Journal ofThermal Analysis. Vol. 51 (1998) 965-972

2,4-~CYCLODEXTRIN COl\1PLEXES Preparation and characterWition by thennal analysis

J. 1. Perez-Martinez', M. J. Arias', J. M. Gines', J. R. Moyano', E. Mo rillo 2, P J. Sanchez-Soto3 and eS. Novae lDepartment of Pharmacy and Pharmaceutical Technology, Faculty of Pharmacy, University of Seville 410 12-Seville 2Institute of Natural Resources and Agrobiology, C. S. I. C. Apdo. 1052, 41080-Seville 3Institute of Material Science and Agrobiology, C. S. I. C. Apdo. 1115, 41 080-Seville, Spain 4Research Group for Technical Analytical Chemistry, of the Hungarian Academy of Sciences, Technical University ofBudapest, St. Gellert ter 4, Budapest, H-1521

Abstract

The aqueous solubility of the pesticide 2,4-D was improved by inclusion complexation with a-CD. The formation of such inclusion compounds was studied via the phase-solubility diagram (solution state) and by DSC and HSM (solid state). 2,4-D presented a typical Bs

Higuchi solubility curve, coprecipitating a 1:2 pesticide-a-CD complex. In order to obtain solid complexes, three processing methods were checked: kneading, coprecipitation and spray-drying. DSC and HSM showed that only the last two ofthese yielded true inclusion com­pounds. Chemical analysis also revealed that the stoichiometry of such solid complexes corre­sponds to a 2,4-D-a-CD ratio of 1 :2.

Keywords: a-cyclodextrin, 2,4-D, DSC, HSM, pesticide, solubility

Introduction

Cyclodextrins (CDs) or cycloamyloses are cyclic oligosaccharides, contain­ing 6 (a-CD), 7 (ß-CD) or 8 ('y-CD) a~ (l,4)-linked glucose units. The CD moleeule has a conical shape, the inside being moderately non-polar. CDs are water-soluble since the free hydroxyl groups ofthe glucose moleeules lie outside the ring. Because of their shape and physicochemical properties, CDs can form inclusion complexes with various hydrophobie compounds. For this reason, CDs have received considerable attention in various applied fields, such as biotech­nology, organic chemistry, drugs, foods and pesticides [1-3]. In particular, inclu­sion complexes involving a-CD are gaining very wide use in preparative and ap­plied chemistry, agriculture and the pharmaceutical industry. Recent publica­tions lend support to the prediction that, in the next few years, a rapid develop­ment can be expected in the application of CDs in pesticide formulations [4-9].

1418-2874/98/ $ 5.00

© 1998 Akademiai Kiad6. Budapest

Akademiai Kiad6. Budapest

Kluwer Academic Publishers. Dordrecht

966 PEREZ-MARTiNEZ ct al.: PESTICIDE-CYCLODEXTRIN COMPLEX

Certain chemicals known as auxins, present in plants, act as accelerators for the growth of cells. Although natural auxins are not commercially available, many synthetic substances, such as 2,4-dichlorophenoxyacetic acid (2,4-D), dis­play similar behaviour and are indispensable in modern agriculture [10, 11]. 2,4-D is a derivative of phenoxyacetic acid, as are monochlorophenoxyacetic acid and 2,4,5-trichlorophenoxyacetic acid. All are applied as pesticides. The compound 2,4-D is widely used too for weed control in cereals and other crops, being re­garded as a systemic herbicide [10, 11]. It is a potential pollutant of soil and groundwater, and is considered a model of an anionic organic pollutant [11]. However, some of the important problems in the application of this chemical are not yet resolved. It has a low aqueous solubility, which might be modified by in­teraction with other molecules.

Experimental

Materials

2,4-D was supplied by Sigma (St. Louis, Missouri, USA) and u-CD by Ro­quette (Lestrem, France). All other materials were of analytical reagent grade.

Preparation of solid complexes

Three methods were tested: kneading, coprecipitation and spray-drying, at a 2,4-D-u-CD stoichiometric ratio of 1 :2. The starting materials were separately spray-dried and kneaded. A physical mixture was also prepared as reference, by simple mechanical mixing during 15 min.

(a) Kneading method: 2,4-D and u-CD were mixed together in a mortar and kneaded. During this process, an appropriate amount of ethanol was added to the mixture in order to afford a suitable consistency, similar to a paste. This process was continued for 45 min and the product was dried at 37°C for 48 h (Selecta oven, model 204). The dried complex was gently ground into a fine powder.

(b) Coprecipitation method: asolid complex of2,4-D and u-CD was obtained by the coprecipitation method, using the conditions derived from the phase-solu­bility diagram in water (Fig. 1). An excess amount of 300 mg of 2,4-D was added to 25 mL of 0.1 M u-CD aqueous solution, and the mixture was stirred for 7 days at room temperature and then filtered. The resulting precipitate was washed with ethanol for purification and dried at 40°C.

(c) Spray-drying method: this was carried out in Büchi 190 M miniSpray­Dryer equipment. For this purpose, 2,4-D was dissolved in 400 mL of ethanol. The amount of u-CD required to obtain a 1:2 molar ratio was separately dis­sol ved in 400 mL of purified water. The solutions were next mixed for 20 min by sonication (Selecta P ultrasons), to produce a clear solution, which was spray-

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14

12 r 2' 1 0 ~~ '0 I

E ,§. 8

Q .;. 6 -N°

4

2 -

0 0 10 20 30 40 50 60 70 80 90 100

Cl -Cyclodextrin (mmol/L)

Fig. 1 Phase-solubility diagram of the 2,4-D-a.-CD system in water at 25°C

dried. The drying conditions were: flow rate, 1000 mL hol; inlet temperature, 116°C; outlet temperature, 67°C; air flow rate, 400 NL hol.

Phase-solubility determinations

These were performed according to the method reported by Higuchi and Con­nors [12]. 50 mg quantities of 2,4-D, an amount that exceeded its theoretical aqueous solubility at 25°C (620 mg L-'), were accurately weighed into 50 mL Erlenmeyer flasks. 10 mL of water containing various concentrations of (X-CD (10-100 mM) was then added. The flasks were sealed and shaken at 25°C for one week, a time considered long enough for equilibrium to be reached [13]. The sampies were next filtered through a 0.22 Ilm Millipore cellulose nitrate mem­brane filter, and appropriately diluted. Sampies were analyzed spectrophotomet­rically at 284 nm (Hitachi V-2000 spectrophotometer).

Stoichiometry in the solid state

This was ca1culated by chemical analysis of the pure solid complexes. An ex­actly weighed amount of powder was washed with 2 mL of cold ethanol. The re­maining product was dissolved in water and analyzed spectrophotometrically at 284 nm by means of a Hitachi V-2000 spectrophotometer.

Differential scanning calorimetry

A Mettler apparatus composed of an FP85 fumace, an FP80 HT temperature­control unit and FP89 HT software was employed. SampIes of 10 mg were put

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into aluminium pans. The lids of these pans were pierced in order to allow the evolved gases to leave during the heating process. The thermal anal('sis was per­formed under static air atmosphere, at a heating rate of IODC min- , in the tem­perature range from 30 to 200De.

Hot-stage microscopy (HSM)

Different observations were made during heating, using a hot-stage device at­tached to an optical microscope (MettIer model FP82HT). Approximately 0.1 mg of sampie was placed on a glass slide with a coverglass and heated at 5DC min- I in the temperature range from 30 to 200DC.

Results and discussion

Inclusion complexation in solution: determination of the complex stoichiometry

The phase-solubility diagram obtained for 2,4-D and <x-CD at 25 DC is shown in Fig. 1. According to Higuchi and Connors [12], the diagram can be c1assified as of Bs type. An insoluble microcrystalline complex was formed from the solu­tion at higher <x-CD concentrations.

The diagram shows a plateau region before the descending part of the curve. From the length of the plateau, it is possible to estimate the stoichiometry of the complex precipitated from solution. The 2,4-D content ofthe complex formed in the plateau region, [Sk, is equal to the totaI2,4-D added to the system [Sh, mi­nus the 2,4-D in solution at the start of the plateau region, [S]s: [Sk=[Sh-[S]s. Hence, [Sk=22.62 mM-12.11 mM=IO.51 mM.

The <x-CD content ofthe complex in the same region, [Lk, is equal to that en­tering the complex during this interval, i.e. [Lk=40 mM-20 mM=20 mM

The stoichiometric ratio of the 2,4-D-<x-CD complex, therefore, is: [S]cI[L]c= 10.51 mM/20 mM=0.5, i.e. a 1:2 stoichiometry. Consequently, the precipitated complex has the formula S I L2: one molecule of 2,4-D is entrapped by two molecules of <x-CD because of the small cavity size of the latter.

The solubility of the <x-CD complex, estimated from the initial straight line portion (8 mM), is significantly different from that at high <x-CD concentrations (4 mM). In accordance with other authors [14], this result may be interpreted in terms of the initial formation of a 1: 1 complex. If the complex responsible for the initial rise in the solubility diagram were the same complex as that finally pre­cipitated, the increase in 2,4-0 concentration would be equal to the final 2,4-0 concentration. This condition is not observed for the 2,4-D-<x-CO system, be­cause these concentrations are 8 mM and 4 mM respectively (Fig. 1). These re­sults suggest the formation of two or more distinct complexes in the system, one

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or two of these being responsible for the initial rise in solubility, while another is precipitated later.

A more adequate description of the initial rise in the solubility diagram (Bs type) may be achieved by assuming the formation of the two complexes SL and SL2 characterized by stability constants Kl:l and K1 :2, according to the following equilibrium:

Kl:l Kl:2 S + L;::::::::!" SL + L I ' SL2

where [S], [L], [SL] and [SL2] are the solubilities of 2,4-D, a-CD, the 1: 1 com­plex and the 1:2 complex, respectively, Kl:l=[SL]/[S][L] and Kl:2=[SL2]/[SL][L].

The material balance equations are

ST = eS] + [SL] + [SL2], with ST = [So], and LT = [L] + [SL] + [SL2],

where ST and LT are the total concentrations of substrate and ligand, respectively. [So] is the equilibrium solubility of 2,4-D in the absence of CD.

lf the above equations are combined, and it is assumed that the extent of com­plexation is fairly smalI, it may be permissible to set [L]:::::[LT ], and obtain the fol­lowing equation [15]:

(ST - So)/LT = KI: 1 So + K1:1 KI:2 So LT

A plot of (ST-So)/LT against LT would then be linear. Thus, the values calcu­'lated for K1:I and Kl:2 from the slope and intercept will be:

K1:1 = 94.5- 10-3 m~1 and Kl:2 = 6.48.10-3 mM-

2

Inclusion complexation in the solid state

Although the results obtained from the solubility study strongly indicate the formation of a true inclusion compound of 2,4-D with a-CD, they do not pre­clude the possibility that the final solid products are simply physical mixtures. For this reason, the powders obtained after each processing method were studied by different techniques, in order to check on the formation of a true solid inclu­sion compound and to determine their stoichiometry in the solid state.

Supporting evidence for complex formation was obtained from thermal analysis studies. DSC records of the pure components, the physical mixture and the kneaded, coprecipitated and spray-dried systems are presented in Fig. 2. The DSC trace of 2,4-D shows one endothermic peak at 145°C, corresponding to its melting point. The DSC trace of a-CD displays two broad endothermic effects, at 85 and 160°C, which are attributed to the loss of the water content of a-CD.

The curves of the physical mixture and the kneaded system show an endother­mic peak at about 140°C, corresponding to the melting of 2,4-D. The presence of

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970 PEREZ-MARTiNEZ cl al.: PESTICIDE--CYCLODEXTRIN COMPLEX

Temperatu re loe

Fig. 2 DSC curves of a) commercial a-CD, b) a 1:2 a-CD-2,4-D physical mixture, c) a kneaded sampIe; d) a coprecipitated complex, e) a spray-dried complex and f) pure 2,4-D

this endotherm, somewhat changed as compared to that for pure 2,4-D, may in­dicate a weak interaction [16]. In contrast, this peak disappears in the cases of the coprecipitated and spray-dried systems. These results clearly indicate that the coprecipitated and spray-dried sampIes are true inclusion complexes and not simple physical mixtures. This is supported by the XRD data obtained for these systems [17].

The two endothermic peaks attributed to the dehydration of a-CD were highly distorted for the coprecipitated and spray-dried sampIes. This provides further evidence for the formation of a true inclusion complex between 2,4-D and a-CD.

Hot-stage microscopy (HSM)

The isolated starting materials were compared with the products obtained by kneading, coprecipitation and spray-drying, by examination under the optical microscope. The feasibility of the formation of an inclusion complex was cor­roborated in the different sampIes by the HSM study. The melting of 2,4-D was observed by HSM in the cases of the physical mixture and the kneaded product.

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However, this event was not detected in the washed coprecipitated sampie, nor in the spray-dried sampIe. Thus, the reality ofthe formation ofinclusion complexes can be deduced only in the cases of the coprecipitation and spray-drying meth­ods.

Stoichiometric study

Once the inclusion compounds had been characterized, chemical analysis was performed to establish their stoichiometry in the solid state. The UV-spec­trophotometric analysis of the solid complexes (coprecipitated and spray-dried) revealed a stoichiometric ratio of 1 mole of pesticide to 2 moles of a-CD. This stoichiometric ratio is in good agreement with that previously calculated from the solubility-phase diagram for the solution state. However, the material ob­tained by the kneading method (see the Experimental) did not seem to be a true inclusion compound. In fact, after washing with ethanol, 2,4-D was practically completely removed, as confirmed by chemical analysis.

ConcIusions

The interaction of the herbicide 2,4-D with a-CD produces inclusion com­pounds in solution and in the solid state. Studies in solution revealed a Bs-type solubility diagram, where a 1:2 2,4-D-a-CD microcrystalline complex precipi­tates. Prior to the formation of this precipitate, a mixture of 1: 1 and 1:2 com­plexes exists in solution. The values calculated for K1:I and K1:2 were: K1:1= 94.5.10-3 mM-1 and K1:2=6.48·1O-3 m~2.

In the solid state, DSC and HSM techniques were applied to investigate the possibility of inclusion complex formation for of sampies obtained by three processing methods (kneading, coprecipitation and spray-drying). These meth­ods revealed that 2,4-D forms an inclusion compound with a-CD only in the cases of coprecipitated and spray-dried sampies. These complexes present a 2,4-D­a-CD stoichiometry of 1 :2.

Utilization ofthe herbicide complexation ability is of great interest in agricul­ture, because it can diminish the risk of 2,4-D as a potential pollutant of ground­water and soil. This maUer is currently the subject of our investigations.

* * * The financial support of the OTKA No 014 550 grant and the Hungarian Scholarship Board is

gratefully acknowledged.

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