Biosurfactants as emulsifying agents for hydrocarbons

Colloids and SurfacesA: Physicochemical and Engineering Aspects 152 (1999) 41–52

Biosurfactants as emulsifying agents for hydrocarbons

G. Bognolo *ICI Surfactants, Everslaan 45, 3078 Everberg, B, Belgium

Received 20 January 1998; accepted 24 July 1998

Abstract

A bibliographic review of hydrocarbon emulsifiers produced through biological processes. © 1999 Elsevier ScienceB.V. All rights reserved.

Keywords: Biosurfactants; Emulsifiers; Micro-organisms; Hydrocarbons

1. Introduction physico-chemical properties with particularemphasis to the emulsification of hydrocarbonsand the existing and potential industrialChemicals with surface active properties are

synthesized by an amazing variety of living bodies, applications.from plants (e.g. saponins) to micro-organisms(e.g. glycolipids) to higher complexity animal struc-tures including the human body (e.g. bile acids) 2. Main classes of biosurfactantsfor intracellular and extracellular activities, thatmay range from emulsification of feedstock to the Biosurfactants are produced mainly by aerobi-transport of material across cell walls to recogni- cally growing micro-organisms in aqueous mediation of cells. from a carbon source feedstock, e.g. carbohy-

Despite their ubiquitous presence and biologcaldrates, hydrocarbons, oils and fats or mixtures

importance, biosurfacants have been the subject thereof. The emulsifiers are secreted into the cul-of systematic research for less than half a century, ture medium during the growth of the micro-on the back of the works aimed at developing new

organism and assist in the transport and transloca-antibiotics, when it was observed that many of the tion of the insoluble substrates across cellcultures had developed a surface-active behaviour membranes.at the end of the growth process. Commercially

All biosurfactants are of the nonionic or anionicthey have not yet achieved a widespread recogni- type. There are no literature reports of cationiction, probably because a number of drawbacks structures, however in some instances the presenceassociated with their industrial manufacturing of nitrogen-containing groups imparts a certainhave not been fully resolved and the application degree of cationic character to parts of the mole-technology has still to be perfected. This paper cule thereby affecting, for example, adsorption onreviews the main classes of biosurfactants, their dispersed solids and particle flocculation. In

common with all surface-active species, biosurfac-* Tel: +32 2 758 9317; Fax: +32 2 758 9008. tants contain one or several lipophilic and hydro-

0927-7757/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved.PII: S0927-7757 ( 98 ) 00684-0

42 G. Bognolo / Colloids Surfaces A: Physicochem. Eng. Aspects 152 (1999) 41–52

Fig. 1. Typical structures of biosurfactants.

43G. Bognolo / Colloids Surfaces A: Physicochem. Eng. Aspects 152 (1999) 41–52

Fig. 1. (continued)

philic moieties. The lipophilic moiety can be a acids have been reported. The hydrophilic moietycan be an ester, an hydroxy, a phosphate orprotein or a peptide with a high proportion of

hydrophobic side chains, but is usually the hydro- carboxylate group or a carbohydrate.Reviews of biosurfactants have been producedcarbon chain of a fatty acid with 10–18 carbon

atoms, although higher molecular weight fatty by Gerson and Zajic [1], Copper and Zajic [2],


Zajic and Seffens [3], Shaw [4], Spencer et al. [5] surfactant can be corrected through the additionof small amounts of lower molecular weightand Asselineau [6 ]. According to Zajic and Seffens

[3], biosurfactants may be classified into five chemicals or biosurfactants.Temperature tolerance: some biosurfactants andgroups:

(1) Glycolipids, e.g. threalose, sophorose and their surface activity are unaffected by temper-atures as high as 90°C [9].rhamnose lipids and mannosylerithritol lipids.

They are involved in the uptake of low polarity Ionic strength tolerance: biosurfactants are notprecipitated or salted-out in up to 10% salinehydrocarbons by micro-organisms.

(2) Liposaccharides, e.g. the high molecular solutions, whilst 2–3% salt is sufficient to deacti-vate chemical surfactants [10].weight, water soluble extracellular emulsifiers

produced by hydrocarbon degrading bacteria Biodegradability: biosurfactants are readilydegraded in water and soil [11].like Acinetobacter calcoaceticus (emulsans).

(3) Lipopeptides, e.g. ornithine lipids and the Emulsion breaking: emulsions made with biosur-factants can be easily split by addition ofsubtilysin produced by Bacillus subtilis,

claimed to be the most effective biosurfactant enzymes. For example, small amounts of theenzyme depolymerase can break hydrocarbon-reported to date.

(4) Phospholipids: although they are present in in-oil emulsions [12].every micro-organism, there are very fewexamples of extracellular production, the mostnotable one being the biosurfactants produced 3. Physico-chemical propertiesby Corynebacterium lepus.

(5) Fatty acids and neutral lipids, e.g. ustilagic Surface tension measurements are a commonacid, the corynomycolic acids, the lipo- tool to monitor the growth of microbial culturestheichoic acids (sometimes classified as glyco- and there is a relative abundance of literature datalipids) and the hydrophobic proteins. on the interfacial or emulsification properties of

On the top of the above classes, a number of culture broths or of the isolated emulsifiers. It iscell cultures have shown interfacial tension reduc- however difficult to compare or rationalize thistion with significant emulsification and demulsifi- data, as in many cases they relate to ill definedcation activity and can therefore be regarded as a products from specific micro-organisms, grown inspecial form of biosurfactants. particular and unrelated culture conditions (type

Structures typical of these classes are given and concentration of nutrients, pH, aeration etc.)in Fig. 1. (Table 1). Despite the scattering of the data, there

As opposed to conventional chemical surfac- are some quite interesting features emerging.tants, biosurfactants have a number of distinctive Table 2 gives an example of surface tension reduc-features and advantages: tion for two specific biosurfactants and shows a

Surface and interface activity: biosurfactants are remarkably fast decrease at very low concen-more effective and efficient than, for example, trations, the data point to a levelling-off of thesulphonated anionics, as they produce lower surface tension for values 5–8 mN m−1 is abovesurface tension at lower surfactant rates [7,8]. synthetic surfactants. Table 3 gives the surfaceHigh molecular weight biosurfacants, e.g. emul- tension of a specific cell extract versus syntheticsan, adsorb at the oil–water interface through anionic and nonionic surface active agents.multiple anchoring points and extend in the For the possible use of biosurfactants, interfacialcontinuous phase stabilizing chains that provide tension is perhaps more useful information andeffective steric stabilization. The large interfacial again biosurfactants appear to have peculiar prop-area covered per adsorbed molecule and the erties. Tables 4 and 5 give the interfacial tensionmultiplicity of the anchoring points ensure that of a synthetic and of a number of biosurfactantsno surfactant desorption occurs on particle colli- differing by origin and chemical structure.sion, thus greatly enhancing the storage stability Although a direct comparison is not possible, the

table shows that low interfacial tensions are attai-of the emulsions. The low mobility of the macro-


Table 1Surface tension of specific biosurfactants

Surface active species Concentration Surface tension (mN m−1) Comments ReferenceWater Electrolyte

Organic crude extract 102 mg l−1 35 30 From Rhodococcus erythropolis [9]grown on n-alkanes

Trehalose dicorynornycolates 102 mg l−1 43 36 From Rhodococcus erythropolis [9]grown on n-alkanes

Trehalose monocorynomycolates 1–102 mg l−1 32 32 From Rhodococcus erythropolis [9]grown on n-alkanes

Phosphatidyl ethanolamiries n.a. 29 38 From Rhodococcus erythropolis [9]grown on n-alkanes

Phosphatidic acids n.a. 33 40 From Rhodococcus erythropolis [9]grown on n-alkanes

Organic crude extract n.a. 26–28 26–28 From Pseudomonas sp. DSM 2874 [13]Rhamnolipid 1 1–10 mg l−1 31 26 From Pseudomonas sp. DSM 2874 [13]Rhamnolipid 2 102 mg l−1 n.a. 25 From Pseudomonas sp. DSM 2874 [13]Rhamnolipid 3 10 mg l−1 31 27–30 From Pseudomonas sp. DSM 2874 [13]Rhamonlipid 4 102 mg/l n.a. 30 from Pseudomonas sp. DSM 2874 (13)

Corynomycolid acid:Mannose monoester 1 mg l−1 n.a. 40 From Arthrobacter Sp. DSM 2567 [14]Glucose monoester 10 mg l−1 n.a. 40 From Arthrobacter Sp. DSM 2567 [14]Maltose monoester 1 mg l−1 n.a. 33 From Arthrobacter Sp. DSM 2567 [14]Maltose diester 10 mg l−1 n.a. 46 From Arthrobacter Sp. DSM 2567 [14]Cellobiose monoester 10 mg l−1 n.a. 35 From Arthrobacter Sp. DSM 2567 [14]Maltotriose triester 102 mg l−1 n.a. 44 From Arthrobacter Sp. DSM 2567 [14]PET 1006 fermentation broth n.a. n.a. 33–31 From PET 1006 [15]

n.a.=not available.

niable with the right choice of biosurfactants, lower the cmc the more efficient the surfactant andthe more favourable are the economics of use inreaching levels quite similar to conventional

anionic species. It is also relevant that the use of industrial processes.a co-surfactant causes a further drop to ultra-lowvalues, which could be exploited for example in 4. Oil-related applicationsenhanced oil recovery or micro-emulsions.

Critical micelle concentration (cmc) data are Compared to the volume of work done on thefew and again difficult to interpret or correlate. identification of the micro-biological streams, onExamples of cmc’s of bio versus chemical surfac- the fermentation processes and on the isolationtants are given in Table 6 and show a much and analytical characterization of the surfacelower cmc for the biosurfactants. In principle, the active species, little has been done on the definition

of the application technology of biosurfactants.Because of their physico-chemical propeeties,Table 2

Tension of biosurfactants at low concentration ([16 ]) biosurfactants are more suitable than many syn-thetic surfacants for applications in the oil indus-

Surface tension mN m−1 at concentration:try, which exlains why the large majority of the

0.0034% 0.0075% 0.01% 0.02% biosurfactants produced (estimated to be of theorder of 400–500 tons year−1, including captive

Biosurfactant 1 46 38 35 35 use for tertiary oil recovery or tank cleaning) areBiosurfactant 2 33 31 30 30

used in petroleum-related applications.


Table 3Surface tension of cell extract of mycobacterium Rhodochrous NCIB 990 versus synthetic anionic and nonionic surface activeagents ([17])

Surfactant concentration (mg l−1) Surface tension (mN m−1)Cell extract Sodium lauryl sulphate Lubrol W Teepol 610

1 70.0 70.7 61.9 61.910 61.2 61.9 49.6 54.7100 51.0 45.2 31.3 45.91000 45.9 32.8 32.1 37.210000 43.7 33.5 32.1 32.8

Table 4Interfacial-tension of petroleum sulphonate of average equivalent molecular weight 420 ([18])

Surfactant concentration (mg l−1) Interfacial tension (mN n−1) in 1% salt

n-Hexane n-Octane n-Decane

10 1.10 0.25 2.0050 0.50 0.10 0.02100 0.30 0.05 0.002500 0.05 0.001 0.0081000 0.02 0.05 0.035000 0.0005 0.40 0.1010 000 0.0002 0.40 0.10

4.1. Tank oil cleaning [11]. Biosurfactants grown on glucose and immisci-ble hydrocarbons by a proprietary bacterial strainwere produced at the semi-technical level and suc-Sludges and heavy oil factions that settle at the

bottom of oil storage tanks are highly viscous or cessfully used to mobilize and clean-up sludges froma crude oil storage tank in Kuwait [15]. In the trialeven solid deposits that cannot be lifted by conven-

tional pumps. Their removal usually requires 1.5 tons of biosurfactant were added to about850 m3 of sludge, along with 750 m3 of cude oilsolvent washing or manual cleaning, both being

hazardous, time consuming and expensive pro- and 2000 m3 of brackish water. Circulation in thetank was initiated by suction at the water–oilcesses. Further, they leave large volumes of oil-

contaminated solids to be disposed of. Several interface and reinjection through the tank bottomand continued uninterrupted for 5 days. By thisattempts were made to develop an alternative clean-

ing process by forming concentrated oil-in-water time the sludge had been dispersed in small dropletsthat on interruption of the circulation and additionemulsions through the use of surface-active agents,

pumping-out the mobilized sludge and recovering of an emulsion breaker separated in an upper oillayer whilst the inorganic contaminants collected atvaluable crude oil after emulsion breaking. Any

solid left behind should carry only limited residual the bottom. At the end of the operation 90% of thesludge originally present was recovered as Kuwait-oil (if any) because of the detergent action of the

surfactants, thus making its disposal far less quality oil, leaving about 75 m3 of easily disposable,non-hydrocarbon materials.problematic.

The suitability of biosurfactants for this processhas been identified already in the early seventies. In 4.2. Oil spill dispersantsthe mid-eighties an extracellular emulsifier from A.calcoaceticus (emulsan) was reported to be used in Crude oil contains mutagenic, carcinogenic,

growth inhibitory compounds which can causesmall-scale cleaning of oil-contaminated vessels


Table 5Tension of specific biosurfactants

Surface active agent Concentration Hydrocarbon Intefacial tension at 40°C (mN m−1) Comments ReferenceWater Electrolyte

PET 1006 fermentation 7.10 g l−1 n.a. 8.1 From PET 1006 [15]broth (dry cell weight)Organic crude extract 102 mg l−1 n-Hexadecane 22 22 From Rhodococcus erythropolis [9]

grown on n-alkanesCorynomycolic acids 102–103 mg l−1 From Rhodococcus erythropolis [9]

grown on n-alkanespH3 n-Hexadecane n.a. 9 From Rhodococcus erythropolis [9]

grown on n-alkanespH9 n-Hexadecane n.a. 6 From Rhodococcus erythropolis [9]

grown on n-alkanesTrehalose dicorynorny- 1–102 mg l−1 n-Hexadecane 18 17 From Rhodococcus erythropolis [9]colates grown on n-alkanesTrehalose mono- 1–102 mg l−1 n-Hexadecane 14 16 From Rhodococcus erythropolis [9]corynomycolates grown on n-alkanesPhosphatidyl 1–102 mg l−1 n-Hexadecane 1 15 From Rhodococcus erythropolis [9]ethanolamiries grown on n-alkanesPhosphatidic acids n.a. n-Hexadecane 17 14 From Rhodococcus erythropolis [9]

grown on n-alkanesCrude C16 derived 1800 mg l−1 Hexane n.a. 0.57 (0.16) [19]glycolipids

Octane n.a. 0.30 (0.05) [19]Nonane n.a. — (0.02) [19]Decane n.a. 0.02 (0.001) [19]Undecane n.a. 0.03 (0.00006) From H13 A [19]Dodecane n.a. 0.09 (0.000012 Figures between brackets refer [19]

to addition of 0.5%Tridecane n.a. — (0.000028) Pentanol [19]Tetradecane n.a. 0.16 (0.06) [19]Hexadecane n.a. 0.25 (0.1) [19]Octadecane n.a. 0.30 — [19]Hexane n.a. — (0.0005) [19]+Hexadecane

Crude C17 derived Octane n.a. 0.29 [13]glycolipids

Decane n.a. 0.33 (0.34) [13]Dodecane n.a. 0.26 [13]Tetradcane n.a 0.23 [13]Hexadecane n.a 0.29 [13]

Organic crude extract n.a. n-Hexadecane <1 <1 From Pseudomonas sp. DSM 2674 [13]Rhamnolipid 1 10–50 mg l−1 n-Hexadecane 8 <1–4 From Pseudomonas sp. DSM 2674 [13]Rhamnolipid 2 102 mg l−1 n-Hexadecane n.a. <1 From Pseudomonas sp. DSM 2674 [13]Rhamnolipid 3 10–50 mg l−1 n-Hexadecane 3 <1–3 From Pseudomonas sp. DSM 2674 [13]Rhamonlipid 4 102 mg l−1 n-Hexadecane n.a. <1 From Pseudomonas sp. DSM 2674 [13]

Corynomycolid acid:Mannose monoester 102 mg l−1 n-Hexadecane n.a. 19 From Arthrobacter sp. DSM 2567 [14]Glucose monoester 10 mg l−1 n-Hexadecane n.a. 9 From Arthrobacter sp DSM 2567 [14]Maltose monoester 10 mg l−1 n-Hexadecane n.a. 1 From Arthrobacter sp. DSM 2567 [14]Maltose diester 10 mg l−1 n-Hexadecane n.a. 13 From Arthrobacter sp. DSM 2567 [14]Cellobiose monoester 10 mg l−1 n-Hexadecane n.a. 1 From Arthrobacter sp. DSM 2567 [14]Maltotriose triester 102 mg l−1 n-Hexadecane n.a. 19 From Arthrobacter sp. DSM 2567 [14]Fructose glycolipid 10 mg l−1 n-Hexadecane 20–22 25 From Arthrobacter paraffineus [20]

ATCC 15591


Table 5 (continued )Tension of specific biosurfactants

Surface active agent Concentration Hydrocarbon Intefacial tension at 40°C (mN m−1) Comments ReferenceWater Electrolyte

Sucrose glycolipid 5–50 mg l−1 n-Hexadecane 2–4 2 From Arthrobacter paraffineus [20]ATCC 15591

Lactonic sophrose lipid 102 mg l−1 n-Hexadecane 3–4 8 From Torulopsis bombicola [20]ATCC 22214

Acidic sophorose lipid 10 mg l−1 n-Hexadecane 15 8 From Torulopsis bombicola [20]ATCC 22214

Rhamnose lipid 15–50 mg l−1 n-Hexadecane 3 3 From Pseudomonas fluoresecens [20]ATCC 15453

Rhamnose lipid 40 mg l−1 n-Hexadecane 5 1 From Pseudomonas sp. MUB [20]

Table 6Examples of critical micelle concentration of biosurfactants compared to chemical surfactants

Surfactants CMC(mg l−1) Comments ReferencesWater Electrolyte

Trehalose dicorynomycolates 0.7 1.7 [9]Trehalose monocorynomycolates 16.5 2 [9]Phosphatidyl ethanolamines 30 5 [9]Phosphatidic acids 70 0.01 [9]Rhamnolipid 1 20 15 5% NACl [13]

20 Deposit watera [13]Rhamnolipid 2 200 Deposit watera [13]Rhamnolipid 3 20 10 5% NaCl [13]

10 Deposit watera [13]Rhamonlipid 4 200 Deposit watera [13]Surfactin 11 [21]Corynomy colid acid:Mannose monoester 5 [14]Glucose monoester 10 [14]Maltose monoester 1 [14]Maltose diester 10 [14]Callobiose monoester 3 [14]Maitotriose triester 20 [14]Linear alkyl benzene sulphonate (LABS) 590 [22]Sodium lauryl sulphate 2000–2900 [22]

aDeposit water composition: NaCl 100 g l−1; CaCl2 18 g l−1; MgCl2 10 g l−1.

severe damage to aquatic and terrestrial environ- damage to marine life (diatoms, ciliates, nema-todes, halophytic plants) than commercially avail-ment. It is estimated that 0.08–0.46% of the total

oil production is wasted to the environment, eventu- able formulations based on synthetic surface activeagents [23,24].ally causing pollution to waters and shores. Surface

active agents assist in the degradation of hydro-carbon polluants by facilitating the desorption from 4.3. Microbial enhancement of oil recovery

(MEOR)the soil and/or by dispersing in small droplets thatare more easily attacked by micro-organisms.

Threalose lipids have been found particularly Only about one third of the crude oil present ina well can be recovered through primary andeffective in dispersing oil residues, causing less


secondary production. Capillary forces trap the It used down-hole Clostridium acetobutylicumrest in the rock pores and winning back this growing on molasses to generate carbon dioxidesignificant volume is the target of enhanced oil and hydrogen and displace the oil. Production roserecovery (EOR) processes. EOR can be performed by 250% and the water-to-oil ratio fell from 650:1through thermal, chemical or displacement to 220:1 at the end of the trial. Although theprocesses. absolute quantity of oil recovered was small, the

Microbial methods represent the latest develop- effect observed was considered genuine and sig-ment, as micro-organisms can produce a variety nificant [25].of materials useful for EOR: e.g. surfactants, poly- Other trials, especially in Eastern Europe, seemmers and gases, using inexpensive feedstocks to to have followed the same ‘‘pressure generation’’replace synthetic chemicals that are derived from approach, apparently applying down-hole injec-crude oil itself and therefore reflect its price. tion methods and molasses as feedstock [26 ]. AllSurfactants can be synthesized by aerobic micro- reported various degrees of success, but the report-organisms in conventional fermenters and be used ing and test controls are scanty and it is difficultinstead of petroleum sulphonates for chemical to draw absolute conclusions. In The Netherlandsflood. These products work only in a narrow range microbial plugging of water-filled regions of theof temperature and ionic strength and it is a major reservoir was used to divert the flow of waterpotential advantage of some biosurfactants that

through regions still containing oil [26 ]. Paralleltheir interfacial activity is unaffected by high tem-trials were conducted at the West Work Field,perature and salinity [9,20].Colorado using two bacterial strains, one to pro-It has been suggested that anaerobic micro-duce biosurfactants, cosurfactants and carbonorganisms can be injected downhole with or with-dioxide, the other to produce polymers to blockout nutrients to produce in-situ surfactants orhighly permeable zones. Although oil productionpolymers or neutral solvents useful in micellarrose nearly four-fold over the 9 month period offlooding, however, only a few micro-organismsthe trial there is still debate on whether this wasgrown anaerobically produce surface activedue to the action of the micro-organisms or tocompounds.improved field management [26 ].Release of oil from rock pores requires that the

Gas generation and lowering of surface tensioncapillary number Nc: induced by biosurfactants have been reported toenable significant oil recovery at acceptable eco-Nc=

gv

s,

nomics, however, in general the paucity of infor-mation does not allow a proper evaluation of the

(where v is the velocity of the displacing fluids, gresults. On the other side the fact in itself thatits viscosity and s the interfacial oil–fluid tension)trials and production appear to continue is perhapsis of the order of 10−2 up from the typical 10−2a sign of a growing confidence that MEOR, ifat the end of the water flooding. This can beproperly designed and performed in controlledacheived by reducing the interfacial tension of 30conditions should not cause environmental or eco-to 10 mN m−1 depending on the oil type andnomic damages whilst at the same time offeringreservoir characteristics. It is indeed easier (and inthe possibility of increased productivity.many cases necessary) to modify interfacial tension

than the fluid velocity and viscosity. The interfacial4.4. Bitumen recovery from tar sandtension range is, as shown above, well within the

reach of biosurfactants, that are also less sensitiveIn the late seventies Gerson et al. [27] conductedto well temperature and salinity.

extensive research to develop microbial strainsTrials with MEOR go back to the early fifties,allowing the recovery of bitumen from Athabascawith the one conducted by Magnolia petrolium intar sand, a huge hydrocarbon reserve estimated to1954 and 1955 at the Lisbon Field, Arkansascontain 1 trilion barrels of oil. The process oper-remaining probably among the best prepared and

reported. ated until then involved treating the tar sand with


hot water (80°C) at high pH (8.5–10 plus), with improved combustion characteristics andhigher calorific yield. Emulsan stabilized O/Wwhich was both expensive and problematic

environmentally. emulsions from high asphaltene content no. 6 fuelhave been prepared, stored and tested in combus-The alternative, low-temperature, low-pH pro-

cesses benefited from the use of synthetic surface tion trials, showing ignition and burning character-istics similar or better than the parent oil as wellactive agents at the rate of 0.006–0.012%, and the

production of surface-active materials by hydro- as improved handling without the need for dilutionwith higher value cutter stock. [28].carbon-degrading micro-organisms was an attrac-

tive target because of the reduced environmentalimpact and the potential economic advantages. 4.7. DemulsificationPromising results were obtained with cultures fromArthobacter, Pseudomonas and Corynebacterium Specific bacterial cells have shown

de-emulsification properties, in model as well as ingenus and Bacillus subtilis, that produced enhancedsurface bitumen release and/or bitumen concen- field hydrocarbon emulsions of both the O/W and

W/O type. Nocardia amarae, Corynebacterium pet-tration in the residual tar sand, but no recentreferences have been found suggesting that the rophilium and Rhodococcus auranticus were the

species showing most promise in the initial works,initial findings translated into industrial scaleoperations. although this highlighted a complex performance

pattern depending on the emulsion nature condi-tions, composition and history, cell growth stage4.5. Heavy oil transportationand growth conditions and contact time [29–31].

Combinations of conventional chemical surfac-tants and emulsans have been used in pilot tests 4.8. Other applicationsto produce a 70% O/W emulsion of high viscosityVenezuelan crude oil and moving it through a Sophorolipids produced by Torulopsis bombicola

from glucose and palm oil were used for furtherphysical distance of 380 miles in a simulated trial.Through emulsification the crude oil viscosity was derivation [32], i.e. esterification of the carboxyl

group and propylene oxide addition. Some of thereduced from the initial 105 mPs to less than70 mPs, with no adverse effects on interruption of derivatives have shown particularly good skin

compatibility and are apparently sold commer-recirculation for 3 days. Also there was no indica-tion of inversion to W/O emulsion, a phenomenon cially as skin moisturizers.

A process for the production of sophorolipidsfrequently observed at high dispersed phase ratiowith classical surface active agents [28]. in industrial quantities has been patented by the

Institut Francais du Petrol and their licensee claimDespite the apparent success of the experiment,the author is not aware of further comercial devel- to have identified interesting applications in cosme-

tology and detergency, but no further details areopments. To the best of his knowledge Venezuelanheavy crude oil from the Orinoco basin is emulsi- available to the author.

European Patent Application 0 209 783 tofied and commercialized as high dispersed phaseO/W emulsions (Orimulsions) stabilized by ethy- Mager, Roethlisberger and Wagner reports formu-

lations for anti-dandruff shampoos, hair gels,lene oxide nonionic sufactants, an indication per-haps of either more favourable cost-effectiveness deodorant sticks, after-shave lotions, hair and

body shampoos and rinse aids based on sophoroli-profile or of better local availability of the surfac-tant or both. pids [33].

Other suggested applications include stabiliza-tion of coal in water slurries by biosurfactants4.6. Heavy-oil combustionproduced by Pseudomonas, Corynebacterium,Nocardia, Arthrobacter, Bacillus and AlkaligenesSmall amounts of water are sometimes emulsi-

fied into heavy fuels to produce W/O emulsions sp. [34], waste water treatment, e.g. for the treat-


ment of waste water from nuclear fuel processing $ optimize the nutrient medium, i.e. provide theright balance of C, N, P and otherplants [35], paints, crop protection formulations,

corrosion inhibition, textile detergents and clean- oligoelements;$ take any given waste by-product and develop/ing agents. Surfactin has been shown to improve

the mechanicai dewatering of peat by more than optimize the micro-organism strain for itsmetabolization.50% at very low concentrations (0.0013 g g−1 of

wet peat) [36 ], however the high synthesis costs $ Yield: choice of strain, biosynthesis control andalteration of the genetic of the producer arelimit its use in biochemical research, where its

properties to inhibit blood coagulum and protein parameters that can significantly affect biosur-factant yields and economics.denaturation and to accelerate fibrynolysis are

being exploited. $ Bioprocess: this can be optimized through reac-tor design operating conditions and recyclingof spent medium.

$ Product isolation/recovery: most of the biosur-5. Economicsfactants technologies originally proposedinvolved more or less elaborated forms of puri-Many of the potential applications that havefication and isolation. The possibility of in-situbeen considered for biosurfactants, as well as angrowth or the use of non-refined fermentationexpansion of the few already established dependbroths can undoubtedly lead to substantial coston whether these can be produced economically inreduction.commercial quantities. Much work is still needed

for process optimization at the biological andengineering level. Typical biosurfactant costs rangefrom about 10 $ mg−1 for purified surfactin (98%

6. Conclusionspurity) for biomedical research to the 2–4 $ kg−1for emulsan formulations proposed in the earlyeighties for tank cleaning/oil recovery. Recent esti- Biosurfactants are a relatively young and largely

unexploited sector of colloid chemistry. Themates (1990) put the costs of biosurfactants at3–20 $ kg−1 [22]. numerous potential advantages that this technol-

ogy offers are at the present obscured by a numberWhilst it is acknowledged that the improvent inthe production technology of biosurfactants has of shortfalls, notably:

$ insufficient awareness of the products and theiralready enabled a 10–20 fold improvement inproductivity, it is likely that a further significant potential;

$ complexity and fragmentation of the synthesisimprovement (albeit possibly of a smaller order ofmagnitude) is required to make the technology process that involves micro-organisms not

always readily accessible and for which thefully commercially viable. In the case of the Kuwaittank cleaning trial it was estimated [15] that the mechanisms of growth and biosynthesis are

often unknown;economics looked favourable compared to alterna-tive methods and it is also possible that benefits $ limited knowledge of the surface-active products

of the biosynthesis and definite lack of data onwere achieved in some of the oil recoveryexperiments. physico-chemical properties and structure–perf-

ormance relationship;The parameters that can be affected to improvethe economics of biosurfactants manufacture $ ill-defined and possibly sub-optimal processes

for industrial scale manufacturing.include:(1) Raw materials, i.e. choice of nutrients, this Further, one should not overlook the instinctive

cautiousness, if not reluctance, of many people tocan take the form of:$ maximizing biosurfactant yield for any given manipulate micro-organisms – sometimes of an

unknown nature with unknown properties – ormicro-organism strain. This requires an eco-nomic assessment of the yield increase versus their metabolites.

It is not surprising then that most of the practicalthe additional cost of nutrients;


[15] I.M. Banat, N. Samarah, M. Murad, R. Hoorne, S.work to date has been of basic research nature,Banerjee, Wld J. Microbiol. Biotech. 7 (1991) 80–88.with much less effort devoted to the application

[16 ] M. Singh, M. Thomas, J. Surf. Sci. Technol. 3 (1)aspects. Consequently it is likely that a significant (1987) 55–62.number of possible end-uses has been overlooked [17] R.S. Holdom, A.G. Tumer, J. Appl. Bact. 34 (1968)

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Biosurfactants as emulsifying agents for hydrocarbons