Subphase pH Research Articles

La3+ and Y3+ interactions with the carboxylic acid moiety at the liquid/vapor interface: Identification of binding complexes, charge reversal, and detection limits

Specific interactions of yttrium and lanthanum ions with a fatty acid Langmuir monolayer were investigated using vibrational sum frequency spectroscopy. The trivalent ions were shown to interact with the charged form of the carboxylic acid group from nanomolar concentrations (<300 nM). Analysis of the spectral features from both the symmetric and the asymmetric carboxylate modes reveals the presence of at least three distinct coordination structures linked to specific binding configurations. Although the same species were identified for both La3+ and Y3+, they display a different concentration dependence, highlighting the ion-specificity of the interaction. From the analysis of the response of interfacial water molecules, the reversal of the surface charge, as well as the formation of yttrium hydroxide complexes, were detected upon increasing the amount of salt in solution. The binding interaction and kinetics of absorption are sensitive to the solution pH, showing a distinct ion speciation in the interfacial region when compared to the bulk. Changing the subphase pH or adding a monovalent background electrolyte that promotes deprotonation of the carboxylic acid headgroup could further improve the detection limit of La3+ and Y3+ to concentrations < 100 nM. These findings demonstrate that nM concentrations of trace metals contaminants, typically found on monovalent salts, can significantly influence the binding structure and kinetics in Langmuir monolayers.

Semi-reversible collapse of preformed cobalt stearate Langmuir monolayer on water surface

Langmuir monolayer of preformed cobalt stearate molecules shows semi-reversible behavior under successive compression-decompression surface pressure versus area per molecule isotherm cycles. However, the conventional cobalt stearate Langmuir monolayer, where cobalt stearate is formed at the air-water interface shows highly irreversible behavior. Films of preformed cobalt stearate molecules are deposited on the hydrophilic silicon substrates by horizontal deposition method at different points of the isotherms to obtain their structure and morphology for the specific surface pressure and subphase pH conditions. Combination of monolayer and different multilayer domains formed by the tilted (layer thickness ≈ 41.0 Å) and untilted (layer thickness ≈ 46.0 Å) molecules are present in the films as obtained from the X-ray reflectivity and atomic force microscopy. Microscopic information implies that in such reversible collapse the multilayer domains are surrounded by the compact monolayer or trilayer domains. Folding from monolayer to multilayer structure with compression and then again reincorporation as a monolayer after decompression on the water surface indicates the presence of long-range connectivity among the preformed cobalt stearate molecules.

Influences of subphase pH and temperature on the interfacial aggregation behavior of poly(lauryl methacrylate)-block-poly(methacrylic acid)

We previously reported that poly(acrylic acid) (PAA) blocks at the air/water interface spontaneously immersed into water at high pH before monolayer compression. To investigate the influence of the hydrophobic methyl groups in poly(methacrylic acid) (PMAA) blocks, in this work, subphase pH and temperature effects on the aggregation behaviors of two pH-responsive amphiphilic diblock copolymers poly(lauryl methacrylate)-block-poly(methacrylic acid) (PLMA-b-PMAA) (PLMA49%-PMAA and PLMA61%-PMAA, composition in weight percent) at the air/water interface were studied. The surface pressure–molecular area isotherms of the two copolymers move to small mean molecular area (mma) as subphase pH increases and a quasi-plateau exists under various subphase conditions, which is quite different from its absence in our previous PAA-containing block copolymer system. The quasi-plateau is attributed to the pancake-to-brush transition as non-ionized PMAA chains submerge into subphase upon monolayer compression. Under acidic condition, PMAA chains of the two copolymers are almost non-ionized, the compression and expansion isotherms show large hysteresis phenomena. Under neutral and alkaline conditions, the isotherms show small hysteresis, which is because more ionized PMAA chains immerse in subphase before compression. The isotherms shift toward small mma with increasing temperature under neutral and alkaline conditions, whereas a slight shift back to large mma is observed under acidic conditions. The plateau surface pressures of the two PLMA-b-PMAA copolymers decrease with increasing temperature under different pH conditions, which is because the micelles move more freely and easier to rearrange at high temperature. The plateau surface pressures of PLMA49%-PMAA are higher than those of PLMA61%-PMAA under the corresponding conditions due to the higher PMAA content in the former. The initial Langmuir–Blodgett (LB) films of the two copolymers transferred under different pH conditions at various temperatures exhibit isolated circular micelles because of the steric hindrance and/or repulsive interactions among the long PMAA chains. Furthermore, temperature has almost no effect on the micelle core diameters of the initial LB films.

Structure, morphology and reversible hysteresis nature of human serum albumin (HSA) monolayer on water surface

Compression-decompression surface pressure (π)-specific molecular area (A) isotherm cycle of human serum albumin (HSA) monolayer is performed on water surface at four different subphase pH conditions, i.e., below and above the isoelectric point (pI ≈ 4.7) of HSA molecule. For all pH conditions, the decompression curve nearly follows the compression curve, however, at pH ≈ 5.0, hysteresis is observed at higher surface pressure. Out-of-plane structures and in-plane morphologies obtained from the X-ray reflectivity and AFM studies show that only the film thickness variation takes place with the change in surface pressure, which is also evidenced from the BAM images. With increase in surface pressure, the oblate-shaped HSA molecules start tilting making an angle with the water surface and as the monolayer is decompressed the molecules regain their initial untilted monomolecular configuration. Depending upon the subphase pH and local surface charge of the specific protein molecule, electrostatic repulsive interaction dominates over the van der Waals attraction and as a result decompression curve follows the compression curve as the molecules repel each other, however, closer to the isoelectric point as strength of the interactions reverses, a hysteresis is obtained at higher surface pressure and accordingly monolayer behaviour modifies on the water surface.

Structure and Elastic Properties of an Unsymmetrical Bi-Heterocyclic Azo Compound in the Langmuir Monolayer and Langmuir-Blodgett Film.

We study the structure and elastic properties of the bi-heterocyclic azo compound at the air–water interface through surface pressure (π)–area (A) isotherm recording followed by transferring them on hydrophilic and hydrophobic Si surfaces by the Langmuir–Blodgett (LB) deposition method. A substantial change in the area/molecule is observed as a function of subphase pH and temperature. Such parameters strongly influence intramolecular interactions within azo molecules and the interactions between azo molecules and water that manifested in higher surface activity at low temperature and high pH, which in turn modifies the elasticity of azo assembly at the air–water interface. A large pH-dependent hysteresis with negative change in entropy, indicating molecular rearrangements, is observed. Molecular assembly formed at the air–water interface is then transferred onto hydrophilic and hydrophobic Si surfaces at two different surface pressures (5 and 30 mN/m) by the LB technique. The structural analysis performed by X-ray reflectivity and atomic force microscopy techniques suggests that the LB films exhibit an abrupt layered structure on hydrophilic Si, whereas an overall rough film is formed on hydrophobic Si. The coverage and compactness of individual layers are found to increase with the deposition pressure (5 to 30 mN/m).

Subphase pH effect on the limiting molecular area of amphiphilic β-diketones in Langmuir monolayers

Theoretical and experimental limiting molecular area values for a series of amphiphilic α- and γ-substituted β-diketones have been analyzed and compared. The investigated β-diketones exist in Langmuir monolayers as mixtures of keto and enol tautomeric forms. The ratio of the forms depends on the subphase pH and affects the shape of surface pressure–area isotherm and the experimentally determined limiting molecular area value for the monolayer.

Collapse of Langmuir monolayer formed by the mixture of short- and long-tailed fatty acid molecules

Surface pressure versus molecular area (π-A) isotherms of stearic acid (SA), behenic acid (BA) and their mixtures are studied at high subphase pH (≈ 9.5) in presence of Ba2+ ions. It is found that the collapse pressure (πC) and dip pressure (πD) points of binary fatty acid monolayer varies depending on the relative amount of the two components and nearly follow the Langmuir growth model. Out-of-plane structures obtained from the X-ray reflectivity and atomic force microscopy analysis imply that SA shows multi-layered structure that gradually decreases with the decrease in SA proportion in the mixed film. The transition from constant pressure to constant area collapse is observed when the weightage of BA becomes ≥66% for the mixed film. The change in the collapse behaviour and collapsed structure is explained using the two-dimensional percolated networks formed by the tail-tail hydrophobic interactions of relatively more hydrophobic long-chained BA molecules.

Evolution of Nickel-Stearate Langmuir Monolayers and the Synthesized NiOx Ultra-Thin Films with pH

Abstract In this work Ni-Stearate (Ni-St) Langmuir-Blodgett (LB) films are oxidized to synthesize uniform ultra-thin films of Nickel oxide (NiOx). Pressure-Area (π-A) isotherms and oscillatory barrier method are used to examine the Ni-St Langmuir monolayers. Increase in the sub-phase pH during Langmuir monolayer formation results in higher compaction and elasticity for the solid phase. NiOx thin films synthesized with 15 layers of Ni-St at pH 7.9 are continuous, uniform and have a band gap of 4.04 eV. Changing the value of pH results in non-continuous films. XRD of the 200 layer films identifies the final phase formation after sintering as Ni2O3.

Lipid-Protein Interactions in Langmuir Monolayers under Dynamically Varied Conditions.

In the Langmuir monolayer technique, a single layer of molecules is formed on a water subphase. This approach was used to mimic the antitumoricidal lipid-protein complex of oleic acid and bovine α-lactalbumin called the BAMLET complex. Our previous studies have shown that at the interface, the BAMLET complex is stabilized by the hydrophobic forces supported by the hydrogen bonds. This study provides an insight into the influence of calcium ions and the experimental conditions (temperature and subphase pH) on the stability of the complex at the interface. The Langmuir technique was expanded using a dosing pump to exchange the subphase and deliver additional substances to the system. We investigated the interactions between oleic acid monolayer and α-lactalbumin in the presence of Ca2+ in the bulk and the effect of varied experimental conditions on the complex stability. The role of calcium ions in this system is important because (in addition to low pH and relatively high temperature) it affects the conformational changes within the protein molecule and facilitates the transition of α-lactalbumin into the molten globule state. A partially unfolded state is required to form the BAMLET complex. We found that the mixed monolayer spread at the interface is stable despite drastic changes in the process conditions and remains stable even after the subphase exchange. This study of molecular interactions explored by the Langmuir technique with peristaltic pump enabled to understand the role of Ca2+ in BAMLET complex formation.

Controlled synthesis and self-assembly of amphiphilic copolymers based on 2,2,3,3,4,4,5,5-octafluoropentyl acrylate and acrylic acid

The reversible addition–fragmentation chain transfer (RAFT) polymerization of 2,2,3,3,4,4,5,5-octafluoropentyl acrylate (OFPA) in the presence of different concentrations of S,S-dibenzyl carbonotrithioate as chain transfer agent (CTA) was investigated. The estimate of Ctr was defined using the size-exclusion chromatography measurements. Copolymers of acrylic acid and OFPA with different microstructure were synthesized by three methods: block copolymerization, copolymerization of acrylic acid and OFPA, and chemical modification of copolymer of tert-butyl acrylate and OFPA. Copolymerization of abovementioned monomer pairs was studied. The reactivity ratios were calculated by using the Fineman–Ross and Kelen–Tudos models, as well as the method of least squares. Aggregation behavior of the obtained amphiphilic copolymers at the air/water interface was defined by the Langmuir monolayer technique. Monomolecular films of amphiphilic copolymers were studied using atomic force microscopy. The effect of the chain microstructure, the acrylic acid content in the copolymer, and subphase pH on the isotherm curves were shown.

Kinetics of interaction between antimicrobial peptide nisin and Langmuir monolayers of DPPC and DPPG molecules.

We have studied the kinetics of the interaction between antimicrobial peptide nisin and Langmuir monolayers of phospholipids DPPC and DPPG at the air-water interface using the surface manometry technique. The charge on the nisin and the lipid molecules is controlled by varying the pH of the subphase, and the interactions between them are studied by measuring the surface pressure of the lipid monolayer as a function of time after injecting the nisin in the subphase. A model based on the diffusion of particles under the influence of a constant force is developed to obtain an analytical expression for surface pressure as a function of time. The expression was found to fit well with the experimental data. The average hydrodynamic radius and the translational diffusion constant of the nisin molecules are calculated from the fit parameters for the different subphase pH solutions.

Effect of Chain Microstructure and Subphase pH on the Surface Properties and Aggregation Behavior of Amphiphilic Copolymers Based on Fluoroacrylates

Copolymerization of 2,2,3,3,4,4,5,5-octafluoropentyl acrylate (OFPA) and acrylic acid (AA), OFPA and tert-butyl acrylate (t-BA) in presence ofdibenzylcarbonotrithioate (BTC) and polymeric RAFT-agents was studied, reactivity ratios were calculated. It was shown that type of RAFT-agent can influence on chain microstructure of obtained polymers. Aggregation behavior of obtained amphiphilic copolymers with different microstructures at the air/water interface was characterized by the Langmuir monolayer technique. The effect of the microstructureand subphase pH on the isotherm curves were shown.

Molecular-level origin of the carboxylate head group response to divalent metal ion complexation at the air–water interface

We exploit gas-phase cluster ion techniques to provide insight into the local interactions underlying divalent metal ion-driven changes in the spectra of carboxylic acids at the air-water interface. This information clarifies the experimental findings that the CO stretching bands of long-chain acids appear at very similar energies when the head group is deprotonated by high subphase pH or exposed to relatively high concentrations of Ca2+ metal ions. To this end, we report the evolution of the vibrational spectra of size-selected [Ca2+·RCO2-]+·(H2O) n=0to12 and RCO2-·(H2O) n=0to14 cluster ions toward the features observed at the air-water interface. Surprisingly, not only does stepwise hydration of the RCO2- anion and the [Ca2+·RCO2-]+ contact ion pair yield solvatochromic responses in opposite directions, but in both cases, the responses of the 2 (symmetric and asymmetric stretching) CO bands to hydration are opposite to each other. The result is that both CO bands evolve toward their interfacial asymptotes from opposite directions. Simulations of the [Ca2+·RCO2-]+·(H2O) n clusters indicate that the metal ion remains directly bound to the head group in a contact ion pair motif as the asymmetric CO stretch converges at the interfacial value by n = 12. This establishes that direct metal complexation or deprotonation can account for the interfacial behavior. We discuss these effects in the context of a model that invokes the water network-dependent local electric field along the C-C bond that connects the head group to the hydrocarbon tail as the key microscopic parameter that is correlated with the observed trends.

Reversible 2D networks of oligo(ε-caprolactone) at the air–water interface

Hydroxyl terminated oligo(ε-caprolactone) (OCL) monolayers were reversibly cross-linked forming two dimensional networks (2D) at the air–water interface. The equilibrium reaction with glyoxal as the cross-linker is pH-sensitive. Pronounced contraction in the area of the prepared 2D OCL films in dependence of surface pressure and time revealed the process of the reaction. Cross-linking inhibited crystallization and retarded enzymatic degradation of the OCL film. Altering the subphase pH led to a cleavage of the covalent acetal cross-links. The reversibility of the covalent acetal cross-links was proved by observing an identical isotherm as non-cross-linked sample. Besides as model systems, these customizable reversible OCL 2D networks are intended for use as pH responsive drug delivery systems or functionalized cell culture substrates.

Effects of Cu(II) on the Formation and Orientation of an Arachidic Acid Langmuir-Blodgett Film.

The influence of the copper(II) ion on the formation, morphology, and organization of an arachidic acid monolayer was investigated using Langmuir-Blodgett (LB) monolayers, Π-A isotherms, and Brewster angle microscopy (BAM). Our findings indicate that a Cu2+-complexed LB film exhibits an order that depends on the subphase pH, analogous to other metal ions. Yazdanian , M. ; et al. Ionic Interactions of Fatty Acid Monolayers at the Air-Water Interface . Langmuir 1990 , 6 , 1093 - 1098 . Kurnaz , M. L. ; et al. Morphology of Microphase Separation in Arachidic Acid-Cadmium Arachidate Langmuir-Blodgett Multilayers . J. Phys. Chem. 1996 , 100 , 11113 - 11119 . The metal ion facilitates the formation of solid-phase films at surface pressures as low as 5 mN/m. The films exhibit a rigid, ordered phase, evidenced by the absence of a collapse point and an increase in surface pressure rather than the typical sharp decrease in surface pressure, indicative of film failure. Amphiphile ionic charge vs pH (i.e., the extent of arachidic acid protonation) plays a role in the observed absence of collapse and the ability of the films to maintain order and cohesion at high surface pressures (ca. 65 mN/m). Additionally, film thickness data suggest that the incorporation of Cu2+ ions induces a change in orientation of the aliphatic chains of the amphiphiles and that amphiphile solubility in the subphase may play a role in the observed film behavior at low surface areas and high pH.

Interfacial Behavior of Acidic Organophosphorus Extractant Monolayer at Air-Water Interface: Subphase pH and Spreading Solvent Effect

Interfacial Behavior of Acidic Organophosphorus Extractant Monolayer at Air-Water Interface: Subphase pH and Spreading Solvent Effect

Effect of Subphase Conditions on the Formation of Graphene Langmuir layers

This paper presents a study of the formation of graphene layers by Langmuir-Blodgett (LB) technique. The effect of subphase pH and temperature on the graphene Langmuir isotherms are investigated. Also, the influence of subphase conditions on the surface potential of a monolayer was reported. A variation in the pH of the water subphase can lead to change in the electrostatic interactions that influence the compression of the layers. The mechanical characteristics of graphene layers variation under the influence of the subphase pH and temperature was investigated. Langmuir layers assembly is a useful method to manipulate the distribution of graphene layers at the air-water interface. Successful deposition of graphene layers at the air-water interface on a silicon substrate was performed by using Langmui-Schaefer (LS) technique.

Charging of Carboxylic Acid Monolayers with Monovalent Ions at Low Ionic Strengths: Molecular Insight Revealed by Vibrational Sum Frequency Spectroscopy

The charging of arachidic acid Langmuir monolayers as a function of subphase pH and monovalent ion concentration below 100 mM was investigated using vibrational sum frequency spectroscopy. Molecular information was obtained by targeting the vibrational modes of the carboxylic acid headgroups, alkyl chains, and water molecules in the immediate surface and diffuse double layers. The surface charge in the monolayer was experimentally determined by monitoring the hydrated carboxylate stretching modes. The charging behavior was found to be in excellent agreement with that predicted by Gouy–Chapman theory using a thermodynamic pKa of 5.1 ± 0.2. This resulted in an apparent pKa of ∼10.8 when the only ions present in solution were those associated with adjusting the pH. Water molecules with a preferred orientation in the immediate surface region were found to primarily interact with the uncharged carboxylic acid moiety, decreasing in number as the monolayer further deprotonated. Contributions from water molecules...

Transparent and Hydrophobic “Reduced Graphene Oxide–Titanium Dioxide” Nanocomposites for Nonwetting Device Applications

Graphene oxide–titanium dioxide (GO–TiO2) nanocomposite sheets were transferred onto Si and quartz substrates by Langmuir–Blodgett (LB) technique at different subphase TiO2 concentrations and pH values. The effects of subphase and heat treatment conditions on the composition, surface morphology, microstructure, and hydrophobic performance of the composite sheets were investigated by XPS, Raman, AFM, HR-TEM, and contact-angle measurements. The wetting behavior of composite sheets before and after vacuum heat treatment was analyzed in conjunction with the extent of TiO2 uptake, distribution of nanoparticles over GO sheets, and overall surface roughness. The degree of subphase ionization significantly affects the uptake and aggregation behavior of TiO2, which tends to be dominated by the interaction of TiO2 with carboxylic acid functional groups at the edges of the GO sheets. Wettability studies revealed improvement in the hydrophobicity of composite sheets compared to the GO sheets, which is attributed prim...

Capacity of cholesteryl hemisuccinate in ion pair/phospholipid complex to improve drug-loading, stability and antibacterial activity of clarithromycin intravenous lipid microsphere.

Current research was to evaluate the capacity and molecular interaction of ion pair/phospholipid complex during preparation of clarithromycin intravenous lipid microsphere (CLA-LM) for improving drug-loading, stability and antibacterial activity. The optimum pH range for the presence of ion pair formed by CLA and cholesteryl hemisuccinate (CHEMS) was found to be between 6.4 and 8.2. CLA-LM prepared by ion pair/phospholipid complex possessed improved drug-loading (5-10 mg/ml), plasma stability, storage stability (24 months at 4 ± 2℃) and better activity for multi-drug-resistant Mycobacterium tuberculosis (MIC = 0.058 μg/ml). The release profile in vitro was pH-sensitive and greater in more acidic condition, which was effective for pathological targeting. CLA-LM presented controlled release and low drug leakage in plasma. Langmuir monolayer showed that the incorporation of CHEMS obviously improved interfacial molecular interactions of the mixed monolayer. The ordering and condensing effect of CHEMS resulted in higher collapse surface pressure and smaller limiting area, as well as reduced compression modulus. Close aggregate network distribution with particular micro domains in atomic force microscopy images reflected the enhanced miscibility and thermodynamic stability. The change of the spatial location and protonation degree of CHEMS resulted from the aqueous subphase pH value from 6 to 8 was observed. More acidic environment allowed for better molecular interaction and interface behavior. These results indicated that CHEMS enhances the phospholipid packing to achieve stable CLA-encapsulation in phospholipid interface and identify the preference of ion pair/phospholipid complex as a valuable delivery strategy to develop CLA formulations for improving drug-loading, stability and antibacterial activity.