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.     

Adsorption of cationic cellulose derivative/anionic surfactant complexes onto solid surfaces. II. Hydrophobized silica surfaces

The effect of the anionic surfactant SDS (sodium dodecyl sulfate) on the adsorption behavior of cationic hydroxyethyl cellulose (Polymer JR-400) and hydrophobically modified cationic cellulose (Quatrisoft LM-200) at hydrophobized silica has been investigated by null ellipsometry and compared with the previous data for adsorption onto hydrophilic silica surfaces. The adsorbed amount of LM-200 is found to be considerably larger than the adsorbed amount of JR-400 at both surfaces. Both polymers had higher affinity toward hydrophobized silica than to silica. The effect of SDS on polymer adsorption was studied under two different conditions: adsorption of polymer/SDS complexes from premixed solutions and addition of SDS to preadsorbed polymer layers. Association of the surfactant to the polymer seems to control the interfacial behavior, which depends on the surfactant concentration. For the JR-400/SDS complex, the adsorbed amount on hydrophobized silica started to increase progressively from much lower SDS concentrations, while the adsorbed amount on silica increased sharply only slightly below the phase separation region. For the LM-200/SDS complex, the adsorbed amounts increased progressively from very low SDS concentrations at both surfaces, and no large difference in the adsorption behavior was observed between two surfaces below the phase separation region. The complex desorbed from the surface at high SDS concentrations above the critical micelle concentration. The reversibility of the adsorption of polymer/SDS complexes upon rinsing was also investigated. When the premixed polymer/SDS solutions at high SDS concentrations (>5 mM) were diluted by adding water, the adsorbed amount increased due to the precipitation of the complex. The effect of the rinsing process on the adsorbed layer was determined by the hydrophobicity of the polymer and the surface.     

AQUEOUS TWO PHASE SYSTEMS FROM CYCLODEXTRIN POLYMERS AND HYDROPHOBICALLY MODIFIED ACRYLIC POLYMERS

In this study, a copolymer of N,N-dimethylacrylamide and hydroxyethyl-methacrylate was synthesized and hydrophobically modified with about 2 mole % of adamantyl groups. The mixing of this modified copolymer with water-soluble β-cyclodextrin polymers reveals thickening properties. Inclusion complexes formation between adamantyl groups and β-cyclodextrin cavities are responsible of intermolecular associations. Above a given total polymer concentration a phase separation occurs, giving a gel phase topped by a liquid phase. We have stated the phase diagram by analysis of the two phases in equilibrium. It has been observed that the two polymers are present in both phases and that the gel phase is more concentrated than the other one. This phase behavior is an original complex coacervation phenomenom which is due to the specific attractive interactions of the two different macromolecules: inclusion complexes formed between the two neutral polymers.     

Clouding and phase behavior of nonionic surfactants in hydrophobically modified hydroxyethyl cellulose solutions

Clouding phenomena and phase behaviors of two nonionic surfactants, Triton X-114 and Triton X-100, in the presence of either hydroxyethyl cellulose (HEC) or its hydrophobically modified counterpart (HMHEC) were experimentally studied. Compared with HEC, HMHEC was found to have a stronger effect on lowering the cloud point temperature of a nonionic surfactant at low concentrations. The difference in clouding behavior can be attributed to different kinds of molecular interactions. Depletion flocculation is the underlying mechanism in the case of HEC, while the chain-bridging effect is responsible for the large decrease of cloud point for HMHEC. Composition analyses for the formed macroscopic phases were carried out to provide support for associative phase separation for the case of HMHEC, in contrast to segregative phase separation for HEC. An interesting three-phase-separation phenomenon was reported in some HMHEC/Triton X-100 mixtures at high surfactant concentrations.     

Adsorption of cellulose derivatives on flat gold surfaces and on spherical gold particles

The adsorption of hydroxyethylcellulose (HEC), ethyl(hydroxyethyl)cellulose (EHEC), and their hydrophobically modified counterparts HM-HEC and HM-EHEC has been studied on planar gold and citrate-covered gold surfaces by means of quartz crystal microbalance with dissipation monitoring (QCM-D), and on citrate-covered gold particles with the aid of dynamic light scattering (DLS). The QCM-D results indicate that larger amounts of polymer are adsorbed from aqueous solutions of HM-HEC and HM-EHEC on both substrates than from solutions of their unmodified analogues. The adsorption affinity for all the polymers, except EHEC, is higher on the citrate-covered surfaces than on the bare gold substrate. This indicates that more adsorption sites are activated in the presence of the citrate layer. The experimental adsorption data for all the polymers can be described fairly well by the Langmuir adsorption isotherm. However, at very low polymer concentrations significant deviations from the model are observed. The value of the hydrodynamic thickness of the adsorbed polymer layer (delta h), determined from DLS, rises with increasing polymer concentration for all the cellulose derivatives; a Langmuir type of isotherm can be used to roughly describe the adsorption behavior. Because of good solvent conditions for HEC the chains extend far out in the bulk at higher concentrations and the value of delta h is much higher than that of HM-HEC. The adsorption of EHEC and HM-EHEC onto gold particles discloses that the values of delta h are considerably higher for the hydrophobically modified cellulose derivative, and this finding is compatible with the trend in layer thickness estimated from the QCM-D measurements.     

Polyelectrolyte-surfactant complexes with long range order

Mixtures of carboxymethyl cellulose (CMC) or hydrophobically modified CMC with an oppositely charged surfactant (benzyldimethyltetradecylammonium chloride) in water were prepared. When the global polymer concentration is 0.18% by weight and the surfactant content is high enough, a precipitate with hexagonal order is formed. The precipitate composition shows practically constancy in its water content and a slight diminution in polymer concentration when the global surfactant content is varied between 0.9 and 23 wt%. The lattice parameter in this phase decreases when the polymer/surfactant ratio in the phase increases; this variation is faster with CMC than with the hydrophobically modified CMC. In this way electrostatic and hydrophobic interactions are far from being additive. From the extrapolation to infinite dilution, the global interaction seems to depend on the substitution degree in the polymer. Additionally, the comparison between the radius at the polar-apolar interface in the cylinders and the lattice parameter as a function of polymer/surfactant ratio in the hexagonal phase is compatible with some of the alkyl chains belonging to the hydrophobically modified CMC being present in the aqueous zone.     

Cellulose–polyacrylonitrile blends, 1: thermodynamic interaction parameters and phase diagram in dimethylacetamide–LiCl

The phase diagram of ternary mixtures composed of cellulose, polyacrylonitrile and dimethylacetamide–7% LiCl was determined at room temperature. Homogeneous solutions were observed at low polymer concentrations. Two demixing areas were experimentally evidenced when the concentration increased: the first corresponds to an equilibrium between isotropic phases, while the second represents the coexistence of a cellulose mesophase with an isotropic phase. The diagram is discussed in terms of the interaction parameters, determined via light scattering.     

Use of cellulose derivatives on gold surfaces for reduced nonspecific adsorption of immunoglobulin G

This study addresses the design of protein-repellent gold surfaces using hydroxyethyl- and ethyl(hydroxyethyl) cellulose (HEC and EHEC) and hydrophobically modified analogues of these polymers (HM-HEC and HM-EHEC). Adsorption behavior of the protein immunoglobulin G (IgG) onto pure gold and gold surfaces coated with cellulose polymers was investigated and described by quartz crystal microbalance with dissipation monitoring (QCM-D), atomic force microscopy (AFM) and contact angle measurements (CAM). Surfaces coated with the hydrophobically modified cellulose derivatives were found to significantly outperform a reference poly(ethylene glycol) (PEG) coating, which in turn prevented 90% of non-specific protein adsorption as compared to adsorption onto pure gold. HEC and EHEC prevented around 30% and 60% of the IgG adsorption observed on pure gold, while HM-HEC and HM-EHEC were both found to completely hinder biofouling when deposited on the gold substrates. Adsorption behavior of IgG has been discussed in terms of polymer surface coverage and roughness of the applied surfaces, together with hydrophobic interactions between protein and gold, and also polymer-protein interactions.     

Network formation of catanionic vesicles and oppositely charged polyelectrolytes. Effect of polymer charge density and hydrophobic modification

In nonequimolar solutions of a cationic and an anionic surfactant, vesicles bearing a net charge can be spontaneously formed and apparently exist as thermodynamically stable aggregates. These vesicles can associate strongly with polymers in solution by means of hydrophobic and/or electrostatic interactions. In the current work, we have investigated the rheological and microstructural properties of mixtures of cationic polyelectrolytes and net anionic sodium dodecyl sulfate/didodecyldimethylammonium bromide vesicles. The polyelectrolytes consist of two cationic cellulose derivatives with different charge densities; the lowest charge density polymer contains also hydrophobic grafts, with the number of charges equal to the number of grafts. For both systems, polymer-vesicle association leads to a major increase in viscosity and to gel-like behavior, but the viscosity effects are more pronounced for the less charged, hydrophobically modified polymer. Evaluation of the frequency dependence of the storage and loss moduli for the two systems shows further differences in behavior: while the more long-lived cross-links occur for the more highly charged hydrophilic polymer, the number of cross-links is higher for the hydrophobically modified polymer. Microstructure studies by cryogenic transmission electron microscopy indicate that the two polymers affect the vesicle stability in different ways. With the hydrophobically modified polymer, the aggregates remain largely in the form of globular vesicles and faceted vesicles (polygon-shaped vesicles with largely planar regions). For the hydrophilic polycation, on the other hand, the surfactant aggregate structure is more extensively modified: first, the vesicles change from a globular to a faceted shape; second, there is opening of the bilayers leading to holey vesicles and ultimately to considerable vesicle disruption leading to planar bilayer, disklike aggregates. The faceted shape is tentatively attributed to a crystallization of the surfactant film in the vesicles. It is inferred that a hydrophobically modified polyion with relatively low charge density can better stabilize vesicles due to formation of molecularly mixed aggregates, while a hydrophilic polyion with relatively high charge density associates so strongly to the surfactant films, due to strong electrostatic interactions, that the vesicles are more perturbed and even disrupted.     

Nonionic surfactant and temperature effects on the viscosity of hydrophobically modified hydroxyethyl cellulose solutions

Nonionic surfactant and temperature effects on the viscosity of hydrophobically modified hydroxyethyl cellulose (HMHEC) solutions are investigated experimentally. Weak shear thickening at intermediate shear rates takes place for HMHEC at moderate concentrations and becomes more significant at lower temperatures. While this amphiphilic polymer in surfactant-free solution does not turn turbid by heating to 95 degrees C, its mixture with nonionic surfactant shows a lower cloud point temperature than does a pure surfactant solution. For some mixture cases, phase separation takes place at temperatures as low as 2 degrees C. The drop of cloud point temperature is attributed to an additional attractive interaction between mixed micelles via chain bridging. With increasing temperature, the viscosity of an HMHEC-surfactant mixture in aqueous solution first decreases but then rises considerably until around the cloud point. The observed viscosity increase can be explained by the interchain association because of micellar aggregation.     

Effects of added surfactants on thermoreversible gelation of associating polymer solutions

The influence of added surfactants on physical properties of associating polymer solutions was examined by a new statistical‐mechanical theory of associating polymer solutions with multiple junctions and by computer simulation. The sol–gel transition line, the spinodal line, and the number of elastically effective chains in the mixed networks were calculated as functions of the concentration of added surfactants. All of them exhibited nonmonotonic behavior as a result of the following two competing mechanisms. One was the formation of new mixed micelles by binding surfactants onto the polymer associative groups. These micelles serve as crosslink junctions and promote gelation. The other was the replacement of polymer associative groups in the already formed network junctions by added surfactants. Such replacement lowers connectivity of junctions and destroys networks. The critical micelle concentration was also calculated. The results are compared with the reported experimental data on poly(ethylene oxide)‐based associating polymers and hydrophobically modified cellulose derivatives. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 733–751, 2004     

Effect of hydrophobically modified polymers on shear-induced multilamellar vesicles

The effects of polymer concentration, polymer molecular weight, and hydrophobe substitution level of modified poly(acrylic acid) polymers on the formation, size, and viscoelastic properties of shear-induced multilamellar vesicles (onions) are studied by rheology and light diffraction. The onions are close-packed, space-filling vesicles formed by shearing aqueous lamellar phases of C12E5 surfactant to produce phases with sufficient order and size uniformity (O(1-3 microm)) to diffract light. The addition of hydrophobically modified polymers enhances the rate of formation, uniformity, and stability independent of hydrophobe substitution level. Onion size decreases with increasing shear rate as observed for pure surfactant onion systems, but the shear-rate dependence is changed by the polymer. The onion phase has a plateau modulus that increases with polymer concentration but is independent of hydrophobe substitution level or molecular weight. The model presented by Panizza et al. that relates the plateau modulus of the onion phase to membrane rigidity and the compression modulus is consistent with independent measurements of membrane properties from SANS.     

pH-dependence of the properties of hydrophobically modified polyvinylamine

A series of N-alkyl or N-benzyl substituted polyvinylamines (PVAm) were prepared and the properties of aqueous solutions were measured as functions of pH. The polymer solutions showed almost no surface activity under acidic conditions whereas surface tension was reduced to 40-50 mN/m around pH 9. Increasing either the degree of hydrophobic substitution or the hydrophobic chain length lowered the pH at which surface tension lowering was observed. Hydrophobic substitution also shifted plots of the degree of ionization versus pH toward lower pH which means lower pH values were required to achieve a given value of polymer charging. The hydrophobically modified PVAm associated in water giving species whose apparent diameter measured by dynamic light scattering decreased with increasing pH, whereas the electrophoretic mobilities of the associated species increased with decreasing pH. Although many hydrophobically modified and pH sensitive polymers have been described in the literature for applications in biomaterials, drug release and as pH sensitive surfactants, the hydrophobically modified PVAms are particularly attractive because they are easily prepared from commercially available polyvinylamines.     

Complexation of hydrophobically modified polyelectrolytes with surfactants: anionic poly(maleic acid/octyl vinyl ether)/anionic sodium dodecyl sulfate

Interactions of surfactants with hydrophobically modified polyelectrolytes in aqueous solutions are important in several applications such as detergents, cosmetics, foods, and paints. Fundamental questions arise on the mechanisms of complexation of the polyelectrolyte and surfactant that control their behavior. In this work, the complexation was studied by examining interactions in aqueous solutions of a hydrophobically modified polymer, poly(maleic acid/octyl vinyl ether) (PMAOVE), with sodium dodecyl sulfate (SDS) by monitoring viscosity, pyrene solubility, light scattering, and analytical ultracentrifugation. When the anionic surfactant SDS was added to aqueous solutions of the similarly charged polymer PMAOVE, the surfactant was incorporated into the hydrophobic nanodomains of PMAOVE even far below the cmc of the surfactant. On the basis of viscosity, pyrene solubility, and analytical ultracentrifugation data, it is proposed that PMAOVE undergoes structural unfolding and at higher SDS concentrations mixed micelles are formed.     

Interaction of surfactants with thickeners used in waterborne paints: a rheological study

Studies of the phase diagram and linear viscoelasticity of aqueous solutions of hydrophobically modified hydroxyethyl cellulose (HMHEC), a thickener used in water-based paints, and SDS reveal that SDS-HMHEC mixed micelles are formed that increase the number of hydrophobic junctions and enhance interpolymer association up to an [SDS]/[HMHEC] ratio. This fact produces a strong increase of viscoelasticity or a phase separation, depending on the [HMHEC]. At higher ratios the excess of micelles with predominant SDS isolates hydrophobes and disrupts the micellar network. Then, viscoelastic functions decrease and HMHEC behaves as a nonassociative polymer. TTAB and Brij30 also interact with HMHEC, but in a different way. No phase separation is observed with these surfactants. TTAB forms mixed micelles and new junction points in the same way as SDS. However, this surfactant does not stabilize the micelles as SDS does, presumably due to the different interaction between the OH from the cellulose and the charged groups. Results seem to indicate that Brij30 enters into the hydrophobic aggregates of HMHEC and stabilizes them, increasing relaxation time, but it does not form new junction points, since it forms quite big micelles.     

Comparison of HPLC enantioseparation of substituted binaphthyls on CD‐, polysaccharide‐ and synthetic polymer‐based chiral stationary phases

Retention and enantioseparation behavior of ten 2,2′‐disubstituted or 2,3,2′‐trisubstituted 1,1′‐binaphthyls and 8,3′‐disubstituted 1,2′‐binaphthyls, which are used as catalysts in asymmetric synthesis, was investigated on eight chiral stationary phases (CSPs) based on β‐CD, polysaccharides (tris(3,5‐dimethylphenylcarbamate) cellulose or amylose CSPs) and new synthetic polymers (trans‐1,2‐diamino‐cyclohexane, trans‐1,2‐diphenylethylenediamine and trans‐9,10‐dihydro‐9,10‐ethanoanthracene‐(11S,12S)‐11,12‐dicarboxylic acid CSPs). Normal‐, reversed‐phase and polar‐organic separation modes were employed. The effect of the mobile phase composition was examined. The enantiomeric separation of binaphthyl derivatives, which possess quite similar structures, was possible in different enantioselective environments. The substituents and their positions on the binaphthyl skeleton affect their properties and, as a consequence, the separation system suitable for their enantioseparation. In general, the presence of ionizable groups on the binaphthyl skeleton, substitution with non‐identical groups and a chiral axis in the 1,2′ position had the greatest impact on the enantiomeric discrimination. The 8,3′‐disubstituted 1,2′‐binaphthyl derivatives were the most easily separated compounds in several separation systems. From all the chiral stationary phases tested, cellulose‐based columns were shown to be the most convenient for enantioseparation of the studied analytes. However, the polymeric CSPs with their complementary behavior provided good enantioselective environments for some derivatives that could be hardly separated in any other chromatographic system.     

Concentration dependence of the interfacial tension for aqueous two-phase polymer solutions of dextran and polyethylene glycol

We studied the interfacial tension between coexisting phases of aqueous solutions of dextran and polyethylene glycol. First, we characterized the phase diagram of the system and located the binodal. Second, the tie lines between the coexisting phases were determined using a method that only requires measuring the density of the coexisting phases. The interfacial tension was then measured by a spinning drop tensiometer over a broad range of polymer concentrations close to and above the critical point. In this range, the interfacial tension increases by 4 orders of magnitude with increasing polymer concentration. The scaling exponents of the interfacial tension, the correlation length, and order parameters were evaluated and showed a crossover behavior depending on the distance to the critical concentration. The scaling exponent of the interfacial tension attains the value 1.50 ± 0.01 further away from the critical point, in good agreement with mean field theory, but the increased value 1.67 ± 0.10 closer to this point, which disagrees with the Ising value 1.26. We discuss possible reasons for this discrepancy. The composition and density differences between the two coexisting phases, which may be taken as two possible order parameters, showed the expected crossover from mean field behavior to Ising model behavior as the critical point is approached. The crossover behavior of aqueous two-phase polymer solutions with increasing concentration is similar to that of polymer solutions undergoing phase separation induced by lowering the temperature.     

Mixed systems of hydrophobically modified polyelectrolytes: controlling rheology by charge and hydrophobe stoichiometry and interaction strength

Rheology and phase separation were investigated for aqueous mixtures of two oppositely charged hydrophobically modified polyelectrolytes. The typical phase separation, normally seen for oppositely charged polymer mixtures, is dramatically reduced by the presence of hydrophobic modification, and phase separation is only detected close to the point of charge neutralization. While the two polyelectrolytes separately can give high viscosities and a gel-like behavior, a pronounced maximum in viscosity and storage modulus with the mixing ratio of the polyelectrolytes is observed; the maximum is located between the points of charge and hydrophobe stoichiometry and reflects a combination of hydrophobic and electrostatic association. Lowering the charge density of the anionic polymer leads to a strengthened association at first, but at lower charge densities there is a weakened association due to the onset of phase separation. The strength of the electrostatic interaction was modified by adding salt. Increased ionic strength can lead to phase separation and to increased or decreased viscosity depending on the polyelectrolyte mixing ratio.     

Characterization of Water-Soluble Cellulose Derivatives in Terms of the Molar Mass and Particle Size as well as Their Distribution

The property profile of cellulose derivatives dissolved in aqueous solvents is not only dependent on the chemical composition (average-, molar- or regiospecific degree of substitution, as well as the substitution along the chain), solvent, temperature and concentration but also on the molar mass and the particle size. All this information can be obtained from the Mark-Houwink-Sakurada-relationship ([;gh]-M-) or the R_G-M-relationship, if these are at hand. These relationships are suitable for a specific degree of substitution. The R_G-M-relationship has only been determined and published for a few water-soluble cellulose derivatives. The prerequisite is the availability of a homologous series of samples with the same chemical composition. In this paper it is shown that only the ultrasonic degradation is able to create such a series. Due to the ability of coupled methods of analysis to acquiring absolute data, molar mass and particle size distributions have been compiled in recent years. Using such methods it was possible to determine molar mass and particle size distributions of several aqueous cellulose derivative solutions by combining a fractionation unit (size exclusion chromatography (SEC) or flow field-flow fractionation (FFFF)) with multi angle laser light scattering (MALLS) for the detection of Mw and R_G and concentration detection (DRI). Results for nonionic cellulose ethers, mixed cellulose ethers, ionic carboxymethyl cellulose, sulfoethyl cellulose, hydrophobically modified hydroxyethyl cellulose were obtained and are partially discussed with focus on the recovery of cellulose derivates after fractionation and the impact on the distribution functions.     

Association of poly-N-[tris(hydroxymethyl)methyl] acrylamide with a water soluble β-cyclodextrin polymer

An acrylic polymer with pendent adamantyl groups was synthesized and its properties in an aqueous solution with a β-cyclodextrin (βCD) epichlorhydrin polymer examined. Viscosity properties of precursor and modified polymers show differences at low concentrations, but not at higher concentration probably due to very important hydrogen bonds which prevent the formation of intermolecular hydrophobic bonds. The association of both complementary polymers through the inclusion of adamantyl groups is evidenced by phases separation occurrence. Phase diagrams were established at two different concentrations of polymers. We have shown a maximal association of both polymers at these two concentrations, for the same ratio βCD moles/adamantyl groups: 2.4. Salt addition favors this association and displaces the two phases zone to smaller concentrations of modified polymer. Further, 4-nitrophenol can be extracted by the concentrated phase resulting from mixture of solutions of guest and host polymers, pointing out the availability of the associated phase to trap organic molecules.