Linear and nonlinear microrheology of dense colloidal suspensions

The length and time scales accessible to optical tweezers make them an ideal tool for the examination of colloidal systems. Embedded high-refractive-index tracer particles in an index-matched hard sphere suspension provide 'handles' within the system to investigate the mechanical behaviour. Passive observations of the motion of a single probe particle give information about the linear response behaviour of the system, which can be linked to the macroscopic frequency-dependent viscous and elastic moduli of the suspension. Separate 'dragging' experiments allow observation of a sample's nonlinear response to an applied stress on a particle-by particle basis. Optical force measurements have given new data about the dynamics of phase transitions and particle interactions; an example in this study is the transition from liquid-like to solid-like behaviour, and the emergence of a yield stress and other effects attributable to nearest-neighbour caging effects. The forces needed to break such cages and the frequency of these cage breaking events are investigated in detail for systems close to the glass transition.

The use of colloidal gas aphrons as novel downstream processing for the recovery of astaxanthin from cells of Phaffia rhodozyma

BACKGROUND: There is an increasing interest in obtaining natural products with bioactive properties, using fermentation technology. However, the downstream processing consisting of multiple steps can be complicated, leading to increase in the final cost of the product. Therefore there is a need for integrated, cost-effective and scalable separation processes. RESULTS: The present study investigates the use of colloidal gas aphrons (CGA), which are surfactant-stabilized microbubbles, as a novel method for downstream processing. More particularly, their application for the recovery of astaxanthin from the cells of Phaffia rhodozyma is explored. Research carried out with standard solutions of astaxanthin and CGA generated from the cationic surfactant hexadecyl. trimethyl ammonium bromide (CTAB) showed that up to 90% recovery can be achieved under optimum conditions, i.e., pH 11 with NaOH 0.2 mol L-1. In the case of the cells' suspension from the fermentation broth, three different approaches were investigated: (a) the conventional integrated approach where CGA were applied directly; (b) CGA were applied to the clarified suspension of cells; and finally (c) the in situ approach, where CGA are generated within the clarified suspension of cells. Interestingly, in the case of the whole suspension (approach a) highest recoveries (78%) were achieved under the same conditions found to be optimal for the standard solutions. In addition, up to 97% recovery of total carotenoids could be achieved from the clarified suspension after pretreatment with NaOH. This pretreatment led to maximum cell disruption as well as optimum conditioning for subsequent CGA separation. CONCLUSIONS: These results demonstrate the potential of CGA for the recovery of bioactive components from complex feedstock. (c) 2008 Society of Chemical Industry.

A rotating disc voltammetry study of the 1,8-dihydroxyanthraquinone mediated reduction of colloidal indigo

Colloidal indigo is reduced to an aqueous solution of leuco-indigo in a mediated two-electron process converting the water-insoluble dye into the water-soluble leuco form. The colloidal dye does not interact directly with the electrode surface, and to employ an electrochemical process for this reduction, the redox mediator 1,8-dihydroxyanthraquinone (1,8-DHAQ) is used to transfer electrons from the electrode to the dye. The mediated reduction process is investigated at a (500-kHz ultrasound-assisted) rotating disc electrode, and the quantitative analysis of voltammetric data is attempted employing the Digisim numerical simulation software package. At the most effective temperature, 353 K, the diffusion coefficient for 1,8-DHAQ is (0.84 +/- 0.08)x10(-9) m(2) s(-1), and it is shown that an apparently kinetically controlled reaction between the reduced form of the mediator and the colloidal indigo occurs within the diffusion layer at the electrode surface. The apparent bimolecular rate constant k (app)=3 mol m(-3) s(-1) for the rate law d[leuco-indigo]/dt = k(app) x [mediator] x [indigo] is determined and attributed to a mediator diffusion controlled dissolution of the colloid particles. The average particle size and the number of molecules per particles are estimated from the apparent bimolecular rate constant and confirmed by scanning electron microscopy.

Colloidal stable silica encapsulated nano-magnetic composite as a novel bio-catalyst carrier

A colloidal stable silica-encapsulated magnetic nano-composite of a controlled dimension is, for the first time, employed to carry beta-lactamase via chemical linkage on the silica overlayer: activity study reflects that this new type of immobilisation allows site (enzyme) isolation, accessibility as good as free enzyme and recovery & reusability upon application of magnetic separation.

Recovery of gallic acid with colloidal gas aphrons (CGA)

In the present paper the potential application of colloidal gas aphrons (CGA) to the recovery of antioxidants from wine-making waste extracts is investigated. CGA were generated by stirring a buffered solution (400 ml) of a cationic surfactant(cetyltrimethylammonium bromide, CTAB) at 8000 rpm for 10 minutes. Trials were carried out on standard solutions (2 ml) of gallic acid (GA) 200 mg/l with varying volumes of colloidal gas aphrons (20-60 ml) generated with varying concentrations of CTAB (2 and 4 mM). Influence of pH, solvent (buffered aqueous solution and ethanol), CTAB to GA molar ratio on recovery were studied. Best recovery (63%) was achieved from an aqueous solution of GA and at a CTAB to GA molar ratio of 16. Separation is mainly driven by electrostatic interactions but pH conditions are to be optimised to preserve the GA antioxidant power.

Recovery of lactoferrin and lactoperoxidase from sweet whey using colloidal gas aphrons (CGAs) generated from an anionic surfactant, AOT

The recovery of lactoferrin and lactoperoxidase from sweet whey was studied using colloidal gas aphrons (CGAs), which are surfactant-stabilized microbubbles (10-100 mum). CGAs are generated by intense stirring (8000 rpm for 10 min) of the anionic surfactant AOT (sodium bis-2-ethylhexyl sulfosuccinate). A volume of CGAs (10-30 mL) is mixed with a given volume of whey (1 - 10 mL), and the mixture is allowed to separate into two phases: the aphron (top) phase and the liquid (bottom) phase. Each of the phases is analyzed by SDS-PAGE and surfactant colorimetric assay. A statistical experimental design has been developed to assess the effect of different process parameters including pH, ionic strength, the concentration of surfactant in the CGAs generating solution, the volume of CGAs and the volume of whey on separation efficiency. As expected pH, ionic strength and the volume of whey (i.e. the amount of total protein in the starting material) are the main factors influencing the partitioning of the Lf(.)Lp fraction into the aphron phase. Moreover, it has been demonstrated that best separation performance was achieved at pH = 4 and ionic strength = 0.1 mol/L i.e., with conditions favoring electrostatic interactions between target proteins and CGAs (recovery was 90% and the concentration of lactoferrin and lactoperoxidase in the aphron phase was 25 times higher than that in the liquid phase), whereas conditions favoring hydrophobic interactions (pH close to pI and high ionic strength) led to lower performance. However, under these conditions, as confirmed by zeta potential measurements, the adsorption of both target proteins and contaminant proteins is favored. Thus, low selectivity is achieved at all of the studied conditions. These results confirm the initial hypothesis that CGAs act as ion exchangers and that the selectivity of the process can be manipulated by changing main operating parameters such as type of surfactant, pH and ionic strength.

An insight into the mechanism of protein separation by colloidal gas aphrons (CGA) generated from ionic surfactants

Colloidal gas aphrons (CGA), which are surfactant stabilised microbubbles, have been previously applied for the recovery of proteins from model mixtures and a few studies have demonstrated the potential of these dispersions for the selective recovery of proteins from complex mixtures. However there is a lack of understanding of the mechanism of separation and forces governing the selectivity of the separation. In this paper a mechanistic study is carried out to determine the main factors and forces influencing the selectivity of separation of whey proteins with CGA generated from ionic surfactants. Two different separation strategies were followed: (i) separation of lactoferrin and lactoperoxidase by anionic CGA generated from a solution of sodium bis-(2-ethyl hexyl) sulfosuccinate (AOT); (ii) separation of beta-lactoglobulin by cationic CGA generated from a solution of cetyltrimethylammonium bromide (CTAB). Separation results indicate that electrostatic interactions are the main forces determining the selectivity however these could not completely explain the selectivities obtained following both strategies. Protein-surfactant interactions were studied by measuring the zeta potential changes on individual proteins upon addition of surfactant and at varying pH. Interestingly strongest electrostatic interactions were measured at those pH and surfactant to protein mass ratios which were optimum for protein separation. Effect of surfactant on protein conformation was determined by measuring the change in fluorescence intensity upon addition of surfactant at varying pH. Differences in the fluorescence patterns were detected among proteins which were correlated to differences in their conformational features which could in turn explain their different separation behaviour. The effect of conformation on selectivity was further proven by experiments in which conformational changes were induced by pre-treatment of whey (heating) and by storage at 4 degrees C. Overall it can be concluded that separation of proteins by ionic CGA is driven mainly by electrostatic interactions however conformational features will finally determine the selectivity of the separation with competitive adsorption having also an effect. (c) 2006 Elsevier B.V. All rights reserved.

Recovery of astaxanthin using colloidal gas aphrons (CGA): a mechanistic study

The aim of this study is to investigate the mechanism responsible for the recovery of astaxanthin using Colloidal Gas Aphrons (CGA), which are surfactant stabilised microbubbles. The latter were produced using different surfactant solutions (Cetyl Trimethyl Ammonium Bromide (CTAB)-cationic, Sodium Dodecyl Sulfate (SDS)-anionic, TWEEN 60-non-ionic and mixtures of TWEEN 60-SPAN 80- non-ionic with varying hydrophobicity) at stirring speed 8000 rpm and stirring time 5 min. Experiments were carried out at varying pH and volumetric ratios of astaxanthin to CGA, and with two different astaxanthin standard suspensions: (i) astaxanthin dispersed in aqueous solutions and (ii) astaxanthin dispersed in ethanolic/aqueous solutions with different compositions of ethanol (20/80 (v/v) and 40/60 (v/v)). When astaxanthin is dispersed in aqueous solutions the separation seems to occur mainly by electrostatic interactions. Therefore the recoveries are higher in the case of the cationic surfactant when astaxanthin particles are strongly negatively charged, as shown by the zeta potential measurements. When ethanol is present, highest recoveries are achieved with CGA produced from the non-ionic surfactant, which indicates that, under these conditions, separation is driven mainly by hydrophobic interactions. In experiments with ethanolic/aqueous suspensions, when the hydrophobicity of the surfactant was increased by increasing volumes of SPAN 80, the CGA produced were less stable; thus higher recoveries of astaxanthin under conditions that favour hydrophobic interactions were not observed. (C) 2008 Elsevier B.V All rights reserved.

Recovery of norbixin from a raw extraction solution of annatto pigments using colloidal gas aphrons (CGAs)

Annatto dyes are widely used in food and are finding increasing interest also for their application in the pharmaceutical and cosmetics industry. Bixin is the main pigment extracted from annatto seeds and accounts for 80% of the carotenoids in the outer coat of the seeds; norbixin being the water-soluble form of the bixin. Typically annatto dyes are extracted from the seeds by mechanical means or solutions of alkali, edible oil or organic solvents, or a combination of the two depending on the desired final product. In this work CGAs are investigated as an alternative separation method for the recovery of norbixin from a raw extraction solution of annatto pigments in KOH. A volume of CGAs generated from a cationic surfactant (CTAB) solution is mixed with a volume of annatto solution and when the mixture is allowed to settle it separates into the top aphron phase and the bottom liquid phase. Potassium norbixinate presented in the annatto solution will interact with the surfactant in the aphron phase, which results in the effective separation of norbixin. Recovery= 94% was achieved at a CTAB to norbixin molar ratio of 3.3. In addition a mechanism of separation is proposed here based on the separation results with the cationic surfactant and an anionic surfactant (bis-2-ethyl hexyl sulfosuccinate, AOT) and measurements of surfactant to norbixin ratio in the aphron phase; electrostatic interactions between the surfactant and norbixin molecules result in the fort-nation of a coloured complex and effective separation of norbixin. (c) 2005 Elsevier B.V. All rights reserved.

Magnetic compliant suspension concept for space robotics

This paper proposes a Dual-Magnet Magnetic Compliance Unit (DMCU) for use in medium sized space rover platforms to enhance terrain handling capabilities and speed of traversal. An explanation of magnetic compliance and how it can be applied to space robotics is shown, along with an initial mathematical model for this system. A design for the DMCU is proposed along with a 4-wheeled DMCU Testing Rig.

Separation of astaxanthin from cells of Phaffia rhodozyma using colloidal gas aphrons (CGA) in a flotation column

The aim of this study is to investigate the separation of astaxanthin from the cells of Phaffia rhodozyma using colloidal gas aphrons (CGA), which are surfactant stabilized microbubbles, in a flotation column. It was reported in previous studies that optimum recoveries are achieved at conditions that favor electrostatic interactions. Therefore, in this study, CGA generated from the cationic surfactant hexadecyl trimethyl ammonium bromide (CTAB) were applied to suspensions of cells pretreated with NaOH. The different operation modes (batch or continuous) and the effect of volumetric ratio of CGA to feed, initial concentration of feed, operating height, and flow rate of CGA on the separation of astaxanthin were investigated. The volumetric ratio was found to have a significant effect on the separation of astaxanthin for both batch and continuous experiments. Additionally, the effect of homogenization of the cells on the purity of the recovered fractions was investigated, showing that the homogenization resulted in increased purity. Moreover, different concentrations of surfactant were used for the generation of CGA for the recovery of astaxanthin on batch mode; it was found that recoveries up to 98% could be achieved using CGA generated from a CTAB solution 0.8 mM, which is below the CTAB critical micellar concentration (CMC). These results offer important information for the scale-up of the separation of astaxanthin from the cells of P. rhodozyma using CGA.

Recovery of gallic acid with colloidal gas aphrons generated from a cationic surfactant

In a previous study we have demonstrated that gallic acid (GA) in its anionic form can be recovered from aqueous solutions using colloidal gas aphrons (CGA) generated from the cationic surfactant cetyltrimethylammonium bromide (CTAB). The aim of the present work is to get a better understanding of the separation mechanism in order to determine the optimum operating conditions to maximise the recovery of GA while preserving its antioxidant properties. Zeta potential measurements were carried out to characterise the surface charge of GA, CTAB and their mixtures at three different pH conditions (both in buffers and in aqueous solutions). GA interacted strongly with CTAB at pH higher than its pKa 3.14 where it is ionised and negatively charged. However, at pH higher than 7 GA becomes oxidised and loses its antioxidant power. GA recovery was mainly affected by pH, ionic strength, surfactant/GA molar ratio, mixing conditions and contact time. Scale-up of the separation using a flotation column resulted in both higher recovery and reproducibility. Preliminary experiments with grape marc extracts confirmed the potential application of this separation for the recovery of polyphenols from complex feedstocks

Colloidal gas aphrons (CGA): dispersion and structural features

Colloidal gas aphrons (CGA) have previously been defined as surfactant stabilized gas microbubbles and characterized for a number of surfactants in terms of stability, gas holdup and bubble size even though there is no conclusive evidence of their structure (that is, orientation of surfactant molecules at the gas–liquid interface, thickness of gas–liquid interface, and/or number of surfactant layers). Knowledge of the structure would enable us to use these dispersions more efficiently for their diverse applications (such as for removal of dyes, recovery of proteins, and enhancement of mass transfer in bioreactors). This study investigates dispersion and structural features of CGA utilizing a range of novel predictive (for prediction of aphron size and drainage rate) and experimental (electron microscopy and X-ray diffraction) methods. Results indicate structural differences between foams and CGA, which may have been caused by a multilayer structure of the latter as suggested by the electron and X-ray diffraction analysis.

Parabens enable suspension growth of MCF-10A immortalized, non-transformed human breast epithelial cells

Parabens (alkyl esters of p-hydroxybenzoic acid) are used extensively as preservatives in consumer products, and intact esters have been measured in several human tissues. Concerns of a potential link between parabens and breast cancer have been raised, but mechanistic studies have centred on their oestrogenic activity and little attention has been paid to any carcinogenic properties. In the present study, we report that parabens can induce anchorage-independent growth of MCF-10A immortalized but non-transformed human breast epithelial cells, a property closely related to transformation and a predictor of tumour growth in vivo. In semi-solid methocel suspension culture, MCF-10A cells produced very few colonies and only of a small size but the addition of 5 × 10-4 M methylparaben, 10–5 M n-propylparaben or 10–5 M n-butylparaben resulted in a greater number of colonies per dish (P < 0.05 in each case) and an increased average colony size (P < 0.001 in each case). Dose-responses showed that concentrations as low as 10–6 M methylparaben, 10–7 M n-propylparaben and 10–7 M n-butylparaben could increase colony numbers (P = 0.016, P = 0.010, P = 0.008, respectively): comparison with a recent measurement of paraben concentrations in human breast tissue samples from 40 mastectomies (Barr et al., 2012) showed that 22/40 of the patients had at least one of the parabens at the site of the primary tumour at or above these concentrations. To our knowledge, this is the first study to report that parabens can induce a transformed phenotype in human breast epithelial cells in vitro, and further investigation is now justified into a potential link between parabens and breast carcinogenesis.