Colloidal gas aphrons for biotechnology applications: a mini review

Colloidal gas aphrons (CGAs) are highly stable, spherical, micrometer-sized bubbles encapsulated by surfactant multilayers. They have several intriguing properties, including: high stability, large interfacial area, and the ability to maintain the same charge as their parent molecules. The physical properties of CGAs make them ideal for biotechnological applications such as the recovery of a variety of: biomolecules, particularly proteins, yeast, enzymes, and microalgae. In this review, the bio-application of CGAs for the recovery of natural components is presented, as well as: experimental results, technical challenges, and critical research directions for the future. Experimental results from the literature showed that the recovery of biomolecules was mainly determined by electrostatic or hydrophobic interactions between polyphenols and proteins (lysozyme, β-casein, β-lactoglobulin, etc.), yeast, biological molecules (gallic acid and norbixin), and microalgae with CGAs. Knowledge transfer is essential for commercializing CGA-based bio-product recovery, which will be recognized as a viable technology in the future.

Aphron applications--a review of recent and current research

Colloidal aphrons are multi-layered stable bubbles (CGAs) or droplets (CLAs), surrounded by a thin surfactant film. The small size of the aphrons creates a system with a high interfacial area which can be pumped like water without collapsing. The high stability of colloidal aphrons due to a thin soapy shell surrounding the core, and high interfacial area make them of interest in many processes such as mineral processing, protein recovery, drilling fluids, separation of organic dyes from waste water, predispersed solvent extraction of dilute streams, clarification and purification of suspensions, soil remediation, material synthesis and immobilization of enzymes. This article aims to provide a comprehensive database in generation, characterization and applications of colloidal gas and liquid aphrons from more than 140 published works so far. The article also reports scale up, industrial applications, technical limitation regarding aphrons application and important future research scopes.

Measurements of interactions between particles and charged microbubbles using a combined micro- and macroscopic strategy

We report the use of atomic force microscopy (AFM) to study the interactions between silica glass colloidal probes and charged microbubbles created using one of two different surfactants: anionic sodium dodecyl sulfate (SDS) and cationic dodecyl trimethylammonium bromide (DTAB) in an aqueous environment. On close approach between the glass probe and a SDS microbubble, an appreciable repulsive force was observed prior to contact. This was not observed when using a DTAB microbubble, where only attractive forces were observed prior to contact. zeta-potential analysis showed that silica surfaces are negatively charged across the pH range of 3-10 when surfactant is not present. Addition of SDS did not alter the zeta-potential significantly, indicating that adsorption onto the particle surface did not occur. Conversely, the addition of DTAB decreased the negativity of the zeta-potential, reversing the sign, indicating that adsorption had occurred. This analysis was used in the removal of fine particles from suspension using charged microbubbles. Silica particles were recovered using positively charged microbubbles from DTAB but not when using negatively charged microbubbles generated from SDS. Taken together, the data suggest that repulsive long-range interactions were responsible for the selective attachment of silica particles to microbubbles in a charge-dependent manner.

Surfactants in microbiology and biotechnology: Part 2. Application aspects

Surfactants are amphiphilic compounds which can reduce surface and interfacial tensions by accumulating at the interface of immiscible fluids and increase the solubility, mobility, bioavailability and subsequent biodegradation of hydrophobic or insoluble organic compounds. Chemically synthesized surfactants are commonly used in the petroleum, food and pharmaceutical industries as emulsifiers and wetting agents. Biosurfactants produced by some microorganisms are becoming important biotechnology products for industrial and medical applications due to their specific modes of action, low toxicity, relative ease of preparation and widespread applicability. They can be used as emulsifiers, de-emulsifiers, wetting and foaming agents, functional food ingredients and as detergents in petroleum, petrochemicals, environmental management, agrochemicals, foods and beverages, cosmetics and pharmaceuticals, and in the mining and metallurgical industries. Addition of a surfactant of chemical or biological origin accelerates or sometimes inhibits the bioremediation of pollutants. Surfactants also play an important role in enhanced oil recovery by increasing the apparent solubility of petroleum components and effectively reducing the interfacial tensions of oil and water in situ. However, the effects of surfactants on bioremediation cannot be predicted in the absence of empirical evidence because surfactants sometimes stimulate bioremediation and sometimes inhibit it. For medical applications, biosurfactants are useful as antimicrobial agents and immunomodulatory molecules. Beneficial applications of chemical surfactants and biosurfactants in various industries are discussed in this review.

Research and application of microbial enzymes--India's contribution

Enzymes have attracted the attention of scientists world over due to their wide range of physiological, analytical and industrial applications. Although enzymes have been isolated, purified and studied from microbial, animal and plant sources, microorganisms represent the most common source of enzymes due to their broad biochemical diversity, feasibility of mass culture and ease of genetic manipulation. With the advent of molecular biology techniques, a number of genes of industrially important enzymes has been cloned and expressed in order to improve the production of enzymes, substrate utilization and other commercially useful properties. Special attention has been focused on enzymes isolated from thermophiles due to their inherent stability and industrial applications. In addition, a variety of methods have been employed to modify enzymes for their industrial usage including strain improvement, chemical modifications, modification of reaction environment, immobilization and protein engineering. A wide range of applications of enzymes in different bioprocess industries is discussed.

Colloidal gas aphrons: potential applications in biotechnology

Colloidal gas aphrons are microbubbles encapsulated by surfactant multilayers. They provide a large interfacial area to adsorb charged and/or hydrophobic molecules; the extent and mechanism of the adsorption depends on the surfactant multilayer. The physical properties of colloidal gas aphrons have recently been characterized for a range of surfactants in order to find the best systems for particular applications. A range of exciting biotechnology applications has been identified, including the recovery of cells, proteins and other biological molecules, and the enhancement of gas transfer in bioreactors and bioremediation.

Colloidal gas aphrons: A novel approach to protein recovery

Sebba (1987) defined colloidal gas aphrons (CGA) as microbubbles stabilized by surfactant layers, which are created by stirring surfactant solutions at speeds greater than a critical value. A high shear impeller is used for stirring and critical values for the impeller speed must be exceeded to create these stable gas liquid dispersions (typically >5000 rpm). Although there have been no previous reports of direct protein recovery using CGA, it is likely that, with appropriate choice of surfactant, proteins should adsorb to these surfactant bubbles by means of electrostatic and/or hydrophobic interactions. This is the basis of this study, in which the use of CGA for protein recovery from aqueous solution is considered. A surfactant which has been characterized previously for generation of CGA was chosen (Jauregi et al., 1997), i.e., the anionic surfactant sodium bis-(2-ethyl hexyl) sulfosuccinate (AOT). Lysozyme, a well-characterized protein, was chosen as the protein to be recovered. Lysozyme was recovered successfully from aqueous solution using CGA generated from AOT. At optimum conditions, lysozyme recovery, enrichment ratio, and separation ratio were 95%, 19 and 302 respectively, with enzyme activity maintained. These results indicate the exciting potential of this technique. A wide range of process conditions including initial concentration of protein and surfactant, surfactant/protein molar ratio, pH, and ionic strength were considered. High recoveries and enrichments were generally obtained at protein concentrations </=0.41 mg/mL, and surfactant concentrations >0.11 mg/mL. However, at high ionic strength (0.29M) poor separation and recoveries were obtained at low protein concentrations (counter-ions diminishing electrostatic interactions between protein and aphrons at this condition). In general, (ns/np)a was determined to be between 10 and 16 for experiments in which high levels of recovery/separation parameters were found. For most conditions, protein precipitation was observed; however, this precipitate could be resolubilized without loss of enzyme activity.