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How 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 Ben J. Boyd,† Calum J. Drummond,* Irena Krodkiewska, and Franz Grieser CSIRO Molecular Science, Private Bag 10, Clayton Sth MDC, 3169, Australia, and Department of Chemistry, The University of Melbourne, Parkville, 3052, Australia Received December 1, 1999. In Final Form: June 2, 2000 A matrix of anomerically pure glucose-based surfactants have been synthesized and their thermotropic and lyotropic liquid crystalline phase behavior, and air-aqueous solution interfacial adsorption were investigated. The surfactants, which represent the major components of the Fischer synthesis products, were the n-octyl, n-decyl and n-dodecyl homologues of alkyl R-D- and β-D-glucoside and alkyl β-D-maltoside. The matrix allowed the investigation of the effects of alkyl chain length, headgroup polymerization, and anomeric configuration on the surfactants’ physicochemical properties. Increasing the alkyl chain length increases the hydrophobicity and the dispersion interaction between surfactant molecules, as one would expect, resulting in greater thermal stability of thermotropic and lyotropic phases. Phase transition temperatures are influenced significantly by the anomeric configuration in the shorter octyl derivatives, but to a lesser extent in the longer alkyl chain derivatives. The effect of increasing the degree of headgroup polymerization from one to two glucose units is to greatly increase the solubility of the surfactant in water and to increase the stability of the thermotropic liquid crystalline state. Changes in the headgroup polymerization and anomeric configuration have very little influence on the air-solution interfacial adsorption of these surfactants, while the effect of alkyl chain length variations was consistent with that expected from a thermodynamic consideration of surfactant self-assembly.
Introduction Surfactants prepared from renewable resources, in particular sugar-based surfactants, can have very desirable environmental, biodegradation, toxicity and dermatological properties. Several reviews1-8 and a book9 have been published on the preparation, properties, and applications of industrially produced glucose-derived surfactants, alkylpolyglucosides (APGs). APGs are complex surfactant mixtures produced by a process known as the Fischer synthesis.10 Typically, the most abundant components in the complex APG mixtures are alkyl monoglucosides and alkyl di-glucosides (maltosides), in both their R- and β-anomeric forms.11-33 * To whom correspondence should be addressed. E-mail: calum.
[email protected]. † Reckitt and Colman Scholar. (1) Schulz, P. Chimicaoggi 1992, August, 38. (2) Siracusa, P. happi 1992, April, 100. (3) Salka, B. Cosmet. Toiletries 1993, 108, 89. (4) Brancq, B. 3rd CESIO Int. Surf. Congr., London, 1992. (5) Balzer, D. Tenside, Surfactants, Deterg. 1991, 28, 6. (6) Platz, G.; Po¨licke, J.; Thunig, C.; Hofmann, R.; Nickel, D.; von Rybinski, W. Langmuir 1995, 11, 4250. (7) Balzer, D. Langmuir 1993, 9, 3375 (8) von Rybinski, W.; Hill, K. Angew. Chem., Int. Ed. Engl. 1998, 37, 1328. (9) Alkyl PolyglycosidessTechnology, Properties and Applications; Hill, K.; von Rybinski, W.; Stoll, G., Eds.; VCH: Weinheim, 1997. (10) Fischer, E. Berichte 1893, 26, 2400. (11) From Waldhoff, H.; Scherler, J.; Schmitt, M.; Varvil, J. in Alkyl PolyglycosidessTechnology, Properties and Applications; Hill, K.; von Rybinski, W.; Stoll, G., Eds.; VCH: Weinheim, 1997. (12) Bury, C.; Browning, J. Faraday Trans. 1953, 49, 209. (13) Shinoda, K.; Yamanaka, T.; Kinoshita, K. J. Phys. Chem. 1959, 63, 648. (14) Zhang, L.; Somasundaran, P.; Maltesh, C. Langmuir 1996, 12, 2371.
Early studies into the thermotropic behavior of glucosebased surfactants established the existence of the so-called “double melting point”.34 The first transition, at temperature Tlc, is a transition from the solid crystalline state into a smectic liquid crystal, and the higher temperature transition, at Tiso, is from the liquid crystal to an isotropic liquid melt. The presence of the thermotropic liquid crystalline phase has been attributed to the hydrogen bonding ability of the glucose headgroups.35 (15) Brown, G.; Dubreuil, P.; Ichhaporia, F.; Desnoyers, J. Can. J. Chem. 1970, 48, 2525. (16) de Grip, W.; Bovee-Guerts, P. Chem. Phys. Lipids 1979, 23, 321. (17) Rosevear, P.; VanAken, T.; Baxter, J.; Ferguson-Miller, S. Biochemistry 1980, 19, 4108. (18) Kameyama, K.; Takagi, T. J. Colloid Interface Sci. 1990, 137, 1. (19) LaMesa, C.; Bonincontro, A.; Sesta, B. Colloid Polym. Sci. 1994, 272, 704. (20) Von La¨sser, H.; Elias, H. Kolloid-Z. u. Z. Polymere 1972, 250, 58. (21) LaMesa, C.; Bonincontro, A.; Sesta, B. Colloid Polym. Sci. 1993, 271, 1165; Roxby, R.; Mills, B. J. Phys. Chem. 1990, 94, 456; Michels, M.; Zana, R., Frindi, M. J. Phys. Chem. 1992, 96, 8137; Itoh, H.; Ishido, S.; Nomura, M.; Hayakawa, T.; Mitaku, S. J. Phys. Chem. 1996, 100, 9047; Von Watterson, J.; Von La¨sser, H.; Elias, H. Kolloid-Z. u. Z. Polymere 1972, 250, 64. (22) Thiyagarajan, P.; Tiede, D. M. J. Phys. Chem. 1994, 98, 10343. (23) Nilsson, F.; Soderman, O.; Johansson, I. Langmuir 1996, 12, 902. (24) VanAken, T.; Foxall-VanAken, S.; Castleman, S.; FergusonMiller, S. Methods Enzymol. 1986, 125, 27. (25) Alpes, H.; Allmann, K.; Plattner, H.; Reichert, J.; Riek, R.; Schulz, S. Biochim. Biophys. Acta 1986, 862, 294. (26) Drummond, C. J.; Warr, G. G.; Grieser, F.; Ninham, B. W.; Evans, D. F. J. Phys. Chem. 1985, 89, 2103. (27) Warr, G. G.; Drummond, C. J.; Grieser, F.; Ninham, B. W.; Evans, D. F. J. Phys. Chem. 1986, 90, 4581. (28) Kano, K.; Ishimura, T. J. Chem. Soc., Perkin Trans. 2 1995, 1655.
10.1021/la991573w CCC: $19.00 © 2000 American Chemical Society Published on Web 08/22/2000
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Figure 1. Structures and abbreviations for the surfactants used in this study.
The lyotropic phase behavior of surfactants is of great importance to the practical use of surfactants in consumer products.36 However, the lyotropic phase behavior of only four of the major components of the Fischer mixture have been reported in detail. The binary surfactant-water phase diagrams are known for the R- and β-anomers of octyl D-glucoside,23,37,38 decyl β-D-glucoside39, and dodecyl β-D-maltoside,27,40 and display a rich array of liquid crystalline phase geometries, which can include hexagonal, cubic, and lamellar phases. The interfacial adsorption of pure alkyl glucoside and maltoside surfactants has been studied more extensively,12-33,41 especially for the more soluble surfactants. However there remains much disparity in the data, possibly due to inconsistencies in anomeric purity, or the complicating effects of phase separation observed with some members of this surfactant type. In this study, a matrix of nine anomerically pure, glucose-derived surfactants have been prepared, to investigate the structure-property relationships for glucosebased surfactants. Specifically, the octyl, decyl, and dodecyl homologues, each with the R-D-glucoside, β-D-glucoside, or the β-D-maltoside headgroup configuration, have been prepared (Figure 1). The thermotropic and lyotropic phase behavior of each surfactant has been investigated by differential scanning calorimetry (DSC) and polarized light optical microscopy. The air/aqueous solution adsorption has been investigated for the matrix of surfactants by surface tension determinations. Experimental Section Materials and Synthetic Detail. Two of the surfactants used in this study, octyl R-D-glucopyranoside (R-C8G1) and octyl β-D-glucopyranoside (β-C8G1), were sourced from a commercial supplier. The remaining surfactants were prepared in-house. (29) Dupuy, C.; Auvray, X.; Petipas, C.; Rico-Lattes, I.; Lattes, A. Langmuir 1997, 13, 3965. (30) Koeltzow, D. E.; Urfer, A. D. J. Am. Oil Chem. Soc. 1984, 61, 1651. (31) Thiem, J.; Boecker, T. Ind. Appl. Surfactants III 1992, 123; Boecker, T.; Thiem, J. Tenside, Surfactants Deterg. 1989, 26, 5. (32) Matsumura, S.; Imai, K.; Yoshikawa, S.; Kawada, K.; Uchibori, T. J. Am. Oil Chem. Soc. 1990, 67, 996. (33) Lorber, B.; Bishop, J.; DeLucas, C. Biochim. Biophys. Acta 1990, 1023, 254. (34) Noller, C.; Rockwell, W. J. Am. Chem. Soc. 1938, 60, 2077. (35) Jeffrey, G.; Bhattacharjee, S. Carbohydr. Res. 1983, 115, 53. (36) Laughlin, R. Aqueous Phase Behaviour of Surfactants; Academic Press: London, 1994. (37) Sakya, P.; Seddon, J.; Templer, R. J. Phys II France 1994, 4, 1311. (38) Nilsson, F.; Soderman, O.; Johansson, I. J. Colloid Interface Sci. 1998, 203, 131. (39) Shinoda, K.; Carlsson, A.; Lindman, B. Adv. Colloid Interface Sci. 1996, 64, 253. (40) Auvray, X.; Petipas, C.; Anthore, R.; Rico-Lattes, I.; Lattes, A. Langmuir 1995, 11, 433. (41) Shinoda, K.; Yamaguchi, T.; Hori, R. Bull. Chem. Soc. Jpn. 1961, 34, 237.
Boyd et al. Three different techniques were employed to determine the purity of the surfactants used in this study, viz. thin-layer chromatography (TLC), nuclear magnetic resonance spectroscopy (NMR), and optical rotation. Refer to section 1 in the Supporting Information for more detail on the methods used for purity determination, and section 2 in the Supporting Information for more detail on the synthetic procedures. Commercially Sourced Materials. Two surfactants were purchased from a commercial source for this study. Octyl R-Dglucopyranoside (R-C8G1) and octyl β-D-glucopyranoside (β-C8G1) were purchased from Calbiochem-Novabiochem GmbH, at nominal purity levels of 99%. The opposite anomers were not detected in the 500 MHz 1H NMR spectrum of either surfactant. β-C8G1. 1H NMR 500 MHz (CD3OD): H-1, δ ) 4.40 (d, 1H); H-2,3,4,5,6,6′, O-CH2, δ ) 3.99-3.40 (8H); CH2, δ ) 1.88-1.73 (m, 2H); -CH2-, δ ) 1.59-1.41 (10H); -CH3, δ ) 1.08 (t, 3H). TLC: (CH3OH:CHCl3, 4:1) Rf ) 0.30, [R]D20 ) -35.0 (c ) 1, MeOH). R-C8G1. 1H NMR 500 MHz (CD3OD): H-1, δ ) 4.92 (d, 1H); H-2,3,4,5,6,6′, O-CH2, δ ) 4.10-3.30 (8H); CH2, δ ) 1.82-1.75 (m, 2H); -CH2-, δ ) 1.51-1.41 (10H); -CH3, δ ) 1.08 (t, 3H). TLC: (CH3OH:CHCl3, 4:1) Rf ) 0.30, [R]D20 ) +120.6 (c ) 1, MeOH). Alkyl Monoglucosides. Decyl β-D-glucopyranoside (β-C10G1) and dodecyl β-D-glucopyranoside (β-C12G1) were both prepared using a multistep, Koenigs-Knorr type synthesis adapted from syntheses previously described by Van Aken et al.24 The final product was lyophilized to give a white powder. No R-anomers were detected in the 500 MHz 1H NMR spectrum of either surfactant. β-C10G1. Yield: 52% from starting glucose. 1H NMR 500 MHz (CD3OD): H-1, δ ) 4.40 (d, 1H); H-2,3,4,5,6,6′, O-CH2, δ ) 3.993.40 (8H); CH2, δ ) 1.88-1.73 (m, 2H); -CH2-, δ ) 1.59-1.41 (14H); -CH3, δ ) 1.08 (t, 3H). TLC: (CH3OH:CHCl3, 4:1) Rf ) 0.50, [R]D20 ) -29.3 (c ) 1, MeOH). β-C12G1. Yield: 58% from starting glucose. 500 MHz 1H NMR (CD3OD): H-1, δ ) 4.92 (d, 1H); H-2,3,4,5,6,6′, O-CH2, δ ) 4.103.30 (8H); CH2, δ ) 1.82-1.75 (m, 2H); -CH2-, δ ) 1.51-1.41 (18H); -CH3, δ ) 1.08 (t, 3H). TLC: (CH3OH:CHCl3, 4:1) Rf ) 0.48, [R]D20 ) -26.8 (c ) 1, MeOH). The two linear alkyl R-D-glucosides, R-C10G1, and R-C12G1, were synthesized, using a modified Fischer synthesis described by Focher et al.42 The R-anomers contained appreciable amounts of the β-anomer, which was subsequently removed by chromatography on Dowex resin as described by Rosevear et al.17 Analysis by 500 MHz 1H NMR revealed that R-C10G1 and R-C12G1 contained 100 °Cs approximately the same composition as that described in this work. However they also report the presence of a cubic QR phase not observed in this work, possibly due to the much longer equilibration times used in this work if the QR phase was meta-stable. In these studies, it was not possible to optically determine the boundary between the anisotropic hexagonal and lamellar phases, and even though a water penetration scan showed the possibility (65) Drummond, C. J.; Wells, D. Colloids Surfaces 1998, 141, 131; van Doren, H.; Wingert, L. Recl. Trav. Chim. Pays-Bas 1994, 113, 260; Herrington, T.; Sahi, S. J. Am. Oil. Chem. Soc. 1988, 65, 1677. (66) Soderberg, I.; Drummond, C. J.; Furlong, D. N.; Godkin, S.; Matthews, B. Colloids and Surfaces 1995, 102, 91.
of a hexagonal phase, the texture was somewhat different to the hexagonal phase displayed by β-C8G1.67 Effect of Glucose Headgroup Polymerization. The nature of the headgroup has a dramatic effect on the lyotropic liquid crystalline phases of glucose-based surfactants. By directly comparing the approximate partial binary surfactant-water phase diagrams of the alkyl β-D-glucoside and β-D-maltoside surfactants in Figure 6, it is apparent that by increasing the number of glucose units in the headgroup to two, the solubility of the surfactant is substantially increased. Unlike the glucosides, there is no phase separation (L2 + L1) observed for the equivalent chain length maltosides. Balzer has studied the effect of headgroup polymerization on the shape of the two-phase L2 + L1 region in commercial polydisperse APG systems.7 By preparing a series of C12/C14 APGs with a number of different average degree of headgroup polymerization between one and two, the dependence of the lower consulate boundary was seen to increase in temperature from around 15 to >100 °C. This behavior is also seen in the present study, where the C12 DP1 compound, β-C12G1, displays its lower consolute boundary at 36 °C, while that of the C12 DP2 version, β-C12G2, is >100 °C. Thus, for all monodisperse glucosederived surfactants the lower consolute boundary will be either at the Krafft boundary or >100 °C, while an intermediate lower consolute boundary can only be (67) See Figure S4 in the Supporting Information for the penetration scan of β-C12G2.
Behavior and Adsorption of Glucose-Based Surfactants
Figure 7. Lyotropic transition temperature for hydrated crystals + water to either L1 or L1 + L2, Tlyo, for the alkyl D-glucosides at 1 wt %. Squares correspond to R-anomers and circles correspond to β-anomers.
attained with a delicate balance of headgroup distribution for a set alkyl chain length. Effect of Alkyl Chain Length. Comparison of the phase diagrams progressively down each column for each of the three headgroup configurations in Figure 6 allows the trends with increasing alkyl chain length to be observed. For the n-alkyl R-D-glucosides, there are three main features to note, as the chain length is increased down the column. First, the crystals + water f L1 or L1 + L2 boundary (Tlyo) rises in temperature as the chain length is increased. This is primarily the result of increased dispersion energy in the hydrated crystals due to the extra chain interaction. The general increases in Tlyo for both the R- and β-glucosides at 1 wt % are illustrated in Figure 7. Second, the miscibility gap widens as the chain length becomes longer. This may be due to the more hydrophobic nature of the longer chained surfactant rendering the L2 phase less tolerant to excess solution, resulting in a narrower range of miscibility. Alternatively, the argument of Nilsson et al.60 that the miscibility gap represents the phase separation of a bicontinuous micellar network from low surfactant content solution, is also supported by these data. The shape of the longer chained glucosides is more cylindrical, and a more planar association of molecules would be favored (i.e., the spontaneous curvature of the surfactant film toward oil would be lower). As the formation of curved interfaces would be even less favorable for the C12 surfactant than the C10 surfactant, the miscibility gap would be expected to exist over a wider composition range. While the data here tend to support the proposed mechanism of phase separation of Nilsson et al.,60 the phenomenon is far from understood, and hence, the description of an L2 phase in this work is a simplified, yet adequate alternative in the absence of further evidence on this matter. Balzer7 also found that an increase in the alkyl chain length had a profound effect on the shape of the lower consolute boundary, giving rise to the phase separation of commercial polydisperse APG mixtures. An increase in average chain length of only 0.3 carbons, induces a change of 65 °C in the lower consolute temperature, around 30 times as sensitive as fatty alcohol ethoxylates. The third point of note is the trend in the composition at which the lamellar phase appears as the chain length is sequentially increased. As the alkyl chain length is increased a more planar assembly of the molecules will be favored, and this is reflected in the sequentially decreasing surfactant content required to induce the
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lamellar phase. This trend is apparent for each of the three different headgroup types. In the case of the three maltoside surfactants, Tlyo is below 0 °C, and the high solubility dictates that no phase separation at low concentrations occurs. The onset of the liquid crystalline phases does follow the same pattern, however, occurring at lower concentrations for the increased chain length surfactants. The phase behavior is almost identical in each case (in the absence of the H1 f LR boundary which could not be determined reliably by the methods used in this study). Effect of Anomeric Configuration. By comparing the first two columns of phase diagrams in Figure 6, the effect of anomeric configuration on the lyotropic liquid crystalline phases can be investigated. For the n-octyl derivatives of the n-alkyl D-glucosides, the phase behavior is quite different. First, the Krafft temperature is much higher for the R-anomer, around 42 °C, compared with that of the β-anomer at less than 0 °C. The decyl and dodecyl glucosides also display higher Tlyo values for the R-anomers. An interesting observation when comparing the Krafft temperatures of the two anomers with increasing chain length in Figure 7, is that the difference between the two anomers is less at longer chain lengths, i.e., the effect of the increased hydrocarbon chain length “washes out” the difference between the R- and β-anomers. Presumably, this is due to the dispersion interaction between the chains dominating over the influence of the headgroup orientation. Second, the β-anomer forms a bicontinuous cubic phase, while the R-anomer does not. This is probably due to the slightly more hydrophilic nature of the β-anomer a point noted by early workers on alkyl glucoside adsorption behavior.15 The β-anomer must have a slightly higher curvature toward oil, which favors the formation of the cubic phase rather than a progression straight from hexagonal to lamellar phase. The thermal stability of the hexagonal phase of the β-anomer is much lower than for the R-anomer, indicating a stronger interaction of the R-anomer micelles in the hexagonal array. Unlike the trend found for increasing alkyl chain length, the anomeric configuration does not substantially affect the composition at which the lamellar phase formation occurs. As mentioned above, with increase in chain length, Tlyo for the two anomers starts to converge, washing out the effect of the anomeric center. The effect on other features of the phase diagrams, such as the miscibility gap at low surfactant concentration and differences in formation of hexagonal and cubic phases, are also washed out with increased chain length. The phase diagrams progress from being quite different for the two octyl anomers, to being almost identical for the dodecyl anomers. This washing out effect is of importance to formulators concerned with the effects of having the two different anomers in a surfactant formulation. At short chain lengths the phase behavior is affected greatly by anomeric content, while at longer chain lengths the anomeric effects are of little or no consequence. Air-Aqueous Solution Interfacial Adsorption. The interfacial adsorption behavior of a number of the surfactants from the matrix has been studied at 25 and 50 °C. Some surfactants were not investigated, as they had either Krafft temperatures that were too high, or they formed a two-phase system at the experimental conditions. Much of the existing literature quotes CMC values where a dispersed L2 phase, rather than micelles exist above the critical concentration where phase separation occurs.
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Table 2. Adsorption of Alkyl Glucosides and Maltosides at the Air-Aqueous Solution Interface γCAC γmin ∆G°agg temp CAC (mN (mN Ao (Å2)a Ao (Å2)a surfactant (°C) (mM) m-1) m-1) linear fit quadratic (kJ mol-1) R-C8G1b R-C8G1 R-C10G1c β-C8G1 β-C8G1 β-C10G1c β-C10G1c β-C8G2 β-C10G2 β-C12G2
25 50 50 25 50 25 50 25 25 25
12.0 17.0 1.60 18.2 11.2 1.96 1.61 19.1 1.99 0.13
35.2 28.0 27.4 31.7 28.2 28.3 27.7 33.4 36.4 35.7
35.2 26.7 27.4 31.3 26.1 27.8 27.1 33.4 36.3 34.4
42 52 44 42 44 42 48 42 44 45
42 49 40 38 42 38 44 38 44 43
-21.0b -21.7 -28.1 -19.9 -22.9 -25.4 -28.1 -19.8 -25.3 -32.1
a The minimum headgroup area per molecule at the interface, Ao, has an error of (5 Å, due to the subjectivity in determining the pre-CAC slope. b At 25 °C, a solubility limit was reached at the CAC value, where crystals existed in the solution. c Data in italics indicates that the surfactant phase separates above the CAC.
Others quote adsorption data where the system is actually below the Krafft temperature of the surfactant without stating that this is the case, making it difficult to reconcile the actual meaning of the data presented. A perfect example illustrating both of these points is the much cited study of Shinoda et al.,41 in which two of the surfactants studied for their surface tension behavior undergo a phase separation above the CMC, and one is studied under conditions well below its known Krafft temperature. For the cases where adsorption isotherms are determined, to avoid the confusion between whether a CMC or a solubility limit exists, an inflection in the adsorption isotherm will be called the critical aggregation concentration (CAC). The occurrence of the phase separation has the unfortunate consequence of limiting the observation of trends over a series of homologous surfactants, nevertheless some trends in the data are apparent. The adsorption isotherms for the surfactants at the airsolution interface are presented in section S5 of the Supporting Information, Figures S5, S6, and S7. The surfactant CACs, headgroup areas (Ao) and free energies of aggregation (∆G°agg) for the surfactants studied are tabulated in Table 2. Note that the data in italics indicates that the surfactant phase separates above the CAC. Note also that the C12 glucosides have not been studied due to their extremely low solubilities and high Krafft temperatures, and the formation of an L2 phase, which introduced impracticalities into the direct measurement of their adsorption behavior at very low concentrations. Some data have been presented for the β-C12G1 in the literature, however there is much disparity between these data, probably due to these complicating effects.31,41 Data for the adsorption at the air-aqueous solution interface for the surfactants that form micellar solutions above the CAC, agree well with the literature.12,14-16,18,24,25,29,31-33,47,68,69 Data for those surfactants which phase separate above the CAC, or whose literature values are obtained at temperatures below their Krafft temperatures, agree less well, but are of the same order of magnitude in CAC, and the headgroup areas agree within the experimental error.31,32,69 This is almost certainly due to the complicating effects aforementioned. Effect of Glucose Headgroup Polymerization. The effect of headgroup polymerization on the interfacial adsorption of monodisperse alkyl glucoside surfactants has been investigated by previous authors.31 They found that the (68) Landauer, P.; Ruess, K.; Lieflander, M. Biochem. Biophys. Res. Commun. 1982, 106, 848. (69) Kutschmann, E.; Findenegg, G.; Nickel, D.; von Rybinski, W. Colloid Polym. Sci. 1995, 273, 3, 565.
degree of polymerization of the headgroup has a very small effect on the adsorption behavior. The CAC values are similar regardless of the headgroup at the different chain lengths. As a consequence, the free energies of aggregation, ∆G°agg, are very similar, indicating similar hydration effects for the two sets of surfactants. This effect has been noted previously, and is one important structural trend for these compounds. Drummond et al.26 noted that the headgroups are highly aqueous-like, and that the hydrated headgroups behave somewhat like bulk water. This is also reflected in the values for the minimum headgroup areas, where the occupied area at the interface is approximately 45 ( 5 Å2 in all cases in Table 2, despite the much larger size of the maltoside headgroup. Effect of Alkyl Chain Length. The length of the alkyl chain in glucose-based surfactants has a large effect on the adsorption from solution at interfaces. It also has a large effect on the aggregation energetics of these surfactants. The effects of alkyl chain length on adsorption properties of a series of surfactants with fixed headgroup are well studied for many surfactant systems.70 It is found that the hydrophobic driving force for interfacial adsorption and self-assembly of surfactants increases as the chain length is increased, as there are more hydrocarbon units requiring minimal contact with the aqueous solution. This manifests itself as a lower CAC and greater free energy of aggregation gain (i.e., more negative) on self-assembly into aggregates. It is also expected to promote the phase separation phenomenon displayed by some of these glucose-based surfactants. A linear relationship between the free energy of aggregation and the alkyl chain length for a homologous series has been given by Klevens,71 which holds well for these surfactants regardless of headgroup, at least for the limited data set available. For the alkyl maltoside surfactant the change per methylene group increment is around 3.1 ( 0.3 kJ mol-1, which is typical for other surfactants of similar type.66,72 The headgroup areas per molecule are relatively invariant with the change in alkyl chain length. However, for the glucosides, the decyl derivatives show a slightly lower minimum surface tension than the octyl counterparts. In the case of the n-alkyl β-D-maltosides, there appears to be no trends in the surface tension or headgroup data, possibly due to the high solubility of all three members of the series. Effect of Anomeric Configuration. The anomeric configuration has little bearing on the interfacial adsorption behavior of alkyl D-glucosides. The literature contains a number of reports in which the R-anomers are seen to have slightly lower CACs than the β-anomers, which suggests that they are slightly more hydrophobic in nature.32,42,47,69 This observation has been attributed to the fact that in the structure of the β-anomer, the different orientation of the headgroup results in the primary alcohol in the C6 position being bent over to slightly shield the first methylene group in the alkyl chain. This results in a slightly enhanced hydrophilicity, which gives rise to the greater CAC.15 The data obtained in this study do not necessarily show this trend, as the CAC for the β-anomer from surface tension measurements is actually lower than that of the R-anomer. However, deriving CACs from surface tension data has inherent subjectivity in determining the exact concentration of break points, and since (70) Rosen, M. Surfactants and Interfacial Phenomena; John Wiley & Sons: New York, 1978; Chapter 3, and references therein. (71) Klevens, H. J. Am. Oil Chem. Soc. 1953, 30, 7, 4. (72) Becher, P. in Nonionic Surfactants; Schick, M., Ed.; Marcel Decker: New York, 1967; Chapter 15.
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the difference between anomers is so small, it is probably not within the resolution of CAC determination in this case. The slope of the adsorption isotherms were within experimental error of each other, so that no difference in headgroup areas per molecule could be distinguished between the anomers. The β-anomers are seen to decrease the interfacial tension fractionally more than the R-anomers, but the difference again is slight. Summary and Conclusions It can be said, in summary, that systematically varying the structure of linear alkyl glucoside and alkyl maltoside surfactants influences the physicochemical properties of the surfactants in many different ways. From these studies a number of general statements about the trends in behavior of glucose-based surfactants can be made. First, changing the degree of polymerization in the headgroup substantially increases the surfactant solubility. In contrast to that of the mono-glucosides, the phase behavior of the di-glucosides is dominated by large micellar regions, irrespective of alkyl chain length. The thermotropic transition temperatures are substantially increased at higher degrees of polymerization; believed to be due to increased hydrogen bonding between headgroups in the solid crystal and liquid crystalline forms. Second, increasing the alkyl chain length has a large effect on the lyotropic liquid crystalline phase behavior due to the increased hydrophobicity of the surfactant, causing phase separation at low surfactant concentrations, and lower composition formation of lamellar phases at high surfactant concentrations. Both the thermotropic and lyotropic transition temperatures increase with the alkyl chain length, due to the increased dispersion interaction between molecules in the solid crystalline, hydrated crystalline, and smectic liquid crystalline states. The alkyl chain length affects the interfacial adsorption by increasing the hydrophobic driving force for self-assembly and
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adsorption at the air-aqueous solution interface. This results in decreasing CACs with increased chain length, as is the behavior for other surfactants. Third, the anomeric configuration has a large impact on the lyotropic phases and lyotropic transition temperatures at short chain lengths, but this effect is washed out at longer chain lengths. The washing out effect at longer chain lengths is also observed in the thermotropic behavior of alkyl D-glucosides, where the transition temperatures and transition enthalpies also converge as the alkyl chain length is increased. In contrast, the anomeric configuration has very little effect on the interfacial adsorption properties; a small effect has been seen by other authors on the hydrophobicity of the glucoside surfactants, resulting in a slightly lower CAC value for R-anomers compared to β-anomers. The anomeric configuration does not substantially affect the surface tensions or calculated headgroup areas. Acknowledgment. We thank Reckitt & Colman Products, Australia, for their support of this work, in the form of a PhD Scholarship for B.J.B. Supporting Information Available: Section S1: description of methods for purity determination. Section S2: synthetic detail for preparation of surfactants used in this paper, including Figure S1, optical rotation values obtained for the surfactants used, and comparison to previous literature values. Section S3: description of method and analysis of data obtained for adsorption at the air-aqueous solution interface. Section S4: Figures S2, S3, and S4, photomicrographs of water penetration experiments for R-C8G1, β-C8G1, and β-C12G2, respectively. Section S5: Figures S5, S6, and S7, plots of surface tension against surfactant concentration, and Figure S8, free energy of aggregation against chain length, for the surfactants studied in this paper. This material is available free of charge via the Internet at http://pubs.acs.org. LA991573W
