Anna-Lena Kjøniksen

Characterization of association phenomena in aqueous systems of chitosan of different hydrophobicity

Abstract Rheological, dynamical and structural properties of aqueous systems of chitosan (UM-chitosan) and hydrophobically-modified chitosans (HM-chitosan) are briefly reviewed. The effects of pH, level of surfactant addition, polymer concentration, and temperature on the rheological behavior, both in the linear and non-linear viscoelastic regime, have been scrutinized. These variables have the strongest impact on the rheological properties of the hydrophobically-modified chitosans. We observe the formation of concentration-induced gels for systems of UM-chitosan and HM-chitosans. Incipient gels are evolved at lower concentrations as the hydrophobicity of the polymer increases. Non-linear shear thinning behavior is found in semidilute solutions of UM-chitosan and HM-chitosan at higher shear rates. The magnitude of this effect depends on factors, such as pH, amount of surfactant, polymer concentration, and hydrophobicity of the polymer. The dynamic light scattering results from semidilute solutions of UM-chitosan and HM-chitosan show that the intermolecular association phenomena in the polymer solutions are promoted by decreasing temperature and increasing polymer concentration and hydrophobicity. The intensity light scattering measurements on semidilute solutions of UM-chitosan and HM-chitosans suggest that the systems have a fractal structure and the fractal dimension is approx. 2.

Stability of chitosan nanoparticles cross-linked with tripolyphosphate.

The physical stability of chitosan nanoparticles cross-linked with sodium tripolyphosphate (TPP) was investigated over a period of 1 month. Special emphasis was placed on changes in the particle size and the particle compactness, which are two important physicochemical parameters of nanoparticulate drug delivery systems. The chitosan-TPP particles were prepared at different ionic strengths, chitosan chloride concentrations, and TPP-to-chitosan ratios. In the presence of monovalent salt, the positive ζ potential of the particles was reduced. In spite of this, the particles were more stable when prepared and stored under saline conditions compared to water. This could be attributed to the smaller particle sizes found in the presence of sodium chloride. Most of the particles prepared in saline solvents were stable with respect to changes in the size and the compactness of the particles. However, instability was observed at the highest cross-linker-to-polymer ratios. Generally, a reduction in the ζ potential and an increase in the particle compactness were observed at increasing TPP-to-chitosan ratios. This combined with the size increase induced by a high concentration of chitosan, increased the aggregation and sedimentation tendency of the particles and reduced the colloidal stability of the particles.

Effects of ionic strength on the size and compactness of chitosan nanoparticles

In this work, chitosan nanoparticles were prepared by ionotropic gelation of chitosan with tripolyphosphate (TPP). The effects of the ionic strength of the solvent employed in the particle preparation on the average size and compactness of the particles were investigated. In addition, the effects of the chitosan concentration and the crosslinker to polymer ratio on the particle characteristics were studied. The chitosan–TPP nanoparticles were characterized by dynamic light scattering, zeta potential, and turbidity measurements. The compactness of the nanoparticles was estimated with a method based on the size of the nanoparticles and the turbidity of the nanoparticle suspension. All the investigated preparation parameters, i.e., the ionic strength of the solvent, the chitosan concentration, and the TPP to chitosan ratio, affected the particle characteristics. For instance, smaller and more compact particles were formed in saline solvents, compared to particles formed in pure water. Further, the addition of monovalent salt rendered it possible to prepare particles in the nanometer size range at a higher polymer concentration. Solvent salinity is thus an important parameter to address in the preparation of chitosan nanoparticles crosslinked with TPP.

Friction in aqueous media tuned by temperature-responsive polymer layers

An atomic force microscope colloidal probe technique has been employed to probe normal and friction forces between silica surfaces coated with adsorbed layers of a diblock copolymer of the composition poly(N-isopropylacrylamide)48-block-poly(3-acrylamidopropyl)trimethylammonium chloride)20, abbreviated PNIPAAM48-b-PAMPTMA(+)20. The interactions between the PNIPAAM48-b-PAMPTMA(+)20-coated surfaces across a 0.1 mM NaCl (pH 6) solution at 25 °C are purely repulsive, due to a combination of steric and electrostatic double-layer forces. However, when the temperature is increased to 35 °C, and subsequently to 45 °C, an attractive force develops at short separations due to the unfavourable PNIPAAM–water interaction at these temperatures. The temperature-dependent polymer–water interaction has implications for the friction force between the layers. At 25 °C a frictional force that increases linearly with increasing load is observed once the surfaces are brought into close contact. At higher temperatures significantly higher friction forces appear as a consequence of attractive segment–segment interactions. Further, a clearly expressed hysteresis between friction forces encountered on loading and unloading is detected. Our results demonstrate that both normal and friction forces between surfaces can be controlled by temperature changes when temperature-responsive polymers are employed, and friction forces can be adjusted as required from low to high.

Studies on pectin coating of liposomes for drug delivery

The present study investigated the surface coating of charged liposomes by three different types of pectin (LM, HM and amidated pectin) by particle size determinations and zeta potential measurements. The pectins and the pectin coated liposomes were visualized by atomic force microscopy. The adsorption of pectin onto positive liposomes yielded a reproducible increase in particle size and a shift of the zeta potential from positive to negative side for all three pectin types, whereas the adsorption of pectin onto negative liposomes did not render any significant changes probably due to electrostatic repulsion. The positive liposomes coated with HM-pectin gave the largest pectin coated particles with the least negative zeta potential, while the opposite was observed for the LM-pectin coated positive liposomes. Furthermore, results from dynamic light scattering revealed narrow size distributions, indicating that the degree of aggregation was low for the pectin coated liposomes. As liposomes are able to encapsulate drugs and pectin has been found to be mucoadhesive, these pectin coated liposomes may be potential drug delivery systems.

Effects of temperature and pH on the contraction and aggregation of microgels in aqueous suspensions.

Chemically cross-linked poly(N-isopropylacrylamide) (PNIPAM) microgels and PNIPAM with different amounts of acrylic acid groups (PNIPAM-co-PAA) were synthesized and the temperature-induced aggregation behaviors of aqueous suspensions of these microgels were investigated mainly with the aid of dynamic light scattering (DLS) and turbidimetry. The DLS results show that the particles at all conditions shrink at temperatures up to approximately the lower critical solution temperature (LCST), but the relative contraction effect is larger for the microgels without acid groups or for microgels with added anionic surfactant (SDS). A significant depression of the cloud point is found in suspensions of PNIPAM with very low concentrations of SDS. The compression of the microgels cannot be traced from the turbidity results, but rather the values of the turbidity increase in this temperature interval. This phenomenon is discussed in the framework of a theoretical model. At temperatures above LCST, the size of the microgels without attached charged groups in a very dilute suspension is unaffected by temperature, while the charged particles (pH 7 and 11) continue to collapse with increasing temperature over the entire domain. In this temperature range, low-charged particles of higher concentration and particles containing acrylic acid groups at low pH (pH 2) aggregate, and macroscopic phase separation is approached at higher temperatures. This study demonstrates how the stabilization of microgels can be affected by factors such as polymer concentration, addition of ionic surfactant to particles without charged acid groups, amount of charged groups in the polymer, and pH.

Temperature-induced formation and contraction of micelle-like aggregates in aqueous solutions of thermoresponsive short-chain copolymers.

A combination of turbidity, light scattering, and steady shear viscosity experiments has revealed that aqueous solutions of an amphiphilic diblock copolymer or a negatively charged triblock copolymer, both containing poly(N-isopropylacrylamide), can undergo a temperature-induced transition from loose intermicellar clusters to collapsed core-shell nanostructures. Turbidity, light scattering, and viscosity results of these short-chain copolymers disclose transition peaks at intermediate temperatures. At high temperatures, the compact core-shell particles from the diblock copolymer aggregate, whereas no renewed interpolymer association is observed for the triblock copolymer or for the solution of the diblock copolymer with added sodium dodecyl sulfate because the electrostatic repulsive interactions suppress the tendency of forming interpolymer clusters. The temperature-induced building up of intermicellar structures and the formation of large aggregates at high temperature in the solution of the diblock copolymer is significantly reduced under the influence of high shear rates.

Temperature-dependent optical properties of gold nanoparticles coated with a charged diblock copolymer and an uncharged triblock copolymer.

We demonstrate that the optical properties of gold nanoparticles can be used to detect and follow stimuli-induced changes in adsorbed macromolecules. Specifically, we investigate thermal response of anionic diblock and uncharged triblock copolymers based on poly(N-isopropylacrylamide) (PNIPAAM) blocks adsorbed onto gold nanoparticles and planar gold surfaces in a temperature range between 25 and 60 degrees C. By employing a palette of analytical probes, including UV-visible spectroscopy, dynamic light scattering, fluorescence, and quartz crystal microbalance with dissipation monitoring, we establish that while the anionic copolymer forms monolayers at both low and high temperature, the neutral copolymer adsorbs as a monolayer at low temperatures and forms multilayers above the cloud point (T(C)). Raising the temperature above T(C) severely affects the optical properties of the gold particle/polymer composites, expelling associated water and altering the immediate surroundings of the gold nanoparticles. This effect, stronger for the uncharged polymer, is related to the amount of polymer adsorbed on the surface, where a denser shell influences the surface plasmon band to a greater degree. This is corroborated with light scattering experiments, which reveal that flocculation of the neutral polymer-coated particles occurs at high temperatures. The flocculation behavior of the neutral copolymer on planar gold surfaces results in multilayer formation. The observed effects are discussed within the framework of the Mie-Drude theory.

Studies on pectin-coated liposomes and their interaction with mucin

Pectin is a polymer with well-known mucoadhesive properties. In this study, liposomes were coated with three different types of pectin. Their properties were characterized and their mucoadhesiveness was estimated by a novel in vitro approach. Two different types of commercially available mucin were investigated in order to choose the best candidate for the method. The effect of pH on the properties of the coated liposomes and the interaction with mucin was also studied. The pectin-coated liposomes and the complexes they formed with mucin were characterized by dynamic light scattering (DLS), zeta potential and turbidity measurements. The zeta potential of the liposomes shifted from positive to negative after coating with pectin. They also exhibited larger diameters, and the liposomes coated with HM-pectin were the largest. After the addition of mucin, the zeta potential shifted to a less negative value and the sizes of the pectin-coated liposomes increased. The complexes formed between mucin and the HM-pectin-coated liposomes were the largest, while the smallest were formed with the LM-pectin-coated liposomes. The pH was found to affect the interaction between the coated liposomes and mucin. DLS was conducted on an ALV goniometer to gain information about the diffusivity of the samples, the relative scattered intensities and to obtain an optimal characterization of the size distributions. The results correlated well with measurements from an automatized light scattering instrument (Zetasizer Nano ZS).

Rheological and structural aspects on association of hydrophobically modified polysaccharides

This review covers recent rheological and structural advances in the interactions between hydrophobically modified polysaccharides (HMP) and surfactants or cyclodextrin compounds in aqueous media. Depending on the surfactant concentration, mixtures of HMPs and a surfactant can form strong associating complexes or disrupted networks at high levels of surfactant addition. By adding cyclodextrin monomers to semidilute solutions of HMPs, the hydrophobic interactions can be deactivated and a looser network is formed. For HMPs in the presence of cyclodextrin polymers, network structures with intriguing morphological and rheological features can be constructed. These complex fluids are of potential interest in the development of systems for drug delivery formulations. These rheology modifiers are also of current interest in many other technological applications.