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Surfactant and Gelation Properties of Acetylsalicylate Based Room Temperature Ionic Liquid in Aqueous Media Nandhibatla V. Sastry, and Dipak K. Singh Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02074 • Publication Date (Web): 19 Aug 2016 Downloaded from http://pubs.acs.org on August 22, 2016

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Surfactant and Gelation Properties of Acetylsalicylate Based Room Temperature Ionic Liquid in Aqueous Media Nandhibatla V Sastry* and Dipak K Singh

Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar – 388120, Gujarat, India

* Correspondence author Email: [email protected]

ACS Paragon Plus Environment

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ABSTRACT: An amphiphilic room temperature ionic liquid (RTIL) containing acetylsalicylate anion of type, 1-dodecyl-1-methylpiperidinium acetylsalicylate, [C12mpip][ AcSa] is synthesized from precursor, [C12mpip][Cl] by ion exchange process. The sample is characterized and its surface active and aggregation behavior in water has been studied and explained. The critical aggregation concentrations (CAC) are determined by variety of methods namely electrical conductivity, surface tension, steady state florescence and isothermal titration calorimetry (ITC) at different temperatures. As compared to its precursor, [C12mpip][AcSa] has low CAC values indicating enhanced favorable interactions between the [alkylmpip]+ cation … bulky [AcSa]- anion and also hydrogen bonding of both the ions with water. The free energy of aggregation, ∆G0a is always negative and while both enthalpy and entropy of aggregation drive the aggregation process. The micelle–like aggregates are ellipsoidal in shape. The aggregation numbers are determined from translational diffusion coefficients and florescence quenching measurements. Aggregates of [C12mpip][AcSa] are larger than that of its precursor IL with chloride anion. Therefore it is evident that the close interactions between the ion pairs of [C12mpip] +….[AcSa] - facilitate packing of more molecules in an aggregate. The steady state and oscillatory rheology measurements in aqueous solutions consisting mixtures of [C12mpip][AcSa] and sodium salicylate (SS), an hydrotope additive were carried out. The analysis of zero shear viscosity and moduli properties as a function of concentration and temperature reveals that the addition of SS promotes the growth of small ellipsoid aggregates into large worm – like structures with a typical viscoelastic gel behavior. The moduli properties vs. temperature profiles are complex and no hysteresis was produced in heating and cooling modes suggesting the thermoirreversibile and complex nature of the network structures. The release of the acetylsalicylate anion from the gels could be triggered by simple dilution and the release occurs due to surface erosion and demicellization.


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1. INTRODUCTION Room temperature ionic liquids (RTILs), commonly referred as room temperature molten salts have been attracting significant attraction and attention due to their unique physicochemical properties such as high viscosity, high electrical conductivity, high thermal and chemical stability and negligible volatility1,2. RTILs are also called as designer solvents cations and anions cam be combined in different combinations to develop ILs with desired properties for a particular application3. The applications of RTILs as catalysts4, green solvents5, in electrochemical and biological processes,

metal extraction, separation

techniques and other chemical analysis are well described and discussed in the literature6-13. The alkylimidazolium cations such as 1-alkyl-3-methylimidazolium ([Cnmim]+), where n ≥ 4, possess an inherent amphiphilicity and therefore ILs based on this cation exhibit novel surfactant properties such as surface adsorption at air/water interface, solubilization, wetting and dispersion etc14-18 and hence such ILs are also known as surface active ionic liquids (SAILs). The amphiphilicity of SAILs can be fine tuned by changing the alkyl chain length, chosing the type of ring cation, and the nature of counter ions. SAILs resemble to well known classical cationic surfactants based on alkyltrimethylammonium halides or linear alkyl chain pyridinium halides etc14. Information about how the SAIL molecules align or adsorb at the air / water interface and also at water / organic solvent interface on one hand and microstructural features of their aggregates on the other hand is highly desirable for inventing their new surfactant based applications that may include wetting, dispersion, solubilization, detergency and soil remediation etc. The micelle formation in aqueous solutions of SAILs based on alkylimidazolium (mim) cations and halide anions, or long alkyl chain sulfates19-25 has been demonstrated. The self organization behavior of alkyl mim cations and other anions such as [BF4]- and [PF6]- has also been reported26-31. The anion chemistry largely influences the aggregation properties of SAILs32-35. In comparison with [C12mim][Br], 1-dodecyl-3-


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methylimidazolium salicylate not only formed hexagonal liquid crystalline phases in aqueous solution but also exhibited cubic liquid crystalline phases in concentrated aqueous solutions. The rheological studies showed that these solutions have impressive viscoelastic behavior36. Attempts were also made to study the effect of the substituent position on the aromatic counter anions (m- and p- hydroxybenzoate) in the rheological behavior of 1-dodecyl-3methylimidazolium based ILs37 in aqueous solutions. Worm like micelles with photoresponsive

viscoelastic

behavior

were

also

described

for

1-hexadecyl-3-

methylimidazolium bromide aqueous solutions in presence of sodium azobenzene-4carboxylate as additive38. Ionic liquids containing anions as active pharmaceutical ingredients are also synthesized and evaluated for solubility, stability and bioavailability39-42. The synthesis of amphiphilic ILs with drug based anions holds a lot of promise and advantage43. Since the anion itself is a drug, the aggregates of ILs themselves carry the drug anion and hence no additional media (such as micelles of classical surfactants or other polymer gels) is required for solubilizing the hydrophobic drugs in water and also as a carrier or adjutant. By this way, amphiphilic ILs with drug anions deliver the drug in as pure form as possible. In spite of this exciting advantage, there are only few studies in the literature on drug anion containing ILs. TourneoPeteilh et al44,45 have synthesized ILs based on short alkyl chain imidazolium cations (Cnmim, where n = 4,6,8), ibuprofenate as anion and investigated their aggregation behavior using several experimental methods such as dynamic light scattering, cryo-TEM and 1HNMR and also atom scale molecular simulations. It was concluded that the increase in the length of the alkyl chain increased not only the amount of imidazolium cations but also mean hydrodynamic diameter of the micellar aggregates. Attractive interactions between the aggregates led to the transition of simple spherical micelles to large globular aggregates and constituted the beginning of phase separation in the aqueous solutions.


The nature of

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interactions among the short chain cations and drug anions, as expected was highly dependent on the concentration of IL. These features indicate that the aggregation of short alkyl chain ILs has to be open type in which step wise formation of dimers, to trimers and to multimers etc occurs successively. Such type of aggregates are different from the usual micelle-like aggregates in which spontaneous and close association of individual monomers into micelle like structures occur at a critical concentration, well known as critical micelle concentration (CMC). Attempts were also made to synthesize pharmaceutically active silica supported ionic liquids, for handling and dosing liquid drug formulations46. Poorly water soluble acidic APIs were converted into TBP ILs to increase the dissolution or solubility of APIs in water47. The challenges and opportunities involved in metathesis approach for obtaining API based ILs have been described48. ILs containing APIs have also been used to electrosynthesize polypyrrole films to control their release kinetics49. β-lactam antibiotics based ampicillin containing novel ILs are also designed to increase the drug water solubility and obtain ILs with low melting point and high thermal stability50. As far as we know, the synthesis of amphiphilic ILs consisting of long alkyl chain (> C8) heterocyclic amine based cations and drug anions that can form stable and usual micelle like aggregates with geometrical features that can be fine tuned by changing the experimental conditions is yet to be reported. Keeping in view, the scarcity of literature studies on the micelle like aggregation behavior of amphiphilic ionic liquids containing pharmaceutically active anions, we report in the present study,

the

synthesis

of

a

new

1-dodecyl-1-methylpiperidinium

acetylsalicylate,

[C12mpip][AcSa] by a two step process. First the precursor IL namely 1-dodecyl-1methylpiperidinium chloride, [C12mpip][Cl] was prepared by usual quarternization of 1methylpiperidine with dodecane. The Cl- was replaced with –OH ion using ion exchange process to get a basic solution. The drug anion, acetylsalicylate is then introduced into the IL by replacing the –OH by direct neutralization with addition of adequate acetylsalicylic acid


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aqueous solution. The synthesized [C12mpip][AcSa] was characterized and its aggregation properties are measured using

variety of techniques such as surface tension, electrical

conductivity, steady state fluorescence spectroscopy, isothermal titration calorimetry (ITC) and particle size analysis. The surface active parameters were calculated and discussed. The thermodynamic parameters of aggregation were determined and explained. The aggregation number of the micelle like aggregates was determined from translational diffusion coefficient and fluorescence quenching methods. The growth of the aggregates into large worm-like micelles in aqueous media was induced by sodium salicylate as hydrotope additive. The aqueous solutions in highly viscous form to gel state were obtained and subjected to the rheological measurements both in steady state as well as dynamic modes to obtain various rheological parameters such as zero shear viscosity, storage and loss moduli, complex viscosity etc. The rheological data were analyzed in terms of Maxwell model with a single relaxation time. 2. EXPERIMENTAL PROCEDURES 2.1 Materials. 1-methylpiperidine (Merck, ≥ 98 %), was dried over potassium hydroxide and freshly distilled prior to use. The fractions corresponding to (379-380.5) K were used. 1chlorododecane (Merck, 95 %), aspirin (Sigma-Aldrich, ≥ 99.0 %), ambersep 900 OH (Sigma-Aldrich, 20-50 mesh size), pyrene (Sigma, 99.0 %), benzophenone (Sigma, 99.0 %) silver nitrate (Spectrochem India, 99 %), ammonia (AR grade, Allied Chemicals, India) were directly used as such. Ethyl acetate and acetonitrile (Merck, ≥ 99.5 %) were further freshly distilled after drying over fused calcium chloride prior to use. 2.2 Synthetic Procedure. Amphiphilic Drug Anion Containing Ionic Liquids. The synthesis was done by a two step procedure. In the first step, surface active ionic liquid (SAIL), namely 1-dodecyl-1-methylpiperidinium chloride, [C12mpip][Cl] was prepared by a


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direct reaction between 1-methylpiperidine and chlorododecane (in excess amount). The emulsions of the reactants were first obtained by dissolving them in acetonitrile. They were allowed to react under stirring at 353.15 K for 48 h under nitrogen atmosphere. The resultant product in solution form was cooled and transferred to a vigorously stirred ethyl acetate at (273.15 – 278.15) K. A two phase mixture was obtained. After a period of 2 h the supernatant of the upper layer was taken out by a syringe. This procedure was repeated thrice until no more dodecane was detected in the supernatant by gas chromatography. The final residual product was vacuum dried at 343.15 K for 24 h. The structure of the SAIL was confirmed by 1H and

13

C NMR spectra in D2O or CDCl3 (The spectral data are listed in Supporting

Information). The chemical structure of the SAIL is shown below:

1-Dodecyl-1-methylpiperidinium chloride The final product was a white solid powder (yield, 93 %) with a m.p. of 320.15 K. The SAIL sample has a water mass fraction of 8.2 × 10-6, as determined by Karl Fischer titrator. The purity of the sample was adjudged by potentiometric titration using argentometric method in which 10 cm3 of its aliquot solution was titrated with a silver nitrate solution standardized against 0.015 moldm-3 sodium chloride using silver chromate as indicator. The SAIL has a mass purity of 99.5 %. The melting point of the sample was determined from differential scanning calorimetry (DSC) trace. Preparation of 1-dodecyl-1-methylpiperidinium acetylsalicylate [C12mpip][AcSa]: Its preparation involves further two steps. In the step 1, 5 cm3 of 100 mM aqueous solution of


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laboratory synthesized [C12mpip][Cl] was passed through a column charged with ambersep 900(OH), a strong basic anion exchange resin. The length of the bed column was kept to be 10 cm. The eluent was checked for presence of free chloride on hourly basis by argentometric titration. Initially, SAIL aqueous solution was added slowly and drop wise. It is found that, a 2 h cycle was sufficient for the exchange of chloride ion with –OH anion. The –OH exchanged ionic liquid, [C12mpip][OH] is obtained as an aqueous solution. The product was subjected to vacuum drying for a period of 24 h. Thus obtained concentrated [C12mpip][OH] aqueous solution was further diluted with 25 cm3 of Milli-Q water. The contents were titrated against known normal HCl aqueous solution using phenolphthalein indicator to determine the percentage of [C12mpip][OH]. The resin bed was purged with 100 cm3 of 1 N NaOH solution over a period of 1 h. Four such cycles were repeated to ensure the regeneration of –OH functionality of the resin. In the subsequent and final step, 100 mM of aspirin aqueous solution was added slowly into 100 mM of [C12mpip][OH] under vigorous stirring. The reaction between the two components yields ammonia gas and the same was removed by rotary evaporator over a period of 12 h. The ammonia free content of the reaction mixture was slowly cooled by storing in a sealed glass vial in the refrigerator for a period of 6 h to allow the excess aspirin to be crystallized. The crystallized aspirin was removed by simple filtration. The final product, [C12mpip][AcSa] aqueous solution was further dried under vacuum in the temperature range of (333.15 to 343.15) K for 24 h to remove the water and traces of moisture to obtain a clear dark brown viscous liquid. The yield of the final product was 98 %. [C12mpip][AcSa] was characterized by FTIR and NMR spectra as listed in Supporting Information.


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Reaction scheme for preparation of [C12mpip][AcSa] 2.3 Nuclear Magnetic Resonance Spectra (NMR). The measurements were carried out immediately after the preparation of the samples to avoid possible deuteration of the acidic H2 of the ILs in D2O. Each solution was transferred from glass bottles to 5 mm NMR quartz tubes through syringe. The 1H NMR spectra of IL solutions prepared in D2O were taken on a Bruker Avance 400 NMR spectrometer operating at 400.13 MHz. The acquisition of spectra and its analysis was handled by TOPSPIN – 13 software. All experiments were performed at 298.15 ± 0.1 K. 2.4 Attenuated Total Reflection - Fourier Transform Infrared Spectroscopy (ATR – FTIR): The ATR-FTIR spectra of samples were acquired using, MB 3000 FTIR spectrometer (ABB Pvt. Ltd., Germany) equipped with a modified deuterated triglycine sulfate (DTGS) detector. FTIR spectra over the wave number ranging from 4000 to 600 cm-1 were collected at room temperature (∼298.15 K). A horizontal Attenuated Total Reflectance (ATR) trough equipped with a ZnSe crystal (refractive index 2.4) with an incident angle of 45° and 16 reflections were employed in the experiments. All the spectra were recorded with a resolution of 8 cm-1 and 16 parallel scans. For each sample, three parallel measurements were taken.


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2.5 Surface Tension Isotherms. Surface tension measurements were performed using a Model DCAT 11 Surface Tensiometer (Dataphysics, Germany) equipped with a Wilhelmy plate. The surface of the plate was cleaned with ethanol followed by water and dried over reducing flame until it gets red hot. After each measurement, the process was repeated to avoid any contamination. The plate was dipped in a concentrated aqueous solution of IL and further dilutions were done by dispersing Milli-Q water (of 0.05

µS) (from a liquid

dispensary unit (LDU)) into the measuring bowl. The amount of water to be added was calculated depending upon the preset concentrations using inbuilt software. The measurements were made from higher to lower concentrations of respective IL. The uncertainty in the temperature during the measurements was ± 0.1 K. 2.6 Electrical Conductivity. Digital conductivity meter (Equiptronics, India) with a conductivity cell made of platinum electrodes was used for electrical conductivity measurements. Aqueous solutions of potassium chloride (0.01 to 1 moldm-3) were used to calibrate the cell and determine the cell constant. Triplet of measurements was made and the mean value of conductivity was taken to calculate the specific conductivity. The uncertainty in the measurements was less than ± 0.4 %. The cell assembly was dipped into glass vial containing the solutions and the same was kept in a thermostatic water bath at a measuring temperature maintained at ± 0.01 K. 2.7 Isothermal Titration Calorimetry. The ITC Experiments were carried out with Nano– Isothermal Titration Calorimeter (TA-Instruments, Germany). The aqueous solutions were prepared using Milli-Q water (with an average conductivity of 0.05 µS). The runs were taken at a given temperature. The uncertainty in the temperature is ± 0.1 K. The sample cell and the reference cell were first filled to 1300 µL capacity with Milli-Q water. A concentrated IL aqueous solution (with a concentration, 10 times above its CAC value, determined from


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surface tension measurements) was loaded into 250 µL syringe as titrant. The contents of the sample cell were stirred (under 250 rpm). 2.5 µL of aliquot was added through the syringe by maintaining 450 sec time interval between successive injections, until a concentration regime corresponding to the CAC is reached. Triplicate measurements were taken for each of the experimental points. Prior to starting the titration experiment, the calorimeter was equilibrated to a base line with a drift of less than 100 nW over a ten minute period. The stock aqueous solution of ILs was degassed for 10 minutes prior to the experiments. The heat evolved during the addition was measured and recorded as enthaplograms using NanoAnalyzeTM software provided by the manufacturer. 2.8 Steady State Fluorescence Spectroscopy. The steady-state fluorescence measurements were carried out with a spectrofluorophotometer (Shimadzu, Japan, Model RF5301PC) with a wave length accuracy of ± 1.5 nm and 150 W Xenon lamp as a light source. Pyrene was used as a fluorescence probe and benzophenone as a quencher. Appropriate volume of pyrene solution in methanol (0.1 mMdm-3) was introduced into a volumetric flask and the solution was purged with a stream of nitrogen. Definite amount of Milli-Q water was added to the flask, that was kept overnight under stirring. The resulting solution was filtered through a 2 µm membrane. This solution was further used as solvent for the preparation of IL aqueous solutions. The excitation wavelength was set at 335 nm, while the emission spectra were scanned from 350 to 450 nm. All the measurements were done at 298.15 K. 2.9 Particle Size Analysis. Particle size measurements along with the size distribution profiles were obtained using a particle size and zeta potential measuring system (Nanopartica SZ-100, HORIBA Ltd, Kyoto, Japan). The scattered light from the micellar aqueous solutions of ILs was collected at an optimum scattering angle of 173o. The IL aqueous solutions prepared in Milli-Q water (with a conductivity of 0.05 µS) were made first dust-free, through


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filtration using 0.2 µm antop membrane filters (Whatman). It was found that 0.2 µm pore-size membrane filters were adequate to remove the dust particles suspended in the solutions, which otherwise may cause obvious side effects during the measurements. 2.10 Rheology measurements. Rheological measurements on the concentrated and also on gel like aqueous solutions of the ILs in absence or presence of an hydrotope were carried out using Anton Paar, Germany MCR 102 Rheometer. Two types of measuring systems were used depending upon the viscosity of the solutions. For low to moderate viscous solutions, a cone and plate measuring system having a cone angle of 10 and diameter of 50 mm was employed. For highly viscous solutions, a plate-plate system, with a diameter of 25 mm was used. The filling of the sample is very crucial. For plate-plate system, the sample was placed just outside the rim of the measuring system. Excess sample was removed at a position just above the measuring position. A 0.5 cm3 of solution/ gel was found to be sufficient. Too much and too little samples were avoided so that no additional errors were introduced during the measurements. Steady shear rate sweep measurements were performed with ascending (from low to high) shear rates in logarithmic intervals. The sample temperature (with an accuracy of ± 0.01 0C) during the measurements was varied by an inbuilt Peltier system. The samples were scanned across a shear rate range of 0.1 to 100 s-1. The viscosity sampling period ranged from 500 to 3000 s depending on the viscosity of the solution and the steady state at each shear rate was defined to be when the three consecutive measurements had variations in the measured viscosity values lower than 5 %. An air bearing system with a minimum torque of 0.05 µNm and a maximum torque of 0.05 µNm was employed. Normal force ranged from 0.01 to 50 N, with a resolution of 1 mN. For linear oscillatory measurements, frequency sweep was performed at given amplitude, γ percentage of 0.1 to 0.5 so that frequency sweep was in the linear viscoelastic domain. The storage modulus (G′), loss modulus (G″) and complex viscosities were measured using a frequency sweep carried across


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the frequency range 100 to 0.1 rad/s at different temperatures. The data were handled with Rheoplus software. 2.11 Determination of [C12mpip][AcSa] by UV spectroscopy measurements. The acetylsalicylate anion is UV active. The λmax of its aqueous solutions was determined using spectrophotometer, model UV-1800 Shimadzu, Japan and was found to be 298 nm. The aqueous solutions of [C12mpip][AcSa], prepared in various concentrations ranging from 0.006 to 0.06 mM were scanned for absorbance. The data of absorbance vs. concentration was fitted to a linear equation (r2 = 0.9993), as shown in Figure S1 of Supporting Information. The unknown concentration of [C12mpip][AcSa] was determined from the calibration equation, 2.12 Drug Anion Release. The release of the drug anion across a dialysis membrane was monitored using Franz diffusion cell apparatus (Model no. EDC 07, India). The diffusion tube was filled with Milli-Q water and the donor compartment was filled with 1 g of IL gel containing drug anion. The semi-permeable dialysis membrane was priorly hydrated in MilliQ water over night. The temperature of the diffusion cell was maintained by an inbuilt thermostat. The release of the drug anion was monitored over a period of 12 h at regular time intervals under constant stirring of the contents of the receptor cell with a rotor speed of 1000 rpm. The samples collected were subjected to UV measurements immediately. The amount of the drug anion released after 48 h is taken as the value corresponding to infinite time. The average of the three consecutive readings for each of the samples was taken.


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3. RESULTS AND DISCUSSION 3.1 Aggregation Behavior and Surface Active Parameters of [C12mpip][AcSa] in Water 3.1.1 Critical Aggregation Concentrations (CAC) and Surface Activity. The CAC values of the [C12mpip][AcSa] were determined from four independent methods namely surface tension, electrical conductivity, steady state fluorescence spectroscopy and isothermal titration calorimetry (ITC). The surface tension or specific conductivity vs. concentration isotherms at temperatures of 298.15, 308.15 and 318.15 K, steady state fluorescence measurements at 298.15 K and ITC curves at 298.15, 308.15 and 318.15 K for aqueous IL solutions are shown in Figure 1. Surface tension measurements (Figure 1a) in aqueous solutions of a [C12mpip][AcSa] provide a two-fold information namely, CAC values and the surface activity at the air/water interface. The surface tension, γ progressively decreases with the increase in the concentration because of the positive adsorption of [C12mpip][AcSa] molecules at the air/water interface. The decrease was smooth initially followed by a sharp fall before reaching a lower limiting value i.e. near constant γ values. The break points corresponding to the intersection between the region of sharp decrease and constant γ values are assigned to the CAC values. We did not notice any minima around the CAC values in the surface tension isotherms, and therefore the presence of any impurities in the [C12mpip][AcSa] samples is ruled out. The perusal of the specific conductivity, κ versus concentration isotherms (Figure 1b) shows that the value of κ increases almost linearly with the increase in the concentration with clear-cut inflections around the maxima gradient in the measured property as determined using Philips definition51. The inflection points corresponding to CAC are marked in the figure. The steep change in the slope of κ vs. concentration isotherms indicates the onset of aggregation process.


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Steady state fluorescence measurements using pyrene as solvatochromic probe are made to investigate the polarity of the microenvironment of the micelles and hence to study the aggregation behavior of surface active ILs in aqueous solutions52-54. Pyrene probe is strongly hydrophobic in nature and its fluorescence emission spectrum exhibits characteristic five bands in the region of 370 to 425 nm (Figure 1c). The pyrene preferentially dissolves into hydrophobic regions of [C12mpip][AcSa]. The first band also known as 0-0 band, peak 1 may be enhanced in a polar microenvironment, due to vibronic coupling, being similar to the Ham effect in the absorption spectra of benzene. The third band is not sensitive to the surrounding environment. Thus, the ratio of the intensities of the first to third band (I1/I3) is useful not only to probe the micro-polarity of the IL aggregates but also when plotted as a function of the concentration, would yield the characteristic break points corresponding to the CAC. The plots showing the variation of I1/I3 ratio with the concentration of [C12mpip][AcSa] in water are shown in Figure 1d. ITC measurements provide a direct determination of CAC value and also the accurate information on the thermodynamic parameters55-57. Figure 1e, 1f and 1g show the representative

isothermal curves obtained from the dilution of a concentrated

[C12mpip][AcSa] aqueous solution into water at a particular temperature. The profiles of enthalpy of dilution as a function of [[C12mpip][AcSa]] are shown in Figure 1h,1i and 1j. These profiles can typically be described in terms of three different concentration regions. The first region (concentrations below the CAC value), is characterized by relatively large and constant enthalpies which can be three different effects namely dilution of the micellar solutions, demicellization of the micelles, and dilution in the concentration of resultant monomer respectively. The second region is characterized by a sharp and decreasing endothermic effect, indicating that the aggregates started forming near the CAC value. The enthalpic effect in the third concentration range (above CAC) becomes small and remains


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almost constant. The difference between the enthalpies of the first and the third region directly corresponds to the enthalpy of aggregation (∆Hagg). The CAC values are obtained from the midpoints as determined from the first order differential of the enthalpy profile curves, shown in Figure 1k, 1l and 1m. The CAC values obtained by four independent methods and the same are collected in Table 1. A perusal of the data indicates that CAC values determined from surface tension, electrical conductivity and steady state fluorescence isotherms in general agree closely with each other. ITC measured CAC values for [C12mpip][AcSa] are slightly higher and for [C12mpip][Cl] are slightly lower than those determined by other three methods. ITC measurement gives a global picture of the aggregation process and also take into account the changes in the thermodynamic functions associated with microstructural changes during the aggregation process57. In contrast, surface tension values vary mainly due to the surface absorption and saturation by monomers, electrical conductivity is determined by the number of charge carriers and relative diffusion of micellar aggregates and monomers in aqueous solutions, in pre and post aggregation stages. Fluorometry, considers only the immediate micro-environment of fluorophore. Therefore, the nature and relative hydration of the cationic head group and also counter anion, along with the cation – anion interactions would contribute to the various physical processes involved in in detecting the CAC of amphiphiles. The average CAC value (in mMdm-3) of 1.25 for [C12mpip][AcSa] at 298.15 K is far less than the CAC values of 17.25, 14.8, 16.00 and 8.2 of [C12py][Cl]59, [C12mim][Cl]18, alkyl quaternary bromides and alkyl sulfates59 respectively under similar conditions. The tendency for self aggregation depends on several factors such as the interactions between the hydrophobic chains, ability of ring cations as well as counter anions to establish hydrogen bonding with water molecules, condensation of counter ions, and the extent of repulsive interactions among the counter ions


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etc. The low CAC values of [C12mpip][AcSa] as compared with other analogous systems (with the same hydrocarbon chain length but different head groups and counter anions), can be accounted and explained by considering the capacity of the ring amine group of the methylpiperidine to hydrogen bond with the water molecules and also with the acidic hydrogens and/or ester hydrogens of acetylsalicylate counter anion. The close proximity of the counter anion to the head group reduces the repulsive interactions among the cationic rings of the [C12mpip][AcSa] and facilitate the formation of aggregates at low concentrations. Moreover, the acetylsalicylate counter anion itself forms hydrogen bonds with water through the participation of its acidic as well as ester parts, resulting its anchoring with the hydrated water around the head group. Such favorable interactions between the ionic head groups and counter ions do not occur in conventional cationic or anionic surfactants. Therefore the combination of a bulky drug counter anion such as acetylsalicylate with the alkylheterocyclic amines yields a surfactant like IL with very low CAC values. The increase in temperature from 298.15 to 318.15 K increased the CAC values. It is well known that the CMC values for ionic surfactants display a U type variation with the increase in temperature and the minimum occurs around 303.15 K60-63. The observed increasing trend of CAC with the temperature (in the range of 298.15 – 318.15 K) can therefore be attributed to two independent factors namely the spreading of electrostatic field around the aggregates causing more repulsions among the head groups of [C12mpip][AcSa] and also to the breakdown of the hydrogen bonded water around the hydrophobic domains of the aggregate. The later effect leads to a decrease in entropy of the structured water surrounding the hydrophobic domains and disfavors the aggregation. 3.1.2 Thermodynamic parameters of aggregation. The thermodynamic parameters namely Gibbs free energy (∆G0a), enthalpy and entropy of aggregation, ∆H0a and ∆S0a have been obtained from ITC measurements and also from the data of CAC at different temperatures.


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ITC is a label-free direct measurement of the heat evolved or absorbed during the aggregation process. The experimental data of change in heats of dilution (please see Figure 1h, 1i and 1j) as a function of concentration was fitted by an independent site binding model based on sigmoidal Boltzman procedure to calculate the enthalpy of aggregation (∆Hagg), binding constant (Ka) and stoichiometry (n). Ka represents the association binding constant64. The enthalpograms were recorded and analyzed using the NanoAnalyzeTM v2.4.1 software supplied with the instrument according to an independent sites binding model58,

65-67

.The

details of this binding model are described elsewhere58,67. In short, in the one-site binding model, all the binding sites are considered to be identical and no distinction between interacting sites is made. Karumbamkandathil et al58 have described that the simple case of one-site model can be written from the definition of an equilibrium constant (Ka) by: Ka = [MoC]/[Mo][C], where [Mo], [C] and [MoC] represent the molar concentrations of the free monomer, counter ions and micelle, respectively. As one can easily see that the quantities [Mo], [C] and [MoC] change constantly, as the titration proceeds, but Ka remains constant. The dissociation constant, Kd, is defined as 1/Ka. The ∆Hagg is obtained from the difference in the enthalpies of dilution at the initial (micellar) and final (monomer) states. The model fit gave the parameters Ka and n. The thermodynamic parameters of aggregation, Gibbs free energy of aggregation, ∆Gagg and entropy of aggregation, ∆Sagg are calculated using well known equations68,69 (The equations are listed in Supporting Information). The summary of various thermodynamic parameters of aggregation at three different temperatures as calculated from both the above described approaches is given in Table 1. The ∆G0agg in general is negative for the aggregation of [C12mpip][AcSa] in water at three different temperatures indicating the spontaneous nature of aggregation of [C12mpip][AcSa]. The negative values of ∆H0agg suggest the exothermic nature of aggregation. The values of ∆S0agg calculated from two different approaches, in general are large and positive and drive the


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aggregation. A comparison of the thermodynamic parameters of aggregation for the precursor [C12mpip][Cl] with the [C12mpip][AcSa] reveals that the negative enthalpies and free energy of aggregation are 7 – 8 and 1.5 times more negative for the [C12mpip][AcSa] over the Clbased precursor SAIL respectively. Therefore it signifies that the bulky acetylsalicylate anion facilitates the aggregation process more favorably. 3.1.3. Surface active parameters. The surface activity of the amphiphiles is always discussed in terms of several parameters namely the area per adsorbed molecule at the air/water interface, aୱଵ (which measures the packing densities in terms of area occupied by molecules in an unit area on the surface), the saturation adsorption values, Γmax at the air/water interface, pC20 (which defines the concentration of an amphiphile at which the surface tension of water is reduced by 20 mNm-1 and gives a measure of the adsorption efficiency at air/water interface) and the surface pressure, πCMC. The pertinent and well reported relations to calculate these parameters are well described elsewhere59 (The relations are listed in Supporting Information). The summary of the surface active parameters for the precursor [C12mpip][Cl]

and

[C12mpip][AcSa] is given in Table 2. Clear-cut differences in various parameters for the precursor SAIL and [C12mpip][AcSa] are noted. It can be seen that pC20 values for the [C12mpip][AcSa] are 2.6 to 2.8 times less than for its precursor SAIL, indicating that the adsorption efficiency of [C12mpip][AcSa] at the air/water interface is higher. This condition automatically leads to the increased surface excess concentration and efficient adsorption of [C12mpip][AcSa] molecules at the surface. Since SAIL and [C12mpip][AcSa] have common alkyl part in the cation and differ only in the nature of the anion, the observed differences in the surface active parameters can best be understood by considering the interactions among the cation and respective anions.


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In order to understand the interactions among the ring cation and drug anion, ATR – FTIR spectra of aspirin, precursor [C12mpip][Cl] and [C12mpip][AcSa] in their pure state as well as in D2O solutions were scanned and are shown in Figure S2 of Supporting Information. The various functional groups and the assigned frequencies are listed in Table S1 of the Supporting Information. The spectra of acetylsalicylic acid has shown characteristic wave numbers (cm-1) corresponding to aromatic C–H bend (768), O–H stretch of carboxylic acid (2400–3200), –C=O of ester group (1680), and C=C stretch of phenyl ring (1603) both in its pure state and in D2O solutions. The spectra of [C12mpip][Cl] displayed similar wave numbers of

(2850–2900) for C–H of the ring in both the states. The spectra of

[C12mpip][AcSa] both in its pure state as well as in solution form showed C–H stretch of aromatic ring (760), –C=O stretch of ester (1710) and C=C stretch of phenyl ring (1588). However the comparison of the spectral features of the acetylsalicylate ion, SAIL and [C12mpip][AcSa]

revealed the following striking differences. The stretching frequency

corresponding to –OH of the carboxylic acid of the acetylsalicylic acid in RTIL is absent indicating the total ionization of acetylsalicylic acid and the formation of [C12mpip]+ …..

–

[acetylsalicylate] combination in the [C12mpip][AcSa]. The –NH stretch corresponding to methylpiperidine part of RTIL and –OH stretch of acetylsalicylic acid are converged into a broad peak around the wave number range of 3000–3650 cm-1. This confirms the formation of strong hydrogen bond between the cation and anion. The C=C stretch of the phenyl ring showed a blue shift (contraction of bonds) and while the C=O stretch displayed a red shift (expansion of bonds) upon [C12mpip][AcSa]

formation.

The above mentioned strong

hydrogen bonding interactions between the cation – anion pair explains the higher surface area for the [C12mpip][AcSa] at the air/water interface as compared to the SAIL in which such interactions are absent. The extent of surface tension reduction of water upon micellar like aggregate formation can be adjudged from the values of the surface pressure, πCAC.


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Higher the γCAC, less would be the πCAC. Increase in the temperature (within the range studied in present work) resulted in a decrease of γCAC , Γmax and increase in aୱଵ and these trends can be attributed to the enhanced molecular mobility at elevated temperatures. 3.1.4 Size and aggregation number. The normalized frequency (in percentage) as a function of the size of the aggregates in aqueous solutions containing different concentrations of [C12mpip][AcSa] and at three different temperatures are depicted in Figure 2a, 2b and 2c. The profiles are characterized by presence of a single peak and therefore it is assumed that the [C12mpip][AcSa] forms aggregates through a close association i.e. the unimer molecules spontaneously aggregate into a single type of micelle-like aggregate structures. The results ഥ for the from particle size analysis also give the average translational diffusion coefficient, D aggregates. In dilute aqueous solutions, the concentration dependence of the translational average diffusion coefficient can be expressed adequately by a first order expansion,

D = Do ( 1 + k D C) , where D o is the z-average diffusion coefficient at infinite dilution and kD is the diffusion second virial coefficient. According to Einstein relation, the D o is inversely proportional to the translational frictional coefficient, ft at infinite dilution by the relation,

Do=

kB T ft

(1)

where kB is Boltzmann constant and T is absolute temperature. The value of ft obtained via eq. 6 can be used for a direct estimation of hydrodynamic radius, Rh of the micellar associates provided they have a spherical shape using the relation, ft = 6πηRh as per the Stokes law. Tanford formula expresses the frictional coefficient for prolate and oblate ellipsoids of revolution with semi axes b and a by,


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ft =

6 π η b (1 − a 2 / b 2 )1 / 2  1 + (1 − a 2 / b 2 )1 / 2  ln   a/b  

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(for prolate ellipsoid)

(2)

(for oblate ellipsoid)

(3)

and

ft =

6 π η a (b 2 / a 2 − 1)1 / 2 tan −1 (b 2 / a 2 − 1)1 / 2

where b is the semi major axis and a is the semi minor axis. By taking trial values of the size parameters in an iterative manner and assuming particular shape for the aggregates, one can reproduce the experimentally observed ft values. Once a close agreement between experimental and fitted values is reached, the shape and dimensions of the micelle like aggregates is determined (please see Supporting Information for more details). The plots of experimental values of average translational diffusion coefficients vs. concentration are shown in Figure 3. Higher the concentration of the [C12mpip][AcSa], less is the diffusion of the micellar particles and the same can be attributed to the increased density of aggregates in an unit volume. The linear extrapolation of the D values to zero concentration gives the average translational diffusion coefficient, തത Dത଴ത. The size parameters of the aggregates were evaluated assuming either spherical or ellipsoidal shape for the aggregates via eqs. 1 – 3. It is found that the experimental frictional coefficient values could best be reproduced only when prolate ellipsoidal shape is assigned for the micellar aggregates. The summary of size parameters for such prolate shaped aggregates of [C12mpip][Cl] and [C12mpip][AcSa] in തതതത଴ values of aqueous solutions and at three different temperatures is listed in Table 3. The D the [C12mpip][Cl] aggregates are larger than that of [C12mpip][AcSa] aggregates. Similarly the aggregates of the [C12mpip][AcSa] are more elongated. The increase in temperature did not alter the size of the micelles indicating that the aggregates are quite stable in the temperature range of present study.


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Aggregation number i.e. number of molecules of the amphiphile in the aggregate is another important parameter that determines not only the shape but also the relative hydrophobicity and stability of the micellar aggregates. The aggregation numbers were calculated from two തതത଴ത and also independent methods using either diffusion coefficients at zero concentration, D fluorescence quenching. In the first method, the aggregation number Nagg is expressed as the ratio of total volume of the aggregate,

ସ ଷ

π ܽଶ ܾ to the volume the alkyl tail of a single

molecule, Vtail. Vtail was calculated from specific densities70 using the Tanford formula71 of Vtail = 27.4 + 26.9 ·nc , where nc = no. of carbon atoms of alkyl part. This approach assumes that the core part of the aggregate structure entirely consists of hydrophobic alkyl chains. Fluorescence quenching technique has also been successfully applied to determine the aggregation number. This technique is based on the principle that emission of pyrene is quenched by addition of benzophenone (a quencher) and the amount of emission reduction is almost linear with quencher concentration. The ratio of fluorescence quenching in a micellar like system is given as52,69, ln(I0/I) = NaggCQ /CAmphi-CAC where I0 and I are the fluorescence intensities of the pyrene in the absence and presence of quencher at a specific wavelength, CQ and CAmphi are molar concentrations of quencher and amphiphile respectively. The change in pyrene fluorescence emission spectra as a function of concentration of benzophenone for [C12mpip][AcSa] is depicted in the inset of Figure 4. A good linear correlation between ln(I0/I) and CQ was found. The values of the aggregation number, as obtained from both the above described methods are also listed in Table 3. It can be seen that the aggregation numbers, determined from diffusion coefficients are higher than those obtained from steady state fluorescence method. This difference can be explained by the fact that the former method gives a number averaged value and while the later gives a mass averaged value. As compared to the micelles of [C12mpip][Cl], the micelles of [C12mpip][AcSa] are larger in size and accommodate more monomers due to close packing of cation – anion pairs in the later.


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Otherwise, the Nagg values decrease with the increase in temperature, as expected usually for the ionic amphiphiles. The increase in temperature increases the thermal motion of the aggregates, increase the possible repulsions among the charged head groups, leading to the decreased participation of molecules in the aggregation state. 3.2 Rheology Measurements 3.2.1 Zero shear viscosities. Rheological measurements on the moderately to highly viscous and also on gel like aqueous solutions of the [C12mpip][AcSa] in presence of sodium salicylate (SS), as an hydrotope additive were carried out both in steady state and dynamic modes. First, the stock aqueous solutions of each of the components namely [C12mpip][AcSa] or SS were prepared and mixed in different volumes so as to obtain mixture solutions across the mole fraction. The concentrations of each of the stock solutions are fixed in the range of 100 to 600 mM so as to cover moderate to highly viscous and gel like regimes. The typical plots of steady state viscosity vs. shear rate are shown in Figure 5. The viscosities of the solutions varied linearly with the applied shear rate. The ηo values were obtained through the fit of the data shown in the figure to Carreau-Yasuda model59 (please see Supporting Information for more details about the model). The effect of the concentration of respective components on zero shear viscosities across the mole fraction for each set of the mixtures is shown in Figure 6 (The data is listed in Supporting Information Table S2). It can be seen that the zero shear viscosities increase with the increase in the concentrations of either of the components and reaches a maximum value of 1250 Pa.s for mixture solutions prepared from 400 mM stock solutions of the respective components. However, the mixture solutions prepared from the next highest concentration of 600 mM were characterized by viscosities of decreased magnitude. In general, for a given set of mixture, the viscosity of the solution increased gradually, peaked to a maximum value followed by a sharp decrease before plateauing

at high X[C12mpip][AcSa]. The observed maximum in zero shear viscosity vs.


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X[C12mpip][AcSa] profiles indicates that SS as an hydrotope additive facilitates the growth of the [C12mpip][AcSa]

micellar aggregates in terms of the formation of large structures

through the overlap of small micellar aggregates. One of the most important and common applications of hydrotopes in the academic research is that, they increase the solubility of surfactants and even dissolve the liquid crystalline phases formed at high surfactant concentration. The mixture solutions of hydrotopes with ionic surfactants are reported to form worm-like micelles, whose solutions themselves are highly viscous and are effective as gelators. Most of the systems that exhibit viscoelastic character are based on cetyltrimethylammonium bromide (CTAB) in aqueous SS solutions71-74. Shear and frequency dependent viscosity (steady and dynamic rheology) measurements help establish the presence of worm or entangled rod-like micellar structures71,72,74,75. It is very interesting to note from the results of present work that the maxima in viscosities and hence maximum micellar growth is observed at compositions lower than the equimolar ratios. Therefore it can reasonably be concluded that besides the charge neutralization, the phenyl ring of the sodium salicylate also help stalk the micellar structures into larger geometry. The large micelles lose their fluidity and therefore the solutions become viscous. In order to understand and correlate the high viscosities of [C12mpip][AcSa] / SS mixture solutions with the type of microstructural organization namely formation of overlapping entangled micelles or viscoelastic worm - like micelles, the mixture solutions corresponding to maximum value of zero shear viscosity were subjected to the frequency sweep measurements. 3.2.2 Dynamic rheology measurements. The oscillatory or frequency sweep measurements were carried out by fixing the shear rate corresponding to linear viscoelastic range (LVE). The mixtures prepared from low concentrations of 100 mM of each of the components were just water like. However, the mixture solutions prepared from high concentrations showed


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frequency dependent response. The variation of loss modulus, G'' and the storage modulus, G' across the frequency range for the three sets of mixture solutions are shown in Figure 7. For the two mixtures containing 200 and 400 mM of each of the components (and X[C12mpip][AcSa] = 0.4), the loss modulus was always higher than the storage modulus across the frequency and both the moduli increased smoothly with the increase in the frequency. These trends suggest that the solutions are predominated by the viscous component alone and the increase in the frequency leads to close organization of the micellar-like aggregates or increase in the number density of the aggregates. For mixture solutions prepared from 600 mM of each of the components (and X[C12mpip][AcSa] = 0.2), the loss moduli predominated over most of the frequency range and the storage moduli increased over the loss moduli at very high frequency. The crossover of moduli clearly indicates the formation of worm-like micelles with viscoelastic gel like behavior. Cole-Cole plots are one of the best ways to verify the viscoelastic worm-like micellar behavior in solutions by determining whether the systems follow Maxwell model or not75-78 The Cole-Cole representation, in which the imaginary part, G''(ω), the loss modulus is plotted against real part G'(ω), the storage modulus. The rheological behavior of worm-like micelles can be described through Maxwell model with a single relaxation time, τR, for which the storage modulus, G'(ω), loss modulus G''(ω) and the magnitude of the complex viscosity [ *] are described by the well known equations (please see Supporting Information). We constructed the Cole-Cole curves for the viscoelastic gel and the same are shown in Figure 8. Our experimental data points fitted well to Maxwell model with a single relaxation time in the range of low and medium frequency, as represented by solid lines in the semicircle, but deviated from the semicircle at high frequencies. The deviations covered a large range of frequency indicating the presence of large worm-like micelles in the system. The formation of worm-like micelles is related to the geometrical packing parameter, which is defined by


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the ratio of the volume of the hydrophobic group to the product of its length and the cross sectional area of the hydrophilic group. The main driving force for the formation of large worm-like micelles is the screening of the electrostatic repulsive interactions between the charged head groups. The overlap and the entanglement of large micellar entities into a three-dimensional network structures lead to the viscoelastic character. Various structural parameters such as micellar contour length, persistent length, and entanglement lengths for the worm-like micelles are characterized by the relations79, (G'α / G"min) ≈ (L / le)

(4)

le = (r)5/3 / (lp)2/3

(5)

r = (kbT / G'α)1/3

(6)

where Lത is the micellar contour length, and le is the entanglement length, lp is the persistent length which gives an estimation of micellar flexibility of worm-like micelles of ionic amphiphiles. lp values depend on the chemical structure and ionic strength of the solution. G'∞ is obtained from the relation, G'∞ ≃ 2 G''max, where G''max is the viscosity modulus (loss modulus) corresponding to shear frequency, ωco or cross over frequency. The typical values for each of the above quantities are: η0 (Pa.s) = 0.591, G0 (Pa) = 337.8, ωco (rad/s) = 32.9, τR (s) = 0.03, G'min(Pa) = 0.40, le (nm) = 44.4 and Lത (nm) = 3080.5 – 2542.7. The large values of le and Lത confirm that the gel solution contains worm-like micellar–like aggregates of large dimension. 3.3.3 Gel temperatures. The gel like solutions of the mixtures, corresponding to the mole fractions at which maxima of zero viscosities were noted, were subjected to temperature scans (in the range of 10 – 60 °C) both in warming and cooling modes to get the G', G'' and [η*] at different temperatures. The variations of these three quantities vs. temperatures are


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shown in Figure 9a, 9b and 9c. Different trends were noted for the three mixtures. It may be recalled that the mixture prepared from 600 mM stock solutions of each of the components exhibited viscoelasticity due to worm-like micellar net work formation, while the first two mixtures were just highly viscous simple gels, predominantly characterized by the loss modulus. For the first mixture, moduli properties showed a Z type decrease (namely a small initial decrease followed by a sharp decrease up to certain temperature before a slow but continuous decrease at elevated temperatures upon warming). The initial decrease in the moduli values is assigned to the initiation of melting of organized structures which is presumed to be completed at temperature beyond which a continuous decrease was noted. For the second mixture, moduli decreased with the increase in the temperature but the profiles displayed three break points. For the viscoelastic gel, the profiles showed U type variation with a broad minimum and four inflections indicating that the viscoelastic gels undergo complex transitions upon warming. The variations in

G', G'' and [η*] under cooling

conditions are quite different The gel solutions at elevated temperatures are characterized by very small moduli values and complex viscosities, which increased linearly with the decrease in the temperature before reaching a first plateau followed by a second linear increase and second plateau region. For the viscoelastic gel, the profiles of variation of moduli and complex viscosities vs. temperature were Z type with two inflections. The moduli properties of gels in general are large when measured in a cooling cycle than in the warming mode. The different nature of moduli vs. temperatures profiles in both the modes indicates that gel solutions formed are thermo-irreversible. The tan δ (G' / G'') profiles as a function of temperature for the viscoelastic gel are given in Figure 10. The profiles showed a sharp peak around 35 oC and 24 oC, corresponding to the gel – sol or sol – gel transition in the warming and cooling modes respectively. Close scrutiny of the data below the gel temperatures reveals that the storage and loss moduli increase sharply and linearly and while the increase


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in complex viscosities was marginal. These results corroborate our earlier conclusion that true gel like structures with viscoelastic properties can be obtained by carefully choosing the right proportion of the [C12mpip][AcSa] and SS in the mixture aqueous solutions. 3.3.4 Release of anionic drug from [C12mpip][AcSa] gels. The release of the acetylsalicylte anion from the gels of [C12mpip][AcSa] – SS mixtures was monitored by simple dilution. We have chosen the gel solution prepared by mixing 600 mM stock solutions of each of the [C12mpip][AcSa] and SS (X[C12mpip][AcSa] = 0.2). This solution gets gelated at 20 oC. The release of the anion therefore was monitored both at 20 oC and 37 oC (close to body temperature). The percentage fractional release was calculated from the equation,

Percentage fractional release =

େಉ ିେ౪ େಉ

× 100

(7)

where Cα = drug released at infinite time; Ct = drug released at time t The percentage fractional release of anionic drug as a function of time at both the temperatures is depicted in Figure 11. The amount of anionic drug released was initially less gradually increased up to 480 minutes and reaches pseudo plateau value at longer times at 20 °C. The data of amount of drug anion released was fitted to various kinetic models to understand the type of release mechanism as per the details reported in the literature80. The summary and relevance of various constants and regression coefficients for each of the models tried are listed (please see Supporting Information and Table S3). It is found that Hixson-Crowell model81 (at 20 oC) and Higuchi model82 (at 37 oC), reproduced our experimental data well. Therefore it can be concluded that the release of anionic drug part from [C12mpip][AcSa] occurs mainly through surface erosion and also contributions from change in surface area and size of the micellar aggregates due to demicellization effects.


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4. CONCLUSIONS Ionic liquid (IL) approach can be used easily to produce acetylsalicylate anion containing RTIL based on 1-dodecyl-3-methylpiperidinium cation, by ion exchange process. [C12mpip][AcSa] is a typical RTIL and has several advantages namely, drug anion is incorporated directly into the RTIL, exhibit surface activity at air/water interface typical of a classical surfactant, form small ellipsoidal shaped micelle – like aggregates in water that can be grown into large worm-like micelles (in presence of sodium salicylate, an hydrotope additive) with a typical gel network and viscoelasticity. The critical aggregation concentrations were determined from four independent methods namely electrical conductivity, surface tension, steady state fluorescence and isothermal titration calorimetry at three different temperatures and the same for [C12mpip][AcSa] are less than those of its precursor SAIL, [C12mpip][Cl], suggesting favorable and strong electrostatic interactions between the cation and bulky drug anion. Detailed ATR-FTIR spectral analysis of acetylsalicylate anion, [C12mpip][AcSa] and [C12mpip][Cl] in pure as well in D2O solution reveals that the acetylsalicylate anion gets totally ionized and gets incorporated as anion into the RTIL. A strong and broad peak around the wave number range of 3000-3650 cm-1 in RTIL system confirms the formation of strong hydrogen bonds between the –NH of the dodecyl mpip cation ring and –O-H of the carboxyl group of acetylsalicylic acid. These strong interactions coupled with the cation – anion attractive forces result into the high adsorbed surface area per RTIL molecule at air/water interface and also low pC20 values. Therefore, by combining a bulky drug anion containing potential sites for hydrogen bonds into a typical alkylheterocyclic amine based cation results into a highly surface active drug anion incorporated amphiphile. The free energy and enthalpy of aggregation of [C12mpip][AcSa] in water are always negative and while the entropy of aggregation is positive. Therefore both enthalpy and


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enthropy of aggregation drive the aggregation process of RTIL in water. As compared to the precursor SAIL, the enthalpy and free energy of aggregation for [C12mpip][AcSa] are less negative due to enhanced cation – anion interactions. The particle size analysis of the micelle – like aggregates of RTIL in water suggests that the aggregates are of prolate ellipsoidal shape and are more elongated than those of its precursor SAIL. The aggregation number i.e. number of molecules in an aggregate are determined both from diffusion coefficients and fluorescence quenching methods. The aggregation numbers for the aggregates of RTIL are almost close to twice that of the same for SAIL. Therefore by incorporating bulky drug anion in place of simple Cl- of an SAIL results into typical amphiphile with larger micellar volume and aggregation number. This means that an aggregate of RTIL reported in the present work can hold double the drug molecules than that of drug molecules solubilized into the aggregates of its counter and simple SAIL with Cl- anion. Another advantage of drug incorporated RTIL, demonstrated in the present work is that the small micellar aggregates can be transformed into large worm – like micellar networks by simply introducing the sodium salicylate as an additive in a mixture solutions. The solutions containing such large micelle like aggregates are found to be gels with typical viscoelastic properties. The analysis of temperature scans of moduli properties of viscoelastic gels reveals that the over lapping of aggregates occur in a complex and irreversible manner. The network of large aggregates occur with different degrees of cross linking depending upon whether the temperature regimes are near or far from the gel points. The drug anion from the gels is released across a semipermeable dialysis membrane by simple dilution with water. The release is caused due to combined effects of the surface erosion and demicellization. The drug formulations based on RTIL gels reported here does not require any other carrier or release matrix system and hence hold a lot of promise for their future evaluation for in vivo


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studies and also for their pharmaceutical activities. The studies of this type are presently underway. ASSOCIATED CONTENT Supporting Information Supporting information includes NMR and MS spectra of synthesized ILs; relations used for calculating surface active properties; equations for calculating thermodynamic parameters; the iteration procedure for obtaining the information about the shape of the micellar aggregates from D o ; relations for Carreau–Yasuda model used to calculate zero shear viscosity; brief description of kinetic models for the release of drug anion; calibrating plot of absorbance versus concentration of RTIL (Figure S1); ATR-FTIR spectra of acetylsalicylic acid, SAIL and RTIL in pure as well as in D2O solutions (Figure S2); assignment of ATRFTIR spectral features (Table S1), zero shear viscosities of mixture solutions (Table S2); fitting coefficients and regression coefficients for model fits of drug release data (Table S3). AUTHOR INFORMATION Corresponding Author: Prof. Dr. N. V. Sastry *E-mail: [email protected] ACKNOWLEDGEMENTS One of the authors, DKS thanks University Grants Commission – Center for Advanced Studies (CAS) program for a research fellowship. The authors also thank DST – PURSE program of Sardar Patel University for mass spectrometry, steady state fluorescence, rheology, particle size analysis and diffusion cell measurements.


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REFERENCES (1) Ryu, H. J.; Sanchez, L.; Keul, H. A.; Raj, A.; Bockstaller, M. R.; Imidazolium‐Based Ionic Liquids as Efficient Shape‐Regulating Solvents for the Synthesis of Gold Nanorods. Angew. Chem. Int. Ed. 2008, 47 (40), 7639–7643. (2) Greaves, T. L.; Drummond, C. J.; Ionic Liquids as Amphiphile Self–Assembly Media. Chem. Soc. Rev. 2008, 37(8), 1709–1726. (3) Triolo, A.; Russina, O.; Bleif, H.; Cola, E. D.; Nanoscale Segregation in Room Temperature Ionic Liquids. J. Phy. Chem. B 2007, 111 (18), 4641–4644. (4) Thomas, W.; Room–Temperature Ionic Liquids Solvents for Synthesis and Catalysis. Chem. Rev. 1999, 99 (8), 2071–2084. (5) Martyn, J. E.; Kenneth, R. S.; Ionic Liquids Green Solvents for the Future. Pure App. Chem. 2000, 72 (7), 1391–1398. (6) 355.

Patel, D. D.; Lee, J.; Applications of Ionic Liquids. Chem. Rec. 2012, 12 (3), 329–

(7) Zhao, H.; Xia, S.; Ma, P.; Use of Ionic Liquids as ‘Green’ Solvents for Extractions. J. Chem. Technol. Biotechnol. 2005, 80 (10), 1089–1096. (8) Zhao, D.; Wu, M.; Kou, Y.; Min, E.; Ionic liquids: Applications in Catalysis. Catal. Today 2002, 74(1–2), 157–189. (9) Liu, J.; Jonsson, J. K.; Jiang, G.; Application of Ionic Liquids in Analytical Chemistry. Trends Anal. Chem. 2005, 24 (1), 20–27. (10) Pandey, S.; Analytical Applications of Room–Temperature Ionic Liquids: A Review of Recent Efforts. Anal. Chim. Acta. 2005, 566 (1), 38–45. (11) Berthod, A.; Ruis–Angel, M. J.; Carada–Broch, S.; Ionic liquids in Separation Techniques. J. Chromatogr. A 2008, 1184 (1–2), 6–18. (12) Sun, P.; Armstrong, D. W.; Ionic Liquids in Analytical Chemistry. Anal. Chim. Acta 2010, 661 (1), 1–16. (13) Plechkova, N. V.; Seddon, K. R.; Applications of Ionic Liquids in the Chemical Industry. Chem. Soc. Rev. 2008, 37 (1), 123–150. (14) Hupka, J.; Thoming, J.; Jungnickel, C.; Self–Organization of Imidazolium Ionic Liquids in Aqueous Solution. Colloids Surf., A 2008, 329 (3), 125–133. (15) Dong, B.; Zhao, X.; Zheng, L.; Zheng, J.; Li, N.; Inoue, T.; Aggregation Behavior of Long–Chain Imidazolium Ionic Liquids in Aqueous Solution: Micellization and Characterization of Micelle Microenvironment. Colloids Surf., A 2008, 317 (1–3),666–672.


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(16) Bai, G.; Lopes, A.; Bastos, M.; Thermodynamics of Micellization of Alkylimidazolium Surfactants in Aqueous Solution. J. Chem. Thermodyn. 2008, 40 (10), 1509–1516. (17) Vaghela, N. M.; Sastry, N. V.; Aswal, V. K.; Surface Active and Aggregation Behavior of Methylimidazolium-Based Ionic Liquids of Type [C n mim][X], n= 4, 6, 8 and [X]= Cl−, Br−, and I− in water. Colloid. Polym. Sci. 2011, 289 (3), 309–322. (18) Sastry, N. V.; Vaghela, N. M.; Aswal V. K.; Effect of Alkyl Chain Length and Head Group on Surface Active and Aggregation Behavior of Ionic Liquids in Water. J. Fluid Phase Equilib. 2012, 327 (0), 22–29. (19) Vanyur, R.; Biczok, L.; Miskolczy, Z.; Micelle Formation of 1–Alkyl–3– Methylimidazolium Bromide Ionic Liquids in Aqueous Solution. Colloids Surf., A 2007, 299 (1–3), 256–261. (20) Goodchild, I; Collier, L; Millar, S. L.; Prokes, I.; Lord, J. C. D.; Butts, C. P. B.; Bowers, J. Webster, J. R. P.; Heenan, R. K.; Structural Studies of the Phase, Aggregation and Surface Behaviour of 1–Alkyl–3–Methylimidazolium Halide + Water Mixtures. J. Colloid Interface Sci. 2007, 307 (2), 445–468. (21) Seoud, O.A. El.; Pires, P.A.R.; Abdel–Moghny, T.; Bastos, E.L.; Synthesis and Micellar Properties of Surface–Active Ionic Liquids: 1–Alkyl–3–Methylimidazolium Chlorides. J. Colloid Interface Sci. 2007, 313 (1), 296–304. (22) Jungnickel, C.; Luczak, J.; Ranke, J.; Fernandez, J. F.; Muller, A.; Thoming, J.; Micelle Formation of Imidazolium Ionic Liquids in Aqueous Solution. Colloids Surf., A 2008, 316 (1–3), 278–284. (23) Baltazar, Q. Q.; Chandawalla, J.; Sawyer, K.; Anderson, J. L., Interfacial and Micellar Properties of Imidazolium-based Monocationic and Dicationic Ionic Liquids. Colloids Surf., A 2007, 302 (1–3), 150–156. (24) Brown, P.; Butts, C. P.; Eastoe, J.; Fermain, D.; Grillo, I.; Lee, H.; Parker, D.; Plana, D.; Richardson, R. M.; Anionic Surfactant Ionic Liquids with 1–Butyl–3–Methyl– Imidazolium Cations: Characterization and Application. Langmuir 2012, 28 (5), 2502–2059. (25) Jiao, J.; Dong, B.; Zhang, H.; Zhao, Y.; Wang, X.; Wang, R.; Yu, R.; Aggregation Behaviors of Dodecyl Sulfate–Based Anionic Surface Active Ionic Liquids in Water. J. Phys. Chem., B 2012, 116 (3), 958–965. (26) Xu, W.; Zang, Q.; Wei, H.; Qin, J.; Yu L., Self-Aggregation of Cationionic Surface Active Ionic Liquids in Aqueous Solutions. J. Surfact. Deterg. , 2015, 18, 421–428. (27) Naderi, O.; Sadeghi, R.; Effect of Temperature on the Aggregation Behavior and Thermodynamic Properties of Surface Active Ionic Liquid 1-Decyl-3-Methylimidazolium Bromide in Aqueous Solutions: Surface Tension, Vapour Pressure Osmometery, Conductivity, Volumetric And Compressibility Study. J. Chem. Thermodynamics 2016, 102, 68–78.


Page 34 of 57

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(28) Borwes, J.; Butts, C. P.; Martin, P. J.; Vergara-Gutirrez, M. C.; Aggregation Behavior of Aqueous Solutions of Ionic Liquids. Langmuir 2004, 20 (6), 2191–2198. (29) Dorbritz, S.; Ruth, W.; Kragl, U.; Investigation on Aggregate Formation of Ionic Liquids. Adv. Synth. Catal. 2005, 347 (9), 1273–1279. (30) Blesic, M. Marques, M. H.; Plechkova, N. V.; Seddon, K. R.; Rebelo, L. P. N.; Lopes, A.; Self–Aggregation of Ionic Liquids: Micelle Formation in Aqueous Solution. Green Chem. 2007, 9 (5), 481–490. (31) Singh, T.; Kumar, A.; Aggregation Behavior of Ionic Liquids in Aqueous Solutions: Effect of Alkyl Chain Length, Cations, and Anions. J. Phys. Chem., B 2007, 111 (27), 7843– 7851. (32) Dong, B.; Li, N.; Zheng, L.; Yu, L.; Inoue, T.; Surface Adsorption and Micelle Formation of Surface Active Ionic Liquids in Aqueous Solution. Langmuir 2007, 23 (8), 4178–4182. (33) Singh, T.; Kumar, A.; Self–Aggregation of Ionic Liquids in Aqueous Media: A Thermodynamic Study. Colloids Surf., A 2008, 318 (1–3), 263–268. (34) Blesic, M.; Lopes, A.; Melo, E.; Petrovski, Z.; Plechkova, N. V.; Lopes, J. N. C.; Seddon, K. R.; Rebelo, L. P. N.; On the Self– Aggregation and Fluorescence Quenching Aptitude of Surfactant Ionic Liquids. J. Phys. Chem., B 2008, 112 (29), 8645–8650. (35) Wang, H.; Wang, J.; Zhang, S.; Xuan, X.; Structural Effects of Anions and Cations on the Aggregation Behavior of Ionic Liquids in Aqueous Solutions. J. Phys. Chem., B 2008, 112 (51), 16682–16689. (36) Xu, W.; Wang, T.; Cheng, N.; Hu, Q.; Bi, Y.; Gong, Y.; Yu, L.; Experimental and DFT Studies on the Aggregation Behavior on Imidazolium-Based Surface Active Ionic Liquids with Aromatic Counterions in Aqueous Solutions. Langmuir 2015, 31, 1272–1282. (37) Wang, T.; Wang, L.; Jin, Y.; Chen, Peng.; Xu, W.; Yu, L.; Study the Effect of Substituted Position in Aromatic Counterion to Self-aggregation of Cation Surface Active Ionic Liquid in Aqueous Medium. J. Mol. Liq. 2015, 204, 90–94. (38) Bi, Y.; Wei, H.; Hu, Q.; Xu, W.; Gong, Y.; Yu, L.; Wormlike Micelles with Photoresponsive Viscoelastic Behavior Formed by Surface Active Ionic Liquid/Azobenzene Derivative Mixed Solution. Langmuir 2015, 31, 3789–3798. (39) Hough, L. W.; Rogers, R. D.; Ionic Liquids Then and Now: From Solvents to Materials to Active Pharmaceutical Ingredients. Bull. Chem. Soc. Jpn. 2007, 80 (12), 2262– 2269. (40) Dobler, D.; Schmidts, T.; Klingenhofer, I.; Runkel, F.; Ionic Liquids as Ingredients in Tropical Drug Delivery Systems. Int. J. Pharma. 2012, 441 (1–2), 620–627. (41) Hough, L. W.; Smiglak, M.; Rodríguez, H.; Swatloski, R. P.; Spear, S. K.; Daly, D. T.; Pernak, J.; Grisel, J. E.; Carliss, R. D.; Soutullo, M. D.; Davis, J. H.; Rogers, R. D.; The


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Third Evolution of Ionic Liquids: Active Pharmaceutical Ingredients. New J. Chem. 2007, 31 (8), 1429–1436. (42) Pinto, P. C. A. G.; Ribeiro, D. M. G. P.; Azevedo, A. M. O.; Justina, V. D.; Cunha, E.; Bica, K.; Vasiloiu, M.; Reis, S.; Saraiva, L. M. F. S.; Active Pharmaceutical Ingredients Based on Salicylate Ionic Liquids: Insights into the Evaluation of Pharmaceutical Profiles. New J. Chem. 2013, 37 (12), 4095–4102. (43) Frisso, C. P.; Gindri, I. M.; Tier, A. Z.; Buriol, L.; Moreira, D. N.; Martins, M. A. P.; Pharmaceutical Salts: Solids to Liquids by Using Ionic Liquid Design. Ionic Liquids–New Aspects for the Future. Dr. Jun–ichi Kadokawa (Ed.), InTech 2013, 557–579. (44) Tourne–Peteilh, C.; Devoisselle, J.; Vioux, A.; Judeinstein, P.; In, M.; Viau, L.; Surfactant Properties of Ionic Liquids Containing Short Alkyl Chain Imidiazolium Cations and Ibuprofenate Anions. Phys. Chem. Chem. Phys. 2011, 13 (34), 15523–15529. (45) Tourne–Peteilh, C.; Coasne, B.; In, M.; Bervet, D.; Devoisselle, A. V.; Viaue, L.; Surfactant Behavior of Ionic Liquids Involving a Drug: From Molecular Interactions to Self– Assembly. Langmuir 2014, 30 (5), 1229–1238. (46) Bica, K.; Rodriguez, H.; Gurau, G.; Cojocaru, O. A.; Riisager, A.; Fehrmann, R.; Rogers, R. D.; Pharmaceutically Active Ionic Liquids with Solids Handling, Enhanced Thermal Stability, and Fast Release. Chem. Commun. 2012, 48 (44), 5722–5424. (47) Balk, A.; Wiest, J.; Widmer, T.; Galli, B.; Holzagrabe, U.; Meinel, L.; Transformation of Acidic Poorly Water Soluble Drugs into Ionic Liquids. Eup. J. Pharma. and Biopharam. 2015, 94, 73–82. (48) Balk, A.; Holzagrabe, U.; Meinel, L.; ‘Pro et contra’ Ionic Liquid Drugs – Challenges and Opportunities for Pharmaceutical Translation. Eup. J. Pharma. and Biopharam. 2015, 94, 291–304 (49) Carquigny, S.; Lakard, B.; Moutarlier, V.; Hihn, J.; Viau, L.; Investigation of Pharmaceutically Active Ionic Liquds as Electrolyte for the Elecrtosynthesis of Polypyrrole and Active Component in Controlled Drug Delivery. Electrochem. Acta, EA 27521. (50) Ferraz, R.; Branco, L. C.; Marrucho, I. M.; Araujo, J. M. M.; Rebelo, L. P. N.; da Ponte, M. N.; Prudencio, C.; Noronha, J. P.; Petroviski, Z.; Development of Novel Ionic Liquids Based on Ampicillin. Med. Chem. Commun. 2012, 3 (4), 494–497. (51) Phillips, J. N.; The Energetics of Micelle Formation. Trans. Faraday Soc. 1955, 51, 561-569. (52) Turro, N. J.; Yekta, A.; Luminescent Probes for Detergent Solutions. A Simple Method for Determination of the Mean Aggregation Number of Micelles. J. Am. Chem. Soc. 1978, 100 (18), 5951–5952. (53) Grieser, F.; Drummond, C. J.; The Physicochemical Properties of Self–Assembled Surfactant Aggregates as Determined By Some Molecular Spectroscopic Probe Techniques. J. Phys. Chem. 1988, 92 (20), 5580–5593.


Page 36 of 57

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(54) Wang, J.; Zhang, L.; Wang, H.; Wu, C; Aggregation Behavior Modulation of 1– Dodecyl–3–Methylimidazolium Bromide by Organic Solvents in Aqueous Solution. J. Phys. Chem. B 2011, 115 (17), 4955–4962. . (55) Galano, P. D.; El Seoud, O. A.; Surface Active Ionic Liquids: Study of the Micellar Properties of 1-(1-Alkyl)-3-Methylimidazolium Chlorides and Comparison with Structurally Related Surfactants. J. Colloids Interface Sci. 2011, 361 (1), 186–194 (56) Galano, P. D.; El Seoud, O. A.; Micellar Properties of Surface Active Ionic Liquids: A Comparison of 1-Hexadecyl-3-Methylimidazolium Chloride with Structurally Related Cationic Surfactants. J. Colloids Interface Sci. 2010, 345 (1), 1–11. (57) Geng, F.; Liu, J.; Zheng, L.; Yu, L.; Li, Z.; Li, G.; Tung, C.; Micelle Formation of Long-Chain Imidazolium Ionic Liquids in Aqueous Solution Measured by Isothermal Titration Microcalorimetry. J. Chem. Eng. Data 2010, 55 (1), 147–151. (58) Karumbamkandathil, A.; Ghosh, S.; Anand, U.; Saha, P.; Mukherjee, M.; Mukherjee, S.; Micelles of Benzethonim Chloride Undergoes Spherical to Cylindrical Shape Transformation: An Intrinsic Fluorescence and Calorimetric Approach. Chem. Phys. Lett. 2014, 593, 115–121. (59) Rosen, M. J., Surfactants and Interfacial Phenomena, 2nd Ed. John Wiley & Sons, 1989, 393–419. (60) Rodriguez, J. R.; Gonzalez-Perez, A.; DelCastillo, J. L.; Czapkiewicz, J., Thermodynamics of Micellization of Alkyldimethylbenzylammonium Chlorides in Aqueous Solutions. J. Colloid Interface. Sci. 2005, 250 (2), 438–443. (61) Chen, L.; Shi-Yow. L.; Chiung-Chang. H.; En-Ming, C.; Temperature Dependence of Critical Micelle Concentration of Polyoxyethylenated Non-Ionic Surfactants. Colloids Surf., A 1998, 135(1–3), 175–181. (62) Mehta, S. K., Bhasin, K. K.; Chauhan, R.; Dham, S.; Effect of Temperature on Critical Micelle Concentration and Thermodynamic Behavior of Dodecyldimethylethylammonium Bromide and Dodecyltrimethylammonium Chloride in Aqueous Media. Colloids Surf., A 2005 255 (1–3), 153–157. (63) Muller, N.; Temperature Dependence of Critical Micelle Concentrations and Heat Capacities of Micellization for Ionic Surfactants. Langmuir 1993, 9 (1), 96–100. (64) Weber, P. C.; Salemme, F. R.; Applications of Calorimetric Methods to Drug Discovery and the Study of Protein Interactions. Curr. Opin. Struc. Bio. 2003, 13 (1), 115– 121. (65) Jaiswal, S.; Mondal, R.; Paul, D.; Mukherjee, S.; Investigating the Micellization of The Triton-X Surfactants: A Non-invasive Fluorometric And Calorimetric Approach. Chem.l Phys. Lett. 2016, 646, 18–24.


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(66) Martinez, J.; Marciani-Calles, J.; Cobos, E.; Lglesias-Bexiga, M.; Luque, I.; RuizSanz, J.; Isothermal Titration Calorimetry: Thermodynamic Analysis of the Binding Thermograms of Molecular Recognition Events by using Equilibrium Models. Intech 2013. (67) Raman-Geurrero, A.; Vernon-Carter, E., Demarse, N.; TA Instruments – Application Note, MCPAN-2010-01 (68) Moroi, Yoshikiyo. Micelles: Theoretical and Applied Aspects. Springer Sci. & Business Media 1992. (69) Carnero Reuiz, C.; Thermodynamics of Micellization of Tetradecyltrimethylammonium Bromide In Ethylene Glycol-Water Binary Mixtures. Colloid Polym. Sci. 1999, 277, 701–707. (70) le Maire, M., Champeil, P., & Møller, J.; Interaction of Membrane Proteins and Lipids with Solubilizing Detergents. Biochimica et Biophysica Acta-Biomembranes 2000, 1508(1), 86-111. (71) Tanford, C.; The Hydrophobic Effect: Formation of Micelles and Biological Membranes 1980, 2d Ed. J. Wiley. (72) Paul, A.; Mandal, P. K.; Samanta. A.; On the Optical Properties of the Imidazolium Ionic Liquids. J. Phys. Chem., B 2005, 109 (18), 9148–9153. (73) Harward, S. J.; Ober, T. J.; Oliveira, M. S. N.; Alves, M. A., Mckinley, G. H.; Extensional Rheology and Elastic Instabilities of a Wormlike Micellar SolutioniIn a Microfluidic Cross-Slot Device. Soft Matter 2012, 8 (2), 536–555. (74) Clausen, T. M.; Vinson, P. K.; Minter, J. R.; Davis, H. T.; Talmon, Y.; Miller, W. G.; Viscoelastic Micellar Solutions: Microscopy and Rheology. J. Phys. Chem. 1992, 96 (1), 474–484. (75) Hartmann, V.; Cressely, R.; Influence of Sodium Salicylate on the Rheological Behavior of an Aqueous CTAB Solutions. Colloids Surf., A 1997, 121 (2–3), 151–162. (76) Aswal, V. K.; Goyal, P. S.; Thiyagarajan, P., Small–Angle Neutron–Scattering and Viscosity Studies of CTAB/NaSal Viscoelastic Micellar Solutions. J. Phys. Chem. B 1998, 102 (14), 2469–2473. (77) Pokhriyal, N. K.; Joshi, J. V.; Goyal, P. S.; Viscoelastic Behaviour of Cetyl Trimethyl Ammonium Bromide/Sodium Salicylate/Water System: Effect of Solubilisation of Different Polarity Oils. Colloids Surf., A 2003, 218 (1), 201–212. (78) Davies, T. S.; Ketner, A. M.; Raghavan, S. R.; Self–Assembly of Surfactant Vesicles that Transform into Viscoelastic Wormlike Micelles upon Heating. J. Am. Chem. Soc. 2006, 128 (20), 6669–6675. (79) Walker, L. M.; Rheology and Structure of Worm–Like Micelles. Curr. Opin. Colloid. Interface. Sci. 2001, 6 (5), 451–456.


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(80) Dash, S.; Murthy, P. N.; Nath, L.; Chaowdhury, P.; Kinetic Modeling on Drug Release from Controlled Drug Delivery Systems. Acta. Pol. Pharm. 2010, 67 (3), 217–223. (81) Hixson, A. W.; Crowell, J. H.; Dependence of Reaction Velocity Upon Surface and Agitation. Ind. & Eng. Chem. 1931, 23(10), 1160–1168. (82) Higuchi, T.; Mechanism of Sustained‐Action Medication. Theoretical Analysis of Rate of Release of Solid Drugs Dispersed in Solid Matrices. J. Pharm. Sci. 52(12), 1145– 1149.


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Table 1. Critical aggregation concentration (CAC), degree of counter ion binding β, Gibbs free energy ∆G0a, standard enthalpy ∆H0a, and standard entropy ∆S0a, of aggregation for aqueous solutions of ionic liquids at different temperatures T/K

Surface Tension CAC

Conductivity CAC

∆G0a

ITC

∆H0a

298.15 308.15 318.15

1.23 ± 0.05 1.44 ± 0.05 1.88 ± 0.05

1.27 ± 0.05 1.51 ± 0.05 1.71 ± 0.05

-45.68 ± 0.13 -46.12 ± 0.13 -47.15 ± 0.17

-18.93 ± 0.08 -20.08 ± 0.09 -20.08 ± 0.09

298.15 308.15 318.15

19.79 ± 1.0 20.40 ± 1.0 21.11 ± 1.0

19.93 ± 0.9 20.13 ± 0.9 20.77 ± 0.9

-30.08 ± 0.17 -30.23 ± 0.17 -30.26 ± 0.16

-2.32 ± 0.06 -2.41 ± 0.06 -2.50 ± 0.06

∆S0a [C12mpip][AcSa] 75.64 ± 0.81 84.51 ± 0.93 89.71 ± 0.94 [C12mpip][Cl] 93.10 ± 0.37 90.29 ± 0.35 87.24 ± 0.33

Fluorescence

β

CAC

∆G0a

∆H0a

∆S0a

CAC

1.52 ± 0.01 1.60 ± 0.01 1.71 ± 0.02

-50.24 ± 3.2 -61.52 ± 6.1 -71.18 ± 8.7

-11.78 ± 0.06 -16.96 ± 0.09 -21.71 ± 0.13

129.0 ± 0.98 144.6 ± 1.5 155.5 ± 1.7

1.28 ±0.05 -----

0.73 0.71 0.61

17.5 ± 0.9 18.3 ± 0.9 19.4 ± 0.9

-26.38 ± 1.3 -25.48 ± 1.1 -26.08 ± 1.2

-0.966 ± 0.01 1.628 ± 0.02 4.033 ± 0.03

85.25 ± 0.76 90.91± 0.81 101.0 ± 0.94

19.50± 1.0 -----

0.53 0.49 0.45

CAC in mMdm-3, ∆G0a and ∆H0a in kJmol-1 and ∆S0a in Jmol-1K-1.


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Table 2. Surface tension at CAC, γCAC, surface pressure at CAC, πCAC, surface tension reduction pC20, maximum surface excess concentration Γmax and surface area per molecule at air/water interface aୱଵ of ionic liquids at different temperatures T/K

γCAC mNm−1

298.15 308.15 318.15

29.0 ± 0.2 28.4 ± 0.2 27.8 ± 0.2

298.15 308.15 318.15

36.5 ± 0.2 35.4 ± 0.2 34.8 ± 0.2

ΠCAC pC20 −1 mNm [C12mpip][AcSa] 42.9 ± 0.2 1.16 41.7 ± 0.2 1.17 40.9 ± 0.2 1.19 [C12mpip][Cl] 35.3 ± 0.2 3.35 34.9 ± 0.2 3.29 34.0 ± 0.2 3.15


Γmax × 1010 mol cm−2

aୱଵ Å2

1.6 ± 0.1 1.4 ± 0.1 1.1 ± 0.1

104.1 ± 1 119.8 ± 1 144.9 ± 1

2.4 ± 0.1 2.1 ± 0.1 1.7 ± 0.1

68.5 ± 1 79.4 ± 1 97.3 ± 1

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Table 3. Average translational diffusion coefficient at zero concentration, തത Dതത଴ , semi major b and semi minor a axis and aggregation number Nagg, for micellar aggregates in aqueous solutions of [C12mpip][AcSa] and [C12mpip][Cl] at different temperatures

Temp 0 C

തതത ‫ܦ‬଴ × 107 cm2sec-1

Prolate ellipsoidal model b a b/a Å Å

Nagg തതത ‫ܦ‬଴

SSF*

[C12mpip][AcSa] 25 1.17 ± 0.05 28 18 1.6 108 120 35 1.55 ± 0.07 26 18 1.4 101 45 2.0 ± 0.09 25 18 1.4 97 [C12mpip][Cl] 25 13.6 ± 0.2 22 16 1.4 86 66 35 17.9 ± 0.3 21 16 1.3 64 45 22.5 ± 0.6 20 16 1.3 61 * steady state fluorescence measurements, the uncertainty in the values of b and a is 10 %.


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Langmuir

CAPTIONS TO THE FIGURES

Figure 1. (a) Surface tension vs. log C isotherms for [C12mpip][AcSa] aqueous solutions; (b) Specific conductance vs. concentration isotherms for [C12mpip][AcSa] aqueous solutions at ((■) 298.15, ( ○) 308.15 and (▲) 318.15) K; (c) Representative pyrene fluorescence emission spectra in aqueous solutions of [C12mpip][AcSa] corresponding to the concentrations at below and above CAC, excitation wavelength: 335 nm, pyrene concentration 2 ×10-6 Mdm-3; (d) Variation of I1/I3 ratio with concentration of aqueous solutions of [C12mpip][AcSa] at 298.15 K; (e), (f) and (g) Raw titration data from ITC in µJs-1, each peak corresponds to a single injection of 15 mMdm-3 aqueous solution of [C12mpip][AcSa] into water; (h), (i) and (j) Enthalpies of dilution of [C12mpip][AcSa ] with respect to the injected number of moles of [C12mpip][AcSa]; (k), (l) and (m) First derivative of heat absorbed with respect to concentration of [C12mpip][AcSa] aqueous solutions at 298.15, 308.15 and 318.15 K respectively. Figure 2. Effect of concentration on the hydrodynamic radii (Rh) of aqueous solutions of [C12mpip][AcSa] at different temperatures (in K): (a) 298.15 (b) 308.15 (c) 318.15. ഥ ) vs. concentration of the aqueous solution of [C12mpip][AcSa] Figure 3. Diffusion coefficient (D at different temperatures (in K): () 298.15, () 308.15 () 318.15. Figure 4. Plots of ln(I/I0) for pyrene vs. concentration of the benzophenone as quencher for aqueous solution of [C12mpip][AcSa] (13 mMdm-3); The inset figure shows changes in the pyrene fluorescence emission spectra as a function of the concentration of quencher, benzophenone in [C12mpip][AcSa] aqueous solutions.


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 57

Figure 5. Shear viscosity vs. shear rate plots at 298.15 K for [C12mpip][AcSa] and SS mixture solutions prepared from initial stock solutions of 200 mM (X[C12mpip][AcSa] = () 0.2, () 0.4, () 0.6, () 0.8); 400 mM stock solutions (X[C12mpip][AcSa] = () 0.2, () 0.4, () 0.6, () 0.8); 600 mM stock solutions (X[C12mpip][AcSa] = () 0.2, () 0.4, ( ) 0.6, ( ) 0.8); points (exp.) and line (fitted). Figure 6. Zero shear viscosity vs. X[C12mpip][AcSa] profiles for mixtures of [C12mpip][AcSa] and SS at 298.15 K: concs. of initial stock solutions (in mM); (a) 100, (b) 200, (c) 400, (d) 600. Figure 7. Variation of the storage modulus (G') (closed symbol) and loss modulus (G'') (open symbol) as a function of frequency for the mixtures of [C12mpip][AcSa] and SS, at X[C12mpip][AcSa] corresponding to maxima in viscosities: (,) X[C12mpip][AcSa] = 0.4 (200 mM stock solutions); (,) X[C12mpip][AcSa]= 0.4 (400 mM stock solutions); (,) X[C12mpip][AcSa] = 0.2 (600 mM stock solutions). Figure 8. Cole-Cole plots (solid lines indicate the best fits of Maxwell Model), points are the experimental data for mixture solutions of [C12mpip][AcSa] and SS; X[C12mpip][AcSa] = 0.2 (600 mM stock solutions). Figure 9. Storage moduli G'(,), loss moduli G''(,) and complex viscosity [η*] (,)versus temperature profiles for the mixtures of [C12mpip][AcSa] and SS, at X[C12mpip][AcSa] corresponding to maxima in viscosities: (a,a’) X[C12mpip][AcSa] = 0.4 (200 mM stock solutions), (b,b’) X[C12mpip][AcSa] = 0.4 (400 mM stock solutions); (c,c’) X[C12mpip][AcSa] = 0.2 (600 mM stock solutions) open symbols (warming mode), close symbols (cooling mode).


Page 45 of 57

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 10. Storage moduli G'(,), loss moduli G''(,) and tan δ ( ) versus temperature profiles for mixtures of [C12mpip][AcSa] and SS,

at X[C12mpip][AcSa] = 0.2 (600 mM stock

solutions) (a) open symbols (warming mode) (b) close symbols (cooling mode). Figure 11. Profiles showing the percentage fraction of [AcSa] anion released as a function of time for gels or solutions prepared from the mixture of [C12mpip][AcSa] and SS at () 293.15 K and () 310.15 K.


Langmuir

65

(a)

60 55 κ /mS cm-1

γ , mNm

-1

50 45 40 35 30 25

1E-4

1E-3

0.01

0.1

C, mMdm

1

260 240 220 200 180 160 140 120 100 80 60 40 20 0 0.0

(b)

0.5

1.0

1.5

2.0

2.5

-3

3.0

3.5

4.0

4.5

C, mMdm

-3

5

2.0x10

1.8

(c)

Below cmc Above cmc

(d)

25

1.7

5

1.5x10

CPS

1.6 5

1.5

(I1/I3)

1.0x10

1.4

4

5.0x10

1.3 1.2

0.0 340

360

380

400

420

440

1.1

460

λ , nm

1

0

10

-3

0

(g)

(f)

(e)

-2

-2

-1

Heat Rate / µJs

0.1

Conc, mMdm

0

-2

-4 -4

-6 -8

-4 -6

-10 -6

0

10000

20000

30000

-8 40000 0

10000

20000

30000

-12 40000 0

10000

20000

30000

40000

t, s 20

20

(h)

∆H/ kJmol

-1

18

20

(i)

18 16

16

14

14

14

12

12

12

10

10

10

8

8

6

6

4

4

∆Hmic

8 4 2

2 0

0

-2

-2

-2

1

2

3

0.5

0

1

2

-3

3

0

1

2

1

(l)

(m)

0.0

0.0

3

C, mMdm 0.5

(k) 6

∆ Hmic

6

∆ Hmic

0 0

(j)

18

16

2

dQ/dc× 10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 57

0

-0.5 -0.5

-1

-1.0

0

1

2

3

1.603

1.527

-1.0

1.065

0

1

2

3

-2 0.5

1.0

1.5

2.0

2.5

-3

C/ mMdm

Figure 1


Page 47 of 57

7

7

7

(b)

(a) 6

6

10

100

3 20 mM

20 mM 2 10 mM

1

5 mM

1

30 mM

30 mM

2

10 mM

0

4

3

20 mM

1

40 mM

4

30 mM

2

5 40 mM

40 mM 4

-1

50 mM

50 mM 5

3

(c) 6

50 mM 5

Frequency, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

5 mM

0

1000

-1 10000 1

10

100

10 mM

1

5 mM

0

1000

-1 10000 1

10

100

1000

Rh, nm

Figure 2


10000

Langmuir

5.5

2 -1

5.0

7

D x 10 , cm s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 48 of 57

4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5

0

10

20

30

-3

40

50

60

Conc, mMdm

Figure 3


Page 49 of 57

0.0

5

3x10

0.02 mM [Q]

5

-0.5

CPS

2x10

0.2 mM 5

1x10

-1.0

ln[I1/I0)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

0 340

-1.5

360

380

400

420

440

460

λ / nm

-2.0 -2.5 0.0

-5

5.0x10

-4

1.0x10

-4

-4

1.5x10

2.0x10

-4

2.5x10

3.0x10

-3

Cbenz / Mdm

Figure 4


Langmuir

10

1

viscosity, Pa.s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 50 of 57

0.1

0.01

1E-3 10

100 -1

Shear rate, s

Figure 5


Page 51 of 57

0.020

(a) 0.015

0.010

0.005

0.000 0.0

0.2

0.4

0.6

0.8

1.0

1.5

(b) 1.0

0.5

0.0

0.0 1500

ηo, Pa.s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

0.2

0.4

0.6

0.8

1.0

(c) 1000

500

0

0.0

0.2

0.4

0.6

0.8

1.0

(d) 6

4

2

0

0.0

0.2

0.4

0.6

0.8

1.0

XIL

Figure 6


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

G' or G", Pa

Langmuir

10

7

10

6

10

5

10

4

10

3

10

2

10

1

10

0

10

Page 52 of 57

-1

0 .0 1

0 .1

1

ω , rad s

-1

10

100

Figure 7


Page 53 of 57

200 180 160 140 120 G", Pa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

100 80 60 40

experimental Fitted

20 0

0

100

200

300

400

500

G', Pa

Figure 8


Langmuir

10

8

10

7

3

10

6

10

2

10

5

10

1

10

4

10

0

10

3

10

-1

10

2

10

-2

10

1

10

-3

10

0

10

5

10

4

10

(a)

10 10

5

10

4

20

30

40

50

(b)

10

8

10

7

10

3

10

6

10

5

10

2 1

10

4

10

10

3

10

0

10

2

10

1

10

0

10

-1

10

-2

10

-3 -4

10

-1

10

10

-2

10

-5 -6

10

-3

10

10

10

5

10

4

10

3

10

2

10

1

10

0

20

30

40

50

10

20

30

40

50

60

10

8

10

7

10

6

10

5

10

4

10

3

20

30

40

50

60

(b')

10

60

(c)

(a')

10

60

[η*],Pa.s

G' or G" ,Pa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 54 of 57

20

30

40

50

60

(c')

10

20

30

40

50

60

o

T, C

Figure 9


Page 55 of 57

5

10

5

6

10

(b)

(a)

0.8 4

10

4

2 2

tan δ

3

10

10

G' or G", Pa

tan δ

4

G' or G", Pa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

0.6

3

10

10

0.4

1

10

0

10

20

30

40

50

60

0 70

o

T, C

0

10

20

30

40

50

60

70

Figure 10


Langmuir

70 % Fractional Drug Realsae

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 56 of 57

60 50 40 30 20 10 0

0

120

240

360

480

600

t, min Figure 11


Page 57 of 57

Graphic for Table of Contents

7

10

6

10

5

10

4

10

G'G", Pa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

3

10

2

10

1

10

0

10

-1

10 0.01

0.1

1

-1

ω, rads

10

100

Moduli vs. frequency curves for gels of RTIL and SS mixture


Surfactant and Gelation Properties of ... - ACS Publications

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