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COLLOIDS AND Colloids and Surfaces A: Physicochemical and Engineering Aspects 131 ( 1998 ) 119 136

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Structure and stability of colloidal liquid aphrons G.J. Lye l, D.C. Stuckey * Department of Chemical Engineering & Chemical Technology, Imperial College g/Science, Technology & Medici, c. London SW7 2BY, UK Received 5 July 1996

Abstract

The structure and stability of colloidal liquid aphrons (CLAs) have been investigated using a variety of experimental techniques, i.e. cryo-TEM, DSC, and light scattering. The findings support the structural model proposed by Sebba who suggested that polyaphron phases resemble a biliquid foam while the individual CLAs, when dispersed in a continuous aqueous phase, consist of spherical, micron-sized, oil droplets surrounded by a thin, aqueous "soapyshell". First-order half-lives of CLAs dispersed in a stirred vessel over a range of continuous phase ionic strengths, pH, and temperatures were also determined for the first time. These allowed quantitative comparison of CLA stability when dispersed under various conditions and of the influence of including various concentrations of lipase or erythromycin-A in the aphron formulation. Based on these results, a mechanism for the breakdown of dispersed CLA structure is proposed which involves destabilisation and loss of the "soapy-shell" followed by coalescence of the oil cores of the aphrons. However, direct evidence for the structure of the surfactant-laden interfaces responsible for the stabilisation of aphrons is still required if the structural model proposed by Sebba is to be fully confirmed. The similarities and differences between CLAs and high-internal-phase-ratio emulsions (HIPREst are also discussed. ~ 1998 Elsevier Science B.V.

Keywords: Aphron; Colloidal liquid aphron; Differential scanning calorimetry; High-internal-phase-ratio emulsion; Stability; Transmission electron microscopy

1. Introduction

Colloidal liquid aphrons (CLAs) are postulated to consist of a micron-sized solvent droplet encapsulated in a thin aqueous film ("soapy-shell"). This structure, as shown in Fig. 1, is stabilised by a mixture of non-ionic and ionic surfactants [1]. Since Sebba's original reports on biliquid foams [2] and subsequently "minute oil droplets encapsu* Corresponding author. ~Present address: The Advanced Centre for Biochemical Engineering, Department of Chemical and Biochemical Engineering, University College London, London WC1E 7JE, UK. 0927-7757 98/$19.00 octanol >

G..L Lye. D. C Stuckey / Colloids Surfaces A." Physicochum. Erie. Asl,ects 131 f1998) 119 136

butyl acetate) and non-ionic surfactants having high HLB numbers (Atlas G1300>Softanol 120 > Softanol 30). In this investigation we used the techniques of cryo-ultramicrotome T E M and DSC to analyse liquid polyaphrons in order to test Sebba's proposed structure for these phases. Results from these methods were also compared with those obtained by light scattering of dispersed CLAs in order to see if any correlation existed. In addition. we extended our initial investigation on dispersed CLA stability to study the influence of continuous phase properties on CLA half-lives, and see to what extent the data could be used to further elucidate the structure of CLAs. Some authors, noticeably Princen [8], refute Sebba's proposed structures for polyaphron phases and CLAs claiming that they are no different from those of highinternal-phase-ratio emulsions, Throughout this work, therefore, our results on the aphron systems under investigation are compared and contrasted with those published for HIPREs. Finally, these are discussed and suggestions are made which may clari~ the current confusion.

2. Materials and methods

The solvents used in this work, decan-l-ol and decane, were purchased from Aldrich and were >99% pure. The anionic surfactant used was sodium dodecyl sulphate (99%, Sigma) whilst the non-ionic surfactants, Softanol 30 and Softanol 120, were of the alcohol ethoxylate type (Honeywill & Stein) and were used as supplied. The non-ionics had EO mole numbers of 3 and 12, respectively. The chloride salts of various cations and glycerol were AnalR grade from Merck. Lipase from CandMa cvlindracea (Type VII) and erythromycin-A (98% pure) were from Sigma. Deionised water from a Purite RO50HP unit had a conductivity of < 0. I laS c m - 1 and was filtered through a 0.2 lam filter. Polyaphron phases were prepared by dropping the organic phase ( 1% (w/v) Softanol in the desired solvent) from a burette into a well-stirred ( ~ 800 rev min- ~), foaming aqueous solution containing 0.5% (w/v) SDS. The initial volume of the

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aqueous phase was typically 1.5 ml and the organic phase was added at an average flow rate of ~ 0 . 5 m l m i n -x until the desired phase volume ratio was reached (PVR= l';rg/l'~q). The organic phase initially disperses easily and can be added at a higher rate, but as the maximum PI"R (PgRma x) is approached [15], the mixture becomes extremely viscous and the rate of addition must be reduced. The polyaphron phases formed in this way had a creamy-white appearance and showed no signs of phase separation over a period of months. When used during aphron formulation, erythromycin was dissolved in the organic phase whilst lipase was dissolved in the aqueous surfactant solution at the desired concentrations. The influence of various solvents, surfactants and formulation procedures on P l/'Rmax and the stability of the polyaphrons have been published previously [15].

2.1. Crvo-ultramicrotomv and eh, ctron microsc¢q~y of polyuphrons Small droplets of the polyaphron phase ( ~ 2 ktl ) were rapidly frozen by plunging them into liquid propane at - 180C (Reichert Jung KF80 immersion cryofixation system). These were subsequently kept in liquid nitrogen before sections, approximately 0.2 jam thick, were cut at --156'C and collected on l\~rmvar-coated copper grids ( Reichert Jung FC40 ultracut system). These were then examined directly in a Philips 400 TEM with cryostage at - 1 6 5 ' C : surface ice. caused by grid transfer, was sublimed at - 1 0 0 C. Initially, there was little contrast between the aqueous and organic areas of the polyaphron. However. it was found that by sublimation at - 8 0 C the water present in the plateau borders of the polyaphron could be removed with no noticeable effect on the organic areas to allow easy differentiation of the lwo domains,

2.2. D((/brential scanning calorhm, trv ~/ polyal)hrons Freezing exotherms of polyaphron phases were obtained using a Perkin Elmer DSC-2 differential scanning calorimeter al a cooling rate of

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G.J, Lye, D.C. Stuckev / Colloids Surfaces A: Physicochon. Eng Aspects 131 (1998) 119-136

5 K min -1, and a scanning range of 220-305 K. Measurements were made under a dry nitrogen atmosphere to prevent condensation of water vapour. Between 2 and 5 mg of the polyaphron phase was sealed in an aluminium pan, an empty sealed pan being used as the reference material. The error in the determination of the onset of the transition temperature was + 0.6 K, whilst that for the heat of phase transition (based upon the peak area) was _+7%.

When polyaphrons were dispersed under conditions which made them unstable, a clear solvent layer could be seen to develop on the surface of the bath. In all cases it was ensured that the speed of the impeller was sufficiently low to prevent re-entrainment of this solvent (or air) back into the dispersion, and hence the sample cell, as this could interfere with the size determination of the CLAs remaining in suspension.

2.3. Light scattering of dispersed CLAs 3. Results and discussion

Measurements were made using a Malvern 2600 (Model 3.0) particle size analyser. For a single determination of particle size, the instrument was fitted with a 15 ml sample chamber containing deionised water. This cell could be mixed by a small magnetic bar to ensure that the polyaphron phase was properly dispersed into CLAs and a measurement was taken within approximately 30 s of dispersion. Results for the CLA diameter, dov, are given here as the Sauter mean diameter which is a measure of the ratio of the total volume of particles to the total surface area. The error in the determination of dov was _+5%. Various routines were available in the software for analysing the scattering pattern obtained from the CLAs. The one which gave the best correlation between experimental and theoretical scattering patterns was that which assumed the sample consisted of spherical particles having a mono-modal size distribution. For determining the stability of the dispersed CLAs, the instrument was fitted with a flowthrough sample cell, and was programmed to take readings over a period of 40 rain. The sample cell was connected to an external stirred vessel in which the temperature (+0. I°C) and pH (by addition of 0.1 M NaOH or HC1) could be controlled, the dispersion being continually passed between the two at a flow rate of ~ 5 0 0 m l m i n - ' using a centrifugal pump. A schematic diagram of this equipment is shown in Fig. 2. In a typical experiment, 0.1 ml of a polyaphron phase was dispersed in 400 ml of the desired continuous phase, creaming of the dispersed CLAs to the surface being prevented by a Rushton-type impeller located near the base of the bath and operated at low speed.

3.1. Cryo-ultramicrotomy and electron microscopy of polyaphrons A photomicrograph of a polyaphron phase prepared at PVR 4 is shown in Fig. 3. This is analogous to a cross-section through a normal gas-liquid foam. In the case of polyaphrons, however, their creamy-white appearance and the small size of the oil cores makes direct visual observation of this structure impossible. The dark areas of the photomicrograph represent the decanol cores of the polyaphron, whilst the white areas are those from which the water in the plateau borders was sublimed; white areas within the decanol cores are due to expansion of the underlying grid during sublimation causing the samples to "tear". Measurements taken from photomicrographs for samples of various PVR, and also those made by light scattering for polyaphrons dispersed in deionised water, are presented in Table 1. The agreement between the aphron diameters, dov, obtained from the two techniques is good, and the increase in dov with decreasing P VR is consistent with previous work [15]. These sizes are similar to those reported for the dispersed-phase oil droplets of HIPREs [19]. The advantage of the cryo-TEM technique is that it has allowed the thickness of the "soapy-shell" to be measured directly for the first time. For polyaphrons at P VR 10, the measured "soapy-shell" thickness of 0.03 gm is very similar to the minimum thickness predicted by Sebba of 10 n m [ 1]. The shape of the dispersed aphrons (CLAs) as determined by analysis of the scattering pattern

G.J. Lye, D.C. Stuckey / Colloids Surfaces A: Physicochem. Eng. Aspects 131 (1998) l 19 136

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