Complexation of β‑Cyclodextrin with a Gemini Surfactant Studied byIsothermal Titration Microcalorimetry and Surface TensiometryMaria Benko,†,§ Laszlo A. Kiraly,† Sandor Puskas,‡ and Zoltan Kiraly*,†

†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, Dom ter 8, H-6720Szeged, 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 byisothermal titration calorimetry (ITC) and surface tension (SFT) measurements.We measured the critical micelle concentration (cmc) of ABSJ in water by thetwo experimental techniques in the temperature range 283−343 K, anddetermined the thermodynamic parameters of the complex formation directly byITC and indirectly by the SFT. The stoichiometry (N), the binding constant(K), and the enthalpy of complexation were determined, and the Gibbs freeenergy and the entropy term were calculated from the experimental data. A novelmethod is presented for the determination of N and K by using surfacetensiometry.

■ INTRODUCTION

β-Cyclodextrin (βCD) is the most stable native cyclodextrin(CD) and is composed of seven (α-1,4)- linked α-D-glucopyranose units.1 The molecule adopts a rigid doughnut-shape. While the outer surface of βCD is hydrophilic, the innersurface is hydrophobic. As a consequence, βCD is water-solubleand can accommodate poorly water-soluble compounds in itscavity, provided that the guest molecule fits geometrically intothe interior of the host molecule. In supramolecular chemistry,such molecular encapsulation is termed inclusion complexformation.2 The guest molecule is stabilized in the interior ofthe host, by dispersion forces. This host−guest complexformation is reversible; under suitable conditions, the guestmolecule can be released from the host.2 Since the inclusioncomplex is water-soluble, this kind of encapsulation has manyadvantages in practice: it prevents the degradation of the guest,dissolution of the guest can be retarded, organic pollutants canbe removed from water, it can be utilized in drug formulation,and so forth. CDs are widely used in the pharmaceutical andfood industries3 and in the field of enhanced oil recovery(EOR).4

Traditional surfactants5−7 are nowadays often replaced bygemini surfactants, in which two monomeric surfactants areconnected through a spacer group.8−11 The critical micelleconcentrations (cmc’s) of gemini surfactants are 1 or even 2orders of magnitude less than those of the correspondingmonomeric derivatives. As a consequence, the desired surfaceactive properties are achieved at lower concentrations andlower amounts of gemini surfactants are therefore required thanof traditional ones. Besides the economic aspects, the reduced

level of environmental pollution is a feature of interest. Thesynthesis of gemini surfactants began some 20 years ago.11−13

There is currently great interest in this new family of surfactantsfrom the aspect of EOR in crude oil exploitation.14,15

In recent years, a number of investigations have beenreported on the interactions between gemini surfactants andCDs. Instrumental studies have involved NMR;16−22 calorim-etry;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 measure-ments.29

In the present work, calorimetric and tensiometric measure-ments of the temperature dependence of the cmc of thecounterion-coupled gemini (cocogem) surfactant bis(4-(2-alkyl)benzenesulfonate)-Jeffamine salt (ABSJ)12 are compared,and a thermodynamic study of the inclusion complex formationbetween ABSJ and βCD is described.

■ EXPERIMENTAL SECTIONMaterials. βCD was a product of Cyclolab Ltd., Budapest,

Hungary. The powder contained 13.1% water as determined with athermogravimetric apparatus (Mettler-Toledo). Knowledge of thewater content was necessary for the preparation of solutions withaccurately known concentrations. ABSJ was synthesized via the 2:1coupling reaction between Lutensite A-LBS and Jeffamine D230, asdescribed earlier.12 The number-average molar mass of the surfactant

Received: April 10, 2014Revised: May 19, 2014

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

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, usinga Microcal VP-ITC power compensation microcalorimeter with a cellvolume of 1.4163 mL. The cell was loaded with Milli-Q water or 0.001M 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 cmcmeasurements, and 10 μL aliquots were taken for the study of theformation of the inclusion complex. For each step, the heat evolved orabsorbed was recorded continuously. The enthalpogram (thecalorimeter power signal versus time recording) composed of a seriesof calorimetric peaks and the summation of the sequential peak areasprovided an S-shaped curve, the integral enthalpy versus concentrationdiagram. Data were analyzed with Microcal Origin 7 software. Theexperimental error in the determination of the reaction enthalpies wasless than 3.0%. In the complexation experiments, corrections for theenthalpy of dilution of ABSJ were made by means of blankmeasurements (titration of water with ABSJ solutions).Surface Tension (SFT) Measurements. The SFT of the solutions

was measured by using a tensiometer (Kruss K100MK2), a computer-controlled apparatus supplied with a thermostat and an automaticburet. 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 (cmcmeasurement) or with 3.4 mM βCD solution (complex formation) inaliquots of 10 mL in 40 steps. Each experimental point was the averageof 5−10 measurements. The standard deviation of the experimentaldata 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 oftemperature have been published previously,12 but this was repeated inthe present study: the agreement was more than satisfactory. Thecomplexation of βCD with surfactant S can be described by thefollowing equation:30,31

β β+ ⇔ −nCD S CD SK

n (1)

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

ββ

=−

K[ CD S ]

[S] [ CD]n

f f (2)

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

β= Δ −Q H V S[ CD ]i ni 0 i (3)

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

∫Δ =H P t t( ) di i (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, thebinding constant (K), and the standard enthalpy of binding (ΔH). Atypical enthalpogram of the thermometric titration and a typical S-shaped enthalpy curve are presented in Figures 2 and 3, respectively, inthe Results and Discussion section.

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

Δ = −G RT Kln (5)

Δ = Δ − ΔT S H G (6)

Surface Tension (SFT) Analysis. Measurement of the surfacetension as a function of concentration is a well-established method fordetermination of the cmc, and this procedure will therefore not bedescribed in detail here. However, the investigation of CD−Sninclusion complex formation by SFT may be regarded as a newexperimental method.19

The total βCD concentration in the solution ([βCD]0) is the sumof the free CD concentration ([βCD]f) and the concentration of thecomplex ([βCD−Sn]):

β β β= + −[ CD] [ CD] [ CD S ]n0 f (7)

A similar equation holds for the surfactant:

β= + −[S] [S] [ CD S ]n0 f (8)

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

Figure 1. Structure and chemical composition of bis(4-(2-alkyl)-benzenesulfonate)-Jeffamine salt (ABSJ).

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 withABSJ solutions. Investigated temperatures: (■) 283 K, (●) 293 K;(▲) 303 K; (▼) 313 K; (◆) 323 K; (◀) 333 K; (★) 343 K. Forclarity, only every second experimental points are indicated.

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β=

−− +

K[S] [S]

([ CD] [S] [S] )[S]0 f

0 0 f f (9)

Equation 9 can be rearranged to give

β− = − − +⎛⎝⎜

⎞⎠⎟K

[S] [S]1 [S]

[S]1 [ CD]0 f

0

f0

(10)

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

β− = −[S] [S] [ CD S ]n0 f (11)

A plot of [S]0 − [S]f against [S]0/[S]f − 1 is linear. Linear regressiongives 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 semiempiricalequation12,28

= + +K PPT

P Tln ln12

3 (12)

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

Δ = − ∂∂

= − −⎜ ⎟⎛⎝

⎞⎠H RT

KT

R P P Tln

( )vH2

2 3 (13)

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

■ RESULTS AND DISCUSSIONOne of the aims of the present study was a comparison of thecmc values obtained at different temperatures from calorimetryand tensiometry.Determination of the cmc of ABSJ. The cmc was

measured in the range 283−343 K. The enthalpogram at 303 Kis shown in Figure 2.The S-shaped curves at selected temperatures are displayed

in Figure 3. The method of evaluation has been describedbefore12,21,28 and will therefore not be described here. For eachtemperature, data analysis with the Microcal Origin 7 softwareprovided the values of the cmc and ΔH. The heat of dilution ofthe micelles (opposite in sign to the heat of micelle formation)changes sign between 293 and 303 K, the micellization processturning from endothermic to exothermic. For each curve, the

position of the inflection point is closely related to the cmc andthe height of the sigmoidal to the enthalpy of micelleformation.12,21,28

A typical SFT titration curve, measured at 303 K, is shown inFigure 4a. The cmc of ABSJ was taken at the intersection point

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

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

Figure 6. Calorimeter signals. Enthalpogram of the dilution of thesurfactant 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.

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

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of the fitted straight lines. The SFT versus log c data in the totaltemperature range studied are displayed in Figure 4b. Asexpected, the SFT decreases with increasing temperature.The temperature dependence of the cmc determined by the

two experimental techniques is given in Figure 5. Theagreement between the results of the two methods issatisfactory, taking into consideration that the cmc values arerather low. The cmc versus T curve passes through a minimumat about 303 K, although the minimum is less clear in the caseof ITC than in the case of SFT. This observation indicates thatthe van’t Hoff enthalpy of micelle formation changes frompositive to negative, that is, the process changes fromendothermic to exothermic at this temperature. A detailedthermodynamic analysis of the micelle formation (changes inenthalpy, entropy and Gibbs free energy) of ABSJ in water wasreported recently.12

As we mentioned earlier, the cmc value of a gemini surfactantmay be 10−100 times less than that of the correspondingmonomeric surfactant.8−11 While the cmc of dodecylbenzene-sulfonate at 298 K is 1.72 mM,12,30 for instance, that of ABSJ isonly 0.065 mM at 298 K.

Investigations of the Formation of ABSJ−βCDInclusion Complexes. In the calorimetric investigation ofcomplexation, an attempt was made to keep the surfactantconcentration below the cmc in order to avoid the super-position effect of the heat of demicellization on the heat ofcomplexation. However, this was not successful because of thelow cmc value of ABSJ (no appreciable calorimeter powersignal could be detected). We therefore we carried out thecomplexation experiment with a surfactant concentration wellabove the cmc (0.51 mM) at each temperature and made abackground correction. There is an option in data analysis withOrigin 7 software which made it possible to eliminate the heatattributable to the dilution of the surfactant. The backgroundcorrection involved subtraction of the heat of dilution of thesurfactant from the total heat measured (complexationaccompanied by surfactant dilution), as shown in Figure 6. Itappears that the correction with the heat of dilution is notnegligible. After the correction, the curve in Figure 7 wasobtained.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 andΔG° and TΔS° were then calculated via eqs 5 and 6. Theresults are reported below. In the SFT measurements ofinclusion complex formation, 50 mL of 1.70 mM surfactantsolution was titrated with βCD solution of concentration of 3.4mM. As a background correction, the surfactant solution wastitrated with water in 40 steps, 10 mL each. The results of theSFT experiments at 298 K (Figure 8) reveal that the SFT (γ)values of the surfactant solutions containing inclusioncomplexes are significantly larger than those of the solutionsin the absence of βCD, but otherwise at the same surfactantconcentration. This is because a proportion of the surfactantparticipates in complexation and the remainder goes to thesurface. On the other hand, surfactants are more surface activethan surfactant−cyclodextrin complexes22,26 and theABSJ−βCD complex is gradually displaced from the surfaceby the surfactant with increasing concentration. The breakpointof the SFT versus log c curve for the complex is attributed tothe cmc of ABSJ in the presence of βCD. The cmc is 1 order ofmagnitude larger for ABSJ−βCD than that for ABSJ alone.After the breakpoint, the two SFT values remain the same, ∼28mN·m−1, which is attributed to surface saturation by adsorbedsurfactant molecules. In fact, it is expected that the SFT doesnot change with increasing surfactant concentration beyond thecmc. The common points after the cmc imply that no βCDmolecules are present in the adsorption layer. There has been adebate in the literature whether CDs interact directly withmicelles. However, it was recently shown that no suchinteractions occur.22 Competing βCD−micelle interactionscan 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 SFTdecreases 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) accom-modate one ABSJ molecule (guest). Since one ABSJ moleculepossesses two long alkyl chains (Figure 1), it may be concludedthat the tails penetrate the hydrophobic cavity of βCD. Thus,

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

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

Table 1. Thermodynamic Parameters of the Formation ofSurfactant−βCD Complexes, Obtained from ITC and SFTMeasurements

T/K K/M−1ΔH°/

kJ mol−1ΔHvH/kJ mol−1

ΔG°/kJ mol−1

TΔS°/kJ mol−1

ITC 288 887 −50.50 − −27.29 −23.21298 1920 −43.51 − −30.16 −13.36308 3080 −42.01 − −32.38 −9.63318 2580 −45.19 − −32.96 −12.23328 1160 −52.84 − −31.81 −21.03338 439 −74.35 − −30.05 −44.30

SFT 288 1519 − −51.42 −28.58 −22.84298 1263 − −53.15 −29.12 −24.04308 1079 − −54.89 −29.69 −25.20318 754 − −56.62 −29.71 −26.92328 651 − −58.36 −30.24 −28.12338 547 − −60.09 −30.67 −29.42

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the average stoichiometry throughout the entire temperaturerange studied was found to be βCD:ABSJ = 2.05 ± 0.05. Asimilar finding was made earlier for gemini−βCD complexationon the basis of density and sound velocity measurements.30 Weare not aware of other reports on the determination of thestoichiometry of inclusion complexation through the use ofsurface tensiometry. The analysis of our titration calorimetricdata led to βCD:ABSJ = 2.04 ± 0.06. The agreement betweenthe 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 exothermicprocess and is accompanied by entropy production because thedegree of freedom of bulk water molecules is larger than that ofcaptured water. The hydrogen-bond density around thesurfactant molecules is larger than that in bulk water, knownas the iceberg structure.32−37 In the next step, therefore, beforethe alkyl chains enter the interior of the βCD, iceberg collapseoccurs: the hydrogen-bond density of the solvating watermolecules decreases to that of bulk water. This process resultsin an increase of entropy and is endothermic. The third factor isthat the dispersion interactions between the alkyl chain and thehydrophobic core of βCD are exothermic, with some loss inentropy. The role of the change in state of the hydrophilicheadgroup upon inclusion is not completely straightforward,but this effect is presumably small because its hydration in thebound state, away from the cavity, must be similar to that in thestate in free solution.28 In any case, the net thermodynamicpotentials ΔH°, ΔG°, and TΔS° are determined by thesuperposition of the above three major factors. The values ofthe binding constant K are given in Table 1 and are plottedagainst 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

to 3100 to 440 M−1. The explanation of the difference betweenthe two sets of results is yet not clear. It may be speculated thatthe SFT method is sensitive to the surface properties of thesystem, while calorimetry is sensitive to the bulk properties.The maximum curve may be related to the change in the depthof penetration of the surfactant into the βCD cavity24 or thechange in the hydration state of the headgroup. In any case,calorimetry as a direct experimental method provides morereliable data than does the SFT method, which providesindirect data. Further differences between the results of the twomethods are given in Table 1 and Figure 10b−d.The thermodynamic potentials ΔHvH, ΔG, and TΔS derived

from SFT measurements are nearly linear functions oftemperature. ΔHvH decreases from −51 to −60 kJ mol−1

(exothermic process). It may be noted that the enthalpy valuesare rather large for the present system. The reason for that isthat gemini surfactants are made up of two alkyl chains, and theheat evolved for a dimeric surfactant is about twice of that forthe corresponding monomeric derivative. TΔS is increasinglynegative, its value changing from −23 to −44 kJ mol−1 (entropyloss). Although complexation is promoted by enthalpy andopposed by entropy, the process is thermodynamically favored:the values of ΔG are between −29 and −31 kJ mol−1. From theITC data, ΔH°, ΔG°, and TΔS° display extremes at about 308K. 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 aminimum from −27 through −33 to −30 kJ mol−1. Thisobservation implies that the change of the heat capacity for thepresent system is positive at low temperature (although itappears to be close to zero). It follows that it is not thehydrophobic effect which is dominating at low temperature. Asmentioned in connection with the binding constant, theextraordinary properties of the thermodynamic potentials mayoriginate from a change in depth of the penetration of thesurfactant tails into the βCD cavity, conformational changes ofthe alkyl chains,24 a change in the hydration state of the

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

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headgroup, or all of these. Phenomenological thermodynamicscannot provide information at the molecular level.

■ CONCLUSIONSThe cmc of the ABSJ gemini surfactant in aqueous solutionpasses through a minimum at T = 303 K and cmc = 0.06 mM inthe temperature range from 288 to 343 K. Good agreement wasfound between the results obtained by SFT and ITCmeasurements. The presence of βCD markedly increased theSFT, attributed to 1:2 ABSJ−βCD inclusion complex formationin the bulk solution. Based on the SFT measurements, a novelmethod has been proposed for the determination of thestoichiometry N and the binding constant K for complexation.The SFT measurements (indirect data) suggested that K andthe thermodynamic potentials ΔH, ΔG, and TΔS are linear anddecreasing functions of T. The calorimetric data (directmethod) indicated that K, ΔH°, ΔG°, and TΔS° pass througha maximum at T = 308 K. Complex formation isthermodynamically favored by enthalpy, but opposed byentropy. The values of ΔG° are in the range from −27 to−33 kJ mol−1.

■ ASSOCIATED CONTENT*S Supporting InformationAdditonal details about the derivation of eq 10. This material isavailable free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: zkiraly@chem.u-szeged.hu.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe project, partially funded by “TAMOP-4.2.2.A-11/1/KONV-2012-0047”, was supported by the European Unionand cofinanced by the European Social Fund, and by MOLHungarian Oil and Gas Plc, Exploration & Production,Integrated Field Applications.

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