Ramping of pH Across the Water-Pool of a Reverse Micelle - Langmuir

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Ramping of pH across the water-pool of a reverse micelle Puspal Mukherjee, Shradhey Gupta, Shahnawaz Rafiq, Rajeev Yadav, Vipin Kumar Jain, Jayraj Raval, and Pratik Sen Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b04429 • Publication Date (Web): 31 Jan 2016 Downloaded from http://pubs.acs.org on February 1, 2016

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Ramping of pH across the water-pool of a reverse micelle Puspal Mukherjee, Shradhey Gupta, Shahnawaz Rafiq§, Rajeev Yadav†, Vipin Kumar Jain, Jayraj Raval and Pratik Sen* Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208 016, India

ABSTRACT In this work we have addressed the problem of “acidity” of the water-pool of a reverse micelle (RM) through the well-known inversion of sucrose reaction as a tool of investigation. This reaction has been performed inside positively and negatively charged RM and the rates are compared with that in bulk water. We propose that the buffer like action in a waterpool is much stronger than expected earlier. Rate of sucrose hydrolysis slowed down in negatively charged AOT reverse micelle while speed up for positively charged CTAB reverse micelle. However temperature dependent measurements showed that the activation energy remained same for all the cases. It has been concluded that a proton gradient exist inside the water-pool of the reverse micelle and it determines the buffer like action of the water-pool which persist till about 2N of HCl in AOT RM of w0 = 10.5.

KEYWORDS Nano water-pool, Acidity, Buffer like action, Sucrose hydrolysis, Proton gradient

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1. Introduction Majority of the chemical and biological reactions take place in aqueous medium and pH of the medium plays a crucial role in their optimization. The idea of pH has been well established over a century and the current IUPAC definition is pH = paH+ = -log aH+ where aH+ is the proton activity.1 The conditions applied with this definition are i) pH range of 2-12 and ii) ionic strength less than 0.1 molL-1. Hence the whole definition is only valid in bulk medium. Naturally the concept of pH is not applicable in a straight forward way when the amount of water becomes very small, e.g. water in a confined environment. A commonly used confined environment is the water-pool of a reverse micelle (RM), which has brought a lot of attentions over a period of time due to its elegancy as biological model.2-4 RM systems are well studied in literature and such a system can form different size of water-pool depending on water loading.5 It has already been demonstrated

that

dioctyl

sodium

sulphosuccinate

(aerosol-OT

or

AOT)

and

cetyltrimethylammonium bromide (CTAB) readily forms well defined RM and the size of the water-pool can be varied easily. Menger et al.6,7 coined the term “water-pool” for water confined in an inverted miceller condition. This water-pool associated with the polar head groups of the surfactants has been characterized as the “bound water” and the maximum amount of bound water in a reverse micelle is given by water-surfactant molar ratio (w0) as [water]

w = [ ]

(1)

It has been proposed that as the size of this water-pool increases, the property of confined water approaches towards the bulk water.8,9 However, several research groups have pointed out that even in RM with w0 = 40, the number of water molecules are too small to be considered as bulk.10-12 So the IUPAC definition of pH is not valid in such cases. Though the explanation of the change in the properties of confined water have been an argument ever since, researchers


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have used the RM as a micro-reactor and performed different reactions.13-15 RMs can act as a micro-reactor and yield different results because of the change in the properties like polarity, viscosity, etc.16-22 Let us now focus on one particular property i.e. “acidity” of the water-pool. As we have discarded the conventional idea of pH inside RM, the words like “acidity” or “basicity” seem more appropriate in such a confined environment. There are several works that has been done in this context. One of the major techniques used is the fluorescence response of organic probe molecules. Probe molecules which show excited state proton transfer are very much useful for such studies. There have been several reports concerning excited state proton transfer studies in the past where different kinds of RMs i.e. anionic, neutral and cationic have been used.23-24 These studies suggested that the proton donation or acceptance ability of confined water changes with the different regions of space inside the RM.1,25-26 Consequently, the water-pool has been classified into two regions. One is the core of the water-pool and termed as “free water” and another is the interfacial region or the “bound water”. Studies have been reported for both the regions with site specific probe molecules. Cohen et al. used 2-naphthol-6,8-disulphonate sodium salt, which resides in the free water part of aqueous AOT RM,27 and showed that the dielectric relaxation of the free water is significantly slower than bulk water in small sized waterpools. On the other hand, Kwon and Jang28 have studied the same RM using 7-azaindole as a probe. This probe tends to locate at the interfacial region of the AOT RM. They have also observed that the rate of proton transfer is slower in small sized RM, which increases with increase in size of the water-pool. According to them the slower rate of proton transfer is because of the increased free energy of solvation of the water shell. These studies used steady state and time resolved fluorescence techniques where the probe concentration was kept very low. So the


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number of ions added to the system was very few to actually observe a movement of ions inside the RM. Nevertheless, one can expect a different concentration of ions in different regions of the water-pool. This essentially means that a concentration gradient of a particular ion exists inside a RM and this gradient depends on the charge and size of the RM. The extent of this phenomenon has not been reported in the literature. Although it was believed that the probe molecule has no effect on the structure of the RM as the concentration is generally very small, however, the change in its overall charge before/after the proton transfer may give rise to ambiguity in the measurement of the acidity of the water-pool. To solve such ambiguity, NMR spectroscopy was used to estimate the acidity of the water-pool.29-30 Haliday et al. have studied the T1 and T2 relaxation time of water by directly probing the water-pool with variation of pH.31 They observed that the T1 relaxation time does not depend on pH but T2 does. For positive and neutral surfactant the T2 relaxation time remains constant for pH greater than 6 but decreases steadily from pH 6 to 3 and then decreases very rapidly. Although they could not determine pH of the water-pool precisely but they proposed a unique way to monitor it in a confined environment like RM. Another NMR probe, highly charged decavanadate polymer, was used by Baruah et al. for the pH dependent study in the AOT RM.32 This probe resides in the free part of the water-pool and by comparing the

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V NMR spectra of AOT RM with bulk water they obtained information

about the pH inside the RM. The bulk pH range used in the study was 3.1 to 8.0. According to them the protons migrate towards the interfacial region and exchanged with the Na+ ions from the surfactant headgroups. Thus the proton concentration in the core region remains small. Besides they have also concluded that even for large RMs the properties of “free water” differs from that of bulk water. Thus the buffer like action of the water-pool was established as was previously proposed by Hasegawa.33 Hasegawa used pH sensing probe pyranine and adjusted the


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pH of the bulk water used for preparing the RM using NaOH/HCl or buffer. The effect of change of pH was not prominent on the spectra of the probe placed inside RM. Thus he concluded about the buffer action of the water-pool. On the contrary, there are reports of increasing acidic activity of dilute acids such as nitric acid and kinetics of enzyme hydrolysis reactions inside reverse micelle which indicate that water-pool of AOT RM has no buffer like action.34-36 Nevertheless, the question that can be raised is, how much changes in the proton concentration can a RM withstand and what is the extent of buffer like action in positively and negatively charged RMs? The idea is to use very strong acidic or basic condition to answer these questions. Instead of using the optical pH sensor, because of its inherent drawbacks as mentioned earlier, we turned our focus to an age-old reaction in chemistry, i.e. the “acid catalyzed hydrolysis of sucrose” or the “inversion of sucrose”. This reaction has been studied thoroughly and is used as a standard reaction for budding chemists all over the world.37 We think the present work can answer some, if not all, of the questions stated before. The hydrolysis of sucrose has been studied under many conditions and using many catalysts.38-41 Acid is one of the commonly used catalyst for this reaction. The rate of this reaction depends on concentration of i) sucrose ii) water and iii) catalyst i.e. HCl in our case. In a situation, where the concentration of water and H+ are too high, they practically remain constant and the reaction transforms into a pseudo first order reaction.42,43 − −

[] []

= ′[][ = []

! "][

#]

(2) (3)

The rate of the reaction can be monitored by the rotation of a polarized light. The equation can be formulated as $% & $' $( & $'

= = e&* (

(4)


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where + , + and +, are the optical rotation at time zero, t and infinity respectively. This reaction also depends on temperature. With the increase in temperature, the rate of this reaction increases and vice versa. The relationship between rate constant and temperature is given by Arrhenius equation as = -e&.//12

(5)

where, k, A, Ea, R and T are the rate constant, pre-exponential factor, activation energy, universal gas constant and temperature in Kelvin, respectively. In the present work we have taken two different surfactant molecules namely dioctyl sodium sulphosuccinate (docusate sodium or aerosol-OT or AOT) and cetyltrimethylammonium bromide (hexadecyltrimethylammonium bromide or CTAB) to form the RM with negatively and positively charged inner interface respectively and studied the hydrolysis of sucrose there in. The idea is to measure the change in the rate of the reaction with change in RM condition and to compare it with the rate in bulk water. If indeed the water-pool has some buffer like action then the rate of the reaction should not match with the bulk water and this can be used as an indicator of the phenomenon. Also the role of differentially charged interface on the buffer like action could be elucidated.

2. Experimental Materials: Analytical grade sucrose, hydrochloric acid (35%), oxalic acid and n-heptane were purchased from Merck, India. AOT and CTAB were purchased from Sigma-Aldrich and dried under vacuum overnight before using. Karl-Fisher titration was performed to estimate the actual water content of the AOT and CTAB used and it was found that the water content is 0.4% w/w in case of AOT and 0.45% w/w in case of CTAB. Sodium hydroxide pellet was of analytical grade


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and purchased from Rankem, India and analytical grade 1-hexanol is from Spectrochem, India. Double distilled water was used throughout the study. Instrument and method: All the optical rotation measurements have been carried out in a Rudolph Research Analytical Autopol IV digital polarimeter coupled with a temperature control unit. The concentration of sucrose stock solution was 20% w/v in water. Exact 2N HCl solution was prepared by dilution from a higher concentrated stock solution. Concentration of AOT was kept at 150 mM and CTAB at 124.7 mM. In case of AOT, n-heptane was used as the bulk medium and in case of CTAB n-heptane mixed with 10% 1-hexanol was used as the bulk medium. As mentioned, the surfactants are not completely dry and we have estimated that in absence of any added water the w0 values are 0.5 and 0.1 for AOT RM and CTAB RM, respectively. The desired w0 values were achieved by adding required amount of water calculated according to equation 1. The hydrolysis of sucrose was studied by measuring the optical rotation of light at different times of a 1:1 mixture of 2N HCl and 20% sucrose in water with a proper zero setting and measurement of time zero optical rotation in bulk and RM systems. Optical rotation at infinite time was recorded by keeping the solution overnight. To achieve the required temperature, stock solutions were left at the same temperature water bath for at least 30 min prior to any measurement. The maximum temperature variation during a particular experiment was ±0.2˚C. Dynamic light scattering (DLS) measurements were done with a Beckman Coulter Delso Nano C, employing dual 30 mW, 658 nm laser diodes equipped with a thermostatted sample chamber maintained at 25°C. The scattering intensity data were processed using the instrumental software to obtain the hydrodynamic diameter (dH) and the size distribution in each sample. For DLS experiment the concentration of the AOT was kept 150 mM and to this reverses micellar solution, required amount of water, sucrose and hydrochloric


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acid were added to obtain desired systems and the corresponding DLS measurements were performed.

3. Results and Discussion Figure 1 a and b depict the kinetic data of sucrose hydrolysis in bulk water, inside AOT RM with w0 = 5.5, 10.5, 15.5 and inside CTAB RM with w0 = 10.1, 15.1, 20.1 at 30˚C. The plot clearly shows the difference in the rate of sucrose hydrolysis in confined environment compared to the bulk water. The data were fitted successfully by equation 4 and hence indicate that there is no change in the overall kinetics of this reaction. More interestingly, the rate of hydrolysis is found to be slower in AOT RM, where as, the rate is faster in CTAB RM, compared to bulk water. In both the cases, the rate approaches to bulk water as the size of the RMs increases (see table 1). The opposite effect of AOT and CTAB RMs on the rate of hydrolysis of sucrose was further compared by same sized water-pool formed by these two surfactants. The water to surfactant ratio corresponding to 10.5 in case of AOT RM has almost the same radius of the water-pool as 10.1 in case of CTAB.44-45 As all the measurements have been done at same temperature, i.e. 30˚C, we found that irrespective of same extent of confinement, the water-pool within the two RMs behaves differently. If the confinement within the cationic or anionic RM changed the activation energy barrier of sucrose hydrolysis in some way then we can expect a change in the rate. To confirm this we have performed a temperature dependent (22ºC, 26ºC, 30ºC and 34ºC) study to calculate the activation energy of the sucrose hydrolysis AOT RM and CTAB RM having same water-pool size as well as in bulk water. In this temperature range the size of the RMs remains almost constant.46-47 It is clear from the figure 2 that with increase in temperature the rate of hydrolysis of sucrose


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increases and the rate constants are summarized in table 2. It is evident that at a particular temperature the rate of hydrolysis of sucrose is maximum in CTAB RM followed by bulk water and AOT RM. The temperature dependent rate constant was used to estimate the activation energy barrier according to Arrhenius formulation as depicted in figure 3 and tabulated in table 3. It can be clearly seen from figure 3 and table 3 that the activation energy values obtained are essentially equal for all the three cases considering the error associated with the measurements. Thus we confirmed that the change of rate of hydrolysis of sucrose in RMs is not associated with change in the activation energy. However, question can be raised whether the activation energy of the reaction remains same for other size of reverse micelle or not. So we have also performed the temperature dependent experiment in AOT RM having a smaller water pool size (w0 =5.5). From the plot (figure 3) the calculated value of the activation energy is found to be 106.3 kJmol-1 (table 3). This fairly confirms that the activation energy value does not change with change in the size of RM, which in turn signifies that the change in the rate of the reaction is due to the character of the reverse micelle itself and there is no change in the reaction energetics inside a water pool of a RM. The hydrolysis of sucrose is essentially a pseudo first order reaction in bulk water and it remain so in the water-pool as the concentration of proton and water is much higher compared to sucrose in the media. So we confer that the reaction did not changed its order. Moreover the same activation energy of hydrolysis of sucrose in bulk water, AOT RM and CTAB RM rules out the possibility of alteration of reaction energetics inside the water-pool. The only other possibility, which can alter the rate of sucrose hydrolysis in AOT RM and CTAB RM compared to bulk water, is the availability of protons inside the water-pool. As HCl catalyzes this reaction and the availability of H+ can change the rate of the reaction, there must be a scarcity or excess of


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H+ ion that changes the rate of the reaction in AOT RM and CTAB RM, respectively. AOT having an anionic sulphonate head group can withdraw excess number of protons from the core part of the water-pool and create a higher concentration of H+ along the periphery of the pool, thus creating a proton gradient. CTAB having a cationic head group had the exactly opposite effect. It repelled protons from the periphery and made a high concentration of them in the core of the water-pool. Therefore in case of AOT RM, sucrose molecules felt a deficiency of catalyst around them and the rate of inversion slowed down. This is to mention that although we have not directly observed the proton gradient within a single reverse micelle, which perhaps not possible using this type of investigation, the clear observation of slowing down of the rate of sucrose hydrolysis with decrease in the size of the water-pool for AOT RMs and vice-versa for CTAB RMs conceivably hint towards the existence of gradient. The water-pool size dependence can be rationalized in the following way. In the smaller sized RM, the number of water molecules trapped inside are less, which eventually reduces the availability of H+. But the buffer like action of AOT RM did not changed much. So the availability of H+ closer to sucrose is less in smaller AOT RM. Therefore we have the slowest rate in AOT RM with w0=5.5. This is to mention that being a neutral molecule, sucrose is expected to stay inside the water-pool. We have estimated that at our experimental condition the number of sucrose molecules present inside a AOT RM of w0 = 10.5 are ~100 compared to ~20500 molecules of water trapped within it.16 This small number of sucrose molecules were not expected to distribute in other layers where its solubility is low. On the other hand, in case of CTAB RM the sucrose molecules experienced a different environment with an increased proton concentration around them. Thus the rate of the reaction increased in this condition than in bulk water. Evidently CTAB RM also show buffer like action but in an opposite way. It actually


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concentrated the protons in the core region and with decrease in size of the water-pool the H+ ions were crammed into a smaller space around the center of the water-pool and the rate of the reaction increased. Thus as the size became smaller the rate increased more and more for CTAB RM. It is interesting to note that in case of CTAB RM, the effect on the rate constant is smaller compared to AOT RM. The rate of acid catalyzed sucrose hydrolysis is known to depend on the polarity of the medium. Previous study reveals that the rate of sucrose hydrolysis decreases with decrease in polarity of the medium.48 On the other hand, it is well known that the polarity of water inside AOT and CTAB RMs are lower than that in bulk water and as the size of the water pool increases the polarity of the water in the pool approaches to that of bulk water. Thus in both AOT and CTAB RMs a slow sucrose hydrolysis rate is expected considering the less polarity of the water-pool. Along with the polarity effect on the slowing down of the sucrose hydrolysis rate inside the AOT RMs, the preferential partitioning of the protons near the RM inner surface (i.e. the buffer action) further slows down the rate of sucrose hydrolysis. Thus in AOT RMs, we have observed an appreciable sowing down in the rate compared to the bulk water. However in CTAB RMs, the concentration of protons in the inner part of the water-pool increases due to the positive nature of the inner surface of the RM. In this case, a faster sucrose hydrolysis is expected which is opposed by the inherent polarity effect of the water-pool. May be because of these two opposing factors in case of CTAB RMs, we have observed a weak dependence in the rate constant in CTAB RMs. Once the idea of the buffer like action of the RM has been established, we searched for the extent of this action. As the effect was more prominent in case of AOT RM than CTAB RM, the H+ concentration dependence was done in AOT RM (w0=10.5) and compared with bulk water (Fig. 4). It is important to mention here that rate constant of this reaction does not follow a linear


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relationship with the concentration of H+ and the same has been observed both in the case of bulk water and in AOT RM. 49 From figure 4 we can see that the rate constant in AOT RM is less than that in bulk water upto a concentration of 2N HCl. But for more acidic solutions, the rate constant became almost same in both conditions. This observation clearly infers that the buffer like action for AOT RM extends upto a H+ concentration of 2N. All the above discussions are valid, if there is no change in the AOT RM system on incorporation of sucrose as well as HCl. To study the effect of sucrose and HCl on the overall structure of AOT RM, we have performed dynamic light scattering (DLS) measurements for AOT RM with only water, AOT RM with 10% sucrose and the same with 10% sucrose + 1N HCl (figure 5). The experimentally measured hydrodynamic diameter values obtained from the DLS study are listed in table 4. We have observed that AOT RM containing sucrose and sucrose + HCl are of similar size to RM containing only water. Thus we can confirm that AOT RMs maintain its character with sucrose and HCl and therefore the change in the rate of sucrose hydrolysis reaction can only arise from the buffer like action of the water pool. The highlight of the present study is that the confinement within a RM did not change the activation energy of the hydrolysis of sucrose. The existence of a H+ gradient is proposed in the micro heterogeneous RM. This gradient is in the opposite direction for AOT and CTAB RM. Moreover highly acidic conditions were kept throughout the study. So the resistivity towards change in the “acidity of the water-pool” is found to be much higher than expected. The charge of the surfactant head groups actually controls the localized acidity of the water-pool. For negatively charged surfactant the interfacial region is found to be more acidic compared to the core of the RM. On the other hand in case of positively charged surfactant the core of the waterpool become more acidic.


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4. Conclusion In this work, we have studied the hydrolysis of sucrose inside the water-pool of AOT and CTAB RMs to comment on the long-standing question of the buffer capacity of RM systems. The unchanged activation energy barrier of sucrose hydrolysis reaction in reverse micelle when compared to that in bulk water indicates that there is a proton concentration gradient in the water-pool. Use of strong acidic condition confirms the extent of buffer like action of the water in confinement and found to be about 2N in case of AOT RM of w0 = 10.5. This study also highlights the strong proton scavenging property of AOT and strong proton repulsive property of CTAB. At last we have concluded that the charged surfactant group actually plays a role in determining the acidity of the different portion of the water-pool in a RM.

AUTHOR INFORMATION Corresponding Author Pratik Sen, Email: [email protected] Present Addresses § Department of Chemistry, Princeton University, Princeton, New Jersey, USA † Department of Chemistry, Bowling Green State University, Bowling Green, USA Notes The authors declare no competing financial interests.


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ACKNOWLEDGMENT PM & RY thank University Grants Commission (UGC) and SG & SR thank Council of Scientific and Industrial Research (CSIR) for awarding fellowship. JR thanks Indian Academy of Science (IAS) for awarding summer project fellowship. Authors thank Dr. Saptarshi Mukherjee and Mr. Nirmal Das of Indian Institute of Science Education and Research, Bhopal, India for DLS experiments. This work is financially supported by SERB, Government of India (Project No. SR/S1/PC-08/2011). REFERENCES (1) Crans, D. C.; Levinger, N. E. The Conundrum of pH in Water Nanodroplets: Sensing pH in Reverse Micelle Water-pools Acc. Chem. Res. 2012, 45, 1637-1645. (2) Bhattacharyya, K. Solvation Dynamics and Proton Transfer in Supramolecular Assemblies Acc. Chem. Res. 2003, 36, 95-101. (3) Pal, N.; Verma, S. D.; Singh, M. K.; Sen, S. Fluorescence Correlation Spectroscopy: An Efficient Tool for Measuring Size, Size-Distribution and Polydispersity of Microemulsion Droplets in Solution Anal. Chem. 2011, 83, 7736–7744. (4) Pileni M.P. et. al. Structure and Reactivity in Reverse Micelle, Elsevier, Amsterdam, 1989. (5) Das, P. K.; Chaudhuri, A.; Saha, S.; Samanta, A. First Simultaneous Estimates of the Water-pool Core Sizeand the Interfacial Thickness of a Cationic Water-in-Oil Microemulsion by Combined Use of Chemical Trappingand Time-Resolved Fluorescence Quenching Langmuir 1999, 15, 4765-4772.


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Table 1. Rate of sucrose hydrolysis in bulk water and in different reverse micelles at 30 ˚C Bulk Water W0 Rate (x10-3 min-1)

AOT Reverse Micelle

30.1

5.5 13.0

10.5 16.5

15.5 25.0

CTAB Reverse Micelle 10.1 34.8

15.1 32.9

20.1 30.6

Table 2. Rate of hydrolysis of sucrose at different temperature in different conditions Temperature (˚C)

Rate Constant (x10-3 min-1) AOT RM AOT RM (w0 = 5.5) (w0 = 10.5) 4.1 4.9 7.5 9.8 13.0 16.5 22.8 28.0

Bulk Water 22 26 30 34

9.0 15.4 30.1 53.8

CTAB RM (w0 = 10.1) 11.9 20.6 34.8 74.4

Table 3. Activation energy of sucrose hydrolysis in different environments Activation Energy (kJ mol-1)

System Bulk water AOT reverse micelle (w0 = 5.5) AOT reverse micelle (w0 = 10.5) CTAB reverse micelle (w0 = 10.1)

113.6 106.3 108.3 113.3

Table 4. Hydrodynamic diameter of AOT reverse micelle of w0 = 10.5 having different composition of polar part Sample water/AOT/n-heptane (w0 = 10.5) 10%sucrose /AOT/n-heptane (w0 = 10.5) 10%sucrose + 1N HCl/AOT/n-heptane (w0 = 10.5)

Hydrodynamic Diameter (nm) 7.2 7.4 7.5


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Scheme 1. Structure of dioctyl sodium sulphosuccinate (docusate sodium or aerosol-OT or AOT) and cetyltrimethylammonium bromide (hexadecyltrimethylammonium bromide or CTAB)


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Figure 1. Plot of normalized optical rotation vs time for hydrolysis of sucrose in different sizes of reverse micelle (a) in n-heptane/AOT/water, (b) in n-heptane/1-hexanol/CTAB/water (magnified region is shown in the inset) at 30˚C.


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Figure 2. Plot of normalized optical rotation vs time for hydrolysis of sucrose at different temperature in different conditions (a) bulk water (b) n-heptane/AOT/water (w0 = 10.5) (c) in nheptane/1-hexanol/CTAB/water (w0 = 10.1) (d) n-heptane/AOT/water (w0 = 5.5)


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Figure 3. Plot of ln k vs 1/T (K-1) for sucrose hydrolysis in different conditions.


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Figure 4. Plot of k vs HCl concentration for sucrose hydrolysis in different conditions


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Figure 5. DLS measurement (a) water/AOT/n-heptane (w0 = 10.5) (b) 10%sucrose /AOT/nheptane (w0 = 10.5) (c) 10%sucrose + 1N HCl/AOT/n-heptane (w0 = 10.5)


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TOC GRAPHICS


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Ramping of pH Across the Water-Pool of a Reverse Micelle - Langmuir

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