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Surface Densities of Adsorbed Layers of Aqueous Sodium Myristate Inferred from Surface Tension and Infrared Reflection Absorption Spectroscopy Xinyun Wen, Jochen Lauterbach, and Elias I. Franses* School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907-1283 Received October 7, 1999. In Final Form: May 15, 2000 Infrared reflection absorption spectroscopy (IRRAS) has been used for probing the surface densities and compositions of adsorbed layers of aqueous sodium tetradecanoate, or myristate, at 25 °C. Aqueous sodium myristate normally becomes protonated (myristic acid) to an extent of about 0.5-1%, yielding a natural pH from 8 to 9, depending on concentration. The myristic acid, and possibly an acid-soap complex, are quite surface active compared to myristate, making impractical the application of the Gibbs adsorption isotherm for determining surface densities from tension data. At pH ) 12 (in 10 mM NaOH), only myristate is expected in the bulk, and the tension is higher; for 2 mM total surfactant concentration, the tension is ∼ 43 mN/m vs 23 mN/m. IRRAS spectra confirm that only myristate is present in the monolayer at pH ) 12. At natural pH (8-9), in addition to the band due to the myristate group, a significant band due to myristic acid is observed. Solutions in D2O were used for observing the carbonyl and carboxylate bands, after eliminating the H2O vapor noise in the polar group region (1800-1300 cm-1), and for having larger reflectance-absorbance intensities due to the smaller complex refractive index of D2O than that of H2O in the hydrocarbon stretching region (2950-2850 cm-1). The surface densities of adsorbed sodium myristate layers at pH ) 12 as determined from tension data by using the Gibbs adsorption isotherm agree to better than 10% to those determined from IRRAS data by using the model of either an isotropic film or an anisotropic film on the surface. The surface densities at pH ) 12 range from 1 × 10-6 to 4 × 10-6 mol/m2 as the concentration increases from 0.05 to 4 mM. At pH ≈ 8-9, the surface density is 8 × 10-6 mol/m2 at 4 mM, as explained by the lower tension. The frequencies of both antisymmetric and symmetric methylene stretching vibration bands are lower at natural pH, indicating more ordered and almost all-trans conformations at the higher surface densities.
1. Introduction Adsorption of amphiphilic molecules from a bulk solution to an air/water interface is important in various applications, such as cleaning products, coating flows, biological membranes, bioprocessing, and lung surfactants which help stabilize the alveoli against mechanical collapse. For understanding the adsorption as well as the nature of the adsorbed surfactant layer, it is important to measure the surface density and the composition of the adsorbed layer. The surface density or concentration of insoluble monolayers at the air/water interface is often directly available from the preparation procedure if there are no significant dissolution, precipitation, or evaporation losses.1,2 However, for soluble surfactants adsorbing from solution (“soluble monolayers”), considerably more effort is needed in this determination, due to the exchange of molecules between the surface and the bulk solution. Traditionally, one uses surface tension measurements for inferring adsorbed densities of surfactants by using the Gibbs adsorption isotherm procedure. The radiotracer technique, developed by Muramatsu, Tajima, and coworkers, is one of the earliest methods which can probe surface density directly.3-5 Recently, neutron reflection, second harmonic generation (SHG), sum frequency gen* To whom correspondence should be addressed. Tel.: (765) 494-4078. Fax: (765) 494-0805. (1) Myrick, S. H.; Franses, E. I. Langmuir 1999, 15, 1556. (2) Myrick, S. H. Ph.D. Thesis, Purdue University, West Lafayette, IN, 1999. (3) Muramatsu, M.; Tajima, K.; Sasaki, T. Bull. Chem. Soc. Jpn. 1968, 41, 1279. (4) Tajima, K.; Muramatsu, M.; Sasaki, T. Bull. Chem. Soc. Jpn. 1970, 43, 1991.
eration (SFG), and ellipsometry have also been used for determining the surface concentration of soluble monolayers.6-13 Comparisons of the strengths and weaknesses of several of these techniques in determining surface coverage, molecular orientation, and hydrocarbon chain conformational state have been given in ref 6. Another technique that has been widely applied to the probing of monolayers at the air/water interface is infrared reflection absorption spectroscopy (IRRAS, sometimes abbreviated as RAIRS), in which one employs an infrared beam incident at an angle φ to the surface normal of a monolayer.14-19 The water surface acts as a reflective element to specularly reflect the IR beam, which is then (5) Muramatsu, M.; Tajima, K.; Iwahashi, M.; Nukina, K. J. Colloid Interface Sci. 1973, 43, 499. (6) Bain, C. D. Curr. Opin. Colloid Surf. Sci. 1998, 3, 287. (7) Manning-Benson, S.; Parker, S. R.; Bain, C. D. Langmuir 1998, 14, 990. (8) Li, Z. X.; Dong, C. C.; Thomas, R. K. Langmuir 1999, 15, 4392. (9) Rasing, Th.; Stehlin, T.; Shen, Y. R.; Kim, M. W.; Valint, P., Jr. J. Chem. Phys. 1988, 89, 3386. (10) Vogel, V.; Mullin, C. S.; Shen, Y. R.; Kim, M. W. J. Chem. Phys. 1991, 95, 4620. (11) Bell, G. R.; Bain, C. D.; Ward, R. N. J. Chem. Soc., Faraday Trans. 1996, 92, 515. (12) Manning-Benson, S.; Bain, C. D.; Darton, R. C. J. Colloid Interface Sci. 1997, 189, 109. (13) Teppner, R.; Bae, S.; Haage, K.; Motschmann, H. Langmuir 1999, 15, 7002. (14) Dluhy, R. A.; Cornell, D. G. J. Phys. Chem. 1985, 89, 3195. (15) Dluhy, R. A. J. Phys. Chem. 1986, 90, 1373. (16) Dluhy, R. A.; Mitchell, M. L.; Pettenski, T.; Beers, J. Appl. Spectrosc. 1988, 42, 1289. (17) Mitchell, M. L.; Dluhy, R. A. J. Am. Chem. Soc. 1988, 110, 712. (18) Flach, C. R.; Brauner, J. W.; Mendelsohn, R. Appl. Spectrosc. 1993, 47, 982. (19) Flach, C. R.; Brauner, J. W.; Taylor, J. W.; Baldwin, R. C.; Mendelsohn, R. Biophys. J. 1994, 67, 402.
10.1021/la991326s CCC: $19.00 © 2000 American Chemical Society Published on Web 07/18/2000
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focused on the IR detector. Surfactants studied, mostly as insoluble monolayers, include certain fatty acids,21,22,24,30 their esters,23 long-chain alcohols,23 and phospholipids.16,17,25,26 Most of these studies focus on the structural characterization of insoluble monolayers.16,17,20-26,30 One focuses on the density and conformation of soluble monolayers of sodium dodecylsulfonate (C12S).27 These authors used solid C12S dispersed in a KBr matrix as a reference point for calibrating their spectral intensities and calculating the surface densities. There are several ways by which IRRAS can be used for probing monolayers, which tend to be oriented and anisotropic. Using polarized incident light provides two pieces of information per incident angle, for each wavenumber and subphase condition. This information, combined with a model with an anisotropic complex refractive index, may be used in principle to determine an average orientation angle and the surface density of the chromophore examined.20,25,27 The average orientation angle can be used for estimating the average monolayer thickness. The measured reflectance-absorbance (RA) depends strongly on the monolayer surface density, the number of chromophores per molecule (see section 3.2.3) and weakly on the monolayer real refractive index. The reflectanceabsorbance also depends on the complex refractive index of the subphase, and is quite different for D2O vs H2O. By using unpolarized incident light, one obtains one piece of information per angle and per subphase (H2O vs D2O), and the average molecular orientation and the film thickness need to be estimated independently. In this paper, we report on the surface concentration and composition of monolayers of a soluble surfactant system, aqueous sodium myristate, as determined with tension measurements and IRRAS. Aqueous sodium myristate solutions have been shown to have low (∼20 mN/m) equilibrium tensions and very low (∼1 mN/m) dynamic tensions at pulsating area conditions.36,37 Without direct information at the air/water interface, such tension behavior cannot be fully understood. With IRRAS, a lot of new information can help resolve certain issues regarding interpretation of certain low equilibrium tensions previously reported for this system.36 For inferring relative surface density, we have used RA data with unpolarized incident light, and the surface density at one concentration, as determined from the Gibbs adsorption (20) Flach, C. R.; Gericke, A.; Mendelsohn, R. J. Phys. Chem. 1997, 101, 58. (21) Gericke, A.; Hu¨hnerfuss, H. J. Phys. Chem. 1993, 97, 12899. (22) Gericke, A.; Hu¨hnerfuss, H. Thin Solid Film 1994, 245, 74. (23) Gericke, A.; Hu¨hnerfuss, H. Langmuir 1995, 11, 225. (24) Simon-Kutscher, J.; Gericke, A.; Hu¨hnerfuss, H. Langmuir 1996, 12, 1027. (25) Gericke, A.; Flach, C. R.; Mendelsohn, R. Biophys. J. 1997, 73, 492. (26) Dluhy, R. A.; Reilly, K. E.; Hunt, R. D.; Mitchell, M. L.; Mautone, A. J.; Mendelsohn, R. Biophys. J. 1989, 56, 1173. (27) Tung, Y.-S.; Gao, T.; Rosen, M. J.; Valentini, J. E.; Fina, L. J. Appl. Spectrosc. 1993, 47, 1643. (28) Fina, L. J.; Tung, Y.-S. Appl. Spectrosc. 1991, 45, 986. (29) Mendelsohn, R.; Brauner, J. W.; Gericke, A. Annu. Rev. Phys. Chem. 1995, 46, 305. (30) Sinnamon B. F.; Dluhy, R. A.; Barnes, G. T. Colloids Surf. A 1999, 146, 49. (31) Dluhy, R. A.; Wright, N. A.; Griffiths, P. R. Appl. Spectrosc. 1988, 42, 138. (32) Gericke, A.; Michailov, A. V.; Hu¨hnerfuss, H. Vib. Spectrosc. 1993, 4, 335. (33) Schopper, H. Z. Phys. 1952, 132, 146. (34) Kuzmin, V. L.; Michailov, A. V. Opt. Spectrosc. 1992, 73, 3. (35) Yamamato, K.; Ishida, H. Appl. Spectrosc. 1994, 48, 775. (36) Wen, X.; McGinnis, K.; Franses, E. I. Colloids Surf. A 1998, 143, 371. (37) Wen, X. M.S. Thesis, Purdue University, West Lafayette, IN, 1998.
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isotherm, as a reference point. For absolute calibration, we have used a monolayer of DPPC (dipalmitoylphosphatidylcholine) with known surface density. From either an isotropic-layer model15 or an anisotropic-layer model,29 and a range of monolayer thicknesses and chain orientations, the surface densities are determined to be in good agreement with the surface tension-based values. Then, the information therein determined has been used to determine total surface densities of mixed monolayers when both myristate and myristic acid are present. 2. Materials and Experimental Methods 2.1. Materials. Sodium myristate (>99 wt % pure) was purchased from Fluka Chemical Co. (Ronkonkoma, NY). NaCl (99.1 wt % pure), NaOH (98.8 wt % pure), and aqueous HCl (37 wt %) were obtained from Mallinckrodt Specialty Chemicals Co. (Paris, KY). Deuterium oxide (D2O, 99.9 at. %) was purchased from Aldrich Chemical Co. (Milwaukee, WI). All materials were used as received. The surfactant solutions were prepared on a weight basis. The pure water used for all samples was first distilled and then passed through a Millipore four-stage cartridge system, which has an organic adsorption column, two mixed ionexchange columns, and an ultrafiltration unit, resulting in water resistivity of 18 MΩ‚cm at the exit port. 2.2. Surface Tension Measurements. A Kru¨ss K8 tensiometer or a KSV Langmuir minitrough with a platinum Wilhelmy plate connected to an electronic microbalance was used for measuring surface tensions of aqueous sodium myristate systems at 25 °C. 2.3. Infrared Reflection Absorption Spectroscopy. The reflection absorption infrared spectra were obtained with a Nicolet Prote´ge´ 460 Fourier transform infrared spectrometer equipped with a liquid nitrogen cooled mercury-cadmium-telluride (MCT) detector and an external reflection attachment (Graseby Specac Inc.), which has a removable Teflon Langmuir trough. The angle of incidence was measured relative to the surface normal, and it can be varied from 20 to 85°. Incident angles of 40 or 45° were used for the H2O or D2O subphase, respectively. For water vapor and carbon dioxide to be reduced in the sample chamber, the instrument was continuously purged with dry air from a Balston purge gas generator. Unpolarized light was generally used. All spectra were collected using 1024 scans at 8 cm-1 resolution with Happ-Genzel apodization and one level of zero filling, resulting in the same data spacing as when the spectra were taken at 4 cm-1 resolution. IRRAS data are reported as plots of reflectance-absorbance (RA) vs wavenumber. RA is defined as -log(R/R0), where R0 and R are the reflectivities of the pure and the film-covered water surfaces, respectively. For identifying the structure of the headgroup, the most informative IR spectral region is between 1800 and 1300 cm-1, as this is the frequency range associated with the carbonyl and carboxylate stretching vibrational modes. However, at the air/ water interface the presence of intense water vapor bands (from water evaporation and incomplete purging) precludes the identification of bands arising from the monolayer headgroups. The signal-to-noise ratio (S/N) did not increase significantly with more scans. Although this vapor influence on the spectrum noise can be reduced with the use of lower resolution scans, certain narrow bands of interest may be broadened excessively and be obscured. Several different experimental approaches have been used for overcoming the problem of water vapor. Mendelsohn and co-workers designed a special accessory in which a small surface film apparatus, consisting of a sample well and a reference well, was externally interfaced to an IR spectrometer. By switching between the two wells during the experiment, they were able to eliminate effectively the influence of the water vapor on the spectra.18-20 Alternatively, Gericke et al. showed that the water vapor bands may be compensated by accurately regulating the humidity in the sample chamber with the help of a low flow of dry nitrogen.32 Finally, Blaudez et al. developed a different IR reflectivity technique based on a rapid modulation of the polarization of the incident electromagnetic field on the sample (38) Ekwall, P. Colloid Polym. Sci. 1988, 266, 279. (39) Lynch, M. L. Curr. Opin. Colloid Surf. Sci. 1997, 2, 495.
Adsorbed Layers of Aqueous Sodium Myristate
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Figure 1. Equilibrium surface tensions with the Wilhelmy plate method at 25 °C of sodium myristate in water (b), 1 mM NaCl (9), 10 mM NaCl (2), 150 mM NaCl ([), 1 mM NaOH (+), and 10 mM NaOH (×). The estimated uncertainty is (2 mN/m or better. interference.45,46
to diminish water vapor For our studies, we used D2O as the subphase for eliminating the H2O vapor bands and for detecting the polar groups of the adsorbed surfactant layers.
3. Results and Discussion 3.1. Surface Densities from Surface Tension and the Gibbs Adsorption Isotherm. Equilibrium surface tensions were measured for aqueous sodium myristate solutions or dispersions with concentrations ranging from 0.001 to 10 mM (Figure 1). For sodium myristate in water the tension decreased with increasing concentration up to about 2 mM, above which the data showed no clear trend, and local minima and maxima were observed. The shape of the γeq(c) curve is unusual. This concentration dependence can be attributed to having a nonbinary system due to the protonation reaction36,37
RCOO- + H2O h RCOOH + OH-
(1)
and subsequent hydrogen-bonding interactions between myristic acid molecules and myristate ions to form acidsoap complexes37-39
RCOOH + RCOO- h acid-soaps
(2)
resulting in having in the bulk solution, and at the interface (as shown in section 3.2.2), not only myristate but also myristic acid or acid-soap. The minimum equilibrium surface tension of sodium myristate in water is ca. 23 mN/m at 2 mM. The increase in pH produced a substantial increase in the surface tension. The minimum (40) Hiemenz, P. C.; Rajagopalan, R. Principles of Colloid and Surface Chemistry, 3rd ed.; Marcel Dekker: New York, 1997. (41) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; John Wiley & Sons: New York, 1989. (42) Born, M.; Wolf, E. Principles of Optics, 4th ed.; Pergamon Press: New York, 1970. (43) Hansen, W. N. J. Opt. Soc. Am. 1968, 58, 380. (44) Hansen, W. N. In Advances in Electrochemistry and Electrochemical Engineering; Muller, R. H., Ed.; Wiley-Interscience: New York, 1973; Vol. 9, Chapter 1. (45) Blaudez, D.; Buffeteau, T.; Cornut, J. C.; Desbat, B.; Escafre, N.; Pezolet, M.; Turlet, J. M. Appl. Spectrosc. 1993, 47, 869. (46) Blaudez, D.; Buffeteau, T.; Cornut, J. C.; Desbat, B.; Escafre, N.; Pezolet, M.; Turlet, J. M. Thin Solid Films 1994, 242, 146.
Figure 2. Dependence of the surface tension of 0.5 mM aqueous sodium myristate on time and the pH of the subphase at 25 °C. The estimated uncertainty is (1 mN/m.
equilibrium surface tension for sodium myristate in NaOH is about 41 mN/m, which is 18 mN/m higher than that of the sample in pure water. The tensions are slightly lower for samples in 10 mM NaOH than those in 1 mM NaOH. This is probably due to the increase in the electrolyte concentration, which often results in a decrease in the surface tension of ionic surfactants. With added NaOH, the shape of the γeq(c) lines become similar (lines downward below the cmc) to those of other single component ionic surfactants, such as alkyl sulfates.49 This might be attributed to the reverse of the soap protonation reaction (eq 1), resulting in a simpler system of essentially only the sodium soap. Moreover, with no added NaOH, the effect of NaCl is different on sodium myristate compared to other ionic surfactants. The addition of NaCl, from 1 to 150 mM, had little effect on the equilibrium surface tension; the tensions changed only slightly from those of myristate in pure water. We infer that (i) the increase in the surface tension caused by the presence of the base is directly connected with the change in pH and not with a counterion effect and (ii) the low equilibrium surface tensions might be attributed to a surface monolayer film containing not only carboxylate salt but probably some acid or acid-soap produced by soap protonation and subsequent complexation of soap and acid. For investigation of the effect of pH on the tension behavior, the surface tension of 0.5 mM sodium myristate was monitored as the pH of the solution was changed. Figure 2 shows that at pH ≈ 8 the equilibrium tension is about 34 mN/m. After NaOH was added to the solution to increase the pH to 12, the tension increased to about 56 mN/m. This result indicates that some surfactant may have desorbed from the interface. Another possibility is that the nature of the surfactant and its surface equation of state changed drastically. After aqueous HCl was added for neutralizing the added NaOH and restoring the original pH, the tension changed back to 34 mN/m. However, when more aqueous HCl was added and the solution pH became about 3, the soap was fully protonated and the tension increased to about 53 mN/m. These results will be discussed further below, in the light of the IR results. (47) Bertie, J. E.; Ahmed, K. M.; Eysel, H. H. J. Phys. Chem. 1989, 93, 2210. (48) Bertie, J. E.; Eysel, H. H. Appl. Spectrosc. 1985, 39, 392. (49) Elworthy, P. H.; Mysels, K. J. J. Colloid Interface Sci. 1966, 21, 331.
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Figure 4. IRRAS spectra of the C-H stretching vibration region for adsorbed sodium myristate layers at pH ) 12. Unpolarized IR radiation is used, and the incidence angle is 40°. The solution concentrations are 0.05, 0.2, 0.5, 1, 2, and 4 mM, from top to bottom. Table 1. Summary of Band Frequencies, Heights, Widths, and Areas of 3 mM Sodium Myristate on D2O
Figure 3. Surface densities of sodium myristate in 1 mM NaOH (b) and in 10 mM NaOH (9). The estimated uncertainty is (5%. Equation 4 is used (see text).
pH
For surfactant concentrations below the cmc, the surface concentrations at the interface may be estimated from the Gibbs adsorption isotherm, which in the absence of added electrolytes is as follows:
9
Γ)-
1 ∂γ mRT ∂(ln c)
(3)
Here Γ is the surface excess concentration, γ is the surface tension, c is the bulk surfactant concentration, and m is the number of species in solution whose surface concentration changes with bulk concentration. For dilute solutions of strongly surface active molecules, the surface excess density is nearly identical to the adsorbed surface density. In the presence of added electrolytes, eq 3 has to be modified in a standard fashion as follows.40,41 For a monomeric ionic surfactant with a supporting monovalent electrolyte with the same counterion and a concentration of cs, and an ideal bulk solution, the Gibbs adsorption isotherm for the surfactant and the counterion becomes
Γ)-
1 c + cs ∂γ RT 2c + cs ∂(ln c)
12
(4)
where the parameter m in eq 3 depends on both salt and surfactant concentrations. Figure 3 shows that, for sodium myristate in NaOH, the surface density Γ increases with increasing concentration up to the cmc (∼2 mM). The maximum surface density is about 4 × 10-6 mol/m2 (about 0.41 nm2/molecule). For sodium myristate in water or in NaCl, since there are more than one component present in the bulk and at the interface due to the protonation of the soap, the surface density of the adsorbed surfactant layer cannot be inferred directly from the tension data. Therefore, a direct probing method is needed to determine surface densities. In addition to probing surface densities, IRRAS provides a direct nonintrusive method for probing surface compositions and conformations in the monolayer and has a strong potential of being used in mixed surfactants and more complex systems without need for special molecular labeling.29 3.2. IRRAS Studies of Aqueous Sodium Myristate. 3.2.1. Results of Sodium Myristate Monolayers at pH ) 12. For understanding the monolayer nature which causes the observed tension behavior reviewed above, the
3
peak center, cm-1
peak height
half width, cm-1
area, cm-1
1552 2854 2923 2962 1468 1554 1699 2850 2918 2958 1469 1697 2848 2916 2954
0.0021 0.0014 0.0022 0.00025 0.00066 0.0012 0.00058 0.0047 0.0064 0.00076 0.0011 0.0036 0.0066 0.0095 0.00076
44 16 23 12 16 48 44 14 17 14 25 31 12 15 12
0.10 0.026 0.061 0.0028 0.011 0.061 0.040 0.074 0.14 0.010 0.025 0.12 0.084 0.19 0.0078
IRRAS technique has been applied to the aqueous sodium myristate systems for studying the composition, conformational state, and surface density of the adsorbed layer. The results for sodium myristate with added NaOH at pH ) 12 will be shown first, because this system is much simpler to analyze than aqueous sodium myristate (with no added base), which contains multiple components due to protonation, as discussed above. Figure 4 shows representative reflection absorption FTIR spectra in the C-H stretching vibration region (3000-2800 cm-1) for samples with bulk concentration ranging from 0.05 to 4 mM. Three bands are usually resolved in the hydrocarbon stretching region. The methyl stretching vibrations are weaker than the methylene stretching vibrations, as expected. The antisymmetric stretching band is visible at about 2960 cm-1, and the symmetric stretching band is typically not seen in monolayer spectra at the air/water interface.30 The strongest band is due to the antisymmetric methylene stretching vibration (νa-CH2), which is centered at about 2924 cm-1. The next strongest band, at about 2854 cm-1, corresponds to the symmetric methylene stretching vibration (νs-CH2). These values are in good agreement with the values of ∼2926 and ∼2855 cm-1 which have been observed for polymethylene chains in the liquid phase,31 or other liquid expanded monolayers,17,30 indicating that the hydrocarbon chains of the sodium myristate monolayers at pH ) 12 may be substantially liquid-like, or disordered. By contrast, the frequencies of the C-H stretching vibration bands for monolayers at pH ≈ 8-9 are lower (2918 and 2850 cm-1, respectively, Table 1), indicating almost all-trans conformations and a closer packing of the hydrocarbon chains (see section 3.2.2). This explains the higher surface tension at pH ) 12. The RA
Adsorbed Layers of Aqueous Sodium Myristate
Figure 5. Dependence of RA intensity and integrated RA intensity of the νa-CH2 band on the concentration of aqueous sodium myristate at pH ) 12. See Figure 4.
Figure 6. Effect of the subphase pH on the reflectanceabsorbance in the C-H stretching vibration region for adsorbed sodium myristate layers, at various times.
intensity and the integrated RA for the νa-CH2 band as a function of concentration are shown in Figure 5. As expected, both values increase with increasing surfactant concentration. At about 2 mM, the RA intensity reaches a nearly constant value of 0.0015. This is consistent with the tension results that surface tension remains constant above 2 mM. 3.2.2. Effect of pH on the Monolayer Spectra. Since sodium myristate becomes partially protonated in water, the pH of the subphase may affect strongly the concentrations of the adsorbing species and, thus, the compositions and surface densities of the adsorbed surfactant layers. For comparison of the spectral results with the tension results, data for 0.5 mM aqueous sodium myristate were used. Figure 6 shows that the intensities of both antisymmetric and symmetric methylene stretching bands decreased as the pH of the subphase increased. After the pH increased, the frequencies of both the νa-CH2 and the νs-CH2 bands changed with time and shifted to higher wavenumbers, from 2918 to 2924 cm-1 for the νa-CH2 band and from 2850 to 2854 cm-1 for the νs-CH2 band. After sufficient aqueous HCl was added to the subphase for restoring approximately the original pH of the subphase, both the νa-CH2 and the νs-CH2 bands shifted back to lower wavenumbers (2918 and 2850 cm-1, respectively), and the band intensities increased (Figure 6). Thus, with IRRAS one can easily detect desorption. When more aqueous HCl was added and the subphase became acidic, the band intensities increased further and the bandwidths decreased. Since the tension of this system under acidic conditions was higher than the value of 34 mN/m observed
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Figure 7. Dependence of the peak wavenumber of the νa-CH2 band on the concentration of aqueous sodium myristate: in water (b); in saline (2); in 10 mM NaOH (9); at pH ) 3 ([).
at pH ≈ 8 (section 3.1), and since myristic acid is sparingly soluble, it appears that the increase in band intensity at low pH values may be not entirely due to a myristate monolayer but partly to some bulk effect, which is under investigation. The frequencies of the methylene stretching vibrations correlate with the packing and the conformational order of the hydrocarbon chains.21 Whereas lower wavenumbers are characteristic of highly ordered almost all-trans conformations, as the number of gauche conformers per chain (a measure of the “disorder” of the chains) increases, the wavenumbers increase. Figure 7 shows that at pH ≈ 9 and high concentrations, the frequencies are about 2917-2918 cm-1, indicating that the adsorbed surfactant layers exhibit a highly ordered, and perhaps all-trans, closely packed, and oriented conformational state. At high pH values, the wavenumbers are higher, 2924 cm-1, implying that there are probably some gauche conformations in the hydrocarbon chains, which may be more loosely packed by virtue of their smaller surface density. The frequency of the νa-CH2 band may be also related to the subcell structure.24 Simon-Kutscher et al. studied the influence of pH and bivalent cations on the properties of the octadecanoic acid monolayer.22,24 They concluded from their results that the variation of the subphase pH value can induce a change of the subcell structure, which in turn can affect the order of the hydrocarbon chains.24 To detect what molecules (soap, acid, or acid-soap) actually adsorb to the surface, one needs to probe the polar group region (1800-1300 cm-1). We used D2O as the subphase to detect the polar groups more easily (section 2.3). For 3 mM sodium myristate in D2O, as the pH of the subphase increased from 9 to 12, the νa-CH2 band frequency increased from 2918 to 2923 cm-1, its width increased from 17 to 23 cm-1, and the band intensity decreased by about 3-fold (Figure 8). At pH ) 12, the band at about 1552 cm-1 is due to the antisymmetric carboxylate stretching vibration (Table 1). This frequency is in good agreement with the value of 1558 cm-1 observed for solid sodium myristate, confirming that only myristate was present in the monolayer. At pH ≈ 9 two major bands are seen in the spectrum. The band at 1554 cm-1 indicates the presence of carboxylate groups, while the band at 1699 cm-1 represents the stretching vibration of the carbonyl groups, which are probably in the doubly protonated
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were obtained from refs 15 and 29. For unpolarized incident light, which can be considered to consist equivalently of 50% s-component and 50% p-component, the reflectivity is
1 Rup ) (Rs + Rp) 2
(5)
and the reflectance-absorbance becomes
(
RA ) - log10
)
Rs + Rp Rs,0 + Rp,0
(6)
The complex refractive index of water was obtained from ref 47 and interpolated to the relevant wavenumbers. Since all the solutions are very dilute, we have ignored the effect of dissolved material on the substrate refractive index. Nonetheless, a small correction can be easily incorporated in a model, if needed. The extinction coefficient kb of a bulk solution can generally be determined from the absorbance in a transmission IR spectrum. From the electromagnetic theory, the extinction coefficient is related to the absorbance by the following expression:48 Figure 8. IRRAS spectra of adsorbed sodium myristate layers on D2O: (a) C-H stretching vibration region; (b) polar group region. The bulk concentration of sodium myristate is 3 mM.
form.29 For solid myristic acid, the carbonyl band is found at about 1698 cm-1. At pH ) 3, only a band representing the carbonyl stretching vibration is observed at about 1697 cm-1, confirming that only myristic acid was present at the air/water interface. (The observation of a higher RA intensity at pH ) 3 than at pH ≈ 9, even though the tension is higher, is under study.) We infer that, at pH ≈ 9, the adsorbed surfactant layer contains not only myristate but substantial amount of myristic acid, and the hydrocarbon chains are highly ordered. As the pH increases, the carboxylate-to-acid molar ratio increases. At pH ) 12, only carboxylate is observed. Stronger repulsive forces between the carboxylate headgroups evidently lead to a lower adsorption density than that at pH ≈ 8-9, where the presence of carboxylic acid groups may help diminish the electrostatic repulsive forces and pack the monolayer more tightly, reducing the tension. 3.2.3. Surface Densities from IRRAS Data. The theory of IRRAS for monolayers at the air/water interface has been reported.15,27-29,32-35,42-44 The measured reflectance-absorbance depends not only on the surface density but also on the thickness, refractive index, and optical anisotropy of the monolayer and also on the complex refractive index of the aqueous subphase. Dluhy used an isotropic-layer model for computer simulations of IRRAS for monolayers of lipids on water.15 To account for the anisotropic properties of the monolayer, several anisotropic-layer models have been developed.27-29,32-35 Three formulations (Schopper,33 Kuzmin and Michailov,34 and Yamamato and Ishida35) have been compared by Mendelsohn et al.29 Even though these authors followed quite different approaches in their derivations and their final expressions look somewhat different, their formalisms give similar results in computer simulations.29 Tung et al. used Schopper’s formulation for the analysis of their data of soluble monolayers of sodium dodecylsulfonate.27 For relating the RA data to the surface density of the adsorbed surfactant layer, we applied both the isotropiclayer model based on Dluhy’s formalism15 and one of the anisotropic-layer models developed by Kuzmin and Michailov.29,34 The equations for s- and p-polarized light
A ) log(I0/I) )
4πnνkbl 2.303
(7)
Here ν is the frequency in cm-1, l is the path length in cm, and I0 and I are the intensities of the beam before and after it passes through the sample, respectively. Calculations for dilute solutions at c e 4 mM indicate that the effects of the solute on the substrate refractive index and the subsequent IRRAS reflectance-absorbance are quite small. For 4 mM sodium myristate, the value of kb at ν ) 2920 cm-1 was calculated to be 0.0159 from transmission IR data. This value is higher by only about 0.0002, or 1.3%, than the value of 0.0157 for pure water.47 This change in the subphase extinction coefficient resulted in less than 0.2% change in the simulated IRRAS reflectance-absorbance, suggesting that the effect of the dissolved absorbing solute on n˜ is negligible. This effect, however, could be important for higher concentrations. The effective thickness (d) of the monolayer depends on the chain length (L) of the molecule and the chain orientation. For sodium myristate with all-trans conformation, the chain length L was calculated to be 1.89 nm by using a commercial modeling software (WebLab ViewerPro, from Molecular Simulations Inc.). The chain with gauche conformation defect was calculated to have a chain length between 1.2 and 1.8 nm, depending upon the number and the position of the gauche conformers. The polar group has an additional length of about 0.2-0.4 nm. Because of the potential variability of the chain length and chain orientation, which may affect the effective monolayer thickness, monolayer thicknesses of 1.2-1.9 nm were used in the isotropic-layer model calculation, and their effects on the reflectance-absorbance were examined to determine the sensitivity of the calculated surface densities on d. For the anisotropic-layer model, we have assumed that the monolayer thickness is determined by d ) L cos θ, where θ is the chain tilt angle (the angle between the molecular axis and the surface normal). For calculations with the isotropic-layer model, the real part of the monolayer refractive index n was taken to be about 1.41 on average, as determined from IR ellipsometry of hydrocarbon chains in the mid-IR region between 2840 and 2960 cm-1.20 This value can be higher than 1.41 and as high as 1.50.29,32 For the anisotropic-layer model, the monolayer refractive index can be separated into its
Adsorbed Layers of Aqueous Sodium Myristate
Langmuir, Vol. 16, No. 17, 2000 6993
Table 2. Values of the Absolute and Normalized Extinction Coefficient, kiso, Obtained from the RA Data for Aqueous Sodium Myristate at pH ) 12, for Various Values of the Monolayer Effective Thickness d, from the Isotropic-Layer Modela kiso for d, nm c,mM
RA
1.2
1.4
1.6
1.8
1.9
0.05 0.2 0.5 1 2 4
-0.0002 -0.0008 -0.0010 -0.0012 -0.0014 -0.0015
0.039 0.104 0.126 0.146 0.168 0.175
0.036 0.092 0.111 0.127 0.147 0.152
0.034 0.082 0.099 0.114 0.131 0.135
0.032 0.075 0.090 0.103 0.118 0.122
0.031 0.072 0.086 0.099 0.113 0.117
c,mM
1.2
1.4
1.6
1.8
1.9
0.05 0.2 0.5 1 2 4
0.23 0.60 0.72 0.83 0.96 1.00
0.24 0.60 0.73 0.84 0.96 1.00
0.25 0.61 0.73 0.84 0.96 1.00
0.26 0.61 0.74 0.84 0.96 1.00
0.27 0.62 0.74 0.84 0.96 1.00
ratio of kiso at c to kiso at 4 mM for d, nm
a
See text.
directional components nx, ny, and nz.29 Results from our simulations revealed that changes in the real part of monolayer refractive index by (0.1 altered the reflectanceabsorbances insignificantly (by less than 6%), as also pointed out by Flach et al.20 Therefore, the value of 1.41 was also used in the anisotropic-layer model. The monolayer extinction coefficient is described by a single value kiso in the isotropic-layer model. For the anisotropic-layer model, by using Fraser and MacRae’s formalism the directional coefficients kx ) ky (a uniaxially symmetric distribution of chain axes is assumed) and kz can be obtained for a given tilt angle (θ) and dipole moment direction angle R (the angle between the dipole moment direction and the molecular chain axis):27,29
kx ) ky ) [f sin2 R/2 + (1 - f)/3]k
(8)
kz ) [f cos2 R + (1 - f)/3]k
(9)
Here f ) (3 cos2 θ - 1)/2. The parameter k is the extinction coefficient for a collection of completely aligned chains and is the parameter that is proportional to the molecular concentration of the monolayer.27 One expects that k is proportional to the surface density Γ. Thus, the reflectance-absorbance can be used for determining Γ, if one can do a proper calibration with films of known surface density and comparable values of the refractive index. The number N of absorbing groups (e.g. CH2 groups) per molecule also needs to be accounted for. Therefore, we used the following equation:
k ) KNΓ
(10)
Here the constant K has the units of m2/mol of the absorbing group. We will first discuss the results using the isotropiclayer model. Even though monolayers are expected to be anisotropic, we used Dluhy’s isotropic-layer model because the frequencies of the methylene stretching vibration bands indicate that the pure myristate monolayer (at pH ) 12) is more liquidlike, or less anisotropic, than those highly ordered monolayers (section 3.2.1). From the RA data at ν ) 2924 cm-1 for sodium myristate in NaOH, the values of kiso for five different monolayer thicknesses d were calculated (Table 2). At a given surfactant concen-
Figure 9. Calculated surface densities of aqueous sodium myristate at pH ) 12. Data are inferred from surface tension (9), IRRAS with the isotropic-layer model (b), or IRRAS with the anisotropic-layer model (2), in which one surface density (4 × 10-6 mol/m2) was assumed the same as determined from the Gibbs adsorption isotherm.
tration, the kiso values are different with different monolayer thicknesses, and the differences increase with increasing concentration. However, the kiso ratios, obtained by dividing the kiso values by that of the value at 4 mM, depend weakly on the thickness. As shown in Table 2, for concentrations ranging from 0.2 to 2 mM the values of kiso ratios vary by less than 4% with the different monolayer thickness. As a first approximation, we assumed that the monolayer thickness does not vary with concentration or that there is similar molecular orientation or conformation at all concentrations. This assumption seems reasonable because the frequency of the antisymmetric methylene stretching vibration band is about the same for concentrations ranging from 0.2 to 2 mM (Figure 4). We then used Γ ) 4 × 10-6 mol/m2 at c ) 4 mM, determined from the Gibbs adsorption isotherm, as a reference (or calibration) point, and the surface densities at other surfactant concentrations were calculated by using the kiso ratios (Figure 9). With the anisotropic-layer model, the values of k for seven different chain tilt angles θ, ranging from 0 to 45°, were calculated (Table 3). Whereas the k values depend strongly on the chain tilt angle θ, their ratios depend weakly on θ. We used the same approach as for the isotropic-layer model. We used the surface density at 4 mM as a reference point and assumed that there is similar molecular orientation at all concentrations. The surface densities inferred from the two models are quite close (Figure 9), differing by less than 5%, which is less than the experimental error of 8%. In addition, we used the spectrum of a DPPC monolayer with known surface density for calibration, using the anisotropic-layer model. At Γ ) 3.4 × 10-6 mol/m2, the values of k and K (eq 10), as determined from the RA value at 2918 cm-1, were found to be 0.6 and 6300 m2/mol, respectively. The chain tilt angle θ of 26° and molecular length L of 2.66 nm, as found in ref 25, were used in the calculation. For aqueous sodium myristate at 4 mM and pH ) 12, the values of K were calculated by taking Γ as 4 × 10-6 mol/m2 from the tension data (Table 4). The value of K as determined from our myristate data and the anisotropic-layer model depends on the tilt angle θ (Table
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Wen et al.
Table 3. Same as Table 2 but with the Anisotropic-Layer Model for L ) 1.9 nm and for Various Values of the Tilt Angle θ of the Chain Relative to the Surface Normala k for θ, deg c, mM
RA
0
10
20
30
35
40
45
0.05 0.2 0.5 1 2 4
-0.0002 -0.0008 -0.0010 -0.0012 -0.0014 -0.0015
0.057 0.134 0.161 0.184 0.211 0.215
0.058 0.149 0.166 0.190 0.218 0.222
0.063 0.151 0.181 0.208 0.239 0.243
0.071 0.175 0.211 0.242 0.278 0.283
0.077 0.193 0.233 0.268 0.308 0.314
0.085 0.216 0.262 0.301 0.347 0.353
0.094 0.246 0.299 0.344 0.397 0.405
ratio of k at c to k at 4 mM for θ, deg c, mM
0
10
20
30
35
40
45
0.05 0.2 0.5 1 2 4
0.26 0.62 0.75 0.86 0.98 1.00
0.26 0.62 0.75 0.86 0.98 1.00
0.26 0.62 0.75 0.85 0.98 1.00
0.25 0.62 0.74 0.85 0.98 1.00
0.24 0.61 0.74 0.85 0.98 1.00
0.24 0.61 0.74 0.85 0.98 1.00
0.23 0.61 0.74 0.85 0.98 1.00
a
See text.
Table 4. Values of K (Eq 10) from Data in Table 3, at Different Monolayer Tilt Angles θ, for Adsorbed Sodium Myristate Layers at c ) 4 mM and pH ) 12 θ, deg
K, m2/mol
θ, degree
K, m2/mol
0 5 10 15 20 25
4484 4518 4621 4800 5063 5426
30 33 35 40 45
5908 6270 6540 7364 8440
4). If we take K ) 6300 m2/mol, from the DPPC data, then θ would be about 33°. If we use K ) 6300 ( 200 m2/mol, we can estimate the error that may be introduced from the assumption that the monolayer has the same thickness at all concentrations. At 0.05 mM, if the average tilt angle is larger than that at 4 mM (45° vs 33°), the calculated surface density would be higher by 20%. Although the Γ values based on the IRRAS data suffer from some uncertainties due to calibration errors and the assumptions of the models used, they provide strong support for independent confirmation of the predictions of the Gibbs adsorption equation. Other confirmations of the Gibbs adsorption equation may be provided by radiotracer data,3-5 neutron reflection data,7,8 and ellipsometry data.12,13 The value of K ) 6300 ( 200 m2/mol was also used as the standard for determining the surface density of 4 mM sodium myristate at pH ≈ 9, at which both myristate and myristic acid are present. The total surface density Γ was found to be 8.1 × 10-6 mol/m2 if θ ) 0° is assumed or 8.5 × 10-6 mol/m2 if θ ) 10° is assumed. These values would indicate a closely packed monolayer, with about 0.2 nm2 per molecular chain,29 close to the minimum area for
closely packed and almost all-trans hydrocarbon chains. For the isotropic-layer model, the value of Kiso ) 2800 ( 100 m2/mol, calculated by taking d ) 1.6 nm and Γ ) 4 × 10-6 mol/m2 at c ) 4 mM and pH ) 12, was used. The total surface density Γ was found to be 8.7 × 10-6 mol/m2 if d ) 1.9 nm is assumed for the myristate monolayer at pH ≈ 9. The exact proportion of each component in the monolayer cannot be determined quantitatively from the intensities of the polar-group bands, because the band intensity depends on the type of molecule and possible hydrogen bonding.39 The relative areas of the carboxylic acid to carboxylate group bands are in the ratio of about 2:3, indicating that roughly 40% carboxylic acid is present. More work is needed for a more precise estimate. The surface densities of aqueous sodium myristate depend substantially on pH. At pH ) 12, stronger repulsive forces between the carboxylate headgroups lead to a lower adsorption density than that at pH ≈ 8-9, where the presence of carboxylic acid groups may help diminish the electrostatic repulsive forces and allow tighter monolayer packing and lower surface tensions. An equivalent alternative description is that the myristate ion is more soluble, more hydrophilic, and less surface active than myristic acid or the mixture of the acid and the myristate. 5. Conclusions Infrared reflection absorption spectroscopy was used for measuring the density and probing the molecularlevel structure and conformation of myristate or myristate/ myristic acid monolayers at the air/water interface. In certain experiments, D2O was used instead of H2O to have a higher RA signal and lower H2O vapor noise in the 18001300 cm-1 spectral region. For aqueous sodium myristate, with increasing surfactant concentration the C-H stretching vibration band frequency decreases and the band intensity increases, indicating a more ordered and closely packed layer. The increasing surface density with increasing surfactant concentration is consistent with the observed tension decrease. The RA intensity and the frequencies of the C-H stretching vibration bands depend on the subphase pH. As the pH of the subphase increases, the monolayer becomes more concentrated in carboxylate than in acid. The stronger repulsions between the carboxylate headgroups lead to a decreased adsorption density. These results, along with the spectra in the polar group region, indicate that the low tensions are not due to adsorption of myristate alone or myristic acid alone but to synergistic adsorption of a mixture of soap and acid. Acknowledgment. This research was supported in part by the National Institutes of Health (Grant HL 5464102) and the National Science Foundation (Grant CTS 96-15649). LA991326S
