J. Phys. Chem. B 2008, 112, 13823–13833
13823
A Low Temperature Phase Transition in Langmuir-Blodgett Films Thomas P. Johansson and Gary W. Leach* Laboratory for AdVanced Spectroscopy and Imaging Research, 4D LABS and Department of Chemistry, Simon Fraser UniVersity, Burnaby, B.C., V5A 1S6 Canada ReceiVed: July 14, 2008
The influences of temperature on the SFG spectra of Langmuir-Blodgett films of cadmium stearate, ferric stearate, stearic acid and octadecanamide are reported. Upon cooling, all films display reversible discontinuous shifts of ∼8 cm-1 in the r+, r- and rfermi modes of the terminal methyl groups at ∼150 K. Reversible changes in the relative intensities of these methyl group peaks, most pronounced in the PPP spectra, are also observed and attributed to a change in the environment of the methyl group that accompanies a discontinuous transition in the ordering of their alkyl chains. The onset of new spectral features at higher frequency is attributed to the observation of ordered water molecules contained within the films. The correlation between the onset of the water features and the onset of the reversible, discontinuous, spectroscopic changes of the amphiphiles argues for a causal connection between the two. In addition to the discontinuous behavior upon cooling, monolayer films of stearic acid and octadecanamide display activity of methylene modes upon exposure to vacuum. Films displaying SFG-active methylene groups at room temperature had them gradually become completely SFG-inactive by 100 K. Heating the films to room temperature revealed that the methylene group activity was reversible. Monolayer films of cadmium stearate and ferric stearate do not display this methylene activity upon exposure to vacuum, suggesting that this behavior may be linked to solvation of the amphiphile’s headgroup. These observations suggest that water plays a key role in the stability and structure of LB supported monolayers, and have important implications to those interested in low temperature (cryogenic) effects of biological systems. Introduction Surface films are of fundamental importance because of their many interesting and useful physical and chemical characteristics.1-5 In addition to their broad current utility and potential for many future applications, surface films represent environments characterized by rich and diverse physical phenomena. As such, they represent model systems of reduced dimensionality and offer the potential for experiment and theory to intersect. Indeed, this potential has sustained an ongoing interest in the phase transitions of two-dimensional surface films.6 The development and application of new and powerful surface characterization techniques, together with new theoretical developments have led to the broader understanding of surface phase transitions and the distinction between melting phenomena in two and three dimensions.7-12 It is now generally accepted that two-dimensional melting processes are not necessarily characterized by a single discontinuous transition, and that the transition between ordered and disordered phases can proceed via intermediate phases described by quasi-long-range orientational ordering. The detailed structure-property characteristics and dynamics of these two-dimensional phase transitions are dictated by the nature of the surface, the size and shape of the adsorbed species in question as well as the type of interaction between the surface and adsorbate.10 To date, the behavior of films composed of molecules of many shapes and sizes have been investigated, including the limiting class of single molecule thickness films formed through self-assembly chemistry.11-13 Previous efforts to examine the low temperature behavior in this type of ordered ultrathin film have focused primarily on * Author to whom correspondence should be addressed. E-mail: gleach@ sfu.ca.
the monolayers formed by self-assembly of n-alkanethiols on gold substrates. The structure and physical properties of these films have been extensively studied using a variety of techniques including electron14 and He diffraction methods,15 transmission electron microscopy,16 scanning tunneling microscopy17 as well as many spectroscopic methods.18,19 In their studies of docosanethiol self-assembled monolayers, Nuzzo et al.12 used infrared spectroscopy to show that these ordered monolayers were characterized by a significant degree of gauche conformations concentrated in the vicinity of the chain termini and that the spectroscopic features observed as a function of temperature were akin to those observed for bulk phase n-alkane crystals. They observed slow continuous spectral changes over a broad temperature range, characteristic of complex phases and heterogeneity in the organic monolayers. Rowntree and co-workers have since undertaken an IR study of SAMs of hexadecanethiol and its deuterated counterpart formed on Au substrates.13 They also observed continuous changes in peak position (on the order of 2 cm-1 spectral shift) and increases in intensity (up to 75%) for both the methyl and methylene modes upon cooling from room temperature to 25 K and attributed the effects to intraand intermolecular interactions of the alkyl chains. The continuous nature of the spectral changes observed in both of these studies suggest that, although there may well exist complex phase transitions of the order-to-order and order-to-disorder type, no phase transitions of a discontinuous nature occur within these n-alkyl SAMs at low temperature. Though the structure of monolayer films deposited via the Langmuir-Blodgett technique is qualitatively similar to those formed by self-assembly, the nature of the adsorbate-substrate interactions are markedly different between the two and may be expected to result in different structural arrangements, degrees
10.1021/jp806226e CCC: $40.75 2008 American Chemical Society Published on Web 10/15/2008
13824 J. Phys. Chem. B, Vol. 112, No. 44, 2008
Johansson and Leach
of freedom, and phase behavior. Indeed, it is well-known that in the vicinity of room temperature, there exists a wealth of complex phase behavior, even for many simple LB films.3 Unfortunately, there is little or no detailed information available regarding the structural characteristics of these films at lower temperatures, despite their relevance to low temperature effects in biological systems, such as the cryogenic freezing of cell membrane structures. The purpose of this work is to investigate the low temperature structural characteristics and phase behavior in ordered amphiphiles deposited by the LB technique. We employ the method of sum frequency generation (SFG) vibrational spectroscopy because of its surface specificity and excellent sensitivity.20
group has C3V symmetry and possesses three nonzero independent elements in its hyperpolarizability tensor. Associated with the r+ mode are Rzzz and Rxxz ) Ryyz, and associated with the rmodes (assumed degenerate) is Rzxx. It should be recognized that one also generally expects a nonzero hyperpolarizability component Rxxx, which, under conditions of free rotation or a random distribution of orientations, averages to zero. Assuming a delta function distribution of orientation angles, θ, an isotropic sample under rotation about the surface normal, and averaging over Euler angles yields23
1 (2) (2) χYYZ,r NR [(R + 1)〈cos(θ)〉 + + ) χXXZ,r+ ) 2 zzz (R - 1)〈cos3(θ)〉] (6)
Theory of SFG The theoretical description of SFG has been presented in detail elsewhere.21 Here, we provide only a brief description. Irradiation from two optical fields E1 and E2 with frequencies ω1 and ω2 generates a second-order nonlinear polarization
P(2)(ω)ω1+ω2) ) χ(2) eff : E1(ω1) E2(ω2)
(1)
where (2) χ(2) eff ) [e(ω3) K(ω3)]χ : [e(ω1) L(ω1)][e(ω2) L(ω2)] (2)
e being the unit polarization vector, L the linear Fresnel factor describing the incident fields at the interface and K, a combination of linear and nonlinear Fresnel factors describing the outgoing SFG field. The geometry of total internal reflection (TIR) was chosen to take advantage of the large Fresnel factors associated with this configuration.22 The macroscopic susceptibility, χ(2), is related to the molecular hyperpolarizability by (2) χeff,IJK )N
∑
〈FijkIJK 〉 Rijk
(3)
i,j,k
where Rijk is the molecular hyperpolarizability in the molecular (2) (ijk) frame, χeff,IJK is the macroscopic susceptibility in the laboratory (IJK) frame, N is the number of surface active molecules and 〈FijkIJK〉 is the function describing the average over molecular orientation. When the IR frequency (ω) is near a vibrational resonance, Rijk can be written as
Rijk ) Rnr +
Λ
∑ ω - ωijk,ν 0,ν - Γν
(4)
where ν represents the νth vibrational mode, ω0,ν is the resonant frequency of that mode, Γν is the damping constant of that mode, Λν represents the transition strength of that mode and Rnr represents the signal contribution from the nonresonant background. Similarly, the macroscopic susceptibility can be written as (2) χIJK ) χ(2) nr +
∑
(2) AIJK,ν ω - ω0,ν - Γν
(5)
(2) in terms of Α(2) ν and χnr , the macroscopic analogues of Λν and Rnr, respectively. If the components χ(2)IJK can be obtained experimentally from the SFG spectrum and the Rijk are known, then the average orientation of the functional groups contributing to that vibrational signal can be deduced. The orientation of the terminal CH3 group can be determined by analyzing its symmetric stretch mode, as well as by its asymmetric stretch mode. In the limit of free rotation, the CH3
(2) 3 χZZZ,r + ) NRzzz[R〈cos(θ)〉 + (1 - R)〈cos (θ)〉]
(7)
1 (2) (2) (2) (2) χYZY,r NR (1 + ) χXZX,r+ ) χZXX,r+ ) χZYY,r+ ) 2 zzz R)[〈cos(θ)〉 - 〈cos3(θ)〉] (8) where R ) Rxxz/Rzzz, N is the number density, and θ is the angle between the C-CH3 axis and the surface normal. Similar expressions hold for each of the CH3 asymmetric stretch modes, ra , rb : (2) (2) χYYZ,r - ) χXXZ,r- )
-1 NRxzx[〈cos(θ)〉 - 〈cos3(θ)〉] 2
(2) 3 χZZZ,r - ) NRxzx[〈cos(θ)〉 - 〈cos (θ)〉]
(9) (10)
1 (2) (2) (2) (2) χYZY,r NR 〈cos3(θ)〉] - ) χXZX,r- ) χZXX,r- ) χZYY,r- ) 2 xzx (11) In many SFG analyses, the CH3 group is considered to be of C3V symmetry and the methyl group ra and rb modes are assumed degenerate and treated as a single r mode. However, attachment of the methyl group to the alkyl chain results in a methyl group of Cs symmetry and removes any degeneracy of the r- modes, because the in-plane and out-of-plane CH bonds are no longer equivalent. Because most SFG analyses involve data obtained with picosecond or femtosecond light sources, their relatively large bandwidth usually precludes resolution of the ra and rb modes and only in relatively few circumstances have the two near degenerate modes been resolved.24,25 In cases of degeneracy, or where the two asymmetric modes cannot be (2) resolved, the corresponding susceptibility is taken to be χIJK,r (2) (2) ) χIJK,ra- + χIJK,rb-. The relative values of Rxxz, Rzzz, and Rxzx are functions of the single bond polarizability ratio. When this ratio has a value of 0.11, the value for R becomes 2.4 and the value of Rxzx/Rzzz becomes 2.0. These values have previously been used with some success in analyzing films of this nature and we employ these assumptions in our subsequent analysis. It should also be noted that the presence of a Fermi resonance band perturbs the intensity of the r+ mode. To evaluate the unperturbed intensity, the following expression has been used.
Aunperturbed(r+) ) √A(r+)2 + A(rfermi)2
(12)
Experimental Section Films were prepared using standard LB protocols. Specifically, amphiphiles were spread from chloroform solutions of 1 mM concentration onto the subphase. The subphase was composed of ultrapure water (18.2 MΩ cm) for films of
Phase Transition in Langmuir-Blodgett Films octadecanamide, stearic acid and ferric stearate; however, the cadmium stearate film was prepared by deposition of stearic acid onto a subphase containing 10-3 M CdCl2, pH adjusted to 6.9.26 A PTFE LB trough (Nima 600) was used for controlling the surface pressure while dipping. Surface pressures were read using a Wilhelmy plate balance. Following chloroform evaporation, the barriers were compressed at a rate of 10 cm2/min. Films were deposited at surface pressures of 35 mN/m and dipped at a rate of 1 mm/min onto a CaF2 prism. The orientation of the CaF2 substrate was uncontrolled. In all cases, the hydrophilic substrate was immersed in the subphase prior to spreading of the film and deposition occurred on the upstroke. Film transfer ratios were 1.0 ( 0.1. The sample was placed in a helium cooled cryostat (APD Cryogenics) equipped with a coldfinger fabricated from red copper. The sample was compressed between two pieces of indium foil to facilitate thermal conductivity between the sample and the thermal finger. The pressure was reduced using a turbomolecular vacuum pump (Leybold-Heraeus). The cryostat system allowed for typical operating pressures ∼(0.5-5) × 10-6 Torr. The cryostat temperature was controlled using a Lakeshore Cryotronics Inc. temperature controller (DRC 80C) and allowed for temperature stability of approximately (5 K. Temperature control to 25 K was routinely achieved under these conditions. A detailed description of the SFG apparatus has been previously reported.27 A brief description is given below. SFG spectra were obtained using tunable, broadband mid-IR light produced by difference frequency mixing of the signal and idler beams produced from a two-stage, β-BBO-based, optical parametric amplifier (OPA) in an AgGaS2 crystal. The OPA was pumped by the output of a regenerative amplifier (Positive Light) operating at 800 nm and a 1 kHz rep rate. A portion of the residual 800 nm light unused in the parametric amplification process was employed for sum frequency generation. Variable resolution was achieved by passing the residual 800 nm beam through a spectral manipulator. Spectral and temporal manipulation of the 800 nm light was performed by reflecting the beam from a diffraction grating and focusing it onto a high reflector. The amplitude of the various frequency components of the 800 nm beam were filtered by placing a slit of variable width in the focal plane. The resulting beam was then redirected along a reverse path toward the diffraction grating where it was spatially reconstructed. The resolution of our instrument, as dictated by the bandwidth of the 800 nm light, was 7 cm-1 for the studies presented here. Sum frequency generation signals were produced by directing the IR and 800 nm light beams collinearly toward the hypoteneuse of a CaF2 right angle prism containing the monolayer of interest. The SFG beam was spectrally filtered and then reflected from a mirror before entering a dual port Oriel spectrograph and detected using a CCD camera (Andor) or photomultipier tube (Hammamatsu R928). Spectra were typically recorded as the sum of two 1 min acquisitions of the CCD camera. Polarization selection was achieved by using a tunable halfwave plate for the IR (Alphalas), a zero-order, half-wave plate for the visible beam (CVI) and a Glan Thompson polarizer (CVI) for the resulting SFG signal. Spectra were taken in the SSP, PPP, SPS and PSS polarization combinations, where the designations refer to the polarizations of the SFG, visible and infrared beams respectively. Without exception, the SPS and PSS spectra were indistinguishable to within the noise level and a constant intensity factor. For simplicity, we report only the SPS spectra. The bandwidth of our mid-IR pulse was taken into account in our analysis by normalizing spectra to the
J. Phys. Chem. B, Vol. 112, No. 44, 2008 13825
Figure 1. Room temperature SFG spectra of cadmium stearate: SSP (lower), PPP (middle) and SPS (upper). Plots are offset for clarity. Solid lines represent best fits to the data.
vibrationally nonresonant response from a spin cast sample of malachite green. Results and Discussion The SFG spectra of a monolayer of each amphiphile was obtained prior to placing each sample into the cryostat. Figure 1 displays the SSP, PPP and SPS spectra of a freshly prepared monolayer of cadmium stearate. The spectra in Figure 1 are characteristic of all of the materials reported here, with at most only minor intensity variations distinguishing the various amphiphiles. The spectra show well-defined peaks at 2880, 2940 and 2966 cm-1. These peaks are associated with the alkyl chain’s terminal methyl groups and have been previously identified as the r+, rfermi and unresolved r- bands, respectively.28 The observation that the intensity of methylene modes associated with the alkyl chain are virtually nonexistent in all polarization combinations is taken as an indication that the alkyl chains of the film possess an all-trans configuration. The solid lines in Figure 1 represent nonlinear least-squares best fits to the data, where the spectral features of all polarization combinations are fit simultaneously. The figure shows that these spectra can be well fitted, in this case, with contributions from three Lorentzian peaks interfering with a small complex nonresonant background signal contribution. This nonresonant signal is generally small (∼3%) relative to the magnitude of the resonant vibrational contributions and it is assumed that all peaks are of the same phase with respect to each other or are 180° out-of-phase.29 Applying eqs 6-11 and following an analysis method published previously,30 we have determined the orientation of the terminal methyl group and hence the tilt of the overall alkyl chain. The ability to obtain the SFG spectra in many polarization combinations makes possible the determination of the averaged orientation by a number of different models and therefore provides a check on the self-consistency of the analysis and interpretation. By examining the susceptibility of the symmetric stretch in the PPP and SSP spectra, we obtain a tilt angle of 38.9° for the methyl group. Alternatively, the ratio of the susceptibilities of the asymmetric stretch in the SPS and PPP spectra yields a tilt angle of 40.3°. A third possible approach employs the ratio of the susceptibilities of the asymmetric stretch to the symmetric stretch in the PPP spectrum, yielding a methyl
13826 J. Phys. Chem. B, Vol. 112, No. 44, 2008
Figure 2. SFG spectra (PPP polarization combination) of stearic acid before (circles) and after (triangles) vacuum pumping for one day. The solid lines represent best fits to the data.
group tilt angle of 34.0°. These angles are self-consistent and indicate that the alkyl chains are oriented approximately perpendicular (150 K) and low (150 K, such splittings are reduced to a few cm-1, and typically not resolved in most SFG spectroscopy. It is a common assumption in most SFG analyses that the methyl group undergoes free rotation, which allows for a simplified analysis in terms of a single r- mode.34-36 However, as others have pointed out, the barrier to internal rotation in these systems is far too large for free rotation, and torsional motion is characterized by rms torsion angles of roughly 10° at room temperature.25,37 To our knowledge, there are only a few reported cases where these bands have been spectrally resolved using SFG,24,25 and this splitting was not always observable.24 Others have inferred the splitting of the degenerate r- mode and the relative phases of the ra and rb modes based on the inability to adequately fit their data to a single r- mode.38-40 In similar fashion, we have attempted to fit our spectra with a single rmode and observe this vibrational contribution to appear to double in line width and drop to half-its initial intensity in the PPP polarization combination, as the temperature is decreased. Note that we have also observed that the increase in line width for the r- mode appearing in the SPS spectrum is very modest. Figure 5a displays the fit of the SPS spectrum at 25 K. The quality of the fit in the vicinity of the r- mode in the SPS spectrum is slightly inadequate. The fitting procedure entails that all spectra are fit simultaneously and, due to the larger line width of the peak observed in the PPP spectrum, it becomes impossible to fit both the PPP and SPS spectra without compromise. Although the observation of a temperature inde-
13830 J. Phys. Chem. B, Vol. 112, No. 44, 2008
Figure 8. Ratio of line strengths, Aeff(fermi)/Aeff(r+), extracted from the spectral fits for the SSP (filled diamonds) and PPP (open squares) polarization combinations, as a function of temperature.
pendent line width in the SPS spectrum may appear to be inconsistent with the observations in the PPP polarization combination and the argument for splitting, it must be recognized that the SPS spectrum probes different susceptibility components than does the PPP spectrum. These seemingly disparate observations may suggest the presence of two peaks, close in frequency, but appearing with different intensities in different polarization combinations. Thus, we attribute these observations to an increase in the splitting of the ra and rb modes at temperatures below 150 K and to differences in their behavior depending upon which polarization combination is used to probe them. To understand the observed spectral changes with temperature in all polarization combinations, it proves useful to consider the nature of the intensity distribution appearing in the CH region of these amphiphiles. It is generally accepted that the Fermi band is an overtone of a bending mode that gains its intensity through Fermi resonance with the symmetric stretch mode.41 These interactions lead to two molecular eigenstates (r+ and rfermi) of mixed character. Within this coupling scenario, the Fermi intensity should therefore be a constant fraction of the intensity of the r+ mode, independent of temperature. Figure 8 displays the ratio of line strengths extracted from fits of the SSP and PPP spectra for the rfermi and the r+ modes (Afermi/Ar+) as a function of temperature for a monolayer of cadmium stearate. This ratio reflects the fractional intensity of the Fermi band relative to that of the r+ band, and on the basis of this argument, one might expect such a ratio to be independent of temperature. However, as shown in Figure 8, the ratios of line strengths display large temperature dependent variations. Specifically, the line strengths observed in both the SSP and PPP polarization combinations appear to be roughly equal and independent of temperature (as expected) from room temperature to ∼150 K, where they both appear to show an abrupt departure from this behavior. In the PPP spectra at temperatures near 150 K, the Fermi band appears to gain substantial intensity relative to r+, whereas in the SSP spectra, the Fermi band appears to lose a small amount of intensity relative to r+. As the temperature is lowered, the ratio of line strengths in both SSP and PPP polarization combinations decrease steadily. The observation of both the ra and rb modes at temperatures below 150 K reflects a significant increase in their splitting
Johansson and Leach relative to higher temperatures. Attachment of the methyl group to the alkyl chain and the concomitant change of methyl group symmetry from C3V to Cs results in the doubly degenerate rmode of E symmetry splitting into two distinct modes, one with A′ symmetry (ra ) and the other mode with A′′ symmetry (rb ). The symmetric ra mode has the same symmetry as the rfermi and r+ modes. The proximity of an additional band to rfermi with the same symmetry as rfermi, could provide an additional avenue of intramolecular coupling. This coupling could take the form of an anharmonic interaction between the Fermi mode and the + ra mode (as well as the r mode), or equivalently, the coupling could be considered a more complex case of Fermi interaction whereby the overtone of the bending mode that gains its intensity through Fermi resonance with the symmetric stretch mode, now also interacts and gains intensity through the ra mode. In this description, the observed vibrational modes are admixtures of what we would normally refer to as pure symmetric stretch, antisymmetric stretch, and bending overtone modes. In such a scenario, one might expect the line strength ratios to change abruptly to reflect an alternative coupling mechanism. Specifically, the additional coupling pathway allows the rfermi peak to increase in intensity at the expense of the ra band, as it borrows intensity. This notion is consistent with our observations. In the PPP polarization combination, at temperatures of ∼150 K, the Fermi band increases its relative intensity to the r+ band (Figure 8) and there is a concomitant inversion in the relative intensities of the rfermi and r- bands (Figure 6c). A similarly consistent observation is seen in the SPS spectrum (Figure 6b), where the r- mode intensity observed above 150 K decreases in response to low temperature. Though the loss of intensity in the rmode(s) and gain of intensity in the Fermi mode are not exactly equivalent in the SPS and PPP spectra, they are very similar. To understand the SSP spectra, it must be recognized that in this polarization combination, the r- mode generally appears as a small contribution with a relative phase opposite to the r+ and rfermi bands and is sometimes observable as a small shoulder on the Fermi band. The mixing of the ra with the Fermi mode at temperatures below 150 K leads to new band positions and intensities for these now mixed modes. The new arrangement of eigenstates leads to a greater destructive interference between the rfermi and ra bands in the SSP spectrum, in the same way that this arrangement of eigenstates leads to an increase in intensity in the PPP polarization combination, where the relative + phase of the ra mode is the same as that for the r and rfermi modes and they constructively interfere. The temperature dependent behavior of the line strength ratio at lower temperatures appears to be more difficult to understand. The ratio of line strengths at temperatures below 150 K shows a steady drop with temperature for both the SSP and PPP polarization combinations. It should be noted that these decreasing ratios reflect a decreasing intensity of the Fermi band with temperature, as the intensity of the r+ band appears to be roughly independent of temperature. Although the splitting of the ra and rb bands is expected to be temperature dependent, the magnitude of this temperature dependence is generally small, and spectral simulation indicates that an explanation of the steady decrease in line strength ratio due only to a temperature dependent splitting is not feasible. It is possible that the temperature dependent splitting leads to temperature dependent + changes in the coupling between the ra , rfermi and r modes sufficient to explain this observation. Alternatively, in their investigation of the temperature dependent spectral changes of alkanethiols self-assembled on gold, Rowntree and co-workers
Phase Transition in Langmuir-Blodgett Films
Figure 9. SFG spectra of cadmium stearate at 25 K: SSP (lower), PPP (middle) and SPS (upper). Plots are offset for clarity. Solid lines represent best fits to the data, assuming two nondegenerate r- modes.
have observed temperature dependent intermolecular as well as intramolecular couplings.13 It is perhaps not surprising that the intramolecular coupling scheme suggested above should be insufficient to explain all of the temperature dependent spectral changes observed, as the observed spectra may be influenced by temperature dependent intermolecular couplings as well. In response to the proposed splitting of the r- band and the resulting coupling scheme, we have attempted to fit the low temperature SFG spectra with a more realistic model. Figure 9 displays fits of the SFG spectra for a monolayer of cadmium stearate at a temperature of 25 K. These spectra are those already displayed in Figures 3a-5a. However, in Figure 9 the data have been fitted with two antisymmetric stretching modes, ra and rb . Although the fits to the data in Figures 3a-5a were reasonably good, a marked improvement in the fits is evident. Peak fitting would suggest that the two r- modes have frequencies of 2955 and 2965 cm-1, each with a width Γ ) 7 cm-1. The splitting of 10 cm-1 obtained from the fits is close to that observed by MacPhail et al.37 The improvement in fitting obtained by inclusion of the splitting of the degenerate antisymmetric stretching modes supports our analysis. We have, thus far, only addressed aspects of the CH region of the spectrum. Upon cooling, the onset of a new feature also appears in the spectrum. The feature appears at energies higher than the CH bands of the amphiphiles (∼3100 cm-1) and appears as a small peak. It should be noted that the frequency range of the spectra reported here are limited by the bandwidth of the IR pulses employed, which had a typical fwhm of 200 cm-1 centered about 2900 cm-1. This new spectral feature appears at the extreme high energy edge of our bandwidth and its observation would indicate substantial intensity. When normalized for incident infrared intensity, the integrated intensity of this new spectral feature is much larger than the signals originating from the methyl groups of the monolayer. Figure 10 displays this new feature, together with the CH bands of the monolayer. Figure 10 was obtained by stitching together separate normalized spectra by scaling them appropriately to common spectral features. When the samples are reheated from 25 K, the intensity of the SFG signal at ∼3100 cm-1 increases dramatically in the vicinity of 165 K, and then decreases dramatically as the temperature is further increased. At room
J. Phys. Chem. B, Vol. 112, No. 44, 2008 13831
Figure 10. Low temperature SFG spectrum (SSP polarization combination) of cadmium stearate. The spectrum is a convolution of two separate spectra, normalized for IR bandwidth, by scaling them appropriately to common spectral features. The solid line represents a best fit to the data.
temperature, following one cooling and heating cycle, there remains a small residual signal at ∼3100 cm-1. Otherwise, this behavior appears to be reversible, with large increases in the intensity of this spectral feature occurring at ∼150 K upon cooling and dramatic decreases in intensity at temperatures greater than ∼165 K upon heating. The 3100 cm-1 feature, and others observed at higher energy, are consistent with the presence of ordered water molecules on the surface.42,43 We have conducted a variety of experiments to elucidate the nature and origin of this signal, including temperature cycling studies, studies on films of various thickness, films of deuterated amphiphiles, mixed films of deuterarated and nondeuterated amphiphiles, and films deposited from a subphase of D2O. The details of these studies are beyond the scope of this manuscript and will be presented in a forthcoming publication. However, it is clear from these studies that the origin of the spectral feature at 3100 cm-1 is due to surface water, and that the presence of water in these films is linked with the deposition method, and not other sources. Further, the abrupt transition from disordered water to ordered water on the surface appears to be reversible and coincides with the observed reversible changes in the SFG spectra originating from the amphiphiles’ methyl groups. Although we have no direct evidence to support the causal nature, we propose that the temperature dependent spectral changes observed in the CH region of all amphiphiles reported in this manuscript, occur in response to the appearance of ordered water molecules in the film. That is, the observed temperature dependent spectral changes reflect a reversible phase transition of surface water molecules. One potential causal link is an abrupt change in the packing of alkyl chains that might accompany the onset of ordering of water molecules within the film. The room temperature structure of fatty acid LB films is typically a pseudohexagonal rotator phase, lacking long-range orientational order between the planes of alkyl chains. A phase transition to a longrange ordered orthorhombic phase would result in a change of packing of the alkyl chains, a change in methyl group interactions, and the potential for the onset of a new vibrational coupling mechanism as outlined above. Consistent with this conjecture and our observation of changes in the spectral features of the asymmetric modes, is the observation by Snyder and co-
13832 J. Phys. Chem. B, Vol. 112, No. 44, 2008 workers that orthorhombic phases show factor-group splitting (Davydov splitting) of the ra and rb modes associated with two molecules in the unit cell.28 To the best of our knowledge, this report represents the first detailed structural study of LB films at low temperature. Previous low temperature studies of LB films have dealt with the paraelectric to ferroelectric or paramagnetic to ferromagnetic phase transitions of specialized amphiphiles; however, these reports have primarily focused on the details of these different ordering phenomena, in some cases over narrow temperature ranges, and have not generally investigated structural aspects of the amphiphiles themselves. Nevertheless, one might expect that on the basis of our findings, others would have observed similar discontinuous phenomena in their studies. Interestingly, Saito et al. have performed ESR experiments on LB films of icosanoic acid mixed with bis(ethylenedioxy)tetrathiofulvalene and decyltetracyanoquinodimethane and observed “an anomalous phase transition” at 140 K.44 The results presented here raise some interesting questions regarding the structure of LB films, the behavior of water associated with these films and the behavior of water at surfaces more generally. The low temperature phase behavior of water is very complex and not entirely well understood. Water is known to exist in many stable and metastable phases depending on its deposition temperature, film thickness and the substrate onto which it is deposited and undergoes a phase transition from a supercooled viscous phase to form amorphous solid water at temperatures in the vicinity of 150 K.45 Films of water and mixed films containing water and other solvents have been reported to undergo a glass transition from a viscous fluid like phase to the amorphous solid phase at comparable temperatures,46 although there has been some disagreement on the glass transition temperature.47 Our results indicate that at comparable temperatures, water deposited in the LB process is altered from an isotropic distribution to a highly ordered structure within these films. This structure may be the result of the anisotropic environment of the amphiphile, or may represent more general behavior of ultrathin water films deposited in this manner. We are currently trying to address this question with further experiments. Conclusion We have investigated the low temperature phase behavior of Langmuir-Blodgett films formed from stearic acid, octadecanamide, ferric stearate and cadmium stearate using sum frequency generation vibrational spectroscopy. Monolayer films of stearic acid and octadecamide display activity of methylene modes upon exposure to vacuum. Their temperature dependence shows slow and gradual change and a complete quenching of activity by 100 K. Subsequent heating leads to the recurrence of the methylene activity, indicating that the behavior is reversible. Monolayer films of the fatty acid salts, ferric stearate and cadmium stearate, however, show no such methylene activity upon vacuum exposure. We have attributed the disparate behavior to differences in headgroup solvation. Removal of weakly bound water molecules by vacuum, induces changes in solvation of the headgroup and the introduction of uncompensated vibrational activity in nearby methylene groups. All amphiphiles display reversible discontinuous spectroscopic changes in the vicinity of 150 K. We have attributed these changes to the onset of a change in the ordering of the alkyl chains within the unit cell and the onset of a new vibrational coupling mechanism that alters the spectral positions and intensities of the methyl group modes at low temperature.
Johansson and Leach The onset of SFG signals originating from ordered water molecules at the surface at the same temperatures have also been reported and suggests a connection between the two. Given the generality of the observation for the range of amphiphiles studied here and similar unexplained observations in other studies, we have proposed that the phase transition manifested in the amphiphilic monolayer CH region is a response to the formation of ordered water molecules contained within the monolayer film. These experiments provide supporting evidence for the idea that the structural stability of these model surfaces rely on the incorporation of solvent transferred from the subphase to the solid substrates upon deposition. Further studies are required to understand the detailed nature of this solvation process and how changes in the degree of headgroup solvation affect the structure and stability of the supported organic monolayer. In addition, the observation of a phase transition of the incorporated water molecules raises interesting questions regarding their character and thermodynamic properties. The onset of order in these surface water molecules may reflect the unique anisotropic solvation environment of the amphiphile or may reflect a more general thermodynamic behavior of water in ultrathin films. Acknowledgment. We thank the Natural Sciences and Engineering Research Council of Canada and Simon Fraser Unversity for financial support and Dr. James W. Hager of MDS Sciex Corp. for the donation of vacuum equipment employed in these studies. Note Added after ASAP Publication. This article posted ASAP on October 15, 2008 with an error in the caption to Figure 7. The final correct version posted October 30, 2008. References and Notes (1) Handbook of Thin Film Technology; Maissel, L. I., Glang, R., Eds.; McGraw-Hill: New York, 1970. (2) Introduction to Surface and Thin Film Processes; Venables, J., Ed.; Cambridge University Press: New York, 2000. (3) Petty, M. C. Langmuir Blodgett Films; Cambridge University Press: New York, 1996. (4) Functional Organic and Polymeric Materials: Molecular Functionality-Macroscopic Reality; Richardson, T. H., Ed.; Wiley,, New York, 2000. (5) Films on Solid Surfaces: The Physics and Chemistry of Physical Adsorption; Dash, J. G., Ed.; Academic Press: New York, 1975. (6) Phase Transitions in Surface Films; Dash, J. G., Ruvalds, J., Eds.; Plenum: New York, 1980. (7) Kosterlitz, J. M.; Thouless, D. J. J. Phys. C 1973, 6, 1181. (8) Nelson, D. R.; Halperin, B. I. Phys. ReV. B 1979, 19, 2457. (9) Young, A. P. Phys. ReV. B 1979, 19, 1855. (10) Hostetler, M. J.; Manner, W. L.; Nuzzo, R. G.; Girolami, G. S. J. Phys. Chem. 1995, 99, 15269. (11) Bhatia, R.; Garrison, B. J. Langmuir 1997, 13, 765. (12) Nuzzo, R. G.; Korenic, E. M.; Dubois, L. H. J. Chem. Phys. 1990, 93, 767. (13) Garand, E.; Picard, J.; Rowntree, P. J. Phys. Chem. B 2004, 108, 8182. (14) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546. (15) Chidsey, C. E. D.; Liu, G. Y.; Rowntree, D.; Scoles, G. J. Chem. Phys. 1989, 91, 3321. (16) Huie, J. C. Smart Mater. Struct. 2003, 12, 264. (17) Volcke, C.; Thiry, P. A. J. Phys.: Conf. Ser. 2007, 61, 1236. (18) Orendorff, C. J.; Gole, A.; Sau, T. K.; Murphy, C. J. Anal. Chem. 2005, 77, 3261. (19) Kondo, T.; Kohei, U. J. Photochem. Photobiol. C Photochem. ReV. 2007, 8, 1. (20) Akamatsu, N.; Domen, K.; Hirose, C.; Onishi, T. Chem. Phys. Lett. 1991, 181, 175. (21) Rao, Y; Comstock, M.; Eisenthal, K. J. Phys. Chem B 2006, 110, 1727. (22) Lo¨bau, J.; Wolfrum, K. J. Opt. Soc. Am. B 1997, 14, 2505. (23) Garand, E.; Picard, J.; Rowntree, P. J. Phys. Chem. B 2004, 108, 8182.
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