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Influence of adsorbed vapors on the photoacoustic spectra of liquids
Influence of Adsorbed Vapors on the Photoacoustic Spectra of Liquids ~
liquid was introduced into the cell using a microsyringe by soaking the inert material. The length of the gas phase above the sample, l~, was not affected in this process. All the experiments were carried out with an l~ ~ 1.5 m m with f = 28.6 Hz. The PA signal increases with time on the introduction of the liquid and reaches a steady value after about 15 min. The results reported (except where mentioned) here are obtained only after the steady-state signal was obtained. The vapor pressure of the liquid (/~iq) within the cell is expected to be equal to the saturated vapor pressure of the liquid (p0q) at the temperature of the cell.
INTRODUCTION The application of photoacoustic spectroscopy (PAS) to the study o f solids and surfaces is well known (I-3). In PAS (1, 2) an intensity-modulated light is absorbed by the sample being investigated. The absorbed light energy is dissipated through nonradiative deexcitation as thermal energy. The periodic heat produced diffuses to the sample surface and then to a coupling gas (usually air), causing a pressure fluctuation and hence an acoustic signal. We have observed (4-6) that when a liquid is introduced into the PAS cell the PA signal is enhanced, the enhancement being linearly related to the vapor pressure o f the liquid. The liquid is introduced without wetting the sample and there is always a gas phase (air and vapor of liquid) in the cell. In the course of our investigations of the enhancement of the signal from liquids we have observed that PAS is surprisingly sensitive to the nature of the liquid-gas or liquid-liquid interface. We are of the opinion that the results could be of interest to those involved in the study of interfaces and surfaces of liquids in particular. We present in this communication a brief report of the most relevant aspects of our PAS studies using aqueous solutions of dyes such as methylene blue or crystal violet in the presence of liquids such as CS2 and ether which are very sparingly soluble in water as well as a liquid such as acetone which is very soluble in water.
RESULTS
EXPERIMENTAL PAS measurements were carried out using a spectrometer built in this laboratory, details of which have been described elsewhere (3, 5). In brief, the spectrometer consisted of a 250-W tungsten lamp, Schoeffel GM 250 monochromator, PAR Model 163 chopper, PAR Model 124A Lock-in amplifier, and GR-1961-1 inch Electret condenser microphone with a flat frequency ( f ) response in the range 5 Hz-2 kHz, The spectrometer showed the expectedf -~ dependence ( 1, 2) of the photoacoustic signal intensity (IpA) with carbon black. The solution under investigation was placed in an aluminum cup (8 m m diam., 1.5 m m deep). This cup was placed in a Teflon holder (13 m m diam., 3 m m deep) containing an inert optically transparent material (in the visible range) such as alumina or silica gel. The desired
Communication No. 230 from the Solid State and Structural Chemistry Unit.
The normalized photoacoustic spectra of a 0.002 M aqueous solution of methylene blue (MB) in the presence of air, CS2, and ether are shown in Fig. 1. The intensities of the various photoacoustic spectra, IPA, have been normalized to be the same at 600 nm. We shall refer henceforth to IpA in the presence of air as 1,~r while that in the presence of a liquid in the cell as I~qu~d. The observed enhancement, Eobs (=l~iJI, i~) at 600 nm are indicated against each curve. Two main absorption maxima are visible at ~605 and ~ 6 6 0 nm which correspond, respectively, to the reported values for the multimer and monomer bands of MB (7). Eob, is large for both ether and CS2. CS2 does not change markedly the features of the spectra while ether shows an enhancement of the absorption band at 660 nm relative to that of the multimer at 605 nm. la~ remains the same when the MB solution is saturated with CS2 or ether. IpA obtained from a saturated solution of MB in pure ether is negligible under these conditions. In Fig. I we have also shown the changes in I, coto,e as a function of time. A time-dependence of the monomer:multimer band intensity is expected as acetone is infinitely soluble in water and the monomer ~ multimer equilibrium is expected to shift to the left with increasing acetone content (7). l,~r obtained with 0.002 M solution of MB in water-acetone system containing different amounts of acetone are shown in Fig. 2a. The m o n o m e r multimer intensity ratio increases with increasing acetone as expected (7). I, ir (as measured from the total area of the normalized PA spectra) increases with increasing acetone content in Fig. 2a being roughly proportional to the vapor pressure of the corresponding water-acetone system. The variation of Iwat~r-aceto,~from a 0.002 M aqueous solution of MB with varying acetone content are shown in
579 0021-9797/84 $3.00 Journal of Colloid and Interface Science, Vol. 101, No. 2, October 1984
Copyright © 1984 by Academic Press, Inc. All rights of reproduction in any form reserved.
580
NOTES
"15-
l
I I I I / I I
10
/ ""
II// 500
600
700
h(nml FIG. 1. Normalized PA spectra of 0.002 M solution of methylene blue in presence of air ( ), CS2 (. • • ), ether (. . . . ), and at various times in presence of acetone (- - -) as indicated. Eo~ values are also given against curve (f: 28.6 Hz; Is: 1.5 mm). Fig. 2b. What is significant is that the changes observed as a function of acetone content in Fig. 2a is roughly the same as that in Fig. 2b. In order to obtain a more quantitative dependence of the PA intensity on the vapor pressure,/~iq, of the introduced liquid we have studied the PA spectra of a solution of crystal violet (~0.002 M) in the presence of various liquids. Eo~ is only roughly linearly related to/~iq (Fig. 3a), a better linear relationship is observed when Eo~ is plotted against b2/3Pliq where "b" is the van der Waal's constant for each liquid (Fig. 3b). We note that Eo~ due to the introduction of CS2 or CH2C12 falls on the same line in Fig. 3a as that observed with other liquids although bulk CS2 and CH2C12 have higher densities than that of water. DISCUSSION The enhancement of the photoacoustic signal by the introduction of a liquid with a high vapor pressure into Journal of Colloid and Interface Science, Vol. 101, No. 2, October 1984
the photoacoustic cell has been attributed to an "adsorbed piston" effect (4-6). An adsorbed multilayer of the molecules of the liquid that is introduced into the cell is assumed to be formed on the surface of the sample. Periodic evaporation and condensation of the adsorbed molecules is likely to take place due to the periodic heat flow to the surface during the photoacoustic experiment. The enhancement is probably due to a more efficient heat transfer between the sample and air molecules in the gas phase by the intermediacy of the desorbed molecules (8) and also perhaps because of the setting up of a pressure wave due to a change in the number of molecules in the gas phase due to the evaporation and condensation. Since the enhancement is dependent on the number of molecules desorbing, we would expect Eo~, to be related to/~q. We note that the observed linear relationship between Eo~ and b2/3pliq (Fig. 3b) can be rationalized if the factor b 2/3 is taken to be proportional to the cross-sectional area of the molecules of the liquid introduced, so that b2/aPliq determines the number of molecules evaporating per unit surface area per unit heat input. Since Eobsis large with CS2 or ether in which methylene blue is very sparingly soluble (9), it is apparent that the thickness of the adsorbed multilayer in these two cases is much less than the thermal diffusion lengths of CS2 or ether at the chopping frequencies employed. We note that the temperature at the surface of the sample during the photoacoustic experiment is very slightly but definitely higher than that of the ambient temperature (10). The relative vapor pressure x (=/~iJp°q) of the liquid introduced into the cell with respect to the surface temperature of the sample is thus slightly less than 1. The statistical number of adsorbed multilayers formed on fiat surfaces such as that of a liquid is then expected to be small. For instance, it has been observed that when 0.95 < x < 1 the number of statistical monolayers adsorbed on surfaces of liquid is between one and two (11, 12). Since the lair from saturated solution of MB in ether is negligible under identical conditions we may assume that the partitioning of methylene blue into the adsorbed layer of ether would have negligible effect on the PA spectra. The adsorbed multilayer may affect the structure and composition of water near the interface and consequently the monomer:multimer ratio at the interface. The effects of the interface is felt (13-15) only to about 5 to 10 molecular layers ( ~ 3 0 A) into the bulk of the liquid. Since the aqueous solution of methylene blue when saturated with ether does not show change in the PA spectra as long as ether is not separately introduced into the cell, we have to enquire whether the change at the interface due to adsorbed ether is sufficient to account for the observed changes in the monomer:multimer band intensity ratio in terms of the conventional PA theory. From the Rosencwaig-Gersho theory (1) the complex envelope Q of the sinusoidal pressure variation due to the photoacoustic effect may be simplified as (see Eqs. (9-31) of Ref. (1))
582
NOTES
to at least the enhanced PA signals is from the interface region instead of the thermal diffusion length as predicted from RG theory. This may be related to the possibility that an adsorbed multilayer is involved in the enhancement. Although the above arguments may be applied to the results obtained with water-acetone systems (Fig. 2) the fact that acetone is infinitely soluble in water poses many problems. We note, however, that lair of methylene blue in pure acetone (Fig. 2a) is nearly the same as that lace~o,e of the 0.002 M aqueous MB solution after 15 min (Fig. 1). If the feature in Fig. 1 is to be interpreted entirely in terms of RG theory it should be argued that during this time a nearly pure acetone layer of thickness of the order of the thermal diffusion length of acetone is formed at the surface into which methylene blue is partitioned. The main features of the above study is therefore the surprising sensitivity of the PA spectra of liquids to the nature of the vapor phase. We are of the opinion that the results communicated here show the potential of the photoacoustic technique in investigating the surface of liquids. Similar studies at higher chopping frequencies (we are restricted by the intensity of our illuminating source for such studies) coupled with other techniques such as photothermal spectroscopy and Fourier-transform infrared photoacoustic spectroscopy would be of immense value in understanding the process involved.
ACKNOWLEDGMENTS The authors are thankful to Professor C. N. R. Rao for the interest and encouragement. They acknowledge the financial assistance from the Indian National Science Academy for the support of the research. One of us (T.S.) is thankful to the Department of Science and Technology for the fellowship. REFERENCES 1. Rosencwaig, A., in "Photoacoustics and Photoacoustic Spectroscopy" (P. J. Elving and J. D. Winefordner, Eds.), Chemical Analysis, Vol. 57. Wiley, New York, 1980.
Journalof Colloidand InterfaceScience,Vol. 101,No. 2, October1984
2. Rosencwaig, A., and Gersho, A., J. Appl. Phys. 47, 64 (1976). 3. Ganguly, P., and Rao, C. N. R., Proc. Indian Acad. Sci. (Chem. Sci.) 90, 153 (1981), and references therein. 4. Ganguly, P., and Somasundaram, T., Appl. Phys. Lett. 43, 160 (1983). 5. Somasundaram, T., and Ganguly, P., 9".Phys. Colloq. (Orsay, Ft.) 44, C6-239 (1983). 6. Somasundaram, T., and Ganguly, P., aT.Appl. Phys., (Communicated). 7. Rabinowitch, E., and Epstein, L. F., J. Amer. Chem. Soc. 63, 69 (1941); Mukerjee, P., and Ghosh, A. K., J. Phys, Chem. 67, 193 (1963). 8. Rosencwaig, A., J. Appl. Phys. 52, 503 (1981), and references therein. 9. Adamson, A. W., Shirley, F. P., and Kunichika, K. T., J. Colloid Interface Sci. 34, 461 (1970); Kloubek, J., J. Colloid Interface Sci. 46, 185 (1974). 10. Rosencwaig, A., in "Photoacoustics and Photoacoustic Spectroscopy" (P. J. Elving and J. D. Winefordner, Eds.), Chemical Analysis, Vol. 57, p. 286. Wiley, New York, 1980. 11. Hauxwell, F., and OttewiU, R. H., J. Colloidlnterface Sci. 34, 473 (1970). 12. Baumer, D., and Findenegg, G. H., J. Colloidlnterface Sci. 85, 118 (1982). 13. Drost-Hansen, W., Ind. Eng. Chem. 57(4), 18 (1965); Drost-Hansen, W., Ind. Eng. Chem. 61(11), 10 (1969). 14. Hauxwell, F., and Ottewill, R. H., J. Colloidlnterface Sci. 28, 514 (1968). 15. Hartkopf, A., and Karger, D. L., Acc. Chem. Res. 6, 209 (1973). T. SOMASUNDARAM P. GANGULY
Solid State and Structural Chemistry Unit Indian Institute of Science Bangalore-560012 India Received September 26, 1983
liquid was introduced into the cell using a microsyringe by soaking the inert material. The length of the gas phase above the sample, l~, was not affected in this process. All the experiments were carried out with an l~ ~ 1.5 m m with f = 28.6 Hz. The PA signal increases with time on the introduction of the liquid and reaches a steady value after about 15 min. The results reported (except where mentioned) here are obtained only after the steady-state signal was obtained. The vapor pressure of the liquid (/~iq) within the cell is expected to be equal to the saturated vapor pressure of the liquid (p0q) at the temperature of the cell.
INTRODUCTION The application of photoacoustic spectroscopy (PAS) to the study o f solids and surfaces is well known (I-3). In PAS (1, 2) an intensity-modulated light is absorbed by the sample being investigated. The absorbed light energy is dissipated through nonradiative deexcitation as thermal energy. The periodic heat produced diffuses to the sample surface and then to a coupling gas (usually air), causing a pressure fluctuation and hence an acoustic signal. We have observed (4-6) that when a liquid is introduced into the PAS cell the PA signal is enhanced, the enhancement being linearly related to the vapor pressure o f the liquid. The liquid is introduced without wetting the sample and there is always a gas phase (air and vapor of liquid) in the cell. In the course of our investigations of the enhancement of the signal from liquids we have observed that PAS is surprisingly sensitive to the nature of the liquid-gas or liquid-liquid interface. We are of the opinion that the results could be of interest to those involved in the study of interfaces and surfaces of liquids in particular. We present in this communication a brief report of the most relevant aspects of our PAS studies using aqueous solutions of dyes such as methylene blue or crystal violet in the presence of liquids such as CS2 and ether which are very sparingly soluble in water as well as a liquid such as acetone which is very soluble in water.
RESULTS
EXPERIMENTAL PAS measurements were carried out using a spectrometer built in this laboratory, details of which have been described elsewhere (3, 5). In brief, the spectrometer consisted of a 250-W tungsten lamp, Schoeffel GM 250 monochromator, PAR Model 163 chopper, PAR Model 124A Lock-in amplifier, and GR-1961-1 inch Electret condenser microphone with a flat frequency ( f ) response in the range 5 Hz-2 kHz, The spectrometer showed the expectedf -~ dependence ( 1, 2) of the photoacoustic signal intensity (IpA) with carbon black. The solution under investigation was placed in an aluminum cup (8 m m diam., 1.5 m m deep). This cup was placed in a Teflon holder (13 m m diam., 3 m m deep) containing an inert optically transparent material (in the visible range) such as alumina or silica gel. The desired
Communication No. 230 from the Solid State and Structural Chemistry Unit.
The normalized photoacoustic spectra of a 0.002 M aqueous solution of methylene blue (MB) in the presence of air, CS2, and ether are shown in Fig. 1. The intensities of the various photoacoustic spectra, IPA, have been normalized to be the same at 600 nm. We shall refer henceforth to IpA in the presence of air as 1,~r while that in the presence of a liquid in the cell as I~qu~d. The observed enhancement, Eobs (=l~iJI, i~) at 600 nm are indicated against each curve. Two main absorption maxima are visible at ~605 and ~ 6 6 0 nm which correspond, respectively, to the reported values for the multimer and monomer bands of MB (7). Eob, is large for both ether and CS2. CS2 does not change markedly the features of the spectra while ether shows an enhancement of the absorption band at 660 nm relative to that of the multimer at 605 nm. la~ remains the same when the MB solution is saturated with CS2 or ether. IpA obtained from a saturated solution of MB in pure ether is negligible under these conditions. In Fig. I we have also shown the changes in I, coto,e as a function of time. A time-dependence of the monomer:multimer band intensity is expected as acetone is infinitely soluble in water and the monomer ~ multimer equilibrium is expected to shift to the left with increasing acetone content (7). l,~r obtained with 0.002 M solution of MB in water-acetone system containing different amounts of acetone are shown in Fig. 2a. The m o n o m e r multimer intensity ratio increases with increasing acetone as expected (7). I, ir (as measured from the total area of the normalized PA spectra) increases with increasing acetone content in Fig. 2a being roughly proportional to the vapor pressure of the corresponding water-acetone system. The variation of Iwat~r-aceto,~from a 0.002 M aqueous solution of MB with varying acetone content are shown in
579 0021-9797/84 $3.00 Journal of Colloid and Interface Science, Vol. 101, No. 2, October 1984
Copyright © 1984 by Academic Press, Inc. All rights of reproduction in any form reserved.
580
NOTES
"15-
l
I I I I / I I
10
/ ""
II// 500
600
700
h(nml FIG. 1. Normalized PA spectra of 0.002 M solution of methylene blue in presence of air ( ), CS2 (. • • ), ether (. . . . ), and at various times in presence of acetone (- - -) as indicated. Eo~ values are also given against curve (f: 28.6 Hz; Is: 1.5 mm). Fig. 2b. What is significant is that the changes observed as a function of acetone content in Fig. 2a is roughly the same as that in Fig. 2b. In order to obtain a more quantitative dependence of the PA intensity on the vapor pressure,/~iq, of the introduced liquid we have studied the PA spectra of a solution of crystal violet (~0.002 M) in the presence of various liquids. Eo~ is only roughly linearly related to/~iq (Fig. 3a), a better linear relationship is observed when Eo~ is plotted against b2/3Pliq where "b" is the van der Waal's constant for each liquid (Fig. 3b). We note that Eo~ due to the introduction of CS2 or CH2C12 falls on the same line in Fig. 3a as that observed with other liquids although bulk CS2 and CH2C12 have higher densities than that of water. DISCUSSION The enhancement of the photoacoustic signal by the introduction of a liquid with a high vapor pressure into Journal of Colloid and Interface Science, Vol. 101, No. 2, October 1984
the photoacoustic cell has been attributed to an "adsorbed piston" effect (4-6). An adsorbed multilayer of the molecules of the liquid that is introduced into the cell is assumed to be formed on the surface of the sample. Periodic evaporation and condensation of the adsorbed molecules is likely to take place due to the periodic heat flow to the surface during the photoacoustic experiment. The enhancement is probably due to a more efficient heat transfer between the sample and air molecules in the gas phase by the intermediacy of the desorbed molecules (8) and also perhaps because of the setting up of a pressure wave due to a change in the number of molecules in the gas phase due to the evaporation and condensation. Since the enhancement is dependent on the number of molecules desorbing, we would expect Eo~, to be related to/~q. We note that the observed linear relationship between Eo~ and b2/3pliq (Fig. 3b) can be rationalized if the factor b 2/3 is taken to be proportional to the cross-sectional area of the molecules of the liquid introduced, so that b2/aPliq determines the number of molecules evaporating per unit surface area per unit heat input. Since Eobsis large with CS2 or ether in which methylene blue is very sparingly soluble (9), it is apparent that the thickness of the adsorbed multilayer in these two cases is much less than the thermal diffusion lengths of CS2 or ether at the chopping frequencies employed. We note that the temperature at the surface of the sample during the photoacoustic experiment is very slightly but definitely higher than that of the ambient temperature (10). The relative vapor pressure x (=/~iJp°q) of the liquid introduced into the cell with respect to the surface temperature of the sample is thus slightly less than 1. The statistical number of adsorbed multilayers formed on fiat surfaces such as that of a liquid is then expected to be small. For instance, it has been observed that when 0.95 < x < 1 the number of statistical monolayers adsorbed on surfaces of liquid is between one and two (11, 12). Since the lair from saturated solution of MB in ether is negligible under identical conditions we may assume that the partitioning of methylene blue into the adsorbed layer of ether would have negligible effect on the PA spectra. The adsorbed multilayer may affect the structure and composition of water near the interface and consequently the monomer:multimer ratio at the interface. The effects of the interface is felt (13-15) only to about 5 to 10 molecular layers ( ~ 3 0 A) into the bulk of the liquid. Since the aqueous solution of methylene blue when saturated with ether does not show change in the PA spectra as long as ether is not separately introduced into the cell, we have to enquire whether the change at the interface due to adsorbed ether is sufficient to account for the observed changes in the monomer:multimer band intensity ratio in terms of the conventional PA theory. From the Rosencwaig-Gersho theory (1) the complex envelope Q of the sinusoidal pressure variation due to the photoacoustic effect may be simplified as (see Eqs. (9-31) of Ref. (1))
582
NOTES
to at least the enhanced PA signals is from the interface region instead of the thermal diffusion length as predicted from RG theory. This may be related to the possibility that an adsorbed multilayer is involved in the enhancement. Although the above arguments may be applied to the results obtained with water-acetone systems (Fig. 2) the fact that acetone is infinitely soluble in water poses many problems. We note, however, that lair of methylene blue in pure acetone (Fig. 2a) is nearly the same as that lace~o,e of the 0.002 M aqueous MB solution after 15 min (Fig. 1). If the feature in Fig. 1 is to be interpreted entirely in terms of RG theory it should be argued that during this time a nearly pure acetone layer of thickness of the order of the thermal diffusion length of acetone is formed at the surface into which methylene blue is partitioned. The main features of the above study is therefore the surprising sensitivity of the PA spectra of liquids to the nature of the vapor phase. We are of the opinion that the results communicated here show the potential of the photoacoustic technique in investigating the surface of liquids. Similar studies at higher chopping frequencies (we are restricted by the intensity of our illuminating source for such studies) coupled with other techniques such as photothermal spectroscopy and Fourier-transform infrared photoacoustic spectroscopy would be of immense value in understanding the process involved.
ACKNOWLEDGMENTS The authors are thankful to Professor C. N. R. Rao for the interest and encouragement. They acknowledge the financial assistance from the Indian National Science Academy for the support of the research. One of us (T.S.) is thankful to the Department of Science and Technology for the fellowship. REFERENCES 1. Rosencwaig, A., in "Photoacoustics and Photoacoustic Spectroscopy" (P. J. Elving and J. D. Winefordner, Eds.), Chemical Analysis, Vol. 57. Wiley, New York, 1980.
Journalof Colloidand InterfaceScience,Vol. 101,No. 2, October1984
2. Rosencwaig, A., and Gersho, A., J. Appl. Phys. 47, 64 (1976). 3. Ganguly, P., and Rao, C. N. R., Proc. Indian Acad. Sci. (Chem. Sci.) 90, 153 (1981), and references therein. 4. Ganguly, P., and Somasundaram, T., Appl. Phys. Lett. 43, 160 (1983). 5. Somasundaram, T., and Ganguly, P., 9".Phys. Colloq. (Orsay, Ft.) 44, C6-239 (1983). 6. Somasundaram, T., and Ganguly, P., aT.Appl. Phys., (Communicated). 7. Rabinowitch, E., and Epstein, L. F., J. Amer. Chem. Soc. 63, 69 (1941); Mukerjee, P., and Ghosh, A. K., J. Phys, Chem. 67, 193 (1963). 8. Rosencwaig, A., J. Appl. Phys. 52, 503 (1981), and references therein. 9. Adamson, A. W., Shirley, F. P., and Kunichika, K. T., J. Colloid Interface Sci. 34, 461 (1970); Kloubek, J., J. Colloid Interface Sci. 46, 185 (1974). 10. Rosencwaig, A., in "Photoacoustics and Photoacoustic Spectroscopy" (P. J. Elving and J. D. Winefordner, Eds.), Chemical Analysis, Vol. 57, p. 286. Wiley, New York, 1980. 11. Hauxwell, F., and OttewiU, R. H., J. Colloidlnterface Sci. 34, 473 (1970). 12. Baumer, D., and Findenegg, G. H., J. Colloidlnterface Sci. 85, 118 (1982). 13. Drost-Hansen, W., Ind. Eng. Chem. 57(4), 18 (1965); Drost-Hansen, W., Ind. Eng. Chem. 61(11), 10 (1969). 14. Hauxwell, F., and Ottewill, R. H., J. Colloidlnterface Sci. 28, 514 (1968). 15. Hartkopf, A., and Karger, D. L., Acc. Chem. Res. 6, 209 (1973). T. SOMASUNDARAM P. GANGULY
Solid State and Structural Chemistry Unit Indian Institute of Science Bangalore-560012 India Received September 26, 1983