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23Na NMR relaxation study of sodium dodecyl sulphate in some aqueous and nonaqueous systems
23Na NMR Relaxation Study of Sodium Dodecyl Sulphate in Some Aqueous and Nonaqueous Systems ANDREA CEGLIE, GIUSEPPE COLAFEMMINA, MARIO DELLA MONICA, LEO BURLAMACCHI,* AND MAURA MONDUZZI *'1 Dipartimento di Chimica, Universitd di Bari, Italy; and, *Dipartimento di Scienze Chimiche, Universitd di Cagliari, Italy Received October 16, 1990; accepted March 19, 1991 The analysis of the Z3Na N M R spin-lattice (Rt) and spin-spin (RE) relaxation rates was performed on several two-, three-, and four-component systems of the surfactant sodium dodecyl sulphate in water, formamide, N-methyl formamide and N,N-dimethyl formamide, n-pentanol or n-octanol was used as cosurfactant, while p-xylene was added as oil to obtain some aqueous and nonaqueous microemulsions. In several systems R~ = R2, thus indicating that Na + ions are not tightly b o u n d to any structured interface. Whenever Rl :b R2 the use of a two-site model gives a reliable estimate of the dynamic parameters characterizing the motion of the Na + ions in the b o u n d state. It is worth noting that while, in the aqueous systems, the Na + ions retain an octahedral coordination (Oh symmetry), a tetrahedral coordination ( Ta symmetry) of the Na + ions by FM, NMF, and D M F molecules has been proposed as a reasonable explanation of the observed high-field shifts and increase in the quadrupolar coupling constant. © 1991 AcademicPress,Inc.
INTRODUCTION
Many scientific papers have dealt with studies concerning the microstrueture and the dynamic behavior of surfactant systems in aqueous solution (1-7). Less attention has been dedicated to surfactant systems in nonaqueous solvents ( 8-14). In particular, some NMR relaxation (15-21 ) and self-diffusion (22-24) studies on the counterions' behavior in surfactant aqueous systems gave evidence of an inhomogeneous distribution of the counterions in micellar systems; it has been suggested that about 60% of the counterions are located in the Stern layer of the charged microscopic interface in order to reduce its high charge density, and about 40% of them are in the bulk of the solution ( 15 ). Furthermore it has been shown that monovalent inorganic counterions retain their inner hydration shells on binding to micelles (19 ), thus involving long range electrostatic forces. In To w h o m correspondence should be addressed.
contrast, to our knowledge, only few data are available on the counterions' behavior in surfactant nonaqueous systems (1 lb). Recently the aggregation phenomena of the surfactant sodium dodecyl sulphate (SDS) in water solution (25-32) and in some nonaqueous polar solvents such as formamide (FM), N-methylformamide (NMF) and N,Ndimethyl formamide (DMF) (14, 33, 34), have been systematically investigated by means of 1H NMR multicomponent self-diffusion and ~3C and 2H NMR spin relaxation measurements. Those data demonstrated the occurrence of spherical micelles in the twocomponent SDS-water system, while in the aqueous microemulsions containing p-xylene (XYL) a bicontinuous structure and a waterin-oil droplet structure were ascertained for addition of pentanol (PEN) and octanol (OCT), respectively (31, 32). In the nonaqueous systems 1H self-diffusion data indicated rather negligible aggregation phenomena (33) while 13C and 2H NMR relaxation rates 363
Journalof ColloidandInterfaceScience,Vol. 146,No. 2, October 15, 1991
0021-9797/91 $3.00 Copyright© 1991by AcademicPress,Inc. All rightsof reproductionin any formreserved
364
CEGLIE ET AL.
of SDS in SDS-DMF, S D S - F M - O C T , and S D S - F M - O C T - X Y L systems indicated the occurrence of some kind of SDS aggregate (33, 34). In this context it seemed interesting to complete the analysis with a study on the behavior of the Na ÷ counterions by means of 23Na N M R relaxation measurements.
glected, the simplified expression o f R B,2 becomes R~B,2 = R e + R],2,
[31
where =
= (27r2/5)X2(1 + 7/2/3 - S2)rfB [4] RIB = ( 2 r 2 / 5 ) ( x S ) 2
THEORY
×
23Na is a quadrupolar nucleus with I = 3Its nuclear spin relaxation is generally due to coupling between its nuclear quadrupole moment and the fluctuating electric field gradients at the nucleus. If the extreme narrowing limit applies, i.e. (wzc) 2 4 1, the spin-lattice (R1) and spin-spin (R2) relaxation rates are given by (15) R1 = R2 = ( 2 7 r a / 5 ) X 2 ( l + 7/2/3)~-c,
+ 0.8J(20 )]
[5]
× [0.3./(0) + 0.5J(~o) + 0.2J(2w)].
[6]
R~B = (27r2/5)(XS) 2
Therefore the difference between the experimental spin-spin and spin-lattice relaxation rates is given by the relation R2 - Rl = p~(27r2/5)(xS)2[ 0 . 3 J ( 0 )
[1] + 0 . 3 J ( w ) - 0.6J(2w)].
where X is the quadrupolar coupling constant, r~ is the reorientational correlation time, and n is the asymmetry parameter for the electric field gradient, which vanishes in symmetric environments. In the presence o f any kind of aggregation, the Na + counterions will sample different environments and thus different relaxation rates. Assuming a rapid two-site exchange between free (F) and bound (B) sites, the relaxation rates become R I .2 -
p v R V1,2 q_ P B R B 2 ,
[2]
where PF and PB are the fractions o f N a ÷ ions in the free and in the bound state, respectively. In the free state the extreme narrowing condition applies, thus R v = R v = R F. In the bound state it has been shown that the hydrated counterions undergo two kinds of motions: an isotropic slow motion, r~cB, due to the tumbling of the aggregate, and a fast, slightly anisotropic motion, r[B, arising from modulation of a small local anisotropy induced by solvent motions ( 15, 20, 35). Thus, provided that the exchange rate between bound and free states and the difference of the chemical shifts of the two species can be neJournal of Colloid and Interface Science, Vol. 146,No. 2, October 15, 1991
[7]
Here the various J(w) are the reduced spectral density correlation functions defined as
J(w)
1
-~- (O)TcSB) 2 '
[ 8]
and ~'[B and rcBs are the fast and slow correlation times, respectively, of the Na ÷ ions in the bound state; S is the residual anisotropy due to preferential orientation of the sodium quadrupolar tensor with respect to the binding site, expressed as S = ½( 3 cos20 - 1 ), where 0 is the angle between the electric field gradient tensor and the local director, normal to the aggregate surface. These equations hold provided that WT~B ~ 1 and ~ fB ~ 7-cBs In order to determine rcB~it is often convenient to extract from Eq. [ 2] the excess relaxation rates Rlex and R2ex, Rlex= R 1 - R
F=pB(R~-R
R2ex= R 2 - R v = p B ( R g - R F ) ,
v)
[9] [10]
which represent a measure of the involvement of the Na + counterions in aggregation phenomena. Introducing Eqs. [ 3 ] - [ 7] and as-
NMR
RELAXATION
suming that R IB = R2fB ~ R v, the ratio Rlex/ R2exis simply given by the relation
Rlcx z~kRex --
R2ex
{ 0.2J(¢o) + 0.8J(2w) } { 0 . 3 J ( 0 ) + 0.5J(w) + 0.2J(2w) } '
[11] from which rib can be obtained in particularly favorable conditions; i.e., extreme narrowing does not hold but both RI and R2 still show exponential behavior. Then introducing rib into Eq. [ 7], the quantity SZpB can be calculated if X is known. Generally ifc0rgB >> 1 relaxation is expected to be bi-exponential (36). In that case the observed resonance signal is given by the superposition of a broad Lorentzian line (fast component) with 60% of the total intensity and a narrow Lorentzian line (slow component) with 40% of the total intensity. Likewise the longitudinal relaxation rate consists of a slow (80%) and a fast (20%) relaxation component. Thus, according to ( 17 ) the decays of the longitudinal and transverse magnetisation are given by
ML(t) -- ML(O) oC 0.2 exp(--tRfB) + 0.8 exp(--tR]B) [12]
M T ( t ) - MT(O) oc 0.6 exp(--tRfB) + 0.4 exp(--tR~B) [13] with RrB = (2zr2/5)X2J(o0)
[14]
RIB = (27r2/5)X2J(2w)
[15]
RfB = (~rz/5)X2{J(O)+ J(c0) }
[161
RS2~= (Tr2/5)xz{J(w)+ J(2c0)}.
[17]
If slow exchange between bound and free states occurs and one or both of the sites is not under conditions of extreme narrowing, relaxation becomes the sum of four exponentials (37).
OF
365
SDS SOLUTIONS EXPERIMENTAL
Materials Sodium dodecyl sulphate (99%, biochemical grade), pentan-l-ol (98%), octan-1ol (99%), formamide (analaR), N-methyl formamide (analaR), N,N-dimethyl formamide, and p-xylene (98.5%) were all from BDH England. All the reagents were used without further purification. The liquid components were stored on molecular sieves (previously activated at 500°C) for at least 24 h before being used, to remove water. The samples for N M R measurements were prepared by weighing sodium dodecyl sulphate directly in the N M R tubes and then by adding the proper amount of the other components. Samples were sealed, homogenized, and then stored for 6 days at room temperature before N M R measurements were taken.
Methods 23Na N M R relaxation measurements were run at 4.7 T on a Varian XL-200 spectrometer at the operating frequency of 52.902 MHz and at 26 + 0.5°C. Some additional measurements were run at 1.88 T on a Varian FT-80A at the operating frequency of 21.161 MHz for 23Na. The partially relaxed spectra were obtained by means of the usual PD-180°-T-90°-AC inversion recovery sequence. The spin-lattice relaxation rates R1 were calculated using a threeparameter nonlinear fitting of 14-20 experimental points. The spin-spin relaxation rates R2 were deduced from 23Na N M R spectra recorded with a 90 ° pulse angle. R2 values were calculated from the bandwidths taken at half height A~I/2 of the 23Na N M R signals after suitable correction for the magnetic field inhomogeneity; R2 = (A~I/2 -- 2xv*)Tr. The inhomogeneity contribution Av* was calculated from the difference between R2 and R~ observed for a 1 M NaC1 aqueous solution measured before and after recording each sample's 23Na spectrum. A maximum error of 1 Hz can be estimated for 23Na Re. Deviations from the Lorentzian bandshape o f the 23Na N M R signals were checked by evaluating the ratio Journal of Colloid and InterfaceScience, Vol. 146, No. 2, October 15, 199 l
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CEGLIE ET AL.
between the bandwidths at ~ and at ½ of the total height of the NMR signal (38). For a theoretical Lorentzian curve AVl/8 = 2.65
X Avl/2. RESULTS AND DISCUSSION Table I shows the composition of the samples and the experimental values of 23Na R1 and R2. The 23Na N M R signals were considered as Lorentzian bandshapes unless deviations greater than 10% from the theoretical value Av~/8/Avl/2 = 2.65 occurred. The SDSH 2 0 - O C T - X Y L system gave a ratio of 4.0, thus indicating the occurrence of a biexponential relaxation. For this system additional measurements were performed at 1.88 T and at 50°C. The experimental data together with the results of our analysis are reported separately in Table II and will be discussed later. Some general considerations arise from a first observation of the experimental data: (i) Only a few systems show significant differences between R1 and R2. (ii) The addition of a third and a fourth component to the SDS-solvent systems induces a decrease of both R~ and R2 in the aqueous systems, but it causes a significant increase of both Rt and R 2 in the nonaqueous systems. (iii) Na + counterions in the various two-component SDS-solvent systems behave rather differently. Figure 1 shows Rlex as a function of the surfactant concentration (wt%). It can be seen that in the aqueous system, largely above the CMC, R~ex slightly increases with increasing SDS concentration, thus indicating a relatively constant distribution of the populations of the bound and free ions, in agreement with previous findings ( 16, 19). On the contrary in the nonaqueous solvents, especially in the SDS-DMF system, g l e x rapidly increases with increasing SDS concentration, thus indicating the occurrence of rapidly evolving systems characterized by labile interfaces rather than the occurrence of specific interactions with a well-defined interface.
Aqueous Systems 23Na NMR relaxation rates of the hexaaquo sodium ions in aqueous NaC1 solution are Journal of Colloid and Interface Science, Vol. 146,No. 2, October15, 1991
usually interpreted by means of Eq. [1] by introducing rc = 10 ps and X = 665.8 KHz (17b). In all aqueous systems the 23Na N M R signals are always relatively narrow, symmetric, and strictly Lorentzian with the exception mentioned above. This suggests that the octahedral coordination sphere of the hexaaquo ions does not experience large distortions of the electric field gradients, in agreement with previous findings (19). Thus X = 665.8 KHz is likely to hold for all aqueous systems, although the R~ and R2 values differ significantly from R1 = R2 = 17.5 s -1. It has been observed that 23Na Rl'S relaxation rates equal R2's (i.e. the extreme narrowing limit applies) in several sodium n-alkyl sulphate surfactant solutions (16). In that case, however, surfactants with shorter chain length were examined, while for SDS only the R2 value was reported. Here a slight, but significant difference between R2 and R1 seems to indicate that in the two-component systems the larger size of SDS micelles causes the Na + ions to occur at the boundary between the extreme narrowing and the slow motion regions. At our operating frequency of 52.902 MHz this limit is given by 1/ w ~ 3 ns. In the SDS-H20 systems "R2-R1" slightly increases with increasing SDS concentration, However, no deviations from a single exponential decay are observed, thus indicating that relaxation rates, to a good approximation, can be described by Eqs. [ 3 ]- [ 11 ]. Table I shows the values of ARox and r~B (rc) obtained from Eq. [11] together with an estimate of the quantity S2pB calculated from Eq. [ 7 ] by introducing X = 665.8 KHz. These roBSvalues account for the observed relaxation rates well. In particular these values agree with the conditions of nonextreme narrowing without implying biexponential behavior of the quadrupolar relaxation. It should be noted that while r ~ is obtained from Eq. [11] without too severe approximations the quantity S2pB includes possible variations not only for S and PB but also for X. The decrease of both R~ and R2 values to-
TABLE I ZaNa N M R R e l a x a t i o n R a t e s a n d C a l c u l a t e d P a r a m e t e r s at 2 6 ° C a n d 4.7 T System and composition (wt%)
Ra (s-~)
NaCI-H20 ~ SDS-H20 a 5-95 10-90 20-80 SDS-H20-PEN 20-43.5-36.5 SDS-H20-PEN-XYL 17.5-35-35-12.5 NaC1-FM b SDS-FM 1-99 4-96 6.5-93.5 SDS-FM-PEN 15-55-30 SDS-FM-OCT a 15-55-30 SDS-FM-PEN-XYL 16-40-32-12 SDS-FM-OCT-XYL 16-40-32-12 NaC1-NMFb SDS-NMF 2-98 8-92 15-85 SDS-NMF-PEN 15-55-30 SDS-NMF-OCT 15-55-30 SDS-NMF-PEN-XYL
17.5
R2 (s-j)
APex
S2pB
9( = 665.8 K H z
z~ (ns)
± 0.08
17.5
0.010
53.48 ± 0.57 5 7 . 8 0 ± 0.33 6 4 . 5 2 ± 0.42
60.7 66.6 76.7
0.8329 0.8208 0.7943
0.028 0.031 0.034
0.980 1.020 1.125
4 0 . 4 9 ± 0.16
43.3
--
--
0.024
42.19 ± 0.18
44.3
--
--
0.025
95.0
± 2.5
97.0
100.0 113.0 123.0
± 3.0 ± 1.3 ± 1.5
100.0 113.0 126.0
----
----
0.039 0.044 0.048
333.0
± 1.1
333.0
--
--
0.130
312.0
± 1.9
329.0
0.9274
0.162
0.590
435.0
+__ 1.9
435.0
--
--
0.170
400.0
+ 1.6
427.0
0.9187
0.218
0.630
130.0
± 0.4
131.0
133.0 150.3 172.0
± 3.6 _+ 3.3 _+ 3.0
135.0 153.0 175.0
----
----
0.019 0.022 0.025
370.0
± 1.4
370.0
--
--
0.053
454.0 500.0
± 2.1 _+ 2.5
454.0 500.0
---
---
0.065 0.071
666.0
+_ 4.4
666.0
--
--
0.095
110.0
± 2.5
113.0
115.0 138.0 172.0
± 1.3 ± 1.2 _+ 1.3
115.0 143.0 184.0
-0,8485 0,8378
0.0027 0.0032 0.0070
0,150 0,920 0.960
244.0
_+ 1.2
251.0
0.9504
0.0245
0.480
278.0
± 1.6
278.0
--
--
0.220
x = 805.6 K H z
0.037
a
c
× = 1332.7 K H z
0.019
16-40-32-12 SDS-NMF-OCT-XYL 16-40-32-12 NaC1-DMF b SDS-DMF d 2-98 5-95 8-92 SDS-DMF-PEN 11-67-22 SDS-DMF-OCT 11-67-22
c
× = 1767.5 K H z
0.009
T h e I M N a C 1 a q u e o u s s o l u t i o n w a s u s e d as r e f e r e n c e to d e t e r m i n e t h e field i n h o m o g e n e i t y c o n t r i b u t i o n t o t h e linewidth, b N a C I h a s a v e r y l o w solubility in t h e n o n - a q u e o u s solvents, t h u s s a t u r a t e d s o l u t i o n s w e r e used. N o a p p r e c i a b l e d e v i a t i o n s f r o m a L o r e n t z i a n s h a p e w e r e d e t e c t e d in the 23Na N M R signals. c H e r e it w a s o b s e r v e d Av~/8]Av~/2 ~ 2.2 w h i c h is i n d i c a t i v e o f t h e o c c u r r e n c e o f a t least t w o different N a + species w i t h different c h e m i c a l shifts a n d in slow e x c h a n g e o n t h e N M R t i m e scale, rc w a s c a l c u l a t e d f r o m R l b y m e a n s o f Eq. [1]. d Eqs. [ 3 ] - [ 1 1 ] w e r e u s e d t h u s h e r e ~-c = riB. 367
Journal of Colloid and InterfaceScience, Vol. 146, No. 2, October 15, 1991
368
C E G L I E E T AL.
R=e x
Is "l}
60-
40
,1~ SDS-H20 •
SDS-FM
• SDS-NMF • SDS-DMF
20
SDS 5
10
15
(wt%} 20
FIG. 1. The excessrelaxationrate R~exas a function of SDS concentration(wt%) in the two-component systems. gether with the observation that R2 approaches R} in the three- and four-component systems with pentanol as cosurfactant indicates a decrease of the degree of aggregation and a decrease of order at the interface as suggested by the rc values of 25 ps reported in Table I. This agrees with the high self-diffusion coefficients ( ~ 10-10 m 2 / s ) of all the components (counterions are not included) which have been measured in these systems and further supports the occurrence of a bicontinous microstructure as previously suggested ( 31-33 ). As for the S D S - H 2 0 - O C T - X Y L system, from the data summarized in Table II, it can be seen that non-Lorentzian bandshapes of the 23Na N M R signals are always observed. At 1.88 T a significant decrease of the ratio A V l / 8 / A / ) } / 2 is found. The same behavior is observed at 50°C. This implies that the reorientational correlation time falls decisively under the slow motion regime. Therefore, neglecting the chemical exchange, Eqs. [ 12 ] and [ 13 ] must be used to rationalize the relaxation rates. Unfortunately, as often observed, the decay of the longitudinal magnetization does not show any appreciable deviations fi'om a single exponential, probably due to a mixing of the two longitudinal recovery rates after the 90 ° pulse of an usual inversion recovery seJournal of Colloid and lnterface Science, Vol. 146, No. 2, October }5, 1991
quence (39a,b) or if it happens that J(co) J(2w) (17b, 40a). In contrast, the deconvolution of the 23Na N M R signal, following the simple procedure proposed by Delville et al. (38), made it possible to estimate the fast and slow components of the transverse relaxation rates (Eqs. [ 16 ], [ 17 ] ), as reported in Table II. Figure 2 shows the result of the deconvolution at 4.7 T and 26°C. A very good agreement between the experimental 23Na signal and the theoretical curve is found. The variations of the fast and slow relaxation components as a function of frequency and temperature are in accord with previous analysis (40b). On the other hand, it is highly improbable that two different chemical environments would happen to occur with relative populations 0.6:0.4 exactly the same as for the two components of a bi-exponential decay (cfr. Eq. [131). The values of roBS obtained either from RfB/R~B ratios at each frequency or from the ratio (R2fB - - R s2B )a.7T / ( R 28 f - R 2sB ) I . 8 8 T s h o w a fairly good agreement. However, the possibility that the exchange rate between bound and free Na + ions may affect relaxation cannot be excluded in this system for which a waterin-oil droplet structure has been proposed (31, 33). A slow exchange between bound and free
NMR
RELAXATION
369
O F SDS S O L U T I O N S
T A B L E II 23Na R e l a x a t i o n Rates o f the S D S - H 2 0 - O C T - X Y L Field (T)
Rx (s-j)
Apl/s/Apx/2
~ S y s t e m at 4.7 a n d 1.88 T at 26 a n d 5 0 ° C b
R~s (s-l)
R~s (s-I)
Rr2rdR~
~s (ns)
T = 26°C 4.7
49.5 +- 0.2
4.0
228.0
55.0
4.15
6.0
1.88
55.9 + 0.6
3.4
214.0
70.0
3.06
8.9
TS c~ = 7.0 ns
S2(pB]pF) = 9.0
(RfB - R2B) s 4.7T/(R2B f -- R2s) s 1.88T = 1.2
~Bc = 5 0 - 6 5 m s
T = 50°C 4.7
37.7 + 0.2
3.3
110.0
36.0
3.06
4.6
1.88
39.7 _+0.2
2.9
107.0
60.0
1.78
5.0
(Rf2B R2a) s 4.7T/(R2a f --
R2B) L88r_ - 1.57 S
s = 4.3 ns r~B
S2(pn/pF)= 4.0
~'Bc = 8-30 ms
The composition (wt%) of the sample is (17.5-35.-35.-12.5). b At higher temperature this system separates into two phases. c The lifetime rB is obtained by introducing into Eqs. [9]-[14], modified followingBull's analysis and assuming that extreme narrowing applies for the free state, x = 665.8 KHz, the quantity S2(pB/pF)= 9 or 4 and rgB = 7 ns or 4.3 ns at 26°C or 50°C respectively and introducing also a dynamic frequency shift of I Hz at 4.7 T and 0.16 Hz at 1.88 T, at both the temperatures. a
N a + ions m a y be justified if we take into acc o u n t that a water droplet radius Rw ~ 25A can be calculated for a m i c r o e m u l s i o n with our composition ( 4 1 ) . In such a condition N a + ions certainly undergo restricted m o t i o n s and thus exchange rate between b o u n d and free sites m a y affect relaxation, Experimentally we did n o t observe any appreciable deviations f r o m a s y m m e t r i c shape in the 23Na N M R signals (cfr. Fig. 2), although the introduction o f a d y n a m i c shift o f 1-4 H z does not m o d i f y the goodness o f the fitting substantially. T h u s assuming slow exchange between F and B sites, where F is u n d e r conditions o f extreme narrowing while B falls in the slow m o t i o n regime, Eqs. [ 12 ] - [ 17 ] were modified, following Bull's analysis ( 3 7 ) , to take into a c c o u n t the dyn a m i c frequency shift a n d the lifetime rB o f the species at the b o u n d site. T h e results o f the calculations are reported in Table II. T o reproduce the experimental data, rather high values o f the quantities S21~/pv must be used; however, again, they are likely to have to compensate for a possible variation o f the X o f the sodium ions b o u n d at the interface. In fact it m u s t be taken into consideration that
octanol, on penetrating the charged interface to induce the droplet closure, certainly determines a higher degree of anistropy o f the sod i u m hydration sphere in the proximity o f the interface with respect to the micellar interface. However, these values of S2pB/Pvand rB seem to be the best ones within a wide range o f possible variations, and it should be remarked that in particular such a value o f rB c a n n o t effect the determination o f riB.
Nonaqueous Systems In these systems the N a + ions exhibit a strong increase o f b o t h RI and R2 values with respect to the aqueous systems. This effect is particularly evident in the three- a n d fourc o m p o n e n t systems for which an increase o f about one order o f magnitude is observed. In m a n y n o n a q u e o u s systems, with the exceptions m e n t i o n e d above, the " R z - R 1 " values a p p r o a c h zero. This m e a n s that m o t i o n s exceeding the extreme narrowing limit do not generally contribute to the total relaxation rates, and the correlation times for the slowest m o t i o n s would be lower than 1 ns. This implies that N a + ions do not experience binding Journal of Colloid and Interface Science, Vol. 146,No. 2, October 15, 1991
370
CEGLIE ET AL.
A
B
,o'oo
~o
5\ ~o
'
700
6~o
s~o
4~o
FIG. 2. Calculated (A) and experimental (B) 23Na NMR signal in the SDS-H20-OCT-XYL system at 4.7 T and 26 °C. Narrow Lorentzian signal (40% of the total intensity) with Avl/2 = 17.5 Hz. Broad Lorentzian signal (60% of the total intensity) with Lxvl/2 = 72.6 Hz.
sites to large aggregates in agreement with previous self-diffusion data, which gave evidence of a negligible self-association degree of SDS molecules in these solvents at room temperature (33). However, it should be mentioned that the occurrence ofSDS micelles in FM has been demonstrated at T above 55°C (13). In the extreme narrowing region an increase of the R~ and R2 values is strictly related to an increase of the quantity X2( 1 + 7 2 / 3 ) (cf. Eq. [1]). However, when symmetric N M R Lorentzian bandshapes are observed, an increase of the quadrupolar coupling constant is more likely to occur. This may be caused by a decrease of the degree of symmetry of the ions' surrounding. In fact, compared with the Journal of Colloid and Interface Science, Vol. 146, No. 2, October 15, 1991
aqueous systems, we observe a high-field shift of ~ 4 ppm in all the nonaqueous systems, independently of the nature of the solvent and of the complexity of the system. Considering that for Na + ions tetracoordinated by tetrahydrofuran or amine molecules (Td symmetry) an increase of the diamagnetic part of the shielding constant of ~ 11 ppm with respect to (Na ÷ )6H20 (Oh symmetry) has been observed (42), here a tetrahedral coordination of Na ÷ ions by FM, NMF, and D M F molecules can be reasonably assumed. In order to extract dynamic information from the experimental relaxation data we can assume that the radius of the hexaaquo sodium ions can be taken as the minimum radius of tetrahedric sodium ions in the nonaqueous solvents. Then from the relation rc = 47r~R3/ 3kT (where ~ is the viscosity of the solvent and R is radius of the solvated Na ÷ ions) and from Eq. [1] we obtain a rough estimate of the values of ~-c and X for the NaC1-FM, - N M F , - D M F systems as shown in Table I. By the use of these X values the r~'s are calculated for all the nonaqueous systems by means of Eq. [1] in the cases o f R l = R2 and by means of Eqs. [ 3]-[1 l] in the cases of Ri 4= R2 (see Table I). As was observed in the aqueous systems either experimental relaxation rates or calculated dynamic parameters are in agreement with previous 2H and 13C relaxation measurements of the same systems (34). In particular, it is evident that the solvent N M F does not favor any kind of aggregation of the counterions or the surfactant molecules (33, 34). The FM solvent in the presence of OCT as cosurfactant induces the SDS molecules aggregation (34) thus forming rather well defined binding sites for counterions, while the role of D M F still remains contradictory. As for the S D S - F M - O C T system it should be mentioned that a ~-~ = 6 ns and an order parameter S = 0.18, estimated for the o~-carbon of SDS (34), are indicative of a significant s 0.6 ns, estimated aggregation. Thus a rob for the counterions, agrees with the occurrence of a well-defined charged interface. In this
NMR RELAXATION OF SDS SOLUTIONS connection it should be mentioned that in the S D S - F M - D e c a n o l system, with concentrations close to those considered here, the occurrence o f a lamellar structure has been demonstrated ( 1 lb). Our samples do not show any birefringence but, in consideration of the slightly shorter chain of OCT, it is likely that some sort of prelamellar organization occurs in the systems. On the other hand, decanol chain length is more similar to that of SDS thus favoring the lamellar structure. The addition of X Y L to the S D S - F M - O C T system does not seem to modify the pre-existing structural features, substantially; thus analogous considerations hold. The S D S - D M F systems also deserve some further comments. At low SDS concentration R1 = R2 and the interpretation of the experimental data can be made either in terms of Eq. [1], which gives rc ~ 10 ps, or in terms of a two-site model, which leads to the figures reported in Table I. In the S D S - D M F system no SDS micelles are likely to occur since a very different trend of Rlex (cf. Fig. 1 ) or of the self-diffusion data (33) with respect to the aqueous micellar system is observed, although in this range of concentration SDS self-diffusion coefficients show an anomalous flexion, that is from D ~ 7 × 10 -1° m 2 / s (2 wt% SDS) to D ~ 4 × 10 -1° m 2 / s (8 wt% SDS). In fact, the low dielectric constant of D M F (e = 36.7) m a y favor some kind of contact ion pairs between sodium ions, differently from FM (E = 109) for which this behavior has been excluded (43). Thus the occurrence of small aggregates, whose formation is strongly dependent on the surfactant concentration can be hypothesized. Similar conclusions can be inferred for the S D S - D M F - P E N system. In particular, it is interesting to note that, analogously to previous findings (34), in D M F octanol acts as a structure breaker while pentanol produces a m i n o r effect, unlike what happens in F M where the former cosurfactant is likely to favor a prelameUar structural organization. In the aqueous solvent and in the presence of the oil, octanol induces the formation of a water-in-
371
oil droplet structure. At present, however, our data on D M F systems do not allow us to deepen this point further. In conclusion, the occurrence of some kind of association among the surfactant molecules, characterized by charged interfaces and by a certain degree of order, m a y be a reasonable explanation for the experimental results also in some of the non-aqueous systems. Whenever RI ~ R2 the use of a two-site model gave a reliable estimate of the dynamic parameters, while a relative uncertainty remains on the structural parameters. In fact the quantities S2/N (or S Z p B / P F ) not only depend on the order parameter and on the degree of counterion binding but also might include the need of compensating a possible variation of the quadrupolar coupling constant. In fact, X m a y increase or decrease as an effect of important distortions of the ideal octahedral or tetrahedral symmetry. These distortions can occur with a lifetime either m u c h longer or m u c h shorter than ~'e. Because of the symmetry of our 23Na N M R signals and the constancy of the chemical shifts, permanent distortions such as the substitution of a solvent molecule in the first coordination shell of the N a ÷ ions are not likely to occur. On the contrary, instantaneous distortions such as strongly anharmonic vibrations of the solvent molecules, due to intermolecular interactions, although they occur within a time scale shorter than re, m a y be effective to modify the fluctuating electric field gradients around the nucleus. Finally it is noteworthy that the 23Na N M R relaxation data while highlighting peculiar dynamic properties of counterions in some SDS systems, they generally agree with a structural picture drawn on the basis of other N M R results (31-34), thus giving further support.
ACKNOWLEDGMENTS We thank Professor Bjorn Lindman for some useful suggestions. Thanks are due to the Italian Council of Research C.N.R. and to Comitato Teenologico(C.N.R.) for partial financial support. Journal of Colloidand InterfaceScience, Vol.146,No.2, October15, 1991
372
CEGLIE ET AL. REFERENCES
1. Prince, L. M. (Ed.), "Microemulsions. Theory and Practice," Academic Press, New York, 1977. 2. Friberg, S. E., and Bothorel, P. (Eds.), "Microemulsions: Structure and Dynamics," CRC Press, Boca Raton, FL, 1987. 3. Rosano, H. L. and Clausse, M. (Eds.), "Microemulsion Systems", Surfactant Science Series, Vol. 24. Dekker, New York, 1987. 4. Shinoda, K., and Lindman, B., Langmuir 3, 137 ( 1987 ), and references therein. 5. Scriven, L. E., in "Micellization, Solubilization, and Microemulsions," (K. L. Mittal (Ed.), Vol. 2, p. 877). Plenum, New York, 1977. 6. Lindman, B., Soderman, O., and Wennerstrom, H., in "Surfactant in Solution. New Methods of Investigation" (R. Zana, Ed.). Dekker, New York, 1987, and references therein. 7. Chachaty, C., Prog. Nucl. Magn. Resort. Spectrosc. 19, 183 (1987). 8. Reinsborough, V. C., and Bloom, H., Aust. J. Chem. 20, 2583 (1967). 9. Singh, H. N., Saleem, S. M., Sing, R. P., and Birdi, K. S., J. Phys. Chem. 84, 2191 (1980). 10. Evans, D. F., and Ninham, B. W., J. Phys. Chem. 87, 5025 (1983). 11. (a) Friberg, S. E.; Liang, Y-C., in "Microemulsions: Structure and Dynamics" (S. E. Friberg and P. Bothorel, Eds.), Chap. 3. CRC Press, Boca Raton. (b) Ward, A. J. I., Rong, G., and Friberg, S. E., Colloids Surf. 38, 285 (1989) and references therein. (c) Friberg, S. E., Sun, W. M., Yang, Y., and Ward, A. J. I., J. Colloid Interface Sci. 139, 160 (1990). 12. Almgren, M., Swarup, S., and Lofroth, J. E., J. Phys. Chem. 89, 4621 (1985). 13. Rico, I., and Lattes, A., J. Phys. Chem. 90, 5870 (1986). 14. Das, K. P., Ceglie, A., and Lindman, B., J. Phys. Chem. 91, 2938 (1987). 15. Wennerstrom, H., Lindblom, G., and Lindman, B., Chem. Scr. 6, 97 (1974). 16. Gustavsson, H., and Lindman, B., J. Am. Chem. Soc. 100, 4647 (1978). 17. (a) Gustavsson, H., Lindman, B., and Bull, T., J. Am. Chem. Soc. 100, 4655 (1978). (b) Chang, D. C., and Woessner, D. E., J. Magn. Reson. 30, 185 (1985). 18. Gunnarsson, G., and Gustavsson, H., J. Chem. Soc. Faraday Trans. 1 78, 2901 (1982). 19. Lindman, B. "NMR of Newly Accessible Nuclei" (P. Laszlo, Ed.), Vol. 1, p. 193 Academic Press, New York, 1983, and references therein. 20. Piculell, L., Lindman, B., and Einarsson, R., Biopolymers 23, 1683 (1984). 21. (a) Halle, B., Wennerstrom, H.; and Piculell, L., ,Z Journal of Colloid and Interface Science, Vol. 146, No. 2, October 15, 1991
22.
23. 24. 25. 26. 27.
28. 29.
30.
31. 32. 33.
34. 35. 36. 37. 38. 39.
40.
41.
42. 43.
Phys. Chem. 88, 2482 (1984). (b) Halle, B., Bratko, D., and Piculell, L., Ber. Bunsenges Phys. Chem. 89, 1254 ( 1985 ) and references therein. Lindman, B., Puyal, M. C., Kamenka, N., Brun, B., and Gunnarsson, G., J. Phys. Chem. 86, 1702 (1982). Nilsson, L. G., Nordenskiold, L., Stilbs, P., and Braunlin, W. H., J. Phys. Chem. 89, 3385 (1985). Bratko, D., and Lindman, B., J. Phys. Chem. 89, 1437 (1985). Stilbs, P., Soderman, O., and Walderhaug, H., J. Magn. Reson. 69, 411 (1986). Ellena, J. F., Dominey, R. N., and Cafiso, D. S., J. Phys. Chem. 91, 131 (1987). Soderman, O., Carlstrom, G., Olsson, U., and Wong, T. C., J. Chem. Soc. Faraday Trans. 1 84, 4475 (1988). Ahlnas, J. T., Thesis, Lund, 1985. Bellocq, A. M., Biais, J., Clin, B., Lalanne P., and Lemanceu, B., J. Colloid Interface Sci. 70, 524 (1979). Soderman, O., Canet, D., Carnali, J., Henriksson, U., Nery, H., Walderhaug, H., and Warnheim, R., in "Microemulsion Systems" (H. L. Rosano and M. Clausse, Eds.), Surfactant Science Series, Vol. 24, p. 145. Dekker, New York, 1987. Ceglie, A., Das, K. P., and Lindman, B., J. Colloid Interface Sci. 115, 115 (1987). Monduzzi, M., Ceglie, A., Lindman, B., and Soderman, 0., J. ColloidlnterfaceSci. 136, 113 (1990). Das, K. P., Ceglie, A., Monduzzi, M., Soderman, 0., and Lindman, B., Prog. Colloid Polym. Sci. 73, 167 (1987). Ceglie, A., Monduzzi, M., and Soderrnan, O., J. Colloid Interface Sci. 142, 129 ( 1991 ). Halle, B., and Wennerstrom, H., J. Magn. Reson. 44, 89 ( 1981 ). Hubbard, P. S., J. Chem. Phys. 53, 985 (1970). Bull, T. E., J. Magn. Resort. 8, 344 (1972). Delville, A., Detellier, C., and Laszlo, P., J. Magn. Reson. 34, 301 (1979). (a) Werbelow, L. G., and Marshall, A. G., J. Magn. Reson. 43, 443 (1981). (b) Marshall, A. G., Wang, T. C. L., Cottrell, C. E., and Werbelow, L. G., J. Am. Chem. Soc. 104, 7665 (1982). (a) Levij, M., De Bleijser, J., and Leyte, J. C., Chem. Phys. Lett. 83, 183 (1981). (b) Lerner, L., and Torchia, D. A., J. Am. Chem. Soc. 108, 4264 (1986). (a) Israelachvili, N., Mitchell, D. J., and Ninham, B. W., J. Chem. Soc. Faraday Trans. 2 72, 1525 (1976). (b) Mitchell, D. J., and Ninham, B. W., J. Chem. Soc. Faraday Trans. 2 77, 601 (1981). Delville, A., Detellier, C., Gerstmans, A., and Laszlo, P., J. Magn. Reson. 42, 14 (198t). Greenberg, M. S., Bodner, R. L., and Popov, A. I., J. Phys. Chem. 77, 2449 (t973).
INTRODUCTION
Many scientific papers have dealt with studies concerning the microstrueture and the dynamic behavior of surfactant systems in aqueous solution (1-7). Less attention has been dedicated to surfactant systems in nonaqueous solvents ( 8-14). In particular, some NMR relaxation (15-21 ) and self-diffusion (22-24) studies on the counterions' behavior in surfactant aqueous systems gave evidence of an inhomogeneous distribution of the counterions in micellar systems; it has been suggested that about 60% of the counterions are located in the Stern layer of the charged microscopic interface in order to reduce its high charge density, and about 40% of them are in the bulk of the solution ( 15 ). Furthermore it has been shown that monovalent inorganic counterions retain their inner hydration shells on binding to micelles (19 ), thus involving long range electrostatic forces. In To w h o m correspondence should be addressed.
contrast, to our knowledge, only few data are available on the counterions' behavior in surfactant nonaqueous systems (1 lb). Recently the aggregation phenomena of the surfactant sodium dodecyl sulphate (SDS) in water solution (25-32) and in some nonaqueous polar solvents such as formamide (FM), N-methylformamide (NMF) and N,Ndimethyl formamide (DMF) (14, 33, 34), have been systematically investigated by means of 1H NMR multicomponent self-diffusion and ~3C and 2H NMR spin relaxation measurements. Those data demonstrated the occurrence of spherical micelles in the twocomponent SDS-water system, while in the aqueous microemulsions containing p-xylene (XYL) a bicontinuous structure and a waterin-oil droplet structure were ascertained for addition of pentanol (PEN) and octanol (OCT), respectively (31, 32). In the nonaqueous systems 1H self-diffusion data indicated rather negligible aggregation phenomena (33) while 13C and 2H NMR relaxation rates 363
Journalof ColloidandInterfaceScience,Vol. 146,No. 2, October 15, 1991
0021-9797/91 $3.00 Copyright© 1991by AcademicPress,Inc. All rightsof reproductionin any formreserved
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CEGLIE ET AL.
of SDS in SDS-DMF, S D S - F M - O C T , and S D S - F M - O C T - X Y L systems indicated the occurrence of some kind of SDS aggregate (33, 34). In this context it seemed interesting to complete the analysis with a study on the behavior of the Na ÷ counterions by means of 23Na N M R relaxation measurements.
glected, the simplified expression o f R B,2 becomes R~B,2 = R e + R],2,
[31
where =
= (27r2/5)X2(1 + 7/2/3 - S2)rfB [4] RIB = ( 2 r 2 / 5 ) ( x S ) 2
THEORY
×
23Na is a quadrupolar nucleus with I = 3Its nuclear spin relaxation is generally due to coupling between its nuclear quadrupole moment and the fluctuating electric field gradients at the nucleus. If the extreme narrowing limit applies, i.e. (wzc) 2 4 1, the spin-lattice (R1) and spin-spin (R2) relaxation rates are given by (15) R1 = R2 = ( 2 7 r a / 5 ) X 2 ( l + 7/2/3)~-c,
+ 0.8J(20 )]
[5]
× [0.3./(0) + 0.5J(~o) + 0.2J(2w)].
[6]
R~B = (27r2/5)(XS) 2
Therefore the difference between the experimental spin-spin and spin-lattice relaxation rates is given by the relation R2 - Rl = p~(27r2/5)(xS)2[ 0 . 3 J ( 0 )
[1] + 0 . 3 J ( w ) - 0.6J(2w)].
where X is the quadrupolar coupling constant, r~ is the reorientational correlation time, and n is the asymmetry parameter for the electric field gradient, which vanishes in symmetric environments. In the presence o f any kind of aggregation, the Na + counterions will sample different environments and thus different relaxation rates. Assuming a rapid two-site exchange between free (F) and bound (B) sites, the relaxation rates become R I .2 -
p v R V1,2 q_ P B R B 2 ,
[2]
where PF and PB are the fractions o f N a ÷ ions in the free and in the bound state, respectively. In the free state the extreme narrowing condition applies, thus R v = R v = R F. In the bound state it has been shown that the hydrated counterions undergo two kinds of motions: an isotropic slow motion, r~cB, due to the tumbling of the aggregate, and a fast, slightly anisotropic motion, r[B, arising from modulation of a small local anisotropy induced by solvent motions ( 15, 20, 35). Thus, provided that the exchange rate between bound and free states and the difference of the chemical shifts of the two species can be neJournal of Colloid and Interface Science, Vol. 146,No. 2, October 15, 1991
[7]
Here the various J(w) are the reduced spectral density correlation functions defined as
J(w)
1
-~- (O)TcSB) 2 '
[ 8]
and ~'[B and rcBs are the fast and slow correlation times, respectively, of the Na ÷ ions in the bound state; S is the residual anisotropy due to preferential orientation of the sodium quadrupolar tensor with respect to the binding site, expressed as S = ½( 3 cos20 - 1 ), where 0 is the angle between the electric field gradient tensor and the local director, normal to the aggregate surface. These equations hold provided that WT~B ~ 1 and ~ fB ~ 7-cBs In order to determine rcB~it is often convenient to extract from Eq. [ 2] the excess relaxation rates Rlex and R2ex, Rlex= R 1 - R
F=pB(R~-R
R2ex= R 2 - R v = p B ( R g - R F ) ,
v)
[9] [10]
which represent a measure of the involvement of the Na + counterions in aggregation phenomena. Introducing Eqs. [ 3 ] - [ 7] and as-
NMR
RELAXATION
suming that R IB = R2fB ~ R v, the ratio Rlex/ R2exis simply given by the relation
Rlcx z~kRex --
R2ex
{ 0.2J(¢o) + 0.8J(2w) } { 0 . 3 J ( 0 ) + 0.5J(w) + 0.2J(2w) } '
[11] from which rib can be obtained in particularly favorable conditions; i.e., extreme narrowing does not hold but both RI and R2 still show exponential behavior. Then introducing rib into Eq. [ 7], the quantity SZpB can be calculated if X is known. Generally ifc0rgB >> 1 relaxation is expected to be bi-exponential (36). In that case the observed resonance signal is given by the superposition of a broad Lorentzian line (fast component) with 60% of the total intensity and a narrow Lorentzian line (slow component) with 40% of the total intensity. Likewise the longitudinal relaxation rate consists of a slow (80%) and a fast (20%) relaxation component. Thus, according to ( 17 ) the decays of the longitudinal and transverse magnetisation are given by
ML(t) -- ML(O) oC 0.2 exp(--tRfB) + 0.8 exp(--tR]B) [12]
M T ( t ) - MT(O) oc 0.6 exp(--tRfB) + 0.4 exp(--tR~B) [13] with RrB = (2zr2/5)X2J(o0)
[14]
RIB = (27r2/5)X2J(2w)
[15]
RfB = (~rz/5)X2{J(O)+ J(c0) }
[161
RS2~= (Tr2/5)xz{J(w)+ J(2c0)}.
[17]
If slow exchange between bound and free states occurs and one or both of the sites is not under conditions of extreme narrowing, relaxation becomes the sum of four exponentials (37).
OF
365
SDS SOLUTIONS EXPERIMENTAL
Materials Sodium dodecyl sulphate (99%, biochemical grade), pentan-l-ol (98%), octan-1ol (99%), formamide (analaR), N-methyl formamide (analaR), N,N-dimethyl formamide, and p-xylene (98.5%) were all from BDH England. All the reagents were used without further purification. The liquid components were stored on molecular sieves (previously activated at 500°C) for at least 24 h before being used, to remove water. The samples for N M R measurements were prepared by weighing sodium dodecyl sulphate directly in the N M R tubes and then by adding the proper amount of the other components. Samples were sealed, homogenized, and then stored for 6 days at room temperature before N M R measurements were taken.
Methods 23Na N M R relaxation measurements were run at 4.7 T on a Varian XL-200 spectrometer at the operating frequency of 52.902 MHz and at 26 + 0.5°C. Some additional measurements were run at 1.88 T on a Varian FT-80A at the operating frequency of 21.161 MHz for 23Na. The partially relaxed spectra were obtained by means of the usual PD-180°-T-90°-AC inversion recovery sequence. The spin-lattice relaxation rates R1 were calculated using a threeparameter nonlinear fitting of 14-20 experimental points. The spin-spin relaxation rates R2 were deduced from 23Na N M R spectra recorded with a 90 ° pulse angle. R2 values were calculated from the bandwidths taken at half height A~I/2 of the 23Na N M R signals after suitable correction for the magnetic field inhomogeneity; R2 = (A~I/2 -- 2xv*)Tr. The inhomogeneity contribution Av* was calculated from the difference between R2 and R~ observed for a 1 M NaC1 aqueous solution measured before and after recording each sample's 23Na spectrum. A maximum error of 1 Hz can be estimated for 23Na Re. Deviations from the Lorentzian bandshape o f the 23Na N M R signals were checked by evaluating the ratio Journal of Colloid and InterfaceScience, Vol. 146, No. 2, October 15, 199 l
366
CEGLIE ET AL.
between the bandwidths at ~ and at ½ of the total height of the NMR signal (38). For a theoretical Lorentzian curve AVl/8 = 2.65
X Avl/2. RESULTS AND DISCUSSION Table I shows the composition of the samples and the experimental values of 23Na R1 and R2. The 23Na N M R signals were considered as Lorentzian bandshapes unless deviations greater than 10% from the theoretical value Av~/8/Avl/2 = 2.65 occurred. The SDSH 2 0 - O C T - X Y L system gave a ratio of 4.0, thus indicating the occurrence of a biexponential relaxation. For this system additional measurements were performed at 1.88 T and at 50°C. The experimental data together with the results of our analysis are reported separately in Table II and will be discussed later. Some general considerations arise from a first observation of the experimental data: (i) Only a few systems show significant differences between R1 and R2. (ii) The addition of a third and a fourth component to the SDS-solvent systems induces a decrease of both R~ and R2 in the aqueous systems, but it causes a significant increase of both Rt and R 2 in the nonaqueous systems. (iii) Na + counterions in the various two-component SDS-solvent systems behave rather differently. Figure 1 shows Rlex as a function of the surfactant concentration (wt%). It can be seen that in the aqueous system, largely above the CMC, R~ex slightly increases with increasing SDS concentration, thus indicating a relatively constant distribution of the populations of the bound and free ions, in agreement with previous findings ( 16, 19). On the contrary in the nonaqueous solvents, especially in the SDS-DMF system, g l e x rapidly increases with increasing SDS concentration, thus indicating the occurrence of rapidly evolving systems characterized by labile interfaces rather than the occurrence of specific interactions with a well-defined interface.
Aqueous Systems 23Na NMR relaxation rates of the hexaaquo sodium ions in aqueous NaC1 solution are Journal of Colloid and Interface Science, Vol. 146,No. 2, October15, 1991
usually interpreted by means of Eq. [1] by introducing rc = 10 ps and X = 665.8 KHz (17b). In all aqueous systems the 23Na N M R signals are always relatively narrow, symmetric, and strictly Lorentzian with the exception mentioned above. This suggests that the octahedral coordination sphere of the hexaaquo ions does not experience large distortions of the electric field gradients, in agreement with previous findings (19). Thus X = 665.8 KHz is likely to hold for all aqueous systems, although the R~ and R2 values differ significantly from R1 = R2 = 17.5 s -1. It has been observed that 23Na Rl'S relaxation rates equal R2's (i.e. the extreme narrowing limit applies) in several sodium n-alkyl sulphate surfactant solutions (16). In that case, however, surfactants with shorter chain length were examined, while for SDS only the R2 value was reported. Here a slight, but significant difference between R2 and R1 seems to indicate that in the two-component systems the larger size of SDS micelles causes the Na + ions to occur at the boundary between the extreme narrowing and the slow motion regions. At our operating frequency of 52.902 MHz this limit is given by 1/ w ~ 3 ns. In the SDS-H20 systems "R2-R1" slightly increases with increasing SDS concentration, However, no deviations from a single exponential decay are observed, thus indicating that relaxation rates, to a good approximation, can be described by Eqs. [ 3 ]- [ 11 ]. Table I shows the values of ARox and r~B (rc) obtained from Eq. [11] together with an estimate of the quantity S2pB calculated from Eq. [ 7 ] by introducing X = 665.8 KHz. These roBSvalues account for the observed relaxation rates well. In particular these values agree with the conditions of nonextreme narrowing without implying biexponential behavior of the quadrupolar relaxation. It should be noted that while r ~ is obtained from Eq. [11] without too severe approximations the quantity S2pB includes possible variations not only for S and PB but also for X. The decrease of both R~ and R2 values to-
TABLE I ZaNa N M R R e l a x a t i o n R a t e s a n d C a l c u l a t e d P a r a m e t e r s at 2 6 ° C a n d 4.7 T System and composition (wt%)
Ra (s-~)
NaCI-H20 ~ SDS-H20 a 5-95 10-90 20-80 SDS-H20-PEN 20-43.5-36.5 SDS-H20-PEN-XYL 17.5-35-35-12.5 NaC1-FM b SDS-FM 1-99 4-96 6.5-93.5 SDS-FM-PEN 15-55-30 SDS-FM-OCT a 15-55-30 SDS-FM-PEN-XYL 16-40-32-12 SDS-FM-OCT-XYL 16-40-32-12 NaC1-NMFb SDS-NMF 2-98 8-92 15-85 SDS-NMF-PEN 15-55-30 SDS-NMF-OCT 15-55-30 SDS-NMF-PEN-XYL
17.5
R2 (s-j)
APex
S2pB
9( = 665.8 K H z
z~ (ns)
± 0.08
17.5
0.010
53.48 ± 0.57 5 7 . 8 0 ± 0.33 6 4 . 5 2 ± 0.42
60.7 66.6 76.7
0.8329 0.8208 0.7943
0.028 0.031 0.034
0.980 1.020 1.125
4 0 . 4 9 ± 0.16
43.3
--
--
0.024
42.19 ± 0.18
44.3
--
--
0.025
95.0
± 2.5
97.0
100.0 113.0 123.0
± 3.0 ± 1.3 ± 1.5
100.0 113.0 126.0
----
----
0.039 0.044 0.048
333.0
± 1.1
333.0
--
--
0.130
312.0
± 1.9
329.0
0.9274
0.162
0.590
435.0
+__ 1.9
435.0
--
--
0.170
400.0
+ 1.6
427.0
0.9187
0.218
0.630
130.0
± 0.4
131.0
133.0 150.3 172.0
± 3.6 _+ 3.3 _+ 3.0
135.0 153.0 175.0
----
----
0.019 0.022 0.025
370.0
± 1.4
370.0
--
--
0.053
454.0 500.0
± 2.1 _+ 2.5
454.0 500.0
---
---
0.065 0.071
666.0
+_ 4.4
666.0
--
--
0.095
110.0
± 2.5
113.0
115.0 138.0 172.0
± 1.3 ± 1.2 _+ 1.3
115.0 143.0 184.0
-0,8485 0,8378
0.0027 0.0032 0.0070
0,150 0,920 0.960
244.0
_+ 1.2
251.0
0.9504
0.0245
0.480
278.0
± 1.6
278.0
--
--
0.220
x = 805.6 K H z
0.037
a
c
× = 1332.7 K H z
0.019
16-40-32-12 SDS-NMF-OCT-XYL 16-40-32-12 NaC1-DMF b SDS-DMF d 2-98 5-95 8-92 SDS-DMF-PEN 11-67-22 SDS-DMF-OCT 11-67-22
c
× = 1767.5 K H z
0.009
T h e I M N a C 1 a q u e o u s s o l u t i o n w a s u s e d as r e f e r e n c e to d e t e r m i n e t h e field i n h o m o g e n e i t y c o n t r i b u t i o n t o t h e linewidth, b N a C I h a s a v e r y l o w solubility in t h e n o n - a q u e o u s solvents, t h u s s a t u r a t e d s o l u t i o n s w e r e used. N o a p p r e c i a b l e d e v i a t i o n s f r o m a L o r e n t z i a n s h a p e w e r e d e t e c t e d in the 23Na N M R signals. c H e r e it w a s o b s e r v e d Av~/8]Av~/2 ~ 2.2 w h i c h is i n d i c a t i v e o f t h e o c c u r r e n c e o f a t least t w o different N a + species w i t h different c h e m i c a l shifts a n d in slow e x c h a n g e o n t h e N M R t i m e scale, rc w a s c a l c u l a t e d f r o m R l b y m e a n s o f Eq. [1]. d Eqs. [ 3 ] - [ 1 1 ] w e r e u s e d t h u s h e r e ~-c = riB. 367
Journal of Colloid and InterfaceScience, Vol. 146, No. 2, October 15, 1991
368
C E G L I E E T AL.
R=e x
Is "l}
60-
40
,1~ SDS-H20 •
SDS-FM
• SDS-NMF • SDS-DMF
20
SDS 5
10
15
(wt%} 20
FIG. 1. The excessrelaxationrate R~exas a function of SDS concentration(wt%) in the two-component systems. gether with the observation that R2 approaches R} in the three- and four-component systems with pentanol as cosurfactant indicates a decrease of the degree of aggregation and a decrease of order at the interface as suggested by the rc values of 25 ps reported in Table I. This agrees with the high self-diffusion coefficients ( ~ 10-10 m 2 / s ) of all the components (counterions are not included) which have been measured in these systems and further supports the occurrence of a bicontinous microstructure as previously suggested ( 31-33 ). As for the S D S - H 2 0 - O C T - X Y L system, from the data summarized in Table II, it can be seen that non-Lorentzian bandshapes of the 23Na N M R signals are always observed. At 1.88 T a significant decrease of the ratio A V l / 8 / A / ) } / 2 is found. The same behavior is observed at 50°C. This implies that the reorientational correlation time falls decisively under the slow motion regime. Therefore, neglecting the chemical exchange, Eqs. [ 12 ] and [ 13 ] must be used to rationalize the relaxation rates. Unfortunately, as often observed, the decay of the longitudinal magnetization does not show any appreciable deviations fi'om a single exponential, probably due to a mixing of the two longitudinal recovery rates after the 90 ° pulse of an usual inversion recovery seJournal of Colloid and lnterface Science, Vol. 146, No. 2, October }5, 1991
quence (39a,b) or if it happens that J(co) J(2w) (17b, 40a). In contrast, the deconvolution of the 23Na N M R signal, following the simple procedure proposed by Delville et al. (38), made it possible to estimate the fast and slow components of the transverse relaxation rates (Eqs. [ 16 ], [ 17 ] ), as reported in Table II. Figure 2 shows the result of the deconvolution at 4.7 T and 26°C. A very good agreement between the experimental 23Na signal and the theoretical curve is found. The variations of the fast and slow relaxation components as a function of frequency and temperature are in accord with previous analysis (40b). On the other hand, it is highly improbable that two different chemical environments would happen to occur with relative populations 0.6:0.4 exactly the same as for the two components of a bi-exponential decay (cfr. Eq. [131). The values of roBS obtained either from RfB/R~B ratios at each frequency or from the ratio (R2fB - - R s2B )a.7T / ( R 28 f - R 2sB ) I . 8 8 T s h o w a fairly good agreement. However, the possibility that the exchange rate between bound and free Na + ions may affect relaxation cannot be excluded in this system for which a waterin-oil droplet structure has been proposed (31, 33). A slow exchange between bound and free
NMR
RELAXATION
369
O F SDS S O L U T I O N S
T A B L E II 23Na R e l a x a t i o n Rates o f the S D S - H 2 0 - O C T - X Y L Field (T)
Rx (s-j)
Apl/s/Apx/2
~ S y s t e m at 4.7 a n d 1.88 T at 26 a n d 5 0 ° C b
R~s (s-l)
R~s (s-I)
Rr2rdR~
~s (ns)
T = 26°C 4.7
49.5 +- 0.2
4.0
228.0
55.0
4.15
6.0
1.88
55.9 + 0.6
3.4
214.0
70.0
3.06
8.9
TS c~ = 7.0 ns
S2(pB]pF) = 9.0
(RfB - R2B) s 4.7T/(R2B f -- R2s) s 1.88T = 1.2
~Bc = 5 0 - 6 5 m s
T = 50°C 4.7
37.7 + 0.2
3.3
110.0
36.0
3.06
4.6
1.88
39.7 _+0.2
2.9
107.0
60.0
1.78
5.0
(Rf2B R2a) s 4.7T/(R2a f --
R2B) L88r_ - 1.57 S
s = 4.3 ns r~B
S2(pn/pF)= 4.0
~'Bc = 8-30 ms
The composition (wt%) of the sample is (17.5-35.-35.-12.5). b At higher temperature this system separates into two phases. c The lifetime rB is obtained by introducing into Eqs. [9]-[14], modified followingBull's analysis and assuming that extreme narrowing applies for the free state, x = 665.8 KHz, the quantity S2(pB/pF)= 9 or 4 and rgB = 7 ns or 4.3 ns at 26°C or 50°C respectively and introducing also a dynamic frequency shift of I Hz at 4.7 T and 0.16 Hz at 1.88 T, at both the temperatures. a
N a + ions m a y be justified if we take into acc o u n t that a water droplet radius Rw ~ 25A can be calculated for a m i c r o e m u l s i o n with our composition ( 4 1 ) . In such a condition N a + ions certainly undergo restricted m o t i o n s and thus exchange rate between b o u n d and free sites m a y affect relaxation, Experimentally we did n o t observe any appreciable deviations f r o m a s y m m e t r i c shape in the 23Na N M R signals (cfr. Fig. 2), although the introduction o f a d y n a m i c shift o f 1-4 H z does not m o d i f y the goodness o f the fitting substantially. T h u s assuming slow exchange between F and B sites, where F is u n d e r conditions o f extreme narrowing while B falls in the slow m o t i o n regime, Eqs. [ 12 ] - [ 17 ] were modified, following Bull's analysis ( 3 7 ) , to take into a c c o u n t the dyn a m i c frequency shift a n d the lifetime rB o f the species at the b o u n d site. T h e results o f the calculations are reported in Table II. T o reproduce the experimental data, rather high values o f the quantities S21~/pv must be used; however, again, they are likely to have to compensate for a possible variation o f the X o f the sodium ions b o u n d at the interface. In fact it m u s t be taken into consideration that
octanol, on penetrating the charged interface to induce the droplet closure, certainly determines a higher degree of anistropy o f the sod i u m hydration sphere in the proximity o f the interface with respect to the micellar interface. However, these values of S2pB/Pvand rB seem to be the best ones within a wide range o f possible variations, and it should be remarked that in particular such a value o f rB c a n n o t effect the determination o f riB.
Nonaqueous Systems In these systems the N a + ions exhibit a strong increase o f b o t h RI and R2 values with respect to the aqueous systems. This effect is particularly evident in the three- a n d fourc o m p o n e n t systems for which an increase o f about one order o f magnitude is observed. In m a n y n o n a q u e o u s systems, with the exceptions m e n t i o n e d above, the " R z - R 1 " values a p p r o a c h zero. This m e a n s that m o t i o n s exceeding the extreme narrowing limit do not generally contribute to the total relaxation rates, and the correlation times for the slowest m o t i o n s would be lower than 1 ns. This implies that N a + ions do not experience binding Journal of Colloid and Interface Science, Vol. 146,No. 2, October 15, 1991
370
CEGLIE ET AL.
A
B
,o'oo
~o
5\ ~o
'
700
6~o
s~o
4~o
FIG. 2. Calculated (A) and experimental (B) 23Na NMR signal in the SDS-H20-OCT-XYL system at 4.7 T and 26 °C. Narrow Lorentzian signal (40% of the total intensity) with Avl/2 = 17.5 Hz. Broad Lorentzian signal (60% of the total intensity) with Lxvl/2 = 72.6 Hz.
sites to large aggregates in agreement with previous self-diffusion data, which gave evidence of a negligible self-association degree of SDS molecules in these solvents at room temperature (33). However, it should be mentioned that the occurrence ofSDS micelles in FM has been demonstrated at T above 55°C (13). In the extreme narrowing region an increase of the R~ and R2 values is strictly related to an increase of the quantity X2( 1 + 7 2 / 3 ) (cf. Eq. [1]). However, when symmetric N M R Lorentzian bandshapes are observed, an increase of the quadrupolar coupling constant is more likely to occur. This may be caused by a decrease of the degree of symmetry of the ions' surrounding. In fact, compared with the Journal of Colloid and Interface Science, Vol. 146, No. 2, October 15, 1991
aqueous systems, we observe a high-field shift of ~ 4 ppm in all the nonaqueous systems, independently of the nature of the solvent and of the complexity of the system. Considering that for Na + ions tetracoordinated by tetrahydrofuran or amine molecules (Td symmetry) an increase of the diamagnetic part of the shielding constant of ~ 11 ppm with respect to (Na ÷ )6H20 (Oh symmetry) has been observed (42), here a tetrahedral coordination of Na ÷ ions by FM, NMF, and D M F molecules can be reasonably assumed. In order to extract dynamic information from the experimental relaxation data we can assume that the radius of the hexaaquo sodium ions can be taken as the minimum radius of tetrahedric sodium ions in the nonaqueous solvents. Then from the relation rc = 47r~R3/ 3kT (where ~ is the viscosity of the solvent and R is radius of the solvated Na ÷ ions) and from Eq. [1] we obtain a rough estimate of the values of ~-c and X for the NaC1-FM, - N M F , - D M F systems as shown in Table I. By the use of these X values the r~'s are calculated for all the nonaqueous systems by means of Eq. [1] in the cases o f R l = R2 and by means of Eqs. [ 3]-[1 l] in the cases of Ri 4= R2 (see Table I). As was observed in the aqueous systems either experimental relaxation rates or calculated dynamic parameters are in agreement with previous 2H and 13C relaxation measurements of the same systems (34). In particular, it is evident that the solvent N M F does not favor any kind of aggregation of the counterions or the surfactant molecules (33, 34). The FM solvent in the presence of OCT as cosurfactant induces the SDS molecules aggregation (34) thus forming rather well defined binding sites for counterions, while the role of D M F still remains contradictory. As for the S D S - F M - O C T system it should be mentioned that a ~-~ = 6 ns and an order parameter S = 0.18, estimated for the o~-carbon of SDS (34), are indicative of a significant s 0.6 ns, estimated aggregation. Thus a rob for the counterions, agrees with the occurrence of a well-defined charged interface. In this
NMR RELAXATION OF SDS SOLUTIONS connection it should be mentioned that in the S D S - F M - D e c a n o l system, with concentrations close to those considered here, the occurrence o f a lamellar structure has been demonstrated ( 1 lb). Our samples do not show any birefringence but, in consideration of the slightly shorter chain of OCT, it is likely that some sort of prelamellar organization occurs in the systems. On the other hand, decanol chain length is more similar to that of SDS thus favoring the lamellar structure. The addition of X Y L to the S D S - F M - O C T system does not seem to modify the pre-existing structural features, substantially; thus analogous considerations hold. The S D S - D M F systems also deserve some further comments. At low SDS concentration R1 = R2 and the interpretation of the experimental data can be made either in terms of Eq. [1], which gives rc ~ 10 ps, or in terms of a two-site model, which leads to the figures reported in Table I. In the S D S - D M F system no SDS micelles are likely to occur since a very different trend of Rlex (cf. Fig. 1 ) or of the self-diffusion data (33) with respect to the aqueous micellar system is observed, although in this range of concentration SDS self-diffusion coefficients show an anomalous flexion, that is from D ~ 7 × 10 -1° m 2 / s (2 wt% SDS) to D ~ 4 × 10 -1° m 2 / s (8 wt% SDS). In fact, the low dielectric constant of D M F (e = 36.7) m a y favor some kind of contact ion pairs between sodium ions, differently from FM (E = 109) for which this behavior has been excluded (43). Thus the occurrence of small aggregates, whose formation is strongly dependent on the surfactant concentration can be hypothesized. Similar conclusions can be inferred for the S D S - D M F - P E N system. In particular, it is interesting to note that, analogously to previous findings (34), in D M F octanol acts as a structure breaker while pentanol produces a m i n o r effect, unlike what happens in F M where the former cosurfactant is likely to favor a prelameUar structural organization. In the aqueous solvent and in the presence of the oil, octanol induces the formation of a water-in-
371
oil droplet structure. At present, however, our data on D M F systems do not allow us to deepen this point further. In conclusion, the occurrence of some kind of association among the surfactant molecules, characterized by charged interfaces and by a certain degree of order, m a y be a reasonable explanation for the experimental results also in some of the non-aqueous systems. Whenever RI ~ R2 the use of a two-site model gave a reliable estimate of the dynamic parameters, while a relative uncertainty remains on the structural parameters. In fact the quantities S2/N (or S Z p B / P F ) not only depend on the order parameter and on the degree of counterion binding but also might include the need of compensating a possible variation of the quadrupolar coupling constant. In fact, X m a y increase or decrease as an effect of important distortions of the ideal octahedral or tetrahedral symmetry. These distortions can occur with a lifetime either m u c h longer or m u c h shorter than ~'e. Because of the symmetry of our 23Na N M R signals and the constancy of the chemical shifts, permanent distortions such as the substitution of a solvent molecule in the first coordination shell of the N a ÷ ions are not likely to occur. On the contrary, instantaneous distortions such as strongly anharmonic vibrations of the solvent molecules, due to intermolecular interactions, although they occur within a time scale shorter than re, m a y be effective to modify the fluctuating electric field gradients around the nucleus. Finally it is noteworthy that the 23Na N M R relaxation data while highlighting peculiar dynamic properties of counterions in some SDS systems, they generally agree with a structural picture drawn on the basis of other N M R results (31-34), thus giving further support.
ACKNOWLEDGMENTS We thank Professor Bjorn Lindman for some useful suggestions. Thanks are due to the Italian Council of Research C.N.R. and to Comitato Teenologico(C.N.R.) for partial financial support. Journal of Colloidand InterfaceScience, Vol.146,No.2, October15, 1991
372
CEGLIE ET AL. REFERENCES
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