1 Introduction

In chemistry and biochemistry, cyclodextrins (CDs) are studied with immense interest due to their unique property of controlled release of molecules through the innovation of inclusion complex (IC) with guest molecules in the hydrophobic cavity.[1] In diverse industries including pharmaceutical, food, textile, pesticides, cosmetics, etc., cyclodextrins have enormous applications.[2] CDs are cyclic oligomer of \(\upalpha \)-D-glucose having numerous of glucopyranose units (6 in \(\upalpha \)-CD, 7 in \(\upbeta \)-CD and 8 in \(\upgamma \)-CD) bound by \(\upalpha \)-(1-4) linkages.[3] CDs are truncated cone-shaped structures with a quite rigid and distinct hydrophobic cavity of varying diameter and two separate rims; all the secondary hydroxyl groups forming a wider rim and all the primary hydroxyl groups forming a narrow rim.[3] Hence, CD is very proficient in forming stable supramolecular host-guest inclusion complex with a variety of molecules for having a hollow-cylindrical shaped cavity.[4] Due to its special structure and prospective application in the discovery of molecular switches, molecular machines and supramolecular polymers, macrocyclic CD molecules are useful in chemistry.[5] In some case, due to micro-encapsulation of guest by CD, various changes in physical and chemical properties occur such as fixation of very volatile substances, masking of the colour, smell and taste of substances, etc.[6] In case of drug delivery devices and nano-sensors, the molecular recognition of the host for the functioning is owing to the conjugation of CD with a variety of nanoparticles.[7]

Tetrabutylammonium iodide \((\hbox {but}_{4}\hbox {NI})\), a quaternary ammonium compound, is a water soluble strong electrolyte which exhibits an assortment of physical, chemical and biological properties. The ionic solid has applications in chemical reactions as surface-active agents (due to the hydrophobic butyl groups), solvents, intermediate emulsifying agents, phase transfer catalyst and for pigment dispersal.[8, 9] Since \(\hbox {but}_{4}\hbox {NI}\) has the tendency of locating at the interface of two phases (liquid–liquid or solid–liquid) for introducing the continuity between two different phases, it is positively a phase transfer catalyst. Due to low cost and low toxicity of \(\hbox {but}_{4}\hbox {NI}\), in recent times it is a promising substitute as a catalyst for functionalization of C–H bonds.[10, 11] In industrial usage, \(\hbox {but}_{4}\hbox {NI}\) acts as active ingredient for conditioners, antistatic agents, detergent sanitizers, softener for textiles, paper products, etc.

Scheme 1
scheme 1

Molecular structures of cyclodextrin molecules and tetrabutylammonium iodide.

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In this article, we explored the creation of host-guest inclusion complexes (ICs) of the ionic solid with \(\upalpha \) and \(\upbeta \)-CD particularly towards their formation, stabilization, carrying and controlled release by diverse methods such as \(^{1}\hbox {H}\) NMR, FT-IR, surface tension, conductivity, density, viscosity and refractive index measurements (Scheme 1). The present study describes both qualitative and quantitative results regarding the formation of ICs of \(\upalpha \) and \(\upbeta \)-cyclodextrin with \(\hbox {but}_{4}\hbox {NI}\). This work highlights the inclusion of ionic solid into the hydrophobic cavity of \(\upalpha \) and \(\upbeta \)-CD.

2 Experimental

2.1 Source and purity of samples

Tetrabutylammonium Iodide \(\hbox {but}_{4}\hbox {NI}\) and cyclodextrins of ‘puriss’ grade were bought from Sigma-Aldrich, Germany and used as purchased. The mass fraction purities of \(\hbox {but}_{4}\hbox {NI}\), \(\alpha \)- and \(\beta \)-cyclodextrin were \(\ge \)0.99, 0.98 and 0.98, respectively.

Fig. 1
figure 1

(a) \(^{1}\hbox {H}\) NMR Spectra of \(\upalpha \)-CD, Tetrabutyl ammonium iodide and 1:1 molar ratio of \(\upalpha \)-CD+ Tetrabutyl ammonium iodide in \(\hbox {D}_{2}\hbox {O}\) at 298.15 K. (b) \(^{1}\hbox {H}\) NMR Spectra of \(\upbeta \)-CD, Tetrabutyl ammonium iodide and 1:1 molar ratio of \(\upbeta \)-CD + Tetrabutyl ammonium iodide in \(\hbox {D}_{2}\hbox {O}\) at 298.15 K.

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2.2 Apparatus and procedures

Stock solutions of \(but_{4}NI\), \(\upalpha \)-CD and \(\upbeta \)-CD were prepared by mass (Mettler Toledo AG-285 with uncertainty 0.0001 g) and by dilution at 298.15 K To change molarity of the solutions to molality, the density of the solutions was used. [32]

Using Bruker AVANCE 400 MHz instrument at 298.15 K. the NMR spectra were recorded in \(\hbox {D}_{2}\hbox {O}\). Signals are cited as \(\updelta \) values in ppm using residual protonated solvent signals as internal standard \((\hbox {D}_{2}\hbox {O} : \updelta \) 4.79 ppm).

With the help of platinum ring detachment technique by a Tensiometer (K9, KRŰSS; Germany), surface tension (Accuracy \(\pm 0.1\,\hbox {mN}\cdot \hbox {m}^{-1})\) of the solution was measured at 298.15 K. Temperature of the system was maintained by circulating thermostated water through a double-wall glass vessel holding the solution.

Conductivities of the solutions were studied by Mettler Toledo Seven Multi conductivity meter having uncertainty 1.0 \(\mu \hbox {S m}^{-1}\). The study was carried out in a thermostated water bath at 298.15 K with uncertainty \(\pm 0.01\hbox { K}\). HPLC grade water was used with specific conductance 6.0 \(\mu \hbox {S m}^{-1}\). The conductivity cell was calibrated using 0.01 M aqueous KCl solution.

The densities \((\rho )\) of the solutions were measured by vibrating U-tube Anton Paar digital density meter (DMA 4500M) having precision of \(\pm 0.00005\hbox { g cm}^{-3}\) and uncertainty in temperature was \(\pm 0.01\hbox { K}\). The density meter was calibrated by standard method.[30]

Viscosities \((\eta )\) were determined by Brookfield DV-III Ultra Programmable Rheometer with spindle size 42. The detail has already been presented in our earlier report.[16] Refractive indexes of the solutions were measured with a Digital Refractometer from Mettler Toledo having uncertainty of \(\pm 0.0002\) units.

To prepare both the solid inclusion complexes (\([\hbox {but}_{4}\hbox {NI}] + \upalpha \)-CD and \([\hbox {but}_{4}\hbox {NI}] + \upbeta \)-CD), 1:1 molar ratio of the \(\hbox {but}_{4}\hbox {NI}\) and cyclodextrin were taken. In both the cases, 1.0 mmol cyclodextrin was dissolved in 20 mL water and 1.0 mmol \(\hbox {but}_{4}\hbox {NI}\) was dissolved in 20 mL ethanol and stirred separately for 3 h. Then, the ethanol solution of the ionic solid was added drop by drop to the aqueous CD solution. The mixture was then allowed to stir for 48 h at 50–55\(^{\circ }\hbox {C}\). It was filtered, then cooled to 5\(^{\circ }\hbox {C}\) and kept for 12 h. The resulting suspension was filtered and the white polycrystalline powder was washed with ethanol and dried in air.

Fourier transform infrared (FT-IR) spectra were recorded on a Perkin Elmer FT-IR spectrometer using the KBr disk technique. Samples were prepared as KBr disks with 1 mg of complex and 100 mg of KBr. The FTIR measurements were performed in the scanning range of 4000–400 \(\hbox {cm}^{-1}\) at room temperature.

3 Results and Discussion

3.1 \(^{1}\hbox {H}\) NMR study establishes inclusion

The inclusion of the guest ionic solid inside the host CD molecule was ascertained by the most reliable NMR study. We have studied the interactions of \(\hbox {but}_{4}\hbox {NI}\) with \(\upalpha \)- and \(\upbeta \)-CD. 1:1 molar ratio, by \(^{1}\hbox {H}\) NMR at 298.15 K (Figure 1). Table S1 (in Supplementary Information) shows the \(^{1}\hbox {H}\) NMR data of the \(\hbox {but}_{4}\hbox {NI}\), two CDs and two ICs. Due to the incorporation of guest ionic solid into the hydrophobic cavity of CD, the protons of the CD molecule show considerable chemical shift.[12] In the structure of CD, the H1, H2 and H4 (Figure 2) protons are at the periphery of CD molecule while the H3 and H5 are located within the cavity; more specifically, the location of H3 and H5 protons are near the wider rim and narrower rim, respectively.[13, 14] The H3 and H5 protons of CD show upfield chemical shift as a result of interaction with the guest in IC. It confirms the formation of host-guest inclusion complexes. The H3 proton located near the wider rim shows higher shift than the H5 proton which is present near the narrower rim at the interior of CD due to insertion of guest ionic solid molecule inside the cavity of host CD through the wider rim (Scheme S1 in Supplementary Information). Due to the peripheral protons, upfield chemical shift is also observed but to a minor extent. The interacting protons of \(\hbox {but}_{4}\hbox {NI}\) also show upfield chemical shift. The shifts of four protons present in each butyl group give four shifts which exemplifies the insertion mechanism. Moreover, the extent of shifts of butyl protons are more in \(\upbeta \)-CD compared to those in \(\upalpha \)-CD, signifying that the binding affinity of the former is higher compared to the latter.

Fig. 2
figure 2

Plot of limiting molar volume \((\phi ^{\textit{0}}_{{V}})\) against mass fraction (w) of aqueous \(\alpha \)-CD (blue) and aqueous \(\beta \)-CD (purple) for \(\hbox {but}_{4}\hbox {NI}\) at 298.15 K.

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3.2 Surface tension study supports inclusion

Surface tension \((\upgamma )\) is an important consideration which supports the inclusion complex formation of Tetrabutyl ammonium iodide with both \(\upalpha \) and \(\upbeta \)-CD.[15] There is no considerable change to the surface tension of water when CD is added to pure water. This indicates that both \(\alpha \)- and \(\beta \)-CD are almost surface inactive compounds.[16] In our present work, the \(\upgamma \) values of aqueous ionic solid have been determined with addition of \(\upalpha \)- and \(\beta \)-CD at 298.15 K (Table S4–S5 in SI). With increasing concentration of CDs, the \(\gamma \) values substantially increase for this ionic solid, \(\hbox {but}_{4}\hbox {NI}\). This is due to the removal of surface active \(\hbox {but}_{4}\hbox {NI}\) molecules from surface of the solution, \(i.e., \,\, \upalpha \)- and \(\upbeta \)-CD form the host-guest inclusion complexes after entry of the hydrophobic tail of \(\hbox {but}_{4}\hbox {NI}\) into the hydrophobic cavity of \(\upalpha \)- and \(\upbeta \)-CD.[17, 18] Figure 3 and Table 1 show the formation of 1:1 inclusion complex of \(\hbox {but}_{4}\hbox {NI}\) and CDs, both the curves show a single break point and after that point, the \(\gamma \) value increases more slowly. Inclusion complex formation with stoichiometry such as 1:2, 2:1, 2:2, etc., occurs only if the surface tension curve shows more than one break point (Scheme S2 in SI).[19, 20] Table 1 shows the values of \(\upgamma \) and the corresponding concentration of CDs and \(\hbox {but}_{4}\hbox {NI}\) at the break point. The breaks have originated at a certain concentration of \(\hbox {but}_{4}\hbox {NI}\) and CD where their concentration ratio in the solution is almost 1:1. Hence, this study proves the formation of 1:1 inclusion complex. In this case, the \(\upgamma \) value at the break point is higher for \(\upbeta \)-CD than that of \(\upalpha \)-CD which suggests that the former is superior to encapsulate the guests than the afterward.

Fig. 3
figure 3

Variation of surface tension of aqueous solutions at 298.15 K. (a) \(\hbox {but}_{4}\hbox {NI}\)-\(\upalpha \)-CD and (b) \(\hbox {but}_{4}\hbox {NI}\)-\(\upbeta \)-CD.

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Table 1 Values of surface tension \((\upgamma )\) at the break point with corresponding concentrations of cyclodextrins and \(\hbox {but}_{4}\hbox {NI}\) at 298.15 K\(^\mathrm{a}.\)
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Fig. 4
figure 4

Variation of conductivity of aqueous systems at 298.15 K. (a) \(\hbox {but}_{4}\hbox {NI}\)-\(\upalpha \)-CD and (b) \(\hbox {but}_{4}\hbox {NI}\)-\(\upbeta \)-CD.

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Table 2 Values of conductivity (\(\kappa \)) at the break point with corresponding concentrations of cyclodextrins and \(\hbox {but}_{4}\hbox {NI}\) at 298.15 K\(^\mathrm{a}.\)
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3.3 Conductivity study notifies inclusion

Conductivity \((\upkappa )\) study validates the host-guest inclusion complex and also provides the stoichiometry of the complex.[21, 22] We have measured the conductivity of the aqueous solution of \(\hbox {but}_{4}\hbox {NI}\) having 10 mM concentration with successive addition of \(\upalpha \)- and \(\upbeta \)-CD at 298.15 K (Table S4-S5 in SI). It was found that the conductivity of the ionic solid decreases on a usual basis with increasing concentration of CDs (Figure 4). Since the guest \(\hbox {but}_{4}\hbox {NI}\) is inserted inside the cavity of the CD molecule, the number of free ions per unit volume reduces. Consequently, the conductivity of the solution decreases. Perusal of Figure 4 and Table 2 shows results analogous with suface tension study, each having a perceptible break point, signifying the formation of \(\hbox {but}_{4}\hbox {NI}\)-CD inclusion complex having stoichiometry 1:1. Table 2 shows the values of \(\upkappa \) and corresponding concentration of \(\hbox {but}_{4}\hbox {NI}\) and CDs at the break point.

3.4 Density study: interaction between host and guest

In this study, important information about the inclusion phenomenon between the ionic solid and CD molecules have been obtained from density measurements. Apparent molar volume \((\phi _{\mathrm{v}})\) and limiting apparent molar volume \((\phi _{\mathrm{v}}^{\mathrm{o}})\) are the two major parameters used for this study. Apparent molar volume \((\phi _{\mathrm{v}})\) mainly describes the summation of the geometric volume of the central solute molecule and changes in the solvent volume as a result of interface with the solute around the co-sphere (here, \(\hbox {solute} = \hbox {but}_{4}\hbox {NI}\) and co-solvent = CD).[23] For this system (ternary phase of \(\hbox {but}_{4}\hbox {NI}\) + aqueous CD system), solute-solvent interaction is conveyed by limiting apparent molar volume. For this study, \(\phi _{\mathrm{v}}\) have been calculated from the density of solutions at 298.15 K by means of eq. (5) as described in Supplementary Information (Table S6). Least squares method was employed to the plots of \(\phi _{\mathrm{v}}\) versus \(\surd m \) using the Masson equation (eq. 6, Table S7 in SI) for obtaining \(\phi _{\mathrm{v}}^{\mathrm{o}}\) value.[24] It was found that \(\phi _{\mathrm{v}}\) values are reduced and \(\phi _{\mathrm{v}}^{\mathrm{o}}\) values are continually enhanced for \(\hbox {but}_{4}\hbox {NI}\) with increasing concentration of both the CDs. It is manifest from this fact that for \(\hbox {but}_{4}\hbox {NI}\), in both the cases of \(\upalpha \)- and \(\upbeta \)-CD, the ion–hydrophilic group interactions are more efficient than ion–hydrophobic group interactions. The inspection of Figure 2 shows that the value of \(\phi _{\mathrm{v}}^{\mathrm{o}}\) increases with increasing mass fractions of both CDs, and greater for \(\upbeta \)-CD than \(\upalpha \)-CD, signifying that the former interacts more with \(\hbox {but}_{4}\hbox {NI}\) than the latter. This may be explained as, in the case of \(\hbox {but}_{4}\hbox {NI}\), two hydrophobic butyl groups can be encapsulated in the cavity of CD and the single positively charged N atom shows higher ion-hydrophilic interaction with the –OH groups of CD. The larger diameter of \(\upbeta \)-CD helps in making a more compact inclusion complex with \(\hbox {but}_{4}\hbox {NI}\) than \(\upalpha \)-CD with a relatively smaller cavity size showing less hydrophobic interactions with \(\hbox {but}_{4}\hbox {NI}\).

3.5 Viscosity study: order of interactions

Viscosity study is also fruitful for the interpretation of interaction between ionic solid and CDs.[25] In the present ternary system \((\hbox {but}_{4}\hbox {NI}\) + aqueous CD), (Table S3 in SI), the viscosity of the solutions show an increasing trend with increasing concentration of \(\hbox {but}_{4}\hbox {NI}\). The size and shape of the solute molecule recommend the solute–solvent interaction (here, \(\hbox {solute} = \hbox {but}_{4}\hbox {NI}\) and co-solvent = CD) which is designated as viscosity B-coefficients (Table S7 in SI).[26] All the viscosity B-coefficient values were found to be positive and depicted in Figure 5. Perhaps due to higher solvation and also greater \(\hbox {but}_{4}\hbox {NI}\)-CD interaction, the viscosity B values increase with increasing concentration of CDs.[17] Since the viscosity B values is again larger for \(\upbeta \)-CD than for \(\upalpha \)-CD, it suggests that inclusion complex is more constructive in the case of former than the latter. The structural features of \(\hbox {but}_{4}\hbox {NI}\) and CDs for which these trends of interactions have relative consequences in viscosity study are analogous to the density study.

Fig. 5
figure 5

Plot of viscosity B-coefficient against mass fraction (w) of aqueous \(\alpha \)-CD (blue) and aqueous \(\beta \)-CD (purple) for \(\hbox {but}_{4}\hbox {NI}\) at 298.15 K.

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3.6 Refractive index shows the compactness of the inclusion complexes

To establish the molecular interaction in the above mentioned ternary solutions, refractive index is one more important parameter to ascertain the molecular interaction.[15] Using the suitable equation (Tables S3 and S6 in SI), refractive index \((n_{\mathrm{D}})\) and molar refraction \((R_{\mathrm{M}})\) values of the solutions have been estimated. The compactness and density of the medium were determined by the greater values of \(R_{\mathrm{M}}\) and the limiting molar refraction \((R^{\mathrm{O}}_{\mathrm{M}})\) (Table S7).[16, 17] In the case of \(\hbox {but}_{4}\hbox {NI}\) in both \(\upalpha \)- and \(\upbeta \)-CD, the \(R^{O}_{M}\) values show an increasing trend with increasing concentration of both CDs, which suggest that the IC of \(\hbox {but}_{4}\hbox {NI}\) with both the CDs are more closely packed, perhaps due to greater hydrophobic as well as ion-hydrophilic interactions between the guest and host as described earlier. The \(R^{\mathrm{O}}_{\mathrm{M}}\) values designate that \(\upbeta \)-CD is more favourable than \(\upalpha \)-CD for formation of the ICs (Figure 6). The density and viscosity studies (Sects. 3.4 and 3.5) are in concurrence with the observations obtained from refractive index data.

Fig. 6
figure 6

Plot of limiting molar refraction \((R^{0}_{M})\) for \(\hbox {but}_{4}\hbox {NI}\) in different mass fractions (w) in aqueous solutions of \(\alpha \)-CD (blue) and aqueous \(\beta \)-CD (purple), respectively, at 298.15 K.

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3.7 FT-IR spectra of solid inclusion complexes

The inclusion phenomena are nicely proved by FT-IR spectrum.[27,28,29] Table S8 (in SI) refers to the characteristic IR frequencies of \(\upalpha \)-CD, \(\upbeta \)-CD, \(\hbox {but}_{4}\hbox {NI}\) and solid ICs. Characteristic broad peaks of –OH at about \(3412.10\pm 0.01 \hbox { cm}^{-1}\) and \(3349.84\pm 0.01 \hbox { cm}^{-1}\) are here in the spectrum for and \(\upbeta \)-CD, respectively. The FT-IR Spectrum of \(\hbox {but}_{4}\hbox {NI}\) shows the presence of peaks for –N-C-H-,-C-H (stretching), -C-H (bending for –\(\hbox {CH}_{3}\) group), -C-H (bending for –\(\hbox {CH}_{2}\) group), etc. (Figures 7(a), (b) and Table S8). Yet, there are several either absent or shifted peaks of the \(\hbox {but}_{4}\hbox {NI}\) due to the change in atmosphere of the guest after inclusion in the CD cavity. The –C-H bending bands for \(\hbox {-CH}_{3}\) and \(\hbox {-CH}_{2}\) of both the \(\hbox {but}_{4}\hbox {NI}\) are shifted in the spectrum of the inclusion complex. The –O-H stretching of \(\upalpha \)-CD is shifted to lower frequency in the spectrum of the IC, probably due to involvement of the –O-H groups of the host molecules in hydrogen bonding with the guest molecule. On the other hand the –O-H stretching of \(\upbeta \)-CD is shifted to higher frequency in the IC, probably due to involvement of the –O-H groups of the host molecules in hydrogen bonding with the guest molecule. In the spectrum of the ICs, the peak for the C-N group of the guest molecule is present. It is an indication that the hydrophobic side chains present on one side of the polar N atom of the guest molecules are encapsulated in the hydrophobic cavity of both the CDs.

Fig. 7
figure 7

(a) FTIR spectra of \(\upalpha \)-CD (top), \(\hbox {but}_{4}\hbox {NI}\) (middle) and \(\hbox {but}_{4}\hbox {NI}\)-\(\upalpha \)-CD inclusion complex (bottom). (b) FTIR spectra of \(\upbeta \)-CD (top), \(\hbox {but}_{4}\hbox {NI}\) (middle) and \(\hbox {but}_{4}\hbox {NI}\)-\(\upbeta \)-CD inclusion complex (bottom).

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3.8 Binding constants: non-linear isotherms determined by conductivity method

1:1 host-guest inclusion complex formation of ionic solid (IS), \(\hbox {but}_{4}\hbox {NI}\) with \(\upalpha \)- and \(\upbeta \)-CD can be represented by this equilibrium:

$$\begin{aligned} IS+CD\mathop {\longleftrightarrow }^{K_{f}} IC \end{aligned}$$
(1)

The corresponding equilibrium constant, \(K_{f}\) is given by,

$$\begin{aligned} K_{_f } =\frac{[IC]}{[IS][CD]}\times \frac{f(IC)}{f(IS)f(CD)} \end{aligned}$$
(2)

where [IC], [IS], [CD] and f represent the equilibrium molar concentrations of the inclusion complex, Ionic solid, CDs and the activity coefficients of the species, respectively. Using the dilute solution system, the activity coefficient of uncharged macrocycle, f(C), can be reasonably assumed as unity. The use of Debye-Hückel limiting law, [30] we conclude that f (IS) \(\sim \) f (CD), so the activity coefficients in Eq. (2) are cancelled. The complex formation constant in terms of the molar conductance, \(\Lambda \), can be expressed as, [31]

$$\begin{aligned} K_f =\frac{[IC]}{[IS][CD]}=\frac{(\Lambda _{IS} -\Lambda _{obs} )}{(\Lambda _{obs} -\Lambda _{IC} )[CD]} \end{aligned}$$
(3)
$$\begin{aligned} \text {where}\quad [CD]=CD_{ad} -\frac{IS_{ad} (\Lambda _{IS} -\Lambda _{obs} )}{(\Lambda _{IS} -\Lambda _{IC} )} \end{aligned}$$
(4)

Here, \(\Lambda _{IS}\) represents molar conductance of the ionic solid before the addition of CD, \(\Lambda _{IC}\) the molar conductance of the inclusion complex, \(\Lambda _{obs}\) the molar conductance of the solution during titration, \(CD_{ad}\) the concentration of added cyclodextrin and [IS] concentration of the ionic solid. The complex formation constant, \(K_f\) and the molar conductance of the complex, \(\Lambda _{IC}\) were evaluated by using Eqs. (3) and (4). Through the application of a non-linear programme, the \(K_f\) values (Table 3) for the ICs were assessed from the binding isotherm.

\(K_f\) values indicate that the \(\hbox {but}_{4}\hbox {NI}\) has higher binding constant in \(\upbeta \)-CD than in \(\upalpha \)-CD. This fact may be due to the larger cavity dimension of \(\upbeta \)-CD than that of \(\upalpha \)-CD. So, inclusion complex formation is highly feasible in \(\upbeta \)-CD than in \(\upalpha \)-CD.

Table 3 Formation constants of \(\hbox {but}_{4}\hbox {NI}\)-cyclodextrin inclusion complexes.
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3.9 Structural influence of cyclodextrin in inclusion complex formation

The formation of host-guest ICs between the Ionic solid and CDs is dependent on the size of the guest molecule and also on the cavity diameter of the host. Between the two CDs, \(\upbeta \)-CD has the higher cavity diameter (6.0–6.5 Å) than that of \(\upalpha \)-CD (4.7–5.3 Å). In view of the compatible size of the \(\hbox {but}_{4}\hbox {NI}\) with \(\upbeta \)-CD, it is found that \(\upbeta \)-CD is more apt to form IC than \(\upalpha \)-CD with \(\hbox {but}_{4}\hbox {NI}\), possibly due to higher surface interaction, escalating the hydrophobic attractions, which is in concurrence with spectroscopic and physicochemical data.[1] In the case of ICs, no covalent bonds are formed or broken, only the hydrophobic alkyl chains of \(\hbox {but}_{4}\hbox {NI}\) are encapsulated in the hydrophobic cavity of CD molecules. This is a very important factor for IC formation that the hydrophobic cavity of CD is engaged by polar water molecule, but it is unfavourable, so the water molecules are effortlessly replaced by more hydrophobic alkyl tails of \(\hbox {but}_{4}\hbox {NI}\). Furthermore, due to the release of the trapped water molecules in the bulk of the solution, entropy of the solution is enhanced and the IC formation becomes spontaneous. Due to the spontaneity, the system becomes energetically very low and stable. So, the ring strain of CD molecule becomes very low. Due to the difficulty of the trapping of another \(\hbox {but}_{4}\hbox {NI}\) by CD molecule, 1:1 host-guest inclusion complex formation occurs. The stability of the IC can be ascribed to structural features such that the N atom present in \(\hbox {but}_{4}\hbox {NI}\) form H-bonds with the –OH groups present at the rim of CD.

4 Conclusions

The above described physicochemical studies help us to conclude that the ionic solid, tetrabutyl ammonium iodide forms host-guest inclusion complexes with \(\upalpha \)- and \(\upbeta \)-CD in solution and in solid state. With the help of \(^{1}\hbox {H}\) NMR study, it is substantiated that inclusion occurs in the apolar cavity of both CD molecules, even as surface tension and conductivity data suggested 1:1 stoichiometry. Density, viscometric and refractometric measurements were also in excellent concord with this, and furthermore, suggested interactions between the ionic solid and CD. The solid state ICs were characterised by FT-IR. In this study, the inclusion phenomenon of ionic solid originated more favourably in case of \(\upbeta \)-CD than in \(\upalpha \)-CD. The binding constants, evaluated using a non-linear programme by the conductivity method, also support the trend in IC formation.