Dicyclohexano-18-crown-6 Research Articles

Study of Complex Formation Between Dicyclohexano-18-Crown-6 (DCH18C6) with Mg 2+, Ca2+, Sr 2+, and Ba2+ Cations in Methanol–Water Binary Mixtures Using Conductometric Method

The complexation reactions between Mg2+,Ca2+,Sr2+ and Ba2+ metal cations with macrocyclic ligand, dicyclohexano-18-crown-6 (DCH18C6) were studied in methanol (MeOH)–water (H2O) binary mixtures at different temperatures using conductometric method . In all cases, DCH18C6 forms 1:1 complexes with these metal cations. The values of stability constants of complexes which were obtained from conductometric data show that the stability of complexes is affected by the nature and composition of the mixed solvents. While the variation of stability constants of DCH18C6-Sr 2+ and DCH18C6-Ba2+versus the composition of MeOH–H2O mixed solvents is monotonic, an anomalous behavior was observed for variations of stability constants of DCH18C6-Mg2+ and DCH18C6-Ca2+ versus the composition of the mixed solvents. The values of thermodynamic parameters (ΔH c °, ΔS c °) for complexation reactions were obtained from temperature dependence of formation constants of complexes using the van’t Hoff plots. The results show that in most cases, the complexation reactions are enthalpy stabilized but entropy destabilized and the values of thermodynamic parameters are influenced by the nature and composition of the mixed solvents. The obtained results show that the order of selectivity of DCH18C6 ligand for metal cations in different concentrations of methanol in MeOH–H2O binary system is: Ba2+>Sr2+>Ca2+> Mg2+.

Unusual extraction behaviour of crown ether when intercalated in bentonite

Dicyclohexano-18-crown-6 (DCH18C6) has been intercalated in bentonite and the product, Ben-Crown, characterized by microelemental analysis, TG-DTA analysis, X-ray diffraction and IR spectroscopy. The permanent negative charge present in the bentonite layer has been exploited for more efficient extraction of cesium and strontium by Ben-Crown at nitric acid concentrations (<1.0 M) that gave only a meagre extraction when either DCH18C6 or bentonite alone were employed. The extraction of cesium and strontium has been investigated as a function of time, temperature, and the concentrations of nitric acid, metal ion and sodium nitrate. An unusual extraction phenomenon was observed at low concentrations of nitric acid (<1.0 M) in the absence of any organophilic agents. Rapid extraction of the metal ion was followed by the establishment of an equilibrium, which occurred after 150 min. Distribution coefficients (Kd) of 599 and 1007 ml g−1 were obtained, respectively, for the extraction of cesium and strontium from 0.1 M nitric acid by Ben-Crown; Kd decreased with an increase in the temperature or in the concentrations of nitric acid and sodium nitrate. The extraction data were fitted by the Langmuir adsorption model and the apparent experimental exchange capacity obtained by linear regression analysis was in good agreement with the amount of crown ether (0.22 mmol g−1) intercalated in bentonite. The enthalpy change (ΔH°) for the extraction of cesium and strontium was found to be exothermic.

Synergistic effects in the facilitated transfer of metal ions into room-temperature ionic liquids

Addition of tri-n-butyl phosphate (TBP) is shown to markedly increase the extraction of strontium from acidic nitrate media into certain 1-alkyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imides by dicyclohexano-18-crown-6 (DCH18C6), the apparent result of the formation of a synergistic adduct between the strontium-DCH18C6 complex and TBP. The magnitude of the synergistic enhancement is shown to depend on the alkyl chain length of the ionic liquid (IL) cation, with the effect diminishing as the cation hydrophobicity increases. The effect also diminishes at high (>50% v/v) TBP concentrations, the likely result of changes in solvent polarity unfavorable to the extraction of metal-crown ether complexes.

Para‐Substituted 1‐Phenyl‐3‐methyl‐4‐aroyl‐5‐pyrazolones as Chelating Agents for the Synergistic Extraction of Thorium(IV) and Uranium(VI) in the Presence of Various Crown Ethers

Para‐substituted 1‐phenyl‐3‐methyl‐4‐aroyl‐5‐pyrazolones, namely, 1‐phenyl‐3‐methyl‐4‐(4‐fluorobenzoyl)‐5‐pyrazolone (HPMFBP) and 1‐phenyl‐3‐methyl‐4‐(4‐toluoyl)‐5‐pyrazolone (HPMTP) were synthesized and utilized for the extraction of thorium(IV) and uranium(VI) from nitric acid solutions. For comparison, studies have also been carried out with 1‐phenyl‐3‐methyl‐4‐benzoyl‐5‐pyrazolone (HPMBP). The results demonstrate that thorium(IV) and uranium(VI) are extracted into chloroform according to: where HX refers to 1‐phenyl‐3‐methyl‐4‐aroyl‐5‐pyrazolone and K ex,0 denotes the equilibrium constant. The equilibrium constants of the extracted complexes have been calculated by nonlinear regression analysis taking into account the aqueous‐phase complexation of the metal ion with inorganic ligands and all plausible complexes extracted into the organic phase. The equilibrium constants of thorium(IV) and uranium(VI) with various 4‐aroyl‐5‐pyrazolones follow the order HPMFBP > HPMBP > HPMTP, which is in accordance with their pK a values. The synergistic solvent extraction of thorium(IV) and uranium(VI) was also investigated with mixtures of HPMFBP and 18‐crown‐6 (18C6), dicyclohexano‐18‐crown‐6 (DC18C6), dibenzo‐18‐crown‐6 (DB18C6), and benzo‐15‐crown‐5 (B15C5). The complexation strength of Th(PMFBP)4 chelate with crown ethers follows the order B15C5 > 18C6 > DC18C6 > DB18C6. The high extraction efficiency of thorium(IV) with B15C5 can be explained on the basis of “size‐fitting effect.” On the other hand, the complexation strength of UO2(PMFBP)2 chelate with various crown ethers follows the order 18C6 > DC18C6 > B15C5 > DB18C6, which can be explained on the basis of the crown ether basicity sequence and steric effects. Solid complexes of these metal ions with HPMFBP and crown ethers have been isolated and characterized by elemental analyses, IR and 1H NMR spectroscopic techniques to further clarify the nature of the extracted complexes.

Electrochemistry of ionophore-coordinated Cs and Sr ions in the tri-1-butylmethylammonium bis((trifluoromethyl)sulfonyl)imide ionic liquid

The selective extraction of Cs + and Sr 2+ from aqueous solutions by using the ionophores calix[4]arene-bis( tert-octylbenzo-crown-6) (BOBCalixC6) and dicyclohexano-18-crown-6 (DCH18C6), respectively, was demonstrated in the hydrophobic, room-temperature ionic liquid (RTIL), tri-1-butylmethylammonium bis((trifluoromethyl)sulfonyl)imide (Tf 2N −). The electrochemistry of Cs + coordinated by BOBCalixC6 and Sr 2+ coordinated by DCH18C6 was examined at a mercury film electrode (MFE) in this ionic liquid by using cyclic staircase voltammetry, sampled current voltammetry at a rotating electrode, and chronoamperometry. Both BOBCalixC6·2Cs + and DCH18C6·Sr 2+ exhibit well-defined reduction waves at approximately −2.4 and −2.9 V versus the ferrocene/ferrocenium (Fc/Fc +) couple, respectively, in which the coordinated ions are reduced to their respective amalgams, permitting the recycling of the ionophores. The diffusion coefficients of BOBCalixC6·2Cs + and DCH18C6·Sr 2+ are (2.7 ± 0.1) × 10 −9 and (2.1 ± 0.1) × 10 −9 cm 2 s −1, respectively, at 30 °C. The coulometric efficiency for the reduction and stripping of Cs at mercury pool electrodes was about 90% and was independent of the deposition time, whereas the efficiency for Sr was slightly less than 90% at short times and decreased with the deposition time, probably due to the formation of a passive layer of Sr(Tf 2N) 2.

The Influence of the Phase Morphology on the Optoelectronic Properties of Light‐Emitting Electrochemical Cells

AbstractWe report on the morphological aspects of thin films prepared from a blue–green light‐emitting conjugated polymer, (methyl‐substituted ladder‐type poly(p‐phenylene, mLPPP)), blended with a solid‐state electrolyte composed either by a crown ether, dicyclohexano‐18‐crown‐6 (DCH18C6), or a high‐molecular‐weight poly(ethylene oxide) (HMWPEO), and a Li salt, lithium trifluoromethanesulfonate (LiCF3SO3, Li triflate (LiTf)), as they have been successfully applied in light‐emitting electrochemical cells (LECs). The surface morphologies of the blend layers were investigated using atomic force microscopy (AFM) in tapping mode, and the ion distribution was probed using X‐ray analysis by means of energy‐dispersive X‐ray spectrometry (EDXS) in the scanning electron microscope (SEM). We show that the two different phase‐separation processes, the complexation tendencies of the ionic species as well as the ionic transport numbers, have tremendous influence on the performances of the corresponding LECs, revealing either rectifying or symmetric optoelectronic characteristics in forward and reverse bias directions. This opens up new possibilities for tuning the optoelectronic properties of ion‐supported organic electronic devices.

Planar polymer light-emitting device with fast kinetics at a low voltage

Polymer light-emitting electrochemical cells containing a ternary mixture of a soluble phenyl-substituted poly(para-phenylene vinylene) copolymer (“superyellow”), a dicyclohexano-18-crown-6 (DCH18C6) crown ether and a LiCF3SO3 salt as the active material have been assembled. Planar Au/{superyellow+DCH18C6+LiCF3SO3}/Au devices, with an interelectrode gap of 50 μm, were initially charged (i.e., electrochemically p- and n-doped in situ) at T=85 °C and then cooled to room temperature under applied voltage. When operated at T=23 °C charged devices show electroluminescence with fast response (&amp;lt; 1 s) at a low applied voltage (V⩾6 V). Charged devices could be stored under open-circuit conditions at room temperature for a prolonged time without detectable changes in device performance, and they can be completely (reversibly) discharged by raising the temperature to 85 °C. The active material mixtures were studied by atomic force microscopy and differential scanning calorimetry. The results demonstrate that superyellow phase separates from a crystalline DCH18C6–LiCF3SO3 complex on a ∼25 nm scale. The superyellow phase exhibits a glass transition at Tg∼180 °C, while the crystalline DCH18C6–LiCF3SO3 phase melts at Tm≈56 °C. Thus, we attribute the stabilization of charged Au/{superyellow+DCH18C6+LiCF3SO3}/Au devices in going from 85 to 23 °C as being directly related to the passage of Tm of the DCH18C6–LiCF3SO3 phase. The ionic distribution related to the p- and n-doped regions is “frozen-in” by this crystallization allowing for the observed fast kinetics at low voltages at room temperature.

Crown ethers as synergists in the extraction of trivalent lanthanoids with 3-phenyl-4-(4-fluorobenzoyl)-5-isoxazolone

SummaryThis paper highlights the results of investigations carried out on the extraction of lanthanoids such as Nd(III), Eu(III) and Tm(III) from nitrate solutions into chloroform with 3-phenyl-4-(4-fluorobenzoyl)-5-isoxazolone (HFBPI) in the presence and absence of various crown ethers (CE); 18-crown-6 (18C6), dicyclohexano-18-crown-6 (DC18C6), benzo-18-crown-6 (B18C6) and dibenzo-18-crown-6 (DB18C6). The results demonstrated that these trivalent metal ions were extracted into chloroform as Ln(FBPI)3with HFBPI alone and as Ln(FBPI)3·CE in the presence of a CE. The equilibrium constants of the above extracted complexes deduced by a non-linear regression analysis were found to increase monotonically with a decrease in ionic radii of these metal ions. The addition of a CE to the metal chelate system significantly improves the extraction efficiency of these metal ions. The complexation strength of trivalent lanthanoids with various CEs follows the order: DC18C6 &gt; 18C6 &gt; B18C6 &gt; DB18C6. Solid complexes of Eu(III) with HFBPI alone and with mixtures of HFBPI and various crown ethers have been isolated and characterised by IR and1H NMR spectral data to further clarify the nature of the extracted complexes.

PVC-based dicyclohexano-18-crown-6 sensor for La(III) ions

A new ion-selective electrode (ISE) based on dicyclohexano-18-crown-6 (DC18C6) as a neutral carrier is developed for lanthanum(III) ions. The electrode comprises of dicyclohexano-18-crown-6 (6%), PVC (33%), and ortho-nitrophenyl octyl ether ( o-NPOE) (61%). The electrode shows a linear dynamic response in the concentration range of 10 −6 to 10 −1 M with a Nernstian slope of 19 mV per decade and a detection limit as 5×10 −7 M. It has a response time of <30 s and can be used for at least 5 months without any significant divergence in potentials. The selectivity coefficients for mono-, di-, and trivalent cations indicate good selectivity for La(III) ions over a large number of interfering cations. The sensor has been used as an indicator electrode in the potentiometric titrations of La(III) with EDTA. The membrane is successfully applied in partially non-aqueous medium. It can be used in the pH range 4–9.

Organic mixed ionic–electronic conductors as active layers in light-emitting electrochemical cells: vibrational spectroscopic, microscopic and electronic characterization

We report on investigations of the morphological aspects of blend layers prepared from a blue-green light-emitting conjugated polymer (methyl-substituted ladder-type poly( p-phenylene), mLPPP), blended with either a crown ether, Dicyclohexano18crown6 (DCH18C6) or a high-molecular-weight poly(ethylene oxide) (HMWPEO), and a Li salt, lithium trifluoromethanesulfonate (LiCF 3SO 3, Litriflate, LiTf for short) as a solid state electrolyte as they were successfully applied as active layers in light-emitting electrochemical cells (LECs). The surface morphologies of the blends were investigated by atomic force microscopy (AFM) in Tapping mode© and the ion distribution with X-ray analysis by energy-dispersive X-ray spectrometry (EDXS) in the scanning electron microscope (SEM). The specific phase morphologies of the blends, the complexation of the ions and the ionic transport numbers were found to have tremendous influence on the performances of the corresponding LECs revealing either rectifying or symmetric optoelectronic characteristics in forward and reverse bias directions.

The influence of spatial disorder of the ion distribution on the surface morphology in thin films of blend based organic mixed ionic-electronic conductors

We report on investigations of the morphology of spin-coated thin films of an organic mixed ionic-electronic conductor consisting of the conjugated polymer methyl substituted ladder type poly(para- phenylene) (mLPPP) blended with a solid state electrolyte (Dicyclohexano18crown6 (DCH18C6), complexed with lithium trifluoromethanesulfonate (LiCF 3SO 3)). This blend system was successfully applied as active layer in light-emitting electrochemical cells (LECs). While thin films blends of the conjugated polymer and the pristine crown ether show a very smooth surface, the addition of LiCF 3SO 3 causes a pronounced surface roughening. Since such a distinct surface roughness can be the reason for a device failure mechanism that limits the device lifetime, this attitude is investigated by tapping mode atomic force microscopy (AFM). These studies are complemented with X-ray analysis by means of energy dispersive X-ray spectrometry (EDXS), in the scanning electron microscope (SEM), in order to get a better insight into the ion distribution within the blend layer and its influence on the surface roughness formed.

Enhanced extraction and separation of trivalent lanthanoids with 3-phenyl-4-(4-fluorobenzoyl)-5-isoxazolone and dicyclohexano-18-crown-6

AbstractThe synergistic extraction of trivalent lanthanoids Nd, Eu and Tm with mixtures of 3-phenyl-4-(4-fluorobenzoyl)-5-isoxazolone (HFBPI) and dicyclohexano-18-crown-6 (DC18C6) in 1,2-dichloroethane from nitrate solutions has been investigated. The results demonstrated that these trivalent lanthanoids were extracted into 1,2-dichloroethane as Ln(FBPI)3with HFBPI alone and as Ln(FBPI)3· DC18C6 in the presence of crown ether. The equilibrium constants of the above extracted complexes have been deduced by non-linear regression analysis. The equilibrium constants of the synergistic species were found to increase monotonically with decrease in ionic radii of these metal ions. The addition of a crown ether to the metal chelate system not only enhanced the extraction efficiency significantly but also improved the selectivites among these trivalent lanthanoids. Solid complexes of these metal ions with HFBPI and with mixtures of HFBPI and crown ether have also been isolated and characterised by IR and1H NMR spectroscopic techniques.

Synergistic Solvent Extraction of Trivalent Lanthanoids with Mixtures of 1‐Phenyl‐3‐methyl‐4‐pivaloyl‐5‐pyrazolone and Crown Ethers

This paper embodies the results of the investigations carried out on the extraction behavior of lanthanoids such as Nd(III), Eu(III), and Tm(III) from perchlorate solutions into chloroform with 1‐phenyl‐3‐methyl‐4‐pivaloyl‐5‐pyrazolone (HPMPP) in the presence and absence of various crown ethers (CE), 18‐crown‐6 (18C6), dicyclohexano‐18‐crown‐6 (DC18C6), and dibenzo‐18‐crown‐6 (DB18C6). The results demonstrate that these trivalent metal ions are extracted into chloroform as Ln(PMPP)3, with HPMPP alone, and in the presence of CE as Ln(PMPP)3·CE. The equilibrium constants of the extracted complexes have been determined by nonlinear regression analysis and are found to increase monotonically with decreasing ionic radii of these metal ions. The addition of a CE to the metal chelate system not only enhances the extraction efficiency but also improves the selectivities among these trivalent lanthanoids, especially in the presence of DB18C6.

Extraction of Lanthanides with Mixtures of Crown Ethers and Heptanoic Acid

The liquid extraction of some lanthanides (Pr, Gd, and Yb) with mixtures of heptanoic acid H3C(CH2)5COOH (HA) and crown ethers (B) benzo‐15‐crown‐5 (B15C5), dicyclohexano‐18‐crown‐6 (DH18C6), and 18‐crown‐6 (18C6) in carbon tetrachloride (CCl4) as a solvent from aqueous chloride medium at constant ionic strength μ = 0.1 is investigated. A synergistic effect in the extraction of Pr, Gd, and Yb with B15C5 and antisynergistic effects in the systems containing the other two crown ethers DH18C6 and 18C6 are observed. For Pr, a formation of mixed complex Pr(HA2)3 · B15C5 is found and the corresponding equilibrium constant is reported, while Gd and Yb form the complexes Gd(HA2)3 · nB15C5 and Yb(HA2)3 · nB15C5, respectively, where n < 1.

Composition of the Organic Phase Species in the Synergistic Extraction of Sr2+ by Mixtures of Di(2‐Ethylhexyl) Alkylenediphosphonic Acids and Dicyclohexano‐18‐crown‐6†

The extraction of Sr2+, Ba2+ and Ra2+ from aqueous nitric acid solutions by P,P′‐di(2‐ethylhexyl) ethylene‐ (H2DEH[EDP]) and butylene‐ (H2DEH[BuDP]) diphosphonic acids in o‐xylene is synergized by the cis‐syn‐cis and cis‐anti‐cis stereoisomers of dicyclohexano‐18‐crown‐6 (DCH18C6). However, no information is available on the composition of the synergistic complexes. In this work a detailed investigation of the composition of the organic phase species formed in these systems has been performed using Sr2+ as a probe, with a variety of techniques such as infrared spectroscopy, vapor pressure osmometry, and distribution methods. The latter methods have been employed under conditions ranging from trace metal concentration to organic phase saturation. The results from these experiments indicate that for the H2DEH[BuDP] system, the synergistic complex contains one Sr2+ ion, one crown ether molecule, and two diphosphonic acid trimers. For the H2DEH[EDP] system, the predominant synergistic complex has the 1:1:1 metal ion:crown ether:diphosphonic acid hexamer stoichiometry. Some of the results, however, indicate that a complex containing two H2DEH[EDP] hexamers can also be formed. †Work performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, U.S. Department of Energy, under contract W‐31‐109‐ENG‐38. The submitted manuscript has been created by the University of Chicago as Operator of Argonne National Laboratory (“Argonne”) under Contract No. W‐31‐109‐ENG‐38 with the U.S. Department of Energy. The U.S. Government retains for itself, and others acting on its behalf, a paid‐up, nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.

Effect of Diluents on the Extraction of Sr2+ from HNO3 Solutions with Dicyclohexano‐18‐crown‐6

Extraction of Sr2+ has been carried out with dicyclohexano‐18‐crown‐6 (DC18C6) in benzene from varying HNO3 concentrations, and the nature of species extracted has been established. The equilibrium constant values for the formation of Sr(NO3)2·DC18C6 have been evaluated, taking into account the aqueous complexing of Sr2+ with NO3 − and with aqueous‐soluble DC18C6, and also the uptake of HNO3 by DC18C6 in the organic phase. Effect of diluents apart from benzene, such as chlorobenzene, dichloroethane, chloroform, 1‐octanol, carbon‐tetrachloride, and nitrobenzene, and various mixtures of benzene and nitrobenzene on the extraction of Sr2+ with DC18C6 has also been studied. On the practical side, the extraction of Sr2+ has been examined with DC18C6 in solvent mixtures of tri‐n‐butyl phosphate (TBP), dodecane, and 1‐octanol. This solvent mixture, at least partially, avoids the disadvantages envisaged while using 1‐octanol alone for the separation and recovery of Sr2+ from minor actinide depleted TRUEX process raffinate.

Spectrophotometric Study of the Complexation of Some Lanthanide (III) Ions with a Series of 18-Crowns-6 in DMSO Solution Using Murexide as a Metallochromic Indicator

The complexation of La(III), Ce(III), Pr(III) and Er(III) with 18-crown-6(18C6), dibenzo-18-crown-6 (DB18C6), dicyclohexano-18-crown-6 (DCY18C6) anddibenzopyridino-18-crown-6 (DBPY18C6) has been studied in dimethylsulfoxide(DMSO) by means of a competitive spectrophotometric method using murexide asa metal ion indicator. The formation constants of the 1 : 1 complexeswere found tovary in the order La(III) > Ce(III) > Pr(III) > Er(III). It was foundthat the structure influences the formation and stability of the resultingcomplexes. The effects ofvarious parameters on complexation are discussed. The order of the stabilityconstants of each lanthanide ion with these macrocycles are18C6 > DC18C6 > DB18C6 > DBPY18C6.

Influence of solvent structural variations on the mechanism of facilitated ion transfer into room-temperature ionic liquids

The predominant mode of strontium ion transfer from acidic nitrate media into a series of 1-alkyl-3-methylimidazolium-based room-temperature ionic liquids containing dicyclohexano-18-crown-6 (DCH18C6) shifts from cation exchange to strontium nitrato-crown ether complex partitioning as the hydrophobicity of the ionic liquid cation is increased.

Preparation and X-Ray Study of Inclusion Complexes of (4Z,5E)-Pyrimidine-2,4,5,6(1 H,3H)-Tetraone 4,5-Dioxime and the Product of Its Cyclization, (1,2,5)-Oxadiazolo(3,4-d)-Pyrimidine-5,7(4 H,6H)-Dione with Two 18-Membered Macrocycles

The novel dioxime, (4Z,5E)-pyrimidine-2,4,5,6(1H,3H)-tetraone 4,5-dioxime (H2-PTD) was obtained by the interaction of 6-amino-5-nitrosopyrimidine-2,4(1H,3H)-dione with hydroxylamine hydrochloride. X-ray structural analysis determined the 4Z,5E-configuration of the corresponding monoanion, pyrimidine-2,6(1H,3H)-dione-4-iminole-5-iminolate in the inclusion complexes with diazonia-18-crown-6 (1,4,10,13-tetraoxa-7,16-diazoniacyclooctadecane) (H2-DA18C6)2+ (complex (1), stoichiometry 2 : 1), and its ammonium salt in the complex with the cis-syn-cis isomer of dicyclohexano-18-crown-6(DCHA) (cis-syn-cis-2,5,8,15,18,21-hexaoxatricyclo (20.4.0.09,14)hexacosane) (complex (2), stoichiometry 1 : 1). X-ray data were also obtained for the complex of the product of (H2-PTD) cyclization, (1,2,5)-oxadiazolo(3,4-d)pyrimidine-5,7(4H,6H)-dione (OPD) with diaqua diaza-18-crown-6 (complex (3), stoichiometry 2 : 2 : 1).

EXTRACTION OF ALKALINE EARTH AND ACTINIDE CATIONS BY MIXTURES OF DI(2-ETHYLHEXYL) ALKYLENEDIPHOSPHONIC ACIDS AND NEUTRAL SYNERGISTS

ABSTRACT The synergistic extraction of alkaline earth (Ca2+, Sr2+, Ba2+ and Ra2+) and actinide (Am3+, UO2 2+ and Th4+) cations from aqueous nitric acid solutions by mixtures of P,P′-di(2-ethylhexyl) methylene-(H2DEH[MDP]), ethylene-(H2DEH[EDP]), and butylene-(H2DEH[BuDP]) diphosphonic acids and neutral extractants in o-xylene has been investigated. The cis-syn-cis and cis-anti-cis stereoisomers of dicyclohexano-18-crown-6 (DCH18C6), the unsubstituted 21-crown-7 (21C7) and dicyclohexano-21-crown-7 (DCH21C7) were used as neutral synergists of the crown ether type. For Am(III) synergistic effects were also investigated using neutral organophosphorus esters, such as, tri-n-butylphosphate (TBP), diamyl amylphosphonate (DA[AP]) and tri-n-octylphosphine oxide (TOPO) as co-extractants. In all systems investigated, no synergistic extraction enhancement was observed for actinide ions. For the alkaline earth cations, synergistic effects were only observed when mixtures of H2DEH[EDP] or H2DEH-[BuDP] with DCH18C6 were used to extract Sr2+, Ba2+ and Ra2+. No synergistic effects were observed for the extraction of alkaline earth cations by H2DEH[MDP] or for the extraction of Ca2+ by any of the diphosphonic acids studied. The synergistic effects obtained with DCH18C6 were significantly higher for the cis-syn-cis than for the cis-anti-cis stereoisomer.