J. Phys. Chem. 1992, 96, 5611-5614
5611
Effect of Counterion on the Size and Charge of Alkyltrimethylammonium Halide Micelles as a Function of Chain Length and Concentration As Determined by Small-Angle Neutron Scattering Stuart Ben,*.+Richard R. M. Jones,$ and James S. Johnson, Jr.s Department of Radiology, MR4 Building- Room 1186, University of Virginia, Charlottesville, Virginia 22908; 3M Company, Industrial & Consumer Sector, Research Laboratory. 201 -4N-01, 3M Center, St. Paul, Minnesota 551 44; and Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (Received: January 21, 1992)
The influence of counterion on micelle structure is examined for a series of trimethylammonium halide surfactants C,TAX = C,,Hb+, N(CH,)!+X- (X = NO3,Br, CH3S04,C1, and OH) by small-angle neutron scattering. The variation of micelle structure as a function of chain length ( n = 12, 14, and 16) and surfactant concentration (0.05,0.1, and 0.2 mol dm-,) is also studied. It was found that the aggregation number, N, increases in the order of NO, > Br > CH3S04> C1 >> OH. This order is roughly correlated to the fractional micellar charge, b, which follows the order OH > Cl > CH3S04 Br NO,. Fractional charge changes very little and in an irregular fashion when n and/or surfactant concentration are increased, but the aggregation number increases with respect to both.
-
-
Introduction Micelles have been the subject of intense investigation for a number of years. Attempts have been made to elucidate micelle structure for numerous surfactantsunder a variety of experimental conditions. Previous studies have indicated that the identity of the counterion has a pronounced effect on the critical micelle concentration (cmc), Kraft temperature, and micelle stru~ture.l-~ These changes are likely a result of differing interactions of the counterion with the charged headgroups in the Stem layer and/or the aqueous solvent molecules. This present work examines the effect of the counterion identity on the molecular structure of a series of alkyltrimethylammonium halides (C,TAX = C,,HWlN(CH3),+X- (where n = 12,14, and 16 and X = NO,, Br, CH3S04,C1, and OH, which are identified as C,TAN, C,,TAB, C,TAMS, C,,TAC, and C,TAH respectively). The aggregation number, N a n d the fractional micellar charge (/3 = Z / N , where Z is the total micellar charge) are determined for the aforementioned surfactants at concentrations of 0.2,O. 1, and 0.05 mol dmP3at 25.0 OC. Experimental Section Surfactants. Bromides. C,TAB were prepared by stirring trimethylamine (Eastman) and the appropriate alkyl bromide (Aldrich, purified by washing with H2S04and then H 2 0 , dried with KzC03,and fractionally distilled, gas chromatography showed the bromides to be >99% pure) in absolute ethanol for 3 days followed by 12 h of reflux. A crop of crystals was obtained from the ethanol. These were recrystallized three times from acetone:ethanol ( 10:1. v:v) . Chlorides. CI6TAC (Kodak) was recrystallized three times from ethanol. CI4TACwas formed by passing an aqueous solution of CI4TABthrough a chloride ion exchange column. The resulting solution was freeze-dried, and the remaining solid was recrystallized three times from ethanol. C12TAC(Eastman) was recrystallized three times from acetone:ethanol (10: 1, v : ~ ) . Nitrates. C,,TAN were formed by combining a solution of the corresponding bromide with equimolar amounts of AgN03. The mixtures were stirred at 60 OC for 5 h in the absence of light. The solutions were filtered and then centrifuged for 12 h. The supernatant was refiltered and then freeze-dried. The solids were tested for Ag+ by dissolving a small portion in water and adding a drop of HCl. No precipitates were observed. The crude C,,TAN 'University of Virginia. This work carried out in part as a Laboratory Graduate Participant with Oak Ridge Associated Universities, Oak Ridge National Laboratory, Oak Ridge, TN 22901, while a graduate student with Wake Forest University, Winston-Salem, NC. t3M Co. 'Oak Ridge National Laboratory.
also tested negative for the absence of Br using AgN0,/HN03. The C,,TAN solids were recrystallized three times form acetone:ethanol (25:1, v:v). Hydroxides. C,TAB and Ag20 (170% excess) were dissolved in D 2 0 (Aldrich 99.8% atom D) in a centrifuge tube under nitrogen. Contact with air was avoided throughout this procedure to prevent the reaction of atmospheric carbon dioxide with the hydroxide surfactant which would result in the formation of the carboxylate. The solutions were stirred for 14 h in the dark. The mixtures were then centrifuged, and the clear supernatant was removed and filtered through a millipore filter into a clean, dry, nitrogen-filled Nalgene screw top container. The solutions tested negative for both Ag+ and Br-. The solutions were titrated with H N 0 3 to determine the molarity. Methyl Sulfates. Methylsulfuric acid was formed by sulfating methanol with an equimolar portion of chlorosulfonic acid (freshly distilled) in dry ethyl ether under nitrogen. The appropriate C,,TAH was formed as above, but using H,O instead of D,O. This C,,TAH was then neutralized to pH 5-6 with methylsulfuric acid. The solution was freeze-dried, brought up in hot acetone:ethanol (lO:l, v:v), and filtered through a millipore filter. A crop of crystals were obtained from this filtered solution. These crystals were recrystallized two more times from acetone:ethanol. Small-AngleNeutron Scattering (SANS) Measurements. All of the final solutions used in neutron scattering experiments were prepared in DzO(Aldrich, 99.8 atom % D). Neutron-scattering measurements were performed on the 30-m (source-to-detector distance) SANS instrument at the High Flux Isotope Reactor of the National Center for Small Angle Scattering Research (NCSASR) at Oak Ridge National Laboratory (ORNL). The sample-to-detector distance was 2.0 m for all runs. The momentum transfer range was 0.03 < Q < 0.30 A-1 (Q = 4 ~ / sin 1 Q, where 2Q is the scattering angle and 1 = 4.75 A is the neutron wavelength). The samples were contained in 0.2-cm path-length Suprasil UV grade quartz cells with tight-fitting Teflon stoppers, sealed with parafilm. The cell temperature was maintained at 25.0 f 0.1 "C by means of an external circulating bath and was monitored by a thermocouple. Scattering intensity from the surfactant solutions were corrected for detector background and sensitivity, empty cell scattering, incoherent scattering, and sample transmission. Solvent intensity was subtracted from that of the sample. The resulting corrected intensities were converted to radial averages versus Q using programs provided by the NCSASR. Absolute cross sections were computed from calculations based on the known scattering of water. This absolute calibration has an estimated uncertainty of 20%. The experimental points are fitted using a nonlinear least-squares routine as described in detail below.
0022-365419212096-5611%03.00/0 0 1992 American Chemical Society
Berr et al.
5612 The Journal of Physical Chemistry, Vol. 96, No. 13, 1992 25
20
+ .-m
?5
0
C,,TAN
Q3
0
C,,TAB
o a
o n
C16TAMS C,,TAC
V
C16TAH
o-m 0
8
00
c Q)
+
c -
10
5 ss
0 0 05
0 10
0 15
0 20
0 Figure 1. SANS intensities as a function of Q for 0.2 mol dm-' CI6TAX in DzO. Peaks occurring at lower Q are indicative of solutions with larger particles. Thus, a qualitative ordering of micellar size can be made as Cl6TAN > C16TAB > CI6TAMS > C16TAC > CI~TAH,e.&, the nitrate counterion results in the largest micelles, followed by bromide, methyl sulfate, chloride, and hydroxide. TABLE I: Values Obtained in a Previous Work lor C,TABO THIK, nm WET A,, nm ClITAX 0.9 4 1.31 CIlTAX 0.8 2.5 I .76 C16TAX 0.7 2.5 2.01
"These values are used in all of the final nonlinear least-squares fits of the SANS data from solutions of C,TAX. Figure 1 for 0.2 M CI6TAXexemplifies the effect of counterion on the SANS patterns. The position of the peak is indicative of micellar size. A solution with larger micelles results in a peak that occurs at lower Q. Thus an initial qualitative ordering of micelle size can be made as CTAN > CTAB > CTAMS > CTAC > CTAH. A more quantitative evaluation can be made as follows. Analysis of SANS Data. The single particle form factor F(Q) was calculated by treating the micelles as ellipses using a model previously described (seeFigure 2).6 It is dificult to distinguish between polydispersity and ellipsoidal micellar shapes. The following analyses were performed assuming monodisperse micelles. It should be noted that, particularly at higher surfactant concentrations, there may be some unaccounted for polydispersity. The parameters of interest are Nand j3. To limit the number of free-fitting parameters, the thickness of the Stern layer (THIK) and the number of wet methylene units per monomer (WET) were set equal to the values found for C,TAB in an earlier work? These values are summarized in Table I. The double axis of the core, A,, is set equal to the extended length of the portion of the chain that is in the micellar core as determined by WET, i.e. A, = 0.295 + 0.127(n - WET) (1) Because A, is a function of n and WET, it is fmed by the values in Table I. The values of A, are also presented in Table I. The single axis of the core, B,, varies as a function of N: B, = 3VC/4rA,2 where V, is the hydrocarbon core volume given by V, = N[VcH, + (n - 1 - WET)Vc,,]
(2) (3)
with VcH,= 0.0543 nm3 and VcH = 0.0269 nm3. The other axes of the micelle are A, = A, T d I K and B, = B, + THIK. The ratio of the axes is ECC = B,/A,. If ECC < 1, the micelle is oblate ellipsoid; if ECC > 1, the micelle is prolate ellipsoid, and if ECC = 1, the micelle is spherical.
+
CHs(CHs)is
Figure 2. Micelle model used in the calculation of the form factor F(Q). The core consists of hydrocarbon only and has dimension A, X A, X B,. A, is assigned the value of the extended length of the portion of the alkyl chain that resides in the core. B, increases to accommodate all of the dry hydrocarbon. The Stern layer, of thickness THIK, contains the cationic head groups, associated counterions, water, and some hydrocarbon. The overall dimensions of the micelle are A, X A, X & The above diagram is for CI6TAX. The models for CI4TAXand ClzTAX are analogous.
The interparticle structure factor S(Q) was accounted for using the rescaled mean spherical approximation as developed by Hayter et al.7-9 This theory is valid if there is no correlation between particle orientation and/or size and interparticle separation. This assumption is reasonable for charged micelles if ECC is not much greater than unity. Strong electrostatic repulsions prohibit close approach by two micelles.
Results and Discussion h t e r i o n Varialim The results of the nonlinear least-squares fits to the data, varying the aggregation number Nand fractional charge j3 are compiled in Table 11. The trends in micelle size with respect to counterion, as indicated by Nand ECC, are the same as those arrived at quantitatively by inspection of the SANS patterns. Nitrate and bromide micelles are similar in size and much larger than the others in this study. Chloride forms quite small micelles, while hydroxide forms extremely small micelles. Most of the micelles are roughly spherical, except C16TANand CI6TABat high surfactant concentrations, which tend toward prolate ellipsoidal, and the hydroxides, which are oblate ellipsoidal. The tendency of CI6TABto form rods and for CI6TACto remain spherical has been previously noted.lOJ1 The change of the fractional charge of the micelle with respect to counterion does not exhibit such definite trends. Nitrate, bromide, and methyl sulfate have similar fractional charges (0.1-0.25). The fractional charge of the chloride micelles is larger (0.2-0.3) than these, and the hydroxides are very highly charged with 4 0 4 5 % of the OH counterions dissociated from the micelles. The sparsity of counterions in the hydroxide micelles results in inadequate screening between charged head groups and prohibits close proximity of head groups. This limits the number of monomers that can pack into the micelle. Hence N is small for these micelles. The same phenomenon exists for chloride micelles, but to a lesser extent. Sufficient NOp,Br, and CH3S04counterions, on the other hand, reside in the Stern layer allowing these micelles to grow quite large. The reasons for the differences in partitioning of the counterions between the bulk water and the Stern region of the micelle are not fully understood. The ability of a particular counterion to promote aggregation appears to be related to its position in the lyotropic series of anions. The lyotropic series for some common anions is F < IO3 < Br03 < C1 < C103 < Br < NO3 < C104 This series is a measure of the ability of the ions to denature proteins, the stronger ions being higher on the list. The order of
The Journal of Physical Chemistry, Vol. 96, No. 13, 1992 5613
Alkyltrimethylammonium Halide Micelles
TABLE II: Results from Nonlinear Least-Squares Fits of Experimental SANS Data in Which the Aggregation Number Nand the Fractional Charne B Are Used as Free-Fittinn Parameters'
XN
NO3 Br CH3SOp CI OH
B
NO3 Br CH3S04
c1
OH ECC
a
NO? Br CH$O( CI OH
0.2 220 195 108 95 51
16 0.1 176 160 106 88 49
0.05 135 104 101 86 45
0.2 98 96 76 63 35
14 0.1 89 85 73 59 32
0.11 0.1 1 0.13 0.20 0.51
0.13 0.13 0.13 0.21 0.44
0.12 0.16 0.16 0.20 0.39
0.22 0.17 0.17 0.28 0.65 1.3 1.3 1.1 1.o 0.7
2.1 1.9 1.2 1.1 0.7
1.7 1.6 1.2 1.1 0.7
1.4 1.1 1.1 1.o 0.6
81 78 71 55 30
0.2 60 55 51 40 24
12 0.1 56 51 47 37 21
0.29 0.23 0.23 0.33 0.63
0.3 1 0.20 0.20 0.33 0.51
0.18 0.22 0.22 0.28 0.62
0.16 0.29 0.29 0.26 0.55
1.2 1.2 1.1 0.9 0.6
1.1 1.1 1.o 0.9 0.6
1.3 1.3 1.2 1.o 0.7
1.3 1.2 1.1
0.05
1.o
0.6
0.05 51 47 45 32
-
0.13 0.24 0.24 0.20
-
1.2 1.1 1.1 0.9 -
ECC is the axial ratio of the major to minor axes. Other micelle parameters are defined in the text.
TABLE III: Volumes, Scattering Lengths, and Hydration of Species Examined in This Work
hydrated species
vol, nm3
bcoh
HYD
vol. nm3
CH2CH3N(CH3)3+ BrCINOJCH3SOL OD-
0.0269 0.0543 0.1023 0.0393 0.0257 0.0366 0.1087 0.0302
-0.083 -0.457 -0.431 0.677 0.96 2.67 2.12 1.245
0 0 1 5 5 6 10 ?
0 0 0.132 0.19 0.177 0.218 0.41 1 ?
aggregation in Table I1 follows the series for the anions studied, where N increases in the order
OH Br > CH3S04 > C1 >> OH. N also increases as the chain length becomes longer or the concentration increases. The fractional charge of the micelle depends upon the counterion, and this in tum influences the size and shape of the micelle. Although the fractional charge does change with chain length and concentration, this change is not regular and is not as great as that which is caused by the identity of the counterion. Registry No. C12TAN,17065-95-1; C12TAB,1119-94-4; C12TAMS, 13623-06-8; CI,TAC, 112-00-5; C,,TAH, 14898-63-6; CI,TAN, 30862-45-4; C,4TAB, 11 19-97-7; Cl,TAMS, 65059-43-0; CIITAC, 4574-04-3; CIPTAH, 84927-25-3; C I ~ T A N371 , 14-85-5; Cl6TAB, 5709-0; C I ~ T A M S65060-02-8; , CIbTAC, 112-02-7; CIeTAH, 505-86-2.
References and Notes (1) Mukerjee, P.; Mysels, K. J.; Kapaian, P. J. Phys. Chem. 1967, 71, 4166. (2) Critical Micelle Concentrations of Aqueous Surfactant Systems; Mukerjee, P.; Mysels, K. J. US.Department of Commerce: NSRDS-NBS 36, Washington, DC, Feb 1971. (3) (a) Szajdzinska-Pietek, E.; Maldonado, R.; Kevan, L.;Jones, R. R. M. J . Am. Chem. SOC.1985, 107, 784. (b) Szajdzinska-Pietek, E.;Maldonado, R.; Kevan, L.; Jones, R.R. M.; Berr, S.S.J. Phys. Chem. 1985,89, 1547. ( c ) Szajdzinska-Pietek,E.;Maldonado, R.; Kevan, L.; Jones, R. R. M. J . Am. Chem. SOC.1984, 106, 4675. (4) Berr, S.S.; Coleman, M. J.; Jones,R. R. M.; Johnson, J. S.,Jr. J . fhys. Chem. 1986, 90, 6492. (5) Berr, S. S.; Jones, R. R. M. Lnngmuir 1988, 4, 1247.
J. Phys. Chem. 1992,96, 5614-5617
5614 (6) (7) (8) 1851. (9)
Berr, S. S. J . Phys. Chem. 1987, 91, 4760. Hayter, J. B.; Penfold, J. Mol. Phys. 1981, 42, 109. Hayter, J. B.; Penfold, J. J . Chem. Soc., Faraday Trans. 1 1981, 7 7 , Hansen, J.-P.; Hayter, J. B. Mol. Phys. 1982, 46, 651.
(10) Reiss-Husson, F.; Luzatti, V. J . Phys. Chem. 1964, 68, 3504. (11) Ulmius, J.; Lindman, 8.;Lindbloom, G.; Drakenburg, T. J . Colloid Inferfuce Sci. 1978, 65, 88. (12) Bruins, E. M. Proc. Acad. Amsterdam 1932, 35, 107. ( 1 3) Gamboa, C.; Sepulveda, L.; Soto, R. J . Phys. Chem. 1981,85, 1429.
Photochemical Switching of Ionic Conductivity in Composite Films Containing a Crowned Spirobenzopyran Keiichi Kimura,' Takashi Yamashita, and Masaaki Yokoyama Chemical Process Engineering, Faculty of Engineering, Osaka University, Yamada-oka, Suita, Osaka 565, Japan (Received: January 21, 1992; In Final Form: March 13, 1992)
Spirobenzopyran derivatives incorporating a monoazacrown ether moiety at the 8-position, Le., crowned spirobenzopyrans, have been applied to composite films that undergo photoinduced switching of ionic conductivity, taking advantage of the photochemical cation-bindingability changes of the crowned spirobenzopyrans. In plasticized poly(viny1 chloride) films containing LiClO, and a crowned spirobenzopyran, which show bi-ionic conducting behavior, isomerizationof the crowned spirobenzopyran to its corresponding merocyanine form proceeded under UV-irradiated or dark conditions. This depressed the Li+ conduction in the film, thus decreasing its ionic conductivity by about a half. Subsequent visible-light irradiation allowed isomerization back to the spiropyran form, restoring the ionic conductivity to the initial value. Much more significant ionic-conductivity switching with about 20-fold changes was realized in composite films of lithium poly(perfluorosulfonate)/oligooxyethylene diacetatelcrowned spirobenzopyran, which are single-ionic conductors, by alternatively turning on and off the visible light.
Considerable attention has been focused on organic ion conductors or polymer electrolytes.',* One main goal for designing organic ion conductors is high electrical conductivity, but there are very few organic ion conductors whose ionic conductivities can be switched by external stimuli. We have already studied the thermo- and photoresponse in ionic conductivities of poly(viny1 chloride) (PVC) composite films containing LiC104 and an azobenzene liquid crystaL3 The azobenzene liquid crystal isomerizes reversibly from its trans to cis forms by photoirradiation and thereby experiences drastic phase transitions. For instance, UV-light irradiation on the composite film at room temperature caused distinct phase transitions from the crystal or liquid crystal to the isotropic liquid states, drastically enhancing ion mobility and, therefore, ionic conductivity in the film. Subsequent visible-light irradiation again diminished the ionic conductivity. It was thus found that ionic conductivity switching is photochemically feasible with the composite films, which are useful for electrostatic printings4 Spirobenzopyran derivatives generally isomerize to their corresponding zwitterionic merocyanine forms upon illumination with UV light and vice versa with visible light. Spirobenzopyran derivatives such as 1, where a monoazacrown ether moiety is incorporated to the &position as a cation-binding site, can act as photochemical controllers of cation binding in s o l ~ t i o n . When ~ NO'
-\ r k i 4
O O ' CH,
OI OCH,
a crowned spirobenzopyran isomerizes to its corresponding merocyanine form in the presence of a metal ion such as Li+ under UV-light-irradiated or dark conditions, the metal ion complexed by its crown ether moiety interacts intramolecularly with a phenolate anion in the merocyanine isomer. Thus, in the crowned merocyanine, the metal ion can be bound more tightly than the corresponding spiropyran isomer due to a kind of additional binding site effect. Visible light brings about the isomerization back to the spiropyran form, attenuating its cation-binding abilities. 0022-365419212096-5614$03.00/0
The photoinduced change in the cation-binding ability of crowned spirobenzopyrans in solution opens up the possibility for photochemically controlling ionic conductivity of films containing a crowned spirobenzopyran as the key material. We recently communicated the photoresponse of ion-conducting composite films made from plasticized poly(viny1 chloride) (PVC), LiC104, and crowned spirobenzopyran lS6 We have also extended the photochemical ionic-conductivity control driven by the crowned spiroknzopyran isomerization to a single-ionic conducting system. Reported herein are the details of the photoinduced ionic-conductivity switching of crowned-spirobenzopyran-containing composite films and its mechanistic study.
Experimental Section Materials. Crowned spirobenzopyrans 1-3 and an acyclic analog 4 were synthesized as reported el~ewhere.~,'PVC (average polymerization degree of 1020) was purified by repeated reprecipitation. Poly(perfluorosu1fonic acid) (Nation) (PPFS) was received as a 5 wt % alcohol solution from Aldrich and its lithium salt was obtained by neutralization with a lithium methoxide methanol solution. 2-Ethylhexyl sebacate (DOS) was purified by distillation in vacuo. Oligooxyethylene diacetate (average molecular weight of 500) (OOEAc) was prepared by treating oligooxyethylene (average molecular weight of 400) with excess acetic anhydride (70 "C, 1 day) and then purified by alumina chromatography. The lithium salts are of analytical reagent grade. Tetrahydrofuran (THF) and N,N-dimethylformamide (DMF) were distilled over Na metal and calcium hydride, respectively. Composite Film Fabrication. PVC-based composite films for measurements of ionic conductivity and absorption spectrum were prepared on indium-tin-oxide-coated (ITO) glasses (2 X 2.5 cm) by spin coating from THF, and then dried overnight at 40 OC. Typically, 100 pL of a T H F solution [60 mg PVC (51.9 wt %), 50 mg of DOS (43.3 wt %), 0.5 mg of LiC10, (0.4 wt %), and 5 mg of crowned spirobenzopyran (4.4 wt %) dissolved in 0.7 mL THF] was used for each spin coating, allowing a film thickness of about 4 pm. PVC composite films of 50-60-pm thickness for isothermal transient ionic current measurements were cast on an I T 0 glass substrate from 1 mL of a T H F solution with the same composition. PPFS-based composite films for measurements of ionic conductivity and isothermal transient ionic current were 0 1992 American Chemical Society
