I
0 Copyright 1990 American Chemical Society
The ACS Journal of
Surfaces and Colloids MARCH, 1990 VOLUME 6, NUMBER 3
Characterization of the Structure of Langmuir-Blodgett Films of 22-Tricosenoic Acid Using X-ray Photoelectron Spectroscopy N. G. Cave,+R. A. Cayless,s L. B. Hazell,$ and A. J. Kinloch*?+ The Department of Mechanical Engineering, Imperial College of Science, Technology and Medicine, Exhibition Road, London, S W 7 2BX, England, and BP Research International, Chertsey Road, Sunbury-on-Thames, Middlesex, TW16 7LN, England Received January 27, 1989. I n Final Form: June 13, 1989 The use of variable electron takeoff angles in X-ray photoelectron spectroscopy (XPS) to clarify the deposition behavior and the physical structure of Langmuir-Blodgett (L-B) films is described. The results confirm the Y-type deposition of both monolayer and bilayer films of 22-tricosenoic acid from an aqueous subphase containing cadmium ions onto evaporated films of aluminum on glass. Surprisingly, no cadmium was incorporated in the interfacial layers in either the bilayer or the monolayer. A trapped monolayer of water appears to be present in these interfacial layers. The inelastic mean free path (IMFP) values necessary to model the film structures have been established. These will assist further studies of L-B film structures. The values are 2.7, 2.1, and 2.8 nm for the A1 2s (1410 eV), 0 1s (955 eV), and C 1s (1201 eV) photoelectrons, respectively. These are discussed with reference to the previously published values. 1. Introduction
The properties of L-B films have been studied for over 50 years, yet it is only recently that they have become the focus of intensive investigation. This interest is due to the potential that L-B films have in areas as diverse as microlithography, solid-state molecular electronics, and biological membranes.' The research is underpinned by the recent application of analytical techniques such as attenuated total reflectance (ATR),2,3grazing angle incidence Fourier transform infrared (FTIR) spectr o ~ c o p y , ~reflection '~ high-energy electron diffraction (RHEED),'-' and, latterly, the near-edge X-ray adsorption fine structure technique (NEXAFS).' These tech-
* Author to whom all correspondence should be addressed.
niques yield information on the orientation and packing of the hydrocarbon chains, the head-group interactions, and the bonding between the films and the substrate. Thus, the effects on the film structure of the deposition conditions and substrate preparation can now be systematically studied. However, these techniques provide information on the average film structure, not on the composition of the individual layers or the degree of film integrity. X-ray photoelectron spectroscopy (XPS) has the potential to give additional information about the structure of Langmuir-Blodgett films. It provides both an elemental analysis and functional group information about the outermost layers of a surface. (It is, however, not sensitive to hydrogen). The surface sensitivity arises from the limited inelastic mean free path (IMFP), A, of the photoelectron, which varies typically between 0.5 and 5
Imperial College of Science, Technology and Medicine.
* BP Research International.
(1) Thin Solid Films 1985, 132-134. (2) Davies, G. H.; Yarwood, J. Spectrochim. Acta 1987, 43A, 1. (3) Kimura, F.; Umemura, J.; Takenaka, T. Langmuir 1986,2,96. (4)Rabolt, J. F.; Burns, F. C.; Schlotter, N. E.; Swalen, J. D. J. Chem. Phys. 1983, 78(2), 946. (5) Allara, D. L.; Swalen, J. D. J. Phys. Chem. 1982, 86, 2700.
(6) Jones, C. A.; Russell, G. J.; Petty, M. C.; Roberts, G. G. Philos. Mag. B 1986, 54(No. 3), L89. (7) Peterson, I. R.; Russell, G. J . Philos. Mag. A 1984, &(No. 3), 463. (8) Prakash, M.: Dutta, P.: Ketterson. J. B.: Abraham, B. M. Chem. Phys. Lett. 1984, 111, 395. (9) Outka, D. A.; Stoehr, J.; Rabe, J. P.; Swalen, J. D.; Rotermund, H. H. Phys. Reu. Lett. 1987,59(No. 12), 1321.
0743-7463/90/24Q6-Q529502.5Q/Q 0 1990 American Chemical Societv
Cave et al.
530 Langmuir, Vol. 6, No. 3, 1990
nm as a function of the electron's kinetic energy and the nature of the material through which it passes. The sampling depth, d, is given by d = -3X sin 0; hence, information about the variation of the composition with depth, and thus the physical structure, can be obtained by varying the electron takeoff angle (ETOA), 8 . XPS therefore provides a means of examining the structure of the initial layers of LB films. It will also be sensitive to the patchiness of the LB film coverage, as this would produce higher than expected substrate elemental intensities. Hazell et al.103'' have developed a model that can be used to deduce the elemental composition and physical structure of the surface region of a flat sample from the experimental elemental intensities obtained as a function of electron takeoff angle. The model assumes that the surface region consists of a number of homogeneous layers of differing compositions and thicknesses that can be iteratively varied to fit the experimental data. It is necessary to input values for the IMFPs for each element and the appropriate number densities for each layer. The validity of the technique was established by using a model system of silicon oxide grown on an ion beam etched single crystal of silicon." Early studies of stearamine L-B films showed these films to be of poor quality, and they suffered degradation under X-ray bombardment. In this study, high-quality films have been obtained on evaporated aluminum on glass substrates. Single-crystal substrates have been avoided to reduce the possibility of photoelectron diffraction or "searchlight" enhancement effects.'* The results of the modeling were correlated with ellipsometry measurements of the overall film thicknesses. The aim of the present work was to elucidate the structures of the monolayer and bilayer L-B films, to further validate the modeling approach, and to derive appropriate IMFPs for organic overlayers.
2. Experimental Section 1. Film Preparation. The films were prepared at Durham
University. The deposition equipment, which has been described previ~usly,'~ was situated in a semiconductor grade, class 10 OOO, clean room. The substrates were glass slides (previously cleaned by refluxing in isopropyl alcohol vapor) onto which 100 nm of aluminum was thermally evaporated in a vacuum system at a pressure of -IO* mbar. 22-Tricosenoic acid was dissolved in chloroform (BDH, Arista) to a concentration of -1 mg mL-' and was spread onto the surface of ultrapure water (obtained by reverse osmosis/ deionization/UV sterilization) at 18 f 2 "C, pH 5.6-5.8,which contained cadmium chloride (-2.5 X M). After compression to 37 mN m-', the fatty acid was transferred to the substrate at a rate of 10 mm min-'. Monolayer samples were fabricated by initially immersing the substrate in the subphase (Le.,before spreading the fatty acid); bilayers were produced by simply dipping and withdrawing the substrate through the condensed floating monolayer. By use of the above technique, high-quality monolayer and bilayer samples were obtained. These fatty acid structures have previously been studied by using both FTIR' and RHEED' techniques; similar layers have also been used as the basis of a number of novel electronic device^.'^ 2. XPS Analysis. XPS analysis of the films was carried out by using a VG ESCALAB Mk 1 spectrometer. Experi(10) Hazell, L. B.; Rizvi, A. A.; Brown, I. S.; Ainsworth, S. Spectrochin. Acta 1986, 40B(No. 5/6), 739. (11) Hazell, L. B.; Brown, I. S.; Freisinger, F. Surf. Interface Anal. 1986, 8, 25. (12) Sian Crawford, E.; Evans, S.; Raftery, E.; Scott, M. D. Surf. Interface Anal. 1983. 5-,. 28. -(1s) Petty, M. C . Polymer Surfaces and Interfaces; Feast, W. J., Munro, H. S., Eds.; Wiley: New York, 1987; p 163.
---.
125
120
115
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Figure 1. Curve-resolved peaks for the A1 2s peak at 3" (A) and 90" (B) showing the enhancement of the bulk aluminum signal (-117 eV) at higher ETOAs compared to the signal from the aluminum in the oxide layer (- 119.5eV). ments were performed in which the electron takeoff angles were varied between 3" and normal to the surface in intervals of approximately constant sin 0. A1 Ka excitation was used at 280 W (14 mbar. At a kV, 20 mA) power at a pressure of less than fixed analyzer energy of 50 eV, each region of the spectrum was repeatedly scanned at 0.1eV/s until a satisfactory signal to noise ratio was obtained. This generally involved total data acquisition times of -5 h, the majority of this spent on the low takeoff angle data. The apparent compositions were obtained from the peak areas obtained at each angle by using elemental sensitivity factors of 0.23, 0.25, 3.5, and 0.66 for A1 29, C ls, Cd 3d,,,, and 0 1s re~pectively.'~The spectra energy scales were calibrated by reference to the C-H peak at 285 eV. Curve resolving was used to define the relative contributions to the total aluminum intensity from the aluminum metal and the aluminum present in the native oxide layer, as illustrated in Figure 1. This shows the relative enhancement of the aluminum oxide contribution with respect to the contribution from the deeper bulk aluminum as the ETOA is reduced from 90" (Figure 1B) to 3" (Fig(14)Wagner, C. D.;
Davis, L. E.; Zeller, M. V.; Taylor, J. A,; Ray-
mond, R. M.; Gale, L. H. Surf. Interface Anal. 1981, 3, 211.
Langmuir, Vol. 6, No. 3, 1990 531
Characterization of L-B Films with XPS ure 1A). Curve resolving of the carbon peak was also performed. This showed that the signal was dominated by the CH signal with very small contributions from the C-0 species. Curve resolving the 0 1s multiplet proved to be problematic. We attribute this to the complexity of a spectrum containing a varying composition of oxygen components with slightly different binding energies. These oxygen-containing species include oxide/ hydroxide from the aluminum substrate, carboxylate/ acid groups from the 22-tricosenoicacid, chemisorbed (not liquid) water, and possibly some organic contamination. All the spectra can be resolved into two basic components. However, the peak position of the low-energy component varies systematically from 530.2 f 0.1 to 531.1 f 0.1 eV as the ETOA increases. The higher energy component shifts in a similar way from 532.2 0.1 to 532.7 0.1 eV. Generally, the low binding energy component can be assigned to a mixture of substrate oxide/ hydroxide and the carboxylate/acid groups of the 22tricosenoic acid film. The high-energy component is chemisorbed water, possibly containing some organic contamination. The information has been of use in constructing the models but cannot be justified on the evidence of the curve resolving alone. 3. Modeling Procedure. The modeling procedure has been described previously.'l In brief, it involves fitting a theoretical model to the apparent compositions (using all the element signals) measured at each angle. A preliminary layer model is constructed with a set of initial values for the thicknesses, composition, and number densities. In the absence of a rigorous mathematicaltreatment of definingthe "uniqueness" of the model, our approach has been to repeatedly remodel the data with different initial parameters. These parameters are then iteratively varied to obtain a reasonable fit between the model and experimental data. The criterion for improvement in each cycle is a lowering of the overall unweighted fit index while creating a model that is plausible and makes physical sense. For fits better than 2% RMS (5% for low-composition elements), the variation in model compositions and layer thicknesses is less than 10% overall. The initial number densities were calculated by using eq 1 to obtain the atom number densities, N,, assuming the layers were perfect
*
ND= (NAPZ)/MW (1) where N A is Avogadro's number, p is an appropriate density for each layer, Z is the number of atoms per molecule, and MW is the molecular weight of the molecule. Appropriate values must be estimated iteratively, as a suitable density for each layer cannot be defined until the layer compositions have been established. For the hydrocarbon layers, the values used were obtained by assuming an aliphatic acid density of 0.85 g/cm3. For the substrate metal and surface oxide, values of aluminum and appropriate aluminum oxides were used. For display purposes, the composition of each layer is renormalized to equal number densities. The film stoichiometries can be calculated from the total number of atoms of each element present, NE, where
N E is the total number of atoms of element E, N o is the number density pertinent to layer D, and ,C is the renormalized concentration of element E in layer D of thickness t,. The values of the IMFPs were arrived at through a similar iterative process, initially based on literature values15 and previous work.lO~llIn the case of the bilayer, these values were then varied so that the total layer thickness correlated with that obtained by using ellipsometry (6.3 0.3 nm). In order to check the suitability of the final set of IMFP values, these were held constant in the modeling of the monolayer results, thus allowing the estimates of film depth from the two techniques to be compared. The modeled overall thickness of the monolayer was found to be -3.7 nm, which compares favor-
*
(15)Seah, M. P.; Dench, W. A. Surf. Interface Anal. 1979, 1 , 2.
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Figure 2. (A, Top) Bilayer carbon and oxygen intensities plotted against electron takeoff angle. Dashed lines refer to the data generated by using the model; points refer to the experimental results. (B, Bottom) Curve-resolveddata for the bilayer aluminum and aluminum oxide intensities plotted against electron takeoff angle. Dashed lines refer to the data generated by the model; points refer to the experimental results.
ably with the 3.9 f 0.3 nm value found from ellipsometry. This close agreement indicates that the optimized IMFP values are reasonable. It was found necessary in this work to include up to 10% of the substrate elements, aluminum and aluminum 3+ from the metal oxide, in the surface layers. This improves the fits significantly and is thought to be a consequence of the need to model the surface roughness and the layer patchiness in terms of homogeneous layers. This does not affect the overall interpretation of the results. The carboxyl groups were not detectable at sufficiently high intensities to allow independent modeling and so were modeled as part of the total carbon and total oxygen. 3. Results 1. The Bilayer. The experimentally determined apparent compositions for the bilayer as a function of electron takeoff angle are shown in parts A and B of Figure 2, compared with the respective best fit obtained from the model. The film structure for the bilayer derived from this modeling procedure is given in Figure 3 (not drawn to scale). The structure is that expected for Y-type deposition of a bilayer onto a hydrophobic surface with the hydrocarbon chains next to the substrate. This should be compared with the results obtained for the monolayer (see Figure 4), where deposition has occurred with the acid head groups next to the surface, as would be expected for Y-type deposition onto a hydrophilic surface. In the bilayer structure (Figure 3), layers three and seven represent the hydrocarbon tails of the 22-tricosenoic acid. Assuming a bond angle of 109.5' a C-C bond length of 0.154 nm, and a C-C bond length of 0.134 nm,16 a total chain length of 21(0.154 sin 54.75) 0.134 = 2.78 nm would be expected for a fully extended chain. This compares to thicknesses of 2.5 and 2.9 nm for lay-
+
(16)The Handbook of Chemistry and Physics; Weast, R. C., Ed.; CRC Press: Cleveland, Ohio.
532 Langmuir, Vol. 6,No. 3, 1990
Cave et al.
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Figure 4. Modeled monolayer structure for 22-tricosenoic acid. The figures in parentheses refer to the total number densities used for each layer (not drawn vertically to scale). ers three and seven, respectively. Layer thicknesses of 0.4 nm were found to be necessary to model the carboxylate head groups. It should be noted that a number of worker^',^,^ using FTIR and RHEED techniques have reported on a different packing for the hydrocarbon chains in the first layer of a fatty acid LB film: this would be expected t o give a slightly different thickness for the first layer than for subsequent layers. T o model the experimental data successfully, an interfacial layer of water with a thickness of 0.2 nm is needed where a layer of cadmium counterions would have been expected. Although some cadmium was detected, no cadmium-rich layer could be modeled at the expected sites in either the bilayer or monolayer films. T h e data in Figure 5 show that the cadmium that was detected was present at the outermost surface of the film. I t could be modeled as a small contribution, -0.3%, in a very thin,
0.1-nm surface Contamination layer. T h e le\& of cadmium were too low to be included in Figurr 3. The high uxygen content of this outer layer indicates that i t is srrongly adsorbed water. probably containing some cadmium oxidr hydroxide. I.ayer 2 has the stoichiometry of AI,O, and represents 2.0 nm of air-furmed oxide on the evaporated aluniinum. A small amount n t AI metal and AI3+ has tu be included in every layer ot both films to account fur the effects of surface roughness (which is signiiicant on this thickness sralei and also iilni patchiness. F n ~ mthe iterative procedure described in section 2.3, the IMFP values used were 2.7 nm i o r the aluminum 2p photoelectron peak at 1.110 eV, 2.1 nni for t h r oxygen Is peak at 955 eV, and 2.8 nm fur the cartion 1s peak at 12U1 eV. These IMFP values are similar t o thuse used in earlier work"'." and are well within the wide scatter of the data compiled by Seah and Denrh.'' The stoichiometry uf the organic material in the bilayer was C,,O,,. T h e deviation from the theoretical value of C 2 , 0 , is probably due to a combination uicntrapped water in rhe hydrocarbon layers increasing the oxygen content and also errors in the relative sensitivity factor values used. 2. The Monolayer. In order to model the monolayer, the optimized parameters developed for the hilayer film were employed. In the resulting structure. shown in Figure 4, the layer representing the hydrocartion tail for the monulayer is 3.U nm thick. In a similar way t o the bilayer, an uxygen IwaterJ rich "contamination' layer containing low levels oi cadmium. -0.4%. is required on the outermost surfare to model the data adequately. The arid head groups are reprrsentrd by a 0.4-nm-thick layer, hut this is slightly richer in oxygen than was necessary for the hilayer. A 0.2-nm-rhick interfacial -water" layer is again found to be necessary in order t o model rhe oxygen data successfully. The calculated stuichiometry of the film was C,0,6. again suggesting water entrapment in the hvdrorarhun chains. It is evident from Figure 4 that the surface oxide on the aluminum has a higher oxygen conrent than expected fin pure A1,0,,. In addition, the oxide thickness has increased to 2.5 nm. This suggests rhar while the unprotected suhstrate was immersed in the aqueous phase prior to film deposition conversion of some of the outermost oxide layers ro an AIOOH phase has uecurred. 8. Sample Degradation Kffecls. Hlrzell et al." indicated in a previous study rhnr I.-Ii films ofstearic acid may not he stahle under prnlonged in vacuo X-ray irradiation. Brundle et a1.I- in their studies of L-R films of cadmium ararhidate I(~H,,I(IH,~,,CO,~;(:~~+ on a vari(17) Brundle, C. 70, 5190.
R.;Hopster, H.; Swale", J. D.J . Chem. Phys. 1979,
Langmuir, Vol. 6, No. 3, 1990 533
Characterization of L-B Films with XPS ety of substrates reported a decrease in the carbon signal and an increase in the substrate signal. To monitor the possibility of sample degradation in the bilayer case, the low-angle spectra were acquired first and then the higher angles, and finally a low-angle experiment was repeated. The bilayer exhibited an increase in the carbon (-5%) and substrate aluminum intensities (-0.5%) at the expense of the oxygen (-5%) and cadmium signals (-0.1%) in the repeated low-angle experiment. The results for the monolayer mirrored this trend, but the differences were smaller, as several scans had been made during the setting-up process before the low-angle experiments were performed. In a separate fixed angle (0 = 65O) experiment, the cadmium was found to decrease from 0.66 to 0.25 atom 5% after 1000 s of irradiation. This was accompanied by a -2.5 atom % reduction in the oxygen signal and a -1.5 atom % increase in both the carbon and the substrate intensities. These results indicate that most of the changes appear to occur in the initial stages of the experiment, and in view of the presence of a thin cadmium- and oxygen-containing contamination layer, we are able to attribute this effect to the sublimation of this contamination layer and then a slower “drying” out of the entrapped water in the films. Other than these minor changes, there was no evidence to indicate that significant degradation had occurred in the bulk of the films during these experiments. 4. Discussion
The results detailed above have confirmed that the structures of the monolayer and bilayer films are consistent with a Y-type deposition mechanism. Within the constraints of the modeling procedure, there appears to be a combined contribution of -10% to film imperfection from effects such as surface roughness and film patchiness. In view of the thickness of these films, it is not surprising that surface roughness is a cause of apparent film imperfection. The model also allows comparison of the experimental results with the predicted results for “perfect” films. This is illustrated for the monolayer carbon compositions in Figure 6. By analogy with aerial photographs of a conifer forest, the deviation may allow distinction between patchiness of the deposited films and substrate roughness. Directly beneath the photographer, the ground (substrate) can be observed between the trees (L-B fim) where firebreaks or roads have been cut (film patchiness). However, any cliff edges where land subsidence has taken place (surface roughness) cannot be seen. At grazing viewing angles, the firebreaks and roads are no longer visible due to shadowing, but the cliff faces are now exposed. The relative deviation between the theoretical and experimental curves for the L-B films is greatest a t low ETOAs, which suggests that surface roughness effects dominate. The presence of a monomolecular layer of water in the interfacial regions and the failure to detect cadmium in these layers strongly indicate that the films have been deposited as the free acid. Recent work by Davies and Yarwood,’ who used ATR to study films of 22-tricosenoic acid on silicon, failed to detect the presence of water. They did, however, note that they used a hydrated silicon crystal to record the reference spectra, and so the water component may have been lost during the subtraction process. We suspect that entrainment of water is related to the film dipping rates, but since inevitably the films “dry out” it is a difficult feature to study with XPS under ultrahigh vacuum. The cadmium that was detected was present a t low levels of - 0 . 3 4 5 % in the outermost contamination layer.
-
0 0
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30 40 50 60 ETOAIDegs.
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80
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Figure 6. Comparison of the experimental monolayer carbon intensities and the intensities that would be expected if the film was perfect. The solid line refers to the intensity expected for a “perfect”film, the dashed line is the data generated by the model, and the points refer to the experimental data. This result is in conflict with the results of Brundle et al.,17 who reported the detection of stoichiometric levels of cadmium in L-B films of cadmium arachidate 1-5 monolayers thick. Oshnishi et a1.l’ have also reported the detection of high levels of cadmium in films of cadmium arachidate. A later study by Clark et al.” on films of cadmium stearate and the cadmium salt of a diacetylene polymer derived from pentacosa-10,12-diyonic acid also found that the films were essentially deposited as the cadmium salt. However, they found that films of 9-nbutyl-10-anthrylpropionic acid were deposited largely as the free acid in spite of cadmium ions being present in the subphase. Pitt and Walpitta2’ have reported a correlation between the subphase pH, the percentage of cadmium included in the films, and the refractive index of multilayer films of cadmium stearate. Support for our conclusion that cadmium has not been included in the interfacial layers in these films is provided by the work of Evans et These workers from Durham University have studied films similar to those used in the present work and reported that films of 22-tricosenoic acid on silicon substrates exhibited no differences between the charge incorporation properties of films deposited with and without cadmium ions present in the subphase. It is clear from the above discussion that the inclusion of metal ions in the deposited films is dependent on a range of parameters which may include chain length, subphase pH, and metal ion concentration in the subphase. The presence of an adsorbed contamination layer which contains the metal ions may, however, have resulted in an overestimation of the degree of metal ion inclusion in some previous XPS studies of L-B films. The successful Y-type deposition of the films on the aluminum substrate is interesting as no precautions were taken to ensure that the surface was hydrophobic or hydrophilic before film deposition. The Y-type bilayer was most probably formed because the aluminum oxide surface was hydrophobic in nature. In the case of the monolayer, the immersion of the substrate in the aqueous phase appears to have generated a hydrophilic surface. As previously pointed out, this was also accompanied by a change in the oxide composition and an increase in the thickness of the oxide, indicating that the outermost surface hydrated, forming A100H. The IMFP values obtained in this work are in general agreement with previously published values. Seah and (18) Ohnishi, T.; Ishitani, A.; Ishide, H.; Yamanoto, N.; Tsubonura,
H,J. phys. them. 1978,82,1989,
(19) Clark, D. T.; Fok, Y. C. T.; Roberta, G. G. J. Electron Spectrosc. Relat. Phenom. 1981,22,173. (20)Pitt, C. W.; Walpitta, L. M. Thin Solid F i l m 1980,68! 101. (21)Evans,N. J.; Petty, M. C.; Robe&, G. G. Thin Sold Films 1988,160,177.
534 Langmuir, Vol. 6, No. 3, 1990 Dench15have produced a compilation of IMFPs in which the range of values for carbon in organic compounds varies between 1.8 and 8 nm. Brundle et al.17 have proposed a value of 3.6 nm for the 1117-eV Ag 3d electron. This was obtained by measuring the substrate intensity attenuation by one, three, and five monolayers of cadmium arachidate. Clark and Thomas22have derived IMFP values in poly(p-xylylene) by measuring the attenuation of the substrate Au 4f signal and the growth of the C 1s signal as a function of depth. They obtained values of 1.4, 2.2, 2.3, and 2.9 nm for the 969-, 1170-, 1202-, and 1403-eV electrons, respectively. However, as Brundle et al. point out, the use of extrapolation techniques in the measurement of the film thicknesses means that these values must be regarded as approximate. In a later study, Clark et a1.I’ derived an IMFP of 4.5 nm for 1170-eV electrons using the same overlayer technique as Brundle et al. They used one, three, and five layers of cadmium stearate deposited onto gold, and the film depths were checked by using capacitance measurements. In the same study, Clark et al. also derived values of 5.7 and 7.0 nm for films of a diacetylene polymer derived from pentacosa-10,12-diyonic acid and for 9-n-butyl-10-anthrylpropionic acid, respectively. Cadman et al.23have also measured the IMFP values for a range of bulk polymeric and fatty acid compounds samples with gold as the reference sample. Using a value of 1.4 nm for the IMFP of gold at 1167 eV,24their data yield IMFPs of 3.7-7.2 nm at 967 eV. However, Brundle et al. noted that they used an experimentally derived value for the Q (Au 4f)/o(C Is) ratio and that the use of the theoretical value for this ratio reduced the range to 2.6-5.1 nm. Measured IMFP values through organic films are generally in the range 2.5-6.0 nm. The spread of results within these limits reflects, we believe, the different nature of the films studied. Clark et al.” concluded that the increase in the IMFP values for the series cadmium stearate < diacetylene polymer (Cd salt) < 9-n-butyl-10-anthrylpropionic acid reflected a corresponding decrease in the relative packing densities. This deviation in the IMFP Values obtained by Clark et al.” for cadmium stearate (4.5 nm), by Brundle et al.” for cadmium arachidate (3.6 nm), and by ns for 22-tricosenoic acid (2.8 nm) in the current study may be explained if increasing chain length results in more efficient packing and higher film densities. using high-energy Ag La and Ti Ka radiRecent tions to study the thickness of silicon oxide on silicon substrates has yielded IMFP values of 3.01 nm at 1610 eV and 4.48 nm at 2661 eV. These values are plotted in Figure 7. Linear extrapolation of the data to lower energies shows that our values for the A1 2p and 0 1s electrons, which are most likely to be dominated by attenuation in the oxide layer, are in accord with this data. As expected, the IMFP for the C 1s electron is above this line since it is dominated by attenuation through organic material. The values we have obtained are well within the scatter of the data from the references discussed above and are shown in Figure 7. The scatter in this data may be due to effects such as the film pressure and possibly the chain length, which affect the packing density of the chains and may therefore contribute to the variations in the IMFP values published to date. A study in which (22) Clark, D. T.; Thomas, H. R. J . Polyrn. Sci., Polyrn. Chern. Ed. 1977,15, 2843.
(23) Cadman, P.; Evans, S.; Scott, J. D.; Thomas, J. M. J. Chern. SOC.,Faraday Trans. 1975, 7, 1177. (24) Cadman, P.; Gossedge, G.; Scott, J. D. J . Electron Spectrosc. Relat. Phenorn. 1978, 13, 1. (25) Kratos Analytical, Manchester, U. K. Applications Note No. A102.
Cave et al.
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Figure 7. Plot of literature inelastic mean free path values against kinetic energies: 0 , ref 17; W, ref 19; 0,ref 22; A, ref 24; 0,ref 25; A,this work. The double-headed arrow indicates the spread in the results (detailed in ref 24).
the film pressure is held constant and the chain length varied or the film pressure varied for a constant chain length would be of great value in resolving this question. The technique described in this study has a significant advantage over previous methods used to determine A. Most of these involved the measurement of the attenuation of a single substrate signal by an overlayer of assumed, but uncharacterized, structure and thickness. The advantage of the modeling technique applied to ETOA experiments is that the structure/thickness of the adsorbed layers is determined, the signals from all the elements present in the surface are used to develop a model, and X values are self-consistent. We have confidence in our results because the film structures make physical sense, and the monolayer film thickness obtained by using the model agreed with that obtained by ellipsometry. 5. Conclusions
The current study has confirmed the value of computer modeling of electron takeoff angle experiments in XPS for the investigation of both elemental composition and physical structure of the near-surface region of a flat solid. The technique possesses the advantage that the location of an element can be defined and so complements the detailed information available from reflectance FTIR and electron diffraction on the head-group interactions and chain packing and orientation. For the LB films examined, a Y-type bilayer is deposited on the “as prepared” aluminum substrate, which has a hydrophobic A1203 oxide surface. In the case of the monolayer, the immersion of the substrate in the aqueous phase generates a hydrophilic surface. This is also accompanied by a change in the oxide composition and an increase in the thickness of the oxide, indicating that the outermost surface hydrated, forming A100H. The hydrophilic nature of the surface facilitates the deposition of a Y-type monolayer. The “degree of perfection” of the films is -90% and is in general limited by the substrate roughness with some patchiness. Failure to detect cadmium in the interfacial layers suggests that the films are not deposited as the acid salt but as the undissociated acid. An interfacial chemisorbed water layer is thought to be responsible for the stabilization of the films. IMFP values of 2.8, 2.7, and 2.1 nm were deduced as optimum for the C 1s electron at 1201 eV, the A1 2s electron at 1410 eV, and the 0 Is electron at 955 eV, respectively. Future work using films deposited onto a range of substrates will allow better definition of the IMFP values appropriate to organic substrates for a range of kinetic
Langmuir 1990,6, 535-538 energies and hopefully clarify the reasons for the deviations in the published data.
Acknowledgment. We thank Dr. M. C. Petty a t the University of Durham for providing the L-B films and for valuable discussions of the results. Thanks are also due to Professor M. Green, Imperial College, for the use of the ellipsometer and Dr. D. L. Perry for valuable dis-
535
cussions of the work and his critical appraisal of the manuscript. We would like to thank the British Petroleum Company plc. for the provision of facilities and permission to publish this work. N. Cave would like to thank both BP and the SERC for their financial support. Registry No. C2302,65119-95-1; Cd, 7440-43-9; Al, 7429-90-
5.
In Vitro Calcification: Effect of Molecular Variables of the Phospholipid Molecule Evangelos Dalas, Panayiotis V. Ioannou, and Petros G . Koutsoukos* Department of Chemistry and Research Institute of Chemical Engineering and Chemical Processes a t High Temperatures, P.O. Box 1239, University Campus, GR-26 110 Patras, Greece Received J u n e 22, 1989. I n Final Form: September 20, 1989 Acidic phospholipids are known to promote biological mineralization through the formation of phospholipid-calcium-inorganic orthophosphate complexes. The role of the fatty acyl and cation content of a simple acidic phospholipid, phosphatidyldiacylglycerol, regarding its capability of inducing hydroxylapatite precipitation was investigated. It was found that the induction times preceding hydroxylapatite precipitation and the subsequent rates of precipitation were strongly affected by the cationic charge, the Pauling radius of the cations contained in the salts, the nature of the acyl content, and its stereochemistry. Hydroxylapatite growth on the phospholipids took place by a polynuclear, surface nucleation mechanism.
Introduction Under physiological conditions, the thermodynamically most stable calcium phosphate salt which is formed in the calcification processes is hydroxylapatite (Ca,(PO,),OH, HAP). A number of more soluble calcium phosphate phases such as tricalcium phosphate (Ca,(PO,),, TCP), octacalcium phosphate (Ca,H(PO,),. 2.5H20, OCP), and dicalcium phosphate dihydrate (CaHP0,.2H20, DCPD) may be formed in supersaturated calcium phosphate solutions. In several cases, the ionic activities of calcium and phosphate ions in biological fluids are sufficiently low as to preclude the formation of any precursor phase other than HAPS1 There is strong evidence that HAP formation in the vertebrates is induced by solvent-extractable complexes, which are composed of acidic phospholipids (PLs), calcium, and phosphate. The acidic PLs, which are known to form such complexes, are phosphatidylserine (PS), phosphatidylinositol (PI), and cardiolipin (CL). The simplest acidic PL, phosphatidic acid (PA), does not form such c~mplexes.~ A five-step mechanism for the formation of the phospholipid-calcium-phosphate complex (PL-Ca-PI) and the subsequent induction of HAP formation have been proposed.2 In this plausible model, the nature of the fatty (1)Newman, W. F.;Neuman, M. W. Chemical Dynamics of Bone Mineral; University Press: Chicago, 1958; p 137. (2)Wuthier, R. E. In The Role of Phospholipid-CalciumPhosphate Complexes in Biological Minemlization;Anghileri, L. J., JuffetAnghileri, A. M., Eds.; CRC Press: Boca Raton, FL, 1982;pp 41-69.
acyl chains and the cation initially bound to the acidic PL, PS, was not considered, implying that these variables may not play a significant role in the induction and the rate of the concomitant HAP formation. Experimental evidence concerning the role of Ca2+ and Mg2+ ions on the HAP and PL-Ca-PI complex formation, respectively,3i4has been considered in Wuthier's model,2implying that in PL-Ca the Ca2+ ion is in an "inactive" state, in the sense that PL-Ca salts do not induce HAP formation. The PL-Mg salt showed more or less the same effect. Bisphosphatidic acid or phosphatidyldiacylglycerol (PDG) are scarce PLs. It has been identified in the yeast Candida tropicalis grown on n-alkanes, where it accounted for 4.5% of the total P L s , ~and in trace amounts (0.02% of the total PLs) in BHK fibroblasts: where it was localized in the lysosomes.' The stereochemistry of this lipid can be sn-3,3-PDG, as that isolated from developing soybean: or sn-3,l-PDG (not found) or sn-1,l-PDG, as that found in BHK cells.g The effect of phospholipases on synthetic sn-3,3-PDG has been studied." Although PDG is a minor PL, its simple structure makes it an attrac(3)Boskey, A. L.Metab. Bone Dis. Rel. Res. 1978,I , 137. (4)Boskey, A. L.;Posner, A. S. Calcif. Tissue Znt. 1980,32,139. (5)Dyatloviskaya,E.V.; Greshnykh,K. P.; Bergelson,L. D. Biochemistry (Engl. Transl.) 1968,33,68. (6)Brotherus, J.; Renkonen, 0. Chem. Phys. Lipids 1974,13,11. (7)Somerharju, P. Biochim. Biophys. Acta 1979,574,461. (8)Morton, W.T.;Stearns, E. M., Jr.; Schmid, H. H. 0.Lipids 1977, 12, 1083. (9)Somerharju, P.; Brotherus, J.; Kahma, K.; Renkonen, 0. Biochim. Biophys. Acta 1977,487,154. (10) de Haas, G. H.; Bousen, P. P. M.; Van Deenen, L. L. M. Biochim. Biophys. Acta 1966,116,114.
0 1990 American Chemical Society
