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Langmuir 1997, 13, 3172-3176
Foam Films Stabilized with Lysophosphatidylcholine: A Comparison of Microinterferometric and Fourier Transform Infrared Spectroscopy Thickness Measurements R. Cohen,† D. Exerowa,*,† T. Kolarov,† T. Yamanaka,‡ and T. Tano§ Institute of Physical Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria, Faculty of Education, Chiba University, Chiba 263, Japan, and Institute for Chemical Research, Kyoto University, Uji, Kyoto-fu 611, Japan Received December 13, 1996. In Final Form: March 19, 1997X Previous studies of foam films stabilized with the soluble phospholipid palmitoyl lysophosphatidylcholine (lyso PC) have demonstrated the effect of Ca2+ ion binding by the phospholipid head group. Two different optical methods have been used in these studiessthe microinterferometric method and Fourier transform infrared (FT-IR) spectroscopy. In both studies some additional treatment specific for the given technique is needed to convert the measured data into real film properties. In the case of the microinterferometric method this is the three-layer sandwich model of the film structure. In the FT-IR spectroscopy method, the Lambert-Beer law with a molar absorption coefficient ) 150 has been used to derive the aqueous core thickness from the film water content. In the present study thicknesses of silver and common black films determined by both optical methods are compared taking into consideration the assumed film structure models. It is shown that the results are essentially similar, but some small discrepancies are observed in the values of the obtained aqueous core thicknesses which can be explained by the necessity of assuming different optical models for the interpretation of the obtained experimental data. It is demonstrated that the use of suitably selected optical parameters can lead to similar equilibrium film thicknesses. The obtained results are used in the analysis of the values and change of the potential of the diffuse electric layer φo and charge density σo with CaCl2 concentration (Cel). The importance of the model assumptions necessary for the determination of parameters characterizing interaction forces in the film is demonstrated. It is shown that some of the previously observed differences in the φo(Cel) and σo(Cel) curves are due to the different models for estimation of aqueous core thicknesses used in microinterferometric and FT-IR spectroscopy investigations.
Introduction Studies1,2 of free liquid (foam) films stabilized with the soluble phospholipid palmitoyl lysophosphatidylcholine (lyso PC) show a remarkable influence of Ca2+ on the thickness of the studied films leading to a Newton black to silver film transition at a CaCl2 concentration (Cel) of 10-3 mol dm-3. The equilibrium thickness of these silver films gradually decreases with increase in Cel until again a transition to Newton black films is observed at Cel ) 0.2 mol dm-3. The thickness transitions in the presence of Ca2+ are considered in these studies1,2 as due to the action of electrostatic long-range repulsive forces caused by Ca2+ adsorption on the phospholipid head groups. The obtained results1,2 are in good agreement with studies of interbilayer interactions of model neutral bilayer phospholipid membranes with divalent cations, e.g., refs 3-10. * To whom correspondence may be addressed: e-mail, exerowa@ ipchp.ipc.acad.bg. † Bulgarian Academy of Sciences. ‡ Chiba University. § Kyoto University. X Abstract published in Advance ACS Abstracts, May 1, 1997. (1) Cohen, R.; Exerowa, D.; Kolarov, T.; Yamanaka, T.; Muller, V. M. Colloids Surf. 1992, 65, 201. (2) Yamanaka, T.; Tano, T.; Kamegaya, O.; Exerowa, D.; Cohen, R. D. Langmuir 1994, 10, 1871. (3) Inoko, Y.; Yamaguchi, T.; Furuya, K.; Mitsui, T. Biochim. Biophys. Acta 1978, 413, 24. (4) McLaughlin, A.; Grathwohl, C.; McLaughlin, S. Biochim. Biophys. Acta 1978, 513, 338. (5) Lis, L. J.; Parsegian, V. A.; Rand, R. P. Biochemistry 1981, 20, 1761. (6) Lis, L. J.; Lis, W. T.; Parsegian, V. A.; Rand, R. P. Biochemistry 1981, 20, 1771. (7) Rand, R. P. Annu. Rev. Biophys. Bioeng. 1981, 10, 277. (8) Oshima, H.; Inoko, Y.; Mitsui, T. J. Colloid Interface Sci. 1982, 86, 57. (9) Marra, J.; Israelachvili, J. Biochemistry 1985, 24, 4608.
S0743-7463(96)02102-6 CCC: $14.00
The lyso PC foam film studies1,2 were carried out by two different optical methodssthe microinterferometric method1 and Fourier transform infrared (FT-IR) spectroscopy,2 at similar conditions concerning lyso PC concentration, Cel, and temperature. In both cases at equilibrium the disjoining pressure in the film was balanced by an external pressure of 35 N m-2. In the case of the microinterferometric method measurements this pressure was the capillary pressure in the meniscus surrounding the horizontal film, and in the case of the FT-IR measurements this was the hydrostatic pressure at the point of measurement. The measurements1 of equilibrium thicknesses of horizontal microscopic foam films with a radius of 0.01 cm were performed using the microinterferometric method, first introduced by Scheludko and Exerowa11,12 (abbreviated hereafter as the MI method). The results were considered in terms of the commonly applied threelayer sandwich model of the film structure in which the hydrocarbon tails of the adsorbed lyso PC molecules form two hydrophobic monolayers surrounding the aqueous film core. The film water content from which the aqueous film core thickness was derived was measured using the FTIR spectroscopy method.2 In this case vertical macroscopic foam films with an area of 0.5-1 cm2 were investigated using a sintered glass frame and a glass tube. The aqueous core thickness was determined from the IR absorbance in the absorption band at 3400 cm-1 using the LambertBeer law. The results obtained using both methods are essentially similar, but some small discrepancies can be found in the derived aqueous core thicknesses. Moreover, the determined values of the potential of the diffuse electric (10) Yamanaka, T.; Hayashi, M.; Matuura, R. J. Colloid Interface Sci. 1982, 88, 458. (11) Scheludko, A.; Exerowa, D. Kolloid Z. 1960, 168, 24. Scheludko, A.; Exerowa, D. Commun. Dep. Chem. (Bulg. Acad. Sci.) 1959, 7, 123. (12) Scheludko, A. Adv. Colloid Interface Sci. 1967, 1, 391.
© 1997 American Chemical Society
Foam Film Thickness Measurements
layer φo and charge density σo do not compare very well. On the other hand both thicknesses should not be directly compared as in both cases some additional treatment is required to convert the measured data in real film properties. This treatment relies on specific assumptions for the film structure and its optical characteristics. However, a comparative analysis of the assumptions made and the results obtained may clarify the reasons for these discrepancies. The aim of the present study is to make a comparison between film thicknesses, determined by both optical methods (FT-IR spectroscopy and MI method), taking into consideration the assumed film structure models. This is important also for the investigations of long-range surface forces in the film and particularly for the estimation of the diffuse electric layer potential φo and charge density σo. Experimental Background Microinterferometric Studies of Foam Films Stabilized with lyso PC. The studies1 of foam films stabilized with lyso PC were performed using the wellknown MI technique, described in more detail in previous papers, e.g., refs 11-15. In this method the processing of obtained values of intensity of monochromatic light reflected by the film allows the so-called “equivalent film thickness” (hw) to be estimated. hw is determined considering the film as homogeneous with an index of refraction equal to that of the solution from which the film is obtained. As above mentioned, the obtained experimental results1 were interpreted assuming the three-layer “sandwich” film structure, where the aqueous film core involving the hydrophilic phospholipid head groups is covered by two hydrophobic monolayers consisting of the hydrophobic hydrocarbon tails of the adsorbed molecules.16-18 In the case of lyso PC stabilized foam films we assumed that
h2 ) hw - 3.6 nm where h2 denotes the aqueous core thickness. This h2 value was assessed in conformity with the detailed investigation19 of the model of a foam film stabilized with a surface active substance with a hydrophobic chain consisting of 16 carbon atoms. This assumption is justified not only by the same length of the hydrocarbon chains but also by the value of the surface concentration Γ of lyso PC,2 which is almost equal to the one in ref 19. On the basis of this three-layer model we can consider also the possibility the hydrophilic head groups to be out of the aqueous core. Then the aqueous core thickness would be equal to the distance between the hydrophilic head groups, that is, the diameter of the lyso PC head groups at both interfaces should be subtracted from h2. The detailed X-ray diffraction and NMR studies20 have shown that the diameter of the lyso PC head groups equals 0.7 nm. (13) 13. Kolarov, T.; Iliev, L. Annu. Univ. Sofia, Fac. Chim. 1974/ 1975, 69, 107. (14) Exerowa, D.; Zacharieva, M.; Cohen, R.; Platikanov, D. Colloid Polym. Sci. 1979, 257, 1089. (15) Kruglyakov, P. M.; Exerowa, D. Foam and Foam films (Russ); Khimia: Moscow, 1990; p 44. (16) Duyvis, E. M. Thesis, State University, Utrecht, 1962, p 19. (17) Frankel, S. P.; Mysels, K. J. J. Appl. Phys. 1966, 37, 3725. (18) Rijnbout, J. B. J. Phys. Chem. 1970, 74, 2001. (19) Donners, W. A. B.; Rijnbout, J. B.; Vrij, A. J. Colloid Interface Sci. 1977, 61, 249. (20) Hauser, H.; Pascher, I.; Pearson, R. H.; Sundel, S. J. Biochim. Biophys. Acta 1981, 650, 21.
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FT-IR Studies of Foam Films Stabilized with lyso PC. In the study2 of vertical macroscopic foam films formed from aqueous lyso PC solutions, IR absorbance data were converted into an aqueous core thickness using the Lambert-Beer law with a molar absorption coefficient of 150. This value of is derived from the FT-IR spectroscopy studies on black soap films prepared from aqueous sodium dodecyl sulfate solutions21 where polarized infrared spectra at various angles of incidence to the film surface have been recorded. It should be noted that in this case a three-layer model of the film structure has also been applied to determine the optical parameters of the film with the aid of thin-layer optics. A validity of Lambert-Beer law and a constant value of was assumed within the whole range of aqueous core thicknesses investigated in this study.2 Because of the effect of the molar extinction coefficient on the determined equilibrium thicknesses, the value of is of utmost importance in foam film thickness studies performed by the FT-IR spectroscopy method. Comparison between MI Method1 and FT-IR Spectroscopy2 Results for Foam Film Core Thicknesses. The analysis of MI1 and FT-IR spectroscopic2 results will be focused on the film thickness range where the film stability is determined by long-range interaction forces. Such films are obtained in the Cel interval of 10-3 to 2 × 10-1 mol dm-3 CaCl2, where thick and common black films are formed. At concentrations lower than 10-3 mol dm-3 and higher than 2 × 10-1 mol dm-3 CaCl2 Newton black films (NBFs) are obtained. As NBFs are considered as bilayer formations,22 the optical three-layer model of the film structure should not be applied to them. On the other hand eventual change of the optical constants in the NBF region could have an effect on the value of as well.21 In the study2 of macroscopic foam films, stabilized with lyso PC with concentration Cs ) 5 × 10-5 mol dm-3 as a function of Cel the aqueous core thickness is derived from the film water content. So, it seems reasonable to define d2 as the distance between the head groups of the adsorbed molecules. The FT-IR spectroscopy results2 obtained with ) 150 are shown in Figure 1a, curve 1. Curve 2 in the same figure shows the determined from interferometric data h2 as a function of Cel. As schematically shown in the figure, h2 includes the hydrophilic head groups. A systematic shift of h2 toward values higher than d2 is clearly observed in the whole studied Cel range. The difference between the h2 and d2 concepts for the aqueous core thickness is clearly seen in the schematic drawing of the film model. Using the data in ref 20 we can approximate both concepts for the aqueous core thickness by subtracting twice the diameter of the adsorbed lyso PC polar head groups from h2. So we obtain that the core thickness in this case should be equal to (h2 - 1.4) nm. The results are shown in Figure 1b, curve 2, where they are compared to the FT-IR measurement data2 as in Figure 1a. As is seen, in this case the agreement between the results for the aqueous core thicknesses is much better. Here, a tendency for higher thickness values obtained from the MI studies is still observed in the Cel area between 10-3 and 10-2 mol dm-3 where thicker films are formed. However, the difference between d2 and h2 in this film thickness range is commensurable with the comparatively bigger experimental scatter observed in this Cel range. This difference between d2 and h2 gradually decreases with increase in Cel, and after Cel ) 10-2 mol dm-3 both curves practically coincide. (21) Umemura, J.; Matsumoto, M.; Kawai, T.; Takenaka, T. Can J. Chem. 1985, 63, 1713. (22) Exerowa, D.; Kashchiev, D.; Platikanov, D. Adv. Colloid Interface Sci. 1992, 40, 201.
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Figure 1. Curve 1: (a) and (b) Aqueous core thickness d2 of thin liquid films stabilized with lyso PC as a function of CaCl2 concentration determined by the FT-IR spectroscopic studies2 with ) 150. Curve 2: (a) Aqueous core thickness h2 of thin liquid films stabilized with lyso PC as a function of CaCl2 concentration determined from the MI studies1 using the threelayer model. (b) Aqueous core thickness h2 corrected with the diameter of the lyso PC head groups. The schematic representation of the film model illustrates the model assumptions.
Another way to demonstrate the effect of the assumed film models on d2 and h2 is to consider the possibility of using a different value in the Lambert-Beer law. The detailed analysis of Smart and Senior23 of the classical IR studies of thin films suspended in air24 suggests that ) 116. So we made a comparison between aqueous core thicknesses d2 of lyso PC stabilized foam films2 determined from absorbance values with this previously found value of ) 11623 and the h2 results shown in Figure 1a, that is, the aqueous core thickness including the polar head groups. The obtained curves are plotted in Figure 2a. In this case a good agreement between both curves is also observed. If we assume that the lyso PC head groups are not included in the aqueous film core and compare them with the FT-IR results2 for d2, determined with ) 116, we obtain the results shown in Figure 2b. In this case the d2 vs Cel curve lies higher than the (h2 - 1.4) nm vs Cel curve. A possible explanation of this result can also be (23) Smart, C.; Senior, W. A. Trans. Faraday Soc. 1966, 62, 3253. (24) Corkill, J. M.; Goodman, J. F.; Ogden, C. P.; Tate, J. R. Proc. R. Soc. London, Ser. A 1963, 273, 84.
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Figure 2. Curve 1: (a) and (b) Aqueous core thickness d2 of thin liquid films stabilized with lyso PC as a function of CaCl2 concentration determined by the FT-IR spectroscopic studies2 with ) 116. Curve 2: (a) Aqueous core thickness h2 of thin liquid films stabilized with lyso PC as a function of CaCl2 concentration determined from the MI studies1 using the threelayer model. (b) Aqueous core thickness h2 corrected with the diameter of the lyso PC head groups. The model assumptions are illustrated by the schematic model of the film.
found in the way d2 is determined from the absorbance band assigned to the OH stretching vibration of liquid water molecules. Some of the latter are, however, distributed between the lyso PC head groups at the film interfaces, and so they are involved in the formation of the adsorbed layers. The usage of the Lambert-Beer law, and the adopted film model, however, treat them as part of the aqueous film core which could lead to the obtaining of somewhat higher d2 values compared to (h2 - 1.4) nm. The above assessments demonstrate that the experimentally found differences between equilibrium thicknesses of foam films studied by different optical methods could be explained with the necessity of assuming optical models for the interpretation of the obtained experimental data. It is seen that the use of suitably selected wellfounded optical parameters can lead to very similar results for the dependence of the equilibrium film thickness on Cel. Comparison between φo and σo Determined from Film Thickness Data Obtained from MI and FT-IR Measurements. The above considered results for film
Foam Film Thickness Measurements
core thicknesses obtained using FT-IR and MI experimental data are of particular importance for the assessment of parameters determining surface forces in such films. According to the classical DLVO theory equations, at equilibrium the disjoining pressure in the film Π ) Πel + Πvw, Πvw and Πel being the van der Waals and electrostatic components of the disjoining pressure, respectively. In the study,2 Πvw was determined according to the Lifshitz theory25 and its extension for triple layer films.26 In these calculations2 d2 is defined as the distance between the centers of the polar head groups and the separation distance between the two polar/nonpolar interfaces, is considered equal to (d2 + 0.7) nm, where d2 is determined with ) 150, and 0.7 nm is the diameter of the lyso PC polar head group.20 The hydrophobic part of the adsorbed layers is assumed to be of thickness 1 nm and refractive index 1.435, corresponding to the properties of bulk hexadecane.27 The potential φo of the diffuse electric layer and the charge density σo are determined following the approach proposed by Oshima et al.8 with solving the complete Poisson-Boltzmann equation. In these calculations,2 the distance d between the film planes of potential φo equals the distance between the centers of the hydrated Ca2+ ions bound by the adsorbed lyso PC head groups. The diameter of the hydrated Ca2+ ion (0.7 nm) has been obtained to average the values, reported in literature.28 So d ) (d2 - 0.7) nm. The obtained results2 show that σo increases with increasing CaCl2 concentration, this increase being steeper in the low Cel range. At the same time φo also increases at low Cel, but after reaching Cel ) 2 × 10-2 mol dm-3 it exhibits saturation behavior up to the highest studied Cel. In the case of film thickness measurements carried out by the MI method,1 analogous computations were carried out to determine φo and σo from the experimental data for Cel, hw, and the capillary pressure. A procedure involving the treatment of nonsymmetrical 2:1 electrolyte within the Poisson-Boltzmann approximation developed on the basis of a previously advanced method for computing the electrical parameters at the film interfaces in the case of 1:1 valent electrolyte29,30 was used in this study.1 The distance d in this case is assumed equal to (h2 - 0.7) nm, which accounts for the shift in the diffuse electric layer planes due to Ca2+ ions binding. Πvw was estimated from the Lifshitz theory and its extension for triple layer films26,27 following the approach developed in ref 27. Unlike the above considered results, in this case both φo and σo increase to rather high values with increase in Ca2+ concentration. For instance, at Cel ) 0.03 mol dm-3 CaCl2, φo equals about 80 mV, a value that is typical for ionic surfactants (e.g., refs 14 and 31). The causes for the difference between the results for φo and σo as a function of CaCl2 concentration can be elucidated if we take into consideration the results of the comparison between the optical models used for estimating foam film core thicknesses. To compare the FT-IR spectroscopy and MI method results, it is important to define in the same way separation distances between the two polar/nonpolar interfaces and the distance d between (25) Dzyaloshinski, I. E.; Lifshitz, E. M.; Pitayevskii, L. P. Adv. Phys. 1961, 10, 165. (26) Ninham, B. W.; Parsegian, V. A. J. Chem. Phys. 1970, 52, 4578. (27) Donners, W. A. B.; Rijnbout, J. B.; Vrij, A. J. Colloid Interface Sci. 1977, 60, 540. (28) Monk, C. B. Electrolytic Dissosiation; Academic Press: New York, 1961. Padova J. J. Chem. Phys. 1964, 40, 691. Nightingale, E. R., Jr. J. Phys. Chem. 1959, 63, 1381. (29) Kolarov, T; Exerowa, D.; Balinov, B.; Martinov, G. A. Kolloidn. Zh. 1986, 48, 1076. (30) Kolarov, T.; Cohen, R.; Exerowa, D. Colloids Surf. 1989, 42, 49. (31) Exerowa, D.; Kolarov, T.; Khristov, Khr. Colloids Surf. 1987, 22, 171.
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Figure 3. Diffuse electric layer potential φo and charge density σo of thin liquid films stabilized with lyso PC as a function of CaCl2 concentration: (9) φo and (0) σo, as determined from the MI film thickness studies;1 (b) φo and (O) σo, as determined from the FT-IR spectroscopic studies.2
the planes of potential φo. In the case of MI measurements1 it was assumed that d ) (h2 - 1.4 - 0.7) nm, which was determined as in Figure 1b, curve 2, taking into account the diameter of the Ca2+, bound at both interfaces. The distance between the two polar/nonpolar interfaces was considered equal to h2. The results for φo and σo as a function of CaCl2 concentration, obtained using the same computational procedure as in ref 1, are shown in Figure 3, curves 1 and 2. The computations were carried out up to Cel ) 0.05 mol dm-3 CaCl2 because of uncertainties in Πvw evaluation for h < 10 nm (e.g., ref 27). Using the same computational method1 we estimated also φo and σo as a function of Cel from the FT-IR spectroscopy experimental data for d2. The d2 data are determined with ) 150. In accordance with the above model assumptions the distance between the two polar/nonpolar interfaces equals (d2 + 1.4) nm and the distance d between the planes of potential φo is determined by d2 minus twice the radius of the adsorbed Ca2+, i.e., d ) (d2 - 0.7) nm. The obtained results for φo and σo are shown in Figure 3, curves 3 and 4. It is seen in Figure 3 that the so obtained φo vs Cel and σ vs Cel curves computed from the microinterferometrically measured thicknesses are similar to those obtained from the FT-IR data. At low Cel, high thicknesses, and low Πvw, the values of φo and σo obtained using both optical methods are practically indistinguishable. Above Cel ) 0.02 mol dm-3, unlike the results obtained in ref 1, in both cases φo reaches a saturation value. Indeed, in the case of MI studies it is about 6 mV higher than that obtained from FT-IR measurements. This is due to the fact that in the case of a 2-1 valent electrolyte, at high Cel, very small differences in film thickness lead to considerable changes in φo. It is important, however, that the applying of this model for determination of separation distances leads to a quite reasonable agreement between the course of the dependencies of φo and σo on Cel, which becomes obvious from the comparison between curves 1 and 3 and 2 and 4, respectively. The above analysis demonstrates the importance of the definition of d and the separation distance between the two polar/nonpolar interfaces for the determination of φo and σo in the case of Ca2+ binding by foam films stabilized with lyso PC. In view of the fact that the exact position of the film planes of potential φo cannot be directly
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measured, it is seen that an appropriate model can bring the different optical method results together. However, the physical aspects of the obtained results need special attention as the formation of common black films found in the Cel range after 5 × 10-3 to 2 × 10-1 mol dm-3 CaCl2 can be considered as an indication of a higher diffuse electric potential at the film interfaces as such a phenomenon has been observed only in the case of foam films stabilized with ionic surfactants.30,31 The above assessments demonstrate the importance of the assumptions for the film model used for the determination of parameters which characterize interaction forces in the film. It is seen that as equilibrium aqueous core thickness is of utmost importance for the estimation of σo and φo, small variation in film thickness can lead to considerable difference in the determined interaction forces. This problem could find its final solution in future
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studies of foam films using other methods which can provide new evidence about film structure, such as smallangle X-ray diffraction,32 dielectric studies,33 and also other techniques some of which have recently been reviewed.34 Acknowledgment. This work is financially supported by the Bulgarian Ministry of Education and Science (Grant No. X-414). LA9621022 (32) Platikanov, D.; Graf, H. A.; Weiss, A.; Clemens, D. Colloid Polymer Sci. 1993, 271, 106. (33) Pissis, P.; Enders, A.; Nimtz, G. In Water-Biomolecular Interactions, Conference Proceedings; Palma, M. U., Palma-Vottorelli, M. B., Parak, F., Eds.; SIF: Bologna, 1993; Vol. 43, p 223. (34) Claesson, P. M.; Ederth, T.; Bergeron, V.; Rutland, M. W. Adv. Colloid Interface Sci. 1996, 67, 119.
