Increasing the water holding capacity and the solubility of cyclodextrins by means of polyvalent metal salts

Abstract

The invention relates to a mixture comprising cyclodextrin and metal salt with polyvalent metal cations, to an aqueous solution containing cyclodextrin and metal salt with polyvalent metal cations, and to the use of the cyclodextrin solutions and the cyclodextrin mixtures for binding pollutants from gases, solids and / or liquids.

Description

[0001] Increasing the water retention capacity and solubility of cyclodextrins using polyvalent metal salts

[0002] The invention relates to a mixture comprising cyclodextrin and metal salt with polyvalent metal cations, an aqueous solution containing cyclodextrin and metal salt with polyvalent metal cations, and the use of the cyclodextrin solutions and the cyclodextrin mixtures.

[0003] Technical background

[0004] Cyclodextrins (CyD) are ring-shaped oligosaccharides consisting of 1,4 linked glucose units and can be enzymatically extracted from starch. Depending on the number of glucose units (6, 7, 8, or 9), they are referred to as α-, β-, γ-, or ε-cyclodextrins. Due to their ring structure and the orientation of the hydroxyl groups of the glucose units, cyclodextrins possess a hydrophilic (polar) outer surface and a hydrophobic (nonpolar) cavity, allowing hydrophobic active and / or pollutant molecules to be bound and encapsulated within the cavity. Due to their classification as sustainable and safe chemicals, cyclodextrins are also used, for example, in the encapsulation of active ingredients in cosmetics, food, or medicine.

[0005] The problem with native cyclodextrins, which consist only of glucose units, lies in two aspects, which make them suitable for use as a formulation ingredient and a cost-effective agent. In order to absorb non-polar active ingredients and harmful substances, the native cyclodextrins must remain moist throughout their entire use phase (good water retention capacity). Once a certain water content is reached and the cyclodextrins dry out, they lose their effectiveness because active ingredient and / or harmful substance molecules can no longer be stored in the cavity for thermodynamic reasons. For this reason, native cyclodextrins are typically used in dissolved form. However, native CyDs have low water solubility. For example, the maximum solubility of ß-CyD (7 glucose units) in water at room temperature is approximately 20 g / L (~2 wt%).The low solubility makes the application of aqueous CyD solutions difficult, since a relatively large volume of solution contains only a small amount of CyD and thus the ability to bind active and / or pollutant molecules is low.

[0006] One way to increase solubility is to chemically modify cyclodextrins. This chemical modification typically occurs by modifying the OH groups of the glucose units with alkyl chains to weaken the strong intramolecular interactions that prevent good solubility (see Gidwani et al., (2015): A Comprehensive Review on Cyclodextrin-Based Carriers for Delivery of Chemotherapeutic Cytotoxic Anticancer Drugs, BioMed research international, p. 198268; Kim et al. (2020): Solubility enhancement and application of cyclodextrins in local drug delivery, J. Pharm.

[0007] Investig. 50 (1), pp. 17-27).

[0008] However, the chemical modification of cyclodextrins has serious disadvantages. Firstly, the modification of cyclodextrins represents an additional process step, which increases production costs accordingly. Secondly, the modification does not increase the water-holding capacity of the CyDs. Furthermore, there are indications that chemically modified CyDs could have negative effects on the human organism and the environment and are therefore not sustainable materials.

[0009] A way to increase the water solubility and water retention capacity of CyD without having to synthetically modify the CyD is therefore desirable and would significantly improve the applicability of CyD.

[0010] Through intensive research in this field, it has now surprisingly been discovered that the water solubility of cyclodextrins is significantly increased by polyvalent metal cations, and the water-holding capacity of a mixture comprising cyclodextrin and metal salt with polyvalent metal cations is also excellent. Summary of the invention

[0011] In a first aspect, the invention relates to a cyclodextrin mixture, characterized in that the mixture comprises a) cyclodextrin, and b) metal salt with polyvalent metal cations M x+ contains; where x is > 2, preferably 2 or 3.

[0012] In a second aspect, the invention relates to an aqueous cyclodextrin solution, characterized in that the aqueous solution contains i) cyclodextrin, and ii) metal salt with polyvalent metal cations M x+contains; where x is > 2, preferably 2 or 3; and wherein the solution contains cyclodextrin in an amount of 5 to 80 wt.%, preferably 10 to 75 wt.%, particularly preferably 30 to 65 wt.%, based on the total weight of the solution.

[0013] A third aspect of the invention relates to the use of the aqueous cyclodextrin solution as described above or below or of the cyclodextrin mixture as described above or below for binding harmful substances and / or active substances from gases, solids and liquids.

[0014] Detailed description of the invention

[0015] In a first aspect, the invention relates to a cyclodextrin mixture which comprises a) cyclodextrin, and b) metal salt with polyvalent metal cations M x+ wherein x is > 2, preferably 2 or 3.

[0016] A mixture within the meaning of the present invention is a homogeneous solid mixture comprising at least two pure substances.

[0017] In order for cyclodextrins to be able to absorb active ingredients and / or harmful substances in non-dissolved form, a certain water content must be present.

[0018] Due to the hygroscopic properties of metal salts with polyvalent metal cations M x+ Water remains in small amounts in the cyclodextrin mixture as crystal water and the effectiveness of the cyclodextrins remains.

[0019] It is preferred that the mixture contains cyclodextrin in an amount of 5 to 95% by weight, preferably 20 to 92% by weight, particularly preferably 50 to 90% by weight, based on the total weight of the mixture.

[0020] All preferred embodiments of the cyclodextrin in the cyclodextrin mixture correspond to the preferred embodiments of the cyclodextrin in the aqueous cyclodextrin solution.

[0021] The amount of metal salt with polyvalent metal cations M x+is preferably from 5 to 95 wt.%, preferably 8 to 50 wt.%, particularly preferably 10 to 40 wt.%, based on the total weight of the mixture.

[0022] Preferably, the cyclodextrin mixture consists of cyclodextrin, metal salt with polyvalent metal cations M x+ and water of crystallization. n(M x+ ')

[0023] The molar ratio n ^ CyD ^) of the polyvalent metal cations M x+ (n(M x+ )) to cyclodextrin (n(CyD)) is preferably in a range of 1.0 to 10.0, preferably 1.1 to 8.0, particularly preferably 1.8 to 7.0.

[0024] The metal salt with polyvalent metal cations M x+ is preferably selected from the group consisting of CaCh, MgCh, ZnCh, AlCh, CuCh, FeCh and combinations thereof, particularly preferably from CaCh, MgCh and combinations thereof, even more preferably the metal salt with polyvalent metal cations M x+ CaCh.

[0025] In a preferred embodiment, the cyclodextrin mixture comprises cyclodextrin and metal salt with polyvalent metal cations in a total amount of > 95% by weight, preferably in a range of 96 to 99.9% by weight, based on the total weight of the mixture.

[0026] Methods for preparing cyclodextrin mixtures as described herein are known to those skilled in the art.

[0027] The mixture can be obtained by mixing cyclodextrin with the metal salt with polyvalent metal cations or by removing the water from an aqueous cyclodextrin solution as described herein.

[0028] Preferably, the cyclodextrin mixture is prepared by removing the water from an aqueous cyclodextrin solution as described herein.

[0029] Cyclodextrin (CyD)

[0030] Cyclodextrins are ring-shaped oligosaccharides made up of glucose units.

[0031] The three most commonly used cyclodextrins are α-, β-, and γ-cyclodextrin. They differ in the number of glucose units in the ring structure, which influences the size and hydrophobicity of the cavity inside the ring.

[0032] Scheme 1: Molecular structures of native α-, β-, and γ-cyclodextrin.

[0033] The cyclodextrin contained in the aqueous CyD solution can be native or modified cyclodextrin, or mixtures thereof.

[0034] Native means that the cyclodextrin consists only of glucose units and has free hydroxyl groups.

[0035] Modified cyclodextrins are cyclodextrins in which the hydroxyl groups at positions 2, 3, and / or 6 of the glucose units are partially or fully substituted. Examples of substituents include alkyl residues, hydroxyalkyl residues, carboxyalkyl residues, aryl residues, or sugar residues.

[0036] Alkyl radicals are preferably C1-C4 alkyl chains, particularly preferably methyl, ethyl, propyl.

[0037] Sugar residues are preferably glucosyl, maltosyl, or panosyl residues. Hydroxyalkyl residues are preferably hydroxymethyl, 2-hydroxyethyl, 2-hydroxypropyl, dihydroxymethyl, 2,2-dihydroxyethyl, and dihydroxypropyl groups. Carboxyalkyl residues are preferably carboxymethyl and 2-carboxyethyl groups.

[0038] Since the modification of cyclodextrins requires additional synthetic effort, it is preferred that the cyclodextrin in the inventive cyclodextrin mixture and / or aqueous cyclodextrin solution is a native cyclodextrin. Particularly preferably, the cyclodextrin is selected from the group consisting of native α-cyclodextrin, native β-cyclodextrin, native γ-cyclodextrin, and mixtures thereof. Even more preferably, the cyclodextrin in the cyclodextrin mixture and / or aqueous cyclodextrin solution is a native β-cyclodextrin.

[0039] Metal salt with polyvalent metal cations

[0040] The cyclodextrin mixture and / or aqueous cyclodextrin solution comprises, in addition to the cyclodextrin, a metal salt with polyvalent metal cations M x+ , where x >2.

[0041] Polyvalent metal cations

[0042] The presence of polyvalent metal cations can significantly increase the water solubility of cyclodextrin. Unlike monovalent metal cations, polyvalent metal cations can form orbital interactions with the hydroxyl groups of cyclodextrin, thereby increasing the water solubility of the cyclodextrin.

[0043] Preferably, the metal cations are divalent or trivalent, and x is 2 or 3, respectively. Metals that can form polyvalent metal cations are alkaline earth metals, metals from groups thirteen to sixteen of the periodic table, and transition metals from groups three to twelve of the periodic table. Polyvalent metal cations of various metals can be present in the cyclodextrin mixture and / or aqueous CyD solution.

[0044] Preferably, the metal of the polyvalent metal cations is a metal from groups 2 to 13 of the third or fourth period in the periodic table.

[0045] Metals of groups 2 to 13 of the third or fourth period in the periodic table are the following metals:

[0046] Mg, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Al, Ga.

[0047] More preferably, the metal of the polyvalent metal cations is selected from the group consisting of Mg, Ca, Fe, Cu, Zn, Al, and combinations thereof.

[0048] When combinations of polyvalent metal cations are included in the cyclodextrin mixture and / or aqueous cyclodextrin solution, each x can independently be a number > 2. For example, if a mixture of MgCh and FeCh is used to prepare the cyclodextrin mixture and / or aqueous cyclodextrin solution, x is 2 for the magnesium cations and 3 for the iron cations.

[0049] To balance the charge of the polyvalent metal cations, the appropriate amount of counterions is present in the cyclodextrin mixture and / or aqueous solution.

[0050] Counterions for polyvalent metal cations are known to the person skilled in the art.

[0051] Preferably, the counterions are selected from the group consisting of fluoride, chloride, bromide, iodide, hydrogen sulfate, sulfate, dihydrogen phosphate, hydrogen phosphate, phosphate, nitrate, dicarbonate, carbonate, hexafluorosilicate, hexafluorophosphate, benzoate, formate, acetate, propionate, butyrate, lactate, gluconate, succinate, oxalate, maleate, fumarate, malate, tartrate, citrate, tosylate, and combinations thereof.

[0052] More preferably, the counterions are selected from the group consisting of fluoride, chloride, bromide, iodide, hydrogen sulfate, sulfate, dihydrogen phosphate, hydrogen phosphate, phosphate, dicarbonate, carbonate, hexafluorosilicate, hexafluorophosphate, benzoate, formate, acetate, propionate, butyrate, lactate, gluconate, succinate, oxalate, maleate, fumarate, malate, tartrate, citrate, tosylate, and combinations thereof.

[0053] Even more preferably, the counterions do not contain hydroxide and / or nitrate.

[0054] Particularly preferably, the anions are selected from the group consisting of fluoride, chloride, bromide, iodide, and combinations thereof; even more preferably, the counterion is chloride. Preferably, the metal salt with polyvalent metal cations is selected from the group consisting of CaCh, MgCh, ZnCh, AlCh, CuCh, FeCh, and combinations thereof.

[0055] Aqueous cyclodextrin solution

[0056] The invention relates in a second aspect to an aqueous cyclodextrin solution which comprises cyclodextrin and metal salt with polyvalent metal cations M x+ includes.

[0057] An aqueous solution within the meaning of the present invention is a homogeneous solution comprising water and substances, liquids and gases dissolved therein.

[0058] By combining polyvalent metal cations with cyclodextrin, the water solubility of cyclodextrin can be significantly increased.

[0059] Accordingly, the aqueous solution contains cyclodextrin in an amount of 5 to 80 wt.%, preferably 10 to 75 wt.%, particularly preferably 30 to 65 wt.%, based on the total weight of the solution.

[0060] In order to achieve a strong increase in the solubility of the cyclodextrin, it is preferred that the solution contains the metal salt with polyvalent metal cations M x+ in an amount of 5 to 60 wt.%, preferably 8 to 50 wt.%, particularly preferably 10 to 30 wt.%, based on the total weight of the solution.

[0061] The amount of metal salt consists of the polyvalent metal cations and the corresponding counterions.

[0062] The molar ratio of the polyvalent metal cations M x+to cyclodextrin influences the possible interactions that can develop between the metal cations and the cyclodextrin. The preferred molar ratio of the amount of polyvalent Metal cations M x+ (n(M x+ )) to the amount of cyclodextrin (n(CyD)) in a range from 1.0 to 10.0, preferably 1.1 to 8.0, particularly preferably 1.8 to 7.0.

[0063] In the embodiments in which combinations of polyvalent metal cations are present in the aqueous cyclodextrin solution, the amount of polyvalent metal cations n(M x+ ) the sum of all amounts of the different, polyvalent metal cations (n(M x+ ) = n(Mi xl+ ) + n(M2 x2+ ) +. . .+ ^Mn^)). For example, if a mixture of CaCh and MgCh is used to prepare the aqueous solution, the amount of polyvalent metal cations corresponds to n(M x+ ) of the sum n(Ca 2+ ) + n(Mg2+ ).

[0064] In the embodiments in which combinations of different cyclodextrins are present in the aqueous cyclodextrin solution, the amount of cyclodextrins n(CyD) is the sum of all amounts of the different cyclodextrins (n(CyD) = n(CyDi) + n(CyÜ2) + . . . + n(CyD n )).

[0065] The polyvalent metal cations in the aqueous solution allow large amounts of cyclodextrin to be dissolved, thus requiring less solution to obtain the desired amount of cyclodextrin. Therefore, it is preferred that the aqueous solution be highly concentrated.

[0066] Accordingly, it is preferred that the aqueous solution contains water in an amount of < 60 wt.%, preferably < 55 wt.%, particularly preferably in a range of 20 to 50 wt.%, based on the total weight of the solution.

[0067] The aqueous cyclodextrin solution may contain other polar solvents, such as methanol and / or ethanol, in addition to water. However, it is preferred that no other solvent be present in the aqueous cyclodextrin solution. Furthermore, other salts, such as salts with monovalent metal cations, may be present in the aqueous cyclodextrin solution. However, it is preferred that no other salt besides the metal salt with polyvalent metal cations be present in the aqueous cyclodextrin solution.

[0068] In a preferred embodiment, the aqueous cyclodextrin solution comprises cyclodextrin, metal salt with polyvalent metal cations and water in a total amount of > 95% by weight, preferably in a range of 96 to 100% by weight, based on the total weight of the solution.

[0069] In a particularly preferred embodiment, the aqueous cyclodextrin solution comprises i) ß-cyclodextrin in an amount of 30 to 65 wt.%, based on the total weight of the solution; ii) metal salt with polyvalent metal cations M x+ in an amount of 10 to

[0070] 30 wt.%, based on the total weight of the solution, wherein the metal salt is selected from the group consisting of CaCh, MgCh, ZnCh, AlCh, CuCh, FeCh and combinations thereof; and iii) water in an amount of 25 to 50 wt.%, based on the total weight of the solution; n (M x+ ) where the molar ratio ( n(-CyD ^) the amount of polyvalent metal cations M x+ (n(M x+ )) to the molar amount of cyclodextrin (n(CyD)) is in a range of 1.8 to 7.0.

[0071] Preparation methods for aqueous cyclodextrin solutions as described herein are known to the person skilled in the art.

[0072] The solution can be prepared by dissolving a metal salt with polyvalent metal cations in water and then adding the cyclodextrin. Applications

[0073] A third aspect of the invention relates to the use of the cyclodextrin mixture as described above or the aqueous cyclodextrin solution as described above for binding harmful substances and / or active substances from gases, solids or liquids.

[0074] Applications of cyclodextrin mixtures and cyclodextrin solutions for binding harmful substances and / or active substances from gases, solids or liquids are known to the person skilled in the art.

[0075] By combining polyvalent metal cations M x+ With cyclodextrin in an aqueous solution, the concentration of cyclodextrin can be significantly increased, which greatly reduces the required volume of solution for cyclodextrin applications.

[0076] In addition, the effectiveness of cyclodextrin in solid mixtures is extended because the water retention capacity is improved by the combination with hygroscopic metal salts with polyvalent metal cations.

[0077] Pollutants within the meaning of the present invention are substances with negative health or odor relevance, for example wood preservatives.

[0078] In particular, a pollutant within the meaning of the present invention is understood to mean substances or mixtures of substances that can be harmful to humans, animals, plants or other organisms as well as entire ecosystems, or that develop an unpleasant odor and thereby make the intended use impossible or hinder it. The legal basis for classification as a pollutant is in particular the current version of the European REACH Regulation (Regulation (EC) No. 1907 / 2006 (REACH)). Examples of pollutants are substances that originate from heating oil, solvents or wood preservatives or that are, for example, polycyclic aromatic hydrocarbons (PAHs) and were previously frequently used in technical and electrical equipment. In particular, problematic pollutants originate from previous or current heating oil damage (such as after a flood), seeped-in industrial solvents (e.g.from industrial and cleaning processes) or are biocidal additives in deliberately applied wood preservatives (including pentachlorophenol (PCP) and lindane (gamma-HCH)); isothiazolinone compounds such as methylisothiazolinone (MIT) or benzoisothiazolinone (BIT)) or are polycyclic aromatic hydrocarbons which are found, for example, in old tar or tar oil treated materials or in rubber products in which they were previously used, for example, as plasticizer oils, or which can also be formed naturally during fires. Most of the problematic pollutants are non-polar, poorly water-soluble substances of low to moderate volatility which retain their harmfulness and their unpleasant odor properties for a long time.

[0079] Most preferably, the pollutants are selected from pollutants relevant to building products, such as PF AS, n-alkanes, VOCs, PAHs, petroleum products, tar products, wood preservatives, biocides, flame retardants, amines, organosulfur compounds, BTXE, odor-active VOCs, etc. In particular, the pollutants are POP substances according to the Stockholm Convention.

[0080] Active ingredients within the meaning of the present invention are substances that have a specific effect in an organism or can cause a specific reaction.

[0081] 1. Binding of pollutants from gases: Application as a surface coating: The aqueous cyclodextrin solution or the cyclodextrin mixture can be applied to surfaces as a coating to bind pollutants and can be used to decontaminate indoor air, supply air or exhaust air.

[0082] The aqueous cyclodextrin solution or the cyclodextrin mixture can be applied to surfaces in enclosed spaces.

[0083] The enclosed space can, for example, be located in a building (residential building, warehouse, office, retail space), motor vehicle (car), rail vehicle, ship or aircraft.

[0084] When the CyD mixture is applied to a filter medium, the air passing through or over the filter material can be decontaminated.

[0085] Use as an aerosol:

[0086] The aqueous cyclodextrin solution can be sprayed as an aerosol and thus used to clean process gases and / or the supply and / or exhaust air in ventilation ducts.

[0087] In particular, substances with negative health or odor relevance should be removed from the supply and exhaust air or minimized.

[0088] 2. Binding of pollutants from solids:

[0089] By coating contaminated building materials with the aqueous cyclodextrin solution or the cyclodextrin mixture, the release of pollutants from the building material or through the material surface can be prevented or at least reduced.

[0090] The aqueous cyclodextrin solution can also be sprayed into the contaminated soil to be treated. Organic pollutant molecules are incorporated into the CyD molecules while they are still present in solution. The bioavailability of the pollutant molecules can be increased by combining them with CyD, thus accelerating their degradation.

[0091] For the application of the metal salt-CyD mixture or metal salt-CyD solution in contact with soil, the mixture or solution can be separated from the soil matrix to be treated by a suitable membrane (pore size up to about 1 nm).

[0092] 3. Binding of pollutants from liquids:

[0093] For the application of the metal salt-CyD mixture or the metal salt-CyD solution in contact with water, the salt-CyD mixture can be separated from the aqueous matrix to be treated by a suitable membrane (pore size up to about 1 nm).

[0094] Application for the removal and recovery of heavy metals from wastewater or process water:

[0095] In this application, the metal salt-CyD mixture or metal salt-CyD solution can function either as an ion exchanger or as an adsorber. In the ion exchanger version, the CyD is complexed with polyvalent metal cations, and the ions are exchanged for heavy metal ions during use.

[0096] Application for the removal of organic pollutants and / or active substances from wastewater or process water:

[0097] The metal salt-CyD mixture can absorb organic pollutants from the wastewater to be treated.

[0098] For example, the metal salt-CyD mixture or the metal salt-CyD solution can be mixed into the water to be treated using a static or dynamic mixer. The organic pollutant molecules are embedded in the CyD molecules while they are still present in solution.

[0099] Due to dilution, which reduces the salt concentration of the metal salt with polyvalent metal cations, the CyD (with the embedded pollutants) can precipitate after a certain time, so that the contaminated precipitate can be separated from the purified water.

[0100] The CyD can absorb organic pollutants from the wastewater being treated and act as an ion exchanger for heavy metals, exchanging magnesium and / or calcium ions for heavy metal ions, for example. The combined removal of organic pollutants and heavy metals represents a particularly efficient application.

[0101] Experimental part

[0102] Preparation of aqueous cyclodextrin solutions

[0103] Preparation of salt solutions SL-A to SL-F

[0104] Highly concentrated salt solutions were prepared by dissolving the respective metal chlorides in water.

[0105] Table 1 shows the compositions of the salt solutions SL-A to SL-F.

[0106] Table 1 : Type and concentrations of the salt solutions prepared. Preparation of ß-CyD salt solutions

[0107] Native ß-cyclodextrin (ß-CyD) was stirred into each of the prepared aqueous salt solutions SL-A to SL-F at room temperature (20-25 °C) until the air had largely escaped from the CyD (reduction of the white color).

[0108] The mixtures were then heated to a temperature between 60 and 90 °C while stirring and stirred at elevated temperature until a clear solution was obtained.

[0109] Table 2 lists the compositions (mass fractions of ß-CyD and salt solution) of the inventive ß-CyD salt solutions CyD-Ll to CyD-LlO, as well as the time until a clear solution was formed.

[0110] Table 2: Compositions of the inventive ß-CyD salt solutions CyD-Ll to CyD-LlO.

Claims

Claims 1. A mixture, characterized in that the mixture comprises a) cyclodextrin, and b) metal salt with polyvalent metal cations M x+ contains; where x is > 2, preferably 2 or 3, characterized in that the molar ratio of the polyvalent metal cations M x+ (n(M x+ )) for the Cyclodextrin (n(CyD)) is in a range of 1.0 to 10.0, preferably 1.1 to 8.0, particularly preferably 1.8 to 7.

0.

2. The cyclodextrin mixture according to claim 1, characterized in that it contains cyclodextrin in an amount of 5 to 95 wt.%, preferably 20 to 92 wt.%, particularly preferably 50 to 90 wt.%, based on the total weight of the mixture.

3. The cyclodextrin mixture according to one of claims 1 or 2, characterized in that it contains the metal salt with polyvalent metal cations M x+in an amount of 5 to 95 wt.%, preferably 8 to 50 wt.%, particularly preferably 10 to 40 wt.%, based on the total weight of the mixture.

4. The cyclodextrin mixture according to one of the preceding claims 1 to 3, characterized in that the metal of the polyvalent metal cations M x+ is selected from the group consisting of metals of groups 2 to 13 in the periodic table, preferably from metals of groups 2 to 13 of the third and fourth period in the periodic table, particularly preferably from Mg, Ca, Zn, Al, Cu, Fe, and combinations thereof.

5. The cyclodextrin mixture according to any one of the preceding claims 1 to 4, characterized in that the counterions of the metal salt are polyvalent metal cations M x+are selected from the group consisting of fluoride, chloride, bromide, iodide, hydrogen sulfate, sulfate, dihydrogen phosphate, hydrogen phosphate, phosphate, dicarbonate, carbonate, hexafluorosilicate, hexafluorophosphate, benzoate, formate, acetate, propionate, butyrate, lactate, gluconate, succinate, oxalate, maleate, fumarate, malate, tartrate, citrate, tosylate, and combinations thereof, preferably from fluoride, chloride, bromide, iodide and combinations thereof, more preferably the counterion is chloride.

6. The cyclodextrin mixture according to any one of claims 1 to 5, characterized in that the metal salt with polyvalent metal cations M x+ is selected from the group consisting of CaCh, MgCh, ZnCh, AlCh, CuCh, FeCh and combinations thereof.

7. The cyclodextrin mixture according to any one of claims 1 to 6, characterized in that the cyclodextrin is selected from the group consisting of native α-, β- and γ-cyclodextrin, modified α-, β- and γ-cyclodextrin, and combinations thereof, preferably of native α-, β- and γ-cyclodextrin, and mixtures thereof, particularly preferably the cyclodextrin is native β-cyclodextrin.

8. An aqueous cyclodextrin solution, characterized in that the aqueous solution contains the cyclodextrin mixture according to one of the preceding claims 1 to 7, wherein the solution contains cyclodextrin in an amount of 5 to 80 wt.%, preferably 10 to 75 wt.%, particularly preferably 30 to 65 wt.%, based on the total weight of the solution.

9. The aqueous cyclodextrin solution according to claim 8, characterized in that the solution contains the metal salt with polyvalent metal cations M x+in an amount of 5 to 60 wt.%, preferably 8 to 50 wt.%, particularly preferably 10 to 30 wt.%, based on the total weight of the solution.

10. The aqueous cyclodextrin solution according to one of the preceding claims 8 or 9, characterized in that the aqueous solution contains water in an amount of < 60 wt.%, preferably < 55 wt.%, particularly preferably from 20 to 50 wt.%, based on the total weight of the solution.

11. Use of the aqueous cyclodextrin solution according to one of the claims 8 to 10 or the mixture according to one of claims 1 to 7 for binding pollutants from gases, solids and / or liquids.

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