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Alkyl Chain Positional Isomers of Dodecyl β-D-Glucoside: Thermotropic and Lyotropic Phase Behavior and Detergency Ben J. Boyd,†,‡ Calum J. Drummond,*,† Irena Krodkiewska,† Asoka Weerawardena,† D. Neil Furlong,† and Franz Grieser§ CSIRO Molecular Science, Private Bag 10, Clayton South MDC, Victoria 3169, Australia, and Department of Chemistry, The University of Melbourne, Parkville, Victoria 3052, Australia Received October 23, 2000 A series of six dodecyl glucopyranoside surfactants, each with a single glucopyranoside headgroup attached sequentially along the chain from carbon 1 to carbon 6, have been prepared in order to systematically simulate “branching” in glucose-based surfactants. The thermotropic and lyotropic liquid crystalline properties of the series have been investigated, as well as the hard soil detergency of these surfactants. The introduction of “branching” into the alkyl chain strongly affects the packing of the molecules into crystals and liquid crystalline structures, thereby influencing the thermotropic properties and the surfactant/ water binary phase diagrams. Attachment of the headgroup at the terminal carbon or at carbon 2 does not appreciably influence the surfactant behavior, while attachment at carbon 3, 4, 5, or 6 results in very different behavior from that of either 1 or 2. This discontinuous change is reflected in the detergency behavior of the surfactants and can be attributed to the change in length and shape of the surfactant molecules and the subsequent changes in intermolecular interaction.
Introduction
* To whom correspondence should be addressed. Email:
[email protected]. † CSIRO Molecular Science. ‡ Reckitt and Colman Scholar. § The University of Melbourne.
For some time, surfactant hydrophobe positional isomers have been used as a means to access surfactant properties that are not available to normal chain surfactants. For example, in some cases the introduction of a headgroup to a nonterminal carbon position has been shown to increase the solubility of some surfactants17 and to destabilize foams by lowering cohesive interactions between hydrophobe chains.18 Nevertheless, there are few systematic studies on the effect of varying the position of the headgroup along the alkyl chain moiety in sugar-based surfactants. The work of Matsumura et al.,19 in which they prepared and studied the interfacial adsorption behavior of a series of sec-decyl glucosides, is the only study in which the position of a glucose headgroup is varied sequentially along the hydrophobe chain. Properties such as static and dynamic surface tensions, critical micelle concentration, occupation area per molecule at the air/aqueous solution interface, and foaming and antifoaming properties were examined. The study concluded that the centering of the headgroup on the chain results in a higher critical micelle concentration (cmc), to the extent that the 3-isomer has a cmc an order of magnitude higher than the n- or straight chain isomer. There is also a trend in the minimum area per molecule, Amin, at the air/aqueous solution interface, where the more the headgroup is centered along the chain, the larger the value of Amin and the lower the foamability of the surfactant.
(1) Schulz, P. Chim. Oggi 1992, August, 38. (2) Siracusa, P. Household Pers. Prod. Ind. 1992, April, 100. (3) Salka, B. Cosmet. Toiletries 1993, 108, 89. (4) Brancq, B. CESIO Int. Surf. Congr., 3rd 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. Chem. Ber. 1893, 26, 2400. (11) Shinoda, K.; Yamanaka, T.; Kinoshita, K. J. Phys. Chem. 1959, 63, 648.
(12) Boyd, B. J.; Drummond, C. J.; Krodkiewska, I.; Grieser, F. Langmuir 2000, 16, 7359. (13) Koeltzow, D.; Urfer, A. J. Am. Oil Chem. Soc 1984, 61, 1651. (14) Nilsson, F.; Soderman, O.; Johansson, I. J. Colloid Interface Sci. 1998, 203, 131. (15) Thiem, J. Tenside, Surfactants, Deterg. 1989, 26, 5. (16) Bikanga, R.; Gode´, P.; Ronco, G.; van Roekeghem, P.; Villa, P. Span. J. Deterg. 1994, 25, 595. (17) Rubinfeld, J.; Emery, E.; Cross, H. Ind. Eng. Chem. Prod. Res. Dev. 1965, 4, 33. (18) Myers, D. Surfactant Science and Technology VCH: New York, 1988; p 265, and references therein. (19) Matsumura, S.; Imai, K.; Yoshikawa, S.; Kawada, K.; Uchibori, T. Yukagaku 1991, 40, 709.
The increasing popularity of commercial surfactants derived from carbohydrate feedstocks stems from, in many instances, their favorable environmental and dermatological properties. The preparation, properties, and applications of alkyl polyglucoside (APG) mixtures in particular have been well studied in recent times.1-9 APGs are complex surfactant mixtures produced by a process known as the Fischer synthesis10 in which the fatty alcohol precursor, which provides the hydrophobic moiety in the product, is often polydisperse, varying in the chain length and sometimes the degree of branching. Early attempts to establish some of the structureproperty relationships for alkyl glucoside surfactants were made by Shinoda et al. in 1961.11 They investigated the effect of changing the alkyl chain length in straight chain alkyl monoglucosides. Since then, there have been various attempts to correlate the observed physicochemical properties of the surfactant with the degree of headgroup polymerization, alkyl chain length, stereochemistry at the anomeric center, and the position of alkylation around the glucose ring.12-16
10.1021/la001484f CCC: $20.00 © 2001 American Chemical Society Published on Web 09/08/2001
Properties of Surfactant Isomers
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Figure 1. (a, left) Stick structures and (b, right) energy-minimized structures for the five secondary R-dodecyl β-D-glucoside surfactants and the primary n-dodecyl β-D-glucoside. Green represents the carbon atoms, red represents the oxygen atoms, and gray represents the hydrogen atoms. The energy-minimized structures were obtained using INSIGHT II, Ver. 4.0.0, 1996 software using the Discover 3 module, and the PCFF2 force field, running on a Silicon Graphics SGI O2 workstation.
More recently, there has been some research into the properties and surface activity of 2-ethylhexyl R-glucoside and isooctyl glucoside (oct-2-yl β-D-glucoside), both branched octyl glucosides. Nilsson14 found that the Krafft temperature of 2-ethylhexyl R-glucoside is about 11 °C higher than the linear analogue, indicating an increased crystal stability. Matsumura19 did not report the melting points or Krafft temperatures for their series of sec-decyl glucopyranosides. Other workers have studied the effect of branching on the interfacial forces between mica surfaces, onto which they adsorbed combinations of linear octyl R- and β-D-glucoside and branched 2-ethylhexyl glucoside.20 They found that there was no significant difference in the intersurface forces when branching of the alkyl chain was introduced, implying that the packing density is independent of the area of the hydrophobe. A similar study into the foaming properties of the same set of surfactants21 did not find any difference between the foaming properties of these surfactants due to branching. There is much work still required to understand the effects of hydrophobe branching and headgroup position on the properties of surfactants with glucose headgroups. In this study, a series of six dodecyl β-D-glucopyranoside surfactants have been prepared, each with a single glucopyranoside headgroup attached sequentially along the chain from carbon 1 to carbon 6 (structures shown in Figure 1). Their thermotropic and lyotropic liquid crystalline properties have been investigated, as well as their (20) Waltermo, A° .; Claesson, P.; Johansson, I. J. Colloid Interface Sci. 1996, 183, 506. (21) Waltermo, A° .; Claesson, P.; Simonsson, S.; Manev, E.; Johansson, I.; Bergeron, V. Langmuir 1996, 12, 5271.
detergency power toward hard soils. Except for the n-alkyl derivative, these surfactants are mixtures of the two possible (+)- and (-)-diastereomers, as a consequence of the secondary carbon in the hydrophobe having four different substituents. Note that in the surfactant industry it is common practice to refer to this type of secondary positional isomer as linear surfactants. We prefer to consider them as effectively “branched” to distinguish them from the primary n-alkyl version. Experimental Section The preparation of the surfactants used in this study followed well-described literature procedures for the preparation of alkyl β-D-glucoside surfactants, using the appropriate secondary dodecanols as the alkylation reagents.12,22-27 The secondary alcohols used for these syntheses were obtained from several sources. (()-2-Dodecanol, (()-4-dodecanol, and (()-5-dodecanol were purchased from Wiley Organics (99%, 99%, and 96% purity, respectively) and were used as received. (()-3-Dodecanol and (()-6-dodecanol were synthesized in-house (purity >99% and >96%, respectively, by gas chromatography/1H NMR). The alkyl β-D-glucosides were prepared and used with anomeric purity and optical rotation as described in Table 1. Surfactants were stored over desiccant in the freezer before use. (22) Boyd, B. J. Ph.D. Thesis, The University of Melbourne 1999. (23) Koenigs, W.; Knorr, E. Chem. Ber 1901, 34, 957. (24) de Grip, W.; Bovee-Guerts, P. Chem. Phys. Lipids 1979, 23, 321. (25) Vanaken, T.; Foxall-Vanaken, S.; Castleman, S.; FergusonMiller, S. Methods Enzymol. 1986, 125, 27. (26) McCloskey, C.; Coleman, G. In Organic Synthesis Collective; J. Wiley & Sons: London, 1955; Vol. 3, p 434. (27) Rosevear, P.; VanAken, T.; Baxter, J.; Ferguson-Miller, S. Biochemistry 1980, 19, 4108.
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Table 1. Anomeric Purity and Optical Rotation of the Series of Dodecyl β-D-glucoside Surfactants Used in This Study surfactant
anomeric purity (%)
[R]20 D (c ) 1, MeOH)
1-isomer (()-2-isomer (()-3-isomer (()-4-isomer (()-5-isomer (()-6-isomer
>99.5 >99.5 94.6 92.3 92.6 90.2
-26.8 -25.8 -21.0 -18.9 -20.1 -12.9
The water used to prepare samples for the lyotropic phase analysis and the detergency studies was obtained by passing deionized tap water through a Milli-Q Plus Ultrapure Water System (Millipore, Australia). Thermotropic liquid crystalline behavior was studied by differential scanning calorimetry (DSC) (Mettler TA3000, 2.5 °C/min, calibrated with indium for temperature and enthalpy) and polarized light optical microscopy on an Olympus inverted IMT2 microscope fitted with crossed polarizing filters and a Mettler FP90 hot stage. For the DSC samples, dry surfactant was weighed into aluminum pans, which were then placed under vacuum of
