J . Phys. Chem. 1986, 90, 1408-1412
1408
Order-Disorder Transltions in Arachidate Monolayers David D. Saperstein IBM, 5600 Cottle Road, San Jose, California 951 93 (Received: July 12, 1985; In Final Form: November 1 , 1985)
Langmuir-Blodgett monolayers of cadmium arachidate have been observed with FT-IRspectroscopy to undergo an order to disorder transition at 110 O C which corresponds to the bulk melting. This paper shows the existence of a glasslike structure of cadmium arachidate which is generated when the monolayers in the melt are exposed to water vapor. This structure is barely distinguishableby FT-IR methods from the melt at temperatures above 110 O C but quite distinct upon cooling whereupon the dry melt appears partially ordered and the glasslike structure shows little or no evidence of orientation or packing order. Some conformational reordering of the backbone methylene is observed in both structures. Water is postulated to disorder the cadmium carboxylate lattice.
Introduction Fatty acid salts form an organized two-dimensional array on smooth when produced by Langmuir-Blodgett methods! Both monolayer and multilayers with preselected thicknesses can be produced by Langmuir-Blodgett techniques. Ordered monolayers have growing application in diverse fields such as catalysis, microelectronics, and lipid membra ne^.^ For example, Langmuir-Blodgett monolayers are ideal for developing ultrathin electronic devices because, in principle, the thickness of a multilayer array can be known to within a nanometer. To meet the expected demand for ordered monolayers and multilayers, studies on the conformational properties of model systems such as cadmium arachidate'S2x6-*need to be investigated thoroughly. Several different structures of arachidate films have been observed on low area metals. Langmuir-Blodgett arrays of fatty acid salts, such as cadmium arachidate, show evidence of orthorhombic p a ~ k i n g ' .with ~ - ~alternating ~~ layers of hydrophobic and hydrophilic groups normal to the metal. Unlike the spectra observed for chemisorbed molecules on metals," spectra of Langmuir-Blodgett monolayers show identical features on metals such as aluminum, silver, and nickel.I2 Langmuir-Blodgett films of cadmium arachidate show apparent structural disorder at temperatures above 110 OC,',* the melting temperature of the bulk. These films partially reorder upon cooling to room temperature. At premelting temperatures between ca. 6 5 and 110 OC,films never melted show partial, conformational disorder when examined in situ. However, unlike the case of the melt, these monolayers show near total reordering after cooling to room temperature.' These observations on the order of thermally treated Langmuir-Blodgett films are summarized in Table I. It is interesting to compare the spontaneously formed films with these Langmuir-Blodgett structures. Under conditions of rapid deposition, ( I ) J. F. Rabolt, F. C. Burns, N. E. Schlotter, and J. D. Swalen, J . Chem. Phys., 78, 946 (1983). (2) D. L. Allara and J. D. Swalen, J . Phys. Chem., 86, 2700 (1982). (3) P. A. Chollet, Thin Solid Films, 52, 343 (1978). (4) G . L. Gaines, Insoluble Monolayers at Liquid-Gas Interfaces, Interscience, New York, 1966. (5) G. G. Roberts, Sens. Actuators, 4, 131 (1984). (6) W. G. Golden, C. D. Snyder, and B. Smith, J . Phys. Chem., 86. 4675 (l982), and references therein. (7) C. Naselli, J. F. Rabolt, and J. D. Swalen, J . Chem. Phys., 82. 2136 (1985). (8) D. D. Saperstein and W. G.Golden, New Surface Science in Catalysis, M. Devinney and J. Gland, Ed., American Chemical Society, Washington, DC, ACS Symp. Ser. in press. (9) R. Snyder and J. Schactschneider. Spectrochim. Acta, 19, 85 (1963). (10) R. G. Snyder, J. Mol. Spectrosc. 7, 116 (1961); R. G. Snyder, J . Mol. Spectrosc., 4,411 (1960); M. Maroncelli, S. P. Gi, H. L. Straws, and R. G. Snyder, J . Am. Chem. Soc., 104, 6237 (1982). and R. G. Snyder, private communication. (1 1) See, for example, Session 1 in Vibrations at Surfaces. C . R . Brundle, and H. Morawitz, Eds., Elsevier, Amsterdam, 1983. ( 1 2 ) Compare spectra in ref 1 . 7, 8, and 14.
0022-3654/86/2090-1408$0l.50/0
TABLE I: Summary of "Phases"Observed
measurement conditions phase ( 1 atm) structure orthorhombic: Langmuir-Blodgett 25-50 O C monolayers two-dimensional array' Langmuir-Blodgett 65-105 O C methylene/methyl backbone premelt conformation disordered' orientation disordered,',* Langmuir-Blodgett I 1 I O O C melt packing partially disordered, CH, conformation'0reorders upon cooling Langmuir-Blodgett 21 I O O C + orientation and packing glasslike water vapor disordered; CH, mayla reorder upon cooling rapid, spontaneous 25 OC partially to totally coating disordered: glasslike? slow adsorption from 25 O C orth~rhombic?'~.'~ dilute solution disordered films are observed.6 Under conditions of slow adsorption, an oriented monolayer may be formedI3J4which appears Langmuir-Blodgett-like. It is intriguing that the rapidly formed, spontaneous monolayer is spectrally different from freshly prepared or melted Langmuir-Blodgett films. This paper presents evidence that the Langmuir-Blodgett arachidate melt contains some local order not readily apparent. This order can be diminished markedly by the addition of water vapor to the hot melt whereupon a new glasslike structure is formed that retains packing disorder upon cooling. Our observations may offer a more complete picture of the stages of disordering of a Langmuir-Blodgett film: at premelt temperatures conformational disorder of the backbone occurs, e.g. trans gauche,I& at melting conformation, orientation and packing disorder occur, and in the presence of water vapor additional packing disorder of the head group occurs. Spontaneous, rapidly formed films are spectrally similar to the glasslike structure and probably have little or no registry of the head groups. Cooling the melted LangmuirBlodgett film shows evidence for conformational reordering of the backbone which was previously7 reported only for the case of the cooled premelt monolayers.
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Experimental Section FT-IRRAS has been described elsewhere. For details the reader should refer to ref 3 and 15 for the conventional fixed polarizer, grazing incidence experiment, to ref 16 and 17 for the polarization (13) J. Gun,R. Iscovici, and J. Saqiv, J . Colloid Interface Sci. 101, 201 (1984), and references therein. (14) D. L. Allara and R. G. Nuzzo, Langmuir, I , 45 (1985); D. L. Allara and R G. Nuzzo, Langmuir, I , 52 (1985). ( 1 5) R. G.Greenler, J. Chem. Phys., 44, 310 (1966); see also N. Sheppard and J Erkelens. Appl Spectrosc., 38, 471 (1984), for a more recent review.
C 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 7, 1986 1409
Arachidate Monolayers
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Figure 1. In situ FT-IR spectra of seven monolayers of cadmium arachidate on aluminum a t (a) 30 "C, (b) 75 "C, (c) 125 OC for 15 h, (d) 125 OC for 20 h, (e) 30 "C immediately after cooling, (f') 30 OC, 18 h after cooling. 4-cm-' resolution; 4000 scans coadded per spectrum (ca. 30 min). 3000 to 2800 cm-'.
modulation addition, and to ref 17 for the polarization modulation addition, and to ref 17 for the polarization/double modulation technique. In order to study low area metal surfaces under reaction conditions, a cell was designed18 for continuous, in situ, grazing angle IR analysis. The chamber can be heated or cooled uniformily and used at reduced or elevated pressure with or without gas flowing. Two thermocouples are used to measure cell temperature reproducibly. In the work reported here, an IBM Instruments IR/85 is used. This I R was equipped with a gold on silver bromide, wire grid polarizer placed before the cell and a mid-band MCT detector. Typically the resolution is set to 4 cm-l with 4000 scans co-added to improve signal-to-noise. The light is incident on the sample at 82' off the normal and is p polarized, i.e., parallel to the plane of reflection and perpendicular to the plane of the metal.I5 (16) W. G. Golden, D. S.Dunn, and J. Overend, J. Phys. Chem., 82,843 (1978); F. Hoffmann and A. M. Bradshaw, Surf. Sci., 72, 513 (1977); J. D. Fedyk and M. J. Dignam in Vibrational Spectroscopies for Adsorbed Species, A. Bell and M. Hair, Eds., American Chemical Society, Washington, DC, 1980, and references therein. P. Hollins and J. Pritchard, Vibrational Spectroscopy for Adsorbed Species, A Bell and M. Hair, Eds., American Chemical Society, Washington, DC, 1980 A. Crossley and D. A. King, Surf. Sci., 68, 528 (1977); M. D. Baker and M. A. Chesters in Vibrations at Surfaces, R. Caudano et al., Eds., Plenum Press, New York, 1982; A. E. Dowry and C. Marcott, Appl. Spectrosc., 36, 414 (1982). (17) W. G.Golden and D. D. Saperstein, J . Electron Spectrosc., 30, 43 (1983), W. G. Golden, D. D. Saperstein, M. W. Severson, and J. Overend, J . Phys. Chem., 88, 574 (1984). (18) D. D. Saperstein, Appf. Spectrosc., 39,615 (1985).
Figure 2. In situ FT-IR spectra of seven monolayers of cadmium arachidate on aluminum alternately purged with dry N, (99.999%) and N, with ca. 12 torr of H20(see text): (a) 30 OC,(b) 75 OC,(c) 125 O C for 60 h without water vapor, (d) 125 OC for 65 h including 3 h with water vapor, (e) 30 OC immediately after cooling, (f) 30 "C, 24 h after cooling, 4-cm-I resolution; 4000 scans per spectrum (ca. 30 min). 3000 to 2800 cm-'.
The IR/85 is nitrogen purged and shows a residual amount of instrumental water vapor in the recorded spectra. This water vapor is as much as 0.01 absorbance units in the 1600- and 3600-cm-' regions and is eliminated digitally with a previously collected spectrum of water vapor in the absence of adsorbate. Cadmium arachidate [Cd(OOC(CH2)18CH3)2]monolayers are prepared by a Langmuir-Blodgett (LB) dipping technique4 (courtesy of Dr. John Rabolt, IBM Research, San Jose, CA). The metal films used, A1 and Ni, are 0.2 km thick and evaporated on to clean glass slides prior to LB monolayer deposition. The aluminum samples have seven monolayers deposited; the first is oriented with the carboxylate oxygens and cadmium proximate to the metal, and the last with the methyl group toward the outer surface. The nickel samples have six monolayers deposited; the first is oriented with the methyl groups proximate to the metal, and the last is oriented with the methyl groups toward the outer surface. In both cases the outer surface should be hydrophobic. The orientation and quality of an LB film preparation can be determined from the IR spectrum. Good preparation^',^ of ordered cadmium arachidate monolayers show the C-H stretching region with five bands, see Figures l a and 2a. These films are also characterized by negligible or absent bands for the antisymmetric carboxylate, the free carboxylic acid, and the methylene bend. The twist/wag region shows a progression of prominent bands indicative of all trans backbone o r i e n t a t i ~ n . ~ . ~ ~ ~ ~ ~ The melting temperature of arachidic acid is approximately 76 "C' which is the temperature used in all spectra b. If there were significant amounts of the free acid in isolated domains, spectral changes in the methylene and the carbonyl stretching regions at this temperature would be expected. That no increase in these bands is observed indicates the film is fairly uniform with respect to its salt content.
1410 The Journal of Physical Chemistry, Vol. 90, No. 7, 1986
Saperstein
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