Langmuir 1994,10,4429-4433
4429
Properties of a Dichained “Sugar Surfactant’’ Julian Eastoe* and Philippe Rogueda School of Chemistry, Cantock’s Close, University of Bristol, Bristol BS8 ITS, U.K.
Bill J. Harrison, Andrew M. Howe, and Alan R. Pitt Kodak Limited, Research Division, Headstone Drive, Harrow HA1 4TY,U.K. Received August 5, 1994. I n Final Form: October 11, 1994@ We have studied the properties of a novel nonionic surfactant possessing two c6 hydrophobic chains and i.e. 7,7-bis[(1,2,3,4,5two nonionic glucamide head groups (C~HI~)~C(CH~NHCO(CHOH)&H~OH)~, pentahydroxyhexanamido)methyll-n-tridecane or di-(CG-Glu). The changes in air-water surface tension y with concentration show a critical micelle concentration (cmc) at 1.9 x mol dm-3. A value for a,, the mean &ea per molecule in the pre-cmc monolayer, of 75 f3A2was obtained using the Gibbs adsorption isotherm. The binary di-(C6-G1u)-water phase diagram has been determined using polarizing optical microscopy and 2HNMR. At 27 “C the phase progression hydrated crystals S lamellar La cubic VI hexagonal HI-micellar solution L1 occurs with decreasing surfactant concentration. In contrast to many polyether nonionics (CiEj) this surfactant does not exhibit cloud-point phase separation between 0 and 100 “C. Analysis of small-angle neutron scattering data at concentrations43,20,and 8 x cmc indicates the presence of cylindrical micelles, 140-200 A long and radius 12 A. The micelle shape and size are not significantly affected by temperature up to 70 “C.
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CHS(CH2)s \
Introduction This Letter describes initial studies of the properties of a novel synthetic alkyl glucamide surfactant.l The material has a n unusual structure for a nonionic surfactant, with two identical glucamide hydrophilic residues and two equal alkyl chains (Figure 1). The systematic name for this compound is 7,7-bis[(192,3,4,5-pentahydroxyhexanamido)methyl]-n-tridecane,abbreviated here to di-(C6-Glu). The presence of these two “sugar))head groups per molecule introduces the possibility of strong intra- and intermolecular hydrogen bonding, in addition to the usual head group-water interactions present with the n-alkyl(poly(ethy1ene oxide)) (CiEj) nonionics. Here we report air-water surface tension measurements, the binary surfactant-water phase diagram, and small-angle neutron scattering (SANS) from post-cmc aqueous solutions. There is currently much interest in surfactants of this type, since they can be derived from renewable resources. To date, all ofthe related glucamide, glucitol, and glucoside surfactant systems studied have consisted of only a single chain and head,2-12 and often unusual behavior, such as Abstract published in Advance ACS Abstracts, November 15, 1994. (1)US Patent No.4,892,806,1990. (2)Shinoda, K.; Kamanaka,T.; Kinoshita, K. J . Phys. Chem. 1969, 63,648. (3)Pfannemuller, B.;Welte, W. Chem. Phys. Lipids 1985,37,22740. (4)Zable, V.; Miiller-Fahmow, A.; Hilgenfeld, R.; Saenger, W.; Pfannemuller,B.; Enklemann,V.; Welte, W. Chem. Phys. Lipids 1986, 39,313-27. (5)Muller-Fahmow, A.; Hilgenfeld, R.; Hesse, H.; Saenger, W.; Pfannemuller, B. Carbohyd. Res. 1988,176,165-74. (6)Fuhrhop, J.-H.; Svenson, S.; Boettcher, C.; Rossler, E.; Vieth, H.-M. J . A m . Chem. SOC.1990,112,4307-312. (7)Hall, C.;Tiddy, G. J. T.; Pfannemuller, B.Liq. Cryst. 1991,9, 527-37. (8)Morley, W. G.;Tiddy, G . J. J . Chem. SOC., Faraday Trans. 1993, 89,2823-31. (9) Okawauchi,M.; Hagio, M.; Ikawa, Y.; Sugihara, G . Bull. Chem. SOC.Jpn. 1987,60,2718. (lo) Denkinger,P.;Kunz, M.;Bouchard,W. ColloidPolym. Sci. 1990, 268,513;Prog. Colloid Sci. 1990,81,257. (11)Kameyama, K.; Takagi, T. J . Colloid Interface Sci. 1990,137, @
1 n * - l*”.
(12)Matsumura,S.;Imai, K.; Yoshikawa, S.; Kawada, K.;Uchibori, T. J . Jpn. Oil Chem. SOC.1991,40,709-14.
0743-7463f94/2410-4429$04.50~0
,
CH2NHCO(CHOH)ICH2OH
’‘ C
CH3(CH2)5
CHzNHCO(CHOH)&H20H
Figure 1. Structure of the surfactant used in this work 7,7bis[(1,2,3,4,5-pentahydroxyhexanamido)methyll-n-tridecane, referred to here as di-(CG-Glu).
gel formation a t low concentration, is o b ~ e r v e d .Here ~ we show t h a t a “sugar surfactant” with two alkyl and two hydrophilic chains exhibits classic surfactant behavior, with a well-defined cmc, discrete micellar structure, and regular lyotropic phase progression. When compared with previous work on single chain materials, the results also suggest that, for surfactants with linear polyhydroxy1 groups in aqueous solution, traditional ideas about the relationship between surfactant structure and micelle stabilitylshape cannot necessarily be applied. Owing to the level of world-wide activity in producing renewable resource surfactants, which have analogous properties to petroleum-derived materials, these results will be of considerable interest. The behavior of nonionic CiE, surfactants has been extensively investigated by a range of techniques.13 The conditions that determine the micellar structure have been well documented and, by and large, can be understood in terms of the simple molecular packing parameter P, as well as a temperature-induced desolvation of the polar groups. P is defined as vla,l, with v the hydrocarbon tail volume, 1 the length, and a, the optimal area per polar group. The origin of the cloud point for these materials is related to desolvation of the ethylene oxide chain. However, there is comparatively little in the literature concerning surfactants with other nonionic head group types. In this respect compounds derived from natural or synthetic sugars are topical due to their greater biocompatibility. These alternative materials are commonly referred to as “sugar surfactants” and contain either cyclic or linear oligomeric saccharides. Shinoda et al. carried out early work and demonstrated the amphiphilic nature of single chain n-alkyl and alkyl aryl glucosides via interfacial tension measurements.2 More recent work has focused on the solid and liquid crystal structures and gel (13)Non-ionicSurfactants; Schick, M.,Ed.;Surfactant Science Series Volume 1; Marcel Dekker: New York.
0 1994 American Chemical Society
Letters
4430 Langmuir, Vol. 10, No. 12, 1994
hydrocarbon micelles. Sample scattering was corrected for cell, DzO,and hydrocarbonincoherent scattering. The counting times were -2 h per sample. Below we summarize the relevant theory. For particle shapes such as spheres, rods, disks, or ellipsoids of volume V, present at number density np and of coherent scattering length density ep,dispersed in a medium of emthe normalized SANS intensity I(Q) (cm-l) may be written
I(&) = np(e, - em)2V~[S(Q)(lP(Q)12) + I(P(Q))l' - (IP(Q)I')l (1) P(Q) is the intraparticle form factor which depends on the size and shape of the particle. S(Q) is the interparticle structure factor. For the dilute systems studied here (micellar volume fraction kc-= 0.05) we find that S(Q) 1 over the accessible Q-range. For shapes like spheres, rods, disks, and ellipsoids P(Q) is defined as 1.0 at Q = 0. The scale factor A (=np(gp- em)2Vp= &dC&2 sincenpVp= &eale) is then a quantitative consistencycheck on the model. ?'rial fits with a number of models show that rigid rod form factor P(Q,r,L)r.odis most appropriate in this case.
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Idi-(C6-Glu)l / mol dm"
Figure 2. Air-water interfacial tensions y for di-(CG-Glu)at 25 "C. The line is a fit for an area per molecule a. = 75.3 f 3
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k . The cmc is 1.9 x
mol dm-3 (0.11 w t %, therefore AmC
1.1 x 10-3).
P(Q,rC),d = formation of single-chain glucamides and methyl glun~~ {sin2[(QLcos 8)/21H4512(Qr sin B)> c a m i d e ~ . ~Less - ~ is known about micelle f o r m a t i ~ n . ~ - ' ~ sinp @ (2) [1/4(QL)' cos' ,8][(Qr)2sin 2 /31
Experimental Section Analysis and Surfactant Properties. A purified sample of di-(CG-Glu)(molecular weight 598.73) was supplied by Kodak Limited.1 Elemental microanalysis confirmed the composition in weight percent as C 52.0 (54.2),H 9.6 (9.031,N 4.67 (4.68),and the remainder 0 33.73 (32.09),with theoretical values given in parentheses. A Purite ion-exchangepurifier was used to provide H2O. IH and13CNMR spectra were recorded in DzO (Fluorochem 99.7% D-atom) on a Jeol GX270 instrument. The measured spectra were all consistent with the molecular structure. Line broadening was observed in the alkyl CHz and CHs region of the 1H spectrum, characteristic of large anisotropic micelles. Airwater surface tensions were measured at 25 "C using a Kriiss K12 tensiometer and Pt-Ir du Nouy ring followingan established experimental protocol.14 The apparatus was also calibrated against a range of standard liquids; excellent agreement with the literature values was found (e.g., air-water 72.3 mNm-l 15). Polarizing optical microscopy and 2H NMR16 were used to determine the surfactant-water phase diagram. Samples of known composition were prepared gravimetrically and homogenized by repeated centrifugation and heating in hermetically sealed glass ampules. The mixtures were subsequently transferred to 10-mm NMR tubes, or glass microscope slides, and sealed. The solvent-penetration microscopy method was used for an initial survey ofthe phase behavior using a Nikon Optiphot I1 polarizing microscope and Linkam hot stage. Samples were then observed microscopically at fxed compositions. 2H quadrupolar splittings, from mesophase-associated 2H20molecules, were measured using a Jeol JNM-FX100spectrometer operating at 15.32 MHz. Before each measurement the sample was equilibrated to zkO.1 "C in the probe for at least 10 min. Small-Angle Neutron Scattering (SANS). SANS measurements were performed on the time-of-flightLOQ spectrometer using the ISIS pulsed neutron source at the EPSRC Rutherford Appleton Laboratory, U.K. The data reduction, normalization procedures, and fitting routines are described e1~ewhere.l~ Samples were contained in stoppered, matched, 2.0 or 5.0 mm path length Hellma cells and thermostated at 25 i: 0.1 "C or 70 & 1.0 "C. DzO was used to contrast the protiated
(14) Simister, E. A.; Thomas, R. K.; Penfold, J.; Aveyard, R.; Binks, B. P.; Cooper, P.; Fletcher, P. D. I.; Lu, J. R.; Sokolowski, A. J.Phys. Chem. 1992,96,1383. (15)CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL. (16) (a) Wennerstrom, H.; Persson, M.; Lindman, B. In Colloidal Dispersions and Micellar Behauiour; Mittal, K., Ed.; ACS Symposium Series 9; American Chemical Society: Washington, DC, 1974. (b) Blackmore, E. S.; Tiddy, G. J. T. Liq. Cryst. 1990,8, 131-51. (17) Eastoe, J.;Heenan, R. K. J.Chem. SOC., Faraday Trans. 1994, 90, 487.
s,
With the radius rand lengthl, J1 is the first-order Bessel function of the first kind and the angle between the rod axis and the scattering vector.18 In the fitting routine the instrument resolution function AQ/Q was taken into account (-8% for Q > 0.05 A-1).
Results Air-Water Surface Tension. The dependence of surface tension y on di-(CG-Glu) concentration at 25 f0.1 "C is shown in Figure 2. The clean break at a cmc is characteristic of a pure surfactant; the measured cmc is 1.9 x mol dm-3. The pre-cmc data were analysed using the Gibbs adsorption isotherm in order to estimate the mean area per molecule in the monolayer a,.19 The fitted line on Figure 2 is for a, = 75 3 Az. Phase Behavior. Figure 3 shows the binary di-(CGG1u)-water temperature phase diagram over the temperature range 0-100 "C. A combination of hot-stage polarizing microscopy and 2HNMR quadrupolar splitting measurements confirmed the locations of the different phase boundaries. For clarity, most bi-phasic LC isotropic regions have been omitted from the diagram. On contacting a sample of pure di-(CG-Glu) with water at room temperature, optical textures characteristic of a phase progression hydrated solid S lamellar L, (mosaic) cubic V1 (viscous isotropic) hexagonal H1 (angular fans) solution L1 (fluid isotropic) are seen with decreasing surfactant concentration when viewed under a polarizing micro~cope.'~ The designation of the cubic phase as bicontinuous V1 is based on its location between H1 and La. The dashed line in Figure 3 represents a clear viscoelastic phase, which flows and appears to shear thin (in contrast to HI). Samples of these compositions are birefringent, giving an interesting "tiled" texture in the polarizing microscope but a single line in the 2H NMR spectrum (see below). This may well be a biphasic region, although at this stage we are not able to assign the phase unambiguously. With decreasing surfactant concentration at 27 "C, the approximate phase boundaries are as follow^: S La, 89%;L,+V1, 75%;Vi HI, 71%; HI
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(18)Cabane, B. Insurfactant SolutwnsNew Methodsoflnuestigatwn; Zana, R., Ed.; Marcel Dekker: New York. (19) Clint, J. Surfactant Aggregation, Blackie Academic: Glasgow, 1992.
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Langmuir, Vol. 10,No.12,1994 4431 -- --
'r ll
-- I
I
7s
v
/
50
1
25
% wt. di(c6-glu) 25
/
50
75
/
Figure 3. Temperature-concentration phase diagram for binary di-(CG-Glu)water mixtures, as determined by optical polarizing microscopy and 2HNMR L1,micellar isotropic solution; HI, hexagonal phase; VI, bicontinuous viscous cubic phase; La, lamellar phase; S, hydrated crystals. For the 2H NMR spectra the x-axis is given in Hz. The dotted line represents compositions that are strongly birefringent but do not give rise t o a quadrupolar splitting. Phase boundaries are f l w t %.
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birefringent viscoelastic phase, 62%; birefringent viscoelastic phase L1, 50% (all to f l wt %). The 2HNMR line shapes are also shown in Figure 3 for various phases and temperatures of interest. By use of this method the presence of anisotropic uniaxial phases is characterized by typical powder spectra where the quadrupolar splitting A (Hz) is equal to 3/4xS with x the quadrupole coupling constant and S the order parameter.16 For a lamellar phase the magnitude of A is generally twice t h a t for hexagonal phases of identical composition (given constant x and We find that a t 27 "C in the La phase (78% by weight di-(CG-Glu)) A 1160 Hz, while for the H1 a t 68%, A 620 Hz. In contrast, isotropic phases (L1 and VI) do not give rise to a splitting but to a rather narrow line of half width ~ l /30-50 ~ Hz. For both the H1 and Laphases we always find a central isotropic line despite
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repeated mixing and heatinglcooling cycles. This indicates that the system is biphasic, consisting of an isotropic phase in coexistence with a n anisotropic mesophase, the relative proportions being indicated by the intensities. The La sample is unusually viscous and could not be melted up to 100 "C, furthermore the close proximity of the viscous isotropic VI phase means t h a t is extremely difficult to prepare a homogeneous sample. Therefore the central isotropic line and quadrupolar splitting in the spectrum may be due to the presence of both La and VI phases, or H1 and V1 phases depending on the composition. It is interesting to note t h a t we do not observe a miscibility loop in the L1 micellar phase. Therefore, in contrast to many CiEj surfactants, this system does not
4432 Langmuir, Vol. 10,No. 12, 1994
Letters neutron contrast he2was calculated to be 36 x 1020cm-4,18 and this value was used in the calculations.
11
Discussion
I(Q) I m '
0.:
0.: 0.05
0.01
0.005
0.009
0.02
QIA '
0.04
0.24
0.07
Figure 4. SANS profiles for di-(CG-Glu)-DzO solutions at 25 "C,error bars included: 4mic = 0.0489, 43 x cmc (A); &ic = 0.0216, 19 x cmc (0);dmiC= 0.0086, 8 x cmc (W). Solid lines are least-squares fits for rigid cylinder form factor model as
described in the text. Table 1. Parameters Derived from SANS Data Analysis Using Rigid Cylinder Model, Eauations 1 and 2= 4mic cylindrical micelle known calculated 1engtWA radiuslA 0.0489 0.0216 0.0086
0.0510 0.0253 0.0085
178 196 143
12.0 12.0 12.0
See Figure 4 for fits. 4micis the volume fraction of micellized surfactant. Comparison ofknown composition and that calculated from the fitted scale factor. Uncertainties in length L f 10 A and radius r i 2 A.
exhibit a cloud point, a t least up to 100 "C. This however does not preclude a desolvation-driven cloud point a t higher temperatures, which are a t present experimentally inaccessible for us. Small-Angle Neutron Scattering (SANS). The volume fraction of micellized surfactant 4mic'is calculated assuming a density of di-(CG-Glu)= 1.0 g (4mic = 4,u1.f The SANS data for 4miC= 0.0489, 0.0261, and 0.0086 di-(CG-Glu) D2O solutions are shown in log[l(Q)l vs log [&I in Figure 4. These samples are 43, 19, and 8 x cmc, respectively. The angular dependence of the scattering is essentially independent of concentration, whereas the intensities simply scale with 4dC,indicating that the structure factor S(Q) 1 over this Q and concentration range. A SANS spectrum of the 43 x cmc solution recorded a t 70 "C was almost identical to the 25 "C profile suggesting t h a t the micellar structure and interactions are not significantly affected by temperature. In the intermediate regime 0.02 A-1 -= Q < 0.17 k l , the intensity scales as Q-l, which is characteristic of cylindrical structures.17J8 The solid lines are least-squares fits to the scattering expected from rigid cylinders (eq 1and 2).17 Fitted parameters are given in Table 1 along with uncertainties in r and L. The range of Q accessed allows both dimensions r and L to be determined to a reasonable accuracy, since (on a log[l(Q)l vs log[&] plot) the steep decay a t high Q is determined essentially by the cross section radius and the turnover at low Q by the finite length of the cylinders. In all cases, excellent quantitative agreement between the fitted model and the observed scatteringwas obtained. It should be noted t h a t the H-0 hydrogen of the glucamide head group is potentially exchangeable with D2O. Given the large excess of solvent, complete D-H exchange was assumed. The effective
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The air-water surface tension y vs In [di(CG-Glu)l exhibits classic behavior for a surfactant; a steady, linear decrease in y , and a clean break with absence of a minimum a t the cmc. The post-cmc tension is 32.4 f 0.1 mN m-l. The use of the Gibbs equation to calculate the mean area per molecule a t the interface a, from interfacial tension measurements is not without errors, and determination by the more direct technique of neutron reflection is clearly preferable. The discrepancies between the two methods have been discussed in detail for the cationic surfactant C14TAl3.l~ For cationics there may be a problem of dewetting of the du Nouy ring; this is not generally the case for nonionics. In order to extract a,,it is usual to fit a polynomial (quartic) to the pre-cmc data and calculate the derivative dyld ln(c) a t the cmc. This method was tested for the data in Figure 2, and owing to the large range of linear behavior, no major differences were found as compared with a linear least-squares fit. For the nonionic C12E3 Thomas et al. have shown that a t the cmc the values ofa, obtained from a simple treatment of surface tension measurements, and a sophisticated analysis of neutron reflectivity data, are in reasonably good agreement, with a, = 36 f2 A2.20This implies that for nonionics in the saturated monolayer, a, is still approximately twice t h a t for a single hydrocarbon chain in close-packed insoluble monolayers, a, 18 Az.19 The value we obtain for the di-(CG-Glu)of 75 f 3 k is ca. twice that found for C12E3. While there are differences in the head group type of the two compounds, the value we obtain for a, is entirely reasonable for a dichained nonionic. The lyotropic phase behavior observed with decreasing surfactant concentration is consistent with a continual increase in net curvature of the aggregates. Since we have not introduced any alkane, and therefore do not expect any significant changes in chain volume, the progression La V1 H1 must be accompanied by a n increase in the mean value of a,. The presence of a birefringent zone in the region of 55-62 % surfactant, which does not give rise to a 2H splitting, is interesting. Since the micelles in L1 are cylindrical (by SANS) and it borders the H1 phase, rigid cylindrical micelles are almost certainly present in this region. The system may also be biphasic a t this point; however samples are clear, and after 4 months no macroscopic phase separation is observed. Unfortunately the range of Q was too low to permit a reliable value of a, for the cylindrical micelles to be extracted from the SANS data (Porod region). If we calculate the maximum value of a, for cylinder micelles (P= 112 19) a value of -74 A2 is obtained. This is very close to that obtained a t the planar air-water interface and suggests t h a t the simple approach for calculating P, considering only the hydrocarbon chain length and volume, is inappropriate in this case. The SANS profiles, and fitting to the rod model, indicate that short (-140-200 cylindrical micelles of approximately 12 radius are present, even at only 8 times the cmc. The micelle radius is consistent with the alltrans length of the CS hydrophobic chains which is calculated to be -11.6 A.1g Apparently the micellar structure is not significantly affected by increasing concentration and/or temperature. Steep low Q scattering can also be attributed to attractive interactions. However the I(Q) profiles are essentially independent of both
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(20)Lu, J. R.; Lee, E.M.; Thomas, R. K.; Penfold, J.; Flitsch, S. L. Langmuir 1993,9, 1352-60.
Langmuir, Vol. 10,No. 12, 1994 4433
Letters parameters, and we see no evidence for a cloud point. Although we are not able to place any significance on the small variations in cylinder length with concentration, it is clear that the radius is constant a t -12 A for all the samples. Cylindrical micelles are to be expected preceding the HI mesophase (-60% see Figure 3), and it is interesting t o note that the cylinders retain their integrity even close to the cmc and at high temperatures. Of course, for higher values of micellar volume fraction, @mie, a n S(Q)term will be needed to model the data. At the low concentrations we have studied, 543 x cmc, good agreement between calculated and known concentrations are found using the noninteracting cylinder form factor (Table 1). Initial analyses of the SANS from post-cmc di-(CB-Glu) di-(C&Glu) solutions suggests a link between n-alkyl chain length, lyotropic phase sequences, and micellar shape in the L1 phase. The systems exhibit a rich phase and structural behavior including a sphere cylinder disk change in L1as the alkyl chain lengths increase from Cg to CS. These results will be reported in a forthcoming paper.
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Conclusion The novel dichained bis(g1ucamide).material di-(C6Glu) exhibits classical behavior in dilute aqueous solution and the usual progression of lyotropic phases in the concentrated regime. However, unlike many polyetherbased nonionic surfactants, di-(CG-Glu) does not exhibit a cloud point up to 100 "C. Tensiometry measurements a t 25 "C give a cmc of 1.9 x mol dm-3 a t 25 "C, and an area per molecule a t the air-water interface a. = 75 f3 k.The latter value is consistent with the head group configuration. SANS measurements provide good evidence for the presence of cylindrical micelles even at 8 x cmc.
Acknowledgment. P.R. thanks Kodak Limited Research Division U.K. and the University of Bristol for provision of ajointly funded Ph.D. studentship. The Royal Society is thanked for a grant toward microscopy equipment. We thank the EPSRC Neutron Beam Committee for an allocation of beam time, and assistance with chemicals and travel. Dr. Richard Heenan is thanked for helpful discussions concerning SANS data modeling.
