Unexpected Si:Al effect on the binary mixtures liquid phase adsorption selectivities in faujasite zeolites

A batch technique was applied to determine the equilibrium adsorption of benzene from its benzene/octane and benzene/octene mixture on NaX (Si:Al 1.20) and NaY (Si:Al 2.79) in liquid phase. Benzene was preferentially adsorbed from both mixtures on NaX and NaY. Regardless whether octane or octene was present in the binary mixture, benzene was adsorbed more selectively on the high-silica NaY compared to NaX, which contained more cations. The presence of cations on the SIII/SIII' site inside the supercages of NaX causes a more difficult hosting of benzene inside the supercages of NaX and especially on the 12-membered ring site. However, at sufficiently high external benzene concentration (>10 mol %), the 12-membered ring site of NaX will be occupied by benzene, leading to a maximum adsorption capacity of five benzenes per supercage on both NaX and NaY. The double bond present inside octene allows the molecules to compete for the adsorption sites with benzene. The competitive effect of octene prevents the fifth benzene molecule to adsorb in the NaX supercage within the investigated concentration range.     

Confinement effect on phase transitions of a discotic liquid crystal: a calorimetric study

Differential scanning calorimetry was used to investigate the confinement effects on the phase transition behaviour of a discotic liquid crystal. The liquid crystal studied is the hexa-n-octanoate of rufigallol (RHO); Millipore membranes of various pore sizes were the confining materials. The polymorphism of RHO is affected by confinement. The transition from an enantiotropic columnar phase (D1) to a monotropic columnar phase (D2) is supressed in membranes with pore sizes 500 A. The transformation from D1 to the crystalline phase is also perturbed, particularly in the membrane having an average pore size of 250 A. In the first case the crystal formed displays a double-melting endotherm, with a distinct structure melting at lower temperatures; in the other, the induction period of isothermal crystallization becomes longer and the global rate of crystallization is slowed. However, confinement shows no effect on the overall crystallization mechanism; a similar Avrami constant of n ~ 3 was obtained for both confined and bulk RHO. An analysis of the results is presented.     

Confinement effect on phase transitions of a discotic liquid crystal: a calorimetric study

Differential scanning calorimetry was used to investigate the confinement effects on the phase transition behaviour of a discotic liquid crystal. The liquid crystal studied is the hexa-n-octanoate of rufigallol (RHO); Millipore membranes of various pore sizes were the confining materials. The polymorphism of RHO is affected by confinement. The transition from an enantiotropic columnar phase (D1) to a monotropic columnar phase (D2) is supressed in membranes with pore sizes 500 A. The transformation from D1 to the crystalline phase is also perturbed, particularly in the membrane having an average pore size of 250 A. In the first case the crystal formed displays a double-melting endotherm, with a distinct structure melting at lower temperatures; in the other, the induction period of isothermal crystallization becomes longer and the global rate of crystallization is slowed. However, confinement shows no effect on the overall crystallization mechanism; a similar Avrami constant of n ~ 3 was obtained for both confined and bulk RHO. An analysis of the results is presented.     

Polymer chain conformation in the phase separation process of a binary liquid mixture

A polymer chain conformation change near the critical point of liquid-liquid phase separation was investigated. Poly(N-isopropylacrylamide) labeled with a small amount of carbazolyl group for a fluorophore (P(NIPA-Cz)) was prepared. A ternary system of P(NIPA-Cz)+cyclohexane+methanol was investigated by the fluorescence spectroscopic technique. A mixed solvent of cyclohexane+methanol (CH/MeOH) shows phase separation at the upper critical solution temperature. Light scattering intensity, fluorescence emission intensity and fluorescence anisotropy ratio, as a function of temperature, were measured with quasi statically approaching to the critical demixing point. The fluorescence intensity of the carbazolyl groups attached to the polymer chain decreases with approaching to the critical temperature. This result suggests that the radius of gyration of the polymer decreases upon approaching to the critical demixing point of the solvent. We discuss the collapse and aggregation processes of the polymer based on the fluorescence quenching method. The rotational diffusion coefficient of carbazolyl groups attached to the polymer chain was estimated by the fluorescence depolarization technique. The rotational motion of carbazolyl groups is slowed down upon approaching the critical point.     

The effect of glass transition on phase separation in binary off-critical blends: different coarsening regimes near the nonequilibrium percolation threshold

The nonequilibrium percolation threshold was shown to take an intermediate position between binodal and mean-field spinodal. Below the nonequilibrium percolation threshold, a bicontinuous phase structure was produced. This percolation structure, depending on supersaturation, breaks down to ramified clusters slowly assuming a spherical droplet form or contracts to the center of the sample. In the latter case, at late stages, secondary phase separation is observed. Accepted: 21 September 1998     

Modelling adsorbent heterogeneity for adsorption from binary liquid mixtures

It is demonstrated that the detailed structure of the surface energy or selectivity distribution function is not critical to obtaining adequate analytical expressions for surface excess isotherms for adsorption from binary liquid mixtures on heterogeneous adsorbents. The gamma and the uniform selectivity distribution functions, which are very different in form, were successfully used in conjunction with the monolayer-pore filling model for adsorption on a homogeneous site to describe adsorption of various binary liquid mixtures on silica gel. Both models described the salient features of the surface characteristics of the silica gel.     

On the formation of multilayers in adsorption from binary liquid mixtures

Summary A new excess isotherm is derived for multilayer adsorption from binary liquid mixtures on solid surfaces. This is done by applying the Myers and Sircar's theoretical procedure, and assuming that the adsorption of single components is described by the BET isotherm equation, as modified by Brunauer, Skalny and Bodor. The derived excess equation is next generalized for the case of patchwise heterogeneous solid surfaces. The illustrative calculations presented here include both model investigations and an investigation of two real adsorption systems. It is drawn as a general conclusion that the heterogeneity effects in adsorption from solutions play a greater role than previously supposed.

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Zusammenfassung Eine neue Excessisotherme wurde für vielschichtige Adsorption aus Zweikomponentlösungen an festen Oberflächen abgeleitet. Das wurde mit Anwendung der theoretischen Methode von Myers und Sircar durchgeführt, wobei angenommen wurde, daß die Adsorption der einzelnen Komponenten durch die BET-Gleichung in der Modifizierung von Brunauer, Skalny und Bodor ausgedrückt wird. Die abgeleitete Excessgleichung wurde dann für heterogene feste Oberflächen generalisiert. Die Berechnungen berücksichtigen beide Untersuchungsmodelle und die Untersuchungen der zwei realen Adsorptionssysteme. Es wurde festgestellt, daß der Einfluß der Heterogenität in Adsorption von Lösungen eine größere Rolle spielt, als es vorher angenommen wurde.

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With 9 figures and 1 table     

Lateral phase separation in cholesterol/diheptadecanoylphosphatidylcholine binary bilayer membrane

We investigated the phase behavior of cholesterol/diheptadecanoylphosphatidylcholine (C17:0-PC) binary bilayer membrane as a function of the cholesterol composition (X(ch)) by fluorescence spectroscopy using 6-propionyl-2-(dimethylamino)naphthalene (Prodan) and differential scanning calorimetry (DSC). The fluorescence spectra showed that the wavelength at the maximum intensity (lambda(max)) changed depending on the bilayer state: ca. 440 nm for the lamellar gel ( [Formula: see text] or L(beta)) and the liquid ordered (L(o)) phases and ca. 490 nm for the liquid-crystalline (L(alpha)) phase. The transition temperatures were determined from the temperature dependence of lambda(max) and endothermic peaks of the DSC thermograms. Both measurements showed that the pre- and main transition disappear around X(ch)=0.05 and 0.30, respectively. The constructed temperature-X(ch) phase diagram resembled a typical phase diagram for a eutectic binary mixture containing a peritectic point. The presence of a peritectic point at X(ch)=0.15 suggested that a complex of cholesterol and C17:0-PC is stoichiometrically formed in the gel phase. Consideration based on the hexagonal lattice model revealed that the compositions of 0.05 and 0.15 correspond to the bilayer states where cholesterol molecules are regularly distributed in different ways. The former is nearly equal to the composition for the membrane occupied entirely with Units (1:18), composed of a cholesterol and 18 surrounding C17:0-PC molecules within the next-next nearest neighbor sites. The latter is represented by a Unit (1:6), including a cholesterol and 6 surrounding C17:0-PC molecules. Further, the disappearance of the main transition at X(ch)=0.30 indicates that the pure L(o) phase can exist in X(ch)>0.30. The eutectic behavior observed in the phase diagram was explainable in terms of phase separation between two different types of regions with different types of regular distributions of cholesterol.     

Conditions of phase separation, both at the interface (occasionally considered as a monolayer) and in solution for a binary liquid vapor system: The adsorption isotherm and the consequences of critical phenomena on the behavior of the system

The effects of modifications in the bulk and superficial phases of a binary mixture in equilibrium with its vapor, on the form of the surface adsorption isotherm, have been studied, and in particular the influence of deviations from ideality and of temperature. One observes, according to the value of the latter as compared to the critical temperatures of the surface and in the solution that a phase separation is possible, which may lead to a system evolving in a different manner from one simply separating into two phases. The monolayer model was used in a part of the work to describe the surface phase.     

The [BMI][Tf2N] ionic liquid/water binary system: a molecular dynamics study of phase separation and of the liquid-liquid interface

We report molecular dynamics (MD) simulations of the aqueous interface of the hydrophobic [BMI][Tf2N] ionic liquid (IL), composed of 1-butyl-3-methylimidazolium cations (BMI+) and bis(trifluoromethylsulfonyl)imide anions (Tf2N-). The questions of water/IL phase separation and properties of the neat interface are addressed, comparing different liquid models (TIP3P vs TIP5P water and +1.0/-1.0 vs +0.9/-0.9 charged IL ions), the Ewald vs the reaction field treatments of the long range electrostatics, and different starting conditions. With the different models, the "randomly" mixed liquids separate much more slowly (in 20 to 40 ns) than classical water-oil mixtures do (typically, in less than 1 ns), finally leading to distinct nanoscopic phases separated by an interface, as in simulations which started with a preformed interface, but the IL phase is more humid. The final state of water in the IL thus depends on the protocol and relates to IL heterogeneities and viscosity. Water mainly fluctuates in hydrophilic basins (rich in O(Tf2N) and aromatic CH(BMI) groups), separated by more hydrophobic domains (rich in CF3(Tf2N) and alkyl(BMI) groups), in the form of monomers and dimers in the weakly humid IL phase, and as higher aggregates when the IL phase is more humid. There is more water in the IL than IL in water, to different extents, depending on the model. The interface is sharper and narrower (approximately 10 A) than with the less hydrophobic [BMI][PF6] IL and is overall neutral, with isotropically oriented molecules, as in the bulk phases. The results allow us to better understand the analogies and differences of aqueous interfaces with hydrophobic (but hygroscopic) ILs, compared to classical organic liquids.     

Relaxation processes in mixtures of liquid crystals and polymers near phase boundaries and during phase separation

We present experimental studies of the relaxation of concentration fluctuations in a semidilute solution of polystyrene (PS) (30% by weight) in 4-cyano-4'-n-octyl-biphenyl (8CB) (70% by weight) using the photon correlation spectroscopy (PCS). In the homogeneous phase there are two modes of relaxation. The slow one (typical time scale is taus = 0.001 s) is due to the diffusion of polymer chains (of molecular mass 65,000) in the LC matrix (of molecular mass 290), while the fast one has the time scale of the order of tauf approximately 0.00001 s. The amplitude of the fast mode is much weaker than the one for the slow mode. Moreover it does not depend on the scattering wave vector, q. The value of the diffusion coefficient, Dc = 1/(tausq2) for the slow mode decreases with temperature according to the Arhenius law until we reach the coexistence curve. Its value close to the coexistence is Dc = 4 x 10(5) nm2/s and the activation energy in the homogeneous mixture is Ec=127 kJ/mol. If we gradually undercool the mixture below the coexistence into the metastable two-phase region without inducing the phase separation we find unexpectedly that Dc does not change with temperature even 4 degrees below the coexistence curve. The characteristic time of the fast mode does not depend on the scattering wave vector indicating that it is related to the transient gel structure. We have shown that it is possible to measure the short time relaxation of concentration fluctuations during the phase separation in the mixture. At low temperature close to the isotropic-nematic phase transition we have observed that the relaxation is well separated in time from the typical time of the domain growth. This relaxation mode is characterized by the large diffusion coefficient D = 2 x 10(8) nm2/s. The mode probably comes from the coupling between the orientational dynamics of liquid crystals and the transient gel structure of polymers.     

Ferroelectric phase transitions near ionic liquid/vacuum interfaces

A simple theoretical model is developed describing ionic liquids as regular solutions. The separation of these ionic mixtures is studied on the base of the Cahn-Hilliard theory coupled with electrostatics. It is shown that the ionic liquids decompose to thin layers of oppositely charged liquids at low temperatures. At larger temperatures the separation occurs only near the ionic liquid/vacuum surface, thus explaining the oscillatory-decaying structure of the electric double layer observed via computer simulations. In contrast to noncharged liquids the ionic ones exhibit two critical temperatures, where the temperature coefficients of all characteristic lengths possess singularities. These second order ferroelectric phase transitions are possible explanations of the experimentally measured via light scattering peculiar temperature dependence of the interfacial dipole moment density on several ionic liquid/vacuum interfaces.     

Effects of solute-solvent and solvent-solvent association in liquid adsorption chromatography with binary mobile phase

Analytical equations for the capacity ratio in liquid adsorption chromatography with a binary mobile phase, involving solute-solvent and solvent-solvent association in the mobile and surface phases, have been derived. Very simple equations have been obtained for higher values of mole fraction of the more efficient eluting solvent.     

Dynamics of adsorption by zeolites from the liquid phase, simulated using data on adsorption statics

A mathematical model for calculation of the dynamics of liquid-phase adsorption in a fixed sorbent bed is constructed on the basis of material balance equations solved by numerical integration on the assumption of a constant mass-transfer coefficient and a limiting inner-diffusion resistance.     

Microfluidic exploration of the phase diagram of a surfactant/water binary system

We investigate the behavior of a binary surfactant solution (AOT/water) as it is progressively concentrated in microfluidic evaporators. We observe in time a succession of phase transitions from a dilute solution up to a dense state, which eventually grows and invades the microchannels. Analyzing these observations, we show that, with a few experiments and a limited amount of material, our microdevices permit a semiquantitative screening of the equilibrium phase diagram as well as a few kinetic observations.     

Thermodynamics of adsorption of binary aqueous organic liquid mixtures on a RPLC adsorbent

The surface excess adsorption isotherms of organic solvents commonly used in RPLC with water as co-eluent or organic modifiers (methanol, ethanol, 2-propanol, acetonitrile and tetrahydrofuran) were measured on a porous silica surface derivatized with chlorotrimethylsilane (C1-silica with 3.92 micromol C1 groups per m2 of SiO2), using the dynamic minor disturbance method. The 5 microm diameter particles were packed in a 150 mm x 4.6 mm column. The isotherm data were derived from signals resulting from small perturbations of the equilibrium between the aqueous-organic solutions and the adsorbent surface. The partial molar surface area of the adsorbed components were assumed to be the same as those of the pure components. The difference sigma-sigmai* between the surface tensions of the adsorbed mixtures and that of the pure liquids was measured as a function of the organic modifier molar fraction. A simple and unique convention for the position of the Gibbs dividing surface was proposed to delimit the Gibbs's adsorbed phase and the bulk liquid phase. The activity coefficients of the organic modifiers and of water and their thermodynamic equilibrium constants between the two phases were measured. The strong non-ideal behavior of the adsorbed phase is mostly accounted for by the surface heterogeneity. Some regions of the surface (bonded -Si(CH3)3 moieties) preferentially adsorb the organic compound while the regions close to unreacted silanols preferentially adsorb water.     

Lateral phase separation gives multiple lamellar phases in a "binary" surfactant/water system: the phase behavior of sodium alkyl benzene sulfonate/water mixtures

We have examined the structure of the lamellar phase (Lalpha) that coexists with a micellar solution (L1) for a commercial sodium alkyl benzene sulfonate (LAS) mixed with water. The surfactant is a mixture containing C10-C13 alkyl chains, having all positional isomers of the benzene sulfonate group present except the 1-isomer. Unusually for ionic surfactants, the difference in compositions between the coexisting L1 and Lalpha phases is large (L1 = approximately 20 wt % LAS; Lalpha = approximately 65 wt %). The main technique employed was X-ray diffraction, supplemented by optical microscopy and differential scanning calorimetry (DSC). At ambient temperatures, the lamellar phase gives a single diffraction pattern with the main reflection (d) at approximately 32.5 A, whatever the composition. However, above 40 degrees C, the diffraction peak becomes broader and moves to higher d values. At higher temperatures still, several distinct and different diffraction peaks are observed, differing in detail according to composition. The largest d values (approximately 42-4 A) are observed for the lowest LAS concentrations, while the largest number of separate reflections (five) occurs for samples with approximately 44-50% LAS, both at the highest temperatures. Although there are some differences in the data between heating and cooling cycles, the d values return to the original value at low temperature. There are no observable transitions in DSC, nor is there any heterogeneity in the lamellar phase observable by microscopy. The data clearly indicate that there is some lateral separation of the different LAS isomers within the bilayers, which results in the formation of local lamellar regions having different surfactant compositions. This lateral phase separation may arise from the presence of an (electrostatic) attractive interaction, which gives rise to an upper consolute loop within the lamellar phase region of a pure LAS isomer. Similar mechanisms may occur in biological membranes and could be responsible for the occurrence of membrane lipid patches.     

Inhomogeneous phase separation kinetics in liquid binary mixtures: Sensitivity to initial local composition 2

The kinetics of phase separation subsequent to a finite temperature quench is assumed to be driven by diffusion on the altered free energy surface and is generally assumed to be slow. The situation can be different in phase separating liquid binary mixtures, especially for systems characterized by the large difference in mutual interactions between solute and solvent molecules. In such cases, the phase separation kinetics could be fast and may get completed within a short time (ns) scale. As a result, in these systems, one may observe diverse dynamical features arising out of local heterogeneity leading to the onset of phase separation through pattern formation, spinodal decomposition, nucleation, and growth. By using a coarse-grained analysis, we examine phase separation kinetics in each spatial grid and indeed observe important effects of initial heterogeneity on the subsequent evolution. Interestingly, we observe slower separation kinetics for those regions that correspond to the composition at the minimum of the high-temperature surface. The heterogeneous dynamics has been captured here through the non-linear susceptibility function, which shows a pattern similar to what is observed in the supercooled liquid. Each grid shows somewhat different dynamics in the three-stage (exponential, power-law, and logarithmic regime) phase separation dynamics. The late stage of phase separation kinetics is usually attributed to the coarsening of the phase-separated domains. However, in a liquid binary mixture, the late-stage power-law decay undergoes a further change. A new dynamical regime arises characterized by a logarithmic time dependence, which is due to the “smoothening” of the rough interface of already well-separated phases. This can also be described as opposite to the roughening transition described by Chui and Weeks [Phys. Rev. Lett. 40, 733 (1978)]. This reverse roughening transition can explain the logarithmic time dependence observed in the simulation.