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Deciphering the Solvent Effect for the Solvation Structure of Ca2+ in Polar Molecular Liquids

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Deciphering the Solvent Effect for the Solvation Structure of Ca2+ in Polar Molecular Liquids Deciphering the Solvent Effect for the Solvation Structure of Ca2+ in Polar Molecular Liquids Guoxi Ren1,2,3, Yang Ha4, Yi-Sheng Liu4, Xuefei Feng4, Nian Zhang1,2, Pengfei Yu1,2, Liang Zhang6, Wanli Yang4, un Feng4, Xiaosong Liu1,2,5,#, Jinghua Guo4,# 1State Ke Laboratory of "unctional Materials for Informatics% Shanghai Institute of Microsyste$ and Information &echnology, Chinese 'cade$ of Sciences% Shanghai 200050, China* 2CAS Center for Excellence in Superconducting Electronics (CE-SE.% Chinese 'cade$ of Sciences% Shanghai 200050, China /0niversit of Chinese Acade$ of Sciences% Bei2ing 100049, China* 3'dvanced Light Source% La5rence Berkele National Laboratory, Berkele % CA 94720, US'* )School of Physical Science and &echnology, Shanghai &ech 0niversit % Shanghai 201210, China* 8Institute of "unctional -ano 9 Soft Materials ,"0-S:M.% ;iangsu Ke Laboratory for Carbon- Based "unctional Materials 9 Devices% Soochow 0niversit % 199 Ren'ai Road, Suzhou 215123, ;iangsu, China* *Corresponding author E<$ail addressA xliu3@$ail.si$*ac.cn, [email protected] 1 ABSTRACT 'lthough the crystal structures for $any inorganic compounds are readil available% researchers are still 5orking hard to understand the relations bet5een the structures and che$ical properties in solutions% because that is 5here $ost of the che$ical reactions take place* ' huge a$ount of effort has been put on $odeling the ion solvation structure% from the perspectives of both experi$ents and theories* In this study, the solvation structures of Ca2+ ion in aqueous and alcohol solutions at different concentrations 5ere carefull evaluated ! Ca K-edge C-ra 'bsorption -ear-Edge Structure ,C'-ES. and Extended C-ra '!sorption "ine Structure (EC'"S.* Densit functional theory ,D"&. calculations 5ere also performed to correlate 5ith the experi$ental data% and then further extended to other si$ilar syste$s* It 5as found that the number of coordination solvent $olecules decreases 5ith increasing Ca2+ concentration and increasing solvent $olecule si?es* "rom the EC'"S data% the Drst solvation shell of Ca 2+ split into t5o Ca<: distances in $ethanol solution and the counter ion Cl< $ight also be 5ithin the Drst shell at high concentrations* "or the Drst ti$e% the effects of solvent 5ith different polarities and si?es on the ion solvation environment were syste$aticall evaluated. KeywordsA Ca2+ ion, solvation structure% concentration dependence% alcohol solution, EC'"S% D"& 2 1. INTRODUCTION Ionic solvation is one of the $ost funda$ental but interesting sub2ects to che$ists% especiall to those 5ho are interested in reactions in solutions* Man ele$ents have been investigated for ionic solvation, e.g* -a+ 1% Mg2+ 2% Cu2+ /% En2+ 3* Calcium% as an abundant ele$ent on earth, pla s an i$portant role in numerous biological% che$ical and environmental processes* "or e+a$ple% calcium chloride can signiDcantl promote the en? $eFactivities in blood and control the G'1'-transa$inase shunt regulation)% 5hile calciu$ oxide is an effective catal st in the production of biodiesel ! transesteriDcation of trigl cerides 5ith $ethanol8* Besides% calcium ions have been identiDed as a promising candidate for transporting $ultiple electrons in secondar batteries because of the high theoretical volu$etric capacit of calcium $etal anode7* Considering the signiDcant i$portance in the various liquid phase% the solvation structure of Ca 2+ has been a sub2ect for various experi$ental and theoretical studiesH<19* 'n in-depth understanding of the solvation structure of Ca2+ is crucial to explain $an che$ical processes occurred in solutions, such as the dyna$ics and kinetics of ion migration influenced by solvation structure20* Jo5ever% it is still not clear about the accurate solvation structure of Ca2+ because of the high disorder of Ca2+ ion in solution, the labile of the solvent molecules surrounding Ca2+, and the lo5 ato$ic number of the atoms involved21* "or e+a$ple% it is still a controversial issue that 5hether there exists concentration dependence on the solvation structure of the Drst shell in water or other polar $olecular liquids* Je5ish e$ al% used neutron diffraction to Dnd that the coordination number and configuration of Ca2+ are strongl dependent on the concentration of hydration structureH* Mean5hile% from the theoretical point of vie5% Chialvo e$ al% thought that calcium hydration number decreases incessantl from H to /*4 5ith concentration varying from dilute to saturation4* a recent stud combined C-ra diffraction and a! Initio stud found that the coordination number decreases 5ith an increase in concentration10* :n the other hand, both Bad al et al e$plo ed neutron diffraction approach11 and 'lbright used C-ray diffraction studies13 suggested that the hydration of Ca2+ is independent of concentration. 'lthough diffraction $ethods are ver popular in solving solvation structures% it focuses on obtaining the long<range 3 order structure for all ion-5ater% ion-ion, and 5ater<5ater interactions* 'nd fe5 studies focus on 5hether concentration dependence exists in other polar $olecular liquids. &herefore 5e also need a s ste$atic stud on the local structure and different solvents to solve this proble$* Moreover% there is still not a clear picture about the solvation structure of Ca2+ in alcohol solutions* :nl a fe5 solvation studies can be found in $ethanol* "or instance% Dang e$ al% suggested that there are 2+ about 8 $ethanol $olecules around the Ca in diluted $ethanolic solution ! $olecular dyna$ics13* Kos?tola´nyi e$ al% also used $olecular dyna$ics to speculate that the Ca2+ ions are surrounded ! H $ethanol $olecules for the diluted solution, 7*7 for the 1 $ol L<1 solution, and extensive Ca<Cl contact ion-pairs in 2 $ol L<1 $ethanol solution1). Lhile Meg es et al. combined diffraction ,C-ra and neutron) stud and $olecular dyna$ics si$ulation to Dnd that in 1 $ol L <1 solutions, the Ca2+ ion is surrounded !y six $ethanol $olecules and in 2 $ol L<1 solution, contact ion-pairs can also be found10% 16* &hrough these literatures% 5e feel there needs a s ste$atic stud on solvation structure in different solvents to bring up the big picture of this field* C-ra a!sorption spectroscop ,C'S. is one of the $ost po5erful tools that provides insight into the electronic structure of selected a!sor!ing atoms* In particular% C'S can give detailed electronic infor$ation on the unoccupied states right near the "er$i level% such as ligand structure and coordination number% 5hich are strongl affected ! the solvation structure around the a!sorption atoms* Calciu$ can be studied ! soft C-ray a!sorption at Ca L-edge C'S22-2) and hard C-ra at Ca K<edge C'S12% 28<27* &he Ca L-edge C'S corresponds to the transition from Ca 2p electron to the unoccupied Ca 3d e$pty orbitals% 5hich $ainl focuses on the a!sor!ing ato$* Lhile the Ca K-edge C'S relates to the transition from Ca 1s electron to the unoccupied Ca 4p states% 5hose spectra called C-ra '!sorption "ine Structure ,C'"S.* &he C'"S is divided into a lo5-energ region (-20–50 eN. called C-ra a!sorption near<edge structure ,C'-ES.% and a high-energ region (50–1000 eN. ter$ed extended C-ra a!sorption Dne structure (EC'"S.* "or relativel large disordered s ste$s% the C'"S spectra can deter$ine the statistical properties of the distribution of atoms relative to the central a!sorbers% 5hich provide the infor$ation of the precise local structures* &his process is highl dependent upon the s $$etr of the local atom 4 shell and hence provides an independent $easure of the local hydration s $$etr * Moreover% unli6e diffraction $ethods providing the long-range order structure for all interactions% C'"S probes the local environment about a speciDc ion of interest 5ithout interfering signals from other atoms in the s ste$* This ma6es C'"S one of the most accurate structure anal sis methods for the Drst<shell region about the ion. &herefore% C'"S has been 5idel applied to investigate the solvation structure/<3% 2H</(* In addition to C'S technolog % quantum che$istr calculations are also frequentl used in $odern che$istr research to provide additional insights into experi$ental results31-/2* 'fter collecting a large a$ount of C'"S spectra% Densit function theor ,D"&. calculations are e$plo ed to calculate both the geometric structures and electronic structures of the $etal site* &he calculated geometric structures are correlated 5ith the EC'"S results% 5hile calculated electronic structures si$ulated spectra are correlated 5ith the C'-ES region and help to assign the peaks//</3* In this 5or6% 5e s ste$aticall investigate and compare the solvation structure of Ca2+ in various polar $olecular solvents% including 5ater% $ethanol% and ethanol% using in-situ% e+-situ C'"S combined 5ith the D"& calculations* Le ai$ to prove the concentration dependence of the Drst shell structure in different solutions% explain the effect of different polar $olecules solvents on the solvation structure* More i$portantl % 5e put for5ard the $ore Dne Drst shell solvation structure picture of Ca2+ in 5ater and methanol% rather than only a coordination number* Moreover% 5e explore the solvation environment for some other $etal ions in aqueous solutions ! D"& to further extend our study to a $ore general level* :ur present stud sheds light on the picture of the solvation shell of Ca2+ and helps to guide the develop$ent of C'"S and D"& in studying other ionic solvation. 2. METHODS 2.1 Sample preparat on Part of the anal sis involved studies of 5ell<characteri?ed reference sa$ples including CaCl2% CaCl2O2H2:% and CaCl2O8J2:* &he anhydrous calcium chloride salt ,P44*44Q purit .
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