addition, the effect of surfactant type. stirring speed and time, on the forr 3stion of ... as a foam with a half-life of 6 min. and that they could be used to separate ...
Colloids arrd SrrrJrrces, 69 (1992) 65-72 Elscvier Scicucc Publishers B.V., Amsterdam
6.5
Predispersed solvent extraction of dilute products using colloidal gas aphrons and colloidal liquid aphrons: aphron prepara.tion, stability and size K. Matsushita’,
AH. Mollah, JXC. Stuckcy, C. de1 Cerro and A.I. Bailey
Department of Chemical Eng,heering Medicine, Eondorz SW7 2.&Z, UK
(Received 25 June 1992; accep;ed
and Chemical Techrtology, Imperial College
ofScience. Techology
and
13 August i992)
Abstract Early work on colloidal gas aphrons (CGAs) and colloidal liquid aphrons (CLAs) has shown that they have considcrablc potential in the field of Predispcrseli solvent extraction (PDSE). While their area of application is potentially very broad, their most promising use is in downstream sey.aration in biotechnology where products are very dilute and occur in complex mixtures. Since tittle wo :!r has been done in this area, this preliminary study examined the iniluence of a range of solvents, varying from non-polar to mildly polar, and a variety of ionic and non-ionic surfac’ants, on CLA size, stability and phase volume ratio (PVR. vobur~z ratio of the dispersed oil phase to the continuous aqueous phase). In addition, the effect of surfactant type. stirring speed and time, on the forr 3stion of CGAs was also studied. The results show that CLAs can bc formulated with quite polar solvents (e.g. pentanol), and their stability increases as the HLB (hydrophilic/lipophilic balancej number of the non-ionic surfactant increases. CLAs could be formulated with PVRs as hig!r as 20 without coalescence, which is markedly higher than with microemulsions. and seems to indicate that the liquid aphrons are stabilised by more than a surfactant monolayer. Finally, it was found that CGAs could be formulated as a foam with a half-life of 6 min. and that they could be used to separate dispersed CLAs effectively from a bulk solution.
Ke~ruor&ls: Colloidal gas aphrons;
colloidal
liquid aphrons;
predispersed
Introduction One of the major drawbacks of most microbialmediated processes arises from the dilute nature of the products obtained during fermentation. This often leads to heavy capita1 and operational costs for downstream processing In relation to the overall cost of the finished product. One conventional downstream separation process which is commonly used is solvent extraction. However, its use in biotechnology has a number of drawbacks, namely Correspondence f~.’ D.C. Stuckey, Dept. of Chemical Engineering and Chemical Technology, Imperial College of Science, Technology and Medicine, London, SW7 2AZ, UK. ‘Present address: Nippon Mining Co., Tokyo, Japan. 0166-6622/92/$05.00
0
1992 -
Elsevicr
Science
Publishers
solvent
extraction.
the capital cost of mixer settlers, the power requirement for solvent dispersion, large solvent inventorie :, and potential toxicity problems if the extracted broth is recycled to the reactor. One novel technique which can circumvent these problems is the use of colloidal liquid aphrons (CLAs) and colloidal gas aphrons (CG.4s) in predispersdd solvent extraction (PDSE). PDSE is the name given to a technique of iiquid-liquid extraction which employs colloidal aphrons as the basis of the process. In PDSE there is no need for tile settling stage, and the ratio of extracting solvent to pregnant solution can be very low, of the order of one to a thousand or even lower [l]. A CLA has been defined as a liquid (oil) core B.V. All rights reserved.
66
globule encapsulated by a soapy (aqueous) shell with colloidal dimensions (L-20 ;~rr?j dispersed in a continuous ~quevus phase [2]. When CLAs are added to water they are easily dispersed homogcneousiy and hence. with CL&, the oil (solvent) is the internal (discontinuous) phase and water is the continuous phase. The requirements for CLA preparation are: (a) the internal phase (oil) bar; to he cithcr iMMiSCible with water, or the solubi!ity has to be very small (the upper solubility limit at this time is not known): (bj the internal phase has to be divided into small globules: (c) the globules Must be cncapsulatcd in a soapy film (Fig. 1). The water-soluble surfactant used to stabi;ize thr: soapy film in CLAs is usual!y anionic, a!though non-ionic surfactants can be cationic and employed. A typical water-phase surfaciant solution is made of sodium dodecyl sulphatc (SDS) or sodium dodecylbenzene ;ulphonate (SDBS). In addition. an oil-soluble surfactant, usually nonionic, is necessary in the internal phase in order to maintain the spreading pressure of oil greater than the surface pressure produced by the surfactant dissolved in water [3]. The dcgrce of hydrophiOUler
surface of
Fig. I. Aphron
shall
structure: [ill.
!ici:y (or hyrl:xphobicity) of non-ionic surfactants is measured by their HLB number, an empirical number which denotes the balance between hydrophilic and lipophilic moietes. A low HLB number Means that the ratio of hydrophilic to hydrophobic groups of the surfactant nlo!ec~!c is small. Such a surfactant would tend to ‘se More soluble in oil than in water, and vice veysa for high HLE num‘;,& TU Make a large volume of CLAs it is important IO provide a large surface area for oil spreading. and this can be achieved by using a gas fo:m [or the initial generarion step. As soon as the first CLAs are formed they prcvide an additicnal interfacial area for producing More CLAs. CLAs can bc produced with a very h;gh phase volume ratio (PVR), which is the volume ratio of the dispersed oil phase to the continuous water phase (e.g., CLA Mixtures containing 50 parts of kerosene and 5 parts of water woulci have a PVR of IO). However, stable CLAs can be generated with PVRs 2,s high as 20 without phase inversion or coalescence, and this is one of the properties of CLAs which distinguish them from Microemulsions. CLA sizes can be varied, by using different surfactant types and concentrations, front submicron to 100 Microns in diameter [2]. A stabie CLA suspension can be stored in a stoppered bottle for years without visible deterioration. CGAs were first made by Sebba [4] under the name of Microfoams, but further experiments proved that the CGA name was more appropriate [3]. The smail size oT the gas bttbbies (25-iO0 ttm), which gives them colloidal propertics, enables them to be pumped from one vessel to another and produces a system of considerable potential in a remarkab!e diversity of applications [5-- 121. The CGAs can be made from solutions with a great variety of ionic surl’actants, and can contain up to 65% gas. Recent!y a method for the characterisation of CGA dispersions has been proposed [13]. and CGAs have been applied successfully in coflotation of metals [14] and solvent sublation processes [ 151. Despite their potential uses in a wide variety 01 applications, e.g. stripping dilute solutes from aque.
K. Matauhira
EI d./Colloids
Surjaccs
69 (1992)
65-72
ous phases (CLA), floating colloidal solids (CGA), improving gas mass transfer in fermenters (CGA), and delivering apolar substrates to aqueous fermentations (CLA), very little is known about the parameters controlling the size and stability of CLAs and CGAs. In this study the effect of various surfactant types and concentrations, solvent types, mixing rates and times were investigated in terms of CLA size and stability. Finally, the influence of surfactant types and concentrations, mixing rate and time, on CGA stability were examined.
67
the solvent disperses easily in the aqueous phase, but after adding about two-thirds of the total so!vent the mixture starts to become highly viscous, and it takes considerably longer to disperse the final volume of solvent. Finally, a white creamy dispersion of CLAs is obtained. The PVR was calculated from the volume ratio of solvent added to the aqueous volume. Different solvents (mildly polar to strongly non-polar) were used to prepare stable collotdal liquid aphrons in this work. In addition, 2 variety of non-ionic and ionic surfactants were evaluated for performance.
iWa:erEals and methods Reage;:ts
The solvents used in this work were decaiin (decahydronapthalene, 98% Aldrich), octane (GPR, BDH), toluene (AnalaR, BDH), P-xylene (GPR, BDH), decanol (decyl alcohol, 99%, Aldrich), 1-actanol (99%, Aldrich), hexan-l-01 (GPR, BDH), I-pentanol (99%. Aldrich), ethyl acetate (GPR, BDH), butyl acetate (99%, Aldrich), amyl acetate (GPR, BDH), hexyl acetate (99%, Aldrich) and cyclohexyl acetate (99%, Aldrich). Surfactants
The surfactants used in this work were CTMAB (cetyltrimethyl ammonium bromide, 95%, Aldrich), DTMAB (dodecyltrimethyl ammonium bromide, 99% Aldrich), SDS (sodium dodecyl sulphate, Aldrich), SDBS (sodium dodecyl benzene sulphate, I3DH), alcohol ethoxylates (Softanol, BP), nonylphenol ethoxylate (Synperonic, ICI), polyoxyethene triglyceride (Atlas 6 1300, ICI), polyoxyethene triglyceride alcohol (Tergital, Sigma). Col!oidal liquid ciphrm preparation
The oil phase (50-200 ml) contaiuing 2 nonionic surfactant was gradually dropped (2-10 ml min- ‘) into 10 ml of a foaming aqueous solution of an anionic surfactant under adequate mixing conditions using a magnetic stirrer. Initially
Colloidal gas aphron preparation
The CGAs were prepared using 2 high-speed stirrer (Silverson R, Model SRT-I), The surfactant solution was stirred at high speed (>4000 rev min- ‘) until a constant volume of creamy CGAs were generated. These CGAs can be kept dispersed under low stirring conditions (around 500 rev min- ‘) and can also be pumped by means of 2 peristaltic pump without breaking. In this work they were characterised by their stability over time, and percentage gas content. A half-life was employed as a measure of CGA stability, where the half-life was the time required for the decrease to half the initial volume of the CGAs with no stirring. The gas content was determined by subtracting the volume of surf2ctant solution from the tot21 volume of CGAs generated in a graduated beaker. Analytical methods
CLA size was determined by laser light scattering using a E4alvern particle size analyzer (Master Particle Sizer M.3.0, Malvern Instruments), and a standard deviation of 10.5% was associated with the particle size measurement. The results obtained are presented here in terms of Sauter mean diameter which is a measure of the ratio of the total volume of particles to the total surface area. The stability of the CLAs was determined by observing
the increase in the clear solvent of the CLA bottle.
layer on the surface
of CLA from wpcntanol
Surfaclant
The CLAs were made using surfactant concentrations of 0.4% (w/v) SDS in the aqueous phase, and 1% (W/V) Softanol 30 in the solvent phase. Very stable CLAs were produced with a variety of non-polar solvents resulting in a maximum PVR of 20 (Table 1). However, as the solvent became more polar, both the stability and the PVR decreased until stable CLAs could :.ot be formed. The influence of HLB number on CLA formation with slightly poIar solvents was studied. Table 2 indicates whether CLA preparation was possible using the polar solvent pcntanol in combination with direrent non-ionic surfactants. it was found that CLAs could be made from pentanol using high HLB number (>16.2) surfactants. CLA preparation from other polar solvents can also be seen in Table 3, where the surfactants SDBS (0.5%
in the oi! phnsc
POE-triglyceride Atlas Gl300
18.1
Yes
POE-nonylphcnol Synpcronic NP20 Synpcronic N P30 Synpcronic NP50
16.0 17.1 18.2
No Yes Yes
POE-POP-nonylphcnol Synpcronic NPE!!?!! Synpcronic NPE-A Synpcronic NPE-B Synpcronic NPE-C
! 4.!
b:‘J
16.3 17.5 IS.7
Yes Yes
7.9- 14.5 16.2
NO
POE-alcohol Softanol-30-SoftanolSynpcronic A20
I20
CLA stability
( wt”; ) wDccane wOctanc iso-Octane rl-Hcxanc
57 ppb 6.6. IO-’ -
YCS
No
CLAs mxde with a PVR of 5 with SDBS (0.5% (w/v)) in water and null-ionic sarlktant (0.5% (w/v)) in solvent.
‘IAELE
3
Preparation
of CL.4 from various Aqueous solubility’
polar solvents CLA formation
PVR obtained
-
( wt?)
of CLA from difkrcrcnt solvcn;s Solubility in wrtcr
surfactants
CLA formation
Solvent
using dilkrcnt
HLB No.
solvent
I
Preparation
2
Preparation
Results and discussion
TABLE
TABLE
CL/\ six (diamctcr)
PVR obtained
Mm)
Butan- l-01 Butan-2-01 rl-Pcntanol rr-Hcxanol wHcptanol rl-Octanol rl-Dccanol
7.45 2.5 2.19 0.706 0.053 Insoluble -
No No Ye!;
YCS Yes
5 5 5 IO I2
Ethyl Butyl AmyI I-lcsyl
8.08 0.68 0.17 0.02
No
-
YCr, Yes
14.0 10.3 9.6 9.6
20
0.00 I13
Very sl;~bls Very stoblc Very s!ablc Very stable
Decalin Kerosene
