| Abstract: | The crystal structure of metal borates has been extensively investigated by X-ray and neutron diffraction,but the structure of aqueous polyborate solutions are still largely unknown. Over the last decade,our group has focused on studying the structure of complex aqueous polyborate solutions of Li,Na,K,Rb,Cs,and Mg using synchrotron radiation X-ray scattering( XRS),EXAFS,Raman,NMR,and DFT,as well as determining the density,conductivity and pH of such solutions. Polyborate species distributions were calculated using pH measurements,and the main species in the solution have been confirmed by NMR and Raman spectra. For alkali-metal metaborates,the dominant species is always B( OH)_4~- in a wide range of concentration,while the presence of others species is negligible. For alkali metal tetraborates,when concentration is in the extreme low range,onlyB( OH)_3 and B( OH)_4~- are present in these solutions. As the total boron concentration increases,B( OH)_3 and B( OH)_4~- polycondensate to form more complex oligomers. Of them,while B_4 O_5( OH)_4~(2-) in the tetraborate solutions is the main species,B( OH)_3,B( OH)_4~-,and B_3 O_3( OH)_4~- are minor species,and B_3 O_3( OH)2-5 and B_5 O_6( OH)_4~- are present only in negligible amounts. As solution continues to concentrate,B_4 O_5( OH)_4~(2-) eventually becomes the dominant species,which is consistent with the congruent compound M_2 B_4 O_7·nH_2 O( M = Li,Na,K,Rb,Cs) in the system M_2 O-B_2 O_3-H_2 O( M =Li,Na,K,Rb,Cs). For alkali metal pentaborates,B( OH)_3 and B( OH)_4~- are the main species at low concentrations. The species distribution,Raman and NMR spectroscopy results verified that the dominant species in concentrated pentaborate solutions with Li and Na is pentaborate B_5 O_6 OH)_4~-,but it is surprising that the main species with K,Rb,and Cs is always the triborate monoanion B_3 O_3( OH)_4~-. Although all MB_5 O_6 OH)_4]·nH_2 O( M = Li,Na,K,Rb,Cs) are congruent compounds in the system M_2 O-B_2 O_3-H_2 O( M = Li,Na,K,Rb,Cs),the main species in aqueous solutions are quite different because of various cation hydration distance( d),hydration number( CN),and configuration,especially charge( Z). For bivalent Mg~(2+),three borate minerals,namely,Inderite( 2 MgO·3 B_2 O_3·15 H_2 O),Hungchaoite( MgO·2 B_2 O_3·9 H_2 O),and Mcallisterite( Mg O·3 B_2 O_3·7. 5 H_2 O),exist in the system Mg O-B_2 O_3-H_2 O at 298 K. Inderite is a congruent compound,but Hungchaoite and Mcallisterite are incongruent compounds. The species distribution and Raman spectra demonstrat that the predominant species in all the solutions with magnesium borates is bivalent triborate B_3 O_3( OH)2-5,while the subordinate species are B( OH)_4~- at a low B_2 O_3/MgO ratio and B( OH)_3 at a high B_2 O_3/MgO ratio,and the otheranions are negligible,as the high Z of the borate anion must match that of Mg~(2+). The disappearance of divalent tetraborate and hexaborate is in agreement with its incongruent nature. The DFT and XRS results showed that tetrahedral Li( H_2 O)_4~+( d = 0. 20 nm,CN = 4),octahedral Na( H_2 O)6+( d = 0. 236 nm,CN = 6),and Mg( H_2 O)_6~(2+)( d = 0. 210 nm,CN = 6) in the first hydration shell belong to Platonic polyhedra. However,K( H_2 O)_8~+( d = 0. 28 nm,CN = 8,XRS and DFT),Rb( H_2 O)_8~+( d = 0. 293 nm,CN = 7. 7-8. 2,EXAFS),and Cs( H_2 O)_8~+( d = 0. 320-0. 326 nm,CN = 7. 6-7. 9,EXAFS) are inclined Voronoi polyhedra. Therefore,the effects of cation Z and d on the structure of polyborates in aqueous solutions are deterministic,while the effect of hydrated-cation symmetry is secondary. Their hydrolysis order was: Mg > Li > Na > K > Rb > Cs,in step with hydration power. Among them,Mg~(2+)and Li+have a strong tendency towards hydrolysis,but Na+scarcely hydrolyzes,especially Rb+and Cs+have a little protonation. X-ray scattering of aqueous alkaline sodium borohydride solutions confirmed that dihydrogen bonds exist in the aqueous solution. The four distinct features of dihydrogen bonds in aqueous solution——unidirectionality,divaricativity,multicentricity,and multidentativity——have been also described here in brief. |
