Phase transition and elastic constants of 4-n-octyl-4′-cyanobiphenyl (8CB) obtained using the light-scattering anisotropy of turbidity

Abstract

A light-scattering technique was used to study the anisotropy of turbidity and the three elastic constants K ₁, K ₂ and K ₃ of 8CB as a function of temperature and sample thickness. The turbidity was measured in the nematic and schematic A phases at sample thicknesses l of 0.02, 0.04, 0.1 and 0.2 cm. The effect of the smectic-like (cybotactic nematic) order was observed near the smectic A-nematic phase transition. Owing to the surface-enhanced cybotactic order, evaluation of the elastic constants and order parameter was possible only from the turbidity data at l = 0.2 cm. From the divergence of both K ₂ and K ₃ near T_{S A N} we estimated an average critical exponent value v of 0.65, suggesting that S_A-N in 8CB is a second-order phase transition. The magnetic-field quenching of director fluctuations showed observed effects on the order of magnitude of the temperature dependence of the turbidities, elastic constants and order parameter.     

Molecular dynamics and alignment studies of silica-filled 4-pentyl-4′-cyanobiphenyl (5CB) liquid crystal

A comprehensive study of the dielectric properties of 4-pentyl-4′-cyanobiphenyl (5CB) liquid crystal filled with silica particles (particle size 30–80 nm, concentration 2, 3, 5, 10 and 15 wt%). Dielectric spectroscopy in the frequency range 100 to 10 7 Hz was applied to investigate the influence of the filler on the dynamic behaviour of the liquid crystal molecules in both the nematic and isotropic phases. In this frequency range one relaxation process is observed (at f>10⁶ Hz). The dynamical behaviour of the 5CB liquid crystal is described by the Cole-Cole relaxation function. The temperature dependence of the relaxation time obeys the empirical Arrhenius equation. The activation energies are approximately 75 kJ mol¹ for the pure 5CB sample in the nematic phase and 50 kJ mol¹ for the 5 wt% silica-filled 5CB sample. These values are compared with the corresponding literature values. The reversible electro-mechanical response of these samples under the influence of an applied a.c. electric field is investigated.     

Mutual influence of cetyltrimethylammonium bromide and Triton X-100 on their adsorption at the water–air interface

On the basis of surface tension values of the aqueous solution of cetyltrimethylammonium bromide (CTAB) and Triton X-100 (TX-100) mixtures measured at 293 K as a function of CTAB or TX-100 concentration at constant TX-100 or CTAB concentration, respectively, the real surface area occupied by these surfactants at the water–air interface was established which is inaccessible in the literature. It appeared that at the concentration of the CTAB and TX-100 mixture in the bulk phase corresponding to the unsaturated monolayer at the water air-interface this area is the same as in the monolayer formed by the single surfactant at the same concentration as in the mixture. In the saturated mixed monolayer at this interface the area occupied by both surfactants is lower than that in the single surfactant monolayer corresponding to the same concentration in the aqueous solution. However, the decrease of the CTAB adsorption is lower than that of TX-100 and the total area occupied by the mixture of surfactants is also lower than that of the single one. The area of particular surfactants in the mixed saturated monolayer changes as a function of TX-100 and CTAB mixture concentration and at the concentrations close to CMC or higher the area occupied by both surfactants is the same. The changes of the composition of the mixed surface monolayer are connected with the synergetic effect in the reduction of the water surface tension by the adsorption of CTAB and TX-100 at the water–air interface. This effect was confirmed by the values of the standard Gibbs free energy of adsorption of both individual surfactants and their mixtures with different compositions in the bulk phase determined by using the Langmuir equation if RT instead of nRT was applied in this equation.     

Large scale molecular dynamics simulations of a 4-n-pentyl-4′-cyanobiphenyl (5CB) liquid crystalline model in the bulk and as a droplet

Molecular dynamics simulations for 4-n-pentyl-4′-cyanobiphenyl (5CB) with as many as 944 molecules are reported. The order-N fast multipole method (FMM) is used to treat the long-range interactions. For a droplet of 944 molecules, the simulation shows a correlation between the droplet shape and the nematic order and a strong surface effect; little nematic order is found in a 118 molecule droplet. Simulations of the bulk system result in similar order parameters for both the 118 and 944 molecular ensembles. Although the nematic–isotropic transition was not observed at temperatures as high as 400 K using the CHARMM force field, a modification of the force field using ab initio determined partial atomic charges lowers the order parameters.     

Monolayers of γ-globulin at the air–water interface

Monolayer formation, of γ-globulin at the air–water interface has been investigated under varying subphase compositions. At pH 7.4, it is found that a stable monolayer is obtained only when the ionic strength is greater than 0.5 M. The magnitude of the collapse pressure increases with increasing ionic strength of the subphase. These data are analyzed in comparison to the literature data. Received: 2 March 1999 Accepted in revised form: 16 June 1999     

Molecular organization and aggregation of mucin at air–water interface in the presence of chromium(III) complexes

The interfacial organization of mucin (glycoprotein) in the presence of chromium(III) complexes has been assessed from the surface pressure–molecular area (π–A) isotherms in Langmuir films at air–water interface and the surface energy of their LB films through contact angle measurements. At pH 7.0, the electrostatic interaction of [Cr(en)₃]Cl₃ with mucin was found to bring about changes in the average surface area from 3.26 to 1.47 nm²; suggesting the possible formation of large aggregates of mucin. Adsorption experiments using surface potential measurements reveal that [Cr(en)₃]Cl₃ binds at a much faster rate to the available binding sites in mucin when compared to [Cr(salen)(H₂O)₂](ClO₄) which binds coordinatively to mucin.     

Nanoplates and nanostars of β-PbO formed at the air/water interface

Nanoparticles with different shapes were prepared at the air/water interface via hydrolysis of Pb²⁺ ions under Langmuir films of poly(N-vinylcarbazole) (PVK) at 30–50 °C. It was found that round or irregular nanoparticles with the size of several to several tens of nanometers were formed when the PbCl₂ aqueous solution with the concentration of 1 × 10^?3 mol L^?1 was used as subphase, while single-crystalline quasi-hexagonal nanoplates, nanostars and dendrites with the size of several hundreds of nanometers were obtained when the subphase concentration was 1 × 10^?4 mol L^?1. Analysis on the selective-area electron diffraction (SAED) patterns revealed that the formed nanoparticles are β-PbO. The formation of the nanostructures should be attributed to the formation and dehydration of lead hydroxide, diffuse-limited growth and aggregation of nanoparticles at the air/water interface.     

Properties of lipophilic nucleoside monolayers at the air–water interface

The capability of self-assembly and molecular recognition of biomolecules is essential for many nanotechnological applications, as in the use of alkyl-modified nucleosides and oligonucleotides to increase the cellular uptake of DNA and RNA. In this study, we show that a lipophilic nucleoside, which is an isomer mixture of 2′-palmitoyluridin und 3′-palmitoyluridin, forms Langmuir monolayers and Langmuir–Blodgett films as a typical amphiphile, though with a smaller elasticity. The nucleoside may be incorporated into dipalmitoyl phosphatidyl choline (DPPC) monolayers that serve as a simplified cell membrane model. The molecular-level interactions between the nucleoside and DPPC led to a remarkable condensation of the mixed monolayer, which affected both surface pressure and surface potential isotherms. The morphology of the mixed monolayers was dominated by the small domains of the nucleoside. The mixed monolayers could be deposited onto solid substrates as a one-layer Langmuir Blodgett film that displayed UV–vis absorption spectra typical of aggregated nucleosides owing to the interaction between the nucleoside and DPPC. The formation of solid films with DNA building blocks in the polar heads may open the way for devices and sensors be produced to exploit their molecular recognition properties.     

Voltammetric characterisation of polyelectrolyte adsorption/transfer at the water∣1,2-DCE interface

The voltammetric characterisation of aqueous soluble polyelectrolytes at the water∣1,2-dichloroethane interface was investigated. The polyelectrolytes studied included poly (diallyldimethylammonium chloride) (PDADMACl) and polyethylenimine (PEI). The adsorption process followed by the transfer of these polyelectrolytes across the interface was characterised. The observable transfer of the monomer cation of the PDADMA⁺, namely diallyldimethylammonium (DADMA⁺) ion, was compared to that of the polyelectrolyte transfer process. Physical data including the diffusion coefficient and the Gibbs energy of transfer across the water∣1,2-dichlorethane interface was evaluated for both the polycation and monocation.     

Infrared and Raman spectroscopies of monolayers at the air–water interface

Infrared and Raman spectroscopies are now currently used to obtain molecular information (orientation, conformation, organization) on monolayers at the air–water interface. In the past year, several original studies were performed on peptides and proteins and their interaction with phospholipidic monolayers.     

Dilational viscoelasticity of anionic polyelectrolyte/surfactant adsorption films at the water–octane interface

In this article, the interfacial tension and interfacial dilational viscoelasticity of polystyrene sulfonate/surfactant adsorption films at the water–octane interface have been studied by spinning drop method and oscillating barriers method respectively. The experimental results show that different interfacial behaviors can be observed in different type of polyelectrolyte/surfactant systems. Polystyrene sulfonate sodium (PSS)/cationic surfactant hexadecanetrimethyl–ammonium bromide systems show the classical behavior of oppositely charged polyelectrolyte/surfactant systems and can be explained well by electrostatic interaction. In the case of PSS/anionic surfactant sodium dodecyl sulfate (SDS) systems, the coadsorption of PSS at interface through hydrophobic interaction with alkyl chain of SDS leads to the increase of interfacial tension and the decrease of dilational elasticity. For PSS/nonionic surfactant TX100 systems, PSS may form a sub-layer contiguous to the aqueous phase with partly hydrophobic polyoxyethylene chain of TX100, which has little effect on the TX100 adsorption film and interfacial tension.     

Electron beam irradiation-induced transformations in the electrical properties of 4’-octyl-4-cyanobiphenyl (8CB)

Changes in the dielectric and thermodynamical properties of electron beam-irradiated 4′-octyl-4-cyanobiphenyl (8CB) were studied. Irradiation-induced changes in the phase transition temperature, dielectric anisotropy, relaxation frequency and activation energy of an observed non-collective relaxation mode corresponding to molecular rotation about the short axis were determined in both nematic and smectic A_d phases. In the nematic phase, dielectric anisotropy increased for a small dose but decreased for a relatively high dose, whereas the relaxation frequency increased due to the irradiation. The pure and irradiated samples were characterised by UV–visible spectroscopy, Fourier transform infrared spectroscopy, gas chromatography, gas chromatography coupled with mass spectroscopy and pulse radiolysis. The observed changes in the dielectric parameters are related to the detachment of the CN group from some of the 8CB molecules due to the electron beam irradiation.     

Interfacial rheology of polyelectrolytes and polymer monolayers at the air–water interface

In this review, we describe interfacial rheology studies of polymer monolayers at the air–water interface. Since polyelectrolytes are usually soluble in water, the formation of surface monolayers requires the presence of a surfactant of opposite charge. The first part of the review is dedicated to these mixed monolayers. The second part is related to neutral monolayers that can be either adsorbed or deposited at the interface. Interfacial rheology studies of these systems are still scarce, despite a considerable interest: insoluble polymer monolayers in two dimensions are suitable model systems for the tests of polymer theories in two dimensions, such as and glass transition. The rheology of soluble polymer monolayers has important connections with the dynamic properties of dispersions stabilized with these polymers.     

pH-Driven adsorption and desorption of fatty acid at the liquid crystal–water interface

The pH-driven adsorption and desorption of fatty acid monolayers at the liquid crystal (LC)–water interface were studied. We doped fatty acids (stearic acid, palmitic acid, myristic acid, dodecanoic acid, and decanoic acid) into 4-cyano-4′-pentylbiphenyl (5CB), and employed sessile LC droplets as our experimental platform. Under a crossed polariser, the LC droplets displayed a bright flower bud-shaped texture at low pH, whereas at high pH, they exhibited a bright four-brush appearance due to desorption of the adsorbed fatty acids at the LC–water interface. Furthermore, we identified the critical transition pH of various concentrations of stearic acid and other fatty acids featuring distinct tail lengths. Based on the interfacial behaviour, we propose a new method to estimate the pK_a of fatty acids, which opens up new possibilities for simple, precise, and reliable measurement of the pK_a of other carboxylic acids. The findings presented herein will greatly facilitate the understanding of the interfacial behaviour of amphiphiles at the oil–water interface.     

Interfacial behaviour of wheat puroindolines: monolayers of puroindolines at the air–water interface

Puroindolines are among the major basic and cysteine-rich lipid binding proteins of wheat seeds. The interfacial properties of puroindoline-a (PIN-a) and puroindoline-b (PIN-b) are important both from a biological and a technological point of view. In the work reported here, the interfacial characteristics of spread monolayers of wheat puroindolines at the air–water interface were studied at varying subphase compositions using a Langmuir–Blodgett film balance. The compression isotherms (π–A _Sp) were recorded at constant barrier speed (3.3 cm/min). It was observed that both PIN-a and PIN-b form stable monolayers at the air–water interface. The stability of the monolayers was found to be dependent on the subphase composition as well as on the concentration of protein in the spreading solution. When the ionic strength of the subphase is below 0.50, the compression isotherms of both PIN-a and PIN-b remains unaffected with the change in the ionic strength of the subphase; however, when the ionic strength is above 0.50, the compression isotherms of both PIN-a and PIN-b undergo significant change with an increase in the ionic strength of the subphase. A gradual increase in the values of the collapse pressure (π_C) and the limiting area (A ₀) was observed due to an increase in the ionic strength of the subphase from 0.5 to 4.0, which may be correlated with the salt-induced conformational changes of the protein molecule. The presence of NaCl and KCl (ionic strength 1.0) in the subphase has a comparable effect on the compression isotherms of both PIN-a and PIN-b; however, the presence of CaCl₂ (ionic strength 1.0) in the subphase leads to an increase in the values of π_C and A ₀. A change in the pH of the subphase from 3.0 to 7.2 was to have a significant effect on the values of π_C and A ₀, which may be due to the pH-induced alteration of the protein conformation. Received: 8 August 2000 Accepted: 15 December 2000     

Coadsorption of sodium dodecyl sulfate and medium-chain alcohols at the air–water interface

The adsorption of sodium dodecyl sulfate (SDS) and n-dodecanol from aqueous solutions of the pure and mixed surfactants at the air–water surface is studied by equilibrium surface pressure measurements, surface pressure transients and Brewster angle microscopy. The adsorption layers of SDS and n-dodecanol show fundamental differences in the phase behaviour. The adsorption parameters of both components are determined. Under appropriate conditions, a phase transition at which condensed phase textures are formed, occurs in the adsorption layers of n-dodecanol. The adsorption layers of surface-chemically purified SDS exist only in a fluid-like state without a phase transition under formation of condensed phase domains. Coadsorption of both surfactants is only investigated in the range of trace amounts of n-dodecanol. Depending on the mixing ratio and the system conditions (bulk concentration, temperature), a phase transition can or cannot occur. At absence of a phase transition, comparable surface concentrations of both components are calculated based on a orthogonal collocation solution for a two-component system. The adsorption properties are completely different when a phase transition occurs. Condensed phase domains of n-dodecanol formed, after the phase transition point, grow finally to a homogeneous condensed phase which replaces completely SDS.     

Photochemical behaviour of polyacryloylacetone;and polyethylacrylacetate monolayers at the air–water interface

The surface organization of enol units of polyacryloylacetone (PAA) and polyethylacrylacetate (PEAA) monolayers at the air–water interface is examined using surface pressure, surface potential and rheological measurements and theoretical calculations based on molecular models. The mechanism and kinetics of the photochemical enol–keto tautomerization of PAA and PEAA polymers organized in a monolayer of closely packed monomer units are studied by measuring the surface area increase at constant surface pressure. The results indicate an increase in the area per unit during the consecutive enol-to-keto photoconversion and the slow interfacial reorganization of these ¶forms to a more favourable state.     

Unified molecular view of the air/water interface based on experimental and theoretical χ(2) spectra of an isotopically diluted water surface

The energetically unfavorable termination of the hydrogen-bonded network of water molecules at the air/water interface causes molecular rearrangement to minimize the free energy. The long-standing question is how water minimizes the surface free energy. The combination of advanced, surface-specific nonlinear spectroscopy and theoretical simulation provides new insights. The complex χ((2)) spectra of isotopically diluted water surfaces obtained by heterodyne-detected sum frequency generation spectroscopy and molecular dynamics simulation show excellent agreement, assuring the validity of the microscopic picture given in the simulation. The present study indicates that there is no ice-like structure at the surface--in other words, there is no increase of tetrahedrally coordinated structure compared to the bulk--but that there are water pairs interacting with a strong hydrogen bond at the outermost surface. Intuitively, this can be considered a consequence of the lack of a hydrogen bond toward the upper gas phase, enhancing the lateral interaction at the boundary. This study also confirms that the major source of the isotope effect on the water χ((2)) spectra is the intramolecular anharmonic coupling, i.e., Fermi resonance.     

Ordered supramolecular assembly of bis[3,4,12,13,21,22,30,31-octa(dodecylthio)-2,3-naphthalocyaninato]erbium at the air/water interface

The supramolecular assembly of bis[3, 4, 12, 13, 21, 22, 30, 31-octa(dodecylthio)-2, 3-naphthalocyaninato] erbium complex (Er[Nc(SC12H25)8]2) was fabricated at the air/water interface by Langmuir monolayer technique. Surface pressure-area isotherm indicates that stable monolayer is formed. The monolayer morphology on mica was investigated by atomic force microscopy (AFM). Columnar domains of ca. 100 nm ×(20-30) nm appearing in the micrograph have ordered orientation. Polarized UV-vis spectra and small-angle X-ray diffraction pattern show that the macrocycles tilt in the monolayer. The orientation angle of the naphthalocyanine ring with respect to the substrate was found to be ca. 54°. Each monolayer is ca. 3.53 nm thick. The columnar domain comprises 4-6 rows of columnar supramolecular assemblies where the molecules stack face-to-face.     

Flow microcalorimetry of adsorption of anionic alkyl surfactants at the solid/liquid interface

Flow microcalorimetry was used to study the adsoption of anionic alkyl surfactants from aque--ous solutions onto silica. It is found that for alkyl sulfate systems the strength of adsorption interactionincreases with increases of the alkyl chain length and decreases as temperature rises. The adsorptiondepends only on monomer concentration of the solution even above the critical micelle concentration(cmc). The assumption is made that the adsorption involves only a transfer of monomers from bulkto surface phase. A different adsorption mechanism is operative for the alkyl carboxylate.