Langmuir 1993,9, 3375-3384
3375
Cloud Point Phenomena in the Phase Behavior of Alkyl Polyglucosides in Water Dieter Balzer Hiils AG, Marl, Germany Received March 24, 1993. In Final Form: September 1 5 , 1 9 9 P A characteristic feature of nonionic surfactants such as, in particular, ethoxylates in aqueous solution is a lower consolute temperature. Alkyl polyglucosides are also nonionic surfactants which, because of their ecological and technicalproperties,are becoming more and more important. So far, no lower consolute temperatures have been observed in aqueous solutions of them, although in some cases at low concentrations there are wide regions of coacervation. These phenomena are particularly pronounced in the presence of electrolytes. Detailed investigations have now shown that, for very narrow ranges of structure, alkyl polyglucosides actuallydo also have lower consolute temperatures. Compared with ethoxylates,the effects of electrolytes are very unusual. Thus, all inorganic salts, with only small differences, result in a distinct reduction in the cloud point, whereas alkalis have a strong hydrotropic effect. The electrolytesensitivity is very much higher than for the ethoxylates. The effects of the alkyl polyglucosides can be explained on the assumption that there is a negative electric charge on the surface of the micelles while the ethoxylates are more or less uncharged. This was demonstrated by the effect of ionic surfactants on the cloud point, and by direct measurements of the potential both in emulsions and in micelles.
Introduction Alkyl polyglucosides (APGs) are nonionic surfactants produced from renewable raw materials-especially starch and fats and their derivatives. Although they have been known for 100years,lways of producing them economically were discovered only a few years ago.2 This led to a massive increase in interest in this group of substances, especially in industry. Evidence of this is that about 380 patents have been filed in the last 10 years, and about 330 of these were application-related, especially for detergents, cleaners, and cosmetics, but also for industrial uses.3 One reason for this great activity is that the ecological and toxicological properties4 of this old/new class of surfactants are extremely favorable, although this is not particularly surprising considering their structural similarity to the glycolipids and other biological surfactantss which, as metabolic products, are very environmentally compatible. Another reason for the overwhelminginterest is that many of the technical properties of the products are o~tstanding.~J In chemical terms, the alkyl polyglucosides are polymeric acetals of glucose and fatty alcohols (Figure 11, although the average degrees of polymerization of the compounds of industrial importance are very low at n < 2. In fact, therefore, it would be more correct to call them oligomers. The main nonionic surfactants, fatty alcohol ethoxylates, display a structural distribution, and so do alkyl polyglucosides. However, in this case a distribution function does not provide adequate analytical characterization because there are three different types of isomerism
* Abstract published in Advance ACS Abstracts, November 1, 1993. (1) Fischer, E. Chem. Ber. 1893,26, 2400; 1895,28,1145. (2) LQders,H.; Balzer, D. Proceedings of the 2nd World Surfactant Congress,Paris,1988;Vol. 2, p81. Anaudia, G.EP 77,167, Sept 11,1985. Wllst, W.; et al. EP 0,362,671,Oct 5,1988. McCurry, P. M.; Pickens, C. A. US 4,950,743,Aug 21, 1990. (3) Balzer, D.; Ripke, N. SOFW 1992, 118, 894. (4) Hofmann, P.;Liiders, H. Proceedingsof the 2nd World Surfactant Congress,Parie, 1988;Vol. 1, p 212. Andree, H.;Middelhauve,B.Tenside 1991,28,413. (5) Wagner, F. Fat Sci. Technol. 1987,89,586. Wagner, F.; et al. DE 26.46 506,Oct 15, 1976. (6) Balzer, D. Tenside Surfactants Deterg. 1991,28, 419. (7) Balzer, D. Proceedings of the 3rd Cesio International Surfactant Congress, London, 1992; Vol. C, p 165.
H
[( O
Od"..
Figure 1. Alkyl polyglucoside: R, c8-C~;n, 1.1-2.
(stereoisomerism,linkage isomerism, and ring isomerism)' which result in a very complex spectrum of products-a fact which is evident, for example, from high-temperature gas chromatography of the silylated products as already described in a previous paper.' Other constituents are polyglucoses and residual fatty alcohols. The products are thus composed of all the chemical entities that can result from the acid-catalyzed acetalization of glucose with fatty alcohol. The surfactant properties of alkyl polygulcosides (cf. Table I) are determined by the low critical micelle concentration values, expected of nonionic surfactants. The relation between the surface tension and the surfactant concentration shows the unusual feature of two break points. This indicates two critical micelleconcentrations: which suggests that there is a transition between two different types of micelles. Whereas the higher cmc2 represents the concentration at which there is formation of rodlike micelles with high axis ratios, as has been shown by measurements of the electrical birefringence:?S the micelles formed at cmcl are presumably spherical. These findings and the high aggregation numbers of APG micelles are in contradiction with the data observed for defined isomers like alkyl &D-glucoside and P-Dmaltoside. In those cases only one cmc,%l1much lower aggregation numbers, and spherical to disklike micellar shape have been observed.12J3 While the cmc range is of about the same order, the surface tension of the alkyl polyglucoside is lower (Table 11). It is remarkable for a (8) KrHmer, K. Ph.D. Thesis, Universitit Bayreuth, 1990. (9) Shinoda, K.;et al. Bull. Chem. SOC.Jpn. 1961, 34, 237.
(10) Bdcker, T.; Thiem, J. Tenside 1989,26, 318. (11) Drummond, C. J.; et al. J. Phys. Chem. 1986,89, 2103. (12) Warr, G.G.;et al. J. Phys. Chem. 1986,90,4581. (13) Bucci, S.;et al. Langmuir 1991, 7,824.
O143-1463/93/ 24Q9-3315$O4.OO/O 0 1993 American Chemical Society
3316 Langmuir, Vol. 9, No. 12, 1993
Balzer
Table I. Surfactant Properties of Alkyl Polyglucosides (Degree of Polymerization 1.3) property C&lO clOc12 ClZCl4 solubility (water 30 "C) soluble soluble dispersible Krafft pointa ("C) 1 1 20 viscositsp (25 O C ) (mPa) 5 40 2006 cmc (25 "C) (ppm) 73/950 40/370 11/80 aggregation n0.c 226 380 420 micellar axis ratiod 14 32 73 surface tensione(25 "C) (mN/m) 30 30 29 interfacial tensionf (25 "C) (mN/m) water/paraffin oil 1.3 0.8 0.4 0.7 0.9 1 water/o-xylene 15.5 14.5 13 HLBe middle-phase rangeh (% NaC1) 4-26 1-26 0-12 foaming' (25 O C ) (mL) 420 330 210 dimineralized water tap water 13"dH 300 200 80 a 100 g/L. * Degree of polymerization 1.7. Small angle light scattering. From relaxation times of electricalbirefringence, 0.5% surfactant. e 1 g/L. f 5g/L. 8 Viathe emulsion method. h SaJinityscan (range of middle-phase microemulsion): n-decane,46%;water,46%; surfactant, 4 % ;isobutanol, 4 % . 'DIN 53902/1, 1 g/L. Table 11. cmc of Anomeric Pure Saccharides and an Alkyl Polyglucoside (26 "C) cmc X 1@ (mol/L) cmc (mN/m) 1.9 39 dodecyl &D-glucoside 0.8* 36 1.5b 39.5 dodecyl a-D-maltoside dodecyl fI-D-maltoside 1.5' 36 2.0b 0.27/1.9d 0.95
37 29 29
Ciz.aH2eAPGi.s CIZ.~H~~APGI.S (1% NaCl) a Shinodaetal? B&kq andThiem.10 Dru"ondetal.ll Table I.
From
nonionic surfactant that in the presence of NaCl the two break points are reduced to one cmc located between cmcl and cmcz. Unusual for aqueous surfactant solutions are the very high viscosity and the very low interfacial tension of alkyl polyglucosides with respect to all organic phases which have been mea~ured.~The latter correlates with the tendency to form middle-phase microemulsions which, interestingly, is independent both of the temperature and of the salt content of the aqueous phase, as shown by the salinity scan values in Table I. This is a unique type of behavior in that it is not shown to this extent even by combinations of surfactantswith antagonistic properties.14 The foaming behavior of the aqueous alkyl polyglucoside solutions is unexpected for nonionic surfactants, being very much similar to that of anionic surfactants, since the values are clearly influenced by the hardness of the water. The same applies to the wetting behavior on hydrophilic surfaces. However, as will be shown, this behavior is not an effect of water hardness but derives from a nonspecific electrolyte effect. Thus, as Table I shows, alkyl polyglucosides do not behave like fatty alcohol ethoxylates, as prototypes of nonionic surfactants, or like anionic surfactants as has been stated elsewhere.l6 This is also true, as will be shown here, of the phase behavior of the aqueous solutions.
Experimental Details Substances. The alkyl polyglucosidesMAFLLOSAN 810,020, and 240 based on C&lo, C10C12, and Cl&l, fatty alcohols were prepared by transglucosylation from butyl glucoside in the (14) Balzer, D.; Kosswig, K.Erdoel Kohle, Erdgos, Petrochem. 1990, 43, 348. (15) Putnic, C. F.; Borys, N.F. Soap, Comet., Chem. Spec. 1986,6, 34. Siracusa, P.A. Household Pers. R o d . Ind. 1992 29, 100.
80
T ('C)
a
b
C
70
60
50
40
30
20 1
SA
10'
0' 0
1
10
20
30
,
1
40
50
60
1
1
70
80
90
100
surfactant (%) Figure 2. Phase diagram of C12C1flGl.s. presence of p-toluenesulfonic acid or HzSOI aa catalyst in a continuously operating pilot plant, and their composition was investigated by thin-layer chromatography and high-temperature gas chromatography. MARLIPAL 24/50 and 24/70 are ClzCld fatty alcohol ethoxylates made by Hiils and having 5.1 and 7.0 EO, respectively, accordingto lH NMR. Correspondingvalues apply to other fatty alcohol ethoxylates. C2, C4, Cb, Ce, and Ca carboxylic acids were from MerckSchuchardt (min 98%). n-Clz and n-Cl4 fatty alcohol sulfates were from Merck-Schuchardt and Aldrich ( N O % ). n-Dodecyl-, n-tetradecyl-, and n-hexadecyltrimethylammonium bromides were from Aldrich (299%). The salts were of analytical grade. These chemicals were used without further purification. Methods. Phase diagrams were constructed by determining the Krafft point by the Shukoff method, and determining the cloud point by inspection during cooling, both with an error of 1 0 . 5 OC, wing a polarizing microscope (Wild M8, with heated stage). The long-term stability of the alkyl polyglucosides had previously been established up to 90 "C at pH 7. Electrophoresis of micelles waa carried out with the Malvern Zeta Sizer 3, and measurements on toluene/water emulsionswere carried out with a Zeiss cytopherometer. The size of the emulsion droplets was about 2 pm. Cloud point and electrophoresis measurements were carried out at pH 5 unless otherwise indicated. Viscosity aa a function of shear rate (D> 108-l) waa measured with a rotary viscometer (Haake, German, RV 20).
Results and Discussion The phase diagram of a C12C14 alkyl polyglucoside with an average degree of glucosidation of 1.3 (C12C14APG1.3) is shown in Figure 2. It does not look as simple as the phase diagram of dodecyl ,9-D-maltoside,l2but there are similarities like the wide range of micellar solution and the existence of a lamellar La phase as the only liquid crystalline phase. The Krafft point curve of the C12C14APG1.3shows relatively little dependence on concentration and remains near room temperature. The broad micellar range is separated by a pronounced miscibility gap into a low-concentration region, LI, and a high-concentration region, L'1. In the range of coacervation between 1%and 25 % ,rapid phase separation is observed as indicated by the tie-line P'P''. The phase boundary line (a) corresponds
Cloud Point Phenomena
Langmuir, Vol. 9, No. 12,1993 3377
to the equilibrium concentration in the upper phase, and (b) to that in the lower phase. In both L1 and Ll', rodlike micelles were observed, in L1 by means of electrical birefringence and of conductivity measurements in orthogonal directions under Couette flow and in L1' by the latter technique. The single-phaseregion is followed,after the usual miscibility gap, by the liquid crystalline region. This is not, as would be expected from the rod micelle structure, a hexagonal phase but a lamellar La phase6which means that near the phase boundary a transition from rodlike to disklike micelles should occur. The most notable point is the broad miscibility gap in the concentration range between about 1% and 20%. No cloud point and no lower consolute temperature, as is so characteristic of ethoxylates, for example, are observed. For comparison, a C&14 ethoxylate with the same HLB has a cloud point at about 70 "C, and a change in the cloud point from about 20 to 100 "C at this alkyl chain length is brought about by a change of about 6 EO units in the degree of ethoxylation. Since alkyl polyglucosideshave different structures with different functional groups, the coacervation behavior is of course not expected to be strictly parallel. Nevertheless, the coacervation range of APGs is highly reminiscent of that of the fatty alcohol ethoxylates, and the lower critical consolute point may be concealed by the heterogeneous region below the Krafft curve. This point is defined by the phases P' and P" characterized by the tie line being identical. The cloud point is then by conventional agreement a point on the lower consolute phase boundary. In principle, it would not be surprising for the aqueous alkyl polyglucosidesolution to show a normal cloud point, i.e., to separate into two isotropic micellar solutions above a particular temperature. This property is by no means confined to fatty alcohol ethoxylates but also occurs with other nonionic surfactants such as alkylphosphine oxides.16 Even ionic surfactants such as cetylpyridinium bromide in the presence of large amounts of NaBr salts'7 or carboxymethylated fatty alcohol ethoxylates in the presence of N a C P display lower critical consolute temperatures. However, even surfactant characteristics are not a prerequisite for this property because it is also shown by higher molecular weight poly(ethy1ene oxides)lgand low molecular weight isobutyl glycol ether. The event of critical phase separation when a particular temperature is exceeded must be a distinct change in the interaction forces between the components, in this case essentially between the surfactant micelles and the water, with temperature. A lower consolute temperature occurs when the interactions which are repulsive at lower temperatures become attractive. The process is accompanied by a certain increase in the size of the micelles as the temperature increases,20but this does not mean that the size of the micelles increases suddenly when the lower consolute temperature is exceeded or that the latter is attributable to an increase in the size of the micelles. Following the models of Degiorgio21 and Kjellander,22 the process can be characterized by an interaction parameter w, i.e., an intermicellar force, and its dependence on temperature. Wdescribes the state of hydration of the micelle shell and the structure of the liquid water. As the (16) Lang, J. C.; Morgan, R. D. J. Chem. Phys. 1980, 73, 5849. (17) Appell, J.; Porte, G. J. Phys. Lett. 1983,44,689. (18) Balzer, D. Unpublished results. (19) Saeki, S.; et al. Polymer 1976,17, 685. (20) Lindman, B.; Karlstr6m, G. 2.Phys. Chem. N.F. 1987,156,199. (21) Degiorgio, V. Physics of Amphiphiles. In Micelles, Vesicles and Microemulsion; Degiorgio, V., Corti, M., Eds.; Amsterdam, 1985; p 303. (22) Kjellander, R. J. Chem. SOC.,Faraday Trans. 2 1982, 78, 2025.
w :
11
0
repulsive interaction (ooe phase)
A+-~ I I
-------
attractive interaction (two phases)
Figure 3. Interaction parameter w versus temperature.
temperature rises, the micelle-water interaction becomes less. The repulsive forces resulting from the hydration and, where electrical charges are involved, from the electrical double layer decrease, and the attractive forces resulting from van der Waalsand hydrophobic interactions become increasingly important. There is an increase in the probability of micelles remaining in the vicinity of one another, which is called critical concentration fluctuation. Finally, W becomes negative (Figure 31, and the lower critical consolute point P, is exceeded. Both enthalpy and entropy contribute to the interaction parameter W. Both increase when the water is transferred from the hydrated surface of the micelle to the bulk phase, and there is a decrease in overall s t r ~ c t u r e .The ~ ~ conse~~~ quence is that W must be dominated by the entropy. These ideas were particularly developed for the interaction between water and fatty alcohol ethoxylate micelles but might also apply to alkyl polyglucosides under analogous conditions. Although no phase separation was observed with the C12C14 alkyl polyglucoside, there is a distinct region of coacervation at concentrations between 1%and 25 % . Examination of a somewhat more hydrophilic alkyl polyglucoside (C1oC12APGl.a)no longer reveals coacervation (Figure 4). The micellar region is extremely broad. Only above 80%is there there a liquid-crystalline Laphase. This compound is therefore a surfactant with a very high solubility in water. For comparison, assuming linear dependence between the cloud point of fatty alcohol ethoxylates and the length of the alkyl chain, a reduction by two C atoms would result in an increase of about 15 "C. By contrast, with alkyl polyglucosides there is a change in the cloud point, when it exists, from below 20 "C to above 100 "C. If small amounts of electrolyte are added to this extremely water-soluble C1OC1APG again something unexpected happens (Figure 5). At low surfactant concentrations there are narrow zones of coacervation. This applies to 1:lelectrolytes as well as to CaCl2, and even at electrolyte concentrations which have no effect on the coacervate behavior of fatty alcohol ethoxylates. A further reduction in the length of the alkyl chain of the alkyl polygulcoside which means a further increase in solubility leads to similar zonea of coacervation, in some cases at a somewhat higher electrolyteconcentration which is, however, far below that having an effect on the coacervation behavior of fatty alcohol ethoxylates. This is shown in Figure 6 for a CaCloAPG1.3. The great effect of electrolytes on the coacervation behavior of alkyl (23) Kjellander, R.; Florin, E. J. Chem. SOC.,Faraday D a m . 1 1981,
77, 2053.
3378 Langmuir, Vol. 9, No.12,1993
Baker
T ("C)
80 r
t
6o 50 40 -
30 -
2o
t
0
/ 10
20
30
40
50
60
70
80
90
100
(%I Figure 4. Phase diagram of C10C12APG1.3.
0.01
0.1
surfactant
80
1
10
surfactant (%) Figure 6. Phase diagram (lower concentrations) of C&APGl.3 in the presence of NaCl (1.0%) and CaC12 (0.07%).
T ?C) 100
T ("C)
90
80
60
70 60
50
40
40
30 20
20 10
0 0 0.01
0 091
1
10
surfactant (%)
Figure 5. Phase diagram of CloC12APG1.3at lower concentrations in the presence of NaCl (0.5%) and CaCl2 (0.07%).
polyglucosides is thus demonstrated, but it is still unclear whether alkyl polyglucosides themselves have a cloud point. Addition of gradually increasing amounts of C&12APG1.3 to the C12C14APG1.3 in Figure 2 results initially in the wide miscibility gap becoming narrower, and finally the coacervation region separates from the region below the Krafft point curve, and a lower critical point is seen (Figure 7). Thus, the relationshipsare analogous to those for fatty alcohol ethoxylates, but the structural rahge for the alkyl polyglucosidesis extremely narrow. In order to (24) Hughes, F.A.; Lew, B.W.JAOCS, J. Am. Oil Chem. SOC. 1970, 47, 162.
5
10
15
20
25
30
surfactant (96)
Figure 7. Phase diagrams (lower concentrations) of alkyl polyglucosides (n= 1.3)versus alkyl chain length. Critical point Pc of C11.&PG1.$ cC, 5 % ; Tc, 25 "C.
increase the cloud point from 25 to 90 "C, it is sufficient
to shorten .the alkyl chain from c11.5to C11.2, i.e., by 0.3 C
atom. Such a tiny change in a fatty alcohol ethoxylate of similar chain length would result in an increase of 2 "C (instead of 65 "C as in this case). Moreover, very careful investigation is necessary to find a cloud point with alkyl polyglucosides. This explainsearlier statements that alkyl polyglucosides have no cloud p ~ i n t s . ~ ~ . ~ ~ For a C11.dPG1.3, the lower consolute temperature Tc is 25 "C and the critical concentration ccis 5 % It is certain that the low-concentrationbranch of the consolute phase b o u n d m is far above the cmc. Compared with the fatty alcohol ethoxylates,the values of ccare somewhat higher.21
.
Langmuir, Vol. 9, No. 12,1993 3379
Cloud Point Phenomena 'C)
T ("C)
100 90
90
80
80
70
70 60 60 50
.........
50
Cl,C14(EO),H
,%
---
,% '
40
30
30
20
20
10
10 0 0
0 ' 0
1
I
I,
I
I
I
I
1
I
5
10
15
20
25
30
35
40
45
50
(%I Figure 9. Phase diagrams (lowerconcentrations)of C1&l&'Gl.e and C&la(EO),H. P,: c,, 5.5%;T,,52 and 54 "C, respectively. surfactant
5
10
15
20
25
30
CIS C14 APG (96)
Figure 8. Phase diagrams(lowerconcentrations)of C12C1APG versus degree of glucosidation.
The findings are very similar when the degree of glucosidation is increased instead of shortening the alkyl chain. This is shown for a C12C14 alkyl polyglucoside in Figure 8. In this case too, even a minute change in structure leads to an enormous increase in P,. A raise of only about 0.2 in the degree of glucosidation increases P, by about 85 "C. For C12C14 fatty alcohol ethoxylates an increase of about eight EO units would be necessary for a similar increase. Thus, alkyl polyglucosides certainly have lower consolute temperatures and cloud points. Moreover, they depend on the length of the alkyl chain and the degree of glucosidation in a way similar to that of the fatty alcohol ethoxylates. There is, however, extreme sensitivity to chemical structure, and this makes it difficult to find the region where there is a lower consolutetemperature. This behavior is very reminiscent of that of N-methyl fatty acid glucamides,defined compounds also with cumulative OH functionalities and which likewise show lower consolute temperatures with similarly great sensitivity to structure in aqueous solutionu.18 It is therefore assumed that this sensitivity to structure is based on the very different interactions between water and cumulative OH groups on the one hand and alkyl ether groups on the other hand. As Drummond et al.ll have put it in investigatingthe interfacialmicroenvironmentof micelles of anomeric alkyl saccharides,the region around the head groups is highly aqueous-like,leading to effectivedielectric constants of the micellar interface that are nearly twice as high as those typical to ethoxylates. The coacervation range of the alkyl polyglucoside (C12C1APG1.8) is quite different from that for a fatty alcohol ethoxylate with about the same P, and a similar dependence on the Krafft point (C1&4(E0)7H) (Figure 9). The very broad asymmetrical miscibility gap for the fatty alcohol ethoxylate contrasts with a narrow and approximately symmetrical miscibility gap for the alkyl polyglucoside. Whereas the cloud points of fatty alcohol ethoxylates are well known to have little dependence on concentration, this has a great effect on the alkyl poly-
glucoside. This is why the coacervation region and thus the cloud point were often overlooked. The critical concentrations are very similar for the two systems and are about 5.5 % . This means that the c, of the industrially manufactured fatty alcohol ethoxylate investigated here is distinctly higher than reported in the literature.21 The different coacervation properties of alkyl polyglycosides and of fatty alcohol ethoxylates will now be shown,taking these two surfactants by way of illustration. The unusual effect of electrolyteson the phase behavior of alkyl polyglucosides has been mentioned. The corresponding dependence of the cloud points of fatty alcohol ethoxylates on the type of electrolyteand its concentration has been known for a long time.25,26 The terms used in this context are salting-out and salting-in effects or lyotropic and hydrotropic electrolyte^,^^ with the specific interaction of EO chains with particular cations being included as a hydrotropic effect, by analogywith the crown ethers.28 Figure 10 summarizes the relationships for the example of C12C14(E0)7H in the presence of Na 1:l electrolytes. The results are substantially consistent with literature data?6 Whereas the anions SCN- and I- increase the cloud point, all the other anions investigated lead to a more or less distinct reduction. Noteworthy, although known for a long time, is the scale of concentration of the electrolyte. Only very high concentrationsmarkedly affect the cloud point of ethoxylates. The effects can be approximately understood on the basis of a balance of the water-ethoxylate interactions, i.e., on the one hand of a highly ordered hydration shell of the EO groups and on the other hand of the more or less strong polarization of the water by the ions, i.e., more or less pronounced water structure compared with bulk ~ a t e r . ~The g very large and weakly polarizing anions Iand SCN- presumably concentrate in the direct vicinity of the ethoxylate group (positive a d s ~ r p t i o n ) which ,~~ contributes to increasing the repulsive electrostatic in~~
(25) Schick, M. J. J. Colloid Sei. 1962, 17, 801. (26) Meguro, K.; et al. Nippon Kagaku Kaishi 1980,394. (27) Kahlweit, M.; Strey, R. Angew. Chem. 1985,97,655. (28) Schott, H.; Royce, A. E. J. Pharm. Sei. 1984, 73,793. Faraday Trans. 1 1984,80,2889. (29) Florin, E.; et al. J. Chem. SOC.,
Balzer
3300 Langmuir, Vol. 9, No. 12,1993
" 0
\
*OI 10
NaF ~
0.3
0.6
0.9
1.2
1.5
electrolyte (mol/l)
Figure 10. Effecta of electrolytes on the cloud point of C12C14-
(E017H ( 5 % ) . CP ("C) 100
90 80
70 60
54 40 30
20 10
NaCl
I ~
0 0
0.02
0.04
0.06
0.08
0.1
electrolyte (mol/l)
Figure 11. Effects of electrolytes (differentanions)on the cloud point of C12HlAPGl.e ( 5 % ).
teraction of the micelles and increases T,. With all the other electrolytes it is probable that the extensive hydration, because the ions are small and strongly polarizing, combined with the high concentration considerably disturbs the entropically unfavorable hydrate structure of the ethoxylate group. The result of this according to KjellanderB is a depletion of electrolyte in the vicinity of the ethoxylate group (negative adsorption) and an attractive interaction of the micelles; P, decreases. The situation with alkyl polyglucosidesis different. As shown in Figure 11 the same salts which display a very different interaction pattern with ethoxylates all lead to a marked depression of the cloud point. The range of
0'
,
I
I
I
1
I
I
0
4
8
12
I6
20
24
28
Surfactant (%I Figure 12. Coacervationranges of C12Cl&F'G1.8 in the presence of electrolytes NaCl, 0.01 mol/L, and NaSCN, 0.01 moVL.
electrolyte concentration is in this case orders of magnitude lower than for several ethoxylates. The usual lyotropic series approximately applies, but now the differences are scarcely perceptible, especially with N d @ , NaC1, and NaF, which normally show marked lyophobic differences. The alkyl polyglucosides differ from the fatty alcohol ethoxylates in that alkalis have a hydrotropic effect, and coacervation is greatly reduced, especially at higher concentrations. The effect of electrolyte on the entire coacervationregion of C12C&PG1.8 is shown in Figure 12 for two electrolytes (NaC1 and NaSCN, 0.01 mol each). The regions become distinctly broader and less symmetrical. The cloud point is very dependent on surfactant concentration. As the critical temperature decreases as a result of the electrolyte, the critical concentration also falls. Whereas there is relatively little difference in the effects of anions, the effects of various cations differ over a relatively wide range (Figure 13). Apart from the unusual effect of alkalis, which is presumably based on a specific interaction, the relationships with alkyl polyglucosides are the converse of those with fatty alkyl ethoxylates.2wo Otherwise, the lyotropic series for the effects of cations shown in Figure 13 is as has been known for a long time for ethoxylates: Li+ < K+< Na+. The surprisingly small effect of Li+, compared with the Hofmeister series, is explained by complexation analogous to crown ethers, which reduces the salting-out effect of the electrolyte.28 It is difficult to consider a similar explanation for the alkyl polyglucosides since the parallel effects occur a t very different electrolyte concentrations. In the presence of multiply charged cations there is a distinct reduction in the cloud point in the case of polyglucosides-in contrast to the behavior of the ethoxylates. As the charge increases there is a great increase in the lyotropic effect, but it is much less than expected with respect to the Schulze-Hardy rule. A plot of the cloud point against the molal strength30 results in about one curve. (30)Maclay, N.W.J. Colloid Sci. 1966, 11, 272.
Langmuir, Vol. 9,No. 12, 1993 3381
Cloud Point Phenomena CP ('C )
60
c
60
*'
,
0 NaCl
40
+c C,,-sulfate-Na 0 C,,-aulfate-Na
30
------ C,,C14(EO),H C12C14-APG1
8
50t
lot 0 0
0,02
0,04
0,06
0,08
091
electrolyte (mol/l)
Figure 13. Effects of electrolytes (different and multivalent cations) on the cloud point of C12ClcAPGl.e ( 5 % ) .
What is the explanation for this very unusual type of coacervation behavior of the alkyl polyglucosides compared with the ethoxylates? The close correlation between the cloud point and the chemical structure can only be due to a completely different type of considerably greater hydration of the cumulative OH groups compared with EO. The effect of electrolytes, which is greater by some orders of magnitude than with ethoxylates, suggests an electrostatic charge on the surface of the alkyl polyglucoside micelle, which is said to be absent in the e t h o x y l a t e ~ . ~ ~ * ~ ~ The situation is therefore similar to ethoxylates to which an ionic surfactant has been added.31 A further clue for an electrostatic charge is the observation mentioned in Table 11, where it is shown that the two cmcs of c12c14APG1.3 are reduced to one cmc in the presence of electrolyte. This fact might be attributedto the screening effect of the electrolyte no longer forming spherical micelles at cmcl. The fact that the interaction of alkyl polyglucosides with electrolyte (effect of cations greater than that of anions) is opposite that observed in ethoxylates, together with the great hydrotropic effect of alkalis (possible dissociation of one or several acidic groups or a specific OH- adsorption), suggests that the surface charge is negative. Information on this should be provided by the effect on the coacervation behavior of adding anionic and cationic surfactants, respectively. Adding Anionic Surfactants. The effect of C12 and C14 alkyl sulfates on the cloud points of alkyl polyglucoside and fatty alcohol ethoxylate is shown in Figure 14. The cloud points are increased to a greater or lesser extent by the addition of small amounts of alkyl sulfates, smaller than their cmcs. Whereas with ethoxylates there is a distinct effect of the alkyl chain length of the anionic surfactant-probably consistent with the difference in tendency toward incorporation in the surfactant micelle-there is no such difference observed with the alkyl polyglucoside, at least not in the range of alkyl chains shown here. It can be concluded from these findings that (32)Valaulikar, B. S.; Manohar, C. J. Colloidlnterface Sci. 1985,108, the alkyl polyglucoside micelles have a comparatively 403. (31)Marszall, L.Langmuir 1988,4, 90.
(33) Sadaghiania, A. S.; Khan,A. H.J . Colloid lnterfoce Sci. 1991, 144,191.
3382 Langmuir, Vol.9,No.12,1993
Baker
CP ( ' C )
90 80
\
0
C,,C,,(EO),H I
h
50
A
40
1 c
-15;
30
20 10 -
,
I
I
I
- 30 L
1
3
4
5
6
7
8
9
PH
Figure 17. {potentialof the toluene-in-wateremulsion versus pH (Zytopherometer,10% toluene, 87.5% water, 2.5% C12Hlr(EOhH or C1~C1flG1.3).
100
3
90
a0 70
0 0
cMCC,,l 0.002
0,004
0,006
1
o,ooa
O,OI
AlkylTMABr (mol/l)
Figure 16. Effects of cationic surfactants (alkyltrimethylammonium bromide) on the cloud points of C1zCll(EO),H and C1&APGl.a: cmc(C12),1.5 X 10-2 mol& cmc(Clr),3.8 X 1V mol/L; cmc(Cle),9.2 X lo-' mol/L, each at 25 O C . = on the data given above for C12C14(EO),H indicates that for C14(TMA)Br it suffices to incorporate one surfactant cation in the ethoxylate micelleto increase the cloud point from 52 toabout 100"C; this assumes that the total amount of ionic surfactant is taken up by the ethoxylate micelles. Since the curve for Cls(TMA)Br is only slightly steeper, whereas that for C12(TMA)Br is considerably less steep, this statement is very probably correct. The situation is entirely different for the alkyl polyglucosides. As shown in Figure 16, cationic surfactants have a lyotropic effect at low concentrations, pass through a broad minimum, and have strongly hydrotropic effects
at higher concentrations. The lyotropic effect seems to be similar to the influence of the salts, but it occurs at much lower concentrations (Figure 13). The gradient of the decrease in the cloud point increases with the chain length of the cationic surfactant, whereas the hydrotropic branch of the curve increases as the alkyl chain becomes shorter. The latter effect is thought to derive from formation of CU- or C14(TMA)Brmicelles in competition with incorporation into the alkyl polyglucoside micelles. The appearance of the hydrotropic branch for Cu(ThL4)Br is surprising because the very gradual increase in the cloud point beyond the minimum, approximately correspondingto that found in the case of uncharged ethoxylate, is followed by an unusually steep rise for (212, a finding which is not yet understood. The cloud point minimum is at approximately the same concentration for all cationic surfactants. It reflects the point of zero charge of the mixed micelle, which in this case is reached on uptake of about 14 molecules of cationic surfactant in the originally negativelycharged alkyl polyglucoside micelle (aggregation number 350). tPotentia1 Meaurements. It is, of course, interesting to study the electrical effects themselves. This was done by classical visual electrophoresis of a toluene-in-water emulsion stabilized with alkyl polyglucoside,for comparison with fatty alcohol ethoxylate (measurements at the Physical Chemistry Institute, University of Bayreuth, Germany). This is based on the assumption that the surfactant films in the oil/water boundary layer of the emulsion drop will behave similarly to the micelles in aqueous solution. The results in Figure 17 show that the electrical charge on the drops stabilized by fatty alcohol ethoxylate is negligibly small, and is probably slightly positive, in the pH range between 3 and 8. Only in the alkaline pH range do the films carry a negative charge. An isoelectric point is at about pH 8. By contrast, alkyl polyglucoside films show throughout the pH range investigated a distinctly negative {potential which increases about linearly with increasing pH. Another measurement technique (Malvern Zetasizer 3) can be used to examine the smaller colloids represented
Langmuir, Vol. 9, No. 12, 1993 3383
Cloud Point Phenomena
c
5i I
-20
\
I 3
4
5
6
7
8
9
PH
Figure 18. {potential of surfactantmicellesversuspH (Zetasizer 3, 0.5% C ~ ~ C I A Por G C12Hl,(EO)sH, ~.~ 1 X 10-9 mol/L KC1). by the larger micelles (meausurements at the Polymer Research Institute, Dresden, Germany). The technique operates by dynamic light scattering using a crossed laser beam which sites the volume to be examined in the stationary plane of the electrophoresis cell. According to the manufacturer, the method detects particles over 5 nm in size and should detect at least the alkyl polyglucoside micelles.' Since this is uncertain for fatty alcohol ethoxylates, the investigations were carried out in both cases in the state of coacervation,which should provide sufficiently large micelles for the ethoxylate C&14(E0)5 in analogy to C12(EO)eH.34 Figure 18 shows the results for the same surfactants used to stabilize the toluene-in-water emulsion in Figure 17. Remarkably, the findings for both are similar to those for the corresponding emulsion films. Whereas the fatty alcohol ethoxylate micelle has no or only a very low charge,the tpotential of the alkyl polyglucoside micelle is distinctly negative, and the effect increases with increasing pH. The drastic hydrotropic effect of alkalis on alkyl polyglucosides is thus understandable. The same is true of the lyotropic effect of common electrolytes being added even in small amounts, which is reflected by a pronounced decrease of the potential of alkyl polyglucoside. The repulsive interaction between the micelles is screened correspondingly so that the attractive forces become more important and the cloud point decreases. The electrolyte sensitivity of ethoxylates by contrast is very small because the electrical potential is virtually negligible. Both effects are important with respect to various modes of application. This is also true of the interaction of alkyl polyglucosides with ionic surfactants. Addition of anionic or cationic surfactants changes the t potential of the alkyl polyglucoside micelles in the expected way (Figure 19). Whereas the alkyl sulfate (SDS) increases the negative potential, the cationic surfactant results in a change of polarity. The point of zero charge at 3 X 10-4 mol/L (DTMA)Br corresponds to about 10 molecules per alkyl polyglucoside micelle. This figure (34) Tanford, C.;et al. J . Phys. Chem. 1977,81, 1555.
.IO
-5
10
-3
surfactant (mol/l) Figure 19. Effect of SDS and (DTMA)Bron the 5 potential of C12C1APG1.3 micelles (Zetasizer 3, 0.5% C1dhAPGl.s, 1 X 1W mol/L KC1). agrees well with the results of cloud point measurements (Figure 16)which were, however, carried out on asomewhat more hydrophilic alkyl polyglucoside (C12C1APGl.e). On the other hand, measurements of the cloud point on C&2APG1.3, i.e., a very much more hydrophilic product, yielded a figure of 9. The agreement between the cloud point and { potential measurements is therefore very good. Thus, whereas many details of the coacervationbehavior of alkyl polyglucosides relative to that of ethoxylates are qualitatively understood, the reason for the negative charge is still uncertain. There is a much better chance that anionic constituents arise not in alkyl polyglucoside synthesis itself but in the subsequent bleaching with H202. Similar to the reactions5 of carbohydrates with HzOz, the free CHzOH group of the glucopyranose could be oxidized to an alkylglucuronicacid. But this would be on no account a single anionic compound but a vast number of isomers which are present in traces and very difficult to identify. Thus, by using 13C NMR techniques for COOH groups, we got the superficial impression that we had detected such compounds, but we are not certain.
Conclusions Aqueous solutions of alkyl polyglucosides show at low concentration lower consolute temperatures which depend much more on the structure than for fatty alcohol ethoxylates. These differences presumably derive from the completely different types of interaction of the head groups with the solvent. The effect of electrolytes on coacervation is several orders of magnitude greater for alkyl polyglucosidesthan for ethoxylates. Salts generally result in a reduction in the cloud point, the effect of (35) Green,J.W. In The Carbohydrates, Chemistryand Biochemistry, 2nd ed.;Pigman, W., Horton, D., Eds.; Academic Press: New York, 1980, Vol. IB, p 1131. Moody, G. J. In Advances in Carbohydrate Chemistry and Biochemistry; Wolfrom, M. L., Ed.; Academic Press: New York, 1964; Vol. 19, p 149. (36) Bayer, 0.; et al. In Surfactants in Solution; Mittal,K. L., Bothorel, P., Eds.; Plenum Press: New York, 1987; Vol. 4, p 343.
3384 Langmuir, Vol. 9, No. 12, 1993
different cations being greater than that of anions. Hydrotropic effects are observed only with alkalis. The fact that the effects differ distinctly from the behavior of ethoxylates suggests that the alkyl polyglucoside micelle has a negative charge. This can be demonstrated both by interactions with anionic and cationic surfactants and by { potential measurements. Both investigations on emulsion droplets stabilized with alkyl polyglucosides and measurements of the coacervate indicate that the alkyl polyglucoside micelle has a distinct negative charge in the pH range between 3 and 9. The potential becomes increasingly negative as the pH increases. By contrast, fatty alcohol ethoxylates have only a very low, probably positive, charge or are uncharged.
Baker Both the effect of electrolytes observed in measurements of the cloud point and the effect of ionic surfactants can be followed by measurements of the {potential. The reason for the negative charge on the micelles is as yet unexplained.
Acknowledgment. Experimental help with cytopherometer investigations, light scattering, and conductivity measurements under flow by H. Renner, Physical Chemistry Institute, University of Bayreuth, and with Zetasizer measurements by Dr. I. Grosse, Polymer Research Institute, Dresden, is gratefully acknowledged. Thanks are due to Prof. Dr. H. Hoffmann, University of Bayreuth, for helpful discussions and for critical reading of the paper.
