5160
J. Phys. Chem. 1992, 96, 5160-5165
Studies on Sodium Ricinoleate. 1. Adsorptlon and Aggregatlon Behavior Neeta Shinde* and K. S. Narayan Hindustan Lever Research Centre, ICT Link Road, Andheri (East), Bombay 400 099, India (Received: October 14, 1991; In Final Form: January 27, 1992)
Adsorption at the air-water interface and aggregation in dilute solutions of sodium ricinoleate (sodium 12-hydroxy-cis-9octadecenoate) (NaR) have been investigated. The mid-chain polarity due to the presence of a double bond and hydroxy group have a profound influence on the dilute solution properties of NaR. The area per molecule obtained at the air-water interface (1.06 nm2) suggests that the molecule takes up a wicket shape with both carboxyl and hydroxyl groups residing at the interface. Studies on the micellar state appear to indicate the formation of premicellar aggregates. NaR fonns small micelles with a low aggregation number (-34). The micelles are probably ellipsoidal and have a high degree of counterion dissociation (77%). These results suggest that NaR behaves like a short chain surfactant and adopts a bent-wicket-like conformation in the micelles. 'Hand 13CNMR of NaR at concentrations below and above the critical micelle concentration (cmc) support the wicketlike conformation of NaR in the micelles.
Introduction Ricinoleic acid (R- 12-hydroxy-cis-9-octadecenoicacid) is an unusual fatty acid in that it is perhaps the only hydroxy fatty acid occurring abundantly in nature (over 80% of the fatty acid in castor oil is ricinoleic acid). Ricinoleic acid is an optically active compound (asymmetric center in the 12th position), and the naturally occurring compound has the R configuration.' Sodium ricinoleate (NaR) with a double bond in the 9-10 position and the hydroxy group in the proximity at position 12 presents an interesting example of a bipolar surfactant with terminal and mid-chain polarity. Not much work has been done on this class of surfactants. Investigations on multipolar surfactants have mainly been confined to di-a and a,B types or the a,w-type bola amphiphiles.2" Castor oil and castor fatty acids are technologically important materials and have been investigated in detail from the applications angle.7 However, fundamental physicochemical studies on NaR and ricinoleic acid are very limited.7 The monolayer properties of ricinoleic acid have been studied in detaL8v9 Some limited amount of work has also been carried out on NaR" and a few derivatives of ricinoleic and hydroxy stearic acids," but the work has been limited to the measurements of surface and interfacial tension and the critical micelle concentration (cmc). The molecular architecture of NaR raises several interesting questions. One would expect that the presence of the double bond at the 9-10 position and assymetric carbon atom in close neighborhood would impose some constraints on the orientation of the molecule, particularly in the micellar state. The energies involved in molecular orientations in such systems are likely to be high.'* As against these stereochemical considerations, one would expect on the basis of simple hydrophilicity-hydrophobicity that NaR would behave as a short chain surfactant with both the mid-chain and terminal polarities residing in the aqueous phase. The size and shape of the micelle and the phase behavior of the NaR-water system would present several interesting possibilities. NaR is a long chain soap, and comparison of the properties of NaR with other saturated and unsaturated soaps would also be of interest. It has been reported that there is a 3-fold increase in cmc by the introduction of a double bond in a c18 carboxylate and a further 3-fold increase by the introduction of a hydroxy group.13 As against this, a 16 times higher cmc has been reported for potassium 9,lO-dihydroxystearate as compared to potassium stearate.'* However, a complete characterization of micelles and liquid crystalline phases of such substituted carboxylates has not been carried out. In detailed physicochemical characterization of NaR in dilute solutions, lyotropic and thermotropic liquid crystalline states of To whom correspondence should be addressed. Present address: Department of Chemistry and Applied Chemistry, University of Salford, Salford MS 4WT. U.K.
0022-365419212096-5160$03.00/0
NaR were, therefore, taken up. The adsorption and aggregation behavior in dilute solutions has been reported here. The other aspects will be discussed in subsequent parts of this series.
Experimental Section Materials. Pure NaR was prepared from castor oil. Castor oil was transesterified with methanoP to obtain the methyl esters of the constituent fatty acids. The methyl esters were separated from glycerol by extraction with solvent ether and repeated washing of the organic phase with water. The methyl esters were recovered by evaporation of the solvent. Pure methyl ricinoleate was obtained by column chromatography of the methyl esters using a 4 cm X 45 cm column loaded with silica gel (60-120 mesh). The 10-g samples of esters were loaded on the the column and washed down with 200 mL of pure hexane. Separation into constituent esters was carried out by elution with a mixture of hexane and solvent ether of increasing polarity. The saturated and unsaturated esters were eluted out with a 93:7 hexane-ether mixture. Methyl ricinoleate was eluted out with a more polar 80:20 mixture of hexane and ether as eluting solvent. TLC of purified methyl ricinoleate showed a single spot with an Rfvalue of 0.37. Methyl ricinoleate was recovered by evaporating off the solvent. The material so obtained had greater than 99.9% methyl ricinoleate by GLC (9-ft SP-1000 column, Shimadzu Gas chromatograph-9A). Methyl ricinoleate was saponified by refluxing with methanolic NaOH, and after refluxing the mixture was left overnight to ensure complete saponification. After evaporation of the solvent, NaR was washed repeatedly with ether and acetone and dried under vacuum at 40 OC. The soap was further purified by dissolving in minimum amount of double distilled water followed by precipitation with acetone and drying. The IR spectrum of purified NaR showed an absence of ester and also other possible impurities expected during the various steps of manipulation. All the chemicals used in the studies were of pure grade. Sodium chloride was further purified by heating for 2 h at 700 OC before use. Water used in all the experiments was doubly distilled in an all glass apparatus over alkaline potassium permanganate. The specific conductance of this water was (1.2-1.3) X 10" mho cm-' and the surface tension at 25 OC was 72.1 mN m-I. All the solvents were distilled before use. The pyrene used was purified by passing a cyclohexane solution through a silica gel column and recrystallized twice from ethyl acetate.16 Cetylpyridinium chloride was used without any further purification. Methods. Surface tension was measured by the Wilhelmy plate method using a Kruss K10 digital tensiometer. The temperature of the sample was maintained at 25 f 0.2 OC by circulating water from a thermostat through the outer jacket. Sufficient time was given for the attainment of temperature and the equilibrium surface tension value. The pH levels of NaR solutions were 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, NO.12, 1992 5161
Studies on Sodium Ricinoleate 7 0 ~
"t
:T ;
E
1 \
A : NO ELECTROLYTE B : O . l M NaCl C : 0 5 M NaCl
TABLE I: Area per Molecule, Saturation Surface Tension, (ST), and
Cmc of NaR NaCl concn, m/dm3
saturation ST, area/molecule, mN m-' nm2
0.0 0.1 0.5
50
38.6 36.4 34.4
cmc X m/dm3 10
1.06 0.12
3 1.2
0.63
OH
I
""4
204 ' ' """ ' ' ' lib 6' CONCENTRATION OF NOR m0Ie5dG3
lb3
" " " ' l
IO2
=&
COONa
/
0
carboxy
0
Hydroxy
Figwe 1. Surface tension as a function of logarithm of concentration for NaR in the presence and absence of electrolyte.
adjusted to 11 to prevent hydrolysis." Conductancewas measured on a Micrometria electrophoreticmass transport analyser (Model 1202). No pH adjustment was made for conductance measurements. Viscosity measurements were camed out with an Ostwald capillary viscometer of appropriate capillary diameter immersed in a thermostat (25 f 0.2 "C). Fluorescence spectra were recorded on a SPEX Fluorolog 1681 022-m spectrometer at an excitation wavelength of 339 nm and emission in the 380-500-nm range. In order to ensure the entry of probe molecules into micelles, a methanolic solution of pyrene (with quencher when required) was added to dry 5-mL flasks and the solvent was evaporated under vacuum at room temperature. NaR solution (5 mL) was added to the thin layer of pyrene in the flask and sonicated. I3C NMR spectra were recorded on a Brucker 500-MHz spectrometer at 125.76 MHz and 23 "C. 3-(Trimethylsilyl)propionic-2,2,3,3-d4acid sodium salt (TSP) in D20 was used as an external standard. The solutions were prepared in double distilled water. 'H NMR spectra were recorded on a Varian XL-300 FT NMR spectrometer at 22 "C. Sodium 2,2-dimethyl-2-silapentane-5-sulfonate(DSS) in D 2 0 was used as an external standard. NaR solutions were prepared in D 2 0 (Aldrich 99.8% enrichment). The pH levels of solutions for NMR measurements were adjusted to 1l.
Results and Discussion Krafft Temperature. Measurement of the clearing temperature of 10%NaR solution showed that the Krafft temperature (KT) was lower than 0 "C, and this is in agreement with the value of 4 OC reported in D20I8(the freezing point of D 2 0is 4 "C). The KT of NaR is lower than that of other sodium CI8soaps, the values being 74,69, and 27 OC for stearate, hydroxystearate, and oleate, respectively.I8 The KT of a surfactant depends on the stability of the crystal rather than the monomer solubility. This, in turn, depends on the effective packing of the surfactant molecules in the solid state.6 The KT values suggest that in the saturated soap the introduction of the hydroxy group hardly alters the packing. However, the introduction of the cis double bond imparts a kink in the molecules and thus reduces the interaction between the molecules. The introduction of a hydroxy group in the unsaturated molecules appears to further weaken the interactions in the solid state. NaR at the Air-Water Interface. Surface tensions (ST) as a function of the logarithm of the concentration at zero, 0.1 M, and 0.5 M added NaCl are given in Figure 1. The saturation ST, the surface area per molecule ( A , obtained by the application of the appropriate form of Gibb's equationI9), and the cmc at the different electrolyte concentrations are presented in Table I. Compared to oleate, which exhibits saturation ST = 27 mN m-l, the corresponding values are higher for NaR." These results suggest that the methylene groups do not pack closely in the monolayer. Due to the presence of a strong polar center in the middle of the chain (double bond and hydroxy group), NaR can
VERTICAL
WICKET
Figure 2. Structure of NaR and molecular conformations of NaR at the air-water interface.
assume several configurations at the air-water interface. These are illustrated schematically in Figure 2, as wicket, vertical, and flat (not shown). The corresponding values of area per molecule estimated from Courtauld models are 0.60, 0.37 and 1.30 nm2. The experimental values of AI for NaR indicate that in the absence of electrolyte the molecule may take up the buckled configuration with the C12alkyl chain arching out of the aqueous phase. In practice, both the terminal alkyl chain and the loop will be removed from contact with water to some extent. With the addition of electrolyte and the consequent shielding of charge, NaR appears to take up a more compact configuration with a hair-pin looping of the CI2alkyl chain. Nagarajan and Shah9 have reported all four configurations in the case of insoluble monolayers of ricinoleic acid. However, they have reported that energies involved in the "wicket" conformation would be too high and that the trans isomer forms such conformation more readily than the cis one. Looping of alkyl chain in bipolar surfactants has been reported earlier by Balasubramanian et in the case of 11-undecenoate and 16hydroxypalmitate, by Goddard et al.z' in the case of mono- and disubstituted hydroxy and keto fatty acids of CI8chain length, and by Tachinaba and Hori22in the case of 12-hydroxystearic acid. Aggregation of NaR in MOIL criticalMicelle Concenhtkn~. The cmc values of NaR as determined by three independent methods, viz.,ST, conductometry, and fluorescencespectroscopy, are 0.01, 0.015,and 0.01 m/dm3, respectively, and are in good agreement. This value is about 10 times that of sodium oleate and 40 times that of sodium stearate and is comparable to the value of potassium dihydr~xystearate.~~ Thus, the effective carbon chain length in NaR appears to be five carbons less compared to that of stearate and about 2.5 carbons less compared to that of oleate. It was felt that because of the prsence of the mid-chain nonionic polarity, electrolyte effects on the cmc of NaR might be different compared to other ionic surfactants. However, it was found that
Shinde and Narayan
5162 The Journal of Physical Chemistry, Vol. 96, No. 12, 1992
'
'
O
i
:I 2
0.14
0 012
0.
B 0.06 a m
0.04
0.q
0 '
as
1.0
1'5
20
CONCENTRATION (g/dL)
Figure 5. Specific viscosity as a function of concentration (5% w/v) of NaR. I.>,
0
Ob1
0.02
0.03
0.04
Oh5
CONCENTRATION (moles d 6 3
Figure 3. Ratio of the intensity of the first and third vibronic bands in the fluorescence spectrum of pyrene ( I 3 / [ , ) as a function of the concentration of NaR.
CONCENTRATION moles dm'3
Figure 4. Specific conductance as a function of molar concentration of NaR.
the plot of log cmc vs log (NaCl concentration) was a straight line with a slope of 0.5 which is commonly found in the case of other anionic surfactants. Premicehr Aggregation. The intensity ratio of the third to first vibronic bands of pyrene fluorescence spectrum (13/11)is very sensitive to the environment of pyrene molecules. Hence, this ratio is used to determine the micropolarity and the cmc of surfactants.16 The plot of 13/11as a function of NaR concentration is given in Figure 3. The 13/11ratio increases sharply from a value of 0.59 and attains a first plateau value of 0.8 at 3 mM, and the value is constant up to 10 mM. With further increase in concentration, I3/ZI increases gradually and attains a second plateau value of 0.9 at 30 mM. The concentration of 10 mM, at which the second increase in 13/Zl begins, corresponds to the value of cmc determined by ST and conductivity measurements. The first sharp increase in 13/11strongly suggests premicehr aggregation, and the further increase in the value with the onset of "proper micelles" suggests that the microenvironment in the premicellar aggregates is more polar. However, further studies are required to confim premicellar aggregation in NaR since the pyrene could act as a nucleus for micelle formation. Such premicellar aggregates have been suggested in the case of short chain surfactants, viz.,sodium hexanoate and Counterion Binding. The plot of specific conductance vs concentration is given in Figure 4. The inflection point is at 15 mM, and the slopes of the postmicellization and premicellization segments are not very different. The degree of micelle ionization (a)calculated from the two is 0.77, and this value is much higher than the value observed for other soaps (0.3-0.5).6bThe higher value of a suggests a small number of ions per unit area and a large head group area which is consistent with the high value of area per molecule observed for NaR at the air-water interface. Large a values have also been reported earlier in the case of and the bola amsodium p(I-pr~pylnonyl)benzenesulfonate~~
phiphile, disodium 1,12-dodecyl disulfatea5 Supporting evidence for a large head group area (Le. looser packing of COT and 12-OH groups) comes from the 13/11value (0.90), which is significantly lower than the value for sodium laurate (0.96).16 This indicates the proximity of polar moieties such as the 12-OH group adjacent to the pyrene. Micelle Shape and Size. The shape of NaR micelles was determined from viscosity measurements. Specific viscosity (v, ) was measured as a function of concentration from 0.05 to 25%. The plot of vsp as a function of concentration is given in Figure 5. The plot consists of two intersecting straight lines, and the point of intersection corresponds to a concentration of 17 mM. It is interesting to note that this concentration corresponds closely to the concentration at which the second plateau starts in the 13/1, vs concentration plot (Figure 3). However, the value is higher than the value of the cmc as determined from either ST or conductivity data. The results suggest that the aggregation behavior of NaR is somewhat complex. The shape factor v was obtained from the second straight line part of the qspvs percent concentration plot by using the following equation:25 y
dvsp = Iplo0) lim 1 + 6 C-o dc
where p = density of solution, 6 = degree of hydration, and c is the concentration (g dL-I). wt of H 2 0 molecules 6= MW of surfactant p was taken as 1 g cm-3 and 6 as 0.56 (eight water molecules per carboxyl and two for the hydroxyl groups). The value of the calculated shape factor of NaR micelles was 6, and this value is greater than the shape factor characteristic of spherical micelles (2.5). This result suggests that the NaR micelles are not spherical but may be ellipsoidal or cylindrical. The axial ratio ( x / y )of this ellipsoidal micelle was (semimajor to semiminor axes) 4.9, obtained from the shape factor using the Simha26equation. The aggregation number of NaR was determined by the fluorescencequenching techniq~e.~'Pyrene (9.9 X lo4 M) was used as the probe and cetylpyridinium chloride ((3-5) X la-4 M) as the quencher. The plot of In (&,/I) versus quencher concentration is given in Figure 6. The plot is a good straight line pasping through the origin. The surfactant concentration used was 0.025 M. An aggregation number A, was calculated using the following equati~n:~' A, = {[SI - cmc)(slope) where [SI is the total surfactant concentration. The aggregation number for NaR was found to be 34 f 3. This value is intermediate between the aggregation number for sodium octanoate (9-17)23 and that for potassium dodecanoate The results presented so far suggest that NaR forms small, nonspherical micelles with a low aggregation number and is unlikely to have an extended chain configuration in the micellar state. It is also most likely that both the carboxyl and hydroxyl groups reside at the micelle-water interface. The ST measurements
The Journul of Physical Chemistry, Vol. 96, No. 12, 1992 5163
Studies on Sodium Ricinoleate
n
A
I/
02
0
7
0
ON CENT RATION moles dn?x lo5
Figure 6. mrene fluorescence quenching in NaR micelles. Plot of the logarithm of IJI as a function of the quencher (cetylpyridinium chloride) concentration. NaR concentration was 0.025 M. TABLE 11: Packing Factors for Different Micelle Shapes orientation V. A3 a. AZ 1. A Vlal micelle shapc >60 21.2 80 9.3 60 AZsince for normal soaps the area is 55 A2, the increase in area is attributed to the bending of molecules due to the cis double bond. In the wicket conformation, the volume of the -OH group was excluded from the hydrophobic part and the area of 20 AZ due to the -OH group was added to the area of the -COO- group (Table 11). Thus, in vertical conformation the NaR micelles could be either spherical/ellipse, and for wicket conformations they would be disc/ellipse. Aggregation numbers of both micelles were calculated theoretically by using the axial ratio obtained from viscometry (Table 111). The micelles with vertical conformation would be too large. On the other hand, the theoretical A, in wicket conformation agrees very well with the experimental A,. NMR chemical shift studies were undertaken to provide further information on the micelle structure. NMR Studies. The IH and 13C NMR spectra of NaR at concentrations below and above cmc are given in Figures 8 and 9, respectively. In the case of IH NMR spectra, the signals were assigned on the basis of chemical shifts, multiplicities, and intensity values. The assignments in the I3CNMR spectra were made on the basis of substituent additivity rules with oleic acid as the basic molecule, and the assignments were in agreement with those reported for methyl r i ~ i n o l e a t eand ~ ~ ricinoleic acid.30 The
0
60 55 5.0 ~ . 5 Lo 3.5 3.0 2.5 2.0 1.5 1.0 0.5 ppm Figure 8. ‘HNMR spectra of NaR in DzO.Spectra A and B are at concentrations below and above cmc, respectively.
chemical shift differences due to micellization and A& were calculated as the difference in chemical shift at pre- and postmicellar concentrations. The assignments, individual chemical shifts, and A6 values are given in Tables IV and V for ‘Hand 13C, respectively. The most significant postmicellization changes in the proton NMR are the splitting of the poorly resolved peak (2) corresponding to eight methylene protons into a set of two well-resolved peaks of equal intensity (Figure 8B), an additional peak corresponding to two protons, and a change in splitting pattern of peak 3 in Figure 8A. These changes suggest significant conformational
Shinde and Narayan
5164 The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 TABLE IV: 'H NMR Chemical Shifts of NaR at Concentrationa above Cmc peak no.
group
1 2
CH3 CH2
3 4 5 6 7 8 9
CH2CHICH2CH2CHOH =CH HC= new peak
bn, P P ~
no. of protons 3
carbon no.' 18 3, 4, 5 , 6 7, 14, 15 16, 17 13 8 2 11 12 9 10
Carbon numbering starts from carboxyl group.
(a#) and below Cmc (q)
2 2 2 2 1 1 1 2
6k
am,
A8Hb
0.86 1.28 1.31 1.38 1.53 2.06 2.17 2.26 3.70 5.48 5.58
0.90 1.30 1.34
+0.04 +0.02 +0.03
1.57 2.07 2.18 2.25 3.64 5.45 5.52 1.47
+0.04 +0.01 +0.01 -0.01 -0.06 -0.03 -0.06
+A6 = positive shift downfield. I'
TABLE V 'JC NMR Chemical Shifts of NaR below Cmc (8:) and above Cmc (8:)
A
Pcak
2 ' L I,
1
I2
I"
190 180 170 160 150 1LO 130 120 110 a0 90 80 70 €0 50 LO 30 20
10
0
PPm
I'
B
no. 1 2 3 4 5 6
group CH,
7 8 9 10 11 12 13
carbon no.' 18 17 3 14 8 5, 6, 7 4, 15 16 2 11 13 12 9 10
16.34 24.90 27.63 28.83 29.67 31.34
8,- PPm :6 16.75 25.41 28.33 29.13 30.14 32.29
+0.41 +os 1 +0.70 +0.30 +0.47 +0.95
33.98 36.91 38.41 40.59 74.70 128.45 136.22
34.61 37.46 39.19 40.79 74.26 128.27 135.20
+0.63 +055 +0.72 +0.20 -0.44 -0.18 -1.02
6:
A62
'Carbon numbering starts from carboxyl group. b+A6 = positive shift downfield.
high cmc, low degree of counterion binding, low aggregation number, and some features of the NMR spectra support the wicket conformation of NaR molecules in the micelles. 180
160
1LO
120
ld0
b PPm
60
LO
20
b
Figure 9. 13C N M R spectra of NaR in H 2 0 . Spectra A and B are at concentrations below and above cmc, respectively.
changes in NaR molecules on micellization and cannot just be explained on the basis of a solvent effecta3* Though the changes in 13CNMR spectra on micellization are not as significant as in 'HNMR spectra, some of the features are similar; e.g. peak 6 corresponding to -CH,-carbons, which in monomeric state exhibits two lines of unequal intensity (Figure 9A), has changed into two peaks of equal intensity in the micellar state (Figure 9B). In both 'Hand 13CNMR spectra, the protons or carbons constituting the *polar*center, viz., 9-12, have shifted in opposite directions as compared to the rest of the atoms (Tables IV and V). If NaR molecules assumed an extended conformation in the micelle, the chemical shifts of all the atoms would have changed in the same direction. The observed changes in 6 and spectral features cannot be solely attributed to the solvent effect but may be due to the significant conformational changes of molecules in micelles. Thus, both IH and I3C NMR results support the wicket conformation. Conclusions From the variety of studies carried out on NaR in dilute solutions, it is evident that the molecule behaves as a bipolar surfactant. The polar center, viz., double bond and hydroxy group in the middle of the alkyl chain, forces the molecules to adopt a wicketlike conformation at the air-water interface. The same conformation appears to persist in the aggregated state of NaR and makes the molecule behave as a short chain surfactant. The
Acknowledgmenr. We are thankful to Professor G. J. T. Tiddy, Unilever Research Laboratory, Port Sunlight, for valuable discussions. We are also thankful to Tata Institute of Fundamental Research, Bombay; Centre for Cellular and Molecular Biology, Hyderabad; Sophisticated Instrumentation Facility, Indian Institute of Science, Bangalore, for NMR facilities; and Indian Institute of Technology, Bombay, for use of spectrofluorimeter. Reglstry No. NaR, 5323-95-5.
References and Notes (1) Markley, K. S. In Farry Acids: Their Chemistry and Properties; Production and Uses;Markley, K. S.,Ed.;Intencience Publishers: New York, 1960; Part I, pp 188-190. (2) Shinoda, K. J . Phys. Chem. 1956, 60, 1439. (3) Malliaria, A,; Palm, C. M. J . Colloid Interface Sei. 1984,101, 364. (4) Menger, F. M.; Wrenn, S.J . Phys. Chem. 1974, 78, 1387. (5) Meguro, M.; Ikeda, K.; Otsuji, A.; Taya, M.; Yasuda, M.; Esumi, K. J . Collotd Interface Sei. 1987, 118, 372. (6) (a) Shinoda, K. In Colloidal Surfactanis; Shinoda, K., Nakagawa, T., Bun-Ichi Tamamushi,Isemura, T., Eds.; Academic Press: New York, 1963. (b) Lindman, B.; Wennerstrom, H. Micelles; Topics in Current Chemistry; Springer-Verlag: Berlin, 1980; p 1 . (7) Narayan, K. S. In Research in Indusrry; Mulky, M. J., Srivastava,H. C., Vatsya, B., Eds.; Oxford and IBH: New Delhi, 1987. (8) Langmuir, J. h o c . Narl. Acad. Sci. 1917, 3, 251. (9) Nagarajan, M. K.; Shah, J. P. J . Colloid Interface Sci. 1981,80, 7. (10) Nambudiry, M.E.N.;Ramachandran, G.;Nayyar, M.; Narayan, K. S.In Surfacranrs in Solution; Mittal, K. L., Ed.; Plenum Press: New York, 1989; Vol. 7, p 359. (11) Markina, 2.N.; Bovkun, 0. P.; Lomonosov, M. V. h o c . I n f . Con/. Colloid Surf. Sei. 1975, 1, 465. (1 2) Mead, J. F.; Howton, D. R.; Nevenzel, J. C. In Comprehenrioe Biochemistry; Florkin, M., Stotz, E. H., Eds.; Elsevier: New York, 1965; Vol. 6, Chapter 1, p 7. (13) Klevens, H. B. J. Am. Oil Chem. Soc. 1953, 30, 74.
J . Phys. Chem. 1992, 96, 5165-5169 (14) Gregory, N. W.; Tartar, H. V. J . Am. Chem. Soc. 1948, 70, 1992. (15) Brown, J. B.; Green, M. D. J. Am. Chem. Soc. 1940, 62, 738. (16) Kalyanasundaram, K.; Thomas, J. K. J . Am. Chem. SOC.1977,99, 2039. (17) McBain, J. W.; Laurent, P.; John, L. M. J. Am. Oil Chem. Soc. 1948, 25, 77. (18) Yang, P. W.; Mantsch, H. H. J. Colloid Interface Sci. 1986,113,218. (19) Suriacrants; Tadrw, Th. F., Ed.; Academic Press: London, New York, 1984. (20) Shoba, J.; Balasubramanian, D. J. Phys. Chem. 1986, 90, 2800. (21) Goddard, E. D.; Alexander, A. E. Biochem. J . 1950,47, 331. (22) Tachibana, T.; Hori, K. J . Colloid Interface Sci. 1977, 61, 398. (23) Lindman, B.; Persson, B. 0.;Drakenberg, T. J . Phys. Chem. 1979, 83, 3011.
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(24) Lianw, P.; Land Lang, J. J . Colloid Interface Sci. 1983, 96, 222. (25) Basandall, L. G.; Moti Lal;Rendall, K.; Tiddy, G. J. T. In Surfacrants in Solution; Mittal, K. L., Ed.;Plenum Press: New York, 1989; Vol. 8. (26) Simha, R. J. Phys. Chem. 1940,44, 25. (27) Turro, N. J.; Yekta, A. J. Am. Chem. SOC.1978, 100, 5951. (28) Israelachvili,J. M.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. I 1976, 72, 1525. (29) Henry, R.; David, W.; Edward, F. Lipids 1979. 14, 81. (30) Berassau, J. M.; Fetizon, M. Tetrahedron 1981, 37, 2105. (31) Specrroscopfc Methods in Organic Chemistry; Williams, D. H., Heming, I., Us.; McGraw-Hill: London, 1973; Chapter 3, p 74. (32) Alhnas, T.; Karlstorm, G.; Lindman, B. J. Phys. Chem. 1987, 91, 4030.
Studies on Sodium Ricinoleate. 2. Lyotropic Liquid Crystals in the Sodium Ricinoleate/Water System K. S. Narayan, N. Shinde? and G.J. T. Tiddy**t Hindustan Lever Research Centre, ICT Link Road, Andheri (East),Bombay 400 099, India (Received: October 18, 1991; In Final Form: January 27, 1992)
The liquid crystals formed by sodium ricinoleate (sodium 12-hydroxy-cis-9-odecenoate) and water have been investigated using optical microscopy, NMR, and low angle X-ray diffraction. Only two mesophases are observed at temperatures in the range 0-100 O C , these being of the hexagonal (H,) and lamellar (L,) varieties. Compositions between HI and Laform a micellar solution rather than the usual bicontinuous cubic (V,)mesophase. Unusually for a C17ionic surfactant, the HI phase is completely melted at 50 O C . Sodium and deuterium (2HzO) NMR measurements indicate that water and sodium ions have order and mobility similar to that found for mixed soap/long chain alcohol mesophases. This, together with the X-ray data, confirms that the 12-OH group resides at the micelle surface.
Introductioo Lyotropic mesophase formation in monovalent ionic surfactants is stereotyped by that of the sodium soap/water systems.' Sodium ricinoleate [sodium 12-hydroxy-cis-9-octadecenoate(NaR)], though a member of the group of soaps, stands apart due to its unique structural features, viz., the presence of a hydroxy group and a double bond in the middle of the alkyl chain. Earlier, we have established the influence of this mid-chain polarity on the dilute solution properties of NaR (adsorption at the air-water interface and micellar aggregation).2 The experimental results indicate that both the -OH and -C02-groups reside in the aqueous region at both the air-water and micelle-solution interfaces; i.e. both the C0,- and 12-OH groups are at the micelle surface with the C2-Cll fragment looping back into the micelle interior (see Figure 7b of ref 2). Moreover, the micelles formed have low aggregation numbers ( 34), are probably ellipsoidal (i.e. short rods), and have a high degree of counterion dissociation (>72%). These results suggest that NaR takes a "bent-wicketlike" conformation in the micelles. Since the micelles are the building blocks of lyotropic mesophases, the unusual orientation of NaR molecules may have effects on its lyotropic behavior as well. In this study we report the first phase studies on the NaR + water system, including a partial phase diagram, information on the hydration, and counterion binding of head groups in various mesophases and X-ray measurementsof mesophase structure. We present our results in two sections. The first section deals with the phase behavior of the NaR + water binary system using mainly polarizing optical microscopy with supplementarydata from X-ray diffraction and NMR spectroscopy. In the second section we N
To whom correspondence should be addressed. Present address: Department of Chemistry and Applied Chemistry, University of Salford, Salford M5 4WT, U.K. *Permanentaddress: Unilever Research Port Sunlight Laboratory, Quarry Rd. East, Bebington, Wirral, Merseyside L63 3JW, U.K.
0022-3654/92/2096-5165$03.00/0
describe details of the micelle structure and molecular interactions (hydration, counterion binding) derived from X-ray and NMR data. Experimental Section Materials. Pure sodium ricinoleate (>99.9%) was prepared by saponifying methyl ricinoleate with sodium hydroxide (Ranabaxy Laboratories Ltd., A-R grade). Pure methyl ricinoleate was obtained by column chromatography of castor esters using silica gel (60-120 mesh, Acme Synthetic Chemicals) as a stationary phase and hexane-ether (8020 (v/v)) as eluent. Sodium ricinoleate was recrystallized from methanol and then reprecipitated from acetone., Samples for X-ray and NMR measurements were weighed directly into glass tubes (7" 0.d.) which were sealed, the contents being mixed by heating and centrifugation. The samples were stored for at least 2 weeks before NMR measurements were made. For X-ray measurements, samples were transferred to 1-mm Lindemann tubes after storage. The normal water used was double distilled over alkaline KMn04, while deuterium oxide (2HzO) was commercially available (Aldrich, >99.7%). NMR and X-ray Measurements. NMR measurements were made using Bruker 3 W and 270-MHz spectrometers with variable temperature probes, operating at 79.38 and 41.46 MHz for 23Na and 2H resonances, respectively. Typical numbers of scans were in the range 2000-5000 and 200-400, respectively. X-ray diffraction measurements were obtained either using the beam line 7.2 at the SERC Synchrotron Radiation Laboratory (Daresbury, U.K.) or using an Anton Parr compact Kratky camera with an INEL position sensitive detector (LPS 50). Optical Micnwcopy. A Carl Zeiss Jena polarizing microscope equipped with a hot stage and a camera were used for optical experiments. The mesophase types and the temperature at which phase changes occurred were observed by slowly warming the stage using circulating water from a Lauda MGW Kruss cryostat. The 0 1992 American Chemical Society
