Three-phase separation in nonionic surfactant/hydrophobically modified polymer aqueous mixtures

Three-phase separation for Triton X-114 or Triton X-100 solutions with addition of hydrophobically modified hydroxyethyl cellulose was investigated experimentally. When the surfactant concentration was high enough, the solution slightly above the cloud point could separate into three macroscopic phases: a cloudy phase in between a clear phase and a bluish, translucent phase. The rate of phase separation was very low with the formation of the clear and cloudy phases followed by the emergence of the bluish phase. The volume fraction of the cloudy phase increases linearly with the global polymer concentration, whereas the volume fraction of the bluish phase increases linearly with the global surfactant concentration. Composition analyses found that most of the polymer stayed in the cloudy phase, as opposed to most of the surfactant in the bluish phase. The interesting phase behavior can be explained by an initial associative phase separation followed by a segregative phase separation in the cloudy phase.     

High-pressure-induced phase transitions in pentaerythritol: X-ray and Raman studies

The high-pressure response of pentaerythritol crystals has been examined to 10 GPa in diamond-anvil cells using angle-dispersive synchrotron X-ray diffraction and Raman spectroscopy. The results reveal two first-order phase transitions: one at 4.8 GPa from phase I, tetragonal I(), to phase II, orthorhombic Pnn2C2v10, with a small approximately 0.5% volume change, and the other at 7.2 GPa to phase III with an unknown crystal structure. We found that phase I exhibits a large crystallographic anisotropy which rapidly decreases with increasing pressure: the ratio of linear compressibilities between two primary crystal axes decreases from betao= 8.1 at 1 atm to betaP = 2.6 at 4 GPa. We suggest that this apparent decrease in crystal anisotropy is due to the disruption of hydrogen bonding in the (001) plane of phase I and eventually leads to an orthorhombic distortion from a quadrilateral network structure in phase I to a quasi one-dimensional structure in phase II. The crystal structure of phase III exhibits a disordered character, and it is likely a conformational variant of phase II.     

EFFECT OF DILUENTS ON CRYSTALLIZATION OF POLY(VINYLIDENE FLUORIDE) AND PHASE SEPARATED STRUCTURE IN A TERNARY SYSTEM via THERMALLY INDUCED PHASE SEPARATION

The phase diagram of a ternary system of PVDF,dibutyl phthalate (DBP) and di(2-ethylhexyl) phthalate (DEHP) was determined in terms of a pseudo binary system with the same polymer concentration and different DBP content in diluent mixture.The experimental results showed that as the DBP content increased in diluent mixture,the phase separation changed from liquid-liquid phase separation to solid-liquid phase separation,and both the cloudy point for L-L phase separation and crystallization temperature shif...     

Structure of mesh phases in a cationic surfactant system with strongly bound counterions

We report the observation of an intermediate mesh phase with rhombohedral symmetry, corresponding to the space group Rm, in a mixed surfactant system formed by the cationic surfactant cetyltrimethylammonium bromide (CTAB) and the organic salt 3-sodium-2-hydroxy naphthoate (SHN). It occurs between a random mesh phase (L(alpha)(D)) and a lamellar phase (L(alpha)) at low temperatures; at higher temperatures, the (L(alpha)(D)) phase transforms continuously into the (L(alpha)) phase with an increasing surfactant concentration (phi(s)). To separate the effects of salt and phi(s) on the phase behavior, the ternary system consisting of cetyltrimethylammonium 3-hydroxy-naphthalene-2-carboxylate (CTAHN), sodium bromide (NaBr), and water was studied. The intermediate mesh phase is found in this system at high NaBr concentrations. The micellar aggregates, both in the intermediate and random mesh phases, are found to be made up of a two-dimensional network of rod-like segments, with three rods meeting at each node. The average mesh size increases with phi(s), and the transition from the random mesh phase to the intermediate phase is found to occur when it is approximately 1.5 times the lamellar periodicity. The intermediate mesh phase is absent in the equimolar dodecyltrimethylammonium bromide (DTAB)-SHN system, indicating the role of the surfactant chain length in the formation of this phase. This system exhibits a random mesh phase over a very wide range of water content, with the average mesh size decreasing upon an increasing phi(s), contrary to the trend seen in the CTAB-SHN system.     

Thermodynamics and kinetics of solute transfer in reversed-phase liquid chromatography

In this study, the thermodynamic and kinetic behavior of a homologous series of fatty acids is examined using a polymeric octadecylsilica stationary phase and a methanol mobile phase. The zone profiles are evaluated as the temperature is varied from 20 to 60 degrees C and the average pressure from 400 to 4570 p.s.i. (1 p.s.i.=6894.76 Pa). The rate constant for solute transfer from mobile to stationary phase (k(ms)) appears to be relatively constant with carbon number. In contrast, the rate constant from stationary to mobile phase (k(sm)) decreases logarithmically with increasing carbon number. This suggests that the mass transport processes become progressively slower, owing to the smaller diffusion coefficients of the larger solutes in the stationary phase. The activation energy decreases slightly in the mobile phase and increases slightly in the stationary phase with increasing carbon number. The activation energy in the stationary phase ranges from 41.6 to 55.9 kcal/mol, while the thermodynamic change in internal energy ranges from -9.8 to -29.0 kcal/mol for C10 to C22, respectively (1 cal=4.184 J). The activation volume increases with increasing carbon number in both the mobile and stationary phase. The activation volume in the stationary phase ranges from 31.7 to 211 cm3/mol, while the thermodynamic change in molar volume ranges from -27.1 to -104 cm3/mol for C10 to C22, respectively. These large changes in activation energy and volume suggest that the solutes do not enter and leave the stationary phase in a single step, but in a stepwise or progressive manner.     

Microscopic phase behavior of supercritical carbon dioxide + non-ionic surfactant + water systems at elevated pressures

The microscopic phase behavior of the supercritical carbon dioxide (scCO₂) + polyethylene oxide-2,6,8-trimethyl-4-nonyl ether (TMN) + water systems at about 3 wt% of TMN were investigated using a synthetic method with a microscope. The two types of TMN (TMN-3 and TMN-10) used in this work had molecular weight distributions caused by the distribution of the number of ethylene oxide groups. Two different types of phase transition were observed when pressure was decreased gradually at a constant temperature from the high pressure at which the transparent phase was observed to the low pressure at which the separate vapor–liquid phases were observed for the scCO₂ + TMN-3 + water system at 3 wt% of TMN-3. The transparent phase was colorless under all experimental conditions and the phase transition from a transparent phase to a turbidity phase with small, dispersed droplets was observed at the higher side phase transition (higher phase transition pressure). As the pressure continued to decrease, another phase transition was observed from the phase with small droplets to a state with an accelerating aggregation of droplets (lower phase transition pressure). In the turbidity phase between the higher and the lower phase transition, the degree of turbidity became higher with decreasing pressure. On the other hand, in the phase observed below the lower phase transition, a new liquid phase adhered to the sapphire windows and the wall inside the optical cell.     

Electron microscopic and light scattering observation on a system with two iridescent phases

Electron microscopic observations and classical light-scattering measurements have been carried out for dodecyldimethylaminoxide/hexanol/water mixtures in the concentration range where iridescent colors occur. This system has two different iridescent phases. The iridescent phase with more hexanol forms quickly, and the phase with less hexanol forms very slowly. Three different isotropic phases which show strong flow birefringence are found near both iridescent phases. The electron microscopic pictures show clearly that only one of these isotropic phases with strong flow birefringence is a bicontinuous sponge phase (L_3h -phase). This is the phase which comes out by adding some alkanol to the upper lamellar phase. The flow birefringent phase below the lower lamellar phase forms unilamellar vesicles. The flow birefringent phase which occurs between both iridescent phases contains multilamellar vesicles and is shown to be a precursor of a lamellar phase.     

Cubic, sponge, and lamellar phases in the glyceryl monooleyl ether-propylene glycol-water system

The phase behavior of 1-glyceryl monooleyl ether (GME) in mixtures of propylene glycol (PG) and water was investigated by visual inspection, polarization microscopy, small-angle X-ray diffraction, and conductance measurements. A phase diagram, based on over 200 samples of the ternary system GME-PG-water, was constructed at 20 degrees C. Without PG, GME forms a reverse micellar phase with up to 10 wt % water and a reverse hexagonal liquid-crystalline phase between 10 and 25 wt % water, a phase that can coexist with excess water. If PG is added in amounts exceeding about 10 wt %, then cubic and lamellar liquid-crystalline phases start to form. A cubic phase, belonging to space group Pn3m, can coexist with excess PG-water mixtures. If even more PG is added, then the cubic phase is transformed into a sponge phase. A lamellar phase forms at water contents between 10 and 15 wt % and with widely differing PG/GME weight ratios. We postulate that the phase behavior is caused by the fact that PG makes the interfacial region between self-assembled GME and PG-water less negatively curved, which in turn allows for the formation of the new phases. The phase behavior obtained for the GME system shows a striking similarity with the phase behavior of the corresponding system in which the GME has been replaced by the ester, 1-glycerol monooleate (GMO), differing only in one extra carbonyl oxygen. The major difference is the lower amount of water present in the GME phases, an effect that is mainly due to the more hydrophobic character of GME compared to that of GMO.     

Nature of phase transitions in crystalline and amorphous GeTe-Sb2Te3 phase change materials

The thermodynamic nature of phase stabilities and transformations are investigated in crystalline and amorphous Ge(1)Sb(2)Te(4) (GST124) phase change materials as a function of pressure and temperature using high-resolution synchrotron x-ray diffraction in a diamond anvil cell. The phase transformation sequences upon compression, for cubic and hexagonal GST124 phases are found to be: cubic → amorphous → orthorhombic → bcc and hexagonal → orthorhombic → bcc. The Clapeyron slopes for melting of the hexagonal and bcc phases are negative and positive, respectively, resulting in a pressure dependent minimum in the liquidus. When taken together, the phase equilibria relations are consistent with the presence of polyamorphism in this system with the as-deposited amorphous GST phase being the low entropy low-density amorphous phase and the laser melt-quenched and high-pressure amorphized GST being the high entropy high-density amorphous phase. The metastable phase boundary between these two polyamorphic phases is expected to have a negative Clapeyron slope.     

Effect of temperature on gradient reequilibration in reversed-phase liquid chromatography

The effect of mobile phase modifier and temperature on gradient reequilibration is examined using three different stationary phases. The stationary phases studied are a traditional C18 phase, a polar endcapped C18 phase, and an alkyl phase with a polar embedded group. It was observed that both temperature and choice of mobile phase organic modifier had an effect on gradient reequilibration volume on both the traditional C18 stationary phase and the polar endcapped phase. On both these phases, at any given temperature, the reequilibration volume was generally smaller when methanol was used as the mobile phase modifier as compared to acetonitrile. As the temperature is increased from 10 to 50 degrees C, significant reductions in reequilibration volume were observed with both mobile phase modifiers. In contrast, neither temperature nor choice of modifier appeared to have much effect on reequilibration volume when the polar embedded group stationary phase was considered.     

Phase behavior of the palmitic acid/palmitin system. A 2H NMR study

The phase behavior of mixtures of palmitic acid (PA) and 1-monohexadecanoyl-rac-glycerol, palmitin, was studied by phase contrast microscopy and deuterium solid-state NMR. At pH 5, mixtures remained precipitated as lumps in solution. The NMR spectrum of the perdeuterated PA (PAd31) at 300 K exhibited a shape and quadrupolar splittings, deltav, characteristic of lipids embedded in a gel phase. The alkyl chains remained in a trans conformation with their long molecular axis oriented at about 15 degrees with respect to the bilayer normal. However, gauche defects were shown to occur at the end of the alkyl chain. At 330 K, the system underwent a phase transition to a hexagonal phase followed by an isotropic phase at 340 K. Upon cooling to 330 K, the spectrum in the hexagonal phase was oriented at 0 degrees showing that the cylinders were oriented with their long axis parallel to the field. Up to 11 positions (from 15) of PAd31 could be assigned. At pH 7 and 9 at room temperature, the mixtures were fully dispersed in a viscous solution of vesicles. The system underwent a phase transition at 320 K from a gel phase to a fluid phase with the bilayer normal oriented at 90 degrees with respect to the field. Analogous experiments performed with PA selectively labeled on carbon C2 allowed for the assignment of deltav for that position and suggested different conformations of the headgroup in the gel and fluid or hexagonal phases. The implications of these findings for the bio-availability of these fatty acids, in the understanding of the contribution of hydroxyl and carboxyl groups in the membrane formation, and for the production of simple self-oriented systems are discussed.     

Probing the interaction of solvents with the stationary phase of C18 high-performance liquid chromatographic column material by variable-temperature dependent 129Xe nuclear magnetic resonance spectroscopy

VT (129)Xe NMR was applied to probe the interactions of solvents having different polarity indices with the stationary phase of a RP-C18 HPLC column material. It was observed that the highly polar ethylene glycol molecules do not mix with the alkyl chains of the RP-C18 stationary phase and the solvent is unable to enter the pores and the spaces between the particles. Three phases in this sample are defined as stationary/xenon phase, xenon gas phase (in the pores and the spaces between the particles) and ethylene glycol/xenon phase. In contrast to ethylene glycol, the nonpolar solvent cyclohexane was observed to be well mixed with the RP-C18 stationary phase. The capillary rise effect allows the solvent to enter the pores and the spaces between the particles. Two phases in this sample are defined as stationary/cyclohexane/xenon phase and cyclohexane/xenon phases. The properties of ethyl acetate are between those of ethylene glycol and cyclohexane. The (129)Xe NMR results show that the rational reversed phases should be conditioned from highly solvating to more polar solvents to remove the trapped air. The (129)Xe NMR results also show that pure stationary phase exists only when a highly polar solvent is used in reversed-phase chromatography. For a solvent with lower polarity, a stationary/solvent phase actually forms. This, together with the mobile phase, determines the selective factor for separating mixtures.     

醋酸水流动相体系中硅胶键合相上生物大分子保留行为的研究

 提出了以醋酸 水作为流动相的体系中 ,在ODS柱上分离生物大分子的反相高效液相色谱 (RPLC)方法。实验结果表明 ,醋酸 水的洗脱能力强于甲醇 水 三氟醋酸体系 ,在一定程度上克服了色谱分离中一些蛋白质的不可逆吸附且具有便于冷冻干燥的优点。用参数Z(1mol溶剂化溶质被溶剂化固定相吸附时从两者接触表面释放出置换剂的摩尔总数 ) ,logI(与 1mol溶质对固定相亲和势有关的常数 )和 j(与 1mol溶剂对固定相亲和势有关的常数 )对 9种蛋白质在此流动相体系中的保留进行了表征。     

Modelling the chemically induced liquid crystalline phases in mixtures of disc-shaped mesogens: the phase diagram of quadrupolar Gay-Berne discs revisited

We investigate the phase behaviour of pure systems and mixtures of quadrupolar Gay-Berne discs. The interaction potential mimics that of disc-shaped molecules which exhibit chemically induced phases with a structure based on columns of alternately stacking species. The phase diagram of the pure system is determined over a range of pressures, and the phase sequence is shown to include tilted columnar and nematic phases. The mixtures, in which the species have equal but opposite quadrupole moments, are investigated over a range of pressures using semigrand canonical simulations, such that the composition of the system is allowed to change. The fact that the composition is not fixed is especially important at the phase transitions, where the compositions of the coexisting phases may not necessarily be the same. In this situation, preparation of a system in the ‘biphasic region’ will lead to phase separation into the two distinct phases of differing compositions. The resulting phase diagram obtained using semigrand canonical simulations indicates that the columnar nematic phase observed in previous fixed composition simulations of this model [Liq. Cryst.,24, 229 (1998)] is not stable with respect to phase separation into an isotropic phase rich in a single component and a hexagonal columnar phase composed of roughly equal quantities of the two components. The structure of the columnar phase for the mixture is shown to be based on the alternate stacking of the different species. The relative concentrations of the different species in this phase may deviate up to approximately 60 : 40 mol%, after which any further material added will separate into the pure isotropic state.     

Modelling the chemically induced liquid crystalline phases in mixtures of disc-shaped mesogens: the phase diagram of quadrupolar Gay-Berne discs revisited

We investigate the phase behaviour of pure systems and mixtures of quadrupolar Gay-Berne discs. The interaction potential mimics that of disc-shaped molecules which exhibit chemically induced phases with a structure based on columns of alternately stacking species. The phase diagram of the pure system is determined over a range of pressures, and the phase sequence is shown to include tilted columnar and nematic phases. The mixtures, in which the species have equal but opposite quadrupole moments, are investigated over a range of pressures using semigrand canonical simulations, such that the composition of the system is allowed to change. The fact that the composition is not fixed is especially important at the phase transitions, where the compositions of the coexisting phases may not necessarily be the same. In this situation, preparation of a system in the 'biphasic region' will lead to phase separation into the two distinct phases of differing compositions. The resulting phase diagram obtained using semigrand canonical simulations indicates that the columnar nematic phase observed in previous fixed composition simulations of this model [ Liq. Cryst ., 24 , 229 (1998)] is not stable with respect to phase separation into an isotropic phase rich in a single component and a hexagonal columnar phase composed of roughly equal quantities of the two components. The structure of the columnar phase for the mixture is shown to be based on the alternate stacking of the different species. The relative concentrations of the different species in this phase may deviate up to approximately 60 : 40 mol %, after which any further material added will separate into the pure isotropic state.     

Stability of nematic and smectic phases in rod-like mesogens with orientation-dependent attractive interactions

The stability of isotropic (I), nematic (N), smectic A (Sm A), and hexatic (Hex) liquid crystalline phases is studied for a fluid of molecules with a rod-like shape and dispersive interactions dependent on orientation. The fluid is modeled with the spherocylindrical Gay-Berne-Kihara interaction potential proposed in a recent work, with parameters favoring parallel pair orientations. The liquid crystal phase diagram is characterized for different molecular aspect ratios by means of Monte Carlo simulations in the isobaric-isothermal ensemble. Three types of triple points are observed, namely, I-Sm A-Hex, I-N-Sm A, and N-Sm A-Hex, leading to island-shape domains for the smectic A phase. The resulting phase diagrams are compared with those derived previously for prolate fluids of ellipsoidal and spherocylindrical symmetry. It is concluded that the stability of the layered phases with respect to the nematic phase is enhanced in the spherocylindrical fluids due to geometrical constraints. Furthermore, the anisotropy of the dispersive interactions induces a stronger dependence of the overall phase diagram on temperature and aids in the energetic stabilization of the hexatic crystalline phase with respect to the fluid smectic A phase.     

An attempt directed toward enhanced shape selectivity in reversed-phase liquid chromatography: preparation of the dodecylaminated beta-cyclodextrin-bonded phase.

With the aim of preparing a stationary phase with a high shape-recognition ability for liquid chromatography, a new bonded phase was synthesized by coupling multiply dodecylamino-substituted beta-cyclodextrin (beta-CD) to 3-glycidoxypropyl-derivatized silica gel. The stationary phase prepared in this way was expected to have increased shape selectivity compared with that of conventional reversed-phase materials, due to solute interactions with the alkyl chain piles built up on the beta-CDs bonded to silica. The separation characteristics of the bonded phase were investigated using polycyclic aromatic hydrocarbons (PAHs) with different molecular shapes and compared with those of monomeric ODS and native beta-cyclodextrin-bonded phases. The newly developed stationary phase was found to be highly selective for PAHs.     

An order-disorder transition in the conjugated polymer MEH-PPV

The poly(p-phenylene vinylene) derivative MEH-PPV is known to exist as two morphologically distinct species, referred to as red phase and blue phase. We show here that the transition from the blue phase to the red phase is a critical phenomenon that can be quantitatively described as a second order phase transition with a critical temperature T(c) of 204 K. The criticality is associated with the trade-off between the gain in the electronic stabilization energy when the π-system of a planarized chain can delocalize and the concomitant loss of entropy. We studied this transition by measuring the absorption and fluorescence in methyltetrahydrofuran (MeTHF) in two different concentrations as a function of temperature. The spectra were analyzed based upon the Kuhn exciton model to extract effective conjugation lengths. At room temperature, the chains have effective conjugation lengths of about five repeat units in the ground state (the blue phase), consistent with a disordered defect cylinder conformation. Upon cooling below the critical temperature T(c), the red phase with increased effective conjugation lengths of about 10 repeat units forms, implying a more extended and better ordered conformation. Whereas aggregation is required for the creation of the red phase, its electronic states have a predominant intrachain character.     

Study on the subgel-phase formation using an asymmetric phospholipid bilayer membrane by high-pressure fluorometry

The myristoylpalmitoylphosphatidylcholine (MPPC) bilayer membrane shows a complicated temperature-pressure phase diagram. The large portion of the lamellar gel (L(β)'), ripple gel (P(β)'), and pressure-induced gel (L(β)I) phases exist as metastable phases due to the extremely stable subgel (L(c)) phase. The stable L(c) phase enables us to examine the properties of the L(c) phase. The phases of the MPPC bilayers under atmospheric and high pressures were studied by small-angle neutron scattering (SANS) and fluorescence spectroscopy using a polarity-sensitive fluorescent probe Prodan. The SANS measurements clearly demonstrated the existence of the metastable L(β)I phase with the smallest lamellar repeat distance. From a second-derivative analysis of the fluorescence data, the line shape for the L(c) phase under high pressure was characterized by a broad peak with a minimum of ca. 460 nm. The line shapes and the minimum intensity wavelength (λ″(min)) values changed with pressure, indicating that the L(c) phase has highly pressure-sensible structure. The λ″(min) values of the L(c) phase spectra were split into ca. 430 and 500 nm in the L(β)I phase region, which corresponds to the formation of a interdigitated subgel L(c) (L(c)I) phase. Moreover, the phase transitions related to the L(c) phase were reversible transitions under high pressure. Taking into account the fluorescence behavior of Prodan for the L(c) phase, we concluded that the structure of the L(c) phase is highly probably a staggered structure, which can transform into the L(c)I phase easily.     

Re-entrant and induced mesophases: Mixed systems showing re-entrant TGBA and re-entrant cholesteric phases

We have investigated the behaviour of chiral carbohydrate-based trioxadecalines with a cyano group in mixed systems. The mixture of a trioxadecaline having a Ch phase and a trioxadecaline having a Ch, S_A and a S*_Cphase sequence induces a re-entrant TGB_A and Ch phase. Below the stabilized S*_C phase, a re-entrant Ch phase is observed. In the mixture of the cholesteric trioxadecaline with CCH7, itself only showing a nematic phase, a TGB_A phase and a broad range S_A phase are induced from two non-smectic compounds. Both mixtures show a blue phase with a pitch ranging from the UV to red depending on the molar fraction.