Iso-branched semifluorinated alkanes in Langmuir monolayers

A series of semifluorinated n-alkanes (SFAs), of the general formula: (CF3)2CF(CF2)6(CH2)nH (in short iF9Hn), n = 11-20 have been synthesized and employed for Langmuir monolayer characterization. Surface pressure and electric surface potential measurements were performed in addition to Brewster angle microscopy results, which enabled both direct visualization of the monolayers structure and estimation of the monolayer thickness at different stages of compression. Our paper was aimed at investigating the influence of the iso-branching of the perfluorinated fragment of the SFA molecule on the surface behavior of these molecules at the air/water interface. It occurred that iF9 SFAs with the number of carbon atoms in the hydrogenated moiety from 11 to 20 are capable of Langmuir monolayer formation. Monolayers from iF9H11 to iF9H13 are instable, whereas those formed by iF9 SFAs with longer hydrogenated chains form stable films at the free surface of water. As compared to SFAs containing perfluorinated chain in a normal arrangement, iso-branched molecules have a greater tendency to aggregate. Lower stability of monolayers formed by iF9 SFAs as compared to F10 SFAs originated from the surface nucleation observed in BAM images, even at the very initial stages of compression. The dipole moment vector for iso-branched SFAs was found to be virtually aligned with the main axis of the molecule, contrary to F10 SFAs, where the dipole moment vector was calculated to be tilted with respect to the main molecular axis. Quantitative Brewster angle microscopy measurements (relative reflectivity experiments) enabled us to monitor the changes of monolayer thickness at different stages of monolayer compression.     

Langmur monolayers of cerebroside with different head groups originated from sea cucumber: Binary systems with dipalmitoylphosphatidylcholine (DPPC)

Surface properties (Langmuir monolayer) of two different cerebrosides which are extracted from the sea cucumber (Bohadschia argus) were investigated. A main difference in chemical structure of cerebroside between BAC-2a and BAC-4 is their head groups (glucose and galactose, respectively). Furthermore, miscibility and interaction between dipalmitoylphosphatidylcholine (DPPC) and cerebrosides (BAC-2a and BAC-4) in the monolayer have been systematically examined. The surface pressure (π)−area (A), the surface potential (ΔV)−A, and the dipole moment (μ)−A isotherms for monolayers of DPPC, cerebrosides, and their binary combinations have been measured using the Wilhelmy method and the ionizing electrode method. BAC-4 forms a stable liquid-expanded (LE) monolayer, whereas BAC-2a has a first-order phase transition from the LE phase to the liquid-condensed (LC) state on 0.15 M NaCl at 298.2 K. The fundamental properties for each cerebroside monolayer were elucidated in terms of the surface dipole moment based on the three-layer model [R.J. Demchak, T. Fort Jr., J. Colloid Interface Sci. 46 (1974) 191–202] for both cerebrosides and the apparent molar quantity change (Δs^γ, Δh^γ, and Δu^γ) for BAC-2a. In addition, their miscibility with DPPC was examined by the variation of the molecular areas and the surface potentials as a function of cerebroside mole fractions, the additivity rule. The miscibility was also confirmed by constructing the two-dimensional phase diagrams. The phase diagrams for the both binary systems were of negative azeotropic type. That is, the two-component DPPC/BAC-2a and DPPC/BAC-4 monolayers are miscible. Furthermore, the Joos equation for the analysis of the collapse pressure of binary monolayers allowed calculation of the interaction parameter and the interaction energy between the DPPC and cerebroside monolayers. The miscibility in the monolayer state was also confirmed by the morphological observation with Brewster angle microscopy (BAM), fluorescence microscopy (FM), and atomic force microscopy (AFM).     

State,electrical and rheological properties of model and dioxan isolated lignin films at the air–water interface

The properties of monolayers of the dioxan lignin and of model dehydrogenation polymers spread at the air-water interface were investigated by the following techniques: surface pressure, surface potential, ellipticity, rheological analysis and molecular modeling. Information about the state of the monolayer was obtained from surface pressure-area and surface potential-area isotherms. At small deposited quantities (1-2 mgm^-2) a quasi-monolayer (2D) structure is formed. The values obtained for the mean area per monomer and for the mean dipole moment are coherent with previously reported data, as well as with the proposed molecular model. At higher deposited quantities (3-6 mgm^-2) a complex 3D network is built. These statements are supported by the results obtained by means of ellipsometry and surface rheology. Ellipsometry provides data for the stability of the 2D structure and for the slightly unstable 3D network. By using an original rheological approach, both structures are distinguished by their mechanical response when a dilatational stress is applied. The quasi-monolayer structure behaves as an elastic 2D medium during the compression, while two relaxation processes with characteristic times of 10 and 100 s were observed. The 3D network behaves as a Maxwell viscoelastic body during the compression, while three relaxation processes with characteristic times of 1, 10 and 100 s were detected.     

Semifluorinated alcohols in Langmuir monolayers--a comparative study

A series of semifluorinated alcohols differing in the proportion of the perfluorinated to hydrogenated chains length was synthesized and investigated in Langmuir monolayers using surface pressure and surface potential measurements. All the investigated semifluorinated alcohols were found to be capable of stable floating monolayer formation. The stability of monolayers was found to be higher upon increasing the length of the perfluorinated segment. A lower stability of the monolayers from alcohols having shorter perfluorinated fragment was attributed to the aggregation process, which was visualized with Brewster angle microscopy (BAM). Most condensed monolayers were formed by compounds with longer perfluorinated moiety, whereas monolayers composed by molecules with an iso-branched perfluorinated segment were found to be more expanded. The change of electric surface potential was negative along the whole compression. The maximum absolute values of DeltaV varies, depending on the number of CF(2) groups, from ca. -400 mV for F6H10OH to ca. -700 mV for F10H10OH. The dipole moments of free molecules were calculated with Hyperchem, and the obtained values were approximately the same (within the experimental error), i.e., 2.8D for all the investigated molecules, independently on the perfluorinated fragment length. The dipole moment vector was found to be virtually aligned to the main molecular axis for the studied compounds. Therefore, the observed differences in the measured values of DeltaV can result from a different dielectric permittivity of a particular monolayer.     

Mixed Monolayers of Amphiphilic Cyclodextrins and Phospholipids: II. Surface Potential–Area Behavior of Per-(6-amino-2,3-di-O-hexyl) β-CD Hydrochloride Salt and 1,2 Dipalmitoyl, 3-sn-Phosphatidic Acid Mixed Monolayers

The dielectric properties of mixed monolayers of per-(6-amino-2,3-di-O-hexyl) β-CD hydrochloride (NH₃-β-CD-OC6) and 1,2 dipalmitoyl, 3-sn-phosphatidic acid (DPPA) have been assessed using surface potential measurements at constant area. From the comparison of these surface potential (ΔV) versus surface density (δ) relationships with those of surface pressure (π) against surface density (δ) it was apparent that the increase in the NH₃-β-CD-OC6 content in mixed films gave rise to a gradual increase in the saturation value of the surface potential (ΔV_max). This potential for pure DPPA was found to be equal to 396 mV and for pure CD 554 mV. The ΔV_maxvalues reflect the onset of reorientation effects that arrive at molar areas before the collapse of these films. Independently of reorientation effects, the obtained results strongly indicate that the dipolar term contributing to the overall ΔVvalue was for NH₃-β-CD-OC6 due to the hydration of its NH⁺₃group. For both DPPA and NH₃-β-CD-OC6 molecules the contribution of the electric double layer (Ψ) was calculated and was found for DPPA and NH₃-β-CD-OC6 to be equal to −249 and +252 mV, respectively. These calculated Ψ values made possible the evaluation of dipole moments for NH₃-β-CD-OC6 and DPPA monolayers which revealed a marked difference in dipolar properties between these two film forming components. In contrast to DPPA which exhibited a decrease in the surface dipole moment (μ) with the decrease inA, NH₃-β-CD-OC6 displayed an increase in μwith the decrease inAforAvalues above 580 Ų. Below this value μdecreases with decreasing molecular area and this variation arises from a change in the polarity of the electric double layer arising from interactions with the complementary anion. The differences in dielectric properties between the two film forming molecules have been attributed to modification, during compression, in the structure of the interfacial water bound to the cyclodextrin.     

Dielectric properties of 1,1,1-trifluoroethane (HFC-143a) in the liquid phase

The relative permittivity (?_r) data of 1,1,1-trifluoroethane (HFC-143a), (CAS N# 420-46-2), a hydrofluorocarbon (HFC) developed as a refrigerant that has zero ozone depletion potential, is reported. The relative permittivity of HFC-143a in the liquid phase was measured using a direct capacitance method at temperatures from T = 218 to 294 K and at pressures up to P = 15 MPa, for a frequency of 10 kHz. The uncertainty of the ?_r measurements is estimated to be better than ±1.2 × 10⁻². A complete set of tables of experimental data as a function of temperature, pressure and density, is presented that covers the dielectric property needs for most engineering applications. To study the dependence of ?_r on density and temperature on a molecular basis, the theory developed by Vedam et al. and adapted by Diguet was applied to analyse the data. The Kirkwood modification of the Onsager equation was used to obtain the value of its dipole moment in the liquid phase (μ*). The apparent dipole moment obtained was μ* = 3.293 D. The effective dipole in the liquid state predicted by the Kirkwood–Frölich theory is 2.530 D. The measured values are compared with density functional and density functional self-consistent calculations (SCIPCM) of the electronic distribution and of the dipole moment of HFC-143a. Finally, the values of the isobaric thermal expansion and isothermal compressibility were estimated from the reported measurements.     

The effect of molecular polarity on nem/catic phase stability in 12-vertex carboranes

Weakly polar–polar isosteric pairs of 12-vertex p-carborane [closo-1,12-C₂B₁₀H₁₂] (1[12]) and monocarbaborate [closo-1-CB₁₁H₁₂]^? (2[12]) nematic liquid crystals, in which the difference in the calculated molecular dipole moment is 11.3 D, were synthesised, and the effect of the dipole moment on nematic phase stability was investigated. The trend observed for the 12-vertex series ([12]) was identical to that of the previously investigated 10-vertex series ([10]) containing [closo-1,10-C₂B₈H₁₀] (1[10]) and [closo-1-CB₉H₁₀]^? (2[10]): the uniform increase in the molecular dipole moment in the pairs of mesogens does not correspond to a uniform change in the clearing temperature, T_NI. This demonstrates the role of a remote substituent in modulating the intermolecular dipole–dipole interactions. The magnitude of such interactions was calculated (using density functional theory methods) for a pair of polar (2[12]d–2[12]d) and an analogous pair of weakly polar (1[12]d–1[12]d) molecules. All results for the 12-vertex series ([12]) were analysed relative to the 10-vertex analogues ([10]).     

Relationship between the structure of compounds and the effect of Pluronic L61 on their permeation through lipid membranes

In view of the earlier observation that the incorporation of Pluronics in lipid membranes enhances their permeability to various drugs, the relationship between the structure of a permeant and the Pluronic-induced acceleration of its membrane transport was studied. The extent of acceleration defined as the ratio of the apparent permeation rate constants in the presence and in the absence of Pluronic, k/k ₀, was determined for 21 weak acids and bases. Multiparameter correlations of the obtained data with various structural characteristics of the solutes transferable were constructed. Twelve structural parameters of the permeants were examined. The best correlation was achieved with a linear combination of the three parameters: the McGowan volume V of a compound, its proton-donating ability A, and the projection of the dipole moment of the molecule onto the normal to the membrane surface μ_Z, ln(k/k ₀) = ?0.87-0.44μ_Z + 0.31 V + 0.28A, R = 0.90. This means that Pluronic preferably accelerates the permeation of large molecules containing proton-donating groups and molecules incorporated in the lipid bilayer in such a manner that the vector of their dipole moment is aligned with the dipole potential of the membrane.     

Dielectric data of binary mixtures of 1,2-butanediol with 2-ethyl-1-hexanol and 1,4-dioxane at T = (298.2, 308.2, and 318.2) K

Relative permittivity measurements were made on binary mixtures of (1,2-butanediol + 2-ethyl-1-hexanol) and (1,2-butanediol + 1,4-dioxane) for various concentrations at T = (298.2, 308.2, and 318.2) K. The molecular dipole moments were determined using Guggenheim–Debye method in the temperature range of (298.2 to 318.2) K. The variations of effective dipole moment and correlation factor, g, with the mole fraction in these materials were investigated using Kirkwood–Frohlich equation. The pure compounds showed a negative and small temperature coefficient of effective dipole moment. In order to obtain valuable information about heterogeneous interaction (interactions between the unlike molecules), the Kirkwood correlation factor, the Bruggeman dielectric factor and the excess permittivity were calculated. In addition, in order to predict the permittivity data of polar-apolar binary mixtures, five mixing rules were applied.     

Induced Collapse in the Formation of Alternating Langmuir–Blodgett Films

Two compounds whose molecules have large dipole moments, strereoregular perfluorinated alkylmethacrylate-co-methacrylic acid (A) and p-octadecylaminoazobenzene-p"-sulfamide (B), were used to assemble Langmuir–Blodgett polar films by the alternating method to achieve high polarization. According to estimates, the dipole moment of the dimeric unit of copolymer A is equal to about 4 D, the dipole moment of compound B, to 12 D. Unusually high (several units) transfer ratio was observed for the monolayer B when preparing the structures of (AB) _n type. The monolayers were stable during their formation process. Upon the pause of a substrate when passing through the monolayer B, the monolayer area remained strictly constant. According to the model proposed, the monolayer looses its stability and locally collapses in the meniscus zone at the contact line of the monolayer B and substrate due to an increase in the electrostatic repulsion between the dipoles of molecules B. The validity of this model is confirmed by the data of small-angle X-ray diffraction and atomic force microscopy for various alternating Langmuir–Blodgett films.     

Classification for molecular aggregation structure of monolayers on the water surface

The melting temperature, T_m, and the crystalline relaxation temperature, Tα_c, of palmitic acid and dipalmitoyl phosphatidylcholine monolayers on the water surface were evaluated by combination of two kinds of measurements: first, the subphase temperature, T_sp, dependence of the monolayer modulus based on the surface pressure-area (π-A) isotherm and second, the T_sp dependence of the electron diffraction, ED patterns of their monolayers. On the basis of their characteristic temperatures of the monolayers, the aggregation structure of the monolayers which were transferred onto a hydrophilic SiO substrate at various surface pressures and T_sps was investigated by means of transmission electron microscopy. The π-A isotherm for the fatty acid monolayer on the pure water surface represented the aggregating process of isolated domains grown right after spreading a solution on the pure water surface. The fatty acid monolayer on the pure water surface was classified into a crystalline monolayer (T_sp < T_m) and an amorphous one (T_sp > T_m). The crystalline monolayer was further classified into two types; crystalline domains were aligned along their crystallographic axes owing to an induced sintering at the interfacial region among monolayer domains by surface compression (T_sp < Tα_c), while not for T_sp > Tα_c. In the case of the phospholipid monolayer, the monolayer was classified into a compressing crystallized monolayer (T_sp < T_m) and an amorphous one (T_sp > T_m). The compressing crystallized monolayer is a monolayer in which crystallization was gradually induced at plateau region on the π-A isotherm by compression. Electron diffraction studies of arachidic acid monolayers in different dissociated states of hydrophilic groups revealed that formation of the compressing crystallized monolayer was attributed to an electrostatic repulsion among ionic hydrophilic groups. It was concluded that the aggregation structure of monolayers on the water surface was systematically classified into ‘the crystalline monolayer’, ‘the amorphous monolayer’ and ‘the compressing crystallized monolayer’, with respect to thermal and chemical (intermolecular repulsive) factors.     

The collision-induced electric dipole moment of H(n=2) in H-He,Ne and Ar collisions

The collision-induced electric dipole moment of H(n=2) in H(1s)-Ne and H(1s)-Ar collisions was measured in an energy range of 1 to 25 keV. For these systems we observe a positive electric dipole moment which corresponds to an electron lagging behind the proton. This behaviour is in contrast to recent measurements for the H-He systems, where a negative dipole moment corresponding to an electron moving in front of the proton was observed. A simple explanation for this difference is given.     

Molecular properties of acetone and some of its chloroderivatives adsorbed at the surface of water

The relations between the electric surface potential (V) and the surface tension () of aqueous solutions of acetone, chloroacetone, 1,3-dichloroacetone, and their concentration were investigated. The vertical components of dipole moments of the above mentioned compounds were determined using the Helmholtz equation. The calculations were carried out on the basis of surface excess values, which were obtained from surface tension measurements and surface potential changes. Once the vertical component of dipole moments were found and the orientation of adsorbed molecules was assumed, the local dielectric permittivities of the surface monolayer were estimated.     

Nitrogen vacancy and ferromagnetism in Cr-doped GaN: First-principles calculations

Nitrogen defects and their effect on the ferromagnetism (FM) in Cr-doped GaN have been systematically investigated by first-principles. Four considered configurations including one N vacancy (V_N), single substitutional Cr, double substitutional Cr, and complex of Cr-V_N are all ferromagnetic. The lowest energy arrangements for double Cr-doped (or Cr-V_N) GaN occur at the nearest Cr-Cr (or Cr-V_N) distance. One Cr contributes 84.3% of the total magnetic moment (2.533 μ_B), while one Cr-Cr pair leads to 5.998 μ_B moment (more than twice moment of one Cr) by the strong d-d spin coupling, which is mediated by two Cr 3d states antiferromagnetically coupling with the “commonly shared” N 2p states, and driven by a double exchange mechanism. The V_N can enhance the FM by adding about 1 μ_B to the Cr moment but reduce the FM spin exchange interaction between the nearest Cr-Cr pairs, so experimentally, high-performance samples may be synthesized by controlling N pressure.     

Langmuir monolayers from perfluorobutyl-n-eicosane

Perfluorobutyl-n-eicosane (abbreviated as F4H20) was spread at the air/water as Langmuir monolayers and studied under different experimental conditions, such as spreading volume, subphase temperature and compression speed. The Langmuir monolayer experiments (π-A isotherms) have been complemented with Brewster angle microscopy results, which enabled direct visualization of the monolayers’ structure and estimation of the film thickness at different stages of compression. It has been found that the molecules are oriented almost vertically (with respect to the interface) in the vicinity of film collapse. The negative sign of the measured surface potential, ΔV, is evidence for the orientation of F4H20 molecules with their perfluorinated parts exposed towards the air. In the case of F4H20 a limited fluorination relative to perfluoroeicosane also results in monolayer formation, in contrast to eicosane itself, which forms lenses.     

The Influence of Polysaccharides/TiO2 on the Model Membranes of Dipalmitoylphosphatidylglycerol and Bacterial Lipids

The aim of the study was to determine the bactericidal properties of popular medical, pharmaceutical, and cosmetic ingredients, namely chitosan (Ch), hyaluronic acid (HA), and titanium dioxide (TiO₂). The characteristics presented in this paper are based on the Langmuir monolayer studies of the model biological membranes formed on subphases with these compounds or their mixtures. To prepare the Langmuir film, 1,2-dipalmitoyl-sn-glycero-3-phospho-rac-(1-glycerol) (DPPG) phospholipid, which is the component of most bacterial membranes, as well as biological material-lipids isolated from bacteria Escherichia coli and Staphylococcus aureus were used. The analysis of the surface pressure-mean molecular area (π-A) isotherms, compression modulus as a function of surface pressure, C_S⁻¹ = f(π), relative surface pressure as a function of time, π/π₀ = f(t), hysteresis loops, as well as structure visualized using a Brewster angle microscope (BAM) shows clearly that Ch, HA, and TiO₂ have antibacterial properties. Ch and TiO₂ mostly affect S. aureus monolayer structure during compression. They can enhance the permeability of biological membranes leading to the bacteria cell death. In turn, HA has a greater impact on the thickness of E. coli film.     

Infrared Spectroscopic and dielectrometric investigation of the molecular geometry of isocyanato- and isothiocyanato-silane derivatives

The molecular geometry of compounds of the series (CH₃)_nSi(NCX)_4-n, (where n  0, 1, 2, 3 X  S, O) were investigated with the objective of determining the magnitudes of the SiNC angles. Measurement of the infrared spectra and dipole moments were carried out. The infrared spectra unambiguously suggest linearity, and this is supported by dipole moment calculations. The deviation from linearity observed by electron diffraction is a consequence of shrinkage effects δ(SiNCX) resulting from low frequency, large amplitude vibrations. From the dipole moment values it is concluded that the N—Si—N angle in the di- and trifunctional isothiocyanates is larger than the tetrahedral angle. The structural differences between the silicon and carbon compounds can be attributed to (dp)π bonding in the former.     

Pd gate MOS sensor for hydrogen detection

The performance of Pd gate MOS hydrogen sensor was studied using C–V and G–V characteristics. The device was fabricated on p-type <100> (1–6Ω cm.) silicon with thermal oxide layer of about 100 Å. The C–V and G–V responses of sensor were measured at different frequencies (1 kHz, 10 kHz, and 100 kHz) upon exposure to hydrogen (conc. 1–8%) at room temperature. It was observed that value of zero bias capacitance decreases with increase in frequency as well as hydrogen concentration. The inversion potential (V_inv.) and flat band voltage (V_FB) of the device approach higher values as frequency is reduced. Interface trap density (N_it) was also determined corresponding to the peak in the conductance curve, using a bias scan conductance method at fixed frequency. N_it was found to be decreasing with increasing concentrations of hydrogen. The sensor showed better sensitivity at lower frequency.     

Effects of lipid chain length on the surface properties of alkylaminomethyl rutin and of its mixture with model lecithin membrane

Three model flavonoid-based bioactive molecules with different lipid chain lengths (RuCn: n=8, 12, 18) were newly synthesized. The surface properties [surface pressure (π)-area (A), surface potential (ΔV)-surface pressure (π) and dipole moment (u(⊥))-surface pressure (π)] of pure RuCn and the lecithin membrane compounds had been investigated by using the Langmuir monolayer technology. The results suggested that the distinctive monolayer behavior of RuCn is strongly dependent on the lipid chain length. The great differences in the monolayer properties brought by the lipid chain length could be attributed to two major factors: (i) the ionization degree of the bulky hydrophilic head group (including hydroxyl and NH groups) alters its local field solely via the surface potential; (ii) tring molecular (or dipole) packing density within monolayers. The excess Gibbs energy (ΔG((ex))) calculated for the RuCn-lecithin mixed monolayers infers that higher stability of the mixed monolayer can be strengthened as the lipid chain length decreases. And the addition of RuCn into lecithin membrane may increase the total u(⊥) of the binary mixed monolayers, which could inhibit the hydration of the lecithin's hydrophilic head groups. The shorter the lipid chain length of RuCn (e.g., RuC8) is, the higher the surface activity can be. Our findings provide a molecular basis for the application of such class of biomolecules in the functional food, cosmetics and medicine.