Thermodynamics of Adsorption of Imidacloprid at Constant Charge

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Thermodynamics of Adsorption of Imidacloprid at Constant Charge Hydrophobic Surfaces: Physicochemical Aspects of Bioenvironmental Activity E. Giannakopoulos, P. Stivaktakis, and Y. Deligiannakis* Department of EnVironmental & Natural Resources Management, UniVersity of Ioannina, Seferi 2, 30100, Agrinio Greece ReceiVed NoVember 29, 2007. In Final Form: January 26, 2008 Adsorption of the insecticide 1-(6-chloro-3-pyridylmethyl)-N-nitroimidazolidin-2-ylideneamine (Imidacloprid) on the hanging mercury drop electrode (HMDE) surface was studied by temperature-dependent stripping voltammetry (TD-SV). At near physiological pH, under reducing conditions, the Gibbs free energy of adsorption, ∆GADS, shows two distinct temperature-dependent regimes. (a) At 0° < T < 10 °C a temperature-independent mechanism occurs with a constant ∆GADS ) -40.5 kJ/mol, resulting in strong chemisorption at high surface coverage. For T < 10 °C a considerable enthalpy gain is estimated, and this represents the driving force for the adsorption of Imidacloprid onto the electrode surface. (b) At T > 10 °C a temperature-dependent mechanism is operative with ∆GADS/∆T ) -91.4 J/K mol, resulting in a rapid weakening of adsorption and low surface coverage. On the basis of the present findings we suggest that the strong chemisorption at T < 10 °C at physiological pH under reducing conditions is related to the high specific insecticide activity of Imidacloprid in cool-blooded insects as contrasted to its low efficiency in warm-blooded organisms.

1. Introduction Imidacloprid [1-(6-chloro-3-pyridylmethyl)-N-nitroimidazolidin-2-ylideneamine], Figure 1, an active ingredient of the chlornicotinyl insecticides, has good systemic properties, lasting action, and low toxicity to warm-blooded animals.1 Imidacloprid causes a blockage to the nicotinergic neuronal pathway that is more abundant in cool-blooded insects than in warm-blooded animals. Therefore, it is selectively more toxic for cool-blooded insects than in warm-blooded animals. This blockage of the nicotinergic neuronal pathway leads to accumulation of acetylcholine, an important neurotransmitter, resulting in the insect’s paralysis, and eventually death.1-4 The ability of imidaclorpid to cause genetic damage was evaluated using calf thymus DNA. DNA detected with particular pesticide metabolites was 32P postlabeled with nuclease P1 enrichment.5 On the basis of these properties, Imidacloprid, introduced by Bayer AG, is currently used extensively for the control of mites present in vegetable crops.6 Its activity and effectiveness at physiological levels have been evaluated by Leicht,6 and its physical, chemical, and toxicological properties have been summarized in pesticide manuals.7 The analytical detection of Imidacloprid is well documented and includes high-performance liquid chromatography (HPLC)8-10 * To whom correspondence should be addressed. E-mail:ideligia@ cc.uoi.gr. (1) Kidd, H.; James, D. Agrochemicals Handbook, 3rd ed.; Royal Society of Chemistry: Cambridge: England, 1994. (2) Jekins, J. J. Use of Imidacloprid for Aphid Control on Apples in Oregon. Potential for Ground and Surface Water Contamination; Department of Agricultural Chemistry, Oregon State University: Corvallis, OR, 1994 (3) Vicete De Linany, C. Farmacologia Vegetal; Agrotecnicas, S. L., Ed.; Madrid, 1997; p 654. (4) Baskaran, S.; Kookana, R. S. Naidu. J. Chromatogr. 1997, 787A (1-2), 271. (5) Shah, R. G.; Lagueux, J.; Kapur, S.; Levallois, P.; Ayotte, P.; Tremblay, M.; Zee, J.; Poirier, G. G. Mol. Cell. Biochem. 1997, 169, 177. (6) Leicht, W. Plfanzenschutz Nachr.: Bayer 1993, 4, 17. (7) Tomlin, C. The Pesticide Manual: A World compendium, 10th ed.; British Crop Protection Council: Croydon, 1994. (8) Placke, E. J.; Weber, E. Plfanzenschutz Nachr.: Bayer 1993, 46, 109.

Figure 1. Molecular structure of Imidacloprid.

and GC-MS methods for determination of Imidacloprid in water and soil11 and vegetables.12 An electroanalytical methodology has been applied by Navalo´n and Guiberteau using sampled DC, differential pulse polarography (DPP),13 and square wave (SW) voltammetry.14 Of pertinence are studies of the adsorptive behavior of the pesticides danazol and fluoxeine on surfaces,15,16 obtaining valuable information providing physicochemical data of clinical interest. Recently, it has been demonstrated that the thermodynamic parameters of the adsorption of dithiocarbamate pesticides (DTC)17 and mitomycin-C (MC)18 on the hanging mercury drop electrode (HMDE) can be determined experimentally by temperature-dependent stripping voltammetry (TD-SV). These studies revealed that radically different physicochemical mech(9) Ferna´ndez-Alba, A. R.; Valverde, A.; Agu¨era, A.; Contreras, M.; Chiron, S. J. Chromatogr. 1996, 725A, 93. (10) Ruiz de Erenchun, N.; Go´mez de Balugera, Z.; Goicolea, M. A.; Barrio, R. J. Anal. Chim. Acta 1997, 349, 199. (11) Vilchez, J. L.; E1-Khattabi, R.; Ferna´ndez, J.; Gonza´lez- Casado, A.; Navalo´n, A. J. Chromatogr. 1996, 746A, 289. (12) Navalo´n, A.; Gonza´lez-Casado, A.; E1-Khattabi, R.; Vilchez, J. L.; Ferna´ndez-Alba, A. R. Analyst 1997, 122, 579. (13) Navalo´n, A.; EI-Khattabi, R.; Gonza´lez-Casado, A.; Vilchez, J. L. Mikrochim. Acta 1999, 130, 261. (14) Guiberteau, A.; Galeano, T.; Mora, N.; Parrilla, P.; Salinas, F. Talanta 2001, 53, 943. (15) Brown, S.; Rowley, G.; Pearson, J. T. Int. J. Pharm. 1998, 165, 227. (16) Atta-Politou, J.; Skopelitis, J.; Apatsidis, I.; Koupparis, I. M. Eur. J. Pharm. Sci. 2001, 12, 311. (17) Giannakopoulos, E.; Deligiannakis, Y. Langmuir 2007, 23, 2453. (18) Pe´rez, P.; Teijeiro, C.; Marin, D. Langmuir 2002, 18, 1760.

10.1021/la7037334 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/07/2008

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Giannakopoulos et al.

Figure 2. Reduction mechanism of Imidacloprid at a mercury drop electrode (pH ) 7.4). (Inset) SW-CSV of 1 µΜ Imidacloprid at pH ) 7.4. Experimental conditions: taccum ) 45 s, T ) 20 ( 0.3 °C, Eaccumulation ) -100 (solid line) and -1060 mV (dotted line) in 10 mM Britton-Robinson buffer solution.

anisms govern the adsorption of the mitomycin-C and dithiocarbamates onto the surface of the HMDE. According to Pe´rez et al. the adsorption of MC is characterized by a negative ∆G ) (-44.1 to -56.7 kJ/mol) over the temperature range T1 ) 283 K to T2 ) 323 K described by a Langmuir isotherm.18 In the case of DTC Giannakopoulos et al.17 found that the adsorption of DTC is characterized by two mechanisms characterized by a temperature-dependent ∆G. The adsorption process was described by the Frumkin isotherm due to the lateral interactions between neighboring adsorbed DTC molecules.17 The work in refs 17 and 18 is a novel contribution toward detailed physicochemical insight into the adsorption mechanisms of molecules with biological and/or environmental importance. In this context the adsorption properties of Imidacloprid is immediately pertinent due to the intriguing selective efficiency for cool-blooded insects vs warmblooded organisms.6,19 In the present work, the thermodynamics of the adsorption of Imidacloprid onto the mercury drop was studied. The study was carried out for a range of surface potentials, pH, and Imidacloprid concentrations. Temperatures between 0 and 30 °C were chosen, which are of pertinence for the temperatures typically occurring in cool-blooded insects’ organism, similar to those typically encountered in biological cell membranes. The aims of this work were to (a) develop an experimental protocol for study of the thermodynamics of Imidacloprid adsorption onto HMDE, (b) obtain numerical values for Gibbs free energy ∆G, entropy ∆S, and enthalpy ∆H of adsorption, and (c) correlate thermodynamic information with biological activity. 2. Experimental Section 2.1. Reagents. All experiments were performed with analyticalgrade chemicals. Stock, working, and standard solutions were prepared with ultrapure water, Milli-Q water, produced by a Millipore Academic system (Millipore, Belford, MS). Imidacloprid was obtained from Riedel-de Hae¨n (purity > 99.9%, CAS Nr. 138261413) and used without further purification. Imidacloprid stock solution in H2O (100 µM ) 25.6 ppm) was prepared by dissolving 1.18 mg of Imidacloprid in 46.07 mL of solution and stored in the dark at 4 °C. Standard solutions were (19) The Agrochemicals Handbook, 3rd ed.; Kidd, H., James, D. R., Eds.; Royal Society of Chemistry Information Services: Cambridge, U.K., 1991; p 10-2.

prepared daily from the stock solution by dilution at the appropriate concentration. Britton-Robinson buffer solution was used, prepared from a stock solution containing 10 mM phosphoric acid, 10 mM boric acid, and 10 mM acetic acid and adjusted to the desired pH with NaOH. This buffer system is commonly used in analogous voltammetry experiments20 because of its ability to stabilize pH in a broad range (i.e., pH 4-10). By comparing the data obtained in Britton-Robinson buffer with data obtained in other buffers, e.g., such as Tris or HEPES, we verified that the observed phenomena were not influenced by the particular buffer used. The ionic strength of the measurement solution was adjusted by the Briton-Robinson buffer with no additional salt. 2.2. Apparatus and Software. An electrochemical analyzer (model TraceLab50, Radiometer Analytical) was used to control the voltage of a three-electrode system as described earlier.17 As a working electrode, a Radiometer hanging mercury drop electrode was used with a drop area of 3 mm2 controlled by a pneumatic connection with nitrogen (99.999%) at p ) 1 bar. A reference electrode (Ag/ AgCl, KCl 3M type TR020) and an auxiliary electrode (platinum, type TM020) Radiometer were used. The system was connected with a Pentium II PC. The pH measurements were carried out with a GLP21, CRISON pH meter. Temperature-Dependent Experiments. All experiments were carried out in a water thermostated double-wall glass temperaturedependent stripping voltammetry (TD-SV) electrochemical cell, CP021 from Radiometer, as described previously.17 The thermostated cell was connected to a temperature-controlled water circulator operating in the temperature range 1-30 °C. The temperature was continuously monitored by a thermometer immersed in the reaction solution. At each temperature setting the system provided a stable temperature within 0.3 °C usually after 20 min of equilibration. In our experiments an equilibration time of at least 30 min was used. 2.3. Experimental Procedure. All the experiments were carried out using 10 mM Britton-Robinson buffer as supporting electrolyte at a pH 7.4. A 10 mL amount of solution was transferred to the electrochemical cell and deaerated by passing a nitrogen stream (99.999%) through it for 10 min. Then polarograms were registered at different temperatures under an inert atmosphere (nitrogen pressure 1 bar) in the cell. The square wave cathodic stripping voltammetry (SW-CSV) technique was used with the following cell parameters: Stirrer 525 rpm, purge time 600 s, waiting time 10 s, Hg drop growth time 0.7 s. The accumulation time was varied as shown in the experimental (20) Davidson, I. E.; Smyth, W. F. Anal. Chem. 1977, 49, 1195.

Thermodynamics of Adsorption of Imidacloprid

Figure 3. Effect of the accumulation time on the E1/2second ) -1350 mV signal at (9) 0.25, (b) 0.5, (2) 0.755, (1) 1, (() 2, (arrow pointing left) 4, (arrow pointing right) 8, (B) 10, and (0) 12 µM concentration of Imidacloprid. T ) 10 °C, Eaccumulation ) -1060 mV. figures. SW signal parameters: Einitial ) -1060 mV, Efinal ) -1450 mV, step duration 0.04 s, step amplitude 1 mV, and pulse amplitude +50 mV.

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Figure 4. Effect of the accumulation time on the E1/2second ) -1350 mV peak current at different temperatures: T ) (9) 1, (b) 5, (2) 10, (1) 15, (() 20, (arrow pointing left) 25, and (arrow pointing right) 30 °C for 1 µM Imidacloprid.

3. Results and Discussion 3.1. Surface Reactions of Imidacloprid Adsorption on the HMD. The square wave-CSV signal for Imidacloprid contained two peaks in the pH range 2.5-11.0, in accordance with reported data.14 Figure 2 (inset) shows a representative signal at pH 7.4 where the first signal is at E1/2first ) -930 mV and the second E1/2second ) -1350 mV. Mechanism of Polarographic Reduction. The two SW-CSV signals in Figure 2 (inset) show that electrochemical reduction of Imidacloprid takes place by a mechanism commonly proposed for the nitro compounds.21-23 For the first wave, at -930 mV (vs Ag/AgCl), the -NO2 group of the Imidacloprid molecule takes four electrons to give the corresponding hydroxylamine derivative,21 and then in the second reduction step this compound takes two electrons in order to be transformed in the corresponding amine derivative (Figure 2). The signal at E1/2second ) -1350mV attained a maximum intensity for pH in the range pH 7-10 and was used throughout the present study of the thermodynamic parameters of Imidacloprid’s adsorption onto the mercury drop surface. 3.2. Effect of Accumulation Time. The effect of the accumulation time (tacc) on the second peak current was studied at various Imidacloprid concentrations at 10 °C. The data presented in Figure 3 show that for increasing tacc times for each concentration studied the peak increased up to a plateau, determining a saturation current (Isat). In our experimental conditions described in Figure 3 at tacc > 100 s practically a saturation coverage of the mercury drop was attained even at the lower concentration of 0.25 µM. In Figure 3 we observed that the saturation current (Isat) increased proportionally with the Imidacloprid concentration up to 8 µM, where at Imidacloprid concentrations > 8 µM the mercury electrode surface was fully covered. Therefore, at [Imidacloprid] > 8 µM a maximum saturation current (Imax) was attained, e.g., near 414 nA for our experimental setup. Taking into account this information, the temperature-dependent experiments were carried out for a fixed Imidacloprid concentration (21) Pezzatini, G.; Guidelli, R. J. Electroanal. Chem. 1979, 102, 205. (22) Squella, J. A.; Borges, Y.; Celedon, C.; Peredo, R.; Nu´n˜ez-Vergara, L. J. Electroanalysis. 1991, 3, 221. (23) Alvarez-Lueje, A. E.; Bastı´as, M.; Bollo, S.; Nu´n˜ez-Vergara, L. J.; Squella, J. A. J. Assoc. Off. Anal. Chem. 1995, 78, 637.

Figure 5. Temperature dependence of the saturation peak current, Isat, for 1 µM Imidacloprid.

of 1 µM concentration, which provides linear dependence of the peak current with tacc. 3.3. Thermodynamics of Adsorption. Effect of Temperature. The peak current dependence on tacc for 1 µM of Imidacloprid was recorded at temperatures varying over the range of 1-30 °C, see Figure 4. At each temperature a saturation current is attained which is inversely proportional with the temperature. This striking observation shows that lower temperature facilitates adsorption of Imidacloprid’s electroactive species on the mercury drop surface. Before proceeding to a more detailed analysis, we notice that this temperature-dependent trend for Imidacloprid is opposite to that previously observed for adsorption of dithiocarbamate pesticides onto a Hg surface.17 A plot of the temperature dependence of the saturation currents, Figure 5, reveals that Isat versus T is in not linear. Instead, we can anticipate two different regimes. One is for temperatures between T ) 1 and 10 °C, where the saturation current is almost temperature independent

∆I/∆T ≈ 0 1 °C e T e 10 °C (mechanism A) and a second region for T ) 30-10 °C, with a negative slope, marked by the solid line in Figure 5, which on average is

∆I/∆T) -8.7 nA/°C 10 °C e T e 30 °C (mechanism B) As we show in the following in the theoretical analysis, the exact temperature dependence of Isat for T ) 30-10 °C is not linear but exponential.

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Giannakopoulos et al. Table 1. Thermodynamic Parameters BADS and ∆GADS for the Adsorption of Imidacloprid on the Hg Surface

Figure 6. Adsorption isotherm of Imidacloprid onto HMDE at T ) 10 °C.

T (K)

BADS105 (M-1)

∆GADS (kJ mol-1)

r

273 278 283 288 293 298 303

10.07 7.36 5.34 3.27 2.12 1.34 0.83

-40.49 -40.51 -40.48 -40.02 -39.66 -39.20 -38.65

0.9989 0.9996 0.9995 0.9987 0.9978 0.9969 0.9991

derived from the slope and intercept, respectively. At 10 οC, this is exemplified by the plot in Figure 6, where we observe that the CIsat-1 versus C is linear with a correlation coefficient r ) 0.9995. Thus, we consider that in the case of the adsorption of Imidacloprid onto HMDE, a Langmuir isotherm is applicable. This is to be contrasted with the case of dithiocarbamates, where the Langmuir isotherm was not applicable.17 Instead, as we described earlier,17 due to strong lateral interactions the Frumkin isotherm was appropriate. Using eq 2 the slope of the line in Figure 6 allows the maximum peak current, Imax, to be calculated Imax ) 454 nA. This is in good agreement with the experimentally obtained value (414 nA), see Figure 2. The parameter BADS can be calculated from the y intercept in Figure 6, yielding BADS ) 5.34 × 105 M-1. This parameter, which reflects the affinity of the adsorbate molecules toward the surface sites, could be expressed as17,24,25

BADS ≡ B0e(-∆GADS/RT) ) Figure 7. Temperature dependence of the ∆GADS for adsorption of Imidacloprid onto HMDE.

We underline that according to the data presented in the previous paragraphs, no change is occurring in the electroanalytical characteristics of the signals as a function of the temperature. Thus, we suggest that the two ∆I/∆T slopes imply two different adsorption mechanisms for Imidacloprid on the HMDE surface, i.e., rather chemical changes. For the sake of brevity, herein we will call the two adsorption mechanisms mechanism A and B, as shown in Figure 5. Mechanism A describes the adsorption at low temperatures, 1 °C e T e 10 °C, and practically, it does not seem to be affected by the temperature, whereas mechanism B describes the adsorption at higher temperatures, 10 °C e T e 30 °C, and is strongly influenced by the temperature. This will be better analyzed together with the thermodynamics parameters in the following discussion. Theoretical Analysis. The present adsorption results can be analyzed by the Langmuir isotherm

θ)

BADSC 1 + BADSC

(1)

θ is the fraction of occupied surface site, C is the concentration of adsorbate, and BADS (M-1) is a constant which depends on the temperature and reflects the affinity of the adsorbate molecules toward adsorption sites.17,24,25 For a given concentration of Imidacloprid, θ can be defined as Isat/Imax. Replacing θ ) Isat/Imax in eq 1 we can obtain the linearized form of the isotherm

1 C C ) + Isat BADSImax Imax

(2)

Adsorption Free Energy, Entropy, and Enthalpy. If the Langmuir isotherm is valid, a plot of CIsat-1 versus concentration C should yield a straight line with parameters Imax and BADS

1 e(-∆GADS/RT) Csolvent

(3)

where R (J mol-1 K-1) is the constant for ideal gases, T (K) the temperature, ∆GADS (J mol-1) the Gibbs free energy of adsorption, and Csolvent the solvent concentration (mol L-1), 55.5 for H2O. Using eq 3 for BADS ) 5.34 × 105 M-1 we calculate

∆G ) -40.48 kJ/mol at T ) 10 °C This is a rather high ∆G value, i.e., comparable with the ∆G values of -60 kJ/mol for dithiocarbamates.17 This might be taken as evidence for strong adsorption of the molecules on the electrode surface, i.e., via chemisorption.25,26 By proceeding in a similar manner, the adsorption data at each temperature shown in Figures 6 and 7 provided isotherms which were then analyzed by the Langmuir isotherm. The parameters calculated from the CIsat-1 versus C plot and eq 3, as described before, are summarized in Table 1. The estimated ∆GADS values in Table 1 are negative at all temperatures. This indicates that the overall adsorption process of the Imidacloprid molecules onto the Hg surface is energetically favorable. Figure 7 is a plot of the temperature dependence of ∆GADS. The ∆GADS vs T relation has two linear domains: one for 0 to 10 °C (y ) -40.49) and the second for 10 to 30 °C (y ) -66.39 + 0.09137x). By comparing Figure 7 with Figure 5, we notice that ∆GADS vs T shows the same nonlinear behavior as Isat with two distinct slopes, i.e.

∆GADS/∆T ≈ 0 1 °C e T e 10 °C and (24) Phillips, R. K. R.; Omanovic, S.; Roscoe, S. Electrochem. Commun. 2000, 2, 805. (25) Adamson, A.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed.; John Wiley & Son: New York, 1997; p 613. (26) Lyklema, J. Fundamentals of Interface and Colloid Science; Academic Press: London, 1995; Vol. 2.

Thermodynamics of Adsorption of Imidacloprid

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∆GADS/∆T ≈ -91.37 J K-1 mol-1 10 °C e T e 30 °C Estimation of ∆SADS and ∆HADS. According to the fundamental thermodynamic relationships 4 and 525,26

(

∆SADS ) -

)

∂∆GADS ∂T

(4)

∆GADS ) ∆HADS - T∆SADS

(5)

Thus, for a reversible process under constant pressure, the slope of the ∆GADS vs T plot corresponds to the entropy ∆SADS of adsorption. Then, for known ∆SADS and T, the enthalpy ∆HADS can be calculated using eq 5. In this way the entropy and enthalpy of adsorption were calculated in the temperature range studied, and the results are summarized in Table 2. The trends in the thermodynamic parameters calculated in Table 2 are visualized in Figure 8. Accordingly, for a temperature range from 0 to10 °C and from 15 to 30 °C the T∆SADS product and enthalpy, ∆HADS, are temperature independent. Thus, for T < 10 °C the adsorption process is enthalpically governed, since the entropic term T∆S makes practically zero contribution. Thus, for T < 10 °C the considerable enthalpy gain represents the driving force for the adsorption of Imidacloprid onto the electrode surface. For T > 10 °C, the entropic term makes a nonzero contribution and the adsorption ∆GADS decreases. This is confirmed by the abrupt enthalpy change and the more positive value of the T∆SADS term versus ∆HADS, see Figure 8. 3.4. Biological-Environmental Implications. The present data reveal that at physiological temperatures of environmental and biological relevance, i.e., for example between 0 and 30 °C, at physiological pH under reducing conditions, adsorption of Imidacloprid onto the charged hydrophobic surface, such as the Hg drop, is more favorable at lower temperatures than at higher temperatures. This is to be contrasted with the behavior of certain dithiocarbamate pesticides17 and mitomycin,18 whose adsorption onto Hg drop is positively correlated with T. In the case of Imidacloprid, the surface coverage data demonstrate that the

Figure 8. Temperature dependence of ∆GADS, T∆S ADS, and enthalpy ∆HADS for the adsorption of Imidacloprid on Hg drop. Table 2. Thermodynamic Parameters ∆SADS and ∆HADS for the Adsorption of Imidacloprid on the Hg Surface T (K)

∆SADS (J K-1 mol-1) ( 0.5

T∆SADS (kJ mol-1) ( 0.8

∆HADS (kJ mol-1) ( 0.8

274 278 283 288 293 298 303

0 0 0 -91.4 -91.4 -91.4 -91.4

0 0 0 -26.3 -26.8 -27.2 -27.7

-40.5 -40.5 -40.5 -66.3 -66.4 -66.4 -66.3

Figure 9. Temperature dependence of Imidacloprid’s adsorption onto Hg drop.

thermodynamic trend has a significant implication on the number of Imidacloprid molecules which are attached on the Hg-drop surface. At T < 10 °C a significant enthalpic gain forces the Imidacloprid molecules to go preferably on the hydrophobic surface, see Figure 9. This bears relevance to the interaction of Imidacloprid with cell membranes during its use as active insecticide.1,2,6 It is has been speculated, though not directly proven, that blood temperature is a key parameter which determines certain insecticides’ action, including Imidaclorpid.27 This is corroborated by the known specific action of Imidacloprid under enzymatic reduction on cool-blooded insects vs its low efficiency toward warmblooded organisms.1,4,19 On the basis of the present data we suggest that the enthalpic gain below 10 °C would strongly favor adsorption of Imidacloprid on hydrophobic surfaces such as membrane cells. Although other more complex enzymatic factors may modulate this effect, the present data show that adsorption thermodynamics play an important role.

4. Conclusion Under reducing conditions, the Gibbs free energy of adsorption, ∆GADS, shows two distinct temperature-dependent regimes. (a) At 0° < T < 10 °C a temperature-independent mechanism occurs with a constant ∆GADS ) -40.5 kJ/mol, resulting in strong chemisorption at high surface coverage. For T < 10 °C a considerable enthalpy gain is estimated, and this represents the driving force for the adsorption of Imidacloprid onto the electrode surface. (b) At T > 10 °C a temperature-dependent mechanism is operative with ∆GADS/∆T) -91.4 J/K mol, resulting in a rapid weakening of adsorption and low surface coverage at increasing temperatures. On the basis of the present findings we suggest that the strong chemisorption at T < 10 °C under reducing conditions is related with the high specific insecticide activity of Imidacloprid in cool-blooded insects as contrasted to its low efficiency in warm-blooded organisms. A comparison of the present results on adsorption thermodynamic of Imidacloprid with other molecules such as dithiocarbammates17 and mitomycin18 reveals important differences with regard to the temperature dependence of ∆G, ∆H, and ∆S values with immediate implications in the physicochemical mechanisms. Thus, the present work together with previous reports17,18 show that the TD-SV technique can provide detailed fundamental physicochemical information. Properly interpreted within the frame of a pertinent biophysical mechanism, this approach can provide key information of immediate environmental and/or biophysical interest. LA7037334 (27) Prosser, C. L. ComparatiVe Animal Physiology; Saunders Co.: Philadelphia, 1952; p 374.