Subphase pH Research Articles

CH3(CH2)nCOOH/Cd2+ System on the Aqueous Cadmium Acetate Solution Investigated in Situ by Polarization Modulation Infrared Spectroscopy

Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) was applied to investigate the classical CH3(CH2)nCOOH/Cd2+ system (abbreviated as CdSt, CdA, and CdB for n = 16, 18, and 20, respectively) in situ on the aqueous cadmium acetate subphase of pH = 6.7 in a Langmuir trough. The alkanoic acids were deprotonated and spontaneously formed crystalline domains after spreading to the subphase. The spectra were recorded at a resolution of 4 cm-1 and the crystal lattice information contained in the CH2 scissoring mode was unambiguously concluded. It is found that the crystal lattice has a systematic dependency on the monolayer temperature and chain length. The CdSt monolayer possesses a hexagonal packing at 293−283 K and pseudohexagonal packing at 274 K. The CdA monolayer possesses a hexagonal unit cell at 293 K and an orthorhombic one at 283−274 K. The CdB monolayer always has an orthorhombic unit cell at 293−274 K. Both temperature lowering and chain lengthening can increase the interch...

Effect of pH on the properties of palmitic acid LB films for gas sensors

In this study, we fabricated the QCM with Langmuir–Blodgett (LB) film deposited at different subphase pH and investigated the resonant frequency response by the injection of organic gas response. In the π-A isotherm, the monolayer on the air/water interface had different limiting areas per molecule and showed more condensed status as increasing the subphase pH. When palmitic acid LB film was deposited on the QCM, the resonant frequency shift was proportional to the deposited layer and had more resonant frequency shift in the case of the higher pH range as expected. In the resonant frequency response for the injection of organic gas, frequency response has been improved in the case of LB film fabricated at the lower subphase pH range and dependent upon the molecular weight of organic gas. The frequency response of QCM for the injection of organic gases was mainly dependent upon the molecular weight of organic gas and the surface morphology of LB film.

Study of the pH dependence of head group bonding in arachidic acid monolayers by polarization modulation infrared reflection absorption spectroscopy

Little information is available on the bonding interaction of fatty acid head groups with solvent molecules and other fatty acid molecules in monolayers at the air–water interface. Recent findings indicate the formation of hydrogen bonds between pairs of neutral fatty acid and carboxylate in the pH range from pH 5.6 to 10. In this work arachidic acid monolayers are investigated on subphases of pH 5.6, 9, 9.7 and 12.0, that contain a variable amount of sodium ions, by means of polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). The fitted peak intensities are related to surface concentrations of singly and doubly hydrogen bonded and non-hydrogen bonded carbonyl and carboxylate. From the change of the concentrations with the pH tentative conclusions are drawn on the bonding situation of fatty acids in monolayers on water and alkaline solution. The degree of dissociation as derived from the peak intensities is compared with calculated results according to the Gouy–Chapman–Stern theory.

Crystal lattice of the cadmium alkanoate monolayer at the air/water interface investigated by polarization modulation infrared spectroscopy

The crystal lattice of an arachidic acid/Cd2+ (CdA) monolayer on the surface of aqueous CdCl2 subphase in a Langmuir trough was investigated in situ by polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). The spectra reveal a hexagonal packing of the hydrocarbon chains on the subphase of pH=6.2 and 293–283 K. Cooling from 283 to 280 K induces the hexagonal unit cell to transform to an orthorhombic one, as evidenced by the splitting of the CH2 scissoring mode. This transition is observed to be reversible with temperature variation within 293–274 K. On the subphase of pH=8.4, the crystal lattice of CdA is orthorhombic even at 293 K. The temperature-dependent crystal lattice transition of CdA is compared with the monotonic hexagonal subcell of a cadmium stearate monolayer within 293–274 K and the orthorhombic subcell of a cadmium behenate monolayer within 293–274 K.

Structure characterization and stability of mixed lipid/protein monolayer at the air/water interface

Abstract A mixed protein/lipid monolayer has been constructed by the protein adsorption from subphase into the spread phospholipid monolayer. A precisely controlled pump was used to exchange the protein solution with different pH values after the protein was ensured to reach the less condensed surface. The domains formed in the coexistence region of D-dipalmitoylphosphatidylcholine (D-DPPC) have been recorded by Brewster angle microscopy (BAM) combined with the film balance before and after the penetration of the protein, human serum albumin (HSA). The subphase was exchanged by gradually increasing or decreasing pH value of the solution. Three isotherms of the mixed D-DPPC/HSA monolayer with the subphase of pH=4.2, pH=7.0 and pH=9.1, respectively, were obtained. It indicated that the area per lipid molecule with protein increased as the subphase pH value was lowed. Simultaneously, morphological dynamic changes caused by the gradual changing on subphase pH were observed. These variations can be ascribed to the conformation change of protein under the fluctuation of pH value. The hydrophobic and electrostatic interactions between the phospholipid and HSA were as considered for the interpretation of domain change based on the current experimental results

Size selective molecular recognition of calix[4]arenes in Langmuir–Blodgett monolayers

A stable and well-ordered monolayer of p-tert-butylcalix[4]arene (Cal-4) has been constructed at the air–water interfaces and transferred on solid substrates under an optimized condition. The surface pressure–area ( Π– A) isotherm of Cal-4 on a basic subphase exhibits a limiting area of 160±10 Å 2 per molecule, which is consistent with results obtained from CPK model and atomic force microscopic (AFM) study. The Π– A isotherms and AFM studies indicated that well ordered two-dimensional crystalline arrangement was obtained only on the basic subphase of pH 10 or higher. Electrostatic repulsion of adjacent phenolic anions at the lower rim and hydrophobic interactions among tert-butyl groups on the upper rim may be responsible for monomeric dispersion and well-aligned packing on the basic subphase. FT-IR and surface-enhanced Raman scattering (SERS) studies indicate that C 4 axis of Cal-4 is oriented parallel to the surface normal at the interface. Size selective molecular recognition of Cal-4 through the hydrophobic upper cavity was demonstrated by using cyclic voltammetry and SERS studies of methyl viologen dication (MV 2+).

Effect of Cations on the Dissociation of Arachidic Acid Monolayers on Water Studied by Polarization-Modulated Infrared Reflection−Absorption Spectroscopy

We report on the variation of the dissociation of a Langmuir monolayer of arachidic acid at the air/water interface as a function of the subphase pH and for several cations (Cd2+, Ca2+, Mg2+, and Na+) with the help of the polarization-modulated infrared reflection−absorption spectroscopy (PM-IRRAS) method. The infrared spectra give access to the relative concentration of acid and salt molecules and allow us to determine the influence of the subphase pH on the acid dissociation reaction for each cation. It is shown that Na+ obeys the purely electrostatic Gouy−Chapman theory quite well, whereas the behavior of Mg2+, and even more so that of Ca2+ and Cd2+, requires the introduction of some complexation constant to be understood.

Spectral properties of stilbazolium merocyanines oriented in stretched polymer films and Langmuir–Blodgett monolayers

Three stilbazolium merocyanines: 1-(6′-hydroxyhexyl)-4-[(4-oxocyclohexa-2,5-dienylidene)ethylidene]-1,4-dihydropyridine; 1-(11′-hydroxyhexyl)-4-[(4-oxocyclohexa-2,5-dienylidene)ethylidene]-1,4-dihydropyridine; 1-(10′-carboxydecyl)-4-[4-oxocyclohexa-2,5-dienylidene)ethylidene]-1,4-dihydropyridine salt HCl located in Langmuir–Blodgett monolayers and deposited on quartz plate as well as embedded in isotropic or stretched polyvinyl alcohol films were investigated. Dyes occur in protonated and free-base forms. The concentration ratio of these forms depends on pH of subphase (for monolayers) or resin addition (in polymer film).The polarized absorption and fluorescence spectra were measured and coefficients of absorption and emission anisotropy were calculated. Dye molecules were in both matrices oriented, but degrees of orientation of various forms of dye were different. The anisotropies of absorption and emission are also different which strongly suggests the occurrence of dye forms with different yield of fluorescence and various orientations. The formation of mixed aggregates of protonated and free-base form is suggested. The orientation of stilbazolium merocyanines in anisotropic matrix is important in the application of these dyes as the sensors for measurements of the local electric field in biological membranes as well as for the formation polarizing absorption polymer film used in some types of two-colours liquid crystal displays.

SNPP and SMAP as Candidate Materials for Langmuir-Blodgett Films for Non-Linear Optics

(S)-N-(4-nitrophenyl)-prolinyl stearate (SNPP) and 2,4-dinitrophenyl-1-alanine stearyl ester (SMAP), stearyl derivatives of the molecules NPP and MAP (which form non-centrosymmetric crystals), have been prepared and studied as candidate materials for Langmuir-Blodgett films for non-linear optics. Pressure-area isotherms were determined under various conditions of sub-phase pH and temperature for both materials, separately and in mixtures with other film-forming species. Surface layers of the pure materials were at best metastable, and Langmuir-Blodgett films deposited from mixtures did not incorporate any SNPP or SMAP. Molecular modelling showed that in SNPP the hydrophobic tail appears to prevent stable film formation by preferring to lie over the weakly hydrophilic nitro group, while in SMAP the orientation of the tail is more favourable but the two nitro groups appear to destabilize the film.

Arachidic Acid Monolayers at High pH of the Aqueous Subphase: Studies of Counterion Bonding

The thermodynamic and morphological properties of arachidic acid monolayers at high pH values (pH 11−13) and temperatures (10 °C ≤ T ≤ 35 °C) are studied. The two-phase coexistence region of the su...

Influence of Cholesterol on Domain Shape and Lattice Structure in Arachidic Acid Monolayers at High pH

Monolayer mixtures of arachidic (eicosanoic) acid with cholesterol are investigated on a subphase of pH 12.0 at 25 °C using grazing incidence X-ray diffraction (GID) and Brewster angle microscopy (...

Spectral behavior and pH dependence of N-hexadecyl-5-iminomethyl-8-hydroxyquinoline

A new kind of amphiphile derived from 8-hydroxyquinoline, N-hexadecyl-5-iminomethyl-8-hydroxyquinoline (HIHQ) was synthesized and characterized by different physical methods. The influence of pH value on UV-vis absorption, fluorescence spectra were investigated and the surface pressure–area isotherms were recorded under different pH subphase. The linear response range between the emission intensity and the concentration of HIHQ was 1–4×10 −6 mol L −1. The strongest emission of HIHQ about 460 nm was obtained at pH 8.2 and the acid–base equilibrium of HIHQ in monolayer and solution was also discussed.

Homogeneous and Heterogeneous Electron Transfer Dynamics of Osmium-Containing Monolayers at the Air/Water Interface

from 3.0 to 7.0. Over the range 3.0 e pH e 7.0 a single pKa of 4.3 ( 0.3 is observed for the unbound Qbpy pyridine nitrogens when confined within a Langmuir monolayer. In solution, over the range 1.6 e pH e 7.0 two pKa so f 3.2( 0.2 and 4.3 ( 0.2 are observed. Horizontal touch voltammetry, performed using microelectrodes, has been used to determine the apparent charge transport diffusion coefficients, Dapp ,a s the mean molecular area is systematically varied. For surface concentrations, i, between approximately 1.0 10 -10 and 1.4 10 -10 mol cm -2 , the monolayer is in a liquid state and Dapp increases linearly with increasing surface concentration. Although surface concentration dependent physical diffusion of the complexes prevents an accurate determination of the self-exchange rate constant, kex, analyzing these data in accordance with a 2-D Dahms-Ruff approach provides a lower bound of 1.0 10 8 M -1 s -1 for kex. When the monolayers are further compressed so that the molecular area is less than approximately 100 A 2 , charge transport proceeds via a percolation mechanism and the maximum value of the apparent diffusion coefficient, Dapp,max, yields a rate constant for electron self-exchange between Os 2+ and Os 3+ of 1.6 10 9 M -1 s -1 . Dapp,max depends on the solution pH in a sigmoidal manner, increasing from approximately 6.6 ( 0.2 10 -6 to 8.9 ( 0.4 10 -6 cm 2 s -1 as the subphase pH is increased from 3.0 to 7.0. To probe the effect of molecular orientation on interfacial electron transfer rates, we have used chronoamperometry conducted on a microsecond time scale to measure the heterogeneous electron transfer rate, k, for both Langmuir and spontaneously adsorbed monolayers. In this way, the effect of making electrical contact through the Qbpy or dpp ligands on k can be probed. Although both rates are large, the rate measured at an overpotential of 50 mV for Langmuir monolayers (1.7 ( 0.2 10 5 s -1 ) is approximately an order of magnitude smaller than that found for spontaneously absorbed systems (1.2 ( 0.3 10 6 s -1 ), suggesting that the local microenvironment, e.g., packing density and molecular orientation, influence the rate of heterogeneous electron transfer in these systems.

Hydrolytic Kinetics of Biodegradable Polyester Monolayers

The rate of hydrolysis of Langmuir monolayer films of a series of biodegradable polyesters was investigated at the air/water interface. The present study investigated parameters such as degradation medium, pH, and time. The hydrolysis of polyester monolayers strongly depended on both the degradation medium used to control subphase pH and the concentration of active ions. Under the conditions studied here, polymer monolayers showed faster hydrolysis when they were exposed to a basic subphase rather than that of acidic or neutral subphase. The basic (pH = 10) hydrolysis of [poly(l-lactide)/polycaprolactone](l-PLA/PCL 1/1 by mole) blend was faster than that of each homopolymer at the initial stage. This result is explained by increasing numbers of base attack sites per unit area owing to the very slow hydrolysis of PCL, a dilution effect on the concentration of l-PLA monolayers. Conversely the hydrolytic behavior of l-lactide-co-caprolactone (1/1 by mole) was similar to that of PCL even though the chemical compositions of the blend and the copolymer are very similar to each other. The resistance of the copolymer to hydrolysis might be attributed to the hydrophobicity and the steric hindrance of caprolactone unit in the copolymer.

Texture features of long-chain fatty acid monolayers at high pH of the aqueous subphase

The thermodynamic properties and the condensed phase textures of Langmuir monolayers of protonated and deprotonated long-chain fatty acids (stearic acid, arachidic acid, and behenic acid) are studied. Surface pressure–area ( π– A) isotherms and Brewster angle microscopy (BAM) images are recorded in the regions of low and high pH values. In the range of pH 11–13, the π– A isotherms of these long-chain fatty acid monolayers show a plateau region indicating a two-phase coexistence region. Under these conditions, segmented domains with highly ordered textures are formed in the stearic acid arachidic acid monolayers, whereas the domains of behenic acid monolayers are irregularly shaped without an inner texture. The best developed domains of stearic acid and arachidic acid are circularly shaped and subdivided into six almost identical segments which do not meet exactly in the centre. The long-range orientational order of the condensed phase domains is discussed. Addition of neutral salt (NaCl) to the subphase improves the stability of the long-chain fatty acid monolayers at high pH values by a salting-out effect. A time-dependent pH drift due to atmospheric CO 2 and desorption of monolayer material are characterised for realising optimal monolayer stability.

An Infrared Study of Molecular Orientation and Structure in One-Layer Langmuir−Blodgett Films of Octadecyldimethylamine Oxide and Dioctadecyldimethylammonium Chloride: Dependence of the Structures of the Langmuir−Blodgett Films on Substrates, Aging, and pH of Water Subphase

Molecular orientation and structure in one-layer Langmuir−Blodgett (LB) films of octadecyldimethylamine oxide (C18DAO) and dioctadecyldimethylammonium chloride (2C18DAC) were investigated by use of infrared (IR) transmission, reflection−absorption (RA), and attenuated total reflectance (ATR) spectroscopy. The IR measurements suggest that the alkyl chains of the one-layer LB film of 2C18DAC on a gold-evaporated glass slide are highly ordered, whereas those of the C18DAO LB film have some gauche conformers. The molecular orientation and structure in both LB films show significant substrate dependence. Probably, the headgroup−substrate interaction controls them. Aging effects on the LB and Langmuir (L) films on an air−substrate interface were studied for the two kinds of amphiphiles. The alkyl chains of C18DAO in the LB film on the gold-evaporated glass slide become more ordered with time whereas the reverse trend is observed for those of 2C18DAC in the film. The effects on the L films were investigated by measuring IR spectra of the LB films prepared from the L films at various stages. The alkyl chains are less ordered in the LB films of C18DAO and 2C18DAC prepared from the aged L films. It is also noted that the molecular orientation and structure in the films of both compounds show strong pH dependence, presumably because of the strong interaction between the headgroups and substrates.

Effect of subphase Ca++ ions on the viscoelastic properties of Langmuir monolayers

It is known that the presence of cations like Ca++ or Pb++ in the water subphase alters the pressure-area isotherms for fatty acid monolayers. The corresponding lattice constant changes have been studied using x-ray diffraction. Reflection-absorption spectroscopy has been used to probe the chemical composition of the film. We report on the first measurements of the time evolution of the shear viscosity of arachidic acid monolayers in the presence of Ca++ ions in the subphase. We find that the introduction of Ca++ ions to the water subphase results in an increase of the film’s viscosity by at least three orders of magnitude. This increase occurs in three distinct stages. First, there is a rapid change in the viscosity of up to one order of magnitude. This is followed by two periods, with very different time constants, of a relatively slow increase in the viscosity over the next 10 or more hours. The corresponding time constants for this rise decrease as either the subphase pH or Ca++ concentration is increased.

Effect of Hydrophobic Surfactant Peptides SP-B and SP-C on Binary Phospholipid Monolayers. I. Fluorescence and Dark-Field Microscopy

The influence of the hydrophobic proteins SP-B and SP-C, isolated from pulmonary surfactant, on the morphology of binary monomolecular lipid films containing phosphocholine and phosphoglycerol (DPPC and DPPG) at the air-water interface has been studied using epifluorescence and dark-field microscopy. In contrast to previously published studies, the monolayer experiments used the entire hydrophobic surfactant protein fraction (containing both the SP-B and SP-C peptides) at physiologically relevant concentrations (∼1 wt %). Even at such low levels, the SP-B/C peptides induce the formation of a new phase in the surface monolayer that is of lower intrinsic order than the liquid condensed (LC) phase that forms in the pure lipid mixture. This presumably leads to a higher structural flexibility of the surface monolayer at high lateral pressure. Variation of the subphase pH indicates that electrostatic interaction dominates the association of the SP-B/C peptides with the lipid monolayer. As evidenced from dark-field microscopy, monolayer material is excluded from the DPPC/DPPG surface film on compression and forms three-dimensional, surface-associated structures of micron dimensions. Such exclusion bodies formed only with SP-B/C peptides. This observation provides the first direct optical evidence for the squeeze-out of pulmonary surfactant material in situ at the air-water interface upon increasing monolayer surface pressures.

A Study on Two-Dimensional Phase Transitions in Langmuir Monolayers of a Carboxylic Acid with a Symmetrical Triphenylbenzene Ring System

A detailed investigation on phase transitions in Langmuir monolayers of 5‘-phenyl-1,1‘:3‘1‘ ‘-terphenyl-4-carboxylic acid (PTCA) at the air−water interface is presented. Surface pressure and surface potential isotherms were obtained using varying experimental parameters such as spreading volume, subphase pH, ionic strength, subphase temperature, and compression speed. The isotherms exhibit two liquid-condensed regions separated by a broad plateau with a 2-fold decrease in area/molecule. PTCA monolayers were visualized by Brewster angle microscopy, transferred as multilayer LB films and characterized by Fourier transform infrared spectroscopy, X-ray diffraction (XRD), and surface potential measurements. On the basis of the analysis of surface pressure and surface potential isotherms, it was not possible to distinguish between bilayer formation and conformational changes of the triphenylbenzene ring system upon compression as the origin of the plateau. The LB transfer from monolayers in the plateau was unsuccessful and thus characterization of LB films does not help in elucidating the possible cause for the transition.

Monolayer and epi-fluorescence microscopy studies of amino acid derivatized diacetylene lipids

Monolayers of amino acid derivatized diacetylene lipids on water were studied by Langmuir trough and epi-fluorescence microscopy (EFM). Examination of the amino acid derivatized diacetylene lipids showed a wide variety of surface characteristics with many novel properties. Many of these new properties are advantageous for film-based biosensor development. To characterize this family of molecules, pressure–area (π–A) isotherms were measured that demonstrate hydrophilic headgroup compounds allow for close-packed monomolecular layer formation in the solid state upon continuous compression. The effect of subphase pH and temperature on monolayer compressibility and phase transition was examined. Decrease of pH results in attenuation of LE–LC phase transition in Gln-PDA monolayers due to charge screening, while temperature affects the monolayers through regular thermal agitation, and the effect is opposite to the thermal anomaly observed for the non-derivatized diacetylene lipids. EFM studies reveal that only the lipids derivatized with hydrophilic headgroups exhibit phase transition and domain formation, while those with hydrophobic headgroups do not easily form polymerizable matrices. The shape and growth of the domains were found to be strongly headgroup dependent. Photo-polymerization of diacetylene lipid domains on water using EFM is discussed.