Thermodynamics of Adsorption of Dithiocarbamates at the Hanging

Nov 20, 2006 - Thermodynamics of Adsorption of Dithiocarbamates at the Hanging. Mercury Drop. Evangelos Giannakopoulos and Yiannis Deligiannakis*...
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Langmuir 2007, 23, 2453-2462

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Thermodynamics of Adsorption of Dithiocarbamates at the Hanging Mercury Drop Evangelos Giannakopoulos and Yiannis Deligiannakis* Lab of Physical Chemistry, Department of EnVironmental & Natural Resources Management, UniVersity of Ioannina, Seferi 2, 30100 Agrinio, Greece ReceiVed July 21, 2006. In Final Form: NoVember 20, 2006 Two dimethyldithiocarbamate (DMDTC) pesticides, thiram and ziram, are adsorbed onto a Hg drop via an entropically driven process. The adsorption isotherms are described by the Frumkin equation. For both molecules, the adsorption is characterized by a nonlinear pseudosigmoid temperature dependence of the Gibbs free energy. For the temperature range of 273-313 K, ∆GADS varies between -43.4 and -56.71 kJ/mol for thiram and -42.60 and -55.67 kJ/mol for ziram. This variation of ∆GADS reveals that the adsorption strength is increased at higher temperatures. During the adsorption of either molecule, strong lateral interactions are developed between neighboring adsorbates, which are severely weakened as the temperature increases. A unified reaction scheme is suggested for both ziram and thiram that predicts the formation and adsorption of a surface complex, (DMDTC)2Hg. In the case of thiram, two DMDTC molecules are formed by the cleavage of the disulfide S-S bond near the Hg electrode. The thermodynamic and structural parameters reveal that there are two limiting thermodynamic regimes for the adsorbed (DMDTC)2Hg species that originate from two limiting adsorption conformations of the adsorbates on the Hg surface. A transition occurs between these two conformations at temperatures in the region of 285-295 K. This transition is accompanied by large entropic and enthalpic changes.

1. Introduction Thiram (C6H12N2S4) [tetra-methyl-thiuramdisulfide] and ziram (C6H12N2S4) [Zn-bis-(dimethyldithiocarbamate)] are among the more widely used dimethyldithiocarbamate (DMDTC) pesticides,1 Figure 1. Because of their high chemical and biological activity and low production cost, they are extensively used as animal repellents to protect fruit trees and ornamentals from damage by rabbits, rodents, and deer or as rubber vulcanization accelerators.1 The biological activity of these compounds is based on the chemical properties of the dithiocarbamate (DTC) group, which can react with sulfur-containing enzymes and coezymes, thus blocking their catalytic activity.2 Enzyme inhibition may also occur by complex formation of the active DTC group with metal ions of metal-containing enzymes.2,3 Once the pesticides are introduced into the environment, then physicochemical processes such as adsorption/desorption and transformation determine their fate.2 Recently we have demonstrated that synergism with other natural factors such as carbonates and carboxylates can decisively affect the physicochemical behavior of thiram.4 Solubility in H2O is poor (e.g., 30 mg/L for thiram and 65 mg/L for ziram4,5), and this makes their elimination from the natural environment difficult. The DTC group can also act as a chelating agent for various metal ions, such as Fe2+, Mn2+, Cu2+, and Ni2+, to form coordination complexes and to act as a fungicide. Toxicity is enhanced significantly if a DTC-metal complex is formed (i.e., as in the case of ziram6). The biological effects of these fungicides * Corresponding author. E-mail: [email protected]. (1) Wauchope, R. D.; Buttler, T. M.; Hornsby, A. G.; Augustijn-Beckers, P. W. M.; Burt, J. P. ReV. EnViron. Contam. Toxicol. 1992, 123, 1-157. (2) Matolcsy, G.; Ndasy, M.; Andriska, V. Pesticide Chemistry; Elsevier: Amsterdam, 1988. (3) Marinovich, M.; Guizzetti, M.; Ghilardi, F.; Viviani, B.; Corsini, E.; Galli, C. L. Arch. Toxicol. 1997, 71, 508-517. (4) Stathi. P.; Christoforidis, K. C.; Tsipis, A.; Hela, D. G.; Deligiannakis, Y. EnViron. Sci. Technol. 2006, 40, 221-227. (5) Worthing, C. R., Ed. The Pesticide Manual, 9th ed.; British Crop Protection Council: Farnham, Surrey, U.K., 1991. (6) Borg, K.; Tjaelve, H. Toxicol. Lett. 1988, 42, 87-91.

Figure 1. Molecular structures of ziram and thiram.

have been described in the literature: induction of sterility in rats7 or thyroid peroxidase inhibition.3 A common denominator in their action is the cell membrane activity, by adsorption on the biological membranes that can determine the surface activity of these molecules.2,5 In a more general context, surface adsorption is widely recognized as a regulatory physicochemical mechanism for the interaction of pesticides with surfaces such as biological membranes8 as well as soil particles.4,9 The hanging mercury drop electrode (HMDE) has been used as a model surface for the physicochemical study of adsorption phenomena of hydrophobic molecules such as aliphatic fatty acids10 and tri-n-butyl phosphate11 and the study of fractal properties of adsorbed linoleic acid.12 In a recent work, the HMDE was used as a model surface for the cellular membrane with regard to the surface activity of the adsorption of the drug mitomycin.13 The interaction of the S-containing molecules with (7) Ema, M.; Itami, T.; Ogawa, Y.; Kawasaki, H. Bull. EnViron. Contam. Toxicol. 1994, 53, 930-936. (8) Schwarzenbach, R. P.; Gschwend, P. M.; Imboden, D. M. EnVironmental Organic Chemistry; John Wiley & Sons: Hoboken NJ, 2003; Chapter 10. (9) Wauchope, R. D.; Yeh, S.; Lindres, J. B. H. J.; Kloskowski, R.; Tanaka, K.; Rubin, B.; Katayama, A.; Ko¨rdel, W.; Gerstl, Z.; Lane, M.; Unsworth, J. B. Pest. Manag. Sci. 2002, 58, 419-445. (10) Ulrich, H.-J.; Stumm, W. EnViron. Sci. Technol. 1988, 22, 37-41. (11) Dogic´, R.; Krznaric´, D. Electroanalysis 2003, 15, 312-318. (12) Risovic´, D.; Gasˇparovic´, B.; CÄ osovic´, B. Langmuir 2001, 17, 10881095. (13) Pe´rez, P.; Teijeiro, C.; Marin, D. Langmuir 2002, 18, 1760-1763.

10.1021/la062147v CCC: $37.00 © 2007 American Chemical Society Published on Web 01/24/2007

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the Hg drop has drawn the continuous attention of scientists over the past 50 years. The pioneering works of Kolthoff and Stricks have contributed to the fundamental understanding of the physical chemistry of the interaction of cysteine14/cystine15 with Hg. Gregg and Tyler paved the way for the polarography of dithiocarbamates.16 A detailed study of the behavior of the S-S bond of cystine and the adsorption of cysteine on the Hg drop has been made by Stankovich and Bard17 and reviewed by Florence18 using cathodic stripping voltammetry. The notoriously complicated reactions of the reduction of the S-S bonds in proteins at the Hg electrode have been reviewed by Honeychurch.19 More recently, the reactions of cystine at the Hg electrode have been discussed in a comprehensive paper by Heyrovsky´ et al.20 In previous works, the HMDE has been employed for the analytical determination of DTC pesticides, including ziram and thiram. These works were analytical in nature, addressing topics such as the polarographic determination of thiram,21 the analytical determination of ziram by anodic stripping voltammetry (ASV),22 and the analytical measurement of thiram by cathodic stripping voltammetry (CSV).23 The aforementioned works were exclusively concerned with the electroanalytical aspects of the detection of dithiocarbamates. Physicochemical surface phenomena such as thermodynamics and adsorption were not studied, with the exception of a study of the adsorption of thiram in lignin24 and more recently our work on the adsorption of thiram on oxides and clays.4 Given the importance of the dithiocarbamate pesticides, this lack of physicochemical data and parameter values calls for a more detailed study of their interaction with hydrophobic constant charge surfaces. In this context, we present here a detailed study of the thermodynamics of adsorption for two representative dimethyl-dithiocarbamate (DMDTC) pesticides on the mercury drop electrode. The main objectives of the present work are (a) to study the thermodynamics of adsorption of DMDTC on HMDE and to calculate the values of the Gibbs free energy, enthalpy, and entropy; (b) to study the physicochemical mechanism of interaction of the dimethyldithiocarbamates with the model HMDE surface at physiological pH; and (c) to provide a comprehensive basis for the correlation of the physicochemical parameters with the surface reactions of the dithiocarbamates. Thiram and ziram were used as dimethyldithiocarbamates because of their common use and the availability of adequate literature on the analytical aspects of their detection. Theoretical Analysis of the Adsorption Isotherms. The simple Langmuir equation for the adsorption is

θ ) BADSC 1-θ

(1a)

where BADS is the adsorption constant that expresses the strength of adsorption.25-27 -∆GADS

BADS ≡ B0e

RT

)

1 Csolvent

-∆GADS

e

RT

(1b)

(14) Stricks, W.; Kollthoff, I. M. J. Am. Chem. Soc. 1953, 75, 5673-5681. (15) Kolthoff, I. M.; Stricks, W.; Tanaka, N J. Am. Chem. Soc. 1955, 77, 4739-472. (16) Gregg, E. C.; Tyler, W. P. J. Am. Chem. Soc. 1950, 72, 4561-4564. (17) Stankovich, M. T.; Bard, A. L. J. Electroanal. Chem. 1977, 75, 487-505. (18) Florence, T. M. J. Electroanal. Chem. 1979, 97, 219-236. (19) Honeychurch, M. J. Bioelectrochem. Bioenerg. 1997, 44, 13-21. (20) Heyrovsky´, M.; Mader, P.; Vesela´, V.; Fedurco, M. J. Electroanal. Chem. 1994, 369, 53-70. (21) Brand, M. J. D.; Fleet, B. Analyst 1970, 95, 1023-1026. (22) Mathew, L.; Reddy, M. L. P.; Rao, T. P.; Iyer, C. S. P.; Damodaran, A. D. Talanta 1996, 43, 73-79.

∆GABS (J/mol) is the Gibbs free energy of adsorbed molecules occupying sites that do not interact with each other. R (J/mol K) is the ideal gas constant, T (K) is the temperature, and Csolvent is the solvent concentration (e.g., 55.5 mol/L in the case of H2O in aqueous solution13,27). C is the equilibrium bulk concentration of the adsorbate. As we show later in the present work, eq 1a was not adequate for the description o the adsorption of DMDTC on the Hg drop. Instead, the Frumkin (Frumkin-Fowler-Guggenheim, FFG) equation (eq 1c), which is a modified Langmuir equation25

θ ) B′C 1-θ

(1c)

was used with

B′ ) B0e

-∆GADS+zωθ RT

) B0e

-∆GADS zωθ RT e RT

zωθ

) BADSe RT (1d)

In brief, the derivation of eq 1d is the following:25,27 The probability of a given site being occupied is N/S, where N is the number of adsorbed molecules and S is the surface, and if each has z neighbors, then the probability of a neighboring site being occupied is zN/S. Therefore, the fraction of adsorbed molecules involved is (1/2)zθ, with the factor of 1/2 correcting for double counting. If the lateral interaction energy is ω, then the added differential energy of adsorption equals zωθ.27 Combining eqs 1c and 1d gives

θ ) B′C ) BADSe(zωθ/RT)C 1-θ θ -(zωθ/RT) e ) BADSC 1-θ θ -(2aθ) e ) BADSC 1-θ

(1e)

where

a≡

zω 2RT

(1f)

Equation 1e is the well-known Frumkin equation,25 with C being the bulk equilibrium concentration of the adsorbate. Herein concentrations are used instead of activities because the concentration of thiram or ziram is at or below the micromolar range. The lateral interaction coefficient a is positive for attractive and negative for repulsive forces.25,26 2. Experimental Section Apparatus. The voltammmetric measurements of thiram and ziram were made by using an electrochemical analyzer (TraceLab50 by Radiometer Analytical) with a three-electrode system. A Radiometer MDE150 hanging mercury drop electrode (type A08C003) was used as the working electrode. The mercury drop was formed at the end of a capillary with an inner diameter of 70 µm (type B18C001). The mercury drop area (0.6-1.8 mm2) was controlled by a pneumatic connection with nitrogen (99.999%) at 1.0 bar. The reference electrode (Ag/AgCl, KCl 3 M, type TR020) (23) Procopio, J. J.; Escribano, M. T. S.; Hernandez, L. H. Fresenius J. Anal. Chem. 1988, 311, 27-29. (24) Rupp, E. B.; Zuman, P.; Sˇ esta´kova´, I.; Horak, V. J. Agric. Food Chem. 1992, 40, 2016-2021. (25) Lyklema, J. Fundamentals of Interface and Colloid Science; Academic Press: London, 1995; Vol. 2. (26) Giles, C. H.; Smith, D.; Huitson A. J. Colloid Interface Sci. 1974, 47, 755-778. (27) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed; John Wiley & Sons: New York, 1997; p 613.

Adsorption of Dithiocarbamates at the HMD and the auxiliary Pt electrode (type TM020) were also from Radiometer. All measurements were carried out with a thermostated double-walled electrochemical cell (type CP021). The temperature of the electrochemical cell was controlled by water circulation via a thermostated circulator operating in the temperature range of 0-50 °C. The temperature was constantly monitored in situ in the measuring cell with a digital thermometer inserted through a sealed opening at the top cap of the measuring cell. At each temperature setting, the system was equilibrated for at least 30 min before measurement. This protocol was found to provide stable temperatures to within (0.3 °C. The temperature-dependent experiments were carried out in triplicate. The pH measurements were carried out with a pH meter (GLP21, Crison). Reagents. All chemicals used were of analytical grade. Stock solutions of thiram and ziram were prepared in methanol because of the low solubility of these compounds in water.4 Standard solutions were prepared in ultrapure Milli-Q water produced by a Millipore Academic system (Millipore, Belford, MS). N,N-Dimethyl-dithiocarbamate (DMDTC) fungicides, thiram (purity >97%, lot S21169404), and ziram (purity >96.65%, lot 45708) were obtained from Aldrich and used without further purification. An N,N-dimethyldithiocarbamate stock solution in methanol (25 µM) was prepared by dissolving 0.30 mg of thiram or 0.38 mg of ziram in 50 mL of methanol, followed by stirring for at least 2 h and then dark storage at 4 °C. Standard solutions were prepared daily from the stock solution by dilution to the appropriate concentration. Thiram is a hydrophobic molecule with low solubility (30 mg/L) in water.4,38 At alkaline pH >9, thiram is rapidly decomposed by nucleophilic attack.4,38 At pH 7 more than 98% of ziram is dissociated in one Zn2+ and two DMDTC- anions. On the basis of this, we conclude that in our CSV experiments that were carried out at pH 7.4 the electrode reaction at the Hg drop will concern only the DMDTC- anion. Electrode Reactions at the Hg-Drop Surface. The electrochemical reactions responsible for the CSV response of DMDTCat the HMDE can be described by the following reactions16,21,24

DMDTC- + Hg0 h (DMDTC-)2Hg + 2e-

(4a)

(DMDTC-)2Hg h 2(DMDTC-) + Hg2+

(4b)

Hg2+ + 2e- h Hg0

(4c)

This is a common mechanism for anionic sulfur species adsorbed at the Hg-drop surface where the electroactive adsorbed species is a sparingly soluble mercuric complex formed between the

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Figure 3. Speciation of ziram (Zn(DMDTC)2) in aqueous solution as a function of pH. At pH >5, almost all ziram molecules are dissociated in Zn2+ cations and DMDTC- anions.

(SERS).39 This is attributed to the high affinity of the DTC moiety to the silver surface.39 Taking into account the very large affinity of the DTC for Hg (see below), the cleavage of the S-S bond at the Hg-drop surface is expected to by strongly favored as well, according to the reaction16,24

DMDTC(S)-(S)DMDTC + 2e- S 2DMDTC- (6a) At the HMDE, this reaction is expected to be catalyzed at the Hg surface, which reacts rapidly with R-S-S-R according to16,24

DMDTC(S)-(S)DMDTC + Hg0 S Hg(DMDTC)2 (6b) Subsequently, Hg(DMDTC)2 is reduced very rapidly at the HMDE electrode16,24

Hg(DMDTC)2 + 2e- S Hg0 + 2DMDTC-

Figure 2. SW-CSV for 0.4 µM (A) ziram and (B) thiram. Experimental conditions: pH 7.4, T ) 24 °C, Eaccum ) -100 mV, and taccum ) 45 s.

anionic sulfur ligand and one Hg2+ atom from the Hg-drop surface.19,20,35 Thiram R-S-S-R Bond CleaVage at the Hg-Drop Surface. The pioneering work of Gregg and Tyler16 showed that the S-S bond of [disulfide-C2H5-dithiocarbamate] (bis-DEDTC, disulfiram) is reduced catalytically at the surface of the Hg drop, resulting in two DEDTC molecules. This occurs at electrode potentials more positive than -400 mV. In a similar manner, the S-S bond of thiram [disulfide-CH3-dithiocarbamate] is cleaved catalytically at the surface of the Hg drop,24 resulting in two DMDTC molecules,21 according to reaction 5

The cleavage of the S-S bond of thiram by metallic surfaces is a common mechanism observed for either the Hg0 or Ag0 surface as evidenced by surface-enhanced Raman spectroscopy

(6c)

Reaction 6a is the overall reaction of 6b + 6c.16,24 An analogous reaction scheme has been suggested15 and is generally accepted for the cystine/Hg system20 and other S-S-bearing systems such as proteins.19 Thus we may consider that the electrode reaction scheme for thiram or ziram at the HMDE is described by the following scheme (Scheme 1). The key observation is that for either ziram or thiram the sulfur species involved is the (DMDTC)2Hg complex. In this context, taking into account Scheme 1, we may understand the similar CSV signals obtained in Figure 2A and B. Ziram gives a CSV signal at -560 mV16 (Figure 2A) due to the reversible reaction 4a-4c16 (i.e., due to the two DMDTC- molecules released). Thiram gives an identical CSV signal (Figure 2B23). This signal does not originate from the direct electrochemical activity of thiram. The reductive cleavage of the S-S bond of thiram requires strong reducing potentials (i.e., near -2000 mV in solution or more negative than -800 mV at the HMD16). At (34) Herbelin, A. L.; Westall, J. C. FITEQL 4.0: A Computer Program for Determination of Chemical Equilibrium Constants from Experimental Data; Report 99-01; Department of Chemistry, Oregon State University: Corvallis, OR, 1999. (35) Wang, J. Stripping Analysis; VCH Publishers: New York, 1985. (36) Scharfe, R. R.; Sastri, V. S.; Chakrabarti, C. L. Anal. Chem. 1973, 45, 413. (37) Aspila, K. I.; Chakrabarti, C. L.; Sastri, V. S. Anal. Chem. 1973, 45, 363-367. (38) Sharma, V. K.; Aulakh, J. S.; Malik, A. K. J. EnViron. Monit. 2003, 5, 717-723. (39) Sa´nchez-Corte´es, S.; Vasina, M.; Francioso, O.; Garcı´a-Ramos, J. V. Vib. Spectrosc. 1998, 17, 133-144.

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Scheme 1. Reaction Scheme for Ziram or Thiram at the Hg-Drop Electrode in Aqueous Solutiona

a The adsorbed species responsible for the CSV signal at -560 mV in Figure 2A,B is the mercuric complex [Hg2+(DMDTC-)2].

the deposition potentials (e.g., -100 mV used in the present CSV experiments), a more complex sequence of reactions take place (Scheme 1), which results in two DMDTC- molecules.

4. Adsorption Studies Effect of Accumulation Time. The effect of accumulation time, taccum, (10-1500 s) on the magnitude of the stripping current was studied at 21 °C at various concentrations of ziram or thiram (Figure 4A and B, respectively). For both molecules, the current increased nonlinearly with increasing taccum. In the experiments described in Figure 4, no signal changes were observed (i.e., peak splittings or shifts) for C < 1 µM. This is in accordance with the monolayer coverage of the Hg drop. At concentrations below 0.8 µΜ, considerably long accumulation times are required to achieve full coverage of the Hg-drop surface. We notice that the current attains its maximum steady value for taccum ) 1500 s for ziram (Figure 4A), which is about 4 times slower than for the case of thiram where the current attains its maximum steady value for taccum ) 250 s (Figure 4B). This reveals that thiram is adsorbed at the Hg surface via a kinetically faster reaction than for ziram. This will be discussed further in the following text, together with the entropy values for adsorption. For a fixed concentration of 0.4 µM thiram and ziram, a series of kinetic curves were recorded as a function of the accumulation time, taccum, at different temperatures over the range of 0-40 °C (Figure 5A for ziram and Figure 5B for thiram). We observe that the maximum peak current varies with temperature, which indicates that a lower temperature facilitates adsorption onto the Hg drop. In addition, the adsorption kinetics is consistently faster for thiram than for ziram at all of the temperatures studied. Overall, the present data show that even at high concentrations, long accumulation times are required for full coverage of the Hg-drop. The kinetic barrier appears to be higher for ziram than for thiram. Taking this information into account, in the following we have measured adsorption isotherms at taccum )1500s where adsorption equilibrium is essentially attained. Adsorption Isotherms. The adsorption of ziram and thiram on the Hg surface, was measured at various temperatures (1, 10, 21, 30, and 40 °C). The obtained isotherms are presented in

Figure 4. Effect of accumulation time on the CSV current at the indicated concentrations, C, for (A) ziram and (B) thiram at T ) 21 °C. The y axes are the peaks at E ) -560 mV.

Figure 6A and B for ziram and thiram, respectively. Two main observations can be made for the isotherms in Figure 6A and B: (a) At low temperatures, the isotherms have a characteristic sigmoid S shape26 that is progressively converted to a Langmuirlike L-shaped isotherm. As discussed by Lyklema25 and Giles et al.26 the S-shaped isotherm is indicative of interactions between the adsorbed molecules. (b) For both ziram and thiram, the isotherms of adsorption follow a characteristic shape transition from the S shape at low temperatures to the L shape at high temperatures (Figure 6A and B26). (c) The onset of full coverage of the Hg drop, manifested by the start of the plateau, shifts to lower concentrations at increasing temperatures. The physicochemical origin of these observations will be analyzed quantitatively in the following text.

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Figure 5. Effect of accumulation time on the 0.4 µM (A) ziram and (B) thiram peak currents at different temperatures: 9, 0; b, 10; 2, 20; 1, 30; and (, 40 °C.

Adsorption Parameters. Because of the S-shape character, the adsorption data do not fit a simple Langmuir equation (eq 1a; see the dashed line in Figure 7). The deviation from the Langmuir isotherm is more clearly manifested at low concentrations as a sigmoid shape (Figure 7). Instead, the use of the Frumkin isotherm (eq 1e) allows a best fit to be achieved; see the solid line in Figure 7. The initial sigmoid part of the adsorption isotherm is sensitive to parameter a, which is a measure of the lateral interactions between neighboring adsorbates.25-27 The fit of eq 1e to the experimental isotherms at each temperature allows estimates of parameters a and BADS to be obtained (summarized in Table 1). The estimated values for the lateral interaction parameter a and the adsorption constant BADS are plotted versus temperature in Figure 8A and B, respectively. We observe that for both ziram and thiram the lateral interaction parameter a has positive values that vary with temperature. According to the

Giannakopoulos and Deligiannakis

Figure 6. Adsorption isotherms for (A) ziram and (B) thiram on the HMDE. Experimental conditions: pH 7.4, supporting electrolyte, 10 mM Britton-Robinson; and accumulation time, taccum ) 1500 s. The solid lines through the experimental points are guide for the eye.

theory, a positive a value, a > 0, indicates an attractive interaction between adsorbates whereas the interactions are repulsive for a < 0.26,27 Thus in the cases of both ziram and thiram, the lateral interactions between neighboring adsorbates are attractive. The adsorption strength BADS also appears to vary nonlinearly with temperature (Figure 8B). For both thiram and ziram, the BADS values are small at low temperatures and increase significantly at T > 295 K. Overall from Figure 8A and B, for both ziram and thiram a complementary trend is observed for the lateral interactions and the surface adsorption strength. At T < 285 K, the interactions between neighboring adsorbates appear to be strong whereas the adsorption strength on the hydrophobic surface of the Hg drop

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Langmuir, Vol. 23, No. 5, 2007 2459

Figure 7. (9) Adsorption isotherm for thiram at 0 °C. (‚‚‚) Best-fit curve using the Langmuir equation (eq 1a). (s) Best-fit curve using the Frumkin equation (eq 1e). Table 1. Thermodynamic Parameters BADS and ∆GADS and Lateral Interaction Parameter Values a for the Estimation of the Adsorption of Ziram and Thiram on the Hg Surface thiram

ziram

a lateral a lateral T interaction BADS ∆GADS interaction BADS ∆GADS -6 -1 (K) parameter (× 10 ) (kJ mol ) parameter (× 10-6) (kJ mol-1) 273 283 294 303 313

1.08 0.89 0.60 0.26 0.01

3.4 5.9 12.1 28.7 48.9

-43.40 -46.28 -49.65 -53.55 -56.71

1.31 1.09 0.70 0.34 0.35

2.4 4.1 8.8 18.0 32.8

-42.60 -45.42 -48.91 -52.371 -55.671

is weak. At higher temperatures, the lateral interactions appear to attenuate significantly whereas the adsorption forces on the surface are strengthened. This interplay between these two phenomena determines the rapid change in the isotherm’s shape from an S sigmoid toward an L shape at T > 20 °C (Figure 6). Adsorption Free Energy, Entropy, and Enthalpy. The Gibbs free energy of adsorption ∆GADS can be calculated from the BADS values according to relation 1b.13,27 The obtained ∆GADS values (Table 1) are negative for all temperatures for both ziram and thiram. This indicates that the overall adsorption process of the DMDTC molecules onto the Hg surface is energetically favorable. Figure 9 shows the temperature dependence of ∆GADS. The ∆GADS versus T relation is not linear; instead, the ∆GADS versus T data in Figure 9 could be fitted to a third-order polynomial. In Figure 9, we notice a pseudosigmoidal variation in the ∆GADS values centered in the temperature range of 285295 K. The significance of this observation will be discussed later. Estimation of ∆SADS and ∆HADS. According to the fundamental thermodynamic relationships

∆SADS )

( ) ∂GADS ∂T

∆GADS ) ∆HADS - T∆GADS

(7a) (7b)

for a reversible process under constant pressure, the slope of the ∆GADS versus T plot corresponds to the entropy ∆SADS of the adsorption; then for known ∆SADS and T, the enthalpy ∆HADS can be calculated by using eq 7b. The nonlinear ∆GADS versus T indicates that the entropy and enthalpy are also temperature-dependent. This is verified by the calculated ∆SADS and ∆HADS values listed in Table 2 for ziram

Figure 8. (A) Temperature dependence of the lateral interaction parameter, a, for the adsorption of (9) ziram and (b) thiram on the Hg drop. The open symbols are representative theoretical calculation using eq 1f assuming an interaction energy of w ) 1 kJ/mol and the number of next-nearest neighbors z ) 1 (O), 2 (]), 4 (3) or w ) 5 kJ/mol and z ) 1 (4). (B) Temperature dependence of the adsorption constant BADS for (9) ziram and (b) thiram.

and thiram. Figure 10 shows a plot of the values for T∆SADS and ∆HADS. Values of ∆GADS are also included for clarity. Overall from Figures 9 and 10 we observe that (a) for both ziram and thiram the ∆GADS values are comparable in both size and sign. Thiram appears to be adsorbed more strongly than ziram at all temperatures. This agrees with the faster adsorption kinetics observed for thiram (e.g., Figures 4 and 5). (b) The entropic term T∆SADS is the dominant factor that determines the sign of ∆GADS. Although at lower and higher temperatures this term is decreased, its contribution remains dominant over ∆HADS. Thus we conclude that the overall adsorption process for both ziram and thiram is entropically driven. (c) In the case of thiram, the entropic term is significantly higher (by 70-100%) than the entropic term for ziram. Thus the slightly stronger free energy of adsorption observed for thiram relative to that for ziram originates from the higher entropic term. The structural implication of this result will be discussed in the following text.

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Figure 9. Temperature dependemce of the Gibbs free energy of adsorption ∆GADS for (9) ziram and (0) thiram. The solid lines are best-fit third-order polynomials that are used for the application of relations 7a and 7b. Table 2. Thermodynamic Parameters ∆SADS and ∆HADS for the Adsorption of Ziram and Thiram on the Hg Surface thiram

ziram

T (K)

∆SADS (J K-1 mol-1)

∆HADS (kJ mol-1)

∆SADS (J K-1 mol-1)

∆HADS (kJ mol-1)

273 283 294 303 313

0.57 0.62 0.53 0.37 0.36

113.4 126.9 100.4 55.9 51.2

0.28 0.31 0.35 0.29 0.23

33.9 43.5 52.8 34.6 16.6

5. Discussion Surface Reactions. The unified reaction scheme (Scheme 1) fits well with the well-established concept that the electrochemical activity of sulfur compounds at the Hg electrode is mainly determined by the interaction of the sulfur-containing groups with the Hg-surface atoms rather than the redox transformation of the sulfur-containing groups themselves.18-20,33 The stability constant for the formation of the mercuric complex of DMDTC by Hg2+ and DMDTC (KHg(DMDTC)2 ) 1036) has been calculated by Bond and Scholtz.40 Although the KHg(DMDTC)2 stability constant is high, we should notice that it about 6 orders of magnitude lower than the stability constant reported for [Hg2+(Cys)2].14 Because of this strong affinity of DMDTC for Hg, sparingly soluble Hg(DMDTC)2 complexes are formed and deposited onto the electrode surface, forming a film.19,20,33 In the case of cysteine, Florence presumed that a multilayer film of mercuric cysteinate could be deposited on the Hg surface; however, only the first layer immediately adjacent to the mercury surface was reducible.18 In the case of ziram or thiram, using a single Hg drop as a working electrode, the insoluble Hg(DMDTC)2 complex is formed under anodic polarization of the electrode in the presence of the reactive DMDTC. Afterward, the insoluble Hg(DMDTC)2 complex plays the role of an electroactive reducible reactant. By applying a cathodic potential scan, the complex can be reduced in a stripping off of the reactive DMDTC from the electrode surface. Thus, as in the case of other sulfur ligands19,20,33 the redox-active center of the adsorbed Hg(DMDTC)2 complex is the Hg/Hg2+ couple, whereas the DMDTC plays the role of complexing ligand. This explains why the E1/2 of reduced species Hg(DMDTC)2 is similar for both ziram and thiram, although this is a drawback for analytical purposes. For example, the CSV signals of thiram and ziram are essentially undistinguishable (Figure 2A and B) because in both cases the same surface species, (DMDTC)2Hg, is reduced. Thus previous reports on the analytical (40) Bond, A. M.; Scholtz, F. J. Phys. Chem. 1991, 95, 7460-7465.

Figure 10. Temperature dependence of the Gibbs free energy ∆GADS, entropy T∆SADS, and enthalpy ∆HADS for the adsorption of (A) ziram and (B) thiram on the Hg drop.

use of the CSV signal at -560 mV for the quantitative determination of thiram23 should be reconsidered with caution in real samples (e.g., in environmental samples, surface waters because many other sulfur pesticides or sulfate ions can give overlapping signals in the same region). Surface ReactionssEnergetics. The comparable ∆GADS values and the temperature-dependence trends observed for both ziram and thiram imply that the fundamental thermodynamic steps determining the adsorption of these molecules onto the Hg drop are similar. This corroborates the reaction scheme (Scheme 1), which indicates that the adsorbed species is essentially the same in both ziram and thiram. We consider that the observed adsorption phenomena reflect the overall process. With the data at hand, it is rather difficult to distinguish the contribution from each intermediate step further. However, we can investigate certain limiting cases that provide further insight into this complicated adsorption mechanism. The reaction scheme (Scheme 1) predicts the formation of the (DMDTC)2Hg complex between one mercuric ion Hg2+ and two DMDTC- anions. The Gibbs free energy of formation for this

Adsorption of Dithiocarbamates at the HMD

complex can be estimated from the thermodynamic stability constant40

K[(DMDTC)2Hg] ) 1036 ∆G[(DMDTC)2Hg] ) -RT ln K[(DMDTC)2Hg] ∆G[(DMDTC)2Hg] ) -8.314 × 298 × 36 (kJ/mol) ) -89.2 kJ/mol at T ) 298 K (8) In the context of the reaction scheme (Scheme 1), ∆G[(DMDTC)2Hg] would correspond to reaction 4b for the complex formation. If we consider that ∆GADS represents the sum of reactions 4a-4c

∆GADS ) ∆G4a + ∆G4b + ∆G4c ∆G4a + ∆G4c) ∆GADS - ∆G4b then the rest of the reactions (4a and 4c) add up to a positive free energy ∆G:

∆G4a + ∆G4c ) -55.4 kJ/mol + 89.2 kJ/mol ) +33.8 kJ/mol at T ) 298 K The reduction of the mercuric ion (reaction 4c) is thermodynamically favorable14

∆G(Hg2++2e-fHg0) ) -FE0(Hg2++2e-fHg0) ) -2 × 96484.6 (C/mol) × 0.854 (V vs SHE) ∆G(Hg2++2e-fHg0) ) -164.8 kJ/mol that is,

∆G4c ) -164.8 kJ/mol Therefore,

∆G4a ) -∆G4c + 33.8 kJ/mol ) +164.8 kJ/mol + 33.8 kJ/mol ∆G4a ) +198.6 kJ/mol That is, surface reaction 4a is energetically disfavored because of the positive ∆G4a. Temperature Dependence of the Lateral Interactions. The comparison of the temperature dependence of the experimental values’ lateral interaction parameter a (Figure 8) with theoretically predicted values (open symbols in Figure 8) reveals that the experimental a values for both ziram and thiram vary more sharply with temperature than predicted by eq 1f. A closer inspection of Figure 8 shows that there is a more abrupt transition of the a values at temperatures of 285-295 K. This indicates that the lateral interaction between adsorbed species undergoes a significant weakening at T > 295 K. A nonlinear change in the entropic term of adsorption also occurs at T ) 285-295 K (Figure 10). Structural Implications. The data in Figure 6A and B indicate that the surface coverage is temperature-dependent (i.e. a significantly higher surface coverage is achieved at lower temperatures). Taking into account the temperature dependence of the lateral interaction value a and the adsorption strength (Figure 8), we suggest in the following a working hypothesis for a unified interpretation of the experimental data. The key observation is that the isotherms of adsorption of DMDTC molecules onto the Hg surface follow a characteristic shape transition from the so-called S shape at low temperatures to the so-called L shape at high temperatures. According to Gilles

Langmuir, Vol. 23, No. 5, 2007 2461

et al.,26 such an S-to-L isotherm transition can be due to a change in the total energy of adsorption.26 According to the foregoing analysis in the case of ziram and thiram, this shape-transition is due to (a) the temperature-dependent weakening of the a value, which exceeds the fundamental thermodynamic effect predicted by eq 1e (Figure 8); (b) the temperature-dependent decrease in surface coverage in which a dense “packing” arrangement of the adsorbed molecules at low temperatures, which is gradually converted to a less dense arrangement at higher temperatures, is the most reasonable physical picture consistent with the experimental results; and (c) the temperature-dependent strengthening of the adsorption force, which is indicative of a phase transition occurring for the adsorbed species at the Hg surface as a function of temperature. The adsorption energy is weak at low temperatures. As the temperature is increased, the surface species adopt a different conformation with stronger adsorption energy and less dense packing, resulting in a smaller number of lateral interactions (i.e., lower a values). Overall the present thermodynamic and structural parameters reveal that there are two limiting thermodynamic regimes for the adsorbed (DMDTC)2Hg molecules. We suggest that the thermodynamic data originate from two limiting adsorption conformations of the adsorbates on the Hg surface. A transition occurs between these two conformations at temperatures in the 285-295 K region. This transition is accompanied by large entropic and enthalpic changes (e.g., see T∆SADS and ∆HADS in Figure 10). Despite the similarity between the ∆GADS values for ziram and thiram, we observe that entropic term T∆SADS for thiram is almost twice that for ziram. Transitions corresponding to adsorption conformations have been reported for the adsorption of p-nitrophenol on silica26 and the adsorption of tri-n-butyl phosphate11 and linoleic molecules onto the Hg surface.12 According to Giles et al.,26 the S isotherm observed for the adsorption of p-nitrophenol on silica corresponds to a conformation where the adsorbate is oriented with its axis of symmetry perpendicular to the Hg surface whereas the L isotherm corresponds to the conformation with the adsorbate’s axis of symmetry being parallel to the Hg surface. More appropriate for our discussion is the work of Heyrovsky´ et. al for cystein-Hg adsorption.41 In the case of Cys-Hg, the hydrophobic end of the Hg-S-CH2- complex is anchored at the Hg-drop surface, and the hydrophilic part is turned toward the solution. This orientation is favored for electrostatic reasons (i.e., electrostatic attraction between oppositely charged NH3+ and COO- groups of the neighboring amino acids). The resulting strong lateral interaction of the adsorbed molecules leads to the formation of a compact monomolecular surface film that further hinders the process when the surface coverage is complete.41 By extrapolation, in the case of ziram/thiram, the DMDTC-Hg might adopt a similar perpendicular orientation with the hydrophobic -S-Hg attached to the Hg-drop surface at low temperatures. The (CH3)-N- moiety of DMDTC lacks the net charges of the amino acid, thus strong electroastatic forces between neighboring adsorbates are not developed. Instead, according to our preceding analysis, in the case of DMDTC-Hg the lateral interactions most likely originate from weaker forces, with energy on the order of 5 kJ/mol, as evidenced by calculations presented in Figure 8. The data at hand preclude a more definitive assignement of these forces. However, if we adopt the physical picture that the adsorption phenomena observed here refer essentially to the displacement of water molcules from the Hg surface by the adsorbates,25-27 then such weak interactions could be H bonds (41) Heyrovsky´, M.; Mader, P.; Vavrˇicˇka, S.; Vesela´, V.; Fedurco, M. J. Electroanal. Chem. 1997, 430, 103-117.

2462 Langmuir, Vol. 23, No. 5, 2007

between the adsorbed DMDTC-Hg molecules bridged by H2O molecules. Indeed, density functional theory calculations show that in aqueous solution the DMDTC moiety tends to be stabilized by H bonds formed with two H2O solvent molecules,4 with an average energy of 12.6kJ/mol per H-bonded H2O.4 In this perspective a set of H bonds holding together neighboring DMDTC-Hg complexes adsorbed at the Hg-drop surface could be responsible for the observed lateral interactions, leading to a denser packing. Increasing temperature (i.e., that due to thermal vibrations and/or thermal expansion of the Hg drop) might disrupt these weak interactions. Additional experiments in solvents other than H2O could provide further information with regard to the nature of the observed lateral interactions.

6. Conclusions A unified reaction scheme is suggested for both ziram and thiram that predicts the formation and adsorption of a surface complex, (DMDTC)2Hg. In the case of thiram, two DMDTCmolecules are formed by the cleavage of the disulfide S-S bond near the Hg electrode. Thiram and ziram are adsorbed onto the Hg drop via an entropically driven process. The adsorption isotherms are described by the Frumkin equation. For both molecules, the adsorption is characterized by a temperature-dependent Gibbs

Giannakopoulos and Deligiannakis

free energy. During the adsorption of either molecule, considerable lateral interactions are developed between neighboring adsorbates at T ≈ 273 K that are severely weakened as the temperature increases. Despite the similarity between the ∆GADS values for ziram and thiram, the entropic term T∆SADS for thiram is almost twice that for ziram. The thermodynamic and structural parameters reveal that there are two limiting thermodynamic regimes for the adsorbed (DMDTC)2Hg species. We suggest that the thermodynamic data originate from two limiting adsorption conformations of the adsorbates on the Hg surface. A transition occurs between these two conformations at temperatures in the 285-295 K region. This transition is accompanied by large entropic and enthalpic changes. Besides a quantitative reproduction and explanation of the experimental data over a range of experimental conditions, the systematic study of the thermodynamics of the adsorption allows us to obtain fundamental information that is generally lacking in the literature and could not have been obtained by considering only the electroanalytical voltammetric data. The experimental data and discussion presented in this work clearly exemplify the need for systematic case studies to describe the physicochemical mechanisms of adsorption on the Hg surface appropriately. LA062147V