Phase Behavior Of Aqueous Solutions Research Articles

Self-assembly of block copolymers derived from elastin-mimetic polypeptide sequences

Protein polymers derived from elastin-mimetic peptide sequences can be synthesized with near-absolute control of macromolecular architecture using genetic engineering techniques. Elastin-mimetic diblock and triblock copolymers have been prepared using this approach in which the individual elastin blocks display different phase behavior in aqueous solution. The selective collapse of the more hydrophobic blocks above the lower critical solution temperature was employed to drive the thermo-reversible self-assembly of elastin-mimetic diblock and triblock copolymer into protein-based nanoparticles and nano-textured hydrogels, respectively. These materials display considerable promise as biomaterials for applications in drug delivery and soft tissue augmentation.

Cloud-Point Phenomena in Wormlike Micellar Systems Containing Cationic Surfactant and Salt

Cationic surfactants with an erucyl (C22, monounsaturated) tail display unusual phase behavior in aqueous solution as a function of temperature and added salt concentration. Low amounts of salts with binding counterions such as sodium tosylate (NaTos) lead to highly viscoelastic wormlike micellar solutions. With further addition of salt, the viscosity decreases, and the solutions cloud on heating (i.e., exhibit lower consolute phase behavior). The cloud-point temperature and the zero-shear viscosity η0 pass in parallel through minima as a function of NaTos concentration. Cloud-point behavior is seen over a wider range of salt concentrations in the case of sodium salicylate (NaSal), which has an even stronger binding counterion. In the case of a weakly binding salt (NaCl), phase transitions are observed only at much higher salt contents, and the phase behavior is predominantly of the upper consolute type. Light and neutron scattering data show evidence of critical concentration fluctuations near the cloud ...

Effects of pressure on the behavior of (hydroxypropyl)cellulose in aqueous solution

The effects of pressure and temperature on the phase behavior of aqueous solutions of (hydroxypropyl)cellulose (HPC) were in¶vestigated at low and high concentrations. In dilute solutions, the transition temperature (Tt) as measured by apparent light scattering increased with an increase in pressure at lower pressures, but decreased with increasing pressure above 100 MPa. At lower temperatures, the transition pressure (Pt) increased slightly with increasing temperature, but decreased with at temperatures above 10–15 °C. Both Tt and Pt showed a moderate polymer-concentration dependence and they also showed strong concentration dependence on salt; Tt and Pt decreased with increasing concentrations of KCl and K2SO4, whereas the addition of KI or KSCN increased Tt and Pt. The apparent absorbance of concentrated solutions (62.5%) of HPC, in a cholesteric (chiral nematic) mesophase, was also measured. The spectrum shifted to a longer wavelength under high pressure. These results could be explained by assuming that the helicoidal pitch was increased under elevated pressure, probably due to a decrease in the angle between the two semiflexible and twisted HPC molecules in adjacent planes under high pressure.

Phase Behavior of Dilute Aqueous Solutions of Lipid−Surfactant Mixtures: Effects of Finite Size of Micelles

We consider the phase behavior of an aqueous solution of a mixture of two amphiphiles having different structural properties: a phospholipid, which tends to self-assemble into extended flat bilaye...

Elongated Aggregates Formed by Cationic Gemini Surfactants

The phase behavior of aqueous solutions of several dissymmetric gemini surfactants (with two hydrophobic chains of different lengths) was studied. From rheological and X-ray scattering studies as well as from freeze fracture imaging, some general patterns of the phase behavior were observed which are remarkably different from those observed for monomeric surfactants. The sequence of phases observed with increasing surfactant concentration was an isotropic wormlike micellar phase, multilayered structures which are first isotropic and then organized with orientational ordering, and an inverted hexagonal phase. The multilayered phase showed a scattering peak corresponding to a periodicity of 40 A indicating the presence of stacks of bilayers without a water layer in between. For some of the samples, cryo-TEM showed that wormlike micelles (WLM) evolve into a ribbonlike structure (elongated bilayer). Upon further increase in concentration, the ribbons transform into multilayered structures with a well-defined width.

Behavior of aqueous solutions of poly(ethylene oxide-b-propylene oxide) copolymers containing a hydrotropic agent

The effect of the hydrotropic agent, sodium p-toluenesulfonate (NaPTS), was evaluated on the micelle formation process and on phase behavior of aqueous solutions containing poly(ethylene oxide-b-propylene oxide) (PEO–PPO) copolymers. We have studied monofunctional diblock copolymers coupled with hydrocarbons groups (R—PEO—PPO—OH and R—PPO—PEO—OH, where R length is linear C4 and C12–14). The critical micelle concentration (CMC) and critical micelle temperature (CMT) values of the aqueous copolymers solutions were obtained from both surface tension versus concentration plots and the dye solubilization method. The influence of the hydrocarbons groups length and PPO segment position in the structure of the copolymers were also analyzed. The same measures were obtained for the aqueous solutions of hydrotropic agent which, in turn, also presented molecular aggregation. The presence of the hydrotropic agent in the aqueous copolymers solutions altered the surface tension of these solutions and the occupied molecular area per copolymer molecule at air–water interface and CMC and CMT values of the copolymers. On the other hand, the aggregation points and the surface tension of the NaPTS solutions were dependent on the copolymer structure and composition. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 2459–2468, 1998

Aqueous biphase formation by mixtures of dextran and hydrophobically modified dextran

A series of modified dextrans was prepared by condensation of straight chain saturated C3, C4 and C6 fatty acids and the phase behavior of aqueous solutions of these materials with unmodified dextran was measured as a function of temperature, concentration and degree of substitution. At a constant degree of substitution the tendency towards aqueous biphase formation increased with the length of the hydrophobic substituent, the temperature and the molecular weight. Fluorescence studies of the modified dextrans with pyrene as a probe indicated the presence of hydrophobic micro-domains. Rheological study showed that there was no large-scale association for C3 and C4 substituted dextran, mainly intramolecular association, however some intermolecular association existed for C6 substituted dextran. The results are compared with the behavior of the classical PEG/dextran biphase systems, and mechanisms driving phase separation are discussed.

Salt effects on the diffusion and release rate of propranolol from poloxamer 407 gels

In this report we show that salts commonly included in the formulation of poloxamer 407 (P407) gels can dramatically change the release rate of a drug from the gel. We used a model drug, propranolol, and measured its diffusion from a 21% P407 gel into an aqueous medium at 37°C through a cellulose membrane. The data followed the Higuchi square root law during the first hour of release. The release rate and diffusion coefficient of propranolol were significantly reduced when NaCl, Na 2SO 4, NaH 2PO 4, MgSO 4 and CaCl 2 were added to the gels. The magnitude of the effect on release rate depends on the nature and concentration of the salt. Since propranolol is expected to be located outside the micelles in the gel, we attribute these salt effects to an increase in microviscosity of the aqueous regions of the gel, rather than to a change in micellization behavior. This is analogous to the change in the aggregation and phase behavior of aqueous solutions of non-ionic surfactants in the presence of salts.

Complex Phase Behavior in Aqueous Solutions of Poly(ethylene oxide)−Poly(ethylethylene) Block Copolymers

The many similarities in morphological behavior exhibited by diblock copolymer melts and lyotropic surfactant suspensions suggest the existence of common physical principles underlying these phenomena. In an effort to identify such principles, we discuss the phase behavior of aqueous solutions of poly(ethylene oxide)−poly(ethylethylene) (PEO−PEE) block copolymers. The molecular weights of these materials are roughly twice that of common poly(ethylene oxide)−poly(propylene oxide) (PEO−PPO) surfactants, leading to microphase separation and a rich mesophase polymorphism in the absence of solvent. Despite the strongly thermotropic nature of the PEO−water interaction, the phase behavior in solutions of high polymer concentration is primarily lyotropic. Mesophase transitions observed in this regime show characteristics similar to those observed in undiluted block copolymers. A crossover to thermotropic behavior occurs at concentrations on the order of 60 wt % polymer, which corresponds to the formation of a stoichiometric complex of three water molecules per ethylene oxide repeat. This crossover is accompanied by changes in the structural dimensions, long-range order, and mechanical characteristics of the mesophases. Transitions in these dilute solutions resemble those previously observed in surfactant systems. These findings suggest that the complex structure of hydrated PEO plays a central role in determining the phase behavior of the system.

Interfacial and capillary properties of a micellar solution model

A prominent feature of the phase behavior of aqueous solutions of nonionic amphiphiles is a phase separation into dilute and concentrated micellar liquids above a lower critical solution temperature T c . Experiment has revealed a strong asymmetry of the coexistence curve, a high sensitivity of the (effective) critical indexes on deviations of the critical composition, and an unusual excess adsorption at the liquid-liquid interface as indicated by ellipsometric measurements. Here, we review and extend the study of a micellar solution model designed to describe amphiphiles in polar solvents and, as we illustrate, it reproduces the properties mentioned above. The model allows for elimination of the degrees of freedom within the aggregates, and this leads to an equivalence with the ordinary Ising model but with temperature and concentration-dependent field and coupling. The latter is identified with the effective intermicellar interaction.

Phase behavior of aqueous solutions containing nonionic surfactant-polyethylene glycol mixtures

The phase separation behavior of two nonionic surfactants, polyoxyethylene (10) oleyl ether (I) and polyoxyethylene (10) cetyl ether (II), was examined in solutions containing polyethylene glycols (PEGs) of various molecular weights. It was found that PEGs decrease the cloud points of both surfactants, and that this decrease is dependent upon the concentration and molecular weight of the PEG. This effect of PEGs on the cloud point can be explained by a depletion flocculation mechanism, which results in a segregative phase separation of the solution into surfactant-rich and polymer-rich phases. The Krafft point of II increased in the presence of PEGs, and this increase was dependent on the concentration of PEG, but independent of its molecular weight.

Phase behavior of aqueous solutions of hydroxypropyl cellulose

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTPhase behavior of aqueous solutions of hydroxypropyl celluloseStefano GuidoCite this: Macromolecules 1995, 28, 13, 4530–4539Publication Date (Print):June 1, 1995Publication History Published online1 May 2002Published inissue 1 June 1995https://doi.org/10.1021/ma00117a023RIGHTS & PERMISSIONSArticle Views706Altmetric-Citations67LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (4 MB) Get e-Alerts Get e-Alerts

Phase behaviour of aqueous solutions of neutral and charged polymer mixtures

A model for the Gibbs energy of mixing for the systems composed of water, neutral polymer and polyelectrolyte is established by combining the theories of Flory-Huggins and Manning, and by treating the free counterions as complementary species. The phase behaviour of two model systems and its dependence on the salt concentration are studied by using the model. The basic features of the phase behaviour of the systems can be captured, and the calculated results are in good agreement with the experimental data.

Branched non-ionic oligo-oxyethylene V-amphiphiles effect of molecular geometry on LC-phase behavior 2

A homologous series of double-chain non-ionic surfactants with the general formula (Cn)2 GEmM, where Cn denotes an alkyl chain, G a triglyceryl, and EmM an oligo-oxyethylene mono-methyl ether are synthesized withn=6–8 andm=6, 8, 10. The branched hydrophobic part of the surfactants introduces a large cross-section of the lipophilic surfactant moiety that is directly reflected in the shape of the micelles and the LC phase behavior in aqueous solution. The phase behavior of these V-amphiphiles is investigated with polarizing microscopy and x-ray diffraction and reveals that the branched lipophilic part strongly stabilizes the lamellar mesophase. This is in agreement with established packing models.

Phase behavior of poly(ethylene oxide)-poly(propylene oxide) block copolymers in aqueous solution

The phase behavior of aqueous solutions of polymers containing ethylene oxide (EO) and propylene oxide (PO) has been modeled on the basis of a mean-field lattice theory for multicomponent mixtures of copolymers with internal states occurring in heterogeneous systems. The regions for monomeric solution, spherical micelles, and elongated rods have been examined for three (EO) x (PO) y (EO) x triblock copolymers. A semiquantitative description of the strong temperature dependence of the phase behavior was obtained. At low polymer concentrations and at low temperatures a monomeric solution was found, whereas at either higher temperatures or higher concentrations a solution of spherical aggregates was present

Phase behavior of aqueous mixtures of hexaethylene glycol monododecyl ether and sodium alkylsulfonates

Addition of low concentrations of sodium alkylsulfonates markedly changes the phase behavior of aqueous solutions of hexaethylene glycol monodecyl ether. Longer chain sulfonates raise the cloud temperature and lead to the formation of new high-temperature phases. One of these phases is colored when illuminated with white light and displays streaming birefringence, while another phase is colored when illuminated.

A differential scanning calorimetric study of the thermotropic phase behavior of surfactant assemblies in water

Differential scanning calorimetry (DSC) has been used to study the phase behavior of aqueous solutions of a homologous series of cationic surfactants, tetradecyl-, hexadecyl-, and octadecyltrimethylammonium bromide. Three separate endothermic transitions were observed by the heating scans of DSC. They are in turn from the higher temperature the transitions (i) from the coagel phase (or the so-called hydrated solid) to the micellar solution; (ii) from the metastable coagel, which is one of the polymorphic forms of the hydrated solid, to the supercooled micelle; and (iii) from the gel phase (metastable) to the supercooled micelle. The metastable gel phase appears when the micellar solution is supercooled and is allowed to stand for a short time at 5°C. A sufficiently long storage time (1–6 days) at 5°C is necessary to complete the transformation to the stable coagel phase. After storage at −20°C the metastable coagel phase appears. The enthalpy and entropy changes were determined for individual transitions. The difference in the enthalpy changes between first and second transitions is attributable to the difference in the dissolution enthalpy from the stable or metastable coagel to the singly dispersed surfactant solution.

CPS LG 315 Correlation between the phase behavior of ternary systems and removal of oil in the washing process

The correlation between the phase behavior of aqueous solutions of nonionic surfactants and the removal of oil from fabrics has been determined. Model washing experiments with pure and technical grade nonionic surfactants indicated that, in the dispersion ranges of liquid crystals, oil removal is substantially more temperature-dependent compared to micellar or surfactant liquid phases. In this phase region, interfacial tension does not seem to play a substantial role. The results are discussed in terms of the macroscopic properties of mesophases.

Water structure and changes in thermal stability of the system poly(ethylene oxide)–water

The phase behaviour of aqueous solutions of poly(ethylene oxide), PEO, is analysed by means of a structural model of the system and a simple statistical-mechanical model based thereupon. The intention is to elucidate the structural questions involved in the water–PEO coupling and to gain some insight in possible consequences of this coupling. The experimental partial molar enthalpy and entropy of water can be reproduced, at least in fairly dilute solutions, if a zone with increased structuring of the water is assumed to exist around the PEO chain. The phase separation that takes place at high temperatures is traced back to the increase in total extension of the zones of enhanced water structure that occurs when the water content is increased. The chain-length dependence of the location of the solubility gap is mainly determined by the combinatorial entropy of the chains. The water solubility of PEO, which is unique in this respect among the polyethers, can be explained in terms of a good strutural fit between the water and the polymer.