Proton sponge and fatty acid interactions at the air-water interface. Thermodynamic, spectroscopic, and microscopic study

Mixed Langmuir and Langmuir-Blodgett (LB) films of a proton sponge, namely, diphenyl bis(octadecylamino)phosphonium bromide, and a fatty acid, docosanoic acid, with different molar ratios have been fabricated. Surface pressure versus area per molecule isotherms were registered, and the excess areas and excess Gibbs energy of mixing were calculated. Strong interactions between the proton sponge and the fatty acid take place at the air-water interface. The existence of a stoichiometric 1:1 acid-base reaction between the two components forming a complex on the water surface at high surface pressures has been demonstrated. Furthermore, the reaction had an efficiency close to 100% at the air-water interface; meanwhile, it hardly takes place in organic solvents such as chloroform or even a mixture of chloroform and dimethyl sulfoxide. The floating films were transferred to solid supports and characterized by means of several techniques including IR spectroscopy, X-ray diffraction, and atomic force microscopy, revealing the presence of highly ordered alkyl chains and a constant architecture along the different layers as well as the presence of different domains in the LB films, except those having a 1:1 proton sponge-fatty acid ratio that are homogeneous. Such domains have been interpreted as the presence of two different phases, the 1:1 complex plus the excess component in the mixture.     

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.     

The equilibria of phosphatidylcholine-fatty acid and phosphatidylcholine-amine in monolayers at the air/water interface

Monolayers of phosphatidylcholine, fatty acid and amine and binary mixtures phosphatidylcholine-fatty acid or phosphatidylcholine-amine were investigated at the air/water interface. Phosphatidylcholine (lecithin, PC), stearic acid (SA), palmitic acid (PA), decanoic acid (DA) and decylamine (DE) were used to the experiment. The surface tension values of pure and mixed monolayers were used to calculate π-A isotherms. The surface tension measurements were carried out at 22°C using an improved Teflon trough and a Nima 9000 tensiometer. The Teflon trough was filled with a subphase of triple-distilled water. Known amounts of lipid dissolved in 1-chloropropane were placed at the surface using a syringe. The interactions between lecithin and fatty acid as well as phosphatidylcholine and amine result in significant deviations from the additivity rule. An equilibrium theory to describe the behaviour of monolayer components at the air/water interface was developed in order to obtain the stability constants of PC-SA, PC-PA, PC-DA and PC-DE complexes. We considered the equilibrium between the individual components and the complex and established that lecithin and fatty acid as well as phosphatidylcholine and amine formed highly stable 1:1 complexes.     

Equilibrium structure and dynamics of (2R, 3R)-(+)-bis(decyloxy)succinic acid at the air-water interface

A twin-tailed, twin-chiral fatty acid, (2R,3R)-(+)-bis(decyloxy)succinic acid was synthesized and its two dimensional behavior at the air-water interface was examined. The pH of the subphase had a profound effect on the monolayer formation. On acidic subphase, stable monolayers with increased area per molecule due to hydrogen bonding and bilayers at collapse pressures were observed. Highly compressible films were formed at 40 degrees C, while stable monolayers with increased area were observed at sub-room temperatures. Langmuir monolayers formed on subphases containing 1 mM ZnCl2 and CaCl2 revealed two dimensional metal complex formation with Zn2+ forming a chelate-type complex, while Ca2+ formed an ionic-type complex. Monolayers transferred from the condensed phase onto hydrophilic Si(100) and quartz substrates revealed the formation of bilayers through transfer-induced monolayer buckling. Compression induced crystallites in 2D from monolayers and vesicle-like supramolecular structures from multilayers were the noted LB film characteristics, adopting optical imaging and electron microscopy. The interfacial monolayer structure studied through molecular dynamics simulation revealed the order and packing at a molecular level; monolayers adsorbed at various simulated specific areas of the molecule corroborated the (pi-A) isotherm and the formation of a hexagonal lattice at the air-water interface.     

Configuration and photochemical reaction of a bolaamphiphilic diacid with a diazo resin in monolayers and Langmuir-Blodgett films

The monolayer formation of a bolaamphiphile, 1,18-octadecanedicarboxylic acid (ODA), on pure water and the subphase containing a positively charged photoactive 2-nitro-N-methyldiphenylamine-4-diazoniumformaldehyde resin (NDR) have been investigated by pi-A isotherms, pi-t curves, and Brewster angle microscopy (BAM) measurements. It has been revealed that although an unstable monolayer was formed by ODA alone, a stable complex monolayer between ODA and NDR could be formed at the interface through electrostatic adsorption and hydrogen bonding. It has been shown that the ODA formed a U-shaped monolayer at a lower pressure and was converted to a stretched configuration upon compression to a higher surface pressure on the subphase containing NDR. Under UV irradiation at the interface photoreaction can occur in the complex monolayer, which causes shrinkage of the monolayer. Photochemical reactions can also occur in deposited Langmuir-Blodgett films. In reactions occurring at the air/water interface, the two ends of ODA can react with NDR to form an ester containing aromatic rings. This makes the compound more hydrophobic and can easily be stretched without any phase transition upon compression. When the film with U-shaped configuration was deposited onto solid substrates, the configuration could be kept even upon photoirradiation.     

Supramolecular assemblies and molecular recognition of amphiphilic schiff bases with barbituric acid in organized molecular films

A bolaform Schiff base, N,N'-bis(salicylidene)-1,10-decanediamine (BSC10), has been synthesized and its interfacial hydrogen bond formation or molecular recognition with barbituric acid was investigated in comparison with that of a single chain Schiff base, 2-hydroxybenzaldehyde-octadecylamine (HBOA). It has been found that while HBOA formed a monolayer at the air/water interface, the bolaform Schiff base formed a multilayer film with ordered layer structure on water surface. When the Schiff bases were spread on the subphase containing barbituric acid, both of the Schiff bases could form hydrogen bonds with barbituric acid in situ in the spreading films. As a result, an increase of the molecular areas in the isotherms was observed. The in situ H-bonded films could be transferred onto solid substrates, and the transferred multilayer films were characterized by various methods such as UV-vis and FT-IR spectrosopies. Spectral changes were observed for the films deposited from the barbituric acid subphase, which supported the hydrogen bond formation between the Schiff bases and barbituric acid. By measuring the MS-TOF of the deposited films dissolved in CHCl3 solution, it was concluded that a 2:1 complex of HBOA with barbituric acid and a 1:2 complex of BSC10 with barbituric acid were formed. On the other hand, when the multilayer films of both Schiff bases were immersed in an aqueous solution of barbituric acid, a similar molecular recognition through the hydrogen bond occurred. A clear conformational change of the alkyl spacer in the bolaform Schiff base was observed during the complex formation with the barbituric acid.     

Proton hopping in phosphoric acid solvated nafion membrane: a molecular simulation study

Molecular simulation studies of the microstructure and of the proton transport properties of phosphoric acid solvated Nafion membrane are carried out. The ab initio calculations show that the phosphoric acid is a good solvent to promote the proton ionization of the sulfonic acid group, and only two phosphoric acid molecules are necessary for the dissociation of one sulfonic acid group. A mechanism of proton hopping between phosphoric acid and protonated phosphoric acid cation in the hydrophilic subphase is also elucidated by ab initio calculations. The molecular dynamics simulations, conducted at a phosphoric acid concentration of 25.4% (wt) which is slightly lower than that of phosphoric acid swollen Nafion, show that the phosphoric acid exists in subphases and that it cannot develop into a continuous subphase. Thus, proton-hopping pathways are interrupted, and the conductivity is expected to be lower than that for pure phosphoric acid. The molecular dynamics simulations, conducted at a phosphoric acid concentration of 45.1% (wt) which corresponds to an unstable state, show that the hydrophobic poly(tetrafluoroethylene) backbones trend to gather together forming hydrophobic clusters and that the phosphoric acid forms a continuous subphase with the sulfonic acid groups located at the hydrophobic/hydrophilic interface. Thus, proton-hopping pathways can develop uninterruptedly like the pure phosphoric acid, and high conductivity is expected. The molecular dynamics study also shows that the hydrogen-bonding characteristics of phosphoric acid and sulfonate anion are similar regardless of the factor that the former can move freely while the latter is attached to Nafion backbone.     

Preparation, characterization, and application of polypropylene-clinoptilolite composites for the selective adsorption of lead from aqueous media

The miscibility, mechanical and morphological properties of mixed Langmuir and Langmuir-Blodgett monolayers prepared from the phospholipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine and the perfluorinated fatty acid perfluorooctadecanoic acid have been studied as a function of film composition and subphase salinity. It was demonstrated here, for the first time, that the extent of surfactant miscibility in mixed phospholipid-perfluoroacid monolayers, and hence the resulting mechanical properties of the monolayer film, can be controlled by altering the concentration of sodium ions in the underlying subphase. Elevated Na(+) concentrations resulted in lower net attractive interactions between film components, likely through specific ion adsorption to the negatively-charged perfluoroacid, along with decreased film elasticities. These results differ significantly from conventional fatty-acid-carboxylate monolayer systems in which film cohesion is typically enhanced through adsorption of cations to surfactant headgroups. Atomic force microscope images of films deposited onto solid mica substrates revealed that the films deposited from pure water formed multimolecular aggregates of surfactant, which could be attributed to the highly cohesive nature of the films, but the use of salt in the subphase diminished aggregate formation and resulted in the production of homogeneous monolayer films.     

Optical and photocatalytic properties of single crystalline ZnO at the air-liquid interface by an aminolytic reaction

Crystalline flowerlike ZnO was synthesized by an aminolytic reaction at the air-liquid interface in an aqueous media at an alkaline pH. A thin visible film was formed at the air-liquid interface by self-assembly of flowerlike ZnO. Diffraction studies show rearrangement of the single crystalline units at the air-liquid interface leading to the formation of nanobelts. These nanobelts overlap systematically to form petals of the flowerlike structure; individual petals get curved with time. Each nanobelt is found to be single crystalline and can be indexed as the hexagonal ZnO phase. The organic product formed in the aminolytic reaction and dissolution-reprecipitation mechanism is the driving force for the formation of flowerlike ZnO at the air-liquid interface. A clear relationship between the surface, photocatalytic, and photoluminescent properties of ZnO is observed. The flowerlike structure exhibits a blue shift (3.56 eV) in the band emission as compared to bulk ZnO (3.37 eV). The photodegradation of methylene blue over the flowerlike ZnO catalyst formed at the air-liquid interface and in the sediments shows enhanced photocatalytic activity. The sub-bands formed due to surface defects facilitate separation of charge carriers increasing their lifetime, leading to enhanced photocatalytic activity of flowerlike ZnO.     

Supramolecular chirality of the hydrogen-bonded complex Langmuir-Blodgett film of achiral barbituric acid and melamine

Complex monolayers of barbituric acid and melamine were formed by spreading a chloroform solution of amphiphilic barbituric acid on the subphase of melamine solution. It was confirmed that the complex monolayer was formed through in situ complementary hydrogen bonding at the air-water interface. It was interesting to find that the complex LB films showed supramolecular chirality although both of the molecules were achiral, as verified by the circular dichroism spectral measurements. It was suggested that the pi-pi stacking of the neighboring barbituric acid and melamine group in a helical sense resulted in the chirality of the molecular assemblies. Due to the directionality of the hydrogen bonding, the BA-M film could form regular aligned nanofibers on the AFM images. Increasing the subphase temperature will lead to the decrease of CD intensity and the change of the morphologies. We suggested that the strength of the hydrogen bonding resulted in the difference.     

Selectivity and stability of organic films at the air-aqueous interface

It has recently been determined that organic compounds represent a significant percentage of the composition of certain atmospheric aerosols. Amphiphilic organics, such as fatty acids and alcohols, partition to the interface of aqueous aerosols. In this way, the air-aqueous interface of an aerosol has the ability to act as both a concentrator and a selector of organic surfactants. Isotherms of nonanoic acid, stearic acid, 1-octadecanol, and a binary of mixture of nonanoic and stearic acids were used to infer the packing ability and molecular orientation of the surfactants at the interface. The selectivity of the air-aqueous interface was studied by monitoring the composition of binary organic films as a function of film exposure time. The films were formed, aged, and collected with the use of a Langmuir trough. The composition of the aged film was determined via GC-MS. Surfactants with differing carbon number and chemical functionalities were studied. These included stearic acid, lauric acid, 1-octadecanol, and octadecane. The stability and packing ability of stearic and lauric acid films were examined as a function of subphase pH. The relevance of these findings as they relate to the composition and structure of organic aerosols as well as recent surface-sensitive aerosol field measurements is discussed.     

Design and interfacial assembly of a new series of gemini amphiphiles with hydrophilic poly(ethyleneamine) spacers

A new series of gemini amphiphiles containing two Schiff base moieties linked by the poly(ethyleneamine) with different lengths were designed, and their interfacial assemblies were investigated. Condensed monolayers were obtained on nearly neutral subphase where the hydrophilic spacers were found to immerse into the subphase. On strong alkaline and acidic subphase, the headgroup and the spacer of the gemini amphiphiles underwent dissociation and protonation, respectively, resulting in the enlargement of the molecular areas. Flat and uniform domains were obtained for the monolayers from nearly neutral subphase; flower-like or dendritic domains were observed for the films transferred from strong acidic subphase. On the other hand, when an anionic tetrakis(4-sulfonatonphenyl)porphine (TPPS) was added into an acidic subphase, an in situ complex formation between the gemini amphiphiles and TPPS occurred. The complex monolayers were transferred onto solid substrate and TPPS existed predominantly as J-aggregate in the complex films. Due to the multisited positive charges in the spacer on acidic subphase, the complex films of gemini amphiphiles with TPPS appeared as short fiber or nanorod structures and formed two-dimensional (2D) conglomerate chiral domains.     

Langmuir-Blodgett films and chiroptical switch of an azobenzene-containing dendron regulated by the in situ host-guest reaction at the air/water interface

An amphiphilic dendron containing an azobenzene ring at the focal point and the l-glutamate peripheral groups was designed. Its monolayer formation and host-guest reaction with cyclodextrins at the air/water interface and the properties of the transferred Langmuir-Blodgett (LB) films were investigated. The individual dendron, although without any long alkyl chains, could still form a stable monolayer at the air/water interface because of the good balance between hydrophilic and hydrophobic parts within the molecule. When cyclodextrin (CyD) was added to the subphase, a host-guest reaction occurred in situ at the air/water interface. The inclusion of the focal azobenzene moiety into the cavity of cyclodextrin decreased the packing of the aromatic ring and also led to the diminishment of the molecular area. Both the films formed at the surface of pure water and aqueous cyclodextrins were transferred onto solid substrates. Nanofiber structures were obtained for the film from the water surface as a result of the packing of the azobenzene groups, and circular domains were obtained for the film transferred from the aqueous CyD phases. The film transferred from the water surface showed an exciton couplet in the absorption band of azobenzene, whereas a negative Cotton effect was obtained for the film from CyD subphases. It was found that the supramolecular chirality in the LB film transferred from water was due to the transfer of the molecular chirality to the assemblies whereas that from the CyD subphase was due to the inclusion of azobenzene into the chiral cavity. Interestingly, the film from the water surface was photoinactive, whereas a reversible optical and chiroptical switch could be obtained for the film from the α-CyD subphase. The work provided a way to regulate the assembly and functions of organized molecular films by taking advantage of the interfacial host-guest reaction.     

TPPS和Gemini表面活性剂的复合膜及其手性的研究

研究了一种新的gemini表面活性剂(C12H24-α,ω-(C12H25N＋(CH3)2Br－)2, （简写为C12-C12-C12）和TPPS在气液界面上形成的复合膜及其手性.实验发现,单独C12-C12-C12不能在纯水表面形成稳定的单分子膜，但当亚相中存在TPPS时，可形成稳定的单分子膜.通过水平提拉法将复合膜转移到固体基板上，发现在适当的pH值条件下，TPPS可在复合膜中形成J-聚集体，并且发现，尽管Gemini表面活性剂和TPPS 都 是非手性的，TPPS的J-聚集体表现出强烈的Cotton效应.另外，gemini表面活性剂的两个正电荷中心对TPPS的J-聚集体的手性并不能表现出协同效应.     

带相反电荷高分子和表面活性剂复合物膜在空气/水界面上的特性

利用原子力显微镜技术、表面压松弛法对带相反电荷高分子和表面活性剂在气/水界面形成的界面复合物膜进行了特性研究.原子力显微镜研究结果表明,部分水解聚丙烯酰胺(HPAM)/十六烷基三甲基溴化铵(CTAB)所形成的界面复合物膜呈现出“团”状聚集体形貌.两性聚丙烯酰胺与CTAB形成的界面复合物膜呈现出“纤维丝”的聚集体形貌.亚相中盐的存在对界面复合物的形貌有很大影响.表面压松弛实验则进一步表明界面复合物膜形貌结构的变化对其膜稳定性有直接的影响.     

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.     

Synthesis and supramolecular self-assembly study of a novel porphyrin molecule in Langmuir and Langmuir-Blodgett films

The self-assembly and supramolecular engineering of porphyrins into ordered arrays have recently attracted much interest because of their promising application potential in molecular and electronic devices, spintronics, energy harvesting and storage, catalysis, and sensor development. We herein report the synthesis and supramolecular self-assembly study of a novel porphyrin molecule, 2Por-TAZ, in Langmuir and Langmuir-Blodgett films. The 2Por-TAZ molecule contains two porphyrin macrocycles attached to a triaminotriazine headgroup. Triaminotriazines are known to form a highly ordered linear supramolecular self-assembly through complementary hydrogen bonding with barbituric acid molecules at the air-water interface. Surface pressure-area isotherm measurements and polarized UV-vis absorption spectroscopic studies indicate that the 2Por-TAZ molecules adopted an edge-on orientation at the air-water interface. Polarized UV-vis absorption study also revealed that the 2Por-TAZ molecules formed linear supramolecular networks on pure water and barbituric acid subphase with porphyrin flat planes facing toward the compression direction. The binding of barbituric acid with 2Por-TAZ molecules was observed from the expansion of the Langmuir monolayer film. Compared to the transferred LB film from pure water subphase, both the UV-vis absorbance and fluorescence emission intensity of the LB film transferred from barbituric acid subphase increased significantly.     

Infrared spectroscopy analysis of the structure of multilayer Langmuir-Blodgett films: effect of deposition velocity and pH

Multiple Langmuir-Blodgett (LB) films of arachidic acid were deposited on germanium (Ge) substrates from subphase solutions of 10(-4) M CdCl2 at different pH values and at different deposition speeds. Attenuated total reflectance infrared (ATR-IR) spectroscopy was used to obtain information on the molecular order and structure of these multilayer LB films. At pHs higher than the pKa of the fatty acid/cation system, transfers took place only during the downstroke, indicating X-type deposition. At pH = pKa and large deposition speeds, deposition partially failed during the downstroke, resulting in Z-type depositions. Analysis of the infrared spectra indicates that multiple LB films deposited only during the upstroke (Z-type) or during downstrokes (X-type) have a centrosymmetric structure typical of films deposited during the upstroke and downstroke, except for a slight decrease in molecular order and tilt angle as the pH increases (X-type). The centrosymmetric structure indicates that rearrangement of layers takes place between cycles. Experimental evidence of such rearrangement occurring in a fatty acid/divalent cation salt subphase is shown here, and rearrangement alternatives are discussed.     

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.     

Controllable growth of straight nanorods and nanowires in the Langmuir films of a bolaamphiphilic par derivative.

A novel bolaamphiphilic compound, 1, 10-bis[3'-hydroxy-4'-(2'-pyridylazo)phenoloxy] decane [(PAR)2C10], was synthesized and its spreading film and in situ coordination with metal ions in the Langmuir monolayer at the air/water interface were investigated. It was found that (PAR)2C10 could be spread on the water surface, and the Langmuir film showed a phase transition from a flat conformation to a U-shaped conformation upon compression. Interfacial coordination between (PAR)2C10 and metal ions could occur in situ in the Langmuir monolayer. Depending on the concentration of the subphase, different coordination modes were observed. A 2:1 (ligand-PAR-to-metal-ion) complex was formed at a lower concentration of the subphase, while a 1:1 complex was obtained on the subphase with a higher concentration of the salt. Interestingly, very straight nanowires, extending to several micrometers, were observed in the AFM images of the films transferred from the subphase containing CuCl2 in higher concentrations. However, tortuous nanowires were observed under the subphase containing Cu(CH3COO)2. The formation of such kind of wire-like structures was only observed for the combination of the bolaamphiphilic PAR derivative with the Cu(II) ions. Other metal ions or the single-chain PAR derivative did not show this behavior. A possible growth mechanism of the nanowires was proposed based on the coordination of the bolaamphiphilic (PAR)2C10 with the Cu(II) ions as well as with the counteranions.