Ion-exclusion controlled size-exclusion chromatography of methacrylic acid-methyl methacrylate copolymers.

Controlled ion-exclusion allows compensation of hydrophobic adsorption in size-exclusion chromatography of negatively charged methacrylic acid-methyl methacrylate (Eudragit) polymers using methanol as a mobile phase. Properly selected low-ionic-strength conditions below 5 mM LiCl provide correct separation in the size-exclusion mode. Possible disturbing effects, mainly related to light scattering, under low-salt conditions are discussed and shown to be negligible if on-line concentration-light scattering detection is used. The absence of these disturbances is checked by a comparison of experiments performed in methanol containing 1.25 mM and 2.5 mM LiCl. Molecular mass averages and distributions identical within the experimental error are obtained.

Multivariate parameter evaluation of pharmaceutically important cellulose ethers.

A set of nonionic cellulose ethers with varying hydrophobicity and molecular weight has been investigated by principal component analysis (PCA). Several experimental variables such as dynamic surface tension, diffusion coefficient, microviscosity as monitored by a fluorescence probe technique, and intrinsic viscosity are included in the analysis. The experimental variables and observations (polymer fractions) are analyzed in models with good predictive capacities. The apparent equilibrium surface tension correlates to the cloud point and to the critical aggregation concentration in the presence of surfactant. The microviscosity is shown to be a predictive parameter for the degree of hydrophobic substitution. The irreversible process of dynamic surface tension is dependent on the diffusion coefficient but to an even larger degree on the polymer concentration, which is well illustrated by the PCA models.

Amphiphilic association of ibuprofen and two nonionic cellulose derivatives in aqueous solution.

The aqueous interaction of the sodium salt of ibuprofen with the cellulose ethers ethyl hydroxyethyl cellulose, EHEC, and hydroxypropyl methyl cellulose, HPMC, has been investigated in the concentration range 0-500 mM ibuprofen and 0.1-1% (w/w) polymer, by cloud point, capillary viscometry, equilibrium dialysis, and fluorescence probe techniques. Ibuprofen forms micelles in pure water, with the critical micelle concentration, cmc, at 180 mM. A combination of time-resolved and static fluorescence quenching shows that micelle-like ibuprofen aggregates are formed in the solution. The average aggregation number of pure ibuprofen micelles in water is about 40. In the presence of EHEC or HPMC the aggregation numbers decrease. The interaction of ibuprofen with cellulose ethers is similar to the normally accepted model for polymer-surfactant interaction, although more complex. Ibuprofen adsorbs to the polymer in the form of mixed polymer-drug micelles, noncooperatively up to cmc and cooperatively when cmc is passed. The interaction starts below 50 mM ibuprofen as monitored by the fluorescent probes pyrene and 1,3-di(1-pyrenyl)propane, P3P, with a maximum in microviscosity below cmc, corresponding to polymer-dense mixed micelles. The study illustrates the importance of a precise apprehension of the aggregation behavior as a background for transport studies in drug-polymer systems.

Diffusion and concentration profiles of drugs in gels.

A versatile membraneless method was used to study the diffusion of acetaminophen, ibuprofen, indomethacin, theophylline, and chlorpheniramine in thermoreversible gels. Two independent ways to calculate the diffusion coefficients and to verify Fickian transport are presented; the most sensitive criterion for Fickian transport being an agreement between the concentration profile for the drug in the gel and the free diffusion model. The diffusion of acetaminophen, ibuprofen, and indomethacin was studied at different temperatures in 1% (w/w) agarose gels. The diffusion coefficients for these drugs were found to be essentially the same as in water, and the apparent diffusion activation energies are close to that for self diffusion of water (4.62 kcal/mol), indicating a common mechanism for the diffusion of these drugs in the gel. The diffusivity of chlorpheniramine was also studied in 4% (w/w) agarose gels or with part of the agarose substituted with other polymers (e.g., chitosan and sodium alginate). These two oppositely charged polymers, mixed together, were found to occupy an "equivalent polymer volume" that was three times larger than the same amount of each of the constituents. When chitosan and gelatin-B were mixed into the agarose gel, non-Fickian transport resulted. Such non-Fickian transport was also observed with theophylline diffusing in a mixture of agarose, chitosan, and sodium alginate at a low pH.

Polyelectrolyte/amphiphile interaction studied by surface tension measurements.

The interaction of kappa-carrageenan with three positively charged drug molecules with amphiphile character has been examined using surface tension measurements. The surface tension was measured by the pendant drop method which makes possible the determination at an apparent steady state which is important for polymeric systems. The results are compared with adsorption isotherms from dialysis equilibrium. The surface tension data, show that the presence of kappa-carrageenan in the amphiphile solutions leads to an increased and pronounced lowering of the surface tension in a low concentration range of amphiphile. It is also shown that not only the hydrophobicity of the amphiphile but also the structure of the polyelectrolyte (charge density and helix-coil structure) largely determine the extent of interaction.

Gel--sol transition in kappa-carrageenan systems: microviscosity of hydrophobic microdomains, dynamic rheology and molecular conformation.

The effect of gel-sol transition in kappa-carrageenan systems on the microviscosity of hydrophobic microdomains, as well as its relation to macroscopic rheology and molecular conformation, was studied in kappa-carrageenan systems. The microdomains were probed by 1,3-di(-1-pyrenyl)propane (P3P) for which the excimer intensity (Ie) provides relative measures of the microviscosity in the immediate probe surroundings. In particular the applicability of P3P to monitor the gel--sol transition was proved, the results showing a dramatic decrease in microviscosity in the vicinity of the transition point. The corresponding changes in rheological properties and carrageenan conformation were investigated by dynamic viscometry (DV) and optical rotation (OR), respectively. The temperature of onset of the transition as indicated by the microviscosity data (T0) was found to correlate well with the OR and DV-results. The application of microviscosity and OR-measurements allowed an estimation of the helical content at T0 to be determined. P3P-data indicate a microenvironment viscosity for the probe sites in the kappa-carrageenan system comparable to that found in SDS micelles.

Temperature and salt optimization of kappa-carrageenan fractionation by DEAE-cellulose.

Carrageenans play an important role in many medical and food technology applications. However, the study of their properties has been hampered by a lack of well-defined samples (fractions). In this paper, a new principle is presented by which well-defined fractions can be obtained. It is based on batch adsorption to DEAE-cellulose combined with careful control of temperature and salt content. The parameters have been pre-optimized in an analytical column.

Rapid transport of macromolecules in polysaccharide systems by means of dissipative structures.

A gravitationally stable boundary between two polymer solutions in a ternary system can develop into an interphase of structured convectional flow. The transport over this interphase is by counter-current distribution and as a result high-molecular weight material moves by bulk-flow and much more rapidly than low-molecular weight material, which is transported essentially by ordinary diffusion.

Thermal diffusion as a mechanism for biological transport.

Accumulated experimental information is used to assess the possible significance of thermal diffusion to mass transport in living matter. Possible thermal gradients across membranes, a single living cell, and an ensemble of such cells (e.g. an organ, tumor, etc.) are estimated. The corresponding model calculations, although not describing the biological process in detail, lead to conclusions about the possibilities for thermal diffusion as follows. Adequate thermal gradients to support substantial thermal diffusion could exist across biological membranes. Thermal diffusive flow would become significant when ordinary Fickian diffusion is sufficiently suppressed, e.g. in more concentrated systems near critical points of solution (i.e. near incipient phase separations). Conditions favorable to thermal diffusion functioning as a mechanism for active transport appear possible. Thermal diffusion appears much more important for transport into and out of an ensemble of cells than into or out of a single cell. Such mass transport by thermal diffusion could assume a sizable magnitude for an ensemble of cells with the dimensions of an organ or a tumor.

Changes in the polyelectrolyte-amphiphile interaction due to helix-coil transition induced by specific counterions or variations in temperature.

For the system kappa-carrageenan/amitriptyline it is shown that the degree of binding of amitriptyline is closely related to the carrageenan conformation as regulated by the counterions (Na+ or K+). The adsorption becomes much more pronounced when the carrageenan molecule is in the helix form (counterion K+) than when it has a coil conformation (counterion Na+). Furthermore, for the helical state the adsorption becomes strongly cooperative. It is also shown experimentally that the release from the adsorbed state has a conversion temperature at about 42 degrees C (helix-coil transition). The effect is also related to the linear charge density. For iota-carrageenan with a higher charge density the adsorption is strong and cooperative both in the presence of Na+ and K+ ions.

The effect of hydrophobic character of drugs and helix-coil transition of kappa-carrageenan on the polyelectrolyte-drug interaction.

PURPOSE: The extent of adsorption of different drug molecules to kappa-carrageenan was investigated in order to evaluate the effect of drug hydrophobicity on the adsorption isotherm. METHOD: Dialysis experiments were used to determine the amount of drug adsorbed to the polyelectrolyte. The amount of drug on both sides of the membrane was determined spectrophotometrically after attaining equilibrium. CMC for the drugs were determined by the dye solubilisation method. RESULTS: It is shown that the small differences in structure between the drug molecules used in this study still leads to considerable difference in adsorption properties, especially the onset of adsorption. It was also found that the slope of the adsorption isotherms among the drug molecules followed the same pattern as the CMC values for drugs. The extent of adsorption of drugs to the helix from of kappa-carrageenan was much higher than to the coil form. CONCLUSIONS: These results suggest that the adsorption of charged drug molecules to an oppositely charged polymer is effected not only by the coulombic interactions, but also by the hydrophobicity of the drug. Furthermore, the adsorption of drug molecules to kappa-carrageenan in the helix form is higher than for the coil form because of the shorter distance between the charges and the thereby enhanced hydrophobic interaction between bound drug molecules.

The effect of polyelectrolyte counterion specificity, charge density, and conformation on polyelectrolyte-amphiphile interaction: the carrageenan/furcellaran-amitriptyline system.

The effect of polyelectrolyte cation specificity, charge density, and conformation on the interaction between furcelleran, kappa, iota, and lambda-carrageenan, respectively, and amitriptyline, an amphiphilic cationic drug molecule, was studied by means of a dialysis equilibrium technique. The carrageenans used in this study-furcelleran, kappa, iota, and lambda-carrageenan-had a charge density corresponding to 0.69, 0.92, 1.53, and 2.07 sulfate groups per disaccharide. In general, the binding isotherms followed the order Li(+) < Na(+) < N(CH3)(4)(+) < K(+) < or = Cs(+) approximately Rb(+), i.e., the binding isotherms were shifted to higher concentrations of free amphiphile according to the sequence indicated. This affinity sequence correlates well with that found for the dextran sulfate-amitriptyline system (A. Hugerth and L.-O. Sundelöf, Langmuir 2000, 16, 313-317). The factor determining the critical aggregation concentration in the presence of Na(+) compared to K(+) was found to be as follows: the flexibility (conformation) in the case of the lower charged carrageenans, i.e., furcelleran and kappa-carrageenan, charge density for iota-carrageenan, and in the lambda-carrageenan case the difference in the ROSO(3)(-)-alkali ion specificity.

A new method for diffusion measurement in polymeric films based on a stacked sheet concept.

PURPOSE: To develop and evaluate a simple, yet well defined, method to measure diffusion in semi-solids, i.e. polymeric materials. METHODS: The method was based on a concept where equivalent discs of polymeric films were cut and stacked on top of each other. The diffusion process was allowed to proceed unidimensionally through the stack of films perpendicular to the film surface. After an appropriate time, the stack was analysed disc by disc with respect to solute content and from the concentration profile so obtained the diffusion coefficient was calculated. RESULTS: An all-in-one device was developed, manufactured in stainless-steel, that cuts circular discs and stores each one successively in a "stack" in the cell compartment. CONCLUSIONS: Data from a silicone based system shows that the method, although simple, is accurate and reproducible.

Diffusion of dextran in concentrated solutions.

A free-diffusion method has been developed for the determination of the intradiffusion coefficient ('self-diffusion coefficient') of a polymer in highly concentrated solutions. A fraction of the polymer is labelled with a small amount of light-absorbing substituent. The diffusion of this labelled species, present in low concentration, is followed in the presence of a high concentration of unlabelled material with the aid of absorption optics in the analytical ultracentrifuge. The diffusion proceeds over a boundary at which the difference in concentration of unlabelled material is varied. The average concentration of total polymer and the concentration of the labelled material are, however, constant. From theoretical considerations it is shown that by extrapolation of the diffusion coefficient so obtained to zero concentration difference of total material, the intradiffusion coefficient of the polymer at that concentration is obtained. The procedure also permits the ordinary translational diffusion coefficient to be estimated. The method has been applied to two dextran fractions with weight-average molecular weights of 19000 and 150000, which were labelled with fluorescein groups. As expected, the intradiffusion coefficient decreases with increasing polymer concentration, the decrease being more pronounced for the high-molecular-weight material. This decrease in the diffusion rate of dextran is, however, less than the corresponding decrease in the sedimentation rate which proteins with similar hydrodynamic parameters experience in dextran solutions. This agrees with the hypothesis that flexible linear polymers move through a network as chains rather than as hydrodynamic spheres. By combining measurements of the ordinary diffusion coefficient and the intradiffusion coefficient, it is possible to calculate the thermodynamic properties (as expressed by the virial expansion) of the system. This method is of particular importance in studies on concentrated solutions of high-molecular-weight polymers.