Langmuir 2008, 24, 14005-14014
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Langmuir Monolayers of Straight-Chain and Branched Hexadecanol and Eicosanol Mixtures Rachel E. Kurtz,† Michael F. Toney,‡ John A. Pople,‡ Binhua Lin,§ Mati Meron,§ Jaroslaw Majewski,| Arno Lange,⊥ and Gerald G. Fuller*,† Department of Chemical Engineering, Stanford UniVersity, Stanford, California 94305, Stanford Synchrotron Radiation Laboratory, Stanford Linear Accelerator, Stanford, California 94025, AdVanced Photon Source, UniVersity of Chicago, Argonne, Illinois 60439, Manuel Lujan Jr. Neutron Scattering Center, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, and BASF SE, 67056 Ludwigshafen, Germany ReceiVed July 31, 2008. ReVised Manuscript ReceiVed September 26, 2008 Langmuir monolayers of straight-chain and branched hexadecanol and eicosanol mixtures were previously studied using surface pressure-area isotherms, Brewster angle microscopy, and interfacial rheology. In this paper, we investigate the structure of these fatty alcohol mixtures using these previous results together with X-ray diffraction and reflectivity measurements, which provide a better understanding of the structure of the monolayer in terms of the phase segregation and location of branched chains. For eicosanol below 25 mN/m, the branched chains are incorporated into the monolayer, yet they are phase-separated from the straight chains. At higher surface pressures, the branched chains are expelled from the monolayer and presumably form micelles or some other aggregate in the subphase. In contrast, the hexadecanol branched chains are not present in the monolayer at any surface pressure. These behaviors are interpreted with the help of the X-ray measurements and density profiles, and are explained in terms of straight-chain flexibility. We will discuss the effect of the monolayer structure on the surface shear viscosity. These studies provide a deeper understanding of the structure and behavior of amphiphilic mixtures, and will ultimately aid in developing models for lipids, micelle formation, and other important biological functions.
1. Introduction Single-component amphiphiles can self-assemble at the air/ water interface to form monolayers with rich phase behavior. These systems are described in detail in the monograph by Gaines.1 The tilted aliphatic hydrocarbon tails often arrange themselves onto hexagonal or quasi-hexagonal lattices. Some phases respond linearly to flow deformation, while other phases behave as smectic liquid crystals. Previous X-ray diffraction studies have established that these monolayers spread to form crystalline and hexatic architectures, allowing for distinctive transport properties.2,3 This powerful tool has been combined with optical methods, such as polarized fluorescence microscopy4 and Brewster angle microscopy,5,6 to assist in constructing phase diagrams of fatty alcohols. Studies using these experimental tools have been reviewed by Kaganer et al.7 While single-component alcohol monolayers have been wellstudied, mixtures have not. Mixtures are widely used as stabilizers for emulsions and foams, and are important as model biological * To whom correspondence should be addressed. E-mail:
[email protected]. Telephone: 650-723-9243. Fax: 650-725-7294. † Stanford University. ‡ Stanford Synchrotron Radiation Laboratory, Stanford Linear Accelerator. § University of Chicago. | Los Alamos National Laboratory. ⊥ BASF SE. (1) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience and John Wiley & Sons: New York, 1966. (2) Shih, M. C.; Bohanon, T. M.; Mikrut, J. M.; Zschack, P.; Dutta, P. J. Chem. Phys. 1992, 97, 4485. (3) Kaganer, V. M.; Brezesinski, G.; Mo¨hwald, H.; Howes, P. B.; Kjaer, K. Phys. ReV. E 1999, 59, 2141. (4) Moy, V. T.; Keller, D. J.; Gaub, H. E.; McConnell, H. M. J. Phys. Chem. 1986, 90, 3198. (5) He´non, S.; Meunier, J. ReV. Sci. Instrum. 1991, 62, 936. (6) Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590. (7) Kaganer, V. M.; Mo¨hwald, H.; Dutta, P. ReV. Mod. Phys. 1999, 71, 779. (8) Brooks, C. F.; Fuller, G. G.; Frank, C. W.; Robertson, C. R. Langmuir 1999, 15, 2450.
membranes. Mixtures involving fatty acids and fatty alcohols have been investigated and are generally immiscible, with some exceptions. This topic is detailed in the review by Kaganer et al.7 Additionally, Ries and Cook9 investigated mixtures of stearic acid with isostearic acid. Isostearic acid differs from stearic acid only by a single methyl side chain, yet with a much larger crosssectional area and a lower collapse pressure. This work presents isotherm measurements only on an equimolar mixture, and the data suggest few cooperative interactions between the two molecules. The extrapolated area per molecule and the collapse pressure are averages of the individual components. Monolayers of fatty acid and fatty alcohol mixtures, particularly that of heneicosanoic acid and heneicosanol, have also been studied using various methods.10-12 This combination allows a direct study of the headgroup influence on the polymorphism of these materials. By evaluating several different mixtures of heneicosanoic acid and heneicosanol, the appearance and disappearance of the next-nearest neighbor (NNN) and nearest neighbor (NN) tilt transition was detailed,10 as fatty alcohols lack a NN tilt. The two materials were found to be miscible in monolayer form, and it was determined how the phase diagram of the acid evolves to that of the alcohol as the composition is adjusted. When the proportion of alcohol in the mixture was increased, the surface pressure at which the swiveling transition, from NN to NNN tilt, occurs decreased. The phase diagrams also showed that the transition between the tilted and untilted phases moves down in pressure with increasing concentration of the alcohol, but it does not move down as quickly as the swiveling transition. Additionally, it was found that the diffraction (9) Ries, H. E.; Cook, H. D. J. Colloid Sci. 1954, 9, 535. (10) Shih, M. C.; Durbin, M. K.; Malik, A.; Zschack, P.; Dutta, P. J. Chem. Phys. 1994, 101, 9132. (11) Fischer, B.; Teer, E.; Knobler, C. M. J. Chem. Phys. 1995, 103, 2365. (12) Teer, E.; Knobler, C. M.; Lautz, C.; Wurlitzer, S.; Kidae, J.; Fischer, T. M. J. Chem. Phys. 1997, 106, 1913.
10.1021/la802467e CCC: $40.75 2008 American Chemical Society Published on Web 11/14/2008
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Figure 1. Schematic diagram of 1-hexadecanol (a), 1-eicosanol (b), and the branched form of hexadecanol (c) and eicosanol (d). The branched form of hexadecanol is a mixture of isomers, as indicated in the text.
pattern of the mixture was not a superposition of the diffraction peaks of the individual components. Rather, the mixed system had a structure intermediate to that of the two individual component monolayers and was distinct from both.10 The objects of this study are 1-hexadecanol (cetyl alcohol) and 1-eicosanol (arachidyl alcohol), which consist of aliphatic tails of 16 and 20 carbon atoms, respectively, with each tail associated with a hydroxyl headgroup. Single-component Langmuir monolayers of each of these fatty alcohols have been studied extensively, and much is known about their phase behavior, structure, and transport properties. This article describes the effects of mixing the straight-chain form of hexadecanol with its branched form. Schematics of the molecular structure of the straight and branched fatty alcohols are shown in Figure 1. Isotherms and rheology data for hexadecanol and eicosanol mixtures were reported elsewhere.13,14 In this work, we developed a model based only on isotherms and rheology that the branched hexadecanol chains were displaced by the straight chains, leaving a monolayer of straight chains. For eicosanol, we suggested the branched chains were integrated in the monolayer up to 25 mN/ m. Now, we use X-ray data to develop a detailed structural model and connect the rheology with the film structure.
2. Experimental Section 2.1. Materials. The straight-chain hexadecanol used in these experiments was acquired from MP Biomedicals and used as received, with a purity of approximately 99%. The straight-chain eicosanol used in these experiments was acquired from TCI America and used as received, with a purity of 96.0% by GC. The purity of these samples was also confirmed by comparing measured Π-A isotherms to those reported elsewhere.1 The branched forms of both hexadecanol and eicosanol (80% 2,4,4,6,6,8,8-heptamethylnonanol-(1); 19% 2,4,4,6,6,8,8-heptamethylnonanol-(3) and 2,4,4,6,6,8,8,10,10-nonamethylundecanol-(1), respectively) were uniquely synthesized by BASF SE according to a procedure by Ivan et al.15 The purity of these samples was verified by BASF SE to be 99% as determined by 1H NMR. These branched fatty alcohols were used as received. Chloroform was used as the spreading solvent for these measurements. All water used in this work was purified to a resistivity of 18.2 MΩ cm using a Millipore Milli-Q system. 2.2. Isotherms and Surface Rheology. Measurements of both Π-A isotherms and surface rheological properties were carried out at room temperature (T ) 22.5 ( 0.5 °C) in Teflon Langmuir troughs and movable Delrin barriers, manufactured by KSV Instruments. A platinum Wilhelmy plate connected to a film balance (KSV Instruments, Finland) was used for surface pressure measurements. To form monolayers, fatty alcohols were dissolved in chloroform, and solutions were added to the air/water interface dropwise using a glass syringe. The solvent was allowed to evaporate approximately 30 min after deposition. To manipulate the surface concentration, the barriers were moved at a rate of 1 mm/min, corresponding to (13) Gavranovic, G. T.; Kurtz, R. E.; Golemanov, K.; Lange, A.; Fuller, G. G. Ind. Eng. Chem. Res. 2006, 45, 6880. (14) Kurtz, R. E.; Lange, A.; Fuller, G. G. Langmuir 2006, 22, 5321. (15) Ivan, B.; Kennedy, J. P.; Chang, V. S. C. J. Polym. Sci. Polym. Chem. Ed. 1980, 18, 3177–3191.
a surface area change of 150 mm2/min in a Langmuir trough of approximately 18500 mm2. Π-A isotherms showed good reproducibility, and changing the barrier speed (0.001-0.002 Å2/molecule/ s) had very little effect on the isotherms. Surface viscoelasticity measurements were performed using the interfacial stress rheometer (ISR) developed by Brooks et al.8 The apparatus uses a magnetic rod held at the interface by means of surface tension. Helmholtz coils are used to generate a magnetic field gradient to move the rod. The interface is sheared as the rod oscillates, and the rheological properties of the film were determined from the stress-strain relationship of the rod. Measurements made using the ISR were performed at constant surface pressure, and the amplitude of the rod’s oscillation was kept small enough to ensure that the system remained in the linear viscoelastic regime and had a maximum shear strain of about 2%. 2.3. X-Ray Measurements. The X-ray diffraction and reflectivity measurements were primarily done at ChemMatCARS Sector 15ID-C of the Advanced Photon Source (APS) at the Argonne National Laboratory.16 Some X-ray experiments were performed at beamline BWl at the HASYLAB, DESY in Hamburg. At the APS, the X-ray wavelength was 1.24 Å corresponding to an energy of 10 keV, while the wavelength was 1.3 Å (9.5 keV) at the HASYLAB. At the APS, the Langmuir trough is mounted on a liquid surface spectrometer. The primary X-ray techniques used with this instrument were reflectivity and in-plane diffraction, or grazing incidence X-ray diffraction (GIXD). Reflectivity provides information about film thickness, surface roughness, and electron density profile normal to the interface. From the reflectivity data, the variation in the electron density of a Langmuir film was obtained in the direction normal to the surface. Simulation of the reflectivity profiles was performed with the Parratt32 and StoichFit17 software package, by the HahnMeitner Institut, Berlin, and the University of Chicago, respectively. In this case, the calculation of optical reflectivity of X-rays was based on Parratt’s recursive algorithm for stratified media using independent layers.18 On the other hand, GIXD measurements reveal information about ordering within the plane of the interface. Intensity is measured as a function of Q, which is typically expressed in terms of its horizontal (in-plane) and vertical components, Qxy and Qz. The molecular tilt and ordering is determined by peak locations in Qz, in the plane perpendicular to the Langmuir trough, and Qxy, in the plane of the Langmuir trough. In our two-dimensional (2D) systems, the monolayers are a mosaic of 2D crystals with random orientation about the direction normal to the subphase and can therefore be described as 2D powders. Due to the lack of restriction on the scattering vector component Qz along the direction normal to the 2D crystal, Bragg scattering extends as continuous Bragg rods in reciprocal space.19 The scattered intensity was measured by scanning over a range of horizontal scattering vectors, (16) Lin, B.; Meron, A.; Gebhardt, J.; Graber, T.; Schlossman, M. L.; Viccaro, P. J. Phys. B 2003, 336, 75–80. (17) Danauskas, S.; Li, D.; Meron, M.; Lin, B.; Lee, K. Y. J. Appl. Crystallogr. Submitted for publication. (18) Parratt, L. G. Phys. ReV. 1954, 95, 359. (19) Als-Nielsen, J.; Jacquemain, D.; Kjaer, K.; Leveiller, F.; Lahav, M.; Leiserowitz, L. Phys. Rep. 1994, 246, 251.
Straight-Chain and Branched Fatty Alcohol Mixtures
2π [cos2(Ri) + cos2(Rf) λ 4π 2cos2(Ri) cos(Rf) cos2(2θxy)]1⁄2 ≈ sin(2θxy ⁄ 2) λ
Qxy ) (Qx2 + Qy2)1⁄2 )
where 2θxy is the angle between the incident and diffracted beam projected onto the horizontal plane, Qxy is the combination of horizontal components Qx and Qy, and Ri and Rf are the incident and the reflected angles, respectively.20 Diffraction peaks were resolved in the Qxy-direction and obtained by integrating the scattered intensity along Qz. Conversely, the Bragg rod profiles were resolved in the Qz-direction, along
Qz )
2π 2π sin(Rf) + sin(Ri)] ≈ sin(Rf) [ λ λ
and obtained by integrating the scattered intensity over Qxy corresponding to the Bragg peak. The Qxy of the maxima of the Bragg peaks allowed for the determination of the repeat distances d for the 2D lattice. From the line shapes of the peaks, corrected for the instrument resolution, it was possible to determine the 2D crystalline in-plane coherence length, Lxy, the average distance in the direction of the reciprocal lattice vector Qxy over which there is “near-perfect” crystallinity. The intensity distribution along the Bragg rod can be analyzed to determine the direction and magnitude of the molecular tilt, the coherently scattering length of the alkyl tail measured along its backbone, Lc, and the magnitude of molecular motion or surface roughness, σ, of the crystallite (Debye-Waller factor). The corresponding full width at half-maximum (FWHM) of the Bragg peaks exceeds the instrumental resolution of FWHMresol(Qxy) ) 0.0075 Å-1 or approximately 0.01 Å-1 for the GIXD measurements in this work. The intrinsic FWHM can be obtained using the equation:
FWHMintrinsic(Qxy) )
[FWHMmeas(Qxy)2 - FWHMresol(Qxy)2]1⁄2 A simple model assumes that the monolayer consists of 2D crystallites that are perfect and have a finite average dimension Lxy, the in-plane coherence length, in the crystallographic direction {h, k}. Using the Scherrer formula,21 the in-plane coherence length can then be calculated from Lxy ≈ 0.9[2π/FWHMintrinsic(Qxy)]. The Langmuir trough was custom-built for these measurements, and a widely used package from Nima Technology Inc. controlled the barrier motion and Wilhelmy plate pressure sensor readout. A near-hermetic sealing of the Langmuir trough allowed for flowing humid helium into the sample environment, thereby minimizing beam damage to the monolayer from ozone production, X-ray absorption, and background scattering. The usable area for X-ray measurements ranged from 120 to 335 cm2, and the highly brilliant X-ray beam passed through polyimide windows of the Langmuir trough cover. All X-ray experiments were performed on a pure water subphase approximately at T ) 22 °C. For the GIXD experiments, the X-ray beam was adjusted to strike the surface at an incident angle of 0.105°, which corresponds to a vertical momentum transfer vector Qz ) 0.85Qc, where Qc ) 0.02176 Å-1 was the critical scattering vector for total external reflection from the liquid subphase. At this angle, the incident X-rays are totally reflected, while the refracted wave becomes evanescent traveling along the liquid surface. Such a configuration maximizes surface sensitivity and minimizes the subphase background. The dimension of the X-ray beam footprint on the liquid surface was approximately 3 mm × 40 mm. For in-plane diffraction measurements, one of two configurations was used. Either a vertically placed Soller collimator with a lateral resolution of ∆Qxy ) 0.01 Å-1 or (20) Kjaer, K. Phys. B 1994, 198, 100. (21) Guinier, A. X-ray Diffraction in Crystals, Imperfect Crystals and Amorphous Bodies; Foley, H. M., Ruderman, M. A., Eds.; W. H. Freeman: San Francisco, 1963.
Langmuir, Vol. 24, No. 24, 2008 14007 two fixed slits were used. In both cases, a vertical one-dimensional position sensitive detector (PSD) with vertical acceptance 0 < Qz < 1.2 Å-1 was used to obtain data for different Qz values. X-rays with a wavelength of about 1 Å can cause significant beam damage to the fatty alcohol monolayer. Overexposure can “burn” the sample, causing a change in the real space structure over time. Prior to these studies, much time was spent establishing how much beam exposure can be tolerated before the sample is significantly altered. To mitigate damage to the film by X-rays, the sample was translated perpendicular to the beam, in between scans (approximately every 5 min), and was only scanned once at each lateral position for all measurements reported. In addition to sample translation, repeat measurements were conducted to minimize the possibility of beam damage artifacts. GIXD scans not only provided a means to determine the change in the lattice spacing with surface pressure but also allowed a means to observe the change in the tilt angle. The tilt angle was determined by calculating the arctangent of the nondegenerate component of Qz/Qxy. The peak location in Qxy was determined by Lorenztian fitting of the data, while the Qz value was determined by constructing contour plots and locating the maximum intensity along the corresponding Qxy value.
3. Results 3.1. Pure Linear-Chain Hexadecanol Monolayers. Isotherms, Brewster angle microscopy images, and interfacial rheology results for straight-chain hexadecanol have been reported elsewhere,13 and these results are summarized as follows. The surface pressure-area (Π-A) isotherm at room temperature for straight-chain hexadecanol has several well-known phase transitions. At an area of 22 Å2 per molecule, the tilted phase is formed with the headgroups in a quasi-hexagonal lattice with the aliphatic tails tilted to the NNN. This cross-sectional area per molecule of hexadecanol estimated from the Π-A isotherm agrees well with previously reported values of about 22 Å2.1 At a surface pressure of 10 mN/m, a kink in the isotherm signals the transition to the untilted phase. Finally, the monolayer collapses at a surface pressure of about 50 mN/m. Interfacial rheology studies of the straight chain demonstrated that the viscous shear modulus is greater than the elastic shear modulus, as these layers are quite fluid.13 The complex surface viscosity exhibits a nonmonotonic response to the surface pressure with a maximum near Π ) 22 mN/m. This surface pressure is close to a slight shoulder in the isotherm, but the microstructural basis for this maximum is not clear. Additionally, the surface viscosity maximum for pure straight-chain hexadecanol of approximately 0.7 mN-s/m occurs in the untilted phase. The phase transitions and positional order of pure linear hexadecanol Langmuir monolayers were first studied by Kaganer et al. by grazing incidence X-ray diffraction3 then by Lee et al. with monolayers of dipalmitoylphosphatidycholine (DPPC) mixtures.22 Both of these studies indicate a sharp, dominant peak for pure hexadecanol at Qxy ) 1.5 Å-1. It has been found that the state of the hexadecanol monolayer at 31.7 °C and that of the tetradecanol monolayer at 31 °C approximately correspond to the states of the octadecanol monolayer at 43 and 53 °C, respectively.3 Kaganer et al. observed higher order peaks in Qxy ) 2.4-3.2 Å-1, but the peaks were very weak. Good agreement between the data and the structure factors confirmed that the broadening was caused by distortions of the lattice.3 Kaganer et al. state that mixing alkanes of different chain lengths weakens the coupling between the layers and makes the behavior of the bulk system more similar to that of a monolayer. It has been found that the addition of small amounts of short-chain fatty (22) Lee, K. Y. C.; Gopal, A.; von Nahmen, A.; Zasadzinski, J. A.; Majewski, J.; Smith, G. S.; Howes, P. B.; Kjaer, K. J. Chem. Phys. 2002, 116, 774.
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acids and n-alcohols, such as hexadecanol, to phospholipid membranes (DPPC), results in higher fluidity.22 In particular, adding hexadecanol to DPPC monolayers increases the temperature of the liquid-expanded to condensed phase transition. Further increase of the amount of hexadecanol beyond a 1:1 molar ratio of DPPC and hexadecanol does not change the lattice or the tilt. Adding hexadecanol to monolayers of DPPC is roughly equivalent to lowering the temperature of a pure DPPC monolayer. The coherence length of the packing, or the extent of ordering, also increases with increasing hexadecanol concentration.22 Lee et al. found only one Bragg peak for hexadecanol, with a corresponding Bragg rod at Qz ≈ 0 Å-1, indicating a hexagonal unit cell with untilted chains and a lattice spacing of 4.82 Å. We have also studied pure films as a baseline, and our results are consistent with these previous results. Some of these data are shown below. 3.2. Pure Linear-Chain Eicosanol Monolayers. Isotherms, Brewster angle microscopy images, and interfacial rheology results for straight-chain eicosanol have also been reported previously14 and are summarized here. The tilted phase is formed over an area of 21 Å2 per molecule with the headgroups in a quasi-hexagonal lattice with the aliphatic tails tilted to the NNN. This value of the cross-sectional area per molecule of eicosanol agrees well with other reported values of approximately 22 Å2.1 At a surface pressure of 15 mN/m, a distinct kink in the isotherm signals the transition to the untilted phase. Finally, the monolayer collapses at surface pressures of at least 50 mN/m. The interfacial rheology data of the straight chain eicosanol show that the viscous shear modulus is significantly larger than the elastic shear modulus, as these layers are quite fluid. The rheological response of eicosanol is quite different from that of hexadecanol. The surface viscosity maximum for eicosanol is observed in the NNN-tilted phase, not the untilted phase. This is consistent with previous studies of eicosanol by Brooks et al.8 Also, the magnitude of the viscous shear modulus of eicosanol is substantially lower than that of hexadecanol. The detailed phase transitions of pure linear eicosanol were first studied by Overbeck et al. using Brewster angle microscopy.23 Lin et al. have studied pure linear eicosanol by X-ray diffraction as well.24 They observed clearly identifiable diffraction peaks above and below the 15 mN/m kink in the isotherm at room temperature. These peaks were weaker than those in lead octadecanoate and tetracosanoic acid by nearly 1 rder of magnitude. While the peak was observed at the same position at 14.5, 28, 39, and 44 mN/m, the monolayer was slightly expanded at the lowest surface pressure studied by Lin et al, 9 mN/m. They suggest that the discontinuous increase in the slope of the eicosanol isotherm at the tilted-untilted phase transition can be due to the fact that “soft” modes of compression of the molecules reach their limit, such that the monolayer suddenly becomes less compressible. This hypothesis is supported by the estimated change in the peak position from 9 to 4.5 mN/m, which gives a compressibility of the same order of magnitude of that calculated from the slope of the surface pressure-area isotherm. This study indicates a sharp, dominant peak for pure eicosanol at Qxy ) 1.55-1.6 Å-1. 3.3. Mixed Hexadecanol Monolayers. As with the straightchain hexadecanol, rheology data for mixed hexadecanol monolayers have been reported previously13 and are summarized here. Our isotherms for spread monolayers of mixtures of straight and branched hexadecanol at room temperature are shown in (23) Overbeck, G. A.; Ho¨nig, D.; Mo¨bius, D. Langmuir 1993, 9, 555–560. (24) Lin, B.; Peng, J. B.; Ketterson, J. B.; Dutta, P. Thin Solid Films 1988, 159, 111–114.
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Figure 2. Pressure-area isotherms of mixtures of straight and branched hexadecanol. From right to left, the branched-chain percentages are 0 (red), 5 (blue), 10 (lavender), 12.5 (light green), 16.7 (dark green), 20 (orange), 25 (green), 33.3 (pink), and 50 (light blue).
Figure 2. An isotherm for the pure branched hexadecanol monolayer, which we define as a film without any straight-chain hexadecanol, is not shown, since it did not form a monolayer that could be reproducibly compressed. The surface pressure for the pure branched material was unstable in time for any compressed surface area. Although the branched chains have the same number of carbon atoms as the straight chains, this suggests that the overall length of the aliphatic tail is critical in forming a stable Langmuir monolayer. In the monograph by Gaines,1 it is suggested that a backbone length of at least 12 carbons is required to form a stable monolayer. For hexadecanol mixtures up to about 30% branched chains, the isotherms have a shape identical to that of pure straight hexadecanol but with a lateral shift toward lower area per molecule. The isotherm slopes are independent of branchedchain fraction. The transition to a NNN-tilted phase occurs at an area per molecule that decreases linearly with increasing branched chain fraction. At higher surface pressure, the linear dependence of the molecular area on the branched-chain fraction holds for all mixtures. The collapse pressure for all mixtures in this study (less than 50% branched) is independent of branched-chain fraction. These trends suggest immiscibility between the straight and branched chains, as confirmed by the X-ray studies discussed below. For branched-chain concentrations of less than approximately 30%, the majority of the branched molecules are segregated from the straight-chain monolayer at the onset of surface pressure rise. Small amounts of the branched chains are likely expelled partly into noncrystalline regions of the monolayer, and the remainder are expelled into micelles in the subphase (see below). Since the collapse pressure for all mixtures studied is independent of branched-chain fraction, this confirms the immiscibility of the two components. If the substances were miscible, the collapse pressure would be expected to vary with composition.1 The dynamic surface viscosities for various mixtures of straightand branched-chain hexadecanol have been reported previously.13 As in the case of the pure straight-chain hexadecanol, the mixtures show a pronounced maximum in the surface viscosity as a function of surface pressure in the untilted phase. The pressure maxima in surface viscosity pass through a significant maximum as a function of concentration at about 12.5% branched chains. GIXD scans were performed for a range of hexadecanol mixtures, including straight hexadecanol and 5, 10, 12.5, 20, and 50% branched hexadecanol. Representative GIXD scans for
Straight-Chain and Branched Fatty Alcohol Mixtures
Langmuir, Vol. 24, No. 24, 2008 14009
Figure 3. GIXD scans for (a) straight hexadecanol at 5 mN/m, (b) straight hexadecanol at 30 mN/m, (c) 20% branched hexadecanol at 5 mN/m, and (d) 20% branched hexadecanol at 30 mN/m, with each showing the raw data of the intensity distribution (top) and the Bragg peak profile or the intensity as a function of Qxy integrated over Qz (bottom) from Qz ) 0 to Qz ) 0.9. The background due to solution scattering and the direct beam scattering have been subtracted.
Figure 4. GIXD scans for 0% branched hexadecanol at (a) 7.5, (b) 15, and (c) 30 mN/m, and 50% branched hexadecanol at (d) 7.5 and (e) 15 mN/m. In plots (a)-(c), no broad peak is present due to the absence of branched chains. In plots (d) and (e), no broad peak is evident, as the branched chains are pushed into the subphase. In plots (a)-(e), the GIXD data are indicated by solid blue lines, and the error bars by red lines at each data point.
straight hexadecanol are shown in Figure 3 at (a) 5 mN/m and (b) 30 mN/m, as well as for 20% branched hexadecanol at (c) 5 mN/m and (d) 30 mN/m. Two overlapping peaks, at Qxy ≈ 1.48 and 1.49 Å-1 and Qz ≈ 0 and 0.2 Å-1, respectively, can be seen in (a) and (c) at 5 mN/m when hexadecanol is in the tilted phase. One intense peak is observed at Qxy ≈ 1.51 Å-1 and Qz ≈ 0 Å-1 in (b) and (d) at 30 mN/m in the untilted phase. GIXD scans for 50% branched hexadecanol and pure straight hexadecanol are shown in Figure 4. Only the strong peak from the straight chain domains is seen for hexadecanol at all surface pressures and all branched fractions. In contrast to eicosanol, there is no weak, low Qxy peak (at approximately 1 Å-1 and due to phase separation of branched chains in the monolayer). Thus, for hexadecanol, the branched chains are pushed into the subphase. As the hexadecanol molecules are compressed, the lattice spacing decreases and Qxy of the Bragg peak increases, as shown in Figure 5a. After the 10 mN/m phase transition, the molecules are in the untilted phase. Based on these data, the lattice spacing is relatively constant with increasing surface pressure. This is
quantitatively consistent with the corresponding isotherm data in Figure 2. The change in the tilt angle as a function of surface pressure for a range of branched hexadecanol mixtures is shown in Figure 5c. These data are consistent with the isotherm data and the transition from the NNN to untilted phases. The in-plane coherence length was generally found to increase from about 100 Å up to 250 Å at the 10 mN/m phase transition and remain relatively constant above this phase transition. Thus, the size of the crystalline domains increases up to the phase transition and then remains constant, as shown in Figure 5e. X-ray reflectivity measurements were done at the APS for 0, 20, 33.3, and 50% branched hexadecanol mixtures at 5, 20, and 35 mN/m to construct electron density profiles. For all hexadecanol mixtures, the reflectivity is independent of branched fraction. This shows that the film forms a single monolayer (specifically, the branched chains are not pushed above the straight-chain monolayer forming a bilayer, since this would have a much different reflectivity). The number of electrons per
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Figure 5. X-ray diffraction results: Lattice spacings, a, for mixtures of straight and branched (a) hexadecanol and (b) eicosanol. Data for 50% branched hexadecanol and 50, 66.7, and 100% branched eicosanol are not shown for clarity; the plot follows the same trend as that for the lower branched fractions. Tilt angles, R, for mixtures of (c) hexadecanol and (d) eicosanol. In-plane coherence lengths, Lxy, for mixtures of (e) hexadecanol and (f) eicosanol. The following branched percentages are shown in plots (a)-(f): 0% ([), 5% (]), 10% (b), 12.5% (O), 20% (9), 33% (2), 50% (0), 66.7% (4), and 100% (/). The dashed line indicates the 10 mN/m phase transition for hexadecanol (plots (a), (c), (e)), and the 15 mN/m phase transition and the 25 mN/m plateau region for eicosanol (plots (b), (d), (f)). The error bars are indicated by thick black lines, and solid black lines have been added to guide the eye.
Figure 6. Normalized number of electrons per molecule of straight chains as a function of branched fraction for (a) hexadecanol and (b) eicosanol. The green line represents the low surface pressure region (5 mN/m for hexadecanol and 7.5 mN/m for eicosanol), the the red line indicates the intermediate surface pressure region (20 mN/m for each), and the blue line indicates the high surface pressure region (35 mN/m for each).
straight chain molecule is relatively constant within experimental error as expected, independent of branched fraction at all surface pressures, indicating that none of the hexadecanol branched chains are incorporated in the monolayer when the surface pressure rises above 0 mN/m. A plot of the number of electrons per straightchain molecule as a function of branched fraction are shown in Figure 6. A schematic of the phase diagram along with a cartoon of the hypothesized structures are shown in Figure 7a. The reflectivity data are consistent with the phase diagrams and our model. 3.4. Mixed Eicosanol Monolayers. As with the straight-chain eicosanol, the isotherms and rheology data for mixed eicosanol
monolayers have been reported elsewhere.14 Isotherms of mixtures of straight and branched eicosanol are shown in Figure 8. An isotherm for a pure branched eicosanol is not shown due to stability, as with the pure branched hexadecanol. The consequences of adding branched chains to straight chain eicosanol are quite different from those for hexadecanol mixtures: adding branched-chain eicosanol causes the liftoff area to increase, and the branched eicosanol is not as easily displaced as the branched hexadecanol. This suggests that regions of pure branched eicosanol chains remain on the monolayer with regions of pure straight-chain eicosanol. The larger molecular cross-sectional area of the branched chains causes the liftoff area to grow
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Figure 7. Schematic phase diagram for mixtures of straight and branched (a) hexadecanol and (b) eicosanol.
Figure 8. Pressure-area isotherms of mixtures of straight and branched eicosanol. From left to right on the x-axis, the branched-chain eicosanol percentages are 0 (navy blue), 2.5, 5, 10, 12.5, 15 (orange), 16.7, 20, 25, 33.3, and 50 (red).
proportionally to their presence.14 This linear progression in the surface area is observed for all branched chain eicosanol mixtures studied, demonstrating that the liftoff area is proportional to the cross-sectional area and fraction of both the straight and branched chains. The straight and branched eicosanol projected areas were estimated from a linear fit to be 21 and 67 Å2, respectively.14 As the surface pressure is increased, the mixtures as well as the pure straight-chain monolayer show a phase transition at approximately 15 mN/m, from the tilted phase to the untilted phase. As the surface pressure is increased further, a second transition at about 25 mN/m occurs. The isotherms shift in the reverse direction toward decreasing area, suggesting the branched chains are ultimately “squeezed out” of the monolayer, leaving the straight chains behind. A slight reduction in collapse pressure for branched fractions higher than 20% indicates a slight miscibility for these mixtures. The dynamic surface viscosities for various mixtures of straight and branched chain eicosanol have been reported elsewhere.14 As with pure straight chain eicosanol, the mixtures exhibit a maximum in the surface viscosity as a function of surface pressure in the tilted phase. As the branched chains are mixed with the straight chains, the maximum value of the surface viscosity occurs from 4 to 7.5 mN/m up to 20% branched chains. This maximum value in surface viscosity is relatively constant (at about 0.09 mN-s/m) until it decays significantly beyond 15% branched chains. Representative GIXD scans for 0, 33, and 50% branched eicosanol are shown in Figure 9. Diffraction was observed from
the phase-separated branched region of the monolayer below 25 mN/m, as shown in Figure 9. The data shown in Figure 9 include two peaks below 25 mN/m: a weak, broad peak at Qxy ) 1 Å-1 and a strong, sharp peak at Qxy ) 1.5 Å-1. The weak peak is absent in the pure straight chain and hence results from phasesegregated domains of weakly ordered branched chains. These adopt an approximately hexagonal structure with a near-neighbor spacing of about 8 Å, corresponding to a molecular area of about 51 Å2, consistent with the liftoff area of 48 Å2 from the isotherm data. This weak peak disappears above 25 mN/m (see Figure 9f and i). The position of the strong peak and its surface pressuredependence are the same as those for pure straight chains, and both are independent of branched fraction. This peak then is due to phase-segregated straight chain domains, as this sharp peak is seen for pure straight chains only, and it has been observed in the literature for these pure straight chains. The contour plot in Figure 10 shows diffraction in the low Qxy region corresponding to the GIXD scans in Figure 9. These data have been background subtracted to more clearly show this weak peak. One observes a weak, broad peak at Qxy ) 1 Å-1 that extends up to Qz ) 0.3 Å-1. Since this peak is extended in Qz (e.g., a Bragg rod), this is due to the diffraction from a monolayer of the branched chains and shows that branched chains are present in the monolayer below 25 mN/m for mixtures of straight- and branched-chain eicosanol. GIXD measurements were performed for several eicosanol mixtures, including straight eicosanol and 5, 10, 12.5, 20, 33.3, 50, 66.7, and 100% branched eicosanol. As the eicosanol molecules are compressed, the lattice spacing decreases, as shown in Figure 5d. After the 15 mN/m phase transition, the molecules are in the untilted phase. Based on these data, the lattice spacing is relatively constant with increasing surface pressure beyond the phase transition. This is quantitatively consistent with the corresponding isotherm data in Figure 8. The tilt angle and the in-plane coherence length were determined in the same manner as that for branched hexadecanol. The change in the tilt angle as a function of surface pressure for a range of branched eicosanol mixtures is shown in Figure 5d. These data are consistent with the isotherm data and the transition from the NNN to untilted phases. The in-plane coherence length was generally found to decrease from about 300 Å to about 100 Å at the 15 mN/m phase transition and remain relatively constant above this phase transition. In contrast to hexadecanol mixtures, the size of the crystalline domains decreases up to the phase transition and then remains constant, as shown in Figure 5f. In addition, nearly all the eicosanol mixtures exhibited an increase
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Figure 9. GIXD scans for 0% branched eicosanol at (a) 10 mN/m, (b) 20 mN/m, and (c) 30 mN/m; 33% branched eicosanol at (d) 10 mN/m, (e) 20 mN/m, and (f) 30 mN/m; and 50% branched eicosanol at (g) 10 mN/m, (h) 20 mN/m, and (i) 30 mN/m. The arrow suggests a weak broad peak shown in the low surface pressure region in (d) and (g). A broad peak is shown by arrows in (e) and (h), indicating that diffraction is observed from the branched molecules in the intermediate surface pressure region, where the straight and branched chains are phase-separated. The branched chains are not as well-ordered as the straight chains. No broad peak at Qxy ≈ 1 Å-1 is observed in the high surface pressure region in (f) and (i), as the branched chains are pushed into micelles. The error bars are indicated by red lines for each data point in the solid blue line.
intermediate surface pressure regions, consistent with our model of branched chains still integrated in the monolayer. On the basis of both the X-ray diffraction and reflectivity data, the majority of the branched chains are most likely squeezed out of the monolayer and form micellar structures in the subphase at surface pressures greater than 25 mN/m. A schematic of the phase diagram and the hypothesized structures are shown in Figure 7b. An irregularly spaced small amount of branched chains may remain on the monolayer at intermediate surface pressures, possibly too small in quantity to be detected by the X-rays, yet significant enough to affect the rheological properties of the material.
4. Discussion Figure 10. Contour plot showing diffraction from the branched chains in Figure 9 for 50% branched eicosanol at 20 mN/m. The figure has undergone a background correction.
in the integrated intensity prior to the 25 mN/m plateau region, indicating a greater degree of ordering up to this transition. X-ray reflectivity measurements were done at the APS for 0, 10, 20, and 33.3% branched eicosanol at 7.5, 20, and 35 mN/m, as well as 50% branched eicosanol at 7.5 and 20 mN/m. Sample X-ray reflectivity data and electron density profiles are shown for branched eicosanol at 7.5, 20, and 35 mN/m in Figure 11. The data are normalized by the Fresnel reflectivity (RF) for a smooth air/water interface. The X-ray reflectivity data and density profiles indicate a trend toward increasing branched fraction at 7.5 and 20 mN/m, while these data and profiles are similar and independent of branched fraction within experimental error at 35 mN/m. A plot of the normalized number of electrons per straight-chain molecule as a function of branched fraction is shown in Figure 6b. The number of electrons per molecule increases as a function of branched fraction in the low and
4.1. Phase Diagram. Our structural model for the fatty alcohol mixtures is summarized by Figure 7, which also illustrates the surface pressure dependence of the branched chains. This is the major result of this study. For mixtures of less than 50% branched fraction, the branched hexadecanol chains are easily displaced by the more efficiently packed straight chains. Although the branched hexadecanol chains contain the same number of carbon atoms as the straight chains, they are effectively only nine carbons long and are more readily expelled from the interface. It is evident that all of the branched chains are forced to leave the air/water surface at the transition from the gaseous phase to the tilted phase, leaving a monolayer of pure straight chains. Above the 10 mN/m phase transition, the hexadecanol straight chains are in the untilted phase, while the branched chains remain below the monolayer of straight chains, most likely as micelles associated with the straight chains. The eicosanol phase diagram (see Figure 7) was found to be qualitatively different from those of hexadecanol mixtures. In the tilted phase, the branched chains are still incorporated in the monolayer with the straight chains. Above the 15 mN/m phase
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Figure 11. X-ray reflectivity data and the scattering length density, F, for 0 (red), 10 (violet), 20 (green), 33 (blue), and 50% (pink) branched eicosanol as a function of the height coordinate, z, at 7.5 mN/m (a and b), 20 mN/m (c and d), and 35 mN/m (e and f). Panels (a), (c), and (e) show Fresnel normalized reflectivity as the points, while the smooth curves are the results of data fitting. Panels (b), (d), and (f) show F as a function of the height coordinate, z, where z ) 0 is the monolayer surface. The 50% branched data are shown for 7.5 and 20 mN/m (a-d) only. The alkyl tail, the hydroxyl head, and the water subphase regions are indicated on the plot. The monolayer thickness is the distance from the headgroup to the monolayer surface (z ) 0). The fits to the lower surface pressures are not as good, most likely due to radiation damage and patchiness of the monolayer.
transition, the branched chains are still incorporated in the monolayer with the straight chains up to the plateau region at 25 mN/m. Phase separation was observed experimentally between the straight and branched chains for each of the fatty alcohol mixtures, due to the very different packing modes of the two phases. Above the plateau region, the eicosanol mixtures are structurally similar to the hexadecanol mixtures in the untilted phase at high surface pressures. In this region, the eicosanol branched chains are below the monolayer of straight chains, also most likely as micelles associated with the straight chains. The lattice spacings calculated from the X-ray data are relatively constant at about 4.8 Å at high surface pressures for hexadecanol and eicosanol mixtures, consistent with pure straight-chain domains. The straight-chain diffraction from all mixtures was also independent of branched fraction. For eicosanol, the Qxy data show the phase separation both in the straight chain peak not changing with composition, and the low Qxy peak appearing at a position consistent with molecular size. Consistent with the diffraction data, the isotherm data suggest that the branched molecules are partly expelled from the monolayer into the subphase to form micelles, and partly into noncrystalline regions of the monolayer, perhaps at grain boundaries, with micellar structures associated with the monolayer of straight chains. Small amounts of branched chains remain on the monolayer at intermediate surface pressures for eicosanol, significant enough to affect the rheological properties. Micelles associated with the
monolayer also affect the rheological properties of eicosanol and hexadecanol mixtures. We have considered the possibility of solubility of the branched fatty alcohol in the aqueous subphase. There must be micelles or some other type of structure that is associated with the monolayer, as the rheological properties of the material are influenced by the presence of the branched alcohols. There have been previous reports of aqueous surfactant-alcohol systems in which micellar size, shape, and interactions have been discussed at length with various alcohols.25 Fatty acids and fatty alcohols are also known to form micelles. Thus, we think that this is the most likely structure that is associated with the straight-chain monolayer, as opposed to another layer (“rafts”) beneath the straight-chain monolayer, or that the branched chains have completely dissolved in the subphase, which does not agree with the rheology data. The reflectivity data show that the branched chains are not pushed above the straight-chain monolayer (i.e., do not form a bilayer). X-ray reflectivity of all hexadecanol mixtures and eicosanol mixtures above 25 mN/m show that the overall monolayer thickness is nearly independent of branched fraction, since the reflectivity is dominated by the straight-chain domains. For hexadecanol mixtures at all surface pressures and for eicosanol at 35 mN/m, the number of electrons per straight chain is independent of branched fraction and is the same as that of the (25) Zana, R. AdV. Colloid Interface Sci. 1995, 57, 1–64.
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pure straight chains (138 and 170, respectively, to within experimental error). This shows that, under these conditions, no branched chains are incorporated in the monolayer. In contrast, for eicosanol at 7.5 and 20 mN/m, the number of electrons per molecule does depend on branched fraction, increasing with higher branched fraction. These results agree with our picture of the surface pressure dependence of the branched chains. 4.2. Straight-Chain Structure. In general, the X-ray data indicate that the straight chains in branched fatty alcohol mixtures behave similarly to the pure straight chains, although there are small differences. All the mixtures pack in 2D quasi-hexagonal or hexagonal unit cells. At low surface pressures, the tails pack in a tilted quasi-hexagonal phase with the tilt angle from the surface normal around 20° and the tilt direction from the nextnearest neighbor. When the pressure increases for both hexadecanol and eicosanol mixtures, the tilt decreases, and for pressures larger than 10 mN/m it is less than 6° for hexadecanol mixtures and 10° for eicosanol mixtures. In addition, the area per molecule, thus the size of the unit cell, is a monotonic function of the surface pressure. In the case of eicosanol and its plateau region in the isotherm, there is not much structural change in the molecular packing above the first phase transition at 15 mN/m. In all of the cases, the molecules are already untilted and perpendicular to the air/water interface at pressures above 15 mN/m. The only obvious packing parameter that is changing with surface pressure for different fatty alcohol mixtures is the in-plane coherence length. This is probably due to the change in the size of the islands of the mixtures in the plateau region. Upon compressing and decompressing of the alcohols, no changes were observed in the straight-chain structure. 4.3. Rheology-Structure Correlation. Based on these data, a correlation between rheology and X-ray diffraction results was observed. The hexadecanol mixtures exhibited an increase in the in-plane coherence length prior to the phase transition and a maximum in the surface viscosity in the untilted phase. On the other hand, the eicosanol mixtures exhibited a decrease in the inplane coherence length up to the phase transition and a surface viscosity maximum in the tilted phase. In addition, the magnitude of the modulus of the hexadecanol mixtures is significantly larger than that of all of the eicosanol mixtures. At low surface pressure, the less viscous hexadecanol monolayers have smaller domains, as measured by the in-plane coherence length. The maximum viscosity is enhanced at low branched fraction, while the viscosity of all mixtures decreases after peaking approximately at 22 mN/ m. At high surface pressure, the maximum viscosity and the domain size are independent of branched fraction. Eicosanol mixtures also exhibit quite different rheological behavior from that of the hexadecanol mixtures. For instance, the film becomes more compliant in the untilted phase after the 15 mN/m phase transition. The viscosity maxima for all eicosanol mixtures occur in the tilted phase, while the hexadecanol maxima occur in the untilted phase, as the hexadecanol mixtures become more viscous in this phase. At low surface pressure, the more viscous eicosanol films have larger domains. The branched eicosanol chains incorporated in the monolayer reduces the viscosity above 12.5% branched chains. Above 15 mN/m, both the domain size and surface viscosity are constant and independent of branched fraction. Overall, the branched fractions have a greater effect on the eicosanol mixtures, since the branched chains are only weakly associated with the hexadecanol films. For both eicosanol and hexadecanol monolayers, the surface viscosity increases with increasing in-plane coherence length: larger monolayer regions or islands are more viscous.
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The qualitative differences in the X-ray diffraction patterns between hexadecanol and eicosanol have provided valuable clues about the differences in rheological behavior between the two fatty alcohols. These studies are valuable in modeling biological systems such as lipids, due to the process of expelling branched chains to form micelles. One application of this system could be used to model the “kiss and run” mechanism associated with synaptic vesicle recycling, where the vesicle briefly connects to the plasma membrane without full collapse.26
5. Conclusion The conclusions from this work are summarized in Figure 7. Since the branched chains affect the viscosity for eicosanol mixtures, some of branched molecules are on or near the interface for all surface pressures. Above 25 mN/m, where branched chains are squeezed out of the eicosanol monolayer, there are likely branched-chain micelles associated with the straight-chain monolayer. For hexadecanol, branched-chain micelles are near the straight-chain monolayer, as the viscosity is slightly enhanced for mixtures. The different behavior between the two mixtures is likely due to the chain length. Eicosanol is longer, more flexible, forcing the branched chains to form an insoluble monolayer. It accommodates the branched chains better than the shorter, stiffer and soluble hexadecanol. Although submerged, the branched hexadecanol evidently interacts with the straight-chain monolayer. The phase separation is likely due to the straight-/branchedchain incompatibility. Since the tails largely determine the monolayer packing,1 the size differences between straight and branched chains suggest that miscibility is unfavorable. These studies have provided a deeper understanding of the structure and behavior of amphiphilic mixtures, since singlecomponent Langmuir films are well-studied, while mixtures are not as well understood. This work is the first reported study of straight-/branched-chain fatty alcohol mixtures. We have used a variety of experimental tools to elucidate how the addition of branched chains can result an enhancement in the surface shear viscosity of the mixture, and how the structure varies with surface pressure and composition. These studies will ultimately aid in developing models for lipids, micelle formation, or other important biological functions. Acknowledgment. The authors gratefully acknowledge support from the National Science Foundation Division of CTS and BASF SE. ChemMatCARS Sector 15-ID-C is principally supported by the National Science Foundation/Department of Energy (CHE-0535644). The Advanced Photon Source is supported by the U.S. Department of Energy, Office of Basic Energy Sciences (W-31-109-Eng-38). The authors thank John Kirkwood, Bridget Ingham, Alice Wong, and Martin Widenbrant for assistance with some of the X-ray diffraction and reflectivity measurements. The authors acknowledge David Schultz for his assistance in developing the Langmuir trough for X-ray studies and Jeff Gebhardt for his assistance at the beamline. Portions of this research were supported in part by the Stanford Synchrotron Radiation Laboratory, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. This research was also facilitated in part by a National Physical Science Consortium Fellowship, by support from Los Alamos National Laboratory, as well as by support from NSF, Grant DMR-0213618. LA802467E (26) Klingauf, J.; Haucke, V. In Protein Trafficking in Neurons, 1st ed.; Bean, A., Ed.; Academic Press: New York, 2006; p 125.
