Langmuir 1988,4, 97-100
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Thermodynamics of Adsorption of Alcohols on a “Modified” Gold Surface G . M. Khan+and M. J. Iqbal* Department of Chemistry, Quaid-i-Azam University, Islamabad, Pakistan Received April 10, 1987 The piezogravimetric method has been used to study the adsorption of various aliphatic alcohols on a gold surface at different temperatures. All the isotherms are of type 11, indicating that the physical adsorption at high pressures is infinite. The cross-sectional areas, the isosteric heats of adsorption, and the film pressures of various alcohols have been calculated.
Introduction Adsorption studies on single, homogeneous, and polished crystals, free from pores, crevices, and other surface imperfections, have obvious points of preference. T h e adsorption data obtained on such surfaces are not complicated by heterogeneity effects and capillary and intergranular condensation. With single-crystal surfaces, it is possible to not only give a definite interpretation of the data in terms of microscopic models but also derive useful information,through comparative studies, about the nature of the surface heterogeneity and mechanism of capillary condensation. However, working with low-specific area solids is not amenable to the conventional techniques because of the high sensitivity required t o determine the amount of adsorbate on limited single-crystal substrate. Even with a Cahn balance, the usual sensitivity that could be relied upon does not exceed lo4 g on solids having areas as low as 200 cm2. Such investigations can, with proper care, be carried out piezogravimetrically, using polish quartz single crystals. The technique was developed by Slutsky and Wade,’ on the suggestion of Sauerbrey,2 and has been used quite successfully for various adsorption studies.= Apart from minor problems in using this technique, it is very suitable for studying adsorption on small surface areas, and mass changes of the order of lo4 g can be determined if sufficient care is taken. Experimental Section The core feature of the piezogravimetric method is that a suitably chosen quartz crystal, capable of resonating at its characteristic frequency between two exciting electrodes, be coupled with an ancillary device for measuring frequency shifts at various pressures of the adsorbate vapor, at equilibrium with the adsorbent phase, in an accuratelytemperature controlled cell. The vacuum system and the circuits used have been described elsewhere in detail.6 For convenience, however, Figure 1shows the details of the experimental cell and the quartz mounting assembly. The gold films, deposited on the faces of the polished quartz crystals, maintained at liquid-nitrogen temperature, were prepared from 0.5-mm wire obtained from Johnson Mathey & Co. (pure grade). The apparatus was thoroughly outgassed for about 10 h and the filaments were heated to as high a temperature as possible without film formation.’ The adsorption vessel containing the quartz crystal was then cooled down by immersing it in liquid nitrogen and the current passing through the filaments was raised. The vessel was kept outgassed through this process. The deposited film was subsequently sintered at 348 K for half an hour. Before use, the cell, the stainless steel components, and the crystal assembly were treated with chromic acid-sulfuric acid mixture followed by repeated rinsing with triply distilled water. t Present
address: JCB,PMA Kakul, Abbotabad, Pakistan.
Table I. Cross-Sectional Area Calculations apparent (geometric) area = 3.08 cm2 calculated surface area (A) = 6.3 cm2,& monolayer capacity, calcd Mc cross-sectional Hz 108/g M, area, ~lO1s/cmz vapor methyl alcohol 62 39.92 32 10.30 16.24 57 36.78 46 ethyl alcohol 53 34.13 60 22.78 n-propyl alcohol 29.17 n-butyl alcohol (i) 50 32.20 74 (ii) 52 33.49 28.70 51 32.80 (iii) 29.23 Before observations were taken, the system was heated to 513 K and evacuated for 24 h. The vacuum line was thoroughly baked at 353 K for about 48 h. When the resonant frequency was stable, the vapor was introduced gradually during each run by increasing the pressure in small steps. The frequency was monitored as a function of time over a period of about 40 min for each run until equilibrium had been attained. After the determination of a particular isotherm, the system was outgassed until a stable value for the resonant frequency was obtained again. The quartz crystal used was AT-cut and highly polished with an apparent (geometric) area of 3.080 cm2and mass of 0.060 g. The crystal and the electrodes were square in shape and were separated by small triangular pieces of inert insulator at the comers of each side of the crystal. The stability of the resonance frequency of the quartz crystal is shown in Figure 2. The adsorbates were obtained from Matheson Coleman and Bell, listed as being of Chromatoquality,which were further dried thoroughly by passing through molecular sieve and finally outgassed by trap-to-trap distillation and a repeated freezing and thawing under vacuum. Argon, with no impurities detectable by mass spectrometer, was supplied by Matheson.
Results and Discussion Figures 3 and 4 show the adsorption of alcohols on a gold-coated quartz surface at different temperatures. A number of general points may be summarized as follows: (1) The adsorption isotherms for alcohols on a gold surface are of type 11; they are reasonably reproducible, particularly at the lower temperature, and display negligible hysteresis. (2) The extent of physical adsorption decreases, at fixed temperatures and relative pressure, as the molecular weight of the adsorbate is raised. (3) T h e extent of physical adsorption of a given adsorbate at fixed relative pressure decreases with increase in temperature. (1)Slutsky, L. J.; Wade, W. H. J. Chem. Phys. 1962, 36, 2688. (2) Sauerbrey, G. 2.Phys. Chem. 1959, 155,206. ( 3 ) Khan, G. M. Can. J . Chem. 1972,50, 125. (4) Khan, G. M. Pak. J . Sci. Ind. Res. 1972, 15, 1. (5) Khan, G. M. 2.Phys. Chem. (Munich) 1973,85, 230. (6) Khan, G. M. Rev. Sci. Instrum. 1972, 43, 117. (7) Saleh, J. M. J . Phys. Chem. 1973, 77,1849.
0743-7463188f 2404-0097$01.50 f 0 0 1988 American Chemical Society
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Figure 4. Adsorption of n-butyl alcohol on a gold surface. 1
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Figure 2. y vs time on a polished quartz crystal having a gold layer at 299.8 K and atmospheric pressure. Surface Area of the Adsorbent. The surface area of the adsorbent was calculated from the monolayer capacities estimated from the isotherms of argon by the “point B” method using 13.7 X cm2 as the cross-sectional area of argon. Its average value was found t o be approximately 6.3 cm2 g-l (rugosity being about 2.0). Cross-Sectional Areas of Adsorbates. Table I shows that the calculated cross-sectional areas are quite consistent, suggesting that the general approach cannot be seriously in error. However, the concordance merely indicates that the adsorbed alcohol molecules in the first layer most probably lie parallel to the adsorbent surface in a close-packed regular array.s
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Figure 5. Plot of log, P vs 1/T for n-butyl alcohol on a gold surface.
Isosteric Heats of Adsorption. The heats of adsorption for the systems investigated were calculated from a
Langmuir, Vol. 4, No. I, 1988 99
Adsorption of Alcohols on a Gold Surface 3 00,
Table 11. Isosteric Heats of Adsorption of a-Butyl Alcohol on a "Modified"Gold Surface HZ Qo/kJ.mol-' Hz Qo/kJ-mo1-l 30 53.1 54 56.5 40 53.1 60 54.8 47 53.1 72 53.1 50 54.4 88 51.5
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series of isotherms a t different temperatures, and the Clausius-Claperon equation was applied in the integrated form Qo =
2.303R log (P2/P1) 1/T' - 1/T,
where R is the gas constant in J mol-' K-I, P1 is the pressure of adsorbate at temperature Tl, P2 is the pressure of adsorbate at temperature T2, and Qois the isosteric heat of adsorption a t constant coverage. Qocalculated from the above equation, however, is not the same as the enthalpy change AH for the adsorption process. The relation between these quantities has been given by HilL9 Nevertheless, the value of Qo and its variation with coverage and temperature will serve as a guide to the value of AH and its variation with the same experimental conditions. The variation of isosteric heats of adsorption against extent of adsorption is illustrated for n-butyl alcohol in Figure 6. No great reliance should be placed upon the actual values reported, however, because of the limitations imposed by the technique itself and by the system, which was studied a t three temperatures. Thus, the ClausiusClapeyron plots were a t best applied to only three temperatures (Figure 5). The reproducibility of the isotherms, coupled with the absence of hysteresis, suggests that the uncertainty should not be very high. The alcohols are adsorbed on the gold surface physically, the interactions being purely dispersive. A close correspondence between the isosteric heat of adsorption of, for example, 1-butanol (53 kJ mol-') and its heat of vaporization (i.e. 46 kJ mol-') is considered to be additional evidence for the occurrence of physical adsorption.1° Previous studies" have suggested that the initial act of adsorption, a t very low relative pressures, is accompanied by a decrease in the heat of adsorption arising from the filling of strong adsorption sites. This is followed by the increase toward the maximum shown in Figure 6. It is of significance that the maximum corresponds closely to the monolayer capacities reported in Table I; the maximum (8) Khan, G. M. Pak. J. Sci. Znd. Res. 1973, 16, 23. (9) Hill, T. L. J. Chem. Phys. 1949, 17, 520.
(10)Cullar, J.; Galan, M. A. J. Sep. Process Technol. 1983,413, 23. (11)Khan, G.M.2.Phys. Chem. (Munich) 1973,83, 179.
Table 111. Film Pressures for the Adsorption of Various Alcohols on a 'Modified" Gold Surface 10711/J.cm-2 absorbate 299.8 K 310.2 K 319.5 K methyl alcohol 391.4 220.5 166.8 ethyl alcohol 364.3 198.4 141.0 n-propyl alcohol 162.6 112.5 332.5 n-butyl alcohol 296.0 122.3 76.8 Table IV. Film Pressure for the Adsorption of Methyl Alcohol on a 'Modified" Gold Surface low/J . C ~ - ~ Plmm 299.8 K 310.2 K 319.5 K 13.6 86.1 48.5 37.3 30.0 160.9 83.4 62.6 54.4 231.7 103.8 76.7 88.2 302.4 147.1 108.3 108.8 337.5 180.0 129.8 122.4 190.5 142.2 360.2
value of Q, is observed a t 54 Hz whereas the maximum monolayer capacity, of, for example, 1-butanol is observed a t 52 Hz. It must be supposed therefore, that the physically adsorbed molecules are a t first randomly deposited on the surface, possessing lateral freedom of motion. As the monolayer fills,lateral interaction between adsorbate molecules results in a parallel and to end orientation leading to the concordance between the "true" surface area of the adsorbent and the regular increase per -CH2 increment in the cross-sectional adsorbate area. The second, third, and susequant layers are then laid down with a degree of orientation which decreases with increasing distance from the surface, finally resulting in the orientation characteristic of bulk liquid. It may here be noted that, in accordance with the views of other workers, the present results lend no support to either the original or the modified BET equation.12J3 Film Pressures. Film pressures, n, were determined by utilizing the values of the areas under the curves obtained by plotting various m / P vs P graphs (see Figure 7) from the equation
(12)Brunauer, S.;Emmet, P. H.; Teller, E. J. Am. Chem. SOC.1938, 60,309. (13) Brunauer, S.;Scalny, J.; Bodor, E. E. J. Colloid Interface Sci. 1969, 30, 546.
100 Langmuir, Vol. 4, No. 1, 1988
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Figure 8. Film pressure II vs PIP, plots for various alcohols on a gold surface at 299.8 K. where M, is molecular weight of the adsorbate, f is the rugosity of the surface, A is the apparent surface area, m is the mass of the adsorbate, and P is the pressure of the adsorbate. The results are summarized in Tables I11 and IV and exemplified by Figure 8, for different alcohols a t 299.8 K. The term film pressure was devised in order to describe the lowering of the surface free energy of a liquid/vapor interface by a partial or complete monolayer and was then extended to liquidlliquid and solid/vapor interfaces. Its use in these situations is both reasonable and helpful; a physical model of matter confined to two dimensions can be visualized. Difficulties arise when attempts are made to extend this physical model to adsorbed multilayers. There is still a reduction in the free energy of the solid surface, but the concept of a two-dimensional pressure becomes increasingly vague. There is, of course, no ambiguity concerning the interpretation of II, a t least a t moderate relative pressures, as being the reduction in the surface free energy of the solid. The "film pressure" model
is used here with this reservation in mind. Representative plots of II vs PIPo at 299.8 K are given in Figure 8, and are of characteristic form. The curves show a marked resemblance to the plots of n-alkanes on anatase and other low-energy surfaces although the magnitude of II values a t any given relative pressure varies considerably.l"16 This confirms the view that the interactions in the present bystem may be regarded as being purely dispersive. The higher II values for alcohols on gold surfaces simply indicate that the alcohols do a good job in "quenching" the unsatisfied valence forces emanating from the adsorbent surface, whereas the sudden increase in II values with PIPo indicates a gradual increase in free energy of alcohol adsorption systems for increasing pressures in conformity with the analogous investigations of Good et al." and Grahamls on other systems. As may be conceived, surface film pressure II is, in fact, the free energy change for the given adsorbate per unit cross section of the adsorbent and we may thus take effectively the II values to be identical in magnitude with changes in Gibbs free energy, AG, values. The surface free energy values obtained for methyl alcohol adsorption on "modified" gold surfaces are given in Table IV, wherein the II values, as a function of various temperatures, have been recorded in order to affect a comparative study of the behavior of the adsorbent toward the adsorbate in hand. It is interesting to see that a t a given pressure, the value of II continuously decreases in magnitude by increasing the temperature. Though no specific conclusions could be drawn from this dependence, it may be suggested that a hydrogen bonded surface complex of the adsorbate species whose sorption mechanism depends on temperature and degree of surface saturation may have been formed. The observed decrease in the value of II with temperature may be due to this effect. The existence of the H-bonded alcoholate and carboxylate surface structures of alcohol adsorbates on other systems has been suggested by various authors.1*21 Registry No. Au, 7440-57-5; methyl alcohol, 67-56-1; ethyl alcohol, 64-17-5;n-propyl alcohol, 71-23-8 n-butyl alcohol, 71-36-3. (14) Harkins, W. D. The Physical Chemistry of Surface Films; Reinhold New York, 1952; p 218. (15) Khan, G. M. Ph.D. Thesis, University of Southampton, 1970. (16) Afzal, M.; Jaffar, M.; Ahmed, J.;Parveen, N. Colloid Polym. Sci. 1978,256, 356. (17) Good, R. J.; Girifalco, L. A.; Kraus, G . J. Phys. Chem. 1958,62, 418. (18)Graham, D.J. Phys. Chern. 1964,68, 2788. (19) Osipova, N. A.;Davydov, A. A.; Kurina, L. N.; Loiko, V. E., Zh. Fiz. Khim. 1985,59, 1479. (20) Vedernikov, V. I.; Gul'yanova, S.G.; Grayznov, V. M.; Tishchenko, A. A. Izv. Vyssh. Uchebn. Zaved., Khim. Khim. Tekhnol. 1985,28, 52. (21) Rczwadowski, M.; Wisniewski, E.; Wojsz, R. Carbon 1984,22,273.
