J . Phys. Chem. 1990, 94, 3702-3706
3702
Micelle Formatlon of Detergent Molecules in Aqueous Media. 4. Electrostatic Features and Phase Behavlor of CetyltrimethylammoniumBromideSalicylic Acid Micellar Solutions Toshiyuki Shikata,*.+Hirotaka Hirata, Department of Physical Chemistry, Niigata College of Pharmacy, Niigata 950-21, Japan
and Tadao Kotaka Department of Macromolecular Science, Faculty of Science, Osaka University, Toyonaka, Osaka 560, Japan (Received: June 23, 1989; In Final Form: October 18, 1989)
Electrostatic features of threadlike micelles formed in aqueous solutions of cetyltrimethylammonium bromide complexed with salicylic acid or sodium salicylate, coded respectively as CTAB:HSal/ W or CTAB:NaSal/ W, were examined through determination of pH and bromide ion concentration in the micellar solutions by varying concentrations CDof CTAB and CAof HSal or Csof Nasal. In the case of the CTAB:NaSal/W system, the degree of dissociation of Br- from CTAB increased with Cs from the value at Cs = 0 up to 100% at Cs = CD. On the other hand, in the CTAB:HSal/W system the degree of dissociation of Br- reached 100%at CA = O X , . The pH of the system correspondingly decreased with increasing CA, changing the slope of pH versus log (C,) around CA= 0.5CD Equimolar CTAB:HSal micelles thus carry net positive charges on their surface. The effect of adding a simple salt, NaBr, to screen the surface charges was examined by observing phase behavior of the CTAB:HSal/W system with varying CD,CA,and the salt concentration C., When NaBr was added beyond a certain level to a viscoelastic solution (liquid phase L,) containing threadlike micelles or to a two-phase system consisting of the L1and crystalline HSal (S) phases, a new liquid (L2) phase emerged that was essentially an NaBr solution of HSal. The volume fraction VLl of the LI phase decreased with CASto an asymptotic value VLImthat was proportional to C, but independent of either CAor CAS.This result suggests that the charged micelles were condensed by addition of the salt to a thicker phase without changing their structure.
Introduction In recent years, it has become popularly known that micelles of cationic detergents in aqueous solution change their shape from spherical to rodlike shape when salts are added. In general, in the transition from sphere to rod, micelles of course change their aggregation number dramatically and grow linearly, keeping their radii constant.'-3 When the salt is a simple one such as NaBr and is of high concentration, long flexible rodlike or threadlike micelles are f ~ r m e d . ~ There -~ have been numerous studies of dilute and moderately concentrated aqueous cationic detergent solutions with simple salts discussing the dynamical behavior of the solutions in connection with network behavior of semidilute solutions of flexible When the added salt has a highly binding counterion such as salicylate, striking behavior is observed in that highly extended stable threadlike micelles are formed at a rather low concentration of detergent or added salt and solutions exhibit striking viscoelastic b e h a ~ i o r . ' ~Systems ,~~ of this type have been extensively studied by Hoffmann et aI.l4J5 rheologically, by Olsson et alei6with NMR methods, and by Brown et aI.l7 with dynamic light scattering experiments. We reported that cetyltrimethylammonium bromide (CTAB) forms long threadlike micelles in aqueous solution of sodium salicylate (Nasal) and the system (CTAB:NaSal/ W) shows unique viscoelastic From nuclear magnetic resonance (NMR) spectroscopy, we confirmed that micelles are essentially 1: 1 intermolecular complexes between CTAB and Nasal, and Nasal is located on the micelle surface beside the ammonium head groups of CTAB.20 Because viscoelastic behavior of the CTAB:NaSal/W system was not influenced by adding a simple salt such as NaBr, the threadlike CTAB:NaSal micelles have no net charges on their surface.22 The tendency of N M R data for aqueous solutions (CTAB: HSal/W) consisting of CTAB and salicylic acid (HSal) is different from that of the CTAB:NaSal/W system.22 From study of their NMR data, it is apparent that HSal molecules assume two kinds of locations in the threadlike micelles. The first location of HSal
'
Present address: Department of Macromolecular Science, Faculty of Science, Osaka University, Toyonaka, Osaka 560, Japan.
is filled up when the concentration C, of HSal has reached only half the concentration CDof CTAB and is essentially the same position as Nasal in the CTAB:NaSal micelles. On the other hand, the second location of HSal is in the micelle interior with a rather nonpolar atmosphere and is filled as CA is increased beyond 0.5CD up to CA = CD + CAsw;CAS. is the saturated concentration of HSal in water.22 Viscoelastic behavior of the CTAB:HSal/W system was strongly influenced by addition of NaBr contrary to the CTAB:NaSal/W system.22 This suggests that the CTAB:HSal micelles have net electric charges on their surface so that their rheological properties are affected by Na+ or Br- ions. However, in the previous work22electrostatic properties of the CTAB:HSal micelles such as the degree of dissociation of Br- ions from CTAB, (1) Ikeda, S.; Ozeki, S.; Tsunoda, M. J. Colloid Interface Sci. 1980, 73, 27. (2) (3) (4) (5) (6) (7) 215. (8) 521. (9)
Ozeki, S.; Ikeda, S. J . Colloid Interface Sci. 1983, 87, 424. Ozeki, S.; Ikeda, S. Colloid Polym. Sci. 1984, 262, 409. Porte, G.; Appell, J.; Poggi, Y. J . Phys. Chem. 1980, 84, 3105. Porte, G.; appell, J. J . Phys. Chem. 1981, 85, 2551. Appell, J.; Porte, G.; Poggi, Y. J . Colloid Interface Sci. 1982,87,492. Imae, T.; Kamiya, R.; Ikeda, S. J . Colloid Interface Sci. 1985, 108, Candau, S. J.; Hirsch, E.; Zana, R. J . Colloid Interface Sci. 1985, 105,
Imae, T.; Ikeda, S. J . Phys. Chem. 1986, 90, 5216. (IO) Candau, S. J.; Hirsch, E.; Zana, R.; Adam, M. J . Colloid Interface Sci. 1988, 122, 430. (11) Imae, T.; Abe, A.; Ikeda, S. J. Phys. Chem. 1988, 92, 1548. (12) Gravsholt, S. J. Colloid Interface Sci. 1976, 57, 575. (13) Ulmius, J.; Wennerstrom, H.; Johansson, L. 9.-A,; Lindblom, G.; Gravsholt, S. J . Phys. Chem. 1979, 83, 2232. (14) Angel, M.; Hoffmann, H.; Loble, M.; Reizlein, K.; Rhurn, H.; Wunderlich, 1. f r o g . Colloid Polym. Sci. 1984, 69, 12. (15) Rehage, H.; Hoffmann, H . J. Phys. Chem. 1988, 92,4712. GuCring, P. J . Phys. Chem. 1986,!?0,5223. (1 6) Olson, U.; Saderman, 0.; (17) Brown, W.; Johansson, K.; Almgren, M. J . Phys. Chem. 1989, 93, 5888. (18) Shikata, T.; Sakaiguchi, Y.; Urakami, H.; Tamura, A.; Hirata, H. J . Colloid Interface Sci. 1987, I 19, 29 1. (19) Shikata, T.; Hirata, H.; Kotaka, T. Langmuir 1987, 3, 1081. (20) Shikata, T.; Hirata, H.; Kotaka, T. Langmuir 1988, 4, 354. (21) Shikata, T.; Hirata, H.; Takatori, E.; Osaki, K. J . Non-Newtonian Fluid Mech. 1988, 28, 171. (22) Shikata, T.; Hirata, H.; Kotaka, T. Langmuir 1989, 5, 398.
0022-3654/90/2094-3102~02.50/0 0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 9, 1990 3703
CTAB:Salicylic Acid Micelles TABLE I: Influence of (TAB Addition on Observed p H with a Glass Electrode for Standard HCI Solutions observed pH at 25 O C [CTAB]/M 0 0.005
0.01
1.92 1.92 1.92
2.43 2.43 2.43
2.89 2.96 2.96
0100
CD80.002M - 5 0
3.34 3.45 3.42
that of H+ ions from HSal, and net charges on the micelles were not considered. In this paper, we report experimental results of determination of Br- ion concentration and pH change for the CTAB:HSal/W system with varying CD and CA and discuss electrostatic and structural features of the CTAB:HSal micelles. Results of observing phase behavior of the CTAB:HSal/W system upon adding a simple salt, NaBr, with varying the concentration CASare also reported to understand the structure of the CTAB:HSal micelles.
Experimental Section Materials. CTAB, HSal, Nasal, NaBr, and water were obtained by the same methods described in previous paper^.'^-^^ Solutions for determination of Br- ion concentration were the CTAB:HSal/W and CTAB:NaSal/W systems with CD= 0.002, 0.005, and 0.01 M and C A and Cs varied from 0 to 3cD. Two types of CTAB:HSal/W solutions with CD = 0.005 and 0.01 M were subjected to pH measurement with varying CAfrom 0.0005 M to the saturated concentration. We observed the phase behavior of aqueous solutions of CTAB, HSal, and NaBr, coded as CTAB:HSal:NaBr/W. Concentrations CDand CAwere varied from 0.01 to 0.12 M and CASfrom 0 to 0.1 M. Br- Concentration Measurement. Br- concentration [Br-] was measured with a Br--sensitive electrode (8005-06T, Horiba, Kyoto). We used a handmade Ag-AgCI electrode inserted into a 3.3 M KCI solution with a salt bridge of 3% agar with 1.0 M NH&l as a reference one. Electric potential between the electrodes was monitored with a pH meter (F-7, Horiba) in a millivolt meter mode. All measurements were carried out at 25 "C. A calibration line between the potential and [Br-] was obtained from aqueous standard NaBr solutions of 0.001, 0.01, and 0.1 M. p H Measurement. pH measurements were carried out at 25 OC with a glass electrode (6026-06T, Horiba), and the same pH meter was used in [Br-] measurements in a pH mode. The pH meter was calibrated with standard buffer solutions of pH = 6.86 and 4.01 at 25 OC. For surfactant solutions with concentration above the critical micelle concentration, cmc, pH values monitored with a glass electrode pH meter often do not represent the true values because of junction potentials caused by adsorption of surfactant molecules on the glass electrode s u r f a ~ e . ~Therefore, ~ - ~ ~ we first made sure of the effects of CTAB molecules on pH values obtained with the pH meter as follows. We first prepared four types of aqueous pure HCI solutions and measured pH values of them with the pH meter to get pH = 1.92, 2.43, 2.89, and 3.43. Then, we added the detergent, CTAB, to the solutions and measured pH values. CTAB was added to the solutions two times as resultant concentrations of it became 0.005 and 0.01 M. Table I shows the observed pH change of the solutions. It is likely that the observed pH shows the true value when pH is below 2.5, but when pH is above 3, the observed pH becomes slightly larger than the true one. From this, the effect of adsorption of CTAB on a glass pH electrode would be negligibly small below pH = 2.5, but not small above pH = 3, so that some correction is necessary for quantitative discussion when pH values greater than 2.5 are obtained in solutions with CTAB. Observation of Phase Behavior. Each CTAB:HSal:NaBr/W sample solution of IO-mL volume was poured into a 15-mL-ca(23) Bunton, C. A.; Minch, M. J. J . Phys. Chem. 1974, 78, 1490. (24) Bates, R. G. Determination of pH, Theory and Practice; Wiley: New York, 1964. (25) Eisenman, G., Ed. Glass Electrodes for Hydrogen and Other Cations; Marcel Dekker: New York, 1967.
0.5CD
CD
1
1
0
I
c r
d 0.' I
2.5
5
0.5 CD
CD
5
1
5
10 IO3 Cn
or
10
7.5
15
O
I03Cs/M
Figure 1. C, or C, dependence of Br- ion concentration [Br-] or degree of Br- dissociation (DD) from CTAB for the CTAB:HSal/W and CTAB:NaSal/W systems. (A) CD= 0.002 M, (B) CD = 0.005 M, and ( C ) CD = 0.01 M.
pacity transparent centrifuging tube with a screw cap. After the sample solutions were heated once in a water bath at about 50 OC to dissolve all the contents, they were stored in an incubator kept at 25 OC for more than 7 days for equilibration. The state of phase separation of the sample solutions was observed, and the volumes of existing phases were measured with scale marked on the tubes. We found that some sample solutions separated into two liquid phases. For the liquid-liquid two-phase solutions, the concentrations of CTAB, HSal, and NaBr of in each phase were determined to draw tie lines in the phase diagrams. Concentration of CTAB was determined by the Orange II-chloroform method26 as follows. With Orange I1 CTAB formed a water-insoluble 1:l complex which was extracted with chloroform. The amount of the CTAB:Orange I1 complex was determined from absorption intensity of visible light at 485 nm, while the concentration of HSal was determined from absorption intensity of ultraviolet light at 296 nm. The concentration of NaBr was on the other hand determined with a Na+-sensitive electrode (86-1 1BN, Orion).
Results and Discussion Degree of Br- Dissociation. Results of free [Br-] measurements for CTAB:HSal/W and CTAB:NaSal/W systems with CD = 0.002, 0.005, and 0.01 M and with varying CAor Cs are shown in parts A, B, and C of Figure 1, respectively. At CA = Cs = 0 M, CTAB forms spherical micelles by itself because CDis much higher than the cmc of CTAB. The degree of dissociation (DD) of Br- is about 50, 30, and 25% in the HSal- or Nasal-free solutions of CTAB with CD= 0.002, 0.005, and 0.01 M. DD for pure CTAB solutions depends on CDand decreases with it. In both of the CTAB:HSal/W and CTAB:NaSal/W systems, [Br-] or DD increases with increasing CAor Cs: In the former, DD reaches nearly 100% at about CA = 0.5CDbut in the latter increases at about C, = CD in solutions with lower C D of 0.002 and 0.005 M as seen in Figure lA,B. In solutions with a higher CD of 0.01 M, increase in DD with CA or C, of both the CTAB:HSal/W and CTAB:NaSal/W systems becomes somewhat dull as seen in Figure 1C. DD of both systems reaches only 80-85% even at CAor Cs = 1.5CD. In the CTAB:HSal/W system, DD linearly increases up to CA= 0.5CAand turns slope, but in (26) Few, A . V . ; Ottewill, R. H. J . Colloid Sci. 1956, I ! , 34.
3104
The Journal of Physical Chemistry, Vol. 94, No, 9, 1990 CTAB:HSal/W
1
25OC
Shikata et al.
t
NaBr
A; L,-L,-s
,L,-L,
I
':\
3.01
- 3.0
- 2.5
, -2.0
Gl 0.01 M -1.5
- 1.0
lOg(Cp, / M I
Figure 2. CAdependence of observed pH change for the CTAB:HSal/W system with C, = 0,0.005,and 0.01 M at 25 OC. Dashed lines represent pH change corrected for the presence of CTAB based on the characteristics of the used electrode listed in Table 1.
the CTAB:NaSal/W system, it turns slope at about Cs = C,. Consequently, the tendency of DD change in the solution with CD = 0.01 M is essentially the same as that of solutions with lower CD. These results imply that two Br- ions are released from CTAB micelles as one HSal is incorporated, but only one Br- ion is released as one Nasal is inserted in the micelles, both forming enormously long threadlike micelles. This difference is very important to comparing electrostatic and structural features of the CTAB:HSal and CTAB:NaSal threadlike micelles. From the previous ' H N M R study of the CTAB:NaSal/W system,20 we concluded that the threadlike micelles are 1 : 1 intermolecular complexes between CTAB and Nasal. Because Nasal dissociates into Na+ and Sal- ions, Br- from CTAB in the micelles should have been completely replaced by Sal- at C, = CDto form threadlike micelles. Consequently, the threadlike micelles of the CTAB:NaSal/W system consists of equal amount of CTA+ and Sal- so that they esentially have no net electric charges. On the other hand, the previous study of the CTAB:HSal/W system revealed that HSal molecules assume two locations:22 Up to C, = O X D HSal is caught in the first location on the micelle surface from which all Br- ions are released. This corresponds well to DD behavior shown in Figure 1 . Therefore, Br- ions are replaced with perhaps ionized HSal molecules (Sal- ions) by the molar ratio of 2:l in the first location of the micelle surface. Since the total molar ratio of HSal to CTAB in the micelles at saturation, CD CAsw, is 1:1, the other half of the HSal molecules are pushed into the second location which is in micelle interior in the concentration range of 0.5CD I C, 5 CD+ CAS,. p H Change. In the threadlike micelles of the CTAB:HSal/W system, a part of the HSal molecules in both or either of the first and second locations must have been dissociated to release H'. Figure 2 shows C, dependence of pH change for the CTAB:HSal/W solutions with CD= 0, 0.005, and 0.01 M. The pK, value of HSal in water is estimated to be 2.96 (at C, = 0.016 M) and 3.10 (at C, = 0.004 M) from pH data of the solution with CD= 0 M. These pK, values are rather similar to that in literature27and reasonable. For the solutions with CD = 0.005 and 0.01 M the slope of pH versus log (C,) changes just below C, = 0.5CDand becomes a smaller constant in the region of C, > 0.5CD. As seen in Table I, the observed pH value for HCI solutions containing CTAB is larger by 0.06 to 0.08 than the true one, when the true pH value exceeds about 3.0. If we correct the observed pH change of the CTAB:HSal/W system, taking this detergent effect on observed pH into account, the pH changes with C, along the dashed lines drawn in Figure 2. The tendency that the slope of pH versus log (C,) changes just below CA = 0.5CD
+
(27) Lange's Handbook of Chemistry, 12th ed.; Dean, J . A,. Ed.; McGraw-Hill: Y e w York, 1979; Section 5.
Figure 3. Phase diagram for the CTAB:HSal:NaBr/W system at 25 "C. L,, L,, and S represent respectively a (viscoelastic) liquid phase, a (low density and low viscosity) liquid phase, and a (needlelike crystalline) solid phase.
can be observed, whether the above correction is employed or not. Because in the region of C, C 0.5CDthe pH value is smaller than that of pure aqueous solution of HSal, dissociation of H + from HSal is presumably accelerated in the stage that the HSal is caught in the first location and releases two Br- ions from the micelle surface. From Figure 2 we can roughly estimate the degree of H+ dissociation from HSal for the solutions at C, = O X D as 60-70%. Since two Br- ions are replaced by one HSal in the first location, positive charges from CTA+ are not neutralized, and at CA= O X D 65-70% of them still remain on the micelle surface. In the region of CA 2 0.5CD,the first location is already filled up with HSal molecules up to O X D , and then, the rest of HSal molecules are pushed into the second location in the micelle interior and the excess, if it exists, remains in the bulk aqueous p h a ~ e . ~ * ? ~ ~ The second location obviously has a nonpolar atmosphere because the micelle interior is filled with n-alkyl chains of CTA+ so that the dissociation of HSal should be strongly suppressed. This would be an essential reason that the slope of pH versus log (C,) becomes smaller in the region of C, 2 OSCD as seen in Figure 2. Consequently positive charges on the micelle surface could not be neutralized by dissociation of HSal caught in the micelles. Phase Behavior. Figure 3 shows a phase diagram in a triangular column shape obtained from observation of phase behavior for the CTAB:HSal:NaBr/W system at 25 OC. The scale in Figure 3 is represented in mole fraction of each component. When the system contains no NaBr (CAS= 0 M), the system has one phase boundary as shown in the triangular base in Figure 3. The region marked L, below the boundary is a viscoelastic liquid phase, L,, containing CTAB:HSal threadlike micelles, and the upper region consists of two phases: L, and a needlelike crystalline solid phase S that should be crystallites of HSal. Since the boundary is parallel with the 1:l composite line between CTAB and HSal, and deviates by the saturated molar fraction of HSal in water, we conclude that HSal is taken into the micelles at maximum up to I : I molar ratio to C T A B . ~ ~ When NaBr is added beyond a certain level to the L, phase, a new liquid phase, L,, emerges which has lower density and lower viscosity than L , . The system separates into two liquid phases L , and L2. This L,-L2 region becomes wider with increasing NaBr. When NaBr is added to the L,-S region beyond the level that can cause phase separation of the L, region into the L,-L2 phases, the L2 phase also emerges, and the system separates into L!-L,-S three phase. Consequently the system is classified into
The Journal of Physical Chemistry, Vol. 94, No. 9, I990 3705
CTAB:Salicylic Acid Micelles
50-
I
H2O
0.001
0.002
0.003CTAB
Figure 4. Phase diagrams for the CTAB:HSal:NaSal/W system with CAS= 0.1 M. Solid lines in the Ll-L2 two-phase region represent tie lines. Part B is a magnification of the small region in part A.
four regions as described with phase boundary lines in Figure 3. To see the L1-L2 two-phase separation of the CTAB:HSal: NaBr/W system in more detail, we pay attention to the solution with CA, = 0.1 M, of which the phase diagram is shown in the triangular diagram at the top of Figure 3. The concentrations of CTAB, HSal, and NaBr of both the L, and L2 phases were determined to draw tie lines on the phase diagram. The concentrations of NaBr in the LI and L2 phases from the same test solution were the same and were identical with the ordinal CAS of 0.1 M within experimental error. Thus, tie lines connecting coexisting L, and L2 phases exist on the same triangle of CAS= 0.1 M, for which we plot phase boundaries and tie lines in Figure 4A. The portion enclosed with the dashed lines in Figure 4A is enlarged in Figure 4B, which shows the compositions of original solutions and resulting LI and L2 phases connected with the tie lines for some of the test solutions. The determined CTAB concentration of the L2 phase for all the solutions was lower than 0.5 mM (9.0 X IO" in mole fraction), which is lower than the cmc of CTAB. The L2 phase is essentially a pure HSal aqueous solution with NaBr and contains very few CTAB molecules. Most of the CTAB molecules exist in the viscoelastic Ll. phase. As CAof the original solution is increased, the tie line increases its slope and gradually reaches the phase boundary between the LI-L2 region and the LI-L2-S region as shown in Figure 4. The composition of the separated L, phase is very close to the equimolar composite line between CTAB and HSal, and the water content of the L, phase decreases with increasing CA. Consequently the separated L l phase containing threadlike CTAB:HSal micelles is condensed by addition of NaBr. The volume fraction VLIof the L, phase of the systems with CAs = 0.1 M but varying CD decreases with increasing CA as shown in Figure 5. It is likely that VL,at high CA reaches an asymptotic value VLlmdepending on CD.Figure 6 shows change of V,, as a function of CAand CASfor the solutions with CD = 0.06 M. From these figures, it is obvious that VLImis dependent only on CD but independent of CA and CAS. Figure 7 shows a relationship between VLlm and CD.Because VLIis perfectly proportional to CDand the L2 phase contains few CTAB molecules, VLlmrepresents the sedimentation volume fraction of the threadlike CTAB:HSal micelles. This VL,change also suggests that the CTAB:HSal micelles have electric charges on their surface and repulsive interaction among them could be screened by addition of NaBr; they could be condensed.
501Ll
i
j I
1
1 ,
:: L 1 L z - S
I
I
;v,
OO
Cn / M
0.15
0.1
Figure 5. CAdependence of volume fraction VL, of the L, phase for the CTAB:HSal:NaBr/W system with C, = 0.1 M and several C, values. 100
50
i
Figure 6. (A) CA and (B) CAS dependence of V,, for the CTAB: HSal:NaBr/W system with the same C, = 0.06 M. VLlmrepresents the
asymptotic value of VLl. The L,-L2 phase separation for this system is qualitatively similar to coagulation p h e n ~ m e n a ~of * . ~hydrophobic ~ colloid particles caused by additive salts. Studies of liquid-liquid phase separation of aqueous surfactant systems were mainly done for nonionic surfactant s y ~ t e m s . ~ " - ~For ' some nonionic systems, (28) Ottewill, R. H.; Shaw, J. N. Discuss. Faraday SOC.1966, 42, 154. (29) Verwey, E. J.; Overbeek, J. Th. G. Theory of Stability of Lyophobic Colloids; Elsevier: Amsterdam, 1948.
(30) Balmbra, R. R.; Clunie, J. S.; Corkil, J. M.; Goodman, J. F. Tram. Faraday SOC.1962, 58, 19661.
J. Phys. Chem. 1990, 94. 3706-3710
3706 30 CTAB:HSal:NoBr / W
C,
/M
Figure 7. Relationship between the asymptotic volume fraction V L I -of the L, phase and C,. additive simple salts induce and enhance the liquid-liquid phase ~ e p a r a t i o n . ) ~Some ~ ’ workers3841reported that simple salts affect the liquid-liquid phase separation in nonionic surfactant systems following the lyotropic series. Recently, Imae et al. reported salt-induced liquid-liquid phase separation in aqueous cationic (31) Clunie, J. S.;Corkill, J. M.; Goodman, J. F.; Symons, P. C.; Tate, J. R. Trans. Faraday SOC.1967, 63, 2839. (32) Ekwall, P. Advances in Liquid Crysrals; Brown, G . H., Ed.: Academic Press: New York, 1975. (33) Lang, L. C.: Morgan, R. D.J. Chem. Phys. 1980, 73, 5849. (34) Corti, M.; Minero, C.; Degiorgio, V. J. Phys. Chem. 1984,88, 309. (35) Imae, T.; Konishi, H.; Ikeda, S. J . Phys. Chem. 1986, 90, 1417. (36) Imae, T.; Ikeda, S. J. Colloid Interface Sci. 1986, 113, 449. (37) Herrmann, K. W . J. Phys. Chem. 1964, 68, 1540. (38) Schich, M. J. J. Colloid Interface Sci. 1962, 17, 801. (39) Tokiwa, F.; Matsumoto, T. Bull. Chem. Soc. Jpn. 1975, 48, 1645. (40) Deguchi, K.; Meguro, K. J . Colloid Interface Sci. 1975, 50, 223. (41) Zulauf, M. Physics of Amphiphiles, Micelles, Vesicles, and Microemulsions; Degiorgio, V., Corti, M. Eds.;North-Holland: Amsterdam, 1985.
surfactant systems,42and they also discussed their experimental results on the basis of the salting-out power and lyotropic series. However, the minimum amount of NaBr necessary for the present CTAB:HSal:NaBr/W system to separate into the Ll-L2 two phases is much smaller than that in Imae’s data; therefore, the mechanism of liquid-liquid phase separation for the CTAB: HSal:NaBr/W system would not be salting-out but coagulation. On the other hand, the CTAB:NaSal/ W system never separated into two liquid-liquid phases in the concentration range equivalent to Figure 3. This again suggests that the CTAB:NaSal micelles have no net charges or electrostatic interactions to be screened by additive NaBr.
Concluding Remarks In the CTAB:HSal/W system with low CD,dissociation of Bris complete at C , = 0.5CD so that Br- from CTAB forming threadlike micelles is replaced with HSal by the mole ratio of 2:l. Dissociation of HSal molecules in the first location on the micelle surface is 60-70% and that in the second location in the micelle interior is rather low. The threadlike CTAB:HSal micelles thus possess net positive charges on their surface. The CTAB: HSal:NaBr/ W system shows liquid-liquid two-phase separation induced by addition of NaBr beyond a certain level. One of the liquid phases contains very little CTAB, but the other viscoelastic phase contains threadlike micelles in a highly condensed state. This phase behavior also suggests that the CTAB:HSal micelles have electrostatic interaction to be screened by an additive salt. On the other hand, threadlike micelles of the CTAB:NaSal/W system have no electric charges, because they consist of essentially 1: 1 complexes between CTA+ and Sal-. Acknowledgment. T.S. thanks the Yukawa Scholarship Association, Faculty of Science, Osaka University, for a scholarship during 1988 that enabled him to carry out this study. (42) Imae,
T.:Sasaki, M.; Abe, A.; Ikeda, S. Langmuir 1988, 4 , 414.
Surface-Enhanced Raman Spectra of 2,2’-Bipyrimidine Adsorbed on Silver Sol G . Sbrana,* Centro Studi sui Composti Eterociclici del CNR, Via G . Capponi 9, 1-50121 Firenze, Italy
N. Neto, Department of Chemistry, Universitri di Potenza, Via N . Sauro 85, I-85100 Potenza, Italy
M. Muniz-Miranda, and M. Nocentini Department of Chemistry, Universitri di Firenze. Via G . Capponi 9, I-50121 Firenze. Italy (Received: June 27, 1989) The surface-enhanced Raman spectrum of 2,2’-bipyrimidine adsorbed on silver sol has been obtained and analyzed by using a vibrational assignment previously determined for this azoaromatic molecule. Evidence was found for Ag-N bond formation with the silver substrate and a model is proposed for the adsorption on the basis of a very close similarity of the SER data with the normal Raman spectrum of a 1:l complex with AgNO,. Normal-mode analysis suggests a planar conformation for the adsorbed species, with the molecule perpendicular to the metal, bound through N atoms. Variation of relative intensities of Raman bands with different exciting lines is consistent with a charge-transfer contribution to the enhancement of the scattering cross section.
introduction Surface-enhanced Raman scattering (SERS) is a well-established method for studying properties of molecules adsorbed on metals like silver, gold, and copper. Through comparison of SER data of the adsorbed species with those obtained from ordinary Raman experiments on solutions or solid samples, information can be gained on the molecular conformation and on the orien0022-3654/90 f 2O94-37O6$02.50f 0
tation of the adsorbate on the metal substrate. Different types of surfaces are commonly used and, among them, colloidal particles are particularly simple to prepare and provide with a substantial enhancement of the Raman cross section interpreted in terms of the electromagnetic theory’ and of a dynamical charge transfer ( I ) Wang, D. S.: Kerker, M.; Chew, H. W . Appl. Opr. 1980, 19, 4159.
0 1990 American Chemical Society
