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 . Philipp Mülle (BASF SE) o help wi h
he in e p e a ion o some o he TEM da a. Pa s o his wo k
we e unded by BASF SE in he amewo k o a collabo a ion
wi h he Uni e si y o Kons anz.
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