Interactions of a hydrophobically modified polymer with oppositely charged surfactants

The interaction of a hydrophobically modified anionic polymer (PMAOVE) with a cationic surfactant (DTAB) was studied using a multi-technique approach: turbidity, surface tension, and viscosity measurements, as well as EPR (5-doxyl stearic acid) and fluorescence (pyrene) probe techniques were used. In the investigated pH range (4-10), the cationic surfactant headgroups interact with the anionic carboxylic groups of the polymer backbone. In addition, nonpolar interactions of the surfactant chains with the n-octyl chains of PMAOVE stabilize the PMAOVE-DTAB complexes. Charge neutralization of the anionic polymer by the cationic surfactant leads to precipitation of the PMAOVE-DTAB complex at a certain DTAB concentration range. Further addition of DTAB causes a charge reversal of the complex and, subsequently, resolubilization of the precipitate. At an acidic pH (pH = 4), a second precipitation was observed, which is probably caused by conformational changes in the PMAOVE-DTAB complex. This second precipitate can be resolubilized by further addition of surfactant. At a neutral and basic pH, this second precipitation is absent. EPR analysis indicates that the surfactants form an ordered structure at the extended polymer chain at a neutral and basic pH, whereas at an acidic pH, a less ordered surfactant layer is formed on the coiled polymer with more hydrophobic microdomains.     

Gel formation in systems composed of drug containing catanionic vesicles and oppositely charged hydrophobically modified polymer

The aim of this study was to explore if mixtures of drug containing catanionic vesicles and polymers give rise to gel formation, and if so, if drug release from these gels could be prolonged. Catanionic vesicles formed from the drug substances alprenolol or tetracaine, and the oppositely charged surfactant sodium dodecyl sulphate were mixed with polymers. Three polymers with different properties were employed: one bearing hydrophobic modifications, one positively charged and one positively charged polymer bearing hydrophobic modifications. The structure of the vesicles before and after addition of polymer was investigated by using cryo-TEM. Gel formation was confirmed by using rheological measurements. Drug release was studied using a modified USP paddle method. Gels were observed to form only in the case when catanionic vesicles, most likely with a net negative charge, were mixed with positively charged polymer bearing lipophilic modifications. The release of drug substance from these systems, where the vesicles are not trapped within the gel but constitute a founding part of it, could be significantly prolonged. The drug release rate was found to depend on vesicle concentration to a higher extent than on polymer concentration.     

Poly(ethylene oxide)s hydrophobically modified. Adsorption and spreading at the air-water interface

A comparative study of spread and adsorbed monolayer of poly(ethylene oxide)s of different molecular weight hydrophobically modified with alkyl isocyanates of different length chain is reported. The modification of the polymer was carried out according to reported procedures. The polymers obtained were studied at the air-water interface by Langmuir isotherms for spread monolayers and by Gibbs isotherms for the adsorption process. Isotherms obtained are interpreted in terms of the hydrophobic and hydrophilic balance of the polymers. Limiting area per repeating unit (A(0)) and collapse pressure (pi(c)) from spread monolayers were obtained. Spread monolayers of the hydrophobically modified polymers show larger collapse pressure values than unmodified polymer monolayers. In the adsorption process the excess surface concentration Gamma(infinity), area per repeat unit sigma, and efficiency of the adsorption were determined. The values of the area occupied per repeat unit in adsorbed monolayer (sigma) were larger than those of the spread monolayer. The efficiency of the adsorption of poly(ethylene oxide)s increases with the hydrophobic modification and with the alkyl chain length.     

Characterization of complexation and phase behavior of mixed systems of unmodified and hydrophobically modified oppositely charged polyelectrolytes

This paper reports turbidity, rheology, zeta potential, and rheo-small angle light scattering measurements on aqueous mixtures of oppositely charged and hydrophobically modified hydroxyethylcellulose derivatives (HM-HEC(?) and HM-HEC(+)) and mixtures of oppositely charged hydroxyethylcellulose (HEC(?) and HEC(+)). The experiments were restricted to the one-phase region, i.e., at mixing ratios before and after the two-phase area. The associative phase separation behavior usually observed when mixing oppositely charged polyelectrolytes was undetectable in the mixtures of the polyelectrolytes without attached hydrophobic groups. Upon modification of HEC by incorporation of pendant hydrophobic groups and by introducing charges of negative or positive sign (HM-HEC(?) and HM-HEC(+)), the mixtures showed phase separation over a certain mixing interval, revealing the existence of large polyelectrolyte complexes. The zero shear viscosity was strongly dependent on both the hydrophobicity of the polymers and the mixing ratio, increasing significantly with hydrophobic modification of polyelectrolytes. The strong enhancement of the turbidity and the viscosity drop as the two-phase area is approached suggest the formation of fragmented non-connected complexes. This work demonstrates that if the oppositely charged polyions have a hydrophilic character, it is not necessary that the attractive Coulombic forces induce insoluble polyelectrolyte complexes.     

Effects of temperature on neutron scattering from aqueous solutions of hydrophobically modified poly(ethylene oxide)

The effect of temperature on the structure of aqueous dispersions of hydrophobically end-capped poly(ethylene oxide) (PEOM) was investigated by small angle neutron scattering (SANS). Polymers with hydrogenated or deuterated n-octadecyl end-groups were studied in heavy water or in a mixture heavy water / water, respectively. In the latter case the PEO chains were selectively matched. In all the cases, the scattering curves were characterised by a main peak which revealed organisation of polymers into micelles consisting of hydrophobic cores surrounded by repulsive PEO coronae. Measurements were performed in the semi-dilute regime where micelles coronae overlap. At constant polymer concentration, an increase in temperature leads to decreasing solvent strength of water for the PEO chains and decreasing repulsion between the PEO coronae. As a result, the intensity of the peak in a mixture of water /heavy water decreases with temperature On the contrary, in heavy water, the peak of the scattered intensity increases with increasing temperature. This scattering behaviour is interpreted on the basis of a scaling theory of the semi-dilute solutions of star-like polymer micelles.     

Protein partitioning in poly(ethylene glycol)/sodium polyacrylate aqueous two-phase systems

The partition of hemoglobin, lysozyme and glucose-6-phosphate dehydrogenase (G6PDH) in a novel inexpensive aqueous two-phase system (ATPS) composed by poly(ethylene glycol) (PEG) and sodium polyacrylate (NaPA) has been studied. The effect of NaCl and Na(2)SO(4), pH and PEG molecular size on the partitioning has been studied. At high pH (above 9), hemoglobin partitions strongly to the PEG-phase. Although some precipitation of hemoglobin occurs, high recovery values are obtained particularly for lysozyme and G6PDH. The partitioning forces are dominated by the hydrophobic and electrochemical (salt) effects, since the positively charged lysozyme and negatively charged G6PDH partitions to the non-charged PEG and the strongly negatively charged polyacrylate enriched phase, respectively.     

Effect of hydrophobically modified polymer on salt-induced structural transition in microemulsions

The phase boundaries of the middle-phase microemulsion for NaCl/SDS/H2O/1-heptane/1-pentanol systems in the absence of polymer and in the presence of unmodified poly(acrylamide) (PAM) and hydrophobically modified poly(acrylamide) (HMPAM) have been determined at varying salt concentrations. These three middle-phase microemulsions (with HMPAM, with PAM, and without polymer) were studied using interfacial tension measurement, steady-state fluorescence, and time-resolved fluorescence quenching. Compared to the polymer-free system and the system with PAM, the addition of HMPAM significantly enlarges the range of the salt concentrations for the formation of the middle-phase microemulison and causes both the excess oil and aqueous phases to increase in volume at the expense of the middle-phase microemulsion. For the middle-phase microemulsion with HMPAM, the interfacial tensions of the microemulsion phase with the excess oil phase and with the excess aqueous phase are all ultralow and exhibit higher values than those with PAM and without polymer. At the same salt concentration, the apparent surfactant aggregation number in the middle-phase microemulsion with HMPAM has the smallest value among these three systems. All results indicate that the strong interaction of surfactant with hydrophobically modified polymer has a large effect on the formation and properties of the middle-phase microemulsion.     

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 hydrophobically modified anionic starch at oppositely charged oil/water interfaces

In this paper we study the adsorption at cationic emulsion droplets of starch which had been hydrophobically modified with octenyl succinic anhydride (OSA), a modification which also renders the starch anionic. Emulsions were formed with didodecyldimethylammonium bromide (DDAB) after which the OSA-starch was added. The emulsions were separated by centrifugation and the surface load of OSA-starch was determined through serum depletion. The results show the adsorbed amounts can become very high, in some cases reaching approximately 40 mg/m2. The surface load correlates positively with the surface charge density of the starch which depends on the degree of substitution, rms radius and molar mass. Furthermore, the surface load obtained depends on the ratio between polymer surface charge density and the interface charge density which could be varied experimentally by combining various amounts of DDAB and dioleoyl phosphatidylcholine (DOPC) in the formation of the emulsion. The very high surface loads should correspond to very thick adsorbed layers. Thus, OSA-starch should be appropriate for encapsulation applications provided a suitable adhesion substance is employed.     

Tripolyphosphate-sensitive egg phosphatidylcholine liposomes incorporating hydrophobically modified poly(ethylene imine)

Liposomes, which release their contents in answer to tripolyphosphate (TPP, a penta-anion), were prepared by immobilizing hydrophobically modified poly(ethylene imine) (HmPEI) on the surface of egg phosphatidylcholine (egg PC) liposome. HmPEI was prepared by covalently attaching decanoyl chloride to PEI through a condensation reaction. According to the ¹H NMR spectrum, the number of decanoyl chloride per one molecule of PEI was about 21, and HmPEI was air/water interface-active. HmPEI could readily complex with TPP in HEPES buffer (30 mM, pH 7.0), confirmed by Fourier transformed infrared spectrophotometer spectroscopy. The complexation increased with increasing the concentration of HmPEI and TPP, investigated through the measurement of optical density and light scattering intensity. Liposomes incorporating HmPEI were prepared by a film hydration and sonication method. The liposomes were multi-lamellar vesicles, observed on transmission electron microscope. Liposomes free of HmPEI did not release calcien when they were mixed with TPP. Liposomes whose egg PC/HmPEI was relatively low (e.g., 20:1 and 20:2) released calcein but not extensively (less than 10%) when mixed with TPP. Liposomes whose egg PC/HmPEI was relatively high (e.g., 20:4 and 20:20) released calcein extensively. For example, when the liposomes with lager amount of HmPEI were mixed with TPP so that HmPEI/TPP weight ratio was 8:1, the release degree in 60 sec was more than 70%. HmPEI can complex with TPP through electrostatic interaction and the complexation was thought to cause perturbation in the liposomal membranes and trigger the release.     

Dynamic interactions and adsorption in systems of hydrophobically modified water-soluble polymers

Associating polymers are prepared from poly(oxyelliylene) and a diisocyanate and terminated by linear alcohols with 8 to 18 C-atoms. Solutions of these polymers in pure water and with surfactants have been investigated by dynamic light scattering and NMR self-diffusion. The diffusion results indicate that the polymers in solution form heterodisperse clusters that show a complex behavior of overlap and coupling to each other, a feature which may be described by the coupling theory of Ngai and coworkers. In addition competitive adsorption on polystyrene latex particle is measured and the results indicate a multilayer adsorption structure that is broken down by surfactant addition. Surfactant adsorption seems to be practically independent of adsorbed polymer, except for an “initial inhibition” concentration that may be connected to the desorption of secondary layers of adsorbed polymer.     

Immobilization of liposomes on hydrophobically modified polymer gel particles in batch mode interaction

Immobilization of liposomal phospholipids onto Sephacryl S-1000 gels that were chemically conjugated with hydrophobic alkyl moieties, octyl, dodecyl and hexadecyl, was examined in batch mode interaction. Compared with the octyl gel, the dodecyl and the hexadecyl gels were found to immobilize the three to four times more phospholipids with the less hydrophobic moieties. The encapsulation of a water-soluble marker, with other evidences, suggests that the majority of the immobilized phospholipids maintained liposomal morphology. As the lipid of the interacting liposomes, egg yolk phosphatidylcholine (eggPC), 1,2-dimyristoylphosphatidylcholine (DMPC) and a mixture of DMPC and 1,2-dimyristamido-1,2-deoxyphosphatidylcholine were examined. At 22 °C, DMPC liposomes showed higher extent of immobilization than at 37 °C but not eggPC liposomes, suggesting that the phase of liposomal membrane could have influence on the immobilization. Exchange between the immobilized liposomes and free ones was found to be small, less than 3%. The gel that had been first interacted with liposomes to apparent saturation could further immobilize the newly added liposomes. The rate of this second immobilization was similar to that of the slow adsorption process; the both could be based on the same mechanism, possibly involving rearrangement of the immobilized liposomes on the gel as proposed by Lundahl. As had been observed in the flow mode, the immobilization had preference for smaller liposomes. In application of the system in batch mode, the size distributions of the immobilized liposomes and of those left in the supernatant may differ from that of the originally added liposomes.     

Structure formation of hydrophobically end-capped poly(ethylene oxide) in the solid state

The conformational transition of hydrophobically end-capped poly(ethylene oxide), HP-PEO-HP [hydrophobic-poly(ethylene oxide)-hydrophobic], was studied using X-ray diffraction (XRD), differential scanning calorimetry (DSC), and Fourier transform infrared spectroscopy (FTIR) methods. Conformational transitions of HP-PEO-HP from a planar zigzag to a 7/2 helical conformation were observed as the molecular weight of the PEO main chain increased. HP-PEO-HP 1(18), with a PEO molecular weight of 1000 and 18 hydrocarbons on each end, has mainly an alpha-helical structure in poor solvents, whereas alpha and beta conformations coexist in good solvents. This means that the alpha-helical structure caused by the hydrogen bonds between the urethane linkages was broken by the high chain mobility caused by the melted adjacent chains of PEO, and instead, the beta-sheet was formed by the interaction of multiple hydrogen bonds. Another indication of hydrogen bonds breaking at increasing temperature is the transition of the N-H stretching peak in the FTIR data. HP-PEO-HP 2(18) and 4(18), which have 18 hydrocarbons on each end and PEO molecular weights of 2000 and 4000, respectively, and consist mostly of PEO, showed spherulites. This result also suggests that the PEO molecule has a 7/2 zigzag helical conformation. In contrast, HP-PEO-HP 1(18), which is composed of less PEO than HP-PEO-HP 2(18) and 4(18), did not show a spherulite structure.     

Cutinase-peptide fusions in thermoseparating aqueous two-phase systems. Prediction of partitioning and enhanced tag efficiency by detergent addition

It is of increasing importance to develop efficient purification methods for recombinant proteins where the number of steps can be minimised. The aim has been to establish a method for predicting the partitioning of the wild-type target protein in an aqueous two-phase system, and with this as basis, develop fusion tags and optimise the phase system for enhanced partitioning of the target protein. The surface of the lipolytic enzyme cutinase from Fusarium solani pisi was investigated with a computer program, Graphical Representation and Analysis of Surface Properties (GRASP). The accessible surface areas for the different amino acid residues were used together with peptide partitioning data to calculate the partition coefficient for the protein. The separation system was composed of a thermoseparating random copolymer of ethylene oxide and propylene oxide. Breox PAG 50A 1000, as top phase forming polymer and a hydroxypropyl starch polymer, Reppal PES 200, as bottom phase polymer. The calculated partition coefficient for the wild-type protein (K= 1.0) agreed reasonably well with the experimentally determined value (K=0.85). Genetic engineering was used to construct fusion proteins expressed in Saccharomyces cerevisiae based on cutinase and peptide tags containing tryptophan, to enhance the partitioning in aqueous two-phase systems. The partitioning of the cutinase constructs could qualitatively be predicted from peptide partitioning data, i.e. the trends in partitioning could be predicted. A spacer peptide introduced between protein and tag increased the partitioning of the protein towards the ethylene oxide-propylene oxide (EOPO) copolymer top phase. The aqueous two-phase system was modified by addition of detergent to increase the partitioning of the cutinase variants towards the EOPO copolymer phase. Triton and a series of C12En detergents selectively increased the partitioning of cutinase constructs with (WP)4-based tags up to 14 times compared to wild-type cutinase. The protein partition could almost quantitatively be predicted from the peptide partition data.     

Molecular-mass characteristics and association behavior of weakly charged hydrophobically modified poly(acrylamides)

The evolution of compositional heterogeneity and molecular-mass distribution in the course of micellar polymerization of weakly charged hydrophobically modified poly(acrylamides) has been studied. The relationship between the molecular characteristics, association behavior, and rheological properties of aqueous solutions and gels of these copolymers has been examined. The molecular-mass distribution broadens with conversion owing to accumulation of a low-molecular-mass fraction depleted of the hydrophobic monomer. A change in the molecular-mass distribution has a strong effect on the viscoelastic behavior of the polymers. The properties of the copolymers are dependent on the pH of the reaction mixture: the copolymers prepared in the alkaline medium show a more pronounced ability to experience hydrophobic aggregation than the copolymers prepared in the acidic medium. The aqueous solutions and the gels of these polymers are characterized by higher viscosities and dynamic moduli. The rheological characteristics of the hydrophobically modified poly(acrylamides) are improved with an increase in both their molecular mass and the length of hydrophobic sequences with a constant molecular mass.     

Nanoparticles of hydrophobically modified dextrans as potential drug carrier systems

Nanoparticles combining a hydrophobically modified dextran core and a polysaccharide surface coverage were elaborated. Their suitability for applications like drug delivery was evaluated. The selected polysaccharide, dextran, was chemically modified by the covalent attachment of hydrocarbon groups (aliphatic or aromatic) via the formation of ether links. According to the extent of modification, either water-soluble or water-insoluble dextran derivatives were obtained. The latter exhibited solubility in organic solvents like tetrahydrofuran or dichloromethane saturated with water. Water-soluble dextran derivatives were used as polymeric surfactants for the control of nanoparticles surface characteristics. Nanoparticles were prepared either by o/w emulsion or solvent-diffusion methods. The size and surface properties of dextran nanoparticles were correlated to processing conditions. The stability of colloidal suspensions was examined as a function of ionic strength and related to the particle surface characteristics. The redispersability of freeze-dried suspensions without the addition of cryoprotectant was demonstrated. Finally, the degradability of modified dextrans was compared to that of starting dextran, after enzymatic hydrolysis in the presence of dextranase.     

Solvent properties governing protein partitioning in polymer/polymer aqueous two-phase systems

Distribution coefficients of various proteins were measured in aqueous Dextran-Ficoll, Dextran-PES, and Ficoll-PES two-phase systems, containing 0.15M NaCl in 0.01 M phosphate buffer, pH 7.4. The acquired data were combined with data for the same proteins in different systems reported previously and known solvatochromic solvent properties of the systems to characterize the protein-solvent interactions. The relative susceptibilities of proteins to solvent dipolarity/polarizability, solvent hydrogen bond acidity, solvent hydrogen bond basicity, and solvent ability to participate in ion-ion and ion-dipole interactions were characterized. These parameters, which are representative of solute-solvent interactions, adequately described the partitioning of the proteins in each system. It was found that the relative susceptibilities of proteins to solvent dipolarity/polarizability are interrelated with their relative susceptibilities to solvent hydrogen bond acidity and solvent hydrogen bond basicity similarly to those established previously for small nonionic organic compounds.     

Thermodynamics of self-assembling of hydrophobically modified cationic polysaccharides and their mixtures with oppositely charged surfactants in aqueous solution

Microcalorimetric techniques, combined with turbidity measurements, were used to study the thermodynamics of self-assembling of hydrophobically modified cationic polysaccharides and their mixtures with oppositely charged surfactants in aqueous solution. The studied polyelectrolytes were a series of polymers based on dextran having pendant N-(2-hydroxypropyl)-N,N-dimethyl-N-alkylammonium chloride groups randomly distributed along the polymer backbone. The parameters for their micellization process are evaluated from the results of the observed dilution enthalpy curves and compared with those of the related cationic surfactants (DTAC and CTAC). The microcalorimetric results for the mixed systems (polyelectrolytes with oppositely charged surfactants) are used along with turbidity measurements to characterize systematically the thermodynamics of their interaction. The phase behavior is described and the interaction enthalpies are derived from the differences between the observed enthalpy curves with and without polyelectrolyte. Therefore, we discuss in detail the effect of changing the alkyl chain length of polyelectrolyte pendant groups, the molecular weight of the dextran backbone, and the temperature of the measurements on the interactions between polyelectrolyte and surfactant.     

Surface plasmon resonance study of the interaction of a beta-cyclodextrin polymer and hydrophobically modified poly(N-isopropylacrylamide)

Surface plasmon resonance (SPR) technique is used to follow, both in real time and in situ, the association between a physically adsorbed polymer of beta-cyclodextrin (pbetaCD) and different hydrophobically modified poly(N-isopropylacrylamide) (PNIPAM) copolymers containing either adamantyl or dodecyl groups. This association is due to the complex formation between the hydrophobic groups and the betaCD cavities. Therefore, the adsorbed amount of PNIPAM onto the pbetaCD layer depends on the substituent and on its substitution level. The association and dissociation rate constants are evaluated from the kinetics of PNIPAM adsorption. An estimation of the association constants leads to values higher than 10(4) M(-1), reflecting the strong interaction between these polymers.     

Temperature effect of hydrophobically modified polyethylene oxide at the air–water interface

Surface pressure (π)–area (A) isotherms of hydrophobically modified polyethylene oxide (HEUR) at the air–water interface was examined. Conformational transitions between pancake, mushroom, and brush states of the hydrophilic backbone influence the intermolecular interaction between the hydrophobic chains. We choose relatively long (18 carbons) hydrophobic ends, which have large hydrophobic interactions, and investigate the main chain effect by change in the length of the hydrophilic PEO chain. At high surface concentration region, the temperature coefficient of surface pressure, dπ/dT, was larger by increasing the portion of the hydrophobicity. This indicates an increase in surface energy and a decrease in surface enthalpy at high surface concentrations. As alkyl chains on both sides of HEURs are anchored at the air–water interface, restriction caused by the alkyl chain would be smaller for the long PEO chain, but the larger for the short PEO chain length.