J . Phys. Chem. 1989,93, 79 13-79 15
7913
Thermodynamic and Transport Properties of Polyelectrolyte-Surfactant Complex Solutions at Various Degrees of Complexationt Joie Skerjanc* and Ksenija Kogej Department of Chemistry, University of Ljubljana, 61000 Ljubljana, Yugoslavia (Received: February 23, 1989; In Final Form: May 31, 1989) The osmotic coefficient, the apparent molar volume, the enthalpy of dilution, and the electrical conductivityof aqueous solutions of poly(sytrenesu1fonicacid), in which various portions of the charged groups on the polyion were blocked by the cetylpyridinium cations, were measured at 25 "C as functions of the polymer concentration and degree of complexation. On the basis of the experimental results it was concluded that the surfactant molecules are bound to the adjacent ionic groups on the polyion. The remaining part of the polymer molecule behaves as a fully charged polyion with unchanged charge density up to 50% coverage. At higher degrees of complexation the shielding of the polyion by the surfactant aggregates becomes more extensive and the measured quantities exhibit a definite change in their behavior.
Introduction Interactions between surfactants and polyelectrolytes have been the subject of some recent studies.14 These interactions have been reported to be specially strong when a polyelectrolyte and an ionic surfactant are oppositely charged. The strong association of surfactant and polymer ions has been attributed to the mutual action of electrostatic and hydrophobic forces between detergent ions and the polyion and between bond surfactant ions, respectively. In a previous paper5 on polyelectrolyte-surfactant interactions we presented the results of experimental studies of the enthalpy and degree of binding of two cationic surfactants, dodecyl- and cetylpyridinium chlorides, by the anionic polyelectrolyte, sodium poly(styrenesulfonate), in aqueous solutions in the presence of added simple salt, NaCI. The results showed that cetylpyridinium cation is almost quantitatively associated with the poly(styrenesulfonate) anion (more than 99%) in a wide concentration range. Utilizing this finding we prepared samples of poly(styrenesu1fonic acid) in aqueous solutions in which various portions of charged groups on the polyion were blocked by surfactant cations. For these solutions the osmotic coefficient, the apparent molar volume, the enthalpy of dilution, and the electrical conductivity were determined as functions of polyelectrolyte concentration. The results of the measurements are given in the present paper. Experimental Section Materials. Poly(styrenesu1fonic acid), HPSS, was prepared from sodium poly(styrenesulfonate), NaPSS, with a molecular weight of about 70 000 and a degree of sulfonation 1.O, supplied by Polysciences, Inc. (Warrington, PA). For purification and preparation of solutions, dialysis and ion-exchange techniques were used as described in detail elsewhere.6 N-Cetylpyridinium chloride (CPC), puriss, obtained from Kemika, Zagreb, Yugoslavia, was repeatedly recrystallized from acetone. The poly(styrenesu1fonate)-cetylpyridinium complex was prepared by mixing equivalent amounts of HPSS and CPC solutions. The resulting white precipitate of PSS-CP complex was washed repeatedly with water to remove HCI and finally dried by lyophilization. By dissolution of weighed amounts of the complex in the HPSS stock solution, the samples with various degrees of free sulfonate groups on the polyion,f, were obtained cf = 0.75, 0.50, and 0.36). We did not succeed in preparing samples with f less than 0.36 due to the limited solubility of the complex. Apparatus. Osmotic pressure measurements were made with a Melabs recording osmometer Model CSM-2. A detailed description of the osmometer and the experimental procedure has been described elsewhere.' Taken in part from a work presented by K.K. to the University of Ljubljana in partial fulfillment of the requirements for the M S c . Degree.
0022-3654/89/2093-79 13$01 .50/0
Calorimetric measurements were carried out in an LKB 10700-1 flow microcalorimeter unit which was built in a thermostated water bath controlled within f0.005 OC. The densities of the solutions were measured with a Paar digital densimeter DMA 02D. An ultrathermostat attached to the instrument controlled the temperature at 25 f 0.002 "C. The conductivity measurements used a Jones conductance bridge, Leeds & Northrup Co. All measurements were done at 25 f 0.002 "C. Results and Discussion Experimental osmotic coefficients, r j , enthalpies of dilution, AHD, apparent molar volumes, a",and molar conductivities, A, are presented as functions of monomalality, m, in Figures 1-4 for various degrees of complexation. The values of r j and AHD for pure acid (f= 1) were taken from previous papers from this laboratory.'+* The osmotic coefficient, which is the ratio of real to ideal osmotic pressure, or
d' = r / * i d
(1)
was calculated from eq 1 using for rid =fmRT The ideal osmotic pressure of a polyelectrolyte solution, consisting of a polyion with a degree of polymerization P and a mixture of monovalent hydrogen ions and multivalent surfactant micelles with an aggregation number z, is given by rid = [ m / P mf m(1 - j ) / z ] R T (3) For high molecular weight polyelectrolytes 1/ P is much smaller than f, except for vanishing values off. Furthermore, by use of the method of Missel et aL9 the aggregation number, z, of CP+ micelle has been estimated to be about 94. Thus eq 3 simplifies to eq 2. The apparent molar volume was obtained from measured densities of solutions and solvent, p and po, respectively, by using eq 4 a" = M / P + 10OO(Po - P)/mPoP (4) where M is the average monomolar mass M = MpSs- + fMHt -+ (1 -j)Mcpt (5)
+
+
( 1 ) Satake, I.; Yang, I. T. Biopolymers 1976, 15, 2263. (2) Malovikova, A.; Hayakawa, K.; Kwak, J. C. T. J . Phys. Chem. 1984, 88, 1930, and references cited therein. (3) Abuin, E. B.; Scaiano, J. C. J . Am. Chem. SOC.1984, 106, 6274. (4) Methemitis, C.; Morcellet, M.; Sabbadin, J.; Francois, J. Eur. Polym. J . 198fjd 22, 619. (5) Skerjanc, J.; Kogej, K.; Vesnaver, G. J . Phys. Chem. 1988,92, 6382. (6) Skerjanc, J.; Pavlin, M. J . Phys. Chem. 1977, 81, 1166. (7) Vesnaver, G.; Dolar, D. Eur. Polym, J . 1975, 11, 657. (8) Vesnaver, G.; Rudei, M.; Pohar, C.; Skerjanc, J. J . Phys. Chem. 1984, 88, 2411. (9) Missel, P. J.; Mazer, N. A,; Benedek, G. B.; Carey, M. C. J . Phys. Chem. 1983, 87, 1264.
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 23, 1989
1914
0.25 cp
t
-
v
h
0 :0.20-
"
0
0
n
-0
-
a
1=0.50
e
V
Skerjanc and Kogej
f=l.Oo f=0.75
Q
U
$ 1 3
f = 0.50 A
1
2
f=0.36
-log m
Figure 1. Osmotic coefficient of aqueous solutions of poly(styrenesu1fonic
f=0.31
acid)-cetylpyridinium complex at 25 OC for various degrees of complexation, ( 1 -A, as function of monomolality. 60P-=-=
0
0.0 4 0.08 m/monomol kg
0.12 1
Figure 4. Concentration dependence of the molar conductivity of the HPSS-CP complex in water at 25 "C. Cloudy solutions: f = 0.31.
In this equation Mpss-, M H t , and Mcpt are the molar masses of the poly(styrenesu1fonate) monomer unit, hydrogen ion, and cetylpyridinium ion, respectively, and f is identical with the mole fraction of the hydrogen counterions, XHt, since, as stated before, the CP+ cation is quantitatively associated with the PSS-anion. Hence
f = XH+ = n H + / ( n H +
+ ncpt)
(6)
In Figures 1-4 we see that the concentration dependence of all properties is similar, as observed earlier for the pure a ~ i d . ' * ~ J ~ As expected, the enthalpy of dilution and molar conductivity decrease with the increasing complexation of the polyion by the surfactant. On the other hand, the apparent molar volume increases with decreasingf. Assuming the validity of the additivity rule
2
3
1
-log
m
Figure 2. Concentration dependence of the enthalpy of dilution of the H P S S C P complex in water at 2 5 OC. Cloudy solutions: f = 0.33 and
f = 0.31.
a plot of the apparent molar volume versusfshould give a straight line with intercept equal to the apparent molar volume of the cetylpyridinium-poly(styrenesu1fonate) complex, @Fppss, and with slope equal to the difference between @.,'s of CP+ and H+counterions. The straight lines observed when the values of @" from Figure 3 are plotted againstfyield @Fppss = 452 mL/monomol and -@ )': = 340 mL/mol at concentrations below 0.01 monomoL/kg. These values agree within the limit of the experimental errors with the literature data for the apparent molar volumes of the hydrogen,1° cetylpyrdinium,Il and poly(styrenesulfonate)12ions: -0, 339, and 113 mL/mol, respectively. The value quoted for the CP+ cation deserves comment. It is wellknownI4 that @" of surfactants increases sharply at the critical micelle concentration. The value 339 mL/mol refers to the concentration range above the cmc, i.e., to the monomer embedded in a micelle, whereas the corresponding value below the cmc, Le., for the free monomer, is 323 mL/mol. The critical micelle concentration of CPC is 6.3 X lo4 M in water.$ Thus, even at the lowest concentrations studied in this paper (monoM) and for the values offlower than 0.4, the surfactant ions exist in the micellar form. Moreover, having in mind the surfactantpolyelectrolyte interaction model,'^^ according t o which the ma-
""I I f
330
0.36
I
1901
1001
3
=
I 2
1
log m
Figure 3. Concentration dependence of the apparent molar volume of the HPSS-CP complex in water at 25 OC. Cloudy solutions: f = 0.31.
(10) Millero, F. J. Chem. Rev. 1971, 71, 147. (11) This value has been obtained from the measured apparent molar volume of CPC13 by subtracting the corresponding value for the Cl- ion, @" = 17.8 mL/mol.'O (12) Tondre, C.; zana, R. J . Phys. Chem. 1972, 76, 3451. (13) Tavzes, L.; Skerjanc, J. Unpublished results. (14) Woolley, E. M.; Burchfield, T. E. J . Phys. Chem. 1984, 88, 2155.
The Journal of Physical Chemistry, Vol. 93, No. 23, 1989 7915
Polyelectrolyte-Surfactant Complex Solutions TABLE I: Relative Values of the Osmotic Coefficient,“Enthalpy of Dilution: and Molar Conductivity”of Aqueous Solutions of HPSSCP Complex at 25 “C Calculated Der Mole of Hvdrogen Ionsc
o.6t f=0.31
1
1
1
1
0.75 0.50 0.36 0.31d
0.95 1.01 1.24
0.96 0.91 0.89 1.56
0.98 0.98 1.12 1.26
“Polymer concentration m = 0.01 monomol/kg of H 2 0 . bInitial concentration m = 0.01 and final concentration m = 0.001 monomol/kg H20.cb*,AH*, and ?* refer to a pure acid, HPSS. “Cloudy solutions.
jority of the surfactant molecules close to the polyion are associated with each other a few orders of magnitude below their cmc, forming many small aggregates or “minimicelles”, one may conclude that this is true also for values off higher than 0.4. A further comment concerns the value of 0, for the CP-PSS complex, OFppss. Since the complex is not soluble in water, the reported value 452 mL/monomol refers to a hypothetical solution of the complex in this solvent and is thus not the subject of a direct experimental determination. In Table I the relative values of the osmotic coefficient, enthalpy of dilution, and molar conductivity calculated per mole of the hydrogen counterions, 4/4*, AHD/fAHD*,and A/fx*, respectively, are presented. The asterisks indicate values for the pure acid. We can see that all relative properties are roughly constant up to 50% coverage, indicating that surfactant molecules are not bound at random, but that they are “trapped” in the form of smaller or larger aggregates3J5 in the regions of high electrostatic potential close to the macroion, making this part of the macroion thermodynamically inactive. The remaining part of the polyion behaves as a fully charged rod with the unchanged charge density. At higher degrees of complexation, Le., forf C 0.5, the shielding of the polyion by the surfactant aggregates becomes more extensive due to the steric factors, and more than the equivalent number of ionic groups on the macromolecule becomes blocked by the detergent aggregates. As a consequence, the measured properties increase. Unfortunately, only a narrow range off is accessible to the measurements due to the limited solubility of the polyelectrolyte-surfactant complex. The same conclusions can be drawn from the results for the fraction of free counterions that can be obtained from electric conductivity measurements following the idea of the association of counterions with the p~lyion.’~*~’ In our notation, the relation between the molar conductivity, A, and the fraction of free counterions, f c , is given by’*
where A, is the monomolar conductivity of the polyion constituent,18and X, is the molar conductivity of the free counterions. The (15) Takagi, K.; Fukaya, H.; Sawaki, Y. J . Am. Chem. SOC.1988, 110, 7469. (16) Huizenga, J. R.; Grieger, P. F.; Wall, F. T. J . Am. Chem. SOC.1950, 72, 2636. (17) Katchalsky, A. Pure Appl. Chem. 1971, 26, 327. (18) Kurucsev, T.; Stell, B. J. Reu. Pure Appl. Chem. 1967, 17, 149.
0.5 fH*
0.4
0.3I 0
I
I
I
0.04 m/monomol
I
0.08 kg-1
I
I
0.12
Figure 5. Concentration dependence of the fraction of free hydrogen counterions at 25 “ C in aqueous solutions of H P S S C P complex. Cloudy solutions: f = 0.3 1.
polyion conductivity, A, = AT,, can be obtained from the combination of the transport number, T,, and conductivity measurements without any approximation.16 In the present computations we used for X, the values that have been obtained for HPSS in this 1aborat0ry.l~ It has to be pointed out that the calculated values off, do not depend considerably on the values of A,, since the second contribution, A,, is much more significant due to the high mobility of the hydrogen ion. In eq 8 A, is still an unknown and may be, to a good approximation,20equated to the corresponding value for the same counterion in a solution of a 1:l electrolyte at a concentration equal to that of the free counterions, f,m. In the computations of A,Cf,m), the dataz1for the molar conductivities and transport numbers of hydrochloric acid solutions were used, and the values o f f , were obtained by iteration. The computed values of the fraction of the free hydrogen ions, presented in Figure 5 , confirm previous observations. Although, as usual,” the values off, are considerably higher than those for 4, the general dependence of both quantities on the degree of complexation, 1 -5 is similar. Up to 50% complexation, and for a given constant concentration, they are constant within the limits of the experimental error. At higher coverage both f, and 4 increase. Acknowledgment. This material is based on work supported by the U.S.-Yugoslav Joint Fund for Scientific and Technological Cooperation, in cooperation with the NSF under Grant 850 9373-JFP 521, and by the Research Community of Slovenia. Registry No. CPC, 123-03-5; HPSS, 50851-57-5. (19) Dolar, D.; Span, J.; Pretnar, A. J . Polym. Sci.: Pur? C 1968,16, 3557. (20) Darskus, R. L.; Jordan, D. 0.;Kurucsev, T. Truns. Furuduy SOC. 1966,62, 2876. (21) Harned, H. S.; Owen, B. B. The Physical Chemistry of Electrolytic Solutions,3rd ed.; Reinhold: New York, 1958; pp 697, 723.
