess isotherms of C6-ol(1)-water(2)/gcb systems were determined at 308.15 ( 0.1 K by static and dynamic methods. GP was used in the immersion experiments. V0 ) 10 cm3 of solution with a solute concentration of c10 was added to a known mass of solid (typically ms ) 1.0 g) in a teflon-sealed adsorption flask. End-to-end stirring was applied overnight, and the sample tubes were immersed for 4 h in a water bath maintained thermostatically at the desired temperature. The supernatant solution was analyzed with a Zeiss interferometer to obtain the equilibrium concentration c1. The volume-reduced surface excess concentration was calculated as Γ1(V) ) V0 (c10 - c1)/msas. The standard deviation of Γ1(V) varied between 2% and 7%, depending on the concentration of the solution. Dynamic Adsorption Measurements. Adsorption excess isotherms on Hypercarb were determined by flow frontal analysis S/L chromatography17,21,32 by using a HPLC system (Merck RI71 differential refractometer, Merck-Hitachi L-6200A intelligent pumps, Merck-Hitachi D-6000 interface, Knauer degasser, Techlab column thermostat, six-way valve, tubings and fittings). The flow rate, Q, varied between 0.3 and 0.7 cm3 min-1, depending on the solution and on the composition. Dead-volume measurement was performed by using a 0.1% D2O solution. The increments of the volume-reduced surface excess amount for a series of concentration steps of ∆c1 ) c1′ - c1′′ were evaluated from the breakthrough curves via the fundamental relation ∆Γ1(V) ) Q (c1′ - c1′′) tCR/msas.11,17,30 The corrected retention time, tCR (retention time minus dead time), was obtained by a numerical integration routine.11 Starting from pure water, ∆Γ1(V) data were summed over the concentration range of interest to construct the adsorption excess isotherm Γ1(V) vs c1. The adsorption isotherms were found by adsorption-desorption cycles to be reversible. The error in the corrected retention volume QtCR was generally less than (0.3%. Flow Sorption Microcalorimetry. The integral enthalpy isotherms of displacement of water(2) by C6-ols(1) on GP were determined by the cumulative method6-8,10,11,14,17-22 by using the flow unit of a Thermal Activity Monitor 2277 microcalorimeter (30) Alcohols with Water; Barton, A. F. M., Kertes, A. S., Eds.; Solubility Data Series, Vol. 15; Pergamon: Oxford, 1984. (31) Solubility of Inorganic and Organic Compounds; Stephen, H., Stephen, T., Eds.; Pergamon: Oxford, 1979; Vol. 1, Parts 1 and 2. (32) Wang, H. L.; Duda, J. L.; Radke, C. J. J. Colloid Interface Sci. 1978, 66, 153.

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Figure 1. Adsorption excess isotherm of phenol(1)-water(2)/gcb at 308.15 K. Symbols: (O) static and (0) dynamic data.

Figure 3. Adsorption excess isotherm of n-hexanol(1)-water(2)/gcb at 308.15 K. Symbols: (O) static and (0) dynamic data.

Figure 2. Adsorption excess isotherm of cyclohexanol(1)water(2)/gcb at 308.15 K. Symbols: (O) static and (0) dynamic data.

Figure 4. Adsorption excess isotherms of phenol (O), cyclohexanol (3), and n-hexanol (0) from binary aqueous solutions on gcb at 308.15 K. Surface excess concentrations are plotted on a relative concentration scale.

(ThermoMetrics) at 308.15 ( 10-4 K. Technical details on this multichannel microcalorimeter have been given elsewhere.33 The calorimetric protocol was essentially identical with that of the HPLC experiment. The liquid-flow adsorption cell, originally designed by van Os,21 was loaded with ca. 0.2 g of solid, and measurements were made at a flow rate of 0.12 cm3 min-1. Calibrations, measurements, and data analysis were all computercontrolled by the Digitam Software. The concentration increments were chosen to be small enough to avoid heat of mixing between the interface of replacing and replaced solutions.11,19 The enthalpy isotherms were found to be reversible in adsorption-desorption runs. The peak areas resulting from the enthalpy changes at the solid/solution interface were measured with a precision between 0.5% and 3%, depending on the magnitude of the heat effects.

Results The adsorption excess isotherms per unit surface area of the solid are shown in Figures 1-3, with inserted magnifications of the lower concentration regimes. In spite of the different origins of the two gcb samples, the static experimental data (on GP) are in excellent agreement with the dynamic data (on Hypercarb) for phenolwater (Figure 1) and cyclohexanol-water (Figure 2) solutions. The agreement is not as good, but still satisfactory, for n-hexanol (Figure 3) at high dilution in water (c1 < 4 mmol dm-3). The isotherms for n-hexanol, cyclohexanol, and phenol are of type S, S, and L, respectively, in the classification of Giles.34 The step in (33) Suurkuusk, J.; Wadso¨, I. Chemica Scripta 1982, 20, 155. (34) Giles, C. H.; Smith, D.; Huitson, A. J. Colloid Interface Sci. 1974, 47, 755.

the S-shaped isotherms (characteristic of cooperative solute adsorption) spreads out over a certain concentration range of the bulk solution. Adsorption data are plotted on a relative concentration scale (equilibrium solute concentration divided by the saturating concentration) in Figure 4. The relative (or reduced) concentration may be regarded as a “corresponding state” for solutes of different limited solubilities in the same solvent. This representation offers a more realistic picture when the adsorption affinities for the solid surface are to be compared. For n-hexanol, a steep rise appears at the end of the isotherm, indicative of multilayer formation as the solubility limit is approached. This behavior also occurs for phenol and cyclohexanol at higher concentrations,1,2 but is not visible in Figure 4 because the whole solubility range is not covered in the present work. Thermal data on the three systems are plotted in Figures 5-8. The displacement of water by C6-ols on gcb is strongly exothermic, in the sequence of phenol > n-hexanol > cyclohexanol (Figure 8). At low relative concentrations, the integral enthalpy isotherms of displacement are closely parallel to the corresponding adsorption excess isotherms. Discussion Calculation of the net enthalpy isotherms of adsorption of C6-ols at 308.15 K requires their respective enthalpies of solution at infinite dilution in water at this temperature. The calorimetric value of -2.73 kJ mol-1 for n-hexanol was taken from the literature.35 A direct calorimetric (35) Halle´n, D.; Nilsson, S.-O.; Rothschild, W.; Wadso¨, I. J. Chem. Thermodyn. 1986, 18, 429.

Sorption onto Graphitized Carbon Black

Figure 5. Integral enthalpy isotherm of displacement of water(2) by phenol(1) onto gcb at 308.15 K.

Figure 6. Integral enthalpy isotherm of displacement of water(2) by cyclohexanol(1) onto gcb at 308.15 K.

Figure 7. Integral enthalpy isotherm of displacement of water(2) by n-hexanol(1) onto gcb at 308.15 K.

value for cyclohexanol was available at 298.15 K: -9.01 kJ mol-1;36 this is in good agreement with our indirect value of -8.87 kJ mol-1, calculated from the temperature dependence of the solubility (eq 13) of cyclohexanol in water.31 This agreement is a good argument that the calculated value of -7.27 kJ mol-1 at 308.15 K, used in subsequent calculations, is reliable. Heats of mixing of phenol with water have been reported at 293 and 298 K (below the melting temperature of Tm ) 314 K) and at four different temperatures above Tm in the range 323358 K.36 Heats of mixing for T > Tm were extrapolated (36) Beggerow, G. In Heats of Mixing; Scha¨fer, K., Ed.; LandoltBo¨rnstein, New Series IV/2; Vol. 2; Springer-Verlag: Berlin, 1976.

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Figure 8. Integral enthalpy isotherms of displacement of water(2) by C6-ols ((O) phenol, (3) cyclohexanol, and (0) n-hexanol) onto gcb at 308.15 K. Enthalpy data are plotted on a relative concentration scale.

to infinite dilution in the water- rich region, according to eq 14. The ∆solh1∞ data were then extrapolated to 308.15 K to yield +4.79 kJ mol-1 for the heat of solution of supercooled liquid phenol in water. The published results for T < 308.15 K were extrapolated to 308.15 K to give a value of +16.31 kJ mol-1 for the heat of solution of solid phenol in water. The difference between the two ∆solh1∞ values, extrapolated to the same temperature but from opposite directions on the temperature scale, is 11.52 kJ mol-1, which is equal to the (endothermic) heat of fusion of phenol at the melting temperature,37 in accordance with eq 15. ∆solh1∞ and Tm data on the C6-ols are collected in Table 1. The theory of hydrophobic hydration has established that two opposite processes take place when a hydrophobic solute molecule is introduced into water.38,39 First, a cavity is formed with a decrease in the hydrogenbonding interaction of the adjacent water. Second, the solute introduced into the cavity organizes the water molecules as an iceberg. At ambient temperature, the small positive or negative enthalpy of solution results from a large positive enthalpy of mixing and a large negative enthalpy of iceberg formation. Although different headgroups make different contributions to the total heat of solution, it is generally observed that in a homologous series of hydrophobic solutes ∆solh1∞ becomes more endothermic with increasing carbon chain length and with increasing temperature. Interestingly, solid phenol has a relatively large positive heat of solution in water (+16.31 kJ mol-1) in contrast with the negative heat of solution of the other two less polar C6-ols. Although a detailed thermodynamic analysis of the phenol-water system has been reported,40,41 the molecular explanation of this large endothermic heat of solution is not readily interpreted. Destruction of the solid crystalline state is not a sufficient reason, and the aggregation of phenol molecules at high dilution can be excluded.42 At any event, the bulk contribution enthalpy, which has exactly the same magnitude, but the opposite sign as the heat of solution, can be subtracted from the enthalpy of displacement on a rigorous thermodynamic basis. The bulk contribution to (37) Organic Solvents. Physical Properties and Methods of Purification; Riddick, J. A., Bunger, W. B., Eds.; Techniques of Chemistry, Vol. 2; John Wiley & Sons: New York, 1970. (38) Shinoda, K. J. Phys. Chem. 1977, 81, 1300. (39) Ruckenstein, E. Colloids Surf. 1992, 65, 95. (40) Schu¨rmann, E.; Diederichs, R. Ber. Bunsen-Ges. Phys. Chem. 1964, 68, 429. (41) Schu¨rmann, E.; Diederichs, R. Ber. Bunsen-Ges. Phys. Chem. 1964, 68, 434. (42) Breslauer, K. J.; Witkowski, L.; Bulas, K. J. Phys. Chem. 1978, 82, 675.

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Table 1. Thermodynamic Parameters of C6-ol(1)-water(2)/gcb Systemsa C6-ol

∆solh1

∞

Tm

A1

∆21h1 (1st)

∆21h1s (1st)

∆21h1 (2nd)

∆21h1s (2nd)

∆fhm

n-hexanol c-hexanol phenol

-2.73 -7.27 +4.79

229 298 314

285 294 326

-12.97 -4.08 -16.09

-15.70 -11.35 -11.30

-0.01 -4.50 -13.74

-2.74 -11.77 -8.95

+15.41 +1.7 +11.52

a ∆ h ∞ (kJ mol-1), enthalpy of solution of liquid C -ol(1) at infinite dilution in water(2) at 308.15 K; T (K), melting temperature of sol 1 6 m pure C6-ol; A1 (m2 mmol-1), molar surface area of adsorbed C6-ol; ∆21h1 (1st), ∆21h1 (2nd)/(kJ mol-1), differential enthalpies of displacement in 1st and 2nd layers, respectively, at 308.15 K; ∆21h1s (1st), ∆21h1s (2nd) (kJ mol-1), differential enthalpies of adsorption in 1st and 2nd layers, respectively, at 308.15 K; ∆fhm (kJ mol-1), enthalpy of fusion at the melting temperature.

Figure 10. Net enthalpy isotherms of adsorption of phenol (O), cyclohexanol (3), and n-hexanol (0) from their binary aqueous solutions onto gcb at 308.15 K. Surface excess concentrations are plotted on a relative concentration scale.

Figure 9. Schematic illustration of the bulk dilution effect superimposed on the net enthalpy of adsorption during the displacement of water(1) by n-hexanol(2) onto gcb.

the displacement process is illustrated on a molecular level in schematic Figure 9 for the n-hexanol(1)-water(2)/gcb system. Statistical thermodynamic calculations show that, in dilute aqueous solutions, the average hydrogen bond density of the water around alcohol molecules is higher than in pure water itself.43 As the alcohol is adsorbed, the cavity disappears and the excess hydrogen bond density falls to the normal bulk water density (Figure 9). The endothermic iceberg collapse is superimposed on the exothermic solid-solute interaction. This mechanistic picture is closely related to eqs 8 and 9, i.e., division of the enthalpy of displacement into bulk and interfacial contributions. The net enthalpies of adsorption of C6-ols from water onto gcb were calculated via eq 11 and are shown on a relative concentration scale in Figure 10. The integral enthalpy isotherms of adsorption run in a different fashion from the corresponding enthalpy isotherms of displacement (Figure 8). For n-hexanol and cyclohexanol, the net enthalpy of adsorption is more exothermic than the enthalpy of displacement, while the opposite holds true for phenol. In the calculations, ∆solh1∞ ) +4.79 kJ mol-1, rather than +16.31 kJ mol-1, was used for phenol, i.e., if solidification occurs in the adsorbed layer, the enthalpy of this liquid/solid phase transition is implicitly included in the net heat of adsorption. Enthalpy of displacement and enthalpy of adsorption data are plotted against surface concentration in Figures 11-13. It should be noted in this context that any small experimental error in the adsorption excess isotherm or in the enthalpy isotherm of displacement is greatly amplified in the enthalpies of adsorption. If the smoothed curves (solid lines) in Figures 1-8 are considered, each of the resulting -∆21hˆ vs Γ1(V) graphs have two markedly (43) Laiken, N.; Ne´methy, G. J. Phys. Chem. 1970, 74, 3501.

Figure 11. Enthalpies of displacement and adsorption plotted against surface concentration for n-hexanol(1)-water(2)/gcb at 308.15 K.

linear sections. The slopes of the straight lines differ from one another, with a transition zone around the break point. The break point may be attributed to the completion of the first adsorption layer. Assuming a close-packed monolayer of the adsorbed solute, the monomolecular adsorption capacities allow calculation of molecular surface areas A1. These values, listed in Table 1, support the assumption that the homogeneous surface of gcb is fully covered by hydrophobic C6-ol molecules before formation of the second adsorption layer begins. For n-hexanol, the value of 285 m2 mmol-1 is in accordance with a geometric model proposed by Groszek.22 According to this model, the adsorption is confined to the basal planes of the graphite structure, and the alcohol molecules are lying flat on the surface in a zig-zag conformation. There is a good geometrical fit of the n-alkanol molecules onto the graphite hexagons, so that each methylene group occupies one hexagon, while the methyl and hydroxy groups equally occupy two hexagons, yielding a value of 283 m2 mmol-1 for n-hexanol. Hydrogen bonding between neighboring alcohol molecules may be expected to stabilize

Sorption onto Graphitized Carbon Black

Figure 12. Enthalpies of displacement and adsorption plotted against surface concentration for phenol(1)-water(2)/gcb at 308.15 K.

Figure 13. Enthalpies of displacement and adsorption plotted against surface concentration for cyclohexanol(1)-water(2)/gcb at 308.15 K.

further the in-registry arrangement of long chain alcohols.44-46 It has been argued that the unique shape of the benzene ring fits the lattice rings of graphite so as to produce localized adsorption.47 If each benzene ring occupies nine unit hexagons, six unshared and six shared equally with neighboring molecules, and the phenol hydroxy group occupies two unit hexagons, the geometric surface area is 345 m2 mmol-1, slightly more than the experimentally observed value of 326 m2 mmol-1. The break point on the -∆21hˆ vs Γ1(V) curve is less pronounced for cyclohexanol, and it can hardly be seen in Figure 13. The surface area is most probably A1 ) 294 m2 mmol-1, as derived from a magnification of the transition zone. This size falls between the values for phenol and n-hexanol. The conformation of adsorbed cyclohexanol is dubious since, unlike phenol, this molecule is not planar. Only three or four methylene groups can make contact with the graphite basal plane unless distortion occurs in the saturated ring. Interpretation of the thermal data will give a picture of this molecular orientation. Nevertheless, the smaller A1 value for cyclohexanol than that for phenol suggests that the alcoholic hydroxy group might have no surface contact but be shifted away toward the aqueous phase. For the present systems, a combination of adsorption data with calorimetric data does not lead to an adequate differentiation between L- and S-curve condi(44) Bien-Vogelsang, U.; Findenegg, G. H. Colloids Surf. 1986, 21, 469. (45) Findenegg, G. H. J. Chem. Soc., Faraday Trans. 1 1972, 68, 1799. (46) Findenegg, G. H. J. Chem. Soc., Faraday Trans. 1 1973, 69, 1069. (47) Pierce, C.; Ewing, B. J. Phys. Chem. 1967, 71, 3408.

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tions. The linear sections in the graphs of Figures 11-13 suggest that the adsorption layer behaves ideally in both the monomolecular and polymolecular adsorption regions. There is no a priori reason to believe that a mixed adsorption layer of a C6-ol with water would behave ideally, unless demixing prevails. Therefore, it is reasonable to assume the formation of a three-phase equilibrium between two coexisting phases, i.e., (nearly) pure water and (nearly) pure C6-ol in the adsorption layer and the bulk solution.6,7,44 If so, the composition of the coexisting phases remains essentially unchanged during the displacement process, only their relative ratio varying. As the monolayer of the adsorbed solute is completed, water is fully displaced from the solid surface and the completed monolayer will provide a new surface for subsequent solute deposition. The adsorption continues on the new surface in a layer-by-layer mechanism. Though the present work hardly extends beyond an adsorbed bilayer, our previous study on related systems (n-butanol(1)-water(2)/nonporous carbons) has indicated no difference between the second and third layers, as far as the composition and the mechanism of formation of the adsorbed phase are concerned.7 The adsorption is expected to become less exothermic in the second and subsequent layers, wherein no direct contact exists between the adsorbate molecules and the solid surface. The differential molar enthalpies of displacement and adsorption, for both first and second layer formation, were calculated from the slopes of the straight lines of the graphs in Figures 11-13. Differential enthalpy data are listed in Table 1. The heats of solution in the bulk liquid phase are comparable in magnitude with the corresponding net enthalpies of adsorption. Therefore, the enthalpy of displacement cannot provide information as to what actually happens within the adsorption layer. In the monolayer region, the displacement of water by phenol is more exothermic (∆21h1 ) -16.09 kJ mol-1) than for the other two C6-ols (-12.97 and -4.08 kJ mol-1 for n-hexanol and cyclohexanol, respectively). However, in terms of adsorption enthalpies, n-hexanol is appreciably more strongly adsorbed (∆21h1s ) -15.7 kJ mol-1) than phenol (-11.30 kJ mol-1) or cyclohexanol (-11.35 kJ mol-1). Though the structure of the delocalized π-electrons of gcb is similar to that of aromatic hydrocarbons,9 a comparison of the enthalpy data for phenol with those for the other two C6-ols suggests only a weak specific interaction between the graphite surface and the benzene ring. This trend is in accordance with the heats of immersion data and the associated volume changes for the wetting of Graphon by pure C6 hydrocarbons possessing the same skeletons as the C6-ols investigated here.48 The absolute surface excess concentrations of pure n-hexane and pure benzene on gcb were found to be positive and negative, respectively, relative to that for pure cyclohexane. On the other hand, the heat of immersion is appreciably higher for n-hexane than for the other two hydrocarbons. These observations indicate a stronger interaction of the graphite basal plane with the n-alkane skeleton than with the cyclic hydrocarbons. Interestingly, the enthalpy of adsorption of n-hexanol in the second layer is as exothermic as the bulk heat of solution (∼-2.7 kJ mol-1) leading to an apparently zero enthalpy of displacement (see eqs 9-11). After formation of the n-hexanol monolayer is completed, the enthalpy of displacement has been reported to turn in an endothermic direction at lower temperature (+2.2 kJ mol-1 at 298.15 K6), which is attributed to the more exothermic enthalpies of dilution at lower temperatures (-6.5 kJ mol-1 at 298.15 K, in contrast with -2.73 kJ mol-1 at 308.15 K). It is an (48) Ash, S. G.; Findenegg, G. H. Spec. Discuss. Faraday Soc. 1970, 1, 105 and general discussion thereafter.

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important observation that, as may be expected, the enthalpy of adsorption is more exothermic at 298.15 K than at 308.15 K, in both the mono- and multilayer regimes (-18.8 and -4.0 kJ mol-1, respectively, at 298.15 K6). An incipient solidification of n-hexanol, under the influence of the graphite surface, some 80 K above the freezing temperature, may be excluded.44-46 The adsorption of a cyclohexanol monolayer is about as exothermic as that for phenol (∼-11.3 kJ mol-1), and both of them are comparable in magnitude with the enthalpy of adsorption of n-hexanol. These findings suggest that the adsorbed cyclohexanol molecules are to some extent distorted on adsorption, so that each methylene group may have surface contact.47 The enthalpy of fusion is 1.7 kJ mol-1 at the melting temperature (298 K). Thus, the enthalpy of solidification, if any, makes a minor contribution to the net enthalpy of adsorption. A liquid/solid transition may well occur for cyclohexanol adsorbing throughout the aqueous phase, because the ∆21h1s data are not significantly different for the first and second layers. As mentioned earlier, the enthalpy of adsorption for phenol was obtained from the enthalpy of displacement by subtracting the heat of solution of supercooled phenol rather than that of solid phenol. The enthalpy of adsorption in the first layer (-11.3 kJ mol-1 at 308.15 K) is very close to the enthalpy of freezing (-11.52 kJ mol-1 at 314 K), so that solidification of phenol would be possible purely on this basis. In fact, it has been found for the vapor-phase adsorption of benzene onto graphite that, at temperatures below the freezing temperature, the heat of adsorption for the first layer molecules is identical (but opposite in sign) to the heat of sublimation.47 The heat of adsorption of phenol is less exothermic in the second layer (∼-9.0 kJ mol-1) than in the first, indicating that the adsorbed phenol molecules are more mobile in the

Kira´ ly et al.

second layer, so that solidification in this layer can be excluded. With the knowledge of the temperature dependence of the adsorption excess isotherms and of the displacement enthalpy isotherms, one can decide whether a liquid/solid transition, or other structuring phenomena due to the presence of the hydroxy groups, occur in the first and subsequent adsorption layers. To answer these questions, adsorption and calorimetric investigations with a systematic variation in temperature are in progress. Conclusions A set of operational formulas has been presented for the analysis of adsorption, and displacement calorimetric data relating to the solid/solution interface. Monomolecular adsorption capacities, net enthalpies of adsorption and other fundamental thermodynamic information can be acquired on the basis of the proposed data treatment. The application of the equations has been demonstrated for the displacement of water by phenol and related compounds from dilute binary aqueous solutions onto gcb. The adsorption of sparingly water-soluble materials from the aqueous phase plays a very important role, among others, in waste-water purification, soil pollution, etc. Though particular attention has been focused on preferential solute adsorption from dilute solutions, the presented thermodynamic analysis can be applied to many practical systems, since no prescription has been given for the adsorption layer to behave ideally. Acknowledgment. This work was carried within the Scientific and Technological Cooperation between Hungary and Germany under Project No. 235.3/OMFB71. LA950390G