Cs+-H,O
Interaction in Lamellar Systems
(2) T. Nakagawa and F. Tokiwa in "Surface anc. Golloid Science", Vol 9, E. Matijevic, Ed., Wiiey, New York, 1976, p 69. (3) C. L. Khetrapal, A. C. Kunwar, A. C. Tracey, and P. Diehl, "Lyotropic Liquid Crystals", Springer-Verlag, Heidelberg, 1975. (4) H. Wennerstrom and B. Lindman, Phys. Rep., 52, 1 (1979). (5) J. C. Eriksson and G. Gillberg, Acta Chem. Scand., 20, 2019 (1966). (6) 8. Lindman and S. ForsBn, "Chlorine, Bromine and Iodine NMR. Physico-Chemical and Biological Applications", Springer-Verlag, Heidelberg, 1976. (7) B. Lindman, G. Lindblom, H. Wennerstrom, and H. Gustavsson, "Mlcellization, Solubilization and Microemulsions", K. L. Mittal, Ed., Plenum Press, New York, 1977, p 195. (8) J. B. Rosenholm, T. Drakenberg, and B. Lindman, J. Colloid Interface Sci., 63, 538 (1978). (9) E. Williams, 8. Sears, A. Allerhand, and E. H. Cordes, J. Am. Chem. SOC.,95, 4871 (1973). (10) U. Henriksson and L. Odberg, CoiioidPolym. Sci., 254, 35 (1976). (11) M.Alexandre, C. Fouchet, and P. Rigny, J . Chim. Phys., 70, 1073 (1973). (12) T. Drakenberg and 8. Lindman, J . Colloid Inferface Sci., 44, 184 (1973).
The Journal of Physical Chemistry, Vol. 83, No. 23, 1979 3015 (13) B.-0. Persson, T. Drakenberg, and 8. Lindman, J. Phys. Chem., 80, 2124 (1976). (14) J. N. Israelachvili, D. J. Mitchell, and B. N. Ninham, J . Chem. Soc., Faraday Trans. 2, 72, 1525 (1976). (15) P. Mukerjee, J. Phys. Chem., 76, 565 (1972). (16) P. Mukerjee in "Micellization, Solubilization and Microemulsions", Vol. 1, K. L. Mittal, Ed., Plenum Press, New York, 1977, p 171. (17) R. J. Tausk and J. Th. G. Overbeek, Biophys. Chem., 2, 175 (1974). (18) I . Danielsson, Fin. Kemistsarmf. Medd., 75, 65 (1966). (19) I.Danielsson and P. Stenius, J. Colbidlnterface Sci., 37, 264 (1971). (20) P. Stenius and C.-H. Zilliacus, Acta Chem. Scand., 25, 2232 (1971). (21) R. Friman, K. Pettersson, and P. Stenius, J . Colloid Interface Sci., 53, 90 (1975). (22) I.Danieisson, J. B. Rosenholm, P. Stenius, and S. Backlund, Progr. Colloid Polym. Sci., 61, 1 (1976). (23) B. Lindman and H. Wennerstrom, "Topics in Current Chemistry", Springer-Verlag, Heidelberg, in press. (24) G. S. Manning, Annu. Rev. Phys. Chem., 23, 117 (1972). (25) S. Engstrom and H. Wennerstrom, J. Phys. Chem., 82, 2711 (1978). (26) B. Jonsson and H. Wennerstrom, Chem. Scripf., in press. (27) G. Gunnarsson and H. Wennerstrom, J. Phys. Chem., in press.
Cesium Ion and Water Interaction in the Lamellar Phase of the I-Monooctanoin-Water-CsCI System. NMR Quadrupole Splittings and Chemical Shift Anisotropies Nils-Ola Persson and Goran Lindblom" Division of Physical Chemistry 2, Chemical Centre, University of Lund, S-220 07 Lund 7, Sweden (Received May 11, 1979) Publication costs assisted by the University of Lund
133Csand 2H magnetic resonance measurements on lamellar phases consisting of 1-monooctanoin, DzO, and CsCl are reported. The spectral parameters measured are 13Wsquadrupole splittings, shift anisotropies and isotropic chemical shifts, and deuteron quadrupole splittings. The dependencies of these parameters on amphiphile and salt concentration are discussed. The deuteron results indicate that a model assuming two kinds of water molecules, bound and free water, can be adapted. The cesium quadrupole splittings and shift anisotropies are in accordance with formerly obtained results for 23Naions. Evidence that the isotropic shifts of the cesium ion give a measure of the ion interaction with the amphiphile polar group is presented. The splittings and chemical shift anisotropies are found to be almost temperature independent. It is shown that changes in the 133Cssplitting and chemical shift anisotropy are both due to changes in the fraction of bound ions and the order parameter for the 1-monooctanoin-water-CsC1system. When a second electrolyte is added changes also in the quadrupole coupling constant and/or the chemical shift tensor occur.
Introduction Lamellar liquid crystals have for a long time received much attention because of their suitability as models for biological membranes.' NMR provides a nondestructive and nonperturbative technique2i3to study molecular dynamics and molecular order in such systems. The ordering of the hydrocarbon chains in the interior of the amphiphile aggregates has been investigated mainly by means of deuteron magnetic resonance on deuterated hydrocarbon
nuclear quadrupole coupling constant or chemical shift tensors, r e ~ p e c t i v e l y ~ (see ~ ' ~ Jbelow). ~ However, often an estimate of the fraction of bound ions can be made from other studies,17 but the order parameter or quadrupole coupling constant are generally not obtained. Here an attempt has been made to improve the ion binding method by studying both quadrupole splittings and chemical shift anisotropies of cesium ions in a welldefined lamellar liquid crystalline system containing 1monooctanoin. A comparison between these NMR paramchain^.^^^ eters, considering the effect of temperature, water, and Information on water and counterion binding in lyoelectrolyte concentration, has then been made to extract tropic liquid crystals has been obtained from studies of information about the factors determining the splittings heavy wateP9 and counterion magnetic resonance methand shift anisotropies. Water orientation has also been o d ~ . ' ~ -The ' ~ NMR parameters measured have been relaxation times" (or line widths), quadrupole ~plittings,~-'~ studied by using 2H NMR on heavy water. or chemical shift anisotropies of both alkali16 (133Cs)and Method halide ions (19F-).'5Unfortunately, these experimental NMR parameters are often difficult to interpret in terms Extensive discussions of quadrupole splittings of quadof molecular interactions since the quadrupole splittings rupolar nuclei in different amphiphile systems have been or shift anisotropies are given by a product of the fraction given previously.'J4 Here only the relevant expressions of bound ions or molecules, the order parameter, and the for the splittings and chemical shift anisotropies for liquid 0022-3654/79/2083-30 15$0 1.OO/O
0 1979 American Chemical Society
3016
The Journal of Physical Chemistry, Vol. 83, No. 23, 1979
11
2A
+u
Flgure 1. Typical powder spectrum from ’%Cs in a lamellar liquid crystal showing both shift anisotropy and quadrupole splitting. The six satellite peaks arise owing to quadrupolar effects ( I = 712 for 133Cs) and the quadrupole splitting (A) is measured as the distance between two adjacent satellites. The chemical shift anisotropy is measured as v L - vII in the figure and the isotropic shift is obtained from (uli -I- 2 u L ) / 3
- ”sample,
crystalline systems will be given. The observed quadrupole splitting, A, for a nonoriented (powder) sample is given by14 A = ICpiVg’S’I (1) where p i is the fraction of the ions at site i, vQi is the effective quadrupole coupling constant, and Siis the order parameter of site i. The chemical shift of a nucleus in an anisotropic liquid crystalline phase is composed of one isotropic and one anisotropic part.lsJ9 In an ordinary liquid only the isotropic part is visible owing to rapid isotropic rotation. The 133Csnucleus has a shift range sufficiently large that in a liquid crystalline sample both the isotropic and the anisotropic shift can be observed. Recently, we reported 133Cs shifts where, also, expressions for the two shift parameters measured were given:16
Persson and Lindblom
Sample compositions were always chosen well within the lamellar phase of the 1-monooctanoin-water systems.21 The samples were heated to the isotropic liquid point and then allowed to cool. The sample homogeneity after equilibration was checked by examining them between two crossed polaroids and by deuteron magnetic resonance spectroscopy. N M R Measurements. The spectrometer used was a modified Varian XL-100 operating in the pulse-Fourier transform mode. The 2H and 133Csresonance frequencies were, respectively, 15.3 and 13.1 MHz. Pulse widths were -30 and 7 ps, respectively, and the required number of transients to get a reasonable signa1:noise ratio were 200-500 and 10000-40000, respectively. The temperature of measurement was the probe temperature 29 f 1 “C, if not otherwise stated. Typical powder spectra were obtained for both quadrupole splittings and 133Csshift anisotropies of the central peaks. The quadrupole splitting was obtained as the distance between adjacent peaks (see Figure 1). Simulations of the shift anisotropy spectra (powder) with a superimposed Lorentz broadening function were performed on a Hewlett Packard 9820 A desk calculator and from these simulations estimations of the magnitude of the shift anisotropies were made. Due to difficulties in getting a correct phasing of the spectra and the fact that the central peak is unsymmetrically superimposed on the smoothly U-shaped central part of the quadrupole splitting of the spectrum, no perfect accordance between simulated and experimental spectra could be obtained and no estimations of Tz* (the “effective” T2)were therefore made. The isotropic part of the chemical shift was measured relative to a CsCl solution in D20 a t infinite dilution.
Results Measurements were performed on three samples series with constant ratio (xA/xD20)between molar fractions of monooctanoin (XA) and water (XD,,-J and various amount of CsCl (or other salt + CsCl in the competition experiwhere p is the relative population of bound counterions ments). The salt content was expressed as molar concenat sites with anisotropic shifts, the o,,s (a: = x , y, or z ) are tration in the aqueous layers. elements of the diagonal chemical shielding tensor at the The deuterium spectra, in all cases, consisted of two site and the superscripts b and f denote bound and free parts, one from the water molecules with a splitting of sites, respectively. BDM and DM are Euler angles specifying 0.5-2.7 kHz (cf. below) and one larger splitting of 3.5-4.5 transformations from the director to the molecular coorkHz from the deuterons on the amphiphile hydroxyl dinate systems. Note that the principal coordinate system groups. In some samples also a third splitting of the of the electric field gradient tensor must not necessarily be the same as that of the shielding tensor. Finally v , , ~ ~ ~ magnitude of half the water splittings was observed. The origin of this third occasionally appearing splitting and vo are the resonance frequency for the sample in in the deuterium spectra is of great interest. The most question and for an infinitely dilute CsCl solution in DzO, reasonable hypothesis is to ascribe them also to amphirespectively. A typical spectrum is shown in Figure 1 philic hydroxyls as there are two nonequivalent hydroxyl where the experimental parameters are defined also. groups in the 1-monooctanoin molecule. Spectra of samExperimental Section ples without CsCl had the same appearance as those with salt. Prolonged centrifugation gave no indication of inhoChemicals. The 1-monooctanoin, i.e., 1-glyceryl 1-octamogeneity. Hydroxylic deuterons on the amphiphile will noate, was synthesized by the Synthesis Service at Chemiexchange rapidly, with the water deuterons if base or acid cal Centre, Lund, according to the procedures in ref 20 with is added. Then only one deuteron splitting should be modifications as follow. Only the second stage from this observed according to eq laz2 This has been tested for one reference was performed from acetone glycerol (solketal), sample, containing a DzO solution with pH 3. The result which was purchased from Aldrich, U.S.A., and the acylaof this experiment was that only one splitting was obtion was performed in dry ether with triethylamine as a served. Furthermore, this splitting was somewhat larger base. Acid hydrolysis was performed in 16% acetic acid than the water splitting for a corresponding salt-free samat 60 “C for 12 h. D 2 0was purchased from CIBA-Geigy, ple at pH 7. The splitting of the acid sample was also in Switzerland, and had an enrichment of 99.7 %. HC1, LiC1, good accordance with the value obtained with eq 1, using NaC1, and CsCl came from the British Drug Houses, Unthe known sample composition and water and amphiphile ited Kingdom, and were all of a purity of 99% or better. splittings from the neutral sample. Sample Preparation. The appropriate amounts of 1The dependences of the main water and the large ammonooctanoin and salt solution in D20were weighted into phiphile splittings on the concentration of cesium salt for ampules which were immediately sealed off. In each of three values of XA/XDzO are shown in parts a and b of the ampules was also a glass bead to facilitate mixing.
The Journal of Physical Chemistry, Vol. 83, No, 23, 1979 3017
Cs+-H20 Interaction in Lamellar Systems
nk
a
0
N
5 n m a
2.0.
0 A
A
A
0
0
A
4 .O
0
"o--T--"\o
0
0
-
1.0.
0
2[CsCIl, M
b
a
Figure 2. Observed deuteron quadrupole splittings from samples in I-monooctanoin-CsCI-D,O vs. concentration (mobr) of CsCl in the water layers for different molar ratios of l-monooctanoin/D2C)( X ~ I x o p ) (A) : X A / X D z O= 0.47; (0)X A / X o z o= 0.19; and (0)XA/XD20 = 0.10. (a) Splittings are from water deuterons and (b) splittings are from hydroxyl deuterons.
0' 0
Figure 3. Observed 133Csquadrupole splittings as a function of concentration of CsCl (mol/dm3) in the water layers for the same samples as in Figure 2. Samples with different XA/XDa are denoted as in Figure 2.
TABLE I: Effect of Replacing One-Half of t h e Cs Ions with Li or Na in Samples with X Ai X n . 0 = O . l g a Aa,
kHz
,''A kHz
uo,
[Cs'l
ppm
ppm
4
1.2 1.0 1.0 1.2 1.1 1.1
0.19 0.15 0.11 0.17 0.14 0.12
-32 -30 --28 -18 -17 -17
-3.0 -3.8 -3.7 -2.0 -3.1 -2.9
AD,
[Li'l
[Na'l
0 0 2 0 0 1 a
0 2 0 0 1 0
2 2 2 1 1
[CsCIl, M 4
Salt concentrations in mol/dmg of solution.
Figure 2, respectively. It is seen that the water splittings show no significant dependence on CsCl content but they increase with increasing amphiphile content. Our water and amphiphile deuteron splittings are in good accordance with previous results.23 Competition experiments where Cs+ ions were replaced with Na+ and Li+ are shown in Table I. Here no significant variation in 2H splitting was observed. The temperature dependences are very weak for the deuterium spectra as can be seen in Table 11. The only significant effect is that the broad amphiphile splitting is diminished by increasing the temperature. The 133Csparameters measured were the quadrupole splittings and the anisotropic and isotropic shifts (cf. Figure 1). Figure 3 shows the effects of the CsCl concentration on the 133Csquadrupole splitting for three molar ratios between amphiphile and water. The shift anisotropies of the same samples are tabulated in Table 111. I t can be seen that these two static NMR parameters increase considerably when the ratio of amphiphile to heavy water is increased. The isotropic shifts (ao) for the samples as a function of molar concentration of CsCl in the water layers are
2
0
ICsCll , M
4
Figure 4. The isotropic part of the 133Cschemical shift (ao)in ppm (with a CsCl solution in D20at infinite dilution as a reference) vs. molar concentration of CsCl in the water layers. Liquid crystalline samples are denoted as in Figures 2 and 3, CsCl in ordinary D,O with ( 0 )and mixtures of ethylene glycol -I-CsCl solution in D20with (X). The molar ratio ethylene glycol to water ratio was 0.19.
shown in Figure 4. It is evident that oo shows a strong and significant variation with the molar concentration of salt. Furthermore, at high salt concentrations there is also O .elucidate a dependence of oo on the ratio X A / X ~ ~ To whether this could be explained by an interaction of Cs+ ions with the polar head group of the amphiphile, the concentration dependence of shifts of CsCl dissolved in an ethylene glycol-water mixture was measured and these results are also depicted in Figure 4. Note that the molecular structure of ethylene glycol is very similar to the polar group of 1-monooctanoin. Figure 4 shows that the chemical shift approaches that of the ethylene glycol-water
TABLE 11: Temperature Dependence of Deuteron Quadrupole Splittings (in k H z ) from Samples in the 1-Monooctanoin-D,O-Alkali Salt Systems _ _ _ _ _ _ I _ _ _ _ _
XA/X,~,
alkali salt (s)
0.47 0.47 0.19 0.19 0.19 0.19 0.19 0.10
CsCl CsCl none HCl CsCl CsCl + NaCl CsCl + LiCl CsCl
salt concn, mol/dm3
_________ 20 i 2 ° C ___ D,O amphiphilic OD
_ _ _ _ _ _ I _ _
0.51 1.0 0 pH 3 0.17 2+ 2 2t 2 1.9
2. 7 4.3 2.7 4.5 1.2 3.5 not measured 1.3 3.1 1.0 5.2 1.0 5.0 0.58 4.3
1.3 1.4 0.5 0.7 0.4
__ 2 6 + 2 ° C
D,O
amphiphilic OD
-____-
2.7 2.7 1.2 1.3 1.2 1.0 1.0 0.58
4.3 4.3 3.2
1.3 1.3 0.9
3.5 5.0 4.6 4.1
0.5
D,O
31 t_ 2 ° C amphiphilic OD
2.1 3.8 2.1 3.1 1.2 1.3 1.2 2.9 1.0 4.7 1.1 4.5 sample melts
1.2 1.3 0.4 1.4
-
3018
The Journal of Physical Chemistry, Vol. 83, No. 23, 1979
Persson and Lindblom
TABLE 111: Chemical Shielding Anisotropy (ppm) of Samples in the l-Monooctanoin-D,O-CsC1 System
systems.6 Thus the molecular ordering at the amphiphilic surface is much lower for the uncharged monoglyceride systems than for a surface containing charges. concn of CsCl in D,O, mol/dmi The origin of chemical shifts of l W s + ions in dilute XA/XD,O 0.171 0.513 0.995 1 . 9 1 3.93 _- --- _--_____________-____ solutions is supposed to be an overlap between the outer 0.10 GO >- 1 -2 -2 -2 orbitals of the ion and those of the solvent or other ions.25 0.19 -2.4 -3.0 -2.9 -2.0 3.0 In solvent mixtures preferential solvation of dissolved ions 0.47 a -4.9 - 5.0 -5.7 8.9 may cause complicated dependences of ion shifts vs. sola Not observed owing t o weak signal intensity. vent compositions.26 A review of factors that determine the 133Cschemical shifts is given in ref 27. The isotropic mixture a t high monooctanoin and salt content. 13Ts shifts of the 1-monooctanoin-DzO-CsC1 samples in The effect on the shift and quadrupole splitting for Figure 4 show mainly the same salt concentration depenreplacement of one half of the Cs+ ions with Li+ or Na+ dence as neat DzO solutions except at the highest CsCl ions is summarized in Table I. On going to a heavier ion concentrations. These results indicate that the alkali ions the quadrupole splitting increases significantly and the at low salt and/or amphiphile concentrations are in an absolute value of the isotropic shift also shows a weak environment similar to that in DzO solutions. A t higher increase. concentrations of both salt and amphiphile more ions inTable IV shows the effect of temperature on the 13Vs teract with the polar groups of 1-monooctanoin and the spectra. shifts of the 13Ts+ions approaches that of the appropriate D20-CsC1-ethylene glycol mixtures. Discussion Studies of the 23Naquadrupole splitting in the system An estimate of the average orientation of the water l-monooctanoin-DzO-NaC1 were recently reported.’O Here molecules bound to the amphiphilic surface can be obit was found that the splitting increased strongly with tained from a simple analysis described p r e v i o u ~ l y . ~ ~ ~X A / X D Bbut was independent of the NaCl content. These Water molecules are assumed to exchange rapidly between findings are in accordance with the present lS3Cssplittings two sites, namely, bound or free. Provided that the free except at high amphiphile content (cf. Figure 3), where the water molecules exhibit zero splitting, the observed deudependence of the salt concentration is significant. The teron splitting (A) will be given by’ (powder spectrum) 23NaSplittingslO were discussed in terms of free and bound ions and the splittings were ascribed to distortions of the hydration shell of sodium ions situated in the bound layers (4) of water. The cesium ion is less hydrated and distortions of the hydration symmetry of this ion should be more where n is the average number of water molecules bound easily induced. A quantitative comparison between data to each amphiphile polar group, vQ is the quadrupole couobtained for the two ions is, however, not possible since, pling constant of DzO, and S is the order parameter (see for example, the symmetry properties of the distorted above). The subscript b indicates bound water. A plot hydration shells of the ions may be quite different. of the water quadrupole splitting vs. XA/XD should give Unambiguous information from quadrupole splittings a straight line through the origin with a sfope equal to of ions is often difficult to extract, since the splittings nlv&‘lb. It was found that this slope was equal to 6.3 f depend on three unknown factors, namely, the fraction of 0.3 and was independent of the CsCl concentration. Takbound ions, the quadrupole coupling constant, and the ing n to be 1.2 as estimated from the phase diagram21as order parameter (see eq 1). Thus, further independent the minimum amount of water required to form the lamethods are needed to be able to interpret splittings in mellar phase and V Q to be 220 kHz? we obtained a value terms of ion binding. It can be expected that studies of of S = 0.03. For samples with 1-monooctanoin 0.5 M both spin relaxation and chemical shielding can be very NaCl in DzO, one also obtains S = 0.03. Thus changing helpful for this purpose, provided that the same interaction the alkali ion has a negligible effect on the water orientasites (binding sites) are responsible for the changes obtion. This is supported by the competition data given in served in the NMR parameters. It seems reasonable that Table I. This is in contrast to previous results where a this latter assumption should be valid at least for quadrudependence on the counterion was obtained for carboxylate pole splittings and chemical shielding anisotropy because both parameters depend on the static properties of the and sulfate surfactants6 but is in line with the behavior of zwitterionic lecithinaZ4 system. For the monoglyceride system studied in this work The hydroxylic deuteron splittings from the amphiphile we found that the 13Ts quadrupole splitting and the shift anisatropy follow the same trend. Furthermore, as Table shown in Figure 2b are much smaller than those observed V shows, the ratio IAu/Al is fairly constant. As can be for decanol hydroxyls in alkali octanoate-decanol-DzO
+
TABLE IV: Temperature Dependence of t h e ‘3’Cs N M R Parameters in _________--___ salt 20 i 2 ° C concn _ _ _ _ _ _ ~ _ _ alkali inD,O, A, Au, o,, IAu/Al, A, XA/XD,O salt(s) mol/dm’ kHz ppm ppm €Iz/Hz kHz
t h e 1-Monooctanoin-D,O-Alkali Salt Systems __
26: 2 ° C
-_.I__-__
__
~____~_________-__ 0.47 0.47 0.19 0.19 0.19 0.19 0.10 0.10
CsCl CsCl CsCl CsCl CSCI + NaCl CSCl i LiCl CsCl csc1
_ _ I _ -
___
34
i
2°C
-.-.---_____I__
Ao,
uo,
IAa/AI,
A,
Ao,
ppm
ppm
Hz/Hz
kHz
ppm
uo,
lAo/Al,
ppm Hz/Hz
0.51 1.0 0.17 1.0 2t 2
0.41 0.41 0.18 0.16 0.15
-5 -5 -2 -2 --3
-7 --11 -4 - 11 -31
0.2 0.2 0.2 0.2 0.2
0.40 0.37 0.19 0.16 0.15
-5 -5 -2 -3 -4
-7 -11 -4 -10 -30
0.2 0.2 0.2 0.2 0.3
0.44 0.45 0.20 0.18 0.17
-5 -5 -2 -3 -4
-8 -11 -3 -10 -32
0.2 0.2 0.1 0.2 0.3
+
0.11
-3
-30
0.4
0.11
-4
-28
0.4
0.14
-3
-29
0.4
0.069 0.071
GO -1
0.079 0.074
90
-
- 2 -33
2
2
0.17 3.93
-6 33
GO
0.2
-2
GO
0.4
samplemelts sample melts
Cs+-H,O
Interaction in Lamellar Systems
The Journal of Physical Chemistry, Vol. 83,
TABLE V: R a t i o b e t w e e n t h e Chemical Shielding A n i s o t r o p y in Hz a n d t h e Quadrupole S p l i t t i n g in Hz for t h e Samples in Table I11 c o n c n of CsCl in D,O.mol/dm3
X A / X D , O 0.171 0.10
a
0.513
90
0.995
1.91
3.93
0.3
0.4
0.2 0.2
0.2 0.2
0.19
0.2
0.2
0.4 0.2
0.47
a
0.2
0.2
GO
Not observed o w i n g t o weak signal intensity.
inferred from Table IV this ratio is not either affected by changes in temperature. Thus even if there is a considerable concentration dependence of both splitting and chemical shielding anisotropy their ratio remains constant. This strongly indicates that the changes observed in splitting and shift anisotropy are a measure of alterations in the fraction of ions bound, i.e., p in eq 1and 2. The arguments for such a conclusion are the following. Both A and Au are products of three factors, P,VQS, and pcsutScs, respectively, where we have used the same symbols as before except that labels q and cs have been hung on the appropriate factors and where ut represents a suitable shielding tensor in the molecular coordinate system. Further it has been assumed that the asymmetry parameter of the electric field gradient tensor14 that determines the nuclear quadrupole coupling constant is zero and that the shielding tensor is axially symmetric so that one order parameter in each case (Le., S, or S,,, respectively) is sufficient to describe the average molecular ordering. Now, it is very unlikely that LQ and ut should have the same concentration dependence, an assumption supported by the competition experiment (see below) and furthermore S, and S, are not equal unless the principal coordinate systems for the electric field gradient and shielding tensors coincide. Thus if lAu/Al is constant changes in Au and A are most probably caused by an alteration in p . It should be noted, however, that if the principal axes of the two tensors involved are the same (a reasonable assumption for monoatomic ions), S, = S,, and changes in Aa and A might be ascribed to changes in the product pS. However, since no temperature dependence is observed for Au and A, S must be constant suggesting that S is probably unaffected also by changes in the salt concentration (CsC1). On the other hand, a replacement of Cs with Li and Na leads to significant changes in the ratio Aula. This can be understood if the chemical shielding and/or the electric field gradient tensors of the cesium ion is affected by addition of a second alkali ion. Since both LiZs and Na10 interact with the amphiphilic surface they can induce a change in the shielding and/or the field gradient tensors, either by ionion interactions or a distortion of the hydration shell of the Cs ions. This conclusion is supported by 133Csand 7Li28 quadrupole splittings, since it was found that in a competition experiment the Li splitting did not change while the Cs splitting decreased when the molar ratio between the two ions was altered. Thus caution is necessary when interpreting quadrupole splittings of a competition exper-
No. 23, 1979 3019
iment in terms of the fraction of bound ions. Finally, possible mechanisms for the cesium ion interaction with the hydroxylic groups of the monoglycerides will be discussed. There are a t least two different kinds of interactions that may occur: (a) the water molecule(s) in the hydration layer of the alkali ion is (are) replaced by the hydroxylic group(s) of the amphiphile or (b) there is hydrogen bonding between water molecules in the hydration layer and the polar groups of the monoglyceride. The present investigation cannot distinguish between these two different possibilities. It should be noted that in the samples most enriched in amphiphile there are only 2.2 water molecules per amphiphile, which also must fill the hydration needs of the Cs+ (and C1-) ions, if not the interaction under (a) is present.
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