Complexation of β-Cyclodextrin with a Gemini Surfactant Studied by

Article pubs.acs.org/Langmuir

Complexation of β‑Cyclodextrin with a Gemini Surfactant Studied by Isothermal Titration Microcalorimetry and Surface Tensiometry Mária Benkő,†,§ László A. Király,† Sándor Puskás,‡ and Zoltán Király*,† †

Department of Physical Chemistry and Materials Science, University of Szeged, Aradi Vt. 1, H-6720 Szeged, Hungary MOL Hungarian Oil and Gas Plc, Exploration & Production, Integrated Field Applications, P.O. Box 37, H-6701 Szeged, Hungary § MTA-SZTE Supramolecular and Nanostructured Materials Research Group, Hungarian Academy of Sciences, Dóm tér 8, H-6720 Szeged, Hungary ‡

S Supporting Information *

ABSTRACT: We report on the inclusion complex formation of β-cyclodextrin (βCD) with a cocogem surfactant (counterion-coupled gemini surfactant; (bis(4(2-alkyl)benzenesulfonate)-Jeffamine salt, abbreviated as ABSJ), studied by isothermal titration calorimetry (ITC) and surface tension (SFT) measurements. We measured the critical micelle concentration (cmc) of ABSJ in water by the two experimental techniques in the temperature range 283−343 K, and determined the thermodynamic parameters of the complex formation directly by ITC and indirectly by the SFT. The stoichiometry (N), the binding constant (K), and the enthalpy of complexation were determined, and the Gibbs free energy and the entropy term were calculated from the experimental data. A novel method is presented for the determination of N and K by using surface tensiometry.

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INTRODUCTION β-Cyclodextrin (βCD) is the most stable native cyclodextrin (CD) and is composed of seven (α-1,4)- linked α-Dglucopyranose units.1 The molecule adopts a rigid doughnutshape. While the outer surface of βCD is hydrophilic, the inner surface is hydrophobic. As a consequence, βCD is water-soluble and can accommodate poorly water-soluble compounds in its cavity, provided that the guest molecule fits geometrically into the interior of the host molecule. In supramolecular chemistry, such molecular encapsulation is termed inclusion complex formation.2 The guest molecule is stabilized in the interior of the host, by dispersion forces. This host−guest complex formation is reversible; under suitable conditions, the guest molecule can be released from the host.2 Since the inclusion complex is water-soluble, this kind of encapsulation has many advantages in practice: it prevents the degradation of the guest, dissolution of the guest can be retarded, organic pollutants can be removed from water, it can be utilized in drug formulation, and so forth. CDs are widely used in the pharmaceutical and food industries3 and in the field of enhanced oil recovery (EOR).4 Traditional surfactants5−7 are nowadays often replaced by gemini surfactants, in which two monomeric surfactants are connected through a spacer group.8−11 The critical micelle concentrations (cmc’s) of gemini surfactants are 1 or even 2 orders of magnitude less than those of the corresponding monomeric derivatives. As a consequence, the desired surface active properties are achieved at lower concentrations and lower amounts of gemini surfactants are therefore required than of traditional ones. Besides the economic aspects, the reduced © 2014 American Chemical Society

level of environmental pollution is a feature of interest. The synthesis of gemini surfactants began some 20 years ago.11−13 There is currently great interest in this new family of surfactants from the aspect of EOR in crude oil exploitation.14,15 In recent years, a number of investigations have been reported on the interactions between gemini surfactants and CDs. Instrumental studies have involved NMR;16−22 calorimetry;16,18,23,24 electric conductivity;17 interfacial tension (IFT)19 and surface tension (SFT) measurements;20,25−27 viscosity, sound velocity,28 and small-angle neutron scattering measurements.29 In the present work, calorimetric and tensiometric measurements of the temperature dependence of the cmc of the counterion-coupled gemini (cocogem) surfactant bis(4-(2alkyl)benzenesulfonate)-Jeffamine salt (ABSJ)12 are compared, and a thermodynamic study of the inclusion complex formation between ABSJ and βCD is described.

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EXPERIMENTAL SECTION

Materials. βCD was a product of Cyclolab Ltd., Budapest, Hungary. The powder contained 13.1% water as determined with a thermogravimetric apparatus (Mettler-Toledo). Knowledge of the water content was necessary for the preparation of solutions with accurately known concentrations. ABSJ was synthesized via the 2:1 coupling reaction between Lutensite A-LBS and Jeffamine D230, as described earlier.12 The number-average molar mass of the surfactant Received: April 10, 2014 Revised: May 19, 2014 Published: May 20, 2014 6756

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where Qi is the heat evolved or absorbed, ΔHi is the molar heat of binding per injection, and V0 is the volume of the calorimeter vessel. The area below each calorimeter peak Pi yielded a single point in the S-shaped reaction enthalpy curve:

was determined with a mass spectrometer (Agilent Technologies) to be 884 Da with a polydispersity index of 1.04. The structure and chemical composition of ABSJ are given in Figure 1.

ΔHi =

∫ Pi(t ) dt

(4)

Step-by-step summation of the peak areas yields the enthalpy curve. Analysis of the S-shaped curves with Microcal Origin 7 software provided the stoichiometry (N) of the complexation reaction, the binding constant (K), and the standard enthalpy of binding (ΔH). A typical enthalpogram of the thermometric titration and a typical Sshaped enthalpy curve are presented in Figures 2 and 3, respectively, in the Results and Discussion section. Figure 1. Structure and chemical composition of bis(4-(2-alkyl)benzenesulfonate)-Jeffamine salt (ABSJ).

Ultrapure water was produced with a Milli-Q purification system (Millipore). Surfactant solutions were made up volumetrically. Methods. Isothermal Titration Calorimetry (ITC). Thermometric titrations were performed in the temperature range 283−353 K, using a Microcal VP-ITC power compensation microcalorimeter with a cell volume of 1.4163 mL. The cell was loaded with Milli-Q water or 0.001 M βCD solution and titrated with the surfactant solution (∼10 × cmc). A volume of 5 μL of surfactant solution was injected at intervals (10 s per injection, 5 min between injections) for the cmc measurements, and 10 μL aliquots were taken for the study of the formation of the inclusion complex. For each step, the heat evolved or absorbed was recorded continuously. The enthalpogram (the calorimeter power signal versus time recording) composed of a series of calorimetric peaks and the summation of the sequential peak areas provided an S-shaped curve, the integral enthalpy versus concentration diagram. Data were analyzed with Microcal Origin 7 software. The experimental error in the determination of the reaction enthalpies was less than 3.0%. In the complexation experiments, corrections for the enthalpy of dilution of ABSJ were made by means of blank measurements (titration of water with ABSJ solutions). Surface Tension (SFT) Measurements. The SFT of the solutions was measured by using a tensiometer (Krüss K100MK2), a computercontrolled apparatus supplied with a thermostat and an automatic buret. The Wilhelmy-plate method was applied. Measurements were made by inverse titration. A volume of 50 mL of a 1.7 mM surfactant solution was titrated with water (cmc measurement) or with 3.4 mM βCD solution (complex formation) in aliquots of 10 mL in 40 steps. Each experimental point was the average of 5−10 measurements. The standard deviation of the experimental data was less than 0.1 mN·m−1. Data Analysis. Thermodynamic Analysis. The results of calorimetric measurements of the cmc of ABSJ as a function of temperature have been published previously,12 but this was repeated in the present study: the agreement was more than satisfactory. The complexation of βCD with surfactant S can be described by the following equation:30,31

Figure 2. (Endothermal) enthalpogram of the heat of dilution of ABSJ (∼10 × cmc) in water at 303 K.

Figure 3. S-shaped curves for the calorimetric titration of water with ABSJ solutions. Investigated temperatures: (■) 283 K, (●) 293 K; (▲) 303 K; (▼) 313 K; (◆) 323 K; (◀) 333 K; (★) 343 K. For clarity, only every second experimental points are indicated.

Equations 5 and 6 allow calculation of the Gibbs free energy and the entropy term of the reaction:30

(1)

where βCD−Sn denotes the host−guest inclusion complex and n is the number of guest molecules inserted into one guest molecule. The binding constant is defined as

K=

[β CD−Sn] [S]f [β CD]f

(2)

where [βCD−Sn]denotes the inclusion complex concentration, [S]f the free surfactant concentration, and [βCD]f the free βCD concentration. The complexation reaction was monitored calorimetrically as

Q i = ΔHiV0[β CD−Sn]i

(5)

T ΔS = ΔH − ΔG

(6)

Surface Tension (SFT) Analysis. Measurement of the surface tension as a function of concentration is a well-established method for determination of the cmc, and this procedure will therefore not be described in detail here. However, the investigation of CD−Sn inclusion complex formation by SFT may be regarded as a new experimental method.19 The total βCD concentration in the solution ([βCD]0) is the sum of the free CD concentration ([βCD]f) and the concentration of the complex ([βCD−Sn]):

K

β CD + nS ⇔ β CD−Sn

ΔG = − RT ln K

[β CD]0 = [β CD]f + [β CD−Sn]

(7)

A similar equation holds for the surfactant:

[S]0 = [S]f + [β CD−Sn]

(3)

(8)

Substitution of eqs 7 and 8 into eq 2 and rearrangement leads to 6757

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Figure 4. SFT titration curves of ABSJ solutions with water at 303 K (a) and at various temperatures (b). For clarity, only every second experimental points are indicated. K=

[S]0 − [S]f ([β CD]0 − [S]0 + [S]f )[S]f

(9)

Equation 9 can be rearranged to give ⎞ 1 ⎛ [S] [S]0 − [S]f = − ⎜ 0 − 1⎟ + [β CD]0 K ⎝ [S]f ⎠

(10)

More details about the derivation of eq 10 are given in the Supporting Information of this paper. [S]0 is known, since this quantity is an initial parameter, and [S]f can therefore be determined indirectly from the fitted calibration curve of the CD-free SFT tension versus log concentration data obtained from surfactant−water titration in the premicellar region:

[S]0 − [S]f = [β CD−Sn]

Figure 5. cmc data obtained from ITC (★) and SFT (∗) measurements plotted against temperature.

(11)

A plot of [S]0 − [S]f against [S]0/[S]f − 1 is linear. Linear regression gives the value of K. The temperature dependence of K can be used for calculations of the Gibbs free energy ΔG and the entropy term TΔS (eqs 5 and 6). The temperature dependence of K is fitted well by the semiempirical equation12,28 ln K = P1 +

P2 + P3 ln T T

(12)

which complies with the van’t Hoff enthalpy of inclusion complex formation:

⎛ ∂ ln K ⎞ ⎟ = − R(P − P T ) ΔH vH = − RT 2⎜ 2 3 ⎝ ∂T ⎠

Figure 6. Calorimeter signals. Enthalpogram of the dilution of the surfactant solution (a) and enthalpogram of the complexation (b). Initial concentration of ABSJ, 0.51 mM; initial concentration of βCD, 0.11 mM; T = 298 K.

(13)

where the Pi’s are fitting parameters. The stoichiometry of the complexation was determined as for the cmc measurements. For the surfactant−βCD system, the SFT was measured by tensiometry. With the SFT represented on the nS/nβCD molar ratio scale, the stoichiometric ratio is obtained at the breakpoint.

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RESULTS AND DISCUSSION One of the aims of the present study was a comparison of the cmc values obtained at different temperatures from calorimetry and tensiometry. Determination of the cmc of ABSJ. The cmc was measured in the range 283−343 K. The enthalpogram at 303 K is shown in Figure 2. The S-shaped curves at selected temperatures are displayed in Figure 3. The method of evaluation has been described before12,21,28 and will therefore not be described here. For each temperature, data analysis with the Microcal Origin 7 software provided the values of the cmc and ΔH. The heat of dilution of the micelles (opposite in sign to the heat of micelle formation) changes sign between 293 and 303 K, the micellization process turning from endothermic to exothermic. For each curve, the

Figure 7. Enthalpies, resulting at 298 K after correction, plotted against the mole number of ABSJ relative to that of βCD (molar ratio).

position of the inflection point is closely related to the cmc and the height of the sigmoidal to the enthalpy of micelle formation.12,21,28 A typical SFT titration curve, measured at 303 K, is shown in Figure 4a. The cmc of ABSJ was taken at the intersection point 6758

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As we mentioned earlier, the cmc value of a gemini surfactant may be 10−100 times less than that of the corresponding monomeric surfactant.8−11 While the cmc of dodecylbenzenesulfonate at 298 K is 1.72 mM,12,30 for instance, that of ABSJ is only 0.065 mM at 298 K. Investigations of the Formation of ABSJ−βCD Inclusion Complexes. In the calorimetric investigation of complexation, an attempt was made to keep the surfactant concentration below the cmc in order to avoid the superposition effect of the heat of demicellization on the heat of complexation. However, this was not successful because of the low cmc value of ABSJ (no appreciable calorimeter power signal could be detected). We therefore we carried out the complexation experiment with a surfactant concentration well above the cmc (0.51 mM) at each temperature and made a background correction. There is an option in data analysis with Origin 7 software which made it possible to eliminate the heat attributable to the dilution of the surfactant. The background correction involved subtraction of the heat of dilution of the surfactant from the total heat measured (complexation accompanied by surfactant dilution), as shown in Figure 6. It appears that the correction with the heat of dilution is not negligible. After the correction, the curve in Figure 7 was obtained. The experimental data in Figure 7 were fitted by means of Origin software to give the values of N, K, and ΔH°,21,28,29 and ΔG° and TΔS° were then calculated via eqs 5 and 6. The results are reported below. In the SFT measurements of inclusion complex formation, 50 mL of 1.70 mM surfactant solution was titrated with βCD solution of concentration of 3.4 mM. As a background correction, the surfactant solution was titrated with water in 40 steps, 10 mL each. The results of the SFT experiments at 298 K (Figure 8) reveal that the SFT (γ) values of the surfactant solutions containing inclusion complexes are significantly larger than those of the solutions in the absence of βCD, but otherwise at the same surfactant concentration. This is because a proportion of the surfactant participates in complexation and the remainder goes to the surface. On the other hand, surfactants are more surface active than surfactant−cyclodextrin complexes 22,26 and the ABSJ−βCD complex is gradually displaced from the surface by the surfactant with increasing concentration. The breakpoint of the SFT versus log c curve for the complex is attributed to the cmc of ABSJ in the presence of βCD. The cmc is 1 order of magnitude larger for ABSJ−βCD than that for ABSJ alone. After the breakpoint, the two SFT values remain the same, ∼28 mN·m−1, which is attributed to surface saturation by adsorbed surfactant molecules. In fact, it is expected that the SFT does not change with increasing surfactant concentration beyond the cmc. The common points after the cmc imply that no βCD molecules are present in the adsorption layer. There has been a debate in the literature whether CDs interact directly with micelles. However, it was recently shown that no such interactions occur.22 Competing βCD−micelle interactions can be neglected in the present case also. The stoichiometry of complexation was determined from the SFT versus molar ratio (nABSJ/nβCD) curve (Figure 9). The SFT decreases with increasing molar ratio until it becomes constant. There is a breakpoint in the curve at a molar ratio of about 0.5, which means that two βCD molecules (host) accommodate one ABSJ molecule (guest). Since one ABSJ molecule possesses two long alkyl chains (Figure 1), it may be concluded that the tails penetrate the hydrophobic cavity of βCD. Thus,

Figure 8. cmc SFT measurement (■) and complexation SFT measurement (●) at 298 K.

Figure 9. Determination of the stoichiometry of complexation of βCD with ABSJ by SFT measurements at 298 K.

Table 1. Thermodynamic Parameters of the Formation of Surfactant−βCD Complexes, Obtained from ITC and SFT Measurements T/K

K/M−1

ΔH°/ kJ mol−1

ΔHvH/ kJ mol−1

ΔG°/ kJ mol−1

TΔS°/ kJ mol−1

ITC

288 298 308 318 328 338

887 1920 3080 2580 1160 439

−50.50 −43.51 −42.01 −45.19 −52.84 −74.35

− − − − − −

−27.29 −30.16 −32.38 −32.96 −31.81 −30.05

−23.21 −13.36 −9.63 −12.23 −21.03 −44.30

SFT

288 298 308 318 328 338

1519 1263 1079 754 651 547

− − − − − −

−51.42 −53.15 −54.89 −56.62 −58.36 −60.09

−28.58 −29.12 −29.69 −29.71 −30.24 −30.67

−22.84 −24.04 −25.20 −26.92 −28.12 −29.42

of the fitted straight lines. The SFT versus log c data in the total temperature range studied are displayed in Figure 4b. As expected, the SFT decreases with increasing temperature. The temperature dependence of the cmc determined by the two experimental techniques is given in Figure 5. The agreement between the results of the two methods is satisfactory, taking into consideration that the cmc values are rather low. The cmc versus T curve passes through a minimum at about 303 K, although the minimum is less clear in the case of ITC than in the case of SFT. This observation indicates that the van’t Hoff enthalpy of micelle formation changes from positive to negative, that is, the process changes from endothermic to exothermic at this temperature. A detailed thermodynamic analysis of the micelle formation (changes in enthalpy, entropy and Gibbs free energy) of ABSJ in water was reported recently.12 6759

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Figure 10. Thermodynamic data of the formation of surfactant−βCD complexes, obtained from ITC and SFT measurements, plotted against temperature. (a) Binding constant, (b) enthalpy change, (c) change in Gibbs free energy, and (d) entropy term.

to 3100 to 440 M−1. The explanation of the difference between the two sets of results is yet not clear. It may be speculated that the SFT method is sensitive to the surface properties of the system, while calorimetry is sensitive to the bulk properties. The maximum curve may be related to the change in the depth of penetration of the surfactant into the βCD cavity24 or the change in the hydration state of the headgroup. In any case, calorimetry as a direct experimental method provides more reliable data than does the SFT method, which provides indirect data. Further differences between the results of the two methods are given in Table 1 and Figure 10b−d. The thermodynamic potentials ΔHvH, ΔG, and TΔS derived from SFT measurements are nearly linear functions of temperature. ΔHvH decreases from −51 to −60 kJ mol−1 (exothermic process). It may be noted that the enthalpy values are rather large for the present system. The reason for that is that gemini surfactants are made up of two alkyl chains, and the heat evolved for a dimeric surfactant is about twice of that for the corresponding monomeric derivative. TΔS is increasingly negative, its value changing from −23 to −44 kJ mol−1 (entropy loss). Although complexation is promoted by enthalpy and opposed by entropy, the process is thermodynamically favored: the values of ΔG are between −29 and −31 kJ mol−1. From the ITC data, ΔH°, ΔG°, and TΔS° display extremes at about 308 K. ΔH° passes through a maximum from −74 through −43 to −51 kJ mol−1, TΔS° also passes through also a maximum from −27 through −33 to 30 kJ mol−1, and ΔG° passes through a minimum from −27 through −33 to −30 kJ mol−1. This observation implies that the change of the heat capacity for the present system is positive at low temperature (although it appears to be close to zero). It follows that it is not the hydrophobic effect which is dominating at low temperature. As mentioned in connection with the binding constant, the extraordinary properties of the thermodynamic potentials may originate from a change in depth of the penetration of the surfactant tails into the βCD cavity, conformational changes of the alkyl chains,24 a change in the hydration state of the

the average stoichiometry throughout the entire temperature range studied was found to be βCD:ABSJ = 2.05 ± 0.05. A similar finding was made earlier for gemini−βCD complexation on the basis of density and sound velocity measurements.30 We are not aware of other reports on the determination of the stoichiometry of inclusion complexation through the use of surface tensiometry. The analysis of our titration calorimetric data led to βCD:ABSJ = 2.04 ± 0.06. The agreement between the results of the two methods is more than satisfactory. As concerns the thermodynamic data, the hydrophobic effect predominates in the net complexation process.21−28,31−35 Complex formation can be divided into three main steps. High-energy water molecules (10−13 molecules per βCD2,24) first leave the inner cavity of the βCD. This is an exothermic process and is accompanied by entropy production because the degree of freedom of bulk water molecules is larger than that of captured water. The hydrogen-bond density around the surfactant molecules is larger than that in bulk water, known as the iceberg structure.32−37 In the next step, therefore, before the alkyl chains enter the interior of the βCD, iceberg collapse occurs: the hydrogen-bond density of the solvating water molecules decreases to that of bulk water. This process results in an increase of entropy and is endothermic. The third factor is that the dispersion interactions between the alkyl chain and the hydrophobic core of βCD are exothermic, with some loss in entropy. The role of the change in state of the hydrophilic headgroup upon inclusion is not completely straightforward, but this effect is presumably small because its hydration in the bound state, away from the cavity, must be similar to that in the state in free solution.28 In any case, the net thermodynamic potentials ΔH°, ΔG°, and TΔS° are determined by the superposition of the above three major factors. The values of the binding constant K are given in Table 1 and are plotted against temperature in Figure10a. The SFT data suggest a linear decrease in K with increasing T: in the range 288−338 K, K drops from 1520 to 550 M−1. From calorimetric data, K passes through a maximum from 890 6760

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(11) Menger, F. M.; Keiper, J. S. Gemini surfactants. Angew. Chem., Int. Ed. 2000, 39, 1906−1920. (12) Páhi, A. B.; Király, Z.; Mastalir, Á .; Dudás, J.; Puskás, S.; Vágó, Á . Thermodynamics of micelle formation of the counterion coupled gemini surfactant bis(4-(2-dodecyl)benzenesulfonate)-Jeffamine salt and its dynamic adsorption on sandstone. J. Phys. Chem. B 2008, 112, 15320−15326. (13) Rist, Ø.; Rike, A.; Jones, L.; Carlsen, P. H. J. Synthesis of novel diammonium gemini surfactants. Molecules 2001, 6, 979−987. (14) Zana, R.; Xia, J. Gemini surfactants: Synthesis, interfacial and solution-phase behavior, and application; Surfactant Science Series, Vol. 117; CRC Press: Dekker, NY, 2004. (15) Weifeng, Lv.; Bazin, B.; Desheng, M.; Liu, Q.; Han, D.; Wu, K. Static and dynamic adsorption of anionic and amphoteric surfactants with and without the presence of alkali. J. Pet. Sci. Eng. 2011, 77, 209− 218. (16) Nilsson, M.; Valente, A. J. M.; Olofsson, G.; Söderman, O.; Bonini, M. Thermodynamic and kinetic characterization of host-guest association between bolaform surfactants and α- and β-cyclodextrins. J. Phys. Chem. B 2008, 112, 11310−11316. (17) Nilsson, M.; Cabaleiro-Lago, C.; Valente, A. J. M.; Söderman, O. Interaction between gemini surfactants, 12-s-12, and β-cyclodextrin as investigated by NMR and electric conductometry. Langmuir 2006, 22, 8663−8669. (18) Sun, D. Z.; Mei Qiu, X. M.; Li, L.; Wei, X. L.; Yin, B. L. A study of α-cyclodextrin with a group of cationic gemini surfactants utilizing isothermal titration calorimetry and NMR. J. Chem. Thermodyn. 2006, 38, 773−777. (19) Sehgal, P.; Mizuki, T.; Doe, H.; Wimmer, R.; Larsen, K. L.; Otzen, D. E. Interaction and influence of α-cyclodextrin on the aggregation and interfacial properties of mixtures of nonionic and zwitterionic surfactants. Colloid Polym. Sci. 2009, 287, 1243−1252. (20) Alami, S. A.; Alami, E.; Eastoe, J.; Cosgrove, T. Interaction between a novel gemini sufractant and cyclodextrin: NMR and surface tension studies. J. Colloid Interface Sci. 2002, 246, 191−202. (21) Carvalho, R. A.; Correia, H. A.; Valente, A. J. M.; Söderman, O.; Nilsson, M. The effect of the head-group spacer length of 12-s-12 gemini surfactants in the host-guest association with β-cyclodextrin. J. Colloid Interface Sci. 2011, 354, 725−732. (22) Carlstedt, J.; Bilalov, A.; Krivtsova, E.; Olsson, U.; Lindman, B. Cyclodextrin−surfactant coassembly depends on the cyclodextrin ability to crystallize. Langmuir 2012, 28, 2387−2394. (23) Bouchemal, K. New challenges for pharmaceutical formulations and drug delivery systems characterization using isothermal titration calorymetry. Drug Discovery Today 2008, 13, 960−972. (24) Guerrero-Martínez, A.; Domínguez-Gutiérrez, D.; Palafox, M. A.; Tardajos, G. Studying the transfer process of a gemini surfactant from water to β-cyclodextrin at a molecular level. Chem. Phys. Lett. 2007, 446, 92−97. (25) Martini, A.; Artico, R.; Civarol, P.; Muggetti, L.; De Ponti, R. Critical micellar concentration shifting as a simple tool for evaluating cyclodextrin/enhancer interaction. Int. J. Pharm. 1996, 127, 239−244. (26) Hernández-Pascacio, J.; Garza, C.; Banquy, X.; Díaz-Vergara, N.; Amigo, A.; Ramos, S.; Castillo, R.; Costas, M.; Pineiro, A. Cyclodextrin-based self-assembled nanotubes at the water/air interface. J. Phys. Chem. B 2007, 111, 12625−12630. (27) Bai, Y.; Xu, G. Y.; Pang, J. Y.; Sun, H. Y.; Hao, A. Y.; Xin, X.; Qiao, M.; Yang, X. D. Comparative study on the effect of NaBr on the interaction between alkyltrimethylammonium bromide and β-cyclodextrin. J. Dispersion Sci. Technol. 2010, 31, 945−953. (28) Qiu, X. M.; Sun, D. Z.; Wei, X. L.; Yin, B. L. Thermodynamic study of the inclusion interaction between gemini surfactants and cyclodextrins by isothermal titration microcalorimetry. J. Solution Chem. 2007, 36, 303−312. (29) Alami, E.; Abrahmsén-Alami, S.; Eastoe, J.; Grillo, I.; Heenan, R. K. Interaction between a nonionic gemini surfactant and cyclodextrins investigated by small-angle neutron scattering. J. Colloid Interface Sci. 2002, 255, 403−409.

headgroup, or all of these. Phenomenological thermodynamics cannot provide information at the molecular level.

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CONCLUSIONS The cmc of the ABSJ gemini surfactant in aqueous solution passes through a minimum at T = 303 K and cmc = 0.06 mM in the temperature range from 288 to 343 K. Good agreement was found between the results obtained by SFT and ITC measurements. The presence of βCD markedly increased the SFT, attributed to 1:2 ABSJ−βCD inclusion complex formation in the bulk solution. Based on the SFT measurements, a novel method has been proposed for the determination of the stoichiometry N and the binding constant K for complexation. The SFT measurements (indirect data) suggested that K and the thermodynamic potentials ΔH, ΔG, and TΔS are linear and decreasing functions of T. The calorimetric data (direct method) indicated that K, ΔH°, ΔG°, and TΔS° pass through a maximum at T = 308 K. Complex formation is thermodynamically favored by enthalpy, but opposed by entropy. The values of ΔG° are in the range from −27 to −33 kJ mol−1.

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ASSOCIATED CONTENT

S Supporting Information *

Additonal details about the derivation of eq 10. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The project, partially funded by “TÁ MOP-4.2.2.A-11/1/ KONV-2012-0047”, was supported by the European Union and cofinanced by the European Social Fund, and by MOL Hungarian Oil and Gas Plc, Exploration & Production, Integrated Field Applications.

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REFERENCES

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dx.doi.org/10.1021/la501386j | Langmuir 2014, 30, 6756−6762