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Predispersed solvent extraction of dilute products using colloidal gas aphrons and colloidal liquid aphrons: Aphron preparation, stability and size
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
Very stable Very stable
13.1 t I.1
‘0 20
p-Xylcnc Toluenc Bcnzenc
0.0156 0.05 I5 0.1791
Stable St*\blc Stable
29.8 30.8 7.2
20 20 20
Chloroform l.2-Dichloroethanc
0.82 0.8 I
YCS
Yes
IO IO
Dccan-l-01 Octan- I-01 n-Pentanol
Insoluble 0.0538 2.19
Stable Stable Unstable
25.1 27.9 -
IO 5 -
Cyclohcxxle
0.02 I3
Yes
IO
CLAs mode with 0.4% (w/v) SDBS in the aqueous 1% (w/v) Softanol-30 in organic phase.
20 20 ‘0
phase
and
acctatc acctatc ncctxtc acc1atc
YCS Yes Yes
5
5 IO
Surfactants used wcw SDBS (0.5% (w/v)) in the aqueous and Atlas G 1300 (!?; (w/v;) in the organic pli:lcc nValues taken from FM. [lSJ
phase
K. Mofsushi!o
er ol./Calloids
Swjiices
69 (1992)
65-72
(w/v) in aqueous phascj ad At!as G!300 (!% (T/Y) in the organic phase) were used. These results show that CLA preparation was not possible when the aqueous solubility of the solvent was somewhere bctwecn 2.2 and 7,45 wt%. It can also be seen that a number of solvents commonly used in solvent extraction in biotechnoIogy, e.g. amyi acetate, can form stabie CiAs. effect of oil-phase mrfactant 011CLA size
type and concentration
Figure 2 shows that the average CLA size decreases with increase in surfactant HLB number (CLAs were made with a PVR of 10 with 0.5% (w/v) SDS in water and 1% (w/v) oil-soluble surfactant in decanol). It has been reported that with an increase in aqueous surlaclani concentration, the CLA size decreases ji6]. Also, there is an empirical rule for surfactant selection for emu:sification which states that the more polar the oil, the higher the HLB number. The HLB number is a measure of the hydrophiiic/lipophihc balance, hence an increase in HLB number will have the same effect as increasing the aqueous-phase surfactant concentration. The Xnarticlc ---_--_ si7e of CLAs with a PVR of 20, 0.4% (w/v) SDES in water and 1% (w/v) Softanoi 30 in decalin was also found to decregsc with an increase in solvent-phase surfactant concen-
69
tmtion up to 2 g 1-l, and then remain mately the same up to 5 g lmi (Fig. 3).
approxi-
Eflect of solvent addition rate on CLd size It is important that the environment surrounding the aphrons should remain aqueous and not be repiaced by oii, otherwise the aphrons will coalesce and no longer offer an interface for spreading. In this work, CLAs (0.5% (w/vj SDS in waier and 1% (w/v) Softanol 120 in decaiin, PVR- 10, mixing speed = 500 rev min- ‘) were prepared at two solvent addition rates (2 ml min-’ and 10 ml min-’ ). The CLA samples were collected at different time intervals and the results were expressed as CLh size as a function of PVR. It was found that the particle size was smaller in the case of a siow solvent addition rate compared with that obtained at a fast rate (Fig. 4). However, as the PVR increased the size differential decreased until it was insignificant at a PVR of 10. At law PVRs the larger particle size at a fast soivcnt addition rate compared with that obtained at a slow solvent addition rate may be due to a difference in energy input level. The amount of energy input in :he case of slow solvent addition was higher than that of t.he fast rate for the same PVR (for the same amount of oil addition) since the addition time was longer. However, as the PVR 30
1
I
o-? 0
I
1
*
I
2
Sr;i
-
,
2
’
I
4
concentration,
’
*
*
5 (w/v)
Fig. 3. Etttct oi cii-phase surraciani conc:r;tration size. (PVR=20; decalin.)
0.4’;: (w/v)
SDBS
in water;
1L
6 %
on CLA Softanol-30 in
K. Mnfs~tshitn
ct nl./Colloicls
Surfkcs
69 (1992) 65-72
higher energy input at higher mixing rates resulted in an increased surface energy by increasing surface area.
Slow rate of oil adition 0
I
Fast rate of oil adition 4
2
8
10
-I 12
PVR6 Fig. 4. Elkct of oil addition 1% (w/v) Soltanol-I 20.)
rate on CLA size. (0.5% (w/v) SDS:
increased the difference between particle sizes became smaller, and this is possibly due to the interracial forces which could control CLA size. At high PVRs the CLA might approach a minimum size or minimum aqtieous film thickness.
To determine the effect of mixing speed on CLA size, CLAs containing 0.5% (w/v) SDS in water and !% (w/v) Softanol 120 in decalin with a PVR of 10 were prepared at three mixing speeds. It was found that with an increase in mixing speed the particle size of the CLAs decreased (Fig. 5). This is probably due to differences in energy input; a
It has been reported that CLAs can be stored in a stoppered bottle for a long time without visible deterioration [23_ To determine whether any coalescence occurred whkh would increase CLA size, the particle size was determined over long intervals of time (Fig. 6). CLAs (PVR= 10, 0.5% (w/v) SDS in water) were made with two oil phase surfactants: Softanol 30 and Softanol 120. It was found that the variation of particle size over time was within the range of measurement error. Hence it was concluded !hat CLAs can be stored for a long time without coalescence. This property of CLAs demonstrates that they are extremely stable, and hence their struciEre is different from monomolecular layer microemulsions. Sebba has postulated that they may contain an encapsulating soapy film which confers considerable stability, since three interfaces have to be broken for coalescence to occur [! I j.
To determine the effect of different types and concentrations, stirring speeds on the stability and air content, CGAs by varying eact-3 parame!er. The results
0
400
500
600
Time,
Mixing speed, rpm Fig. 5. Effect of mixing on CLA size. (PVR= SDS; 1% (w/v) Softanol-I20 in dccalin.)
IO; 0.5% (w/v)
Fig. 6. Variation SDS in water: decanol.)
surfactanl and times .jvere made are showr
C;LAmadeafSoRanol-12
day
of CLA size with time. (PVR = 10; 0.5% 1% (w/v) Softanol-30 and Softanol-120
(IV/~
i
in Tables 4 and 5. The air content of the CGAs and their stability in terms of half-life increased substantially when the stirring speed was increased from 5000 to 5500 rev min- ’ (Table 4). With further increases in stirring speed u:i to 8500 rev min-’ there were only small increases it; both air ccntent and siability. increased stability at a higher air content meant that small CGAs were more stable. Increases in stirring time did nc,i beem to have a significant effect on CGA stability, however, the air content in the CGAs increased with an TABLE Elkct
4 of stirring -.-.-.
Stirring speed (rev mine’)
iimc and speed
Air content
(min)
(min; ___--
(vol.%)
5000 5500 6000 6500 7000 8500
2.0 2.0 2.0 2.0 7.0 2.0
3.5 5.0 5.0 5.4 5.x 5.9
34 57 62 70 68 69
8500 Y500 8500 8500 --
0.5 1.o 7.0 5.0
5.4 5.7 5.9 5.5
60 63 69 73
TABLE
CTMAB
time
CL.4 jiotatim
H:!l:-lik
Surfactxnt:
Stirring
on CGA preparation
0.5% (w/v) in the aqueous
phase.
5
Effect of surfxtant
type clnd conccntralion
on CGA
In these expr
propcrtics
-Cont.
Half-lift
Air content
(g I-‘)
(min)
(vol. ‘2)
CTMAB CTMAB CTMAB
0.25 0.50 I.0
3.6 5.9 6.6
56 69 68
CTMAB
2.5
8.1
68
CPB”
(is
7.1
6X
DTMAR
0.5
2.9
50
SDS
0.5
5.2
67
SDBS Gl300
0.5 0.5
6.8 2.4
68 42
0.5
7.7
FO
Surlhctnnt
increase in stirring time. In addition, surfactant type had a great influence on CGA stability (Table 5). When the concentration of CTMAB was increased above 0.5 g 1-l ihc air content remained the same, but the stability increased markedly. A concentration of 0.25 g l- 1 CTMAS a;jj)eers to be unsuirable for stabIe CGA preparation since itc half-life was very short. It is interesting :o note that small changes in the surfactant (DTbIAB to CTMAf:! ;li times resulted in large increases in both half-life and air content, while with other surfactants (SDS to SDBS) these changes were not so great.
o;
120
a 5
loo-
;
8o:
60 z 40
Tcrgital
(15-S-30)
Stirring
speed
“CPB,
8500 rev min-’
cctylpyridium
bromide.
for 2 min.
-
5
E g ~5
CGA added in column 3.5 cm diE
-a--
Natural in column 3.,X cm &I
20-
Natural in flask, 250 ml
I.--Z+,: .;-_-‘_“._’
1 100
Tims,
200
min.
Fig. 7. CLA flotation. For CLA: PVR= IO; 0.5% (w/v) 0.3% (w/v) G1300 in dccsnol: 0.5% (w/v) CLA in water. CGA: 0.5X- (w/v) DTMAB in w!atcr: 60% gas.
SDS FOI
72
CLAs from suspension, and possibly break them due to charge neutralisation, is an important prcperty of aphrons which may be exploited in separation processes.
6
Acknowledgement
9
7 8
IO
The authors are greatly indebted to the SERC for financing Dr AH. Mollah, and to Nippon Mining Co. Tokyo, Japan for sponsoring K. Matsushita’s postgraduate work. References I 1 3
4 5
F. Scbba. Sep. Purif. Methods, I?( I ) ( 1985) I27- 148. F. Scbba. Foams and Biiiquid Foams-Aphrons, Wiicy. 1987. F. Scbba. Coiioidai dispersions and micciiar behnviour, in K.L. Mittai (Ed.), ACS Symp. Ser. 9, American Chemical Society. Washington, DC. 1975. pp. 18-39. F. Sebba, J. Coiioid intcrfacc Sci.. 35 (1971) 643-646. F. Scbba and S.M. Barnctt. Proc. 2nd Int. Congr. Chcm. Erg, Montreal, 4-9 October 1981, Vol. IV. pp. 27-31.
II I2
13 14 15 16 17 18
S. Cirieiio. S.M. Barnett and F.J. Dciuisc, Sep. Sci. Tcchnol.. 17 (1982) 521-534. S.S. Honcycut. D.A. Wallis and F. Scbbn. Biotcchnoi. Biocng. Symp.. 13 (1983) 567-575. F. Sebba and R.H. Yoon, in B. Yarar nnd D.J. Spottiswood (Eds), Interfacial Phenomena in Mineral Processing, The Foundation, New York, 1982. pp. 161-172. D.L. Michelsen, D.A. Wallis and F. Scbbn, Environ. Prog., 3 (7) (1994) 103-107. W.L. Auicn and F. Scbbn, The USCof colloidal gas aphrons (CGAs) for the removal of siimcs from water by floe flontation. in J. Gregory (Ed.), Solid and Liquid Scparations. Ellis-Horwood. Chichcstcr, 1984. pp. Il41-I 1.52. D.A. Waliis. D.L. Michelsen, F. Sebba. J.K. Carpcnlcr and D. Houlc, Biotcchnoi. B’iocng. Symp.. I5 (1985) 399-408. F. Sebba, Novel separations using aphrons, in J.F. Scamchorn and J.H. Hnrweil (Eds), Surhctant-based Separation Processes. Marcel Dekkcr. New York, 1989. pp. 91-i 17. M.C. Amiri and E.T. Woodburn. Trans. inst. Chem. Eng., Part A, 68 (1990) 154-160. J.M. Diuz. M. Crtballcro and J.A. Pcrcz-Bustamantc. Analyst (London), I I5 (1990) 1201-1205. M. Caballero, R. Ccln and J.A. Perez-Bustumantc, Sep. Sci. Tcchnoi.. 24 (1989) 679-640. F. Scbba, Coiloid Polym. Sci., 257 (1979) 392-396. F. Sebba, Chcm. Ind. 21 (1984) 367-372. J.A. Riddick, W.B. Bungcr and T.K. Sakano (Eds), Organic Solvents: Physical Propcrtics and Method of Purification. Wiley, Vol. 2, 1986.
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
Very stable Very stable
13.1 t I.1
‘0 20
p-Xylcnc Toluenc Bcnzenc
0.0156 0.05 I5 0.1791
Stable St*\blc Stable
29.8 30.8 7.2
20 20 20
Chloroform l.2-Dichloroethanc
0.82 0.8 I
YCS
Yes
IO IO
Dccan-l-01 Octan- I-01 n-Pentanol
Insoluble 0.0538 2.19
Stable Stable Unstable
25.1 27.9 -
IO 5 -
Cyclohcxxle
0.02 I3
Yes
IO
CLAs mode with 0.4% (w/v) SDBS in the aqueous 1% (w/v) Softanol-30 in organic phase.
20 20 ‘0
phase
and
acctatc acctatc ncctxtc acc1atc
YCS Yes Yes
5
5 IO
Surfactants used wcw SDBS (0.5% (w/v)) in the aqueous and Atlas G 1300 (!?; (w/v;) in the organic pli:lcc nValues taken from FM. [lSJ
phase
K. Mofsushi!o
er ol./Calloids
Swjiices
69 (1992)
65-72
(w/v) in aqueous phascj ad At!as G!300 (!% (T/Y) in the organic phase) were used. These results show that CLA preparation was not possible when the aqueous solubility of the solvent was somewhere bctwecn 2.2 and 7,45 wt%. It can also be seen that a number of solvents commonly used in solvent extraction in biotechnoIogy, e.g. amyi acetate, can form stabie CiAs. effect of oil-phase mrfactant 011CLA size
type and concentration
Figure 2 shows that the average CLA size decreases with increase in surfactant HLB number (CLAs were made with a PVR of 10 with 0.5% (w/v) SDS in water and 1% (w/v) oil-soluble surfactant in decanol). It has been reported that with an increase in aqueous surlaclani concentration, the CLA size decreases ji6]. Also, there is an empirical rule for surfactant selection for emu:sification which states that the more polar the oil, the higher the HLB number. The HLB number is a measure of the hydrophiiic/lipophihc balance, hence an increase in HLB number will have the same effect as increasing the aqueous-phase surfactant concentration. The Xnarticlc ---_--_ si7e of CLAs with a PVR of 20, 0.4% (w/v) SDES in water and 1% (w/v) Softanoi 30 in decalin was also found to decregsc with an increase in solvent-phase surfactant concen-
69
tmtion up to 2 g 1-l, and then remain mately the same up to 5 g lmi (Fig. 3).
approxi-
Eflect of solvent addition rate on CLd size It is important that the environment surrounding the aphrons should remain aqueous and not be repiaced by oii, otherwise the aphrons will coalesce and no longer offer an interface for spreading. In this work, CLAs (0.5% (w/vj SDS in waier and 1% (w/v) Softanol 120 in decaiin, PVR- 10, mixing speed = 500 rev min- ‘) were prepared at two solvent addition rates (2 ml min-’ and 10 ml min-’ ). The CLA samples were collected at different time intervals and the results were expressed as CLh size as a function of PVR. It was found that the particle size was smaller in the case of a siow solvent addition rate compared with that obtained at a fast rate (Fig. 4). However, as the PVR increased the size differential decreased until it was insignificant at a PVR of 10. At law PVRs the larger particle size at a fast soivcnt addition rate compared with that obtained at a slow solvent addition rate may be due to a difference in energy input level. The amount of energy input in :he case of slow solvent addition was higher than that of t.he fast rate for the same PVR (for the same amount of oil addition) since the addition time was longer. However, as the PVR 30
1
I
o-? 0
I
1
*
I
2
Sr;i
-
,
2
’
I
4
concentration,
’
*
*
5 (w/v)
Fig. 3. Etttct oi cii-phase surraciani conc:r;tration size. (PVR=20; decalin.)
0.4’;: (w/v)
SDBS
in water;
1L
6 %
on CLA Softanol-30 in
K. Mnfs~tshitn
ct nl./Colloicls
Surfkcs
69 (1992) 65-72
higher energy input at higher mixing rates resulted in an increased surface energy by increasing surface area.
Slow rate of oil adition 0
I
Fast rate of oil adition 4
2
8
10
-I 12
PVR6 Fig. 4. Elkct of oil addition 1% (w/v) Soltanol-I 20.)
rate on CLA size. (0.5% (w/v) SDS:
increased the difference between particle sizes became smaller, and this is possibly due to the interracial forces which could control CLA size. At high PVRs the CLA might approach a minimum size or minimum aqtieous film thickness.
To determine the effect of mixing speed on CLA size, CLAs containing 0.5% (w/v) SDS in water and !% (w/v) Softanol 120 in decalin with a PVR of 10 were prepared at three mixing speeds. It was found that with an increase in mixing speed the particle size of the CLAs decreased (Fig. 5). This is probably due to differences in energy input; a
It has been reported that CLAs can be stored in a stoppered bottle for a long time without visible deterioration [23_ To determine whether any coalescence occurred whkh would increase CLA size, the particle size was determined over long intervals of time (Fig. 6). CLAs (PVR= 10, 0.5% (w/v) SDS in water) were made with two oil phase surfactants: Softanol 30 and Softanol 120. It was found that the variation of particle size over time was within the range of measurement error. Hence it was concluded !hat CLAs can be stored for a long time without coalescence. This property of CLAs demonstrates that they are extremely stable, and hence their struciEre is different from monomolecular layer microemulsions. Sebba has postulated that they may contain an encapsulating soapy film which confers considerable stability, since three interfaces have to be broken for coalescence to occur [! I j.
To determine the effect of different types and concentrations, stirring speeds on the stability and air content, CGAs by varying eact-3 parame!er. The results
0
400
500
600
Time,
Mixing speed, rpm Fig. 5. Effect of mixing on CLA size. (PVR= SDS; 1% (w/v) Softanol-I20 in dccalin.)
IO; 0.5% (w/v)
Fig. 6. Variation SDS in water: decanol.)
surfactanl and times .jvere made are showr
C;LAmadeafSoRanol-12
day
of CLA size with time. (PVR = 10; 0.5% 1% (w/v) Softanol-30 and Softanol-120
(IV/~
i
in Tables 4 and 5. The air content of the CGAs and their stability in terms of half-life increased substantially when the stirring speed was increased from 5000 to 5500 rev min- ’ (Table 4). With further increases in stirring speed u:i to 8500 rev min-’ there were only small increases it; both air ccntent and siability. increased stability at a higher air content meant that small CGAs were more stable. Increases in stirring time did nc,i beem to have a significant effect on CGA stability, however, the air content in the CGAs increased with an TABLE Elkct
4 of stirring -.-.-.
Stirring speed (rev mine’)
iimc and speed
Air content
(min)
(min; ___--
(vol.%)
5000 5500 6000 6500 7000 8500
2.0 2.0 2.0 2.0 7.0 2.0
3.5 5.0 5.0 5.4 5.x 5.9
34 57 62 70 68 69
8500 Y500 8500 8500 --
0.5 1.o 7.0 5.0
5.4 5.7 5.9 5.5
60 63 69 73
TABLE
CTMAB
time
CL.4 jiotatim
H:!l:-lik
Surfactxnt:
Stirring
on CGA preparation
0.5% (w/v) in the aqueous
phase.
5
Effect of surfxtant
type clnd conccntralion
on CGA
In these expr
propcrtics
-Cont.
Half-lift
Air content
(g I-‘)
(min)
(vol. ‘2)
CTMAB CTMAB CTMAB
0.25 0.50 I.0
3.6 5.9 6.6
56 69 68
CTMAB
2.5
8.1
68
CPB”
(is
7.1
6X
DTMAR
0.5
2.9
50
SDS
0.5
5.2
67
SDBS Gl300
0.5 0.5
6.8 2.4
68 42
0.5
7.7
FO
Surlhctnnt
increase in stirring time. In addition, surfactant type had a great influence on CGA stability (Table 5). When the concentration of CTMAB was increased above 0.5 g 1-l ihc air content remained the same, but the stability increased markedly. A concentration of 0.25 g l- 1 CTMAS a;jj)eers to be unsuirable for stabIe CGA preparation since itc half-life was very short. It is interesting :o note that small changes in the surfactant (DTbIAB to CTMAf:! ;li times resulted in large increases in both half-life and air content, while with other surfactants (SDS to SDBS) these changes were not so great.
o;
120
a 5
loo-
;
8o:
60 z 40
Tcrgital
(15-S-30)
Stirring
speed
“CPB,
8500 rev min-’
cctylpyridium
bromide.
for 2 min.
-
5
E g ~5
CGA added in column 3.5 cm diE
-a--
Natural in column 3.,X cm &I
20-
Natural in flask, 250 ml
I.--Z+,: .;-_-‘_“._’
1 100
Tims,
200
min.
Fig. 7. CLA flotation. For CLA: PVR= IO; 0.5% (w/v) 0.3% (w/v) G1300 in dccsnol: 0.5% (w/v) CLA in water. CGA: 0.5X- (w/v) DTMAB in w!atcr: 60% gas.
SDS FOI
72
CLAs from suspension, and possibly break them due to charge neutralisation, is an important prcperty of aphrons which may be exploited in separation processes.
6
Acknowledgement
9
7 8
IO
The authors are greatly indebted to the SERC for financing Dr AH. Mollah, and to Nippon Mining Co. Tokyo, Japan for sponsoring K. Matsushita’s postgraduate work. References I 1 3
4 5
F. Scbba. Sep. Purif. Methods, I?( I ) ( 1985) I27- 148. F. Scbba. Foams and Biiiquid Foams-Aphrons, Wiicy. 1987. F. Scbba. Coiioidai dispersions and micciiar behnviour, in K.L. Mittai (Ed.), ACS Symp. Ser. 9, American Chemical Society. Washington, DC. 1975. pp. 18-39. F. Sebba, J. Coiioid intcrfacc Sci.. 35 (1971) 643-646. F. Scbba and S.M. Barnctt. Proc. 2nd Int. Congr. Chcm. Erg, Montreal, 4-9 October 1981, Vol. IV. pp. 27-31.
II I2
13 14 15 16 17 18
S. Cirieiio. S.M. Barnett and F.J. Dciuisc, Sep. Sci. Tcchnol.. 17 (1982) 521-534. S.S. Honcycut. D.A. Wallis and F. Scbbn. Biotcchnoi. Biocng. Symp.. 13 (1983) 567-575. F. Sebba and R.H. Yoon, in B. Yarar nnd D.J. Spottiswood (Eds), Interfacial Phenomena in Mineral Processing, The Foundation, New York, 1982. pp. 161-172. D.L. Michelsen, D.A. Wallis and F. Scbbn, Environ. Prog., 3 (7) (1994) 103-107. W.L. Auicn and F. Scbbn, The USCof colloidal gas aphrons (CGAs) for the removal of siimcs from water by floe flontation. in J. Gregory (Ed.), Solid and Liquid Scparations. Ellis-Horwood. Chichcstcr, 1984. pp. Il41-I 1.52. D.A. Waliis. D.L. Michelsen, F. Sebba. J.K. Carpcnlcr and D. Houlc, Biotcchnoi. B’iocng. Symp.. I5 (1985) 399-408. F. Sebba, Novel separations using aphrons, in J.F. Scamchorn and J.H. Hnrweil (Eds), Surhctant-based Separation Processes. Marcel Dekkcr. New York, 1989. pp. 91-i 17. M.C. Amiri and E.T. Woodburn. Trans. inst. Chem. Eng., Part A, 68 (1990) 154-160. J.M. Diuz. M. Crtballcro and J.A. Pcrcz-Bustamantc. Analyst (London), I I5 (1990) 1201-1205. M. Caballero, R. Ccln and J.A. Perez-Bustumantc, Sep. Sci. Tcchnoi.. 24 (1989) 679-640. F. Scbba, Coiloid Polym. Sci., 257 (1979) 392-396. F. Sebba, Chcm. Ind. 21 (1984) 367-372. J.A. Riddick, W.B. Bungcr and T.K. Sakano (Eds), Organic Solvents: Physical Propcrtics and Method of Purification. Wiley, Vol. 2, 1986.