Influence of Molecular Organization of Photo-active Azo-phanes on the Reactivity in Monolayers at the Air–Water Interface

The molecular organization of monolayers at the air–water interface of an amphiphilic azo-phane with unbranched n-dodecyl substituents differs from that of the analog with bulky ‘tert’-octyl substituents as seen in the area per molecule. Complexation with sodium ions from the aqueous subphase, as deduced from measurement of the surface potential, is facilitated by closer approach of the macrocycles for the n-dodecyl-substituted azo-phane, since two macrocycles form the complex with Na⁺. The compensation of the positive charge after complexation in the case of a two-component monolayer of the n-dodecyl-substitued azo-phane and octadecanoic acid, molar ratio azo-phane to acid = 2:1, enhances even more the complexation. The complex equilibrium constants and the contributions of the hydrophilic head group region to the surface potential are evaluated from Langmuir isotherm fits to the dependencies of the surface potential on the NaCl concentration in the aqueous subphase for the three monolayer systems investigated, i.e., the two different pure azo-phane monolayers and the two-component monolayer.     

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

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

Sorption of aliphatic alcohols from aqueous solutions by starch cryotextures

Capillary GLC was applied to study the sorption ofn-butanol,n-hexanol,n-octanol, and linalood from aqueous solutions by com starch cryotextures. The concentrations of alcohols in aqueous solutions were varied from 0.5 to 15 mmol L⁻¹. The sorption of alcohols by crytotextures formed of sols containing these alcohols and the coefficients of their. distribution depend on the initial concentration and structure of the alcohols.n-Butanol is not sorbed by the corn starch cryotextures over the range of 0.5–80 mmol L⁻¹. The sorption of other alcohols increases with increasing length of the alkyl substituent and the concentration of the alcohol. The highest sorption capacity of the cryotexture (88%) was observed forn-octanol. The sorption of linalool decreases due to the double bonds and branching in its molecules. Translated fromIzestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 734–737, April, 1999.     

Monomeric tert-Butanol in Benzene and Cyclohexane Solutions: A Fourier-Transform Infrared Spectroscopy Study

The dependence of the hydroxyl stretching frequency of monomeric tert-butanol in cyclohexane and benzene solutions on concentration was studied at different temperatures. The –OH group in the monomeric form of the alcohol appears to experience very small or negligible alcohol concentration effects. This is attributed to the ability of tert-butanol to form dimers by hydrogen bonding, which in turn shield their polar groups from monomeric tert-butanol. As a result, the environment that the latter interacts with consists of weakly polar or nonpolar species (cyclohexane or benzene), and the nonpolar (methyl) groups of dimeric tert-butanol.     

Dynamic Behaviors and Morphology Change of Anionic Phospholipid DPPG Monolayer Caused by Bovine Serum Albumin at Air-Water Interface

The interaction between bovine serum albumin (BSA) and the anionic 1.2-dipalmitoyl-snglycero- 3-(phospho-rac-(1-glycerol)) (sodium salt) (DPPG) phospholipid at different subphase pH values was investigated at air-water interface through surface pressure measurements and atomic force microscopy (AFM) observation. By analyzing surface pressure-mean molecular area (π-A) isotherms, the limiting molecular area in the closed packing state-the concentration of BSA (A_lim-[BSA]) curves, the compressibility coefficient-surface pressure (C_S^-1-π) curves and the difference value of mean molecular area-the concentration of BSA (ΔA-[BSA]) curves, we obtained that the mean molecular area of DPPG monolayer became much larger when the concentration of BSA in the subphase increased at pH=3 and 5. But the isotherms had no significant change at different amount of BSA at pH=10. In addition, the amount of BSA molecules adsorbed onto the lipid monolayer reached a threshold value when [BSA]>5×10^-8 mol/L for all pHs. From the surface pressure-time (π-t) data, we obtained that desorption and adsorption processes occurred at pH=3, however, there was only desorption process occurring at pH=5 and 10. These results showed that the interaction mechanism between DPPG and BSA molecules was affected by the pH of subphase. BSA molecules were adsorbed onto the DPPG monolayers mainly through the hydrophobic interaction at pH=3 and 5, and the strength of hydrophobic interaction at pH=3 was stronger than the case of pH=5. At pH=10, a weaker hydrophobic interaction and a stronger electrostatic repulsion existed between DPPG and BSA molecules. AFM images revealed that the pH of subphase and [BSA] could affect the morphology features of the monolayers, which was consistent with these curves. The study provides an important experimental basis and theoretical support to understand the interaction between lipid and BSA at the air-water interface.     

Formation of micelles of cetyltrimethylammonium chloride and cetyltrimethylammonium bromide in N-methyl acetamide—alcohol and N,N-dimethyl acetamide—alcohol mixtures

The critical micelle concentrations of cetyltrimethylammonium chloride and cetyltrimethylammonium bromide in solutions of in N-methylacetami de and in N,N-dimethyl acetamide with added methanol, ethanol, n-propanol, n-butanol and n-pentanol were determined using electrical conductivity and surface tension measurements at various temperatures. Both methods show that micelles are formed in N-methyl acetamide and N, N-dimethyl acetamide solutions in a presence of n-alcohols. Critical micelle concentrations were also determined as functions of concentration of added alcohol. The data suggest that alcohol adding leads to an enhancement of penetration of alcohol into the micelle external shell that depends on the alcohol chain length. Thermodynamic parameters for micellar systems in a presence of n-alcohols were also calculated.     

EXCITATION TRANSFER BY CHLOROPHYLL a IN MONOLAYERS AND THE INTERACTION WITH CHLOROPLAST GLYCOLIPIDS*

In mixed monolayers with purified chloroplast glycolipids and other colorless lipids, chlorophyll a fluorescence exhibits a decrease in quantum efficiency with increasing chlorophyll concentration. The fluorescence, which is strongly polarized in dilute films, becomes progressively depolarized as the area fraction of chlorophyll increases, and it is completely depolarized in a pure chlorophyll a monolayer. The observed behavior is consistent with an inductive resonance mechanism of energy transfer among the chlorophyll molecules with a critical transfer distance of 20–90 Å, depending on the model chosen for the energy transfer mechanism. The purified glycolipids–mono-and digalactosyl diglycerides and sulfoquinovodiglyceride–separately form stable, compressible monolayers of the liquid-expanded type on an aqueous subphase and in an atompshere of nitrogen. At maximum compression the three glycolipids occupy areas of 55, 80 and 47 A²-molecule^-1, respectively, in the monolayer. Mixed monolayers of chlorophyll a with, separately, the monogalactolipid and the sulfolipid behave upon compression as two-dimensional solutions. The fluorescence polarization at high chlorophyll concentrations in mixed monolayers indicates that several of the lipid diluents facilitate local ordering of the pigment molecules.     

Effect of different butanols on the adsorption of sodium dodecylsulfate on alumina

The adsorption of sodium dodecylsulfate onto alumina from aqueous solutions (0.3 M NaCl) was determined in the presence of short-chain alcohols,i.e.,1-, 2-,iso-, andtert-butanol. All adsorption isotherms of the surfactant show the same trend upon the addition of the different short-chain alcohols. The isotherm reaches its saturation level at lower surfactant equilibrium concentrations than without alcohol, and the saturation adsorption density level decreases. The reason for these changes is attributed mostly to the lowering effect of butanols on the critical micelle concentration of sodium dodecylsulfate in the aqueous bulk phase, but also the competitive adsorption of the alcohols is considered.     

Study on the Film Properties and Recognition for Metal Ions of Calix[4]arenes Derivative by Langmuir Monolayer

Calixarene molecules are very powerful ligand for ions and small molecules, and have been studied with several techniques as models for host‐guest systems. In this approach, the formation of Langmuir monolayer properties of three kinds calix[4]arene derivative were characterized and one of them, p‐tert‐butylthiacalix[4]arene (TCA), was chosen as object to study its Langmuir monolayer affected by different subphase conditions. The purpose of this study is to investigate the molecular recognition ability of TCA for metal ions at the water‐air interface. Changing the composition of aqueous subphase (containing various metal ion solutions respectively) produced strong variations on the monolayer parameters, indicating a different selectivity of the TCA ligand for the different metal cations. In particular, high selectivity for transition metal ions was found. Limiting area values are discussed in relation to the orientation of the cone‐shaped molecules at the water‐air interface.     

Viscosity studies of solutions of water inn-aliphatic alcohols at various temperatures

Viscosity measurements have been made at 25°C on solutions of water inn-propanol, and at 15, 25, 35, and 45°C on solutions of water inn-butanol,n-pentanol, andn-hexanol over the respective solubility ranges. For most of the systems, water decreases the viscosity of the dry alcohols, while for the lower members of the series literature data report an increase in viscosity on addition of water. These results are rationalized in terms of two kinds of interaction between water molecules and alcohols: participation of water molecules in chain formation for the lower alcohols and formation of water-centered complexes for butanol and higher alcohols.     

Solubility of benzilic acid in select organic solvents at 298.15 K

Experimental solubilities are reported for benzilic acid dissolved in ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol, 1-heptanol, 2-propanol, 2-butanol, 2-methyl-1-propanol, diethyl ether and methyl tert-butyl ether at 298.15?K. Results of these measurements reveal that the observed solubilities in the nine alcohol solvents fall within a fairly narrow mole fraction range of each other. Benzilic acid is also very soluble in the two ether solvents studied.     

Adsorption of 1-pentanol on bismuth single-crystal plane electrodes

Cyclic voltammetry, impedance and chronocoulometry have been employed for the quantitative study of 1-pentanol (n-PenOH) adsorption at the bismuth single-crystal plane | aqueous Na₂SO₄ solution interface. The adsorption isotherms, Gibbs energies of adsorption ΔG _A ^∘, the limiting surface excess Γ_max and other adsorption parameters, dependent on the crystallographic structure of the electrodes, have been determined. The adsorption of n-PenOH on Bi single-crystal planes is mainly physical and is limited by the rate of diffusion of organic molecules to the electrode surface. Comparison of the adsorption data for n-PenOH with 1-propanol (n-PrOH), 1-butanol (n-BuOH), cyclohexanol (CH) and 1-hexanol (n-HexOH) shows that the adsorption characteristics depend on the structure of the hydrocarbon group. The adsorption activity of adsorbates at the bismuth | solution interface increases in the sequence n-PrOH < n-BuOH < CH ≤ n-PenOH < n-HA as the adsorption activity at the air | solution interface increases. For all the compounds studied, the adsorption activity increases in the sequence of planes (111)<(001)<(011ˉ). Received: 1 July 1998 / Accepted: 2 October 1998     

Glycoprotein adsorption into bilirubin/cholesterol mixed monolayers at the air-water interface

The adsorption of α₁-acid glycoprotein into bilirubin/cholesterol mixed monolayers with various component molar ratios is investigated using surface pressure-area (π-A) isotherms and (dπ/dA)-A curves. The results showed that the surface area per molecule increased after the adsorption/insertion of glycoprotein molecules into the monolayers. The compressibility of mixed monolayers increased as a result of hydrogen bonding between bilirubin and glycoprotein molecules, while the interactions between bilirubin and cholesterol are weakened. The adsorption of glycoprotein into a monolayer induced changes in molecular surface area depending on the surface pressure and molar fraction of bilirubin. The transmission electron microscopy of mixed monolayers confirmed the insertion of glycoprotein particles of spherical shape with an average diameter of about 80 nm into the monolayer. The text was submitted by the authors in English.     

Effects of Calcium Ions on Thermodynamic Properties of Mixed Bilirubin/Cholesterol Monolayers

The mixed monolayer behavior of bilirubin/cholesterol was studied through surface pressure-area (?-A) isotherms on aqueous solutions containing various concentrations of calcium ions. Based on the data of ?-A isotherms, the mean area per molecule, collapse pressure, surface compressibility modulus, excess molecular areas, free energy of mixing, and excess free energy of mixing of the monolayers on different subphases were calculated. The results show an expansion in the structure of the mixed monolayer with Ca2+ in subphase, and non-ideal mixing of the components at the air/water interface is observed with positive deviation from the additivity rule in the excess molecular areas. The miscibility between the components is weakened with the increase of concentration of Ca2+ in subphase. The facts indicate the presence of coordination between Ca2+ and the two components. The mixed monolayer, in which the molar ratio of bilirubin to cholesterol is 3:2, is more stable from a thermodynamic point of view on pure water. But the stable 3:2 stoichiometry complex is destroyed with the increase of the concentration of Ca2+ in subphase. Otherwise, the mixed monolayers have more thermodynamic stability at lower surface pressure on Ca2+ subphase.     

The inhibition of N2O5 hydrolysis in sulfuric acid by 1-butanol and 1-hexanol surfactant coatings

Gas-liquid scattering experiments are used to measure the fraction of N2O5 molecules that are converted to HNO3 after colliding with 72 wt % H2SO4 containing 1-hexanol or 1-butanol at 216 K. These alcohols segregate to the surface of the acid, with saturation coverages estimated to be 60% of a close-packed monolayer for 1-hexanol and 44% of a close-packed monolayer for 1-butanol. We find that the alkyl films reduce the conversion of N2O5 to HNO3 from 0.15 on bare acid to 0.06 on the hexyl-coated acid and to 0.10 on the butyl-coated acid. The entry of HCl and HBr, however, is enhanced by the hexanol and butanol films. The hydrolysis of N2O5 may be inhibited because the alkyl chains restrict the transport of this large molecule and because the alcohol OH groups dilute the surface region, suppressing reaction between N2O5 and near-interfacial H3O+ or H2O. In contrast, the interfacial alcohol OH groups provide additional binding sites for HCl and HBr and help initiate ionization. These and previous scattering experiments indicate that short-chain alcohol surfactants impede or enhance sulfuric acid-mediated reactions in ways that depend on the chain length, liquid phase acidity, and nature of the gas molecule.     

Scanning force microscopic study of protein adsorption on the surface of organosilane monolayers prepared by the Langmuir‐Blodgett method

The n‐octadecyltrichlorosilane (OTS, CH₃(CH₂)₁₇SiCl₃), 18‐nonadecenyltrichlorosilane (NTS, CH₂=CH(CH₂)₁₇SiCl₃), [2‐(perfluorooctyl)ethyl] trichlorosilane (FOETS, CF₃(CF₂)₇CH₂CH₂SiCl₃) monolayers, and their mixed monolayers were used as the model substrates for the study of protein adsorption mechanism. Surface plasmon resonance (SPR) spectroscopy was applied to analyze the protein adsorption behavior onto the surface of the monolayers. Atomic force microscope (AFM) was used to observe the monolayer surfaces after exposure of these monolayers to bovine serum albumin (BSA) and γ‐globulin(IgG) solution. AFM observation revealed that the charged protein either below or above the isoelectric point was preferentially adsorbed onto the FOETS phase of the (OTS/FOETS) mixed monolayer. SPR revealed that the amount of adsorbed protein in the charged state was lower than that in the neutral state. These results indicate that the preferential adsorption of protein onto the FOETS phase for the mixed monolayer systems at either below or above pI is due to (1) the minimization of interfacial free energy between the monolayer surface and the buffer solution, and (2) the electrostatic repulsion among protein molecules bearing charges.     

Adsorption of n-Butanol on Pb–Ga in Aqueous Solutions: Effect of Metal on the Adsorption of Organic Compounds in the Absence of Metal–Water Chemisorption Interaction

The potential dependence of the differential capacitance is measured by an ac bridge at 420 Hz and 32°C at Pb–Ga/H₂O interface in 0.05 M Na₂SO₄ solutions containing n-butanol in different concentrations. Adsorption parameters for n-butanol are determined using a regression analysis and compared with those for Hg, Tl–Ga, and Bi–Ga. As follows from this comparison, though there is no chemisorption interaction between these metals and water, the energy of n-butanol adsorption at these metals depends on the metal nature. The Pb–Ga data fit a common correlation dependence of the electronic capacitance of different electrodes (C _m ^–1)_phys on the energy of the n-butanol molecules adsorption thereon in the absence of a metal–water chemisorption interaction. This finding evidences that the dependence of the energy parameters for the adsorption of organic compounds on the metals' electronic properties, when characterized by (C _m ^–1)_phys, is of general nature.     

Langmuir monolayers as models to study processes at membrane surfaces

The use of new sophisticated and highly surface sensitive techniques as synchrotron based X-ray scattering techniques and in-house infrared reflection absorption spectroscopy (IRRAS) has revolutionized the monolayer research. Not only the determination of monolayer structures but also interactions between amphiphilic monolayers at the soft air/liquid interface and molecules dissolved in the subphase are important for many areas in material and life sciences. Monolayers are convenient quasi-two-dimensional model systems. This review focuses on interactions between amphiphilic molecules in binary and ternary mixtures as well as on interfacial interactions with interesting biomolecules dissolved in the subphase. The phase state of monolayers can be easily triggered at constant temperature by increasing the packing density of the lipids by compression. Simultaneously the monolayer structure changes are followed in situ by grazing incidence X-ray diffraction or IRRAS. The interactions can be indirectly determined by the observed structure changes. Additionally, the yield of enzymatic reaction can be quantitatively determined, secondary structures of peptides and proteins can be measured and compared with those observed in bulk. In this way, the influence of a confinement on the structural properties of biomolecules can be determined. The adsorption of DNA can be quantified as well as the competing adsorption of ions at charged interfaces. The influence of modified nanoparticles on model membranes can be clearly determined. In this review, the relevance and utility of Langmuir monolayers as suitable models to study physical and chemical interactions at membrane surfaces are clearly demonstrated.     

Effect of alcohols on the micellar properties in aqueous solution of alkyltrimethylammonium bromides

The effect ofn-butanol,n-propanol, andn-hexanol on the critical micelle concentration (CMC) and degree of ionisation of the micelles of dodecyl-, tetradecyl- and hexadecyltrimethylammonium bromides in aqueous solution has been determined by conductimetric techniques. Increase of the molality of added alcohol over the concentration ranges examined (up to 0.3 mol kg^–1 butanol, 0.07 mol kg^–1 pentanol and 0.025 mol kg^–1 hexanol) caused a progressive decrease of CMC and increase of the degree of ionisation for each surfactant-alcohol system. At a constant molality of added alcohol the degree of ionisation increased with a) an increase of the chain length of the surfactant for each alcohol and b) an increase of the chain length of the alcohol for each surfactant. The distribution of each alcohol between the aqueous and micellar phases and the free energy of solubilization were determined from the change of CMC with molality of added alcohol.