Hydrolysis of Surfactants Containing Ester Bonds: Modulation of

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Hydrolysis of Surfactants Containing Ester Bonds: Modulation of Reaction Kinetics and Important Aspects of Surfactant Self-Assembly Dan Lundberg*,†,‡,# and Maria Stjerndahl§,# †

Department of Chemistry, University of Coimbra, Rua Larga, 3004-535 Coimbra, Portugal Division of Physical Chemistry, Center for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden § Akzo Nobel Functional Chemicals AB, SE-444 85 Stenungsund, Sweden ‡

ABSTRACT: The effects of self-assembly on the hydrolysis kinetics of surfactants that contain ester bonds are discussed. A number of examples on how reaction rates and apparent reaction orders can be modulated by changes in the conditions, including an instance of apparent zero-order kinetics, are presented. Furthermore, it is shown that the examples on reaction kinetics display parallels and connections to important physicochemical aspects of surfactant aggregates, namely, the “reservoir function” of micelles and the fact that the headgroup region of micelles constitutes an aqueous environment largely distinct from the bulk solution. The examples presented can be used in teaching organic as well as physical chemistry. The text is written with the intention to be largely self-contained, in order to make it accessible for readers with different background experience. KEYWORDS: First-Year Undergraduate/General, Upper-Division Undergraduate, Analytical Chemistry, Physical Chemistry, Textbooks/Reference Books, Esters, Kinetics, Micelles, Surface Science

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urfactants, from surface active agents, are amphiphilic molecules that contain both hydrophilic (water-loving) and hydrophobic (water-hating) moieties. This dual nature underlies the many characteristic properties of surfactants, including their propensity for accumulation at interfaces, which is the basis of their ability to reduce the surface tension of aqueous solutions as well as the self-assembly into micelles and other supramolecular aggregates. In modern life, it is virtually impossible to avoid daily encounters with products containing surfactants, which are present in, for instance, soap, shampoo, laundry detergent, and toothpaste, as well as many pharmaceutical formulations and food products. Furthermore, surfactants fulfill crucial functions in paints and numerous industrial processes. Considering the importance of surfactants in everyday life, they generally take a rather inconspicuous position in the chemistry curriculum. Traditional surfactants have good chemical stability. For a long time, optimal performance was practically the only goal when developing surfactants, and to make the substances functional in a wide range of environments and to minimize degradation upon storage, reactive linkages were avoided in their molecular structures. However, the high chemical stability of traditional surfactants often has the consequence of high resistance to biodegradation, which, in turn, gives the compounds long life times in nature. As a result of the increased concern for the environment, efforts have been made to develop surfactants that are more benign to nature, and the negative attitude toward surfactants containing cleavable bonds has changed.1 Furthermore, it has been recognized that there are sometimes technical Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.

incentives for using surfactants containing cleavable bonds. For example, there are occasions when a surfactant is required in one part of a process but can cause serious complications, such as foaming or formation of undesirable stable emulsions, at a later stage. In such cases, the use of a surfactant that can easily be cleaved into nonsurface-active compounds at an intermediate stage may offer a way to circumvent the problems.1 Most cleavable surfactants have a functional group susceptible to chemical or enzymatic hydrolysis inserted in-between the hydrophilic and hydrophobic moieties. A type of hydrolyzable bond that has found commercial significance in surfactants is the ester group. Ester surfactants are relatively easy to produce and show high susceptibility to hydrolysis by, for instance, lipases, that is, the class of enzymes that catalyze the hydrolysis of fats.1 We have been involved in several studies on different types of ester surfactants, where hydrolysis kinetics as well as physicochemical properties of the compounds have been investigated. During this work, we have encountered a number of phenomena that constitute elegant illustrations of important concepts in both reaction kinetics and the physical chemistry of surfactants. In the following text, we share some observations that can be useful as examples in teaching situations.

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Figure 1. Molecular structure of tetra(ethylene glycol)mono-2,2-dimethyl hexanoate, A.

Figure 3. The half-life of A versus the initial surfactant concentration. The reactions were carried out at pH 13.7 Redrawn from data presented in ref 2.

first-order kinetics, r ¼
ðd½Ester=dtÞ ¼ k1 ½Ester

ð2Þ

where k1 is the pseudo-first-order rate constant, which, in turn, can be described by Figure 2. Illustration of the situation in a micellar solution, where individually dissolved surfactant molecules coexist with surfactant aggregates. Micelles are dynamic entities. The residence time of a monomer in a micelle is typically of the order of microseconds to milliseconds.

k1 ¼ k2 ½OH


’ HYDROLYSIS OF A NONIONIC ESTER SURFACTANT: ZERO-ORDER KINETICS AND THE RESERVOIR FUNCTION OF THE SURFACTANT MICELLE Polyethylene glycol (PEG) chains of varying length are commonly used as polar head groups for nonionic surfactants. This is also the case for cleavable surfactants. As a consequence of the production procedures, commercial PEG esters are typically mixtures of substances with a wide range of structures. For a study with the goal of establishing structure
property relationships for PEG ester surfactants, a series of well-defined and pure PEG ester surfactants were specially prepared.2 One of these compounds is shown in Figure 1. Among the experiments performed on A, the kinetics of basecatalyzed hydrolysis was investigated. In this study, the reaction rates were determined in solutions with initial surfactant concentrations below and above the critical micelle concentration (CMC). Below the CMC, the surfactant molecules are individually dissolved, whereas above this concentration, surfactant monomers coexist with supramolecular aggregates called micelles, which commonly consist of 50
100 surfactant molecules.3,4 Micelle formation is a highly cooperative process. This has the consequences that the onset of aggregation is typically rather abrupt at the CMC and that the size distribution of the micelles is narrow.5 The concentration of monomers remains essentially unchanged when a surfactant is present at a concentration above the CMC. The situation in a micellar solution is illustrated in Figure 2. Alkaline hydrolysis of an ester is a second-order reaction that proceeds with a rate r,

where [Ester]0 is the initial ester concentration. A convenient approach for identifying the (apparent) order of a reaction is to assess the dependence of the half-life t1/2, that is, the time required to reduce the amount of remaining reactant to half of the initial amount. Inserting [Ester] = [Ester]0/2 into eq 4 and rearrangement gives that

r ¼ k2 ½Ester½OH


ð1Þ

where k2 is the second-order rate constant, [Ester] is the concentration of ester, and [OH
] is the concentration of hydroxyl ions.6 If the reaction is carried out with a large excess of hydroxyl ions, the reaction is expected to proceed according to

ð3Þ

The integrated first-order rate law is ln ½Ester ¼
k1 t þ ln ½Ester0

t1=2 ¼ ðln 2Þ=k1

ð4Þ

ð5Þ

Thus, for a first-order reaction, the half-life is independent of the starting concentration of the reactant. The half-life of A as a function of the initial concentration at a large excess of hydroxyl ions is shown in Figure 3. With lower initial concentrations, up to about 50 mM, t1/2 is, as expected from the above discussion, invariable. At higher concentrations, however, there is a practically linear increase in t1/2 with an increasing initial concentration of A. Interestingly, the concentration where t1/2 begins to increase coincides with the CMC of A at the experimental conditions used. The finding that t1/2 increases with initial surfactant concentration suggests that the apparent reaction order is lower than first-order. For a zero-order reaction, that is, a reaction that is independent of the initial concentration of the reactant so that r = k0, where k0 is the zero-order rate constant, the half-life is t1=2 ¼ ½A0 =ð2k0 Þ

ð6Þ

In fact, it is found that for the initial surfactant concentrations higher than the CMC, the data presented in Figure 3 fit well with zero-order kinetics. The results thus imply that the effective concentration of reactant is constant above the CMC. As was discussed above, the concentration of individually dissolved surfactant molecules is practically unchanging above the CMC, which suggests that only single molecules of A are susceptible to hydrolysis. In other words, the results indicate that the micelles act as inert reservoirs of reactive surfactant monomers, which are “fed” into the surrounding solution as the reaction proceeds; when the concentration of remaining intact surfactant goes 1275

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Figure 4. The molecular structure of dodecyl betainate, B.

below the CMC, the reaction should proceed with first-order kinetics.8 How is the surfactant protected against hydrolysis when residing in micelles? There are a number of tentative mechanisms for the prevention of hydrolysis of the aggregated molecules: (i) Steric hindrance. In a micelle of an oligoethoxylene surfactant, the headgroup region constitutes a crowded corona that can potentially obstruct transport of the hydroxyl ions to the “reaction site”. (ii) Repulsion between ethylene oxide chains and hydroxyl ions. Different ions show significantly different interactions with ethoxylene chains, some are attracted to the chains whereas others are repelled.3 Hydroxyl ions are found to belong to the latter category.9 (iii) Electrostatic repulsion. Hydrolysis of A produces a fatty acid, which is deprotonated and thus negatively charged at the conditions in question. This will reinforce repulsion of the attacking hydroxyl ions from the micelles.10 The relative importance of the respective mechanisms is not established; it is likely that they act simultaneously. To summarize the discussion so far, the results from hydrolysis experiments on A illustrate how the apparent order of a reaction can be modulated by the conditions: A second-order reaction is pared down to pseudo-first-order reaction when it proceeds at a large excess of one of the reactants and further reduced to apparent zero-order reaction when A is present at concentrations above the CMC. Examples of reactions following zero-order kinetics are rare and usually occur when there is some sort of bottleneck in the mechanism. A typical example is a reaction that depends on a heterogeneous catalyst that is present in a limited amount. When the reactant is present in excess, that is, when the catalyst is saturated, the reaction rate will be practically independent of reactant concentration. The herein discussed situation is related to this example in the sense that the CMC represents the point where the solution is “saturated” with individually dissolved surfactant monomers. The protection toward hydrolysis of aggregated A and the reservoir function of the micelle have a partially incidental but elegant parallel in the consequences of micelle formation on the surface activity of a surfactant. In a surfactant solution, only molecules present as monomers contribute to the decrease in surface tension. Micelles are not surface active. They can be regarded as highly “hydrophilized” oil droplets with no attraction to the hydrophobic air interface; in a surfactant solution, they do, in direct parallel to the situation in the hydrolysis reaction discussed above, provide a stock of monomers “ready for action”. Thus, a plot of surface tension versus surfactant concentration will show a gradual decrease in surface tension with increasing concentration up to the CMC, above which the surface tension levels out and remains practically constant.

’ HYDROLYSIS OF A CATIONIC ESTER SURFACTANT: THE MICELLAR PSEUDO-PHASE, SURFACTANT AGGREGATES AS CATALYSTS, AND MICROSCOPIC ION-EXCHANGERS An example of a different type of ester surfactant, a long-chain ester of the amino acid betaine, or trimethylglycine, is shown in

Figure 5. Concentration dependence of the pseudo-first-order rate constants at pH 7.5 and 37 °C for two betaine esters:15 (Δ) dodecyl betainate (B); (O) ethyl betainate (a nonsurfactant reference substance). Redrawn from data in ref 14.

Figure 4. Owing to structural features, including the presence of an electron-withdrawing cationic charge in close proximity to the carbonyl group, betaine esters are exceptionally susceptible to alkaline hydrolysis. In fact, they are often degraded at a significant rate by the base-catalyzed mechanism even at neutral pH.11,12 Using a similar approach as for the nonionic surfactant A, the hydrolysis rates for compound B and related substances with different lengths of the hydrophobic tail have been investigated at a range of conditions.13,14 Because betaine esters are much more susceptible to alkaline hydrolysis than A, it would be difficult to monitor a reaction performed with a large excess of hydroxyl ions (the reaction proceeds too fast). Thus, the hydroxyl ion concentration is instead kept constant by the presence of a buffer. Equations 2
4 apply also in this case. The rate of hydrolysis of B shows strong dependence on the initial surfactant concentration (Figure 5). At low initial surfactant concentrations, the reaction rate of B is the same as that for a nonsurfactant reference compound, ethyl betainate. With increasing initial concentration, however, the reaction rate shows an abrupt increase, goes through a maximum, and then a gradual decrease. The sharp increase in hydrolysis rate can be linked to the onset of micellization.13,14 Thus, in contrast to the case with A, the hydrolysis of B is promoted by aggregation. The increase in hydrolysis rate on aggregation can be understood from the local increase in concentration in the micellar region not only of the surfactant molecules that make up the micelles, but also of the negatively charged hydroxyl ions, which are attracted to the highly charged micellar surface (compare to eq 1). It has been long known that the rates of chemical reactions can be affected by the presence of surfactant assemblies, and the phenomenon has been extensively studied from the 1960s on.16
18 An increase in reaction rate induced by micelles, as is observed in the hydrolysis of B, is often referred to as micellar catalysis (although it should be noted that the herein discussed case is atypical in that the constituent of the “catalyst”, that is, the surfactant, is itself one of the reactants). The protection against hydrolysis of aggregated A could correspondingly be called “micellar inhibition”, but this term is not well established. In general, micellar rate effects are, as in the discussed cases, consequences of compartmentalization of the reactants between the aqueous bulk and the micellar environment, which includes the micelle and its immediate aqueous surrounding. The micellar environment is often referred to as the micellar pseudophase. Most quantitative treatments of micellar rate effects are based on so-called pseudophase models, in which the partitioning of the 1276

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aqueous environment largely distinct from the bulk solution environment.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes Figure 6. Schematic representation of the two-site principle of the pseudophase model applied to a bimolecular reaction. X and Y are the reactants and subscripts w and m denote the aqueous bulk and the micellar pseudophases. k2w and k2m are the second-order rate constants in the respective pseudophases.

reactants between the bulk and micellar sites is considered. The general principle of the pseudophase model is illustrated in Figure 6. From Figure 6 and the finding that hydrolysis of betaine ester surfactant B is promoted by micellization, one could expect a monotonic increase in reaction rate with increasing surfactant concentration above the CMC. However, as was seen in Figure 5, this is not the case. The decrease in hydrolysis rate observed at higher surfactant concentrations can be attributed to a competition between the reacting hydroxyl ions and the chloride surfactant counterions for binding to the micellar surface. As the surfactant concentration increases, there is a parallel increase in the concentration of chloride ions, whereas the hydroxyl ion concentration remains unchanged, which leads to an increase in the ratio of chloride to hydroxyl ions. Furthermore, because the chloride ions are more polarizable and less hydrated than the reacting hydroxyl ions, they compete more effectively for the micellar surface. Consequently, the locally elevated hydroxyl ion concentration in the micellar pseudophase will decrease with increasing surfactant concentration, which in turn explains the gradual decrease in hydrolysis rate. In this context, it can be noted that the addition of salt to the solution can completely eliminate micellar catalysis. It has been found that the effects of charged micelles on the rate of reactions between an ion and a molecule that is distributed between the bulk and micellar pseudophases can be successfully treated by viewing the surface of the micelle as a selective ion-exchanger, using the so-called pseudophase ionexchange model.17,18 The discussion on micellar catalysis illustrates that not only the hydrophobic core, but also the hydrophilic surface domain of surfactant micelles in many respects constitute a distinct environment in a micellar solution and that the properties of this environment can change substantially with rather small changes in the overall conditions.

’ CONCLUSIONS Effects of micellization on the hydrolysis rates for two different types of ester-based cleavable surfactants have been discussed. The examples illustrate that (i) self-assembly of a reactant (or, in more general terms, inclusion of a reactant in a pre-existing aggregate) can have dramatic effects on reaction kinetics; depending on the conditions, a reaction can be everything from completely inhibited to substantially promoted when a reactant resides in an aggregate; (ii) a surfactant micelle largely functions as an inactive reservoir of active surfactant monomers; and (iii) the headgroup region of a surfactant micelle constitutes an

#

The original experimental work was performed by the authors at Applied Surface Chemistry, Department of Chemical and Biological Engineering, Chalmers University of Technology, SE-412 96 G€oteborg, Sweden.

’ ACKNOWLEDGMENT Olle S€oderman is acknowledged for the initial suggestion to write this article and Krister Holmberg for important contributions to the original work. D.L. receives funding from Fundac) ~ao para a Ci^encia e a Tecnologia (Portuguese Science Council) (Contract SFRH/BPD/48522/2008). ’ REFERENCES (1) Lundberg, D.; Stjerndahl, M.; Holmberg, K. Surfactants Containing Hydrolyzable Bonds. In Interfacial Processes and Molecular Aggregation of Surfactants (Advances in Polymer Science Vol. 218); Narayan, R., Ed.; Springer Verlag: Berlin-Heidelberg, 2008; p 57. (2) Stjerndahl, M.; Holmberg, K. J. Surfactants Deterg. 2003, 6, 311–318. (3) Holmberg, K.; J€onsson, B.; Kronberg, B.; Lindman, B. Surfactants and Polymers in Aqueous Solutions; Wiley: London, 2002. (4) These are representative aggregation numbers for typical spherical micelles. Certain surfactants show a strong tendency to form elongated or worm-like micelles, which can have average aggregation numbers of up to several thousands. (5) The latter is true for spherical micelles. Worm-like micelles typically show a significant polydispersity in size. (6) March, J. Advanced Organic Chemistry: Reactions, Mechanisms and Structure, 4th ed.; John Wiley & Sons: New York, 1992; p 378. (7) The reaction was performed in deuterium oxide, D2O, so to be fully correct one should state that the pD, rather than pH, was 13. However, this has no qualitative consequences for the present discussion. (8) It should here be noted that the degradation product might affect the effective CMC of the surfactant. However, since the produced fatty acid has a CMC that is higher than that of A, the change in the effective CMC should not be substantial. (9) Hey, M. J.; Jackson, D. P.; Yan, H. Polymer 2005, 46, 2567–2572. (10) One can note that at the high pH used in the hydrolysis experiments the terminal hydroxyl groups is expected to be partially deprotonated and thus also give a minor contribution to the negative micellar charge. (11) Wright, M. R. J. Chem. Soc. B 1968, 548–550. (12) Wright, M. R. J. Chem. Soc. B 1969, 707–710. (13) Thompson, R. A.; Allenmark, S. Acta Chem. Scand. 1989, 43, 690–693. (14) Lundberg, D.; Holmberg, K. J. Surfactants Deterg. 2004, 7, 239–246. (15) Also in this case the reaction was carried out in D2O; compare to note 7. (16) Menger, F. M.; Portnoy, C. E. J. Am. Chem. Soc. 1967, 89, 4698. (17) Romsted, L. S. Micellar Effects on Reaction Rates and Equilibria. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; Vol. 2, pp 1015
1068. (18) Bunton, C. A.; Nome, F.; Quina, F. H.; Romsted, L. S. Acc. Chem. Res. 1991, 24, 357–363.

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