Publications (11)34.83 Total impact
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Article: Characterization of interactions in aqueous mixtures of hydrophobically modified alginate and different types of surfactant
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ABSTRACT: Rheology, turbidity, and small-angle neutron scattering (SANS) measurements have been carried out on semidilute alginate and hydrophobically modified (HM)-alginate solutions in the presence of different surfactants. The turbidity and SANS result indicate that the cationic surfactant hexadecyltrimethylammonium bromide (CTAB) and Gemini induce the formation of large inhomogeneous structures in the alginate solutions, whereas their effects on the structure of HM-alginate is less marked. The addition of the nonionic surfactant Brij 35 leads to amended solvent conditions for both alginate and HM-alginate solutions. Compared to the changes induced by cationic and nonionic surfactant, the effect of SDS on the structure of the solutions is less marked. The rheological measurements show generally small viscosity changes upon addition of the surfactants to unmodified alginate. The HM-alginate system shows a different behavior, with a marked viscosity enhancement upon addition of ionic surfactants in the surfactant concentration range probed. Most pronounced viscosification is demonstrated when CTAB and Gemini are added into the solution. This is ascribed to the cooperative effects of hydrophobic association and electrostatic attractions. In contrast, the nonionic surfactant Brij 35 leads to a decrease in the solution viscosities, indicating the break-up of hydrophobic association due to the inclusion of the hydrophobic pendants by Brij 35 micelles.Colloids and Surfaces A Physicochemical and Engineering Aspects 04/2007; 293(1-3):105-113. · 2.24 Impact Factor -
Article: Characterization of thermally sensitive interactions in aqueous mixtures of hydrophobically modified hydroxyethylcellulose and cyclodextrins.
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ABSTRACT: Effects of beta-cyclodextrin (beta-CD) or hydroxypropyl-beta-cyclodextrin (HP-beta-CD) addition and temperature on thermodynamic, rheological, and structural features of semidilute solutions of hydroxyethylcellulose (HEC) and its hydrophobically modified analogue (HM-HEC) are reported. Differential scanning calorimetric (DSC) measurements revealed a thermally induced crystal melting transition of beta-CD at high concentrations in solutions of HEC and HM-HEC. No transition with HP-beta-CD was observed in aqueous solution. Viscosity results indicated that at a cosolute concentration of 2 mm, the beta-CD units are threaded onto hydrophobic tails of HM-HEC (C16 groups) to form columnar structures. This arrangement is more effective in the encapsulation of the hydrophobic chains than the monomer hydrophobic deactivation accomplished by the HP-beta-CD units. At cosolute concentrations above 8 mm, no further decoupling of the hydrophobic interactions occurs for any of the cosolutes. Small-angle neutron scattering (SANS) experiments on HM-HEC/beta-CD mixtures suggest that the large-scale association structures in HM-HEC/D(2)O solutions are reduced upon addition of beta-CD, and an interesting temperature effect is observed at 2 mm beta-CD addition. At high beta-CD concentrations and low temperatures, the formation of large beta-CD clusters or crystallites generates cross-links in the HEC and HM-HEC networks, resulting in a viscosity enhancement of several orders of magnitude. This strong temperature effect is not reflected in the structural features probed by SANS.Langmuir 11/2006; 22(21):9023-9. · 4.19 Impact Factor -
Article: Effects of the Quantity and Structure of Hydrophobes on the Properties of Hydrophobically Modified Alginates in Aqueous Solutions
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ABSTRACT: Alginate is an important polysaccharide with applications in food industry, cosmetics and pharmacy. In this study, three hydrophobically modified alginates with different amount of various hydrophobic groups have been synthesized. Rheology and turbidity measurements have been carried out on polymer solutions in the concentration regime 0.005–2.0wt %. The rheological results indicate that the introduction of hydrophobic groups induce the formation of intra- or intermolecular associations in the aqueous solutions depending on the polymer concentration and the hydrophobicity. The concentration dependency of the viscosity exhibit different power-law behaviors depending on the concentration regime and the hydrophobic modification. This has been scrutinized in view of theoretical scaling laws presented in the literature.Polymer Bulletin 07/2006; 57(4):563-574. · 1.53 Impact Factor -
Article: Characterization of interactions in aqueous solutions of hydroxyethylcellulose and its hydrophobically modified analogue in the presence of a cyclodextrin derivative.
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ABSTRACT: The formation of associative networks in semidilute aqueous solutions of hydrophobically modified hydroxyethylcellulose (HM-HEC) is dependent on intermolecular hydrophobic interactions. Addition of hydroxypropyl-beta-cyclodextrin (HP-beta-CD) monomers to the system provides decoupling of these associations via inclusion complex formation with the polymer hydrophobic tails. Results from viscosity, polymer NMR self-diffusion, and dynamic light scattering (DLS) measurements show that the hydrophobic interactions in HM-HEC solutions are effectively suppressed when the level of HP-beta-CD addition increases. Small-angle neutron scattering (SANS) results reveal that the large-scale association complexes in HM-HEC solutions are strongly diminished when the concentration of HP-beta-CD rises. The time correlation data obtained from the DLS experiments unveiled the existence of two relaxation modes: one single exponential at short times followed by a stretched exponential at longer times. The fast mode is always diffusive, whereas the slow mode exhibits progressively stronger wavevector dependence as the intensity of the hydrophobic interactions increases. This feature, as well as the accompanying drop of the stretched exponential beta as the HP-beta-CD concentration decreases, is attributed to enhanced hydrophobic interactions and can be well rationalized in the framework of the coupling model of Ngai.The Journal of Physical Chemistry B 05/2006; 110(13):6601-8. · 3.70 Impact Factor -
Article: Interaction of unmodified and hydrophobically modified alginate with sodium dodecyl sulfate in dilute aqueous solution: Calorimetric, rheological, and turbidity studies
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ABSTRACT: Interactions between hydrophobically modified alginate (HM-alginate) and the anionic surfactant sodium dodecyl sulfate (SDS) have been investigated by microcalorimetry, rheology and turbidimetry. Significant polymer–surfactant interactions were observed between HM-alginate and SDS, whereas for mixtures of unmodified alginate and SDS only a weak synergism was detected. The intrinsic viscosity of HM-alginate solutions with added SDS showed a strong upturn at low SDS concentrations, indicating a polyelectrolyte effect. The zero-shear viscosity of the semidilute HM-alginate solution was found to increase at a low level of SDS addition, and then decrease with increasing SDS concentration. The turbidity curves exhibit the same profile as those of the viscosity. The maximum of the turbidity is located at the same SDS concentration as the viscosity maximum for the semidilute solution. This picture is consistent with the conjecture of the formation and breakup of the associations as the SDS concentration increases.Colloids and Surfaces A Physicochemical and Engineering Aspects 04/2006; 278(1-3):166-174. · 2.24 Impact Factor -
Article: Characterization of Gelation of Aqueous Pectin via the Ugi Multicomponent Condensation Reaction
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ABSTRACT: Turbidity and rheological features during gelation of semidilute solutions of pectin via the Ugi multicomponent condensation reaction have been investigated at different polymer and cross-linker concentrations at ambient temperature. The gelation time of the system decreased with increasing polymer and cross-linker concentrations. At the gel point, a power law frequency dependence of the dynamic storage modulus (G’∼ωn’) and loss modulus (G”∼ωn”) was observed with n’=n”=n. The value of the power law exponent is about 0.6∼0.7 for all the gelling systems, which is close to that predicted (0.7) from the percolation model. The elastic properties of the gels continue to grow over a long time in the post-gel region, and a solidlike response is observed at later stages during the gelation process. The turbidity of the gelling system is higher as the polymer and cross-linker concentrations rise, but the turbidity is virtually unaffected during an extended time after the formation of the incipient gel. It was demonstrated that polymer and cross-linker concentrations could be utilized to tune the physical properties, such as transparency and viscoelasticity, of the Ugi hydrogels.Polymer Bulletin 01/2006; 56(6):579-589. · 1.53 Impact Factor -
Article: Rheological properties of pH-induced association and gelation of pectin
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ABSTRACT: Pectin is an important polysaccharide with applications in food industry, cosmetics and pharmacy. In recent years, there has been an increased use of hydrogels in pharmaceutical formulations in connection with drug-delivery administration. In this study of aqueous pectin systems, association and gelation have been accomplished by changing pH of the samples to an acidic pH. This is carried out by adding glucono-δ-lactone (GDL), which will gradually reduce the pH of the system. As the pH is reduced, the carboxyl acid groups on the pectin chains are neutralized, and the polyelectrolyte character of the polymer is reduced promoting association and gelation of the system. The viscoelastic properties of the system are monitored by oscillatory shear measurements as a function of time after the addition of GDL. The system exhibits very long gelation times and a pronounced viscosity increase in the vicinity of the gel points.Polymer Bulletin 01/2006; 56(2):239-246. · 1.53 Impact Factor -
Article: Effects of surfactant and temperature on rheological and structural properties of semidilute aqueous solutions of unmodified and hydrophobically modified alginate.
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ABSTRACT: The dynamic and structural perturbations that result from the interactions between the anionic surfactant sodium dodecyl sulfate (SDS) and the hydrophobically modified biopolymer alginate (HM-alginate) have been studied with the aid of rheological methods, turbidimetry, and small-angle neutron scattering (SANS). The rheological results for a semidilute HM-alginate solution in the presence of SDS disclose strong interactions between HM-alginate and SDS at a low level of surfactant addition, and this feature is accompanied by enhanced turbidity. At higher surfactant concentrations the association complexes are disrupted. A strong temperature effect of the viscosity is observed in HM-alginate solutions at moderate SDS concentrations, where an elevated temperature leads to enhanced chain mobility, which promotes a breakup of the association complexes. The SANS results reveal a pronounced peak in the plot of scattered intensity versus wavevector q at intermediate q values for SDS concentrations above the critical micelle concentration (cmc). With contrast-matching conditions, using deuterated SDS instead of SDS, no interaction peak appears but an "upturn" in the scattered intensity is observed at small q value. The magnitude of this effect decreases with increasing surfactant concentration, showing clearly that SDS is capable of breaking up the large aggregates created.Langmuir 12/2005; 21(24):10923-30. · 4.19 Impact Factor -
Article: Effects of pH on dynamics and rheology during association and gelation via the Ugi reaction of aqueous alginate
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ABSTRACT: Effects of solution pH on the physical properties of a semidilute aqueous alginate solution without cross-linker agent and during gelation via the Ugi multicomponent condensation reaction at a fixed cross-linker concentration have been investigated. Both rheology and turbidity results on the alginate solution without cross-linker revealed enhanced associations at low pH. In the course of the cross-linker reaction, the time of gelation is shortest at pH = 3.5 and at pH values above 3.8 no gel is formed but only a viscosification of the solution is observed. The turbidity during the cross-linking reaction rises as the pH increases from 3.5 to 5. Furthermore, the initial change of the turbidity in the course of the cross-linking process is more pronounced at higher pH. The dynamic light scattering (DLS) results of the reaction mixture at pH = 4.0 (ergodic features at this condition) show that the chain relaxation is slowed down as the reaction proceeds. The effect of pH on the kinetics of the Ugi reaction is discussed.European Polymer Journal 04/2005; 41(8):1708-1717. · 2.74 Impact Factor -
Article: Rheological characterization of photochemical changes of ethyl(hydroxyethyl)cellulose dissolved in water in the presence of an ionic surfactant and a photosensitizer.
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ABSTRACT: The effects of addition of the photosensitizer riboflavin (RF) to semidilute solutions of the systems ethyl(hydroxyethyl)cellulose (EHEC)/water, EHEC/sodium dodecyl sulfate (SDS), and EHEC/cetyltrimethylammonium bromide (CTAB) on the turbidity and the linear viscoelasticity are studied. The turbidity behavior and the cloud point (CP) are influenced by the addition of RF to the EHEC/SDS system, whereas no discernible change is observed for the other systems. The rheological features of all systems are affected by the presence of RF at lower temperatures, whereas at temperatures close to the CP, only a slight effect is detected. Both the EHEC/SDS and EHEC/CTAB systems evolve thermoreversible gels at the same temperature (37.5 degrees C), but in the presence of RF, the EHEC/CTAB system does not form a gel, whereas the gel temperature for the EHEC/SDS system is depressed (32.5 degrees C). Light irradiation of RF in the EHEC/SDS/RF system causes fragmentation of the network and a higher temperature is required to re-form the incipient gel network. The photochemical degradation of EHEC gives rise to a decrease in the dynamic moduli and the complex viscosity for all of the three systems. The effect is strengthened at higher temperatures and it is most pronounced for the EHEC/SDS system.Biomacromolecules 05/2004; 5(2):610-7. · 5.48 Impact Factor -
Article: Rheological and structural properties of aqueous alginate during gelation via the Ugi multicomponent condensation reaction.
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ABSTRACT: Turbidity, structure, and rheological features during gelation via the Ugi multicomponent condensation reaction of semidilute solutions of alginate have been investigated at different polymer and cross-linker concentrations and reaction temperatures. The gelation time of the system decreased with increasing polymer and cross-linker concentrations, and a temperature rise resulted in a faster gelation. At the gel point, a power law frequency dependence of the dynamic storage modulus (G' proportional, variant omega(n)(')) and loss modulus (G' ' proportional, variant omega(n)(' ')) was observed for all gelling systems with n' = n' ' = n. By varying the cross-linker density at a fixed polymer concentration (2.2 wt %), the power law exponent is consistent with that predicted (0.7) from the percolation model. The value of n decreases with increasing polymer concentration, whereas higher temperatures give rise to higher values of n. The elastic properties of the gels continue to grow over a long time in the postgel region, and at later stages in the gelation process, a solidlike response is observed. The turbidity of the gelling system increases as the gel evolves, and this effect is more pronounced at higher cross-linker concentration. The small-angle neutron scattering results reveal large-scale inhomogeneities of the gels, and this effect is enhanced as the cross-linker density increases. The structural, turbidity, and rheological features were found to change over an extended time after the formation of the incipient gel. It was demonstrated that temperature, polymer, and cross-linker concentrations could be utilized to tune the physical properties of the Ugi gels such as structure, transparency, and viscoelasticity.Biomacromolecules 05/2004; 5(4):1470-9. · 5.48 Impact Factor
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Institutions
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2005–2007
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University of Oslo
- Department of Chemistry
Oslo, Oslo, Norway
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