Phase separation in semidilute aqueous poly(N-isopropylacrylamide) solutions

The phase separation mechanism in semidilute aqueous poly(N-isopropylacrylamide) (PNIPAM) solutions is investigated with small-angle neutron scattering (SANS). The nature of the phase transition is probed in static SANS measurements and with time-dependent SANS measurements after a temperature jump. The observed critical exponents of the phase transition describing the temperature dependence of the Ornstein-Zernike amplitude and correlation length are smaller than values from mean-field theory. Time-dependent SANS measurements show that the specific surface decreases with increasing time after a temperature jump above the phase transition. Thus, the formation of additional hydrogen bonds in the collapsed state is a kinetic effect: A certain fraction of water remains as bound water in the system. Moreover, H-D exchange reactions observed in PNIPAM have to be taken into account.     

Phase separation behavior of aqueous solutions of a thermoresponsive polymer

Cloud‐point and binodal curves of the LCST type were obtained for aqueous solutions of a thermoresponsive polymer, poly [2‐(2‐ethoxy)ethoxyethyl vinyl ether], poly(EOEOVE). The cloud‐point curve obtained was very flat except in a dilute region, that is the cloud‐point temperature was insensitive to the polymer concentration, resembling the cloud‐point curve for aqueous solutions of poly(N‐isopropylacrylamide). On the other hand, the binodal curve obtained was parabolic, and located within the two‐phase region of the cloud‐point curve. Accompanied with the phase separation, a sharp endothermic peak was observed in a region including the cloud‐point and binodal temperatures. The reciprocal of the osmotic compressibility ?Π/?c obtained by sedimentation equilibrium indicated that water changes from a good to poor solvent for poly(EOEOVE) with increasing temperature. Analyzing the ?Π/?c data by a thermodynamic perturbation theory, we determined the interchain interaction parameters, the hard‐core diameter d and the depth ε of the square‐well potential. Theoretical binodal and endothermic curves calculated by the perturbation theory using the estimated interaction parameters reproduced experimental ones semiquantitatively, but the theoretical binodal disagreed with the experimental flat cloud‐point curve. The disagreement at high concentrations was in the opposite direction to that expected from the sample polydispersity in the molecular weight. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2937–2949, 2005     

Phase separation kinetics of polyelectrolyte solutions

The kinetics of phase separation of aqueous solutions of sodium-poly(styrene sulfonate) (NaPSS) containing barium chloride (BaCl(2)) is studied by static and dynamic light scattering. We report a novel mechanism of phase separation, where an enrichment of polymer aggregates of well-defined size occurs in the very early stage of nucleation, which is then followed by a growth process in the formation of the new phase. In the latter stage, the polymer aggregates formed in the early stage act as the templating nuclei. Even in the homogeneous phase at higher temperatures above the upper critical phase boundary, polymer aggregates are present in agreement with previously reported results. Upon rapidly cooling the system below the phase boundary, the number concentration of the aggregates increases first by maintaining their size to be relatively monodisperse, before the growth process takes over at later times. The size and fractal dimension of aggregates in the homogeneous phase and the early nucleation stage of phase separation and the dependence of nucleation time and growth rate on quench depth and salt concentration are determined. The hydrodynamic radius (R(H)) of the unaggregated chains is of the order of 1-10 nm depending on the molecular weight of NaPSS, while R(H) of aggregates is of the order of 100 nm independent of the molecular weight of NaPSS. Unaggregated chains follow good solution behavior with a fractal dimension of 5/3 while the fractal dimension of aggregates is larger than 3.5 suggesting the branched nature of aggregates. Nucleation time is sensitive to quench depth and salt concentration. Increasing a quench depth or increasing BaCl(2) concentration shortens the nucleation time. After the nucleation time, during the growth period, the size of aggregates grows linearly with time, with growth rate being higher for deeper quench depths and higher BaCl(2) concentrations. The mechanism of phase separation of aqueous solutions of NaPSS and BaCl(2) is seen to proceed by utilizing the already-existing aggregates to nucleate the new phase, in marked contrast to hitherto known results on phase separation in uncharged polymer systems.     

Phase separation of aqueous solution of methylcellulose

 As we determined visually by the temperature cloud point method, the coexistence phase curve of methylcellulose in aqueous solution belongs to the LCST (low critical solution temperature) type. Rheological dynamic measurements reveal the existence of three gel domains. The gel (I) localized in the homogeneous phase at low concentration and low temperature, is a very weak gel, where the cross-links are attributed to pairwise hydrophobic interactions between the most hydrophobic zones of the backbone: the trimethyl blocks. The second gel (II) was revealed in the high concentration regime and below the LCST, it may be attributed to the formation of crystallites which play the role of cross-linking points. The third gel was concomitant to the micro-phase separation. In these turbid gels, syneresis develops slowly with time: the higher the temperature and the lower the concentration, the faster the syneresis. Near the three sol–gel transitions, a power law frequency dependence of the loss and storage moduli was observed and the viscoelastic exponent Δ(G′∼G″∼ω^Δ) was found to be 0.76 and 0.8 and to reach 1 at high concentration. Received: 18 July 1996 Accepted: 17 February 1997     

Phase separation dynamics of aqueous solutions of thermoresponsive polymers studied by a laser T-jump technique

Poly(N-isopropylacrylamide) and poly(vinyl methyl ether) are well-known thermoresponsive polymers. The aqueous solutions of these polymers exhibit a phase transition followed by phase separation with LCST approximately 305-310 K. In the present study, the dynamic behavior of the phase separation was analyzed by a laser T-jump method. Two different T-jump methodologies were employed: the first was a dye-photosensitized T-jump technique (indirect heating) using 532 nm laser pulses, while the other was a direct heating T-jump technique using 1.2 mum laser pulses. Both methods gave similar results. The time constants (tau) of the phase separation were systematically determined for 1-10 wt % aqueous solutions of the polymers, and a hydrodynamic radius (R) dependence for tau was clearly observed. The values of tau increased linearly with increasing square of R. The present behavior is interpretable in the framework of Tanaka's model for the volume phase transition of a gel, since each of the polymer chains are entangled in the present sample solutions, which can be regarded as approximating to a gel in solution.     

Light scattering studies of amphiphilic drugs promethazine hydrochloride and imipramine hydrochloride in aqueous electrolyte solutions

Two amphiphilic drugs, promethazine hydrochloride (PMT) and imipramine hydrochloride (IMP), have been studied using both static and dynamic light scattering techniques. Due to having rigid tricyclic hydrophobic moieties in their molecules, these drugs show interesting association behavior. The static light scattering (SLS) measurements show that the self-association commenced above a well-defined critical micellar concentration ( cmc), which decreases with increasing NaBr concentration. The Gibbs energy of micellization, DeltaG(0)M, in all cases, is negative. The colloidal stability of the system in terms of the interparticle interaction at different NaBr concentrations was studied using the dynamic light scattering (DLS) technique. The experimentally evaluated interparticle interaction parameter ( k D ) was compared with the Derjaguin-Landau-Verwey-Overbeek (DLVO) model. Interestingly, these two drugs with similar molecular structure show difference in their interparticle interactions, e.g., PMT showed complete agreement with the DLVO model whereas IMP showed clear deviation from this model at lower concentrations and agreement at higher concentrations of NaBr.     

Phase separation of micellar solutions at extreme dilution: A study of polystyrene-b-poly(sodium acrylate) micelles in aqueous NaCl solutions

Micellar solutions of polystyrene-b-poly(sodium acrylate) copolymers in aqueous NaCl were studied by static light scattering (SLS). It was found that micellar solutions of the copolymer, at concentrations of NaCl at, or above, 2.0 mol dm⁻³, became turbid on dilution at constant salt concentration and at constant temperature. Turbidity arose from highly dilute solutions (typically at a concentration three orders of magnitude lower than the overlap concentration of the micelle, C*), but at concentrations above the expected critical micellization concentrations (c.m.c.s). The observed turbidity was attributed to the phase separation of the micellar phase. A systematic investigation of the phase separation phenomenon was performed. The effects of various parameters on the solution behavior of the micellar solutions were studied, including the effect of the concentration of NaCl, the effect of temperature, and the effect of the length of the hydrophilic, corona-forming poly(sodium acrylate) block. Phase separation was attributed to the presence of a very large excess of NaCl in the dilute micellar solutions. It was proposed that phase separation arose because of the reduced hydration of the polyion, the decreased electrostatic repulsion between the micelles, and the increase in the amount of ion binding, which occur in highly dilute salt solutions. © 1996 John Wiley & Sons, Inc.     

Microphase separation in poor-solvent polyelectrolyte solutions: Phase diagram

Microphase separation in solutions of weakly charged polyelectrolytes in poor solvents is studied in the weak segregation limit within the framework of the mean field approximation using a method first developed by Leibler. As a result a complete phase diagram of the solution near the critical point is obtained. The regions of the stability of the disordered, homogeneous phase and of body-centered cubic (bcc), triangular and lamellar microdomain structures, as well as the phase separation regions are determined. The most striking difference in comparison with the corresponding diagram for block-copolymer melts is the existence of broad phase separation regions even for monodisperse systems. As the quality of solvent becomes poorer, the triangular microdomain structure remains the most stable among microdomain phases of other symmetry.     

Thermoreversible gelation and phase separation in aqueous methyl cellulose solutions

We derived typical phase diagrams for aqueous solutions of methyl cellulose (MC) of different molecular weights via micro‐differential scanning calorimetry, small‐angle X‐ray scattering, and visual inspection. The phase diagrams showed the cooccurrence of gelation and phase separation and qualitatively agreed with the theoretically calculated diagrams. The sol–gel transition line and phase separation line of a lower critical solution point type shifted toward lower temperatures and lower concentrations with an increase in the MC molecular weight. The sol–gel transition line intersected at a temperature higher than the critical point of the phase separation; therefore, both sol–gel phase separation and gel–gel phase separation were possible, depending on the temperature. Specifically, through visual inspection of a high molecular weight MC sample in the critical temperature region, we observed phase separation into two coexisting gels with different polymer concentrations. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 91–100, 2001     

Phase separation of polymer solutions on a solvent surface

Study of the dynamics of liquid droplets of dilute and semidilute polymer solutions on the surfaces of liquid subphases representing solvents for corresponding polymers has shown that a spot of a rather stable layer is formed on an air–liquid interface. The spot spreads over a liquid subphase surface to yield a monomolecular polymer layer. At the same time, the solvent passes into the solution, so that the polymer or its concentrated solution remains on the subphase surface. The polymer does not dissolve in the bulk subphase for several hours. The stability of the polymer spot has been explained under the assumption that the interfacial surface possesses elastic properties and hinders the penetration of macromolecules into the bulk subphase. Desolvation of macromolecules followed by phase separation occurs on the surface. The initial rate of the phase separation of the solution is rather high, while the time dependence of the diameter of the spreading spot is described by a scaling law with an exponent almost equal to 2/3.     

Flotation separation of hafnium(IV) from aqueous solutions

A simple, rapid method for the separation of hafnium from aqueous solutions has been investigated using^(175+181)Hf tracer. Cationic hafnium complex ions were floated from dilute acid solutions with sodium lauryl sulfate (SLS) and anionic hafnium complexes were floated from basic and oxalic acid solutions with hexadecyltrimethyl ammonium bromide (HTMAB). The conditions necessary for quantitative recovery of the metal and mechanisms of flotation are described.Author to whom correspondence should be addressed.     

Americium and samarium determination in aqueous solutions after separation by cation-exchange

The concentration of trivalent americium and samarium in aqueous samples has been determined by means of alpha-radiometry and UV–Vis photometry, respectively, after chemical separation and pre-concentration of the elements by cation-exchange using Chelex-100 resin. Method calibration was performed using americium (²⁴¹Am) and samarium standard solutions and resulted in a high chemical recovery for cation-exchange. Regarding, the effect of physicochemical parameters (e.g. pH, salinity, competitive cations and colloidal species) on the separation recovery of the trivalent elements from aqueous solutions by cation-exchange has also been investigated. The investigation was performed to evaluate the applicability of cation-exchange as separation and pre-concentration method prior to the quantitative analysis of trivalent f-elements in water samples, and has shown that the method could be successfully applied to waters with relatively low dissolved solid content.     

The density,viscosity and structural properties of aqueous ethambutol hydrochloride solutions

Ethambutol (EMB) is a bacteriostatic antimycobacterial drug prescribed to treat tuberculosis. It is bacteriostatic against actively growing TB bacilli. The density and viscosity of aqueous ethambutol hydrochloride solutions have been studied at 298.15, 301.15 and 304.15 K and at different concentrations (0.255, 0.168, 0.128, 0.087, 0.041, and 0.023 mol dm⁻³). The apparent molar volume of these solutions for different temperatures and concentrations was calculated from the density data. The relative viscosities of drug solutions have been analysed by Jones-Dole equation. The limiting apparent molar volumes have been evaluated for different temperatures. The different properties have been used to study structural properties, structure formation and breaking properties of drug and solute-solvent interactions in solutions.     

Phase behavior in mixtures of cationic and anionic surfactants in aqueous solutions

Phase behavior of cationic/anionic surfactant mixtures of the same chain length (n=10, 12 or 14) strongly depends on the molar ratio and actual concentration of the surfactants. Precipitation of catanionic surfactant and mixed micelles formation are observed over the concentration range investigated. Coacervate and liquid crystals are found to coexist in the transition region from crystalline catanionic surfactant to mixed micelles.The addition of oppositely charged surfactant diminishes the surface charge density at the mixed micelle/solution interface and enhances the apparent degree of counterion dissociation from mixed micelles. Cationic surfactants have a greater tendency to be incorporated in mixed micelles than anionic ones.     

The foam separation of zirconium(IV) from aqueous solutions

Summary The foam separation of zirconium(IV) from chloride solutions has been investigated over the 1.8–12 pH range using sodium lauryl sulphate or cetyl(trimethyl)ammonium bromide as collectors. The effects of gas flow rate, bubbling time, collector and zirconium(IV) concentrations, ageing of the metal ion, and ionic strength have been studied and the results are discussed in relation to the hydrolytic behaviour of zirconium(IV). Under optimum conditions,ca. 99.5% removal can be achieved.     

Modeling and simulation of butanol separation from aqueous solutions using pervaporation

A study was conducted to separate butanol from an aqueous solution using pervaporation. A specially designed and manufactured cell was used to separate the butanol from butanol/water solutions of different butanol concentrations (6-8-11-16-20-50) g/l. A 250 cm³ butanol mixture at 33 °C was used to feed the cell, while the pressure of permeation side was about 0 bar. Results revealed that butanol concentration changes non-linearly during the first 3 h, and then proceeds linearly. The percentage of butanol removal increases with increasing feed concentration. The permeability of the used membrane was determined experimentally. A resistance in series model was used to simulate the pervaporation step. The butanol concentration in the feed during the pervaporation step was predicted by using the developed model. There is a fair agreement between butanol concentration in feeding tank of pervaporation cell both experimentally and predicted from the developed model.     

Phase diagram for microphase separation transition in poor solvent polymer solutions

In the present paper, we consider the possibility of microphase separation transition in poor solvent polymer solutions. It is shown that this phenomenon can take place if the following two conditions are fulfilled: i) there is a large entropic contribution to the entropy of polymer/solvent mixing, i.e., solvent acts like a plastisizer; ii) this entropic contribution is nonlocal. Both conditions are met below the glass transition temperature for the pure polymer near the so-called Berghmans point when the glass transition curve intersects the liquid-liquid phase separation curve for polymer solutions. The phase diagram for the microphase separation transition is calculated within the framework of weak segregation approximation first proposed by Leibler for block-copolymer systems. The regions of stability of different microdomain structures (lamellar, triangular, body-centered-cubic) are obtained. It is shown that under certain conditions the phase diagram can have two critical points related to the macro- and microphase separation respectively.This paper is dedicated to Prof. E. W. Fischer on the occasion of his 65th Birthday.This work was done in the course of the Humboldt Research Award stay of A.R. Khokhlov at the Max-Planck-Institute for Polymer Research in Mainz. During this stay A.R.K. greatly benefited from numerous discussions with Professor E.W. Fischer who introduced him to the fascinating field of glass transition in polymer systems and formulated several new directions for future research.     

Adsorption and separation of amino acids from aqueous solutions on zeolites

The adsorption of various amino acids on zeolites with different structures was studied with regard to dependence of the pH value of the solution and the aluminum content of the zeolite in order to design tailor-made adsorbents for amino acid separations.     

Kinetics of thermoelectric signals under nanosecond laser heating of mercury in aqueous electrolyte solutions

Acetaldehyde oxidation to form acetic acid is enhanced by oxygen adsorbing ad-atoms to a great extent. At the same time, the oxidation is enhanced by the S_hole control by Bi, Te, Se and S ad-atoms, which do not adsorb oxygen. The number of unoccupied Pt sites isolated by these inactive ad-atoms (S_hole), which is available for reactions, is less than that required for the formation of poisoning species but equal to that required for the oxidation, resulting in the inhibition of the poison formation reaction and the enhancement of the oxidation. The same type of enhancement, that is, the enhancement both by the S_hole control by ad-atoms and by oxygen adsorbing ad-atoms, was previously found in the enhancement of formaldehyde oxidation by ad-atoms.     

Phase separation mechanism in gelling aqueous biopolymer mixture probed by light scattering

Using time-resolved static and dynamic light scattering (DLS) we have studied the kinetics of phase separation in an aqueous gelatin/maltodextrin mixture upon fast cooling. The time evolution of the droplet radius is modelled for the monodisperse case under reaction-limited and diffusion-limited conditions and compared with the observed evolution of the mode associated with the droplet diffusion. For quenches to above the gelatin ordering temperature, nucleation and rather reaction-limited than diffusion-limited growth and late-stage coalescence of droplets with diameters up to 90 μm were concluded. Quenches to well below the gelatin ordering temperature seem to induce diffusion-limited growth or (delayed) spinodal decomposition (SD) to a phase-separated microstructure with slow late-stage coarsening. In deep quenches, a second slow SD or diffusion-limited cluster aggregation (DLCA) process becomes apparent from the evolution of the static structure factor; the process seems to be related to the maltodextrin gelation in the composite.