Experimental Osmotic Pressure Research Articles

Core-Shell Colloids and Hollow Polyelectrolyte Capsules Based on Diazoresins

Hollow polyelectrolyte microcapsules containing diazoresins (DZR) were fabricated by the layer-by-layer self-assembly of a polycation, DZR, in alternation with poly(styrenesulfonate) (PSS) onto polystyrene (PS) particles, followed by dissolution of the PS core by tetrahydrofuran (THF). The multilayer film buildup on the colloids was observed by UV-visible spectroscopy, single particle light scattering (SPLS), and transmission electron microscopy (TEM). The data confirmed regular and stepwise layer formation of DZR and PSS on the colloid particles, with a thickness of about 10 nm for each DZR/PSS bilayer when exposed to aqueous solution, and approximately 5 nm in the “dry state”. The photosensitive nature of the DZR layers was exploited to construct highly stable, covalently attached (polymerized) films by exposure of the ionic self-assembled DZR/PSS multilayer films to UV-irradiation. TEM and atomic force microscopy (AFM) confirmed the formation of hollow DZR/PSS multilayer capsules. Osmotic pressure experiments followed by confocal laser scanning microscopy revealed a high mechanical stability of the hollow DZR/PSS capsules. The mechanically robust polymerized multilayer films on the colloids and as free-standing three-dimensional hollow capsules are more stable in various chemical environments (i.e., resistant to etching by solvents) than their ionically linked counterparts.

A Self-Consistent Multicomponent Activity Coefficient Model for Ionic Micellar Surfactant Solutions

The proposed model incorporates a distribution of micellar sizes self-consistently. The model is based on a mass-action equilibrium approach that includes micelle−micelle interactions as a function of size for a multicomponent surfactant solution consisting of micellar aggregates, monomer, counterions, and added electrolyte. The primary solution nonidealities are accounted for in the multicomponent model with excluded-volume and electrostatic interactions as a function of aggregate size. In addition, the model accounts for the Donnan equilibrium existing between an ionic surfactant solution and the electrolyte solution from which it is separated by a semipermeable membrane. Surfactant solutions of sodium dodecyl sulfate (SDS) and cetylpyridinium chloride (CPC) in 0.01 M NaCl are studied over the concentration range from the critical micelle concentration up to volume fractions of 0.19 (0.87 M) for SDS and 0.16 (0.56 M) for CPC. In comparison to the predictions of an ideal solution multiple-chemical equilibrium constant model, the activity-coefficient model predicts increased growth and polydispersity of the aggregates for both CPC and SDS at higher surfactant concentrations. Micellar interactions enhance the growth of the micelles due to excluded-volume effects that favor growth and electrostatic repulsions that oppose it. Micellar aggregates are found to be slightly globular for both SDS and CPC; however, a clear spherical to globular transition is predicted for the CPC micelles. The multicomponent model reflects experimental osmotic pressure data successfully.

Osmotic pressure measurements of dissociating bovine hemoglobin

While the dissociation of hemoglobin, in general, has been studied extensively using various techniques under both alkaline and acidic conditions, the dissociation under alkaline conditions using membrane osmometry has not been previously reported. An equilibrium mathematical model describing the two-stage dissociation is presented in this paper and agrees with the experimental osmotic pressure measurements of bovine hemoglobin at pH's above 7.4. Results of this study show that the pH dependency under alkaline conditions at 37°C can be regressed to give approximate values of the dissociation constants for both the dissociation of the tetramer to the dimer and the dimer to the monomer as a function of pH. The data also indicate that, in the presence of the borate buffer used in these experiments, the liberation of approximately one proton occurs for the dissociation of the tetramer into the dimers; approximately two protons are liberated for the dissociation of the dimer into the monomer. The dissociation constants reported here for bovine hemoglobin are in reasonable agreement with those obtained under similar conditions using other techniques.

Protein interactions in the calf eye lens: interactions between beta-crystallins are repulsive whereas in gamma-crystallins they are attractive.

Non-specific interactions in beta- and gamma-crystallins have been studied by solution X-ray scattering and osmotic pressure experiments. Measurements were carried out as a function of protein concentration at two ionic strengths. The effect of temperature was tested between 7 degrees C and 31 degrees C. Two types of interactions were observed. With beta-crystallin solutions, a repulsive coulombic interaction could be inferred from the decrease of the normalized X-ray scattering intensity near the origin with increasing protein concentration and from the fact that the osmotic pressure increases much more rapidly than in the ideal case. As was previously observed with alpha-crystallins, such behaviour is dependent upon ionic strength but is hardly affected by temperature. In contrast, with gamma-crystallin solutions, the normalized X-ray scattering intensity near the origin increases with increasing protein concentration and the osmotic pressure increases less rapidly than in the ideal case. Such behaviour indicates that attractive forces are predominant, although we do not yet know their molecular origin. Under our experimental conditions, the effect of temperature was striking whereas no obvious contribution of the ionic strength could be seen, perhaps owing to masking by the large temperature effect. The relevance of the different types of non-specific interactions for lens function is discussed.

Osmotic pressure measurements of ovalbumin and lysozyme mixtures.

Ovalbumin and lysozyme have been reported to undergo a mixed association in solutions of low ionic strength. Osmotic pressure experiments were performed on ovalbumin and on lysozyme solutions in 0.06 M cacodylate buffer (I = 0.02, pH = 5.8) at 30 and at 37 degrees C. The individual proteins did not undergo any self-associations at either temperature; these measurements indicated that each of the solutions was nonideal. Osmotic pressure experiments on three blends of lysozyme and ovalbumin at 30 and 37 degrees C could be interpreted in two ways. One interpretation was that a nonideal, nonassociating mixture of A and B was present; for the three solutions the mixed nonideal term BAB was negative. A negative nonideal term is usually interpreted as indicating an association. The other interpretation of the data was as a quasi-ideal mixed association of the type A + B in equilibrium AB.

Transient diffusion through a membrane with a selective skin

A model is introduced for a membrane with a skin. The isothermal nonstationary diffusion through the membrane is described for a system with, on one side of the membrane, a binary solution with a constant concentration, and, on the other side, a binary solution with an initially uniform concentration, contained in a closed cell with a finite volume. The orientation of the asymmetric membrane toward the external phases is taken into account. Diffusion of solute through the membrane results in a decay of the experimental osmotic pressure across the membrane. This pressure depends not only on the concentrations on both sides of the membrane but also on the concentration profile in the membrane. Practical equations are given allowing the determination of the average reflection coefficient and of the solute permeabilities of the selective skin and the supporting layer from observed pressure-time curves. The influence of the volume flow history on the initial concentration profile is taken into account. Special attention is paid to the occurrence of asymmetry pressures (osmotic pressures at zero concentration difference over the membrane, which are induced by the initial volume flow).

Osmotic properties of a modified cellulose acetate membrane: The reflection coefficient and its dependence on the volume flow history

The selectivity of a membrane toward a permeable solute is described by the reflection coefficient, which is defined as the ratio of the experimental osmotic pressure Δπ exp to its theoretical value Δπ. The systems investigated consisted of a modified cellulose acetate membrane and aqueous solutions of NaCl at various concentrations and BaCl 2. Modification of an electronic membrane osmometer permitted direct measurements of Δπ exp at zero volume flow for such a membrane. The values of Δπ exp depend not only on the external concentrations but also on the concentration profile in the membrane established by the preceding volume flow. The significant influence of the sign and magnitude of these flows demonstrates the heterogeneity of the membrane. This memory for the volume flow history becomes manifest in Δπ exp measured at equal external concentrations. The experimental results can be interpreted semiquantitatively by describing the modified cellulose acetate membrane as a bilayer.

Nonstationary diffusion through a membrane separating two finite volumes of stirred or unstirred solutions

The decay of the osmotic pressure with time is treated theoretically and experimentally for the case of a solute with low molecular weight permeating through a membrane. The influence of the boundaries of the external phases has been taken into account. The solutions of the diffusion equations for the composite system yield the solute concentration as a function of position and time everywhere in the system. Expressions are derived for the experimental osmotic pressure (i.e. the pressure needed to make volume flow through the membrane vanish) with the external solutions either stirred or unstirred and including a number of experimentally interesting initial conditions. For sufficiently large values of time, even in the unstirred situation, these expressions predict a simple exponential decay with time, reflecting that the system has arrived at a quasi-stationary state. The pressure-time curves were measured with a high-speed osmometer equipped with a cellulose membrane separating two unstirred solutions of glycerol trioctadecanoate in toluene. A single curve allows the determination of at least two transport parameters: the reflection coefficient σ and the membrane diffusion coefficient D. It is shown that without loss of accuracy a simplified evaluation of these parameters is possible by using only the simple exponential part of the decay curve.

Sedimentation coefficients of self-associating species

Abstract Under the same solution conditions, the apparent weight average sedimentation coefficient, swa, and some quantities obtained from it can be combined with the equilibrium constant or constants, Ki, and the monomer concentration, cI, obtained from sedimentation equilibrium, light scattering or osmotic pressure experiments on the same self-associating solute, so that the individual sedimentation coefficients, si, of the self-associating species, and also the hydrodynamic concentration dependence parameter,g or g , can be evaluated. Using two different models for the hydrodynamic concentration parameter, four different methods are presented for the evaluation of the si's. Methods for evaluating g or g , once the si's are known, are presented. A method for obtaining the number average sedimentation coefficient, sN, and its application to self-associations is presented. Methods are shown for the evaluation of Z average properties, xzc, as well as number average properties,xNc, of a self-associating solute from its weight average properties, xwc.

Chemically reacting systems of the type A + B ⇄ AB. II. Osmometry

AbstractThe osmotic pressure equation for nonideal, associating systems of the type nA +mB ⇄ AnBm, has been derived, by using the assumption yAnBm/yAnyBm = 1. This treatment can also be applied to related associations such as nA + mB ⇄ AB + AB2 + A2B + …. From osmotic pressure experiments on the pure reactants it is possible to obtain the molecular weights (MA and MB) of the reactants and also the virial coefficients (BAA and BBB) of the reactants. The osmotic pressure of a nonreacting mixture of A and B can be calculated from these measurements. It can be used along with osmotic pressure measurements on equilibrium mixtures of A and B to obtain expressions containing the equilibrium constant (or constants) and the cross‐virial coefficients (BAB and BBA). Several procedures are described for the evaluation of the equilibrium constant (or constants) and the BAB or BBA terms. It appears that this procedure is a general one which is applicable to associations of the type nA + mB ⇄ AB + A2B + AB2 + …. By correcting for nonideal behavior, one should then be able to apply it to any method available for analyzing ideal associations of the types considered here. In addition it is possible, subject to certain restrictions, to analyze associations of the type 3A + B ⇄ A2 + AB.

Stress Relaxation in Concentrated Solutions of Polymethyl Methacrylate: Molecular Weight Dependence

The measurement of stress relaxation in concentrated solutions of polymethyl methacrylate has revealed a relation between the spectrum of relaxation times, the weight average molecular weight, and the molecular weight distribution. The spectrum of relaxation times and the essentially identical theories developed by Rouse, Bueche, and Zimm are used to calculate an average relaxation time which is directly proportional to the weight average molecular weight. It is then shown that a first approximation to the distribution of molecular weights may be calculated if one assumes that the friction factor increases with the 1.5 power of the molecular weight. This distribution for selected samples of polymethyl methacrylate is compared with the Schulz distribution in which the parameters were obtained from light scattering and osmotic pressure experiments. The ratios Mz/Mw calculated from the theory and stress relaxation data on a rough fraction, a whole polymer, and a blend of two whole polymers are found to be 1.3, 1.7, and 1.9, respectively.

THE ISOELECTRIC POINT OF GELATIN AT 40°C

Measurements have been made at 40 degrees C. of the osmotic pressure and viscosity of 1 per cent gelatin solutions containing varying amounts of hydrochloric acid or sodium hydroxide. Each property was found to exhibit a decided minimum near pH 4.7. In the osmotic pressure experiments the pH of the inside solutions was greater than that of the outside solutions at pH values below 4.7, while it was less than that of the outside solutions at values above pH 4.7. These results indicate that gelatin at 40 degrees C. retains its isoelectric point at about pH 4.7.