Measurement of the density distributions in quenched polycarbonate specimens by a quantitative schlieren optical technique

A new technique for the determination of spatial density distributions is described. The density is derived from the refractive index gradient as determined with a quantitative Schlieren optical technique. The advantages of this technique are a high spatial resolution and sensitivity. Gapwise profiles can be measured without slicing the sample so that more reliable data are obtained quickly. This is demonstrated for the gapwise density distributions in quenched polycarbonate specimens. It is shown that the density distribution is determined by a competition between the local cooling rate variation and the cooling stress distribution. The residual density is predominantly determined by the coolant temperature. The density in the core is higher than at the surface for high coolant temperatures, whereas for low coolant temperatures the density in the core is lower than at the surface. The observed effects are explained semiquantitatively by a simple model using calculated cooling stresses and cooling rates.     

Quantitative effect of nonionic surfactant partitioning on the hydrophile-lipophile balance temperature

Phase behaviors of water/nonionic surfactants/isooctane systems are determined experimentally in temperature-global surfactant concentration diagrams. The surfactants are monodistributed polyoxyethylene glycol n-dodecyl ether. They are used as model mixtures of two, three, or five compounds or as constituents of a commercial surfactant. It is found that the phase diagrams of these systems are bent gradually toward the highest temperatures as the global surfactant concentration decreases. Each phase diagram is well-characterized by the curve of the HLB (hydrophile-lipophile balance) temperature versus the global surfactant concentration. For any fixed global surfactant concentration, this temperature is the middle temperature of the three-phase region; it can be calculated from an additive rule of the HLB temperatures of the surfactants weighted by their mole fractions at the water/oil interface. These mole fractions are determined through the pseudophase model using surfactant partitioning. Calculations require the knowledge of the critical micelle concentration, the partition coefficient between water and oil, and the HLB temperature of each surfactant of the mixture. This treatment can be used to correctly predict the variation of the HLB temperatures of the surfactant mixtures studied versus the global surfactant concentration. Furthermore, these calculations show that the observed curvature of the phase diagrams at the lowest global concentrations is due to the most favorable partitioning toward the oil of the lowest ethoxylated surfactant molecules.     

Hydrogen-bond-induced inclusion complex in aqueous cellulose/LiOH/urea solution at low temperature.

It was puzzling that cellulose could be dissolved rapidly in 4.6 wt % LiOH/15 wt % urea aqueous solution precooled to -12 degrees C, whereas it could not be dissolved in the same solvent without prior cooling. To clarify this important phenomenon, the structure and physical properties of LiOH and urea in water as well as of cellulose in the aqueous LiOH/urea solution at different temperatures were investigated by means of laser light scattering, 13C NMR spectroscopy, differential scanning calorimetry, Fourier transform infrared spectroscopy, wide-angle X-ray diffraction, and transmission electron microscopy (TEM). The results reveal that a hydrogen-bonded network structure between LiOH, urea, and water can occur, and that it becomes more stable with decreasing temperature. The LiOH hydrates cleave the chain packing of cellulose through the formation of new hydrogen bonds at low temperatures, which result in a relatively stable complex associated with LiOH, water clusters, and cellulose. A channel inclusion complex (IC) hosted by urea could encage the cellulose macromolecule in LiOH/urea solution with prior cooling and therefore provide a rationale for forming a good dispersion of cellulose. TEM observations, for the first time, showed the channel IC in dry form. The low-temperature step played an important role in shifting hydrogen bonds between cellulose and small molecules, leading to the dissolution of macromolecules in the aqueous solution.     

Freezing on heating of liquid solutions

We report a reversible liquid-solid transition upon heating of a simple solution composed of a-cyclodextrine (alpha CD), water, and 4-methylpyridine. These solutions are homogeneous and transparent at ambient temperature and solidify when heated to temperatures between 45 degrees and 75 degrees. Quasielastic and elastic neutron scattering show that molecular motions are slowed down in the solid and that crystalline order is established. The solution "freezes on heating." This process is fully reversible, on cooling the solid melts. A rearrangement of hydrogen bonds is postulated to be responsible for the observed phenomenon.     

Physicochemical investigations of microemulsification of coconut oil and water using polyoxyethylene 2-cetyl ether (Brij 52) and isopropanol or ethanol

The microemulsification of coconut oil/polyoxyethylene 2-cetyl ether/2-propanol or ethanol/water was investigated. The phase behaviors of the mixed system were examined. The shear viscosity at different temperatures was measured to derive activation parameters for the viscous flow. The diffusion coefficient of the microemulsions at different compositions was determined by the DLS method. The energetics of solubilization of water into oil + Brij + alkanol as well as of oil into water + Brij + alkanol forming w/o and o/w microemulsions, respectively, were calorimetrically determined.     

Polymorphism of progesterone

Solid dispersions are used in pharmaceutical technology in order to improve solubility and/or dissolution kinetics of poorly water soluble drugs [1, 2, 3]. A preliminary study concerning progesterone structure after melting revealed the existence of a drug polymorphism after cooling, and gave the opportunity to specify the manufacturing conditions in order to obtain the stable form of this hormone [4]. In this work, two different types of progesterone solid dispersion have been compared. The first one is obtained by a slow cooling rate of the drug in the presence of polyoxyethylene glycol 6000 and the second one after quenching in the presence of saccharose distearate. DSC and radiocrystallographic studies of the solid dispersions served to specify the nature of the compounds obtained and to characterize the physical structure of the hormone in the solidified melts.     

Heat capacity of tetrahydrofuran clathrate hydrate and of its components, and the clathrate formation from supercooled melt

We report a thermodynamic study of the formation of tetrahydrofuran clathrate hydrate by explosive crystallization of water-deficient, near stoichiometric, and water-rich solutions, as well as of the heat capacity, C(p), of (i) supercooled tetrahydrofuran-H2O solutions and of the clathrate hydrate, (ii) tetrathydrofuran (THF) liquid, and (iii) supercooled water and the ice formed on its explosive crystallization. In explosive freezing of supercooled solutions at a temperature below 257 K, THF clathrate hydrate formed first. The nucleation temperature depends on the cooling rate, and excess water freezes on further cooling. The clathrate hydrate melts reversibly at 277 K and C(p) increases by 770 J/mol K on melting. The enthalpy of melting is 99.5 kJ/mol and entropy is 358 J/mol K. Molar C(p) of the empty host lattice is less than that of the ice, which is inconsistent with the known lower phonon frequency of H2O in the clathrate lattice. Analysis shows that C(p) of THF and ice are not additive in the clathrate. C(p) of the supercooled THF-H2O solutions is the same as that of water at 247 K, but less at lower temperatures and more at higher temperatures. The difference tends to become constant at 283 K. The results are discussed in terms of the hydrogen-bonding changes between THF and H2O.     

The Physicochemical Behavior of Phytosterol Ethoxylates

In this work the physicochemical behavior of a series of phytosterol ethoxylates in water is presented. The influence of the length of the polyoxyethylene chain is studied. The surfactant solutions have been examined by means of birefringent microscopy, surface tension, self-diffusion 1H NMR, dynamic and static light scattering, and rheology. The surfactants with a hydrophilic chain of 10 oxyethylene units or more gave a micellar region. The CMC values were generally very low and a reverse relationship between the CMC value and the polyoxyethylene chain length was obtained. The time required to reach equilibrium surface tension was very long, more than 150 min. For the hydrophobic surfactants large lamellar regions appeared while for the more hydrophilic surfactants cubic and hexagonal structures were present which remained stable up to temperatures of 100 degrees C. In the micellar region prolate aggregates were formed which showed "ghostlike" behavior, consisting of cross-linked micelles with very fast relaxation times. Copyright 1999 Academic Press.     

Microphase separation of PEG-modified urethane acrylate and the swelling behavior of its gels

The viscosity of PEG-modified urethane acrylate (PMUA) showed peculiar behavior in the course of soap-free emulsification. Moreover, the viscosity change with added amounts of water was influenced by the reaction molar ratio of polyethylene glycol (PEG). The rate of increase in viscosity slowed and the ratio of increase in viscosity increased as the reaction molar ratio of PEG increased. This peculiar viscosity behavior was due to the microphase separation between hydrophilic and hydrophobic segments of PMUA, and the orientation of polyoxyethylene groups at O/W interface which influenced droplet size of the soap-free PMUA emulsion. The location of polyoxyethylene groups of this resin at O/W interface was confirmed using the adsorption isotherm measurement of PMUA molecules containing polyoxyethylene groups at water/benzene interface. The microphase separation behavior of PMUA between hydrophilic and hydrophobic segments could apply to the preparation of the PMUA gels containing peculiar structure. PMUA gels were prepared using dioxane (UAG) and the swelling behavior of these gels were compared to that of gels prepared using water (UAHG) in the same medium. In the same medium, the swelling behavior of UAHG gels differed from that of UAG gels because of the difference in the microstructure of gel due to the microphase separation between hydrophilic and hydrophobic segments. This phase separation in the course of gelation in water could be confirmed using contact angle measurement.     

Infrared analysis of CO ice particles in the aerosol phase

Fourier transform infrared extinction spectra of a variety of CO ice aerosols, generated at low temperatures in a liquid helium cooled collisional-cooling cell, have been analyzed. Different operation modes of the cooling system were used for the generation of spherical and nonspherical CO nanoparticles at temperatures between 5 and 35 K and with diameters between 10 and 1000 nm. In contrast to the predominantly amorphous CO films described in the literature the presented CO particles are (poly)crystalline. A Mie inversion iterative scheme is presented and used to infer the optical constants of CO ice for the cases compact particles have been produced. The spectra of nonspherical CO aerosol particles are interpreted by modeling the extinction using the discrete dipole approximation procedure combined with the retrieved optical constants. A global positive matrix factorization scheme allows us to infer the dominant shapes in the observed particle distribution and can be used as a guide for further experiments. Near 25 K a pronounced shape evolution of smaller particles from spherical toward longish structures was observed at low buffer-gas pressure over 400 s.     

Incorporation of disodium alkyl polyoxyethylene ether sulfosuccinate inside styrene droplets: mechanism and its application for preparation of multihollow polymer spheres

Emulsion polymerization of styrene was carried out using two kinds of alkyl polyoxyethylene ether sulfosuccinates as surfactant: disodium cetyl polyoxyethylene (25) ether sulfosuccinate (CPS) and octyl-phenol polyoxyethylene (10) ether sulfosuccinate (OPS). In experiments, the incorporation of CPS or OPS inside styrene droplets and polystyrene particles was clearly observed. Based on this phenomenon, multihollow polymer spheres are prepared in a one-step reaction and this strongly supports the proposed incorporation mechanism. CPS is more effective than OPS during the preparation of multiporous spheres. This difference between the two surfactants mainly contributes to the difference of the length of the EO (polyoxyethylene) group, which can determine the affinity among surfactant, styrene, and water molecules.     

Helium induced pressure broadening and shifting of HCN hyperfine transitions between 1.3 and 20 K

We have measured the helium induced pressure broadening and shifting of the distinct hyperfine components of the j = 1 <-- 0 and j = 2 <-- 1 transitions of HC14N at temperatures between 1.3 and 20 K. The HCN molecules were cooled to these temperatures using the collisional cooling technique. As a test of this cooling technique we measured the Doppler contribution to the spectral lines, and these measurements confirm that the molecules are at the same temperature as the walls of the spectroscopic cell. We observed that the hyperfine components of the 2 <-- 1 transition have distinct broadening coefficients that differ from one another by as much as 5%. The measured differences are in reasonable agreement with theoretical predictions. We have also performed molecular scattering calculations on three He-HCN potential energy surfaces in order to compare our results with theoretical expectations. At the lowest temperatures these calculations predict broadening coefficients that are considerably larger than the measured coefficients. We have previously found a similar discrepancy for two other molecules at these low temperatures, and we discuss possible experimental and theoretical origins for this persistent discrepancy.     

Co-solvent effects on drag reduction, rheological properties and micelle microstructures of cationic surfactants

Some quaternary cationic surfactants, when mixed with a counterion, are known to self-assemble into threadlike micelles in water. Such behavior causes drastic changes in rheological properties of even very dilute solutions, allowing them to be used as drag reducing agents (DRA) in turbulent pipe flow circulating systems, such as district cooling/heating systems. Surfactant self-assembly is a physicochemical phenomenon whose character depends on surfactant nature and concentration, nature of the solvent, temperature and type and concentration of counterions. This study investigates drag reduction (DR) and rheological properties of two cationic surfactants, Ethoquad O/12 (oleyl bis(hydroxyethyl)methylammonium chloride) and Ethoquad O/13 (oleyl tris(hydroxyethyl) ammonium acetate), with excess salicylate counterion (NaSal), in mixed solvents containing 0 to 28 wt% ethylene glycol (EG) and water. The addition of EG to the solvent had greater effects on solutions' DR ability, shear viscosity, apparent extensional viscosity and viscoelasticity at 25 degrees C than at approximately 0 degrees C. Cryo-TEM images show threadlike micelle in these systems. DR at low temperatures in solutions containing moderate amount of EG can be utilized in a new approach to energy saving in district cooling systems using EG-water based mixtures as the cooling fluids.     

Gelation process of amylose-DMSO-water system

Amylose gel is obtained by cooling amylose aqueous solution at a certain cooling rate. In order to clarify the gelation process of amylose in the mixed solvent of water and DMSO, the characteristics of turbidity and dynamic viscoelasticity of the solution as a function of temperature have been studied by optical and rheological methods, respectively. Accordingly, cloud temperature (Tc) at which the aggregation of amylose occurs, and gelation temperature (Tgel) at which the elasticity began to appear were obtained. Tc and Tgel were strongly dependent on cooling rate, and these values shifted to higher temperatures with decreasing cooling rate. However, there was difference between Tc and Tgel, and Tc was higher than Tgel, indicating the gelation process occurs in two stages.     

The influence of surfactant mixing ratio on nano-emulsion formation by the pit method

The formation of O/W nano-emulsions by the PIT emulsification method in water/mixed nonionic surfactant/oil systems has been studied. The hydrophilic-lipophilic properties of the surfactant were varied by mixing polyoxyethylene 4-lauryl ether (C12E4) and polyoxyethylene 6-lauryl ether (C12E6). Emulsification was performed in samples with constant oil concentration (20 wt%) by fast cooling from the corresponding HLB temperature to 25 degrees C. Nano-emulsions with droplet radius 60-70 nm and 25-30 nm were obtained at total surfactant concentrations of 4 and 8 wt%, respectively. Moreover, droplet size remained practically unchanged, independent of the surfactant mixing ratio, X(C12E6). At 4 wt% surfactant concentration, the polydispersity and instability of nano-emulsions increased with the increase in X(C12E6). However, at 8 wt% surfactant concentration, nano-emulsions with low polydispersity and high stability were obtained in a wide range of surfactant mixing ratios. Phase behavior studies showed that at 4 wt% surfactant concentration, three-liquid phases (W+D+O) coexist at the starting emulsification temperature. Furthermore, the excess oil phase with respect to the microemulsion D-phase increases with the increase in X(C12E6), which could explain the increase in instability. At 8 wt% surfactant concentration, a microemulsion D-phase is present when emulsification starts. The low droplet size and polydispersity and higher stability of these nano-emulsions have been attributed, in addition to the increase in the surface or interfacial activity, to the spontaneous emulsification produced in the microemulsion D-phase.     

Melt processing of hexa-peri-hexabenzocoronene on the water surface

A discotic polycyclic aromatic hydrocarbon, hexa-peri-hexabenzocoronene, was oriented by slow cooling from the isotropic phase on a water surface as a film. For melt processing at low temperatures, an HBC derivative with long swallow-tailed alkyl side chains was chosen. The supramolecular organization in the resulting thin layer was investigated by electron microscopy. In high-resolution mode, the structural study showed large domains in which the columnar structures were oriented uniaxially with an edge-on arrangement of the hydrophobic molecules. The length of the stacks exceeded several hundred nanometers without obvious defects. The small-area analysis by TEM allowed the direct visualization of individual packed molecules. Electron diffraction revealed a high in-plane order of the columnar superstructures in which the discs were tilted by ca. 40° with respect to the stacking direction. This is the first example of a discotic system melt processed on the water surface yielding a pronounced order.     

Interfacial composition, thermodynamic properties, and structural parameters of water-in-oil microemulsions stabilized by 1-pentanol and mixed surfactants

The present study is focused on the evaluation of the interfacial composition, thermodynamic properties, and structural parameters of water-in-oil mixed surfactant microemulsions [(cetylpyridinium chloride, CPC+polyoxyethylene (20) cetyl ether, Brij-58 or polyoxyethylene (20) stearyl ether, Brij-78)/1-pentanol/n-heptane, or n-decane] under various physicochemical environments by the Schulman method of cosurfactant titration of the oil/water interface. The estimation of the number of moles of 1-pentanol at the interface (n(a)(i)) and bulk oil (n(a)(o)) and its distribution between these two domains at the threshold level of stability have been emphasized. The thermodynamics of transfer of 1-pentanol from the continuous oil phase to the interface have been evaluated. n(a)(i),n(a)(i), standard Gibbs free energy (ΔG(t)(0)), standard enthalpy (ΔH(t)(0)), and standard entropy (ΔG(t)(0)) of transfer process have been found to be dependent on the molar ratio of water to surfactant (ω), type of nonionic surfactant and its content (X(Brij-58 or Brij-78)), oil and temperature. A correlation between (ΔH(t)(0)) and (ΔS(t)(0)) is examined at different experimental temperatures. Bulk surfactant composition dependent temperature insensitive microemulsions have been reported. Associated structural parameters, such as droplet dimensions and aggregation number of surfactant and cosurfactant at the droplet interface have been evaluated using a mathematical model after suitable modifications for mixed surfactant systems. In light of these parameters, the prospect of using these microemulsion systems for the synthesis of nanoparticles and the modulation of enzyme activity has been discussed. Correlations of the results in terms of the evaluated physicochemical parameters have been attempted.     

Kinetics of nonisothermal polymer crystallization

We adopt a cluster size distribution model to investigate the kinetics of nonisothermal polymer crystallization. The time dependencies of polymer concentration, number and size of crystals, and crystallinity (in Avrami plots) are presented for different cooling rates. The incubation period is also investigated at different cooling rates and initial temperatures. The relationship between cooling rates and incubation time is presented graphically and compared with experimental measurements. The initial temperature (relative to melting point) has a significant effect on nonisothermal crystallization. A comparison of moment and numerical solutions of the population balance equations shows the influence of Ostwald ripening. Agreement between modeling results and experimental measurements at different cooling rates supports the application of the distribution kinetics model for nonisothermal crystallization.     

Morphology transitions in nonionic surfactant adsorbed layers near their cloud points

The structure of adsorbed layers of several polyoxyethylene alkyl ether (C(n)E(m)) nonionic surfactants on silica and graphite surfaces has been imaged using atomic force microscopy as a function of temperature up to their cloud points. For all surfactants with a cloud point within the experimentally accessible range, the adsorbed layer morphology on silica evolved from globules at low temperatures first into rods and then a mesh with increasing temperature. This mesh structure was retained even when the solutions were heated above their cloud points into the two-phase coexistence region. Only C(12)E(3) was observed to form a laterally unstructured bilayer. On graphite, all surfactants formed straight, parallel hemicylinders at all temperatures examined.     

Conformation and dynamics of polyoxyethylene lauryl ether (Brij-35) chains in aqueous micellar solution studied by 2D NOESY and 1H NMR relaxation

Spin-lattice relaxation time, spin-spin relaxation time and two-dimensional nuclear Overhauser enhancement spectroscopy (2D NOESY) experiments of polyoxyethylene lauryl ether (Brij-35) micelles in aqueous solutions at a concentration of 100 times the critical micellar concentration (cmc) give direct evidence that the hydrophilic polyoxyethylene chains, staying in the exterior of the micellar core, are coiled, bent and aligned around the micellar core with a certain number of water molecules included. This hydrophilic layer is in contact with the solvent, water, keeping the micellar solution stable. 1H NMR relaxation time measurements show that the first oxyethylene group next to the alkyl chain participates in the formation of the surface area of the micellar core. The motion of the hydrophilic polyoxyethylene chains is less restricted as compared with the hy-drophobic alkyl chains.