Th.F. Tadros

Surface Modification of Poly(lactide-co-glycolide) Nanospheres by Biodegradable Poly(lactide)-Poly(ethylene glycol) Copolymers

The modification of surface properties of biodegradable poly(lactide-co-glycolide) (PLGA) and model polystyrene nanospheres by poly(lactide)-poly(ethlene glycol) (PLA:PEG) copolymers has been assessed using a range of in vitro characterization methods followed by in vivo studies of the nanospheres biodistribution after intravenous injection into rats. Coating polymers with PLA:PEG ratio of 2:5 and 3:4 (PEG chains of 5000 and 2000 Da, respectively) were studied. The results reveal the formation of a PLA: PEG coating layer on the particle surface resulting in an increase in the surface hydrophilicity and decrease in the surface charge of the nanospheres. The effects of addition of electrolyte and changes in pH on stability of the nanosphere dispersions confirm that uncoated particles are electrostatically stabilized, while in the presence of the copolymers, steric repulsions are responsible for the stability. The PLA:PEG coating also prevented albumin adsorption onto the colloid surface. The evidence that this effect was observed for the PLA:PEG 3:4 coated nanospheres may indicate that a poly(ethylene glycol) chain of 2000 Da can provide an effective repulsive barrier to albumin adsorption. The in vivo results reveal that coating of PLGA nanospheres with PLA:PEG copolymers can alter the biodistribution in comparison to uncoated PLGA nanospheres. Coating of the model polystyrene nanospheres with PLA:PEG copolymers resulted in an initial high circulation level, but after 3 hours the organ deposition data showed values similar to uncoated polystyrene spheres. The difference in the biological behaviour of coated PLGA and polystyrene nanospheres may suggest a different stability of the adsorbed layers on these two systems. A similar biodistribution pattern of PLA:PEG 3:4 to PEG 2:5 coated particles may indicate that poly(ethylene glycol) chains in the range of 2000 to 5000 can produce a comparable effect on in vivo behaviour.

Investigations into the mechanisms of electrohydrodynamic spraying of liquids: I. Effect of electric field and the environment on pendant drops and factors affecting the formation of stable jets and atomization

Abstract The effect of an applied electric field on pendant and flowing drops (through a capillary) was studied by observing the droplet profile as a function of applied voltage. With pendant drops of hexadecane, application of the electric field caused a reduction in the apparent surface tension, γ app . In surrounding gases such as N 2 , CO 2 , and Ar, droplet profiles similar to those in air were observed. While gases such as He and Ne have low breakdown potentials, no change in droplet profile upon application of an electric field was detectable. The glow discharge in He was investigated for oil (insulator), water, glycerol, and a brass tip, for both positive and negative potentials. Conducting liquids such as water and glycerol permitted discharge to take place from the surface of the droplet, whereas insulating oils forced the discharge to occur at the metal tip, where the drop is attached. With flowing drops, the liquid conductivity was a major factor in the electrostatic disruption of the liquid surface. With insulating liquids such as paraffinic oil, no disruption occurred due to the lack of sufficient free ions in the bulk liquid. With conducting liquids such as water, very unstable streams were produced. Stable jets having a conical base were only produced with semiconducting liquids (nonpolar liquids with dissolved ionic materials). The cone angle at the base of the jet increased, whereas its length decreased with an increase in applied voltage. At higher voltages, secondary jets were produced from the primary one, whose number increased with an increase in the applied field. The effect of liquid conductivity, applied voltage, flow rate, and capillary diameter on the stability of jets was investigated by measuring the critical voltage, φ c , at which transition from the pulsating mode to the stable jet mode occurred. By measuring the current carried by the jet, the charge-to-mass ratio could also be calculated. Some measurements of droplet size distribution were made using a practical sprayer and a particle measuring system for measuring droplet diameters in flight. These measurements were made as a function of applied voltage, conductivity, and flow rate. The results obtained clearly demonstrate the importance of applied voltage, liquid conductivity and flow rate in the formation of stable jets and the subsequent process of electrohydrodynamic atomization. At a given voltage and flow rate an optimum conductivity range is necessary for producing the most stable jet, the narrowest size distribution, and the smallest droplet size. This could be accounted for in terms of the electric forces acting on the liquid, which are related to the relaxation time of the liquid. The influence of flow rate on the production of stable jets and the subsequent atomization could also be understood in terms of the inertial and electrostatic forces which act in the same direction. At a given conductivity and voltage, stability is enhanced by increasing the flow rate, but at the expense of producing larger droplets.

Fundamental principles of emulsion rheology and their applications

Abstract This review starts with an introduction summarizing the factors that affect the flow characteristics (rheology) of emulsions. These factors are the volume fraction of the disperse phase, the viscosity of the disperse droplets, the droplet size distribution, the viscosity and chemical composition (pH, electrolyte concentration, etc.) of the medium, the interfacial rheology of the emulsifier film and the concentration and nature of the emulsifier. A section on interfacial rheology describes the basic equations and the methods that may be applied to the measurement of interfacial elasticity and viscosity. This is followed by a discussion on the correlation of interfacial rheology with emulsion stability. Examples are given showing the correlation between interfacial viscosity and elasticity with emulsion coalescence. The second part of the review deals with the bulk rheology of emulsions. It starts with a section on the viscosity-volume fraction relationship, which clearly showed that emulsions stabilized with surfactant films behave like hard sphere dispersions provided the adsorbed layer thickness is much smaller than the droplet radius. The viscoelastic properties of concentrated sterically stabilized oil-in-water (o/w) and water-in-oil (w/o) emulsions are then described. These emulsions show a transition from predominantly viscous to predominantly elastic response as the frequency of oscillation exceeds a critical value. This allows one to obtain the relaxation time of the emulsion system, which was shown to increase with increase in the volume fraction φ. Plots of the complex modulus (G*), the storage modulus (G′) and the loss modulus (G″) showed a transition from predominantly viscous to predominantly elastic response at a critical φ value. This reflected the steric interaction which increased with increase in φ. When the surface-to-surface separation between the droplets became smaller than twice the adsorbed layer thickness, the steric repulsion became strong, increasing in magnitude with increase in φ. The last part of the review describes the viscoelastic properties of weakly flocculated emulsions. These were obtained by addition of a “free” (non-adsorbing) polymer to a sterically stabilized emulsion (depletion flocculation). Again the viscoelastic behaviour reflected the weak attraction between the droplets in the emulsion. Above a critical volume fraction of free polymer (which decreased with increase of the molecular weight of the polymer) there was a rapid increase in the storage modulus and this determined the onset of depletion flocculation. The results obtained were similar to those obtained with suspensions, thus showing that viscoelastic measurements can be applied to investigate the interaction between emulsion droplets in the same manner as for suspensions.

The settling of particles through Newtonian and non-Newtonian media

Abstract An experimental investigation has been made of the settling rate of polystyrene latex dispersions containing particles of radius = 1.55 μm in salt solutions and in shear thinning polymer solutions. The volume fraction of the dispersion was varied from dilute, ca. 0.01, to concentrated, ca. 0.5. The experimental data are compared with theoretical predictions for dispersions of hard spheres. It was found that the relative velocity of settling U U o for concentrated systems could be represented by the equation U U 0 = 1 — φ/p) kp with p = the latex volume fraction at close packing, φ = the volume fraction of the latex, and k a constant. In shear thinning polymer solutions it was concluded that the zero shear rate viscosity controlled the rate of settling of the particles.

Influence of pH, electrolyte, and poly(vinyl alcohol) addition on the rheological characteristics of aqueous dispersions of sodium montmorillonite

Abstract The rheological characteristics of sodium montmorillonite dispersions were investigated as a function of pH and electrolyte (NaCl) concentration. Steady state and low shear rheology and instantaneous modulus measurements were used to obtain the yield value τβ, the plastic viscosity ηPL, the residual viscosity η0, and the modulus G0. With the exception of the plastic viscosity, the other rheological parameters showed a minimum at pH = ∼7 increasing rapidly as the pH was increased above or decreased below this value. Moreover, τβ, η0, and G0 increased with increase of NaCl concentration (within the range 10−3 to 10−1 mole dm−3). These results indicate increase in gelation of aqueous montmorillonite dispersions in acid and alkaline conditions. This gelation becomes more pronounced the higher the NaCl concentration within the range investigated. The various possible interactions between the clay platelets and their dependence on pH and electrolyte concentration were considered assuming that the isoelectric point (iep) of the particle edges was around pH = 7. At pHs below the iep, edge-to-face attraction was considered to dominate the interaction. On the other hand, at pHs above the iep of the edge, the increase in gelation with increase in pH was considered to be due to an increase in double-layer repulsion as the net charges on the edges increase. Preliminary results on the effect of addition of PVA at pH = 7 showed a gradual increase in structure on the addition of polymer, reaching a maximum at an optimum adsorption value (corresponding to ∼1 4 coverage) above which further increase in polymer concentration results in reduction of this structure. This gelation at an optimum PVA concentration was considered to be due to polymer bridging.

Investigations into the mechanism of electrohydrodynamic spraying of liquids: II. Mechanism of stable jet formation and electrical forces acting on a liquid cone

Abstract The mechanism of stable jet formation was investigated by observing the motion of the liquid in a jet close to the capillary tip from which it emerges under the influence of an electric field. Tracer particles of lycopodium were inserted in the liquid and streak photographs were taken of a stable jet formed under the application of 10.5 kV and a flow rate of 30 ml/h. These photographs show an axisymmetric circulation of the liquid in the conical base of the jet, invalidating previous theories that have assumed a uniform velocity profile in the liquid cone. This axisymmetric motion of the liquid in the jet was explained in terms of interfacial electrical shear stresses. Due to the semi-insulating nature of the liquid, there will exist a potential difference between the base of the capillary and the tip of the cone. This potential drop ensures that the interface is subjected to a tangential electric field in the direction of flow and hence an electric shear stress on the surface of the cone. Both tangential and normal fields on the cone were calculated from a knowledge of the jet profile and current. The tangential field on the surface of the cone was calculated by considering it to be a section of a sphere and dividing it into five sections. Each section was assumed to form an equipotential surface normal to the direction of flow and to have a constant current flowing through it. The normal field was calculated numerically using a computer program that estimates the potential distribution within a region subject to given boundary conditions using the finite element method. The results of calculation showed that the shear force acting on the cone was about 5 to 10 times smaller than the normal force. However, the normal force was found to be fairly constant and independent of applied voltage, within the range studied. In contrast, the shear force showed a tendency to decrease with an increase in applied voltage. It was also observed that as the voltage was increased, the length of the jet decreased. This could be explained in terms of the reduction of tangential shear force with an increase in applied voltage, thus reducing jet stability.

Adsorption of polyvinyl alcohol on silica at various ph values and its effect on the flocculation of the dispersion

Abstract The adsorption of polyvinyl alcohol (PVA) from water onto various forms of silica and the flocculation of the dispersions by the polymer were found to depend on the nature and history of the silica sample, and the pH of the solution. Maximum adsorption of PVA, accompanied by maximum flocculation, occurs at the point of zero charge (pzc) of the oxide, above which there is a progressive decrease of adsorption and flocculation with increasing pH. The adsorption of PVA also increases as the temperature of the heat pretreatment of the silica is increased, reaching a maximum at ∼700°C; thereafter the adsorption decreases progressively with increasing temperature. This maximum and the marked pH influence indicate that both “isolated” silanols and siloxane bonds are adsorption sites for PVA adsorption, an optimum ratio for maximum adsorption being obtained by heating silica at 700°C.

The influence of temperature on the adsorption and adsorbed layer thickness of various molecular weight fractions of poly(vinyl alcohol) on polystyrene latex particles

Abstract A comparison of the adsorption, at 5, 25, 37, and 50°C, of poly(vinyl alcohol) (PVA) on polystyrene latex particles from water has been made, in terms of the adsorbed amount (Γ) and the adsorbed layer thickness (δ), for various PVA molecular weight fractions (13,600 to 65,100). Solution properties of these PVA fractions (i.e., the radius of gyration and the χ-parameter), at various temperatures, have also been determined from viscometry studies. Γ would appear to increase slightly with temperature (at least for the higher molecular weights), while δ decreases. However, there is evidence for multilayer adsorption of PVA, and, therefore, it is considered that the average volume fraction (φ) of segments in the adsorbed layer (obtained from a combination of Γ and δ) is a better indication of the role of temperature on adsorption. φ is found to increase significantly with increasing temperature, for all the molecular weight fractions studied. This reflects the decreasing solvency of water for PVA with increasing temperature; this effect is also apparent in the trend in the values of the radius of gyration and the χ-parameter.

Viscoelastic properties of aqueous concentrated polystyrene latex dispersions containing grafted poly(ethylene oxide) chains

Abstract The viscoelastic properties of aqueous concentrated polystyrene latex dispersions containing grafted poly(ethylene oxide) (PEO) chains were investigated using oscillatory and steady-state shear stress-shear rate measurements. The relative viscosity-effective volume fraction results were fitted to the Dougherty-Krieger equation for hard spheres by adjusting the value for the adsorbed layer thickness Δ. The latter was found to decrease with an increase in volume fraction of the dispersion and near to close-packing considerable compression of the chains occurred. From the oscillatory measurements, the complex modules G ∗ , storage modules G′, and loss modulus G″ were obtained as a function of frequency at various latex volume fractions. The results showed that the dispersion changes from being more viscous (G″ > G′) to more elastic (G′ > G″) over a narrow range of volume fraction φ of the dispersion, i.e., when φ is increased from 0.465 to 0.5. Within the range of increase of φ, it is likely that the chains undergo some compression and interpenetration of the peripheries. When φ increased significantly above 0.5, the system becomes predominantly elastic and significant interpenetration and compression of the chains occur. Indeed when φ is further increased to 0.585 and 0.62, the moduli increase by several orders of magnitude and so does the dynamic viscosity. Under these conditions, the latex behaves as an elastic “gel.”

Rheological studies on concentrated polystyrene latex sterically stabilized by poly(ethylene oxide) chains

The rheological properties of aqueous concentrated polystyrene latex dispersions for three particles with core diameters of 155 nm, 612 nm, and 1004 nm were investigated using steady-state shear stress—shear rate and oscillatory measurements. The relative viscosity—effective volume fraction results were fitted to the Dougherty—Krieger equation for hard spheres for dispersions with various particle sizes by adjusting the value for the adsorbed layer thickness Δ. The latter was found to decrease with an increase in volume fraction of the dispersion which is caused by considerable compression of the chains when the volume fraction of the particles approaches close-packing. The value of Δ was also found to increase with increases in particle size. From the oscillatory measurements, the complex modulus G∗, the storage modulus G′, and the loss modulus G″ were obtained as a function of frequency at various latex volume fractions. The results showed that the dispersion changed from being more viscous (G″ > G′) to more elastic (G′ > G″) over a narrow range of volume fraction of the dispersion. Within this range of φ, it is likely that the chains undergo some compression and interpenetration of peripheries. When φ was increased significantly above a certain value, the system became predominantly elastic and significant interpenetration and compression of the chains occurred. If φ was further increased, the moduli increased by several orders of magnitude as did the dynamic viscosity. Under these conditions, the latex dispersion behaves as an elastic “gel” which can be fitted by a power law equation G′ = kφm where m is related to the compressibility of particles which is determined by the ratio of adsorbed layer thickness to radius of particle, ΔR.