Langmuir 1996, 12, 963-965
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Reactions in Monolayers: Oxidation of 1-Octadecanethiol Catalyzed by Octadecylamine Jamil Ahmad* Department of Chemistry, University of Bahrain, P.O. Box 32038, Isa Town, Bahrain Received July 17, 1995. In Final Form: November 6, 1995X The oxidation of monolayers of 1-octadecanethiol to dioctadecyl disulfide over a subphase of potassium hexacyanoferrate(III) was investigated. The reaction was accelerated by the presence in the monolayer of even a minute amount of octadecylamine, which acted as a catalyst. As the amount of amine in the monolayer was increased, the rate of the reaction increased. A stage was reached at which a further increase in the catalyst concentration did not cause a commensurate increase in the rate. This has been interpreted as indicating a clumping of amine molecules in the monolayers at higher concentrations. The reaction is also catalyzed by OH- contained in the subphase. Over both acidic and basic solutions, however, the effectiveness of the amine as a catalyst is reduced compared to its effectiveness over the neutral subphase.
In a number of previous papers,1-7 we have reported reactions in monomolecular films at a liquid-air interface. Such systems offer a novel opportunity to examine and control the reactivity of molecules. Since the reacting molecules are constrained to a plane and the surface pressure determines how they are oriented in space, compressing the film may affect their reactivity. If, for example, the reaction is between the monolayer and the underlying liquid phase, and if the reacting functionality of the molecule in the monolayer is remote from the hydrophilic head group, this functionality may be made inaccessible to the subphase by compressing the film to a higher surface pressure.1,2 The monolayer will then be less reactive in the compressed state in comparison to the expanded one. If, however, the reacting functionality cannot be made inaccessible by compression, a change in surface pressure does not affect the reactivity.3-5 One system of interest is the oxidation of a thiol to a disulfide at the water-air interface. Thiols and disulfides are important compounds in many biological redox systems. Lipoic acid, for example, is a cofactor in biological oxidation, while the thiol group in cysteine plays a unique role in protein structure owing to its facile oxidation to cystine thus allowing the cross-linking of protein chains. We have previously reported7 the oxidation of 1-octadecanethiol (1) to dioctadecyl disulfide (2) at the liquidair interface by potassium hexacyanoferrate(III) provided by the subphase. It was found that the reaction was slower at high surface pressures than at low pressures. This result was ascribed to the difficulty of orienting the octadecyl chain in compressed films to form the preferred skew conformation of the disulfide product.
We now report the effect on this reaction of a catalyst, octadecylamine (3), in the monolayer. The presence of as * Telephone: (+973) 683716 (home); (+973) 688343 (work). FAX: (+973) 681612. E-mail:
[email protected]. X Abstract published in Advance ACS Abstracts, February 1, 1996. (1) Ahmad, (2) Ahmad, (3) Ahmad, (4) Ahmad,
J.; J.; J.; J.;
Astin, Astin, Astin, Astin,
K. K. K. K.
B. B. B. B.
J. Am. Chem. Soc. 1986, 108, 7434. J. Am. Chem. Soc. 1988, 110, 8175. Langmuir 1988, 4, 780. Langmuir 1990, 6, 1098.
0743-7463/96/2412-0963$12.00/0
much as 2% amine 3 in the monolayer increases the rate of reaction 30-fold. While catalyzed reactions in monolayers have been studied before,4,5,8,9 the catalyst (H+ in alcohol dehydration or OH- in ester hydrolysis) was present in the subphase rather than in the monolayer. This is the first such investigation dealing with a catalyst contained in the monolayer. Experimental Section Materials. All water used was distilled through a Pyrex still and then doubly distilled by using a two-stage Heraeus Destamat quartz still. The subphase was prepared by dissolving the required mass of potassium hexacyanoferrate(III), Fluka (Puriss), in water to get a solution of the required concentration. 1-Octadecanethiol was commercially available (Riedel-de Hae¨n AG) and recrystallized from ethanol before use. Dioctadecyl disulfide was prepared by the oxidation of thiol 1 (7 mmol) by iodine affording the disulfide (1.9 mmol). Octadecylamine was commercially available (Riedel-de Hae¨n AG), 99%, and used without further purification. The hexane used was HPLC grade from Fluka AG. 1-Octadecanol, which was used in the control experiment, was commercially available (Riedel-de Hae¨n AG), 99%, and was used without further purification. Apparatus. The apparatus used in this study has been fully described. The reaction was carried out in a circular multicompartmental trough10 (manufactured by Mayer-Feintechnic, Guttengin). This is essentially a modified Langmuir trough, made from PTFE, circular in shape and with dimensions 24 cm o.d., 12 cm i.d., and 0.3 cm depth. The trough is fixed onto an aluminum base through which water can be circulated from a thermostated bath. Two PTFE barriers are connected through separate shafts to an axle and can be moved independently to vary the enclosed surface area, whose value is displayed digitally, or moved in concert from one part of the trough to another. Near one barrier was a Wilhelmy plate, made of a filter paper, connected to the core of a linear variable differential transformer, where the output signal could be calibrated in tems of the surface tension acting on the plate. The surface pressure was digitally displayed and can be read to 0.1 mN m-1. The trough could be operated at constant surface pressure whereby the area enclosed between the barriers varies automatically to maintain the surface pressure at a predetermined initial value. One of the compartments had a deep rectangular well (6 cm × 1.5 cm and 4.5 cm deep) originally meant for the (5) Ahmad, J.; Astin, K. B. Langmuir 1990, 6, 1797. (6) Ahmad, J.; Astin, K. B. Angew. Chem., Int. Ed. Engl. 1990, 29, 306. (7) Ahmad, J.; Astin, K. B. Colloids Surf. 1990, 49, 281. (8) Alexander, A. E.; Schulman, J. H. Proc. R. Soc. London, Ser. A 1937, 161, 115. (9) Alexander, A. E.; Rideal, E. K. Proc. R. Soc. London, Ser. A 1937, 163, 70. (10) Fromherz, P. Rev. Sci. Instrum. 1975, 46, 1380.
© 1996 American Chemical Society
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Langmuir, Vol. 12, No. 4, 1996
deposition of Langmuir-Blodgett films. It was found convenient to quench the reaction mixture by sweeping the monolayer material into this reservoir containing water and 0.4 mL of hexane. The hexane layer was then removed by Pasteur pipet for analysis by high-performance liquid chromatography (HPLC). Kinetics. Separate 2.4 × 10-3 M solutions of the thiol 1 and amine 3 were made, the former in hexane and the latter in an 80% hexane, 20% 2-propanol mixture. Calculated volumes of these two solutions were mixed using a microsyringe to get mixtures of required composition for spreading on the surface. The trough was filled with the potassium hexacyanoferrate(III) solution at 25 ( 0.1 °C, and the surface as swept by moving the compression barrier over the entire surface and applying suction to remove any adsorbed impurities. The barriers were then positioned to get the maximum possible area between them. The octadecanethiol/octadecylamine solution was added dropwise to the surface using a Hamilton microsyringe until a surface pressure of 6 mN m-1 was obtained. The instrument was set at the constant surface pressure mode. After reaction of the film for a given time, the reaction mixture was swept into the reservoir containing 0.4 mL of hexane and water by the coupled motion of the barriers. The hexane solution was withdrawn by a Pasteur pipet and analyzed by HPLC. HPLC analyses were performed using an LKB system with an LKB 2151 variable wavelength detector and an LKB 2220 recording integrator. To study the effect of the pH of the subphase, the experiment was repeated using the hexacyanoferrate(III) solution containing, in turn, 1 × 10-3 M NaOH (pH ) 11) and 1.0 × 10-2 M HCl (pH ) 2), with and without the amine. A separate control experiment was also carried out in which the thiol was mixed with 1-octadecanol (in a ratio of 100:1) instead of the amine to see how a nonbasic long-chain alcohol would affect the reaction. The reaction mixture could be resolved by isocratic elution with 95% ethanol/5% water on a 25 cm Supelcosil Lc-8 5 µm column. Both the thiol and disulfide were detected in the UV at 200 nm, and peak areas were measured electronically, allowing the concentration to be determined using molar response factors calculated from a standard solution of both of the substances. Repeat injections of the same sample were reproducible to within 2%. Duplicate analyses of kinetic runs were reproducible to within 5%.
Results and Discussion The reactions were carried out in compressed monolayers (π ) 6 mN m-1) at constant surface pressure. Compared to the earlier reported7 uncatalyzed reaction, the concentration of the oxidant in this study (1 × 10-5 M) was much lower. The monolayer was spread directly on the oxidant solution instead of being spread on water prior to being transferred over to the oxidant. The latter method would cause a layer of water to be dragged along with the monolayer, introducing an uncertainty as to the role of diffusion of the oxidant to the surface. At the low concentrations of oxidant employed in the study, this could be a source of considerable error. It was therefore found to be more expedient to spread the monolayer directly on the oxidant solution. Since the rate of the reaction depends on the surface pressure as reported earlier,7 it is necessary to eliminate the effects of variation in surface pressure by keeping it constant during the reaction, in order to study the effect of the catalyst. But to maintain a constant surface pressure, the surface area available to the film has to be decreased progressively as the reaction proceeds, since two molecules of the thiol are replaced by one molecule of the disulfide, which occupies a smaller area. The result of this continuous decrease in the surface area, arising from the need to maintain a constant surface pressure, is that the amount of thiol present in the reaction mixture as determined by HPLC at the end of each time interval does not reflect its concentration at the surface at that time. In order to determine the actual concentration, a series of measurements of area of the monolayer against
Ahmad
Figure 1. Variation of the area of the monolayer with time at a constant surface pressure of 6 mN m-1. Monolayer composition at start: 200:1 thiol/amine. Subphase: potassium hexacyanoferrate(III), 1.0 × 10-5 M.
Figure 2. Area of the monolayer vs percent conversion of thiol to disulfide. Monolayer composition at start: 200:1 thiol/amine. Subphase: potassium hexacyanoferrate(III), 1.0 × 10-5 M.
time was made (Figure 1). Then in several other runs, a set of values of the percent conversion of thiol to the disulfide (determined by HPLC) at various time intervals was obtained. Combining these two sets of data gives a linear graph (Figure 2) between the area and the percent conversion, which can be used to calculate the actual surface concentration if the percent conversion is known. A plot of ln(thiol concentration) against time is a straight line with slope ) -k, the pseudo-first-order rate constant. The disulfide by itself does not form a stable film, and at the high surface pressure used in our earlier study, it contributed negligibly to the surface area, making it possible to treat the surface concentration of the thiol as essentially constant and use equations for zero-order kinetics. At the surface pressures (around 6 mN m-1) used in this study, however, the disulfide appears to form a mixed film with the thiol, or at any rate it seems to desorb from the surface slowly compared to the time scale of the experiment. Thus even when 80% of the thiol has been used up in a kinetic run (as revealed by HPLC), the measured film area is not reduced by 80% (as would be the case if all the disulfide were desorbed and made no contribution to the surface area), but only by about 50%. Measurements of π vs area, in a separate experiment, on films spread from a standard mixture of synthetic disulfide and thiol revealed that disulfide does not contribute to the area in such a monolayer and, indeed, such a mixed
Oxidation of 1-Octadecanethiol
Langmuir, Vol. 12, No. 4, 1996 965 Table 1. Pseudo-First-Order Rate Constants for the Oxidation of Octadecanethiol to Disulfide with and without 1% Octadecylamine over Neutral, Acidic, and Basic Subphases at 25 °C subphase 1×
10-5
M K3Fe(CN)6
1 × 10-5 M K3Fe(CN)6 + 1 × 10-3 M NaOH 1 × 10-5 M K3Fe(CN)6 + 1 × 10-2 M HCl
Figure 3. Observed first-order rate constant vs percent amine in the monolayer at the start of the reaction at 25 °C. Subphase: potassium hexacyanoferrate(III), 1.0 × 10-5 M.
film has a limiting area/molecule of thiol which is less than what would be expected if disulfide were not mixed in with the thiol. The disulfide does not form a stable monolayer by itself, the molecule not being amphiphilic, but the fact that the presence of the disulfide actually decreases the area/molecule of the thiol seems to suggest that the former remains as bulk floating on the surface and partially dissolves the latter. We intend to carry on further investigations of this effect. Our conclusion of relevance to the present study is that the disulfide produced within the monolayer as a result of the reaction behaves differently from that spread from without, and the most likely explanation is its rate of desorption. An attempt to determine the dependence of the observed first-order rate constant, k, on the amine concentration did not yield any simple relationship. Figure 3 shows the variation of k with percent amine in the monolayer when it has just been spread. At low concentrations of the catalyst, the observed rate constant is directly proportional to the concentration of the amine as is the expected behavior in bulk reaction. However, as the concentration of the amine in the monolayer exceeds 2%, the increase in k becomes less steep. This indicates that, above this concentration of the catalyst, a clumping together of similar molecules occurs, leading to a nonhomogeneous film, consisting of separate islands of amine and the thiol. The result of this is a reduced efficiency of the catalyst. Assuming that the aforementioned explanation for the decrease in the steepness of the curve in Figure 3 is reasonable, it is possible to speculate on the amount of the amine unavailable for catalysis owing to the formation of islands. If the straight line portion of the graph is extended, then the ratio of the measured k to its extrapolated value should give an estimate of the fraction of amine available for catalysis.11 Thus in a monolayer with 10% amine, roughly only half of it is available for catalysis; the rest presumably forms the land area. An additional complication in studying the effect of the amount of amine is that, during the course of a run, the surface concentration of the amine actually increases because of the condition of constant surface pressure and the consequent decrease in area as the reaction proceeds. (11) The author thanks a reviewer for pointing out this possibility.
monolayer
k (min-1)
pure thiol 100:1 thiol/amine pure thiol 100:1 thiol/amine pure thiol 100:1 thiol/amine
3.74 × 10-3 4.55 × 10-2 1.54 × 10-1 3.01 × 10-1 2.61 × 10-3 7.70 × 10-3
The control experiment whereby the thiol was mixed with 1-octadecanol instead of the amine showed that the presence of the alcohol does not affect the rate of the reaction. It is therefore the basic nature of the amino group that is responsible for the catalytic effect. This is not surprising since the reaction is also catalyzed by OHpresent in the bulk. At pH ) 11, and without the amine, the reaction is about 40-fold faster than over a neutral solution. At pH ) 2, on the other hand, the reaction is slightly slower than over neutral solution. Moreover, the efficacy of the amine catalyst is markedly reduced over both acidic and basic solutions. The results are summarized in Table 1. This is consistent with the mechanism whereby R-SH is deprotonated to give RS-, R being the octadecyl chain, followed by nucleophilic attack to give R-S-S-R. The amine molecule on the surface facilitates the deprotonation, and so does the OH- in the bulk. Over a neutral solution, the presence of R-NH2 provides a means for the deprotonation of R-SH, increasing the rate of the reaction dramatically. Over a basic solution, however, such a means already exists because of the presence of OH-; the amine does not enhance the reaction as markedly as it does over a neutral solution. Over the acidic solution the amine is expected to exist as R-NH3+, reducing its ability to deprotonate the thiol group. This explains its reduced effectiveness as a catalyst at lower pH. A possible extension to this work would be to study the reaction with a secondary or a tertiary amine, and also to investigate the effect of the chain length of the amine on catalytic activity. Conclusion The oxidation of monolayers of 1-octadecanethiol to dioctadecyl disulfide over a subphase of potassium hexacyanoferrate(III) proceeds faster in the presence of even minute amounts of octadecylamine, which acts as a catalyst. As the amount of amine in the monolayer is increased, the rate of the reaction increases. A stage is reached at which a further increase in the catalyst concentration does not cause a commensurate increase in the rate. This has been interpreted as indicating a clumping of amine molecules in the monolayers at higher concentrations. The reaction is also catalyzed by OHcontained in the subphase. Over both acidic and basic solutions, however, the effectiveness of the amine as a catalyst is reduced compared to its effectiveness over the neutral subphase. Acknowledgment. I am grateful to Dr. K. B. Astin of the University of Bournemouth, England, for many helpful suggestions during the course of this work. LA950585O
