J. Phys. Chem. 1992, 96,4621-4633
4621
Lyotropic Mesophase Structure in the System (2-Ethylhexyl)oxypropionate-Water K. C. Gounden, P. Ganguli, and G. J. T.Tiddy* Hindustan Lever Research Centre, Andheri (East), Bombay 400 099, India (Received: September 18, 1991)
Optical microscopy and low-angle X-ray diffraction and multinuclear NMR ('H, 2H, 23Na,I3C) measurements have been carried out on the various phases formed by the novel surfactant sodium (2-ethy1hexyl)oxypropionate (NaEHOP) and water. Three liquid-crystalline phases occur, an hexagonal phase (H,) at the highest water content and below 55 "C, an extensive lamellar region (L,), and at the highest surfactant concentrations an "intermediate" phase (S). NaEHOP is in the S state with no added water. Further work is required to determine the structure of S. The HI phase is separated from L, by a "reentrant" micellar solution phase rather than cubic (VI) or intermediate mesophases. Hydrocarbon tail packing constraint concepts suffice to account for the limited stability of the HI phase. X-ray measurements show that the L, phase swells laterally on addition of water, rather than with an increase of water layer thickness. NMR measurements of water (2H20) and sodium quadrupole splittings are consistent with the head-group conformation becoming markedly more disordered at higher water concentrations.
Introduction The relationship between surfactant molecular architecture and lyotropic mesophase structure is a t least qualitatively understood for the small-micelle cubic phase (Il), normal and reversed hexagonal phases (HI, H2 respectively), and lamellar phase (L,).'" A number of important problems remain, including the factors responsible for thermotropic liquid crystal formation by surfactants,and what happens at the hexagonal-lamellar transition. This is particularly so in the case of short-chain surfactants. These appear to exist as thermotropic mesophases at room temperature, if the hydrocarbon is branched.' A well-known example is sodium bis(2-ethylhexyl) sulfosuccinate (AOT) which takes the reversed hexagonal (H2) structure., Moreover, instead of forming bicontinuous cubic (V,) or "intermediate" mesophase structures in the composition region between HI and La,1,4,6short-chain surfactants either show a direct H1-L, transition or give a "reentrant" micellar solution. There is little systematic information on the structural feature of the surfactant responsible for these differences. As a contribution toward solving these complex problems, we report the lyotropic and thermotropic (anhydrous) mesophase behavior of sodium (2-ethylhexy1)oxypropionate [CH3(CH2),CH(C2H5)CH20CH2CH2C02Na](NaEHOP). It is well-known that soaps give thermotropic mesophases, but these usually occur a t elevated temperatures.' The surfactant NaEHOP was selected for study because both the head group and the alkyl chain contain structural features that were thought likely to cause thermotropic mesomorphism a t ambient temperature. The branched chain should prevent effective packing in the crystal as for AOT, while the replacement of CH2 group by an ether oxygen gives an additional flexibility. Moreover, there is an extensive literature on the lyotropic mesophases of sodium ~ o a p s ' +so ~ Jthat ~ the influence of the different substituents can be assessed, at least to some extent. It is of interest to examine if the ether oxygen is part of the hydrophobic core of the surfactant (i.e., the surfactant is similar to a Clo-CIlhomologue) or whether it resides in the water (C, homologue). In this study we have employed polarizing microscopy, multinuclear NMR, and X-ray diffraction not only to examine the liquid-crystal structure, but also to elucidate the ion-water interactions with the head group, and the head-group conformation. Experimental Section (i) Materials. Sodium (2-ethylhexyl)oxypropionate(NaEHOP) was synthesized from highly pure (>98%) 2-ethylhexanol and acrylonitrile." The intermediate ether nitrile was hydrolyzed to the ether acid, which on neutralization with NaOH gave the sodium salt of (2-ethylhexy1)oxypropionate. The soap was purified Address correspondence to this author at: Unilever Research Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral, Merseyside L63 3JW, UK.
by repeated precipitation in acetone (purity as methyl ester >98%). The pure soap was vacuum-dried at ambient temperature initially followed by drying at 68 OC. Our material was a gift from Dr. M. E. N. Nambudiry of this laboratory; it was from the same batch used in earlier studies." Heavy water (2H20,99.8%) was obtained from Aldrich and used without further purification. In initial experiments a sample of significantly lower purity was employed (see below). (ii) Techniques/Measurements. Optical Microscopy. A Docuval polarizing microscope (Carl-Zeiss) with hot stage having an external temperature control water bath (0-90 "C) was used to characterize the phase structures using standard mesophase textures (oily streaks/mosaic textures for lamellar phase, nongeometric/fan textures for hexagonal phase."J0J2 High-temperature microscopy was carried out using a Reichert Neopan microscope with attached Koffler hot stage.6J0 NMR Measurement. High-resolution proton N M R was performed on Bruker WP-80spectrometer operating at 80 MHz, for IH, at 298 f 0.5 K. The 13C spectra were recorded on a Bruker AM 500 spectrometer operating at 125 MHz, at 298 f 0.5 K. Quadrupole splitting measurements for 2H and 23Nawere carried out on Bruker 270-MHz high-resolution and 300-MHz solid-state spectrometers, respectively, operating a t 41 -46 MHz (*H) and 79.38 MHz (23Na)using the Bruker V.T. probe. Typical numbers of scans in the above experiments were in the range of 200-400 and 2000-10000 to ensure a satisfactory S / N ratio. The spin-spin relaxation times (Tkff)of the surfactant protons were measured from the fids obtained from a Bruker Minispec PC 20B operating at 20 MHz for 'H, at 313 f 0.5 K. Simple 90" pulse excitation followed by data acquisition was performed. (1) Luzzati, V. In Biological Membranes; Chapman, D. Ed.; Academic Press: London, 1968;Chapter 3, p 71. (2) Winsor, P. A. Chem. Rev. 1968, 68, 1. (3) Ekwall, P. In Advances in Liquid Crystals; Brown, G . H., Ed.; Academic Press: London, 1971;Vol. 1, Chapter 1, p 1. (4)Tiddy, G. J. T. Phys. Rep. 1980,57, 1. (5) Tiddy, G.J. T. In Modern Trends of Colloid Science in Chemistry and Biology; Eicke, H. F., Ed.; Birkhausser Verlag: Basel, Switzerland, 1985;p 150. (6) Blackmore, E.S.;Tiddy, G. J. T. J. Chem. Soc., Faraday Trans. 2 1988,84, 1115. (7) Winsor, P. A. Liquid Crystals and Plastic Crystals; Gray, G. W., Winsor, P. A., Eds.; Ellis Horwood Ltd.: Chichester, England, 1974;Vol. 1, Chapter 5, p 199. (8) Rogers, J.; Winsor, P. A. J. Colloid Interface Sci. 1969,30, 241, (9) McBain, J. W.; Sierichs, W. C. J. Am. Oil Chem. Soc. 1948,25, 221, Vold, R. D.;Reivere, R.; McBain, J. W. J . Am. Oil Chem. SOC.1941, 63, 1293. (IO) Rendall, K.;Tiddy, G. J. T.; Trevethan, M. A. J. Chem. Soc., Faraday Trans. 1 1983,79, 637. (1 1 ) Nayyar, N.; Rao, R. M.; Nambudiry, M. E. N.; Narayan, K. S. To be published. (12) Rosevear, F. B. J. Am. Oil Chem. Soc. 1954, 31, 628.
0022-3654/92/2096-4627%03.00/0 0 1992 American Chemical Society
Gounden et al.
4628 The Journal of Physical Chemistry, Vol. 96, No. I I , I992 + 100
k, t- t
I 1
I I
I 1
I
I io
o
zo
30
40 150 io io W€Wl PERCENI OF No€HOP
e0
--
-
90
TABLE I: Vicinal Coupling Constants (Hz) for CH and CH2Groups in NaEHOP Solutions (298 K) J(CH-CH) I 0% mouv 1% OCH2CH2C026.84 f 0.05 7.08 f 0.05 -cH, -0
5.3 f 0.1
I
W
100
Figure 1. Phase diagram of the sodium (2-ethylhexyl)oxypropionatewater (2H20) system. LI = aqueous solution; HI = hexagonal phase; La = lamellar phase; 's'= waxy, semisolid phase formed by surfactant alone. Dotted lines indicate boundaries where concentrations are less accurately determined. Note that two-phase regions are generally small and have not been shown.
:>cH
I
co 0)
I 6
I
5
4
3
-6
2
1
0
(PPM)
Figure 2. Proton high-resolution N M R spectrum of NaEHOP (1 and 10%) in 2 H 2 0at 25 OC. TSP is the SiMe3 resonance of trimethylsilyl
propionate added a s a reference. a)
Le4
4
L1
5.2 f 0.1
The "effective" T2value (T M ) was taken as the time for the signal to decay to l / e of the original value.I0 X-ray Difftaction. Most of the X-ray data were determined with a Siemens (Model Kristalloflex 8 10) diffraction camera equipped with a scintillationcounter at 298 f 1 K using Ni filtered Cu Ka,radiation. A few preliminary X-ray measurements were obtained on the high-intensity, high-resolution X-ray equipment (line 8.2) at the SERC Synchrotron Laboratory, Daresbury, UK. The bilayer (dk)and water layer (d,) thickness as well as the area per head group ( a ) have been calculated from the repeat distances, making use of the partial specific volumes of the components in each sample.' For NaEHOP the density was calculated from the known volumes of CH, CH2, and CH3 groups. Sample Preparation. The samples were prepared by weighing the appropriate amount of NaEHOP and water (IH20or 2H20) into Pyrex glass tubes, sealing them, and homogenizing by repeated heating and centrifugation. Most of the measurements except for the high-resolution 13CNMR spectra were carried out on the same samples containing 2H20. Before any measurements, the samples were thermostat4 at 40 OC for at least a week. The 13C spectra were obtained on a sample of significantly lower purity.
Results and Discussion (i) Phase Diagram. A partial binary phase diagram of the NaEHOP/water system is given in Figure 1. This is based on an overview of all the results from the different techniques used. The phase regions are micellar solution (Ll), normal hexagonal phase (HI), lamellar phase (La), and "waxy solid" phase (S).The L,,HI, and Laphases have the usual structures: but the structure of S has not been fully elucidated. This is discussed further below. (ii) Solution Phase (L,). The critical micelle concentration (cmc) of NaEHOP (0.14 M 3.14 wt %) has been determined elsewhere in a separate study" and is typical of that for a C8rather than a CII-CI2soap. This is excellent evidence to show that the DCH2CH2C02-group is surrounded by water even in micelles. As a further check we have examined the high-resolution IH NMR spectrum of NaEHOP (in 2H20)at 1% and 10% (Figure 2 and Table I). The spectrum shows well-resolved resonances from the propionate OCH2and CH2C02groups (AA'XX') and from the 1-CH20group (doublet) as well as overlapping alkyl and methyl peaks. The vicinal coupling 3JHHof -CHCH- moieties are strongly dependent on the dihedral angle between C-H bonds of adjacent carbons (projected along the C-C bond), being ca. 7 Hz larger for a trans conformation than for the gauche conformation (the Karplus ru1e).l3 If the ether oxygen exists in the micelle
0
Figure 3. (a, top) Optical microscope (crossed polars) penetration scan for NaEHOP/water at 20 OC. The bands corresponding to lamellar and hexagonal phases are clearly distinct (X 100). (b, bottom) Optical texture (crossed polars) for the waxy solid S phase of NaEHOP at 25 OC (X100).
interior, one might expect significant conformation alterations between monomers and micelles. The data in Table I indicate that the conformations of the CI-C2bond are almost unaltered on micelle formation while for the propionate group the slight (but significant) increase in J(CH2CH2)is consistent with an increased population for the conformation with trans-0 and C02groups. This obviously arises from the steric/electrostatic repulsions between C02groups which can be somewhat relaxed by movement of the C 0 2groups away from the alkyl chain/water surface (sited (1 3) Jackman, L. M.; Sternhall, S. Applications of Nuclear Magnetic Resonance Spectroscopy in Organic Chemistry; Pergamon Press: Oxford,
England, 1969; Chapter 4-2, p 280.
The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4629
NaEHOP Liquid Crystal Phases
TABLE 11: Water (2H20) and Sodium (UNa) Quadrupole Splittings (A) for Sodium (2-Ethylhexyl)oxypropionateat 25 "C wt % molar ratio NaEHOP (S/W) A(*H), Hz A(23Na),kHz 40 0.0595 693 1.52 42 0.0615 745 1.27 44 46
0.0701
0.0760
787 802
(1.71, 35 " C ) 1.io
1.17
(+BO) approximately at Cl-O). Again, this fully supports the proposition that the oxypropionate group is surrounded by water. (iii) The Normal Hexagonal Phase (HI). The H1 phase was detected in the optical microscopy penetration scan (Figure 3a) as a distinct band separated by an isotropic liquid region from the L, phase. It melts to L1 on raising the temperature above 55 OC, and reappears on cooling, showing that it is an equilibrium phase. The existence range is ca. 39-46% at 25 OC. Preparation of bulk samples confirmed that the region between HI and L, is an isotropic liquid and not a V I cubic phase. The region of existence of the H1 phase varies markedly for different batches of NaEHOP available in the laboratory. In the purest material (>98%) the HI phase melted as shown in Figure 1 at 55 "C. However, in other batches which contained 1-2'37 of 2-ethylhexanol this melting temperature was reduced by up to 20". These levels of impurities have no influence on the occurrence of the L'l phase nor on the N M R and X-ray measurements. Hexagonal phase formation is common with linear sodium soaps of C8 and longer chain length.1-3J0Usually the phase melts to L1 at temperatures well above 100 OC. Micelle shapes and liquid-crystal phase structures can be rationalized on the basis of the 'packing constraints" d e s ~ r i p t i o n . ~This ~ J ~ approach gives the limiting value of head group area (a) for a surfactant to pack into spherical, rod or disk (bilayer) micelles assuming that the smallest micelle dimension cannot exceed the maximum chain extension (i.e,, the all-trans chain length). It is also assumed that the alkyl chain/water interface is smooth. For linear chains the a values are as follows: s herical micelle, 2 7 0 AZ;rod micelle, 247 A2;disk micelle, 1 2 3 The same values apply for the limits of stability of small micelle cubic phase (II), H I , and L, mesophases. With a 2-ethylhexyl chain these values are no longer valid because the 2-ethyl group contributes to the micelle volume without a proportional increase in micelle radius. The values for the 2-ethylhexyl chain can be easily calculated from the known volumes of CH3, CHz, and -CHCH2-CH3 groups (54, 27, and 77 AS, respectively), and the all-trans length of the chain ( I , = 8.0 A).sJ5This gives spherical micelles or I, phase with a 2 90 AZ,rod micelles or H1phase with a 2 60 A2, and disk micelles or L, phase with o 1 30 A2. For sodium soaps on a value of ca. 55 AZhas been measured by X-ray diffraction on the highest water concentration hexagonal phase. The major factors that determine this value are the forces that occur at the micelle surface, and hence a similar a value would be expected for sodium 3-ethyloctanoate. Thus, with NaEHOP we expect that the increased size of the head group will increase this somewhat. The a value calculated for the low surfactant concentration L, phase from X-ray data (see below) is 60 A2, in agreement with this. (Again, this confirms that the oxypropionate group is in the aqueous region.) Thus,from the packing constraint theory we expect that HIphase will barely be stable at all, as we observe in practice. Once again this demonstrates the remarkable success of this simple theory14 in rationalizing lyotropic crystal formation, despite all the inherent weaknesses. Quadrupole splittings (A) were measured for several H, samples. These are listed in Table I1 and are discussed with the data on the L, phase below.
oJ
:;I
;
5
0
0
1
2
3
L
5
6
7
0
9
NO.OF MOLES OF 2H20 PER MOLE OF NaEHOP-
Figure 4. Bilayer repeat spacing (d), aqueous layer thickness (dw),hydrocarbon layer thickness (&), and area per head group ( a ) as a function of NaEHOP/water mole ratio from X-ray diffraction measurements at 25 "C. L4iS
Ld
4 OCC
w2.
(14) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (15) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L. R.; Bostock, T.; McDonald, M. P. J. Chem. Soc., Faraday Trans. I 1983, 79, 975.
-
P
Qa0
io 1
0
,
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I
'0
'01
'0.2 '03 '06 0 5
'0.6 '07
'08
MOLE RATIO OF NaEWX/ 2H20
0 9 10"
'LOO
+
Figure 5. Effective spinspin relaxation time ( TkR)of alkyl chain protons in NaEHOP/water mixtures at 40 "C.
(iv) The Lamellar Phase (La)-Microscopy and X-ray Data. This is the most extensive mesophase region, occurring over the range 50-185% surfactant concentration, according to temperature. The La phase extends to well above 100 "C, but is not observed with the anhydrous surfactant up to the decomposition temperature (ca. 250 OC, see below). The optical textures of the phase are the typical oil streaky mosaic patternsS (Figure 3b). X-ray diffraction on this phase generally gave three reflections in the ratio 1:1/2:1/3 as expected. While there is some scatter of the do values, which was reproduced on duplicate runs, the values themselves are remarkably invariant with water content (Figure 4). We have calculated the hydrocarbon layer thickness (dhc),the aqueous layer thickness including the sodium oxypropionate group (d,), and the area per molecule (a) using standard formulae and assuming densities of 1.1 and 0.705 kg dm-3 for the aqueous and hydrocarbon regions, respectively. The layer thickness do shows changes in the expected direction for variation of concentration (i.e., d, decreases and dh, increases with reduced water concentration). However, the major variation is in the a value (60A2 at 45% 2Hz0to 43 A2at 17.5% ZH20). This indicates a dramatic increase in the population of more linear head-group conformations as water content is reduced. Note that even a t very high water levels, the d, value (- 16 A) is only ca. 3 A wider than the length of two all-trans hydrated head groups.
4630 The Journal of Physical Chemistry, Vol. 96, No. 11, I992 Thus, the lateral repulsions between adjacent surfactant molecules are the dominant repulsion forces in the bilayers, rather than interbilayer electrostatic repulsions. Note that the head-group volume is larger than that of water at S / W > 0.3. (v) NMR Measurements: The Liquid-Crystalline Phases. Figure 5 shows the proton T2cffvalues for the surfactant as a function of concentration in the L, and S phases. This T2value is a rough and ready measure of the average degree of order within a mesophase. For many systems we have found that typical La phases have TtCffN 120 ps, while HI phases give TzenN 180 ps (see, for example, ref 10). The values for NaEHOP (140 f 20 ps) are slightly larger than those typical of La,but the presence of the 2-ethyl group and the flexibility of the -OCH2CH2C02 group are expected to contribute to a larger T2 value (Le., lower order parameters). The real surprise in these data is that the S phase gives TzCff values which are remarkably similar to those of the L, phase (see below). The major N M R studies were carried out using ,H and ,jNa quadrupole splittings (A(2H), A(23Na))to investigate details of water and counterion binding to the head groups. In previous work we have ~ h o w nthat ~ ~the ' ~only contribution to A(2H) arises from the fraction of head group bound water (Pb). The A value of the bound water (Ab) is also a function of the order parameter (S) of the bound water and the quadrupole coupling constant (x[,H]). The latter depends on both nuclear properties and the magnitudes of the electric field gradients of the nucleus.
Because of the different orientations of the surface with respect to the liquid crystal axis, when molecular ordering is identical in hexagonal and lamellar phases we expect Sb(lam) = -2Sb(hex). If more than one type of bound water occurs (Le., several binding sites) then A(,H) also is a weighted average of the different types. Recently, we have proposed that &, can pass through a maximum if the head group/water binding is governed by an equilibrium of the type16-1s
(nb is the number of bound water molecules per surfactant molecule; s is the free surfactant and S(H20)nbis the surfactant/water complex). The parameter p b is linearly dependent on the surfactant/water (S/W) molar ratio at low concentration, and passes through a maximum when s/w = 1/(nb - l).i63'7 Obviously this can only be observed when & is invariant with concentration. Values of nb obtained by this technique show good agreement with those determined by other techniques for the sodium salts of C12soaps and C,,sulphates, for various C,, poly(oxyethy1ene) derivatives, and for C trimethylammonium ch1oride.l8 As yet this treatment should still be regarded with caution since it seems remarkable that such a simple model can apply in complex concentrated mixtures like surfactant mesophases. Figure 6 shows the dependence of A(,H) for NaEHOP as a function of concentration at three temperatures. The values are somewhat larger than those observed for the L, phase of sodium laurate.I0 Note that a few data points are shown for samples also containing the S phase. Remarkably, only a single powder doublet spectrum was observed across the whole concentration range up to 90% surfactant even for samples that are clearly identified as L, + S from X-ray measurements. The values of A show an initial steep increase with concentration at all three temperatures followed by a sharp decrease at 293 K, a slower decrease at 323 K, and a further gradual increase at 358 (16) Carvell, M.; Hall, D. G.; Lyle, I. G.; Tiddy, G. J. T. Faraday Discuss, Chem. SOC.1986,81, 223. (17) Rendall, K.; Tiddy, G. J. T. J . Chem. SOC., Faraday Trans. I 1984, 80,3339. (1 8) Blackmore, E. S.; Tiddy, G. J. T. Liq. Crysr. 1990, 8, 1 3 1. (19) Lindblom, G.; Lindman, B.; Tiddy, G. J. T. J . Am. Chem. Soc. 1978, 100. 2299.
Gounden et al.
29fK
0'5
t
L
0
. 0.1
.
6.2
,
O"3 6 b 65 0.6 MOLE RATIO OF NoEHOPi0 2 : '
0.8
0.9
1.0
Figure 6. Water quadrupole splittings [A(2H)] as a function of NaEHOP/*H20 mole ratio at various temperatures: 0 , = compositions where the phase S is present.
K. The initial steep increase is much larger than expected for an increase in Pb; hence it must arise from an increase in Sb (A change in X [ ~ Hof ] this magnitude is most unlikely since it is not observed in other soapi0 or polyetherl' surfactants.) There are three separate binding sites for water in the NaEHOP head group, the ether oxygen, the carboxylate group, and the sodium ion. (For sodium laurate the head-group hydration is estimated to be nb = 10.io,18)The sodium ion is expected to be fully hydrated at s / W 0.1, but S b will be small because of the high symmetry and rotational freedom of the hydrated ion. We expect at most only two weakly H bonded water molecules for the ether oxygen. Hence for NaEHOP we expect nb = 12. Thus for a constant Sb, A(,H) vs surfactant/water mole ratio should show an initial linear increase followed by a shallow maximum at mole ratio 1:11 (i.e., nb - 1). In practice, this expected linear portion falls in the L, region, while the maximum occurs a t a NaEHOP/water ratio of 1:7.3. As water binding to the ether oxygen is weaker than binding to either of the ionic groups, reduction of the S/W ratio below 1:12 will first result in loss of the ether bound water. This will change the ether group from a hydrophilic region to a hydrophobic one, resulting in movement of the sodium carboxylate moieties away from this location and into the midplane of the aqueous layer. Thus the major contribution to S b probably arises from water bound to the carboxylate ion. Hence the sharp increases in A(,H) reflect the shift in the oxypropionate conformation toward a more extended form as the sodium carboxylate moves to the aqueous midplane. This will also reduce the area per headgroup (a) in agreement with the X-ray data. The subsequent decrease a t 293 K above S/W = 0.16 reflects a decrease in the fraction of water bound to the carboxylate group. If the simple theory described in eq 2 is applied, then from the value of S / W a t A(,H) (maximum) we could calculate &, (total) as 7.3. However, as discussed above because there are several different binding sites, and because Sb is varying, this simple theory no longer applies. In fact, this maximum probably reflects the concentration above which addition of more surfactant causes a reduction of the fraction of water bound to carboxylate ions in favor of water bound to sodium ions. Then the temperature variation of A(,H) above S / W 0.2 indicates a change in these two bound populations with temperature, with a decrease in the sodium hydration on increasing temperature. For the hexagonal phase the data show a much simpler pattern of behavior (Table 11). The values of A(,H) are those expected assuming that nb and S b are the same in the HI phase and the highest water concentration Laphase (except that S b is reduced by half in HIdue to the change in the orientation of the micelle surface with respect to the director). This is excellent evidence that the hydration of the head group is the same at these compositions, indicating that the H I instability and the reentrant L, do not occur because of a water shortage.
-
-
The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4631
NaEHOP Liquid Crystal Phases 90
"
80
"
'
CH2 C H3
'
CH3 CHZCHZ CH2 CH C H 2 0 CHZCH2 C02 No 70-
b
d
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e
/
t
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kl(d~l83.3PPM)
60-c
50-
?
60.'
30. 20. 10 '
0
0.05 0.10 0.15 0.20 0 2 5 0 3 0 0.35
10
MOLE RATIO OF NaEHOP *H20-
Figure 7. Sodium quadrupole splittings [A(*jNa)] as a function of NaEHOP mole ratio at various temperatures.
The measured sodium quadrupole splittings are shown in Figure 7 and Table 11. Typical powder pattern spectra with a central sharp line were observed at the highest water levels, but the spectra became progressively broader and the powder features were more difficult to discem as the NaEHOP concentration and temperature increased. Above ca. 77% NaEHOP it was impossible to measure A(23Na) at any temperature, and only a fairly broad central transition was observed. Sodium quadrupole splittings in lyotropic mesophases arise from the presence of a net electric field gradient at the sodium ion due to an asymmetric hydration sheath. This occurs only for the fraction of ions within -2-3 A of the head groups [Pb(Na)]. For the ions far from the interface A(23Na) = 0. Hence the observed values are given by A(23Na) = 3/4Pb(Na)S(Na)x(Na)
-
80 70
60
50 60
3
d(PPM)
(3)
where Sb(Na) is the order parameter describing the orientation of the electric field gradient at the sodium ion and x(Na) is the nuclear quadrupole coupling constant (as for 2H, see above). When no water is present (surfactant thermotropic mesophases), these electric field gradients are very large, giving A(23Na)values of 1 MHz or more.2' When the ions are hydrated, the A(23Na) values are much smaller, typically in the range 20-50 kHz for La phases.10s21-23 In the lamellar phase A(23Na) increases very sharply with concentration and less so with temperature. The increase with concentration up to S / W N 0.16 has a similar shape to that of the A(2H) vs S / W curve, suggesting that it has a similar origin, Le., an increase in Sb(Na) due to extension of the head group. In addition, the increase in A(23Na)with temperature in this region may reflect an increase Sb(Na) due to local reorientation of the sodium ions with respect to the carboxylate ions since this is observed for sodium octanoate/octanol lamellar phaseI9 and sodium laurate.I0 The dramatic increase in A(23Na) above S/W = 0.25 can be attributed to an increase in x ( ~ ~ due N ~to) gradual dehydration of the sodium ions. In neat NaEHOP, we expect a A value of 1 MHz, as for neat phase sodium soaps?O and hence the changes are unsurprising. A different pattern is observed for the HI phase. Here A(Z3Na) decreases with increasing concentration while increasing with N
(20) Phillips, M . L.; Jonas, J. J . Chem. Phys. 1987, 86, 4294. (21) Wennerstrom, H.; Lindblom, G.; Lindman, B. Chem. Scr. 1974, 6, 97, Halle, B.; Wennerstrom, H. J . Chem. Phys. 1981, 75, 1928. (22) Kahn, A.; Jonsson, B.; Wennerstrom, H. J . Phys. Chem. 1985, 89, 5180. (23)
90
Wennerstrom, H.; Lindman, B.; Lindblom, G.; Tiddy, G. J. T. J . Chem. Soc., Faraday Trans. I 1979, 75, 663.
I
90
80
70 60
50
-
LO
30
20
10
0
6 (PPM)
Figure 8. Carbon-13 high-resolution NMR spectra of NaEHOP in 2 H 2 0 at 25 O C : (a, top) 5 wt %; (b, bottom) 70 wt %.
temperature. This behavior is observed for other soap systemd0J9 where it has been attributed to a change in the location of the sodium ions with respect to carboxylate ions (Le., variation of Sb(Na)). The A(23Na)values decrease by more than a factor of 2 at the Lato H1transition, again indicating a change in Sb(Na). Any alteration in the location of the sodium ions is not accompanied by a concurrent change in A(2H); hence the conformation of the oxypropionate group is unaffected. Finally, in this section we report some preliminary 13CN M R spectra for the L,and Laphases. At 5 wt % NaEHOP we obtain well-resolved peaks for all the carbons, with chemical shifts as given in Table 111. Surprisingly, even a t 70% of NaEHOP we can still observe distinct resonanw from the terminal alkyl groups albeit somewhat broadened, while the head group and the 1- and 2-carbon peaks are too broad to be observed (Figure 8). We suspect that the low power of IH double resonance employed is insufficient to remove IH-l3C dipolar coupling; hence line broadening probably arises from this source. In future work we
4632 The Journal of Physical Chemistry, Vol. 96, No. 11, 1992
Gounden et al.
20 -.
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. 16
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8 4
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Figure 9. X-ray diffraction curves obtained on high-resolution line 8.2 using Daresbury synchrotron source: (a, left top) 30 "C; (b, left middle) 45 "C; (c, left bottom) 70 "C; (d, right top) 100 O C ; (e, right bottom) cooled to 27 "C.
hope to obtain details of order parameter changes from I3C line widths to check on the variations in head group conformation at different water concentrations. The S Phase. The sample of NaEHOP employed here (containing
