Article pubs.acs.org/JPCC
Transfer of Ordered Phospholipid Films onto Solid Substrates from a Drained Foam Film Naif Alshehri,† Trystan Bennett,‡ Gregory F. Metha,‡ and Gunther G. Andersson*,† †
Flinders Centre for NanoScale Science and Technology, Flinders University, Adelaide SA 5001, Australia Department of Chemistry, University of Adelaide, Adelaide SA 5005, Australia
‡
ABSTRACT: Bubbles have been formed from solutions of 1palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) in formamide. The bubbles were transferred onto a hydrophobic substrate after drainage time of 0−20 min. Drainage time of 20 min is sufficient to generate fully drained bubbles. The structure of the transferred films was investigated with X-ray photoelectron spectroscopy and metastable induced electron spectroscopy. Films transferred after a drainage time of 5 min or less show random orientation of the molecules in the films. Films transferred after a drainage time of 10 min or longer result in films forming a mixture of two different types of structures. The smaller fraction of the transferred film shows random orientation of the molecules. The larger fraction shows preferred orientation of the POPC molecules. In the fraction with preferred orientation the polar part of the POPC molecules forms to a large degree the outermost layer which is different to surfaces of surfactant solutions and to the surface of foam films formed by surfactant molecules. A surface formed by polar groups has a higher surface energy than a film formed by nonpolar groups. The possible reasons for the orientation of the molecules with the polar group pointing to the surface are discussed.
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INTRODUCTION
layers becomes weaker than that of CBFs due to the presence of salt in surfactant solution.6 The self-organization properties of surfactant layers of foam films have been used for the controlled assembly of nanomaterials. Assembling nanoparticles in a controlled way is of interest in a number of fields in nanotechnology. Benattar and co-workers reported that bubble deposition method (BDM) is beneficial not only for the transfer of pure freestanding thin films onto surfaces3,7,8 but also for inserting different types of nanoparticles such as single-walled carbon nanotubes,9 gold nanoparticles,10 proteins, mixed oxide nanowires,11 and graphene oxide12 and silica nanoparticles13 into freestanding films. In this paper, we transfer a surfactant double layer from a thin foam film onto a hydrophobic substrate (hydrophobized silicon wafer). The thickness and composition of the transferred film and orientation of the surfactant molecules forming the film is investigated for a range of drainage times of the foam film. Two surface sensitive techniques are employed: X-ray photoelectron spectroscopy (XPS) and metastable induced electron spectroscopy (MIES). XPS allows us to determine the atomic composition of elements present in the samples and to gain information about the thickness of the films. MIES provides details of the valence band structure present at the outermost layer of the transferred films, and thus information on the molecular orientation of the surfactant molecules. Computational calculations are used to correlate the
Foam films are thin liquid films stabilized by surfactant molecules. The liquid surfaces forming the foam film come in close contact, with a distance of a few to a few tens of nanometers. The coverage of the liquid surface with surfactant molecules is slightly higher at the surface of a foam film compared to the surface of a bulk liquid with the same concentration of the surfactant solution.1,2 This leads to a reorientation of the surfactant molecules upon foam film formation, often with the headgroup of the surfactant molecules moving to a larger depth and the alkyl chain being oriented more parallel to surface normal.1,2 As a consequence, the layer of surfactant molecules at the surface of a foam film show a higher order than the surfactant layer at the surface of the bulk solution. The organized bilayers of surfactants can be used to form two-dimensional nanostructures on solid surfaces through transfer of the foam film. The thickness of the solvent layer inside the film ranges from a few angstroms to a few nanometers.3 The foam films can exist in two states of equilibrium depending on the choice of surfactant and the concentration of ions: common black films (CBFs) and Newton black films (NBFs).4,5 CBFs have a thickness of a few 10 nm,5 have a rather large solvent core,3 and are stabilized by a balance of the internal osmotic pressure and attractive van der Waals forces. The first effect is usually attributed to the overlap of electrostatic double layers creating a repulsive force between the surfaces.5,6 The NBFs, in contrast to CBFs, are much thinner, contain only residual solvent in the core upon maximal drainage,7 and can be obtained when the electrostatic repulsion generated between the two surfactant © 2015 American Chemical Society
Received: July 12, 2015 Revised: September 1, 2015 Published: September 8, 2015 22496
DOI: 10.1021/acs.jpcc.5b06689 J. Phys. Chem. C 2015, 119, 22496−22503
Article
The Journal of Physical Chemistry C peaks in the MIE spectra to the functional groups of the compounds investigated. In the present work, we use the phospholipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine with the chemical formula C42H82NO8P (POPC), and formamide as solvent with the chemical structure CH3NO (FA). FA is used as the solvent due to its low vapor pressure. The low vapor pressure reduces the evaporation of solvent from the foam film and was found to lead to longer lifetimes of the foam films compared to foam films based on aqueous solutions. POPC was used because phospholipids are known to form closed layers at liquid surfaces.14 Sodium Iodide (Nal) was added to the foam films in order to investigate whether NaI remains in the film during the drainage of the film and can thus be found in the foam film transferred to the silicon surface.
Figure 2. Closed cell of the experiment showing the major components including a glass rod, a porous plate, and a closed chamber of plastic.
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EXPERIMENTAL SECTION Surfactant Solution. Foam films of POPC solutions in FA with a concentration of 0.03 mM/kg (about five times the POPC cmc) were investigated. POPC (structure shown in Figure 1) was purchased from Avanti Polar Lipids Inc. and used without further purification. FA was purchased from Acros Organics (>99.5%) and was used without further purification.
The tray was filled with surfactant solution to a level corresponding to about half of the height of the porous disc. The porous disc had a height of 4 mm. A cleaned and hydrophobized silicon wafer was placed onto the tip of a rod that could be moved through a hole in a transparent cover. The cover was placed over the porous disc loaded with the foam film. The function of the cover is first holding and guiding the movement of the rod and second shielding the foam film from disturbance through air movement. After a given drainage time the silicon wafer was lowered until it touched the bubble. Upon contact the bubble covered the substrate (usually almost the entire silicon wafer is covered) and also formed a cylinder of a single foam film between the porous substrate and the silicon substrate. After a few seconds, the bubble had burst and the remaining solvent in the film transferred to the substrate evaporated. Subsequently, the substrate was removed from the rod for characterization with XPS and MIES. Computer Calculations POPC. All calculations, including geometry optimizations and harmonic vibrational frequency calculations, were undertaken utilizing the B3LYP functional15 and cc-pvdz basis set,16 as implemented in the Gaussian 09 suite of programs.17 All calculations were performed as closedshell species. The optimization was undertaken in the C1 point group, with all numerical integral grids and optimization cutoffs left at the default. The optimization of the POPC molecule was followed by a harmonic frequency calculation to confirm the geometry was a true minimum with no imaginary frequencies. Density of states (DOS) spectra were extracted from the calculation files utilizing GaussSum 2.2.18 Electron Spectroscopy (XPS and MIES). The investigation of the samples were conducted using XPS and MIES in an ultrahigh vacuum (UHV) system built by SPECS (Berlin, Germany) with a base pressure of a few 10−10 mbar. He* were generated in a two stage cold cathode gas discharge from MFS (Claustal-Zellerfeld, Germany.) A nonmonochromatic X-ray source (Mg Kα) was used to generate Kα radiation. The emitted electrons are detected via a hemispherical Phoibos 100 energy analyzer from SPECS (Berlin, Germany). MIES and XPS were conducted with a pass energy of 10 eV. A bias of 10 V was applied to the samples for the MIES measurements. The angle of He* and X-ray source radiation and the analyzer are both 54° with respect to the sample normal. High resolution XP spectra were fitted with combined Gaussian−Lorentzian components and a correction for the Shirley background.19 MIES uses metastable helium atoms (He*) to electronically excite the valence electron orbitals forming the outermost layer
Figure 1. Structure of the 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) molecule.
Cleaning and Hydrophobization of Substrate. The substrate used was n-type Si(100) wafers with a thickness of 0.525 mm and size of 1 cm × 1 cm and a resistivity of
