Langmuir 1997, 13, 3349-3354
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Physical-Chemical Properties of Some Branched Alkyl Glucosides Frederik Nilsson* and Olle So¨derman Physical Chemistry 1, Center for Chemistry and Engineering, Lund University, P.O. Box 124, S-221 00 Lund, Sweden
Ingega¨rd Johansson Akzo Nobel Surface Chemistry, S-444 85 Stenungsund, Sweden Received October 1, 1996. In Final Form: February 10, 1997X
Four different alkylglucosides have been examined with respect to their physicochemical properties, viz. 2-ethylhexyl glucoside, isooctyl glucoside, 2-propylheptyl glucoside, and isodecyl glucoside. The substances have been examined by means of microscopy, crossed polarizers, 1H-NMR self-diffusion, 2H-NMR quadrupolar splittings, and small angle X-ray scattering. Properties such as critical micelle concentrations, solubilities, Krafft points, aggregation structures, and liquid crystallinity have been determined. For all substances, the only phases appearing in the binary (surfactant/water) system are the micellar and lamellar phases. For the two substances with eight carbons in the hydrophobic tail, there is a broad micellar region, a minor two-phase region, and a lamellar crystalline phase. The isodecyl glucoside, with 10 carbons in the hydrophobic tail, shows a liquid-liquid coexistence region from low surfactant concentrations up to ca. 18 wt % surfactant. In the case of the 2-propylheptyl glucoside, there is an extended two-phase region up to ca. 67 wt % surfactant consisting of a mixture of an isotropic solution and a lamellar phase. Above 67 wt % there is a lamellar phase. The isotropic solutions of 2-ethylhexyl glucoside, isooctyl glucoside, and isodecyl glucoside have been studied with self-diffusion 1H-NMR to obtain structural information about the micelles present. The concentration dependence of the surfactant self-diffusion follows the pattern that can be expected of spherical micelles. SAXS measurements show that the liquid crystalline phase formed by all the four surfactants has a lamellar structure. The SAXS results have been analyzed in terms of repetition distances and area/headgroup in the liquid crystalline region.
Introduction The increased environmental awareness among consumers constitutes a driving force to develop nontoxic, biocompatible, and biodegradable surfactants and emulsifiers. In addition, since many of our natural resources have a finite availability, there is presently an interest in synthesizing such substances from raw material taken from renewable resources. The word nonionic surfactant is frequently associated with those based on ethylene oxide. However, there are other nonionic surfactants, and one such group of surfactants is constituted by alkyl glucosides. Alkyl glucosides (henceforth referred to as AG), or alkyl polyglucosides as they are often called due to the fact that they frequently have more than one glucose unit, are made from different fatty alcohols and glucose. There are several applications where alkyl glucosides may substitute common and less environmentally friendly surfactants. In many applications, e.g. in machine washing, wind screen cleaning, and detergent formulations with high pH (>10), their performance is sometimes superior compared to other surfactants.1,2 The two components of AGs are linked together by a glucoside linkage.3,4 The glucoside linkage is often found in nature, e.g. in starch and cellulose. The building/ breakdown of glucoside linkages are, in nature, enzy* To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, June 1, 1997. (1) Balzer, D. Tenside Surf. Det. 1991, 28, 419. (2) Nilsson, F. INFORM 1996, 7, 490. (3) McMurry, J. Fundamentals of Organic Chemistry, 1st ed.; Brooks/ Cole Publishing Company: Pacific Grove, CA, 1986. (4) Pine, S. H. Organic Chemistry, 5th ed.; McGraw-Hill International Editions: New York, 1987.
S0743-7463(96)00964-X CCC: $14.00
matically controlled by different glucosidases.5 The presence of the glucoside linkage is one of the reasons for the biodegradability of the AGs. Industrialy produced AGs often have complex structures as a result of the many possible isomers formed when two or several glucose molecules are linked together. The physical-chemical properties of AGs, such as the critical micelle concentrations, packing parameters, Krafft points, and solubilities appear to be determined to a large extent by whether there is an R- or β-linkage between the glucose unit and the hydrophobic tail.6-9 In the case of industrially produced AGs, there is always a mixture between the two enantiomers, and generally one observes physicalchemical properties that are in between those of the two pure enantiomers. Another important factor is the degree of glucosidation. The Fisher synthesis10,11 gives a mixture of enantiomers and oligomers that results in a mixture of monoglucoside surfactants up to penta- and hexaglucoside surfactants. The degree of glucosidation is an important parameter to control in the production of hydrophilic/hydrophobic balanced surfactants. The glucose molecule is a highly hydrophilic headgroup, and when the degree of glucosidation increases the surface active properties will decrease. (5) Stryer, L. Biochemistry, 3rd ed.; W. H. Freeman and Company: New York, 1988. (6) Brown, G. M.; Dubreuil, P.; Ichhaporia, F. M.; Desnoyers, J. E. Can. J. Chem. 1970, 48, 2525. (7) Focher, B.; Savelli, G.; Torri, G.; Vecchio, G.; McKenzie, D. C.; Nicoli, D. F.; Bunton, C. A. Chem. Phys. Lett. 1989, 158, 491. (8) Shinoda, K.; Yamanaka, T.; Kinoshita, K. J. Phys. Chem. 1959, 63, 648. (9) Shinoda, K.; Yamaguchi, T.; Hori, R. Bull. Chem. Soc. Jpn. 1961, 34, 237. (10) Fischer, E. Chem. Ber. 1893, 26, 2400. (11) Fischer, E. Chem. Ber. 1895, 28, 1145.
© 1997 American Chemical Society
3350 Langmuir, Vol. 13, No. 13, 1997
Nilsson et al.
The work described here is based on four different AGs. All are surfactants with branched hydrophobic tails and degrees of glucosidation of around 1.5. Presented are the (pseudo) binary surfactant/water phase diagrams for all substances. In addition, the micellar phases have been characterized by means of 1H NMR self-diffusion, and the liquid crystalline phases have been investigated by means of small angle X-ray scattering (SAXS). Finally, the partial ternary phase diagram isooctyl glucoside/water/octanol is given. Experimental Section Materials. The surfactants used in this work have been synthesized at Akzo Nobel Surface Chemistry AB, Stenungsund (Sweden), from 2-ethylhexanol, 2-propylheptanol, and the commercial methyl branched mixture sold under the brand name Exxal-8 (isooctanol) and Exxal-10 (isodecanol). The alcohols were reacted with glucose according to the Fisher technology.10,11 The surfactants are mixtures of R- and β-AGs, and there is a distribution in the number of glucose units per headgroup. The degrees of glucosidation have been calculated from standard gas chromatography analysis, using silylated compounds. The four different AGs are 2-ethylhexyl glucoside (1.5), isooctyl glucoside (1.4), 2-propylheptyl glucoside (1.4), and isodecyl glucoside (1.4). The numbers within parentheses are the degrees of glucosidation. They will be referred to as 2EHG, IOG, 2PHepG, and IDG, respectively. The AGs have been purified from the excess alcohols used in the synthesis by column chromatography using a precolumn Apex Prepsil ODS 5 µm, main column Apex Prepsil ODS 8 µm 250 × 25 mm, and detector RI ERC-7510. The remaining alcohol after the purification is less than 0.1 wt % in all substances. All samples have been prepared in heavy water obtained from Dr. Glaser, AG, Basel (>99.8%), except for the samples used in the SAXS measurements. In the latter case Millipore water was used. Densities and volume fraction used in the calculation have been obtained from molecular volumes given in refs 12 and 13. Experimental Methods. The binary phase diagrams were determined by visual inspection of samples between crossed polarizers. The textures of the different phases were obtained by using a Zeiss D-7082 Oberkochen Axioplan Universial Microscope equipped with crossed polarizers. The phase boundaries in the binary phase diagrams have been determined to within (0.5 wt %. The isotropic solutions appearing in three of the four binary phase diagrams were characterized with the pulsed gradient spin echo proton NMR (PGSE-1H NMR) method to determine the surfactant self-diffusion coefficients. The experiments were performed in 5 mm NMR tubes on a Varian Unity 400 spectrometer, equipped with a 360 MHz (8.45 T) Oxford wide bore magnet. The system is equipped with a field gradient probe and driver unit, which are designed “in-house”. The probe generates field gradients of strength 0.18 T m-1 A-1. The experimental procedures were as recommended in ref 14. An ordinary spin-echo sequence was used. In all the samples studied, the echo decay could be rationalized in terms of a single diffusion coefficient, D, which was obtained by fitting eq 1 to the obtained NMR data.
I ) I0e{-(γGδ) D(∆-δ/3)} 2
(1)
In eq 1, I is the observed echo intensity, I0 is the echo intensity without field gradient pulses, γ is the magnetogyric ratio, G is the field gradient strength, δ is the length of the gradient pulse, and ∆, is the time between the leading edges of the gradient pulses. The ternary phase diagram of IOG/heavy water/octanol has been determined by means of crossed polarizers and heavy water 2H NMR quadrupole splittings. The 2H NMR splittings were (12) Reiss-Husson, F.; Luzzati, V. J. Phys. Chem. 1964, 68, 3504. (13) Handbook of Chemistry and Physics, 70th ed.; CRC Press: Boca Raton, FL, 1990. (14) Stilbs, P. Prog. Nucl. Magn. Reson. Spectrosc. 1987, 19, 1.
Figure 1. The temperature-composition phase diagrams of 2EHG (upper left), IOG (upper right), 2PHepG (lower left), and IDG (lower right) in water. Isotropic is a micellar solution, anisotropic means a birefringent lamellar phase, and 2φ is a two-phase region. All phase boundaries have been determined to an accuracy of (0.5 wt %. The dashed horizontal line appearing in the four phase diagrams is the lowest temperature where measurements have been performed. used to determine the presence of one- and two-phase regions.15-17 2H NMR splitting experiments were performed on a Bruker MSL
100 spectrometer. The liquid crystalline phases were examined by means of small angle X-ray scattering (SAXS) performed on a SEIFERT IsoDebyeflex 3000 X-ray equipment. The samples were prepared in vacuum-tight cells with thin mica windows, and the spectra were recorded in vacuum. Unless otherwise stated, the different experiments were carried out at a temperature of 25 °C and by using flame-sealed sample containers.
Results and Discussion The Binary Phase Diagrams. A first step in the characterization of a novel surfactant is to determine the binary phase diagram. The binary phase diagrams of the four different substances are presented in Figure 1. The phase diagrams of 2EHG and IOG are similar in appearance with an isotropic one-phase region extending to 80 and 75 wt %, respectively. Above those concentrations there is a ca. 5 wt % two-phase region which separates the isotropic phase from a liquid crystalline phase. A common factor for these surfactants is that they both have eight carbons in the alkyl chain. The 2EHG has a welldefined branched structure in its hydrophobic tail, whereas the IOG has a random branched structure (mostly methyl branchings) in its tail. The other two surfactants under study both have 10 carbons in the tail. The 2PHepG is well-defined in its (15) Khan, A.; Fontell, K.; Lindblom, G.; Lindman, B. J. Phys. Chem. 1982, 86, 4266. (16) Ulmius, J.; Wennerstro¨m, H.; Arvidson, G. Biochemistry 1977, 16, 5742. (17) Wennerstro¨m, H.; Lindblom, G.; Lindman, B. J. Phys. Chem. 1974, 6, 97.
Physical-Chemical Properties of Alkyl Glucosides
branching, and IDG has a random structure with methyl branching similar to the one in IOG. The 2PHepG has an anisotropic liquid crystalline phase above 67 wt %. Below 0.05 ( 0.002 wt %, there is an isotropic one-phase region. Between these concentrations there is a two-phase region of a dilute surfactant solution and the liquid crystalline phase, which is of lamellar symmetry (vide infra). In the case of IDG there is a one-phase isotropic region between 18 and 62 wt % surfactant, a 17 wt % wide twophase region, and finally above 79 wt % an anisotropic liquid crystalline phase. At very dilute surfactant concentration (
