Langmuir 1990,6, 1495-1498
1495
Solubilization of Water-Insoluble Dye by a Cooperative Binding System of Surfactant and Polyelectrolyte Katumitu Hayakawa,* Takeshi Fukutome, and Iwao Satake Department of Chemistry, Faculty of Science, Kagoshima University, Korimoto-1, Kagoshima, J a p a n 890 Received January 31, 1990. I n Final Form: April 13, 1990 Solubilization of a water-insoluble dye, oil yellow OB (OY), is examined in poly(viny1 sulfate)alkyltrimethylammonium complexes as a function of the degree of surfactant binding. Solubilization correlates strongly to the degree of cooperative binding of surfactant. The binding isotherms of surfactant shift to lower equilibrium concentration of free surfactant in the presence of solubilized dye. A concerted cooperative binding model of dye and surfactant is proposed for this solubilization. The model predicts the solubilization of OY and surfactant binding in the presence of OY. The free energy of transfer of OY from the solid phase to polyion/surfactant complex is found to be 3.1 kJ mol-' from the solubilizing capacity.
Introduction Polymer-surfactant mixtures are well-known to show solubilization of water-insoluble dyes below the critical micelle concentration of surfactant and greater solubilizing capacity per surfactant ion.'-" While several papers have discussed the physicochemical properties and mechanism of the solubilization in nonionic polymer-ionic surfactant systems4+3and dye solubilization was used to study interactions of protein with surfactant,' only a few papers have been devoted to the phenomenological properties of solubilization by polyion-ionic surfactant of opposite charge,8-11 and no mechanistic study has appeared yet for this system. In the present paper, we discuss our measurement of the solubilization of oil yellow OB, l-(m-tolylazo)-2naphthylamine (OY), by complexes of poly(viny1 sulfate) (PVS) and alkyltrimethylammonium (C,TA, n = 10 and 12) and of binding isotherms of surfactant by PVS. We also discuss the mechanism of a proposed concerted cooperative binding model. Experimental Section Materials. Potassium poly(viny1sulfate) (PVS, n = 1500, Wako, specially prepared for colloid titration) was dissolved in 1 mol dm-9 (M) NaCl. The solution was dialyzed against water until no chloride was detected by silver nitrate. The ionic concentration was determined by colloid titration with methyl glycol chitosan (n > 400,Wako, standard solution for colloid titration). Alkyltrimethylammonium bromides (CloTAB and C12TAB, Tokyo Kasei, GR) were repeatedly recrystallized from acetone including a little water. Oil yellow OB (OY, Tokyo
* Author to whom correspondence should be addressed.
(1) Saito, S. Kolloid 2. 1957,154, 19. (2) Saito, S. Kolloid 2. 1958,158, 120. (3) Jones, M. N. J. Colloid Interface Sci. 1968,26, 532. (4) Arai, H.; Murata, M.; Shinoda, K. J. Colloid Interface Sci. 1971, 37, 223. (5) Lang, H. Kolloid 2.2.Polym. 1971,243, 101. (6) Murata, M.; Arai, H. J. Colloid Interface Sci. 1973,44,475. Tokiwa, F.; Taujii, K. Bull. Chem. SOC.Jpn. 1973,46, 2684. (7) Blei, I. J. Colloid Sci. 1960,15, 370. Breuer, M.; Straws, V. P. J. Phys. Chem. 1960,64,228. Steinhardt, J.; Stocker, N.; Carroll, D.; Birdi, K. S. Biochemistry 1974,13,4461. (8) Goddard,E. D.; Hannan, R. B. In Micellization, Solubilization and Microemulsions; Mittal, K. L., Ed.; Plenum Press: New York,1977; Vol. 2, p 835. (9) Ohbu, K.; Hiraishi, 0.;Kashiwa, I. J. Am. Oil Chem. SOC.1982,59, 108. (10) Leung, P. S.; Goddard, E. D.; Han,C.; Glinka, C. J. Colloids Surf. 1985, 13, 47. (11) Ananthapadmanabhan, K. P.; Leung, P. S.; Goddard, E. D. Colloids Surf. 1985, 13, 63.
0743-7463/90/2406-1495$02.50/0
I
'P
8 U
0.5
0.0 0.0
I 0.5
,
I .o
CC oT A 81/mM
Figure 1. Dependence of absorbance at 450 nm, Am, by OY on total CloTAB concentration at constant PVS concentration. [PVS], 0.5 mM; [NaCl], (a) 0, (b) 5, (c) 10,(d) 20 mM. Kasei Chemicals) was used without further purification. PVS concentration is represented by the ionic concentration. Measurements. A small amount of OY powder was carefully added to PVS/C,TAB mixed solutions to avoid irreversible precipitation of PVS/C,TAB complex. Test solutions were shaken at 25 O C for 2 days and allowed to stand for at least 12 h at the same temperature. Shaking for 2 days equilibrated the solubilization of OY, because absorbance by solubilized OY increased no more after 2 days. The colored supematant was used to measure the absorption spectrum and electromotive force. Absorption spectra of OY were recorded on a Hitachi spectrophotometer 228 with a thermostated cell holder. A comparison of the spectrum of OY in PVS/C,TAB mixed solutions with the spectrum obtained in some organic solvents indicated that the microenvironment of OY in PVS/surfactant solutions was close to that in ethanol. The molar absorption coefficients e = 13 300 M-l cm-l at 450 nm in ethanol was, therefore, used for e8timation of the quantity of solubilized OY in PVS/C,TA complexes. Binding of surfactant ion by PVS was obtained potentiometrically in both the presence and absence of OY by use of a plastic membrane electrode responsive to surfactant ion. This electrode showed an excellent Nernstian response even in the presence of excess NaC1.12
Results and Discussion Solubilization of OY. OY is insoluble in both water and PVS solution and also in surfactant solution below the critical micelle concentration (cmc). Figure 1shows a typical example of the dependence of absorbance a t 450 nm, A450, on surfactant concentration in the presence of (12)Shirahama,K.;Yuasa,H.;Sugimoto, S.Bull. Chem. Soc. Jpn. 1981, 54, 375.
0 1990 American Chemical Society
1496 Langmuir, Vol. 6, No. 9,1990
Hayakawa et al.
CC,,TAB1 / mM
Figure 2. Degree of binding BS of CloTAB and Arw by OY as a function of total CloTAB concentration. [PVS],0.5 mM;[NaCl], 10 mM. (a) Bs without OY; (b) BS with OY; (c) Am. 1.0
IB a"
P
a"
t
I
0.5 0.0 0.00
0.03
0.06
CC12TABlF/mM 0.0
0.0
0.4
0.8
tCl2TA6lF/mM
Figure 3. Binding isotherms of surfactant by PVS in the absence of OY and curves calculated by eq 1. (A) CloTAB. (B)%TAB. [PVS], 0.5 m M ;[NaCl], (a) 0,(b) 5, (c) 10, (d) 20 mM.
0.5 mM PVS a t varying concentrations of NaC1. The behavior of Am indicates that (1) a certain amount of &TAB is required for solubilization of OY, (2) the solubilized quantity of OY increases linearly with surfactant concentration beyond a critical concentration where solubilization starts, (3) the critical concentration for solubilization shifts to higher concentration with increasing NaCl concentration, and (4) the slope of the linear part of the curves tends to decrease with increasing NaCl concentration in PVS/CloTAB mixed systems. Binding Isotherms. Binding of C,TAB (n = 10 and 12) by PVS was measured potentiometrically in both the absence a n d presence of OY a t varying CloTAB concentrations. The degree of surfactant binding (6s = [ S ] ~ / [ P ] t o t a l without ) OY and with OY is, as an example, plotted against the total CloTAB concentration together with A450 in Figure 2. The fact that A450 increases with 6s with OY clearly indicates the solubilization of OY by PVS/CloTAB complex. Comparison between 6s without OY and 6s with OY indicates that the solubilized OY promotes the surfactant binding a t a low surfactant
Figure 4. Binding isotherms of surfactant by PVS in the presence of OY and curves calculated by eq 12. (A) CloTAB. (B) CI~TAB. [PVS],0.5 mM; [NaCl], (a) 0,(b) 5, (c) 10, (d) 20 mM. concentration but induces early saturation of surfactant binding a t a low fls value. This trend was also observed in PVS/C12TAB mixed system and will be discussed in the following section. Figures 3 and 4 show the binding isotherms of surfactant, the plot of Bs versus the concentration of free surf a c t a n t [ S l f , in t h e absence a n d presence of OY, respectively. Comparison between Figures 3 and 4 indicates that, as stated above, OY induces shift of the isotherms to lower concentration of free surfactant, early saturation a t low degree of binding, and lowering of the slope a t the steepest rise. The steep rise a t a critical concentration of free surfactant points t o a highly cooperative process which is characteristic of the binding of ionic surfactant or dye by polyions of opposite charge.I3-l6 Applying the model derived by Schwarz for dye stacking on polymer17 and by Satake and Yang for surfactant binding by dissociative polypeptides,'* we obtain the expression for the degree of binding of surfactant by polyion (6s) as eq 1 in the absence of dye
ps =
;( 1+
)
s-l 4s/u]"2 s and s / u in this equation are represented by
(1)
s / u = [os]/[ool= K[Sl, s = [SS]/[SO] = KU[S]f for the following equilibria of surfactant binding:
(2)
[(s - 1 ) 2
+
(3)
(13) Hayakawa, K.; Ayub, A. L.; Kwak, J. C. T. Colloids Surf. 1982, 4 , 389.
(14) Schwarz, G.; Klose, S.; Balthaaar, W. Eur. J. Biochem. 1970,12, 454. (15) Hayakawa, K.; Kwak, J. C. T. J . Phys. Chem. 1982,86,3866. (16) Satake, I.; Hayakawa, K.; Komaki, M.; Maeda, T. Bull. Chem.Soc. Jpn. 1984,57, 2985. (17) Schwarz, G. Eur. J. Biochem. 1970,12,442. (18) Satake, I.; Yang, J. T. Biopolymers 1976, 15, 2267.
Langmuir, Vol. 6, No. 9, 1990 1497
Solubilization in Polymer-Surfactant Complex Table I. Cooperative Binding Parameters and Thermodynamic Parameters of Solubilization of OY in PVS/C.TAB Complexes
0 5 10 20
3800 2320 1680 1180
0
33000 (48 OOO)' 26000 18300 14500
5 10 20 0
1200 250 200 200
0.41 0.39 0.40 0.38
0.52 0.37 0.20 0.13
PVSf Clz TAB 50 (200)' (0.45)' 0.35 150 400 0.29 700 0.25
-'-I
0.29 0.28 0.29 0.28
3.1 3.1 3.1 3.2
0.34 0.25
3.4
0.40 0.29 0.39 0.28 0.47 0.32
3.1 3.1 2.8
0.00.00
c
a t ps = 1 / 2
(d&/d In [SI,) = u'/'/4
a t Bs = 1 / 2
(6) (7)
Figure 3 includes the curves calculated (solid lines) by eq 1. The values Ku and u are listed in Table I for PVS/ CloTAB and PVS/C12TAB systems. The large value of u indicates a highly cooperative binding of these surfactants by PVS a s found in many polyion/surfactant systems.12-16Jg A Concerted Cooperative Binding Model. In the presence of insoluble dye OY, we represent, for example, a polyion site sequence as follows: - 0 0 s S S D S S S O S O S D D S O 1 1 sfus s d s s s 1 sful slud d s 1 (8)
where D stands for a dye-bound site. Since OY is insoluble in polyion solutions, the site D must be among surfactantbound sites. When we represent the statistical weight by 1 for the site 0, by s for the site S having a neighboring S site, by s / u for the isolated or starting S site, and by d for the site D, the partition function of a site, w, is written by
w=[$
1 1 s d d
-
1
0.06
+
-S 0- D $ 4 D-
(4)
(5) where 0 stands for an empty site, S for a surfactantbound site, and [ i j ] for a concentration of an ij pair sequence in polyion/surfactant complexes. K is the equilibrium constant for surfactant binding to form an isolated or starting S site, and Ku is the equilibrium constant for surfactant binding to a vacant site next to the site already occupied by surfactant. The parameter u measures the cooperativity of surfactant binding: u > 1 for cooperative binding, u = 1for noncooperative binding, and u < 1 for anticooperative binding. The values of Ku and u are calculated by the following relationships:
Ku = l/[S],
J
Figure 5. Degree of binding of ClzTAB (Bs) and OY (BD) as a function of concentration of free CIzTAB and their curves calculated by eqs 1,12, and 13. [PVS],0.5 mM;[NaCl], 10 mM. (a) @s without OY; (b) j3s with OY; (c) PD.
K
sKu -s 0- + s * -s s-
,
0.0 3 CCI~TABIF/~M
Values for the broken line a in Figures 4B and 6B.
-00-+ s e - 0
"
r .
(9)
Note that the statistical weights s and s / u are equivalent to those defined by eqs 2 and 3, respectively. Therefore, d stands for the following equilibrium: (19) Hayakawa, K.; Santem, J. P.; Kwak, J. C. T. Macromolecules 1983, 16, 1642.
-D 0-+ D
-D D-
(10) (11)
Applying an eigenvalue method of matrix developed by Zimm and Bragg for coil-helix transition of biopolymer,m we obtain the expressions of the degree of binding of surfactant 0s and dye OD:
Since d stands for dye binding (eqs 10 and ll),it will be a function of concentration of free dye in bulk solution. In the present systems, however, it is constant because dye always saturates the aqueous bulk solutions. Using Ku and u obtained in Figure 3, we calculate the binding isotherms of surfactant (&) and solubilization of OY (OD) by using an optimum d value for the best fit to experimental data. Figure 5 shows an example for the PVS/ClzTAB system a t 10 m M NaC1. The calculated curves (solid lines) fit well the experimental data for Bs and PD in this case. Solid lines in Figures 4 and 6 are calculated by eqs 12 and 13. In the PVS/CloTAB system, the curves in Figure 4A reasonably fit the experimental data in the "rise" region. Precipitation, however, reduced the coincidence even a t low degree of binding, in particular for OY solubilization. The coincidence in Figure 4 is much better for PVS/C12TAB systems than for PVS/CloTAB systems. However, we could not find a fitting curve for the data in the absence of NaCl by use of Ku and u for curve a in Figure 3B. The broken lines in Figures 4B and 6B are calculated by use of the values in parentheses in Table I. This may possibly be due to adsorption of ClzTAB by OY powder. The values of d used are given in Table I. The present theoretical approach assumes a site binding of dye molecule and a linear Ising approximation with only nearest-neighbor interactions, which are inconsistent with a concept of "solubilization" by polyion/surfactant complexes. Poor agreement of the experimental results with the calculated curves in Figure 6 suggesta a limitation of this theoretical approach. This model may be applicable to a territorial binding of ionic dye molecule. This model, however, predicts quantitatively facilitation and early saturation of cooperative binding of surfactant in the ~~~
(20) Zimm, B.
~
~~
H.;Bragg, J. K.J. Chem. Phys. 1959, 31, 526.
1498 Langmuir, Vol. 6, No. 9, 1990
Hayakawa et al. and solubilized OY in polyion/surfactant complexes is given by
Ks = rBD/(Ps + PD) = P S / ( l + Ps) = exp(-AGo/RT) (15) where AGO (=p"~(P/s) - p'D(so1id)) stands for the free
1 e .
0.0
I
0.0
[C12TABIF/mM
Figure 6. Degree of binding of OY, @D, as a function of the concentration of free surfactant and curves calculated by eq 13. (A) CloTAB. (B) C12TAB. [NaCl], (a) 0, (b) 5,(c) 10, (d) 20 mM.
presence of solubilzate and qualitatively saturation in OD a t 19s < 1. These facts are inconsistent with the concept of solubilization by surfactant cluster, where the amounts of solubilizate are proportional t o host (bound surfactant) concentration. Thermodynamics of Solubilization. The ratio PD/ j3s stands for the solubilizing capacity PSwhich measures the amount of solubilized dye per bound surfactant ion in polyion/surfactant complexes. From the linear plots of OD versus Os, Ps was calculated for each NaCl concentration and is given in Table I. This solubilization is represented by OY(so1id)
OY(bu1k) s OY(P/S complex) (14)
Therefore, the equilibrium constant Ks between solid OY
energy of transfer of OY from solid phase to polyion/ surfactant complex. Table I includes the values of Ks and AGO. We obtain nearly constant Ks (0.29 f 0.02) and therefore AGO (3.1 f 0.2 kJ mol-') for both PVS/C,TAB systems. This result indicates that OY is solubilized in similar environments in PVS/C,TAB complexes, because the standard chemical potential of solid OY is common in both systems. In the previous concerted cooperative binding model, we obtained decreasing values of d with increasing NaCl concentration. Since d stands for the equilibria (10) and (11) and the standard chemical potential of OY in both PVS/C,TAB complexes is i n d e p e n d e n t of NaCl concentration, this decreasing d is ascribed to decreasing activity of OY in bulk solution with increasing NaC1. This is consistent with the decreased solubility of hydrophobic materials in solutions containing added salt. Conclusion Measurements of solubilization of OY and binding of C,TAB by PVS-C,TAB mixed systems demonstrate that (1)a certain amount of surfactant is required for solubilization, (2) the solubilized amount of OY is proportional to the quantity of bound surfactant, (3) an apparent interference of solubilization by added NaCl is ascribed to the salt effect on surfactant binding, and (4) the solubilizing capacity per bound surfactant ion is independent of added NaCl concentration and cooperativity of surfactant binding. The concerted cooperative binding model proposed in the present paper predicts these behaviors of solubilization of OY and of cooperative binding of surf a c t a n t in t h e presence of OY. Thermodynamic consideration leads to a value of the free energy of transfer of OY from the solid phase to PVS/C,TA complex of 3.1 kJ mol-'. Acknowledgment. This work was supported in part by Grant-in-Aid for General Scientific Research No. 01540382 from the Ministry of Education, Science and Culture. Registry No. OY,131-79-3;PVS, 25191-25-7;CIZTAB, 111994-4; CloTAB, 2082-84-0.
