1930
J. Phys. Chem. 1984,88, 1930-1933
Surfactant-Polyelectrolyte Interactions. 4. Surfactant Chain Length Dependence of the Binding of Alkylpyridlnlum Cations to Dextran Sulfate Anna Malovikova; Katumitu Hayakawa,t and Jan C. T. Kwak* Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, Canada (Received: January 19, 1984; In Final Form: March 16, 1984)
Isotherms for the binding of undecyl-, dodecyl-, tridecyl-, and tetradecylpyridinium cations to the anionic polyelectrolyte sodium dextran sulfate in aqueous solutions containing excess NaCl are reported. All measurements are at 30 “C. Solid membrane electrodes selective for each surfactant cation are used to determine the free surfactant concentration. Surface tension and cmc values are reported for each of the surfactants; for the CI1and C13 pyridinium bromides these appear not to have been reported earlier. The polymer concentrationis 5 X lo4 quiv/kg of H20in all cases, the added NaCl concentration varies from 5 X to 5 X lo-’ m. The binding isotherms show a highly cooperative character, indicating a large effect of the hydrophobic interactions between bound surfactants. The chain length dependence of the free energy of binding at the half-bound point indicates a contribution per CHz group of 1.29kT, in close agreement with more isolated observations in other surfactant-polymer systems, and with the case of micellar aggregation.
Introduction Aqueous solutions containing polymers and surfactants are of importance in a wide variety of biological and industrial systems. In addition, the study of such systems can give information about the influence of hydrophobic and electrostatic effects on polymer-surfactant interactions and on surfactant aggregation mediated by the polymer. A recent review by Robb stresses the importance of determining binding isotherms in polymer-surfactant systems, in particular in systems containing polyelectrolytes.’ Since the first determination of polyelectrolyte-surfactant ion binding isotherms, using surfactant ion-selective electrodes as analytical method, by Satake and Yang? the difference between the surfactant binding behavior of charged and neutral polymers has been clearly established. In the case of neutral polymers binding occurs only close to the cmc of the surfactant, but with charged polymers binding starts at free surfactant concentrations often orders of magnitude below the cmc of the oppositely charged ionic surfactants3 In previous publications, we have determined the influence of polymer charge density, polymer structure, surfactant headgroup, added electrolyte, and temperature on the binding of cationic surfactants by anionic polyeIe~trolytes.~-~ Thermodynamic parameters derived from measurements of the temperature and chain length dependence of the overall binding constant were described in terms of a model where aggregation of bound surfactants is governed by parameters closely resembling those of micelle formation in a polymer-free solution.69 Because of experimental limitations the surfactant chain length dependence in these studies could only be varied to a limited degree for a given polymer and a given ionic strength. In the present paper we present a more extensive study of the surfactant chain length dependence of the binding of a cationic surfactant by the same polyelectrolyte anion, Le., dextran sulfate, over a wide range of ionic strengths. Alkylpyridinium halides, with alkyl chain lengths C, I-C14, are the cationic surfactants; added NaCl concentrations vary from (0.5-4) X m (undecylpyridinium bromide, rn (tetradecylpyridinium bromide, CIIPyBr) to (4-50) X C14PyBr). All measurements were performed at 30.0 O C . Experimental Section Alkylpyridinium halides were either commercial products (C,,PyCl) or synthesized (CiI, C,,, C14). In either case final purification is extremely important. Dodecylpyridinium chloride, obtained from Tokyo Kasei Kogyo Co., Ltd., was purified by repeated recrystallization from acetone followed by decolorization Institute of Chemistry, Slovak Academy of Sciences, Bratislava, Czechoslovakia *Department of Chemistry, Kagoshima University, Kagoshima 890, Japan. *Author to whom correspondence should be addressed. Department de chimie, Universitt de Sherbrooke, Sherbrooke, Quebec, Canada J1K 2R1.
0022-3654/84/2088-1930$01.50/0
with active carbon. Undecyl-, tridecyl-, and tetradecylpyridinium bromides were synthesized from the corresponding 1-bromoalkanes (Kodak Laboratory Chemicals) previously purified by fractional distillation, following a standard procedure.I0 The crude products were extracted with diethyl ether, followed by up to six recrystallizations from acetone and treatment with active carbon. Sodium dextran sulfate (Pharmacia) was purified and characterized as All aqueous solutions were prepared by weight from polyelectrolyte, surfactant, and NaCl stock solutions in distilled and deionized water. All concentrations as given are in molality units (mol/kg of H20); for the polymer this denotes moles of sulfate anionslkg of HzO. Free surfactant concentrations were determined by means of surfactant-selective solid membrane (PVC gel) electrodes developed for each of the surfactants. Electrode preparation was as described b e f ~ r e with , ~ the carrier complexes prepared by reacting each alkylpyridiniumbromide with highly purified sodium dodecyl sulfate, followed by extensive recrystallization of the complex from acetone. A titration technique as described earliers,9 was used to determine both calibration curves of emf vs. total surfactant concentration and polymer binding curves. For the polymer binding curves, a solution of surfactant with added NaCl (concentration m,) was added to the polymer solution (monomolal concentration mf also with added NaCl, concentration ms),but for each surfactant addition an identical amount of polymer solution, polymer concentration 2m,, NaCl concentration m,, was added as well, to maintain a constant polymer concentration. The emf was measured with a Keithley 616 digital electrometer (10.1 mV), interfaced to a laboratory microcomputer. The microcomputer checks for constancy of emf (normally defined as C 0.2-mV variation over, e.g., 5 min), actuates the piston burets, and accumulates the emf data. (1) I. D. Robb In “Anionic Surfactants, Physical Chemistry of Surfactant Action”; Lucassen-Reynders, E. H., Ed.; Marcel Dekker: New York, 1981; Surfactant Science Series Vol. 11, Chapter 3. (2) Satake, I.; Yang, J. T. Biopolmers 1976, 15, 2263. (3) (a) Goddard, E. D.; Hannan, R. B. J . Colloid Interface Sci. 1976, 55, 73. (b) J . Am. Oil Chem. SOC.1977, 54, 561. (4) Hayakawa, K.; Kwak, J. C. T. J . Phys. Chem. 1982, 86, 3866. (5) Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1983,87, 506. (6) Hayakawa, K.; Santerre, J. P.; Kwak, J. C. T. Biophys. Chem. 1983, 17, 175. (7) Hayakawa, K.; Santerre, J. P.; Kwak, J. C. T. Macromolecules 1983, 16, 1642. (8) Santerre, J. P.; Hayakawa, K.; Kwak, J. C. T. CoNoids SurJ, in press. (9) Malovikova, A.; Hayakawa, K.; Kwak, J. C. T. In “Relation between Structure and Performance of Surfactants”; Rosen, M. J., Ed.; ACS Syrnp. Ser., in press. (10) Knight, A.; Shaw, B. D. J . Chem. SOC.1938, 682. (11) Joshi, Y. M.; Kwak, J. C. T. Biophys. Chem. 1978, 8, 171.
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88,No. 10, 1984 1931
Letters
NaDxS
0.0 O!1
- Cl,PyBr
-4
3
log mb
Figure 3. Binding isotherms for sodium dextran sulfate (NaDxS) ( 5 m)-C,,PyBr-NaC1 at 30 'C. NaDxS
io
Figure 1. Surface tension of alkylpyridinium bromides (C,,, C13,C14, CI6)and chloride (CI2)as a function of surfactant concentration, mD. Arrows indicate literature values (Table I). NaDxS
- ClZPyCl
X
- c,,PyCI .-
4
1 0.005m NaCl 2 OOimNaCl
3 0.02mNaCl
05
4 0.04mNaCl
I
5 0 0 6 m NaCl
I
6 0 i o m NaCl
>
E
2
e
v v v
v v v v v ~ v v v v v 1v v v
00
v v v v
-5
-4
log mb
V D
100-
v "A.
Figure 4. Binding isotherms for NaDxS-C12PyC1-NaC1.
.
.V
NaDxS
- C,3PyBr
v
1.0 i 0 0 0 5 m NaCl
I 0
2 002
m NaCl
1
P I
1
1
,
05
1
0.02m NaCl
2
0 0 4 m NaCl
3
0 0 8 m NaCl
4
O l O m NaCl
5
0 2 0 m NaCl
Figure 2. Observed emf in the system sodium dextran sulfate ( 5 X m)-C,,PyCI-NaCl: (0)mp = 0.
Results and Discussion The purity of all alkylpyridinium halides was checked by means of surface tension measurements at 30 OC using a Du-Nouy ring tensiometer. Results are shown in Figure 1, where our previous results for C16PyBr9have been included. No minima are apparent in the curves for CI6,C14,and C13;however, the C l l curve shows a small but unfortunately persistent minimum. As was discussed before9 there may also be a very slight minimum in the curve for C1?PyCl, but in this case the deviation for the first surface tension point beyond the cmc is barely significant. The cmc values obtained for the CL2to C16 surfactants are in very reasonable agreement with literature values (Table 1,12 some additional lit(12) Mukerjee, P.; Mysels, M. J. Natl. Stand. ReJ Data Ser., Natl. Bur. Stand. 1971,No. 36. (13) Trap, H. J. L.; Hermans, J. J. Proc. K. Ned. Akad. Wet., Ser. B., 1955,58,77. (14) Ford, W. P. J.; Ottewill, R. H.; Parreira, H. C. J . Colloid Interface Sci. 1966,21, 522. (15) Rosen, M. J.; Dakanayake, M.; Cohen, A. W. ColloidsSurf. 1982, 5, 159. (16) Hoffmann, H.; Nagel, R.; Platz, G.;Ulbricht, W. Colloid Polym. Sci.
1976,254,812. (17) Weiner, N. D.; Lografi, G . J . Pharm. Sci. 1965,54,436. (18) Venable, R. L.; Nauman, R. V. J . Phys. Chem. 1964,68, 3498. (19) Evers. I. C.: Kraus. C . A. J . Am. Chem. SOC.1948,70,3049. (20) Benton, D.;Sparks, B. Trans. Faraday SOC.1966,62, 3244. (21) Miola, L.; BlottaAbakerli, R.; Ginani, H. F.; Filho, P. B.; Toscano, V. G.; Quina, F. H. J . Phys. Chem. 1983,87,4417.
0 0 .. -5
-4
log mb
Figure 5. Binding isotherms for NaDxS-C,,PyBr-NaCl. erature values have been included). The literature value for CllPyBr quoted in the compilation of ref 1213 seems too high considering the trend in cmc values for alkylpyridinium bromides, as is obvious from a plot of In cmc vs. the number of carbon atoms in the alkyl chain. On the other hand, the value of 1.95 (fO.lO) X lo-* reported here fits in well with the data for the other bromides. The methods used to obtain binding isotherms, Le., plots of the degree of binding /3 vs. log mDf,where
mDis the total surfactant concentration, mDf is the free surfactant concentration, have been described in The calibration curves, Le., emf vs. log mD in polymer-free solution are linear from well below m to the cmc for all four alkylpyridinium salts studied, and the slopes show no significant variation with the ionic strength (Le., NaCl concentration). A typical case is shown in Figure 2. The slope of the calibration curve equals 59.9 f 0.2 mV/decade where the theoretical slope is 60.2 mV/decade. (22) Paluch, M. J . Colloid Interface Sci. 1978,66, 582.
1932 The Journal of Physical Chemistry, Vol. 88, No. 10, 1984 TABLE I: Critical Micelle Concentrations for Alkylpyridinium Halides cmc surfactant this work"
CllPYBr Cl2PYCl
1.95 (fO.lO) 1.40 (f0.04)
C13PyBr C14PyBr
4.57 (f0.06) x 10-3 2.65 (f0.05) X
C16PyBr
6.2 (h0.1) x 10-4
X X
Letters
lit.
remarks OC, turbidity OC, surf tension OC, conductance OC, light scattering
4.2 X 1.46 X 10-z,c 1.62 X 10-2,d1.4 X 1.48 X 10-2,d1.78 X 10-zf 1.47 X
30 25 25 25
2.57 x 10-3g 2.63 x 10-3.r 5.8 x 10-4; 6.2 x 10-4' 7.1 x 10-4; 7.05 x 10-4.r 6.6 X
30 "C, surf tension 30 OC, conductance 25 OC, conductance 30 OC, conductance 25 OC, surf tension
"Surface tension, 30 OC. bReference 13. CReference 14. dReference 15. eReference 17. fReference 16. gReference 18. *Reference 19. Reference 20. Reference 2 1. Reference 22. J
NaDxS
- C,4PyBr
1.0
I
P
1
0.5 -
00
I
004111 NaCl
2
0 0 8 m NaCl
3
010 m NaCl
4
020mNaCl
5
O50mNaCl
I
-5
log rn;
-4
Figure 6. Binding isotherms for NaDxS-C14PyBr-NaC1. Observed slopes were 59.8, 59.9, 59.7, and 59.9 mV/decade for CIIPy+to CI4Py+,respectivkly, all k 0.2 m v . All measurements were performed at NaCl concentrations at least one and often several orders of magnitude above the total surfactant concentrations, thus the fact that for the C l l , CI3,and CI4cases we use the alkylpyridinium bromides whereas for C12we use the chloride should make no difference, C1- being the major anion in all cases. The resulting binding isotherms are shown in Figure 3-6. Note that although we are able to obtain at least the steeply rising part of the binding isotherm of CI6Pyas well, at ionic strengths between 0.1 and 0.5 m NaCl, binding occurs at such low free surfactant concentrations (see discussion below) that no points can be obtained in the region before the first critical point, Le., in the precooperative region. Thus the results for C16Py, although generally in agreement with the results and trends reported here for the Cl1-CI4 pyridinium salts, are not sufficiently accurate to be included in this paper. The binding isotherms all show the highly cooperative character which now has been well documented for polyion-surfactant ion interaction^.^,^ The "matrix method" developed by Zimm and Bragg to describe the helix-coil transition in a polypeptidesz3has proven particularly convenient in the description of a cooperative binding process by a linear polymer hai in.^,^^ In this method, the overall binding parameter Ku, where K is the binding constant of a surfactant with an isolated site, and u a cooperativity parameter expressing the extra binding term included when binding occurs at a site adjacent to an already occupied site, is given by KU
=
(m~~)l/l'
(2)
(mD3112is the free surfactant concentration at the half-bound point, lee.,where half of the binding sites are occupied. Although such a nearest-neighbor model is obviously unsatisfactory for the description of a binding process where both polymer configurational changes and possibly surfactant aggregation into distinct Ku still provides aggregates may be i n v o l v e d , 9 ~the ~ ~parameter ~~ (23) Zimm, B. H.; Bragg, J. K. J . Chem. Phys. 1959, 31, 526. (24) Schwarz, G. Eur. J . Biochem. 1970, 12, 442.
Figure 7. Binding constants Ku as a function of added NaCl concentration, 111,: (V)dodecyltrimethylammonium br~rnide.~ for a convenient comparison between the binding isotherms, in particular to describe the ionic strength and surfactant chain length d e p e n d e n ~ e . ~The , ~ dependence of log Ku, Le., log ( m D f ) l , 2on , the added NaCl concentration is shown in Figure 7. We observe a linear relationship between log Ku and log mNaCI,the observed slopes vary only slightly, Le., from 0.77 f 0.02 for CI1to 8.66 f 0.02 for C14. Although this ionic strength dependence of log Ku, and therefore of log (mDql12closely parallels the ionic strength dependence of the cmc of ionic surfactants, the physical basis for such a correspondence is not that clear, given that in the case of micelle formation the slope of the log cmc vs. log m, relation is determined by the fraction of bound counterions, where in the present case the surfactant charge is neutralized by the polymer ionic groups. Note that the binding constant for surfactants with a pyridinium headgroup is higher than that for the corresponding trimethylammoqium surfactants, the difference in In Ku being about 0.8 between CIzPy+DTA'. It seems most likely that this reflects a closer approach of the surfactant positive charge to the polyion fixed charge in the case of pyridinium cations. A clear indication of the importance of surfactantsurfactant interactions is obtained from the chain length dependence of Ku (Figure 8). The increase in In Ku for each CH2 group added to the surfactant chain is independent of the chain length and of the ionic strength and amounts to 1.29 (f0.03) k T per CH2 group when expressed as kT In Ku ( k is the Boltzmann constant, T the temperature, i.e., 303 K for the present case) for n varying from 11 to 14 and ionic strength from 0.005 to 0.5 m NaCl. This compares to earlier reported chain length dependences of In Ku of 1.18kT per CH2 group for alkylpyridinium binding to various polysaccharides and ( 2 5 ) Dubin, P. L.; Oteri, R. J . Colloid Interface Sci. 1983, 95, 453. (26) Dubin, P. L.; Davis, D. Colloids Surf., in press. ( 2 7 ) Cabane, B. Colloids Surf., in press.
1933
J . Phys. Chem. 1984,88, 1933-1935 I
I
I
I
a
Y
,a04m
-0
or for that matter other processes involving the transfer of an alkylchain from an aqueous to a hydrocarbon environment. In previous publications we have also compared the cooperativity parameter u as determined from the slope of the binding isotherm at the half-bound
,A
0
(dp/d In mof)1,2 = u1I2/4
(2)
for different surfactant-polymer combinations. In the Schwarz and Satake-Yang treatments u expresses an equilibrium constant for the aggregation of bound surfactants, Le., for a process 20D -,00 DD:
6
+
A/ / P /
u =
/Lz
(DD)(OO) (DD)*
4
I
I
I
I
11
12
13
14
C”
Figure 8. Binding constants Ku as a function of surfactant chain length at various NaCl concentrations, m.
p~lyacrylate,~ and from a comparison of dodecyl- and tetradecyltrimethylammonium binding, 1.23kT per CH2 group for binding to DNA: and 1.lOkTto 1.32kTfor binding to a number of carboxylated p ~ l y i o n s .From ~ Figure 8, we predict log Ku = 6.1 for C16Py+and dextran sulfate in 0.1 m NaCl, and about 5.6 in 0.5 m NaCl, corresponding to values of 8 X lo-’ and 2.5 X 10” m, respectively. These values are too low to allow for the determination of accurate binding isotherms, but less accurate data for c&+ not reported here are in agreement with this prediction. In the case of polysaccharides of lower charge density binding isotherms for c&+ can be determined.g We may conclude that for a wide range of surfactant chain lengths the free energy of surfactant binding by polyelectrolytes shows a chain length dependence closely resembling the case of micelle forma-
(3)
where 00 represents two neighboring free surfactant sites and D an occupied site. Thus the chain length dependence of the free energy of binding, i.e., the 1.29kT difference per CH2 group calculated earlier, should also be reflected in the difference in u . For the present case, however, the cooperativity is so high that u cannot be calculated with the precision necessary to make such a comparison meaningful (we find u = 900 f 200 for CIIPy+, 1700 f 600 for CI2Py+,and >2000 for CI3Py+and CI4Py+). In addition, such an interpretation of the slope of the binding isotherm depends on the correctness of the nearest-neighbor model. It seems preferable therefore to use chain length dependence data to calculate the free energy associated with the hydrophobic interaction of the bound surfactant chains. Acknowledgment. We are grateful to the Natural Sciences and Engineering Research Council of Canada and the Czechslovak Academy of Sciences for the award of a scholarship under the auspices of the scientific exchanges agreement between the Council and the Academy, and to the Killam Foundation for the award of a postdoctoral fellowship to A.M. This research is supported by the Natural Sciences and Engineering Research Council of Canada. (28) Shinoda, K.;Nakagawa, T.; Tamamushi, B.; Isemura, T. ‘Colloidal Surfactants”; Academic Press: New York, 1963. (29) Mukerjee, P. Adv. Colloid Interface Sci.1967, 1, 241.
Submicron Structures on Diacetylene Slngle-Crystal Surfaces by Electron Beam Irradiation H. Niederwald,* H. Seidel,? W. Guttler, and M. Schwoerer Universitat Bayreuth, Physikalisches Institut, 0-8580Bayreuth, West Germany (Received: February 8, 1984)
Submicron structures on the surface of single crystals of 2,4-hexadiynylene bis(p-toluenesulfonate) (TS) have been produced by electron beam irradiation and etching techniques. The first results reported here show that structures up to a spatial frequency of 5000 lines/mm can be generated. From this a polymer chain length in the monomer crystal of 400 monomer units is derived. These surface structures combined with the well-known special properties of diacetylene single crystals bear interesting applications.
Introduction The well-known solid-state polymerization of dia~etylenesl-~ can be initiated by heat or by UV, y, or, as is done in these experiments, electron beam irradiation. An old and not yet satisfactorily answered question is that of the chain length of the polymer created in the monomer matrix. The lower limit is given *Present address: Chemistry Department, University of California, Los An eles, CA 90024 gSiemens AG, ZFEAM 22, Giinther Schwarowsky-Str. 2, D-8520 Erlangen, West Germany
0022-3654/84/2088-1933$01.50/0
by the number of observed intermediates at about 20 monomer units. The upper limit was found to be 0.6 wm or about 1200 monomer units according to an experiment which created polymer structures of more than 1600 lines/mm in monomer single crystals by UV-optical interferences4 (1) G. Wegner, “Molecular Metals”, W. E. Hatfield, Ed., Plenum Press, New York, 1979, pp 209ff. (2) D. Bloor, “Developments in Crystalline Polymers”, D. C. Basset, Ed., Applied Science Publishers, London, 1982. (3) R. H. Baughman and R. R. Chance, Ann. N.Y.Acad. Sci. 313, 705
(1978).
0 1984 American Chemical Society
