Metastability in Monolayer Films Transferred onto Solid Substrates by the Langmuir-Blodgett Method: IR Evidence for Transfer-Induced Phase Transitions

Abstract

The rate at which a monomolecular film is deposited onto a solid substrate in the Langmuir-Blodgett process of preparing supported monolayer films influences the final structure of the transferred film. Attenuated total reflectance infrared spectroscopic studies of monolayers transferred to germanium substrates show that the speed at which the substrate is drawn through the air/water interface influences the final conformation in the hydrocarbon chains of amphiphilic film molecules. This transfer-induced effect is especially evident when the monolayer is transferred from the expanded region of surface-pressure-molecular-area isotherms at low surface pressures; the effect is minimized when the film molecules are transferred from condensed phases at high surface pressures. This phenomenon has been observed for both a fatty acid and a phospholipid, which suggests that these conformational changes may occur in a variety of hydrocarbon amphiphiles transferred from the air/water interface. This conformational ordering may be due to a kinetically limited phase transition taking place in the meniscus formed between the solid substrate and aqueous subphase. In addition, the results obtained for both the phospholipid and fatty acid suggest that the structure of the amphiphile may help determine the extent and nature of the transfer-speed-induced structural changes taking place in the monomolecular film.

Interaction of the cationic peptide bactenecin with mixed phospholipid monolayers at the air–water interface

“…4a (not shown) shows less than 5% height values larger than 2.5 nm, which means that the repulsion effect of the mica on the negative phospholipid monolayer is low. The effect of the monolayer lifting due to substrate repulsion was not observed, demonstrating previous observations [36] for condensed phases. At this respect, it is necessary to bear in mind that monolayer transfer might affect the structure of the monolayer films.…”

“…At this respect, it is necessary to bear in mind that monolayer transfer might affect the structure of the monolayer films. The transference can be effectively affected when it is performed at the gaseous and LE phases [36]. However, several authors have found that the transfer of the first lipidic monolayer is relatively easy at surface pressures above the LE state [33][34][35], even when the solid substrate and the monolayers may carry the same kind of electrical charge, probably because of the monolayer film greatly reduces the surface energy of the hydrated substrate in air [33].…”

“…Sample-tip forces can be introduced by hydrophobic interaction occurring between the tip and the different zones of the surface, often due to differences in water accumulation [32]. Some authors have investigated the effect of the charge of solid substrates on the lipid and phospholipd monolayers and bilayers structure [33][34][35], and others have noticed the influence of the rate at which the monolayer is deposited at a solid substrate on the final structure of the transferred film [36]. However, this technique has proven to be a useful additional method to elucidate the structure of supported phospholipid monolayers and bilayers, being its major advantage the high resolution on the nanoscopic scale that enables to investigate proteins or peptides that intercalate into the different phospholipids [11,12,[37][38][39].…”

Human stratum corneum lipid organization as observed by atomic force microscopy on Langmuir–Blodgett films

“…However, due to diVerent adhesive interactions of the monolayer transferred to a solid substrate with respect to the monolayer Xoating on water, the mixing behaviour of Xoating and transferred monolayers is diVerent, leading to a possible substrate-mediated condensation of monolayers upon transfer (Spratte and Riegler, 1994). This eVect is however minimized when the Wlm molecules are transferred from condensed phases at high surface pressures (Rana et al, 1994). Furthermore, the recorded condensation into close-packed islands during transfer of fatty acids in the liquid expanded state is less pronounced at low pH (Sikes and Schwartz, 1997).…”

Raman spectroscopy of model membrane monolayers of dipalmitoylphosphatidic acid at the air-water interface using surface enhancement from buoyant thin silver films

“…Studies of phospholipid films in dispersions, Langmuir layers, and Langmuir–Blodgett layers are well documented as biological membrane models 1. These assemblies have been studied by a wide range of techniques including NMR spectroscopy,2–14 X‐ray and electron diffraction,15–17 fluorescence spectroscopy,18–32 optical and spectroscopic microscopy,20, 22, 24, 33 scanning probe microscopies,34–37 and the vibrational spectroscopies FTIR,38–51 Raman,42, 52–63 and vibrational sum‐frequency generation 64–66. The vibrational spectroscopies provide much needed data about molecular structure and interactions between lipid molecules in such assemblies, particularly for the alkyl chain.…”