Interafacial tension and surfactant stabilisation during membrane emulsification

A new insight on the interaction between complex fluids and solid surfaces is given in this study. It is demonstrated how these liquid-solid interactions can greatly influence the droplet generation during membrane emulsification and, therefore, resultant emulsion size and uniformity. These aspects of the process are often underestimated and poorly comprehended by, specially, industrial colleagues. Thus, that unawareness can lead to poor results and to a disbelief in this upcoming technology. A novel membrane emulsification system is reported consisting of a tubular metal membrane, periodically azimuthally (tangentially) oscillated with frequencies up to 50 Hz and 7 mm displacement in a gently cross flowing continuous phase. Using an azimuthally oscillating membrane, oil-in-water (o/w) emulsions were experimentally produced with a diameter of 20 120 µm, and a coefficient of variation (CV) of droplet size of 8%. The drop size was correlated with shear stress at the membrane surface using a force balance. In a single pass of continuous phase, it was possible to achieve high dispersed phase concentrations up to 38% v/v. A vertical oscillation membrane emulsification was used to study the influence of dynamic interfacial tension in membrane emulsification: drop size can be tuned between 35 and 85 µm by changing the surfactant concentration in the continuous phase. In addition, a method to determine the percentage of active pores during membrane emulsification is demonstrated. This method links knowledge acquired in the surfactant adsorption dynamics and drop expansion rate. This study reinforces the importance of dynamic interfacial tension which must be considered in process design, and modelling purposes, particularly in two liquid phase systems using membranes such as membrane emulsification at high injection rates. Hydrophobization of metal surfaces is reported based on silanization reactions to broaden the use of metal porous membranes to water-in-oil (w/o) emulsion production. The developed procedure is shown to be a straightforward hydrophobization method, with minimal cost to apply, reproducible, stable and the possibility of reuse of the membrane after losing hydrophobicity by simply reapplying the hydrophobization method. On the other hand, formation of water-based droplets can be achieved using a hydrophilic porous metal membrane. To investigate this, water-based droplets were produced using a hydrophilic membrane and wetting experiments were also carried out: sessile droplets were used to determine static contact angles and a rotating drum system was used to determine contact angles under dynamic conditions. In the latter case the three-phase contact line was observed between the rotating drum and two immiscible liquids. It was observed that the oil phase can preferably wet a hydrophilic surface in case when a surfactant is present in the oil phase, and above a certain concentration, even in the presence of the water phase. Apart from membrane emulsification, spontaneous formation of water droplets in kerosene was observed, facilitated by the presence of an oil soluble surfactant: Span® 80. This process was characterized and the influence of chemical potential between both phases was evaluated. Nano-sizing analyses were performed for a set of experiments, where the influence of the surfactant concentration in the organic phase, as well as the influence of NaCl concentration in the aqueous phase, was studied. Water droplets between 100 and 400 nm were measured in kerosene. Interfacial tension between both phases, was not lower than 4 mN/m. Therefore, ultra-low interfacial tension was not required for this process to occur spontaneously. Continuous production of spherical polymeric particles, via membrane emulsification, was demonstrated. An o/w emulsion was used as precursor and, by solvent removal, solid particles were obtained. In addition, production of polycaprolactone (PCL) particles containing encapsulated (entrapped) protein model was also demonstrated via a double emulsion method

Membrane emulsification: Formation of water in oil emulsions using a hydrophilic membrane

It is shown that formation of water based droplets in an immiscible (i.e. oil) continuous phase can be achieved using a hydrophilic porous metal membrane without prior hydrophobic treatment of the membrane surface. This avoids the need for "health and safety approval" of typical hydrophobic treatments for the membrane, which often use chemicals incompatible with pharma or food applications. To investigate this, wetting experiments were carried out: sessile droplets were used to determine static contact angles and a rotating drum system was used to determine contact angles under dynamic conditions. In the latter case the three-phase contact line was observed between the rotating drum, water and the continuous phase used in the emulsification process; a surfactant was present in the continuous phase which, in this process, has a double function: to assist the wetting of the membrane by the continuous phase, and not the disperse phase, and to stabilize the droplets formed at the surface of the porous membrane during membrane emulsification

Spontaneous emulsification of water in oil at appreciable interfacial tensions

Spontaneous formation of aqueous droplets in kerosene was observed, which was facilitated by the presence of an oil soluble surfactant: Span® 80 at concentrations above CMC. Kerosene/water interfacial tension under all conditions studied was not lower than 4 mN/m. Therefore, ultra-low interfacial tension was not required for this process to occur spontaneously. The process was caused by a transfer of water molecules to swollen reversed micelles. The influence of both the surfactant concentration in the organic phase and NaCl concentration in the aqueous phase on spontaneous aqueous droplet formation was investigated. Nano-sizing analyse of the drops was performed, which showed the droplets sizes in between 100 and 400 nm. It is proven that the presence of salt in the aqueous phase inhibits droplet formation. It is shown that big sessile aqueous droplets deposited on a hydrophobic substrate inside the kerosene phase were dissolved in kerosene through formation and growth of droplets, which form an aqueous film at the droplet base

Water in oil emulsions from hydrophobized metal membranes and characterization of dynamic interfacial tension in membrane emulsification

Hydrophobization of metal surfaces is reported based on silanization reactions. The aim was its application to metal porous membranes for the production of water in oil emulsions using a process known as membrane emulsification. A vertical oscillating membrane system was used to carry out drop formation experiments. It is shown that drop size can be tuned between 35 and 85. μm by changing just the surfactant concentration in the continuous phase. In addition, a method to determine the percentage of active pores during the membrane emulsification process is demonstrated. This method links knowledge acquired in the surfactant adsorption dynamics and drop expansion rate. Using this approach, pore velocity can be determined, which will help in determining the boundary between dripping and jetting from a pore. This study reinforces the importance of dynamic interfacial tension which must be considered in process design, and modelling purposes, particularly in two liquid phase systems using membranes such as membrane emulsification

Azimuthally oscillating membrane emulsification for controlled droplet production

A novel membrane emulsification system is reported consisting of a tubular metal membrane, periodically azimuthally (tangentially) oscillated with frequencies up to 50 Hz and 7 mm displacement in a gently cross flowing continuous phase. A CFD analysis showed consistent axial shear at the membrane surface, which became negligible at distances from the membrane surface greater than 0.5 mm. For comparison, CFD analysis of a fully rotating membrane emulsification system showed local vortices in the continuous phase leading to a variable shear along the axis of the membrane. Using an azimuthally oscillating membrane, oil-in-water emulsions were experimentally produced with a median diameter of 20-120 µm, and a coefficient of variation of droplet size of 8%. The drop size was correlated with shear stress at the membrane surface using a force balance. In a single pass of continuous phase it was possible to achieve high dispersed phase concentrations of 40% v/v