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Mixed Micellar Systems of Cleavable Surfactants Dan Lundberg,†,‡ Maria Stjerndahl,*,† and Krister Holmberg† Applied Surface Chemistry, Department of Chemical and Biological Engineering, Chalmers University of Technology, SE-412 96 Go¨ teborg, Sweden, and Camurus AB, Ideon Science Park, Gamma 2, So¨ lvegatan 41, SE-223 70 Lund, Sweden Received April 29, 2005. In Final Form: July 6, 2005 Previous studies have shown that the alkaline hydrolysis of cleavable ester surfactants is strongly affected by aggregation. The alkaline hydrolysis of the cationic species decyl betainate (DB) is strongly enhanced by micellization, whereas the nonionic species tetra(ethylene glycol)mono-n-octanoate (TEO) is virtually protected when residing in aggregates. In the present work, mixtures of DB and TEO were studied at concentrations above the critical micelle concentration, and the rate of hydrolysis of each surfactant in the presence of the other was assessed. The micellar interaction parameter (β) was determined from the critical micelle concentrations of various mixtures of the two surfactants. The result (β ) -2.4) indicates a moderate net attraction. The hydrolysis of the surfactants was monitored using 1H NMR. It was shown that the hydrolysis of DB exhibits the main characteristics of the pseudophase ion-exchange model and that the reaction rate decreases with an increasing molar ratio of TEO. There are indications that the hydrolysis rate parallels the expected total counterion binding to the mixed micelles. The hydrolysis of TEO was not affected by the presence of DB. However, complementary experiments showed that it is possible to accelerate or retard the hydrolysis of TEO by coaggregation with stable cationic or anionic surfactants, respectively.
Introduction In recent years, cleavable surfactants (i.e., surfactants having a labile bond deliberately included in the molecule) have attracted considerable attention.1-3 The main incentive for the development of such surfactants is environmental concerns, but practical considerations are also important. For instance, a surfactant may be needed at one stage of a process but may cause problems with foaming or the formation of unwanted, stable emulsions at later stages. The possibility of having a controlled degradation of the surfactant, induced by, for instance, a change in pH, irradiation with ultraviolet light, or ozone treatment, can in such cases be advantageous. Cleavable bonds may also be introduced into surfactants for more sophisticated reasons. For instance, a double-chain vesicleforming surfactant with a labile bond between the headgroup and one of its hydrophobic tails can be transformed into a micelle-forming amphiphile by cleavage of the labile bond.4 One of the most common labile bonds used in cleavable surfactants is the ester bond. Depending on the molecular structure in the vicinity of the ester bond, the sensitivity toward alkaline and acid hydrolysis can be modulated. In our group, we have previously studied the effects of aggregation on the rate of alkaline hydrolysis of two different classes of ester surfactantssthe cationic betaine ester surfactants5 and the nonionic tetra(ethylene glycol) * Corresponding author. E-mail:
[email protected]. Tel: +46317722983. Fax: +4631160062. † Chalmers University of Technology. ‡ Camurus AB. (1) Jaeger, D. A. Supramol. Chem. 1995, 5, 27-30. (2) Hellberg, P.-E.; Bergstro¨m, K.; Holmberg, K. J. Surfactants Deterg. 2000, 3, 81-91. (3) Stjerndahl, M.; Lundberg, D.; Holmberg, K. In Novel Surfactants, 2nd ed.; Holmberg, K., Ed.; Marcel Dekker: New York, 2003; Vol. 114, p 317-345. (4) Jaeger, D. A.; Sayed, Y. M.; Dutta, A. K. Tetrahedron Lett. 1990, 31, 449-450. (5) Lundberg, D.; Holmberg, K. J. Surfactants Deterg. 2004, 7, 239246.
Figure 1. (a) Structure of decyl betainate, chloride salt (DB). (b) Structure of tetra(ethylene glycol)mono-n-octanoate (TEO).
ester surfactants.6 It was found that the effect was completely different for the two substances. Whereas for the cationic surfactants aggregation had a strongly accelerating effect on hydrolysis, for the nonionic surfactants the molecules residing in micelles were virtually protected from degradation. The purpose of the present work was to investigate the characteristics of micellization and the effect of aggregation on alkaline hydrolysis for different mixtures of decyl betainate (DB, Figure 1a) and tetra(ethylene glycol)monon-octanoate (TEO, Figure 1b). These two surfactants have similar critical micelle concentrations and can therefore be expected to form mixed aggregates with a composition close to the overall solution composition. We were particularly interested in how the hydrolysis rate for each of the surfactants will be affected by the presence of the other. Will the tetraoxyethylene headgroup of TEO slow the attack by hydroxide ions on the labile ester bond of the betaine ester? Will the hydroxide ion attracted by the positive charge of DB accelerate the breakdown of the nonionic ester surfactant? To our knowledge, this is the first study on the degradation characteristics of mixtures of cleavable ester surfactants. The behavior of surfactant mixtures containing cleavable surfactants is not only interesting from a fundamental point of view but may also have important practical implications. For instance, the possibility of having surfactant aggregates with a time-dependent (6) Stjerndahl, M.; Holmberg, K. J. Surfactants Deterg. 2003, 6, 311318.
10.1021/la051162h CCC: $30.25 © 2005 American Chemical Society Published on Web 08/11/2005
Mixed Micellar Systems of Cleavable Surfactants
Langmuir, Vol. 21, No. 19, 2005 8659
surface charge can allow for dynamic interactions between these and various kinds of charged surfaces. It also deserves to be noted that in most practical applications of surfactants, mixtures of surface-active species are used. Experimental Procedures Materials. Chemicals. Decyl and ethyl betainates (both >98%) and tetra(ethylene glycol)mono-n-octanoate (>98%) were prepared using the procedures described in previous publications.5,6 Dodecyltrimethylammonium chloride (>99%) and sodium decyl sulfate (>99%) were obtained from Fluka. Deuterium oxide (99.8 at. % D) was purchased from Dr. Glaser AG. Sodium deuterioxide (40 wt % solution in D2O) and boric acid-d3 (98 at. % D) were purchased from Aldrich. Sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium chloride were of analytical grade. All chemicals were used without further purification. All water solutions were prepared using water obtained from a milli-Q water purification system. Buffer Solutions. The pD values of the buffer solutions were determined using a glass electrode (Metrohm Solitrode) calibrated in H2O-based buffers. It was empirically verified that this electrode obeys the linear relationship pD ) pH(meter reading) + 0.4 at pD
