Physicochemical Behaviors of Cationic Gemini Surfactant (14-4-14

Article pubs.acs.org/Langmuir

Physicochemical Behaviors of Cationic Gemini Surfactant (14-4-14) Based Microheterogeneous Assemblies Sibani Das,† Indrajyoti Mukherjee,† Bidyut K. Paul,‡ and Soumen Ghosh*,† †

Centre for Surface Science, Department of Chemistry, Jadavpur University, Kolkata 700 032, India Surface and Colloid Science Laboratory, Geological Studies Unit, Indian Statistical Institute, 203, B.T. Road, Kolkata 700108, India

‡

S Supporting Information *

ABSTRACT: A comprehensive study of micellization and microemulsion formation of a cationic gemini surfactant (tetramethylene-1,4bis(dimethyltetradecylammonium bromide; 14-4-14) in the absence or presence of hydrophobically modified polyelectrolyte, sodium carboxymethylcellulose (NaCMC), has been conducted by conductometry, tensiometry, microcalorimetry, and fluorimetry methods at different temperatures. Both critical micelle concentration and degree of ionization of the surfactant have been observed to increase with increasing temperature. The interfacial and thermodynamic parameters were evaluated. The standard Gibbs free energy of micellization (ΔGm ° ) is negative, which decreases with increase in temperature. Larger entropic contribution is observed compared to the enthalpy. The interaction of 14-4-14 with NaCMC produces coacervates which was determined from turbidimetry method. The pseudoternary phase behavior of the microemulsion systems comprising water (or NaCMC as additive), 14-4-14, isopropanol (IP) or n-butanol (Bu) as cosurfactant, and isopropyl myristate (IPM) were studied at 298 K. Phase diagrams reveal that IP derived microemulsions (in the absence of NaCMC) offer a large isotropic region compared to Bu-derived systems at comparable physicochemical conditions. Increasing the concentration of IP or Bu decreases the isotropic region in the phase diagram. NaCMC influences the microemulsion zone, depending upon its concentration, and type of cosurfactant and surfantant/cosurfactant ratio. Dynamic light scattering and conductometric measurements show the size of the droplet, threshold temperature of percolation, scaling parameters, and activation energy of the percolation process of 14-4-14/IP or Bu derived microemulsion systems without/with NaCMC at various physicochemical conditions. Bu exerts a greater effect to reduce θt than IP as a cosurfactant (in the absence of NaCMC) at comparable ω. On the other hand, IP showed better percolating effect than Bu in the presence of NaCMC. Bu and IP (as cosurfactant) and NaCMC (as additive) influenced the microemulsion droplet size (Dh) to different extents under comparable conditions. Temperature insensitive microemulsions have been reported at the studied temperature range (298−353 K). 14-414/IP (1:2)-derived microemulsion showed a fractured surface at fixed ω = 15, where ω is the water and surfactant molar ratio, and temperature (298 K); whereas, large scale mesospheres comprising multiple closely winded nanoslices and spheroid morphology were formed in 14-4-14/IP and 14-4-14/Bu microemulsions, respectively, in the presence of 0.01 g % NaCMC, at comparable conditions. These systems revealed good antimicrobial activity toward the strains of Gram-positive Bacillus subtilis and Gram-negative Escherichia coli bacteria at 298 K, and inhibitory effect was governed by ω, type of cosurfactant, and bacterial strains.

1. INTRODUCTION Geminis (twin soaps) can self-aggregate easily at almost 100 times lower concentrations than the corresponding conventional monomeric amphiphiles and hence are superior to them.1−3 Gemini surfactant consists of two polar head groups covalently attached by a spacer group. Due to unique characteristic features, for example, low critical micelle concentration (cmc) and Krafft temperature, high surface activity, better wetting capacity, and so forth, such surfactants have a lot of applications in the fields of food and cosmetic industry, gene transfection, drug delivery, tertiary oil recovery, polymerization, antimicrobial material designing, and so forth.4−8 Moreover, during interaction with polymer and © 2014 American Chemical Society

macromolecules, gemini surfactants help one to understand their physicochemical behaviors and the underlying mechanism of interactions.9,10 With micellization, these amphiphiles can form stable microemulsion. Surfactant−oil−water combinations can form different types of microemulsions. The phase behavior of microemulsion containing gemini surfactants are available in literature.11−13 Zana and In12 have reported the comparative phase behavior of cationic gemini surfactant with monomeric Received: July 3, 2014 Revised: September 17, 2014 Published: September 21, 2014 12483

dx.doi.org/10.1021/la5025923 | Langmuir 2014, 30, 12483−12493

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To the best of our knowledge, microemulsion stabilized by gemini surfactant in IPM (as oil) has not been reported to date. In addition, temperature induced percolation of conductance and droplet size of the microemulsions without or with anionic polymer (NaCMC) at different concentrations under various physicochemical environments (for example, temperature, water and surfactant molar ratio, ω and surfactant and cosurfactant mass ratio) were investigated from conductivity and dynamic light scattering (DLS) techniques. Morphology or shape of microemulsion droplets was investigated by DLS and field emission scanning electron microscopy (FESEM) techniques. Thermal stability of these systems is also investigated. The antibacterial activity of the microemulsion phases was tested toward two bacterial strains such as Grampositive Bacillus subtilis and Gram-negative Escherichia coli by taking the diameter of the inhibition zone (“diz”). It is believed that the findings of this study are expected to improve the basic understanding of the formation and characterization of gemini surfactant-derived micelles and microemulsions.

conventional surfactant. Similar type of phase behavior was obtained for both the surfactants, but the results indicated that the length of spacers and also their chemical nature denote them as very effective parameters for tuning the thermotropic and lyotropic behaviors. Effect of spacer group on the microemulsion phase behavior was also reported by Chen et al.13 Formation of a middle phase microemulsion by dimeric anionic gemini amphiphile and butanol or hexanol as cosurfactant and comparison of its solubilization ability to the corresponding monomeric surfactants is available in literature.14 Alcohols (viz., medium chain length alcohols and/or branched short chain alcohols) can act as cosurfactants to tune different characteristic features of microemulsions, such as phase behavior, solubilization, and stability. Generally, cosurfactants mix with the surfactant molecules in aqueous medium and form a microemulsion in the presence of oil.15,16 In the presence of medium or branched short chain alcohols, the interfacial tension decreases and helps to form an isotropic microemulsion phase. Tieke and Dreja17,18 first reported microemulsion based on cationic gemini surfactant. Their focus was on the polymerization of styrene in the microemulsion media. Recently, the incorporation of charged and uncharged polymer and their influence on the phase behavior and structure of microemulsion has drawn considerable attention.19,20 Effect of molar mass on polymer-modified microemulsion was reported.21 Incorporation of the polymer in the individual microemulsion droplets or polymer induced cluster formation depends on the size of the microemulsion. Addition of oppositely charged polymers into water-in-oil (w/o) microemulsion systems increases the stability of the film formation of the surfactant. Such polymer-induced microemulsion can form a template for the synthesis of nanoparticle and drug delivery.22−24 In view of these, we investigate the formation of both micelles and microemulsions stabilized by a cationic gemini surfactant [tetramethylene-1,4-bis(dimethyltetradecylammonium bromide (14-4-14)] in the absence or presence of an anionic polymer, sodium carboxymethylcellulose (NaCMC), to unravel physicochemical and transport properties, thermal stability, microstructure, and morphology of these compartmentalized systems with a prospect of new applications. In this report, we have presented physicochemical behaviors of 14-4-14 in water at different temperatures via interfacial and bulk routes by employing tensiometry, conductometry, microcalorimetry and fluorimetry. During such study, coacervation was found through mutual interaction of cationic, 14-4-14 with anionic NaCMC, which was characterized by turbidity study. In a subsequent section, phase behavior of pseudoternary microemulsion systems consisting of (a) water/14-4-14 + cosurfactant (isopropanol, IP)/oil (isopropyl myristate, IPM), (b) water/14-4-14 + cosurfactant (butanol, Bu)/IPM, (c) aqueous solution of NaCMC/(14-4-14 + IP)/IPM, and (d) aqueous solution of NaCMC/(14-4-14 + Bu)/IPM, has been investigated by determination of the pseudoternary phase diagrams at different physicochemical conditions at a fixed temperature. Choice of IPM as oil is not arbitrary. Formation and characterization of biocompatible microemulsions involving IPM as oil are reported for pharmaceutical, drug delivery, and biological applications.25 Further, recent reports on the formation and thermodynamic properties of water-in-IPM microemulsions stabilized by single (CTAB, SDS)26 and mixed surfactants (SDS + Brij-58 or Brij-78)27,28 and also by APSA-80 (an useful pesticide spray adjuvant)29 are available in literature.

2. EXPERIMENTAL SECTION 2.1. Materials. 1,4-Dibromobutane and N,N-dimethyltetradecyl amine were purchased from Aldrich. Initially, 1,4-dibromobutane was refluxed with N,N-dimethyltetradecyl amine in dry ethanolic medium for 48 h, and after evaporating the solvent the crude product tetramethylene-1,4-bis(dimethyltetradecylammonium bromide) (14-414) was obtained. Then, recrystallization of cationic gemini surfactant was performed from ethanol/acetone mixture.30 Its structure was confirmed by 1H NMR spectroscopy. Again, no minima in surface tension (γ) versus log [14-4-14] plot also provide evidence of the purity of this gemini surfactant. Cetylpyridinium chloride (CPC) and sodium carboxymethylcellulose (NaCMC) were AR grade products from Aldrich and used as received. NaCMC has a viscosity-average molecular weight of 208 000 and average degree of substitution (i.e., average number of carboxymethyl groups per cellulose monomer) of greater than 0.4. Pyrene, purchased from Aldrich, was recrystallized three times from hexane−water and then used. Ethanol, AR grade (99.9%), was purchased from Chanhu Yangyuan Chemical. Isopropylmyristate (IPM) (≥ 98%) was purchased from Fluka (Switzerland). Both isopropanol (IP) and n-butanol (Bu) were AR grade products obtained from SRL (India) and Merck (India), respectively. Peptone (5 g/L) was purchased from S. D. Fine Chem. Ltd. (India), which can be applied as nutrient in microbiological experiments. Beef extract powder (3 g/L) was bought from Loba Chemie Pvt. Ltd (India). The specific conductance of double distilled water at 298 K used for all sample preparations was 2−4 μS cm−1. 2.2. Methods. 2.2.1. Conductometry. The conductivity measurements were taken in an Eütech (Singapore) conductometer with a cell of cell constant of 1 cm−1. The conductivity values of the microemulsions, with four different compositions at a fixed ω ([water]/[surfactant] = 15) were determined within the temperature range of 283−313 K at regular 2 K intervals. The detailed procedure of measurements was reported elsewhere.8,27,31 2.2.2. Tensiometry. Surface tension measurements were performed with a calibrated du Noüy tensiometer (Krüss, Germany) by the platinum ring detachment method. The detailed procedure is available in literature.8,29,32 The volume change during the addition of surfactant was very small. The measurements were duplicated to get reproducible results. The accuracy of determined values of surface tension (γ) was within ±0.1 mN m−1. 2.2.3. Microcalorimetry. Thermodynamic measurements have been performed with the help of an OMEGA isothermal titration calorimeter (ITC) of Microcal (Northampton, MA) at the temperature range of 298−323 K. Detailed discussion of the procedure is available in literature.8,32,33 The measurements have been done repeatedly for getting reproducible data. 12484


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2.2.4. Fluorimetry. Steady state fluorescence experiments were performed in a PerkinElmer LS55 fluorescence spectrophotometer with a quartz cell of unit path length. Pyrene was used as a probe. The temperature was maintained at 298 K. Details of the technique can be obtained in our previous publication.8 It is considered that the probe and the quencher (CPC) are located in the same environment of micelle and dispersed among aqueous and micellar pseudophases following Poisson statistics. 2.2.5. Turbidimetry. The turbidimetric measurements have been performed with the help of a Shimadzu, 1601 UV-VIS spectrophotometer (Japan) which operates in dual beam mode. The spectra were recorded in transmittance (% T) mode for accuracy, and the plot of (100 − %T) versus [14-4-14] has been drawn. Further description has been made in previous publications.32,34 2.2.6. Phase Diagram. For characterization of different phases in gemini-based microemulsions, the pseudoternary phase diagrams in the absence or presence of a polyelectrolyte were constructed using conventional titration technique under varied physicochemical conditions at isothermal condition. The pseudoternary systems consisting of different constituents are as follows: (a) water/(14-414 + IP)/IPM, (b) water/(14-4-14 + Bu)/IPM, (c) aqueous solution of NaCMC/(14-4-14 + IP)/IPM, and (d) aqueous solution of NaCMC/(14-4-14 + Bu)/IPM. A definite amount of surfactant with cosurfactant (IP or Bu) in fixed weight ratio was mixed in various amounts of oil, IPM, in sets of dry test tubes. Subsequently, water (in (a) and (b)) or aqueous solution of polymer (in (c) and (d)) was gradually added with thorough mixing with a vortex shaker through visual observation at 298 K. The resulting mixtures were settled after reaching the equilibrium condition, and then the stability of different phases was studied with time. The compositions of different components (wt %) of microemulsions were evaluated, and the plot of the data was drawn on a triangular coordinate (Gibbs triangle) for preparing the phase diagram.28 2.2.7. Dynamic Light Scattering (DLS). The hydrodynamic diameter of the microemulsion droplets was evaluated at a 90° angle with a Malvern Zetasizer Nano ZS (U.K.) DLS instrument with a He− Ne laser having a wavelength of 632.8 nm and a Peltier attachment. Before the experiment, three times filtration of the microemulsion solutions was done through a cellulose acetate filter paper having a pore size of 0.45 μm. 2.2.8. Field Emission Scanning Electron Microscopy (FESEM). A field emission scanning electron microscope (FESEM, HITACHI S4800) was used to study the morphology of the microemulsion system. High vacuum (∼10−7 Torr) field emission setup has been applied to deposit the thin film of microemulsion on the glass plates. The details of the technique are available elsewhere.35 2.2.9. Antibacterial Activity. The antibacterial properties of microemulsions were investigated using two rod-shaped bacterial strains, Bacillus subtilis (Gram-positive, obtained from soil) and Escherichia coli (Gram-negative, available in the lower intestine of warm-blooded organisms), in the case of three different compositions: (1) water/(14-4-14 + IP, 1:2)/IPM (ω = 15), (2) water/(14-4-14 + IP, 1:2)/IPM (ω = 25), and (3) water/(14-4-14 + Bu, 1:2)/IPM (ω = 15). The procedure of investigation details was stated in our earlier publication.36

Figure 1. (a) Tensiometric, (b) conductometric, and (c) microcalorimetric profiles of 14-4-14 in water at 298 K.

measurements and microcalorimetry. The profile of specific conductivity (κ) as a function of concentration of 14-4-14 in water is shown in Figure 1b. The cmc of the gemini in aqueous medium is found to be lower than that of the corresponding monomer TTAB. The cmc of the gemini and its corresponding monomer TTAB are 0.153 and 3.75 mM,37 respectively, in water at 298 K. The cmc was determined from the inflection point, and the degree of ionization (α) was determined from the ratio of post- and premicellar slopes. The cmc and degree of ionization increase gradually with increasing temperature. The effect of temperature on micellization can be interpreted by two opposite effects. The water structure surrounding the hydrophobic chains can be destroyed when the temperature of the system increases, resulting the inhibition of micellization. Again, the degree of hydration around the hydrophilic groups is decreased with increase in temperature, favoring micellization. In our investigation, the second effect is suppressed by the first one in the temperature range of 298−323 K.8,38,39 The values of cmc and α obtained are quite close to literature data40 (Table 1). The cmc of 14-4-14 determined from microcalorimetry is higher compared to that determined from conductometry and tensiometry. The cmc values obtained from microcalorimetric method show a similar trend with temperature as that of conductometric values. Analysis of the effect of temperature on cmc and α can be done from the data of the corresponding Gibbs free energy of micellization (ΔG°m) of 14-4-14 determined from the following equation41 ΔGm° = 2(1.5 − α)RT ln Xcmc

3. RESULTS AND DISCUSSION 3.1. Solution Behavior of 14-4-14 (Interfacial and Bulk). The amphiphilic behavior of 14-4-14 in water was determined by tensiometry, conductometry, and fluorimetry methods. 14-4-14 is highly soluble in water. The critical micellar concentration (cmc) of 14-4-14 was determined using tensiometry from the sharp break in the surface tension (γ) versus log [14-4-14] plot at 298 K [Figure 1a]. The break point corresponds to the threshold concentration of the amphiphile where the air/solution interface is saturated by the monomer. The cmc values of the surfactant at different temperatures in the range of 298−323 K have been evaluated by conductance

(1)

where Xcmc denotes the cmc in mole fraction unit, and R and T have their usual meanings. The Gibbs free energy of micellization is negative and becomes less negative with increasing temperature, indicating that micellization becomes less favorable with increasing temperature. The corresponding enthalpy change for the micellization (ΔH°m) is obtained from microcalorimetry, denoting that the micellization process is exothermic (Figure 1c). It can be proposed that negative values of ΔH°m indicate evidence of London-dispersion interaction which represents the synergistic interaction toward micellization.37 Table 1 denotes that with increasing temperature such interaction has a significant effect on micellization.37 From the 12485


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Table 1. Determination of cmc, Degree of Ionization (α), and some Thermodynamic Parameters (ΔG0m, ΔH0m and ΔS0m) of 14-414 in Water cmc (mM)a T/(K) 298 303 308 313 318 323 a

ST

microcal.

α

−ΔGm ° /kJ mol−1

−ΔHm ° /kJ mol−1

−ΔSm ° /kJ K−1 mol−1

0.157 (0.161)

0.240 0.240 0.240 0.220 0.250 0.270

0.33 0.35 0.37 0.41 0.43 0.45

74.22 73.77 73.34 71.61 71.14 70.63

16.21 19.50 21.90 24.90 28.61 30.80

0.195 0.179 0.168 0.149 0.134 0.123

cond. 0.153 0.164 0.174 0.183 0.192 0.201

(0.146) (0.151) (0.172) (0.176) (0.175)

The cmc values are taken from refs 8 and 40 (parentheses).

data of ΔGm ° and ΔHm ° , we can calculate the entropy of micellization (ΔS°m) by using the equation41 ΔSm° =

(ΔHm° − ΔGm° ) T

(2)

The values of ΔG°m, ΔH°m, and ΔS°m at different temperatures are listed in Table 1. With increase in temperature, the enthalpy of micellization becomes more negative and the entropy of micellization becomes less positive, indicating an enthalpy− entropy compensation for the process of micellization of 14-414 in water.41 Literature shows that ΔGm ° values do not change significantly, whereas ΔHm ° and ΔSm ° values follow the same trend with our results (Table 1).40 The values of different interfacial properties, the maximum surface excess (Γmax), the minimum surface area per molecule (Amin), and the surface pressure at the cmc (Πcmc), at 298 K were obtained by using the following equations31 and are listed in Table S1 in the Supporting Information. Γmax = −

A min =

⎡ dγ ⎤ 1 ⎢ ⎥ 2.303nRT ⎣ d log C ⎦

1018 NA Γmax

Πcmc = γsol − γcmc

Figure 2. Profile of ln(I0/I) versus [Q] of 14-4-14 in water. Inset: corresponding basic spectra.

4-14 in aqueous medium where only one break is observed. However, after gradual addition of [14-4-14] to aqueous NaCMC solution, four distinct regions are evidenced in the tensiometric profile (Figure 3a). At region I, with low [14-4-14]

(3)

(4) (5)

where NA, γsol, and γcmc are Avogadro’s number, surface tension of pure water, and surface tension of surfactant solution at the cmc, respectively. R and T have their usual meanings. The aggregation number of the surfactant in water is obtained by using the following equation:31 Nagg[Q ] ⎛I ⎞ ln⎜ 0 ⎟ = ⎝ I ⎠ [S] − cmc

(6)

where I0 and I denote the fluorescence intensities of 14-4-14 without and with quencher CPC, respectively. [S] and [Q] are the concentrations of 14-4-14 and CPC, respectively. The aggregation number, Nagg, of 14-4-14 can be determined from the fitting of the fluorescence intensity data at different concentrations of CPC (Figure 2). Nagg of 14-4-14 in water is 24 which is much lower than that of TTAB (60).38 3.2. Interaction of 14-4-14 with NaCMC in Water. 3.2.1. Tensiometry. For polymer−surfactant interaction, the interfacial and bulk behaviors of the amphiphile can be revealed distinctly from tensiometry. NaCMC has no surface activity below 0.7 g %.42 Concentrations of NaCMC used in our study are 0.005, 0.0075, and 0.01 g % which is much lower than the reported value. Figure 1a shows the tensiometric profile of 14-

Figure 3. (a) Tensiometric, (b) conductometric, and (c) turbidimetric profiles as a function of 14-4-14 due to its interaction with NaCMC.

(from 0.0016 to 0.026 mM for 0.0075 g % NaCMC), a shallow decrease in slope of γ versus log [14-4-14] profile was found. This implies binding of cationic part of the 14-4-14 with anionic sites of the NaCMC. At a lower [14-4-14] than its cmc, the surfactant molecules started to form small aggregates up to a certain concentration, called critical aggregation concentration (cac = 0.027 mM for 0.0075 g% NaCMC). In our previous report, the cac for the interaction of CTAB with 0.0075 g % 12486


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Figure 4. Pseudoternary phase diagram of (a) water/IPM/14-4-14 + IP system, (b) water/IPM/14-4-14 + Bu system, (c) aqueous NaCMC/IPM/ 14-4-14 + IP system, and (d) 0.01 g % NaCMC/IPM/14-4-14 + Bu system at 298 K.

NaCMC was 0.047 mM which was ≈21-fold lower than the cmc (1.00 mM).32 Similar types of polyion-oppositely charged amphiphile interactions, for example, interaction of DTAB with sodium carboxymethylcellulose, poly(acrylic acid), and methacrylic acid/ethyl acrylate copolymer, were reported earlier by Guillot et al. and Wang and Tam.42,43 In the region after the cac (region II), the micelles started to bind along the chain of the polymer and prevented movement of the cationic part of the surfactant toward the surface, resulting a constant γ value in the range of 0.027−0.063 mM (for 0.0075 g % NaCMC). At this region, the conformation of the polymer changed.34 This process is continued up to Cs, the polymer saturation concentration (0.063 mM for 0.0075 g% NaCMC). At the concentrations ≥ Cs (region III), strong interaction occurs between polymers and micellar aggregates, resulting in escape of the polymer−surfactant complex from the interface and it being buried in the bulk media.34 Upon subsequent addition of surfactant, monomers are adsorbed at the air−solution interface, resulting in a decrease in γ, and the process was completed at Cf (extended cmc). Beyond this point (region IV), free micelles are formed in solution and gamma (γ) values remain unaltered on further addition of surfactant monomers. The values of Cs and Cf increase with increasing concentration of polymer, due to the mass balance consideration,34 as shown in Figure 3(a. The results are given in Table S2 in the Supporting Information. 3.2.2. Conductometry. The bulk interaction between 14-414 and NaCMC can be determined from measurement of conductivity of the mixed solutions as a function of [14-4-14], and the results are presented in Figure 3b. It can be seen that the conductometric profile of the mixtures 14-4-14 and NaCMC at different contents have evidenced two distinct breaks, which fairly matched with the Cs and Cf values obtained from tensiometry (Figure 3a). But no breaks were found near the cac. Initially, the conductance values increase linearly with the increase in [14-4-14], and after Cs the slope increases. Near Cs,

a cloudy solution or turbidity starts indicating that binding and formation of the aggregates has been initiated. Due to the mutual interaction at all concentrations of NaCMC, the Cf values are higher than the values obtained in absence of the polymer. Thus, it indicates higher propensity toward aggregation of the surfactant with polymer molecules. With increasing polymer concentration, the Cf values also increase because of higher availability of reactive binding sites for the surfactant monomers.44 Hence, more surfactants are required to bind with the polymer sites completely. After Cs , coacervation takes place and the system becomes turbid.34 This colloidal solution has evidenced enhanced conductivity and thus increases in slope of the post Cs region. Beyond Cf, free normal micelles start to form with a fair degree of counterion binding. At higher [14-4-14], the coacervates obstruct the ion transport, and thus, a reduced slope of the post Cf region is obtained. The degree of ionization α2 increases with increasing concentration of polymer. 3.2.3. Turbidimetry. Interaction of 14-4-14 with anionic polymer (NaCMC) produces turbidity due to coacervation. The turbidimetric plot for (14-4-14)−NaCMC interaction in water at various concentrations of polymer is presented in Figure 3c, which shows three inflection points, T1, T2, and T3. The values of T1 and T3 are close to Cs and Cf, respectively in the tensiometric plot. Beyond T1, the turbidity is clearly visible and the values increase sharply. The inflection point corresponds to the completion of adsorption of surfactants on polymeric sites. T2, the turbidimetric maximum, is quite lower than the Cf value obtained tensiometricaly. T3 values increase with increasing concentration of polymer. The turbidity slightly reduces beyond T3. The results are given in Table S3 in the Supporting Information. Here, free micelle is formed, solubilizing the (14-4-14)-NaCMC coacervates, and turbid solution is converted partially to clear one.45 3.3. Formation and Characterization of w/o Microemulsion with Gemini Surfactant 14-4-14. 3.3.1. Phase Behavior of Water (or NaCMC)/14-4-14/IP or Bu/IPM 12487


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Microemulsions. In this report, the phase behavior of the pseudoternary gemini-based microemulsions (14-4-14) in the absence or presence of a polyelectrolyte has been investigated to underline the extent of variation in single phasic microemulsion zone as a function of surfactant/cosurfactant mass ratio, type of cosurfactant, and content of polyelectrolyte at a constant temperature, in view of possible uses of these surfactants in formulations.11−24 Limitation of these phase diagrams is that detailed mapping of different phases (such as solid-liquid, viscous, gel, liquid crystalline etc.) of gemini surfactant-containing microemulsions that are valued from the viewpoint of expected behavior of a formulation in its conditions of application are not depicted. These systems are mainly composed of water, IPM, and [14-4-14] as polar, nonpolar, and surfactant components, respectively, along with varying cosurfactants, for example, IP or Bu (at different mass ratios with [14-4-14]) in the absence and presence of a polyelectrolyte additive NaCMC at 298 K and are presented in Figure 4. The percent areas of different phases at a fixed ω are presented in Table S4 in the Supporting Information, and distinct variations in phase boundaries between clear and turbid zones are presented in Figure 4, which indicates system’s specificity. Figure 4a depicts a phase map of the pseudoternary system [water/(14-4-14 + IP)/IPM] with varying surfactant/ cosurfactant mass ratios as 1:1, 1:2, and 1:4. With increasing IP proportion, single phase clear microemulsion zone decreases from 34.5% to 31.7%. Similar observations have been found when Bu was employed as cosurfactant (Figure 4b). However, microemulsion zones have been reduced from 14.6% to 9% with increasing surfactant/cosurfactant mass ratios from 1:2 to 1:6 for the Bu-stabilized system. It may be due to the lowering in mutual solubility of the components with an increase in alkanol proportion, which is well supported by the results obtained by Chen et al.46 According to them, greater interfacial flexibility played a significant role for generating a larger monophasic domain with lower amount of cosurfactant. Hence, it can be summarized that the single phase clear microemulsion zone in system (a) is almost 2-fold larger than that of system (b). This is because IP facilitates the bending of the IPM/water interfacial film more as compared to Bu, indicating superior interaction with 14-4-14 due to special structural characteristics of IPM, a polar lipophilic oil, compared to linear hydrocarbons (discussed in subsequent subsection 3.3.3). At this juncture, further studies are required to understand precisely the effect of alkanol chain length and it’s polarity on bending of the oil/ water interface. Thus, increase in content of IP, in system (a), facilitates to achieve more negative curvature of the interfacial film composed of 14-4-14 + IP, which results in higher microemulsion zone in w/o microemulsion at higher proportions of IP (depicted at the left-hand part of the phase diagram). On the other hand, smaller microemulsion zone (at right-hand portion of the phase diagram) have been observed with higher cosurfactant proportions at o/w microemulsion region. But, the competitive bending in two different regions, leads to an overall decrease in microemulsion zone of the system (a) with increase in cosurfactant proportions. Further, it is evident from the phase diagrams that the microemulsion zone started to form at 30% 14-4-14 + IP (1:1) at the right-hand corner of the phase diagram which corroborates well with the result reported by Chen et al.13 Addition of the polymer NaCMC to the microemulsion system exhibits (c); a minor decrease in the microemulsion zone as compared the absence of it has been

observed (Figure 4c and Supporting Information Figure S1a). On the other hand, the microemulsion zone increased when NaCMC was added to microemulsion system (d). In system (c), the increase in cosurfactant proportion increases the clear zone with a fixed [NaCMC] of 0.0075 g % (Supporting Information Figure S1b), but at a fixed surfactant versus cosurfactant ratio (1:1 for system (c) and 1:2 for system (d)), an increase in [NaCMC] decreases the clear zone (Figure 4c). Strong interaction between oppositely charged polymer and surfactant reduces the net charge of the formed complex, and subsequently, hydrophobicity and interpolymer attraction increase. After further addition of [NaCMC], binding with the surfactant increases, leading to precipitation. Under these constraints, the microemulsion is not formed; instead, a surfactant−polymer complex is formed as a result of visible observation of phase separation. The phase behaviors from aqueous NaCMC/IPM/14-4-14 + Bu systems (Figure 4d) are quite different from those of NaCMC/IPM/14-4-14 + IP systems. 3.3.2. Dynamic Light Scattering (DLS). The DLS technique is applied to measure the distribution of size and the interaction between the microemulsion droplets in the organic phase. Regarding this, Stokes−Einstein equation helps to correlate the hydrodynamic diameter (Dh/nm) of droplets with viscosity of solvents (η), diffusion coefficient (D/m2 s−1), and temperature (θ) of the medium. That equation is as follows: Dh =

kBθ 3πηD

(7)

where kB is the Boltzmann constant. Here, DLS method was employed to determine the values of Dh of water or 0.01 g % NaCMC/(14-4-14 + IP)/IPM or Bu microemulsions at different 14-4-14/cosurfactant ratios (1:1, 1:2, 1:4, and 1:6), and water content, ω (8, 15, 20, and 42). Table S5 in the Supporting Information represents the values of Dh with polydispersity index (PDI) of selected samples with different compositions. Polydispersity index has significance to measure structural characteristics of the system using DLS method. For a monodisperse sample, the PDI value is less than 0.08, and for a midrange polydispersity this value is 0.08−0.7.47 The range of PDI values of the studied systems (Table S5, Supporting Information) is mainly 0.15−0.35, signifying a fairly polydispersed system. Such type of report of fair polydispersity in the case of w/o microemulsions is available in the literature.48 Size distributions of different microemulsion systems having different ω and with different 14-4-14/IP or Bu mass ratios are shown in Figure 5 and Supporting Information Figures S2 and

Figure 5. Size distribution of water/IPM/14-4-14 + IP at a fixed 14-414/IP ratio (1:2) with different ω. Inset: Size distribution of water/ IPM/14-4-14 + IP at a fixed ω (15) with different 14-4-14/IP ratios. 12488


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obtained when IP is replaced with Bu at a fixed ratio of 14-414/IP and ω = 15. 3.3.3. Morphology of Microemulsions without or with Polymer. The morphology of the three microemulsion systems without or with polymer (NaCMC) were investigated by employing FESEM and are presented in Figure 6. Micro-

S3. For water/(14-4-14 + IP)/IPM and water/(14-4-14 + Bu)/ IPM systems, the diameter of the droplet increases with increasing ratio of 14-4-14/IP or Bu at a fixed ω = 15. The average droplet dimension ranged between 6.23 and 17.43 nm and 10.44 and 18.50 nm, respectively, which indicates that cosurfactant (IP or Bu) plays different roles to underline droplet size of these microemulsions. It was reported that, in the presence of alcohol as a cosurfactant, the interfacial free energy vis-à-vis interfacial tension reduces due to its adsorption into the amphiphilic interface and this is more prominent for short chain alcohols.49 As a result, various shapes of the interfacial film are observed for the formation of a balanced microemulsion. More precisely, a short chain alcohol as cosurfactant is more effective for such a job. 1-Butanol having its straight chain can be easily incorporated into the interfacial layer following the reduction of the interfacial tension.49 Again, the hydroxyl group in isopropanol is located toward the aqueous environment of the surfactant film, and the branched isopropyl chain is likely to be incorporated in the amphiphilic film. However, because of this branching, IP might be more difficult to accommodate in the interface. Owing to the difference in steric constraints arising out of the chemical structures (shorter and wider shape of branched hydrocarbon chain of IP than Bu) of both cosurfactants, IP can better stabilize the interfaces of the microemulsion droplets.50 So, the droplet size of the microemulsion is larger for IP than Bu as cosurfactant. The same type of report is available in the literature in the case of water/AOT/ alcohol (ethanol−pentanol)/IPM reverse micelles.51 Further, at a fixed 14-4-14/IP or Bu ratio of 1:2, droplet size decreases moderately from 18.40 to 10.10 nm and 18.62 to 7.30 nm, where ω values are in the range 8−42 and 8−20, respectively. With increase in water content (ω), more and more water molecules are solubilized in the microemulsion core instead of accommodating near the micellar core. Further, increase in water content leads to more favorable H-bond interaction with polar head groups of 14-4-14. Hence, strong H-bonding interaction with water promotes the polar head groups of 14-4-14 to reside much nearer to the micellar core, which is manifested by lowering of hydrodynamic diameter of the microemulsions.52 Gao et al.53 reported the same type of phenomena during the addition of water in 1-butyl-3methylimidazolium tetrafluoroborate, [bmim][BF4]/Triton X100/cyclohexane microemulsions. Further, it is interesting to note that the droplet size increases with increasing ratio of 144-14/IP or Bu (1:1−1:6) for both IP and Bu containing microemulsion systems at a fixed ω. Generally, droplet size of water-in-oil microemulsions decreases with increasing concentration of alkanol, mainly for long chain alkanol as cosurfactant.51 However, a plausible explanation for this reverse trend in the present report is due to the solubilization capacity of alcohols in the confined environments. Both IP and Bu penetrate easily into the micellar interface due to their small molar volumes51 and shorter alkyl side-chain length of IP (branched isopropyl) and Bu (linear butyl). Their polar head (hydroxyl group) can be immersed in the water pool, or can stay in the surroundings of amphiphilic headgroup area or palisade layer of the interface. As a result, the micellar interface will be more fluidic with increase in their concentration due to the expansion of the interface for both of these solubilization regions.54 Consequently, droplet size increases for both systems. Further, the dimension of the droplet decreases with increasing polymer concentration, but the reverse trend is

Figure 6. FESEM images of (A) water/14-4-14 + IP (1:2)/IPM, (B) 0.01 g% NaCMC/14-4-14 + IP (1:2)/IPM, and (C) 0.01 g% NaCMC/14-4-14 + Bu (1:2)/IPM systems (at ω = 15) and 298 K.

emulsion of water/(14-4-14 + IP) (1:2)/IPM system (at ω = 15) at 298 K exhibits a fractured surface (Figure 6A) which increases with the addition of 0.01 g % NaCMC (Figure 6B). On the other hand, the morphology of microemulsion system containing 0.01 g % NaCMC looks like a large scale mesosphere, which is composed of multiple closely winded nanoslices. In a w/o microemulsion system, the droplets are spherical and the radius of the water pool is linearly correlated with the water content. Figure 6C depicts the morphology of the NaCMC (0.01 g%)/(14-4-14 + Bu) (1:2)/IPM microemulsion system, which is quite similar to the spheroid morphology of the 3D-Ni(OH)2 microemulsion of water/ cetyltrimethylammonium bromide (CTAB).55 3.3.4. Temperature Induced Percolation of w/o Microemulsion in Absence and Presence of Polymer. Generally, conductance measurement is applied to determine the interaction between microemulsion droplets and their stability.56 As such, a process of percolation signifies the appearance of large synergistic interaction between microemulsion droplets. Electrical conductivity (k) of water or NaCMC (0.01 g%)/(14-4-14 + IP or Bu, 1:2 wt %)/IPM microemulsion has been measured as a function of temperature (θ = 283−313 K) at a fixed ω (equal to 15), and the results are presented in Figure 7 and Supporting Information Figure S4. Results show the gradual ascending of the conductance value with increasing temperature for these systems. This may be owing to the “transient fusion−mass transfer−fission” process involving 14-4-14, because of the movement of ions among the droplets in these w/o microemulsion systems.33,49 However, a nonlinear sharp increase in conductivity has been evidenced above a certain θ in all systems, as observed from Figure 7 and Supporting Information Figure S4. The sharp enhancement in conductivity at higher temperature may be due to temperatureinduced percolation in conductance in such systems.56 12489


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k = P(θ − θt)n

(8)

where k and θ are specific conductance and temperature, respectively. n and P are the exponent and constant terms, respectively. Least-squares analysis of the profile of ln k versus ln(θ − θt) helps to evaluate n and ln P from the slope and the intercept, respectively. In the present study, the values of ln P are positive and the range is 3.8−6.3. The values of n vary from 0.09 to 0.04 (without additives) and 0.06 to 0.14 (in the presence of 0.01 g % NaCMC). Again, both P and n values are influenced by the amount of water in the system, composition of the studied systems, nature of the additives and oil added affecting the clustering of droplet, and transfer of ion during dynamic percolation. The clustering process of percolation in conductance can be determined by activation energy, Ep, which in mixed reverse micelles can be evaluated by an Arrhenius type equation48

Figure 7. Temperature induced percolation profile of water/14-4-14 + Bu, 1:2 wt % /IPM microemulsion system at ω (equal to 15). Inset: Temperature percolation profile of aqueous 0.01 g % NaCMC/14-414 + Bu, (1:2 wt %)/IPM microemulsion systems at the same ω.

According to previous reports, percolation in conductance shows that the surfaces of the microemulsion droplets are much fluidic to coalesce during the collisions, resulting in material exchange fusion at fixed water content with progress of temperature. Percolation denotes the increase in size of the droplet, synergistic interaction between droplets, and rate of exchange of materials between the droplets with increasing temperature.56 The temperature-induced percolations of all five systems were sigmoidal in nature. Thus, sigmoidal Boltzmann fitting (SBF) procedure is used to evaluate the threshold temperature of percolation (θt) for each case. Results are presented in Table 2. Bu has been found to be a better percolating cosurfactant than IP for microemulsion system in the absence of NaCMC. In other words, θt for the water/(14-414 + Bu)/IPM system (292.27 K) is lower compared to that for the water/(14-4-14 + IP)/IPM system (297.24 K) at constant ω (= 15). This difference in θt for IP and Bu may be due to their different structures and resulted in different capabilities of stabilization toward the curved interfaces along the barriers between the continuous and dispersed pseudophases.57 Consequently, the randomness with higher kinetic energy due to thermal motion is possible for Bu containing systems and resulted in lower θt compared to IP containing systems. Figure 7 (inset) represents the data of the influence of additives on the percolation of conductance in water/(14-4-14 + Bu or IP)/IPM systems at ω = 15 versus temperature. Figure S4 (Supporting Information) and Table 2 show that addition of 0.01 g % NaCMC reduces θt for water/(14-4-14 + IP)/IPM system, whereas reverse effect is evidenced for water/(14-4-14 + Bu)/IPM system. The influence of NaCMC similar to other organic molecules58 is assumed of two different types: (a) the incorporation of polymer into the amphiphilic film and (b) the substitution of water molecules from the surface.49 After completing the percolation process, this phenomenon at a constant ω obeys the scaling law of the following form

⎡ ⎛ E ⎞⎤ k = A exp⎢ − ⎜ P ⎟⎥ ⎣ ⎝ RT ⎠⎦

(9)

where A is a constant. The slope of the linear portion of the profile of ln k versus (1/T) provides the value of Ep. The results are depicted in Table 2. Without and with additive (NaCMC), the values of Ep vary from 5.33 to 9.20 kJ mol−1. The reported value of Ep varies from 338 to 876 kJ mol−1 for pure AOT/Brij56 mixed reverse micelles having different hydrocarbon oils (C6−C10).59 Low and moderate values of Ep (176 and 397 kJ mol−1) have also been reported there for AOT/Brij-56 and AOT/Brij-58 respective systems stabilized in IPM. Also, the Ep value was found to be in the range of 70−330 kJ mol−1 for water/AOT/Tween-85/EO or IPM or IPP systems without and with additives.48 In the present paper, the observed values of Ep are lower than that of any studied hydrocarbon oil and conventional AOT based surfactant systems. In this report, both gemini surfactant (14-4-14) and polar lipophilic oil (IPM) are structurally different from their physicochemical properties and also from chemical structures of that of conventional anionic surfactant and linear hydrocarbon oils, respectively, and hence, the percolation of conductance in the studied systems is totally different compared to other systems. 3.3.5. Thermal Stability of Microemulsions. The thermal stability, that is, temperature sensitivity of phase behavior of all microemulsion systems having the same composition as adopted in DLS measurements, was examined in the temperature range of 298−353 K by 2 K increase in each step. All the systems were stable and transparent up to 353 K. Hence, it can be concluded that all the systems are on the whole temperature insensitive in the studied temperature range. 3.3.6. Antibacterial Activity of Microemulsions. The results of antibacterial activity of 14-4-14-based microemulsions of three different compositions, namely, (1) water/(14-4-14 + IP,

Table 2. Threshold Temperature of Percolation (θt), Scaling Equation Parameters (ln P and n), and Activation Energy (EP) of Different Microemulsion Systemsa

a

systems

θt/K

ln P

n

EP/kJ mol−1

water/14-4-14 + IP/IPM (ω = 15) water/14-4-14 + Bu/IPM (ω = 15) 0.01 g % NaCMC/14-4-14 + IP/IPM (ω = 15) 0.01 g % NaCMC/14-4-14 + IP/IPM (ω = 25) 0.01 g % NaCMC/14-4-14 + Bu/IPM (ω = 15)

297.24 292.27 296.09 297.04 296.33

5.17 3.83 4.90 6.25 5.96

0.09 0.037 0.064 0.136 0.097

6.29 5.33 6.18 7.27 9.20

The ratio of 14-4-14:IP or Bu is 1:2 for all the above microemulsion systems. 12490


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phase diagrams that the microemulsion system composed of water/14-4-14 + IP/IPM offers a large isotropic region compared to the water/14-4-14 + Bu/IPM system, and hence, IP derived microemulsions depict larger water solubilization capacity than Bu-derived systems. The isotropic region throughout the phase diagram decreases with increasing concentration of either IP or Bu. Addition of NaCMC (of 0.0075% and 0.01%) to the IP derived microemulsion system leads to a minor decrease in the microemulsion zone. On the other hand, the microemulsion zone mildly increased when NaCMC was added to Bu derived systems. DLS measurements demonstrate that the droplet size of IP derived systems is larger than that of Bu derived systems at comparable conditions. Further, when ω increases, size of the microemulsion droplet decreases and this is more pronounced in the Bu system than in the IP system at a fixed 14-4-14/IP or Bu ratio of 1:2. Droplet size increases with increasing ratio of 14-4-14/IP or Bu. Droplet size decreases with increasing [NaCMC], but the reverse trend is observed when IP is replaced by Bu at a fixed ratio of surfactant/cosurfactant (1:2) and ω =15. The Bu derived system was better at percolating than the IP derived system, whereas the reverse trend was observed in the presence of NaCMC at comparable conditions. However, the presence of NaCMC decreases the energy of activation (Ep) in the IP derived system, whereas the reverse trend is observed for the Bu derived system at comparable conditions. FESEM reveals different morphologies depending upon the type of cosurfactant as well as polymer and its concentration for these microemulsion systems. Microemulsions have been found to be temperature insensitive in the studied temperature range (298− 353 K). These systems show good antimicrobial activity against the strains of B. subtilis (Gram-positive) and E. coli (Gramnegative) bacteria, and inhibitory effect depends on ω, type of cosurfactant, and bacterial strains.

1:2 wt %)/IPM (ω = 15), (2) water/(14-4-14 + IP, 1:2 wt %)/IPM (ω = 25), and (3) water/(14-4-14 + Bu, 1:2 wt %)/IPM (ω = 15) toward the strains of Gram-positive B. subtilis and Gram-negative E. coli bacteria are shown in Figure 8

Figure 8. Antimicrobial activity of (A) water/14-4-14 + IP (1:2, wt %)/IPM, ω = 15, (B) water/14-4-14 + Bu (1:2, wt %)/IPM, ω = 15, and (C) water/14-4-14 + IP (1:2, wt %)/IPM, ω = 25 against E. coli.

and Supporting Information Table S6. However, it can be noted that no antibacterial activity was observed in the case of both pure gemini surfactant, 14-4-14 and IPM. When aqueous NaCMC is used in the same system instead of water, no change is observed in the inhibition zone. The diameter of inhibition zone (diz) produced by system (1) against E. coli is 13 mm (Figure 8A), which decreases to 10 mm for the same system at ω = 25 (system 2) (Figure 8C). On the other hand, diz of system (3) is 11 mm (Figure 8B), which is lower than that of system (1). These results indicate that the diz decreases with an increase in ω and type of cosurfactant at comparable compositions against E. coli (Gram-negative) bacteria. Further, from comparative analysis of the results, it can be seen that diz produced by water/(14-4-14 + IP)/IPM system increases from 8 to 10 mm with increasing ω from 15 to 25 against B. subtilis (Supporting Information Figure S5). Hence, it can be inferred that inhibitory effect of these microemulsions depends on ω, type of cosurfactant, and bacterial strain. It is quite likely that the interaction between microemulsion and bacterial membrane decreases surface hydrophobicity of the bacterial cell, resulting in leakage of cytoplasmic constituents and quick loss of bacterial validity.60

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ASSOCIATED CONTENT

S Supporting Information *

Interfacial parameters for micellization and interaction characteristics of 14-4-14 with NaCMC obtained from conductometric, tensiometric and turbidimetric method, percent area of different microemulsion phases, droplet size distributions of the corresponding microemulsion systems, results of antimicrobial activity test, and their corresponding graphs. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

4. CONCLUSIONS The present report focuses on systematic investigation on both the micellization and microemulsion formation of a cationic gemini surfactant, 14-4-14, in the absence and presence of hydrophobically modified polyelectrolyte cellulose, NaCMC, of different contents at different physicochemical conditions. In aqueous solution, it shows very low cmc and high surface activity compared to the conventional surfactant. In the case of interaction of 14-4-14 with NaCMC, both Cs and Cf increase with increasing concentration of polymer. Coacervation is also observed in the system without any additives. It reveals from

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.D. thanks CSIR, Government of India, New Delhi, for Senior Research Fellowship. We also thank Prof. S. C. Bhattacharya, Department of Chemistry, Jadavpur University for DLS measurements and Dr. Dipankar Haldar, Department of Food Technology and Bioengineering, Jadavpur University for antimicrobial activity. 12491


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