3616
J. Phys. Chem. 1983, 87,3616-3619
Unusual Monolayer Behavior of a Geminally Disubstituted Fatty Acid. Characterization via Surface Plasmons and X-ray Photoelectron Spectroscopy Study Charles A. Brown,’ F. C. Burns,+ W.
J. D. Swalen,’
IBM Research Laboratory, San Jose, California 95 193
and A. Fischer Physik Department €22, Technische Universltat Miinchen, D-8046 Garching, Federal Republic of Germany (Received: October 25, 1982; In Final Form: April 6, 1983)
Preparation and characterization of Langmuir-Blodgett films of a geminally branched lipid acid have been achieved for the first time. Pressure-area isotherms of cadmium a,a-dimethylarachidate (2,2-dimethyl-l-eicosanoate) exhibit two condensed phases, one at an area consistent with a single layer expanded somewhat from cadmium arachidate by the methyl groups, the other at a much smaller area which could be consistent with trilayer formation. The isotherms are quite distinct considering the discontinuity in packing expected a priori by the presence of the geminal dimethyl substitution. Transfer to a solid surface from the air-water interface was accomplished at the less condensed phase and the resulting films were characterized as a monolayer, slightly expanded relative to cadmium arachidate. This characterization was made by a combination of two techniques: attenuated total internal reflectance shifts in the excitation of surface plasmons and electron mean free path of escape from X-ray photoelectron spectroscopy;both techniques were required as neither alone gave complete understanding. Transfer at the more condensed phase produced a layer somewhat thicker than that produced at the less condensed phase, but not as thick as expected from film area; expansion upon transfer apparently occurred, and this prevents full characterization. However, the more condensed phase is reproducible and is apparently a stable phase. Preparation of the substituted arachidic acid by an unambiguous route and characterization of the monolayer assemblies using the two techniques are discussed.
Introduction Monomolecular layers (Langmuir-Blodgett films) of lipids are of interest in a variety of applications including the preparation of very thin controlled films for interfaces in solid-state electronic devices.’I2 The presence of a site radiation-induced polymerization has been used to provide increased film stability and has been described as an application for high-resolution electron beam lithography for So~ ~far only polymthe fabrication of m i c r o c i r ~ u i t r y . ~ erizable sites have been described, and monomolecular layers involving radiation-cleavable sites appear unreported. One approach to such a site is the inclusion of a quaternary center and/or a heavy element in the chain to enhance electron beam and X-ray cross section. Essentially nothing appears to be known about the film-forming properties of lipid acids containing a heavy element such as Si, Sn, Ge, or Pb in the chain. Because of the lability of M-H bonds to oxygen, substitution by methyl groups is essential. Such substitution produces a “bulge” in the chain and could be anticipated to profoundly disrupt packing. Herein we report on the first study of the film formation of a lipid acid containing such a quaternary site. There has been substantial investigation of the effect of deviation from saturated straight chains upon the monolayer-film-forming tendencies of lipid The common deviations in structure studied have been unsaturation along a chain in, for example, a fatty acid or substitution along the chain, such as hydroxyl groups. Singly branched carbons have been incorporated at one or more points (e.g., 2-methylstearic acid, phytanic acid) along the chain, but the effect of a doubly branched carbon IBM Postdoctoral Fellow 198C-1981. IBM World Trade Postdoctoral Fellow 1980-1981. Permanent address: Physik Department E22, Technische Universitiit Munchen, D-8046 Garching, Federal Republic of Germany.
apparently has not been reported. Such a branch (e.g., in 2,2-dimethyleicosanoic acid, i.e., a,a’-dimethylarachidic acid) eliminates the possibility for chirality found in 2methylalkanoic acids but in models appears to increase hindrance at the branch point and might be expected to produce a skewing of the chain conformation (Figure l), thus interfering with packing of the condensed phase. Consequently, monolayer formation of such a doubly branched structure may be unusual and quite different from other monolayer assemblies. Synthesis and monolayer studies of a geminally branched acid-a,a-dimethylarachidic acid-were undertaken to assess the feasibility of forming stable films in such a system, films which should possess enhanced radiation sensitivity and which provide a model for ones from acids containing the M(CH& subunit. The monolayer behavior on a water surface was observed by the pressure vs. area isotherms. Surprisingly, the quaternary center did not markedly disturb formation of monomolecular films, and the films formed and transferred to a solid surface were studied by optical and X-ray photoelectron spectroscopic measurements. As will be elaborated below, unexpected phenomena were encountered, most notably the (1) G. L. Gaines, Jr., “Insoluble Monolayers at Liquid-Gas Interfaces”, Interscience, New York, 1966. (2) P. S. Vincett and G. G. Roberts, T h i n Solid Films, 68, 135 (1980). (3) A. Barrand, C. Rosilio, and A. Ruandel-Teixier. Solid S t a t e Technol., 22, 120 (1979); J. Colloid Interface Sci., 62, 509 (1979); Thin Solid Films, 68, 91, 99 (1980). (4) M. K. Nagarajan and J. P. Shah, J. Colloid Interface Sci., 80, 7
(1981).
(5) B. M. J. Kellner and D. A. Cadenhead, J. Colloid Interface Sci., 63, 452 (1978). (6) B. M. J. Kellner and D. A. Cadenhead, Chem. Phys. Lipids, 23,41 (1979).
( 7 ) I. M. Jalal, G. Zografi, A. K. Rakshit, and F. D. Gunstone, J. Colloid Interface Sci., 76, 146 (1980).
0022-3654/83/2087-36 16$01.5010 0 1983 American Chemical Society
The Journal of Physical Chemistry, Vol. 87,No. 19, 1983 3617
Behavior of a Geminally Disubstituted Fatty Acid
R
R
H
C
C
C
' 0 '0
' 0 '0
0 ' '0
iai
(bi
IC1
= CH,
Flgure 1. Conformations of staggered arachidate ion (a), staggered dimethylarachidate ion (b), gauche dimethylarachidate ion (c).
apparent existence of a reproducible, highly condensed phase exhibiting an unusually small molecular area. Experimental Section The sample of a,a-dimethylarachidic acid (2,2-dimethyleicosanoic acid) was prepared by alkylation of the dianion of isobutyric acid8 with carefully purified l-iodooctadecane. This route was chosen to avoid the ambiguities and purification problems associated with introduction of the methyl groups into arachidic acid. After purificationgaa white solid (mp 64-5 "C) was obtained which possessed analytical and spectroscopic datagbcompletely consistent with the assigned structure. GLPC analysis (of the methyl ester) showed the material to be >99.5% pure. Solutions (in hexane-ethyl alcohol, 91) of arachidic acid (HAA) and a,a-dimethylarachidic acid (u(CH3)zAA)were spread on an air-water interface. The aqueous phase consisted of M CdCl, solution at pH 6.3 and temperature control to better than f0.5 "C. The air in the air phase was filtered and temperature controlled. The experimental apparatus and procedure have been described previously.'JO Calibration of the pressure-area isotherms was performed with palmitic acid. At a lateral pressure of about 30 dyn/cm, monolayer assemblies of cadmium arachidate (Cd(AA),) and cadmium a,a-dimethylarachidate (Cd((CH,),AA),) were transferred to substrates by using the Langmuir-Blodgett technique' at (8) P. L. Creger, J. Am. Chem. SOC.,89, 2500 (1967). (9) (a) In a 125-mL round-bottom flask (oven dried and purged with argon and equipped with a septum closure and a TFE-covered magnetic s t i r r i i bar) were placed 25 mL of dry tetrahydrofuran and 20 m o l (2.02 g) of dry diisopropylamine. With stirring and ice cooling, 12.5 mL of 1.6 M n-butyllithium in hexane was added dropwise over a 5-minperiod; the mixture was then stirred at room temperature for 2 h to complete the formation of lithium diisopropylamide. To this clear solution at 0 "C was added isobutyric acid from a freshly opened bottle (10 mmol,O.88 g). The reaction mixture was stirred for 2 h to complete the formation of the dianion, and then 10 mmol (3.60 g) of n-octadecyl iodide, twice recrystallized and dissolved in 10 mL of tetrahydrofruan, was added. The resulting mixture was subsequently stirred overnight at 25 "C and the poured into a separatory funnel containing 50 mL of 10% H$04 and 100 mL of ether. The organic phase was washed with a 15% NaCl solution and dried for 30 min over MgSO1. Evaporation yielded 3.20 g, i.e., 94%, of a crude off-white solid. This was purified by flash column chromatography on SiOz with a solvent mixture of 5% (1:3 acetic acid-ethanol) and 95% (1:2 ether-hexane). After evaporation, 2.93 g of a white solid was obtained at a yield of 86%, with a melting point of 63-64 "C (sintering at 60 "C). Recrystallization from ether-pentane gave a solid with melting point of 64-65 "C (sintering at 60 "C). (b) Infrared spectroscopy of the melt at 80 "C gave bands at 34W2400 (OH stretch) and 1709 (C=O stretch) cm-'. Proton NMR at 80 MHz (solvent CDC1, with MelSi reference) gave resonances at 6 0.88 for the terminal CH, group and 6 1.26 for the chain methylene group. Carbon-13 NMR at 50 MHz (solvent CDC13 with Me4Si reference) showed resonances at 6 183.1 for the carbonyl carbon; at singlet at 6 42.1 for the quaternary carbon; triplets at 6 40.7, 32.0, 30.2, 29.7, 29.5, 29.3, 24.9, and 22.7 for the methylene carbon chain; the quartets at 6 25.0 for the geminal methyl groups and 6 14.0 from the terminal CH3. Based on the areas of the signal assigned to the quaternary and the terminal methyl carbon (using gated decoupling to suppress NOE), the total assigned area of the spectrum, exclusive of the carbonyl carbon, was ca. 21 carbons. Anal. Calcd: C, 77.58; H, 13.02. Found C, 77.69; H, 12.89. We are indebted to Mr. Mark Johnson of IBM Instrument Systems for obtaining the high-field 13C NMR spectra. (10) 0. Albrecht and E. Sackmann, J.Phys. E, 13, 512 (1980).
I
6°C
I~
0
40
20
60
Area a l A 2 Figure 2. Pressure vs. area for cadmium dimethylarachidate on a water surface at five different temperatures. The curves have been displaced upward for clarity. Note that, starting at large areas, the liquid transition Is encountered at 40 A2, and then less condensed phases appear at 20 and 22 A2. The flrst probably occurs when the methyl groups first touch and the second might involve a rearrangement of the methyl group, is., relative displacement and/or tilt. Finally, the more condensed phase is reached at 7 A2 where molecules may have been forced out of the flrst layer and possibly flipped over to form a trilayer system.
room temperature. These substrates were glass slides (n = 1.516) coated with 500 A of silver by evaporation. Attenuated total reflectivity (ATR) measurements were performed with a 6328-A line of a He-Ne laser and prism coupling in the Kretschmann cod1guration.l' The angular scans of the reflectivity were performed under computer control, as described earlier,', and optical thicknesses of the dielectric overcoatings (see Discussion) were determined by fitting the experimental curves with our standard computer programs.12 Silver films for the XPS studies were prepared by evaporating lo00 A of Ag onto clean glass slides, which had been cut to fit into the XPS spectrometer's sample holder. Monolayers of Cd(AA)2and Cd(CH3)2AA)2were transferred onto the silver films as described in the previous paragraph. XPS spectra were taken on an HP 5950B spectrometer using monochromatized A1 K a radiation. Each time XPS data were to be collected, an untreated silver film,a silver film with n monolayers of Cd&Q2, and a silver film with n monolayers of Cd((CH3),AA), were mounted on the sample probe. It was found that an electron flood gun current of 0.04 mA was sufficient to prevent any charging effects. During data collection, the samples were kept at 15 f 1 "C. For each of these samples, the Ag 3d5,2 core level was measured and repeated measurement of this signal indicated that there was no deterioration of the sample during data collection. The geometric arrangement between the incident X-rays and (11) E. Kretachmann and H. Raether, 2. Nuturforsch. A, 23, 2135 (1968). (12) I. Pockrand, J. D. Swalen, J. G. Gordon 11, and M. R. Philpott, Surf. Sci., 74, 237 (1978).
3618
The Journal of Physical Chemistry, Vol. 87,No. \lumberof Layers
0 1
3
5
Brown et al.
19, 1983
7
External Angle O/deg.
Figure 3. Reflectivity minima is attenuated total reflection (ATR) on the base of a right-angle prism coated with a 500-A film of silver and various monolayer assemblies. The minima from left to right are for films of bare silver and silver with one, three, five, and seven layers of cadmium dimethylarachidate.
ejected electrons was described previ0us1y.l~
Results The pressure vs. area isotherms are shown in Figure 2 for a series of temperatures. A liquidlike phase is seen around 40 A2 and three condensed phases are seen around 7,20, and 22 A2. The phases at 20, 22, and 40 A2, based on previous work as discussed above, are expected. The phase at 7 A2, however, seems to be very condensed and may involve the expulsion of some molecules from the assembly. To investigate these structures, we first attempted to transfer a layer to a solid substrate for analysis by ATR and X-ray techniques. Efficient and regularly behaved transfer took place at the 20-A2 point on the isotherm. These results will be discussed below. The transfer films at the 7-A2 point on the isotherm seemed to allow only a transfer of less than an equivalent area of the 7-A2 condensed phase with an in and out dipping procedure. Probably partial transfer is occurring at the withdrawing step. The energy-momentum relation of plasmon surface polaritons (PSP) excited at an air-metal interface can be substantially modified by thin dielectric overcoatings.12 For example, the minimum in reflectivity of a silver surface observed at an angle where the incident laser light couples resonantly to the PSP states of the metal is shifted 0.4' by only one layer of Cd(AA)zwith respect to the uncoated interface. This shift, shown in Figure 3, can be used to calculate by the Fresnel equations the optical thickness of the overlayer which depends on n, the index of refraction, and d , the geometrical thickness of the overlayer. For Cd(AA)2,the thicknessid, of one deposited monolayer is accurately known from X-ray and optical data from multilayer assemblies. Knowing this and the number of layers, 1, we can calculate the index of refraction, n,. For Cd((CH3),AA),, the optical thickness was calculated to be smaller than that of Cd(AA)2. One has to allow, however, that both the index of refraction, n2, as well as the geometrical thickness of one monolayer, Cd((CH3)zAA)z,d,, can be different from those of Cd(AA)2. If, for example, the molecules are tilted with respect to the layer normal, both the thickness of the layer and the lateral density p will decrease. The close chemical similarity, however, (13) C. R. Brundle, H. Hopster, and J. D.Swalen, J. Chem. Phys., 70, 5190 (1979).
I 1
I 2
I 3
I 4
I
5
I 6
Number of Monolayers Flgure 4. X-ray photoelectron (XPS) signal intensity as a function of the number of layers.
between Cd(AA), and Cd((CH3)2AA)2 allows us to calculate the index of refraction if one introduces a lateral dilution factor, x , which accounts for the fact that the Cd((CH,),AA), monlayer can be looser packed than the Cd(AA),monolayer. One obtains, then, for the index of refraction
n2 = xn,
+ (1 - x )
(1)
+1
(2)
or n2 = x(nl - 1)
The optical thickness, therefore, depends only on two parameters: the geometrical thickness, dz,and the dilution factor, x , and the shift in the reflectivity minima gives an almost linear relation between d, and x . Now X-ray photoemission measurements (XPS) have also been shown to be another powerful tool for the study of metal over coating^.'^ It has, for example, been demonstrated that a thin overlayer like a Cd(AA), monolayer attenuates the substrate XPS signal and that this attenuation yields information,about the molecular organization of the overlayer. Let Io be the Ag(3d) (XPS) intensity at a bare silver surface. A thin overcoating consisting of 1 layers of Cd(AA), attenuates this signal
Z, = Z , p
(3)
where CY,
= @plldl/sin y
(4)
p1 is the lateral density of the Cd(AA)2 layer, d, is the thickness of one monolayer, y is the angle of observation (which in our case was 38O), and @ is a molecule-dependent proportionally constant. For Cd((CH3),AA),, the attenuation was less than for Cd(AA), (see Figure 4). Nevertheless, because of the nearly identical chemical nature of Cd(AA)2 and Cd((CH3)2AA)2, we assumed that p is the same for both compounds because both are long-chain acids. That means that the different attentuation of the Ag signal has to be attributed to a different thickness d z and/or a different lateral density, p2, of the Cd((CH3)2AA)2 layers. This possible lower lateral packing of the Cd-
The Journal of Physical Chemistry, Vol. 87,
Behavior of a Geminally Disubstituted Fatty Acid
((CH,),AA), layer, as compared to Cd(AAI2layer, is taken into account again by the dilution factor x , i.e. Pz = PlX
(5)
a2 = fixplld2/sin y
(6)
Consequently, we get Dividing eq 6 by eq 4 and solving with respect to dz, we get ~ d =z d l ~ ~ z / a l a second relation between d2 and x . These two relations, one from the optical ATR measurements and one from the ESCA measurements, lead to a determination of d, and x. The values that we found were d2 = 25.2 A and x = 0.72, showing that the film is slightly thinner and less compact than Cd(AA)2.
Discussion The results clearly indicate that the geminal dimethyl branching at the head of the fatty acid does not present typical monolayer behavior, i.e., formation and transfer of a condensed phase. This phase consists, no doubt, of long aliphatic chains separated from one another, more than in the case of arachidic acid, due to the increased steric effects of the methyl groups. This increased separation permits some possible inclination or tilt of the alkyl chains and results in a slightly reduced thickness: 25.2 A/layer for cadmium a,a-dimethylarachidate, compared to 26.8 Allayer for cadmium arachidate. The dilution factor, x , is also consistent with increased chain separation. These conclusions were possible only through the combination of optical and XPS data; neither alone was sufficient. The formation of such layers allows investigation of saturated lipid layers with a well-defined site of potentially enhanced radiation lability, as well as the potential for forming layers with heavy atoms incorporated into the chains. It is also apparent that the alternative conformation (c) in Figure 1 does not exert a significant perturbation upon the packing in the layer. However, there are two peculiarities in the results with a,a-dimethylarachidic acid. First, there is a surprisingly small effect of the two methyl groups on the area on water, which is not consistent with the dilution factor determined for the transferred film. Second, there exists a highly condensed phase which is apparently stable and quite reproducible though not capable of intact transfer under the conditions examined. The work of Weitzel et aLl4 leads to the expectation of an increase in area due to a-methyl substitution of at least 4 A2 (from comparison of stearic and a-methylstearic acids). Thus, it is quite surprising to find that a,a-dimethylarachidic acid exhibits an area of 20-22 A2 under conditions where arachidic acid exhibits an area of 18-20 A2. On the other hand, if one calculates from the dilution factor, x , the area should be 25-28 A2, consistent with two methyl groups. It is possible that the enantiomericmixture present in a-methylstearic acid may have significantly less close packing on the water surface than would a pure enantiomorph, thus leading to an overly large effect attributed to the steric effect of the methyl group alone. Ad(14)G.Weitzel, A. M. Fretzdorff, s. Helter, and s. Graeser, Kolloid-2.
127, 110 (1952);ref 1, pp 238-40.
No. 19, 1983 3619
ditionally, there may be a distortion of the film on water with some vertical displacement of a fraction of the chains. This could take the form of individual displacements or a more extensive curvature such as those reported in phospholipid 1a~ers.I~ The rigidity of the solid underphase on transfer could then eliminate these effects and produce the larger area determined from the dilution factor. Arachidic acid has a single condensed phase which shows slight compression upon increased pressure until a collapsed phase forms. a,a-Dimethylarachidic acid shows similar behavior until a pressure of 20-25 dyn/cm is applied and then behavior suggestive of a slightly expanded phase is seen until a new, highly condensed phase is apparently formed which exhibits behavior of a stable condensed phase, not a collapsed phase. However, upon transfer to a solid surface this phase underwent expansion, and the resulting film was observed to be only slightly thicker than that transferred from the phase at 22 A2. Thus,these measurements of a,a-dimethylarachidic acid have given rise to two unanticipated results: the apparent formation of a highly condensed phase, and the small area increased on water from the geminal dimethyl substitution. We believe it unlikely-though possible-that one or both of these effects are the result of artifacts. One possibility for error is a loss of material during application of the lipid solution to the surface of the aqueous phase in the tank. (The hexane-ethanol 9:l solvent mixture used has the advantage of being lighter than water.) Such loss would have to be adventitiously consistent in different experiments, but we do not believe there was any significant loss in our experiments. Further recrystallization of the acid did not alter the observations and GLPC analyses indicated a single pure compound. The method of synthesis precludes contamination by monomethyl-substituted acid or from positional isomers. Chain length purity is beter than 99%, probably better than 99.5%. Thus, the phenomena observed would not appear to result from impurities. We originally considered this phase to be a collapsed, i.e., relatively unordered, phase. Such phases are usually observed under high rates of compression; it proved, however, to be quite reproducible in several experiments in two laboratories, with different apparatus, and under various rates of compression. The unusually small area per molecule is suggestive of a trilayer17or other multilayer structures. In summary, a,a-dimethylarachidic acid has proven surprisingly amenable to formation of a monomolecular film which can be transferred and studied conventionally. This clearly indicates that a quaternary site can be accommodated in a stable layer. However, this material has shown two currently unanticipated and unexplained phenomena: a surprisingly small effect on the area (on water) of the two geminal methyl groups, and the appearance of a highly condensed phase of unusually small area. We are currently pursuing further studies on this and related compounds. (15)(a) E.J. Luna and H. M. McConnell, Biochim. Biophys. Acta,
466, 381 (1977);470,303 (1977);C. Gebhardt, H. Gruler, and E. Sachmann, Z . Naturforsch. C, 32, 581 (1977);( c ) see also discussions by S.
Doniach, J. Chem. Phys., 70,4587 (1977). (16)Reference 1, pp 150-1. (17)H. Sobotka, M.Demeny, and M. Avnaki, J.Colloid Sci., 14, 281 (1959),and references therein.
