Alkaline Hydrolysis of Methyl Decanoate in Surfactant-Based Systems

Article abstract of DOI:10.1021/acs.joc.8b00247

Surfactant-modified reaction systems are one approach to perform organic reactions with water as the solvent involving hydrophobic reactants. Herein, the alkaline hydrolysis of the long-chain methyl decanoate in cationic and nonionic surfactant-modified systems is reported. The physicochemical behavior of the reaction mixture and the performance of the alkaline hydrolysis were systematically investigated. In water as the solvent, the reaction is slow, but at elevated temperatures, the alkaline hydrolysis of methyl decanoate is accelerated because the reaction product sodium decanoate acts as an ionic surfactant, leading to an increased solubility of methyl decanoate in the aqueous phase. The rate can be significantly increased by the addition of surfactants as solubilizers. In nonionic TX-100 solutions, the reaction rate can be increased by a factor of about 100 for a surfactant concentration of 5 wt %. If cationic surfactants are applied, the reaction rate can be further increased due to the electrostatic interaction between the hydroxide ions in solution and the charged head groups of the cationic micelles.

Full text of DOI:10.1021/acs.joc.8b00247

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Article
Alkaline Hydrolysis of Methyl Decanoate in Surfactant based Systems
Marcel Schmidt, Johannes Deckwerth, Reinhard Schomaecker, and Michael Schwarze
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b00247 • Publication Date (Web): 15 May 2018
Downloaded from http://pubs.acs.org on May 18, 2018
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Alkaline Hydrolysis of Methyl Decanoate in
Surfactant based Systems
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Marcel Schmidt†*, Johannes Deckwerth†, Reinhard Schomäcker†, and Michael Schwarze††
† Department of Chemistry, Technische Universität Berlin, Str. des 17. Juni 124, Sekr. TCꢀ8, Dꢀ
10623, Germany
†† Department of Process Engineering, Technische Universität Berlin, Str. des 17. Juni 135, Sekr.
TKꢀ01, Dꢀ10623, Germany
*Eꢀmail: marcel.schmidt@tuꢀberlin.de
KEYWORDS: Surfactant, Hydrolysis, Triton Xꢀ100
ABSTRACT:
Surfactantꢀmodified reaction systems are one approach to perform organic reactions with water
as the solvent involving hydrophobic reactants. Herein, the alkaline hydrolysis of the longꢀchain
methyl decanoate in cationic and nonionic surfactantꢀmodified systems is reported. The
physicochemical behavior of the reaction mixture and the performance of the alkaline hydrolysis
were systematically investigated. In water as the solvent, the reaction is slow, but at elevated
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temperatures, the alkaline hydrolysis of methyl decanoate is accelerated because the reaction
product sodium decanoate acts as an ionic surfactant, leading to an increased solubility of methyl
decanoate in the aqueous phase. The rate can be significantly increased by the addition of
surfactants as solubilizers. In nonionic TXꢀ100 solutions, the reaction rate can be increased by a
factor of about 100 for a surfactant concentration of 5 wt%. If cationic surfactants are applied,
the reaction rate can be further increased by a factor of 30 due to the electrostatic interaction
between the hydroxide ions in solution and the charged headꢀgroups of the cationic micelles.
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INTRODUCTION
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Surfactantꢀbased reaction media have attracted much attention during the last years as
alternatives to conventional organic solvents. Their outstanding properties, such as water as
solvent, increased interfacial area, and advanced separation concepts, have classified them as
green solvents. Manifold stoichiometric, nobleꢀmetalꢀ as well as bioꢀcatalyzed reactions were
successfully performed in these media.1–3 Above the critical micelle concentration (CMC),
surfactant monomers spontaneously aggregate into micelles that act as nanoꢀsized carriers able to
solubilize hydrophobic reactants in water. In many stoichiometric reactions involving
surfactants, a significant increase in the reactions rate is observed for which the term micellar
catalysis was introduced.4 Mubofu and Engberts studied DielsꢀAlder reactions and in comparison
to pure water as the solvent, the rate increased by a factor of about 80 using SDS (10 mM, pH 2)
as the surfactant.5 Bahrami et al. synthesized benzimidazoles and got almost quantitative yield
using SDS whereby in pure water the yield was only 23%.6 The main reason for the higher
reaction rate is the locally higher concentration of reactants. In both examples, the hydrophobic
reactants are solubilized in the core of the micelles, where they accumulate and react. In other
examples, especially hydrolysis reactions involving OHꢀ ions, one reactant accumulates at the
surface of the micelles, if cationic surfactants are applied.7–10 One group of reactants where this
phenomena can be followed visually are triarylmethane dyes, where the solution at full
conversion becomes colorless. Several studies for hydrolysis reactions catalyzed by cationic
micelles are shown in the literature.11–13 The effects that surfactants can have on the rate of
chemical reactions are reviewed by Samiey et al.14 From the huge number of commercially
available surfactants, sodium dodecyl sulfate (SDS), linear alkyl sulfononate (LAS),
cetyltrimethylammonium bromide (CTAB), and (tertꢀOctylphenoxy) Polyethoxyethanol
(TXꢀ100) are most frequently used. In addition, new soꢀcalled designer surfactants, e.g. PTS,
PQS or TPGSꢀ750, have been synthesized by the group of Prof. Lipshutz at the University of
California, Santa Barbara. These surfactants possess higher solubilization capacity and show
good performance, e.g. in nobleꢀmetal catalyzed coupling15,16 or metathesis17,18 reactions.
Besides the solubilization of hydrophobic reactants, micellar reaction media allow for the
recycling of expensive nobleꢀmetal catalyst complexes in their active state, e.g. by micellar
enhanced ultrafiltration19,20 or phase separation.21,22 Recently it was shown that such advanced
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separation concept can be applied on a technically relevant scale in a continuously operated mini
plant, e.g. for the hydroformylation of longꢀchained olefins.23
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In this contribution, we study the hydrolysis of methyl decanoate in aqueousꢀmicellar solutions
of various cationic and nonionic surfactants. The investigations on the alkaline hydrolysis are
part of collaborative research project called InPROMPT "Integrated Chemical Processes in
Liquid Multiphase Systems". A strong focus of InPROMPT is catalytic reactions in multiphase
systems to recycle the homogeneously dissolved catalyst by a simple phase separation under
simultaneously discharge of the product.24–26 The formation of long chained acids like decanoic
acid can be carried out with a oneꢀstep sequence (hydroxycarbonylation) or with a twoꢀstep
sequence (methoxycarbonylation with consecutive hydrolysis) (see Figure 1).
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Figure 1. Reaction network for the synthesis of decanoic acid.
To evaluate the advantages and disadvantages of the two reaction pathways, investigations on the
alkaline hydrolysis are indispensable. The reaction can be catalyzed by a base, involving only
irreversible reaction steps. Generally, water is used as the solvent for the alkaline hydrolysis
process to dissolve both the ester and the base. However, methyl decanoate is poorly soluble in
water and the obtained twoꢀphase system provides only a small interfacial area, resulting in low
reaction rates. The addition of surfactants as solubilizers will lead to an increased solubility of
methyl decanoate in water. But for the micellar system, the properties of the surfactant and the
reaction rate depend on the composition of the reaction mixture and the temperature; therefore,
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we determined first the impact of the reaction conditions on the CMC and the phase behavior.
Thereafter, we investigated the hydrolysis of methyl decanoate using TXꢀ100 and alkyl
trimethylammonium
bromide
surfactants
with
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(CH3(CH2)nN(CH3)3Br, n = 9 – 15). Detailed investigations of the reaction rate as a function of
surfactant type and concentration were performed to select the minimum concentration of
surfactant for the reaction mixture and to facilitate the subsequent product isolation.
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RESULTS AND DISCUSSION
Reaction
The alkaline hydrolysis is an important reaction for the synthesis of carboxylic acids from the
corresponding esters. We selected the poorly waterꢀsoluble long chained methyl decanoate as a
model substrate which reacts with sodium hydroxide as the base to sodium decanoate (Figure 2).
Figure 2. Reaction scheme for the alkaline hydrolysis of methyl decanoate with sodium
hydroxide (NaOH) to sodium decanoate.
With aqueous NaOH solutions, methyl decanoate will form a twoꢀphase system where the
reaction will slowly take place at the interphase. To increase the reaction rate, we applied
different surfactants. The main surfactant characteristics needed to select an appropriate additive
to the reaction mixture is the CMC of the surfactant and the phase behavior of the reaction
mixture. Both were evaluated prior to the stoichiometric reaction to relate the observed kinetics
to the properties of the reaction system.
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Determination of the critical micelle concentration (CMC)
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Since TXꢀ100 is frequently investigated as a solubilizer for organic reactions in water, e.g. for
coupling reactions15,27, and available with high purity, it was chosen as the benchmark surfactant.
The CMC of the surfactant is a crucial parameter for organic reactions in water. Hence, the CMC
of the applied surfactant TXꢀ100 has been measured with a bubble pressure tensiometer at room
temperature. In addition, the CMC of sodium decanoate and mixtures of sodium decanoate and
Triton Xꢀ100 have been determined, since the alkaline hydrolysis of methyl decanoate forms
sodium decanoate. Because of its amphiphilic structure, the sodium decanoate may act as a
surfactant, leading to the formation of single micelles or mixed micelles with TXꢀ100.
Consequently, the CMC could vary significantly, depending on the conversion of methyl
decanoate hydrolysis. For TXꢀ100, the determined CMC in water is 4.1·10ꢀ4 mol/L, which is in
accordance with the literature.28–30 The pure sodium decanoate, which can be classified as an
ionic surfactant, has a CMC of 0.1 mol/L, which is significantly higher compared to TXꢀ100.
The CMC of a mixture of sodium decanoate and Triton Xꢀ100 with a mole fraction of 0.5 is
5.8·10ꢀ4 mol/L. With lower mole fraction of TXꢀ100 (x=0.25) the CMC is further increased to
6.9·10ꢀ4 mol/L. As expected, the CMC values of the mixtures are between the values of the pure
components (see Figure 3). However, the CMC of the mixtures resemble predominantly the
CMC of the nonionic surfactant TXꢀ100, which is typical for mixed micelles consisting of an
ionic and nonionic surfactant.31,32 The electrostatic repulsion of the anionic head groups of
sodium decanoate is drastically reduced with the addition of TXꢀ100, causing micellization and a
significant decrease in the CMC value already for low mole fractions of TXꢀ100. The results
show that the formed sodium decanoate molecules aggregate with TXꢀ100 molecules in aqueous
solutions. In summary, the formation of sodium decanoate leads to an increase of the CMC,
especially at low mole fractions of TXꢀ100, which has to be kept in mind for the evaluation of
the reaction performance for the alkaline hydrolysis of methyl decanoate.
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Figure 3. CMC of TXꢀ100, sodium decanoate and mixtures of TXꢀ100 and sodium decanoate at
room temperature, standard deviation 3%.
Investigation on phase behavior
Besides the impact of aqueous mixtures on the CMC, knowledge about the phase behavior,
especially the cloud point, is essential for the investigation of the alkaline hydrolysis in aqueous
TXꢀ100 solutions, particularly for the separation of the applied surfactant TXꢀ100. Since the
hydrolysis takes place under alkaline conditions, the impact of the sodium hydroxide
concentration and additionally, the impact of the formed sodium decanoate on the cloud point of
TXꢀ100 has been investigated. Figure 4 (left) shows the cloud point of aqueous TXꢀ100 solutions
(5 wt%) as a function of the concentration of sodium hydroxide in a range of 0 to 1 mol/L. The
cloud point of TXꢀ100 (5 wt%) in an aqueous solution without sodium hydroxide was
determined to be 67.2°C and is in good agreement with the literature.33 With increasing
concentration of sodium hydroxide, the cloud point decreases linearly to 36.8°C at 1 mol/L of
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sodium hydroxide. As expected, sodium hydroxide act as an electrolyte and leads to a salting out
process decreasing the cloud point with increasing electrolyte concentration. The sodium
hydroxide molecules are cosmotropic ions, binding strongly to water and thus belonging to the
"waterꢀstructureꢀmaking" ions. Hence, the strong interaction of the electrolyte molecules with
water molecules leads to a dehydration of the polyethylene glycol chains of the applied
surfactant, resulting in a lower cloud point, which is known as salting out effect.34 Based on
these findings the sodium hydroxide concentration for all further experiments was fixed to
0.2 mol/L.
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Next, the effect of sodium decanoate on the cloud point has been investigated (Figure 4, right).
Interestingly, sodium decanoate strongly influences the cloud point of the mixtures. The cloud
point increases drastically from 58.1°C to 93.6°C with only 0.03 mol/L of sodium decanoate,
showing a linear tendency. The sodium decanoate act as an anionic cosurfactant forming mixed
micelles with TXꢀ100. The incorporation of the anionic molecules into the micelles, formed by
TXꢀ100, leads to a negative surface charge of the micelles. Thus, the incorporation of charged
cosurfactants forces a repulsion between the micelles, resulting in an increased water solubility,
which makes the micelles more hydrophilic.35 Consequently, the cloud point of the mixtures is
drastically increased. We have to mention that a full conversion of methyl decanoate corresponds
to a sodium decanoate concentration of 0.2 mol/L. In Figure 4 (right), the concentration of
sodium decanoate was varied between 0 and 0.03 mol/L, which corresponds to a conversion of
methyl decanoate in the alkaline hydrolysis between 0 and 15%. As the cloud point is increased
to very high temperatures with increasing conversion of methyl decanoate, a oneꢀphase solution
is expected after the reaction.
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Figure 4. Effect of sodium hydroxide (left) and sodium decanoate (right) on the cloud point of
an aqueous TXꢀ100 solution. Experimental conditions: V=20 mL, cTX100=5 wt%, right:
cNaOH=0.2 mol/L.
Alkaline hydrolysis of methyl decanoate in water
Initially, the alkaline hydrolysis of methyl decanoate in an aqueous sodium hydroxide solution
has been studied at different temperatures, which is shown in Figure 5. For 40 °C and 60 °C the
conversion after 20 h reaction time is 3% and 5%, respectively. As expected, the low solubility
of methyl decanoate in water causes the low reaction rates, resulting in low conversions.
Surprisingly, the conversion obtained after 20 h reaction time at a reaction temperature of 80°C
is 82%, although no further additives were used. Furthermore, the slope of the conversion plot
for the reaction temperature of 80°C increases with proceeding reaction time, which is an
untypically trend for a second order reaction. Apparently, the conversion of methyl decanoate to
sodium decanoate accelerates the reaction. The determined CMC of sodium decanoate at room
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temperature is 0.1 mol/L, which is equal to 50% conversion of methyl decanoate. Hence, the
formation of micelles, initiated by the formation of sodium decanoate during the reaction,
increases the solubility of methyl decanoate, resulting in an enhancement of the reaction rate
with proceeding reaction time. Obviously, the slope of the conversion plot is slightly increasing
at a conversion of 40% (see Figure 5), which confirms this assumption, since the CMC of
sodium decanoate is reached. We have to keep in mind that the CMC of sodium decanoate is
slightly temperature dependent, which could lead to a small mismatch.
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Figure 5. Effect of the temperature on the conversion plot of the alkaline hydrolysis of methyl
decanoate in water. Experimental conditions: V=20 mL, cmethyl decanoate=0.2 mol/L,
cNaOH=0.2 mol/L.
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Alkaline hydrolysis of methyl decanoate in aqueous TX-100 solution
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Based on the findings of the critical micelle concentration and the phase behavior, the alkaline
hydrolysis of methyl decanoate was performed at three different temperatures. To perform the
reaction below and above the cloud point of the reaction mixture, we decided to operate at 40°C,
60°C and 80°C. The twofold amount of the determined CMC for TXꢀ100, which was measured
at room temperature, has been used as TXꢀ100 concentration for the temperature variation. As
seen in Figure 6 the addition of TXꢀ100 is crucial for the reaction performance of the alkaline
hydrolysis of methyl decanoate comparing both reaction systems, with and without phase
transfer agent (Figure 5). Already the addition of small amounts of TXꢀ100 leads to a significant
increase in the reaction rate. Comparing the reaction at 80°C with and without added TXꢀ100,
the reaction rate, which is proportional to the slope of the conversion plot, is increased by a
factor of 30. Moreover, the conversion reaches 100% after 8 h reaction time. If the temperature is
lowered to 60 or 40°C, the reaction is still much faster than in the absence of the surfactant. Even
for the reaction performed at 40°C noticeable conversions of 40% are obtained after 20 h
reaction time. Since the added TXꢀ100 forms micelles above the CMC, methyl decanoate can be
solubilized in the aqueous phase and the reaction rate is significantly enhanced. If hydrolysis
reactions are performed under conventional twoꢀphase conditions, the rate is often influenced by
film diffusion, resulting in apparent low activation energies. The application of surfactants with
concentrations above the CMC will not only increase the solubility of the hydrophobic reactants
in the aqueous phase; moreover, it will provide a high interfacial area for mass transport. In this
case, the temperature dependency of the reaction using the Arrhenius law should result in much
higher apparent activation energy. The activation energy of the alkaline hydrolysis of methyl
decanoate was calculated from Figure 6 to be 56.4 KJ/mol, which indicates a kinetic control of
the reaction in aqueous TXꢀ100 solution that results in a stronger acceleration of the reaction at
elevated temperature.
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Figure 6. Effect of the temperature on the conversion plot of the alkaline hydrolysis of methyl
decanoate in aqueous TXꢀ100 solution. Experimental conditions: V=20 mL,
cmethyl decanoate=0.2 mol/L, cNaOH=0.2 mol/L, cTXꢀ100=2xCMC.
Additionally, the impact of the TXꢀ100 concentration has been studied with regard to the
reaction performance, especially the initial rate of reaction. As it can be seen in Figure 7 the
amount of surfactant plays a crucial role for the reaction rate of the alkaline hydrolysis of methyl
decanoate in aqueous systems. The reaction temperature was kept constant at 80°C. As expected,
the reaction rate increases substantially with increasing concentration of the phase transfer agent,
for instance an increase of the surfactant concentration from 4.1 mmol/L, which is equal to
10xCMC, to 133.3 mmol/L (5 wt%) leads to an enhancement of the reaction rate by a factor of 5.
On the one hand, the higher the concentration of TXꢀ100, the higher is the concentration of
methyl decanoate in the aqueous phase since more micelles can be formed. As a result the
reaction rate is enhanced. On the other hand the amount of surfactant increases the interface
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between water and oil, which could result in higher reaction rates. However, the increase of the
reaction rate is not proportional to the increase of the surfactant concentration. Obviously, the
reaction rate converge a maximum rate with increasing surfactant concentration since the rate is
limited by the concentration of methyl decanoate in the aqueous phase. At high TXꢀ100
concentrations all of the substrate can be solubilized in the micelles. A further increase of the
surfactant concentration could lead to a decrease of the reaction rate since the methyl decanoate
is diluted with higher amount of surfactant. This trend was also shown by other researchers, e.g.
for the fading of dyes.36,37
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Moreover, no correlation can be found between the reaction rate and the phase behavior of the
systems. Starting above the cloud point (80°C), indicated by a white and turbid reaction mixture
after injecting of the substrate, the mixture turns into a clear solution at the end of the reaction,
indicating a micellar solution below the cloud point. As mentioned, the formed sodium
decanoate has a strong impact on the cloud point of the mixture, leading to an increase of the
cloud point with increasing conversion of methyl decanoate. However, no change in the
conversion plot can be observed, concluding no correlation between reaction performance and
phase behavior. This is another indication that no mass transfer limitation is involved in the
reaction kinetics.
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Figure 7. Effect of the TXꢀ100 concentration on the initial rate of the alkaline hydrolysis of
methyl decanoate. Experimental conditions: T=80°C, V=20 mL, cmethyl decanoate=0.2 mol/L,
cNaOH=0.2 mol/L, initial rate determined at a conversion of 10%.
Interestingly, the electrical conductivity of the reaction mixture drops drastically using high
concentrations of Triton Xꢀ100 (5 wt%), resulting in an unexpected minimum at the beginning of
the reaction. Figure 8 shows the conductivity and the calculated conversion over time for a
Triton Xꢀ100 concentration of 5 wt%. Starting above the cloud point (Figure 9, A), the reaction
mixture passes through different phase states with increasing conversion, herein expressed by the
proceeding reaction time. Firstly, a blue transparent reaction mixture can be observed after 12
minutes reaction time (Figure 9, B), indicating a phase transition to a lamellar phase. Due to the
bilayer structure of the lamellar phase, the movement of the ions is restricted, which explains the
minimum in the conductivity plot.38 The distance of the surfactant layers is in the range of the
wavelength of visible light, which describes the blue color of the solution. After that, the reaction
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mixture turns back into a turbid biphasic reaction mixture, leading to an increase of the
conductivity (Figure 9, C). At the end, with increasing conversion the solutions are transparent,
since the cloud point increases strongly due to the formation of sodium decanoate (Figure 9, D).
Due to these overlapping effects of the concentration of ionic reactants and phase behavior of the
system, the conductivity measurement has to be corrected to monitor the progress of the reaction.
Hence, the minimum of the electrical conductivity, due to the formation of a lamellar phase, was
not considered to calculate the conversion of methyl decanoate.
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Figure 8. Conversion and conductivity plot of the alkaline hydrolysis of methyldecanoate.
Experimental
conditions:
T=80°C,
V=20 mL,
cTX100=5 wt%
cNaOH=0.2 mol/L,
cmethyl decanoate=0.2 mol/L.
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Figure 9. Reaction mixture with proceeding reaction time, A=0 min, B=12 min, C=15 min,
D=180 min.
Effect of the type of surfactant
Besides the widely used TXꢀ100, several nonionic and cationic surfactants for the alkaline
hydrolysis of methyl decanoate have been tested (see Figure 10). To increase the hydrophobicity
of the surfactant, TXꢀ114 has been chosen which has a lower amount of ethoxy units as the polar
head group, but the same hydrophobic alkyl chain as TXꢀ100. Furthermore, the nonionic
surfactants Marlipal 24/70 and Marlosol TA3070 have been tested, which have a linear alkyl
chain without any aromatic groups as hydrophobic part. In addition to the nonionic surfactants,
alkyltrimethylammonium bromides with an alkyl chain length from 10 to 16 have been
investigated as cationic surfactants to evaluate the impact of the hydrophobicity of the surfactant
on the reaction performance. The cationic surfactants have been applied as they frequently show
the potential to increase the reaction rate of hydrolysis reactions.39–41
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Figure 10. Chemical structures of the applied surfactants.
As seen in Figure 11 (left), the type of nonionic surfactant has only a minor impact on the
reaction performance since the conversion plots are nearly identical. Interestingly, the same
reaction rate is observed using TXꢀ100 and Marlipal 24/70 as phase transfer agents, which have
the same cloud point around 60°C and thus, almost an identical hydrophobicity. In contrast, the
more hydrophobic surfactants TXꢀ114 and Marlosol TA3070, indicated by their lower cloud
points around room temperature, show a slightly lower reaction rate compared to TXꢀ100 and
Marlipal 24/70. Apparently, the hydrophobicity of the surfactant has a low impact on the reaction
performance. Since the more hydrophobic surfactants TXꢀ114 and Marlosol TA3070 exhibit a
lower cloud point, the solubility of methyl decanoate is lower, resulting in a lower reaction rate
compared to TXꢀ100 and Marlipal 24/70. However, the more or less equal reaction profiles
clarify that the surfactants are utilized as phase transfer agents as the substrate is nonꢀcharged
and nonꢀaromatic further interactions between the substrate and the surfactant are not expected.
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Figure 11. Variation of the type of surfactant. Experimental conditions: T=80°C, V=20 mL,
cNaOH=0.2 mol/L, cmethyl decanoate=0.2 mol/L, left: nonionic surfactants (5wt%), right: cationic
surfactants (10xCMC).
Additionally, several cationic surfactants have been investigated (see Figure 11, right).
Compared to the nonionic surfactants, much less surfactant is needed to obtain similar reaction
rates. Since the head groups of the alkyltrimethylammonium bromides carry a positive charge, an
electrostatic attraction of the hydroxide ions is obtained, increasing the local concentration of
hydroxide ions at the waterꢀoil interface and enhancing the reaction rate.
Since the CMC of cationic surfactants is much higher compared to the applied nonionic
surfactants, the effect of the CMC towards the reaction performance has to be investigated.
Therefore, the amount of surfactant was varied in two series of experiments, in which firstly the
tenfold CMC and secondly a constant cꢀCMC value of 10 mM was used. The results are given in
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Table 1 expressed as the initial rates which were calculated from the conversion plots. For a
better comparison, also the normalized rates (based on CTAB with the lowest observed reaction
rate) are given. Additionally, the CMC values were determined by bubble pressure tensiometer
and conductivity measurements. As expected, with increasing chain length of the alkyl chain
from 10 to 16, the CMC decreases significantly from 60 mmol/L to 1 mmol/L (see Table 1),
indicating a higher hydrophobicity of the cationic surfactant. Using the tenfold CMC, the initial
rate decreases from 3839 mmol/(L·h) with DeTAB to 322 mmol/(L·h) with CTAB as surfactant.
Since the CMC increases from CTAB to DeTAB, the amount of surfactant in the reaction
mixture increases drastically using the tenfold CMC, resulting in higher solubilization of methyl
decanoate and thus, in a much higher initial reaction rate. Hence, experiments were carried out
using the same concentration of the surfactant but corrected with the determined CMC (cꢀ
CMC=10 mM). As a result, the molar concentration for micellization is constant which allows
for a fairer comparison of the used cationic surfactants. Interestingly, the same trend can be
observed whereby the initial rate decreases from 1023 mmol/(L·h) for DeTAB to
291 mmol/(L·h) for CTAB. From the normalized reaction rates it is obvious that for the same
micellar concentration, the rates are closer together, showing that the CMC can’t be neglected in
the screening of ionic surfactants. After correction of the surfactant concentration by its CMC,
the more hydrophilic cationic surfactant still shows a higher reaction rate. Two more effects have
to be considered to discuss the observed reactions rates: a) the solubilization of methyl decanoate
and b) the local concentration of hydroxide ions at the waterꢀoil interface. Considering the
aggregation number for the investigated surfactants, which decreases from CTAB to DeTAB by
a factor of about two,42,43 the number of micelles for DeTAB as the phase transfer agent is
higher. Hence, the concentration of methyl decanoate is higher in the water phase which explains
the higher initial reaction rate. Nevertheless, to understand the role of the surfactant in detail,
investigations of the aggregation behavior, solubilization capacity and phase behavior is
essential.
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Table 1. CMC values and initial reaction rates for the cationic surfactants.a
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d
d
Initial rate r0
[mmol/(L·h)]
10xCMC
Initial rate r0
CMCb
Normalized
rate
Normalized
rate
CMCc
Surfactant
[mmol/(L·h)]
[mmol/L]
[mmol/L]
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cꢀCMC=10 mM
DeTAB
DTAB
TTAB
CTAB
ꢀ
61.1
14.4
5.8
1.4
ꢀ
62.1
14.5
4.6
1.0
ꢀ
3839
2813
703
322
5
11.9
8.7
1023
632
461
291
ꢀ
3.5
2.2
1.6
1
2.2
1
0,015
ꢀ
a CMC was measured at 25°C.
b Determined by bubble pressure tensiometer, standard deviation 3%.
c Determined by conductivity measurements, standard deviation 3%.
d
Determined at a conversion of 10% from the conversion plot.
Experimental conditions: T=80°C, V=20 mL, cNaOH=0.2 mol/L,
cmethyl decanoate=0.2 mol/L.
CONCLUSION
In summary, the presented results show the impact of the surfactant type and concentration on
the reaction performance of the alkaline hydrolysis of methyl decanoate in surfactant modified
systems. The surfactant is needed to increase the waterꢀoil interface, resulting in higher reaction
rates compared to system without additive. The physical properties of the mixtures like phase
behavior and CMC are essential to understand the role of the surfactant. Interestingly, the
formation of sodium decanoate has a strong influence on the phase behavior and CMC due to its
surface active properties. Hence, an autocatalytic effect of the product can be observed.
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EXPERIMENTAL SECTION
Chemicals
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The substrate methyl decanoate (99%), sodium decanoate (98%) for the calibration to evaluate
the reaction progress and the surfactants (tertꢀOctylphenoxy) Polyethoxyethanol degree of
ethoxylation (EO)=9ꢀ10 (Triton Xꢀ100), (tertꢀOctylphenoxy) Polyethoxyethanol EO=7ꢀ8 (Triton
Xꢀ114), dodecyltrimethylammonium bromide (DTAB, 98%), and tetradecyltrimethylammonium
bromide (TTAB, 99%) were purchased from SigmaꢀAldrich. Sodium hydroxide (NaOH) as base
with a purity of 99% and the cationic surfactant hexadecyltrimethylammonium bromide (CTAB)
with a purity of 99% were acquired from Roth. ABCR has delivered the cationic surfactant
decyltrimethylammonium bromide (DeTAB, 99%). The technical grade nonionic surfactants
Marlipal 24/70 and Marlosol TA3070 were purchased from Sasol. Deionized water was used in
all experiments. All chemicals were used without further purification.
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CMC determination
The electrical conductivity method and the bubble pressure tensiometer were used to determine
the critical micelle concentration (CMC) of the alkyltrimethylammounium bromides. Aqueous
solutions with different concentrations of the corresponding surfactants were prepared and
measured with both methods. For the electrical conductivity method the conductivity was plotted
versus the concentration of the surfactant and the CMC was obtained from the intersection of the
two resulting linear slopes. In addition, the surface tension of the solution was measured using a
bubble pressure tensiometer, depending on the surface age of the produced bubbles. At a surface
age of 14000 ms, when the surface tension remains constant, the surface tension value was taken
and plotted versus the concentration of the surfactant to determine the CMC. Also, the CMC of
Triton Xꢀ100, sodium decanoate and 1:1 mixture of Triton Xꢀ100 and sodium decanoate was
determined with the bubble tensiometer. All reported values were measured at 25°C and are
indicated as moles of surfactant per volume.
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Investigation on the cloud point
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The influence of sodium hydroxide and the surface active product sodium decanoate on the cloud
point of the aqueous TXꢀ100 solution was investigated in a temperature range from 25°C to
95°C. The concentration of TXꢀ100 was fixed at 5 wt% and the concentration of sodium
hydroxide was varied in the range from 0.1 to 1.0 mol/L. In addition, the cloud points of the
nonionic surfactants Triton Xꢀ114, Marlipal 24/70 and Marlosal TA3070 were examined. To
determine the impact of sodium decanoate on the cloud point, the concentration of sodium
hydroxide and TXꢀ100 was fixed at 0.2 mol/L and 5 wt%, respectively and the amount of sodium
decanoate was varied between 0 and 0.04 mol/L. The mixtures, prepared in graduated cylinders,
were added to a water bath and the temperature was raised in one degree steps waiting one
minute to reach the equilibrium for each temperature step. The resulting phase behavior was
observed, mainly indicated by the turbidity of the solution.
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General procedure of the alkaline hydrolysis
The main part of the setup for the alkaline hydrolysis is a 40 mL doubleꢀwall reactor to adjust the
temperature via a thermostat. The substrates can be injected with a twoꢀway valve at reaction
temperature, whereby the mixture is mechanically stirred. To follow the reaction progress the
conductivity is measured in-situ using a conductivity electrode from the company WTW. The
thermostat and the conductivity probe are connected to a computer to monitor the reaction
progress. To perform the reaction, a stock solution containing the surfactant and sodium
hydroxide (4 mmol, 1 eq.) was filled into the reactor, stirred mechanically and heated up to the
desired reaction temperature. Immediately after reaching the reaction temperature, the substrate
methyl decanoate (4 mmol, 1 eq.) was injected via a syringe into the reactor. The bulk volume
was 20 mL for each experiment. The reaction was stopped when the conductivity remains
constant, or latest after 24 hours.
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Evaluation of reaction progress
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Since the conversion depends linear on the conductivity of the mixture, the conductivity was
measured to determine the conversion of methyl decanoate to sodium decanoate. For that,
sodium hydroxide, which mainly contributes to the conductivity of the mixture, and the product
sodium decanoate were mixed, adapted to the concentrations from the previous section,
simulating different conversions between 0% and 100%. After waiting four minutes for
temperature equilibrium, the corresponding conductivity was measured for 40°C, 60°C and 80°C
and plotted versus the associated conversion. A twoꢀpoint calibration was done for the cationic
surfactants, mixing the tenfold critical micelle concentration (CMC) of the corresponding
surfactant, sodium hydroxide, and sodium decanoate and measuring the conductivity at 80°C.
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AUTHOR INFORMATION
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Corresponding Authors
*Eꢀmail: marcel.schmidt@tuꢀberlin.de
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Author Contributions
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The manuscript was written through contributions of all authors.
ACKNOWLEDGMENT
This work is part of the Collaborative Research Center "Integrated Chemical Processes in Liquid
Multiphase Systems" (subproject A2) coordinated by the Technische Universität Berlin.
Financial support by the German Research Foundation (Deutsche Forschungsgemeinschaft,
DFG) is gratefully acknowledged (TRR 63).
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