682
J. Phys. Chem. 1980, 84, 662-670
Order Profiles of Host Decyl Sulfate and Decylammonium Chains and Guest Carboxylic Acids and Carboxylates in Aligned Type I1 DM Lyomesophases 6. J. Forrest, F. Y. Fujiwara,+ and L. W. R e e v e s * Department of Chemistry, University of Waterloo, Waterloo, N2L 36 1, Ontario, Canada; Instituto de Quimlca, Universidade de Sa0 Paulo, Sa0 Paulo, Brazil (Received May 9, 1979) Publication costs assisted by the National Research Council of Canada
Lyotropic liquid crystals type I1 (DM) have been synthesized from sodium decyl sulfate, decanol, water, and sodium sulfate with low concentrations of lauric, palmitic, and stearic acids and their conjugate anions. A wide variation of specificallydeuterated decyl sulfate chains has been incorporated in low concentration as well as perdeuterated decyl sulfate, carboxylicacids, and their anions. Degrees of order profiles have been extracted from the study of deuterium quadrupole splittings in the corresponding NMR spectra of aligned mesophases. The ratios of segmental order along the C-D bonds for host decyl sulfate chains are independent of water content (within the region of the DM phase), a temperature variation of 3 O C , and the presence of small quantities of the guest amphiphiles. The invarianceof the kink/jog motions in the decyl sulfate chain as the water content is varied has been interpreted in terms of the invariancein internal packing of host chains inside the disk micelle as these entities are diluted with more interstitial water. The decrease in absolute degrees of order of the decyl sulfate segments as water is added can be explained by increases in the amplitude of micelle motion because the water layer is increased in thickness and perhaps also because the mean disk diameter may decrease with added water. The guest amphiphile chains do not conform in length to the half bilayer thickness of the disk micelle. Several types of behavior have been noted: (a) The guest head group anchoring at the interface determines the form of the order profile of the guest chain to a very large extent within the half bilayer distance; i.e., the form of the order profiles near the anchored end depends on the head group and not on the chain length for long chains. (b) The guests are uniformly of higher order in the first segments at the impurity site than the host segments which form the solvent bilayer. (c) In the case of carboxylic acid guests, the degree of order profile in the region of high probability for single gauche rotations follows very closely that of the solvent SDS/decanol bilayer. (d) The degree of order profiles of carboxylate guests, on the other hand, are quite distinct from the host segments, A higher order of the guest chains persists well beyond the half bilayer distance. (e) A third situation arises for carboxylic acid guests in a decylammonium chloride type I1 DM mesophase. In this case the fall in order of the guest chains occurs well wiithin the plateau region of the host chains. (f) The excess chain lengths of palmitic, myristic, and stearic acids and anions participate in the disordered region at the center of the disk bilayer structure having low and steadily decreasing order. Nevertheless, the ratio of CD2to CD3splitting the nonpolar end of the chain falls close to a theoretical ratio of 3 for a rigid chain for the anion guests.
Introduction At the symposium on “Magnetic Resonance in Colloid and Interface Science” of the American Chemical Society in Sept 1976, a preliminary verbal report was made with respect to a degree of order profiles of guest amphiphile chains in aligned mesophases of type 111which were prepared with a bilayer thickness in the disklike micelle^^,^ too short to accommodate the guests without some perturbation in the order profiles. We have recently shown that the type I1 mesophases (negative diamagnetic anisotropy) may be of two forms. The classical La lamellar phase is type 11, as is a disk-shaped micelle nematic mesophase, “DM”, with which it can occur in eq~ilibrium.~ The DM mesophase has a long-range orientational order of disks but no positional order. The finite disk mesophase based on the mixture S D S / H , O / d e ~ a n o l / N a , S 0 ~is~ composed of micelles of bilayer thickness 38 A,which are disposed in a water matrix of about 90 A in average thickness and have a dimension greater than 1000 A.5 This structure is illustrated in Figure 1. There is no positional order correlation of the centers of gravity of the disk^,^,^ *Visiting Professor, Universidade de Sao Paulo, since 1967. de Quimica, Universidade Estadual de Campinas, Campinas S.P. Brazil. t Instituto
and the lamellar mesophase La can coexist in equilibrium with the DM type I1 mesophase and nucleates from it by gaining one positional degree of ordera3 The misfit chain aspect of this present study has been examined previously.6 Specifically deuterated alcohols, a-deuterated carboxylic acids, and carboxylate amphiphiles of widely varied chain length were included as guests of small concentration in mesophases of type I1 DM, and the degree of order of the guests at the cu-CDz position were measured by the quadrupole splittings of the deuterium magnetic resonance spectra. The increase in degree of order of the (r-CD2of the acids and carboxylate amphiphiles per CH2segment added was found to be linear over a region of chain lengths less than the half-bilayer distance of the host mesophase.6 At chain lengths for the guests which approach the host half-bilayer thickness, there is no further increase in degree of order at the a position. At short chain lengths of the carboxylates, the distribution into the interstitial water is appreciable and leads to deviations from the linear dependence.6 The a-CD2segments of the carboxylate amphiphiles in the host micelle bilayer of sodium decyl sulfate (SDS),decanol, water, and sodium sulfate are lower in degree of order than those of the carboxylic acids, except a t very long chain lengths of 12 or more carbons. In this previous study6 there was no
0022-3654/80/2084-0662 $0 1.0010 0 1980 American Chemical Society
Aligned Type
The Journal of Physical Chemistry, Vol. 84, No. 6, 1980
I1 DM Lyomesophases
Magnetic Fjeld
’
DLrec tor
I
Figure 1. Schiematic representation of the type I1 DM meosphase. The size of the disks in the upper part of the diagram may have a more or less broad distribution. No attempt has been made to include this feature. There is motion of the disks in the aqueous medium, but the mean direction of the normal to each disk is along the director at the left. This dlrector tends to align in a direction perpendicularto an applied magnetic field. An expanded diagram of an individual disk micelle is given in the lower part with distances estimated from low-angle X-ray diffraction studkse The mean thickness of the aqueous layer between disks is about ‘30 A.’
attempt to study the order in the segments of the host detergents or to investigate the behavior of the full-order profile of the guests. The present work is thus an extension of the previous study along these lines. Charvolin’~~ has independently studied the effect of heterogeneity of chain length on the polymorphism of lamellar L, and hexagonal H, lyotropic liquid crystals. These studies included X-ray diffraction at low angle and deuterium magnetic resonance. The polar head groups in his study were uniform in chemistry, and mesophase structural changes’s8 were observable as well as differences in the chain-order profile. The philosophy that separates our studies from the investigations of the classical mesomorphic p h , ~ e slies ~ * in ~ our attempts to incorporate chemical variety into the bilayer solution chemistry. The study of carboxylates and carboxylic acids in the same host mesophase as guest species becomes accessible by changes in pH of the water, whereas in mesophases containing only the carboxylate salts, this is not possible.
Experimental Section Perdeuterated lauric, palmitic, and stearic acids were purchased from Merck Sharp and Dohme Ltd. and were greater than 99% deuterated. Sodium decyl sulfate was prepared and purified as published previ~usly.~ Specifically and perdeuterated sodium decyl sulfates were synthesized from labeled decanols a~ described in our previous work.g The preparation of mesophases of type I1 “DM” requires the use of pure components to achieve reproducible liquid crystalsa2The procedures have been described in detai1.3v4*6$’ The assignment of decyl sulfate deuterium splittings wais achieved by preparing the mesophases first with the decyl sulfate label and secondly with the perdeuterated carboxylic acids or their salts. The deuterium-labeled compounds were required in small amounts;
663
in the case of guests, pure deuterated compound was used, but in the case of host decyl sulfate ions the labeled chains were highly diluted in normal decyl sulfate. The formation of carboxylic acids as guests in the bilayer was assured by using pH 2 for the water of the liquid crystal, which also contained 0.1 % D20 to verify water degree of order in the mesophases. The carboxylate ions were included as guests by using the sodium salts with the water at pH 10. The composition of the mesophases with labeled SDS was standardized with one sample which contained labeled acid or carboxylate. For example, standardized mesophases contained the following mol % of the various components for lauric, palmitic, and stearic acids: HzO (pH 2,0.1% DzO) 92,21, decanol 1.45,carboxylic acid guest 0.1, sodium decyl sulfate 5.15 (may include -0.48 mol % deuterated SDS), and sodium sulfate 1.08. Some dilution studies over the stability range of the DM mesophase were important. The composition of the mesophases was obtained by dilution of the standardized mesophases with weighed quantities of water. A series for palmitic acid contained 92.21,92.76,93.26,98..80, and 94.22 mol % water in a mesophase where the other components had the same mole ratios as in the standardized mesophase above. Some experiments were completed with a decylammonium chloride type I1 mesophase;1° however, the stability of this mesophase deteriorates sufficiently at higher pH values to render the study of carboxylate guests inaccessible. These studies were restricted to carboxylic acid guests at relatively low pH. The temperature of all measurements can be assumed to be 31.2 f 0.1 “ C or 34 f 0.1 “ C , as noted in the text. The spectrometers used were a Varian XL-100 with gyrocode option and a Bruker SXP 60 locked externally at 60 MHz by a proton signal in a Varian 14.1-kG magnet.
-
Results SDS Host-Order Profile Ratios as a Function of Water , ~ have ~ ~ ~shown ~ ~ that degrees of Content. P r e v i o u ~ l ywe order of chain segments, measured along the C-D bonds from deuterium quadrupole splittings, are proportional to one another in the same chain, as the mesophases are changed in water content over the region of stability of type I CM or type I1 DM systems. These mesophases are composed, respectively, of finite cylindrical micelles (CM) or of finite disk micelles (DM) with long-range orientational but no positional order.2 The proportionality results from cancellation of common motions in the ratios, e.g., oscillation of the whole chain about the perpendicular to the interface of the micelles, rocking and translation of micelles, diffusion of whole chains, or, indeed, any motion affecting all segments equally. The differences in the ratios of quadrupole splittings along a given chain occur because the hydrocarbon chains have rotational motions of the trans/gauche type along the chain, and this affects each segment differently, at least in principle.11J2 The degree of order profile ratios are thus a comparison of the relative amounts of trans/gauche motion in two segments.13 The fact that these profile ratios are independent of the water content over the range of stability of the mesophase indicates that the relative trans/gauche motions are unchanged by dilution of micelles and that the decrease in absolute degrees of order is a reflection of more extra-chain motion of the types indicated above. The present investigation includes a study of order profile ratios as a function of chain length of guests and hosts in a bilayer of decyl sulfate/decanol amphiphiles. It became important to verify further the proportionality of the degrees of order
664
The Journal of Physical Chemistry, Vol. 84, No. 6, 1980
of CD2 segments with change in water content for the amphiphiles used in this study. The host decyl sulfate profile ratios in type I1 DM mesophases have been studied previo~sly,~ but the data required here for all of the host mesophases is much more extensive, and thus a statistical analysis of the ratios of quadrupole splittings is warranted. The guest amphiphiles used in the present work are all of greater length in the hydrocarbon chains than the host amphiphiles, decanol and decyl sulfate ions. In the case of lauric acid and laurate ions, the excess length is only one or two segments, but for the stearic and palmitic acids considerable excess chain length is present. A careful analysis of the decyl sulfate chain profiles in all mesophases measured in this work shows that the host profile ratios are not perturbed within experimental errors by any guest additive in the low concentrations used in this investigation. The temperature variation of 3 "C is also not significant. Results obtained in Brazil tend to be at 34 f 0.1 "C, and those in Waterloo at 31.2 f 0.1 "C. We have previously shown that order profile ratios in the same chain are essentially the same over a considerable temperature range of 20 "C or so.l0 I t is reasonable that the defect equilibrium in a single chain is not a sensitive function of temperature. In a consideration of a larger number of results, it was important to seek out a deviation from simple proportionality in terms of a linear relation. The results have been fitted to eq 1. Avcl is the quadrupole splitting of Avcl = mAvc, + c (1) some standard segment. This standard segment in the chain is preferably chosen as one which has no overlap in the spectrum with other splittings. For the decyl sulfate chain, because of the alternating order near the head group, which has been thoroughly investigated by specific deuteriation procedures both here and previo~sly,~ we have chosen the first segment. In the mesophases prepared here the combination of specifically deuterated chains with the first, third, fifth, and sixth segment overlapping has been avoided as much as possible. Avc is the quadrupole splitting of an arbitrary segment x in the chain. The slope m and intercept c are derived so as to accentuate deviations from simple ratios of proportionality. The relationship to the graphical figures to be presented here and the ratios presented previously may be found by taking l / m as the slope and -c/m as the intercept. For decyl sulfate chains the results are presented in Table I, at the foot of which the inverted slopes l / m are compared directly with the previous values obtained from simple ratiosag The quadrupole splittings which provided the basis for the data are precise to f20 Hz at worst and ki5 Hz at best, these errors being random ones. The statistical analysis of the broader data of this work listed in Table I indicates that the quadrupole coupling slope ratios for segments in the SDS chain are independent of water content of the mesophase over the type I1 DM region of stability, independent of small concentrations of guests, and independent of small temperature changes. The intercepts c are very small, especially when expressed graphically as -c/m. The inverse ratios of slopes m in the table compare well with the simple ratios determined in ref 9 for uncontaminated SDS type I1 me so phase^.^,^ The ratios (AvCg/Avc,,)determined from the linear relation fall very close to the theoretical value 3 for a rigid rotating chain.l0 Degree of Order Profile of Guests as a Function of Water Content. The dependence of deuterium quadrupole splittings in the guest and host chain segments on the
Forrest et al.
TABLE I: Quadrupole Splittings A. Relation A V C , = m A v C , t C for Decyl Sulfate'
X
2 3 4 5,6 7 8 9 10
m C 0.926 f 0.009 0.19 f 0.11 1.002f 0.011 0.04 f 0.12 0.963 f 0.009 0.17 f 0.11 1.00 always overlaps position x 1.127 f 0.022 -0.41 f 0.20 1.269 f 0.018 -0.34 f 0.16 1.633 f 0.016 -0.21 f 0.12 4.971 i 0.070 0.18 2 0.16
no. correla- of tion values
0.9997 9 0.9997 7 8 0.9997 =1 0.9994 5 0.9991 10 0.9995 12 0,9990 12 B. Comparison of l l m with Ratios of Ref g b AvilAvi m;/mj ratios of ref 9 1.080 1.086 Av,/Av, 4.971 5.131 A v l / Av lo 3.780 4.110 Av8/AvIo 3.134 Av9/AvIo 3.045 5.630 A.v,/Av I o 5.370 ' The quadrupole splittings measured for all mesophases in the SDS component fitted the equation A V C ,= m A v c x t C
where A v C l is the splitting for the first segment in kHz and A vcx is that for the other segments in the left hand column according to x . Values of m and C with least-squares fit errors are placed in the second and third columns. The correlation for the data is in the fourth column, and the number of available values in the fifth. The mesophases vary in water content, in identity of the low concentration of guests, and over a 3 "C temperature difference, having no effect on the linearity of the data. Appropriate manip ulation of the values in the tables is required to compare with graphical data represented as Av = ( A v c , / m )- ( C / m ) CX Comparison of the inverse slopes l / m with the ratios of quadrupole splittings determined in ref 9 and based on data is insufficient for statistical treatment.
water content of the type I1 DM phase is plotted in Figure 2. The plots chosen are quite typical and have the same form for all guests. The fall in degree of order of water (DOH) and chain segments with water content has been demonstrated rather thoroughly in a previous study of the type I1 DM decylammonium chloride mesophase.1° The guest chains are thus similarly decreasing in degrees of order as water is added to the system. The case of segments in the decylammonium chains'O is one where there is at first a linear fall in order with percent water, but for both decyl sulfate host chains and all guest chains there is no linear dependence at the lower water contents. It is interesting to note at this point that the range of water content sustained by the decylammonium chloride system is about the same as that of the SDS/decanol type I1 DM phase.1° The degrees of order in Figure 2B for the decyl sulfate chain are always lower than that for the corresponding segment in any fatty acid or fatty acid salt guest. The guest is thus always more highly ordered than the host, indicating as before14that the hydration and anchoring of chemically different head groups in the same bilayer seem largely independent of the host bilayer, though specific effects resulting mainly from Coulombic forces between head groups have been reported.16 This aspect of solution bilayer chemistry is still under study in our laboratory. For lauric acid and laurate ion, the linear relationship between degrees of order of C-D bonds for different segments in the same chain, as the water content is varied, is illustrated in Figure 3. The plots do extrapolate to zero order for all segments of both species plotted. There is,
The Journal of Physical Chemistry, Vol. 84, No. 6, 1980 665
Aligned Type ]:I DM Lyomesophases
F
S.D.S. -d - 2 1
Palmitic Acid do31 Guest
2,394
(Palmitate
'*\
Host
d -31 Guest)
I
-
L ! ! "I '''"' 16
-e-*.
92
M o l e 96
93 H2O
94
92
93
94
Figure 2. The quadrupole splittings in CD, segments of a typical guest and of the host decyl sulfate, (with a guest) as a function of water content over the range of stability of the type I1 DM mesophase. (A) Palmitic-d,, acid guest chain; 34 'C. (B) Decyl sulfate host chain in the system whlch had palmitate ion guest. The assignments were made from specific deuteriation studies; 31.1 'C. The ordinates are calibrated with the splittings of the, ith segment as Ava. The values of lare placed against each curve. The scales of both graphs are identical and may be read from the left on A. A
A /c5
I _ _
I Sodium Lnurote Guerf 34%
Lauric Acid Guest 34Y
Hz. 300
1
t
y6 H
,.IC12
-100 , :/
,
,
,
,
,
,
10
0
Arc2
,
,
20
K.Hz. -
Figure 3. The ratios of quadrupole splttings for segments of lauric acid guest (A) and laurate ion guest (B) in sodium decyl sulfate/decanol/ H,O/sodium sulfate type I1 DM mesophases. The ordinates are quadrupole splittings for arbitrary segments AC,, and abcissae are quadrupole spliwtings measured for the first segment of the chain ACp, both in kHz. The quadrupole splittings for DOH in the aqueous compartment are ailso plotted with points by using the right-hand scales in Hz. The water content is varied in the mesophases.
of course, a substantial error in the long extrapolation, but least-squares fits do confirm zero intercept well within this statistical plot. The degree of order of DOH is not proportional to or linear with the splitting near the polar head group for the guest species. Since the interface water is largely adsorbed at the host sulfate head groups, these species being in the large majority, a simple relation to the guest chain order is not expected. There is, at higher water contents, a reasonable linear dependence between the ol-CD2degree of order in the host decyl sulfate chain and the degree of order of the IDOH. Since we have shown re~ently'~~'' that quadrupole splittings of water (DOH) in mesophases with
Figure 4. The quadruple splittings for segments Aucl of palmitic acid and palmitate ion plotted against corresponding splittings of the overlapping segments Aut,, and first segment Avc,, respectively. The DOH splittings measured in'the same mesophases were also plotted by using the right-hand scales in Hz. Segment number iis placed against each plot.
decyl sulfate and carboxylate groups are rather similar and indicate strong binding of interface water, it is interesting that the linearity of the DOH splittings with the laurate guest a-CD2 is much more developed than for the lauric acid guest. Compare Figure 3, A and B. The water is evidently bound quite differently by carboxylic acids (e.g., at a different angle; more or less strongly) than sulfate or carboxylate head groups. Palmitic acid and palmitate ion with chains of 16 carbon atoms are considerably longer than the host bilayer of decyl sulfate/decanol. In Figure 4 the plots of quadrupole splittings for these guest species are displayed in a manner similar to those of Figure 3. The experimentswith palmitic acid guest show that segments beyond the half-bilayer distance, that is, beyond the C-11 segment, behave differently. Segments 5-10 show linearity of order with water
666
The Journal of Physical Chemistry, VoL 84, No. 6, 1980
Forrest et al.
18 r---
17 ,515 3x0
b
,
~
I
‘7 I
4
lL,l
STEAHIC -cID i Gu est)
3
I ,
1 ;
’
I
2
kHz.
6 5 1 /
7 8
g
I?
I
-twJ
Figure 5. The quadrupole splittings for segments of Aut, stearic acid guest chains plotted against the splittings (which overlap) of C-2,-3, and -4, A V ~ , ~The , . DOH splittings are also plotted and refer to the right-hand scale. The numbers in brackets against each plot refer to the segment number C,.
content as observed for guest lauric acid and laurate ion, but beyond this point the plots become definitely curved with an undetermined intercept. This deviation from linearity is well above experimental error. It is evident that the trans/gauche equilibria in the region C-11 to C-16 are affected in a regular manner as the disk micelles are diluted with more interstitial water. With palmitate ion guest chains (Figure 4B), however, the segments beyond C-9 manifest a linear rather than proportional dependence of degrees of order one on another. In addition, the intercept on the Auc2 axis is negative, so that in the hypothetical state in which the order of segment C2is zero, there remains a small residual order in the long tail of the chain. In agreement with the laurate guest system, the residual order in the DOH is linear with the degree of order of the a-CD2 of the palmitate chain, whereas there is no simple relation for the water order and that of palmitic acid segments (Figure 4A). Therefore, the chain length in excess of the half-bilayer for the acid and carboxylate guests do behave differently as water content is varied. The corresponding situation for stearic acid as guest is plotted in Figure 5. Once again, some curvature of the plots is evident in the region of excess chain length, but this deviation from linearity is not as pronounced as the case of palmitic acid. The DOH quadrupole splitting, on
ff
LI I
-8b2
kHz.
0
Figure 6. Deuterium magnetic resonance spectrum of the perdeuterated stearic acid guest. The low frequency half of the spectrum is reproduced with a little of the high frequency half in order to make clear the assignment of Cl@, and DOH peaks. The high frequency half is remote from the carrier frequency and therefore of lower intensity.
the other hand, has an even more accentuated nonlinear dependence on the quadrupole splittings for superposed C-2, C-3, and C-4 segments than those observed for lauric and palmitic acids. Insofar as can be estimated, the intercepts of the plots involving the excess segments tend to be negative but are rather irregular. For the segments C-13 to C-18 the nonlinearity is sufficient to render extrapolation procedures uncertain. The stearic acid values in Table I1 indicate simple ratios of quadrupole couplings in segments, without any tests or proportionality or linearity as in the data for other chains. The typical deuterium magnetic resonance spectra are similar to that in Figure 6. In this presentation, the spectrum of guest stearic-d36acid, where many of the individual doublets are separately resolved, is reproduced. 3. Direct Comparison of Order Profiles. The standardized mesophases, defined in the Experimental Section, present order profiles for lauric, palmitic, and stearic acid as shown in Figure 7. Sections of the host SDS profile are reproduced on the same graph. For the case of the acids the initial segments have a very similar profile. This fact is reemphasized in a quantitative manner by the slopes l / m cited in Table 11. It is quite clear that the carboxylic
TABLE 11: Ratios of Quadrupole Splittings for All Guest Segments Expressed as A L J C , / A V ~ , ~ X
lauric acid
3 4
1.00 1.00
5 6 7 8 9
0.934 0.878
10 11
12 13 14
0.750 0.616 0.357 0.336 0.08135
palmitic acid
stearic acid
1.00 1.00 0.932
1.00 1.00 0.916
0.891 0.776 0.640
0.891 0.718 0.594 0.468 0.336 0.242 (0.188)
0.511 0.391 (0.297) (0.245) (0.213)
laurate 10.888 10.845 0.814 0.766 0.668 0.59 1 0.422 0.135
palmitate
1
0.852
t -0.759
stearate
1
0.867
0.561 0.472 0.347 0.267
0.808 0.733 0.614 0.512 0.378 0.285
0.179 0.052
0.198 0.159
(0.184) 15 16
(0.180) (0.053) (0.164)
17 18
(0.061) 2.68
0.156 0.038 3.03
3.11 3.40 4.13 3.37 Crl-1 IC, * These are derived from least-squares fits t o the linear relation A U C , = A U C , ~+ C, where possible. The bracketed numbers involve some curvature in the plot, and so the mean ratio is taken over all measurements in these cases. The ratio (Cn-l,Cn)refers to the ratio of the quadrupole splitting observed for the penultimate CD, t o ultimate CD, in the chain.
The Journal of Physical Chemistry, Vol. 84, No. 6, 1980
Aligned Type 1’1 DM Lyomesophases
667
A!
hcs t
it,c
m> [
5
10
Segment no.
-
I5
Figure 8. The quadrupole splittings for segments Aut, of guest palmitate-d,, and the ~tearate-d,~ion in SDS mesophases. Compositions of mesophases are the following: (a) 92.21 mol % H20 (0.1% D20), 1.45% decanol, 0.10% sodium palmitate-d,,, 5.15% SDS, and 1.08% sodium sulfate; (b) 92.22 mol % H20 (pH 12, 0.1 % D20),‘1.45% decanol, 0.10% sodium ~tearate-d,~,5.15% SDS, and 1.08% sodium sulfate. The temperature of the experiments was 31.1 OC. The profile of SDS-d,, was obtained from a mesophase without guests, but essentially apart from this, of the same composition. The five profiles plotted can be identified by points or lines: (1) SDS profile X; (2) stearic acid profile repeated from Figure 5 for comparison; (3) palmitiic acid profile from Figure 5; (4) palmitate ion profile (A);(5) stearate ion profile (0). The ions are considerably higher in order at almost all segments, the first segment being higher than the plateau of order in each case.
-
l
5
,
.
,
,
10
Segment no,
l
,
~
-
l
.
l
~
.
.
15
Figure 7. The quadrupole splittings for segments Auc,of guest lauric, palmitic, and stearic acids perdeuterated in SDS mesophases. Compositions of mesophases (mol % ) are the following: (a) 92.01 mol % H,O (pH 2, 0.1 % D,O), 0.12 % lauric-d,, acid, 1.40 % decanol, 5.43% SDS, 1.04% sodium sulfate; (b) 92.21 % H20 (pH -2, 0.1 % D,O), 1.45% decanol, 0.10% palmitic-d,, acid, 5.15% SDS, 1.08% , (pH 2, 0.1% D20), 1.45% decanol, sodium sulfate; (c) 92.2’1 % HO 0.10% steariod, acid, 5.21 % SDS, 1.08% sodium sulfate. The profile of SDS was obtained by preparing a mesophase with the same composition but with position!; 1, 2, 3, 4, 8, 9, and 10 labeled with deuterium in a small percent of the detergent. The profile of SDS is not changed appreciably by lhe small concentration of guests. Points can be identified as (A)palmitic: acid, (0) stearic acid, (0)lauric acid, and (X) SDS segments. The first carbon segment of SDS is plotted as segment 2.
-
-
acid head group determines the order profile of the segments within the half-bilayer (to approximately C-11),and this must be the result of the specific anchoring of this acid group at the interface. We have previously shown6 that the excess chain length beyond the half-bilayer has no further influence on the degree of order of the cu-CD2 position. The SDS profile has the characteristic alternation of order in the first four segments; but, more important, the degree of order of the first seven segments is much lower than that of the guest species. The fall in degree of order starting at C!-7 arises because of an increased probability of single gauche rotations near the end of the chain.I1 In this region the guests and the host follow each other quite closely in order profile. The excess length of guest chains provokes deviations of the profiles from one another, palmitic acid having higher order than stearic acid in this region. The llow quadrupole splittings in this region suggest that the excess lengths participate in the central disordered region of the bilayer rather than insert into the other side of bilayer structure. In Figure 8 the profiles for standardized mesophases containing palmitate and stearate ions are reproduced along with t:he complete profile of the SDS host chains. For comparison, the order profiles of the corresponding protonated species are sketched in from Figure 7. The order profiles of the carboxylates are in contrast with those of the parent acids. Points of difference may be summarized as follalws. (a) As in all previous cases, the carboxylate group is associated with a higher degree of order in the a-CDPposition, and this is followed at the third carbon segment on Iby a uniform plateau of order.18 (b) Unlike the acid amphiphiles, the fall in degree of order occurs deeper into I;he bilayer (C-9), and thus the carboxylate
I
I
Segment no.
----3*
Figure 9. The degree of order profiles of lauric, myristic, and palmitic acids as guests in a type I1 DM mesophase of decylammonium chloride/electrolyte/water. Profiles are designated by numbers where they differ sufficiently on the scale of the graph: (1) decylammoniuim-d,,, (2) lauric-d,, acid, (3) myristic-d,, acid, and (4) palmitic-d,, acid. For decylammonium chloride the a-CD, group is plotted as segment 2, etc., taking account that the first segments in the acid chains are C2 in the chain. Points for the host detergent are circled; experimentad temperature, 31.1 OC; composition of mesophases: 5.70 mol % decylammonium chloride, 1.48% ammonium chloride, 0.28% of guest fatty acid, and 92.54% of H20 (pH -2, 0.1% D,O).
amphiphiles tend to be of higher order except in the third and fourth carbon segments. (c) The fall in degrees of order toward the nonpolar tail of the chain does not follow that of the host detergent closely. Consequently, the quadrupole splittings for the acids and the corresponding carboxylates become similar in value only at the chain termini. In Figure 9 the degree of order profile of the acids as guests is compared with a decylammonium host amphiphile. In this case the first six segments of the guests are more highly ordered than the host, but the fall in order profile of the guests occurs at lower segment numbers than the host in contrast to both acids and amphiphile salts in SDS mesophases. In the region of excess chain length for the half-bilayer, lauric acid segments retain the highest order, palmitic acid the lowest.
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The Journal of Physical Chemistry, Vol. 84, No. 6, 1980
Discussion 1. Relationships between Micelle and Mesophase Structure. In order to understand in a qualitative manner the relationships of micelle order and host and guest orders, it is necessary to clarify the nature of these mesophases in the manner suggested in a recent review article based on the magnetic evidence of diamagneticanisotropic2 phase diagram studies3 and confirmations by low-angle X-ray diffraction work.5 The type I1 DM mesophase is a nematic state, according to the definition of de Gennes,ls which in the present case has negative diamagnetic anisotropy. The molecules of thermotropic nematic crystals are re laced by micelles of a disk shape5with a thickness of 38 and a planar extension exceeding 1000 A. These micelle entities are dispersed in an aqueous bath whose mean thickness is about 80-130 A,5 and isotropic motion is inhibited by intermicelle forces, which become repulsive at shorter distances. The lack of spherical symmetry of the micelle entities implies a diamagnetic anisotropy of the individual units, and orientational long-range order may be influenced by an applied magnetic torque.lg The nematic state does not have long-range positional order, but there must be long-range orientational order. The water, when residing in the interstitial region, is not anisotropic in properties, but both water and ions contained in it can gain anisotropic motions by adhering to the interface of the micelle and oscillating with it.20t21There is a rapid exchange on the NMR time scale between free isotropic water or ions and an adsorbed layer on the micelle. The situation is diagramatically illustrated in Figure 1. The effect intended is that of an instantaneous picture of the positions of the disk micelles. The dynamic mean of the motions of the disk micelles is such as to align the normal to the disk along the uniaxial direction “director” of the liquid crystal. The circular shape has been given to the disks, but the information available so far5 does not bear on the actual shape of the disk; in fact, the disks may not all be of uniform shape and size. The diagram also assumes that only one of the perpendicular directions to the magnetic field is assumed by the mesophase. The reprersentation in Figure 1corresponds to a sample aligned in a magnetic field with the spinning axis perpendicular to the field. This spinning axis becomes the unique direction of the director.lg 2. Simple Proportionality in Order for Host Segments. The lack of structural information about type I1 mesophases has led to some erroneous speculationslO about the role of structure in determining the degree of order of CD bonds. However, with the structural data now clear on the nature of the type I1 DM lyomesophase, summarized in the introduction to this paper, the arguments previously usedlo can be reformulated in real structural terms. The degree of order profile ratios do indeed reflect the intramolecular contributions to hydrocarbon chain order, and nothing that has been said9J0J3in this regard needs modification since it is based on the fact that the degree of order of C-D bond axes is given by eq 2. (So), is the
g:
(So), = (St/g),Sc
(2)
experimentally observed degree of order of the nth segment of a chain along the C-D bond. This is the product of (St/g),, the degree of order of the segment n arising simply from trans/gauche motions in the chain, and Sc, the degree of order of the pseudorigid chain.2J0 Motions of the entire chain as a pseudorigid body are identical for every segment of an individual chain, and thus taking a ratio we get eq 3, where the subscripts m and n refer to
(3) two different segments of the same chain. The fact that these ratios within the same chain are independent of water content of the type I1 DM mesophase, as found here and reported previously,2J0has a more profound meaning than just a comparison of intrachain motions in the ratio.2A10s13 The degress of order of C-D bonds in the chain segments have the contribution of S, which is itself a product of angular oscillations of the chain, as we have considered previously.1° The structural information a ~ a i l a b l eplus ,~ the fact that the ratios of orders in segments are independent of water content, suggests that the micelle units do not change in the internal packing structure of the host chains but that on dilution of the mesophase the freedom of motion of the micelle as a body increases in the sense of allowing larger amplitudes of motion of the micelle in the aqueous bath. If the ratios of degrees of order with change in water content were not a constant, it would imply that the gauche/ trans segmental motions are affected by dilution of the micelles. Since this is contrary to the experimental fact, we adopt the point of view that, at constant temperature, the packing structure of the host chains in the micelle which dictates the nature of gauche/trans segmental motions is not changed with water content. As the disk micelles are diluted with water, there are two principal mechanisms which can lead to the observed decrease in order (Figure 2):1° (1)A larger mean distance between the finite micelles allows a larger angle of oscillation of the micelle as a body. (2) The mean diameter of the micelle may decrease as water is added, allowing the disks more freedom of angulqr motion. The separation of the angular movement of a pseudorigid chain about the perpendicular to the interface of the disk from angular displacements of the normal to the disk, as previously noted,1° requires special experiments, not included in this work. We hope to separate these motions in experiments to be reported soon. The important conclusion drawn here is that packing of host chains within the micelle does not vary for the type I1 mesophase as water content is changed. The change in quadrupole coupling of amphiphile chain deuteronsz2when there is coexistence of L, lamellar and type I1 DM mesophase suggests a rather large angle for micelle oscillations (-27’) in the DM mesophase. 3. Behavior of Guest Amphiphiles. The guest amphiphiles are in sufficiently small concentration that we may consider them a dilute impurity site in the host micelle made up of decyl sulfate/decanol amphiphiles. The deuterium magnetic resonance spectra of the guest chains carry information about these impurity sites only and not about the main solvent body of the micelle. The neighbor effect on decyl sulfate and decanol entities is lost and averaged in the rapid diffusive exchange processes in the micelle bilayer. So much is this so that there is no detectable difference in the SDS chains between uncontaminated mesophases and those containing the guest chains. Figure 3 shows that lauric acid and laurate ion impurity sites follow the simple proportionality in segment order with water content. This leads to the conclusion that these guest impurity sites are not affected by dilution of the micelles with additional water. In the cases of the carboxylate guests (Figures 3 and 4) the quadrupole coupling of DOH is linear with the a-CD2 deuterium splitting. This can be interpreted in terms of a similar binding strength for water at both sulfate and carboxylate groups,16which
Aligned Type I1 DM Lyomesophases
has been noted before. The overall water order is transferred at the sites of sulfate head groups, which are in the vast majority at the interface. If the chain of the impurity center and the order of the water are dependent linearly, it follows that the water at the impurity head group does not differ in order greatly from that at the host head group. In contrast, all three situations with the Carboxylic acid impurity (Figures 3,4, and 5) show that the degree of order of the interface water as a whole is much higher than would be expected from the values at low water content if a linear relation helld with a small positive intercept. (See cases of carboxylates, Figures 3B and 4B.) The carboxyl group therefore in a stronger binding site for water than the sulfate or carboxylate. The small amount of decanol has an exchangeable proton at its polar end and does contribute somewhat to increasing the overall order observed for the water, as indeed does the exchange with the carboxyl proton. These are, however, small and constant effects, because the composition of the disklike micelle itself is invariant as the mesophase is diluted with water. The impurity sites of palmitic acid and palmitate ion have a behalvior distinct from those of the C12acid and ion (Figure 4A and B). For palmitic acid chains the segments up to Cll are linear in order as the mesophase is diluted. From C12to C16there is a distinct curvature in the plots (Figure 4A); thus, there is a change in the trans/gauche equilibria along the chain end which extends into the disordered region of the center of the bilayer. Since the trans/gaucliie equilibria of the host do not change with dilution as shown earlier, then the bilayer thickness does not depend on dilution with water.ll The continuous changes in the tail of the carboxylic acid amphiphile with water conteat are not large but do increase in amplitude from Cll to C16. This part of the guest chain has no effect on the order of the cy-CD2position, because a recent studyu conclusively shows; no increase in order at the a-CD2 position as the carboxylic acid guest has chain lengths increased beyond Cgin the same host mesophase (Figure 3, ref 23). The end of the chain, C-12-C-16, moves independently of the head group in the sense that the trans/gauche equilibria are much more like a free hydrocarbon chain in a liquid. The variation with water content of the mesophase of the chain motion at the nonpolar end is difficult to explain in simple terms. The palmitate ion does have a linear dependence between order in segments, which becomes a simple proportionality within the ordered region of the bilayer C2-C-8. A small negative intercept a t the nonpolar end, C-10-C-16, is associated with a region of the chain which has little influence on the degree of order of the cy-CD2 position (Figure 3, ref 23). For the ion there is no change in the qualiiy of the chain motion with mesophase dilution. The differential anchoring by the hydration changes with dilution appears to be the only other detectable effect a t the impurity site in the bilayer. Stearic acid impurity sites are associated with changes in the quality of chain motion only in the center region of the bilayer, and it is assumed that stearate ion would behave similarly to palmitate ion on dilution. 4 . GuesL Motion Profiles Compared to SDS Host Chains. A general question with regard to the degree of order profile of guest chains which are longer than the half-bilayer is whether the order of the guest follows the plateau region of the host and then falls off at the same point as the host chains. This would show a strong influence of the surrounding order on the guest chain. The present study gives two diametrically opposed answers: yes for the acid guests and no for the carboxylate guests. Figure 7 shows the affirmative answer in a rather spec-
The Journal of Physical Chemistry, Vol. 84, No.
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tacular manner. For the carbon segments 8-11 thle host and guest degrees of order are practically indistingui~ghable. The degrees of order in the plateau region near thle head group separate into two groups. In one group, the! order of the segments C-2-C-7 for the carboxylic acid guests is very similar to and of higher order than the host. The excess chain length has no influence on the a position (Figure 3, ref 23) or, as it appears from this work, on any of the first seven segments. The second group is tbe host, whose segmental order is much lower and has a t(ypica1 alternating e f f e ~ t . ~The J ~ degrees of order in the excess segments beyond C-11 are all low, indicating that single gauche rotations are highly probable at all positions. The excess chain length participates in the disordered region at the center of the bilayer and does not insert into the ordered plateau region across the bilayer. The diametrically opposed result for palmitate and stearate ions vividly illustrates the extent to which a deprotonation of a head group influences the interface anchoring effect. In all cases so far studied, the cy position of carboxylate chain has a degree of order above that of the plateau of uniform order. A special anchoring of the carboxylate group must occur such that the participation of the a-CD2 group in the formation of kinks and jogs is impeded.” Along the uniform plateau of order the probability of gauche rotations of the king/jog type must be equal at each segment. The anchoring of the carbolxylate head groups may occur by the use of hydrogen-bond bridges through water between head groups. Indeed, some evidence of cooperative hydration of adjacent head groups has already been found.16 Such cooperative hydration effects are not stereochemically favorable when the carboxyl group is protonated, but this suggestion is very speculative. A special feature of note for the host decylammlonium chloride bilayer is the ill-developed and very short region over which the guest segments have close to uniform order. The guest chains are very much influenced in this case by the bilayer solvent but do not follow the order in segments of the host. The hydration anchoring of adjacent NH3+ and COOH groups must be the specific cause of these changes in average motion of the guest chain. The variation of the length of the chain does not influence the form of the order changes in the first nine segments appreciably.
Acknowledgment. F.Y.F. appreciates the support of the Banco Naciond de Desenvolvemento Economico (BNDE). The work was otherwise financed by the operating grant to L.W.R. by the National Research Council of Canada (NRCC). References and Notes (1) L. W. Reeves, F. Y. Fujiwara, and M. Suzuki, ACS Symp. Ssr., No.
34, 55 (1976). (2) F. Y. Fujiwara, L. W. Reeves, M. Suzuki, and J. A. Vanin, in “,Solution Chemistry of Surfactants”, Vol. I, K. L. Mittal, Ed., Plenum Press, New York, pp 63-79. (3) F. Y. Fujiwara and L. W . Reeves, J. Pbys. Chem., preceding article in this issue. (4) K. Radley, L. W. Reeves, and A. S.Tracey, J . Pbys. Chern., 80, 174 (1976). (5) Lia Queiroz de Amaral, C. Pimentel,and M. Tavares, privale communication, 1977; M. R. Tavares, MSc. Dissertation, University of Sao Paulo, 1978; Acta Cystalbgr., Sect. A, Suppl., 34, S188 (1978); L. Q. Arnaral, C. A. Pimentel, M. R. Tavares, and J. A. Vanln, J . Cbem. Pbys., 71, 2940 (1979). (6) D. M. Chen, F. Y. Fujiwara, and L. W. Reeves, Can. J . Chern., 5 5 , 2404 (1977). (7) J. Charvolin and B. Mely Mol. C y s t . Liq. Cyst., 41, 209 (1978). (8) B. Mely and J. Charvolin, “Physical Chemistry of Amphiphilic Compounds”, communication presented to the CNRS symposium, Bordeaux, June 1978, CNRS.
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(9) L. W. Reeves and A. S. Tracey, J. Am. Chem. Soc.,97,5729 (1975). (10) F. Fujiwaraand L. W. Reeves, J. Am. Chem. Soc., 98, 6790 (1976). (11) (a) S. Marcelja, Nature (London), 241, 451 (1973); (b) Biochim. bbphys. Acta, 367, 165 (1974); (c) J. Chem. Phys., 60, 3599 (1974). (12) (a) J. Seelig and W. Niederberger, Biochemistry, 13, 1565 (1974); (b) J. Am. Chem. Soc., 96, 2069 (1974). (13) F. Y. Fujiwara, L. W. Reeves, A. S. Tracey, and L. A. Wilson, J. Am. Chem. Soc., 96, 5249 (1974). (14) B. J. Forrest and L. W. Reeves, Chem. Phys. Lipids, 24, 183 (1979). (15) M. Acimls and L. W. Reeves, submitted for publication in Can. J . Chem . (16) L. W. Reeves, A. S. Tracey, and Y. Lee, Can. J. Chem., in press.
(17) L. Hecker, L. W. Reeves, and A. S. Tracey, Mol. Cryst. Liq. Cryst., 53, 77 (1979). (18) P. 0. de Gennes, "The Physics of Liquid Crystals", Clarendon Press, Oxford, 1974. (19) F. Y. Fujiwara and L. W. Reeves, Can. J. Chem., 56, 2178 (1978). (20) L. W. Reeves, J. Sauches de Cara, M. Suzuki, and A. S. Tracey, Mol. Phys., 25, 1481 (1973). (21) L. W. Reeves, A. S. Tracey, and M. M. Tracey, J. Am. Chem. Soc., 95, 3799 (1973). (22) 8. J. Forrest and L. W. Reeves, Mol. Cryst. Liq. Cryst., in press. (23) D. M. Chen, F. Y. Fujiwara, and L. W. Reeves, Can. J. Chem., 55, 2396 (1977).
Measured Dependence of Photovoltage on Irradiance in the Gold-Rhodamine B Photoelectrochemical Cell T. I. Quickenden" and G. K. Yim' Department of Physical and Inorganic Chemistty, University of Western Australia, Nediands, W.A., 6009, Australia (Received May 23, 1979; Revised Manuscript Received December IO. 1979)
A linear dependence of open circuit photovoltage on irradiance has been observed for the gold-rhodamine B photoelectrochemical cell in the investigated irradiance range of (0-1.2) X lo" einsteins m-2 s-l. Care was taken to ensure that thermogalvanic effects did not contribute significantly to the measured photovoltages. The observations conformed with the predictions of a previously published comprehensive photoelectrochemical model which predicts linearity under the low photovoltage and low irradiance conditions of the present investigation.
Introduction Despite a growing i n t e r e ~ t l -in~ the possible use of photoelectrochemical effects for the conversion and storage of solar energy, photoelectrochemical studies are still relatively undeveloped. This is partly due to the complexities introduced into the t h e ~ r y of ~ -photoelectro~ chemical effects by the fusion of the diverse fields of photochemistry and electrochemistry and partly due to the shortage of reliable data on the electrodic and photolytic properties of the fluorescent dyes commonly used in photoelectrochemical cells. In particular, it is surprising that so little experimental data are available on the fundamental dependence of photovoltage on irradiance in cells containing fluorescent dyes. We have previously derived6t7theoretical relationships between open circuit photovoltage, V and irradiance, E, for several situations and, recently, have developed a comprehensive model for cells containing inert electrodes and a fluorescent dye. This model predicts a relationship of the form (1)
where the constant K is a function of the dark concentrations of the electrolyte species and of the rate constants for excitation, fluorescence emission, intersystem crossing, *To whom reprint requests should be directed. On leave from the University of Western Australia until February 1980, as a Visiting Scientist in the Department of Physical Chemistry,at the University of Cambridge, Cambridge, CB2 lEP, U.K. Nire Tan. Recently appointed to a Postdoctoral Research Assistanship in the Department of Physical Chemistry, University of Cambridge. The present work was carried out at the University of
Western Australia.
0022-3654/80/2084-0670$01 .OO/O
TABLE I : Classification of Photoelectrochemical Cells type I inert metal electrodes immersed in a nonelectronically excited liquid electrolyte type I1 inert metal electrodes coated with a solid fluorescent dye and immersed in a
nonelectronically excited liquid electrolyte type I11 inert metal electrodes immersed in a liquid electrolyte which is electronically excited by the incident light type IV semiconductor electrodes immersed in a nonelectronically excited liquid electrolyte type V semiconductor electrodes coated with a solid fluorescent dye and immersed in a nonelectronically excited liquid electrolyte type VI semiconductor electrodes immersed in a liquid electrolyte which is electronically excited by the incident light
photochemical reaction, the various types of quenching, diffusion, and electrodic and nonelectrodic electron transfer. R is the gas constant, T is the absolute temperature, and F is the Faraday. At low photovoltages, eq 1 simplifies8 to the linear relationship RT V,, = -KE (2) F It is the purpose of this present paper, to test the above relationships for the cell Aulrhodamine B, FeCl,, FeCl,, HC1, HzOIAu where one of the two identical, semitransparent, gold electrodes is illuminated and the other gold electrode is kept in the dark. At the same time, it is desired to obtain reliable V,, vs. E dependences for this cell, unaffected by electrode heating and photochemical decomposition which may have unfortunately complicated the few previous experimental determinations in this area. 0 1980 American Chemical Society
