New insights into the nucleation of magnesium hydroxide and the influence of poly(acrylic acid) during the early stages of Mg(OH)2 crystallisation

New insigh s in o he nuclea ion o magnesium hyd oxide and he in luence o poly(ac ylic acid) du ing he ea ly s ages o Mg(OH) 2 c ys allisa ion† J. Scheck,‡ a J. K. Be g,‡ a M. D echsle , b A. Kemp e , c A. E. S. Van D iessche, d H. Cöl en, a D. Gebaue * e and M. Kelle meie * Nuclea ion is a unique p ocess wi h b oad ele ance ac oss a wide ange o scien i ic disciplines and applica ions. While conside able p og ess in he unde s anding o he mechanisms unde lying he nuclea ion o mine als om solu ion has been made o popula model sys ems such as calcium ca bona e, co esponding de ailed insigh s a e s ill missing o o he , less p ominen mine als. He e, we p esen a po en iome ic i a ion-based me hod ha allows he ea ly s ages o he c ys allisa ion o b uci e, Mg(OH) 2 , o be moni o ed and quan i ied. Toge he wi h complemen a y cha ac e isa ion p o ided by (c yogenic) ansmission elec on mic oscopy, he collec ed da a shed no el ligh on he species occu ing p io o, du ing, and a e nuclea ion o b uci e. In he second pa o he wo k, he newly de eloped app oach was applied o in es iga e he e ec s o added poly(ac ylic acid) on he di e en s ages o he c ys allisa ion p ocess. The polyme is ound o s abilise b uci e nanopla ele s and co- p ecipi a e wi h he ino ganic phase, yielding a composi e ma e ial. The me hodology es ablished in his s udy can eadily be used o sc een o he chemis ies o hei abili y o p e en magnesium hyd oxide scaling and/o a o d b uci e nanoma e ials wi h ailo ed p ope ies. In oduc ion O e he pas en yea s, ou unde s anding o he mechanisms unde lying he c ys allisa ion o ino ganic mine als om aqueous solu ions has ad anced signi ican ly hanks o a la ge numbe o pionee ing, and some imes con o e sial, s udies. 1–5 While many o he key disco e ies we e made o p ominen mine al sys ems such as calcium ca bona e, 6–8 calcium phospha e, 9–11 o i on (oxy)(hyd ) oxides, 12–14 o he ino ganic compounds ha e ecei ed much less a en ion. In his con ex , one a he neglec ed example is magnesium hyd oxide. I s mine al o m, known as b uci e, is mode a ely abundan in na u al se ings, whe e i mainly occu s in ul ama ic ocks and o ms ia me amo phosis o magnesium- ich phases like dolomi e o pe iclase. 15 F om an indus ial poin o iew, in e es in magnesium hyd oxide de i es om i s po en ial use as lame e a dan , 16,17 addi i e in he p ocessing o pape and pulp, 18 neu alising agen in he ea men o acidic was ewa e , 19,20 o o he ab ica ion o e ac o y ma e ials. 21 As he cons i uen ions a e omnip esen in seawa e , Mg(OH) 2 has also been conside ed as an a i icial sca old o co al colonies. 22 An ob ious ad an age o magnesium hyd oxide in any o i s po en ial applica ions is he ac ha i does no con ain CO 2 and hence can be conside ed a clima e-neu al al e na i e o o he mine als like lime o soda. Con e sely, b uci e has also a oused in e es in he con ex o new echnologies o ca bon dioxide seques a ion. 23,24 Howe e , he e a e also indus ial se ings whe e he o ma ion o magnesium hyd oxide is highly undesi ed, mos no ably in p ocesses in ol ing aqueous media a high empe a u es and/o alkaline pH alues. Due o he low solubili y o Mg(OH) 2 unde hese condi ions, mine alisa ion may eadily occu and lead o inc us a ion o echnical su aces, which will educe hea 7718 a Physical Chemis y, Uni e si y o Kons anz, Uni e si ä ss . 10, D-78464 Kons anz, Ge many b Ba a ian Polyme Ins i u e (BPI), Keylab “Elec on and Op ical Mic oscopy”, Uni e si y o Bay eu h, Uni e si ä ss . 30, D-95440 Bay eu h, Ge many c Ino ganic Ma e ials & Syn hesis, BASF SE, Ca l-Bosch-S . 38, D-67056 Ludwigsha en, Ge many d Ins i u o Andaluz de Ciencias de la Tie a (IACT), CSIC –Uni e si y o G anada, E-18100 A milla, G anada, Spain e Ins i u e o Ino ganic Chemis y, Leibniz Uni e si y Hanno e , Callins . 9, D- 30167 Hanno e , Ge many. E-mail: gebaue @acc.uni-hanno e .de; Fax: +49 (0)511 762 3006; Tel: +49 (0)511 762 18855 Ma e ial Science, BASF SE, Ca l-Bosch-S . 38, D-67056 Ludwigsha en, Ge many. E-mail: ma hias.kelle meie @bas .com; Fax: +49 (0)621 66 43388; Tel: +49 (0)621 60 43388 †Elec onic supplemen a y in o ma ion (ESI) a ailable: Plo o he a ios o bound ions du ing i a ion wi h and wi hou added polyme (Fig. S1) and esul s o solu ion specia ion calcula ions o he concen a ion o di e en solu es (Fig. S2) and hei ac i i y coe icien s (Fig. S3) du ing he i a ion expe imen . ‡These au ho s con ibu ed equally o his wo k. Kons anze Online-Publika ions-Sys em (KOPS) URL: h p://nbn- esol ing.de/u n:nbn:de:bsz:352-2-1ohsw xledj3l5 E schienen in: C ys EngComm ; 24 (2022), 44. - S. 7718-7726 h ps://dx.doi.o g/10.1039/D2CE00896C 7719 ans e and clog pipes. 25–28 B uci e scaling is pa icula ly c i ical in he desalina ion o seawa e ia e apo a i e p ocesses, o en in combina ion wi h o he mine als such as a agoni e. Due o his b oad ele ance, he c ys allisa ion o magnesium hyd oxide om solu ion has been in es iga ed ecu en ly in he pas . Many o hese p e ious s udies de e mined he a es o Mg(OH) 2 nuclea ion and/o g ow h as a unc ion o a ious solu ion pa ame e s (like supe sa u a ion), o unde he in luence o di e en he e ogeneous subs a es, 25,29–35 o en wi h he aim o iden i y expe imen al windows o he o ma ion o uni o m b uci e pa icles wi h con olled mo phologies. O he wo k ocused on al e a ions o exis ing Mg(OH) 2 su aces, o example in con ac wi h oxic me al ca ions, 36 du ing ca bona ion, 37 o upon ans o ma ion o laye ed double hyd oxides. 38 To da e, de ailed insigh s in o he e y ea ly s ages o b uci e o ma ion om solu ion and he po en ial ole(s) o c ys allisa ion modi ie s in his p ocess a e s ill missing. In he p esen wo k, we ha e add essed his aspec and in es iga ed Mg(OH) 2 nuclea ion in he absence and p esence o poly(ac ylic acid) (PAA) as an a che ype o polyme ic scale inhibi o s. Inspi ed by p e ious s udies o o he mine als like calcium ca bona e 6,7,39,40 o calcium hyd oxide, 41,42 we ha e de eloped a po en iome ic i a ion assay, du ing which magnesium chlo ide solu ion is dosed in o a ese oi o alkaline wa e a cons an pH (wi h and wi hou added PAA), while he ac ual concen a ion o ee Mg 2+ ions is moni o ed by an ion-selec i e elec ode (ISE). P og essi e hyd olysis o MgCl 2 wi h slowly inc easing supe sa u a ion e en ually leads o Mg(OH) 2 nuclea ion and g ow h. Du ing hese ea ly s ages o c ys allisa ion, samples we e d awn a di e en imes and analysed by (c yogenic) ansmission elec on mic oscopy (TEM), while he inally o med solid phase was cha ac e ised using in a ed (IR) spec oscopy and powde X- ay di ac ion (PXRD). The esul s ob ained in his way shed no el ligh on he p ocesses p eceding and con olling he nuclea ion o b uci e, and show how polyme ic addi i es in e ac wi h he mine al p ecu so s and, hus, allow he cou se o c ys allisa ion o be modi ied. The gained insigh s may help in he design o mo e e icien Mg(OH) 2 an iscalan s, as well as in he syn hesis o b uci e (nano)ma e ials wi h ailo ed p ope ies. Expe imen al sec ion Po en iome ic i a ion expe imen s In o de o induce nuclea ion o Mg(OH) 2 , a 20 mM solu ion o magnesium chlo ide hexahyd a e (Sigma-Ald ich, ≥99.0%) was added con inuously a a a e o 0.01 mL min −1 o 50 mL wa e ha was p e iously adjus ed o pH 11.0 using 100 mM NaOH solu ion (Me ck, Ti ipu ®). Du ing addi ion, he pH in he eac ion essel was measu ed by an imme sed elec ode (Me ohm, No. 6.0256.100) and kep cons an by au oma ic coun e - i a ion o 100 mM NaOH. Simul aneously, he ac ual amoun o ee Mg 2+ ions was moni o ed by means o an ion-selec i e elec ode (ISE; C-CIT Senso s AG, model MT050). Read-ou o he wo senso s and con olled addi ion was achie ed wi h a i a ion uni om Me ohm (905 Ti ando), o which wo dosing uni s (800 Dosino) o he MgCl 2 and NaOH solu ions we e a ached. To s udy he in luence o added poly(ac ylic acid) (Sigma- Ald ich, a e age molecula weigh : M w = 5100 Da, wi h PDI = M w /M n ≈2.7), 10 ppm polyme we e dissol ed in he ini ial 50 mL wa e be o e s a ing he i a ion. All solu ions we e p epa ed using wa e aken om a Millipo e®sys em and pu ged wi h ni ogen o emo e any dissol ed ca bon dioxide. Du ing i a ion, he eac ion essel was lushed wi h a gen le s eam o wa e -sa u a ed ni ogen o a oid ca bona e o ma ion. The pH elec ode was calib a ed wi h comme cial bu e solu ions (Me le -Toledo, pH alues 7.00, 9.21 and 11.00) p io o each expe imen . Calib a ion o he magnesium-selec i e elec ode was ca ied ou by i a ing MgCl 2 solu ions in o wa e a neu al pH unde o he wise iden ical condi ions, usually immedia ely be o e an ac ual p ecipi a ion assay due o he impo ance o a p ope calib a ion o he Mg-ISE and i s known changes upon ageing. All i a ion expe imen s we e pe o med a leas in iplica e. Ex si u cha ac e isa ion P ecipi a es o med a he end o he i a ion expe imen s we e isola ed by cen i uga ion and subsequen epea ed washing wi h e hanol. A e d ying in ai , he composi ion o he ma e ial was s udied by a enua ed o al e lec ion in a ed spec oscopy (ATR-IR) using a Pe kin-Elme Spec um 100 ins umen , while powde X- ay di ac ion (PXRD) pa e ns we e eco ded wi h a Hube G670 Imaging Pla e Guinie Came a sys em using Cu-K α adia ion. Fo he la e pu pose, he isola ed p ecipi a es we e moun ed on a Myla ilm wi h a hickness o 6 μm. Fo ansmission elec on mic oscopy (TEM) and elec on di ac ion (ED), a d op o he eac ion mix u e was placed on a ca bon-coa ed coppe g id and blo ed wi h a il e pape . A e d ying, pa icles p esen on he g id we e in es iga ed on a JEOL JEM-2200FS mic oscope ope a ed a 200 kV. To gain u he insigh s in o he ea ly s ages o Mg(OH) 2 c ys allisa ion, aliquo s we e d awn om he eac ion essel a di e en s ages o he i a ion expe imen s and in es iga ed by means o c yogenic ansmission elec on mic oscopy (c yo-TEM). Fo his pu pose, he liquid samples we e applied o lacey ca bon- ilmed coppe g ids and blo ed using il e pape . The emaining hin ilm was immedia ely i i ied by plunging he g ids in o liquid e hane a i s eezing poin . The esul ing specimens we e s udied on an EM922 Omega mic oscope om Zeiss/LEO, equipped wi h a cooled sample holde ha kep he sample a ca. 90 K. Collec ed images we e p ocessed wi h a backg ound-sub ac ion ou ine and, op ionally, a Bu e wo h smoo hening il e . 7720 Resul s & discussion Moni o ing he p og ess o he ea ly s ages o p ecipi a ion in eal ime was ealised by a i a ion-based assay, in which MgCl 2 solu ion is added con inuously o alkaline wa e a a cons an pH o ypically 11.0, gi ing cha ac e is ic p o iles o he p e- and ea ly pos -nuclea ion s ages as shown in Fig. 1. The amoun o ee magnesium ions in solu ion (n ee (Mg 2+ )), acked by an imme sed ion-selec i e elec ode (Fig. 1a), inc eases as mo e and mo e MgCl 2 is added up o a poin whe e c i ical condi ions a e eached and nuclea ion occu s. Subsequen ly, n ee (Mg 2+ ) dec eases as solid pa icles ha e o med and con inue g owing. P io o he maximum, he de ec ed amoun o ee magnesium ions is signi ican ly lowe (ca. 20–25%) han he added quan i y (n added (Mg 2+ ); do ed line in Fig. 1a), indica ing ha ion pai s, complexes, and/o la ge clus e s exis in no iceable ac ions in he one- phase sys em du ing he p e-nuclea ion egime –in ull analogy o p e ious obse a ions o o he mine als. 6,7,39–42 A n added (Mg 2+ )≈5μmol, he slope o n ee (Mg 2+ ) becomes s eepe , which sugges s a change in he s oichiome y o p e- nuclea ion ion associa ion. Fu he insigh s in o he binding beha iou can be gained by e alua ing he olume o base needed o keep he pH cons an , om which he amoun o hyd oxide ions bound in associa ed species (n bound (OH − )) can be de i ed ia mass balance conside a ions and compa ed o he espec i e bound amoun o magnesium ions (n bound (Mg 2+ )). In e es ingly, he esul ing binding p o iles (Fig. 1b) show ha he n bound (Mg 2+ )/n bound (OH − ) a io inc eases om ini ially ca. 0.3 o le els highe han 1 soon a e MgCl 2 i a ion has been s a ed (see Fig. S1 in he ESI†). As nuclea ion is app oached, he excess o magnesium ( ela i e o hyd oxide) in bound s a es becomes mo e and mo e p onounced o each alues o a ound 2.2 a he maximum in n ee (Mg 2+ ). This means ei he ha he associa ed species a e posi i ely cha ged on a e age, and/o ha o he anions (e.g. chlo ide) pa icipa e in complex o ma ion. The la e no ion is suppo ed by solu ion specia ion calcula ions pe o med using he PHREEQC so wa e package, 43 which p edic s he coexis ence o [MgOH] + and [MgCl] + a all composi ions du ing he p e-nuclea ion egime (see Fig. S2 in he ESI†). Howe e , [MgOH] + should be much mo e abundan (by abou wo o de s o magni ude) and consequen ly, he da a collec ed in he p esen wo k canno be explained by such mononuclea complexa ion p ocesses alone. Mos likely, Fig. 1 In si u moni o ing o he nuclea ion o magnesium hyd oxide by po en iome ic i a ions o aqueous solu ions a a cons an pH o 11.0. a) Mola amoun o ee Mg 2+ ( ull line) de ec ed by an ion- selec i e elec ode du ing addi ion o 20 mM MgCl 2 solu ion o 50 mL wa e a a a e o 0.01 mL min −1 . The do ed line ep esen s he added amoun o MgCl 2 . A ows ma k he ime (o ime in e al) a which samples we e d awn o ambien - empe a u e and c yogenic ansmission elec on mic oscopy (c . Fig. 2). b) Co esponding p o iles o bound magnesium (n bound (Mg 2+ ), ull line) and hyd oxide ions (0.5·n bound (OH − ), dashed line) as compa ed o he dosed amoun o MgCl 2 (do ed line) (see Fig. S1 in he ESI† o a plo o he e olu ion o he n bound (Mg 2+ )/n bound (OH − ) a io in he cou se o he i a ion expe imen ). c) F ee ion p oduc calcula ed om he measu ed ee concen a ions o magnesium and hyd oxide ions. Fig. 2 De elopmen o he amoun o ee magnesium ions de ec ed upon i a ion o 20 mM MgCl 2 solu ion in o 50 mL o pu e wa e ( ull black line) and 1 mM NaCl solu ion ( ull g ey line). The do ed black line ep esen s he amoun o dosed MgCl 2 . 7721 mixed species comp ising mul iple Mg 2+ ions as well as OH − and Cl − coun e ions will be o med. Pa icipa ion o chlo ide ions in p e-nuclea ion ion associa ion is u he subs an ia ed by i a ion expe imen s, in which 1 mM sodium chlo ide was added o he s a ing olume. Co esponding da a (Fig. 2) show ha he slope o n ee (Mg 2+ ) dec eases in he p esence o added NaCl, indica ing ha mo e magnesium ions a e bound a highe chlo ide concen a ions as he equilib ium is shi ed owa ds associa ed s a es such as [MgCl x ] (2−x) o [MgCl y OH z ] (2−y−z) . Indeed, he ends obse ed in Fig. 2 a e con olu ed o some ex en wi h ac i i y e ec s caused by he inc eased ionic s eng h; howe e , any such e ec s should lead o s eepe slopes 39 and a e likely negligible as he le el o n ee (Mg 2+ ) a e nuclea ion, which is a measu e o he solubili y p oduc o he o med solid phase, is e y simila o ha o he e e ence expe imen in pu e wa e . A e nuclea ion, he p o iles o n bound (Mg 2+ ) and 0.5·n bound (OH − ) exhibi e y simila slopes and un pa allel o he dosed amoun , which con i ms he o ma ion and g ow h o Mg(OH) 2 a e nuclea ion as expec ed (Fig. 1b). O se s o he h ee p o iles along he y-axis a e likely caused by he assump ion o ideal condi ions du ing da a e alua ion, which is a guably no ul illed a he gi en elec oly e concen a ions (see Fig. S3 in he ESI† o a plo o changes in he ac i i y coe icien s o Mg 2+ and OH − du ing i a ion, which was calcula ed using PHREEQC 43 and shows ha highe amoun s o appa en ly bound magnesium, as obse ed in Fig. 1b, a e expec ed o ideal ea men o inc easingly non-ideal da a). Howe e , he de i ed ion p o iles should s ill p ope ly e lec he associa ion p ocesses occu ing in solu ion, as demons a ed p e iously o he calcium ca bona e sys em. 39 Finally, he measu ed Mg 2+ and OH − concen a ions can be used o calcula e ac ual ee ion p oduc s, i.e. c ee (Mg 2+ )·c ee2 (OH − ). The esul ing p o iles (Fig. 1c) exhibi classical LaMe - ype beha iou and show ha he ee ion p oduc app oaches a cons an le el a e nuclea ion, om which he appa en solubili y o he nuclea ed phase(s) can be in e ed. The measu ed solubili y p oduc (ca. 9.9 ×10 −11 M 3 ) is mo e han one o de o magni ude highe han epo ed alues o c ys alline b uci e (5.6 ×10 −12 M 3 ). 44 These di e ences may a ise om he ideal ea men o he da a in he p esen wo k, al hough any co ec ion o non-ideali y should ac ually no a ec he esul ing solubili y p oduc s (o , i any, lead o e en highe alues). 39 The e o e, i is mo e likely ha c ys alline b uci e coexis s wi h a mo e soluble phase (e.g. amo phous magnesium hyd oxide) a his s age, as obse ed o o he mine als. 6 Fig. 3 Cha ac e isa ion o pa icles o med du ing po en iome ic i a ion in he absence o addi i es by means o ansmission elec on mic oscopy in (a–c) d y s a e a ambien empe a u e and (d–g) i i ied s a e unde c yogenic condi ions. Samples we e d awn nea he maximum in n ee (Mg 2+ )(c . Fig. 1a; sampling in e al indica ed by dashed lines) (a, b and d–g) o a e he subsequen d op o a cons an pla eau as ma ked by he a ow in Fig. 1a (c). The inse ed elec on di ac ion pa e ns iden i y he pa icles as being ei he amo phous (a) o c ys alline (b and g), wi h he obse ed e lec ions ma ching hose expec ed o b uci e in he la e case. No e ha he c oss-like a ia ions in elec on con as obse ed o he c ys al in (b) may a ise due o local di e ences in hickness ( o a hexagonal-bipy amidal habi ) and/o bending con ou s caused by ensions in he basal plane. 7722 In o de o u he elucida e he na u e o he species ele an o b uci e nuclea ion, samples we e d awn om he eac ion essel a ound he maximum in n ee (Mg 2+ ) and cha ac e ised using ansmission elec on mic oscopy (Fig. 3). In TEM analyses ca ied ou a ambien empe a u e (Fig. 3a), ne wo ks o mo e o less sphe ical nanopa icles we e epea edly obse ed in small amoun s ac oss he g id su aces, wi h p ima y pa icle sizes o 10–20 nm and no dis inc c ys alline o de as indica ed by ED pa e ns. In e es ingly, such de ined amo phous pa icles we e no p esen in analogous i i ied samples, whe e diso de ed ma e ial a he occu ed in he o m o ill-de ined and ex ended “clouds”wi hou clea -cu bounda ies (Fig. 3d and e). This sugges s on he one hand ha amo phous p ecu so s a e in ol ed in Mg(OH) 2 nuclea ion, as o many o he mine al sys ems. 1,2,7–11,40–43 On he o he hand, his ma e ial is appa en ly no s able enough o be p ese ed upon d ying and ans o ms in o (a guably less hyd a ed) nanopa icles. C yo-TEM images u he e eal ha c ys alline s uc u es s a o eme ge wi hin he diso de ed ne wo ks (Fig. 3d– ), p esumably ia in e nal eo ganisa ion p ocesses. E en ually, his yields well-de ined hexagonal pla ele s wi h diame e s anging om 200 o 400 nm and di ac ion pa e ns consis en wi h c ys alline b uci e, 20 as obse ed independen ly by bo h ambien - empe a u e (Fig. 3b) and c yogenic TEM (Fig. 3g). As mo e and mo e MgCl 2 is added and he amoun o ee Mg 2+ has dec eased o a cons an le el a e nuclea ion, he abundance and ma u i y o hese hexagonal pla ele s inc eases and pa icle agg ega ion is obse ed (Fig. 3c). The de eloped me hodology can be eadily applied o in es iga e he in luence o addi i es on he ea ly s ages o b uci e c ys allisa ion om solu ion. He e, we chose poly(ac ylic acid) as an a che ype o polyme ic scale inhibi o s o a pilo s udy. P e ious wo k has shown ha polyca boxyla es, such as PAA, can ha e mul iple e ec s du ing mine al p ecipi a ion a beyond he well-known phenomena o ion complexa ion, h eshold inhibi ion, and bulk pa icle dispe sion. 40–42,45–48 Mo eo e , polyme ic addi i es bea ing ca boxyla e g oups we e ound o a ec he p ope ies o he o med solid p ecipi a es and may hus po en ially lead o new (hyb id) ma e ials wi h in e es ing mo phologies and s uc u es. 49–51 Typical i a ion p o iles ob ained in he p esence o 10 ppm PAA (ini ial concen a ion) a e summa ised in Fig. 4 ( ed cu es), in di ec compa ison o a e e ence expe imen wi hou added polyme (black cu es). F om he p og essions o n ee (Mg 2+ ) (Fig. 4a) and he n bound (Mg 2+ )/ n bound (OH − ) a io (Fig. 4b), i becomes e iden ha PAA does no in e e e no iceably wi h ion binding pa e ns in he p e- nuclea ion egime, as he espec i e slopes a e almos indis inguishable om hose o he e e ence expe imen . The mos s iking e ec o PAA on he c ys allisa ion beha iou is a d as ic delay o he maximum in n ee (Mg 2+ ), i.e. nuclea ion is s ongly inhibi ed. Plo s o he ee ion p oduc (Fig. 4c) show ha abou wice he nominal le el o supe sa u a ion is equi ed o b uci e o nuclea e unde he in luence o 10 ppm PAA. Du ing he inhibi ion pe iod, he a io o bound ions inc eases u he o each a h ee old excess o Mg 2+ a he ime o nuclea ion (see Fig. S1 in he ESI†), which sugges s ha associa ion o chlo ide (and polyac yla e) ions plays an e en mo e impo an ole han in he absence o he polyme . The o ma ion o Mg(OH) 2 a e nuclea ion is again con i med by a s ong inc ease in Fig. 4 Resul s o po en iome ic i a ion assays in he p esence o 10 ppm poly(ac ylic acid) ( ed cu es) as compa ed o a e e ence expe imen wi hou any addi i es (black cu es) a a cons an pH o 11.0. a) Mola amoun o added (do ed line) and de ec ed ee ( ull lines) Mg 2+ ions. A ows and dashed lines ma k he ou ime in e als du ing which samples we e d awn o ambien - empe a u e and c yogenic ansmission elec on mic oscopy (c . Fig. 4). b) Co esponding p o iles o bound magnesium (n bound (Mg 2+ ), ull lines) and hyd oxide ions (0.5·n bound (OH − ), dashed lines) as compa ed o he dosed amoun o MgCl 2 (do ed line) (see Fig. S1 in he ESI† o a plo o he e olu ion o he n bound (Mg 2+ )/n bound (OH − ) a io in he cou se o he i a ion expe imen ). c) F ee ion p oduc s calcula ed om he measu ed ee concen a ions o magnesium and hyd oxide ions. 7723 n bound (OH − ) owa ds he expec ed 1 : 2 mola a io (Fig. 4b). This ansi ion in ion binding s oichiome y appea s o be slowe a he onse o polyme -con olled nuclea ion, indica ing a ba ie o dissocia ion o [MgCl x ] (2−x) and/o [MgCl y OH z ] (2−y−z) o yield pu e hyd oxide species. Ano he key ea u e induced by added PAA is he ac ha he amoun o ee Mg 2+ (as well as he ee ion p oduc ) does no d op o a cons an le el immedia ely a e nuclea ion. Ins ead, a local minimum is obse ed, subsequen o which n ee (Mg 2+ ) e- inc eases and eaches a seconda y maximum (c . Fig. 4a). In he ollowing, ano he such maximum occu s un il he amoun o ee magnesium ul ima ely app oaches he inal le el measu ed in he e e ence expe imen . These wa e-like pos -nuclea ion pa e ns signi y p onounced g ow h inhibi ion, whe e poisoning by he polyme impedes inco po a ion o u he ions and s abilises he o med pa icles a small sizes, as also obse ed o calcium ca bona e c ys allisa ion in he p esence o PAA. 40,46,47 A e ini ial nuclea ion, con inued addi ion o MgCl 2 and NaOH hen soon leads o a e-inc ease in he amoun o ee ions, un il c i ical condi ions a e again es ablished and a second (o hi d) e en o nuclea ion akes place. The eby, he le el o supe sa u a ion equi ed o nuclea ion o occu dec eases om s ep o s ep, as polyme is p og essi ely consumed upon pa icle o ma ion ( educing he ne inhibi o y e ec ) and he e ogeneous nuclea ion on al eady exis ing b uci e s uc u es is likely associa ed wi h lowe ba ie s. F ee ion p oduc s de e mined a he end o his i e a i e p ocess a e e y close o hose ob ained immedia ely a e p ima y nuclea ion in he e e ence expe imen (c . Fig. 4c), sugges ing ha PAA does no a ec he na u e o he o med phase(s). Fig. 5 shows a selec ion o c yo-TEM mic og aphs aken om samples ha we e d awn a di e en s ages o he i a ion assay wi h added PAA, namely nea he i s /main maximum o n ee (Mg 2+ ) (Fig. 5a), a e he i s d op (Fig. 5b), nea he second maximum (Fig. 5c), and a e he second d op (Fig. 5d). This sequence o images e eals ha Mg(OH) 2 nuclea ion in he p esence o poly(ac ylic acid) a o ds e y ine pla ele s, which ini ially a e signi ican ly smalle (<100 nm) and mo e agg ega ed han in he e e ence expe imen (Fig. 5a), p obably due o pa icle c oss- linking by he o ganic polyme . Despi e hei de ined shapes, hese pla ele s do no exhibi cohe en sca e ing in ED pa e ns a he beginning. On he o he hand, he e was no e idence o he p esence o (di use) diso de ed s uc u es (like in Fig. 3d) o any polyme -induced s abilisa ion o amo phous p ecu so s –in con as o e ec s obse ed o PAA in CaCO 3 c ys allisa ion. 40 Wi h ime and con inued addi ion o MgCl 2 and NaOH (Fig. 5b–d), he pla ele s unde go a ipening p ocess and g ow in size (up o ca. 200 nm in diame e ), bu ne e each he dimensions o he indi idual hexagons in he e e ence expe imen (c . Fig. 3b). In pa allel, he o al numbe o pa icles as well as hei densi y in agg ega ed s uc u es inc eases, accompanied by he occu ence o single spo s and inally almos con inuous ings in co esponding di ac ion pa e ns, which can all be assigned o c ys alline b uci e. 20,52 C yo-TEM images Fig. 5 Cha ac e isa ion o pa icles o med du ing po en iome ic i a ion in he p esence o poly(ac ylic acid) by means o ansmission elec on mic oscopy in (a–e and g) i i ied s a e unde c yogenic condi ions and ( and h) d y s a e a ambien empe a u e. Samples we e d awn nea he ollowing cha ac e is ic poin s in he co esponding n ee (Mg 2+ ) p o ile (c . Fig. 4a, whe e he ime in e als o sampling a e indica ed by dashed lines and a ows): (1) i s /main maximum (a), (2) i s minimum (b, e and ), (3) second maximum (c), and (4) second minimum (d, g and h). The inse ed elec on di ac ion pa e ns iden i y he pa icles as being ei he amo phous (a) o c ys alline wi h single spo s (b and c), a cs ( ) o ull ings (d and h). All obse ed e lec ions can be assigned o c ys alline b uci e. 7724 acqui ed a lowe magni ica ion o samples d awn nea he i s (Fig. 5e) and second (Fig. 5g) minimum o n ee (Mg 2+ ) illus a e ha he abundance and size o he agg ega ed s uc u es inc eases wi h ime and e e y new nuclea ion e en , as expec ed. This no ion is u he co obo a ed by TEM analyses pe o med on he same samples a ambien empe a u e a e d ying (Fig. 5 and h), which show dense ne wo ks wi h almos powde -like di ac ion pa e ns. The ole o PAA in Mg(OH) 2 c ys allisa ion unde he p esen condi ions hus appea s o be h ee old: i s , he polyme inhibi s ini ial phase sepa a ion – o easons ha equi e u he s udies, al hough i may be specula ed ha he ansi ion o chlo ide-con aining magnesium complexes o pu e hyd oxide-based nuclea ion p ecu so s is hinde ed by he polyelec oly e. Second, PAA has a minia u ising e ec on he o med b uci e pa icles, a guably due o he g ow h- inhibi ing in luence obse ed in po en iome ic i a ions (c . Fig. 4) and he ac ha a he ime when he sys em becomes uns able agains phase sepa a ion, he nominal le el o supe sa u a ion is u e ly high and many nuclei will hus be o med in a bu s -like e en . I is also possible ha a he ime o nuclea ion, he polyca boxyla e ca ies la ge amoun s o magnesium ions (much like a “sponge”) and hus p o ides local domains en iched in ele an species, whe e nuclea ion will p e e en ially ake place –in analogy o he calcium sponges o sul a ed (mac o)molecules epo ed o be loca ed a ound he nuclea ion si es in he o ma ion o nac e o ma ion. 53 This migh explain he absence o mo e s able/ ex ensi e amo phous mine al s uc u es in he polyme - con aining sys em, as ipening owa ds mo e o de ed s a es should p oceed as e in a mic oen i onmen o locally enhanced supe sa u a ion. Thi d, he polyme igge s ample agg ega ion o b uci e pla ele s in o ex ended ne wo k-like s uc u es, p esumably ia adso p ion on Mg(OH) 2 su aces and b idging o mul iple pa icles (like a “glue”), which should e en ually yield a hyb id ma e ial o ino ganic magnesium hyd oxide mine al and o ganic polyme (no e ha a he ime o he i s maximum in Fig. 4a, he mola a io o Mg 2+ and monome ic AA uni s is abou 10 : 1, ende ing he ole o he polyme as “glue” easonable). Finally, p ecipi a es p esen a he end o he i a ion expe imen s shown in Fig. 1 ( e e ence) and Fig. 4 (PAA) we e isola ed by il a ion and analysed a e d ying wi h espec o s uc u e and composi ion using X- ay di ac ion and IR spec oscopy (Fig. 6). Powde XRD pa e ns con i m ha he o med ma e ials consis o c ys alline b uci e, 20,52 in line wi h obse a ions made by elec on di ac ion (c . Fig. 3 and 5), in bo h he p esence and absence o PAA (Fig. 6a). As all occu ing e lec ions can be assigned o b uci e, he e is no e idence o any o he phases in he da a, such as magnesium ca bona e o mixed hyd oxide–ca bona es (which may esul om in-di usion o a mosphe ic CO 2 ). Mo eo e , he peaks in he di ac og ams o he PAA-con aining ma e ial a e gene ally b oade han in he e e ence sample, which is consis en wi h he smalle p ima y pa icle sizes induced by he polyme as obse ed by TEM (c . Fig. 5). IR spec a o p ecipi a es isola ed om he e e ence expe imen wi hou addi i e (black cu e in Fig. 6b) a e domina ed by he sha p and in ense O–H s e ching ib a ion in c ys alline b uci e a 3698 cm −1 . 54 Alongside, a smalle band loca ed a ca. 1575 cm −1 is obse ed, which likely o igina es om bending ib a ions o associa ed wa e molecules. Fu he mo e, he spli peak a 1450/1410 cm −1 and he band a ca. 1040 cm −1 indica e he p esence o ce ain amoun s o amo phous ca bona e phases, 55 esul ing om appa en ly ine i able up ake o CO 2 by he alkaline mix u es despi e he used p o ec i e ni ogen a mosphe e. Fo p ecipi a es o med in he p esence o PAA ( ed cu e in Fig. 6b), IR spec oscopy p o ides clea e idence ha he polyme is associa ed o he ino ganic phase in ela i ely la ge amoun s: nex o he O–H s e ching ib a ion o b uci e, he spec um shows se e al in ense bands ha a e cha ac e is ic o he polyca boxyla e, such as he alipha ic s e ching modes o he ac ylic backbone be ween 3000 and 2800 cm −1 as well as mul iple o he peaks in he ange o 1800–600 cm −1 caused by C–H de o ma ion and di e en COOH- ela ed ib a ions. 56 This demons a es ha PAA does no only beha e as a po en modi ie o he p og ess o b uci e c ys allisa ion om solu ion, bu also becomes an essen ial pa o he o med solid ma e ial ia spon aneous co-p ecipi a ion –leading o an o ganic/ ino ganic composi e whose s uc u e and p ope ies wa an deepe in es iga ions in he u u e. Fig. 6 Cha ac e isa ion o solid p ecipi a es isola ed a he end o po en iome ic i a ion expe imen s wi h ( ed cu es) and wi hou (black cu es) added poly(ac ylic acid) by means o a) powde X- ay di ac ion and b) ATR-IR spec oscopy. 7725 Summa y & conclusions In his wo k, we ha e es ablished a new i a ion-based assay ha allows he ea ly s ages o magnesium hyd oxide c ys allisa ion om solu ion o be moni o ed in eal ime and a cons an pH. The me hod uses an ion-selec i e elec ode and a pH senso o ack he ee (and bound) amoun s o he ele an ions (Mg 2+ and OH − ) p io o, du ing and a e nuclea ion. Combined wi h powe ul ex si u cha ac e isa ion by means o (c yogenic) ansmission elec on mic oscopy, his app oach deli e s deep insigh s in o he species and p ocesses in ol ed in Mg(OH) 2 nuclea ion and g ow h bo h in he nea mine al sys em and unde he in luence o addi i es. Speci ically, po en iome ic i a ions pe o med in he absence o addi i es showed ha , while he gene al ea u es o he ob ained p o iles esemble hose epo ed o o he mine als like calcium ca bona e, ion binding s oichiome ies in he p e-nuclea ion egime a e complex in he Mg(OH) 2 sys em and sugges associa ion o Mg 2+ wi h bo h hyd oxide and chlo ide, leading o clus e s ha may bea ne cha ges. In u he analogy wi h o he mine als, a diso de ed (amo phous) phase is in ol ed in he nuclea ion o solid magnesium hyd oxide, which howe e exhibi s a highly ansien cha ac e and soon gi es way o hexagonal pla ele s ha hen g ow in size. Addi ion o poly(ac ylic acid) as a ypical an iscalan led o a d as ic delay o Mg(OH) 2 nuclea ion (as expec ed), bu did no a ec ion associa ion equilib ia in solu ion o any no iceable ex en . A e nuclea ion, he polyme showed s ong in e ac ions wi h he o med solid phase and inhibi ed g ow h, which esul ed in a downsizing o p ima y pa icles and yielded b uci e nanopla ele s ha we e in ima ely in e spe sed wi h co- p ecipi a ed o ganic polyme . This la e obse a ion appea s p omising in he con ex o he syn hesis o b uci e nanoma e ials wi h con olled sizes o a ious applica ions, 15–19 while he spon aneous hyb idisa ion o he ino ganic mine al wi h o ganic polyme may gi e ise o addi ional in e es ing p ope ies ha equi e u he in es iga ions. Apa om ha , he me hodology de eloped in he p esen s udy is now a ailable o u he in-dep h analyses o p e- and ea ly pos -nuclea ion phenomena in he Mg(OH) 2 sys em, which may be a ge ed a condi ions ha a e mo e ele an o b uci e scaling in indus ial se ings (e.g. highe empe a u es, di e en pH alues, e c.) 24–27 o used o sc een po en ial al e na i e (i.e. mo e e icien and/o sus ainable) chemis ies o an iscaling and o unde s and hei mode(s) o ac ion. Au ho con ibu ions Concep ualisa ion: AK, HC, DG & MK; in es iga ion: JS, JKB, MD, AESVD, HC, DG & MK; me hodology: JS, JKB, MD, DG & MK; o mal analysis: JS, JKB, MD, AESVD & MK; p ojec adminis a ion: AK, HC & MK; supe ision: HC, DG & MK; isualisa ion: JS, JKB, MD, AESVD & MK; w i ing (o iginal d a ): JS, JKB & MK; w i ing ( e iew & edi ing): JS, JKB, MD, AK, AESVD, HC, DG & MK. Con lic s o in e es The e a e no con lic s o decla e. Acknowledgemen s The au ho s hank D . 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