Thermally induced micro-domain formation in fatty acid Langmuir/Blodgett films observed by attenuated total reflectance infrared spectroscopy

Transcription

1 Colloids and Surfaces A: Physicochem. Eng. Aspects 219 (2003) 125/145 Thermally induced micro-domain formation in fatty acid Langmuir/Blodgett films observed by attenuated total reflectance infrared spectroscopy Keith M. Faucher 1, Richard A. Dluhy * Department of Chemistry, University of Georgia, Athens, GA , USA Received 15 May 2002; accepted 13 January 2003 Abstract We have utilized attenuated total reflectance infrared (ATR-IR) spectroscopy to study the temperature dependence of conformational transitions and micro-domain structure in single- and binary-component Langmuir /Blodgett (LB) films. The single-component films such as lignoceric (C24), stearic (C18), and perdeuterated stearic (C18-d 35 ) acids, as well as binary-component films of 4:1 H:D C24:C18-d 35 and 4:1 H:D C18:C18-d 35 were studied. For C24:C18-d 35 binary films, changes in the peak wavenumber of the n a CH 2 band after heating above the main chain melting temperature (T m ) reflect a thermally induced de-mixing and phase separation of the C24 component from C18-d 35. For C18:C18-d 35 LB film, heating above T m shows that C18 and C18-d 35 components do not phase segregate and retain a degree of conformational flexibility, even at low temperatures. The carbonyl C/O stretching region between 1800 and 1600 cm 1 showed that each LB film contained a mixture of cis /trans ring dimer conformations at all temperatures below T m. Above T m, the LB films converted to the higher energy cis isomer, while upon further cooling the more stable trans isomer predominated. The shorter chain length C18:C18-d 35 LB film more easily crystallized the trans ring dimer after heating above T m. The band splitting of the dch 2 vibration at /1470 cm 1 was used to monitor micro-domain phase separation in these samples. For the C24:C18-d 35 binary mixture, heating the sample above T m before cooling crystallizes the C24 chains into phase-separated micro-domains. In contrast, the C18:C18-d 35 binary mixture shows that a much smaller domain size is calculated, indicative of a higher degree of chain miscibility. The shorter C18 hydrocarbon chain length produces a smaller dch 2 band splitting in the binary C18:C18-d 35 sample, and hence is less ordered and less crystalline than the C24 binary LB film. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Fatty acids; Langmuir/Blodgett films; Micro-domain structure; Infrared Abbreviations: ATR, attenuated total reflectance; C24, lignoceric acid; C18, stearic acid; C18-d35, perdeuterated stearic acid; FA, fatty acid; IR, infrared; LB, Langmuir/Blodgett; T m, main acyl chain melting temperature; SA, self-assembled. * Corresponding author. Tel.: / ; fax: / addresses: dluhy@chem.uga.edu, dluhy@sunchem.chem.uga.edu (R.A. Dluhy). 1 Current address: Department of Surgery, Emory University School of Medicine, Woodruff Memorial Building, 1639 Pierce Drive, Atlanta, GA 30322, USA /03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi: /s (03)00027-x

2 126 K.M. Faucher, R.A. Dluhy / Colloids and Surfaces A: Physicochem. Eng. Aspects 219 (2003) 125/ Introduction Langmuir/Blodgett (LB) films have been widely studied to provide insight into the molecular structure and organization of organic films that are used in a wide variety of applications. Some of these applications include the use of LB films in lubrication and corrosion inhibition as well as in the fabrication of microelectronic devices, nonlinear optics, and chemical sensors [1/4]. They can also be used as a model to further understand complex biological membranes [5]. One important consideration in the use of LB films is an understanding of the structural changes that occur at elevated temperatures. An understanding of the thermal stability is particularly important to the diffusion and permeability characteristics of LB films. X-ray reflection and neutron reflectivity measurements have been used to investigate the changes in structure of fatty acid and fatty acid salt LB films during annealing [6 /11]. Asmussen and Riegler [9] studied the structure of varying thickness behenic acid LB films before and after heating using X-ray reflection. They discovered that before heating the behenic acid LB films the molecules at the substrate/film interface were oriented approximately normal, whereas the molecules at some distance from the interface ranged in tilt angles from 218 to 368. After heating below the main phase transition, the orientation and packing of the first LB layer remained the same, however, the molecular packing and topology of subsequent layers were greatly altered and changed irreversibly with a uniform molecular tilt angle of 368. From these studies, the authors concluded that for five LB layers or less the behenic acid multilayers dewetted from the substrate surface (five layers), whereas for thicker films thermally induced uniform restructuring occurs. Infrared reflection/absorption spectroscopy has also been widely used to investigate thermal stability in LB films [12,13]. For example, order / disorder transitions for a 7-monolayer thick cadmium arachidate (CdA) LB film on silver were studied [13]. This study showed that if the CdA film was heated below the main transition temperature (T m ), intensity variations occurred for the C/H region (3000/2800 cm 1 ), but the vibrations due to the head group (1600/1400 cm 1 ) remained constant. This was attributed to a pretransitional disordering of the hydrocarbon chain packing. It was noted that no loss in infrared spectral band intensity occurred when cyclically heating the CA film to temperatures below T m and then back to room temperature. When the CA film was heated above T m, however, significant changes in the intensity of both the C/H and head group stretching regions were affected. Additionally, upon cooling to room temperature there was a permanent loss of infrared band intensity. This loss in spectral band intensity was attributed to an irreversible disordering in the packing of the cadmium head group lattice of the CA film. Since this early work there have been several reports that have utilized infrared spectroscopy [14 /24] and Raman scattering [25,26] to investigate the thermal stability and order /disorder transitions of LB and self-assembled (SA) monolayer films. Many of these studies employ polarized external reflection infrared reflection / absorption spectroscopy using silver or gold substrates [2,14,15,17,18,20 /24]. This external reflection method is highly sensitive to conformational and orientational changes since the parallel polarized radiation is responsive to specific molecular structure perpendicular to the surface. In spite of the work in this area, there is still no comprehensive picture of the exact mechanism of the structural changes that occur in monolayers as the temperature is increased, although several general conclusions can be made. One is that the monolayer structural changes in the pretransition melting phase (below T m ) are due to the hydrocarbon chains alone (i.e. changes in the chain tilt and gauche-trans isomerization of the hydrocarbon chains). Another general conclusion is that heating above T m irreversibly damages the monolayer film so that a breaking of the head group lattice and/or desorption occurs. In this work, we have used attenuated total reflectance infrared (ATR-IR) spectroscopy to investigate the structural changes that occur during order /disorder transitions of fatty acid LB films. The fatty acid films studied were 4-monolayer thick and were composed of single and

3 K.M. Faucher, R.A. Dluhy / Colloids and Surfaces A: Physicochem. Eng. Aspects 219 (2003) 125/ binary mixtures of lignoceric (C24), stearic (C18) and perdeuterated stearic acids (C18-d 35 ). ATR-IR spectra were acquired above and below T m for the fatty acid LB films to investigate the differences in hydrocarbon chain conformation as well as the carboxylic acid cis and trans ring dimer conformational isomers. In addition to studying the monolayer conformational structure, we have also used these temperature-dependent ATR-IR spectra to calculate the micro-domain size for each component in binary fatty acid LB films above and below T m. The calculation of micro-domain size relies on an infrared method first developed for studying threedimensional bulk solutions of binary n-alkane mixtures using infrared spectroscopy. The spectroscopic basis for this method involves monitoring the crystal field splitting of the dch 2 or dcd 2 scissoring vibrations in binary protiated /deuterated mixtures. This method has been applied to studying the effects that differences in temperature, H /D isotope composition, and chain length have on the microphase separation of n-alkane binary mixtures [27 /30]. We have previously utilized this method in order to calculate microdomain phase segregation in two-dimensional binary-component fatty acid LB films at room temperature [31]. The research described in this paper represents the first study that uses IR spectroscopy to calculate temperature-dependent micro-domain structure in fatty acid thin films. By monitoring changes in hydrocarbon chain conformation, cis / trans ring dimer isomer configurations, and microdomain size, we are able to add further insight into the complex structural changes that occur during thermally induced order/disorder transitions in two-dimensional fatty acid LB films. 2. Experimental 2.1. Preparation of fatty acids Lignoceric (C24), stearic (C18) and acyl chain perdeuterated stearic (C18-d 35 ) fatty acids were purchased from Matreya, Inc. (State College, PA) at a stated 99% purity and were used without further purification. Stock solutions of each fatty acid at 3.5/4.0 mg ml 1 were prepared by dissolving the proper amount in HPLC-grade chloroform. The 4:1 (mol:mol) H:D C24:C18-d 35 and 4:1 (mol:mol) H:D C18:C18-d 35 binary mixtures were prepared by combining aliquots of the individual fatty acid stock solutions in the proper molar ratio Substrates for LB film deposition Fatty acid monolayers were deposited onto monocrystalline, trapezoidal germanium (Ge) attenuated total reflectance (ATR) elements. The Ge ATR elements (Spectral Systems, Hopewell Junction, NY) had dimensions of 50 mm/10 mm/2 mm with 458 face angles, and a total surface area of 12.6 cm 2. Ge crystals were cleaned prior to LB film deposition by sonication for 15 min in a 6:4:1 chloroform:methanol:water solution, followed by sonication twice for 15 min each in ultrapure water obtained from a Barnstead (Dubuque, IA) ROpure/Nanopure reverse osmosis/deionization system (nominal resistivity of 18.3 MV cm 1 ). Chloroform and methanol used in the washing steps were HPLC-grade reagents LB film deposition Fatty acid monolayers were transferred onto the Ge ATR crystals using a Joyce-Loebl Ltd. (Gateshead, UK) LB trough that utilizes a constantperimeter PTFE-coated fiberglass tape to control the size of the trough area. The subphase used in these experiments was 18 MV cm 1 ultrapure. The subphase temperature was held constant to 219/ 1 8C by flowing thermostatted water through the hollow body of the PTFE-coated aluminum trough. Surface pressure measurements were recorded by differential weight measurements with a filter paper (Whatman No. 1) Wilhelmy plate suspended from a microbalance. The surface of the LB trough was determined to be clean if the surface pressure was less than 0.2 mn m 1 with the barriers fully closed. To prepare a transfer, a Ge ATR crystal was fully immersed into the subphase through a clean surface. The fatty acid sample was then applied to

4 128 K.M. Faucher, R.A. Dluhy / Colloids and Surfaces A: Physicochem. Eng. Aspects 219 (2003) 125/145 the trough surface and allowed to equilibrate for 15 min. The fatty acid monolayer was compressed at 10 cm 2 s 1 to a final surface pressure of 30.0 mn m 1. The Ge crystal was vertically raised from the subphase through the monolayer film at a rate of 20 mm min 1 while the surface pressure was held constant to 9/1.0 mn m 1. This procedure resulted in the first monolayer of the fatty acid to be deposited with its carboxyl head group supported by the Ge crystal surface. This procedure was repeated until a total of four monolayers had been deposited onto the surface of the Ge crystal. With the Ge crystal still immersed in the subphase after the deposition of the 4th monolayer, the remaining surface film was removed via suction until a surface pressure reading of 0.0 mn m 1 was obtained. The barriers were then fully opened and the Ge crystal was raised from the subphase. This procedure was performed to ensure that further deposition of fatty acid monolayers would not occur. The transfer ratio of the LB film balance was recorded for each transfer. Transfer ratios were found to be in the range 1.2/1.0 for a upward deposition of the monolayer film to the Ge crystal and 0.8/0.9 for the downward deposition of the LB film ATR-IR spectroscopy of transferred films Infrared spectra were acquired using a BioRad/ Digilab (Cambridge, MA) FTS-40 spectrometer equipped with a wide band, LN 2 -cooled HgCdTe detector. Spectra were recorded with 512 co-added scans at 2 cm 1 resolution using triangular apodization and one level of zero filling. The Ge ATR crystal was mounted into a horizontal ATR accessory (CIC Photonics, Inc., Albuquerque, NM). Absorbance spectra were acquired using backgrounds of the Ge crystal at every temperature point being studied just prior to LB film deposition. The temperature of the Ge crystal was changed using a custom designed stage that consisted of two thermoelectric heating/cooling (TEC) elements (Melcor Electronics, Trenton, NJ), a water jacket heat sink and an AD590 temperature sensor. The temperature sensor and TECs were wired to an Alpha Omega Instruments (Cumberland, RI) series 2 TC 2 TEC temperature controller that allowed the temperature to be monitored and controlled to within 9/0.1 8C. The temperature was allowed to settle for 7 min before spectra were acquired. ATR-IR spectra were baseline corrected using the Grams/32 spectral software package (ver 5.04, Galactic Industries, Inc., Salem, NH). Otherwise, the spectra have not been smoothed or further processed. For binary-component LB films, the band heights of the CH 2 or CD 2 methylene symmetric and anti-symmetric stretching vibrations were used to determine the fractional amount of C24, C18, or C18-d 35 fatty acid transferred to the Ge ATR crystal. Vibrational band heights, wavenumber peak positions, and integrated intensities were calculated using a 5-point center-ofgravity algorithm [32], written in our laboratory for the Grams/32 environment. 3. Results and discussion ATR-IR spectroscopy was used to study the temperature-dependent structural changes that occur in single- and binary-component fatty acid monolayers. Fatty acid films were transferred to Ge ATR crystals using the LB technique until four monolayers were deposited. Spectra of singlecomponent films of lignoceric (C24), stearic (C18), and deuterated stearic (C18-d 35 ) acids were obtained and compared with previous literature studies [18,19,33,34]. The observed IR bands as well as the vibrational band assignments for the protiated and deuterated single-component monolayer films are shown in Tables 1 and 2; these spectra were acquired immediately after LB film deposition at 23 8C. Two different binary-component fatty acid mixtures were selected for study: 4:1 H:D C24:C18-d 35 and 4:1 H:D C18:C18-d 35. These particular mixtures were selected to study the effects of temperature on two separate types of fatty acid monolayers: (1) identical chain lengths (C18:C18- d 35 ) and (2) varying chain lengths (C24:C18-d 35 ). These particular monolayers were studied in our previous investigations of micro-domain size in two-dimensional LB films at room temperature [31]. For both types of fatty acid monolayers, the

5 K.M. Faucher, R.A. Dluhy / Colloids and Surfaces A: Physicochem. Eng. Aspects 219 (2003) 125/ Table 1 Infrared bands with assignments observed in spectra of singlecomponent fatty acid LB film Vibrational assignment Lignoceric acid (C24) Stearic acid (C18) n a CH 3 (asymm stretch) n a CH 2 (asymm stretch) n s CH 2 (symm stretch) nc/o (stretch) (shoulder) dch 2 (scissoring) nc/o/doh / dc a H 2 (scissoring) dch 3 (methyl umbrella) / vch 2 (wagging) (/1360/1180 cm 1 ) nc/o/doh nc/o doh (bend) gch 2 (rock) Spectra were obtained from samples immediately after LB deposition at 23 8C. mole ratio of protiated /deuterated components (i.e. H:D) was 4:1. The observed IR bands as well as the band assignments for a binary film are presented in Table 3; these spectra were acquired immediately after LB transfer at 23 8C. The vibrational assignments presented in Tables 1/3 were based on prior literature [12,17 /19,33,35/ 38]. ATR-IR spectra were acquired both above and below T m so that the changes in hydrocarbon chain conformation, cis /trans carboxylic acid conformational isomers and micro-domain size of the FA films could be characterized as a Table 2 Infrared bands with assignments observed in spectra of deuterated stearic acid (C18-d 35 ) Vibrational assignment Observed (cm 1 ) n a CD 3 (asymm stretch) n a CD 2 (asymm stretch) n s CD 2 /2dCD 2 (Fermi resonance) n s CD 2 (symm stretch) n s CD 3 (symm stretch) nc/o (stretch) n s COO (asymm) n s COO (symm) doh coupled to C/D stretching nc/o coupled to doh nc/o dcd 2 (scissoring) n a dcd 3 (methyl umbrella) doh Spectra were obtained from sample immediately after LB deposition at 23 8C. function of temperature. Two types of temperature profiles were used. In temperature profile #1, the sample was heated above its T m and then cooled to /10 8C (i.e. 23 8C0/T G 0//10 8C, where T G is a temperature greater than T m for each individual sample). T G differed depending upon the type of sample; the T G temperatures used were: 90 8C for C24, 80 8C for C24:C18-d 35, and 65 8C for the C18, C18-d 35, and C18:C18-d 35 LB films. Using temperature profile #1, spectra were collected as a function of decreasing temperature from T G to / 10 8C. In temperature profile #2, the sample was first analyzed at room temperature, then heated to just below its T m, and subsequently cooled back to room temperature (e.g. 23 8C0/60 8C0/23 8C). For this profile, temperature-dependent spectra were collected first at room temperature after LB transfer (the initial spectrum), and then as a function of increasing temperature from 23 8C to just below T m Hydrocarbon conformational analysis Fig. 1 presents a three-dimensional representation of the temperature dependence of the unpolarized ATR-IR spectra of fatty acid LB monolayers transferred onto Ge ATR crystals. Both the C/H (3000/2800 cm 1 ) and C/D

6 130 K.M. Faucher, R.A. Dluhy / Colloids and Surfaces A: Physicochem. Eng. Aspects 219 (2003) 125/145 Table 3 Infrared bands with assignments observed in spectra of binary component fatty acid LB films Vibrational assignment 4:1 H:D C24:C18-d 35 4:1 H:D C18:C18-d 35 n a CH 3 (asymm stretch) n a CH 2 (asymm stretch) n s CH 2 (symm stretch) n a CD 3 (asymm stretch) n a CD 2 (asymm stretch) n s CD 2 /2dCD 2 (Fermi resonance) n s CD 2 (symm stretch) n s CD 3 (symm stretch) nc/o (stretch) dch 2 (scissoring) nc/o/doh dc a H 2 (scissoring) dch 3 (methyl umbrella) vch 2 (wagging) (/1360/ 1180 cm 1 ) nc/o/doh nc/o dcd 2 (scissoring) n a dcd 3 (methyl umbrella) doh (bend) gch 2 (rock) Spectra were obtained from samples immediately after LB deposition at 23 8C. (2250/2050 cm 1 ) stretching regions are shown. The spectra are presented decreasing in temperature starting above T m. Figs. 1(A) and (B) show the C /H stretching region for the C24 and C24:C18-d 35 fatty acid samples. Figs. 1(C) and (D) show the analogous plots of the C/D stretching region for the C18 and C18:C18-d 35 LB films. The spectra for the LB film samples in Fig. 1 show an increase in band height and a shifting of the C/ H or C/D stretching modes to lower wavenumber values as the temperature decreases. This trend is consistent with the LB film undergoing a phase transition from a disordered (liquid) to an ordered (solid) phase. We have measured the changes in wavenumber position for the C /H and C /D stretching bands to follow the conformational order of the protiated and deuterated chains as a function of temperature. The peak wavenumber position reflects the intramolecular conformation of the acyl chains, where a higher wavenumber value is indicativeofa disordered chain, while a lower wavenumber value reflects an ordered, trans acyl chain segment. Fig. 2 presents the wavenumber changes in the n a CH 2 stretching mode (/2920 cm 1 ) and the n a CD 2 stretching mode (/2190 cm 1 ) for the C24 samples (i.e. C24 and 4:1 H:D C24:C18-d 35 LB films). This figure presents data after sample heating using both temperature profiles #1 and #2. Profile #1 used the heating program 23 8C0/ T G 0//10 8C, where T G was 90 and 80 8C for the C24 and C24:C18-d 35 LB films, respectively. Profile #2 used the heating program 23 8C0/ 60 8C 0/23 8C for both C24 films. The temperature-dependent wavenumber plot for the C/H region in C24 and C24:C18-d 35 LB films investigated using heating profile #2 (i.e. below T m ) is presented in Fig. 2(A). Below T m,at 23 8C, the initial wavenumber value is greater for the C24:C18-d 35 film ( cm 1 ) than the C24 film ( cm 1 ), indicating increased disorder in the binary mixture relative to the pure component. With increasing temperature, however, the frequency of the C24:C18-d 35 LB film decreases, indicating an ordering of the hydrocarbon chains (Fig. 2(A)). In contrast, the wavenumber value of the single-component C24 film increases, indicating a conformational disordering, as expected for a single-component with increasing temperature. The two values appear to converge to an average value slightly below T m. The corresponding temperature-dependent wavenumber plot for the C /H region in the C24 samples investigated using heating profile #1 (i.e. above T m ) is shown in Fig. 2(B). Above T m, the peak values (/2920 cm 1 ) reflect the fluid and disordered nature of the acyl chains. As the

7 K.M. Faucher, R.A. Dluhy / Colloids and Surfaces A: Physicochem. Eng. Aspects 219 (2003) 125/ Fig. 1. Representative ATR-IR spectra in the C/H (3000/2800 cm 1 ) and C/D (2250/2050 cm 1 ) stretching regions presented as a function of temperature. Spectra are shown of 4 LB fatty acid monolayers transferred onto Ge ATR crystals for the following samples: (A) C/H region of C24 fatty acid, (B) C/H region of the C24 component in a 4:1 H:D C24:C18-d 35 binary mixture, (C) C/D region of C18-d 35 fatty acid, and (D) C/D region of the C18-d 35 component in a 4:1 H:D C18:C18-d 35 binary mixture. temperature decreases, the peak wavenumber values for both C24 samples also decrease in a similar fashion. At /10 8C both the C24 and C24:C18-d 35 films have ac/h peak wavenumber value below 2918 cm 1, indicating conformationally ordered acyl chains [39]. For the C24:C18-d 35 film, the final wavenumber value of the n a CH 2 band at /10 8C decreases by almost two wavenumbers when compared with the band value just after film deposition, cm 1 vs cm 1 (Figs. 2(A) and (B)). This differs from the C24 film alone, where the wavenumber value at /10 8C after heating above T m is almost identical to the value obtained immediately after film deposition, cm 1 vs cm 1. The decrease in the wavenumber value after heating above T m for the C24:C18-d 35 film reflects a thermally induced de-mixing and phase separation of the C24 component from the C18-d 35 component. Further support for this conclusion will be presented later in this work. The analogous wavenumber vs. temperature plots for the n a CD 2 stretching mode (/2190 cm 1 ) of the C18-d 35 component are presented in Figs. 2(C) and (D). Fig. 2(C) illustrates the effect of temperature profile #2 (heating below T m ) on the C18-d 35 component in C18-d 35 and C24:C18-d 35 LB films, while Fig. 2(D) shows the effect of temperature profile #1 (heating above T m ) on the C18-d 35 films. As seen in Fig. 2(C), the two C18-d 35 films result in nearly identical wavenumber values when heated to just below T m. When heated above T m and cooled, the wavenumber values of the two C18-d 35 films are separated by /2.0 cm 1 at 65 8C (Fig. 2(D)), converge with decreasing temperature, and are nearly the same

8 132 K.M. Faucher, R.A. Dluhy / Colloids and Surfaces A: Physicochem. Eng. Aspects 219 (2003) 125/145 Fig. 2. Temperature dependence of the wavenumber values for the anti-symmetric C/H (/2920 cm 1 ) and anti-symmetric C/D (/ 2190 cm 1 ) stretching modes in C24-containing fatty acid LB monolayers. (A) Effect of heating below T m on the C24 component in C24 (m) and 4:1 H:D C24:C18-d 35 (k) LB films. (B) Effect of heating above T m on the C24 component in C24 (m) and 4:1 H:D C24:C18-d 35 (k) LB films. (C) Effect of heating below T m on the C18-d 35 component in C18-d 35 (j) and 4:1 H:D C24:C18-d 35 (I)LB films. (D) Effect of heating above T m on the C18-d 35 component in C18-d 35 (j) and 4:1 H:D C24:C18-d 35 (I) LB films. (within 0.5 cm 1 )at/10 8C. The wavenumber value of the C18-d 35 component in the binary film at /10 8C ( cm 1 ) is only slightly smaller than in the binary film initially after LB deposition ( cm 1 ). These results indicate that the C18- d 35 component of the C24:C18-d 35 LB film does not appreciably change structure after temperature cycling. Fig. 3 presents the wavenumber changes in the n a CH 2 stretching mode and the n a CD 2 stretching mode for the C18 samples (i.e. C18 and 4:1 H:D C18:C18-d 35 LB films). This figure presents data after sample heating using both temperature profiles #1 and #2. The CH 2 data for the C18 samples heated using profile #2 (below T m ) are presented in Fig. 3(A). The results for the temperature dependence of the wavenumber shift seen in Fig. 3(A) resemble the results obtained for the C24 LB films in Fig. 2(A). In both cases, the initial difference in wavenumber values between the single- and binary-component mixtures is eliminated upon heating to below T m. Fig. 3(B) presents the wavenumber changes in the n a CH 2 stretching mode for the C18 and

9 K.M. Faucher, R.A. Dluhy / Colloids and Surfaces A: Physicochem. Eng. Aspects 219 (2003) 125/ Fig. 3. Temperature dependence of the wavenumber values for the anti-symmetric C/H (/2920 cm 1 ) and anti-symmetric C/D (/ 2190 cm 1 ) stretching modes in C18-containing fatty acid LB monolayers. (A) Effect of heating below T m on the C18 component in C18 (m) and 4:1 H:D C18:C18-d 35 (k) LB films. (B) Effect of heating above T m on the C18 component in C18 (m) and 4:1 H:D C18:C18-d 35 (k) LB films. (C) Effect of heating below T m on the C18-d 35 component in C18-d 35 (j) and 4:1 H:D C18:C18-d 35 (I)LB films. (D) Effect of heating above T m on the C18-d 35 component in C18-d 35 (j) and 4:1 H:D C18:C18-d 35 (I) LB films. C18:C18-d 35 films after heating using temperature profile #1 (above T m ). These experiments were performed in the same manner as the experiments using the C24 and C24:C18-d 35 films, except that the highest temperature reached was 65 8C. The decrease in the wavenumber values upon cooling to /10 8C for the C18 and C18:C18-d 35 films follow a similar pattern to the data previously presented for the C24 single- and binary-components in Fig. 2(B). In both plots, there is a sharp initial decrease in value followed by a constant, smaller decrease towards the final value. The final wavenumber value of the C18 component in the C18:C18-d 35 film at /10 8C is cm 1, which indicates that the hydrocarbon remains slightly disordered [39,40]. The wavenumber vs. temperature plots for the n a CD 2 stretching mode (/2190 cm 1 ) of the C18- d 35 samples are presented in Figs. 3(C) and (D). Fig. 3(C) illustrates the effect of temperature profile #2 (heating below T m ) on the C18-d 35 component in C18-d 35 and C18:C18-d 35 LB films,

10 134 K.M. Faucher, R.A. Dluhy / Colloids and Surfaces A: Physicochem. Eng. Aspects 219 (2003) 125/145 while Fig. 3(D) shows the effect of temperature profile #1 (heating above T m ) on the C18-d 35 films. In a similar fashion seen for the C24 samples (Fig. 2(C)), the two C18-d 35 films converge to a similar wavenumber value when heated to below T m (Fig. 3(C)). When heated above T m and cooled, the wavenumber values of the two C18-d 35 films are separated by /1.6 cm 1 at 65 8C, but do not converge with decreasing temperature and have nearly the same separation (/1.5 cm 1 ) at / 10 8C. The implication from this data is that the C18-d 35 component in the C18:C18-d 35 binary film does not phase segregate, and that the well-mixed nature of this system allows the deuterated component some conformational flexibility cis /trans Ring dimer conformations The cis /trans ring dimer in fatty acid dispersions has been characterized by infrared spectroscopy [35,41]; these studies have shown that cis / trans ring dimer conformations influence the frequency and position of several IR vibrations in the region from 1800 to 650 cm 1. (Note: the nomenclature is such that the fatty acids are cis and trans with respect to the C/O and C a /C b bonds.) The existence of the cis /trans ring dimer was further confirmed in stearic and arachidic acid LB films [18,19,33,42], where it was shown that the stability of the configurations was dependent on film thickness and temperature [18,19,33]. A previous study [19] has looked at the changes that occur in the cis /trans ring dimer conformations in fatty acid LB films after heating. In this work, we have further investigated the effects that heating both above and below T m have on the cis /trans ring dimer configurations in single- and binary-component LB films. Figs. 4(A) and (B) present ATR spectra between 1800 and 1600 cm 1 for C24 and 4:1 H:D C24:C18-d 35 films using temperature profile #1, i.e. heating from room temperature to above T m with subsequent cooling to below zero (i.e. 23 8C0/80 8C0/ /10 8C). In these figures, ATR spectra at three different temperatures are presented. Initially, just after LB film deposition, the C/O band is quite broad due to a mixture of cis and trans ring dimer isomers and is centered at 1700 cm 1 (Fig. 4(A), 23 8C initial). However, after heating above T m (80 8C), there is a narrowing and shifting of the C/ O band to 1712 cm 1. This narrowing and shifting of the C /O band has been assigned to the conversion of the ring dimer predominantly to the cis conformation [35,41,43]. After cooling down to /10 8C there is a shifting of the C/O band to lower wavenumber values and a splitting of the C/O vibration into two bands at 1702 and 1686 cm 1. This splitting is associated with the domination of the trans dimer conformation [35,41,43]. Although both the C24 and C24:C18-d 35 LB films (Fig. 4(B)) produce the band splitting at /10 8C, it is much more pronounced in the single-component C24 film. Thus, it appears that the C18-d 35 component could be interfering with the formation of the trans dimer in the C24:C18-d 35 LB film after cooling. Another difference in the spectra presented in Figs. 4(A) and (B) is that the band intensity of the C /O vibration in the C24 film decreases less after heating than does the C/O band intensity in the spectra of the C24:C18-d 35 LB film. This decrease in spectral band intensity has previously been attributed to a breaking of the head group lattice and/or desorption of the fatty acids from the film [12,13,18,19]. It has also been suggested that the decrease in band intensity after heating is due to a change in the packing density of the film [20]. The greater amount of lost band intensity, as well as the decreased value of T m for the C24:C18-d 35 sample relative to C24 (80 8C vs. 90 8C), indicates that the presence of the C18-d 35 reduces the thermal stability of the C24 in the C24:C18-d 35 LB film. Figs. 4(C) and (D) present the spectra for C24 and 4:1 H:D C24:C18-d 35 LB films from 1800 to 1600 cm 1 acquired using temperature profile #2, i.e. heating from room temperature to below T m and back to room temperature (i.e. 23 8C0/ 60 8C0/23 8C). Unlike the spectra acquired for the films above T m, the C/O vibration in the C24 or C24:C18-d 35 LB films does not change wavenumber position or convert to one particular cis / trans dimer conformation during the temperature profile. This indicates that below the melting point the cis /trans dimer conformers remain a mixture, similar to what is observed just after LB film

11 K.M. Faucher, R.A. Dluhy / Colloids and Surfaces A: Physicochem. Eng. Aspects 219 (2003) 125/ Fig. 4. Representative ATR-IR spectra in the C/O stretching region (1800/1600 cm 1 ) of C24-containing fatty acid monolayers presented as a function of temperature. Spectra are shown of 4 LB fatty acid monolayers transferred onto Ge ATR crystals for the C24 and 4:1 H:D C24:C18-d 35 samples. (A) Spectra of C24 LB film when heated above T m. (B) Spectra of 4:1 H:D C24:C18-d 35 LB film when heated above T m. (C) Spectra of C24 LB film when heated below T m. (D) Spectra of 4:1 H:D C24:C18-d 35 LB film when heated below T m. deposition. There is an additional decrease in the spectral band intensity for the C24:C18-d 35 LB film (Fig. 4(D)) that is not observed for the C24 alone (Fig. 4(C)). The C18-d 35 component, due to its hydrocarbon chain mismatch, is undoubtedly altering the packing density of the film; these results support the theory that a change in the packing density is causing the decrease in band intensity after heating [20]. Other spectral regions also provide evidence for micro-domain phase separation in the C24:C18- d 35 LB film. Fig. 6 illustrates the CH 2 wagging band progressions between 1350 and 1150 cm 1 for the FA LB films. These CH 2 wagging bands are highly coupled vibrational modes that exist only in ordered hydrocarbon systems. For the case of the C24:C18-d 35 LB film, the spectra show that after heating and cooling, a wagging mode band progression appears in the spectra of the binary mixture around 1300 cm 1 that is not present just after transfer (Figs. 6(A) and (B)). These additional wagging modes agree in number and have similar intensity to the wagging modes seen in the spectra of the pure component C24 film. Further support for the phase separation and de-mixing of the C24 from the C18-d 35 components from one another in the binary mixture is seen by an increase in the scissoring band separation from 7.9 to 9.7 cm 1 after heating and cooling (discussed in detail in Section 3.3). The increase of the scissoring band separation and the appearance of the additional wagging modes are due to increased

12 136 K.M. Faucher, R.A. Dluhy / Colloids and Surfaces A: Physicochem. Eng. Aspects 219 (2003) 125/145 intermolecular interaction by neighboring C24 molecules. The temperature-dependent ATR spectra of C18 and 4:1 H:D C18:C18-d 35 LB films from 1800 to 1600 cm 1 are presented in Figs. 5(A) and (B). In this set of experiments, the transferred films were subjected to temperature profile #1, i.e. heating from room temperature to above T m with subsequent cooling to below zero (i.e. 23 8C 0/65 8C 0//10 8C). There are similar trends in the formation of cis and trans ring dimer conformers for the C18 LB films (Figs. 5(A) and (B)) as there was for the C24 LB films (Figs. 4(A) and (B)). The main difference in the cis /trans dimer conformations between the two binary mixtures is that there is a slightly increased band splitting observed in the C/O vibration (due to the trans conformer at 1702 and 1686 cm 1 ) after cooling in the 4:1 H:D C18:C18-d 35 LB film (compare Fig. 5(B) with Fig. 4(B)). This indicates that the C18 and C18:C18-d 35 LB films could more easily crystallize the trans dimer configuration. This is probably due to the shorter chain length of the C18 film and greater mixing in the 4:1 H:D C18:C18-d 35 film. There is also a greater loss of spectral intensity in the C18 and C18:C18-d 35 spectra after heating as well, due to decreased thermal stability and increased breaking apart of the head group lattice and packing density [12,13,19,20]. Figs. 5(C) and (D) present spectra from 1800 to 1600 cm 1 for C18 and 4:1 H:D C18:C18-d 35 LB Fig. 5. Representative ATR-IR spectra in the C/O stretching region (1800/1600 cm 1 ) of C18-containing fatty acid monolayers presented as a function of temperature. Spectra are shown of 4 LB fatty acid monolayers transferred onto Ge ATR crystals for the C18 and 4:1 H:D C18:C18-d 35 samples. (A) Spectra of C18 LB film when heated above T m. (B) Spectra of 4:1 H:D C18:C18-d 35 LB film when heated above T m. (C) Spectra of C18 LB film when heated below T m. (D) Spectra of 4:1 H:D C18:C18-d 35 LB film when heated below T m.

13 K.M. Faucher, R.A. Dluhy / Colloids and Surfaces A: Physicochem. Eng. Aspects 219 (2003) 125/ films heated to below T m. In this set of experiments, the transferred films were subjected to temperature profile #2, i.e. heating from room temperature to below T m with subsequent cooling to room temperature (i.e. 23 8C 0/50 8C 0/23 8C). In a similar fashion to that observed for the C24 films, only intensity changes occur in the C /O band; no dramatic differences in the cis /trans dimer configurations are observed. A comparison of the spectra obtained for the C18 and C18 binary-component films aboveand below the T m reveals that there is no evidence for phase separation in the C18:C18-d 35 film. Although there is a slight increase in the scissoring band splitting after heating for the C18:C18-d 35 film (7.7 cm 1 after heating as opposed to 7.1 cm 1 before heating*/data discussed in detail below), there is no increase in number or intensity in the wagging mode progression in the C18 binary film. Figs. 6(C) and (D) show that after temperature cycling, the number and intensity of the CH 2 wagging modes seen in the spectra of the C18:C18- d 35 LB films are the same as the number seen for single-component C18 film alone. This indicates that the C18-d 35 introduces less perturbation to the structure of the C18:C18-d 35 binary film than to the C24:C18-d 35 film, presumably due to the miscibility of the two identical chain lengths Micro-domain size determination Infrared spectroscopy has been successfully employed to study the micro-domain phase separation in binary alkane mixtures. This method was originally developed as a means to study the growth kinetics of microphase separation in threedimensional bulk solutions of binary n-alkane mixtures. Infrared spectroscopy is able to monitor microphase separation due to the fact that the crystal field splitting of the methylene scissoring (dch 2 ) vibration at /1470 cm 1 is very sensitive to the crystallographic subcell packing of the hydrocarbon chain s next nearest neighbor. For example, orthorhombic packing of a hydrocarbon chain assembly is associated with a splitting in the dch 2 scissoring band, whereas hexagonal packing of the hydrocarbon matrix is associated with a dch 2 scissoring band singlet. A diagrammatic representation of these limiting cases is presented in Fig. 7. In practice, it is possible to calculate the microdomain sizes for individual components in a binary mixture as long as one component is deuterated and the crystal field splitting of the dch 2 or dcd 2 band vibration is observed. The addition of deuterated alkane chains to a protiated hydrocarbon matrix disrupts the H:H interaction constants among next nearest-neighbor alkane chains and results in a decrease in the splitting of the dch 2 scissoring band. In addition to calculating micro-domain size, splitting of the dch 2 and dcd 2 scissoring vibrations has also been used to monitor interlayer diffusion of cadmium arachidate through polymer films [21]. We have previously used this method in determining microdomain size in two-dimensional binary-component LB films at room temperature [31]. In order to calculate the micro-domain size of fatty acid LB films at varying temperatures, the normalized splitting, R, must be obtained. R is determined from the wavenumber separation of either the dch 2 or dcd 2 bands in a single- or binary-component mixture with the corresponding pure component at its maximum wavenumber separation: R Dn Dn 0 : (1) In the above equation, Dn is the magnitude of the dch 2 or dcd 2 band splitting (in cm 1 ) in the single- or binary-component sample and Dn 0 the magnitude of the splitting of the dch 2 or dcd 2 band in the corresponding pure component at maximum separation, typically in a highly ordered conformational state. We use the magnitude of the dch 2 splitting for the pure component at /10 8C as our estimate of Dn 0. This value for Dn 0 allows us to calculate the micro-domain size for C24 and C18 single-component films at any temperature greater than /10 8C. Since the C18-d 35 component is always the minor component in the C24:C18-d 35 and C18:C18-d 35 binary mixtures (4: 1 H:D), no splitting is observed for the dcd 2 band in these binary-component LB films. Therefore, micro-domain sizes were calculated only for the

14 138 K.M. Faucher, R.A. Dluhy / Colloids and Surfaces A: Physicochem. Eng. Aspects 219 (2003) 125/145 Fig. 6. Representative ATR-IR spectra of the CH 2 wagging band progressions (1800/1600 cm 1 ) for fatty acid monolayers presented as a function of temperature. (Note: An asterisk indicates the middle of the wagging mode progression.) Spectra are shown of 4 LB fatty acid monolayers transferred onto Ge ATR crystals. (A) Spectra of C24 LB film at room temperature and three spectra of C24:C18-d 35 LB film when heated below T m,238c0/60 8C0/23 8C. (B) Spectra of C24 LB film at room temperature and three spectra of C24:C18-d 35 LB film when heated above T m,238c0/80 8C0//10 8C. (C) Spectra of C18 LB film at room temperature and three spectra of C18:C18-d 35 LB film when heated below T m,238c0/50 8C0/23 8C. (D) Spectra of C18 LB film at room temperature and three spectra of C18:C18-d 35 LB film when heated above T m,238c0/65 8C0//10 8C. protiated C24 and C18 components using the splitting of the dch 2 band. The equation that relates the normalized splitting of the dch 2 mode to the two-dimensional micro-domain size is the following: p 2 N pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 : (2) 2(1 R) In the above equation, N is the number of chains in a two-dimensional micro-domain. A detailed derivation of Eq. (2) from the zone center vibrational frequencies is presented elsewhere. While the R value mainly reflects the average size of domains, it is also affected by domain shape. Thus, for a constant value of N to reflect micro-domain separation, the normalized splitting should increase as the domain sizes become more compact. The experimental data described below show that this assumption is true for the C24:C18- d 35 and C18:C18-d 35 binary LB films studied here. The normalized splitting was observed to be greater for the C24 binary mixture (8.0 cm 1 ) compared with the C18 mixture (7.0 cm 1 ), as expected, since the longer chain length is more ordered and able to form a more compact microdomain. Figs. 8(A) and (B) present the dch 2 scissoring band spectra collected from 1490 to 1440 cm 1 for C24 and 4:1 H:D C24:C18-d 35 LB films acquired using temperature profile #2, i.e. heating from room temperature to below T m and back to room temperature (i.e. 23 8C0/60 8C0/23 8C). At

15 K.M. Faucher, R.A. Dluhy / Colloids and Surfaces A: Physicochem. Eng. Aspects 219 (2003) 125/ Fig. 7. (A) Diagrammatic representation of hexagonal chain subcell packing arrangement and its characteristic IR dch 2 methylene scissoring band spectrum. (B) Diagrammatic representation of orthorhombic chain subcell packing arrangement and its characteristic IR dch 2 methylene scissoring band spectrum. 23 8C, the splitting of the dch 2 band for the single C24 film is greater than that for the C24 component in the C24:C18-d 35 binary mixture (9.5 cm 1 vs. 8.0 cm 1, see Fig. 8(C)). As the temperature increases, however, the dch 2 band splitting for the C24 film decreases steadily while the splitting for the C24 component in the C24:C18-d 35 mixture remains relatively constant so that the dch 2 wavenumber splitting for both films is within approximately 0.5 cm 1 at 65 8C. Figs. 9(A) and (B) present the dch 2 scissoring band spectra for C24 and 4:1 H:D C24:C18-d 35 LB films collected using temperature profile #1, i.e. heating from room temperature to above T m with subsequent cooling to below zero (i.e. 23 8C0/ 90 8C 0//10 8C). In this case, for both samples, the scissoring band collapses to a singlet at /1464 cm 1 above T m. This singlet indicates the absence of an ordered matrix with the hydrocarbon chains now in a hexagonal subcell arrangement. As the temperature is decreased, the splitting of the dch 2 vibration reappears for both films at 65 8C; both the C24 film and the binary C24:C18-d 35 film have nearly identical values (within 0.5 cm 1 ) for the wavenumber splitting of the dch 2 band below 60 8C. The spectra of the dch 2 band for the C24:C18-d 35 film show an increased band intensity in the center of the dch 2 profile that indicates the presence of a middle peak (Fig. 9(B)). A band in this area has been previously assigned to a protiated chain surrounded by deuterated chains. The calculation of domain size reflects the extent of phase separation in these samples. The hydrocarbon domain size calculations for the samples heated to below T m (Fig. 8(D)) show that, upon cooling, the C24 film increases its domain size (from /30 to 138 hydrocarbon chains) while the domain size of the C24 component in the C24:C18-d 35 film remains constant (/ 15/18 chains). After heating above T m, however, the domain sizes for both the single and binary samples increase steadily upon cooling up to / 10 8C (Fig. 9(D)). When compared with the domain size of a sample obtained at room temperature immediately after LB film deposition, the C24 domain size increases by /35 chains after

16 140 K.M. Faucher, R.A. Dluhy / Colloids and Surfaces A: Physicochem. Eng. Aspects 219 (2003) 125/145 Fig. 8. Effect of heating to below T m on the splitting of the dch 2 methylene scissoring band in C24-containing fatty acid monolayers. Spectra are shown of 4 LB fatty acid monolayers transferred onto Ge ATR crystals for the C24 and 4:1 H:D C24:C18-d 35 samples. (A) dch 2 spectra of C24 LB film when heated below T m. (B) dch 2 spectra of C24 component in 4:1 H:D C24:C18-d 35 LB film when heated below T m. (C) Splitting of the dch 2 methylene scissoring band on the C24 component in C24 (m) and 4:1 H:D C24:C18-d 35 (k) LB films when heated below T m. (D) Calculated micro-domain size for C24 component in C24 (j) and 4:1 H:D C24:C18-d 35 (I) LB films when heated below T m. heating above T m, but the domain size for the C24:C18-d 35 increases by /60 chains. This difference indicates that temperature cycling above T m more efficiently crystallizes C24 chains into phaseseparated micro-domains in a C24:C18-d 35 binary film than does heating to below T m. Figs. 10(A) and (B) present the dch 2 scissoring band spectra collected from 1490 to 1440 cm 1 for C18 and 4:1 H:D C18:C18-d 35 LB films acquired using temperature profile #2, i.e. heating from room temperature to below T m and back to room temperature (i.e. 23 8C 0/60 8C 0/23 8C). The wavenumber splitting of the dch 2 band decreases with increasing temperature (as shown in Fig. 10(C)) in a fashion similar to that observed for the C24 LB films (Fig. 8(C)). In the case of the C18 films, the difference in wavenumber splitting ( / 1.3 cm 1 ) between the single C18 film and the C18:C18-d 35 binary mixture is maintained through the temperature profile, whereas for the C24 films, the separation tended to decrease with increasing temperature (Fig. 10(C) vs. Fig. 8(C)). Figs. 11(A) and (B) present the scissoring band spectra for C18 and 4:1 H:D C18:C18-d 35 LB films

17 K.M. Faucher, R.A. Dluhy / Colloids and Surfaces A: Physicochem. Eng. Aspects 219 (2003) 125/ Fig. 9. Effect of heating to above T m on the splitting of the dch 2 methylene scissoring band in C24-containing fatty acid monolayers. Spectra are shown of 4 LB fatty acid monolayers transferred onto Ge ATR crystals for the C24 and 4:1 H:D C24:C18-d 35 samples. (A) dch 2 spectra of C24 LB film when heated above T m. (B) dch 2 spectra of C24 component in 4:1 H:D C24:C18-d 35 LB film when heated above T m. (C) Splitting of the dch 2 methylene scissoring band on the C24 component in C24 (m) and 4:1 H:D C24:C18-d 35 (k) LB films when heated above T m. (D) Calculated micro-domain size for C24 component in C24 (j) and 4:1 H:D C24:C18-d 35 (I) LB films when heated above T m. collected using temperature profile #1, i.e. heating from room temperature to above T m with subsequent cooling to below zero (i.e. 23 8C0/70 8C0/ /10 8C). As previously seen for the C24 films (Fig. 9), heating above T m results in a collapse of the dch 2 scissoring band to a singlet at /1464 cm 1. For the C18 films, the dch 2 wavenumber splitting begins to reappear at 50 8C in both films as the sample is cooled (Fig. 11(C)). As the temperature is lowered below 40 8C, the dch 2 wavenumber splitting increases similarly for the two C18 films, although the difference in the value of the dch 2 splitting between the C18 films (/1.0 cm 1 ) is larger than the difference observed for the C24 films (/ 0.5 cm 1, Fig. 9(C)). For the C18 samples heated using temperature profile #2 (i.e. to below T m ), the domain size calculations show that there is a increase in domain size as the sample is cooled, from /27 to 140 for the C18 single film and /10 to 20 chains for the C18:C18-d 35 binary-component film, respectively (Fig. 10(D)). This is in contrast