Polyhydroxyl-based surfactants and their physico-chemical properties and applications

ELSEVIER

Current Opinion in Colloid & Interface Science 4 (2000) 391-401 www.elsevier.nl/locate/cocis

Polyhydroxyl-based surfactants and their physico-chemical properties and applications Olle Sodermana2*,Ingegard Johanssonb aDepartmentof Physical Chemistry 1, Centerfor Chemistry and Chemical Engineering, Lund University, P.O. Box 124, S-22100 Lund, Sweden bAkzo Nobel Surface ChemistryAB, S-444 85 Stenungsund, Sweden

Abstract

This contribution deals with the physical-chemical properties of surfactants carrying hydroxyl groups in the polar part. Topics discussed include surface and colloid properties, micelles (both single and multi-component), general phase behaviour and microemulsions. A general conclusion is that our understanding of polyhydroxy surfactants is increasing, but that further work is needed to unravel certain aspects of this important class of surfactants. Examples of such aspects are the origin of the liquid-liquid phase separation and the adsorption to solid surfaces of polyhydroxy surfactants. 0 2000 Elsevier Science Ltd. All rights reserved. Keywords: Polyhydroxy surfactants; Micelles; Phase diagrams; Microemulsions; Adsorption

1. Introduction

The rationale for the ever-increasing interest in polyhydroxy surfactants, PHS (by which term we mean surfactants with hydroxyl groups in the polar head group) can be found in the fact that they can be made from renewable sources. In addition, the surfactants are biodegradable and considered dermatologically safe [l]. Typical hydrophilic building blocks are glycerol and carbohydrates but we will focus on saccharide derivatives. PHS can be made by linking the hydrophilic and hydrophobic parts in different ways which would imply varying stability and application areas of PHS. Esters are sensitive towards hydrolyses, especially on the alkaline side. Amides have the same hydrolyses profile but need much tougher conditions to become hydrolyzed. They are usually considered

+

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* Corresponding author. Tel.: 46-46-2228603; fax: 46-462224413. E-mail addresses: [email protected] (0.Soderman), [email protected] (I. Johansson).

rather stable in water solutions. Glucosides are stable in alkaline surroundings but hydrolyze with acidic catalysis. This means that esters are preferably used as emulsifiers and stabilizers under neutral conditions in personal care, food and pharmaceutical applications. Glucamides and glucosides are used in cleaning under mild conditions, such as in personal care, dishwashing liquids, etc., but also for more or less alkaline situations. PHS are non-ionic surfactants and as a consequence it seems natural to compare them to the traditionally used alkyl polyglycol ether non-ionics, C,Ej. A crucial question is then in what sense the PHS behave differently when compared to the alkyl ethoxylate based type, and if we can rationalize these differences in terms of molecular properties. As noted by Shinoda et al. [2], PHS have the property of showing both stronger lipophobicity and hydrophilicity as compared to surfactants with oligo ethylene oxide headgroups. In addition, the temperature dependence of the solution properties of the former is much less pronounced than those of the latter. Thus one would

1359-0294/00/$ - see front matter 0 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 0 2 9 4 ( 0 0 ) O 0 0 1 9 - 4

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perhaps expect that the use of PHS in industrial applications will be even more abundant than what it is today. This review covers the literature concerned with the general class of PHS. We do not purport to cover all literature on PHS that have appeared over the last few years, but have made a selection biased by our own preferences. We have limited ourselves to papers dealing with physical-chemical aspects of PHS. In addition, we will treat some papers dealing with applications of PHS. Thus reports on syntheses of PHS are left out. Some reviews that cover earlier work on PHS can be found in [3,4]. Throughout the text we will sometimes use abbreviations when referring to a specific surfactant. We follow the most commonly used nomenclature in the literature. C,G, refers to alkyl p-D-ghcopyranosides, while C,G, refers to alkyl p-D-maltosides. Other surfactants are either named on the basis of these abbreviations (for instance, dodecyl a-D-maltoside is named C,,-a-G,) or referred to by their full name. Some representative PHS are presented in Fig. 1.

2. Basic properties and PHS at interfaces In this section we will refer to investigations of basic surface and colloid chemistry properties of PHS and to studies of PHS at interfaces. Aveyard and co-workers [5] have performed a thorough investigation of the surface tension and interfacial tension as well as the distribution of (mainly) decyl p-D-gluco-

side between toluene and water. The influence of salt on these properties is also discussed. One interesting result is that the cloud point of C,,G, depends strongly on the surfactant concentration at low concentrations. In addition, the cloud point is also very sensitive to small concentrations of NaCl. It is suggested that this effect is caused by the presence of a small charge on the micelles, although the origin of this charge remains unclear. We note that C,,G, is less strongly partitioned into the oil-phase than C,,E, at 298 K, showing that the PHS is more lipophobic than C,,E,. Drummond and Wells present some basic physicochemical properties of lactose and lactitol monoesters with varying alkyl chain lengths [6]. The chemical structures are given in Fig. 2. No big differences due to the closed and open structures of the lactose derivatives were seen. They both behave similarly to sucrose esters with the same hydrocarbon chain length though a tendency towards closer packing at the air-water interface for the open chain type was noted. Dynamic surface tension, DST, is an important property of surfactants in many industrial applications. Eastoe and co-workers present DST data for a number of different non-ionic surfactants including dichain gluconamides, a new type of PHS containing two identical alkyl chains as well as two gluconamide parts, all joined at the same carbon (cf. Fig. 3). Combining the tension data with self-diffusion data (obtained from pulsed field gradient NMR spectroscopy) for the monomeric surfactants they show that the final stage of the DST decays is consistent with an activation-diffusion controlled mechanism [7,8'].

0 (CH21&&

wlqj( OH

AJkyl glucotids

An

Sucrose ester

I

Sorbitan

ester

M e t h y l glocamlde

Fig. 1. Some commercially produced PHS (reprinted with permission from von Rybinski and Hill MI.

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393

trostatic nature (since variations in pH imply a variation of the charge density of the surface). The adsorption behavior of C,, P-maltoside is contrary to that that of polyethylene glycol surfactants which adsorb on silica but not on alumina. Complete dispersion of hydrophobic graphite occurs and the amounts adsorbed correspond to a monolayer. A more detailed study of adsorption on hydrophilic silica by C,E, and CaG1 has been performed by Kiraly et al. [lo] using calorimetry. A striking difference between the two species was seen in that the enthalpy of displacement was 12-13 times higher for the ethoxylate, i.e. the adsorption was much stronger. A similar phenomenon is described by Jha et al. [11] in their calorimetric study of technical grade APG (alkyl polyglucoside) compared to pure C,G, and C12E8. It was found that the enthalpy of micellization was approximately one order of magnitude higher for the ethoxylate than for any of the two glucosides. In conclusion, there is a rather large difference in the adsorption behavior to various hydrophilic surfaces between PHS and ethylene oxide based surfactants. It seems clear that this difference is caused by the differences in the hydrophilic headgroups of the two classes of surfactants. It would seem likely that the formation of hydrogen bonds between the head group and the surface is important in this regard (although there must also be other effects involved since the surfactants compete with water for the hydrogen bonding sites on the surface). One difference is that the OH groups of the sugars must bind to an 0-group at the surface while the ether bond of the ethoxy group must form a H-bond to an OH group. Perhaps the differences in adsorption behavior of the surfactants can be traced back to a difference in the density of these OH groups on the various surfaces? We end this section by mentioning a study by Briggs et al., in which the structure and thermal stability at the solid/water interface of the same dichain gluconamides as mentioned above [7,8'1 are investigated by sum-frequency vibrational spectroscopy [12]. The interface is hydrophobized gold-coated chromium-primed silicon wafers. An important finding is that the monolayer of the PHS is exceptionally stable with respect to elevated temperatures.

%&do 0

no

0

\

no-on

OH

Lactose Esters

Lactitol Esters Fig. 2. The structures of lactose and lactitol mono-ester surfactants. The asterisks denote alternative locations for the fatty acid substitution (reprinted with permission from Drummond and Wells 161).

Adsorption of surfactants at interfaces is often used for stabilization purposes in practical applications. The interactions between C,, P-maltoside and hydrophilic as well as hydrophobic solids have been studied by Zhang et al. [9']. It was found to adsorb at room temperature on alumina, silica and titania in amounts corresponding roughly to a double layer (although other geometries may equally well explain the adsorbed amounts) but much less on silica. No pH influence was found, which was taken as an indication that the adsorption mechanism is not of an elec-

3. Micelles

HO

OH Fig. 3. Molecular structure of the double-chained gluconamide (reprinted with permission from Briggs et al. [12]).

The structure and properties of micelles formed by n-alkyl-P-D-glucopyranosidecontinues to attract interest. In a careful X-ray and S A N S study, Zhang et al. have presented a detailed picture of micelles in C7,8,9Gl [13']. They interpret their scattering data in terms of a cylindrical core-shell form factor. The length of the cylinders depends strongly on the num-

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ber of carbons in the alkyl chain as well as on the concentration and isotopic composition of the solvent, while the cylinder radius grows in proportion to changes in the alkyl chain length only. Although the result quoted for C,G, with a length of the cylinder roughly equal to the diameter may appear somewhat odd, the overall dependence on the dimensions of the aggregates on alkyl chain length, concentration and the use of H 2 0 or D 2 0 provide valuable information. Of interest is also the anomalous decay at high q, which was modeled in terms of fluctuations in the position of the surfactant in the micelle. The proposition put forth by Nilsson et al. that C,G, and Cl,G, forms a network of branched micelles is worth mentioning in this context. This conclusion is reached on the basis of self-diffusion NMR data and TRFQ data D41.

On the basis of static fluorescence data, aggregation numbers of 54 and 105 are given for CgG, close to CMC and at a concentration twice that of the CMC, respectively [15]. These aggregation numbers are bigger by roughly a factor two as compared to the ones presented by Zhang et al. [13']. The work of Pastor also includes thermodynamic measurements (density and speed of sound) of the C,G, system as well as the influence of salts on the investigated parameters. The addition of Ca- or Zn-chlorides leads to a decrease of CMC and an increase in the aggregation numbers. The authors speculate that this could be due to a shielding effect of the repulsive forces between the head groups in very much the same way as with anionic surfactants. However, this appears less likely since C,G, is non-ionic. An alternative explanation would be that the headgroups are de-hydrated on account of the increased osmotic pressure of the bulk-solution. Three different PHS: C,-glucoside, C12-maltoside and a C,-carbamoyl methyl glucoside, have been investigated by Aoudia et al. [16'] by means of fluorescence probing and time-resolved fluorescence quenching. The temperature dependence of the CMC was found to be rather small, typically the strongest effect was seen for C,-glucoside where the CMC changed from 23 to 33 mM when the temperature was altered from 40 to 2°C. Aggregation numbers at concentrations considerably higher than the CMC values were found to be rather invariant and approximately 125 for CI2G2,105 for C,Gl and 92 (in agreement with results presented in [15]) for the C,-carbamoyl glucoside. It was pointed out that these rather high values are far above the minimum values compatible with the existence of spherical micelles. The micelle architecture for a number of different PHS has been addressed in two contributions from Dupuy and coworkers [ 17,181. Information is obtained from SAXS and S A N S . Studied are lactose-based

surfactants (N-dodecylamino-lactitol, N-dodecylamino-lactobionamide, N-acetyl N-dodecyl-lactosylamone and a- and P-dodecylmaltosides), the structures of which are given in Fig. 4. The micellar structure is suggested to be either spherical or oblate ellipsoids. It is argued that the conformation at the anomeric center is important in determining the micellar architecture. In Dupuy et al. [181 an odd behavior of the lactobionamide is reported, showing similar CMC and minimal surface tension values t u t a two to threefold larger area/molecule (120 A2) as compared to the other lactose derivatives. This is discussed in terms of the amide function. However, the acetyl lactosylamine also contains an amide bond, though not linking the hydrophobe to the sugar but adjacent to the cyclic sugar structure whereas the other two are open rings. The explanation may be found in the total steric arrangement thovgh it is not clear how. Lower values (approx. 40 A2) for the area/molecule of the lactobionamide are presented by Arai et al. [19] and Syper et al. [20] which are of the same order of magnitude as for the C12-lactitolin [18], i.e. no deviation from the expected. Further information on the N-alkanoyl glucamines (alkyl glucamides) as well as N-alkyl gluconamids can be found in several papers. Both types of surfactants are difficult to handle since they have low water solubility and a strong tendency to crystallize. Different substituents on the nitrogen function in order to OH

ClI20H

OH

CH2OH

CH 2 OH

OH

OH

CH~OH OH

CH20H

OH

CH~OH

Fig. 4. Schematic structures of (a) (N-dodecy1amino)lactitol; (b) (N-dodecyl)lactobionamide,and (N-acetyl N-dodecy1)lactosylamine (reprinted with permission from Dupuy et al. [HI).

0.Sodemzan, I. Johansson /Current Opinion in Colloid & Interface Science 4 (2000)391 -401

Cqz-NILA

395

C,-LA, where n= 10 and 12

Fig. 5. Structures of some N-alkylaldonamides(reprinted with permission from Syper et al. [20]).

create a steric disturbance around the amide bond have been considered. An interesting series of aldonamide (cf. Fig. 5) has been investigated by Syper et al. [20]: C,,-glucon amide is insoluble in water, C12N-methyl gluconamide has a CMC of 0.148 mM, C,, lactobionamide shows a CMC at 0.251 mM and C , , N-methyl lactobionamide at 0.338 mM. All the soluble ones occupy approximately the same space at the air/water interface. The effect of an extended sugar chain was studied by Zhang and Marchant [21'] where a C,,-malton amide is compared with C,,-dextran aldonamide (nine glucose units) (cf. Fig. 6 for chemical structures) resulting in approximately the same CMC (0.31-0.32 mM) but different area/molecule, 43 A2 vs. 60 A,. However, the properties as emulsifier were not stronger for the dextrane derivative, which one might have expected since the steric stabilization should have been increased as compared to the malton amide. Another group of amide-containing products that has shown to be of great commercial interest is the N-alkanoyl-N-methyl glucamines usually called alkyl glucamides. Again the methyl at the nitrogen contributes to the water solubility. Zhu et al. have investigated the effect of the amount of OH-groups in the head group [22], going from four in the glucamides to three in a xylamide and one in a glyceramide. The CMC for the C,, derivatives decreases from 0.347 to 0.331 and 0.234 mM. The are:/molecule follows the same trend (30, 27.6, 26.1 A,). The Krafft point

changes from 51 to below O'C, which is not self-evident since the size of the hydrophile is decreased. In conclusion, it would appear that the intermolecular interactions between the amide groups in the PHS containing such units need further investigations in order to increase the understanding of the behavior of these surfactants.

MAL-C(n+ I ). where n = 5.7.9. lLlZ

DEX9-C12 Fig. 6. Chemical structures of N-dodecylgluconamide (GLU-C12), N-alkylmaltonamide [MAL-C(n + l)], and the dodecylaldonamide of dextrane (DEC9-Cl2) (reprinted with permission from Zhang and Marchant [21']).

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A number of studies have been performed on mixed micellar system of PHS and other surfactants and additives. The effect of addition of butanol to C,G1, C,,G, and C,,G, have been investigated. Butanol is found to be enriched at the interface and not solubilized in the micellar core [23',24]. The micellar shape is inferred to be a prolate ellipsoid. Several reports concerned with the mixed micellar properties of PHS and various surfactants have appeared [25-271. In general, the approach is to measure the CMC as a function of the mixing ratio of the two surfactants. The data is subsequently analyzed by means of a regular solution approach (see Barzykin and Almgren [28] for an illuminating discussion of this topic), the analysis of which yields an interaction parameter, P, which is zero for the case of ideal mixing in the micelles and negative for the case of an attraction between the surfactants in the mixed micelles. In general, reported interaction parameters are negative for mixed micelles of PHS and other nonionic surfactants and also in mixtures with anionic or cationic surfactants. An anionic character of C,,G, is suggested by Sierra and Svensson [27] as based on the rather strong (as judged by the value of the P-parameter) attraction with DTAB. This is, however, difficult to reconcile with the negative p-value reported for C,,G,/SDS micelles. It is also shown that the attractive interaction between C,,G, and C,,G, is unexpectedly high. Since the commercial APGs contain a distribution of different alkyl oligomers dominated by the mono and di-glucoside the possible interactions within the mixture as such may influence the behavior in practical formulations. A discussion in terms of counterion effects is taken up by Kameyama et al. [29] for the combination of C8G,and SDS. When adding salt the CMC is lowered, more so for pure SDS than for pure C8G,. This is interpreted as counterion binding for SDS and as a salting-out effect for the glucoside. When the two surfactants are mixed the stronger counterion effect does not dominate until the mole ratio of SDS exceeds 0.6, when a sudden increase in the counterionbinding coefficient is seen. This behavior is different from what has been found with anionic/alkoxylate mixtures where the change is more gradual. Arai et al. [30] study alkyl lactobionamides (LABA) and their interaction with the fluorinated surfactant lithium perfluorooctane sulfonate. In the case when the pure surfactants have similar CMCs and aggregation numbers, the mixtures show a strong synergy, with P values of approximately -5. This is found when the hydrophobes are similar, i.e. for c8 and C,, chained PHS. With C,,-LABA the interaction is weak. It is also shown that the saccharide derivative interacts in a different way as compared to a corresponding ethoxylate.

The salt effects and the interactions between surfactants in solution take us into the area of practical applications of surfactants where formulations with high amounts of electrolytes as well as combinations of varying types and amounts of surfactants are used. Electrolytes such as complexing agents and/or pH raising additives are often introduced to make cleaning systems more efficient and robust for different water qualities. This creates problems when abundant degreasing agents like alkyl ethoxylates are used since their solubility then is limited. To overcome this state of affairs, hydrotropes are added for which purpose PHS can be used with very good results. Comparisons of efficiency and mechanisms have been made [31,32] for alkyl glucosides as hydrotropes with different chain lengths. The C,-glucoside was found to be especially promising, being a compromise between the mixed micelle forming C,-glucoside and the liquid-crystal destabilizing C,-glucoside. In this context it was also found that the weakly acidic hydroxyl functions of the PHS might help to solubilize the conventional non-ionics in extremely high alkaline solutions. 4. Phase behavior Binary PHS/water phase diagrams for several different PHS have appeared over the last few years. A much-studied system is constituted by c,-P-D-glUCOpyranoside, the binary composition vs. temperature phase diagram with water of which has been presented in several articles [33-351. The presented phase diagrams are in agreement with earlier work, and show the expected behavior of a short chain rather hydrophilic surfactant: a micellar phase followed by a hexagonal, normal cubic and lamellar phase. At temperatures above room temperature the hexagonal phase melts and the cubic phase is in equilibrium with a concentrated micellar region. The cubic phase is bicontinuous [33] and of space group Ia3d [33,36']. Upon increasing the chain length of the alkyl chain the hexagonal phase is suppressed to lower temperatures until for the C,,G, it disappears below the freezing temperature of water [35,37']. For the C,,G, case, the phase diagram is dominated by a micellar region and a lamellar phase at high surfactant concentrations. An additional feature is the liquid-liquid phase separation that occurs in the micellar region, where the micellar solution is split into a concentrated and a dilute phase [33,37']. Both solutions contain micelles. Based on time-resolved fluorescence quenching, Nilsson et al. show that the dilute phase consists of discrete micelles with an aggregation number between 200 and 400 (the uncertainty stems from the fact that the distribution of the quencher between

0.Sodemzan, I. Johansson /Current Opinion in Colloid & Interface Science 4 (2000) 391-401

S O F . . . . . . . . . . . . . . . . . . . . . . . . ~ 0

40 0 CornlHwt

20 I

100

8o

O

80

loo

/-u

1

t t

80

r1.c

L,

Ll

10 0

40 20

40

M CMclHwt

80

loo

397

the micellar sub-phase and the surrounding aqueous media is not accurately known), while the concentrated phase consists of micelles with an aggregation number in excess of 600. It is suggested that the micellar structure in the concentrated phase is a network of branched micelles, and that it is the free energy penalty caused by the increase in the curvature of the surfactant film when diluting this phase with water that causes the phase separation [37’]. A similar miscibility gap is seen for the dichain gluconamides discussed above [38] when the alkyl chain contains seven or eight carbons (total 14 and 16). It is interesting to note that the miscibility appears as an upper critical solution temperature for C,-gluconamide. Phase diagrams for the whole series C = 5,6,7 and 8 are given as well as basic characteristics such as CMC, area/molecule, etc. The phase diagrams are reproduced in Fig. 7. Several other binary PHS/water systems have also appeared. Sakya et al. present phase diagrams of nine mono-alkyl-glucosides in the high concentration range (60-100 wt.%) [36’]. This study provides a large amount of useful data, allowing for a deeper understanding of which properties (a-and P-anomers, chain length, whether a sulfur or an oxygen links the alkyl chain to the sugar ring, number of OH-groups, etc.) that are important in determining the phase behavior of PHS. Nilsson et al. present phase diagrams of four different octyl glucosides, viz. 2-ethylhexyl-a- and p-Dglucoside and n-octyl-a- and P-D-glucoside [391. The phase diagrams are reproduced in Fig. 8. Effects of type of anomeric carbon and branching of alkyl chain on the phase behavior is discussed. A general result is that presence of the a-linkage gives rise to an increased crystal stability and, as a consequence, to an increased Krafft boundary. For the surfactants with branched chains, only lamellar liquid crystalline phases are found, whereas the surfactants with straight chains also form hexagonal and cubic phases. 5. Microemulsions

Fig. 7. Binary temperature composition phase diagrams of some dichain gluconamides (reprinted with permission from Eastoe et al. [381).

An important property of surfactants is the ability to stabilize (one-phase) solutions of non-polar substances and water. Such systems are generally called microemulsions. PHS are in general too hydrophilic to form three-component microemulsions. Therefore, two routes have been taken to form microemulsions with PHS. In the first, a co-surfactant is used, and the fraction of co-surfactant in the co-surfactant/surfactant mixture is the ‘tuning’ parameter of the surfactant film in these microemulsions. In the second approach polar oils are used.

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0.Sodemzan, I. Johansson /Current Opinion in Colloid & Interface Science 4 (2000) 391-401

I

U

Lam.

5

Mic.

--Liq. + Cryst.

50 Conc/wt%

----.o

Cone /wt%

501

;

A

50

Lam.

--

I Liq. + Cryst.

Mic.

Liq.+ Cryst.

Fig. 8. Binary temperature composition phase diagrams of four different octyl glucosides (reprinted with permission from Nilsson et al. [56]).

N-Decyl-P-D-glucopyranoside has been used to form microemulsions with butanol [401and decanol [411as co-surfactants. In both cases mineral oils (n-octane and n-decane) were used as non-polar substances. Both studies show the importance of the surfactant to co-surfactant ratio in determining the properties of the microemulsions. Sucrose-alkanoates as stabilizers of microemulsions have been the topic of several studies. Pes et al. show that sucrose mono dodekanoate (SMD) forms temperature-insensitive microemulsions if hexanol is used as a cosurfactant [42].Compared to the oligoethylene-based surfactants the temperature dependence of the maximum solubilization point is marginal. It is shown that the composition of the film is rather temperature independent. The same group investigated the effect of added salt on three-phase behavior in SMD microemulsions [431.It was found that the three-phase boundary was moved to higher mixing ratios by the addition of NaCl and KSCN, a result which is different from the effect observed when the same salts are added to oligoetyl-

ene oxide based surfactants, where KSCN shifts the three-phase body to higher hexanol/surfactant mixing ratios and NaCl does the opposite. Evidently, in the case of SMD microemulsions the salts have the effect of decreasing the fraction of hexanol in the film, more so for KSCN than for NaCI. Microemulsions of sucrose alkanoates carrying one or more alkyl chains, a non-toxic co-surfactant and ethyl- or cetyl 2-(hexylethyl)-2-hexanoate as oil have been studied by a number of techniques, including S A N S , and freeze fracture electron microscopy [44]. Ryan et al. have discovered a very interesting phase behavior when adding n-alkyl P-D-glucosides to systems of n-alkyl polyglycol ethers, water and octane [45’]. The three-phase boundary in these systems is dominated by a ‘chimney’ (see Fig. 9) in which the extension of the three-phase boundary is almost independent of the temperature. The reason for the existence of such a chimney can be traced to a combination of oil-solubility of C,E, and oil-insolubility of CmGl. In a series of papers, Ryan and Kaler reported on

0.Sodemzan, I. Johansson /Current Opinion in Colloid & Interface Science 4 (2000) 391-401

show that the microemulsion system can be tuned by means of the ratio of the surfactant to the co-surfactant; the typical progression of oil-in-water to waterin-oil via a bicontinuous microemulsion is obtained when increasing the ratio of geraniol to C,G,. Of interest in this context are two reports by Kahlweit et al. who show how to form microemulsions from commercial blends of APG, using saturated or unsaturated fatty acid alkyl esters, and alkane 1,2diols as co-surfactants. [51,52]. Commercial uses of microemulsions are manifold - from cosmetic, mild formulations [53], over more aggressive cleaning fluids for industrial purposes, to emulsification [54] of oil spill and microemulsification of chlorocarbons [55].

80

60

V

c-.

399

40

80

60

40

6. Conclusion

20

0 0

10 __c

20

30

40

ylwt%

Fig. 9. Vertical sections through a pseudoternary phase diagram of water-octane-C,E,-CloGl at equal amounts of water and octane and with varying amounts of CIoG, (as indicated by 8 ) . y denotes the total amount of C,E, and CloGl (reprinted with permission from Ryan et al. [45.1).

systematic investigations of phase behavior of oxygenated oils (alkyl ethylene glycol ethers, C,OC,OC,), n-alkyl glucosides and water. The oxygenated oil is polar enough so that microemulsions are formed in the absence of a co-surfactant. The Winsor phase sequence 2 - 3 - 2 is observed as the temperature is increased, showing that the surfactant is partitioned into the oil phase with increasing temperature. The influence of the hydrophobicity of the oil (the value of k ) , of addition of salt and changes in the glucose headgroup and alkyl chain is reported [46,47]. In addition, the microstructure of the microemulsions is investigated by means of S A N S [48]. Stubenrausch et al. have used a co-surfactant of interest to the perfume industry, namely the doubly unsaturated monoterpene geraniol to form microemulsions using C,G, and cyclohexane as the oil component. Again the important tuning parameter is the composition of the surfactant film, which quantity is determined using a procedure due to Shinoda and Kunieda. The composition of the balanced interface is roughly 25, expressed as the ratio of geraniol to surfactant [49']. An important finding is that if the oil phase is saturated with geraniol and C,G, the threephase body is symmetric with respect to the composition of the balanced interface. In a second contribution, the microstructure of the geraniol system was investigated by means of the NMR self-diffusion approach [501. The data clearly

It seems clear that the rather intense studies of PHS over the last decade have significantly increased our understanding with regard to interfacial properties and phase behavior of this class of surfactants. However, it is fair to state that the general understanding of the behavior of PHS is not on a par with that of ethylene oxide based surfactants. This is to some extent a function of the fact that PHS is a much more diverse class of surfactants on account of the much larger possibility to vary the headgroup. When it comes to applications of PHS, this fact is probably an advantage since PHS surfactants can be tailor-made for a given application to a larger extent than the situation for their ethylene oxide-based counterpart. One might ask, what are the topics that require attention in the future? Below we list some of the topics we believe are important in this regard. The origin of the liquid-liquid phase separation (in, e.g. C,,G,/water) needs to be clarified. In the development of our understanding of ethylene oxide based surfactants, the thermodynamics of the solution properties of the ethylene oxide headgroup has been important. Perhaps a reasonable starting point for PHS is to investigate the thermodynamics of sugar/ water solutions? The very strong dependence of the properties of PHS on whether H,O or D,O is used as a solvent needs further investigation. The mastering of making and tuning the properties of PHS-based microemulsions need to be further improved. Here it is important to draw on the extensive work performed on the phase behavior of ethylene oxide based microemulsions. The driving forces that govern the adsorption processes of PHS to various solid surfaces needs to be clarified. Systematic studies using different PHS and both hydrophilic and hydrophobic surfaces need to be performed. Again, the corresponding properties of

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ethylene oxide based surfactants may serve as useful points of departure. Finally, when is comes to applications, the environmental properties need to be further addressed. Also important is the performance of formulations based on PHS. One area where the relative temperatureinsensitive properties of the PHS may be an advantage is in energy-efficient low-temperature applica tions. To achieve this, attention has to be focussed on ways to lower the Krafft-boundary of PHS. Acknowledgements

It is a pleasure to acknowledge the co-operation within the Center of Competence for Surfactants from Natural Products (SNAP) on the studies of PHS. The Center receives financial support from the Swedish Board for Industrial and Technical Development (NUTEK) and from a number of companies active in the Center. References and recommended reading of special interest of outstanding interest Garcia MT, Ribosa I, Campos E, Leal JS. Ecological properties of alkylglucosides. Chemosphere 1997;35:545-556. 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; 64253-271. von Rybinski W. Alkyl glycosides and polyglycosides. Curr Opin Colloid Interface Sci 1996;1:587-597. von Rybinski W, Hill K. Alkyl polyglycosides - properties and applications of a new class of surfactants. Angew Chem Int Ed 1998;371328-1345. Aveyard R, Binks BP, Chen J et al. Surface and colloid chemistry of systems containing pure sugar surfactants. Langmuir 1998;14:4699-4709. Drummond CJ, Wells D. Nonionic lactose and lactitol based surfactants: comparison of some physico-chemical properties. Coll Surf 1998;141:131-142. Eastoe J, Dalton JS, Rogueda PGA, Crooks ER, Pitt AR, Simister EA. Dynamic surface tensions of nonionic surfactant solutions. J Colloid Interface Sci 1997;188:423-430. Eastoe J, Dalton JS, Rogueda PGA, Griffiths PC. Evidence for activation-diffusion controlled dynamic surface tension with a nonionic surfactant. Langmuir 1998;14979-981. An interesting account of dynamic- surface tension, where the surface tension data is combined with self-diffusion data for the surfactant. [9] Zhang L, Somasundaran P, Maltesh C. Adsorption of ndodecyl-P-D-maltoside on solids. J Colloid Interface Sci 1997;191:202-208. Adsorption data for C,,G, on several different hydrophilic and hydrophobic surfaces are presented. Useful starting point for discussions on adsorption of PHS to solid surfaces. [lo] KirBly Z, Borner RHK, Findenegg GH. Adsorption and aggregation of C,E, and C,G, nonionic surfactants on hydrophilic silica studied by calorimetry. Langmuir 1997;13: 3308-3315.

[ l l ] Jha BK, Svensson M, Holmberg K. A titration calorimetry study of a technical grade APG. Prog Colloid Polym Sci 1998;110:230-234. [12] Briggs AM, Johal MS, Davies PB, Cooke DJ. Structure and thermal stability of dichain sugar surfactants at the solid/water interface studied by sum-frequency vibrational spectroscopy. Langmuir 1999;15:1817-1828. [13] Zhang R, Marone PA, Thiyagarajan P, Tiede D. Structure and molecular fluctuations of n-alkyl-b-D-glucopyranoside micelles determined by X-ray and neutron scattering. Langmuir 1999;15:7510-7519. Interesting observations on the differences in properties when water is replaced with heavy water. Nilsson F, Soderman 0, Hansson P, Johansson I. Physical-chemical properties of C,G, and C,,G,-P-alkylglucosides. Phase diagrams and aggregate size/structure. Langmuir 1998;14:4050-4058. Pastor 0, Junquera E, Aicart A. Hydration and micellization processes of n-octyl-P-D-glucopyranosidein aqueous solution. A thermodynamic and fluorimetric study in the absence and presence of salt. Langmuir 1998;14:2950-2957. Aoudia M, Zana R. Aggregation behavior of sugar surfactants in aqueous solutions: effects of temperature and the addition of nonionic polymers. J Colloid Interface Sci 1998;206:158-167. This contribution presents a lot of useful data with regard to the micellization process of PHS. Dupuy C, Auvray X, Petipas C, Rico-Lattes I, Lattes A. Anomeric effects on the structure of micelles of alkyl maltosides in water. Langmuir 199713:3965-3967. Dupuy C, Auvray X, Petipas C, Anthore R, Rico-Lattes I, Lattes A. Influence of structure of polar head on the micellization of lactose-based surfactants. Small angle X-ray and neutron scattering study. Langmuir 1998;14:91-98. Arai T, Takasugi K, Esumi K. Micellar properties of nonionic saccharide surfactants with amide linkage in aqueous solution. Coll Surf 1996;119:81-85. Syper L, Wilk KA, Sokolowki A, Burczyk B. Synthesis and surface properties of N-alkylaldonamides. Progr Colloid Polym Sci 1998;110199-203. Zhang T, Marchant RE. Novel polysaccharide surfactants: the effect of hydrophobic and hydrophilic chain length on surface active properties. J Colloid Interface Sci 1996177: 419-426. Discusses structural changes of PHS in order to control steric stabilization. [22] Zhu Y-P, Rosen MJ, Vinson PK, Morrall SW. Surface properties of N-alkanoyl-N-methyl glucamines and related materials. J Surfactants Detergents 1999;2357-362. [23] Moller A, Lang P, Findenegg GH, Keiderling U. Location of butanol in mixed micelles with alkyl n-glucosides studied by SANS. J Phys Chem B 1998;102:8958-8964. Interesting study of the location of a cosurfactant in a mixed micellar system. Moiler A, Lang P, Findenegg GH, Keiderling U. Micellar solutions of octyl monoglucoside in the presence of butanol: a small angle and light scattering study. Ber Bunsenges Phys Chem 1997;101:1121-1128. Hines JD, Thomas RK, Garrett PR, Rennie GK, Penfold J. Investigation of mixing in binary surfactant solutions by surface tension and neutron reflection: anionic/nonionic and zwitterionic/nonionic mixtures. J Phys Chem 1997101: 9215-9223. Esumi K, Arai T, Takasugi K. Mixed micellar properties of octyl-b-D-glucoside and lithium perfluorooctane sulfonate. Coll Surf A 1996;111:231-234. Sierra ML, Svensson M. Mixed micelles containing alkylgly-

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[42] Pes MA, Aramaki K, Nakamura N, Kunieda H. Temperature-insensitive microemulsions in a sucrose monoalkanoate system. J Colloid Interface Sci 1996;178: 666-672. [43] Aramaki K, Kunieda H, Ishitobi M, Tagawa T. Effect of added salt on three-phase behavior in sucrose monoalkanoate systems. Langmuir 1997;13:2266-2270. [44] Bolzinger-Thevenin MA, Grossiord JL, Poelman MC. Characterization of a sucrose ester microemulsion by freeze fracture electron micrograph and small angle neutron scattering experiments. Langmuir 1999;15:2307-2315. [45] Ryan LD, Schubert K-V, Kaler EW. Phase behavior of microemulsions made with n-alkyl monoglucosides and n-alkyl polyglycol ethers. Langmuir 1997;13:1510-1518. A very interesting work on microemulsions formed in PHS systems. In particular, the existence of a ‘chimney’, i.e. a region where the extension of the three-phase boundary is almost independent of temperature is interesting. [46] Ryan LD, Kaler EW. Role of oxygenated oils in n-alkyl p-D-monoglucoside microemulsion phase behavior. Langmuir 1997;13:5222-5228. [47] Ryan LD, Kaler EW. The effect of anomeric head groups, surfactant hydrophilicity, and electrolytes on n-alkyl monoglucoside microemulsions. J Colloid Interface Sci 1999;210:251-260. [48] Ryan LD, Kaler EW. Microstructure properties of alkyl polyglucoside microemulsions. Langmuir 1999;15:92-101. [49] Stubenrauch C, Paeplow B, Findenegg GH. Microemulsions supported by octyl monoglucoside and geraniol. 1. The role of the alcohol in the interfacial layer. Langmuir 1997;13: 3652-3658. The phase behavior of a microemulsion with PHS and a co-surfactant is discussed. The importance of the composition of the surfactant film is stressed. [50] Stubenrauch C, Findenegg GH. Microemulsion supported by octyl monoglucoside and gerianol. 2. An NMR self-diffusion study of the microstructure. Langmuir 1998;14:6005-6012. [51] Kahlweit M, Busse G, Faulhaber B. Preparing nontoxic microemulsions with alkyl monoglucosides and the role of alkanediols as cosolvents. Langmuir 1996;12(4):861-862. [52] Kahlweit M, Busse G, Faulhaber B. Preparing nontoxic microemulsions. 2. Langmuir 199713:5249-5251. [53] Comelles F. Alternative cosurfactants and cosolvents to prepare microemulsions suitable for practical applications. J Dispersion Sci Technol 1999;20:491-511. [54] Kim J-Y, Song M-G, Kim T-S, Kim J-D. Effectiveness of a new water-based oil spill dispersant comprised of an alkyl polyglucoside. J Surfactants Detergents 1999;2539-544. [55] Baran Jr JR, Pope GA, Wade WH, Weerasooriya V. Water/chlorocarbon Winsor I 111 I1 microemulsion phase behavior with alkyl glucamide surfactants. Environ Sci Techno1 1996;30:2143-2147. [56] Nilsson F, Soderman 0, Johansson I. Physical-chemical properties of some branched alkyl glucosides. Langmuir 1997;13:3349-3354.