© Copyright 1999 American Chemical Society
NOVEMBER 9, 1999 VOLUME 15, NUMBER 23
Letters Molecules in a Thin Bubble Membrane Arun Chattopadhyay* Department of Chemistry, Indian Institute of Technology, Guwahati, Guwahati 781 001, India Received December 31, 1998. In Final Form: August 31, 1999 A simple method for generation of a stable bubble membrane from an aqueous solution of sodium dodecyl sulfate (SDS) is reported. The method involves repeated boiling of an aqueous solution of SDS with addition of fresh SDS in the end. An air bubble is blown over a small inverted beaker containing a portion of the solution. With time the bubble loses water and other components and results in a thin membrane. The membrane thus obtained is stable for minutes. Time-dependent UV-vis spectroscopic measurements are used to ascertain the change in thickness of the bubble with time. FTIR spectroscopic measurements have been utilized to follow the change in chemical composition of the bubble. The bubbles containing Rhodamine B and Rhodamine 6G dyes reveal that these dyes remain in the bubbles even if they become very thin. The visible absorption spectra of these dyes indicate that these dyes are possibly in the aqueous layer which may be present in the thin bubble membrane.
Introduction Mimicing the cell membrane is a challenging research area with a vast opportunity to study the processes occurring in the cell membrane and with application potential in biomimetic systems. Conventional membrane mimetic systems are made of supported bilayer lipid membranes on solid and liquid surfaces, micelles, vesicles, and biopolymers.1-4 The architecture of these systems is based on the assemblies of amphipathic molecules in water as they occur in the natural environment. On the other hand soap films constitute a different class of model membranes where the orientation of surfactant molecules is opposite that in a lipid bilayer, with the hydrophobic parts pointing to the air and water molecules between the hydrocarbon layers. The possibility of incorporation of additional molecules into these films opens up newer avenues to study molecular properties and chemical * E-mail:
[email protected]. (1) Fuhrhop, J. H.; Koning, J. Membrane and Molecular Assemblies: The Synkinetic Approach; The Royal Society of Chemistry: London, 1994. (2) Biomimetic Polymers; Gebelein, C. G., Ed.; Plenum Press: London, 1990. (3) Sackmann, E. Science 1996, 271, 43. (4) Menger, F. M.; Angelova, M. I. Acc. Chem. Res. 1998, 31, 789.
reactions in thin soap films. The fundamental challenge lies in obtaining thin stable soap films for scientific and practical applications.5 The key to stabilization of a soap film lies in understanding the time evolution of these films and the forces leading to film ruptures. The literature regarding the study of kinetics of drainage of foams and foam films is vast.6-8 In addition, foams have been the subject of various scientific studies owing to their scientific and practical applications such as in cosmetic products, fire retardants, food products, and oil recovery.9 One traditional approach of investigating foams is to make a film from a soap solution and study various time-independent and time-dependent properties.10-12 Study of the thinning of vertical foam films (5) Cohen, R.; Exerowa, D.; Kolarov, T.; Amanaka, T.; Tano, T. Langmuir 1997, 13 (12), 3172. (6) Exerova, D. R.; Krugliakov, P. M. Foams and Foam Films: Theory, Experiment, Application; Studies in Interface Science, Volume 5; Elsevier: New York, 1997. (7) Prud’Homme, R. K.. Khan, Saad A., Eds.; Foams: Theory, Measurements and Applications; Surfactant Science Series, Volume 57; Marcel Dekker: New York, 1995. (8) Clunie, J. S.; Goodman, J. F.; Ingram, B. T. Surface and Colloid Science; Matijevic, E., Ed.; Wiley: New York, 1971; Vol. 3, p 167. (9) Foams: Physics, Chemistry and Structure; Wilson, A. J., Ed.; Springer-Verlag: Berlin, 1989. (10) Huibers, P. D. T.; Shah, D. O. Langmuir 1997, 13, 5995.
10.1021/la981774k CCC: $18.00 © 1999 American Chemical Society Published on Web 10/15/1999
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Figure 1. A part of the of the experimental setup containing the bubble: B, bubble; S, sapphire window.
has revealed various aspects of time evolution of film drainage leading to a Newton black film.13-15 UV-vis and IR spectroscopic methods have been used to study the time-dependent changes in thickness of foam films and to obtain information about the structure and chemical composition of black films.5,10,16 Here a new method for obtaining stable soap bubbles and bubbles embedded with laser dyes is reported. UVvis and FTIR spectroscopic methods are used to follow the time-dependent changes in thickness and chemical composition, respectively. Bubbles made from an aqueous solution of sodium dodecyl sulfate (SDS), which has been boiled repeatedly, are found to be quite stable even after the thickness of the bubble film (also referred to here as bubble membrane) reaches a value less than 10 nm. Interference patterns observed in the UV-visible absorption spectra indicate that the thickness of the bubble film changes slowly. FTIR spectroscopic measurements indicate different time scales for the loss of water and hydrocarbon components. Furthermore, time-dependent visible spectra of Rhodamine B (Rh B) and Rhodamine 6G (Rh 6G) dyes in these bubbles show that they drain gradually and they remain embedded even when the film becomes very thin. The visible absorption spectra of the dyes, in thin bubble membranes, indicate the likelihood of the dye molecules being in the aqueous layer of the bubble membrane. These results may open up new avenues of membrane mimetic studies by spectroscopic techniques where the probe system is in transmission mode. Experimental Section Bubbles from SDS. The procedure to make a bubble membrane is as follows. About 0.325 g (0.11 M) of SDS (EMerck, Germany) was dissolved in 10 mL of distilled water and then boiled until the solution was a slurry (nearly a dry paste). To the cooled paste 10 mL of water was then added, and the solution was boiled to slurry again. This process was repeated four to six times. Finally, 10 mL of water was added and a colloidal solution was obtained. About 0.11 g (0.04 M) of SDS was then added to the solution. A clear solution was obtained upon warming. About 0.5 mL of this solution was then poured over an inverted beaker which was enclosed in another beaker. An air bubble was blown on top of the beaker and spectroscopic investigations were carried out. A part of the experimental configuration is shown in Figure 1. It may be noted here that ordinary bubbles made from an ordinary solution of SDS in water are not stable and burst in a few seconds. Also, the air inside the beaker is saturated with water vapor as the bubble-making chamber contains a few milliliters of the detergent solution in the large beaker (spillover from the smaller beaker).
Figure 2. Time-dependent UV-visible spectra of a bubble: A ) 0-2 min; B ) 2-4 min, d ) 1132 nm; C ) 4-6 min, d ) 805 nm; D ) 6-8 min, d ) 610 nm; E ) 8-10 min, d ) 453 nm; F ) 10-12 min, d ) 356 nm, G ) 12-14 min, d ) 257 nm; H ) 14-16 min; I ) 16-18 min; J ) 33-35 min. The unit of absorbance is 0.2 between the serrated line and the top line for each box. d ) estimated thickness of the bubble. Bubbles with Dyes. About 0.012 g (2.5 mM) of Rhodamine B and 0.160 g (0.055 M) of SDS were dissolved in 10 mL of water. The solution was boiled until a slurry formed as in the case of ordinary SDS solution mentioned above. Ten milliliters of water was added to the cooled mixture. This process was repeated 12 times, and finally 10 mL of water was added to the mixture. About 0.059 g (0.02 M) of SDS was then added to the solution. The solution was ready for making bubbles. Experimental arrangements for the spectroscopic probes are identical with the setup for ordinary bubbles. The procedure for making bubbles with Rhodamine 6G is same as that with RhB. About 0.008 g (1.7 m M) of Rh6G and 0.168 g (0.058 M) of SDS were dissolved in 10 mL water, and the solution was repeatedly boiled as described above. Finally to 10 mL of the solution 0.056 g (0.019 M) of SDS was added, and the solution was ready for experiments.
Results and Discussion (11) Yamanaka, T. Bull. Chem. Soc. Jpn. 1975, 48, 1755. (12) Yamanaka, T. Bull. Chem. Soc. Jpn. 1975, 48, 1760. (13) Stoyanov, S. D.; Paunov, V. N.; Basheva, E. S.; Ivanov, I. B.; Mehreteab, A.; Broze, G. Langmuir 1997, 13 (6), 1400. (14) Sentenac, D.; Schalchli, A.; Nedyalkov, M.; Benattar, J. J. Faraday Discuss. 1996, 104, 345. (15) Evers L. J.; Shulepov, S. Y.; Frens, G. Farday Discuss. 1996, 104, 335. (16) Umemura, J. Can. J. Chem. 1985, 63, 3 (7), 1713.
Bubbles from SDS. Soap bubbles from a repeatedly boiled solution of SDS are found to be more stable than those obtained from an ordinary solution of SDS. One of the reasons for the enhanced stability of the bubbles may be due to the presence of 1-dodecanol, which is generated by hydrolysis of SDS upon prolonged boiling. As a test, the following experiments were performed. Assuming that
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Figure 3. Time-dependent FTIR spectra of a bubble. Each spectrum is an average of eight scans. (A) The peak due to the OH stretching vibration of water at 3400 cm-1. The average time separation between two consecutive spectra is 33 s for the first 18 spectra. The first spectrum is recorded 9 s after the formation of the bubble. The time separation between the next three spectra is 12 s followed by a separation of 36 s. The last three spectra were recorded with a gap of 12 s. (B) The time evolution of the peaks due to C-H stretching vibration occurring at 2920 cm-1.
all the SDS was converted into 1-dodecanol upon boiling, an equivalent amount of 1-dodecanol (0.11 M) was mixed with 0.04 M of SDS in 10 mL of water. They did not mix well. A cloudy solution was obtained which did not become clear upon warming. On the other hand it was noticed that when 0.094 M of SDS was mixed with 0.011 M of 1-dodecanol in 10 mL of water, a clear solution was obtained upon warming. The bubbles obtained from this solution were stable up to 10 min but burst in a few seconds after they became thin (after losing color). Thus the enhanced stability of the bubbles may be due to the structure of the organized assemblies of surfactants than a mere mixture of surfactants. Further experiments to find the exact reasons for the enhanced stability are in progress.
The interference of the partially reflected visible light from a thin soap film makes the UV-vis absorption spectra of a soap bubble appear as a set of maxima and minima, when the thickness of the bubble film is of the order of a wavelength of visible light. From these interference maxima and minima the thickness of a bubble film can be obtained. This method has been described by Huibers and Shah10 in which they have used a fast diode array UV-vis spectrophotometer to follow the time-dependent changes in thickness of a vertical soap film down to 270 nm. The same principle is used in the present system where the time-dependent absorption spectra were recorded by repeatedly scanning the wavelength range of an ordinary dispersive UV-vis spectrophotometer. The difference here is that the probe light travels through two
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such films in a bubble. As the drainage of components from the bubble is vertically symmetrical, the thickness of two films on opposite sides is taken as equal at all times. Hence the thickness obtained from the spectrum is a measurement of thickness of the bubble film. The time-dependent changes in the thickness of the bubble film were monitored by recording the changes in the UV-vis absorption maxima and minima in the region 200-1100 nm. A collection of such spectra, for a spherical bubble (truncated at the top of the inverted beaker) of approximately 2.5 cm diameter, is shown in Figure 2. The time interval between two scans is about 2 min. The reported thickness is only an approximate measurement as the film thickness may change considerably during the course of measurement. The refractive index of the bubble film was taken as 1.33310 to calculate the thickness. It is clear from the figure that in the beginning there are no interference fringes, indicating the thickness of the bubble to be much higher than the wavelength of visible light. After about 2 min, the thickness of the bubble is about 1100 nm. It takes a little over 14 min for the thickness to reach less than 200 nm, as evident from the disappearing fringes. With time, the interference fringes disappear completely and a constant spectrum with nearly zero absorbance is obtained (Figure 2I-J). These spectra are similar to the simulated spectrum of a 4 nm thick soap film, reported by Huibers and Shah.10 A conservative value of thickness of the order of 5-10 nm may be assigned as the final thickness of the bubble film. The bubble film may now be called a thin bubble membrane. This membrane is stable up to an hour or so. Time-dependent FTIR spectroscopic investigation with the bubble suggests that water content in the bubble decreases rapidly compared to the hydrocarbons. One such measurement is shown in Figure 3. The change in water content is followed by following the changes in the OH stretching peak intensity occurring at 3400 cm-1. The timedependent changes in the surfactant concentrations are followed by the change in the peak intensity due to C-H stretching, occurring at 2920 cm-1. Individual bubbles differ in their lifetimes and the rates of change of compositions. On the other hand, a general trend may be observed in terms of relative changes in component concentrations and variation of thickness. For example, as shown in Figure 3, the rate of decrease of OH peak intensity is fast compared with that of the C-H stretching peak. This indicates that water is lost at a much faster rate than the hydrocarbon components of the bubble. Evaporation may play a crucial role in the preferential water loss from the bubble. Finally, water absorption vanishes completely from the bubble resulting in a thin membrane containing mainly surfactants. One important question that may be asked here is whether the final thin membrane contains water or not. One possible answer to this question comes from the spectroscopic observations with dyes in the bubbles. The results, as described below, indicate that water may be present in the final structure of the bubble membrane. Bubble with Rhodamine Dyes. The spectroscopic properties of a molecule doped on a bubble membrane carry structural information about the bubble. These properties would depend on the structure and chemical compositions of the bubble and also the interactions between the doped molecule with the components of the bubble. In the present study the inclusion of Rhodamine dyes in the bubble has two goals. The first being a test of the possibility of having dopands even in the thin membrane with further possibilities of putting more versatile dopands. The second objective is to study the
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Figure 4. Visible absorption spectra of Rhodamine B and Rhodamine 6G in different media: A, 4.1 µM RhB in water; B, RhB in surfactant solution (using a 25 µm spacer cuvette); C, RhB in the bubble at 0-2 min after the formation and at 20-28 min after the formation (superposition of five spectra taken one after another) (the background spectrum is taken after bursting the bubble); A′, 4.1 µM Rh 6G in water; B′, Rh 6G in surfactant solution (using a 25 µm spacer cuvette); C′, Rh 6G in the bubble at 0-2 min after the formation and at 33-38 min after the formation of the bubble (superposition of three spectra taken consecutively) (the background spectrum is taken after cracking the bubble).
effect of the environment on the molecular properties of dopands whereby the structural information of the membrane can also be extracted. To pursue these objectives, water-soluble Rhodamine B and Rhodamine 6G dyes were incorporated in soap bubbles and the time evolution of the visible absorption spectra of the dyes was followed. Both dyes have high oscillator strengths and thus can be observed even at low thickness of the bubble using an ordinary dispersive UV-vis spectrophotometer. Figure 4 shows the absorption spectra of Rh B and Rh 6G in water (A, A′), in the bubble-making solution (B, B′), and in the bubble at the time of formation and at the time when the bubble is very thin and has reached its final stable thickness (C, C′). For both dyes, all the spectra have the same absorption features having one major peak with a shoulder. However, it is clear that the peak positions of the dyes in the bubble as well as in the bubble-making solution are red shifted compared to the their peaks in water. The shift is definitely due to the presence of surfactants in the solution as well as in the bubble. The shift disappears when the solution is diluted. An additional point to note here is that when the bubbles are very thin the dye peak positions are the same as those when the
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due to the solvation of the dyes in water layers attached to surfactants either in the membrane layer or in the micelles (vesicles) in a concentrated surfactant solution. The absorption spectral features of the dyes indicate that they may be present in the water layer of the thin bubble membrane rather than being embedded in the hydrocarbon layer. In a nutshell, RhB and Rh6G are dissolved in the inner water core of the thin film of the soap bubble surrounded by surfactants whose hydrophobic ends are pointed to the air. The time evolution of the dye peak intensities reveals that their concentration changes in the bubble are gradual and slow. This is shown in Figure 5. Also, the timedependent spectra reveal that during the course of the measurement and at different bubble film thickness there is no observable shift in the peak position of any of the dyes. This means that the dyes are dissolved in an aqueous environment which does not change with the thickness of the film. As the absorption spectra are superimposed on the interference absorption minima and maxima, the exact time-dependent changes in the intensity of the spectral absorptions are not quantified for various film thickness. It may be pointed out here that to monitor the dye absorption in a thin bubble, a high initial dye concentration is needed. If the concentration of the dye in the parent solution is halved, very little or no peak is observed. Conclusion
Figure 5. Time evolution of the spectra of RhB and Rh 6G in bubbles. RhB: A, 0-2 min; B, 2-4 min; C, 4-6 min; D, 6-7 min; E, 19-32 min (seven consecutive spectra) and the background taken after the bubble was cracked. Rh 6G: A′, 0-2 min; B′, 4-6 min; C′, 6-8 min; D′, 10-12 mim; E′, 28-36 min (three spectra) and the background taken after the bubble was cracked. The y axis is absorbance.
bubbles were initially formed. Thus the red shifts may be
Sodium dodecyl sulfate bubbles are stabilized by repeatedly boiling aqueous solution of SDS and then adding fresh SDS to the solution. The time evolution of these bubbles indicates that the thickness of the bubble changes slowly compared to the bubbles generated from an ordinary aqueous SDS solution. Finally, a stable bubble with a film thickness of less than 10 nm is obtained. Bubbles doped with Rh B and Rh 6G dyes indicate that water may be present in the final thin bubble membrane. The dyes are likely to be dissolved in the aqueous layer of the bubble as the spectra of the dyes in the bubble membrane resemble those in aqueous solution.
Acknowledgment. The author wishes to thank Dr. Anumita Paul for helpful discussions, Professor Mihir Chowdhury for inspiration, and Mr. J. N. Bharali and Mr. D. Kalita for help. LA981774K
