Relaxation of bimolecular layer films on water surfaces

Ferric stearate, a three-tailed amphiphile, forms bimolecular layers on water surfaces. Molecules in the lower layer are in an "asymmetric" configuration, Fe-containing heads touching water and three hydrocarbon tails in air, while molecules in the upper layer are in a "symmetric" configuration, in pairs of "Y and inverted Y" disposition of tails about the Fe-bearing head. Pressure relaxation at constant area (pi- t curves) and area relaxation at constant pressure ( A- t curves) of this bimolecular layer can be modeled as a sum of three exponential decay terms with distinct time constants and weight factors. Relating the long-term decay with desorption of the total film thus indicates a remarkable long-term stability of the bimolecular layer film. An X-ray reflectivity study of the bimolecular films deposited horizontally on Si(001) at various conditions of relaxation shows no further growth along the vertical of any other layer. Under pressure relaxation molecules are transferred from the upper layer to the lower layer with a change from symmetric to asymmetric configuration, while under area relaxation the transfer is from the lower layer to the upper layer with a configurational change from symmetric to asymmetric.     

A study of the properties and composition of stearic acid monolayers on an aqueous subphase containing cadmium ions

Compression isotherms and chemical compositions of stearic acid (HSt) monolayers applied onto an aqueous subphase containing cadmium ions have been studied in a wide pH range. The data obtained have been used to calculate the dependences of surface elasticity E_2.5, change ΔA_2.5 in the surface area per molecule at a constant two-dimensional pressure in the monolayer of 2.5 mN/m, and change Δ|ζ| in the absolute value of the ζ potential of particles formed from the material of a monolayer collapsed on the surfaces of aqueous solutions with the same compositions on the pH of the subphase. The obtained data are in good agreement with each other and unambiguously attest to the dependences of the surface (E_2.5, ΔA_2.5) and electrosurface (Δ|ζ|) characteristics of the monolayers on their chemical composition. This leads us to make the reasonable suggestion that two alternative reactions may occur between cadmium ions and the HSt monolayer with the formation of two salts, CdSt₂ and Cd(OH)St.     

Interaction of DMPC liposomes with a DMPA monolayer: related study of Langmuir-Blodgett films

The formation of a phospholipidic layer was achieved in two steps: (1) a dimyristoyl-l-α-phosphatidic acid (DMPA) Langmuir monolayer was formed by spreading a chloroform/methanol DMPA solution onto an aqueous subphase; after a 10 min period, the monolayer was compressed at 5mNm⁻¹; and (2) keeping the area of the DMPA monolayer constant, a dimyristoyl-l-α-phosphatidylcholine (DMPC) liposomal suspension was added. The progressive incorporation of DMPC molecules into the DMPA monolayer was studied by monitoring the variation of surface pressure with time at constant film area. Three parameters involved in the formation of the interfacial layer DMPA/liposomal DMPC (DMPA/ lip-DMPC) were studied: liposome addition, aqueous subphase composition and initial surface pressure of the DMPA monolayer. The transfer of this mixed layer was controlled through a traceable fluorescent probe incorporated in the liposomes. The thickness and homogeneity of the Langmuir-Blodgett films thus obtained were assessed through Fourier transform infra-red spectroscopy and Nomarski microscopy, respectively. This study shows that the DMPA/lip-DMPC monolayer could be transferred without dragging of aggregates or mesophases.     

亚相pH对气水界面上VE/DPPC单分子膜的影响

通过表面压－分子面积等温线的测定，考察了亚相ｐＨ对气水界面上的维生素Ｅ（ＶＥ）／二棕榈酰基磷脂酰胆碱单分子膜的影响。亚相ｐＨ降低不改变ＤＰＰＣ单分子膜的崩裂压，但使ＶＥ单分子膜的崩裂压明显增大，不改变ＶＥ单分子膜的平均分子面积，但使ＤＰＰＣ单分子膜凝缩，低表面压下，ＶＥ对ＤＰＰＣ单分子膜的膨胀作用在纯水上很小，在ｐＨ为１的亚相上则很明显，这提示在低ｐＨ的亚相上，ＶＥ／ＤＰＰＣ单分子膜中的极性头基间     

Molecular Dynamic Studies on Langmuir Monolayers of Stearic Acid

Compression isotherm for stearic acid was obtained by means of molecular dynamic simulation and compared to experimentally measured values for the Langmuir monolayers. Compared to the previous simulation, the present simulation has provided a method to reproduce the compression of the monolayer. The result is consistent with other experimental results. By analyzing the alkyl tails, the configuration of stearic acid molecules during the compression process was studied and a uniform monolayer was obtained after compression. Stearic acid molecules were observed to form fine organized monolayer from completely random structure. Hexatic order of the arrangement has been identified for the distribution of stearic acid molecules in the monolayer. At the end of the compression, the stearic acid molecules were tightly packed in the gap of two other molecules. At last, the hydrogen bonds in the system were analyzed. The main hydrogen bonds were from stearic acid-water interaction and their intensities constantly decreased with the decreased of surface area per molecule. The weak hydrogen bond interaction between stearic acid molecules may be the reason of easy collapse.     

Effect of compression of a stearic acid monolayer on interfacial binding of copper ions and cluster formation

The effect of compression of a stearic acid Langmuir monolayer on the interaction of copper ions with the monolayer and on the formation of interface clusters has been studied as functions of pH and ionic content of the water subphase. Copper binding was estimated by direct electron paramagnetic resonance measurements of ion concentrations in the water subphase. A scanning tunnelling microscopy study of monolayer Langmuir-Blodgett films, deposited on graphite substrates, gave evidence for the formation of nanosized clusters on the monolayer surface. The data obtained showed that the interaction of the monolayer with copper ions and the accompaning cluster formation processes were determined by the arrangement, order and mobility of the stearic acid molecules in the monolayer and the electrostatics at the interface.     

Protein-lipid interactions at the air/water interface

Surface pressure measurements and external reflection FTIR spectroscopy have been used to probe protein-lipid interactions at the air/water interface. Spread monomolecular layers of stearic acid and phosphocholine were prepared and held at different compressed phase states prior to the introduction of protein to the buffered subphase. Contrasting interfacial behaviour of the proteins, albumin and lysozyme, was observed and revealed the role of both electrostatic and hydrophobic interactions in protein adsorption. The rate of adsorption of lysozyme to the air/water interface increased dramatically in the presence of stearic acid, due to strong electrostatic interactions between the negatively charged stearic acid head group and lysozyme, whose net charge at pH 7 is positive. Introduction of albumin to the subphase resulted in solubilisation of the stearic acid via the formation of an albumin-stearic acid complex and subsequent adsorption of albumin. This observation held for both human and bovine serum albumin. Protein adsorption to a PC layer held at low surface pressure revealed adsorption rates similar to adsorption to the bare air/water interface and suggested very little interaction between the protein and the lipid. For PC layers in their compressed phase state some adsorption of protein occurred after long adsorption times. Structural changes of both lysozyme and albumin were observed during adsorption, but these were dramatically reduced in the presence of a lipid layer compared to that of adsorption to the pure air/water interface.     

From monomolecular films to bulk liquid crystals

Molecules forming thermotropic liquid crystal bulk phases, were investigated at the air-water interface. With nematic and smectic substances, compression of the film resulted in an increase of the lateral pressure, provided the molecules had an amphiphilic character. Two types of π-A-diagrams were found for areas below a threshold value: (i) A single coexistence region (constant pressure at different areas) is obtained for molecules forming nematic liquid crystal bulk phases. The original monomolecular film grew in the third dimension to form a nematic liquid crystal at the interface. (ii) The π-A-diagram showed several coexistence regions with constant pressures if the molecules had the capacity to form smectic bulk phases. The “collapses” were at areas in the relation 1 : 1/2 : 1/3 etc. suggesting that smectic layers have been formed. The aqueous subphase is found to make a large contribution to the latent heat. The initial layer (together with the subphase) is found to be of lower symmetry than the upper layer(s). The transition is first order at low temperature (< 30°C) and second order at high temperatures. Evidences are given for a smectic Shubnikov phase where the layer-to-layer material flow is concentrated in flux lines.     

Effect of pH on the control of molecular orientation in monolayer of bent-core liquid crystal materials by Langmuir–Blodgett method

Control of molecular orientation at the substrate surface is significant to understand the surface science. Langmuir films of bent-core liquid crystals having alkyl chains at both ends were deposited on silicon substrate. Studies were carried out on air–water interface by changing pH of the subphase. On compression, molecules were arranged in stacks at high pH where as uniform monolayer was formed at lower pH. Limiting area increased at low pH, which resulted in the formation of monolayer after attaining a sustainable surface pressure. Langmuir films were transferred to silicon substrate, and atomic force microscopy images showed appropriate height profiles.     

Monolayer studies of single-chain polyprenyl phosphates

The monolayer properties of some single-chain polyprenyl phosphates (phytanyl, phytyl, and geranylgeranyl phosphates), which we regard as hypothetical primitive membrane lipids, were investigated at the air-water interface by surface pressure-area (pi-A) isotherm measurements. The molecular area/ pressure at various pH conditions dependence revealed the acid dissociation constants (pKa values) of the phosphate. The pKa values thus obtained at the air-water interface (pKa1 = 7.1 and pKa2 = 9.4 for phytanyl phosphate) were significantly shifted to higher pH than those observed in the bilayer state in water (pKa1 = 2.9 and pKa2 = 7.8). The difference in pKa values leads to a stability of the phosphate as both monolayer and bilayer states in a pH range of 2-6. In addition, the presence of ions such as sodium, magnesium, calcium, and lanthanum in the subphase significantly altered the stability of the polyprenyl phosphate monolayers, as shown by the determination of monolayer collapse and compression/expansion hysteresis. Although sodium ions in the subphase showed only a weak effect on the stabilization of the monolayer, addition of magnesium ions or of a small amount of calcium ions significantly suppressed the dissolution of the monolayer into the subphase and increased its mechanical stability against collapse. In contrast, the presence of larger amounts of calcium or of lanthanum ions induced collapse of the monolayers. Based on these experimental facts, a plausible scenario for the formation of primitive cell membrane by transformation of a monolayer to vesicle structures is proposed.     

Insoluble mixed monolayers : II. Protolytic equilibria and the influence of the pH on the collapse pressure

A general thermodynamic treatment is given for the protolytic equilibria in an insoluble monolayer, containing surfactant molecules with n ionizable protons and able to accept m more protons, and being spread at the liquid/gas interface. The correlation between the pH of the subphase liquid and the collapse pressure of the monolayer is discussed. By using the approximation of perfect solutions and of binary surface systems (protonated and deprotonated molecular species of the surfactant) several methods are proposed for deriving apparent surface acidity constants from experimental collapse pressure vs pH curves, in the case of both miscible in monolayer miscible in collapsed bulk phase and miscible in monolayer immiscible in collapsed bulk phase, type systems. Some of these methods are based on a complete perfect solution approximation (CA) taking into account the molar fraction of the subphase liquid in the monolayer and the others use “surfactant” approximation (SA), neglecting this molar fraction. The methods proposed are tested on monolayers of carotenoid pigments spread at aqueous solution/air interfaces. Results obtained by the different methods are rather close to each other, but the approximation CA is better than SA. The apparent surface acidity constants of different carotenoids are compared with each other and discussed in terms of molecular structure and electronic effects.     

The controlling effect of pH on oxidation of Cr(III) by manganese oxide minerals

Effects of the subphase temperature on the surface pressure (pi)-area (A) isotherms of mixed monolayers of miltefosine (hexadecylphosphocholine), a potential anticancer drug, and cholesterol were investigated at the air/water interface, which were supplemented with Brewster angle microscopy (BAM) observations. Comparison of the collapse pressure values, mean molecular areas, excess areas and excess free energy of mixing between the mixed monolayer at various molar ratios and the pure component monolayers showed that, regardless of the subphase temperature, the investigated miltefosine-cholesterol system is much more stable than that the pure component monolayers, suggesting strong attractive interactions between miltefosine and cholesterol in mixed monolayers. As a consequence, it was postulated that stable "complexes" of the two components could form at the interface, for which stoichiometry may vary with the subphase temperature. Such "surface complexes" should be responsible for the contraction of the mean molecular area and thus the high stability of the mixed monolayer.     

Metal ion-induced H-aggregation of a water-soluble anionic dye Congo red (CR) in Langmuir–Blodgett (LB) film

Present communication reports the preparation of Langmuir monolayer of water-soluble anionic dye Congo red (CR) by allowing it to adsorb from the aqueous subphase onto the preformed Langmuir monolayer of anionic stearic acid using divalent metal cations Mg²⁺ as mediator. Isotherm and compressibility studies of SA-Mg-CR hybrid monolayer gave valuable information about the molecular organisation in the Langmuir monolayer. Absorption spectroscopic studies revealed the formation of H-aggregates in the hybrid Langmuir–Blodgett (LB) films fabricated at lower salt concentration in the subphase. Atomic Force Microscopic image gave visual evidence of distinct nanocrystalline domains in the LB monolayer film.     

Formation of silver nanoparticles at the air-water interface mediated by a monolayer of functionalized hyperbranched molecules

Nanofibrillar micellar structures formed by the amphiphilic hyperbranched molecules within a Langmuir monolayer were utilized as matter for silver nanoparticle formation from the ion-containing water subphase. We observed that silver nanoparticles were formed within the multifunctional amphiphilic hyperbranched molecules. The diameter of nanoparticles varied from 2-4 nm and was controlled by the core dimensions and the interfibrillar free surface area. Furthermore, upon addition of potassium nitrate to the subphase, the Langmuir monolayer templated the nanoparticles' formation along the nanofibrillar structures. The suggested mechanism of nanoparticle formation involves the oxidation of primary amino groups by silver catalysis facilitated by "caging" of silver ions within surface areas dominated by multibranched cores. This system provides an example of a one-step process in which hyperbranched molecules with outer alkyl tails and compressed amine-hydroxyl cores mediated the formation of stable nanoparticles placed along/among/beneath the nanofibrillar micelles.     

Cd2+-induced interfacial structural changes of Langmuir-Blodgett films of stearic acid on solid substrates: a sum frequency generation study

The molecular structures and their stabilities at the outmost-layer of the Langmuir-Blodgett (LB) films of stearic acid on solid substrates have been investigated by a highly surface-sensitive spectroscopic technique, sum frequency generation (SFG), in air and in aqueous solution, using the combination of both normal and deuterated stearic acid. Peaks observed in the SFG spectra are mainly attributed to the terminal methyl group at the outmost layer of the LB films. The SFG spectra in air are virtually identical and are independent of the odd-even property and thickness (1-12) of the LB films, indicating that the even-numbered LB film changes its surface structure after passing through the interface between the water subphase and air, especially when the Cd2+ cation was included in the water subphase. Furthermore, we have demonstrated for the first time using in situ SFG measurement that the interfacial molecular structure at the LB bilayer of stearic acid on the hydrophilic substrates significantly change with immersion in the water subphase containing Cd2+ cation while such a structural change has not been observed in the water subphase without Cd2+. These results clearly indicate that a reorganization process takes place on the surface of the stearic acid bilayer induced by the Cd2+ cation. The electrostatic interaction between the carboxylate headgroup of stearic acid via the Cd2+ cation seems to play an important role in the surface reorganization process both in air and in solution.     

Memory effects on the interfacial characteristics of dioctadecyldimethylammonium bromide monolayers at the air-water interface

The structural and dynamic characteristics of dioctadecyldimethylammonium bromide (DODAB) monolayers on a pure water subphase were investigated by surface film balance, Brewster angle microscopy, and relaxation in area and surface pressure at constant surface pressure and area, respectively. The first compression-expansion cycle of the monolayer is not reversible and the second pi-A compression isotherm deviates to larger molecular areas relative to the first one. At a microscopic level this hysteresis may be assigned to an irreversible hydration of the ammonium groups of DODAB. The morphology and reflectivity of DODAB monolayers during compression and expansion on the monolayer depend on the monolayer history. Bright domains randomly dispersed were observed during compression before collapse. Surprisingly, this random distribution of domains changes into a fractal-like structure during the monolayer expansion in a narrow range of surface pressures. This morphology does not form when the monolayer is previously compressed above the collapse surface pressure. 2D foam-like structure is often observed when the film is expanded at maximum area. Relaxation phenomena in DODAB monolayers are attributed to monolayer reorganization and nucleation of liquid-condensed domains from the liquid-expanded phase. These time-dependent processes are irreversible.     

十八胺单分子膜的研究

通过测定在纯水和CdCl2溶液亚相上十八胺单分子膜的平衡、循环π～A等温线及其动态弹性,发现在亚相中加入Cd2+可以使膜的液态相凝聚性增强,固态相凝聚性减弱.液态的单分子膜在两种亚相上有较好的可回复性,而在固态膜中则不然.这可归因于在水面上十八胺分子间可形成氢键,而在CdCl2水溶液亚相上的十八胺则与Cd2+发生配位,形成了多配位络合物,两种情况下十八胺分子在高膜压区都会发生稳定的聚集.静、动态弹性的比较表明,膜障的振动不利于十八胺分子与Cd2+间的配位作用.     

Formation of langmuir-blodgett films containing two-dimensional monoatomic arrays of rare-earth cations

The work is focused on the preparation and characterisation of rare-earth containing stearic acid Langmuir-Blodgett (LB) films. Stearic acid monolayer behavior on the rare-earth containing aqueous subphase have been analyzed by thermodynamic measurements. The compression isotherm shape changed substantially under varying the anion type at the constant rare-earth cation content in aqueous subphase. The structure of formed multilayer rare-earth containing LB films was studied using small angle X-rays diffraction. The considerable structural defects in multilayer films were found when rare-earth chloride solution was exploited as aqueous subphase, caused presumably by electrostatic interactions at the monolayer surface under high binding of rare-earth cations. The use of monodentate complexones like acetic acid was proposed to form bulk phase quasi-neutral rare-earth complexes. The electroneutral ligand exchange reactions of such complexes with stearic acid monolayer allowed to neutralise monolayer surface under rare-earth cations binding and to form condenced monolayer below the triple-point temperature. The compression of such monolayer to the state of minimal compressibility and subsequent monolayer deposition resulted in the formation of high ordered LB films with high content of adsorbed multivalent rare-earth cations arranged in two-dimensional monoatomic arrays.     

聚苯胺Langmuir－Blodgett膜的电聚合制备及电化学性质

在加有苯胺的亚相上,铺展硬脂酸分子,在气液界面上可形成苯胺－硬脂酸Ｌａｎｇｍｕｉｒ膜．随膜压增大至５．０×１０￣（－６）Ｎ时,苯胺与硬脂酸间的氢键会发生重排,形成另外一种构型的Ｌａｎｇｍｕｉｒ膜,分子所占面积减小,膜增加一个苯环的厚度．将膜转移到ＳｎＯ＿２导电玻璃基底上,制成硬脂酸－苯胺Ｌａｎｇｍｕｉｒ－Ｂｌｏｇｄｅｔｔ膜,经电化学氧化,苯胺可聚合成薄膜。膜夹在两层硬脂酸分子间,它的电化学行为不仅受到离子膜中传输过程的影响,同时受到硬脂酸极性亲水端（－ＣＯＯ￣－）的作用．