Langmuir 1997, 13, 6951-6955
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Fluorescence Probe Study of the Interactions between Nonionic Poly(oxyethylenic) Surfactants and Poly(acrylic acid) in Aqueous Solutions Marilena Vasilescu* and Dan F. Anghel Institute of Physical Chemistry, Romanian Academy, Splaiul Independentei 202, 77208 Bucharest, Romania
Mats Almgren and Per Hansson Department of Physical Chemistry, University of Uppsala, Box 532, S-751 21 Uppsala, Sweden
Shuji Saito Nigawa-Takamaru 1-12-15, Takarazuka 665, Japan Received January 2, 1997. In Final Form: August 20, 1997X Hexaethylene (C12E6) and octaethylene (C12E8) glycol dodecyl ethers in dilute aqueous solutions, with or without poly(acrylic acid) (PAA), were studied by steady-state fluorescence and time-resolved fluorescence quenching methods, using pyrene as probe and dimethylbenzophenone as quencher. The polarity parameter (I1/I3) pointed out a critical aggregation concentration lower than the critical micelle concentration of the surfactant. On polymer addition, the fluorescence lifetime increased, indicating that the polymer wraps around the micelle-like clusters of surfactants. The aggregation numbers of clusters were smaller than those of free micelles. For 10.2 mM PAA and surfactant concentrations higher than around 1 mM, free micelles appeared in solution, but cluster formation continued until polymer saturation. At the surfactant concentration selected (1 mM for C12E6 and 0.8 mM for C12E8) and over the temperature range chosen (8-45 °C for C12E6 and 8-60 °C for C12E8), the presence of polymer does not very much affect the temperature dependence of aggregation number.
Introduction Surfactant-polymer interactions in aqueous solutions are important from basic as well as technologic viewpoints, and several reviews1-6 have appeared. Special attention has been paid to interactions of ionic surfactants with ionic polymers with opposite charge to that of the surfactant.7-12 However, the interactions between nonionic surfactant and ionic polymers have attracted less attention and are often dismissed as negligible. Saito et al.13 were the first to study the interaction of nonionic poly(oxyethylenic) (PEO) surfactants with poly(acrylic acid) (PAA). Their viscosity measurements showed13 configurational changes in the poly(acrylic acid) induced X Abstract published in Advance ACS Abstracts, December 1, 1997.
(1) Robb I. D. In AnionicSurfactants. Physical Chemistry of SurfactantActions; Lucassen-Reynders, E. H., Ed.; Marcel Dekker: New York, 1981; p 109. (2) Molyneux, P. In Water Soluble Synthetic Polymers: Properties and Behavior;, CRC Press: Boca Raton, FL, 1984; Vol. 2. (3) Goddard, E. D. Colloids. Surf. 1986, 19, 301, 255. Goddard, E. D. In Interactions of Surfactants with Polymers and Proteins; Goddard E. D.; Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993; p 219. (4) Lindman, B.; Karlstro¨m, G. In The Structure, Dynamics and Equilibrium Properties of Colloidal Systems; Bloor, D. M., Wyn-Jones, E., Eds.; Kluwer Academic Publishers: Dordrecht, 1990; p 131. (5) Brackman, C. J.; Engberts, J. Chem. Soc. Rev. 1993, 22, 85. (6) Piculell, L.; Lindman, B. Adv. Colloid Interface Sci. 1992, 41, 149. (7) Hayakawa, K.; Kwak, J. In Cationic Surfactants: Physical Chemistry; Rubingh, D., Holland, P. M., Eds; Surfactant Science series 37; Marcel Dekker: New York, 1991; Chapter 5. (8) Guillemet, F.; Piculell, L. J. Phys. Chem. 1995, 99, 9201. (9) Hansson, P.; Almgren, M. J. Phys. Chem. 1995, 99, 16694. (10) Hansson, P.; Almgren, M. J. Phys. Chem. 1995, 99, 16684. (11) Hansson, P.; Almgren, M. J. Phys. Chem. 1996, 100, 9038. (12) Anthony, O.; Zana, R. Langmuir 1996, 12, 1967. (13) Saito, S.; Taniguchi, T. Kolloid Z. Z. Polym. 1971, 248, 1039. Saito, S.; Taniguchi, T. J. Colloid Interface Sci. 1973, 44, 114. Saito, S. Colloid Polym. Sci. 1979, 257, 266.
S0743-7463(97)00002-4 CCC: $14.00
by the surfactant. Hydrophobic attraction of the surfactants to PAA and hydrogen bonding between the acidic groups of the polymer and the polar headgroups of the surfactants were suggested to give important contributions to the interaction in such systems.13-16 The later is reminiscent of the well-known attractive interactions between PEO and PAA.17-21 The surfactant concentration required to induce binding to the polymer, referred to as the critical aggegation concentration (cac), is lower than the critical micelle concentration (cmc) in polymer-free solution, the difference is a measure of the strength of polymer-surfactant interaction. It is particularly pronounced for polyelectrolytes with charge opposite to that of the surfactant.7,22 The behaviour of poly(oxyethylenic) nonionic surfactants in the presence of PAA was recently investigated by different physical methods.23 In addition to the cac, the starting point of micellar aggregation on the PAA chain and the critical concentration where free micelles appeared in the solution were determined. (14) Saito, S. In Nonionic Surfactants. Physical Chemistry; Schick, M. J., Ed.; Surfactant Science series Vol. 23; Marcel Dekker: New York, 1987; Chapter 15. (15) Saito, S. J. Am. Oil Chem. Soc. 1989, 66, 987; Rev. Roum. Chim. 1990, 35, 821. (16) Saito, S. J. Colloid Interface Sci. 1993, 158, 77. (17) Antipina, A. D.; Baranovsky, V. Yu.; Papisov, I. M., Kabanov, V. A. Vysokomolek. Soedin. 1972, A14, 941. (18) Osada, Y. J. Polym. Chem. Ed. 1979, 17, 3485. (19) Kabanov, V. A.; Papisov, I. M. Vysokomolek. Soedin. 1979, A21, 243. (20) Baranovsky, V. Yu.; Litmanovich, A. A.; Papisov, I. M.; Kabanov, V. A. Eur. Polym. J. 1981, 17, 969. (21) Petrova, T.; Rashkov, I.; Baranovsky, V. Yu.; Borisov, G. Eur. Polym. J. 1991, 27, 189. (22) Thalberg, K.; Van Stam, J.; Lindblad, C.; Almgren, M.; Lindman, B. J. Phys. Chem. 1991, 95, 8975. (23) Anghel, D. F.; Saito, S.; Iovescu, A.; Baran A. Colloids Surfaces A 1994, 90, 89.
© 1997 American Chemical Society
6952 Langmuir, Vol. 13, No. 26, 1997
In this paper we used the fluorescence probe technique to obtain information about the structure and size (aggregation number) of micelle-like clusters formed by the POE nonionic surfactants on the PAA chain. Pyrene was employed as a fluorescent probe, and one resorted to the dependence on the microenvironment of its fluorescence lifetime and of the ratio of the vibrational band intensities. Time-resolved24-29 fluorescence quenching (TRFQ) has proved to be useful for the measurement of mean aggregation number, N, of micelles. The method is applied in this contribution to the nonionic surfactantpolymeric acid systems in water; earlier it was applied successfully to ionic surfactant-ionic or -neutral polymer systems and recently to nonionic surfactant-neutral polymer mixtures.30,31 For comparison, the aggregation numbers of the C12E6 and C12E8 micelles in aqueous solutions were determined. The influence of the PEO chain length of the surfactant upon the N value and the strength of surfactant-polymer interaction was studied. The effect of temperature upon the size of aggregate was also investigated.
Vasilescu et al. Fluorescence steady-state spectra were recorded on a SPEX Fluorolog 1680 combined with a SPEX Spectroscopy Laboratory Coordinator DM1B. The measurements, except those that sought the effect of temperature variation, were performed at 25 °C. The natural lifetime τ0 )1/k0 was determined in solutions without quencher (DMBP), where a monoexponential fluorescence decay was observed
F(t) ) F0 exp(-k0t)
(1)
where F0 is the fluorescence intensity at time t ) 0, and k0 is the unquenched decay rate. The TRFQ method in microheterogeneous solutions is well described in the literature,35,36 and it is based on the equation proposed by Infelta et al.37
ln[F(t)/F(0)] ) -A2t + A3[exp(-A4t) - 1]
(2)
A2 ) k0 + kqk-n/(kq + k-)
(3)
Experimental Section
A3 ) nkq2/(kq + k-)2
(4)
Materials and Preparation of Samples. Nikko Chemical hexaethylene (C12E6) and octaethylene (C12E8) glycol n-dodecyl ether were used without further purification. Poly(acrylic acid) (PAA) with a polymerization degree ofabout 2100, from Wako Pure Chemical Industries, Ltd., Japan, was used as received. Throughout the experiments, the PAA was kept constant at 10.2 mM. The pH of this solution was 3.6. PAA concentration was selected according to the conclusions that resulted from previous studies,13,14,16,23 regarding the effect of pH upon nonionic surfactant-PAA interaction. This concentration and the resulting pH value are appropiate for the investigation of this interaction; the pH value lies within the optimal pH range (3.2-3.6), which ensures the formation of the complex. Pyrene (Py) and dimethylbenzophenone (DMBP), from Aldrich Chemical Co., were recrystallized from ethanol. Adequate volumes of probe and quencher (DMBP) in stock ethanol solutions were transferred to empty flasks and the solvent was carefully evaporated. The surfactant or surfactant-polymer solutions were then added. In order to ensure equilibrium, the solutions were measured after 48 h of storage in a cool environment. The pyrene concentration was kept low enough ( 40 °C. As was concluded also by Alami et al.44 (for another nonionic surfactant), the N variation with temperature depends on the surfactant concentration and the Tc - T difference. At small concentrations the micelles are almost spherical and, at the beginning, N increases slightly with temperature; then the increase of N is faster (41) Lo¨froth, J.; Almgren, M. In Surfactants in Solutions; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; Vol. I, p 627. (42) Corti, M.; Degiorgio, V. Phys. Rev. Lett. 1980, 45, 1046; J. Phys. Chem. 1981, 85, 1442. (43) Binana-Limbele, W.; Zana, R. J. Colloid Interface Sci. 1988, 121, 81. (44) Alami, E.; Kamenka, N.; Raharimihamina, A.; Zana, R. J. Colloid Interfacace Sci. 1993, 158, 342.
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Figure 5. Temperature dependence of the aggregation number values obtained for C12E6 (1 mM) (0) and C12E8 (0.8 mM) (O) without polymer (9) and with PAA (10.2 mM) addition (b).
at a temperature T1, which depends on concentration. At low concentrations, T1 - Tc is small and increases at higher concentrations. Similar conclusions were reached by other methods.45-47 Addition of polymer decreases the aggregation number but brings about no marked effect upon the shape of the curves. Conclusions The fluorescence lifetime of the probe increases on polymer addition owing to the polymer coiling around the micelles and screening of O2 from water. Therefore, the quenching of pyrene fluorescence is smaller. The value of the lifetime decreases at higher surfactant concentrations, because free micelles appear in the system. The variation of I1/I3 ratio suggests that on polymer addition the polarity sensed by pyrene is about the same as that in normal micelle. Addition of the polymer shifts the critical aggregation concentration toward lower values. The aggregation numbers are smaller for polymer bound aggregates than for free micelles at the same surfactant concentration and change little with surfactant concentration. The micelle size grows with temperature, more so for C12E6. Addition of polymer lowers N and slightly reduces the temperature dependence. The quenching (by DMBP) rate constant kq is lower in polymer bound than in free micelles indicating an intimate interaction between the PAA and the micellized surfactant; that is a binding of the polymer occurs at micellar headgroups surface leading to a reduction of the surfactant mobility in the clusters and thus to the cluster microviscosity modification. The lower aggregation number of the cluster and higher τ0 support also this conclusion. Acknowledgment. This contribution is the result of a cooperation between the Royal Academy of Sweden and the Romanian Academy. M.V. is grateful for financial support from the University of Uppsala and Romanian Academy (Grant 127/1995). We are indebted to Dr. J. Alsins for his assistance in our experimental endeavor and to A. Baran and A. Iovescu for preparation of initial surfactant solutions. LA9700026 (45) Nilsson, P. G.; Wennerstro¨m, H.; Lindman, B. J. Phys. Chem. 1983, 87, 1377. (46) Zulauf, M.; Weckstro¨m, K.; Hayter, J. B.; Degiorgio, V.; Corti, M. In Surfactants in Solution; Mittal, K. L., Bothorel P., Eds.; Plenum Press: New York and London, 1986; Vol. IV, p 131. (47) Magid, L. J.; Triolo, R.; Caponetti, E.; Johnson, J. S. In Surfactants in Solution; Mittal, K. L., Bothorel P., Eds.; Plenum Press: New York and London, 1986; Vol. IV, p 155.
