Water Subphase Research Articles

Interfacial Water Structure and Effects of Mg2+and Ca2+Binding to the COOH Headgroup of a Palmitic Acid Monolayer Studied by Sum Frequency Spectroscopy

The interfacial hydrogen-bonding network that uniquely exists in between a palmitic acid (PA) monolayer and the underneath surface water molecules was studied using vibrational sum frequency generation (VSFG) spectroscopy. Perturbations due to cation binding of Mg(2+) and Ca(2+) were identified. The polar ordering of the interfacial water molecules under the influence of the surface field of dissociated PA headgroups was observed. Only a fraction of PA molecules are deprotonated at the air/water interface with a neat water subphase, yet the submonolayer concentration of negatively charged PA headgroups induces considerable polar ordering on the interfacial water molecules relative to the neat water surface without the PA film. Upon addition of calcium and magnesium chloride salts to the subphase of the PA monolayer, the extent of polar ordering of the interfacial water molecules was reduced. Ca(2+) was observed to have the greater impact on the interfacial hydrogen-bonding network relative to Mg(2+), consistent with the greater binding affinity of Ca(2+) toward the carboxylate group relative to Mg(2+) and thereby modifying the interfacial charge. At high-salt concentrations, the already disrupted hydrogen-bonding network reorganizes and reverts to its original hydrogen-bonding structure as that which appeared at the neat salt solution surface without a PA monolayer.

The Equilibrium of Phosphatidylcholine–Amino Acid System in Monolayer at the Air/water Interface

Monolayers of phosphatidylcholine, tyrosine, and phenylalanine and binary mixtures phosphatidylcholine-tyrosine or phosphatidylcholine-phenylalanine were investigated at the air/water interface. Phosphatidylcholine (lecithin, PC), tyrosine (Tyr), and phenylalanine (Phe) were used in 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 amino acid result in significant deviations from the additivity rule. An equilibrium theory to describe the behavior of monolayer components at the air/water interface was developed in order to obtain the stability constants of PC-Tyr as well as PC-Phe complexes. We considered the equilibrium between the individual components and the complex and established that lecithin and amino acid formed highly stable 1:1 complex.

Reorganization and Caging of DPPC, DPPE, DPPG, and DPPS Monolayers Caused by Dimethylsulfoxide Observed Using Brewster Angle Microscopy

The interaction between dimethylsulfoxide (DMSO) and phospholipid monolayers with different polar headgroups was studied using "in situ" Brewster angle microscopy (BAM) coupled to a Langmuir trough. For a 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) monolayer, DMSO was shown to significantly impact the structure of the liquid expanded (LE) and gaseous phases. The domains reorganized to much larger domain structures. Domains in the liquid condensed (LC) phase were formed on the DMSO-containing subphase at the mean molecular area where only gaseous and LE phases were previously observed on the pure water subphase. These results clearly demonstrate the condensing and caging effect of DMSO molecules on the DPPC monolayer. Similar effects were found on dipalmitoyl phosphatidyl ethanolamine, glycerol, and serine phospholipids, indicating that the condensing and caging effect is not dependent upon the phospholipid headgroup structure. The DMSO-induced condensing and caging effect is the molecular mechanism that may account for the enhanced permeability of membranes upon exposure to DMSO.

Self-organized dendritic patterns in the polymer Langmuir–Blodgett film

We report the formation of a self-organized dendritic pattern of nanometer thickness in polymer Langmuir–Blodgett (LB) films. Poly(N-dodecylacrylamide) (pDDA)/chloroform solution was spread on a water surface to form a stable polymer monolayer. A pDDA monolayer was deposited onto a hydrophilic silicon substrate by upward deposition from a water subphase, and a second layer was then deposited by downward deposition. The substrate with the two layers was withdrawn from a clean water surface at a high speed to form the dendritic pattern, which was imaged by atomic force microscopy. The height of the pattern, 3.5nm, corresponds to the height of a bilayer pDDA LB film, suggesting that the pattern forms when the deposited outermost layer overturns by meniscus oscillation. A similar dendritic structure of narrower width and lower height was fabricated on a hydrophobic silicon substrate.

Hydrophobic to hydrophilic transition of HF-treated Si surface during Langmuir–Blodgett film deposition

HF-treated Si surface, which is hydrophobic in nature and quite stable in air and inside pure water, can become completely hydrophilic during nickel arachidate Langmuir–Blodgett film growth. Such transition is clearly evident from the structures of films deposited by different number of strokes and can be understood by considering partial oxidation of Si surface inside subphase water and further or complete oxidation in air, in presence of Ni head-groups. Attached Ni head-groups weaken the nearby Si–Si covalent bonding and easily oxidize those Si atoms. Depending upon the amount of those head-groups and its distribution, oxidation or transition can even complete.

Effect of trehalose on the interaction of Alzheimer's Aβ-peptide and anionic lipid monolayers

The interaction of amyloid β-peptide (Aβ) with cell membranes is believed to play a central role in the pathogenesis of Alzheimer's disease. In particular, recent experimental evidence indicates that bilayer and monolayer membranes accelerate the aggregation and amyloid fibril formation rate of Aβ. Understanding that interaction could help develop therapeutic strategies for treatment of the disease. Trehalose, a disaccharide of glucose, has been shown to be effective in preventing the aggregation of numerous proteins. It has also been shown to delay the onset of certain amyloid-related diseases in a mouse model. Using Langmuir monolayers and molecular simulations of the corresponding system, we study several thermodynamic and kinetic aspects of the insertion of Aβ peptide into DPPG monolayers in water and trehalose subphases. In the water subphase, the insertion of the Aβ peptide into the monolayer exhibits a lag time which decreases with increasing temperature of the subphase. In the presence of trehalose, the lag time is completely eliminated and peptide insertion is completed within a shorter time period compared to that observed in pure water. Molecular simulations show that more peptide is inserted into the monolayer in the water subphase, and that such insertion is deeper. The peptide at the monolayer interface orients itself parallel to the monolayer, while it inserts with an angle of 50° in the trehalose subphase. Simulations also show that trehalose reduces the conformational change that the peptide undergoes when it inserts into the monolayer. This observation helps explain the experimentally observed elimination of the lag time by trehalose and the temperature dependence of the lag time in the water subphase.

Synthesis of 3-tetradecylpentane-2,4-dione and investigation of its compression isotherms by the Lengmuir-Blodgett method

By alkylation of acetylacetone in anhydrous methanol 3-tetradecylpentane-2,4-dione was prepared. Its individuality and structure were confirmed by IR and NMR spectroscopy and mass-spectrometry. Compression isotherms of the Lengmuir monolayers on water subphase surface depending on the tautomeric form of 3-tetradecylpentane-2,4-dione were studied.

Effect of water and air–water interface on the structural modification of Ni-arachidate Langmuir–Blodgett films

Nickel arachidate (NiA) Langmuir-Blodgett (LB) films have been deposited on hydrophilic Si(0 0 1) substrates by three (up-down-up) and five (up-down-up-down-up) strokes. During deposition, substrates were kept inside the water subphase for different times after each down stroke. Structural information of all the LB films have been obtained from X-ray reflectivity (XRR) studies. One and two symmetric monolayer (SML) was deposited on top of the asymmetric monolayer (AML) in three and five stokes respectively. All the preformed LB films were then used to go through the air-water interface with the same speed that was used at the time of film deposition. Structural information obtained from the XRR studies show that mainly the top layer density decreases after passing through the air-water interface but the layered structure remains the same. Information obtained from both the XRR and atomic force microscopy (AFM) studies suggest that molecules peeled from the top SML layer do not reincorporate with the LB film through tail-tail hydrophobic interaction. Our study shows that NiA LB film has better stability compared with cadmium arachidate LB film inside the water subphase without forming any out-of-plane molecular reorganization.

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.

Highly ordered monolayer formation of silica beads assisted by ultra-sound and microscopic barrier at air/water interface

We fabricated closely packed monolayer of hydrophobically modified non-porous silica (NPS) beads at air/water interface. Hydrophilic silica beads were modified by hydrocarbon that render hydrophobic on their surface. Ultra-sound applied to the water subphase, inducing large scaled two-dimensional ordering of the beads by reducing three-dimensional irregular cluster formations while spreading solvent evaporated. Furthermore, surface pressure generated by polymeric surface micelle effectively pushes the silica beads and reduces voids between the 2D silica clusters.

Langmuir and Langmuir−Blodgett Films of a Maleic Anhydride Derivative: Effect of Subphase Divalent Cations

We report the study of the equilibrium and dynamic properties of Langmuir monolayers of poly(styrene-co-maleic anhydride) partial 2-buthoxyethyl ester cumene terminated polymer and the effect of the Mg(NO(3))(2) addition in the water subphase on the film properties. Results show that the polymer monolayer becomes more expanded when the electrolyte concentration in the subphase increases. Dense polymer films aggregate at the interface. The aggregates are transferred onto silicon wafers using the Langmuir-Blodgett methodology and the morphology is observed by AFM. The structure of aggregates depends on the subphase composition of the Langmuir film transferred onto the silicon wafer.

Excess fibrinogen adsorption to monolayers of mixed lipids

Adsorption of fibrinogen to the monolayers of mixed lipids, dipalmitoyl phosphatidyl choline (DPPC) and eicosylamine (EA) was measured at a surface pressure of 20 mN/m by an in situ surface plasmon resonance technique. Pressure–area isotherms of DPPC + EA mixtures on water and buffer subphases indicated good lipid miscibility and some contraction of the monolayers at intermediate and higher surface pressures. Surface electric potential of the DPPC + EA monolayers showed excess values for intermediate DPPC:EA ratios. Fibrinogen adsorption and its adsorption rates from a dilute solution (0.03 mg/ml) were proportional to the fraction of EA in the monolayer indicating that protein binding was primarily driven by electrostatic interactions between positive EA charges in the monolayer and a net negative protein charge. At a higher protein concentration (0.06 mg/ml) both the fibrinogen adsorbed amount and its maximum adsorption rate showed excess values relative to the pure EA for 1:1, 2:1 and 3:1 DPPC + EA monolayers. This excess adsorption could be explained, in part, by the contraction of the monolayers with intermediate DPPC:EA ratios which resulted in an excess surface electric potential.

Molecular Assemblies of 4-(Hexadecyloxy)-N-(pyridinylmethylene)anilines at the Air−Water Interface and Cu(II)-Promoted Vesicle Formation via Metal Coordination

The molecular assemblies of 4-(hexadecyloxy)-N-(pyridinylmethylene)anilines (HPA) at the air-water interface on pure water and aqueous Cu(II) subphases have been investigated using in situ infrared reflection absorption spectroscopy (IRRAS). The Schiff base units were oriented with their long axes almost perpendicular to the water surface, and both imine and pyridinyl nitrogen atoms of the Schiff base units were coordinated to Cu(II) ions together with their geometrical conversions. The alkyl chains in the monolayers were quantitatively determined on the assumption that the HPA monolayers at the air-water interface were composed of sublayers of alkyl chains and Schiff base units, and the chain orientation angle on pure water was 30 +/- 2 degrees and increased to 37 +/- 2 degrees on the aqueous Cu(II) subphase. The HPA amphiphiles could not be dispersed in pure water but could self-organize into vesicles with metal-coordinated headgroups and interdigitated-packed alkyl chains in the presence of Cu(II) ions in aqueous solution. Transmission electron microscopy (TEM), differential scanning calorimetry (DSC), UV-vis spectroscopy, and small-angle X-ray diffraction (XRD) were used to investigate the aggregate structures and specific properties of the coordinated vesicles.

Polymer Behavior at the Air-Water Interface

AbstractThis article reviews polymer behavior at the air-water interface with a focus on the Langmuir technique and spreading processes to gain insight into this interface. The influence of the solvent and the water subphase on some polymer monolayers is discussed. The surface pressure–area isotherms (π–A) are described in terms of the chemical structure and the hydrophilic-hydrophobic balance of selected polymeric systems, with an emphasis on the behavior of the specific diblock copolymer films at the air-water interface. In some cases, molecular dynamic simulation studies can be used to visualize the organization and orientation of polymers at the air-water interface and to identify the main molecular interactions involved in such processes.

Lipid–polyelectrolyte complexes at the air–water interface for different lipid packing

In presence of calcium ions in the water subphase, DNA and carboxymethyl cellulose (carboxyMC) can form complexes with the zwitterionic lipid dipalmitoyl- sn-glycero-3-phosphocholine (DPPC) at the air–water interface. Depending upon the lipid packing, these polyelectrolytes can penetrate the lipid monolayer or can stay below the lipid monolayer after forming lipid–polyelectrolyte complexes, which are evidenced from the variation of the surface pressure with time at a constant area. Variation of surface pressure shows that at relatively less packing of DPPC molecules, (area per molecule is 119.5 Å 2 in presence of DNA and 101.1 Å 2 in presence of carboxyMC) surface pressure increases with time following the Langmuir growth relation. At intermediate packing, in presence of DNA, surface pressure decreases slowly compared to that of pure DPPC monolayer, whereas pressure remains nearly constant in presence of carboxyMC in the water subphase. At dense packing, i.e., at lower area per molecule (nearly 53 Å 2 in presence of both DNA and carboxyMC), surface pressure decreases slowly for both DNA and carboxyMC, compared to that of pure DPPC monolayer. Mostly all the decay curves fits well with the stretched exponential function or with the sum of one constant and two exponential decay terms, indicating the existence of different reorganization processes of the lipid and such lipid-polyelectrolyte complexes at the air-water interface.

Pattern Formation in Monolayer Transfer Systems with Substrate-Mediated Condensation

The formation of regular stripe patterns during transfer of surfactant monolayers onto solid substrates is investigated. Two coupled differential equations describing the surfactant density and the height profile of the water subphase are derived within the lubrication approximation. If the transfer is carried out in the vicinity of a first order phase transition of the surfactant, the interaction with the substrate plays a key role. This effect is included in the surfactant free-energy functional via a height-dependent external field. Using transfer velocity as a control parameter, a bifurcation from a homogeneous transfer to regular stripe patterns arranged parallel to the contact line is investigated in one and two dimensions. Moreover, in the two-dimensional case, a secondary bifurcation to perpendicular stripes is observed in a certain control parameter range.

On the origin of iron-oxide nanoparticle formation using phospholipid membrane template

In this report, we have studied the formation of iron-oxide nanoparticle at biologically relevant phospholipids, DPPC Langmuir monolayer at air/water interface. Water subphase contains FeCl 3. Adsorption and agglomeration of Fe 3+ ions at DPPC head group have being monitored by Langmuir and Langmuir Blodgett (LB) technique. Adsorption kinetics (π– t) as well as the surface pressure area (π– A) isotherms measurement demonstrate the incorporation of Fe 3+ ion at DPPC monolayer. The amount of incorporation of Fe 3+ to the DPPC monolayer is FeCl 3 concentration and time dependent. This reaction kinetics is well fitted by single exponential association equation. The composite monolayers transferred to different substrates are characterized by UV–vis absorption spectroscopy and electron microscopy (FE-SEM and HR-TEM). Study shows the formation of monodisperse Fe 3O 4 nanoparticle having size ∼20 nm coated with DPPC mono or multilayer. The overall study indicates that the formation as well as assembly of iron-oxide nanoparticle in two dimensions is possible using lipid monolayer as a template.

Langmuir monolayers of fractions of cork suberin extract

The wide variability in composition and molecular weight of natural polymers has hampered understanding of their physicochemical properties and ultimately their use in noble applications, especially in the cases where surface properties need to be probed at the molecular level. A useful approach to analyse data from surface monolayers of complex mixtures is to try distinguishing the effects from the distinct fractions in such mixtures. The cork suberin extract investigated here is known to contain aliphatic monomers with terminal carboxylic acid and methyl ester groups as well as long esterified aliphatic chains dispersed in a polymeric aliphatic matrix. The role of such terminal groups was studied and the results showed that depending on the nature of the terminal groups the monolayers present distinct isotherms due to the different interactions with the water subphase. Fractionation strategies based on different solubilities of the cork suberin components in chloroform were also employed to probe their effect on the monolayer characteristics. From the two sets of experiments it is clear that the presence of monomers with terminal carboxylic acids in the suberin extract affects considerably the monolayer-forming ability. This approach may be used as a complementary, relatively simple route to assess suberin genetic engineering strategies towards resistance to environmental stress.

Influence of (phospho)lipases on properties of mica supported phospholipid layers

The effect of enzymes: lipase from Candida cylindracea (L Cc ), phospholipase A 2 from hog pancreas (PLA 2) and phospholipase C from Bacillus cereus (PLC) to modulate wetting properties of solid supported phospholipid bilayers was studied via advancing and receding contact angle measurements of water, formamide and diiodomethane, and calculation of the surface free energy and its components from van Oss et al. (LWAB) and contact angle hysteresis (CAH) approaches. Simultaneously, topography of the studied layers was determined by Atomic Force Microscopy (AFM). The investigated lipid bilayers were transferred on mica plates from subphase of pure water by means of Langmuir–Blodgett and Langmuir–Schaefer techniques. The investigated phospolipid layers were: saturated DPPC (1,2-dipalmitoyl- sn-glycero-3-phosphocholine), unsaturated DOPC (1,2-dioleoyl- sn-glycero-3-phosphocholine), and their mixture DPPC/DOPC. The obtained results revealed that the lipid membrane degradation by the enzymes caused increase in its surface free energy due to the amphiphilic hydrolysis products, which may accumulate in the lipid bilayer. In result activity of the enzymes may increase and then break down the bilayer structure takes place. It is likely that after dissolution of the hydrolysis reaction products in the bulk phase, patches of bare mica surface are accessible, which contribute to the apparent surface free energy changes. Comparison of AFM images and the free energy changes of the layers gives better insight into changes of their properties. The observed gradual increase in the layer surface free energy allows controlling of the hydrolysis process to obtain the surfaces of defined properties.

Study of the interaction between cardiolipin bilayers and methylene blue in polymer-based Layer-by-Layer and Langmuir films applied as membrane mimetic systems

An increase of the reports involving mimetic systems has been observed. Briefly, these systems use biological phospholipids to exploit specific interactions between membrane-models and drugs. Here, the Layer-by-Layer (LbL) and Langmuir techniques were used to investigate the interaction between cardiolipin (CLP—negative phospholipid) and a cationic-like drug methylene blue (MB). Supported by a cationic polyelectrolyte (PAH), LbL films containing PAH/(CLP + MB) and PAH/(CLP + MB + AgNP) were grown up to 14 bilayers. The optical microscopy analysis revealed a decrease of the CLP vesicle sizes in the presence of MB as a possible consequence of the MB action onto the mechanical properties of the CLP membrane. From FTIR spectra, changes mainly related to peak position and band intensity and shape were observed in the spectra from PAH/CLP when in the presence of MB. The latter supports that the interactions between the phosphate and amine charged groups from CLP and PAH, respectively, established during the LbL film fabrication, besides the CLP hydrocarbon environment, are influenced by the presence of MB. Using the micro-Raman technique, a chemical mapping was build based on MB spectrum by resonance Raman scattering (RRS) and surface-enhanced resonance Raman scattering (SERRS). The later phenomenon was activated by Ag nanoparticles (AgNPs) trapped within the LbL film allowing collecting spectra for a single bilayer of PAH/(CLP + MB + AgNP). A rough estimation showed a SERRS amplification of 10 3 in comparison to RRS spectra. As a complementary approach, Langmuir films of CLP in the presence of co-spread MB were investigated through surface pressure vs mean molecular area ( π– A) isotherms. The results showed that for concentrations of MB below 100 mol%, the drug is expelled to water subphase for high values of surface pressure (condensed phase). For concentration at 100% and higher, the MB keeps bound to CLP floating monolayer.