Studies on Cesium Uptake by Phenolic Resins

Abstract

The selective removal of cesium by phenolic ion-exchange resins from highly salted alkaline radioactive solutions was studied. The resins were synthesized by alkaline polycondensation of phenol, resorcinol, catechol, and a resorcinol-catechol mixture with formaldehyde and characterized for their moisture regain, ion-exchange (H⁺→Na⁺) capacity, and distribution coefficient (K_D ) for cesium. The effects of open and sealed curing of the polymers on their properties were studied. The effect of Na⁺, NaOH, and Cs⁺ concentration on the uptake of cesium by resorcinol-formaldehyde resin was investigated, in particular. The chemical, thermal, and radiation stabilities of the polymers were also studied.     

Synthesis of poly(hemispherand) via cyclopolymerization of diepoxide

Summary Cyclopolymerization of 2,6-bis[3-(4,5-epoxy-2-oxapentyl)-2-methoxy-5-methylphenyl]-4-methylanisole was carried out with cationic, anionic, and coordination catalysts. The polymers obtained with BF₃·OEt₂ or SnCl₄ in dichloromethane and with t-BuOK in DMSO were soluble in benzene, chloroform, and THF. The mole fractions of the cyclic units in these polymers were from 0.65 to 0.75. The resulting poly(hemispherand) bound alkali metal cations and the selectivity was in the order of Rb⁺>K⁺>Cs⁺>Na⁺>Li⁺.     

The adiabatic compressibility of poly(acrylic acid) and polyacrylamide in aqueous solution

The results of adiabatic compressibility measurements of poly(acrylic acid) and polyacrylamide along with their corresponding monomers and two poly(sodium acrylates) obtained by neutralizing the polyacid 25% and 100% with sodium hydroxide have been described. The total adiabatic compressibility of poly(acrylic acid) solution is higher than that of the corresponding salt solutions or of polyacrylamide solutions. The unneutralized acid does not dissociate much, even in dilute solution, and the magnitude of electrostriction in polyamide is greater than in acid. The ΦV₂ and ΦK₂ values for monomers and polymers are seen to be almost concentration independent, and so are the sodium salts of the polyacid. Poly(acrylic acid) and poly(acrylamide) are structurally closely related polymers, and water must be bound to them through polar groups either by hydrogen bonding or by dipole attraction. The hydrophobic part of the solute, because of compact orientation of water and solute in the boundary region, causes a decrease in solvent volume and therefore in the values of ΦV₂ and ΦK₂. On the other hand, intermolecular hydrogen bonding between the polar groups increases the volume and counterbalances the hydrophobic effect. Because of these two counteracting effects, the observed ΦV₂ and ΦK₂ values are seen to be concentration independent. Contrary to the observation with poly(methacrylic acid)¹ and its sodium salts, the solvated counter-ions in case of poly(sodium acrylates) make no special contribution in the dilute region. In 100% neutralized polyacid, the dissociation of counterions is complete, and the magnitude of electrostriction is highest in this case. Accordingly, lowest ΦV₂ and ΦK₂ values (37.0 cc/mole and ?50.50 × 10^?3 cc bar^?1 mole^?1) are observed. However, the dissociation and therefore the magnitude of electrostriction are somewhat reduced in the presence of 1.0M NaCl solution; and, accordingly, the values increase to 42.80 cc/mole and ?33.0 × 10^?4 cc bar^?1, mole^?1, respectively. The limiting values for the apparent molal volume and the apparent molal compressibility for the polymers show a considerable decrease over those of the monomers. The values of ΦV₂⁰ and ΦK₂⁰ per methyl group are less in the polymers than in the monomers, and this has been attributed to water clusters that become stronger and better formed as the molecules grow larger and larger. The molar volumes of acrylic acid and methacrylic acid are decreased, while those of acrylamide and methacrylamide are increased when dissolved in water to form an infinitely dilute solution.     

Synthesis,characterization, and gamma radiation effects over well‐defined poly(vinylsiloxanes) copolymers

Anionic ring‐opening polymerization (AROP) was employed for the controlled synthesis of linear model block copolymers of 1,3,5,7‐tetrametil‐1,3,5,7‐tetravinyl(cyclotetrasiloxane) (V₄) and 1,3,5‐dimethyl(cyclotrisiloxane) (D₃) monomers by using sec‐butyl lithium (sec‐Bu^?Li⁺) as initiator, and high‐vacuum anionic polymerization techniques. V₄ copolymerization was promoted by employing D₃ and sec‐Bu^?Li⁺ producing living silanolates that open the stable V₄ ring. For this purpose, two strategies were applied: (a) sequential addition of monomers, and (b) one‐step copolymerization at different reaction temperatures. According to the experimental results, higher levels of V₄ incorporation (～ 18.14 mol %) were obtained by mixing both co‐monomers and performing the reaction at high temperature (80°C). This strategy allowed the control of the V₄ incorporation into the copolymer structure, giving the opportunity of synthesizing model vinyl‐siloxane polymers. The gamma radiation of these materials showed that lower doses are needed to achieve the same gel content as in a model poly(dimethylsiloxane) (PDMS). In such a sense, these results constitute one of the first reports regarding the effect of gamma radiation on vinyl‐containing silicon polymers, and may be of fundamental importance if a biomedical cross‐linked rubber‐type PDMS is needed at earlier doses of sterilization. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Adiabatic compressibility of polyelectrolytes: The influence of solvents on ionic and nonionic polymers

The adiabatic compressibility of two ionic polymers; namely, poly(acrylic acid) (PAA) and poly(N-dimethylaminoethyl methacrylate) (PDAM) in methanol and dioxane solutions and of a nonionic polymer, poly(vinyl pyrrolidone) (PVP) in aqueous, methanol, and dioxane solutions has been studied. The φV₂ and φK₂ values for the three polymers and their corresponding monomers in methanol and dioxane solutions are found to be concentration independent. There is a marked difference in φK⁰₂ and φV⁰₂ values between monomer and polymer in all three solvents. In aqueous solution, the difference in φV⁰₂ is, on an average, 16.1 cm³/mol, while in methanol and dioxane solution, the same is 24.0 and 20.5 cm³/mol (average), respectively. All three monomers in dilute aqueous solution show a contraction of volume and decrease of adiabatic compressibility which are comparatively small in methanol and dioxane solutions. The φV⁰₂ for PAA, PDAM, and PVP were found to have increased by 0.8, 11.0, and 1.5 cm³/mol, respectively, in dioxane solution over that of the value of the aqueous solution. It is interesting to note that in methanol solution, PAA, PDAM, and PVP show a decrease of φK⁰₂ and φV⁰₂ values by 68.8 cm³/bar/mol and 8.2 cm³/bar/mol, 32.7 cm³/bar/mol, and 4.3 cm³/mol, and 36.6 cm³/bar/mol and 5.8 cm³/mol, respectively, compared to the values obtained from aqueous solution. This has been ascribed to geometric effect since the void space around the molecules is smaller in methanol than in water.     

The association constant of macrocyclic ether–cation interactions in 1,4-dioxane/water mixtures. III. Effect of temperature on the binding

The association constant, K_a of Na⁺ with [12]crown-4, [15]crown-5 and [18]crown-6 crown ethers were determined in a binary mixture, 1,4-dioxane/water (50/50) using a Na⁺ ion selective electrode at different temperatures. K_a values were determined with the relationship, 1/K_a [L_o]^n+m–1 = (1–nP′)ⁿ(1–mP′)^m/P′, for various stoichiometries, (n:m), where P′ is the mole fraction of the complexed cation. The exothermic association constants and the thermodynamic data for cation–macrocycle complexes explained in terms of Eigen–Winkler binding mechanism are given. The binding power found for Na⁺, however, was the highest with [18] crown-6.     

Synthesis,characterization, and thermotropic properties of side-chain liquid crystalline polysiloxane polymers with an oligo(ethylene oxide) unit in the side chain

A series of new alkene and octenyloxy monomers containing 4′-[oligo(ethylene oxide)] n monomethyl ether 4-biphenyl ether carboxyl benzoate [MS3BDBE_n] (n = to 3) and 1- (p-methoxydiphenyl)–(carboxyl benzoate) [oligo(ethylene oxide)]n [MS_m+2BEnDB] (m = 1, 6; n = 1 to 3) as end groups were synthesized. The molecular structure of the monomers was charaterized using nuclear magnetic resonance (NMR) spectroscopy. These monomers were grafted onto poly(methylhydrosiloxane)s (PMHS) by the platinun-catalyzed hydrosilylation process. The thermal transition temeratures and mesophase textures of the monomers and the polysiloxane polymers have been determined by diffential scanning calorimetry (DSC) and by polarized optical microscopy. The effect of changes in chemical structure on the mesophase properties, glass transition temperature, isotropic temperature, and mesophase texture of the monomers and the polysiloxane polymers is discussed. Polymers PS3BDBEn showed smectic and nematic phases which were not analogous to their precursor nematic monomers MS3BDBEn. Both monomers MS_m+2BE_nDB and their polymeric homologous PS_m+2BE_nDB did not exhibit mesophase properties. This demonstrated that the polymer effect could not stabilize the mesophase obtained from mesogenic core which contained a flexible oligo(ethylene oxide) unit interconnecting aromatic group. © 1995 John Wiley & Sons, Inc.     

Interactions of Some Hofmeister Cations with Sodium Dodecyl Sulfate in Aqueous Solution

In this work, we have investigated the influence of some alkali metal ions on the Krafft temperature (T_K) and critical micelle concentration (CMC) of a classical ionic surfactant, sodium dodecyl sulfate (SDS), over a wide range of temperature. The alkali metal cations such as Li⁺, Na⁺, Cs⁺, and K⁺ are found to affect the solubility and hence the T_K of the surfactant. It was observed that kosmotropic Li⁺ lowers the T_K of the surfactant. Due to the common ion effect, the solubility of SDS decreases in the presence of Na⁺, resulting in an increase in the T_K. On the other hand, chaotropic K⁺ and Cs⁺, capable of forming contact ion pairs with the chaotropic dodecyl sulfate ion, lower the solubility and hence elevate the T_K. In terms of decreasing the T_K, the ions follow the trend: Li⁺ > Na⁺ > Cs⁺ > K⁺ except for 0.0025 M CsCl. The added cations screen the charge of the micelle surface and facilitate closer packing of the surfactant with a consequent decrease in the CMC. In terms of the effectiveness in lowering the CMC, the ions follow the order: Cs⁺ > K⁺ > Na⁺ > Li⁺. In the presence of added electrolytes, the γ_CMC values are found to be lower than the corresponding values in pure water. The thermodynamic parameters (Gibbs free energy, enthalpy, and entropy changes) of micellization were calculated to gain insights into the mechanism of the process.     

Synthesis and phase behavior of cholesteric liquid crystal polymers containing glutamic acid in main‐chain

Four polymers (P₀–P₃) containing peptide chain as polymer backbone were synthesized by condensation reaction with bis(trichloromethyl)carbonate and triethylamine. The chemical structures of the monomers M₀–M₃ were confirmed by FTIR and ¹H‐NMR. The structure–property relationships of the monomers and polymers are discussed. Their phase behavior and optical properties were investigated by differential scanning calorimetry, thermogravimetric analysis, and polarizing optical microscopy. Monomers M₁–M₃ and polymers P₁–P₃ displayed cholesteric phases. The results demonstrated that the melt temperature and clear point of monomers (M₁–M₃) and polymers (P₁–P₃) decreased with the increase of the flexible spacer length in the side‐chain, and the mesophase temperature range of the polymers increased with the increase of the flexible spacer length. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Electrochemical studies of titanium in fluoride-chloride molten salts

The interaction between titanium and Ti⁴⁺ ions (K₂TiF₆), the electroreduction reaction of Ti⁴⁺ ions and the anodic reaction of Ti in KCl–NaCl–KF melts with K₂TiF₆ at 973 K were studied by means of electrochemical and physical measurements. It was found that the fluoride ions played a very important role in these reactionsIn KCl–NaCl-3 wt % K₂TiF₆ molten salts with less than 3 wt % KF, the interaction reaction was considered to proceed as Ti⁴⁺+Ti=2Ti²⁺. If the bath contained more than 10 wt% KF, the reaction 3Ti⁴⁺+Ti=4Ti³⁺ occurred.The electrochemical reduction of Ti⁴⁺ (K₂TiF₆) ions in the molten salts with less fluoride ions was observed to proceed according to three reaction steps, i.e. Ti⁴⁺+e=Ti³⁺, Ti³⁺+e=Ti²⁺, Ti²⁺+2e=Ti. In the case of the fluoride ion concentration being higher, two reduction steps, i.e. Ti⁴⁺+e=Ti³⁺, Ti³⁺+3e=Ti were suggested.     

Selectivity Sequence of Multivalent Lanthanides for their Separation on Antimonate Based Exchangers

The adsorption of trivalent lanthanide ions, Ln³⁺ (La³⁺, Pr³⁺, Nd³⁺, Gd³⁺, and Er³⁺), using two antimonate based exchangers, namely, silicon antimonate (SiSb) and poly(acrylamide titanium antimonate), PAmTA, was studied from their nitrate solutions at pH 3.5 ± 0.1. The formation constants in Ln³⁺/SiSb and Ln³⁺/PAmTA systems, logK _S1 and logK _S2 , were calculated for the above mentioned lanthanides. The K _d values of the selected lanthanides were calculated at 30, 40, 50, and 60°C reaction temperatures. A fractionation is observed that due to the selective sorption between the heavy rare earths (HREEs) and the light rare earths (LREEs) at various reaction temperatures, the HREEs are being more sorbed than the LREEs. The selectivity order increased with increasing atomic number of these ions. Log K _d -Z plots revealed the presence of the M-type tetrad phenomenon in the system. Furthermore, the calculated separation factors showed that, binary separations for Er⁺³/La⁺³, Er⁺³/Pr⁺³, Er⁺³/Nd⁺³, and Gd⁺³/La⁺³ on SiSb, while, Er⁺³/Gd⁺³, Gd⁺³/Pr⁺³, Gd⁺³/Nd⁺³, Nd⁺³/La⁺³, and Nd⁺³/Pr⁺³, side by side to the mentioned separations could be accomplished on PAmTA at 30°C. The separation efficiency was found to increase with increasing in reaction temperatures. On PAmTA (at 40°C and 50°C) and on SiSb (at 50°C), Er⁺³/(La⁺³, Pr⁺³, Nd⁺³, Gd⁺³), Gd⁺³/(La⁺³, Pr⁺³, Nd⁺³), Nd⁺³/(La⁺³, Pr⁺³) fractionations were also noticed, while on the surface of SiSb at 40°C, Gd⁺³/La⁺³ and Gd⁺³/Pr⁺³ could be added to the list of binary separations at 30°C. Nevertheless, La⁺³/Pr⁺³ break through could not be obtained at any reaction temperature. Kinetic investigations were carried out; drawing an analogy, the pseudo second-order model was more agreeable to fit the data.     

Use of chitosan derivatives as solid phase extractors for metal ions

This paper shows a comparative study between the two radiation grafted chitosan derivatives viz. cross-linked chitosan (CRC) and cross-linked chitosan after hydrolysis (CRCH). These chitosan derivatives were used as solid phase extractors for several radionuclides. The uptake of ¹³⁷Cs, ^85,89Sr, ¹⁵²Eu, ²⁴¹Am, ²³⁴Th and ²³³U by CRCH and CRC was studied using batch and column methods. The K_d, exchange capacity, breakthrough capacity for different metal ions with the functionalized polymers were determined. The uptake followed the following trend: UO₂²⁺>Th⁴⁺>Cs⁺>Eu³⁺>Am³⁺>Sr²⁺ for both sorbents. It was seen that CRCH has a greater uptake of metal ions compated to CRC but CRC was more selective of the two.     

Synthesis and characterization of 3,4‐dihydroxyphenylacetic acid imprinted polymers

Eight molecularly imprinted polymers (MIP1–MIP8) were synthesized with different functional monomers and porogens using 3,4‐dihydroxyphenylacetic acid (DOPAC) as a template. Thermal, radical bulk polymerization was employed in the presence of ethylene glycol dimethacrylate as a cross‐linker. A computational analysis indicated that complexes with four molecules of 4‐vinylpyridine, 1‐vinylimidazole and acrylonitrile had high positive enthalpies of formation. The polymers synthesized with these monomers showed an imprinting factor below 1. Polymer MIP8 synthesized with allylamine as the functional monomer, with the highest energy of interaction with DOPAC, was characterized by the highest imprinting factor equal to 1.91. Examination of the binding ability of DOPAC and a group of structurally related compounds showed that the strong interactions between amine groups in the polymer and carboxylic groups in the analyte governed the recognition mechanism. The Langmuir adsorption model and the pseudo‐second‐order mechanism properly evaluated the MIP8 and non‐imprinted polymer 8 adsorption characteristics. Scatchard analysis revealed that MIP8 had two classes of heterogeneous binding sites with K_d(1) = 0.12 µmol L^?1 and K_d(2) = 1.46 µmol L^?1. Finally, the potential application of MIP8 for separation of DOPAC was demonstrated. Copyright © 2011 Society of Chemical Industry     

An unprecedented bonding mode for potassium within a PCP-pincer palladium hydride–K-Selectride complex

The reaction of (PCP)^i-PrPdCl and the commonly-used reductant K-Selectride^® solution [K(sec-Bu₃BH) in THF] does not yield a simple (PCP)^i-PrPdH species, but rather an adduct of the Pd–H that contains bound K(sec-Bu₃BH). This adduct has been characterized by X-ray crystallography and shown to be a centrosymmetric dimer in the solid state. The most unique feature of the structure is that the tri-coordinate K⁺ ion bonds only to the terminal palladium hydride and the two bridging boron hydrides. Despite being prepared in THF, no other donor ligands are bound to K⁺. This [H₃K]⁺ bonding mode for K⁺ ions has not been previously reported.     

Synthesis,characterization, metal sorption,and biological activity of poly(N‐heterocylic acrylamide)

N‐heterocyclic acrylamide monomers were prepared and then transferred to the corresponding polymers to be used as an efficient chelating agent. Polymers reacted with metal nitrate salts (Cu²⁺, Pb²⁺_, Mg²⁺, Cd²⁺, Ni²⁺, Co²⁺_, Fe²⁺) at 150°C to give metal‐polymer complexes. The selectivity of the metal ions using prepared polymers from an aqueous mixture containing different metal ion sreflected that the polymer having thiazolyl moiety more selective than that containing imidazolyl or pyridinyl moieties. Ion selectivity of poly[N‐(benzo[d]thiazol‐2‐yl)acrylamide] showed higher selectivity to many ions e.g. Fe³⁺, Pb²⁺, Cd²⁺, Ni²⁺, and Cu²⁺. While, that of poly[N‐(pyridin‐4‐yl)acrylamide] is found to be high selective to Fe³⁺ and Cu²⁺ only. Energy dispersive spectroscopy measurements, morphology of the polymers and their metallopolymer complexes, thermal analysis and antimicrobial activity were studied. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42712.     

Structures,Stabilities and Electronic Properties of Endo- and Exohedral Dodecahedral Silsesquioxane (T<Subscript>12</Subscript>-POSS) Nanosized Complexes with Atomic and Ionic Species

The structures of endohedral complexes of the polyhedral oligomeric silsesquioxane (POSS) cage molecule (HSiO_3/2)₁₂, with both D _2d and D _6h starting cage symmetries, containing the atomic or ionic species: Li⁰, Li⁺, Li⁻, Na⁰, Na⁺, Na⁻, K⁰, K⁺, K⁻, F⁻, Cl⁻, Br⁻, He, Ne, Ar were optimized by density functional theory using B3LYP and the 6-311G(d,p) and 6-311 ++G(2d,2p) basis sets. The exohedral Li⁺, Na⁺, K⁺, K⁻, F⁻, Cl⁻, Br⁻, He, Ne, Ar complexes, were also optimized. The properties of these complexes depend on the nature of the species encapsulated in, or bound to, the (HSiO_3/2)₁₂ cage. Noble gas (He, Ne and Ar) encapsulation in (HSiO_3/2)₁₂ has almost no effect on the cage geometry. Alkali metal cation encapsulation, in contrast, exhibits attractive interactions with cage oxygen atoms, leading to cage shrinkage. Halide ion encapsulation expands the cage. The endohedral X@(HSiO_3/2)₁₂ (X = Li⁺, Na⁺, K⁺, F⁻, Cl⁻, Br⁻, He and Ne) complexes form exothermically from the isolated species. The very low ionization potentials of endohedral Li⁰, Na⁰, K⁰ complexes suggest that they behave like “superalkalis”. Several endohedral complexes with small guests appear to be viable synthetic targets. The D _2d symmetry of the empty cage was the minimum energy structure in accord with experiment. An exohedral fluoride penetrates the D _6h cage to form the endohedral complex without a barrier.     

Effects of Zeolite Structures, Exchanged Cations, and Bimetallic Formulations on the Selective Hydrogenation of Acetylene Over Zeolite-Supported Catalysts

Supported PdAg bimetallic catalysts were evaluated for the selective hydrogenation of acetylene in the presence of ethylene. The effects of different zeolite structures and cations were investigated using flow reactor studies, with K⁺-β-zeolite supported PdAg showing the lowest activity but highest selectivity comparing to the γ-Al₂O₃ support and other alkaline metal exchanged β-zeolite supports. The K⁺ promoter effect on γ-Al₂O₃ was also tested, which showed that adding K⁺ to γ-Al₂O₃ increased activity and selectivity. Bimetallic catalysts consisting of Pd and a Group IB metal were also compared. It was found that the PdAg bimetallic catalyst had similar activity but better selectivity comparing to PdCu, while the PdAu catalyst showed the highest activity but lowest selectivity.     

New soluble azophenol polymers prepared by oxidative polycondensation

New azophenol polymers (P₁, P₂ and P₃) were synthesised by the oxidative polycondensation (OP) reaction of three different azophenol monomers in aqueous alkaline medium with NaOCl as the oxidant. The monomers and the polymers were characterised by elemental analyses, and UV‐visible, Fourier transform infrared, ¹H NMR and ¹³C NMR spectroscopic studies, which revealed that the polymers synthesised by OP are composed of oxyphenylene (C? O? C) and phenylene (C? C) units. The polymers obtained are soluble in dimethylformamide and dimethylsulfoxide. Average molecular weights of the polymers were determined by gel permeation chromatography. Additionally, P₂ and P₃ are soluble in water and methanol. On the basis of thermogravimetric analyses, 5 and 50% weight‐loss temperatures of the polymers were found to be 218, 700 (P₁), 263, 609 (P₂) and 100, 809 °C (P₃), respectively, suggesting a high thermal stability. Thermal analyses using differential scanning calorimetry revealed that the azophenol polymers are highly amorphous, and melting peaks were not observed in the heating cycles. This suggests that all the polymers are highly amorphous. The azophenol polymers show a reversible trans–cis–trans isomerisation process. These properties of the polymer could be promising for their technological usage. Copyright © 2007 Society of Chemical Industry     

Photoresponsive Functionalized Vinyl Cinnamate Polymers: Synthesis and Characterization

A series of functionalized vinyl cinnamate monomers were synthesized by the reaction of hydroxyethylmethacrylate and various substituted cinnamoyl chlorides. Electron donating and accepting functional groups such as −OCH₃, −Cl and −NO₂ were introduced at the para position of cinnamoyl chloride. Homopolymerization of the synthesized monomers were carried out in dimethylformamide using azobisisobutyronitrile as a free radical initiator at 80°C for 12h. The structures of the synthesized monomers and their polymers were characterized using Fourier transform infrared (FTIR), ¹H and ¹³C nuclear magnetic resonance spectroscopic techniques. Solid-state crosslinking of the above photosensitive polymers was studied by UV and FTIR spectroscopic techniques. The effects of various functional groups and the addition of sensitizer (benzophenone) on the photocrosslinking nature of the polymers were studied. The mechanism of photocrosslinking is a (2+2)π electron cycloaddition and not cis–trans isomerization in the functionalized polyvinylcinnamates. © 1997 SCI     

A Selective Na+ Aptamer Dissected by Sensitized Tb3+ Luminescence

A previous study of two RNA‐cleaving DNAzymes, NaA43 and Ce13d, revealed the possibility of a common Na⁺ aptamer motif. Because Na⁺ binding to DNA is a fundamental biochemical problem, the interaction between Ce13d and Na⁺ was studied in detail by using sensitized Tb³⁺ luminescence spectroscopy. Na⁺ displaces Tb³⁺ from the DNAzyme, and thus quenches the emission from Tb³⁺. The overall requirement for Na⁺ binding includes the hairpin and the highly conserved 16‐nucleotide loop in the enzyme strand, along with a few unpaired nucleotides in the substrate. Mutation studies indicate good correlation between Na⁺ binding and cleavage activity, thus suggesting a critical role of Na⁺ binding for the enzyme activity. Ce13d displayed a K_d of ～20 mm with Na⁺ (other monovalent cations: 40–60 mm ). The K_d values for other metal ions are mainly due to non‐specific competition. With a single nucleotide mutation, the specific Na⁺ binding was lost. Another mutant improved K_d to 8 mm with Na⁺. This study has demonstrated a Na⁺ aptamer with important biological implications and analytical applications. It has also defined the structural requirements for Na⁺ binding and produced an improved mutant.