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Nakach M.,Sanofi S.A. | Authelin J.-R.,Sanofi S.A. | Tadros T.,89 Nash Grove Lane | Galet L.,Ecole des Mines dAles | Chamayou A.,Ecole des Mines dAles
International Journal of Pharmaceutics | Year: 2014

This paper describes a systematic approach to select optimum stabilizer for the preparation of nano-crystalline suspensions of an active pharmaceutical ingredient (API). The stabilizer can be either a dispersant or a combination of dispersant and wetting agent. The proposed screening method is a quick and efficient way to investigate a large number of stabilizers based on the principles of physical-chemistry and employs a stepwise approach. The methodology has been divided in two main parts; the first part being focused on the qualitative screening with the objective of selecting the best candidate(s) for further investigation, the second part has been focused on quantitative screening with the objective to optimize the ratio and amount of wetting and dispersing agents, based on wettability, surface charges measurement, adsorption evaluation, process-ability evaluation and storage stability. The results showed clearly that SDS/PVP 40/60% (w/w) (sodium dodecyl sulfate/poly(vinyl pyrrolidone)) at a total concentration of 1.2% was the optimum stabilizer composition, at which the resulting nanosuspensions were stable for more than 50 days at room temperature. © 2014 Elsevier B.V. All rights reserved. Source

Naden B.J.,Wilton Center | Naden B.J.,Imperial College London | Kessell L.M.,Wilton Center | Luckham P.F.,Imperial College London | Tadros T.F.,89 Nash Grove Lane
Colloids and Surfaces A: Physicochemical and Engineering Aspects | Year: 2015

TiO2 nanoparticles are becoming increasingly important as sunscreens due to their propensity to absorb UV light. It is crucial that the particles are nanoscale so that scattering of visible light is minimal and UV attenuation is optimised, thus stabilisation is essential to prevent particles from aggregating. In this study we investigate the adsorption of dispersant based on stearic acid to TiO2 nanoparticles by adsorption isotherm determination. The methods used to determine dispersant concentration (acid value titration and gel permeation chromatography, GPC) provided information regarding the chemical nature of the polymers and insights into the way the molecules interacted with particle surfaces in different solvent conditions. It was observed that the smaller dispersant molecules adsorb preferentially, contrary to the behaviour of adsorbing homopolymers that display a direct relationship between molecular weight and strength of adsorption. Adsorption in this case is a result of single point adsorption that occurs via interaction of the carboxylate group of the dispersant to the TiO2 surface, the thermodynamics of which favours low molecular weight components. This is clearly demonstrated by GPC which was able to resolve and quantify the non-adsorbed molecules by size. Discrepancies between GPC and acid value measurement for dispersant concentration reinforce this observation. © 2015. Source

Tadros T.,89 Nash Grove Lane
Advances in Colloid and Interface Science | Year: 2015

Abstract The interaction forces between emulsion droplets containing adsorbed polymeric surfactants and the theory of steric stabilisation are briefly described. The results for the viscoelastic properties of O/W emulsions that are stabilised with partially hydrolysed poly(vinyl acetate) that is commonly referred to as poly(vinyl alcohol) (PVA) with 4% vinyl acetate are given. The effect of the oil volume fraction, addition of electrolytes and increasing temperature is described. This allows one to obtain various parameters such as the adsorbed layer thickness, the critical flocculation concentration of electrolyte (CFC) and critical flocculation temperature (CFT) at constant electrolyte concentration. The viscoelastic properties of O/W emulsions stabilised with an A-B-A block copolymer of polyethylene oxide (A) and polypropylene oxide (B) are described. These emulsions behave as viscoelastic liquids showing a cross-over-point between G′ (the elastic component of the complex modulus) and G″ (the viscous component of the complex modulus) at a characteristic frequency. Plots of G′ and G″ versus oil volume fraction ℙ show the transition from predominantly viscous to predominantly elastic response at a critical volume fraction ℙc. The latter can be used to estimate the adsorbed layer thickness of the polymeric surfactants. Results are also shown for W/O emulsions stabilised with an A-B-A block copolymer of polyhydroxystearic acid (PHS, A) and polyethylene oxide (PEO, B). The viscosity volume fraction curves could be fitted to the Dougherty-Krieger equation for hard-spheres. The results could be applied to give an estimate of the adsorbed layer thickness Δ which shows a decrease with increase of the water volume fraction. This is due to the interpenetration and/or compression of the PHS layers on close approach of the water droplets on increasing the water volume fraction. The last section of the review gives an example of O/W emulsion stability using an ABn graft copolymer of polyfructose (A) to which several C12 alkyl chains are grafted. The emulsions are stable both at high temperature and in the presence of high electrolyte concentrations (2 mol dm- 3 NaCl). This high stability is due to the strong adsorption ("anchoring") of the graft copolymer with several C12 alkyl chains and the strong hydration of the polyfructose chains both in water and in the presence of high electrolyte concentrations and temperature. Evidence for this high stability is obtained using disjoining pressure measurements which show a highly stable film between the emulsion droplets and absence of its rupture up to high pressures. © 2015 Elsevier B.V. Source

Amine C.,BASF | Dreher J.,BASF | Helgason T.,BASF | Tadros T.,89 Nash Grove Lane
Food Hydrocolloids | Year: 2014

The emulsifying properties and emulsion stability of medium chain triglyceride (MCT) oil in water emulsions stabilized by plant proteins (potato, soy, and pea) as well as those extracted from milk (sodium caseinate and whey) have been investigated at two different pH values (7 and 10). The effect of protein type, concentration, pH and temperature has been studied using droplet size distribution, interfacial tension, and interfacial elasticity measurements. The interfacial tension is measured as a function of concentration ranging from 0.01% to 2.5% using the pendant drop method. The interfacial elasticity is also measured using the oscillating drop technique. The stability of the resulted emulsions is assessed using a stress test whereby the emulsions are placed at 60 or 40°C for 24h by measuring the droplet size distribution before and after the stress test. The interfacial tension decreases with increase of the pH and increase of the protein concentration pH value. A higher elasticity at the oil/water interface is obtained at pH=10 when compared to that at pH=7 for both Na-caseinate and pea proteins. Fine and stable emulsions are obtained at pH=10, showing good correlation between interfacial tension and emulsion formation as well as between interfacial elasticity and emulsion stability for milk and plant proteins. © 2014 Elsevier Ltd. Source

Tadros T.,89 Nash Grove Lane
Advances in Colloid and Interface Science | Year: 2011

The interparticle interactions in concentrated suspensions are described. Four main types of interactions can be distinguished: (i) "Hard- sphere" interactions whereby repulsive and attractive forces are screened. (ii) "Soft" or electrostatic interactions determined by double layer repulsion. (iii) Steric repulsion produced by interaction between adsorbed or grafted surfactant and polymer layers. (iv)and van der Waals attraction mainly due to London dispersion forces. Combination of these interaction energies results in three main energy-distance curves: (i) A DLVO type energy-distance curves produced by combination of double layer repulsion and van der Waals attraction. For a stable suspension the energy-distance curve shows a "barrier" (energy maximum) whose height must exceed 25 kT (where k is the Boltzmann constant and T is the absolute temperature). (ii) An energy-distance curve characterized by a shallow attractive minimum at twice the adsorbed layer thickness 2δ and when the interparticle-distance h becomes smaller than 2δ the energy shows a sharp increase with further decrease of h and this is the origin of steric stabilization. (iii) an energy-distance curve characterized by a shallow attractive minimum, an energy maximum of the DLVO type and a sharp increase in energy with further decrease of h due to steric repulsion. This is referred to as electrosteric repulsion. The flocculation of electrostatically and sterically stabilized suspensions is briefly described. A section is devoted to charge neutralization by polyelectrolytes and bridging flocculation by polymers. A distinction could be made between "dilute", "concentrated" and "solid suspensions" in terms of the balance between the Brownian motion and interparticle interaction. The states of suspension on standing are described in terms of interaction forces and the effect of gravity. The bulk properties (rheology) of concentrated suspensions are described starting with the case of very dilute suspensions (the Einstein limit with volume fraction φ ≤ 0.01), moderately concentrated suspensions (0.2 > φ ≥ 0.1) taking into account the hydrodynamic interaction and concentrated suspensions (φ > 0.2) where semi-empirical theories are available. The rheological behavior of the above four main types of interactions is described starting with "hard-sphere" systems where the relative viscosity-volume fraction relationship could be described. The rheology of electrostatically stabilized suspensions was described with particular reference to the effect of electrolyte that controls the double layer extension. The rheology of sterically stabilized systems is described using model polystyrene suspensions with grafter poly(ethylene oxide) layers. Finally the rheology of flocculated suspensions was described and a distinction could be made between weakly and strongly flocculated systems. © 2011 Elsevier B.V. All rights reserved. Source

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