Aggregation Behavior of Sodium Dioctylsulfosuccinate in Aqueous

Oct 16, 2012 - Department of Chemistry, North-Eastern Hill University, Shillong, 793022, India. Langmuir , 2012, 28 (45), pp 15762–15769. DOI: 10.10...
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Aggregation Behavior of Sodium Dioctylsulfosuccinate in Aqueous Ethylene Glycol Medium. A Case of Hydrogen Bonding between Surfactant and Solvent and Its Manifestation in the Surface Tension Isotherm D. Das, J. Dey, A. K. Chandra, U. Thapa, and K. Ismail* Department of Chemistry, North-Eastern Hill University, Shillong, 793022, India S Supporting Information *

ABSTRACT: The dependence of critical micelle concentration (cmc) of sodium dioctylsulfosuccinate (AOT) on the amount of ethylene glycol (EG) in water + EG medium was reported to be unusual and different from that of other surfactants to the extent that the cmc of AOT in EG is lower than in water. It is yet to be understood why AOT behaves so in water + EG medium, although AOT is known to have some special properties. Hence in the present study cmc of AOT in water + EG medium in the range from 0 to 100% (by weight) EG is measured by using surface tension and fluorescence emission methods. In contrast to what was reported, this study revealed that with respect to EG amount the cmc of AOT follows the general trend and AOT has higher cmc in EG than in water. On the other hand, it was surprisingly found that a break in the surface tension isotherm occurs in the premicellar region when the amount of EG exceeds 50% rendering a bisigmoidal shape to the surface tension isotherm. UV spectral study showed that AOT and EG undergo hydrogen bonding in the premicellar region when the EG amount is ≥50% and this hydrogen bonding becomes less on adding NaCl. The density functional theory calculations also showed formation of hydrogen bonds between EG and AOT through the sulfonate group of AOT providing thereby support to the experimental findings. The calculations predicted a highly stable AOT-EG-H2O trimer complex with a binding energy of −37.93 kcal mol−1. The present system is an example, which is first of its kind, of a case where hydrogen bonding with surfactant and solvent molecules results in a surface tension break

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INTRODUCTION Solvent plays a significant role in aggregating surfactant molecules into micelles, vesicles, etc. An illustration of the importance of solvent property on the aggregation phenomenon of surfactants can be found in the study of Seguin et al.,1 wherein it was shown that the micellization of nonionic surfactants in a mixture of ethylene and propylene glycols could be switched on or off by adjusting the composition of the mixed glycol media. Quantifying the role of solvent in the aggregation process of surfactants has however remained an unsettled fundamental issue. In this respect, studies on the aggregation behavior of surfactants in various single and mixed solvents provide useful results, and accordingly interest has always been shown in investigating the micellization characteristics of surfactants in mixtures of solvents as these mixtures provide solvent media of varying polarities. Ethylene glycol (EG) is considered to be a water-like solvent in terms of hydrogen-bonding ability and is an important solvent due to its use as antifreeze and in cryobiology studies.2 Nevertheless, the water + EG system is reported to be a potential medium for removal of SO2 from flue gas.3 The surface excess of some nonionic surfactants is reported3 to be © 2012 American Chemical Society

unexpectedly equal in water and EG. The phase diagram of the water + EG system indicates the formation of a 1:1 molar complex at 77.5 wt % EG.5 The conductivity of the water + EG system exhibits peculiarity with a maximum and minimum at ∼13 and 87 wt % EG, respectively.6 Thus, the water + EG mixture is an important and interesting medium for studying the aggregation and adsorption behaviors of surfactants. Sodium dioctylsulfosuccinate (AOT) is a double-chained anionic surfactant known to exhibit some special properties,7,8 e.g., it forms microemulsion without the presence of a cosurfactant6 and it has a special counterion binding behavior (SCB),7 viz. the value of its counterion binding constant undergoes a sudden 2-fold increase in aqueous NaCl solution when NaCl concentration is about 0.015 mol kg−1. Therefore, we were interested in studying the properties of AOT in a water + EG mixture with a view to examining the effect of solvophobicity on the micellization of this special surfactant. Received: July 17, 2012 Revised: October 1, 2012 Published: October 16, 2012 15762

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Table 1. Reported cmc Values of Some Surfactants in Different Media surfactant AOT

medium

temp./K

diel. const.

cmc/mM

method

refs

propylene glycol

298 303 303 303

78.5 76.8 76.8 30.2

ethylene glycol (EG)

303

33.3

Wa + EG (48:52 wt %)

303

60

Wa + EG (28:72 wt %)

303

52

dioxane (Dx)

303

2.66 2.23 6.00 5.60 3.00 1.50 0.90 5.5 5.0 3.6 6.5 6.4 1.80 1.60 2.2 2.5 >118 within 13.1 to 17.3 0.92 230 10.3 220

surface tension calorimetry conductometry calorimetry spectrophotometry calorimetry spectrophotometry calorimetry spectrophotometry conductometry calorimetry conductometry calorimetry spectrophotometry calorimetry conductometry SANSa MEKCb surface tension surface tension calorimetry surface tension

8, 11 9, 10 10 9 9 9, 10 9, 10 10 10 10 10 10 9, 10 9, 10 10 10 12 13 14, 15 15 14 16

water (Wa)

Wa + Dx (25:75 wt %)

CPCc SDSd

formamide (FA) Wa EG Wa FA

2.2 12.7

298 300 300 333 333

109 77.8 33.3

a

SANS = small-angle neutron scattering. bMEKC = micellar electrokinetic chromatography. cCPC = cetylpyridinium chloride. dSDS = sodium dodecylsulfate. Wilhelmy plate method using K11 Krüss tensiometer. A Hitachi F4500 FL spectrophotometer was used to record the fluorescence emission intensities of pyrene. Conductance measurements were made using Wayne Kerr B905 automatic precision bridge and a dip-type cell. The UV absorption spectra of AOT solution were recorded using Hitachi 2900 model spectrophotometer and 1 cm quartz cells. The FTIR spectra were recorded using a Shimadzu IR Affinity-1 spectrometer with a Diamond ATR (attenuated total reflection) operating at a resolution of 0.5 cm−1, and 20 scans were done. The temperature was controlled during the different measurements by using a Haake DC 10 circulation bath. More details about the experimental methods are given elsewhere.8,11 For the computation work, we have used the density functional theory (DFT) based B3LYP method combined with the 6-31G(d,p) basis set. The geometries of the isolated AOT and EG were optimized. The calculated hydrogen bonding energy values include the zero-point energy (ZPE) correction.

The main motivation for choosing the AOT + water + EG system for study stems from the unusual observations reported9,10 in this system, viz., (i) above 63 wt % EG the critical micelle concentration (cmc) of AOT shows a decreasing trend with increasing amount of EG, which is in contrast to the expected trend, and (ii) the cmc of AOT in EG is lower than in water, which is also surprising because in nonaqueous solvents cmc generally has a higher value. Some of the reported cmc values of AOT and other surfactants in media of different dielectric constants are listed in Table 1.8−16 It is not yet known why AOT behaves so differently in water + EG media. It is also not yet confirmed whether this is another type of special behavior of AOT since AOT is already known to have some special properties.7,8 The reported values of cmc of AOT in EG and water + EG media were obtained by employing calorimetry, spectrophotometry, and conductometry techniques.9,10 The conductance method is used very often to determine the cmc of ionic surfactants. However, BinanaLimbele and Zana17 have reported that extreme caution must be exercised while determining cmc of ionic surfactants in nonaqueous solvents owing to both ion−ion interactions and micellization causing slope changes in the conductivity versus concentration plot. In the present work we therefore used surface tension and fluorescence emission methods to measure the cmc of AOT in water + EG medium in the range from 0 to 100% (by weight) EG. For comparison purposes, cmc values were determined from the conductance method as well. UV spectral study and density functional theory (DFT) calculations have also been made to elucidate hydrogen bonding between AOT and EG.

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RESULTS AND DISCUSSION Nature of Surface Tension Isotherms. The measured surface tension of AOT + H2O + EG systems as a function of AOT concentration at 25 °C and at varying amounts of EG (0 to 100 wt %) is shown in Figure 1. From Figure 1 it is apparent that, when the media contain more than 50 wt % EG, a new inflection in the surface tension isotherm of AOT occurs much below the cmc. A more clear view of such a break is shown in Figure 2. The values of AOT concentration and the surface tension corresponding to these inflections are listed in Table 1. Such inflections in the surfactant isotherms look similar to those occurring in surfactant−polymer systems due to interaction between surfactant and polymer.18 When the amount of EG in the mixed solvent is more than 50%, added AOT through oxygen of its SO appears to form hydrogen bonds with EG which are sufficient for the manifestation of break in the surface tension isotherm in the premicellar concentration region. When such interaction occurs between AOT and EG, the added surfactant molecules can remain in the bulk without going to the air − solution interface causing

EXPERIMENTAL METHODS

AOT (Sigma, > 99%), EG (SRL, > 99%), and pyrene (Fluka, > 97%) were used without further purification. Milli-Q grade water was used for making samples. Surface tension measurements were made by the 15763

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surface tension isotherms are given in Table 1 and also shown in Figure 3. The present work therefore shows that the cmc of

Figure 1. Surface tension of the AOT + H2O + EG system at 25 °C as a function of AOT concentration.

Figure 3. Variation of cmc of AOT with weight % of EG and dielectric constant. The inset (B) shows reported data on an expanded scale.

AOT increases with an increase in EG % or a decrease in the solvophobicity of the medium in accordance with the general trend. In Figure 3 we also plotted the cmc as a function of dielectric constant of the medium22 since solvent properties control the micellization phenomenon. In this plot an inflection is observed at a dielectric constant of ∼61 which corresponds to 60% EG. However, the cmc values of AOT in EG + water media reported by Mukherjee et al.9,10 had unusual trends as cited above in the Introduction section, and these reported cmc values are compared with the present values in Figure 3. We are of the view that, similar to the breaks occurring in surface tension isotherms due to hydrogen bonding, inflections occur in the calorimetry (one of the methods used by Mukherjee et al.9,10) curves of AOT in EG + water media due to hydrogen bonding leading to pseudocmc values. We also made an attempt to determine cmc values of AOT from the fluorescence intensity ratios of pyrene probe. Plots of I1/I3 versus AOT concentration are shown in Figure 4. I1 and I3 refer to the intensities of the fluorescence emission spectra of pyrene at 374 and 384 nm, respectively. The cmc values were determined from the I1/I3 data by two methods. In one method, derivative plots were drawn and AOT concentration

Figure 2. Surface tension of the AOT + NaCl + water + EG system at 25 °C as a function of AOT concentration. The weight % of EG and the corresponding NaCl concentrations in mol kg−1 are indicated against the plots.

thereby less change in surface tension. This may be responsible for the initial inflection in the surfactant isotherm below cmc. Another probable cause for this could be the formation of premicellar aggregates. However, despite premicellar aggregates being known to form in many surfactant solutions, no such inflection in surface tension isotherm has been reported. As far as interaction with water and EG is concerned, extensive hydrogen bonding between them has been reported19 at around 70% EG. It has also been reported3 that in EG + H2O medium containing 70 to 90% EG the solubility of SO2 is optimum due to hydrogen bonding between SO2 with EG. Similar to the AOT-EG interactions expected in the present system of study, EG-AOT and EG- H2O-AOT interactions are reported in reverse micelles of AOT in heptane and isooctane, and it is also reported that these interactions are stronger than water-AOT interactions.20 Durantini et al.21 studied the interactions of the polar headgroup of AOT with different polar solvents such as EG, glycerol (GY), propylene glycol (PG), dimethylformamide (DMF) and dimethyacetamide (DMA) in nonaqueous AOT/n-heptane reverse micelles by using noninvasive FTIR spectroscopy and also reported hydrogen bonding between the different nonaqueous solvents and the headgroup of AOT. To the best of our knowledge the present system is the first example in which the hydrogen bonding interaction is manifested in the surface tension isotherm. The hydrogen bonding interaction between AOT, EG and H2O is discussed more in the subsequent sections. cmc from Surface Tension and Fluorescence. The cmc values of AOT in EG + water media determined from the

Figure 4. Variation of the fluorescence intensity ratio I1/I3 of pyrene with AOT concentration at 298 K in water + EG media. The red lines represent calculated values obtained by fitting the data to eq 1. 15764

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Table 2. Values of Cmc of AOT in Water − EG Media at 25 °C, and Values of AOT Concentration and Surface Tension at the Pre-Micellar Break in Surface Tension Isotherm cmc/mmol kg−1 weight % EG 0 10 20 30 40 50 60 70 80 90 100 a

from surface tension 2.61 2.88 3.10 4.30 7.10 9.10 16.2 27.3 50.3 86.3 122.5

corresponding to first break in surface tension isotherm

from fluorescence 2.53 2.98 3.93 4.94 7.22 9.45 17.3 29.9 51.7 92.3

a

(2.82) (3.00) (3.74) (4.88) (7.27) (14.3) (18.6) (37.9) (58.3) (87.1)

b

from conductance

AOT conc./mmol kg−1

surface tension/mN m−1

no inflection no inflection 6.6 7.0 8.0 10.0 9.0c (17.3)d 15.0 (27.6) 16.0 (50.0) 85.3 (185.0) 54.0 (161.2)

0.25 0.20 0.17 0.55 3.00

48.0 47.0 45.5 44.0 42.0

From derivative method. bFrom fitting the data to eq 1. cFirst inflection. dSecond inflection.

of added salts, temperature and pressure on the hydrogen bonding in water and other hydrogen bonded compounds.25−28 These studies have revealed that the hydrogen bonding in water decreases with increase in the concentration of the added salts such as NaCl and KCl.26 In the present study, we added NaCl to the AOT + water + EG mixtures and measured the variation of surface tension. The surface tension isotherms obtained by varying the EG composition from 60−100% in the presence of NaCl are shown in Figure 2. From Figure 2 it can be seen that the hydrogen bonding decreases by the addition of NaCl. In 60 and 70% EG, the first break due to hydrogen bonding disappeared completely by adding 0.10 and 0.21 mol kg−1 NaCl, respectively. The NaCl concentration required for removing the first break in the surface tension isotherm is found to be dependent on the EG amount, more the EG % higher the NaCl concentration needed. Thus, disappearance of surface tension break by the addition of NaCl confirms that these premicellar breaks in surface tension isotherm are due to hydrogen bonding between AOT, EG and water. Such an effect of NaCl also appears to rule out the possibility of premicellar aggregate formation being responsible for the initial inflection in the surface tension isotherm because it is reported29 that conditions facilitating micelle formation also favor formation of premicelles. UV Spectra. To understand the nature of interactions between AOT and EG, which cause breaks in the surface tension isotherms in the premicellar region, we recorded the UV spectra of AOT in water + EG medium by varying both AOT and EG amount. The observed UV band corresponds to the n → π* transition in AOT. Representative plots of the UV spectra are shown in Figure 5. From these plots it is clear that when the amount of EG in the mixture is 50% or more a red shift in λmax begins to occur in the premicellar concentration region of AOT and the shift increases with increase in AOT concentration. In fact, Mukherjee et al.9,10 also observed similar red shift in λmax with increasing concentration of AOT in EG and water + EG medium. A plot showing the variation of λmax with the concentration of AOT in 50 to 100% EG media is shown in Figure 6. This red shift in the λmax of the AOT band is indicative of EG−H2O−AOT and EG−AOT interactions through hydrogen bonding. Such shift in λmax due to hydrogen bonding has been reported in acetic acid and water system also.30 In 50% EG, hydrogen bonding between AOT and EG is however not manifested in premicellar surface tension break.

corresponding to the deep minimum in the derivative plot was taken as the cmc value. In the other method the data were fitted to a sigmoidal equation of the form23 A1 − A 2 I1 = A2 + I3 1 + exp[(c − x 0)/b]

(1)

In eq 1, c represents the surfactant concentration, x0 is the value of c corresponding to the center of the sigmoid, A1 and A2 are the upper and lower limits of the sigmoid, respectively, and the term b reflects the range of c wherein sudden change of I1/I3 occurs. The fitted value of x0 is taken as equal to cmc. The calculated values of I1/I3 from eq 1 are compared with its experimental values (Figure 4) and the fitting is found to be good. It is observed that as the amount of EG increases the shape of the plots shown in Figure 4 deviates from the sigmoidal shape and in neat EG sigmoidal shape completely disappears. Due to this reason, cmc of AOT in pure EG could not be determined from the I1/I3 data. The values of cmc determined from I1/I3 data by using the two methods are listed in Table 1 and the agreement between the two sets of values are poor from 50% EG onward, which may be attributed to the fact that x0 is not always equal to cmc.23,24 The measured conductivity values are shown in Figures S1 and S2 as plots of conductivity versus AOT concentration. No slope change is observed in water and 10% EG (Figure 4) within the concentration range of conductivity measurement. In 20 to 50% EG single slope change is noticed, whereas in 60 to 100% EG slope changes occurr at two concentrations of AOT. The concentrations of AOT corresponding to the inflections in the conductivity plots (Figures S1 and S2) are listed in Table 2. From Table 2 it is clear that the concentrations corresponding to some of the inflections only are comparable with the cmc values from the surface tension. These observations from the conductivity data are in accordance with the points made by Binana-Limbele and Zana17 that in nonaqueous media slope changes in the conductivity versus concentration plots occur at more than one places and the surfactant concentrations at the inflections depend upon the concentration range of conductivity measurement. Thus, conductance method may provide sometimes incorrect cmc values. Effect of NaCl on the Hydrogen Bonding in AOT + Water + EG Mixtures. Added electrolytes are known to affect the hydrogen bonding interactions in water. A large number of theoretical studies have been devoted to understand the effect 15765

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Figure 7. Fitted Gaussian curves corresponding to the UV spectra of AOT (0.24 mmol kg−1) at 25 °C in water + EG media. Weight % values of EG are indicated in the insets.

Figure 5. UV absorption spectra of AOT in water + EG media at 298 K. Weight % of EG is indicated on each layer. AOT concentrations in mmol kg−1 are as follows: (i) in water: (a) 0.4, (b) 1.1, (c) 1.8, (d) 3.6, (e) 7.2, (f) 10.8, (g) 14.4; (ii) in 20% EG: (a) 0.6, (b) 1.2, (c) 1.8, (d) 2.4, (e) 3.0, (f) 3.6, (g) 4.2, (h) 4.8, (i) 5.4, (j) 6.0; (iii) in 30% EG: (a) 2.0, (b) 2.5, (c) 3.0, (d) 3.5, (e) 4.0, (f) 4.5, (g) 5.0, (h) 5.5; (iv) in 50% EG: (a) 0.8, (b) 1.2, (c) 2.3, (d) 5.0, (e) 9.0, (f) 16.0, (g) 30.0, (h) 50.0; (v) in 80% EG (data unsmoothed): (a) 0.3, (b) 0.7, (c) 0.8, (d) 1.2, (e) 2.3, (f) 5.0, (g) 10.0, (h) 16.0, (i) 33.0, (j) 50.0, (k) 100.0, (l) 290.0; and (vi) in EG (data unsmoothed): (a) 0.5, (b) 0.8, (c) 1.2, (d) 2.3, (e) 5.0, (f) 9.0, (g) 16.0, (h) 32.0, (i) 50.0, (j) 100.0, (k) 160.0. Spectra of AOT in 60, 70, and 90% EG are not shown.

considered the AOT concentration corresponding to the break in the plot of absorbance of AOT versus EG content as the cmc. In the light of the above analysis of the uv spectra of AOT, it has become clear that hydrogen bonding, besides micellization, also causes a bend in the plot of absorbance of AOT versus EG content, leading to pseudocmc values. DFT Calculations. In order to have qualitative understanding about the hydrogen bonding interactions of AOT with EG and H2O and the relative strength of such interactions, we performed simple model calculations using B3LYP/6-31G(d,p) DFT. We first optimized the structures of isolated AOT, EG, and H2O and the (1:1) hydrogen bonded complexes of AOT with EG and H2O. At each stationary point vibrational frequencies were calculated to confirm the minimum energy structure having all positive frequencies. The optimized structures of AOT, EG and hydrogen bonded structures are shown in Figure 8 and their energies are listed in Table 2. The energy difference between the monomers and the hydrogenbonded complexes gave the hydrogen bonding energy, ΔEHB (listed in Table 2). The basis set superposition error (BSSE) correction was not done since we are interested only in relative hydrogen bonding strength and validity of this correction is recently questioned.31 From the Table 2, it is clear that the hydrogen bonding in AOT + EG system is much stronger (by almost 5 kcal/mol) than the hydrogen bonding in AOT + H2O. The primary reason behind this stronger interaction for EG is the strong OH···O interaction as evidenced from the distance (1.738 Ǻ ) and secondary C−H···O interaction. From the structure of the hydrogen-bonded complex it is also clear that the hydrogen bonding between AOT and EG takes place primarily due to interaction between the SO3− group of AOT and the −OH groups of EG. This supports our observation made from the surface tension measurements. Next we examined the hydrogen bonding between EG and H2O and the effect of the presence of H2O on AOT−EG hydrogen bonding (Table 3). Our results for (1:1) complexes show that EG has strong hydrogen bonding interaction with H2O. In fact, the EG−H2O interaction energy (−12.50 kcal mol−1) is more than double the interaction energy of water dimer. This is due to the presence of two strong O−H···O hydrogen bonding in EG−H2O complex as shown in Figure 8. We thought that it would be interesting to find out whether presence of a water molecule with EG strengthen or weaken the

Figure 6. Shift in λmax of AOT in water + EG media as a function of AOT concentration.

We analyzed the spectra by fitting them to a Gaussian function. Such fitted spectra at fixed AOT concentration (0.24 mmol kg−1) and varying amount of EG are shown in Figure 7. We made two observations from Figure 7: (i) Below 50% EG the spectra fitted to one Gaussian curve only with λmax = 206 ± 2 nm. Therefore, at this fixed AOT concentration when the medium contains less than 50% EG the hydrogen bonding of AOT with EG is weak. (ii) In the range from 50 to 100% EG, the spectra fitted to two Gaussian curves. In 50−90% EG medium, the lower intensity Gaussian curve has λmax at 207 ± 2 nm, whereas in neat EG this curve has λmax at 213 nm. This band is attributed to AOT that had not undergone hydrogen bonding with EG. The λmax of the second higher intensity Gaussian curve varied from 211 to 222 nm as the EG content changed from 50 to 100% (Figure 7), and this band is due to the hydrogen bonded AOT. Thus, the analysis of the AOT spectra using Gaussian function reveals that AOT takes part in hydrogen bonding with EG. It may be pointed out here that in the spectrophotometric method used by Mukherjee et al.9,10 for determining cmc of AOT in water + EG medium, they 15766

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Figure 8. B3LYP/6-31G(d,p) optimized structure of (A) AOT (H-atoms are not shown), (B) EG, (C) AOT−H2O complex, (D) AOT−EG complex, (E) EG + H2O + AOT, and (F) EG + H2O. Color code for atoms: dark gray/black, carbon; light gray, hydrogen; red, oxygen; yellow,− sulfur.

strong cooperative effect. The cooperativity in a molecular trimer containing A, B, and C molecules is given by the threebody term ΔEcoop which can be defined32,33 as the difference between the total interaction energy Eint(ABC) and the sum of the pairwise or two-body interaction energies E2(AB), E2(BC), E2(AC), ΔEcoop = Eint (ABC) − E2(AB) − E2(BC) − E2(AC). Here the E2 values correspond to the two-body contributions at the trimer geometry calculated with the same basis set. Our results provide ΔEcoop value of −6.94 kcal mol−1, which is almost 18% of the interaction energy. We believe that our simple model calculations suggest that in actual experimental medium AOT remains preferentially hydrogen bonded with EG, which is stabilized further due to hydrogen bonding with H2O and the resulting significant cooperative effect on AOT− EG binding.

Table 3. Hydrogen Bond Energies (ΔEHB) of the Complexes between AOT, H2O, and EG system

ΔEHB/kcal mol−1

EG + H2O AOT + H2O AOT + EG AOT + EG + H2O

−12.50 −17.23 −21.96 −37.93

AOT−EG binding. This study is relevant since our experiment was carried out in a mixed solvent of EG and H2O. Our results predict a highly stable AOT−EG−H2O trimer complex with a binding energy of −37.93 kcal mol−1. The structure of the AOT−EG−H2O trimer complex is shown in Figure 8. It is interesting to observe that hydrogen bond lengths between EG and AOT is considerably shortened in the trimer indicating a 15767

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FTIR Spectra. To confirm further hydrogen bond formation in water + EG + AOT system, we recorded the FTIR spectra of water +70% EG and water +70% EG + AOT (0.01 mol kg−1) systems which are shown in Figure 9. The vibrational spectra of

of calorimetric and uv absorbance data appear to be the pseudocmc values caused by the hydrogen bonding. In fact, in mixed solvent and nonaqueous media the conductance method of determining cmc has also been reported17,36 to give pseudocmc values due to ion−ion interactions. Therefore, this work envisages that in nonaqueous and mixed solvent media the common methods of cmc determination may provide pseudocmc values and it is necessary to ascertain whether the estimated cmc values are true or pseudo. The results of the present study have shown that pseudocmc caused by hydrogen bonding can be distinguished from true cmc by the addition of NaCl. This study has demonstrated that hydrogen bonding between surfactant and solvent can lead to surface tension break in the premicellar region.

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ASSOCIATED CONTENT

S Supporting Information *

Plots of conductivity versus AOT concentration in water + EG media (Figures S1 and S2). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

Figure 9. FTIR spectra of water, water + 70% EG, and water + 70% EG + 0.01 mol kg−1 AOT.

*E-mail: [email protected]; [email protected]. Telephone: 91-364-2722610. Fax: 91-364-2550486. Notes

water + EG system is in good agreement with the reported34 spectra. The strong band at 3265 cm−1 corresponds to the OH stretching vibration of water. The total peak area of the OH stretching band of water decreases on adding EG and the decrease is considerable in the presence of EG + AOT. This kind of decrease in the peak area of OH stretching band was reported in water/AOT/isooctane microemulsion35 on decreasing water content or increasing AOT content (decrease in the mole ratio of water to AOT). Therefore, it may be inferred that when AOT is added to water + EG more hydrogen bonds are formed. It is difficult to monitor the changes in the SO stretching frequency of AOT19,33 which occurs at about 1050 cm−1 since C−O stretching of EG also falls in this region. The decrease in the peak area of the C−O stretching band on adding AOT may be attributed to SO...H−O−C hydrogen bonding.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support received from the DST, New Delhi. D.D. and U. T. acknowledge the UGC, New Delhi for the research fellowship. J.D. acknowledges the CSIR, New Delhi for the Research Associateship. We are thankful to Mrs. Judy K. Ajish, Radiation and Photochemistry Division, BARC, Mumbai for FTIR studies of our samples.

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REFERENCES

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CONCLUSIONS Cmc is an important characteristic property of a surfactant and a surfactant is considered good or bad based on its cmc value. The results of this work has settled the controversy about the cmc of AOT in EG and water + EG media. The cmc of AOT monotonically increases with increasing amount of EG in the water + EG mixed solvent and follows the general trend. The cmc of AOT in EG is 122.5 mmol kg−1, which is about 47 times higher than the cmc in water. The UV study and DFT calculations have confirmed that there is strong hydrogen bonding between AOT, EG, and H2O. The binding energy for the AOT−EG−H2O trimer is predicted to be −37.93 kcal mol−1. Such hydrogen bonding is responsible for the break in the surface tension isotherm of AOT in the premicellar concentration region and it also causes inflection in the plots of uv spectral data (plots of absorbance and λmax versus AOT concentration). Such strong hydrogen bonding is expected to cause inflection in the plot of calorimetric data as well. Therefore, the unusually low values of cmc of AOT in EG and above 63% EG reported by Mukherjee et al.9,10 on the basis 15768

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