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Langmuir 1998, 14, 5988-5993
Specific Nonthermal Chemical Structural Transformation Induced by Microwaves in a Single Amphiphilic Bilayer Self-Assembled on Silicon Rivka Maoz,*,† Hagai Cohen,‡ and Jacob Sagiv*,† Department of Materials & Interfaces and Chemical Services Unit, The Weizmann Institute of Science, Rehovot 76100, Israel Received February 25, 1998. In Final Form: July 20, 1998 We demonstrate a new approach to the study and application of microwave-induced chemical reactions, using a purpose-designed molecular film as a discrete ultrathin antenna that can utilize absorbed electromagnetic energy to directly drive a specific chemical transformation. Exposure of such a bilayer “antenna” self-assembled on silicon to common microwave radiation in a domestic oven is shown to reproducibly generate a depleted top monolayer structure with molecular-size vacancies that can incorporate and control the further surface manipulation of various gap-fitting guest species. The nonthermal nature of this process is unequivocally demonstrated, as the irradiated bilayer cannot store heat, while conventional heating causes its irreversible structural deterioration, before an equivalent thermally activated transformation could occur. These findings shed new light on the much disputed issue of nonthermal microwave effects, suggesting, beyond obvious implications for basic research in this area, that microwave radiation could be rationally utilized to achieve specific (nondestructive) transformations in properly designed supramolecular systems and could be utilized particularly as an attractive new synthetic tool in molecular surface engineering.
The expanding use of microwaves as a source of energy in food cooking,1 materials processing,1-6 medical therapies,7 communications,7 and, more recently, chemical synthesis3,8-12 has prompted renewed interest in the elucidation of the various possible mechanisms of interaction of such radiation with matter. As quanta of elec* To whom correspondence should be addressed. E-mail:
[email protected]. Fax: +972-8-9344138. † Department of Materials & Interfaces. ‡ Chemical Services Unit. (1) (a) Metaxas, A. C.; Meredith, R. J. Industrial Microwave Heating; Peter Peregrinus: London, 1983. (b) Microwave Processing of Materials; Sutton, W. H., Brooks, M. M., Chabinsky, I. J., Eds.; MRS Symposium Proceedings Vol. 124; MRS: Pittsburgh, PA, 1988. (c) Introduction to Microwave Sample Preparation; (Skip) Kingston, H. M., Jassie, L. B., Eds.; American Chemical Society: Washington, DC, 1988. (2) (a) Thuillier, F. M.; Jullien, H.; Grenier Loustalot, M.-F. Polym. Commun. 1986, 27, 206. (b) Beldjoudi, N.; Gourdenne, A. Eur. Polym. J. 1988, 24, 53. (3) Mingos, D. M. P.; Baghurst, D. R. Chem. Soc. Rev. 1991, 20, 1 and references therein. (4) Marand, E.; Baker, K. R.; Graybeal, J. D. Macromolecules 1992, 25, 2243. (5) Lewis, D. A.; Summers, J. D.; Ward, T. C.; McGrath, J. E. J. Polym. Sci., Part A: Polym. Chem. 1992, 30, 1647. (6) Binner, J. G. P.; Hassine, N. A.; Cross, T. E. J. Mater. Sci. 1995, 30, 5389. (7) IEEE Trans. Microwave Theory Technol. 1996, 44, Special Issue on Medical Applications and Biological Effects of RF/Microwaves. (8) (a) Gedye, R.; Smith, F.; Westaway, K.; Ali, H.; Baldisera, L.; Laberge, L.; Rousell, J. Tetrahedron Lett. 1986, 27, 279. (b) Giguere, R. J.; Bray, T. L.; Duncan, S. M.; Majetich, G. Tetrahedron Lett. 1986, 27, 4945. (9) (a) Caddick, S. Tetrahedron 1995, 51, 10403 and references therein. (b) Strauss, C. R.; Trainor, R. W. Aust. J. Chem. 1995, 48, 1665 and references therein. (c) Jacob, J.; Chia, L. H. L.; Boey, F. Y. C. J. Mater. Sci. 1995, 30, 5321 and references therein. (d) Galema, S. A. Chem. Soc. Rev. 1997, 26, 233 and references therein. (e) Bose, A. K.; Banik, B. K.; Lavlinskaia, N.; Jayaraman, M.; Manhas, M. S. CHEMTECH 1997, 18. (10) (a) Laurent, R.; Laporterie, A.; Dubac, J.; Berlan, J.; Lefeuvre, S.; Audhuy, M. J. Org. Chem. 1992, 57, 7099. (b) Shibata, C.; Kashima, T.; Ohuchi, K. Jpn. J. Appl. Phys. 1996, 35, 316. (11) (a) Loupy, A.; Pigeon, P.; Ramdani, M. Tetrahedron 1996, 52, 6705. (b) Baldwin, B. W.; Hirose, T.; Wang, Z.-H. Chem. Commun. 1996, 2669. (12) Stuerga, D.; Gonon, K.; Lallemant, M. Tetrahedron 1993, 49, 6229.
tromagnetic energy in the spectral range characteristic of microwave and radio frequencies (rf) are far too small for direct excitation of electronic or vibrational transitions in molecules, it is generally accepted that the only effective mode of energy dissipation associated with the absorption of such radiation in condensed phases is by generation of heat.13 This so-called “dielectric heating”1,3,7 is a consequence of hindered oscillatory motions of charged particles (ions and dipolar species), driven by the alternating electric field of the radiation. In a condensed phase, multiple collisions with neighbor molecular species cause rapid randomization of this initially field-driven molecular agitation, thus raising the heat content of the irradiated material.1,3 According to this effective thermalization model, chemical transformations occurring upon exposure of a dielectric material to microwave/rf radiation are generally regarded as thermally activated processes, in principle realizable by the use of any thermal source capable of generating an equivalent heat profile in the investigated material. Despite the widespread acceptance of the essentially thermal nature of chemical transformations induced by microwaves, there have been repeated reports of unusual rate enhancements2-6,9-11 as well as significant differences in the distribution of reaction products2,3,9,12 in various microwave-driven reactions, when compared with presumably equivalent control systems heated conventionally. Yet, many of these apparently “specific microwave effects” were later on shown to originate in improper monitoring of the real local temperatures of reaction sites in the investigated systems, as uneven absorption of the microwaves within a bulk dielectric may frequently give rise to sharp thermal gradients, superheating (in liquids), and localized hot spots (in solids).3,9 Such thermal inhomogeneities, peculiar to the dielectric heating, may remain unnoticed when measuring the temperature of an (13) It should be understood that the processes considered here do not include plasmas produced by microwave or rf-induced discharge in a gas phase.
S0743-7463(98)00223-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/24/1998
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Figure 1. Schematic view of the microwave-induced conversion of a self-assembled amine-acid salt bilayer (left) to a depleted imide bilayer (middle) and the insertion of oriented long chain alkane molecules into the intermolecular vacancies left in the imide top layer (right). The irradiation was carried out in a turntable domestic microwave oven (Brother 850, 10 discrete power levels, 800 W maximal radiation output), in the presence of microwave-absorbing dummy loads consisting of four glass bottles, each filled with ∼100 g of a mixture of poly(vinyl alcohol) flakes and alumina.21 The bilayer was prepared by the self-assembly of stearylamine (C18) or arachidylamine (C20) from respectively 5.0 × 10-2 and 1.0 × 10-2 M solutions in pure bicyclohexyl (∼2 min immersion at the ambient temperature) on the carboxylic acid outer surface of a functionalized long chain silane monolayer (NTSox) selfassembled on each (polished) side of a silicon wafer substrate (Virginia Semiconductor, 4 × 2 × 0.038 cm, 〈111〉 orientation, resistivity g50 Ω cm, undoped). For clarity, all paraffinic tails were drawn eight carbon atoms shorter. The bottom silane monolayer (NTSox) was prepared as described before,15,16 except for the cleaning of the silicon wafer, which was now performed by its exposure to intense microwave radiation in air for ∼2 min (a detailed account of this new surface cleaning method will be given in a separate publication).
irradiated bulk phase, particularly if it is chemically and dielectrically inhomogeneous.3,5,9,10 Beyond technical difficulties, the assessment of the effective local temperatures of molecular reaction centers in a microwave-irradiated bulk material represents a fundamentally difficult problem, as any thermometric device would normally respond to the average temperature of a sufficiently large portion of the examined system, while the preferential interaction of the radiation with polar molecular components of a complex material may lead to significant sub-microscopic thermal inhomogeneities. Thus, particularly large local deviations (on a molecular scale) from the measured average temperature can be expected in systems that are far from molecular homogeneity, such as many of those for which “nonthermal” microwave effects were reported. Given the unavoidable generation of heat in any macroscopic body absorbing microwaves and the inherent uncertainty associated with the determination of real molecular-scale temperatures in such systems, the authenticity of a large part of the reported “nonthermal” effects remains doubtful. If genuine, nonthermal effects of the microwave radiation are of considerable theoretical and practical interest, both in the context of chemical synthesis, and in relation to their possible influence on living organisms, through specific chemical transformations directly affecting the metabolism or, indirectly, through effects resulting from the consumption of microwave-treated foods by humans and animals. Currently, this important issue continues to stir considerable controversy and confusion, largely because of the ubiquitous nature of thermal effects and the mentioned ambiguity associated with an unequivocal identification of eventual nonthermal effects, when superimposed on a large thermal background that tends to mask or alter their paths. Here we demonstrate a promising new approach (free of the inherent difficulties mentioned above) to the study of microwave-driven chemical transformations, based on
the direct examination of a single bimolecular layer of reactive species, designed to simultaneously function as both efficient microwave absorbers and potential reaction centers. To achieve unambiguous decoupling of nonthermal from possible thermal effects, the reactive ultrathin film is held in intimate thermal contact with a practically infinite, nonreactive thermal sink (cold, microwavetransparent environment), so as to prevent any significant accumulation of heat at the reaction centers during the irradiation. In this manner, both the absorption of the electromagnetic energy and any chemical transformation induced by it are confined to a discrete, predefined section of the experimental system which, by virtue of its negligible thickness, cannot store heat, thus totally eliminating the possible occurrence of a thermally activated process. We have anticipated that the selective local interaction of microwaves with the polar groups of a reactive amphiphilic bilayer14 might induce a chemical transformation localized in the bilayer polar region, while, unlike in a regular thermal process, the nonpolar (microwavetransparent) paraffinic tails are not directly affected and so would tend to preserve the overall molecular organization of the system. This would conceivably happen if the microwave excitation energy can be directly utilized to drive a chemical transformation,4 before it degrades into heat and is thus lost to the environment. To test this hypothesis, we investigated a prototype model system consisting of an amphiphilic bilayer self-assembled on silicon, in which a top amine monolayer is attached ionically to the exposed carboxylic acid groups of a surfacefunctionalized silane monolayer15,16 that provides a firm anchor to the underlying substrate (Figure 1, left). This selection was guided by (i) the expected efficient interaction (14) Daniel, V. V. Dielectric Relaxation; Academic Press: London, 1967; pp 217-221. (15) Maoz, R.; Sagiv, J.; Degenhardt, D.; Mo¨hwald, H.; Quint, P. Supramol. Sci. 1995, 2, 9. (16) Maoz, R.; Matlis, S.; DiMasi, E.; Ocko, B. M.; Sagiv, J. Nature 1996, 384, 150.
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of such an amine-acid bilayer with electromagnetic fields, considering the extremely high polarizabilities associated with the hydrogen-bond/proton transfer in related amineacid systems,17 the large pyroelectric activity,18,19 and other dipole-moment-dependent electrical effects20 observed in some Langmuir-Blodgett amine-acid alternate layer films, as well as previous reports regarding the efficient microwave absorption by some ammomium carboxylate salts;11 (ii) considerations of chemical reactivity, as the possible formation of amidic bonds, through the radiationinduced coupling of adjacent amine and acid moieties, is a fundamental chemical transformation of wide scientific relevance; (iii) analytical considerations, as both chemical and structural transformations occurring in a bilayer system of this kind can be conveniently followed using a number of powerful methods of surface characterization now available for the study of such solid-supported films. Highly ordered amine-acid bilayer specimens prepared with either stearyl (C18) amine (Figure 1, left) or arachidyl (C20) amine were exposed to low levels of 2.45 GHz radiation in a domestic microwave oven, in air, or immersed in n-hexadecane (C16)san inert, microwavetransparent oil. Irradiation of such films for 2 × 10 s at the oven power setting #3 (on a scale of 10) or for about 2 × 10 min at the setting #1, in the presence of additional dummy loads, was found to result in a permanent chemical-structural transformation, no further changes taking place upon additional prolonged microwave irradiation under these conditions21 (Figure 1). Evidence from quantitative FTIR (Fourier transform infrared) spectra, XPS (X-ray photoelectron spectroscopy) data, and wettability observations, combined with a series of postirradiation derivatization experiments(vide infra), demonstrates that the microwave irradiation effected, in all investigated samples, the virtually quantitative formation of imide bilayers, as depicted schematically in Figure 1, middle. One may note that the steric constraints arising from the dense packing and perpendicular orientation of the paraffinic tails of the unaffected bottom layer15 would permit covalent bonding between the amine nitrogens and nearest neighbor carbonyls only. The bilayer structure created in this manner should thus be rather unusual, in the sense that although the top layer is depleted of half of its initial content, the molecules left on the surface are fixed at predefined positions by virtue of their covalent anchoring to pairs of nearest neighbor molecules belonging to the undepleted bottom layer. This should preclude clustering or other lateral redistribution processes char(17) (a) DeTar, D. F.; Novak, R. W. J. Am. Chem. Soc. 1970, 92, 1361. (b) Lindeman, R.; Zundel, G. J. Chem. Soc., Faraday Trans. 1977, 73, 788. (18) Jones, C. A.; Petty, M. C.; Roberts, G. G.; Davies, G.; Yarwood, J.; Ratcliffe, N. M.; Barton, J. W. Thin Solid Films 1987, 155, 187. (19) Davies, G. H.; Yarwood, J.; Petty, M. C.; Jones, C. A. Thin Solid Films 1988, 159, 461. (20) Jones, C. A.; Petty, M. C.; Davies, G.; Yarwood, J. J. Phys. D: Appl. Phys. 1988, 21, 95. (21) It should be noted that domestic microwave ovens operate such that the power setting represents a time average.3 The adjustment of the power affects the fraction of the nominal irradiation time during which microwaves are actually generated by the megatron, but not the field strength of the radiation. Thus, mentioning the power settings employed in these experiments must not be taken as an indication of the amount of microwave energy absorbed by the reacting bilayer. Moreover, since the bilayer sample can obviously absorb only a minor fraction of the total microwave energy supplied to the oven cavity, a dummy load had to be used in order to avoid overheating of the megatron by the back-reflected radiation.3 Experimental data relating the conversion to the time of irradiation (at a selected power setting of the microwave oven) were recorded; however, since the very simple experimental setup employed in this study does not allow us to determine the actual amount of electromagnetic energy absorbed by the reacting bilayer, we cannot offer, at this stage, a quantitative analysis of the kinetics of the observed microwave-induced chemical transformation.
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Figure 2. Quantitative FTIR Brewster’s angle spectra15 of an amine-acid salt bilayer (double-side-coated Si wafer) with C20 top amine monolayer (lower curves) and the corresponding microwave irradiation product (upper curves). The spectra were acquired as described before,15,16 with a Nicolet 730 FTIR system at a resolution of 4 cm-1.
acteristic of partial monolayer coverages, thus preserving molecular-size vacancies potentially useful for subsequent incorporation of appropriate gap-fitting guest species (Figure 1, right).22 Given this unique expected property, we followed the evolution of each of several investigated film samples along an entire sequence of operations as those shown in Figure 1. Here we briefly discuss some representative data selected from a larger body of available evidence, with the quadruple purpose of demonstrating that (i) the microwave irradiation generates the imide bilayer structure depicted in Figure 1, (ii) this unusual film structure cannot be generated by conventional heating, (iii) no measurable thermal effect, in the sense normally attributed to heat, is involved in this microwaveinduced transformation, and (iv) the molecular free “pockets” created in the top monolayer by the microwave irradiation may be used to further generate a variety of unusual surface architectures that cannot be easily produced by other means. Figure 2 shows that the microwave irradiation of an amine-acid salt bilayer causes the disappearance of the broad IR bands centered around 1408, 1558, and 1648 cm-1, assigned to vibrations of the -COO- and -NH3+ groups18,19 (with possible contributions from hydrogenbonded -COOH and -NH2 species, and water of hydration picked by the hygroscopic amino functions),17 and the appearance of two overlapping bands around 1693 and 1734 cm-1, consistent with the formation of imide (OdC-N-CdO) functions.23 The -CH3 and -CH2stretch and -CH2- bending bands, around 2900 and 1467 cm-1, respectively, decrease in their intensities upon irradiation, while the -CH2- components around 2916 and 2849 cm-1 shift to higher wavenumbers and broaden, thus pointing to a loss of paraffinic tails from the surface, accompanied by significant disorientation15,24 of the residual film material. Since no depletion or disordering of silane monolayers directly anchored to silicon was observed under identical conditions of microwave irradiation, and the bottom monolayer (NTSox) in an irradiated (22) (a) Sagiv, J. Isr. J. Chem. 1979, 18, 346. (b) Wulff, G. Angew. Chem., Int. Ed. Engl. 1995, 34, 1812. (c) Kallury, K. M. R.; Thompson, M.; Tripp, C. P.; Hair, M. L. Langmuir 1992, 8, 947. (23) Hargreaves, M. K.; Pritchard, J. G.; Dave, H. R. Chem. Rev. 1970, 70, 439. (24) Casal, H.; Cameron, D. G.; Mantsch, H. Can. J. Chem. 1983, 61, 1736.
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Figure 3. Difference infrared spectra (derived from quantitative FTIR spectra measured as in Figure 2) showing the net spectral contribution of the top layer (see text) in two different amine-acid bilayers, before and after their microwave irradiation, and after their postirradiation refilling with various long-tail guests: curve a, C20 top layer before irradiation; curve b, C20 top layer after irradiation; curve c, irradiated C20 top layer refilled with C20 amine (after 1 min immersion in a 1.0 × 10-2 M solution of the amine in BCH); curve d, C18 top layer before irradiation; curve e, C18 top layer after irradiation; curve f, irradiated C18 top layer refilled with C18 amine (after 3 min of immersion in a 5.0 × 10-2 M solution of the amine in BCH); curve g, irradiated C18 top layer refilled with n-hexadecane (C16) (after 3 min of immersion in the neat C16 liquid); curve h, irradiated C18 top layer refilled with 1-octadecene (V-C18) (after 3 min of immersion in the neat V-C18 liquid). All curves represent the same C-H stretch spectral region and are laterally shifted with respect to one another in order to enable easy visual comparison of their relative heights. Due to the high receding contact angles (see text), all refilled film samples emerged totally dry from the respective liquids, with no extra material sticking to their outer surfaces.
bilayer sample could be recovered intact following harsh acidic hydrolysis of the imide functions, we can safely conclude that only the structure of the top amine monolayer was affected. Therefore, by subtracting mathematically the spectrum of the unaffected bottom layer from spectra of the bilayer taken before and respectively after irradiation, it is possible to direcly evaluate the net changes induced in the paraffinic tails region of the top monolayer. Figure 3 shows a series of such difference spectra obtained for two bilayer specimens, with C20 and C18 top amine monolayers. In addition, spectra of postirradiation refilled top monolayers (as in Figure 1, right), with the respective C20 and C18 amines and with n-hexadecane (C16) and 1-octadecene (V-C18), are also displayed for comparison. Within the accuracy of our spectral data (∼4% of a complete C18-C20 monolayer),16 the -CH2peak absorbance intensities of both the initial (a, d) and irradiated (b, e) layers are seen to be proportional to the numbers of -CH2- groups (19/17) in the corresponding C20 and C18 paraffinic tails. As expected, the peak intensities after irradiation drop to ∼50% of their initial values; however, because the disorder of the tails in the depleted layers leads to additional interactions of the incident infrared beam (in the Brewster’s angle configuration15 used here) with out-of-plane vibrational components,15,25 the integrated intensities of the broader imide bands (b, e) actually exceed the 50% figure (∼60-65% of the corresponding a, d curves) expected for a fully reacted imide layer. This situation is seen to be remedied when the layers are refilled with appropriate long-tail guest species (c, f), which largely restores the initial dense packing and order of the respective undepleted hydrocarbon cores and thus provides a direct comparative means for the quantification of the surface coverage in the (25) Frydman, E.; Cohen, H.; Maoz, R.; Sagiv, J. Langmuir 1997, 13, 5089.
depleted state. A comparison of the intensities of the C-H stretch bands obtained with refills having tails with respectively 19, 17, 14, and 15 -CH2- groups (curves c, f, g, and h), indicates that, within the mentioned 4% uncertainty, the conversion to the depleted half-layer imide structure appears to be virtually complete in all investigated cases. They also confirm the structural stability of the bottom layer under the irradiation. We note that because of the weak binding forces exerted by the vacant pockets in the imide layer, the incorporation of the long-tail guests is reversible; n-hexadecane slowly evaporates from the surface, the characteristic imide (C18) spectrum being reestablished within ∼35 min after withdrawal from the neat C16 liquid, whereas the incorporated amines could be ejected either by conventional heating (at 125 °C, for the C20 amine in a C20 imide film) or by exposure to the microwave radiation, however for times considerably longer than those required for the initial formation of the imide structures. Judging from the widths and peak positions of the imide -CH2stretch bands (Figure 3 b,e), the disorder of the depleted layers approaches that of a liquid hydrocarbon,24 so that the vacant “pockets” suggested in Figure 1 (middle) must be rather poorly defined. However, when interacting with similar long tail molecules supplied from a bulk liquid phase, the site-immobilized imide tails successfully control the reconstruction of a well-oriented, complete monolayer. This is a remarkable process of two-dimensional crystallization induced by weak dispersion forces provided by a quasi-liquidlike template that encodes the instructions required for the reproduction of a preexisting solidlike surface structure. The infrared spectral evidence is corroborated by XPS data which, subject to some of the experimental limitations of this method (Table 1),25 confirm the partial loss of total carbon upon formation of the imide layer, the higher
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Table 1. Elemental Composition Obtained from XPS Dataa sample
C (total, %) (285 eV)
Cd0/C (%) (288-289 eV)
N/C (%) (399-401 eV)
N/CdO
amine-acid salt (C20) imide (C20) amine-refilled imide (C20)
64.2 ( 0.8 56.5 ( 0.7 65.9 ( 0.8
3.57 ( 0.40 5.66 ( 0.50 3.71 ( 0.40
3.23 ( 0.30 3.25 ( 0.30 3.67 ( 0.30
0.91 ( 0.20 0.57 ( 0.10 0.99 ( 0.20
a XPS data were obtained on a Kratos AXIS-HS spectrometer using a monochromatic Al KR X-ray source, at an analyzer pass energy of 40 eV. To minimize the radiation-induced damage (which is non-negligible even with monochromatized radiation),25 data were collected by repeatedly exposing fresh spots to the X-ray beam (for at most 2 min each) within a homogeneously coated sampling area of the order of ∼4 cm2. Even with these precautions, the amine-acid salt had to be cooled to about -10 °C in order to minimize its deterioration. The stability of each measured film was checked by comparing infrared spectra taken before and after the XPS measurement.25 All binding energies are referenced to the C1s main line, which was fixed at 285 eV (an electron flood gun was used to minimize line shifts due to surface charging).
relative CdO carbon contribution in the depleted imide film, and the expected variation in the measured N/CdO ratios, from ∼1 to ∼0.5 and back to ∼1, on proceeding from the initial bilayer to the imide and then to the aminerefilled imide.26 The completeness of the imidization reaction was further confirmed by examining the possible generation of carboxylate (-COO-) surface groups in microwave-irradiated samples. Thus, Cu2+-acetate from an aqueous solution is adsorbed into the free “pockets” of imide layers with full preservation of the carbonylic imide bands around 1700 cm-1 (Figure 2) and without visible formation of monolayer carboxylates, whereas following acidic hydrolysis of the imide functions, a similar treatment with aqueous copper acetate results in the quantitative conversion of the liberated -COOH groups (IR peak at ∼1707 cm-1) into the corresponding copper carboxylates (IR peaks at ∼1533 and ∼1433 cm-1). To assess the possible effect of heat, we examined thermal transformations induced in amine-acid salt bilayers as well as in microwave-generated imide samples upon their heating in a large conventional oven between the ambient temperature and 220 °C, under variable conditions of temperature rise and times of exposure (2 min to several hours) to various selected temperatures in this range, including experiments in which samples were directly exposed for 2-3 min to the desired temperature in the preheated oven. It was found that regardless of the mode of heating, the proton-transfer equilibrium in the C18 amine-acid system is reversed already at ∼28 °C (with the formation of the free acid and free amine),19 and there is a gradual loss of top layer material and irreversible disordering of the residual film, starting at 70-80 °C and ending up with practically total loss of the top layer at ∼170 °C. For comparison, the corresponding microwavegenerated imide is perfectly stable up to ∼210 °C, and only at 220 °C could some initial loss of film material be detected. Dramatic differences between microwave-generated imide and amine-acid bilayers heated conventionally were revealed by their wetting behavior. We measured contact angles for water and two organic hydrocarbons with comparable surface tensions but totally different molecular shapessthe bulky bicyclohexyl (BCH) and rodlike n-hexadecane (C16) that can fit well into the free “pockets” of the imide27 (Figure 1, right). Before being irradiated or heated, all bilayer samples displayed very high contact angles, characateristic of highly perfect -CH3 outer film surfaces, with no measurable hysteresis (114(26) Considering the depth-dependence of the XPS signals, the stoichiometric and experimentally observed N/CdO ratios should be closely related only if N and CdO reside at comparable depths below the outer film surface, as expected for each of the three samples listed in Table 1 (see Figure 1). (27) (a) Bartell, L. S.; Ruch, R. J. J. Phys. Chem. 1959, 63, 1045. (b) Gun, J.; Sagiv, J. J. Colloid Interface Sci. 1986, 112, 457.
115° for H2O, 54-55° for BCH and 51-52° for C16).27,28 As expected, loss of material and disordering of the top layer and increased accessibility of interlayer polar groups cause a marked drop in all contact angles, accompanied by considerable hysteresis between higher advancing (a) and lower receding (r) values, with the striking exception of C16 on the imide surfaces, for which the receding angle reproducibly exceeded the advancing one, reaching the value measured before irradiation. For example, the contact angles on a C18 imide heated for ∼ 2 h at 170 °C remained almost the same as before heating, H2O (96°a; 65°r), BCH (41°a; 37°r), and C16 (47°a; 51°r), whereas a C18 amine-acid bilayer similarly heated at only 90 °C gave H2O (84°a; 58°r), BCH (33°a; 28°r), and C16 (25°a; 0°r) and was totally wetted (0°a,r) by both BCH and C16 after heating at 170 °C. Thus, the C16-refilled hydrocarbon core of a thermally stable imide layer obviously mimics the undepleted surface of the initial bilayer,27 whereas the amine-acid bilayer is irreversibly damaged by heat, so as to render impossible the incorporation of oriented C16 molecules. This is fully confirmed by infrared spectroscopy, as no evidence could be found for the incorporation of either C16 or the C18 and C20 amines in any thermally depleted amine-acid bilayer, even after hours of immersion in n-hexadecane or in solutions of the respective amines in BCH (compare with Figure 3). Likewise, IR spectroscopy confirms the lack of incorporation of bulky BCH molecules in the imide structures. The maximal surface temperatures during the microwave irradiation of bare as well as bilayer-coated Si wafers were straightforwardly evaluated, by watching the range of melting of tiny grains of a homologous series of microwave-transparent crystalline hydrocarbons (with sharp, gradually increasing melting points) placed on the wafer surface, when irradiated in air, or by measuring the temperature of the C16 liquid near the wafer surface immediately after the irradiation, when the irradiation was performed under liquid. Surface temperatures determined in this manner did not exceed ∼50 °C in air and ∼40 °C under liquid, for either coated or uncoated wafers. This moderate temperature rise, obviously caused by the microwave energy dissipated in the Si wafer itself,29 cannot explain the formation of the imide structure. (28) Maoz, R.; Yam, R.; Berkovic, G.; Sagiv, J. In Thin Films; Ulman, A., Ed.; Academic Press: San Diego, CA, 1995; Vol. 20, p 41. (29) Simple considerations of heat capacity and flow would rule out the possibility that a molecular stratum of microwave-active reaction centers (the amine-acid pairs in Figure 1, left) positioned by means of inert hydrocarbon spacers (the paraffinic tails) at a distance of ∼25 Å from both the silicon substrate and its outer environment could store heat in amounts that would give rise to a significant temperature gradient across the spacer layers. Under the given irradiation conditions, such a stratum of reaction centers must therefore be thermally equilibrated with both the surface of the silicon substrate and its near outer environment, implying that the surface temperatures estimated by the direct procedures employed cannot differ significantly from the actual maximal temperatures felt by the reaction centers themselves.
Letters
While the utilization, in this preliminary study, of a commonly used domestic microwave oven confers general practical relevance to our present findings, the complexity of the radiation field generated within the cavity of such an oven does not allow us to reach at this stage a definitive conclusion as to the detailed molecular-scale mechanism of the observed microwave-induced reaction. However, the available evidence conclusively rules out the occurrence of a trivial thermal process associated with a significant measurable rise in reactants’ temperature.29 It is conceivable that the predetermined arrangement of the amine and acid moieties in a juxtaposed double layer configuration facilitates their radiation-induced covalent coupling, following excitation by the microwaves of collective dipolar oscillations of the adjacent -NH3+ and -COO- strata30 (Figure 1, left). Such field-driven headon collisions of the reacting groups can occur with full preservation of their initial mutual alignment (presumably needed for a facile coupling), being favored by the uniaxial perpendicular orientation of the nonpolar paraffinic tails. On the other hand, the overall alignment rigidity conferred by the tight packing of the oriented tails is expected to hinder the rotation of individual dipoles (which would promote molecular disorder) as well as oppose thermalization processes (i.e., the random redistribution of energy) that would damp down the microwave-excited dipolar oscillations. In other words, the specific supramolecular architecture of the highly dipolar amine-acid bilayer seems to play a crucial triple role in this imidization reaction, first, by facilitating the absorption of the microwave energy through collective dipolar oscillations of the reacting species that can be excited by such lowfrequency radiation,31 second, by channeling the microwave excitation energy into an entropically favored path for the amine-acid covalent coupling, and, finally, by reducing the efficiency of thermalization processes through which the excitation energy would be lost to the environment. Rapid lateral rearrangement of the reacting top layer molecules, possibly facilitated by the nonlocalized nature of the ionic amine-acid bonding, could also be an important factor that contributes to the observed high yield of this surface reaction. The central role ascribed to the proper prearrangement of the reacting functions is consistent with the fact that, within the limits of the analytical tools employed, the imide was always the only (30) One may expect the microwave electric field to be significantly influenced by the presence of the silicon substrate. Even though the undoped silicon employed in this work is a poor conductor, it is conceivable that the electric field will adopt a preferential perpendicular orientation at the Si/air or Si/oil interface, which coincides with the direction of the oscillating amine-acid dipoles. Enhancement of the radiation field in the vicinity of the Si surface is also possible. (31) It is of interest to note in this context the much longer microwave irradiation time that was needed to completely remove the C20 amine incorporated as refill in a C20 imide bilayer, as compared with that required for the generation of the imide itself. This would suggest that, unlike the initial ionic amine-acid bilayer, a refilled covalent imide bilayer is a poor microwave absorber, the ejection of the incorporated amine possibly being assisted by the heat generated in the Si substrate, in addition to the direct microwave action on the ejected molecules. This interesting point remains to be clarified in future studies.
Langmuir, Vol. 14, No. 21, 1998 5993
reaction product observed under irradiation, despite the use of a poorly defined radiation field,30 as well as with the failure of our repeated attempts to realize the same transformation under conditions of conventional thermal activation, which clearly indicates that the irreversible structural deterioration of the bilayer caused by heat occurs before the minimal temperature presumably required for any such thermal reaction to occur can be reached. These findings suggest attractive possibilities for the assembly of organized supramolecular systems designed to provide efficient paths for the direct (nonthermal) utilization of microwave/rf energy in specific chemical transformations. Such systems could be very useful as models in the study of nonthermal microwave effects, while offering interesting new options in materials synthesis. Applied to surface engineering, specific transformations induced by microwaves appear particularly promising as a tool for the achievement of unusual surface modifications, en route to the fabrication of new types of organized film structures. Thus, we currently explore the use of refillable bilayers generated by the present process as templates for the incorporation and alignment of various organic molecules (as in Figure 1, right) and inorganic species that may subsequently be reacted to create novel organicinorganic nanocomposites. For example, incorporation of gap-fitting guest molecules with sufficiently long paraffinic tails into the free molecular “pockets” of an imide layer could be used to induce epitaxial self-assembly of interdigitated multilayers, and the reduction or further reaction with H2S of incorporated copper, gold, or silver salt species might yield metallic or semiconductor (metal sulfide) nanoparticles whose size and lateral distribution are controlled by the free “pockets” of the organic imide matrix. A number of additional monolayer chemical modifications, different from the one described here, were successfully carried out by us with microwave irradiation and will be reported in future publications. Finally, implications of the present results with regard to possible nonthermal transformations induced by microwaves in structurally related biological membranes remain to be explored. Acknowledgment. We thank Dr. K. Ogawa of Matsushita Co. (Osaka) for supplying the vinyl-terminated silane used in the preparation of the bottom monolayers and Dr. A. Segal (W.I.S.), Professor R. Naaman (W.I.S.), and Professor F. Keilmann (M.P.I. fu¨r Biochemie, Martinsried) for helpful discussions on the efficiency of heat transfer across the bilayer and on the mechanism of the observed effect. R.M. and J.S. thank Tova Sagiv for the microwave oven present which triggered our interest in the subject of this work. Support by the G.M.J. Schmidt Minerva Center on Supramolecular Architectures and by the Consortium of German Chemistry Association and German Friends of the Weizmann Institute is acknowledged. LA980223R
