Natural surfactants - Chimica UniPD

Current Opinion in Colloid & Interface Science 6 Ž2001. 148᎐159

Natural surfactants Krister HolmbergU Sweden Department of Applied Surface Chemistry, Chalmers Uni¨ ersity of Technology, SE-412 96 Goteborg, ¨

Abstract The ever-increasing environmental concern about surfactants triggers an interest in natural surfactant. This review, which has an emphasis on work published since 1998, covers three categories of natural surfactants: amphiphiles produced by yeast or bacteria, amphiphiles containing a natural polar headgroup and amphiphiles containing a natural hydrophobic tail. Microorganisms produce both high molecular weight and low molecular weight surfactants. Only the low molecular weight compounds are included in the review. Sugars and amino acids are the two most important examples of surfactant polar headgroups of natural origin. The research is particularly intense in the area of sugar surfactants and the review covers three types: alkylglucosides, alkylglucamides and sugar esters. Surfactants based on two types of natural hydrophobic tails are included: fatty acid monoethanolamides and sterol ethoxylates. Routes of preparation as well as physico-chemical properties are discussed for the surfactants prepared by organic synthesis. 䊚 2001 Elsevier Science Ltd. All rights reserved. Keywords: Surfactant; Natural; Biotechnology; Sugar; Polyol; Amino acid; Fatty acid; Sterol

1. Introduction The term ‘natural surfactant’ is not unambiguous. Taken strictly a natural surfactant is a surfactant taken directly from a natural source. The source may be of either plant or animal origin and the product should be obtained by some kind of separation procedure such as extraction, precipitation or distillation. No organic synthesis should be involved, not even as an after-treatment. There are in fact not many surfactants in use today that fulfil these requirements. Lecithin, obtained either from soybean or from egg yolk, is probably the best example of a truly natural surfactant. The main reason why natural surfactants in the real sense of the word are so scarce is not a lack of availability. Amphiphiles are abundant in both the

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plant and the animal kingdom where they are often referred to as polar lipids. In biological systems the surface active agents are used in very much the same way as surfactants are employed in technical systems: to overcome solubility problems, as emulsifiers, as dispersants, to modify surfaces, etc. The factor that works against production of truly natural surfactants is the cost of work-up. The products are usually present in small quantities and the separation process tends to be tedious. In most instances the cost of separationrisolation will by far exceed the manufacturing cost of equivalent synthetic surfactants. The unfavorable cost situation for truly natural surfactants may change if fermentation processes can be developed that yield biosurfactants in high yields. Both yeast and bacteria can be efficient producers of surface active agents and there is considerable current interest in biotechnological processes for production of amphiphilic compounds. These substances can be either low molecular weight, such as acylated

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K. Holmberg r Current Opinion in Colloid & Interface Science 6 (2001) 148᎐159

oligosaccharides, or high molecular weight, most often lipopolysaccharides. So far most interest has focused on the high molecular weight products, with 䊛 Emulsan, a highly charged heterolipopolysaccharide with a molecular weight of approximately 10 6 , as the prime example w1x. The term ‘natural surfactant’ is often used in a broader sense than that discussed above, however. Surfactants synthesized from natural raw materials are usually referred to as natural surfactants. Proper examples of surfactants belonging to this category are fatty acid esters of sugars and fatty acid esters or amides of amino acids. It is, in fact, common practice to be even more generous in the use of the term ‘natural surfactant’. A surfactant with one of the main building blocks, the polar headgroup or the hydrophobic tail, obtained from a natural source is often referred to as a natural surfactant. For example, alkyl glucosides which are prepared from a ‘natural’ sugar unit and a ‘non-natural’ fatty alcohol are often regarded as natural surfactants. ŽSo-called ‘natural fatty alcohols’ should not be seen as natural starting materials, although this is often done. Fatty alcohols derived from coconut oil and other triglycerides are made by two consecutive chemical reactions: methanolysis of the triglyceride followed by reduction of the methyl ester.. In this review the term ‘natural surfactant’ is used in its broadest sense. The paper covers true natural surfactants prepared by fermentation, synthetic surfactants with a natural polar head group and synthetic surfactants with a natural hydrophobic tail. Only low molecular weight surfactants with a well-defined structure are included. The ‘natural surfactants’ used to treat the respiratory distress syndrome, a chapter of its own, are in reality mixtures of amphiphiles with structures not fully elucidated w2x and are, hence, not covered. The review is organized correspondingly.

2. Surfactants prepared by fermentation 2.1. Acylpolyols

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Fig. 1. A trehalose ester.

hydroxy fatty acids connected to disaccharides by ester bonds. They are extracellular compounds produced by actinomycetes such as Mycobacterium, Corynebacterium and Bre¨ ibacterium w3x. The acylpolyols are abundant in bacterial cell walls. A typical example of an acylpolyol is the trehalose ester shown in Fig. 1. 2.2. Glycolipids Glycolipids are usually hydroxy fatty acids attached to a sugar via a glycosidic bond. Sophorolipids and rhamnolipids are well known examples, produced by Candida and by Pseudomonas, respectively w4,5x. Fig. 2 shows a cyclic and an acyclic sophorolipid. Rhamnolipids are attractive in that they can be efficiently produced during growth on either hydrocarbon or carbohydrates as the sole carbon source. There has been considerable interest in the molecular genetics of rhamnolipids and this topic has recently been reviewed in this journal w6x. A sucrose lipid produced by Serratia marcescens has recently been isolated and characterized w7 x. The product was found to be an excellent emulsifier for a 䢇

Acylpolyols prepared by fermentation are usually

Fig. 2. A cyclic and an acyclic sophorolipid.

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Fig. 3. Surfactin from Bacillus subtilis.

wide range of hydrocarbons including crude oil. It has been suggested as a surfactant for cleaning of oil tanks.

3. Surfactants based on a natural polar headgroup

2.3. Acylpeptides

Various types of sugars or polyols derived from sugar have been used as surfactant polar headgroup for many years. Sorbitane alkanoates and ethoxylated sorbitane alkanoates, well known under the trade names of 䊛 Span and 䊛 Tween, respectively, have been around for a long time. In recent years there has been a focus on three classes of surfactants with sugar or a polyol derived from sugar as polar headgroup: alkyl polyglucosides ŽAPGs., alkyl glucamides and sugar esters. Representative structures of the three surfactant types are shown in Fig. 4. There is currently a very strong interest in exploring alkyl polyglucosides ŽAPGs. as surfactant for several types of applications. APGs are synthesized by direct reaction of glucose with fatty alcohol, using a large excess of alcohol in order to minimize sugar oligomerization. Alternatively, they are made by transacetalization of a short chain alkyl glucoside, such as ethyl or butyl glucoside, with a long chain alcohol. An acid catalyst is used in both processes. Either glucose or a degraded starch fraction is used as starting material. Fig. 5 illustrates the synthesis. Alkyl

Acylpeptides or lipopeptides, are usually cyclic compounds based on a hydroxy acid and a short peptide chain. By far the most studied compound is 䊛 Surfactin, produced from Bacillus subtilis w8,9x. Acylpeptides, such as Surfactin and Lichenysin produced by Bacillus licheniformis, are efficient amphiphiles when it comes to surface tension reduction, etc. Surfactin is one of the few biosurfactants that has found commercial use. It is used in a variety of pharmacological applications w10x. The formula of Surfactin is shown in Fig. 3. Surfactin and rhamnolipids are the only two biosurfactants for which there is any regulatory information and the molecular genetics of Surfactin has recently been reviewed w6x.

Fig. 4. Examples of polyol surfactants. From top to bottom: an alkyl glucoside, an alkylglucamide, and the ethyl acetal of a fatty acid glucose ester.

3.1. Sugar as polar headgroup

Fig. 5. Pathway for APG synthesis Žfrom von Rybinski and Hill w18x with permission..


glucosides can also be made by enzymatic synthesis, using ␤-glucosidase as catalyst which yields only the ␤-anomer Žand gives low yield.. The corresponding ␣-anomer can more readily be obtained by ␤-glucosidase catalyzed hydrolysis of the racemate. There are considerable differences between the ␣␤ mixture obtained by organic synthesis and the pure enantiomers obtained by the bio-organic route. The ␤-anomer of n-octyl glucoside has found use as a surfactant in biochemical work. Alkyl glucosides are stable at high pH and sensitive to low pH where they hydrolyze to sugar and fatty alcohol. A sugar unit is more water-soluble and less soluble in hydrocarbons than the corresponding polyoxyethylene unit; hence, APGs and other polyol-based surfactants are more lipophobic than their polyoxyethylene-based surfactant counterparts w11x. This makes the physico-chemical behavior of APG surfactants in oil᎐water systems distinctly different from that of conventional non-ionics. Furthermore, APGs do not show the pronounced inverse solubility vs. temperature relationship that normal non-ionics do. This makes an important difference in solution behavior between APGs and polyoxyethylene-based surfactants. The main attractiveness of APGs lies in their favorable environmental profile: the rate of biodegradation is usually high and the aquatic toxicity is low w12,13x. In addition, APGs exhibit favorable dermatological properties, being very mild to skin and eye w14x. The mildness makes this surfactant class attractive for personal care products but APGs have also found a range of technical applications. APGs have been the topic of several excellent overviews w15᎐18x. In addition, the physico-chemical properties of APGs, as well as other polyhydroxylbased surfactants, have recently been reviewed in this journal w19x. Only a few brief notes on recent developments in the field will therefore be given here. The solution behavior of APGs, both anomeric mixtures, prepared by organic synthesis, and pure enantiomers have been studied in detail w20 ,21᎐23x. The micellar phase region is usually large and at higher concentrations the normal pattern of liquid crystalline phases appears. It interesting to note, as was described in a previous review w19x, that some APG surfactants exhibit a miscibility gap in the micellar region. As with polyoxyethylene-based non-ionics two micellar phases appear in this region, one dilute and one concentrated. For a pure decyl glucoside surfactant it was found that the dilute phase consisted of micelles of aggregation number in the range 200᎐400 while the concentrated phase contained larger aggregates, probably branched micelles that formed a network through entanglement w20x. A characteristic feature of the liquid crystalline region which appears at higher surfactant concentration is that the 䢇

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borders between the different crystalline phases in a temperature vs. surfactant concentration diagram are almost vertical, indicating a temperature-independent behavior. This is very different from the behavior of polyoxyethylene-based non-ionics. Mixed micelle formation of two APGs with different length of the alkyl chain with both anionic and cationic surfactants have been studied w24 x. The experimental data were fitted to Rubingh’s model for non-ideal mixed micelle formation w25x. A value of the interaction parameter, ␤ is obtained and the value of ␤, positive or negative, indicates whether there is a repulsive or an attractive interaction involved in the formation of mixed micelles between the two surfactant types. Combination of an APG with the cationic surfactant dodecyltrimethylammonium bromide gave a value of the ␤-parameter of y4.1 indicating rather strong attractive interaction. The combination of the same APG with the anionic surfactant sodium dodecylsulfate gave a ␤-value of y2.3, indicative of a smaller attraction. These results are interesting and not easily explained. They tend to support the view, proposed by several workers, that APGs carry a negative net charge, the origin of which is unclear w24 x. It is interesting that whereas a combination of an APG and an alcohol ethoxylate gave a value of the ␤parameter of zero, the same APG when combined with another alkyl glycoside, an alkyl maltoside of the same alkyl chain length ŽC10. gave a ␤-value of y2.0. This attractive interaction was tentatively explained as due to favorable packing of the headgroups of the two surfactants w24 x. The majority of work on glycoside surfactants has been made on alkyl glucosides, i.e. surfactants based on glucose or slightly oligomerized glucose as polar headgroup. Recently, alkyl maltosides have been studied with a specific focus on its behavior in mixed micelles together with anionic and non-ionic surfactants w26,27x. A combination of a decyl maltoside with sodium dodecylbenzene sulfonate gave a value of the ␤-parameter of y2.1 which can be compared with the value of y3.3 for a combination of an alcohol ethoxylate, octaŽethylene glycol.monodecyl ether ŽC10E8. with the same anionic surfactant. A combination of the alkyl maltoside with the alcohol ethoxylate gave a very small ␤-value Žy0.3.. A synergistic effect with regard to surface tension reduction was also found for combinations of either of the non-ionic surfactants and sodium dodecylbenzene sulfonate. Thus, the results obtained with the maltoside surfactant agree very well with those previously obtained with glucoside surfactants of the same alkyl chain length. A full explanation of the results still remains, however. Alkylglucamides Žsee Fig. 4., or more strictly named as N-alkanoyl-N-metylglucamines, are commercially important products. The product sold in large quanti䢇

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ties for the detergent sector is N-dodecanoyl-N-methylglucamines, i.e. the C12-derivative. The product is prepared from glucose, metyl amine, hydrogen and methyl laurate by a two-step reaction. Alkylglucamides have been the subject of an extensive review recently w28x. Few major investigations on this surfactant class seem to have been made recently. In a paper by Zhu et al. a glucamide was compared with polyol surfactants of similar structure, a xylamide and a glyceramide, all having a C12 alkyl chain w29 x. The glucamide has four hydroxyl groups in the polar head, the xylamide three and the glyceramide one. This was reflected both in the area per molecule values and the CMC values which were 30, 28 and 26 ˚2 and 0.347, 0.331 and 0.234 mM, respectively. A There is considerable current interest in sugar esters Žsee Fig. 4. and a recent review covers the topic until 1998 w30x. Esters of glucose can be made either by enzymatic synthesis, using a lipase catalyst, or by an organic chemical route. Using the right enzyme the bio-organic route can give esterification almost exclusively at the 6-position of the sugar moiety. The organic synthesis requires extensive use of protecting groups to obtain high selectivity. Selective enzymatic synthesis of other sugar esters than glucose esters have been difficult to achieve without the use of protective groups. Starting from sugar acetals and fatty acids, monoesters of sugars were obtained in good yield, after a deprotection step, from several starting materials, both mono- and disaccharides w31,32x. The advantage of the organic synthesis over the enzymatic route is that the sugar unit can be varied at will. The group of Drummond has recently made important contributions in this area with a specific focus on degradation mechanisms w33 ,34 ,35x but also covering physico-chemical behavior w36 x. A series of sugar ester surfactants were synthesized having different headgroup size Žglucose, sucrose and raffinose, consisting of one, two and three anhydroglucose rings, respectively. and with C12 and C16 acyl chains. Sulfonated sugar esters were also prepared. It was found that all unsubstituted sugar esters degraded rapidly. Variations in the sugar headgroup size or the acyl group chain length did not significantly affect the biodegradability. The anionic derivatives degraded more slowly, however w33 x. A detailed study on the mechanism of biodegradation of the unmodified and the sulfonated sugar esters showed that sucrose laurate biodegradation occurs via initial ester hydrolysis. The sucrose ␣-sulfonyl laurate, on the other hand, degrades by initial alkyl chain oxidation. This indicates that the ester hydrolysis pathway is blocked by the sulfonyl group adjacent to the ester bond so that biodegradation is forced to proceed via the slower alkyl chain oxidation pathway 䢇

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w34 x. An ethyl substituent in ␣-position instead of the sulfonate group also slowed down the biodegradation rate, probably for the same reason. The two degradation routes are shown in Fig. 6. Base catalyzed hydrolysis was also investigated. Similar to the results from the biodegradation, the unsubstituted sugar esters degraded faster than the derivatives with either a sulfonate or an ethyl group in the ␣-position. The difference in hydrolysis rate between the three surfactants sucrose laurate, sucrose ␣-sulfonyl laurate and sucrose ␣-ethyl laurate, varied with temperature. It was concluded that at higher temperature, when steric hindrance is the dominant factor, hydrolysis of ␣-sulfonyl laurate is slowest. By contrast, at lower temperature, when activation energy is more important, hydrolysis is faster in the presence of an electron-withdrawing ␣-sulfonyl group than in the presence of an electron-attracting ␣-ethyl group w35x. Dimeric and trimeric sugar ester surfactants have been synthesized by a combination of enzymatic and organic chemical methods w37 x. An impressive series of products were prepared with different spacer units between the sugar rings. For instance, a dimeric Žgemini. glucose ester with C12 acyl chains attached at positions 6 of the two anhydroglucose units and with the two moieties connected via a six carbon spacer was prepared in high yield. The product is shown in Fig. 7. 䢇䢇

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3.2. Amino acid as polar headgroup Amino acids and short peptides constitute an alternative to sugars as a natural polar headgroup for surfactants. Several amino acids have been investigated for this purpose with the majority of papers dealing with basic amino acids such as arginine and lysine from which cationic surfactants can easily be prepared. Most synthetic procedures are based on organic synthesis but enzymatic processes have also been explored w38,39x. Arginine-based surfactants have recently been reviewed w40x. Arginine surfactants, such as N-dodecanoylarginine metyl ester salt are conveniently prepared by reaction between the oil soluble dodecanoic acid and the water soluble arginine methyl ester hydrochloride using dicyclohexylcarbodiimide as condensing agent. The reaction is normally carried out in an aprotic, polar solvent such as dimethylformamide ŽDMF.. The reaction is performed at room temperature and works well in a laboratory scale. However, DMF is an unsuitable solvent for large scale operations, partly due to cost and toxicity and partly due to difficulties in removing it from the product by simple vacuum evaporation. An alternative process, using a highinternal-phase-ratio emulsion, has been investigated


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Fig. 6. Degradation pathways of a sugar fatty acid ester Žfrom Baker et al. w34x with permission..

for the synthesis w41 x. Such emulsions can be formulated with a large amount, sometimes above 99%, of internal phase and with very low amount of surfactant, often less than 0.5%. The structure of water-inoil emulsions of this type is that of closed-packed 䢇䢇

Fig. 7. A sugar fatty acid gemini surfactant Žfrom Gao et al. w37x with permission..

water droplets of a broad size distribution separated by a thin film of the continuous oil phase. The viscosity is usually high and such emulsions are sometimes referred to as gel emulsions. It was demonstrated that the concentrated emulsions were good reaction media with yields equivalent to those obtained in DMF. The yield depended on the waterto-oil ratio. Too high water content at constant surfactant concentration gave large water droplets with corresponding small interfacial area which was found to be unfavorable. A papain-catalyzed synthesis of arginine-based surfactants have been reported w42,43x. Papain deposited on a solid support was used as catalyst for the amide and ester bond formation between arginine methyl ester and various long-chain alkyl amines and fatty

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Fig. 8. Route of preparation of a tryptophan-based surfactant Žfrom Pegiaduo et al. w46x with permission..

alcohols in organic solvent. High yield of arginine amide was obtained from reaction in acetonitrile. For the synthesis of the arginine fatty acid ester the best yield was obtained in a solvent-free system kept at a temperature above the melting temperature of the fatty alcohol. Using a combination of organic and bioorganic synthesis arginine-based gemini surfactants have been synthesized by the group of Infante w44x and the solution properties have been investigated w45x. The gemini surfactants consisted of two N-acylarginine moieties connected via the arginine carboxyl groups by a diamino spacer molecule, such as 1,3-diaminopropane or 1,3-diamino-2-hydroxypropane. A N-acyl-L-arginine alkyl ester was used as starting material. This compound was first reacted with the diamino spacer to form the amino-amide, i.e. the spacer reacted with one of its amino group to form a N-acyl-L-arginine amide. In a second step, catalyzed by papain, the free amino group reacted with another molecule of N-acyl-L-arginine alkyl ester to form the diamide product. Overall yields of the order of 65% were reported.

A series of tryptophan-based surfactants have been synthesized w46 x. The surfactants were synthesized by reaction of tryptophan with the epichlorohydrine adduct of a fatty alcohol, as shown in Fig. 8. The series of surfactants based on fatty alcohols with from 9 to 16 carbon atoms was evaluated with respect to CMC, surface tension at CMC and area per molecule at the air᎐water interface. The results are summarized in Table 1. As can be seen, the CMC values are low, considerably lower than for simple alkanoate surfactants of the same hydrocarbon chain length. For instance, sodium laurate has a CMC of 20 mM to be compared with the value of 0.038 mM of the C12 surfactant of Table 1. The surface tension values are generally low, as expected for these hydrophobic surfactants. The areas per molecule are within the expected range, the increase in area for the shorter chain length surfactants most probably reflecting a less ordered packing at the surface. The amino acid-based surfactants are possible candidates for pharmaceutical applications and the hemolytic action of some lysine-based anionic surfactants has been investigated. The surfactants were salts 䢇


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Table 1 Physico-chemical data for arginine-based surfactants having the structure given in Fig. 8 Ždata taken from Pegiaduo et al. w46x. Compound

CMC ŽmM.

Surface tension at CMC ŽmNrm.

Area per molecule ˚2 . ŽA

Rs C9H19 Rs C10H21 Rs C11H23 Rs C12H25 Rs C13H27 Rs C14H29 Rs C15H31 Rs C16H33

0.5 0.36 0.065 0.038 0.022 0.020 0.017 0.012

35 33 32 32 32 31 30 30

78 67 53 51 64 64 66 68

of N␣ ,N␧-dialkanoyl lysine. In one series of experiments the alkyl chain lengths were varied w47x. In another work the surfactant was N␣ ,N␧-octanoyl lysine and the counterion was varied w48x. As expected, the hemolytic activity depended on the alkyl chain length and on the choice of counterion. With lysine as counterion to the lysine-based anionic surfactant a protective effect against hypotonic hemolysis was obtained.

4. Surfactants based on a natural hydrophobic tail 4.1. Fatty acid as hydrophobic tail Fatty amide ethoxylates are easily prepared by ethoxylation of the fatty amide monoethanolamide. The ethanolamide, in turn, is prepared by aminolysis of the fatty acid metyl ester by ethanolamine. The fatty amide ethoxylates are of interest as an alternative to fatty alcohol ethoxylates for several reasons: Ži. they biodegrade readily to fatty acid and aminoterminated polyŽethylene glycol.; Žii. the amide bond in the structure may improve surfactant packing due to hydrogen bond formation; and Žiii. double bonds in the fatty acid chain can be preserved in the product Žalthough during the ethoxylation step 1,3-cis,cis-double bonds undergo migration to conjugated trans,cis structures .. A series of fatty amid ethoxylates, all based on C18 fatty acids but with varying number of double bonds, has been synthesized and evaluated with regard to the effect of the amide bond and the double bonds on physico-chemical properties w49 ,50 x. Surfactants with different polyoxyethylene chain lengths were prepared and the corresponding saturated fatty alcohol ethoxylates, stearyl ethoxylates, were used as references. It was found that the presence of the amide group led to a decrease in CMC, possibly due to hydrogen bond formation. The surface tension at the CMC was higher when an amide bond was present, however, which was assumed to reflect the increase in 䢇

size of the polar group Ždecrease in the critical packing parameter value. which is unfavorable for close alignment of surfactant molecules at the air᎐water interface. The same effect was seen in adsorption at a hydrophobic, solid surface, as studied by ellipsometry. The presence of unsaturations in the hydrophobic tail gave an increase in CMC. This may partly be due to the surfactants becoming more hydrophilic and partly to the increased bulkiness of the chains rendering packing into closed aggregates and formation of hydrogen bonds between amide groups more difficult. The presence of one double bond, cis Žoleyl. or trans Želaidyl., did not much affect the molecular cross-sectional area at the surface. Two double bonds gave much larger areas, as can be seen from Fig. 9. Self-diffusion NMR was used to study the effect of temperature and surfactant concentration on the size of the surfactant aggregates. It was found that at both

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Fig. 9. Area per molecule for a series of fatty amide ethoxylates with 10, 15 or 20 oxyethylene units. The number of double bonds in the acyl chain vary from zero Žstearyl. to one, cis Žoleyl. and one, trans Želaidyl. to two Žlinoleyl.. Stearyl alcohol erhoxylates ŽC18En. are included as references.


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high and low surfactant concentration the hydrodynamic radius of a stearyl amide ethoxylate was somewhat larger than the radius of the stearyl alcohol ethoxylate with the same number of oxyethylene units. For the higher surfactant concentration, where the micelles are non-spherical, this may be due to the larger polar headgroup size of the amide surfactants Žsee discussion above. giving slightly less elongated micelles. The results are more difficult to rationalize for the low surfactant concentration where both amphiphiles give spherical micelles. 4.2. Sterol as hydrophobic tail Sterol-based surfactants have been reviewed recently w51x and a new review is in progress w52x. Sterol-based surfactants are of interest because of the large hydrophobic group of fully natural origin which, due to its rather planar four ring structure, may induce good packing at interfaces. Two recent papers deal with phytosterol ethoxylates w53 ,54x. Phytosterols are sterols of plant origin and their structure is similar to that of cholesterol, the prime example of sterols from animal sources. The sterols contain a secondary hydroxyl group that can be ethoxylated. The alcohol is sterically hindered, however, and the reaction with ethylene oxide is not straightforward. The best procedure seems to be to start the ethoxylation with a Lewis acid catalyst, cease the reaction after 3᎐5 mol of ethylene oxide have been added, and continue with KOH as initiator w55x. The structure of a representative sterol ethoxylate is shown in Fig. 10. Surface tension plots of a series of phytosterol ethoxylates with 10, 20 and 30 oxyethylene units showed that Ži. the CMC values were very low; and Žii. the CMC increased with increasing polyoxyethylene chain length. The latter, which has previously been observed by others as accounted for in Folmer et al. w53 x, is unexpected and is difficult to explain. The values are collected in Table 2. Another interesting observation is that the time to reach surface tension equilibrium is very long, more than 2 h. The decay of surface tension with time is shown in Fig. 11. Most likely, the long equilibrium time is due to ex䢇

Table 2 Physico-chemical data for sterol-based surfactants having the structure given in Fig. 10 Ždata taken from Folmer et al. w53x. Number of oxyethylene

CMC Ž ␮ M.

Surface tenstion at CMC ŽmNrm.

10 20 30

10 7 3

31 34 42

change reactions of sterol ethoxylates with varying polyoxyethylene chain length at the air᎐water interface and to slow conformational changes of individual molecules at the surface. The multiring structure of sterols is rigid and the time required to adapt a favorable conformation at an interface may be long. The phase behavior of the sterol ethoxylates has been studied in some detail w53 ,54x. Fig. 12 shows phase diagrams for phytosterols with 5, 10, 20 and 30 oxyethylene units. The surfactant having 10 oxyethylene units gives a lamellar phase over a very wide concentration range at elevated temperature. The phase diagrams of the more hydrophilic surfactants with large headgroups are dominated by hexagonal and cubic phases. The high thermal stability of the liquid crystalline phases is noteworthy. It is particularly striking to see a cubic phase for a non-ionic surfactant that extends above 100⬚C. Cubic phases are normally seen as being trapped in a three-dimensional lattice with no possibility of movement. Usually when the temperature is increased a cubic structure therefore melts at a relatively low temperature. The high thermal stability of the cubic phase formed by the ethoxylated sterols is probably related to an efficient packing of the sterol moieties. Self-diffusion NMR measurements of samples from within the cubic 䢇

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Fig. 10. ␤-Sitosterol ethoxylates.

Fig. 11. Surface tension for phytosterol with 20 oxyethylene units as a function of time at fixed concentration Ž3.44= 10y6 M9 Žfrom Folmer et al. w53x with permission..


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Acknowledgements Dan Lundberg at Chalmers University of Technology is acknowledged for help with the literature search. This review on natural surfactants and some of the work referred to are part of the activities within SNAP, the Centre for Surfactants from Natural Products which is sponsored by VINNOVA and by several companies. References and recommended reading 䢇䢇䢇

Fig. 12. Phase diagrams for phytosterol with Ža. five oxyethylene units; Žb. 10 oxyethylene units; Žc. 20 oxyethylene units; and Žd. 30 oxyethylene units. L1, L2, L ␣ , L ␤ , I1, Nc , and H1 indicate the following phases: micellar, reversed micellar, lamellar, gel, closedpacked micellar cubic, nematic, and hexagonal, respectively Žfrom Folmer et al. w53x with permission..

phase has shown the presence of elongated aggregates with an axial ratio of 1.6 w54x.

5. Conclusions

The review has covered work performed in recent years in three areas, i.e. surfactants produced from bacteria and yeast, surfactants based on either sugar or amino acid as polar headgroup, and surfactants based on either fatty acid or sterol as hydrophobic tail. All three areas are important and subject to considerable research activity. The natural surfactants obtained by these approaches are generally well characterized and the performance are up to the standards of conventional surfactants. The biotechnologically produced surfactants suffer from high cost due to relatively low yield in the fermentation and cumbersome work-up procedures. The surfactants based on either a natural polar headgroup or a natural hydrophobic tail show great promise for the future.

of special interest of outstanding interest

w1x Kosaric N. Biosurfactants. Surfactant Science Series 48. New York: Marcel Dekker, 1993. w2x Halliday HL. Controversies: synthetic or natural surfactant. The case for natural surfactant. J Perinatal Med 1996;24:417᎐426. w3x Haferburg D, Hommel R, Claus R, Kleber H-P. Extracellular microbial lipids as biosurfactants. Adv Biochem EngrBiotechnol 1986;33:53᎐93. w4x Weber L, Stach J, Haufe G, Hommel R, Kleber H-P. Elucidation of the structure of an unusual cyclic glycolipid from Torulopsis apicola. Carbohydrate Res 1990;206:13᎐19. w5x Guerra-Santos LH, Kappeli O, Fiechter A. Dependence of Pseudomonas aeruginosa continuous culture biosurfactant production on nutritional and environmental factors. Appl Microbiol Biotechnol 1986;24:443᎐448. w6x Sullivan ER. Molecular genetics of biosurfactant production. Curr Opin Biotechnol 1998;9:263᎐269. w7x Pruthi V, Cameotra SS. Novel sucrose lipid produced by 䢇 Serratia marcescens and its application in enhanced oil recovery. J Surf Det 2000;3:533᎐537. Sucrose lipids with excellent ability to emulsify hydrocarbons, including crude oil, were produced. The surfactant was a mixture of two hydroxyalkanoic acid esters of sucrose. w8x Zuber P, Nakano MM, Marahiel MA. Peptide antibiotics. In: Zuber P, Nakano MM, Marahiel MA, editors. Bacillus subtilis and other gram-positive bacteria. Washington DC: American Society for Microbiology, 1993:897᎐916. w9x Sen R, Swaminathan T. Application of response-surface methodology to evaluate the optimal environmental conditions for the enhanced production of surfactin. Appl Microbiol Biotechnol 1997;47:358᎐363. w10x Vollenbroich D, Ozel M, Vater J, Kamp RM, Pauli G. Mechanism of inactivation of enveloped viruses by the biosurfactant surfactin from Bacillus subtilis. Biologicals 1997;25:289᎐297. w11x Shinoda K, Carlsson A, Lindman B. On the importance of hydroxyl groups in the polar headgroup of nonionic surfactants and membrane lipids. Adv Colloid Interface Sci 1996;64:253᎐271. w12x Garcia MT, Ribosa I, Campos E, Leal JS. Ecological properties of alkylglucosides. Chemosphere 1997;35:545᎐556. w13x Uppgard L, Sjostrom ¨ ¨ M. Multivariate quantitative structureactivity relationships for the aquatic toxicity of alkyl polyglucosides. Tenside Surf Det 2000;37:131᎐138. w14x Busch P, Hensen H, Tesmann H. Alkylpolyglycoside-eine neue Tensidgeneration fur ¨ die Kosmetik. Tenside Surf Det 1993;30:116᎐121. w15x Hill K, von Rybinski W, Stoll G. Alkyl polyglycosides: technology, properties and applications. Weinheim, Germany: VCH, 1997.

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w16x von Rybinski W. Alkyl glycosides and polyglycosides. Curr Opin Colloid Interface Sci 1996;1:597. w17x von Rybinski W, Hill K. Alkyl polyglycosides ᎏ properties and applications of a new class of surfactants. Angew Chem Int Ed 1998;14:4699᎐4709. w18x von Rybinski W, Hill K. Alkyl polyglycosides. In: Holmberg K, editor. Novel surfactants. New York: Marcel Dekker, 1998:31᎐85. w19x Soderman O, Johansson I. Polyhydroxyl-based surfactants ¨ and their physicochemical properties and applications. Curr Opin Colloid Interface Sci 2000;4:391᎐401. w20x Nilsson F, Soderman O, Reimer J. Phase separation and ¨ 䢇 aggregate᎐aggregate interactions in the C9G1rC10G1 ␤-alkylglucosiderwater system. A phase diagram and NMR selfdiffusion study. Langmuir 1998;14:6396᎐6404. The paper discusses the mechanism behind the liquid᎐liquid phase separation that occurs in the micellar region; this region is split into one concentrated and one dilute phase, both containing micelles. w21x Nilsson F, Soderman O, Johansson I. Four different C8G1 ¨ alkylglucosides. Anomeric effects and the influence of straight vs. branched hydrocarbon chains. J Colloid Interface Sci 1998;203:131᎐139. w22x Dorfler HD, Gopfert A. Lyotropic liquid crystals in binary systems n-alkylglucosidesrwater. J Dispersion Sci Technol 1999;20:35᎐58. w23x Hantzschel D, Schulte J. Thermotropic and lyotropic properties of n-alkyl-␤-D-glucopyranoside surfactants. PCCP 1999;1:895᎐904. w24x Sierra ML, Svensson M. Mixed micelles containing alkylgly䢇 cosides: effect of the chain length and the polar head group. Langmuir 1999;15:2301᎐2306. Formation of micelles in mixtures of alkylglycosides and a conventional surfactant was studied. A strong negative value of the interaction parameter, ␤ , indicating attractive interaction, was obtained for the mixture of alkylglycoside and the cationic surfactant. A somewhat smaller negative ␤-value was obtained for the mixture of a glycoside surfactant with the anionic surfactant. No interaction was seen for the alkylglycoside᎐alcohol ethoxylate mixture. w25x Holland PM, Rubingh DN. Mixed surfactant systems. ACS Symp Ser 501. Washington: American Chemical Society, 1992. w26x Liljeqvist P, Kronberg B. Comparing decyl ␤-maltoside and octaŽethylene glycol.mono n-decyl ether in mixed micelles with dodecylbenzene sulphonate-1. Formation of mixed micelles. J Colloid Interface Sci 2000;222:159᎐164. w27x Liljeqvist P, Kronberg B. Comparing decyl ␤-maltoside and octaŽethylene glycol.mono n-decyl ether in mixed micelles with dodecylbenzene sulphonate-2. Interaction of mixed micelles with polyvinylpyrrolidone. J Colloid Interface Sci 2000;222:165᎐169. w28x Laughlin RG, Fu Y-C, Wireko FC, Scheibel JJ, Munyon RL. The physical science of N-dodecanoyl-N-metylglucamine and its aqueous mixtures. In: Holmberg K, editor. Novel surfactants. New York: Marcel Dekker, 1998:31᎐85. w29x Zhu Y-P, Rosen MJ, Vinson PK, Morral SW. Surface proper䢇 ties of N-alkanoyl-N-metylglucamines and related materials. J Surf Det 1999;2:357᎐362. The effect of the number of hydroxyl groups in polyol surfactants on physico-chemical parameters such as CMC, area per molecule and Krafft point was investigated. The values of all three parameters were reduced with decreasing number of hydroxyl groups. w30x Allen DK, Tao BY. Carbohydrate-alkyl ester derivatives as biosurfactants. J Surf Det 1999;2:383᎐390. w31x Fregapane G, Sarney DB, Vulfson EN. Facile chemoenzymatic synthesis of monosaccharide fatty acid esters. Biocatalysis 1994;11:9᎐18.

w32x

Sarney DB, Kapeller H, Fregapane G, Vulfson EN. Chemoenzymatic synthesis of disaccharide fatty acid esters. J Am Oil Chem Soc 1994;71:711᎐714. w33x Baker IJA, Matthews B, Suares H et al. Sugar fatty acid ester 䢇 surfactants: structure and ultimate aerobic biodegradation. J Surf Det 2000;3:1᎐12. A series of sugar esters with different sized sugar headgroups were investigated with respect to ultimate aerobic biodegradation. Sugar esters with either a sulfonate or an ethyl group in ␣-position of the fatty acyl chain were also included. It was found that variations of the sugar headgroup did not much affect the rate of biodegradation. The ␣ substituted surfactants degraded at a lower rate. w34x Baker IJA, Willing I, Furlong DN, Grieser F, Drummond CJ. 䢇䢇 Sugar fatty acid ester surfactants: biodegradation pathways. J Surf Det 2000;3:13᎐28. The paper discusses biodegradation pathways for sugar ester surfactants. It was shown that normal nonionic sugar esters degraded via initial ester bond hydrolysis. Sugar ester surfactants with either a sulfonate or an ethyl group in ␣-position of the fatty acyl chain degraded by another, slower mechanism, ␻-oxidation of the acyl chain. w35x Baker IJA, Furlong DN, Grieser F, Drummond CJ. Sugar fatty acid ester surfactants: base-catalyzed hydrolysis. J Surf Det 2000;3:29᎐32. w36x Boyd BJ, Drummond CJ, Krodkiewska I, Grieser F. How 䢇 chain length, headgroup polymerization, and anomeric configuration govern the thermotropic and lyotropic liquid crystalline phase behavior and the air᎐water interfacial adsorption of glucose-based surfactants. Langmuir 2000;16: 7359᎐7367. The effect of alkyl chain length, headgroup polymerization and anomeric configuration of a series of sugar esters on physico-chemical properties were investigated. w37x Gao C, Millqvist-Fureby A, Whitcombe MJ, Vulfson EN. 䢇䢇 Regioselective synthesis of dimeric Žgemini. and trimeric sugar-based surfactants. J Surf Det 1999;2:293᎐302. An impressive series of gemini sugar surfactants Žand some trimeric surfactants . were synthesized by a combination of enzymatic and organo-chemical routes. w38x Vulfson E. Enzymatic synthesis of surfactants. In: Holmberg K, editor. Novel surfactants. New York: Marcel Dekker, 1998:279᎐300. w39x Valivety R, Jauregi P. Chemo-enzymatic synthesis of amino acid-based surfactants. J Am Oil Chem Soc 1997;74:879᎐886. w40x Infante MR, Peres ´ L, Pinazo A. Novel cationic surfactants from arginine. In: Holmberg K, editor. Novel surfactants. New York: Marcel Dekker, 1998:87᎐114. w41x Pinazo A, Infante MR, Izquierdo P, Solans C. Synthesis of 䢇䢇 arginine-based surfactants in highly concentrated water-in-oil emulsions. J Chem Soc Perkin Trans 2000;2:1535᎐1539. A highly concentrated water-in-oil emulsion was used as a reaction medium for acylation of arginine methyl ester with dodecanoic acid. It was demonstrated that such emulsions should be seen as an alternative to apropic, polar solvents such as dimethylformamide and also to microemulsions. w42x Clapes P, Moran C, Infante MR. Enzymatic synthesis of arginine-based cationic surfactants. Biotechnol Bioeng 1999;63:333᎐343. w43x Piera E, Dominguez C, Clapes P, Erra P, Infante MR. Qualitative and quantitative analysis of new alkyl amide arginine surfactants by high performance liquid chromatography and capillary electrophoresis. J Chromatogr A 1999;852:499᎐506. w44x Piera E, Infante MR, Clapes P. Chemo-enzymatic synthesis of arginine-based gemini surfactants. Biotechnol Bioeng 2000;70:323᎐331.

K. Holmberg r Current Opinion in Colloid & Interface Science 6 (2001) 148᎐159 w45x Pinazo A, Wen XY, Perez L, Infante MR, Franses EI. Aggregation behavior in water of monomeric and gemini cationic surfactants derived from arginine. Langmuir 2000;15: 3134᎐3142. w46x Pegiaduo S, Perez L, Infante MR. Synthesis, characterization 䢇 and surface properties of 1-N-L-tryptofan-glycerol-ether surfactants. J Surf Det 1999;3:517᎐525. A new type of tryptophan-based surfactants were synthesized and characterized with respect to CMC, surface tension and area per molecule at the air᎐water interface. The tryptophane surfactants showed very low CMC values. w47x Vives MA, Macian M, Seguer J, Infante MR, Vinardell MP. Hemolytic action of anionic surfactants of the diacyl lysine type. Comp Biochem Physiol C Pharmacol Toxicol Endocrinol 1997;118:71᎐74. w48x Vives MA, Infante MR, Garcia E, Selve C, Maugras M, Vinardell MP. Erythrocyte hemolysis and shape changes induced by new lysine-derivate surfactants. Chem Biol Interact 1999;118:1᎐18. w49x Folmer BM, Holmberg K, Gottberg Klingskog E, Bergstrom ¨ 䢇 K. Fatty amide ethoxylates; synthesis and self-assembly. J Surf Det 2001 Žin press.. A series of fatty acid monoethanolamide ethoxylates with varying number of double bonds in the acyl chain and with varying number of oxyethylene groups in the polar headgroup was investigated. The influence of the amide bond and of the unsaturations on adsorption at the air᎐water interface and on self-assembly was investigated. w50x Folmer BM, Nyden ´ M, Holmberg K. Micellization and ad-

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sorption of a series of fatty amide ethoxylates. J Colloid Interface Sci 2001; in press. A series of fatty acid monoethanolamide ethoxylates with varying number of double bonds in the acyl chain and with varying number of oxyethylene groups in the polar headgroup was investigated. The influence of the amide bond and of the unsaturations on adsorption at a hydrophobic solid surface and on micellization was investigated by ellipsometry and by NMR. w51x Svensson M. Surfactants based on sterols and other alicyclic compounds. In: Holmberg K, editor. Novel surfactants. New York: Marcel Dekker, 1998:179᎐200. w52x Folmer BM. Sterol surfactants; from synthesis to applications. In: Karsa DR, editor. Annual surfactant reviews, Vol. 3. Sheffield, UK: Sheffield Academic Press, 2001 Žin press.. w53x Folmer BM, Svensson M, Holmberg K, Brown W. The physic䢇 ochemical behavior of phytosterol ethoxylates. J Colloid Interface Sci 1999;213:112᎐120. A series of sterol ethoxylates was investigated. It was found that the time to reach equilibrium surface tension was very long, more than two hours. This probably reflects both exchange reactions at the surface and a slow reorientation of molecules at the air᎐water interface. w54x Folmer BM, Nyden ´ M. Structure and dynamics in the micellar and cubic phase of an ethoxylated phytosterol surfactant. Langmuir Žsubmitted.. w55x Folmer BM. Ph.D. Thesis. Royal Institute of Technology, Stockholm, Sweden, 2000. 䢇