An electrochemical approach to study water–d (−)fructose interactions

Electrochimica Acta 97 (2013) 231–237

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An electrochemical approach to study water–d(−)fructose interactions Md. Tariful Islam Mredha a , Chanchal Kumar Roy a , M. Muhibur Rahman b , M. Yousuf A. Mollah a , Md. Abu Bin Hasan Susan a,∗ a b

Department of Chemistry, University of Dhaka, Dhaka 1000, Bangladesh University Grants Commission of Bangladesh, Agargaon, Dhaka 1207, Bangladesh

a r t i c l e

i n f o

Article history: Received 25 November 2012 Received in revised form 25 January 2013 Accepted 19 February 2013 Available online 5 March 2013 Keywords: Ferrocenecarboxylic acid Cyclic voltammetry Cetyltrimethylammonium bromide Critical micelle concentration d(−)Fructose

a b s t r a c t Water–d(−)fructose interactions have been studied for the first time by applying electrochemical technique using a redox-active probe ferrocenecarboxylic acid (FCA). Cyclic voltammetry was employed to study electrochemical behavior of FCA in aqueous solution of cetyltrimethylammonium bromide (CTAB) both in absence and presence of d(−)fructose. A three electrode system with a glassy carbon electrode as working electrode was used for this purpose. The concentrations of CTAB, d(−)fructose and FCA were varied in order to correlate electrochemical responses with the dissolved states to interpret water–d(−)fructose interactions. The anodic and the cathodic peak current, as well as, the apparent diffusion coefficient of FCA in micellar solution of CTAB increase with added d(−)fructose at low concentrations; while a reverse trend is observed at high d(−)fructose concentrations. The additions of d(−)fructose at low concentrations raise the critical micelle concentration (CMC) of CTAB, while high concentrations of d(−)fructose favor micellization. Analyses of experimental results indicate that d(−)fructose at lower concentrations behaves as a structure breaker of the water cluster, while that at higher concentrations acts as a structure maker. We have been able to successfully demonstrate that FCA can serve as a standard electro-active probe for studying the interaction of a non-electro-active guest with water. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction The complex structure of water has been one of the most delicate problem to unveil the mystery of nature [1,2]. Different degrees of hydrogen bonding among water molecules are considered to be the origin of different water clusters [3]. The presence of dissolved substances viz. urea, carbohydrates, lower alcohol etc. in water can modify the structure of water [4]. Several analytical methods such as X-ray, proton nuclear magnetic resonance, Raman, infrared and near infrared spectroscopy, photo-physical study, diffusion measurement, dielectric relaxation and many other techniques have been employed [5–14] to study these modifications of water structure. But the origin of structural modification of water with added substances is yet to be well understood. Intermolecular interactions among the supramolecules in aqueous media depend on the structure of water cluster i.e. surrounding atmosphere where they are dissolved. Therefore, any variation in regular water structure can cause significant change in the

∗ Corresponding author. Tel.: +880 29661920x7162; fax: +880 28615583. E-mail addresses: [email protected], [email protected] (Md.A.B.H. Susan). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.02.123

self-association of amphiphilic materials, consequently bringing about changes in the physicochemical properties of the dissolved states. Parameters representing micellization behavior, such as, critical micelle concentration (CMC), micellar solubility, micellar structure and aggregation number depend on the surfactant type, nature of the solvent, presence of additives and the temperature. Therefore, it would be possible to predict water-solute interactions by monitoring any physicochemical changes due to variation in micellization behavior. Carbohydrate, a typical non-electrolyte with the accumulation of hydroxy group at concentrations higher than any other biomolecules, is one of the most interesting additives to influence the water structure. Biomolecules of this variety provide specific [15] and unusual [16,17] interaction with water that influence the aggregation behavior of surfactants. A surfactant, when dissolved in water, can lead to the formation of micelles, which have significant difference in polarity and activity at an electrode interface. These amphiphiles are expected to respond to electrochemical changes and are potentially able to trigger reversible changes of self-assembly by creation or depletion of a charge via a redox reaction of the redox-active moiety of a redox-active surfactant [18–29] or an electroactive species solubilized in the reaction medium [29–36]. It is, therefore, expected that d(−)fructose will

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influence the electrochemistry of redox-active surfactant or redoxactive substance in conventional surfactant systems when there is a change in the regular water structure. The use of an electro-active probe in a conventional surfactant solution appears to have bright prospect for the ease of analysis and ferrocene derivatives have proved to be the most important electro-active probes due to the simple, stable and reversible electrochemistry of ferrocene group [34–36]. The central theme of the work is to use the response of a simple outer sphere redox probe (a ferrocene derivative) to infer the structure of the medium in which it resides. In this study, we employed cyclic voltammetry to investigate the electrochemical behavior of ferrocenecarboxylic acid (FCA) in aqueous medium and in a cationic surfactant, cetyltrimethylammonium bromide (CTAB) both in the absence and presence of d(−)fructose. The electrochemical results have for the first time been analyzed in terms of the micellization behavior of CTAB in aqueous solution with varying d(−)fructose concentration to understand the influence of d(−)fructose on the water structure for a consequent change in electrochemistry of FCA in aqueous medium.

a

3. Results and discussion Cyclic voltammetric behavior of FCA in 0.025 mol dm−3 KCl aqueous solution was studied over a wide range of CTAB concentrations, from below CMC to far above CMC both in the absence and presence of d(−)fructose. The cyclic voltammogram (CV) of FCA in aqueous solution (Fig. 1a) shows a single oxidation peak in the positive scan (0.1–0.6 V) and a corresponding reduction peak in the reverse scan (0.6–0.1 V). The shape agrees with those reported in the literature [37,38]. The electrochemical reaction can be shown as given in Scheme 1. Electrochemical responses of FCA are quite sensitive to the concentrations of CTAB, d(−)fructose and FCA (Fig. 1). CTAB and d(−)fructose affect the magnitude of the peak currents and the

c

d

e

(a) cathodic wave anodic wave

(b)

1 A

(c)

(d)

2. Materials and methods Analytical grade ferrocenecarboxylic acid, FCA (TCI, Japan), potassium chloride, KCl (Merck), cetyltrimethylammonium bromide, CTAB (Merck) and d(−)fructose (Merck) were used as received without further purifications. All the aqueous solutions were prepared with Puric-S grade deionized water (R = 2.0 M cm, Organo Co., Tokyo). Spectral measurements were carried out in a double-beam Shimadzu UV-Visible spectrophotometer (model UV-1650 PC). Rectangular quartz cells of path length 1 cm were used throughout the investigation. A sonicator (LU-2 Ultrasonic cleaner, LABNICS Equipment, USA) was used to clean the glass wares and to prepare stock solution of FCA in water. The CMC of CTAB was determined by electrical conductivity method using a conductimeter, TOA CM5S (TOA Electronics, Japan) with a dip-type pre-calibrated cell. A computer controlled electrochemical analyzer (Model CHI 600D; CH Instruments, USA) was employed for the cyclic voltammetric measurements. A single compartment three electrode cell containing a glassy carbon electrode (GCE) with geometric area of 0.071 cm2 (Bioanalytical Systems, BAS) as the working electrode, a silver–silver chloride (Ag/AgCl) electrode as the reference electrode, and a platinum wire as the counter electrode was used for electrochemical measurements in this study. The surface of the working electrode was polished with 0.05 ␮m alumina (Buehler) before each run. The electrochemical measurements were carried out using 0.025 mol dm−3 KCl aqueous solution as the supporting electrolyte. The potential sweep rate, () was between 0.01 and 0.50 V s−1 . The electrochemical and spectral measurements were conducted at 30 ± 0.5 ◦ C.

b

[FCA], mmol.dm-3 0.25 0.25 0.25 0.25 0.40 0.00 1.20 4.00 1.20 1.20 [CTAB], mmol.dm-3 -3 [D(-)fructose], mol.dm 0.00 0.80 0.80 2.00 0.80

Current, i( A)

232

(e)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Potential,E (V vs. Ag/AgCl) Fig. 1. Cyclic voltammograms of FCA in 0.025 mol dm−3 KCl aqueous solution (v = 0.02 V s−1 ).

position of the peak potentials (both anodic and cathodic) of the FCA system. As concentration of CTAB rises from 1.2 mmol dm−3 to 4.0 mmol dm−3 , the peak current decreases and the peak potential increases (Fig. 1b and c). Similar effect has been observed when d(−)fructose concentration increased in presence of 1.2 mmol dm−3 CTAB solution (Fig. 1b and d). In the case of high FCA concentration, the peak current becomes more prominent but the potential remains almost constant (Fig. 1b and e). Fig. 2 depicts the detail analyses of the effect of CTAB on the anodic peak potential (Epa ), the cathodic peak potential (Epc ) and the half wave potential (E1/2 , taken as the average of the Epa and Epc ) and the magnitude of the anodic and the cathodic peak currents (ipa and ipc ) of FCA. Addition of surfactant causes a decrease in the potentials up to the CMC value of CTAB followed by an increase with increasing CTAB concentration (Fig. 2a). The Epa and Epc moves together with the concentration of CTAB and their separation (E, the difference between Epa and Epc ) is very close to 65 mV. The electron transfer process of FCA at low scan rate is very rapid, reversible and CTAB does not affect the electrode kinetics of FCA. The total variation of E1/2 with CTAB concentration is not very high as it is within 10 mV. Therefore the effect of CTAB on E1/2 is also negligible. CTAB affects the diffusion of FCA in the solution phase rather than the electrode kinetics of electrode-solution interface which is clear from Fig. 2b

COOH Fe2+ Ferrocenecarboxylicacid FCA

COOH

Oxidation Reduction

Fe3+

+ e-

Ferriciniumcarboxylic acid FCA+

Scheme 1. Electrochemical reaction of ferrocene carboxylic acid, FCA.

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0.38

233

1.4 (a)

(b) Epa E1/2 Epc

ipa ipc

1.2

0.34

i, A

E, V (vs. Ag/AgCl)

0.36

1.3

1.1

0.32

1.0 0.30

0.9

0.28 0

1

2

3

4

5

0.8 0

1

[CTAB], mmol.dm-3

2

3

4

5

[CTAB], mmol.dm-3

Fig. 2. Variation of (a) anodic peak, cathodic peak and half wave potential (Epa , Epc and E1/2 ); (b) anodic and cathodic peak current (ipa , ipc ) of 0.25 mmol dm−3 FCA in 0.025 mol dm−3 KCl aqueous solution with concentration of CTAB in presence of 0.8 mol dm−3 d(−)fructose (v = 0.02 V s−1 ).

that an increase in the concentration of CTAB leads to the decrease in both ipa and ipc . The sharp decrease in currents below CMC of CTAB can be explained by strong electrostatic interaction between FCA and monomer of CTAB. This is supported by UV spectroscopic results (vide infra). As concentration of CTAB increases, aggregation of monomer results in the formation of micelle. Consequently, penetration of FCA inside the core of micelle leads to lower diffusivity of the redox-active probe in micellar media. In aqueous solution, UV spectrum of FCA exhibits two absorption bands. The band at 217 nm corresponds to the ␲→␲* transition of aromatic rings and 259 nm corresponds to the n→␲* transition of carboxylate ion present in FCA. When CTAB is added to aqueous solution of FCA, absorbance at 259 nm is noticeably changed as apparent from Fig. 3. The polar ammonium head group of CTAB might have electrostatic interaction with the carboxylate part of FCA, which makes the excitation process for the lone pair electrons of oxygen in carboxylate ion of FCA difficult, resulting in decrease in the absorbance. Analyses of cyclic voltammetric data (log ipa vs. log v gives straight lines with slope ≈ 0.5, Figure not shown) indicate that the electrochemical process of FCA in CTAB solution on GCE is dominated by diffusion and adsorption of FCA has no significant contribution to the overall electrochemical process. In the potential range studied, the adsorption of d(−)fructose or the surfactant species on the electrode surface has also been found not to influence the electrochemical process as apparent from the

2.5 [CTAB], mmol.dm-3

Absorbance

2.0

0.2 0.8 2.0 4.0

1.5

1.0

0.5

0.0 220

240

260

280

300

320

340

wavelength, nm Fig. 3. Absorption spectra of 0.25 mmol dm−3 FCA in 0.025 mol dm−3 KCl aqueous solution at different concentrations of CTAB in presence of 0.8 mol dm−3 d(−)fructose.

background current (data not shown). Apparent diffusion coefficients (Dapp ) of FCA in aqueous solution at different concentrations of CTAB have been estimated using Randles–Sevcick equation [39] from the slopes of the plot, ipa vs. 1/2 and the values are presented in Table 1. It is worth noting that the Dapp value of FCA in 4.0 mmol dm−3 exhibits about 2-fold decrease compared to that in the aqueous solution (Table 1). This confirms that transport of the micelle-bound hydrophobic probe is much slower than in the bulk. However, the decrease is less compared to around 10-fold decrease reported in the literature for CTAB concentration of 30 mmol dm−3 [40–42]. In this study, we used relatively low surfactant concentration so that the influence of water structure modifier additive can be monitored easily by voltammetric measurements and due to the use of low surfactant concentrations just above the CMC, we could not obtain large decrease in apparent diffusion coefficient. The Dapp is strongly dependent on concentration of the surfactant and the value increases sharply for decreasing surfactant concentration to approach the diffusion of free electrochemical probe in aqueous solution; while at very high concentration the Dapp approaches micellar diffusion coefficient [18,24,27,32]. Rusling and coworkers [42] also reported that the diffusion coefficient of CTAB increases sharply with decreasing surfactant concentration. Therefore, the Dapp value in our case is justified. It may be noted that the change in Dapp of 0.25 mmol dm−3 FCA with CTAB concentration shows similar trend to that of i vs. [CTAB] profile (Fig. 2b). Addition of d(−)fructose brings about changes in the organization of regular water structure and therefore in the micellization behavior of CTAB which is evidenced by the electrochemical results of FCA. In a micellar solution of CTAB, a slight decrease in potential is apparent at low concentrations of d(−)fructose; while with further addition of d(−)fructose, the potentials increase (Fig. 4). The peak separation, E is very close to 65 mV and hence the electrochemical process is almost reversible. The total variation of E1/2 with d(−)fructose concentration is not very high. d(−)fructose affects the regular water structure and micellization of CTAB and ultimately on the diffusion of FCA which brings about change of peak potential of FCA. On the other hand, increase in viscosity of the medium with added d(−)fructose leads to lower diffusion of FCA Table 1 Electrochemically estimated apparent diffusion coefficient, Dapp of 0.25 mmol dm−3 FCA in 0.025 mol dm−3 KCl aqueous solution at various concentration of CTAB in presence of 0.8 mol dm−3 d(−)fructose. [CTAB] (mmol dm−3 )

0.0

0.2

0.4

0.6

0.8

1.2

2.0

4.0

Dapp × 10 10 (m2 s−1 )

4.07

4.94

4.71

4.14

3.85

3.75

2.85

2.06

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0.40

1.2 1.1

CMC ofCTAB, mmol.dm-3

E, V (vs. Ag/AgCl)

0.38 0.36 0.34 0.32 0.30

Epa E1/2 Epc

0.28 0.26 0.0

0.5

1.0

1.5

2.0

2.5

to increase the potential. This may also be interpreted in terms of the fact that d(−)fructose acts as a water structure maker at higher concentrations which favors micellization. The variation in ipa with increasing d(−)fructose concentration for a fixed CTAB concentration is shown in Fig. 5. At concentration (0.8 mmol dm−3 ) below the CMC of CTAB, d(−)fructose does not cause any considerable change in current up to 0.8 mol dm−3 , after that lowering of current is observed with the addition of d(−)fructose. This can be explained by the fact that d(−)fructose can change the organization of surfactant media by promoting regular water structure. Enhancement of water structure favors the micellization process of CTAB; eventually increases the possibility of FCA to be trapped inside the core of micelles. At concentrations (1.2 mmol dm−3 and 2.0 mmol dm−3 ) above the CMC of CTAB, the peak current increases up to certain concentration of d(−)fructose and then decreases. This is due to the fact that d(−)fructose at lower concentrations behaves as a water structure breaker, while at higher concentrations acts as a structure maker [11]. To clearly understand the effect of d(−)fructose on water structure as well as the CMC of CTAB, we have also performed conductivity measurement of aqueous CTAB solution both in absence and in presence of different concentrations of d(−)fructose which also leads to the same conclusion. The change in the CMC of CTAB with concentrations of added d(−)fructose is 1.3 [CTAB], mmol.dm-3 0.8 1.2 2.0

ipa, A

1.1 1.0 0.9 0.8

0.5

1.0

1.5

0.8

0.6 0.0

0.5

1.0

1.5

2.0

2.5

[D(-)fructose], mol.dm-3

Fig. 4. Variation of peak potentials (Epa , Epc , E1/2 ) of 0.25 mmol dm−3 FCA in 0.025 mol dm−3 KCl aqueous solution with d(−)fructose concentration in presence of 1.2 mmol dm−3 CTAB (v = 0.02 V s−1 ).

0.7 0.0

0.9

0.7

[D(-)fructose], mol.dm-3

1.2

1.0

2.0

2.5

[D(-)fructose], mol.dm-3 Fig. 5. Anodic peak current, ipa of 0.25 mmol dm−3 FCA in 0.025 mol dm−3 KCl aqueous solution at various concentrations of d(−)fructose in presence of fixed CTAB solution (v = 0.02 V s−1 ).

Fig. 6. The CMC of CTAB as a function of concentrations of added d(−)fructose at 30 ◦ C.

represented in Fig. 6. The CMC of CTAB gradually increases with the addition of d(−)fructose at lower concentrations; while at higher concentrations CMC decreases. The CMC attains a maximum value at about 0.8 mol dm−3 of d(−)fructose concentration. The CMC variation of CTAB can be explained by the fact that d(−)fructose at low concentrations behave as a structure breaker of the water cluster; while at high concentrations acts as a structure maker [11]. Any structural or environmental factors that may affect solvent–lyophobic group interactions or interactions between the lyophobic groups in the interior of the micelle will therefore affect the free energy of micellization and consequently the value of the CMC. Structure breaker can lower the hydrophobic effect in a solution that is considered to be the driving force for micellization. The entropy of micellization is positive and the reasonable explanation of this process is that the water molecules surrounding surfactants are more ordered. During the micellization, the ordered structure of water molecules collapses that leads to an increase in disorder in the system and thereby entropy increases. The aggregation number (N) of CTAB in aqueous solution has been reported to decrease with increased addition of d(−)fructose. The values reported by us for 50 mmol dm−3 CTAB aqueous solution are 59 and 56 in presence of 0.25, 2.0 mol dm−3 d(−)fructose, respectively [43]. The concentration of micellized surfactant ([CTAB]-CMC) is significantly higher than the probe concentration (0.25 mmol dm−3 ) in all cases above the CMC and concentration of CTAB micelles (Concentration of micellized surfactant/N) is also sufficient to solubilize FCA in the hydrophobic core of micelles, which intrinsically exhibit high solubilization capacity to hydrophobic groups. Although the N of CTAB varies depending on concentration, it is apparent that above the CMC, most of the FCA species can be incorporated inside the core of micelles easily. In the presence of d(−)fructose, water-water interaction is replaced by water–d(−)fructose interaction in aqueous solution to some extent. The addition of d(−)fructose into the cluster induces the breaking of bonds in the periphery and inside the water cluster (structure breaking effect) [11]. It is the binary interaction between water-water and water-hydroxyl groups of d(−)fructose in the hydration layer. The introduction of foreign co-solvent molecules, like d(−)fructose, into the hydration layer is likely to interrupt the H-bond network which can disturb the water molecules ordering in surfactant solutions, and decreases the entropy of micellization. The free energy of micellization is always negative and it may become less negative as the concentration of d(−)fructose in the mixed solvent system increases. This indicates that the formation of micelles becomes less spontaneous at low concentrations

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235

Scheme 2. Micellization process of CTAB with added d(−)fructose.

CTAB, the trend of Figs. 5 and 7 remains unchanged. This is due to the fact that d(−)fructose at high concentration acts as water structure maker and reduces the amount of free surfactant in the system. Consequently, in presence of 1.2 and 2.0 mmol dm−3 of CTAB (above CMC of CTAB), current and diffusion coefficient values follow the same trend for all d(−)fructose concentrations due to reversible electrochemistry of FCA. The trends for the Dapp vs. concentration of d(−)fructose profiles for CTAB concentrations above the CMC in Fig. 7 are similar, although different trend for 0.8 mmol dm−3 of CTAB (below the CMC) can be observed. Although the large dominance of diffusion current over the whole electrochemical process is apparent, little contribution from the adsorption current in presence of 0.8 mmol dm−3 CTAB (below CMC) gives exaggerated values and the extent decreases with increasing d(−)fructose concentration. At higher concentrations, d(−)fructose promotes micellization of CTAB resulting in the increase in the possibility of FCA to be incorporated inside the micelle core to lower the diffusion current. The overall action of d(−)fructose on the micellization process of CTAB can be portrayed as in Scheme 2. The possibility of contribution of change in viscosity with added d(−)fructose in aqueous solution of CTAB cannot be overruled. According to Stokes–Einstein relationship, the diffusion coefficient should also be influenced by viscosity. In fact, diffusion coefficient depends on size and shape of molecule, interaction with solvent and viscosity of solvent. Rampp et al. reported that the viscosity of a medium increases with increasing d(−) fructose concentration [46]. Electrochemically estimated diffusion coefficients are only apparent values and in the surfactant concentration ranges

10 2 -1 (m .s )

5

Dapp × 10

of d(−)fructose. As a result, the amount of free FCA available in the media increases. Consequently, current increases with increasing d(−)fructose concentration in fixed CTAB solution (1.2 mmol dm−3 and 2.0 mmol dm−3 ). The peak current-[d(−)fructose] profiles for high CTAB concentrations (above the CMC; 1.2 mmol dm−3 and 2.0 mmol dm−3 ) show a gradual decrease in peak current to attain a value observed in absence of d(−)fructose. This suggests that solution behavior tends to restore a situation similar to pure water. Above ca. 0.8 mol dm−3 of d(−)fructose concentration solute–solute interactions take place. As a consequence, water molecules are reorganized in a more stable cluster and can serve as a structure maker to decrease the CMC value (Fig. 6) and consequently, current decreases (Fig. 5). An alternative explanation for the action of d(−)fructose in the case of ionic surfactant is based on the reduction of the dielectric constant of the aqueous solution that they produce. The dielectric constant of the medium is reduced by addition of d(−)fructose [44,45], which causes the repulsion effect among the ion head groups to increase. This factor may affect micellization and cause the CMC to increase at low d(−)fructose concentrations. As peak current (Fig. 5) and CMC of CTAB (Fig. 6) reaches maximum at a concentration of d(−)fructose of ca. 0.8 mol dm−3 , dielectric constant of the medium is minimum here. On further increase in d(−)fructose concentration, the dielectric constant of the medium may increase slightly as d(−)fructose predominates in the reaction mixture and cause the CMC to decrease again resulting in increase in the probability of FCA to be trapped inside the micelle core to lower the current. Analyses of cyclic voltammetric data indicate that the electrochemical process of FCA in d(−)fructose solution on GCE is dominated by diffusion. Apparent diffusion coefficients (Dapp ) of 0.25 mmol dm−3 FCA have been estimated and plotted against concentration of d(−)fructose in Fig. 7. The slight difference in the trend for current (Fig. 5) and diffusion coefficient (Fig. 7) at low d(−)fructose concentrations can be marked. Such a variation is not, in fact, surprising. Fig. 5 represents the anodic peak currents only at a low potential scan rate (0.02 V s−1 ). For the determination of apparent diffusion coefficient (Fig. 7) we have used anodic peak currents at different scan rates and influence of factors other than diffusion like minor adsorption has been averaged through linear regression and also uncertainty in diffusivity for measurement from single individual measurement has been minimized. Furthermore, the FCA deviated from reversibility to some extent in presence of free surfactant species. The presence of free CTAB might induce slight irreversibility of FCA in the system due to the electrostatic interaction of cetyltrimethylammoniun and ferronce carobyxylate ions. At low concentrations, d(−)fructose acts as water structure breaker, and free surfactant species dominate the system to widen the scope for such interaction. But when d(−)fructose concentration increases beyond 0.8 mol dm−3 in presence of 0.8 mmol dm−3

[CTAB], mmol.dm-3 0.8 1.2 2.0

4

3

2

0.0

0.5

1.0

1.5

2.0

2.5

[D(-)fructose], mol.dm-3 Fig. 7. Dependence of apparent diffusion coefficients (Dapp ) of 0.25 mmol dm−3 FCA in 0.025 mol dm−3 KCl aqueous solution on the concentration of d(−)fructose in the presence of CTAB of fixed concentrations.

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A clear understanding of the influence of different additives on the water structure would allow designing and fabricating new materials in these reaction medium. The work would create ample opportunity to establish the fundamental principle of the electrochemical switching in surfactant (a very cheap material)-based organized media by controlling the surrounding water structure using carbohydrates and open routes for further development.

2.5

2.0

ipa ipc

i, A

1.5

1.0

Acknowledgements 0.5

0.0 0.0

0.1

0.2

0.3

0.4

0.5

[FCA], mmol.dm-3 Fig. 8. Peak currents (ip ) for different concentration of FCA in 0.025 mol dm−3 KCl aqueous solution in presence of 1.2 mmol dm−3 CTAB with 0.80 mol dm−3 d(−)fructose (v = 0.02 V s−1 ). Background current has been subtracted and corrected baseline has been used for evaluation of both anodic and cathodic peak currents.

studied (just above the CMC), they are much smaller than the micellar diffusion coefficient. Although apparent diffusion coefficient should also have influence from the viscosity of the medium, this is more related to the redox behavior and dynamic equilibrium between the micellar and monomeric forms of surfactants. The effect of viscosity is much smaller than the effect by change in micellization process. For instance, the Dapp of 0.25 mmol dm−3 FCA in 0.025 mol dm−3 KCl aqueous solution is 4.36 × 10−10 m2 s−1 , which falls to 4.07 × 10−10 m2 s−1 when 0.8 mol dm−3 d(−)fructose is added to it. The similar system in the presence of 1.2 mmol dm−3 CTAB causes a 38.4% enhancement of the Dapp with 0.8 mol dm−3 d(−)fructose.These indicates that although viscosity has influences on the diffusivity, the micellization behavior has much more prominent role. The effect of viscosity does not affect the trend of change of electrochemical behavior with added d(−) fructose and for the sake of simplicity may therefore be ignored. Fig. 8 reveals that both the ipa and the ipc increase sharply with increase in the concentration of FCA. As concentration is increased, more electro-active FCA is diffused to the electrode surface; consequently, current increases. In the presence of 0.8 mol dm−3 d(−)fructose, disruption of micelles becomes significant. Presumably, there are no more micelles in the medium under this condition as apparent from the linear increase in current with increasing FCA concentration, which otherwise might show non-linear increase because of entrapped FCA in the hydrophobic core of micelles. d(−)Fructose behaves as a water structure breaker at low concentrations resulting in the disruption of micelles and the maximum influence is noticeable at 0.8 mol dm−3 of d(−)fructose concentration. 4. Conclusions Water–d(−)fructose interactions have been interpreted by analyzing the details of the electrochemical behavior of a redox-active probe FCA in aqueous solution of CTAB. The CMC of CTAB increases with added d(−)fructose at low concentrations, while at high concentrations the value decreases until it attains the value obtained in the absence of d(−)fructose. The electrochemical behavior of FCA in CTAB solution is therefore, fairly dependent on d(−)fructose concentration. d(−)fructose at low concentrations behaves as a structure breaker of the water cluster, while at high concentrations acts as a structure maker. Our study demonstrates that FCA is a standard electro-active probe for studying the interaction of a non-electro-active guest with water.

The authors gratefully acknowledge financial support for a sub-project (CP-231) from the Higher Education Quality Enhancement Project of the University Grants Commission of Bangladesh financed by World Bank and the Government of Bangladesh. The research was also supported in part by a grant for a research project from the University Grants Commission of Bangladesh.

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