Simultaneous Measurement of Infrared Spectra and Adsorption Isotherins for the Adsorption of Phenol on Silica at the Solid/Liquid Interface BY KENNETH MARSHALL AND COLIN H. ROCHESTER* Department of Chemistry, University of Nottingham, Nottingham NG7 2RD Received 15th April, 1975 An infrared cell is described with which spectra of species adsorbed at the solid/liquid interface and the corresponding adsorption isotherms can be determined simultaneously. The system has been tested by a study of the adsorption of phenol onto silica immersed in carbon tetrachloride. Isolated hydroxyl groups on the oxide surface form hydrogen bonds with adsorbed phenol molecules. The combined adsorption isotherm and spectroscopic results have enabled an estimate to be made of the popblation of isolated hydroxyl groups on the sample of silica studied.A simple cell used for the measurement of infrared spectra of several hydrogen bond acceptor molecules ' 7 adsorbed on silica at the solid/ liquid interface has been described previous1y.l Although easy to use the cell had some disadvantages which made it unsuitable for the simultaneous determination of adsorption isotherms and infrared spectra. The liquid system was neither thermo- statted nor stirred and quantitative measurement of the uptake of the adsorbate by the oxide disc was impossible. The aim of the present work was to construct and test an infrared cell in which the liquid phase was adequately thermostatted and stirred, and differences between the added and equilibrium concentrations of adsorbate in solution could be determined reliably.Carbon tetrachloride was chosen as solvent because of its spectroscopic properties. Isotherms for the adsorption of phenol on silica immersed in carbon tetrachloride have been reported by Boehm and Gromes and by Davis et aL5 and carboxylic acids 2 - EXPERIMENTAL The infrared cell is shown in fig. 1. The basic design of that part of the cell which contained the oxide disc immersed in solutions of adsorbate was identical to that of the cell previously described.l Unless otherwise stated the materials used in the canstruction of the two cells were the same. The base section of the new cell was made from stainless steel because brass, which was used before, had occasionally reacted with the adsorbates being studied. The use of a copper gasket as a seal between the flanges holding the fluorite optical windows (diameter 4 cm) considerably improved both the dynamic and static vacuums attainable. There was no detectable contamination of the solution or oxide sample resulting from the use of copper, rather than Teflon,l as the gasket material.A subsidiary optical compartment was constructed from two stainless steel flanges into which fluorite windows of diameter 3 cm were fixed with Araldite and which were bolted together via a copper gasket. The main and subsidiary optical compartments were made demountable for ease af internal cleaning. The subsidiary compartment contained adsorbate in carbon tetrachloride at the same solution composition as was contained in the main compartment in equilibrium with the oxide sample.Equilibrium was maintained throughout the system by the anticlockwise (fig. 1) circulation of solution through the two compartments in series from a Pyrex reservoir containing a rotating paddle which provided the pumping mechanism, Solution contained 2478K . MARSHALL AND C. H. ROCHESTER 2479 in the reservoir was thermostatted by a glass jacket through which water was passed from a conventional thermostat bath. Temperature probes in the form of copper-constantan thermocouples in the reservoir and at the base of the cell showed that the temperature of the circulating solution varied at 25°C by < +0.2"C. Unlike the systems involving a reference cell adopted by Low and Hasegawa 6*7 the spacing of the main and subsidiary cell compart- ments was such that when the main compartment was positioned in the sample beam of the infrared spectrometer then the subsidiary cell was clear of the spectrometer reference beam, The reference beam passed through the space between the two compartments. The aim was to measure the separate spectra of the oxide disc immersed in a solution and of the copper.qa5ke.t f l u o r i t e window; stirrer motor t h z r rnoco u p ie , well --I-- ---per mar e r t mognzt --Te I f o r4 bea r i n q soft iror! piece CI amping w e II2480 solution itself. The optical path lengths of the main and subsidiary compartments were 2.8 mm and 3.8 mm respectively. The purification of carbon tetrachloride, the preparation and admission of solutions to the infrared cell, the preparation and pretreatment of silica discs (surface area 176 m2 g-I), and the measurement of infrared spectra of the discs immersed in solutions of adsorbate were as before.' Phenol was recrystallised twice from light petroleum, b.p.40-60°C. Spectra were measured of phenol solutions of known concentration in the subsidiary cell compartment. Linear calibration graphs of the absorbances of the bands at 3608 and 3045 cm-' due to OH and CH stretching vibrations, respectively, against phenol concentra- tion were plotted. For the 3608 cm-I band the graph was only linear up to - 18 mmol dm-3 as phenol was partially dimerized at higher concentrations. Equilibrium concentrations of phenol solutions in adsorption experiments were thence deduced from the corresponding spectra of the solutions in contact with oxide discs.The differences between the added and equilibrium concentrations gave a measure of the amount of phenol adsorbed by the oxide sample and enabled adsorption isotherms to be calculated. Spectra of the oxide immersed in phenol solutions were also recorded. The optimum weight of the disc for spectroscopic study was - 80 mg. This weight was insufficient to give a reliably measurable difference between the added and equilibrium concentrations of phenol in solution. A second heavier ( ~ 0 . 6 5 g) disc of silica was therefore supported directly above the first disc such that the former did not interfere with the optical beam. The two discs were subjected to identical thermal pretreatment conditions and both were completely immersed in the liquid phase during a series of adsorption experiments.The attainment of equilibrium was established by confirmation that spectra of the oxide disc and the circulating solution did not vary with time. ADSORPTION OF PHENOL ON Si02 RESULTS Spectra of a silica disc immersed in solutions of phenol in carbon tetrachloride are shown in fig. 2. The band at 3686 cm-1 due to isolated silanol groups perturbed by immersion of the oxide in carbon tetrachloride was broadened and shifted to -3400 cm-' when phenol was adsorbed onto the surface. The exact position of 4 600 3500 3000 2500 FIG. 2.-Spectra (c)-(f) immersed wavenumber /cm-' of silica (a) after evacuation at 480'C for 64 h, (b) immersed in carbon tetrachloride, in solutions of phenol at equilibrium concentrations of 3.3,10.3, 19.7 and 40.8 mmol d ~ n - ~ respectively.K .MARSHALL AND C. H. ROCHESTER 248 1 the broad maximum shifted from 3430 to 3390 cm-I as the equilibrium concentration of phenol in solution was increased. The loss of intensity of the band at 3686 cm-I was a linear function (fig. 3) of the increase in the intensity of the broader band at 0.6[ I I I f 0.2 0.4 0.6 0.8 optical density at 3400 cm-I FIG. 3.-Linear relationship (slope -1.1) between the intensities of the infrared bands due to free and perturbed silanol groups on silica when phenol adsorbs from carbon tetrachloride solution. -3400 cm-l. These spectroscopic changes are characteristic of a hydrogen bonding interaction between surface silanol groups and adsorbed phenol molecules.A band at 3613 cm-l is assigned to the OH stretching vibration of phenol monomer in solution. Phenol existed entirely as its monomer at concentrations up to - 18 mmoldm-3. At higher concentrations some dimer (band at 3485cm-') was also phenol adsorbed/mg g-' FIG. 4.-Linear relationship between the growth of the absorption band due to perturbed sikmol groups and the weight of phenol adsorbed.2482 present. The highest concentration studied was 40 mmol dm-3 for which 3 % of the phenol existed as dimer. Infrared bands at 3080, 3049 and 3024cm-l were due to the CH stretching vibrations of phenol molecules both in solution and in the adsorbed state. The positions and relative intensities of the three bands were identical to those for phenol in solution in carbon tetrachloride.The bands were apparently unaffected by the adsorption of phenol on the surface of silica. The variation in the intensity of the broad infrared band at -3400 cm-l as a function of the equilibrium concentration of phenol in solution was of similar form to the corresponding variation in the weight of phenol adsorbed per gramme of silica. The latter was evaluated from the differences between the added and equilibrium concentrations of phenol. The similar shapes of the two curves are emphasized by the approximately linear relationship (fig. 4) between the weight of phenol adsorbed and the increase in intensity of the absorption maximum at 3400cm-'. Adsorbed phenol was completely desorbed from the surface of silica by flushing a disc immersed in a phenol solution with pure carbon tetrachloride solvent.The disc remained in contact with liquid phase throughout the flushing procedure. There was no spectroscopic evidence for chemisorption of phenol on the oxide surface. ADSORPTION OF PHENOL ON Si02 DISCUSSION The perturbation of the infrared band at 3686 cm-l and the concomitant growth of a broad band at -3400 cm-1 which occurred when phenol adsorbed onto silica showed that adsorbed phenol molecules were involved in hydrogen bonding inter- actions with isolated surface silanol groups. In contrast to the corresponding results for cyclohexanone the linearity of the plot in fig. 3 suggests that phenol was not adsorbed onto adjacent interacting silanol groups at low surface coverages. Furthermore the linear relatioilship (fig.4) between the growth of the broad band at -3400 cm-l and the weight of phenol adsorbed suggests that for concentrations <40 mmol dm-3 there was no phenol adsorbed onto sites other than surface silanol groups. The shifts to -3400 crn-l of the infrared band due to isolated silanol groups is too great to be caused by hydrogen bonding interactions between surface hydroxyl groups and the benzene rings of adsorbed phenol molecules.' Isolated silanol groups were hydrogen bonded to the hydroxyl groups of phenol molecules. A single inter- action may be represented by structure A (R = phenyl) in which the figures represent the shifts AVOH in the positions of the bands due to the stretching vibrations of silanol and phenol hydroxyl groups which occurred on adsorption. The shifts may be compared with those for n-decanol and n-propanol (structure B ; R' = alkyl) for which two broad infrwed bands appeared on adsorption of the alcohol molecules.2 For phenol only one broad band appeared presumably because the absorption maxima for the perturbed silanol and phenolic hydroxyl groups coincided.The difference between the shifts for the alcohols and phenols may be rationalized as follows. Phenol is a stronger acid but a weaker base than either of the alcohol^.^ For two hydrogen bond donors the one of higher acidity gives the greater shift AVOHcm-' on forming a hydrogen bond to a particular acceptor Similarly the shift AVOH for alcohols or phenols acting as hydrogen bond acceptors is greater the more basic is the acceptor molecule.In structure A the hydrogen bond a would therefore be expected to be stronger and the hydrogen bond b would be weaker than in structure B. In general the shifts in the bands due to OH-stretching vibrations are greater if the hydroxyl groups are acting as hydrogen bond donors rather than acceptors.10 Thus the shift ABoN for the adsorbate molecules will be primarily influenced by the strength of hydrogen bond a but for the silanol groups the prime and linolenic acidK . MARSHALL A N D C. H. ROCHESTER 2483 influence will be hydrogen bond b. It follows that AToH for the adsorbate molecules should be greater and AToH for the silanol groups should be less when phenol rather than n-propanol or n-decanol are adsorbed on silica. These conclusions are consistent with the experimental observations.R 1 213 cm - l/o..**.. H H R’ I I 76 cm - l/o*=*.. H H a ’***..0/286 cm- 1 a “.*-.O/~X cm- 1 I Si I Si (4 / I \ (B) / I \ The linearity of the plots in fig. 3 and 4 and the evidence that phenol was only adsorbed onto isolated surface silanol groups suggests that the present spectroscopic and adsorption isotherm data provide a method for the estimation of these groups. Combination of the two linear graphs shows that a 50 % reduction in the intensity of the infrared band at 3686 cm-l due to isolated silanol groups resulted from the adsorption of 28.2 mg of phenol per gramme of silica (surface area 176 m2 g-I). The total number noH of isolated hydroxyl groups per unit area on the oxide surface may be calculated via eqn (1) where NA is Avogadro’s number, A is the surface area of the oxide, M is the molecular weight of phenol, and w is the weight of phenol adsorbed on m g of silica to give 50 % coverage of the silanol sites.Hence noH was 2.05 nm-* for a silica sample which had been evacuated at 480°C. This figure is in good agreement with a value of - 2.2 nm-2 interpolated from data for the total surface silanol concentration. The small difference between the two values would be consistent with a small residual concentration of adjacent interacting surface silanol groups. The latter were respon- sible for the shoulder at -3680 cm-1 in fig. 2a. The adsorption isotherm at 25°C measured here is compared in fig. 5 with the isotherm at 35°C obtained by Davis et aL5 In the figure no is the total number of nOH = (2NAw/MmA) (1) 0 0.I 0.2 0.3 0.4 0.5 1Oy.x:) FIG. 5.-Isotherms for the adsorption of phenol on silica from carbon tetrachloride solution, 0 aerosil silica, 25”C, present study ; 0 silica gel, 35”C, ref. ( 5 ) (see text),2484 ADSORPTION OF PHENOL ON SiOz moles of solute plus solvent in contact with rn g of silica with surface area A, x i is the equilibrium mole fraction of phenol, and Axv is the difference between the added and equilibrium mole fractions of phenol. The points plotted for the isotherm at 35°C were calculated from the adsorption parameters deduced by Davis et aL5 from their experimental data, numerical details of which were not published. The curve therefore represents a smoothed isotherm. The two isotherms (fig. 5) are similar in shape but differ in magnitude.The difference must arise in part because of the significantly different properties, particularly with respect to pore structure, of silica gel and Aerosil silica. However the prime effect must be that the sample studied by Davis et aLs had been evacuated at 120°C prior to immersion in solutions of phenol and therefore retained a much higher residual concentration of surface silanol groups and molecular water than the oxide evacuated in the present study at 480°C. The authors thank Tioxide International Ltd. for the award of a Fellowship (to K. M.). D. M. Griffiths, K. Marshall and C. H. Rochester, J.C.S. Faraday I, 1974,70,400. K. Marshall and C. H. Rochester, Faraday Disc. Chem. SOC., 1975,59, in press. K. Marshall and C.H. Rochester, J.C.S. Faraduy I, 1975,71, 1754. H. P. Boehm and W. Gromes, Angew. Chem., 1959,71,65. K. M . C. Davis, J. A. Deucher and D. A. Ibbitson, J.C.S. Far&y I, 1973,69,1117. M. J. D. Low and M. Hasegawa, J. Colloid Interface Sci., 1968, 26,95. ’ M. Hasegawa and M. J. D. Low, J. Colloid Interface Sci., 1969, 29, 593. P. G. Rouxhet and R. E. Sempels, J.C.S. Faraduy I, 1974,70, 2021. C. H. Rochester in The Chemistry of the Hydroxyl Group, ed. S . Patai (Interscience, London, 1971), p. 327. lo A. Hall and J. L. Wood, Spectrochim. Acta, 1967, 23A, 2657. li R. Bode, H. Ferch and H. Fratzscher, Properties and Applications of Aerosil (manufacturers handbook, Degussa, Frankfurt). Simultaneous Measurement of Infrared Spectra and Adsorption Isotherins for the Adsorption of Phenol on Silica at the Solid/Liquid Interface BY KENNETH MARSHALL AND COLIN H.ROCHESTER* Department of Chemistry, University of Nottingham, Nottingham NG7 2RD Received 15th April, 1975 An infrared cell is described with which spectra of species adsorbed at the solid/liquid interface and the corresponding adsorption isotherms can be determined simultaneously. The system has been tested by a study of the adsorption of phenol onto silica immersed in carbon tetrachloride. Isolated hydroxyl groups on the oxide surface form hydrogen bonds with adsorbed phenol molecules. The combined adsorption isotherm and spectroscopic results have enabled an estimate to be made of the popblation of isolated hydroxyl groups on the sample of silica studied. A simple cell used for the measurement of infrared spectra of several hydrogen bond acceptor molecules ' 7 adsorbed on silica at the solid/ liquid interface has been described previous1y.l Although easy to use the cell had some disadvantages which made it unsuitable for the simultaneous determination of adsorption isotherms and infrared spectra.The liquid system was neither thermo- statted nor stirred and quantitative measurement of the uptake of the adsorbate by the oxide disc was impossible. The aim of the present work was to construct and test an infrared cell in which the liquid phase was adequately thermostatted and stirred, and differences between the added and equilibrium concentrations of adsorbate in solution could be determined reliably. Carbon tetrachloride was chosen as solvent because of its spectroscopic properties.Isotherms for the adsorption of phenol on silica immersed in carbon tetrachloride have been reported by Boehm and Gromes and by Davis et aL5 and carboxylic acids 2 - EXPERIMENTAL The infrared cell is shown in fig. 1. The basic design of that part of the cell which contained the oxide disc immersed in solutions of adsorbate was identical to that of the cell previously described.l Unless otherwise stated the materials used in the canstruction of the two cells were the same. The base section of the new cell was made from stainless steel because brass, which was used before, had occasionally reacted with the adsorbates being studied. The use of a copper gasket as a seal between the flanges holding the fluorite optical windows (diameter 4 cm) considerably improved both the dynamic and static vacuums attainable.There was no detectable contamination of the solution or oxide sample resulting from the use of copper, rather than Teflon,l as the gasket material. A subsidiary optical compartment was constructed from two stainless steel flanges into which fluorite windows of diameter 3 cm were fixed with Araldite and which were bolted together via a copper gasket. The main and subsidiary optical compartments were made demountable for ease af internal cleaning. The subsidiary compartment contained adsorbate in carbon tetrachloride at the same solution composition as was contained in the main compartment in equilibrium with the oxide sample. Equilibrium was maintained throughout the system by the anticlockwise (fig.1) circulation of solution through the two compartments in series from a Pyrex reservoir containing a rotating paddle which provided the pumping mechanism, Solution contained 2478K . MARSHALL AND C. H. ROCHESTER 2479 in the reservoir was thermostatted by a glass jacket through which water was passed from a conventional thermostat bath. Temperature probes in the form of copper-constantan thermocouples in the reservoir and at the base of the cell showed that the temperature of the circulating solution varied at 25°C by < +0.2"C. Unlike the systems involving a reference cell adopted by Low and Hasegawa 6*7 the spacing of the main and subsidiary cell compart- ments was such that when the main compartment was positioned in the sample beam of the infrared spectrometer then the subsidiary cell was clear of the spectrometer reference beam, The reference beam passed through the space between the two compartments.The aim was to measure the separate spectra of the oxide disc immersed in a solution and of the copper. qa5ke.t f l u o r i t e window; stirrer motor t h z r rnoco u p ie , well --I-- ---per mar e r t mognzt --Te I f o r4 bea r i n q soft iror! piece CI amping w e II2480 solution itself. The optical path lengths of the main and subsidiary compartments were 2.8 mm and 3.8 mm respectively. The purification of carbon tetrachloride, the preparation and admission of solutions to the infrared cell, the preparation and pretreatment of silica discs (surface area 176 m2 g-I), and the measurement of infrared spectra of the discs immersed in solutions of adsorbate were as before.' Phenol was recrystallised twice from light petroleum, b.p.40-60°C. Spectra were measured of phenol solutions of known concentration in the subsidiary cell compartment. Linear calibration graphs of the absorbances of the bands at 3608 and 3045 cm-' due to OH and CH stretching vibrations, respectively, against phenol concentra- tion were plotted. For the 3608 cm-I band the graph was only linear up to - 18 mmol dm-3 as phenol was partially dimerized at higher concentrations. Equilibrium concentrations of phenol solutions in adsorption experiments were thence deduced from the corresponding spectra of the solutions in contact with oxide discs. The differences between the added and equilibrium concentrations gave a measure of the amount of phenol adsorbed by the oxide sample and enabled adsorption isotherms to be calculated.Spectra of the oxide immersed in phenol solutions were also recorded. The optimum weight of the disc for spectroscopic study was - 80 mg. This weight was insufficient to give a reliably measurable difference between the added and equilibrium concentrations of phenol in solution. A second heavier ( ~ 0 . 6 5 g) disc of silica was therefore supported directly above the first disc such that the former did not interfere with the optical beam. The two discs were subjected to identical thermal pretreatment conditions and both were completely immersed in the liquid phase during a series of adsorption experiments.The attainment of equilibrium was established by confirmation that spectra of the oxide disc and the circulating solution did not vary with time. ADSORPTION OF PHENOL ON Si02 RESULTS Spectra of a silica disc immersed in solutions of phenol in carbon tetrachloride are shown in fig. 2. The band at 3686 cm-1 due to isolated silanol groups perturbed by immersion of the oxide in carbon tetrachloride was broadened and shifted to -3400 cm-' when phenol was adsorbed onto the surface. The exact position of 4 600 3500 3000 2500 FIG. 2.-Spectra (c)-(f) immersed wavenumber /cm-' of silica (a) after evacuation at 480'C for 64 h, (b) immersed in carbon tetrachloride, in solutions of phenol at equilibrium concentrations of 3.3,10.3, 19.7 and 40.8 mmol d ~ n - ~ respectively.K .MARSHALL AND C. H. ROCHESTER 248 1 the broad maximum shifted from 3430 to 3390 cm-I as the equilibrium concentration of phenol in solution was increased. The loss of intensity of the band at 3686 cm-I was a linear function (fig. 3) of the increase in the intensity of the broader band at 0.6[ I I I f 0.2 0.4 0.6 0.8 optical density at 3400 cm-I FIG. 3.-Linear relationship (slope -1.1) between the intensities of the infrared bands due to free and perturbed silanol groups on silica when phenol adsorbs from carbon tetrachloride solution. -3400 cm-l. These spectroscopic changes are characteristic of a hydrogen bonding interaction between surface silanol groups and adsorbed phenol molecules. A band at 3613 cm-l is assigned to the OH stretching vibration of phenol monomer in solution.Phenol existed entirely as its monomer at concentrations up to - 18 mmoldm-3. At higher concentrations some dimer (band at 3485cm-') was also phenol adsorbed/mg g-' FIG. 4.-Linear relationship between the growth of the absorption band due to perturbed sikmol groups and the weight of phenol adsorbed.2482 present. The highest concentration studied was 40 mmol dm-3 for which 3 % of the phenol existed as dimer. Infrared bands at 3080, 3049 and 3024cm-l were due to the CH stretching vibrations of phenol molecules both in solution and in the adsorbed state. The positions and relative intensities of the three bands were identical to those for phenol in solution in carbon tetrachloride. The bands were apparently unaffected by the adsorption of phenol on the surface of silica.The variation in the intensity of the broad infrared band at -3400 cm-l as a function of the equilibrium concentration of phenol in solution was of similar form to the corresponding variation in the weight of phenol adsorbed per gramme of silica. The latter was evaluated from the differences between the added and equilibrium concentrations of phenol. The similar shapes of the two curves are emphasized by the approximately linear relationship (fig. 4) between the weight of phenol adsorbed and the increase in intensity of the absorption maximum at 3400cm-'. Adsorbed phenol was completely desorbed from the surface of silica by flushing a disc immersed in a phenol solution with pure carbon tetrachloride solvent. The disc remained in contact with liquid phase throughout the flushing procedure.There was no spectroscopic evidence for chemisorption of phenol on the oxide surface. ADSORPTION OF PHENOL ON Si02 DISCUSSION The perturbation of the infrared band at 3686 cm-l and the concomitant growth of a broad band at -3400 cm-1 which occurred when phenol adsorbed onto silica showed that adsorbed phenol molecules were involved in hydrogen bonding inter- actions with isolated surface silanol groups. In contrast to the corresponding results for cyclohexanone the linearity of the plot in fig. 3 suggests that phenol was not adsorbed onto adjacent interacting silanol groups at low surface coverages. Furthermore the linear relatioilship (fig. 4) between the growth of the broad band at -3400 cm-l and the weight of phenol adsorbed suggests that for concentrations <40 mmol dm-3 there was no phenol adsorbed onto sites other than surface silanol groups.The shifts to -3400 crn-l of the infrared band due to isolated silanol groups is too great to be caused by hydrogen bonding interactions between surface hydroxyl groups and the benzene rings of adsorbed phenol molecules.' Isolated silanol groups were hydrogen bonded to the hydroxyl groups of phenol molecules. A single inter- action may be represented by structure A (R = phenyl) in which the figures represent the shifts AVOH in the positions of the bands due to the stretching vibrations of silanol and phenol hydroxyl groups which occurred on adsorption. The shifts may be compared with those for n-decanol and n-propanol (structure B ; R' = alkyl) for which two broad infrwed bands appeared on adsorption of the alcohol molecules.2 For phenol only one broad band appeared presumably because the absorption maxima for the perturbed silanol and phenolic hydroxyl groups coincided.The difference between the shifts for the alcohols and phenols may be rationalized as follows. Phenol is a stronger acid but a weaker base than either of the alcohol^.^ For two hydrogen bond donors the one of higher acidity gives the greater shift AVOHcm-' on forming a hydrogen bond to a particular acceptor Similarly the shift AVOH for alcohols or phenols acting as hydrogen bond acceptors is greater the more basic is the acceptor molecule. In structure A the hydrogen bond a would therefore be expected to be stronger and the hydrogen bond b would be weaker than in structure B. In general the shifts in the bands due to OH-stretching vibrations are greater if the hydroxyl groups are acting as hydrogen bond donors rather than acceptors.10 Thus the shift ABoN for the adsorbate molecules will be primarily influenced by the strength of hydrogen bond a but for the silanol groups the prime and linolenic acidK .MARSHALL A N D C. H. ROCHESTER 2483 influence will be hydrogen bond b. It follows that AToH for the adsorbate molecules should be greater and AToH for the silanol groups should be less when phenol rather than n-propanol or n-decanol are adsorbed on silica. These conclusions are consistent with the experimental observations. R 1 213 cm - l/o..**.. H H R’ I I 76 cm - l/o*=*..H H a ’***..0/286 cm- 1 a “.*-.O/~X cm- 1 I Si I Si (4 / I \ (B) / I \ The linearity of the plots in fig. 3 and 4 and the evidence that phenol was only adsorbed onto isolated surface silanol groups suggests that the present spectroscopic and adsorption isotherm data provide a method for the estimation of these groups. Combination of the two linear graphs shows that a 50 % reduction in the intensity of the infrared band at 3686 cm-l due to isolated silanol groups resulted from the adsorption of 28.2 mg of phenol per gramme of silica (surface area 176 m2 g-I). The total number noH of isolated hydroxyl groups per unit area on the oxide surface may be calculated via eqn (1) where NA is Avogadro’s number, A is the surface area of the oxide, M is the molecular weight of phenol, and w is the weight of phenol adsorbed on m g of silica to give 50 % coverage of the silanol sites.Hence noH was 2.05 nm-* for a silica sample which had been evacuated at 480°C. This figure is in good agreement with a value of - 2.2 nm-2 interpolated from data for the total surface silanol concentration. The small difference between the two values would be consistent with a small residual concentration of adjacent interacting surface silanol groups. The latter were respon- sible for the shoulder at -3680 cm-1 in fig. 2a. The adsorption isotherm at 25°C measured here is compared in fig. 5 with the isotherm at 35°C obtained by Davis et aL5 In the figure no is the total number of nOH = (2NAw/MmA) (1) 0 0. I 0.2 0.3 0.4 0.5 1Oy.x:) FIG.5.-Isotherms for the adsorption of phenol on silica from carbon tetrachloride solution, 0 aerosil silica, 25”C, present study ; 0 silica gel, 35”C, ref. ( 5 ) (see text),2484 ADSORPTION OF PHENOL ON SiOz moles of solute plus solvent in contact with rn g of silica with surface area A, x i is the equilibrium mole fraction of phenol, and Axv is the difference between the added and equilibrium mole fractions of phenol. The points plotted for the isotherm at 35°C were calculated from the adsorption parameters deduced by Davis et aL5 from their experimental data, numerical details of which were not published. The curve therefore represents a smoothed isotherm. The two isotherms (fig. 5) are similar in shape but differ in magnitude. The difference must arise in part because of the significantly different properties, particularly with respect to pore structure, of silica gel and Aerosil silica. However the prime effect must be that the sample studied by Davis et aLs had been evacuated at 120°C prior to immersion in solutions of phenol and therefore retained a much higher residual concentration of surface silanol groups and molecular water than the oxide evacuated in the present study at 480°C. The authors thank Tioxide International Ltd. for the award of a Fellowship (to K. M.). D. M. Griffiths, K. Marshall and C. H. Rochester, J.C.S. Faraday I, 1974,70,400. K. Marshall and C. H. Rochester, Faraday Disc. Chem. SOC., 1975,59, in press. K. Marshall and C. H. Rochester, J.C.S. Faraduy I, 1975,71, 1754. H. P. Boehm and W. Gromes, Angew. Chem., 1959,71,65. K. M . C. Davis, J. A. Deucher and D. A. Ibbitson, J.C.S. Far&y I, 1973,69,1117. M. J. D. Low and M. Hasegawa, J. Colloid Interface Sci., 1968, 26,95. ’ M. Hasegawa and M. J. D. Low, J. Colloid Interface Sci., 1969, 29, 593. P. G. Rouxhet and R. E. Sempels, J.C.S. Faraduy I, 1974,70, 2021. C. H. Rochester in The Chemistry of the Hydroxyl Group, ed. S . Patai (Interscience, London, 1971), p. 327. lo A. Hall and J. L. Wood, Spectrochim. Acta, 1967, 23A, 2657. li R. Bode, H. Ferch and H. Fratzscher, Properties and Applications of Aerosil (manufacturers handbook, Degussa, Frankfurt).