1. INTRODUCTION:

The alternative proposed in this investigation for the remediation of acid herbicides involves calix[4]pyrrole derivatives as receptors. Pesticides are found in various parts of the environment in quite small concentrations, but they accumulate and thus become a threat to human health and life. Pesticides are, in fact, among the most important environmental pollutants because of their increasing use in agriculture [1].

Therefore, selective binding of pesticides is of great importance to researchers and a variety of receptor systems have been put forward in this regard. of all groups of herbicides, five kinds have been selected (which contain in their structure a carboxylic acid as a common characteristic) to carry out this investigation (Table 1) [2] [3]. The design and synthesis of receptors to recognize, sense and bind the anions are an important area of Supramolecular Chemistry [4]-[8]; a brief description of Supramolecular Chemistry is given. Supramolecular Chemistry is based on the molecular assemblies and intermolecular bond covering mainly structures, properties and reactions of molecular species. Jean-Marie Lehn introduced the concept of Supramolecular Chemistry in 1978, as an interdisciplinary field of science “covering the chemical, physical and biological characteristics of the chemical species of greater complexity than molecules themselves that are associated by intermolecular (non-covalent) binding interactions” [9].

Table 1.Group of herbicides selected to perform this investigation.

Herbicides	pKa values at 20 ^ᵒC in water
P1	2.30
2	2.84
P3	2.88
P4	2.73
P5	4.23

Calix[4]pyrroles are conformationally flexible macrocycles [10] [11] of significant importance due to their binding under different conditions with anions [12], neutral substrates [13] and metal ions [14]. The complexation behavior of calix[4]pyrroles with anions and cations has been widely studied using fluorescent [15], colorimetric [16] and electrochemical signaling [17] devices.

Calix[4]pyrrole is the simplest member of this class of receptors.The calixpyrroles (meso-octasubstituted porphyrinogens) are a considerable class of macrocycles, first synthesized by Baeyer in the nineteenth century by condensing pyrrole and acetone in the presence of an acid (to produce meso-octamethylcalix[4]pyrrole) [18]. Interest in these macrocycles was renewed in the 1990s by the extensive work of Floriani and co-workers on the metallation and attendant synthetic chemistry of deprotonated calixpyrroles [19]. Our interest in calixpyrroles is a result of the discovery in the mid-1990s that the NH group present in these species can act as a binding site for anionic and neutral guest species [12] [13]. Calix[4]pyrroles are easy to make and functionalize. As such, they have been employed in the production of separation media for anionic and neutral species.

In our research, it can be observed that calix[4]pyrrole derivatives bind weakly with acid pesticides. In order to modulate the anion binding behavior, the core size of calix[4]pyrroles was extended through the induction of suitable spacer units as a rigid wall. Thus, this study concerns the design of receptors which are able to interact with these pollutants. The whole idea of this investigation is to proceed with fundamental studies as the basis for the design of receptors and their incorporation in solid supports for the production of new and recyclable materials for the removal of these pollutants from the water. The prevalent research studies concentrate on the binding between calixpyrrole and toxic metals and pay little attention to the interaction between the calixpyrrole derivatives and organic compounds such as pesticides.

In this study, a number of calix[4]pyrrole derivatives were tested for their ability to the interaction with acid pesticides (Table 2). Initially, four receptors of calix [4] pyrrole derivatives were prepared, namely calix [4] pyrrole [meso - octamethyl – calix [4] pyrrole], 1, meso -tetra methyl - tetrakis - (4- hydroxyl phenyl) calix [4] pyrrole, 2, meso – tetra methyl – tetrakis - (3- hydroxyphenyl) calix [4] pyrrole, 3, and meso - tetramethyl - tetrakis - [(4- N,N – diethyl acetamide)

Phenoxy methyl] calix [4] pyrrole, 4. Then a new receptor was prepared, named, meso - tetramethyl-bis (diethyl amino) ethoxy – bis - (4 – hydroxyl phenyl) calix [4] pyrrole, 5.

Table 2.Includes the structures name and symbol of receptors which are used to study the interaction with herbicides.

From ¹H NMR studies, it is concluded that receptor, 5, interacts with acid pesticides. Therefore, in this study, it is essential to investigate the factors of why other receptors did not interact with these pesticides. The presence of the amino groups (basic) in the lower rim for calix[4]pyrrole makes these ligands attractive to explore their complexation with acid pesticides. This receptors was characterized by elemental analysis and ¹H NMR as well as ¹³C NMR and ¹³C NMR-DEPT which confirmed that this receptor was successfully synthesized. ¹H NMR studies were used to determine the interaction of these receptors with pesticides by calculating the chemical shift changes after the addition of excess amounts of the pesticide. The ¹H NMR studies provided evidence that significant chemical shift changes were observed for receptor, 5, upon addition of the pesticide.

Conductometric measurements were performed for receptor, 5, with the aim of determining 1) the composition of the ligand-acid pesticides interaction and gaining information regarding the type and the strength of interaction of the ligand with the acid pesticides in acetonitrile at 298 K; 2) the degree of association of these acids in non-aqueous media. Standard thermodynamics parameters of complexation ((log K_s, ΔH^ᵒ_c, ΔS^ᵒ_c, ΔG^ᵒ_c) for receptor, 5, with pesticides in MeCN were determined using the Nano ITC (isothermal titration calorimetry).

2. EXPERIMENTAL PART:

2.1. Chemicals:

All chemicals were obtained from Sigma-Aldrich, Fluka and Fisher UK Scientific and were either analytical or reagent grade. Solid chemicals were used as received without further purification. All solvents were dried and purified as described in the literature [20].

2.2. ¹H NMR Measurements:

¹H NMR measurements were used to characterize the calix[4]pyrrole derivatives and to provide information about its interaction with acid herbicides and whenever possible to establish the site of interaction of the ligand. ¹H NMR measurements were recorded at 298 K on A Bruker DRX-500 pulse Fourier Transform NMR Spectrometer. The operating conditions involved pulse or flip angle of 30˚, spectra width (SW) of 15 ppm, spectral frequency (SF) of 500.150 MHz, delay time of 0.3 s, acquisition time (AQ) of 3.17 s, and line broadening of 0.3 Hz. Solutions of the samples of interest (3.00 × 10⁻³ mol∙dm⁻³) were prepared in the appropriate deuterated solvent. These were placed in 5 mm NMR tubes using TMS (tetramethylsilane) as the internal reference.

2.3. Conductance Measurements:

A Wayne-Kerr Autobalance Universal Bridge type B642 was used for conductometric measurements. The Wayne-Kerr is connected to a platinum glass bodied electrode housed in a cylindrical glass vessel where the reaction takes place. A thermostated bath circulating water in the vessel jacket was used to maintain the temperature of the vessel at 298.15 K. A magnetic stirrer was used to keep homogeneous the solutions throughout the time of the experiment. The conductance cell was a Russell type glass bodied electrode with a cell constant (determined using 0.10 mol∙dm⁻³ aqueous KCl solution). For these experiments, the vessel was filled with the acid pesticides in the appropriate solvent (25 cm³) and the conductance of the solution was measured. Then, a known volume of solution of calix[4]pyrrole derivative in the same solvent was added stepwise into the vessel and the conductance measured after each addition.

2.4. Nano ITC (Isothermal Titration Calorimetry):

The basic principle of ITC is simply to measure the heat released or absorbed in a liquid sample after the addition of another liquid sample. This heat is proportional to the total amount of binding that occurs within the calorimeter cell. The instrument has a pair of identical cells (1.4 ml), denoted as the reference and sample cells. These cells, along with access stems, are enclosed in a temperature-controlled thermal jacket. The reference and sample cells (and stems) are filled with identical Ligand solutions. The power (or heat) difference between the sample and reference cells is used to determine n, Ka, and ΔHº. To determine the accuracy of measurements carried out in the Nano ITC, a chemical calibration should be performed. The following equilibrium is established upon the addition of barium chloride to 18-crown-6 ether in water at 298.15 K [21]. The sample cell was filled with an aqueous solution of 18-Crown-6 and titrated incrementally from the burette stirring system with BaCl₂ (0.015 mol∙dm⁻³).

2.5. Synthesis:

2.5.1. Synthesis of Calix[4]pyrrole[meso-octamethyl-calix[4]pyrrole], 1

The preparation of this derivative was achieved the procedure reported in the literature [12].

2.5.2. Synthesis of Meso-tetramethyl-tetrakis-(4-hydroxyphenyl)cali[4]pyrrole, 2

The preparation of this derivative was achieved the procedure reported in the literature [22] [23].

2.5.3. Synthesis of Meso-tetramethyl-tetrakis-(3-hydroxyphenyl)calix[4]pyrrole, 3

The same procedure to prepare 2 was used to prepare αβαβ isomers by successive re crystallisation. Re-crystallisation of the mixture obtained from acetic acid. The product obtained was dissolved in chloroform and followed by precipitation from water.

2.5.4. Synthesis of Meso- tetramethyl- tetrakis- [(4- N,N- diethyl acetamide) phenoxy methyl] calix [4] pyrrole, 4

The preparation of this derivative was achieved by procedure reported in the literature [23].

2.5.5. Synthesis of Meso - tetra methyl- bis (diethyl amino) ethoxy - bis (4 - hydroxyl phenyl) cali [4] pyrrole, 5

Meso-tetramethyl-tetrakis-(4-hydroxy phenyl) calix [4] pyrrole (0.93 g, 1.25 mmol), potassium carbonate (0.9 g, 6.5 mmol) and 18-crown-6 (0.09 g, 0.34 mmol) were vigorously stirred and refluxed for 1h in freshly refluxed acetonitrile (MeCN) (150 ml) and under a nitrogen atmosphere. Then 2- (diethyl amino) ethyl chloride hydrochloride (0.36 gm, 2.5 mmol) was added. The mixture was refluxed overnight at 80˚C. The reaction was monitored by TLC using DCM/MeOH (9:1) as the developing solvent system. After cooling down the reaction, the solvent was removed under pressure. The solid obtained was dissolved in dichloromethane and extracted several times with water to remove the excess of potassium carbonate. The dichloromethanic phase was separated and dried over anhydrous magnesium sulphate, then filtered. Dichloromethane was rotary-evaporated and the oily product was recrystallized from acetonitrile to yield 70% of yellow powder. The product was purified by column chromatography (SiO₂, ethyl : acetate : hexane = 1:1). ¹H-NMR (CD₃CN, 500 MHz) ; Ϩ(ppm) = 8.34 (s, 4 H, NH); 5.66 (d, 8 H, pyrrole-H); 1.71 (s, 12 H, CH3); 6.94; (d, 8 H, ArH); 6.60 (d, 8 H, ArH); 7.47 (s, 4 H, OH); 6.52 (d, 2H, H-7); 6.40 (d, 2H, H-8) 3.66 (t, 2H, H-9); 2.73 (t, 2H, H-10); 2.40 (q, H-11), 1.28 (t, H-12). 13C NMR: Ϩ(ppm): 140.90 (C1), 107.17 (C2), 105.48 (C3), 139.01 (C4), 30.87 (C5), 71.93 (C6), 134.01(C7), 127.80 (C8), 120.01 (C9), 155.48 (C10), 30.95 (C11), 72.88 (C12), 135.90 (C13), 128.55 (C14), 122.15 (C15), 157.17 (C16), 52.56 (C17), 58.72 (C18), 47.61 (C19), 11.96 (C20). 13C NMR-DEPT: Ϩ(ppm): 107.15 (C2), 105.47 (C3), 30.89 (C5), 127.81 (C8), 120.04 (C9), 30.97 (C11), 128.58 (C14), 122.17 (C15), 52.57 (C17), 58.74 (C18), 47.42 (C19), 11.97 (C20). Elemental analysis, % found for C, 76.80 and H, 7.35, N, 8.89, % calculated, C, 76.75, H, 7.46 and N, 8.95.

3. RESULTS AND DISCUSSIONS:

3.1. ¹H NMR Characterization of the Receptor 5

The NMR spectrum of this receptor in CD₃CN was recorded (Appendix A).

3.2. ¹³C NMR and ¹³C NMR-DEPT of Receptor 5 in CDCl₃ at 298 K

¹³C NMR and ¹³C NMR-DEPT spectra of receptor 5 in CDCl₃ at 298.15 K are shown in (Appendix A).

3.3. ¹H NMR Studies on the Interaction of 1, 2, 3, 4, 5 with Herbicides in Deuterated

Solvent at 298 K

¹H NMR spectra of the acid pesticides-receptors complexes at 298 K were recorded (Appendix A). The relevant ¹H NMR chemical shift changes of the protons observed by the addition of acid pesticides (P1, P2, P3, P4, P5) to calixpyrrole derivatives, in deuterated solvent at 298 K are listed in Tables 3-5. From Δδ values in Tables 3-5, it can be seen that no significant chemical shift changes are observed for calixpyrrole derivatives 1, 2, 3 upon addition of the pesticide. Therefore, it can be concluded that there is no evidence of complexation of these receptors with acid pesticides. This may be attributed that these compounds are partially substituted and do not contain any active group which are able to bind acid pesticides by hydrogen bonding. Therefore, attempts were made in this study to expand the cavity of these receptors by using the receptor 4. Table 4 also, it can be observed that no significant change in the chemical shift of the protons of the receptor 4 was observed. This may be attributed to the steric hindrance effects because this receptor contain carbonyl groups which prevents or reduce formation hydrogen bonding. Also, the lack of interaction between calixpyrroles derivatives and these pesticides must be due to the fact that in acetonitrile the pesticides are almost fully associated and therefore the interaction between these receptors and the anion is not possible. As for receptor, 5, Table 5, the interaction between the receptor 5 with pesticides was investigated in acetonitrile at 298 K. Approximately, all protons for ligand 5 have been affected upon the addition of the pesticides in CD₃CN at 298 K. Some protons have shielding effects and others have deshielding effects. It can be noted that the protons closest to the nitrogen and the oxygen atoms such as H-9, H-10, H-11 and H-12 have considerable chemical shift changes relative to others. This is an indication that the lower rim groups of the receptors interact with the pesticides. Protons such as H-1, H-2, H-3, H-4, H-5, H-6, H-7 and H-8 have been also affected as a result of this interaction. Changes in ¹H NMR of receptor 5 in CD₃CN before and after the addition of pesticides provide useful information regarding of active sites of the receptor interacting with the pesticides.

Table 3. Chemical shift changes for receptor 1 and receptor 2 after addition of an excess amount of appropriate Herbicides in CD₃CN at 298 K (0.09 mol of the receptor + 0.9 mol of herbicides)

Receptor 1	δ Δδ (ppm) P1(ppm)	δ Δδ (ppm)P2 (ppm)	δ Δδ (ppm) P3 (ppm)	δ Δδ (ppm) P4 (ppm)	δ Δδ (ppm)P5 (ppm)
N-H(1)	……. ……..	……. …….	…… ……	…… ……	…… …..
Ar-H(2)	5.85 0.00	5.78 0.05	5.83 0.00	5.79 0.04	5.82 0.01
CH₃(3)	1.50 0.01	1.54 -0.02	1.50 0.00	1.51 0.00	1.51 0.00
Receptor 2
N-H(1)	8.74 0.00	8.75 0.04	8.75 0.04	8.76 0.00	8.75 0.00
Ar-H(2)	5.96 0.00	5.96 0.04	5.96 0.04	5.98 0.00	5.96 0.00
CH₃(3)	1.82 0.00	1.82 0.02	1.82 0.03	1.84 0.00	1.82 0.00
Ar-H(4)	6.76 0.00	6.76 0.03	6.78 0.04	6.80 0.00	6.78 0.00

The chemical shifts δ (ppm) for the protons of the receptor 1 in CD₃CN at 298 K are: δ H₁ = 7.41, δ H₂ = 5.83, δ H₃ = 1.51 ppm. The chemical shifts δ (ppm) for the protons of receptor 2 in acetone at 298 K are: δ H₁ = 8.74, δ H₂ = 5.96, δ H₃ = 1.81, δ H₄ = 6.78, δ H₅ = 6.66 ppm.

Table 4. Chemical shift changes for receptor 3 and receptor 4 after addition of an excess amount of appropriate herbicides in acetone at 298 K (0.09 mol of the receptor 3, 4 + 0.9 mol of herbicides).

Receptor 3	δ Δδ (ppm)P1 (ppm)	δ Δδ (ppm)P2(ppm)	δ Δδ (ppm)P3 (ppm)	δ Δδ (ppm)P4 (ppm)	δ Δδ (ppm)P5(ppm)
H(1)	8.78 0.00	8.80 -0.01	8.78 0.00	8.78 0.00	8.79 0.00
H(2)	6.01 0.00	6.03 -0.02	6.01 0.00	6.01 0.00	6.02 -0.01
H (3)	1.80 -0.05	1.86 -0.02	1.84 0.00	1.84 0.00	1.85 -0.01
H(Benzene)(O)	6.44 0.00	6.38 -0.05	6.44 0.00	6.44 0.00	6.39 -0.05
H(8)	8.01 0.07	…….	8.00 0.07	………	8.07 0.00
Receptor 4
H(1)	7.97 0.00	7.97 0.00	7.96 0.01	7.97 0.00	………..
H(2)	5.99 0.00	5.99 0.00	5.99 0.01	5.99 0.01	6.01 -0.01
H (3)	1.83 0.00	1.83 0.00	1.82 0.00	1.83 0.00	1.84 -0.01
H(4)	6.83 0.00	6.82 0.01	6.82 0.00	6.83 0.00	6.80 0.03

The chemical shifts δ (ppm) for the protons of the receptor 3 in acetone at 298 K are: δ H₁ = 8.78, δ H₂ = 6.01, δ H₃ = 1.84, δ HAr(O) = 6.44, δ HAr(P) = 6.55, δ HAr(m) = 7.06, δ OH = 8.07 ppm. The chemical shifts δ (ppm) for the protons of receptor 4 in acetone at 298 K are: δ H₁ = 7.99, δ H₂ = 6.00, δ H₃ = 1.83, δ H₄ = 6.81, δ H₅ = 6.76, δ H₆ = 4.68, δ H₇= 3.34, δ H₈ = 1.20 ppm.

Table 5. Chemical shift changes for receptor 5 after addition of an excess amount of appropriate Herbicides in CD₃CN at 298 K (0.09 mol of the receptor 5 + 0.9 mol of herbicides)

Receptor 5	δ Δδ (ppm)P1(ppm)	δ Δδ (ppm)P2(ppm)	δ Δδ (ppm) P3 (ppm)	δ Δδ (ppm)P4(ppm)	δ Δδ (ppm)P5(ppm)
H(1)	7.81 - 0.42	............	..............	..............	.............
H(2)	5.88 0.22	5.75 0.09	5.81 0.15	5.78 0.12	5.70 0.04
H(3)	1.50 - 0.21	1.65 - 0.06	1.55 - 0.16	1.60 - 0.11	1.67 - 0.04
H(4)	7.27 0.33	7.03 0.09	7.17 0.19	7.11 0.17	7.04 0.10
H5)	6.35 - 0.25	6.73 -0.10	6.93 -0.20	6.85 -0.19	6.75 -0.09
H(6)	..............	............	..............	...............	...............
H(7)	6.30 - 0.22	5.40 - 0.10	6.35 - 0.18	6.41 - 0.11	6.47 - 0.01
H(8)	6.68 0.28	6.50 0.10	6.55 0.15	6.53 0.13	6.51 - 0.06
H(9)	3.50 - 0.16	3.56 - 0.10	3.53 - 0.13	3.51 - 0.15	3.56 - 0.10
H(10)	3.12 0.39	2.83 0.10	3.03 0.30	2.92 0.19	2.84 0.11
H(11)	2.73 0.33	2.50 0.10	2.60 0.20	2.55 0.15	2.51 0.11
H(12)	1.15 - 0.17	1.20 - 0.08	1.18 - 0.10	1.18 - 0.10	1.23 - 0.05

The chemical shifts δ (ppm) for the protons of the free ligand in acetone at 298 K are: δ H₁ = 8.23, δ H₂ = 5.66, δ H₃ = 1.71, δ H₄ = 6.94, δ H₅ = 6.60, δ H₆ = 7.47, δ H₈ = 6.40, δ H₉ = 3.66, δ H₁₀ = 2.73, δ H₁₁ = 2.40, δ H₁₂ = 1.28 ppm.

Table 6. Thermodynamic parameters of BaCl₂ binding to 18-crown-6 by Nano-ITC in aqueous medium deionized water) at 298.15 K

Log K_s	Δ_cH^ᵒ(KJ. mol^-1)	Δ_cG^ᵒ(KJ. mol^-1)	Δ_cS^ᵒ(J. mol^-1.K^-1)	Ref
3.63	-30.82	-20.73	-33.8	This work
3.77	-31.39	-21.49	-33	[24]

Table 7. Thermodynamic of complexation of receptor 5 and herbicides in acetonitrile at 298.15 K

Complexes	Log K_s	Δ_cH^ᵒ (KJ. mol^-1)	Δ_cG^ᵒ (KJ. mol^-1)	Δ_cS^ᵒ (J. mol^-1.K^-1)	n
5 + P1	4.49	-41.04	-25.65	-51.6	2. 18
5 + P2	3.81	-21.37	-20.47	-16.4	2.12
5 + P3	4.18	-38.90	-23.86	-50.45	2.17
5 + P4	4.14	-32.42	-23.64	-29.43	2.21
5 + P5	3.42	- 17.51	-19.53	6.77	2.07

3.5.2. Thermodynamic Parameters of Complexation of the Receptor 5 with Herbicides in

Acetonitrile at 298.15 K

Thermodynamic data for the complexation of the receptor 5 with the pesticides in acetonitrile are summarized in Table 7. The complexation between the receptor 5 and pesticides in acetonitrile was investigated at 298.15 K.

Inspection of stability constant data (expressed as log K_s) shows that this ligand interacts selectively with pesticides in acetonitrile following the sequence:

P1 > P3 > P4 > P2 > P5

This decrease may be attributed to the presence of the phenol groups which may either 1) lead to steric effects by which the phenol units may form rigid walls restricting the easy access of the pesticides to interact with the pyrrolic protons or 2) to electronic effects, since the aromatic phenol rings may form an induced magnetic field which may act as a repulsive force for these anionic guests. It can be observed that the selective behaviour of receptor 5 for P1 relative to other acid pesticides in this solvent. This is corroborated by the ¹H NMR data where significant chemical shift changes were found in H-2, H-3, H-4, H-5, H-7, H-8, H-10 and H-11 upon complexation of receptor 5 with these acid pesticides.

A general analysis of the thermodynamic parameters shows that the complexation process is favored in terms of enthalpy ΔH^ᵒ < 0) but not in terms of entropy (ΔS^ᵒ < 0) in all the above systems. Therefore, the complexation process is enthalpically controlled. The only exception is P5 which shows the opposite behaviour (entropy controlled). This may be attributed to the higher desolvation that the pesticide undergoes upon complexation. The data in Table 7 show the ΔG^ᵒ values are obtained for the receptor 5 and the different pesticides studied in acetonitrile are close to each other. A linear relationship was in fact found when ΔH^ᵒ values were plotted against ΔS^ᵒ values as shown in Figure 3. Therefore, these data suggest the presence of enthalpy-entropy compensation effects possibly due to solvent reorganization upon complexation. These effects have been discussed in detail by Grunwald and Steel [24]. The slope of this plot representing the experimental temperature (298 K) is 287 K, the correlation coefficient is 0.95 while the intercept of the fitted line, ΔG^ᵒ, is −18.54. According to Choppin [25] a good linear relationship is often found between the values of the enthalpy and the entropy of complexation for the same ligand and the different pesticides.

Figure 3.Plot of Δ_cH^ᵒ vs Δ_cS^ᵒ for the complexation of herbicides by receptor 5 in acetonitrile at 298.15 K.

4. CONCLUSION:

From the above discussion on the calix[4]pyrrole derivatives, the following conclusion can be drawn. The ligand under investigation, 5, was successfully synthesised in good yields and characterized by ¹H NMR. From ¹H NMR studies, it is concluded that the receptor 5 interacts with acid pesticides. Therefore, in this study, it is essential to investigate the factors of why other receptors did not interact with these pesticides. The presence of the amino groups (basic) in the lower rim for calix[4]pyrrole derivatives makes these ligands attractive for exploring their complexation with acid pesticides. The ¹H NMR technique was successfully used for establishing the binding sites upon complexation with the pesticides. The interaction of meso-tetramethyl-bis (diethylamino) ethoxy-bis-(4-hydroxyphenyl) calix [4] pyrrole with several acid herbicides was carried out in acetonitrile at 298 K. From Table 5, the results obtained seem to indicate that the sites of interaction of this ligand with the acid herbicides are amine group and ethoxy group. Indeed significant chemical shift changes in the proton close to the amine group and ethoxy group were observed. Conductometric measurements were carried out with the aim of determining the composition of the receptor-acid pesticides interaction and gaining information regarding the type and the strength of interaction of these receptors with acid pesticides in acetonitrile at 298 K. From the conductance measurements of the acid herbicide at different concentrations, it was concluded that these acids are highly associated in non-aqueous solvents. As far as the interaction process is concerned, conductometric measurements are performed in order to derive equilibria data for the process involving a neutral receptor and a highly associated acid (acid herbicide) to give 1:2 (receptor:herbicide) adducts. Nano-isothermal titration calorimetry is the most powerful tool to determine the enthalpies of binding of various reactions, including pesticides- ligand binding. Isothermal titration calorimetry (ITC) provides the most accurate and direct measurement of the enthalpy of any reaction under isothermal and isobaric conditions. It is also the only method capable of determining the enthalpy, the entropy, and the Gibbs free energy of a reaction in a single titration experiment. Future work will involve the attachment of the receptor 5 to a solid support to generate recyclable materials for pesticide removal.

5. ACKNOWLEDGEMENTS:

The authors thank the Iraqi Government, Ministry of Higher Education and Basrah University for the financial support provided for this work.

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