High-Resolu ion Ul asonic Spec oscopy: Looking a he
In e polyelec oly e Neu aliza ion om a Di e en Pe spec i e
Tin Klacic,*Adam Jugl, Milosla Peka , and Da o Ko ace ic*
Ci e This: Mac omolecules 2023, 56, 1434−1445
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sı Suppo ing In o ma ion
ABSTRACT: In his s udy, he high- esolu ion ul asonic spec oscopy (HR-US)
echnique was applied o examine in e polyelec oly e neu aliza ion. The
men ioned me hod was es ed on he example o complexa ion be ween
poly(allylammonium) ca ions and poly(ac yla e) anions in aqueous solu ions a
pH = 7. I was con i med by HR-US ha he ype o i a ion (s epwise o ab up ),
he di ec ion o i a ion, and he ype o backg ound sal a ec he ou come o
in e polyelec oly e neu aliza ion. The ob ained esul s we e explained on he
basis o ul asonic eloci y and a enua ion changes in he con ex o suspension
comp essibili y, a pa ame e ha is ex emely sensi i e o molecula o ganiza ion and in e molecula in e ac ions. Mo eo e , he
esul s o HR-US measu emen s p o ed o be consis en wi h p e ious esul s ob ained by mo e adi ional me hods such as dynamic
ligh sca e ing, mic ocalo ime y, and elec okine ics. This esea ch demons a es ha HR-US is a con enien and eliable me hod
ha can be employed o he in es iga ion o in e polyelec oly e neu aliza ion and polyelec oly e- ela ed p ocesses.
1. INTRODUCTION
Polyelec oly es a e mac omolecules (e.g., syn he ic polyme s
o biopolyme s) wi h ionic o ionizable unc ional g oups ha
can ca y posi i e o nega i e cha ges in solu ion. Mixing o
opposi ely cha ged polyelec oly es in aqueous solu ion may
esul in polyelec oly e complex (PEC) o ma ion. The
occu ence o PECs o en goes along wi h phase sepa a ion,
esul ing in ei he solid−liquid o liquid−liquid phase
sepa a ion. In he o me case, dense polyelec oly e
p ecipi a es a e o med, while he la e case esul s in he
o ma ion o polyelec oly e complex coace a es wi h liquid-
like p ope ies.
1−4
As his sel -assembly p ocesses o many
polyelec oly e pai s a e gene ally isoen halpic, i is belie ed
ha PECs a e o med due o an inc ease in en opy caused by
he elease o s uc u ed wa e molecules and coun e ions
associa ed wi h polyme chains.
5−12
The mo phology,
s uc u e, composi ion, and physicochemical p ope ies o
o med PECs will depend on nume ous assembly pa ame e s
such as concen a ion and cha ge densi y o polyelec oly es,
13
empe a u e, pH, and ionic s eng h o he medium.
5,12,14−16
One in e es ing ac o ha also a ec s he s uc u e and
p ope ies o PECs is he ype o used backg ound sal . As has
been ecen ly shown by ou and Schleno g oup,
7−10
he
in luence o suppo ing anions on PEC composi ion and
ene ge ics o in e polyelec oly e neu aliza ion can be
easonably well co ela ed wi h he posi ion o he coun e -
anion in he Ho meis e se ies. The Ho meis e se ies was
disco e ed in p o ein p ecipi a ion expe imen s,
17,18
and his
se ies classi ies anions and ca ions acco ding o hei inc easing
p ecipi a ion powe as ollows:
< < < < < < <SCN ClO I NO B Cl F H PO
4 3 2 4
< < < < < <
+ + + + + + +
Li Na K Rb Cs NH (CH ) N
4 3 4
This lis o ions ank-o de ed in e ms o how s ongly hey
modula e p o ein solubili y p o ides a pa h o he
modi ica ion o PEC p ope ies simply by changing he ype
o sal du ing hei p epa a ion. Clea ly, PECs a e e sa ile
ma e ials ha could possibly be used in a wide ange o
applica ions, such as ma ices o enzyme immobiliza ion,
19
biocompa ible coa ings o d ug deli e y,
20
and ec o s o
gene he apy,
21,22
o name a ew.
The p ocess o in e polyelec oly e neu aliza ion is
commonly s udied by ligh sca e ing me hods, u bidime y,
conduc ome y, po en iome y, spec opho ome y, and calo-
ime y.
5−13,23
Each o hese expe imen al echniques gi es
aluable in o ma ion abou he examined sys em. Po en iom-
e y, o example, p o ides in o ma ion on he p opo ion o
pai ed monome s by moni o ing coun e ion ac i i y in
solu ion. Howe e , i is limi ed o complexa ion eac ions o
“pu e” polyelec oly es (wi hou excess coun e ions). Calo im-
e y p o ides aluable in o ma ion on he ene ge ics o
in e polyelec oly e neu aliza ion. Howe e , as men ioned
ea lie , in e polyelec oly e neu aliza ion p ocesses a e mos ly
isoen halpic,
5−12
which limi s he applica ion o calo ime y in
esea ch. In addi ion o he abo e, measu able changes in
Recei ed: No embe 17, 2022
Re ised: Janua y 31, 2023
Published: Feb ua y 14, 2023
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en halpy always indica e ha a chemical p ocess is aking place.
The only p oblem is o de e mine which p ocess i is. Me hods
ha moni o he sca e ing o ligh and elec opho e ic
mobili y o pa icles p o ide aluable in o ma ion on he size
and s uc u e o PECs. Howe e , hey a e limi ed o he mola
a ios o i an and i and be o e he onse o loccula ion.
UV/ is spec opho ome y is sui able o de e mining he
p opo ion o opposi ely cha ged monome s in he eac ion
p oduc s. Spec opho ome y can also be used o in es iga e
he con e sion o me as able p oduc s o equilib ia.
7
Howe e ,
o pe o m a spec opho ome ic expe imen , i is necessa y o
use spec opho ome ically ac i e polyelec oly es such as
poly(sodium 4-s y enesul ona e) (PSS) o a leas label he
polyelec oly es wi h some kind o a ma ke . F om all o he
men ioned, i can be concluded ha he p ocesses o
in e polyelec oly e neu aliza ion a e bes s udied by a
combina ion o complemen a y echniques, bu e en hen, as
shown by Philipp e al.,
13
di e en me hods o en indica e
di e en “ i a ion endpoin s” o he same pai o polyme s.
High- esolu ion ul asonic spec oscopy (HR-US) is a
ela i ely no el spec oscopic echnique o ma e ial analysis.
A de ailed desc ip ion o his me hod can be ound in ecen ly
published e iew pape s.
24,25
B ie ly, his echnique is based on
he measu emen s o eloci y and a enua ion (ene gy losses
ha include abso p ion and sca e ing con ibu ions) o
ul asonic comp ession wa es p opaga ing h ough he
analyzed sample. These ul asonic wa es p obe he elas ic
p ope ies o ma e ials de e mined by in e molecula in e -
ac ions and molecula o ganiza ion. Thus, HR-US o e s a
numbe o ad an ages o e adi ional me hods such as UV/ is
spec opho ome y. Fo example, i is a nondes uc i e
echnique ha does no equi e op ical ma ke s and can be
used on concen a ed and opaque samples. Ano he ad an age
is ha i is ela i ely easy o change he wa eleng h o he
ul asonic wa e. Unlike spec opho ome y whe e he elec o-
magne ic wa e o igina es in a ligh sou ce and he e o e needs
special e o o ob ain he equi ed spec al pu i y, ul asonic
wa es a e gene a ed elec onically. The e o e, a ypical
ul asonic spec ome e can co e a b oad ange o equencies
( om 1 o 20 MHz). In addi ion, HR-US measu emen s can
be pe o med wi h esolu ion down o 10−5% o ul asonic
eloci y in small sample olumes ( ypically 1 mL and down o
0.03 mL) a ambien o ele a ed p essu es o e a b oad
empe a u e ange (−40 o 130 °C) and in a ious measu ing
egimes such as au oma ic i a ions and measu emen s in low.
Also, HR-US measu emen s can be pe o med in di e en
media anging om dilu e solu ions o semi-solid ma e ials
such as waxes and gels. The e o e, HR-US is a e y e sa ile
me hod ha can be used o analyze a b oad ange o molecula
p ocesses. Cu en applica ions o his echnique include
analysis o agg ega ion, micelliza ion, and gela ion phenom-
ena,
26−30
phase diag ams o mic oemulsions,
31
con o ma ional
ansi ions in polyme s and biopolyme s,
32
in e ac ions
be ween polyelec oly es and su ac an s o amino acids,
33−35
s abili y o emulsions and suspensions,
36−38
polyme adso p-
ion on o su aces,
39
enzyma ic hyd olysis o p o eins and
ca bohyd a es,
25,40−43
and many o he s. Despi e being
employed o he cha ac e iza ion o nume ous chemical and
physical p ocesses, he e is a lack in he li e a u e ega ding he
po en ial use o HR-US o s udying he o ma ion o
polyelec oly e complexes.
The aim o his wo k is o use, o he i s ime, high-
esolu ion ul asound spec ome y in a i a ion egime o gain
a mo e de ailed insigh in o in e polyelec oly e neu aliza ion
o poly(allylamine hyd ochlo ide) (PAH) and poly(ac ylic
acid) (PAA) in di e en sal media, namely, NaF, NaCl,
NaClO4, LiCl, and (CH3)4NCl. To he bes o ou knowledge,
he e is no epo published ye on using his me hod o
in es iga e he o ma ion o polyelec oly e complexes. In ou
p e ious s udy,
9
complexa ion be ween he same pai o
mac oions was examined by means o elec okine ics, mic o-
calo ime y, and dynamic ligh sca e ing (DLS). I was ound
ha he p esence o di e en elec oly es a ec ed he PAH/
PAA in e polyelec oly e neu aliza ion conside ably, leading
o ion-speci ic agg ega ion o polyelec oly e pa icles. He e,
we expand he esea ch o HR-US measu emen s, which a e
discussed also wi h espec o he ea lie s udy.
9
2. EXPERIMENTAL SECTION
2.1. Ma e ials. Poly(allylamine hyd ochlo ide) (Mw≈17.5 kDa)
and poly(ac ylic acid) (Mw≈1000 kDa) we e pu chased om Sigma-
Ald ich. The monome unc ionaliza ion deg ees ( ), also known as
deg ees o subs i u ion,
44
we e de e mined by po en iome ic
i a ions wi h s anda dized NaOH (Me ck) and AgNO3(Kemika)
solu ions. The alues ob ained we e 0.88 ±0.01 o PAH and 0.97 ±
0.02 o PAA ( o mo e de ails abou he p ocedu e o de e mining ,
see he Suppo ing In o ma ion). All polyelec oly e solu ion
concen a ions we e co ec ed acco dingly o - alues and a e
exp essed wi h espec o he monome epea ing uni (deno ed cm
in he ex ). The polyelec oly e solu ions used o HR-US and
densi y in es iga ions we e p epa ed by dissol ing a ce ain amoun o
he polyelec oly e, 3-(N-mo pholino)p opanesul onic acid (MOPS)
bu e (Sigma-Ald ich), and a backg ound sal in deionized wa e .
Be o e dissol ing hem, PAH and PAA we e d ied a 60 °C, and sal s
we e d ied a 110 °C o abou 200 min. To ensu e he maximum
cha ge densi y o bo h mac oions,
45
he pH o polyelec oly e
solu ions was adjus ed o 7.0 ±0.1 wi h a 1.0 M NaOH solu ion
(Pen a). Fo ha pu pose, a pH me e (888 Ti ando, Me ohm)
equipped wi h a combined glass mic oelec ode (6.0262.100,
Me ohm), p ecalib a ed wi h s anda d bu e s (Hamil on) wi h pH
alues o 4.01, 7.00, and 9.21, was used. NaCl, NaClO4, NaF,
(CH3)4NCl, and LiCl sal s o analy ical pu i y we e pu chased om
Sigma-Ald ich and we e used wi hou u he pu i ica ion excep o
(CH3)4NCl. Te ame hylammonium chlo ide (he ea e e e ed o as
Me4NCl), a sal o a e ame hylammonium ca ion Me4N+and a
chlo ide anion, was pu i ied by ec ys alliza ion om a mix u e o
me hanol (Honeywell Riedel-de-Haen) and ace one (Honeywell
Riedel-de-Haen) as desc ibed in he li e a u e.
46
The e hanol used o
cleaning he ube o he densi ome e was pu chased om Pen a. The
deionized wa e used in all expe imen s was p epa ed in a PURELAB
Classic pu i ica ion sys em (ELGA) and had an ini ial conduc i i y
lowe han 0.056 μS/cm.
2.2. High-Resolu ion Ul asonic Spec oscopy Measu e-
men s. High- esolu ion ul asonic spec oscopy measu emen s we e
pe o med in a i a ion egime on an HR-US spec ome e (model
102T) equipped wi h a i a ion accesso y p oduced by Ul asonic
Scien i ic (I eland). All expe imen s we e ca ied ou a six selec ed
equencies (2.8, 5.3, 8.0, 11.7, 12.3, and 14.9 MHz) and 25.00 °C.
Tempe a u e con ol was achie ed by a Haake PC300/A28 wa e
ba h (The mo Scien i ic), which p o ided a empe a u e s abili y o
0.01 °C. Be o e he measu emen s, each solu ion was degassed by a
5452 miniSpin cen i uge (Eppendo ) o 5 min a 3500 pm. A e
ha , he e e ence cell was illed wi h 1 mL o deionized wa e and he
sample cell wi h 1 mL o polyca ion o polyanion solu ion. Then, he
sample cell was closed wi h a s oppe , which inco po a ed a line o
he injec ion o i an solu ion. The i a ion accesso y was equipped
wi h a 50 μL Hamil on sy inge and he i an solu ion was injec ed
in o he sample cell au oma ically e e y 13 min in s eps con olled by
Pump Moni o so wa e (Ul asonic Scien i ic). A e each injec ion,
he i and solu ion was igo ously s i ed o 3 min using a double
s i ing sys em ha p o ided e ec i e mixing. A e wa d, he
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polyelec oly e mix u e was no s i ed un il he nex addi ion o
i an . The di e ences in ul asound eloci y and a enua ion
be ween he cells we e moni o ed con inuously h oughou he
i a ion. Da a a e he e o e epo ed he e in e ms o eloci y
di e ence (Δu) and a enua ion di e ence (ΔN) be ween he sample
and e e ence cells.
=u u u(sample) (wa e )
(1)
=N N N(sample) (wa e )
(2)
The measu ed da a we e p ocessed using Ti a ion Analysis so wa e
( e sion 4.50.27.35, Ul asonic Scien i ic) in a way ha means alues
o ul asonic pa ame e s we e calcula ed based on da a measu ed in
he las 100 s be o e he nex i an addi ion. Each measu emen was
made a leas in iplica e, and in he ex and ables, a e age alues o
measu ed quan i ies wi h hei s anda d e o o he mean a e
epo ed. Howe e , o cla i ica ion, only a e age alues o ul asound
eloci y and a enua ion a e epo ed in he igu es. The highes
s anda d e o s o eloci y and a enua ion we e 0.08 m/s and 0.36
m−1, and he highes ela i e e o s we e 1.1% and 78%, espec i ely
( o examples o i a ion p o iles wi h s anda d e o s, see Figu e S5).
The e o e, ep oducibili y was much be e in he case o ul asound
eloci y han ha in a enua ion. Howe e , his did no a ec he
conclusions o his esea ch because a enua ion was used only as an
auxilia y quali a i e pa ame e and only he qui e ep oducible shapes
o he measu ed a enua ion cu es we e ele an . The ul asound
equency had no e ec on he measu ed da a (Figu e S6);
consequen ly, only esul s o 14.9 MHz a e epo ed he e. Also,
he e ec o dilu ion o he i and solu ion on measu ed ul asound
pa ame e s was negligible (Figu e S7).
I should also be men ioned ha isual obse a ion o he i a ed
sys ems was also pe o med ou side he HR-US spec ome e in a es
ube wi h a bo om s i e , using he same s eps o he i a ion
p ocedu e pe o med in he spec ome e o ob ain a be e insigh
in o he i a ion p ocess.
2.3. Densi y Measu emen s. The densi y o polyelec oly e
solu ions was measu ed a 25.000 °C using a densi ome e DSA 5000
M (An on Paa ). The densi ome e calib a ion was pe o med a
25.000 °C using ai as a s anda d. Be o e he measu emen s, he
samples we e degassed as desc ibed ea lie , and hen hey we e
ca e ully injec ed in o a U-shaped bo osilica e glass cell so ha no gas
bubbles we e p esen . To pe o m he measu emen s, he U-cell was
elec onically exci ed o ib a e a i s cha ac e is ic equency. A e
he measu emen s, he U-cell was ho oughly cleaned i s wi h 10 mL
o a 5.0 M NaClO4solu ion o emo e any esidues o adso bed
polyelec oly es and hen wi h a la ge amoun o deionized wa e and
e hanol. Densi y measu emen s we e made on h ee samples o he
same composi ion in duplica es, and he densi ies o polyelec oly e
solu ions p esen ed he e a e a e age alues o six measu emen s ±
s anda d e o s o he mean.
3. RESULTS AND DISCUSSION
3.1. Complexa ion o PAH and PAA wi hou he
Added Suppo ing Elec oly e: S epwise Ti a ion.
Ul asonic eloci y (u) is one o he main ou pu s o HR-US
measu emen . Fo liquids, his pa ame e s ongly depends on
he densi y (ρ) and adiaba ic comp essibili y (βS) o he
sample acco ding o he well-known New on−Laplace
equa ion.
=u
1
S
(3)
The comp essibili y is mainly de e mined by he molecula
o ganiza ion and in e molecula o ces in he medium.
The e o e, ul asonic measu emen s a e usually accompanied
by measu emen s o densi y o enable calcula ions o
comp essibili y om ul asonic eloci y and densi y o he
sample. He e, in Table 1, we p esen he esul s o densi y and
eloci y measu emen s ca ied ou wi h solu ions o poly-
(allylamine hyd ochlo ide) and poly(ac ylic acid) in he
p esence o MOPS bu e .
The densi y and ul asonic eloci y o bo h polyelec oly e
solu ions a e simila , which means ha he s udied sys ems
ha e much he same comp essibili y. These esul s sugges ha
he con o ma ion o PAH and PAA polyme chains in solu ion
is almos iden ical, as well as he s uc u e o he ionic
a mosphe e and he hyd a ion shell a ound cha ged monome
uni s. Howe e , i should be poin ed ou ha he bu e
concen a ion was 10 imes highe han ha o polyelec oly es
o main ain he pH o he solu ions cons an . As a pH = 7.0,
app oxima ely hal o MOPS molecules a e p esen in
zwi e ionic and ano he hal in he anionic o m,
48
i is o
be expec ed ha MOPS molecules a e associa ed wi h bo h
PAH and PAA cha ged segmen s. This means ha MOPS
molecules bound o polyelec oly e chains can e ec i ely
elimina e he elec os a ic epulsions be ween g oups o he
same cha ge so ha mac oions adop globula shape
con o ma ion. Fu he mo e, he comp essibili y o p epa ed
solu ions is p obably mos a ec ed by he s a e o ee bu e
molecules because MOPS is in a la ge su plus. As will be
e iden om he esul s o HR-US measu emen s made on
polyelec oly e solu ions in he p esence o di e en back-
g ound sal s, he comp essibili y o he solu ion is o en
de e mined by he mos abundan species.
To u he expand ou esea ch, he o ma ion o PEC was
moni o ed by he HR-US echnique. Fo his pu pose, he
in e polyelec oly e neu aliza ion o PAH and PAA in aqueous
MOPS bu e solu ions was ca ied ou a pH = 7.0. The
ul asonic eloci y and a enua ion o PAH/PAA complexes,
p epa ed by s epwise addi ions o PAH solu ion o PAA
solu ion and ice e sa, a e shown in Figu e 1.
As can be seen in Figu e 1, h ee egions can be iden i iable
o he i a ion p o iles measu ed in bo h di ec ions. In he
i s egion, eloci y and a enua ion di e ences linea ly
inc ease as he i an - o- i and mola a io enla ges. Visually,
he mac oion mix u e in his pa changed om clea o milky
clouded a e he i s i an addi ion, and hen he u bidi y
e ol ed wi h new i an addi ions (Figu e 2). In egion II,
which expands e y na owly a ound he equi alence poin , he
eloci y and a enua ion di e ences suddenly dec ease wi h he
addi ion o i an solu ion. In his pa , he o ma ion o laky
whi e p ecipi a es can be obse ed, which sedimen o e ime
(Figu e 2). Chollakup and co-wo ke s
49,50
also epo ed ha
polyelec oly e complexa ion be ween PAH and PAA in he
absence o sal esul s in a p ecipi a e (no he coace a e),
independen o he mixing a io, pH (7.0 and 8.6), and he
Table 1. Densi y, Ul asonic Veloci y, and Comp essibili y
o 0.005 M Polyelec oly e Solu ions a pH = 7.0 (0.05 M
MOPS Bu e ) and 25.0 °C
a
PAH PAA
ρ(kg/m3) 1001.07 ±0.02 1001.22 ±0.06
u(m/s) 1502.77 ±0.06 1502.61 ±0.11
βS(10−13 Pa−1) 4423.3 ±0.3 4423.8 ±0.9
a
The ul asonic eloci ies we e calcula ed om he measu ed
di e ence in eloci y be ween he sample and e e ence cells a 14.9
MHz using he eloci y o pu e wa e a 25.0 °C (u= 1496.687 m/s)
om he li e a u e.
47
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molecula weigh o he employed PAA. Finally, he hi d
egion o i a ion p o iles is cha ac e ized by a einc ease in
ul asound eloci y and a enua ion upon he addi ion o
i an solu ion. This pa is somewha simila o he i s
egion o he i a ion p o iles, bu he slope o eloci y and
a enua ion dependence on he polyme mola a io is sligh ly
smalle han ha in he i s egion. Visually, his s age o he
i a ion p o ile co esponds o a clea solu ion wi h locs ha
ha e been sedimen ed (Figu e 2).
Ob ained eloci y i a ion p o ile (Figu e 1a) can be
explained easonably well wi hin he amewo k o he
New on−Laplace ela ionship (eq 3) by conside ing he
sequence o e en s ha occu upon s epwise addi ion o one
polyelec oly e in o he solu ion o he o he , as p oposed by
Fuoss and Sadek.
51
Fo simplici y, his sequence o e en s is
illus a i ely shown o he case o in e polyelec oly e
neu aliza ion o PAH wi h PAA in Figu e 2.
When he i s d op o PAA is added o PAH solu ion, he
polyelec oly es in e ac , o ming a s able colloidal dispe sion.
The esul ing polyme agg ega es, deno ed he e as p ima y
complexes (o en called nons oichiome ic
52,53
o quasi-
soluble
54
complexes), a e compac clus e s c oss-linked by
elec os a ic in e ac ions. Since PAH is ini ially in excess, he
in e io o hese p ima y complexes is mos ly composed o
PAA molecules, while he ex e io con ains posi i ely cha ged
PAH chains. O e all, as epo ed ea lie ,
9
PAH/PAA p ima y
complexes a e posi i ely cha ged, and hei hyd odynamic
diame e is app oxima ely cons an o e a wide ange o
polyion mola a ios ( < 0.8). The e o e, each subsequen
addi ion o i an p ima ily causes he o ma ion o new
elec ically cha ged p ima y complexes. Consequen ly, in his
pa o he i a ion p o ile ( egion I in Figu e 1a), he
ul asonic eloci y, as well as u bidi y (Figu e 2), o he colloid
solu ion inc eases. As men ioned be o e, ul asonic eloci y is
de e mined by he comp essibili y and he densi y o he
medium. Gene ally, he comp essibili y esponse, which is
ex emely sensi i e o molecula o ganiza ion and in e mo-
lecula in e ac ions, is domina ing.
24
When a polyelec oly e
molecule encoun e s a mac oion o opposi e cha ge, he s ong
a ac i e ield be ween hem causes elec os a ic binding o
hei monome segmen s, and many o he coun e ions a e
displaced. A ound displaced ions, new hyd a ion shells
composed o less comp essible wa e molecules a e o med,
and hus, he comp essibili y o he sys em dec eases.
Figu e 1. Veloci y (a) and a enua ion (b) di e ence ob ained by
s epwise i a ions o PAH solu ion wi h PAA solu ion (blue squa es)
and ice e sa ( ed ci cles) in MOPS bu e (c= 0.05 M) a pH = 7.0,
25.0 °C, and 14.9 MHz equency. The i an - o- i and mola a io
o epea ing uni s was calcula ed on he basis o he added olume o
i an solu ion (cm= 0.05 M) o i and solu ion (cm= 0.005 M, V0=
1.0 mL). Dashed lines ha e no physical meaning and we e added as
guides o he eye.
Figu e 2. Illus a ion o he sequence o e en s ha occu upon s epwise addi ion o PAA solu ion in o PAH solu ion accompanied wi h images o
he mix u e a di e en PAA- o-PAH mola a ios deno ed .
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The e o e, he linea inc ease in ul asonic eloci y in egion I
o he i a ion p o ile (Figu e 1a) is an ou come o he
o ma ion o compac and s i PAH/PAA p ima y complexes,
he elease o wa e and associa ed coun e ions (in ou case
MOPS ions) om cha ged polyelec oly e monome s, and he
o ma ion o new hyd a ion shells a ound PECs and coun e -
ions.
As he a io o i an o i and monome s u he inc eases
and app oaches equimola alue, he p ima y complex co ona
cha ge is compensa ed by i an molecules. Consequen ly,
opposi ely cha ged seconda y complexes bea ing he excess o
PAA monome s a he su ace a e o med. Thei in e ac ions
wi h he p ima y complexes lead o mac oscopic loccula ion
and o e a sho pe iod o sedimen a ion o locs (Figu e 2).
The loccula ion is accompanied by he educ ion o o al solid
su ace a ea and by he expulsion o s i hyd a ion wa e om
he su ace o PECs, which inc eases he comp essibili y and
dec eases he ul asonic eloci y o he polyelec oly e mix u e.
This sha p dec ease in he eloci y is isible in Figu e 1a
( egion II), ega dless o he polyelec oly e addi ion o de .
Howe e , i is in e es ing o no e ha he s eep eloci y and
a enua ion dec ease in egion II occu s a di e en mola
a ios depending on he di ec ion o i a ion. In he case o he
PAH o PAA i a ion di ec ion, i appea s a a mola a io o
0.80, and in he case o he opposi e di ec ion, i appea s a a
highe mola a io o 0.95. Mo eo e , his s ep dec ease is
la ge o he case o PAA addi ion o PAH solu ion han ha
o PAH addi ion o PAA solu ion. The obse ed dependency
o he loccula ion poin on he di ec ion o i a ion is in good
ag eemen wi h he esul s o DLS measu emen s epo ed
ea lie ,
9
and i was also ound o PECs p epa ed by o he
polyelec oly es.
55−57
Mainly, his mixing o de e ec can be
explained by misma ches in kine ically con olled monome
pai ing. A e all, nons oichiome ic complexes, especially hose
ha depend on he o de o mixing, a e a signa u e o kine ic
con ol in polyelec oly e associa ion.
8,58,59
O he easons o obse ed i a ion endpoin disc epancy
could be associa ed wi h he deg ee o ioniza ion, deg ee o
unc ionaliza ion, and molecula weigh o polyelec oly es
used in his esea ch (Table 2). Acco ding o he li e a u e,
45
unde condi ions o he pe o med expe imen s (pH = 7.0),
PAH has a highe deg ee o ioniza ion han PAA. I means ha
ewe polyca ion molecules a e needed o o e cha ge p ima y
complexes ha con ain an excess o he polyanion, and hus,
phase sepa a ion akes place a lowe mola a ios. I also
means ha he mola a io p esen ed in Figu e 1 is no equal o
he cha ge a io o polyelec oly es. Howe e , Li e al.
15
ha e
ecen ly demons a ed ha polyelec oly e pai ing in PAH/
PAA complexes is no only achie ed by elec os a ic
in e ac ions bu also by he hyd ophobic in e ac ions o he
alipha ic polyme backbone and he in e polyme hyd ogen
bonding o nonionized ac ylic monome uni s. The o me wo
in e ac ions a e p onounced especially a acidic pH. The e o e,
aking hyd ogen bonding and hyd ophobic in e ac ions in o
accoun makes i di icul o ely only on he cha ge a io o
exp essing he i a ion endpoin .
Fu he mo e, he eal monome a ios can also be di e en
om hose gi en in Figu e 1 due o expe imen al e o s in he
s anda diza ion o polyelec oly es. On he o he hand, e en i
he unc ionaliza ion deg ees o bo h polyme s a e co ec ly
de e mined, he un unc ionalized monome s a e andomly
dis ibu ed among he PAH and PAA chains. This a ec s he
alue o he co ec ed mola a io a which p ecipi a ion
occu s, especially in he case o low unc ionaliza ion deg ees.
Howe e , he unc ionaliza ion deg ee o he polyca ion and
polyanion used in he he ein in es iga ed case is la ge enough
(Table 2) ha i s de ia ion om uni y p obably does no a ec
he pai ing o cha ged g oups. One should also keep in mind
ha he e e sed mixing o de canno be expec ed o yield
PECs o he same s uc u e, since PAH used in he ame o
ou in es iga ion has a much smalle molecula mass (sho e
chains) han PAA, which mus induce somewha di e en
oppo uni ies o assembly du ing he PEC o ma ion p ocess.
The ecen in es iga ion o PAH/PAA in e polyelec oly e
neu aliza ion in solu ion conduc ed by Ga dlund and co-
wo ke s
60
suppo s his s a emen .
Re u ning o he esul s in Figu e 1a, one can see ha a e
he locs a e o med, ul asonic eloci y inc eases upon u he
addi ion o i an solu ion ( egion III o i a ion p o ile).
Al hough subsequen inco po a ion o i an chains in o he
locs may con ibu e o he solu ion comp essibili y and hus o
he eloci y, we belie e ha eloci y inc eases wi h new i an
addi ions due o he inc easing numbe o ee i an
molecules wi h hyd a ion shells composed o less comp essible
wa e molecules. In suppo o his conclusion, we s a e he
ollowing: Fi s , he slope o he linea dependence o
ul asonic eloci y on he mola a io a e he loccula ion
poin ( egion III) was no he same as be o e loccula ion
( egion I) when he i an molecules we e inco po a ed in o
he polyelec oly e complex. Fu he mo e, in he ea lie s udy,
9
he co esponding eac ion was in es iga ed calo ime ically,
and no measu able hea e ec s abo e equi alence we e
obse ed. This inding indica es ha i an molecules do no
pa icipa e in u he build-up o PECs, as he esul s o
calo ime ic measu emen s we e co ec ed o he hea s o
i an dilu ion.
In he las ew pa ag aphs, i was demons a ed how
measu ing ul asonic eloci y du ing he cou se o polyelec-
oly e complexa ion by he HR-US spec ome e can p o ide
a be e insigh in o his p ocess based on he comp essibili y
changes o he sys em. Like ul asonic eloci y, measu ing
a enua ion du ing in e polyelec oly e neu aliza ion can be o
g ea impo ance o a be e unde s anding o PEC o ma ion.
Ul asonic a enua ion ep esen s he ene gy losses in
comp essions and decomp essions o he medium in ul asonic
wa es, and i is mainly a ec ed by he he e ogenei y o he
medium (in his case, his means he o ma ion o colloidal
pa icles in a con inuous liquid medium).
25
In a ew sen ences
Table 2. Deg ee o Ioniza ion (α), Deg ee o
Func ionaliza ion ( ), Weigh -A e age Molecula Weigh
(Mw), Deg ee o Polyme iza ion (DP), and Polydispe si y
Index (PDI) o Polyelec oly es Used in This Resea ch
a
PAH PAA
α(%) ≈85 ≈65
(%) 88 ±1 97 ±2
Mw(kDa) ≈17.5 ≈1000
DP 187 13 877
PDI 4.3
a
Values o αco espond o pH = 7.0 and we e aken om da a
published by Choi and Rubne .
45
Deg ees o unc ionaliza ion we e
de e mined by po en iome ic i a ions as explained in Sec ion 2.
Molecula weigh s o polyme s, deg ees o polyme iza ion, and
polydispe si y indexes (excep o PAH) we e p o ided by he
supplie .
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ha ollow, we will explain he esul s o a enua ion
measu emen s ob ained by he HR-US de ice, which a e
shown in Figu e 1b. The inc eased a enua ion in egion I o
he a enua ion i a ion p o ile e lec s he o ma ion o new
he e ogenei ies, i.e., s able nons oichiome ic PAH/PAA
nanoassemblies. Then, close o he equi alence poin ,
a enua ion dec eases s eeply ( egion II). The s ep dec ease
co esponds o he coalescence o p ima y and seconda y
complexes ha sedimen in he absence o s i ing. As a esul ,
he mix u e clea s up ( egion II in Figu e 2), and he e a e only
se e al unse led pa icles on he way o ul asonic wa e
p opaga ion ha accoun o he ene gy losses. This beha io is
consis en wi h he esul s o u bidi y measu emen s ca ied
ou by Chen e al.
56
They no ed a d op in u bidi y nea he
equi alence poin du ing i a ion o poly-
(diallyldime hylammonium chlo ide) (PDADMAC) solu ion
wi h po assium sal o poly( inylsul onic acid) (PVSK) and
explained i by agg ega ion and sedimen a ion o polyme
pa icles. A e he sha p dec ease in a enua ion in he icini y
o he equi alence poin ( egion II), he a enua ion again
mode a ely inc eases wi h new addi ions o he i an . An
inc ease in a enua ion in egion III indica es inc easing
he e ogenei y o he sys em, as he concen a ion o ee i an
molecules ha sca e he ul asonic wa e inc eases.
3.2. Complexa ion o PAH and PAA wi hou he
Added Suppo ing Elec oly e: Ab up Ti a ion. A he
beginning o he s epwise i a ion, he monome s o i and
a e in conside able excess compa ed o he added monome s
o i an , i.e., he p ima y complexes con ain excess monome s
o i and. As he i a ion p og esses, he ee chains o i and
un ou , which leads o he o ma ion o seconda y complexes
ha ins an ly loccula e wi h he p ima y ones. I he cha ge
in e sion is linked exclusi ely o he excess o one o he
polyelec oly es, only seconda y complexes should o m om
he p ima y complexes wi hou mac oscopic phase sepa a ion
when a la ge excess o i an molecules is suddenly added o
he co esponding suspension. To es his hypo hesis by HR-
US, posi i e PAH/PAA p ima y complexes we e p epa ed in
MOPS bu e (pH = 7.0) by s epwise i an o i and
addi ions up o a 0.6 monome mola a io. This p ocedu e
was ollowed by he addi ion o he equi alen amoun o
i an o achie e a mola a io o 1.6. The esul s o he ab up
i an addi ion in excess ollowing he s epwise addi ion
p ocedu e a e depic ed in Figu e 3.
As can be seen in Figu e 3, he ou come o he ab up ype
o PAA o PAH i a ion was di e en om he ou come o he
s epwise i a ion expe imen . Al hough he ul asonic eloci y
and a enua ion inc eased equally up o a mola a io o 0.6 in
bo h ypes o i a ion due o he p ima y complexes o ma ion,
no d op in ul asonic pa ame e s nea equi alence was
obse ed in he case o ab up i a ion. Ins ead, a e he
addi ion o an equi alen amoun o PAA in o a suspension o
p ima y PECs a a mola a io o 0.6, he ul asound eloci y
and a enua ion inc eased almos linea ly compa ed o he
alues be o e his addi ion. Visual inspec ion o PEC
suspension a a mola a io o 1.6 e ealed no mac oscopic
phase sepa a ion, jus a milky clouded appea ance (inse in
Figu e 3a).
Judging by he ob ained esul s, i can be s a ed ha he
ab up i an addi ion in excess led o he p ima y PEC
o e cha ging and hus o he o ma ion o PAH/PAA
seconda y complexes. Nega i ely cha ged seconda y complexes
did no ans o m in o spa sely soluble p ecipi a es due o
elec os a ic s abiliza ion h ough epulsions be ween pa icles.
The e o e, a e he ab up i an addi ion, he ul asonic
eloci y inc eased due o he coexis ence o igid seconda y
complexes and ee PAA molecules in s able colloidal
suspension, and he a enua ion inc eases as a consequence
o he inc ease in he e ogenei y. In addi ion o he PAH/PAA
pai , he o e cha ging o p ima y PECs achie ed by ab up
i an addi ion in excess was p e iously obse ed by DLS and
elec opho e ic me hods in he case o PDADMAC-PSS and
PAH-PSS pai s.
9
I , he e o e, seems ha he phenomenon o
cha ge in e sion o p ima y polyelec oly e complexes is
uni e sal, jus like he su ace cha ge in e sion ha occu s
du ing he deposi ion o each polyelec oly e laye in he
p ocess o p epa ing polyelec oly e mul ilaye s (PEMs).
61−64
The desc ibed esul s a e he addi ional s ong e idence o he
al eady obse ed
65
co ela ion be ween he p ocess o
in e polyelec oly e complexa ion in solu ion and he o -
ma ion o PEMs on he su ace.
3.3. Complexa ion o PAH and PAA in Va ious
Suppo ing Elec oly es. The complexa ion o PAH and
PAA was also s udied by HR-US in NaF, NaCl, NaClO4, LiCl,
and Me4NCl aqueous solu ions. The selec ion o hese sal s
was based on ema kable anion- and ca ion-speci ic e ec s
obse ed ea lie .
9
As in sal - ee expe imen s, i a ions we e
Figu e 3. Veloci y (a) and a enua ion (b) di e ence ob ained by
ab up i a ion o PAH solu ion (cm= 0.005 M, V0= 1.0 mL) wi h
PAA solu ion (cm= 0.05 M) in MOPS bu e (c= 0.05 M) a pH =
7.0, 25.0 °C, and 14.9 MHz equency ( ed ci cles). The esul s o
co esponding s epwise i a ion a e gi en o compa ison (blue
squa es). The PAA- o-PAH mola a io o epea ing uni s was
calcula ed on he basis o added olume o PAA solu ion o PAH
solu ion. Dashed lines ha e no physical meaning and we e added as
guides o he eye. Image in (a) shows he s able colloidal solu ion o
PAH/PAA seconda y complexes ob ained a a mola a io o 1.6.
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ca ied ou in MOPS bu e solu ions a pH = 7.0 wi h i and
and i an monome concen a ions o 0.005 and 0.05 M,
espec i ely. To ensu e comple e subs i u ion o PAH and PAA
coun e ions, which coun e balanced he mac oion cha ge in
co esponding bu e solu ions wi h he ions o he in oduced
suppo ing elec oly e, a conside able excess o sal wi h
espec o he bu e was added o polyelec oly e solu ions.
Solu ions o bo h polyions in all cases con ained equal
concen a ions o he backg ound sal (c= 0.1 M). The sal
concen a ion was also kep high, since he in luence o
coun e ion ype on he cou se o in e polyelec oly e
neu aliza ion can be obse ed solely in concen a ed sal
solu ions.
10
Be o e p esen ing he esul s o i a ion expe imen s, he
adiaba ic comp essibili y o p epa ed PAH and PAA i and
solu ions in di e en suppo ing elec oly es will be conside ed
(Figu e 4). As be o e, co esponding comp essibili y alues
we e calcula ed om eq 3 based on he measu ed ul asonic
eloci y and densi y da a (see Table S1).
Se e al impo an conclusions can be d awn om he
ob ained esul s. Fi s , he comp essibili y o polyelec oly e
solu ions in he p esence o all in es iga ed backg ound sal s is
lowe han he comp essibili y o he same solu ions wi hou
sal s (Figu e 4). This means ha he p esence o ions in
polyelec oly e solu ions causes changes in molecula o gan-
iza ion and in e molecula o ces in he solu ion. These
s uc u al changes a e p ima ily associa ed wi h he ac ion o
ions’ elec ic ield on wa e and changes in hyd ogen bonding
among wa e molecules.
66−68
Mo eo e , he obse ed
comp essibili y alues a e p ac ically he same o PAH and
PAA solu ions p epa ed in he same sal . This ac indica es
ha he ions o he suppo ing elec oly e ha e he g ea es
in luence on he comp essibili y o solu ions. Tha is no so
unexpec ed conside ing ha he e is a 20 imes highe
concen a ion o sal in solu ions han he concen a ion o
polyme monome uni s. The e o e, he con ibu ion o he
dissol ed mac omolecules o he comp essibili y can be
neglec ed in u he conside a ion. Also, he ype o back-
g ound sal a ec s he comp essibili y o polyelec oly e
solu ions. Conside ing he sodium sal s used, he in luence o
anions on he comp essibili y o solu ions inc eases in he
o de o F−< Cl−< ClO4
−. This ank o de o ions is in
acco dance wi h he posi ion o hese ions in he Ho meis e
se ies.
17,18
The in luence o ca ions on he comp essibili y o
solu ions also ollows he posi ion o he ions in he
Ho meis e se ies, so he comp essibili y inc eases in he
sequence Me4N+< Na+< Li+.
The adiaba ic comp essibili y o he elec oly e solu ion can
be use ully in e p e ed in e ms o ions’ con ibu ions o he
s uc u e o he wa e ne wo k. In his manne , ion−wa e and
wa e −wa e in e ac ions should be dis inguished.
66−68
Cha ged ions in luence adjacen wa e molecule dipoles by
causing s ong elec os a ic con ac ion. This phenomenon,
usually called elec os ic ion,
69
causes a pa ial loss in he
mobili y o wa e molecules loca ed nea ions. As a esul ,
hyd a ion shells a ound ions a e o med. Gene ally, hyd a ion
wa e is less comp essible han bulk wa e . The e o e, solu ions
o ions wi h less hyd a ion wa e , i.e., hinne hyd a ion shells,
will ha e highe alues o comp essibili y. Howe e , one should
no neglec he e ec o hyd ogen bond eo ganiza ion induced
by he p esence o ions on he o e all comp essibili y o he
solu ion. Acco ding o Ma cus,
70,71
he ex en o hyd ogen
bonding in wa e can be sa is ac o ily desc ibed by a nume ical
alue o ΔGHB. Tha physical pa ame e ep esen s he a io o
s anda d mola Gibbs ee ene gy o ans e o he solu e om
ligh (H2O) o hea y (D2O) wa e and he mola di e ence in
he hyd ogen bonding ene gies o ligh and hea y wa e . Ions
wi h posi i e ΔGHB alues a e known as wa e s uc u e
make s, and ions wi h nega i e ΔGHB alues a e known as
wa e s uc u e b eake s. In p inciple, solu ions ha con ain
la ge ions, which ha e a hin hyd a ion shell and nega i e
ΔGHB alues, will be mo e comp essible han solu ions ha
con ain small ions, which ha e a hick hyd a ion shell and
posi i e ΔGHB alues. In Table 3,ΔGHB alues o ions p esen
in PAH and PAA solu ions and he hickness o hei hyd a ion
shells a e lis ed.
By inspec ion o Table 3, one can see ha he ela i e
ela ionship o obse ed anions comp essibili ies (F−< Cl−<
ClO4
−) is in acco dance wi h he a ionale explained abo e.
Howe e , ha is no he case o ca ions. Solu ions o well-
hyd a ed Li+ion ha e he highes comp essibili y alues o all
examined sal s (Figu e 4), bu hey should ha e he lowes
comp essibili y acco ding o Ma cus o malism.
70−72
One
easonable explana ion o his unexpec ed esul (also obse ed
by o he s
73,74
) could be he o ma ion o clus e s in LiCl
solu ion. As Thomas and Elcock
75
no iced in molecula
dynamics (MD) simula ions, Li+and Cl−ions o m ionic linea
clus e s (s ings). The o ma ion o hese clus e s is
accompanied by he expulsion o s i hyd a ion wa e
molecules o he bulk space. Consequen ly, solu ion’s
comp essibili y should inc ease. In e es ingly, he same au ho s
ound ha he p esence o bulky Me4N+ca ions in solu ion
inc eases he a e age numbe o hyd ogen bonds pe wa e
Figu e 4. In luence o di e en backg ound sal s (c= 0.1 M) on
adiaba ic comp essibili y o poly(allylamine hyd ochlo ide) and
poly(ac ylic acid) solu ions (cm= 0.005 M) a pH = 7.0 (0.05 M
MOPS bu e ) and 25.0 °C.
Table 3. Thickness o Hyd a ion Shells (Δ ) and ΔGHB
Values o Selec ed Ions
a
ion Δ (pm) ΔGHB
F−79 0.08
Cl−43 −0.61
ClO4
−19 −1.01
Li+172 0.28
Na+116 −0.03
Me4N+14 −0.47
a
Ion pa ame e s we e aken om da a published by Ma cus.
70,72
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molecule compa ed o he a e age numbe o hyd ogen bonds
in pu e wa e . This s uc u e-making beha io o Me4N+ions is
no in acco dance wi h he Ma cus classi ica ion o ions
70,71
and could be an explana ion o he obse ed lowe
comp essibili ies o Me4NCl solu ions han expec ed. To
uly demons a e ha he comp essibili y o LiCl and Me4NCl
solu ions is he excep ion a he han he ule, i would be
necessa y o de e mine βS alues o a se ies o solu ions
con aining di e en ca ions. Un o una ely, his is ou o he
scope o his pape because he aim was o explo e how
di e en backg ound sal s a ec he o ma ion o PAH/PAA
complexes by HR-US.
In Figu e 5 one can see eloci y p o iles ob ained by
i a ions o PAH solu ion wi h PAA and ice e sa in he
p esence o examined sal s. Fo he sake o easy isual
compa ison, he cu es ha e been shi ed e ically along he y-
axis by adding a cons an so ha all cu es s a a he same
poin . As can be seen o all sal s, he ul asonic eloci y
changes wi h he mola a io o polyelec oly es in he same
ashion as in sal - ee expe imen s. In he beginning, eloci y
inc eases linea ly wi h addi ions o i an , hen i suddenly
dec eases a ound he equi alence poin , and inally again
inc eases. The simila i y be ween i a ion p o iles ob ained in
expe imen s pe o med wi h and wi hou suppo ing elec o-
ly es indica es ha o med PECs a e mo e likely p esen in he
o m o a p ecipi a e han in he o m o a coace a e (p obably
due o he low concen a ion o sal in solu ions). This
conclusion is well suppo ed by he li e a u e
49,50
and by
addi ional op ical mic oscopy in es iga ion. Fo example, in
he op ical mic og aphs, we ob ained, o s oichiome ic PECs
p epa ed in NaCl solu ion, a dis inguishable o m o p ecipi a e
(Figu e S8).
Al hough he cu es p esen ed in Figu e 5 look e y simila ,
he e a e some impo an di e ences be ween hem. Fo
example, in some cases, he sha p d op in ul asonic eloci y
occu s a lowe mola a ios as compa ed o he “sal - ee”
cu e. In he uppe panel o Figu e 5, he anion in luence on
he cou se o in e polyelec oly e neu aliza ion is depic ed. In
he case o PAA o PAH i a ion, a sudden dec ease in he
ul asonic eloci y o posi i ely cha ged PAH/PAA complexes
a a mola a io o 0.6 (NaClO4) and a mola a io o 0.9
(NaCl and NaF) can be obse ed. Howe e , his kind o anion-
speci ic e ec is no obse ed in he case o i a ion o
nega i ely p ima y complexes (PAH o PAA i a ion).
The obse ed e ec o anions on he o ma ion o he PECs
can be unde s ood as ollows. No mally, a cha ge o
polyelec oly e epea ing uni s is balanced ei he by he
complemen a y polyelec oly e (in insic cha ge compensa-
ion) o by coun e ions (ex insic cha ge compensa ion).
76
The inc ease in he ionic s eng h o polyelec oly e solu ions
esul s in a mo e p onounced coun e ion-polyelec oly e ype
o binding.
10
The coun e ion-polyelec oly e binding cons an
is he one ha exhibi s ion speci ici y, i.e., polyelec oly es and
di e en sal ions a e expec ed o in e ac wi h di e en
s eng hs. Recen ly, i was ound ha PAH p e e s he binding
o weakly hyd a ed oxyanions such as pe chlo a es o e
s ongly hyd a ed halide anions (e.g., Cl−and F−).
9
The e o e,
i is o be expec ed ha posi i ely cha ged PAH/PAA p ima y
complexes p epa ed in he p esence o NaClO4will ha e a
highe con en o compensa ed cha ges in he co ona han he
same PECs p epa ed in he p esence o NaCl o NaF. This
Figu e 5. In luence o di e en backg ound anions (a, b) and ca ions (c, d) o 0.1 M concen a ion on he eloci y di e ence ob ained by s epwise
i a ions o PAH solu ion wi h PAA solu ion (a, c) and ice e sa (b, d) in MOPS bu e (c= 0.05 M) a pH = 7.0, 25.0 °C, and 14.9 MHz
equency. The i an - o- i and mola a io o epea ing uni s was calcula ed on he basis o added olume o i an solu ion (cm= 0.05 M) o
i and solu ion (cm= 0.005 M, V0= 1.0 mL). Dashed lines ha e no physical meaning and we e added as guides o he eye. The esul s o
co esponding sal - ee expe imen s a e gi en o compa ison (do ed line).
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h ps://doi.o g/10.1021/acs.mac omol.2c02349
Mac omolecules 2023, 56, 1434−1445
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leads o he seconda y agg ega ion and inally o he
loccula ion o posi i e PEC pa icles a lowe mola a ios.
On he con a y, when nega i e PEC pa icles a e p epa ed by
i a ion o PAA wi h PAH, he e is no such e ec because he
su ace cha ge o his ype o complex is no a ec ed by he
anions. Ins ead, he excess o PAA monome s a hei su ace is
sc eened by Na+ions ha a e common o all in es iga ed
solu ions.
The he ein-desc ibed HR-US esul s con i m he p e iously
epo ed anion-speci ic agg ega ion o PAH/PAA complexes
obse ed by DLS measu emen s.
9
In ha s udy, agg ega ion o
posi i e complexes was de ec ed by a la ge inc ease in he
pa icle size a mola a ios o 0.6 and 0.8 in he p esence o
NaClO4and NaCl, espec i ely (Figu e 6a in e 9). Mo eo e ,
he anion-speci ic agg ega ion o PAH/PAA complexes was
u he explo ed by i a ion o posi i e p ima y complexes
wi h simple elec oly e solu ions. Compa ed o NaCl, he
obse ed alue o c i ical elec oly e concen a ions needed o
he onse o posi i e PAH/PAA complex agg ega ion was
signi ican ly lowe when he suspension was i a ed wi h
NaClO4. The analogous beha io was no iced in he case o
posi i e PAH/PSS complexes.
7
Undoub edly, bo h PAH/PAA
and PAH/PSS complexes bea an excess o polyca ion
monome s a hei su ace, and ClO4
−ions mo e e ec i ely
sc een his cha ge han Cl−ions, which esul s in lowe alues
o loccula ion endpoin s.
Opposi e o he in luence o anions, he e ec o
coun e ca ion ype on he PAH/PAA in e polyelec oly e
neu aliza ion is no so p onounced, i.e., he obse ed esul s
o all in es iga ed suppo ing elec oly es a e ela i ely simila .
As can be seen in Figu e 5c,d, he sudden dec ease in
ul asonic eloci y o measu emen s pe o med in NaCl, LiCl,
and Me4NCl occu s a a simila mola a io as in he case o
sal - ee expe imen s, i espec i e o i a ion di ec ion. The
obse ed loccula ion a a ela i ely high i an - o- i and a io
(≈0.8) indica es ha monome pai ing in hese suppo ing
elec oly es p oceeds as e icien ly as in MOPS bu e solu ions.
Howe e , by close examina ion o he p esen ed da a, one can
see ha he e is a li le shi in he loccula ion onse owa d
lowe monome mola a ios in he case when he polyca ion
was added o he polyanion solu ion in he p esence o LiCl
and Me4NCl. This inding can be in e p e ed as elec oly e-
induced nega i e complex agg ega ion. Namely, Li+and
Me4N+ions mo e e icien ly sc een he excess o nega i e
cha ge a he PEC su ace han Na+ions and hus induce
agg ega ion o pa icles.
To summa ize, PAH/PAA in e polyelec oly e neu aliza-
ion in sal solu ions seems o esul in bo h posi i e and
nega i e p ima y complex agg ega ion due o he masking
e ec o ions. This e ec leads o asymme ic in e polyelec-
oly e neu aliza ion, esul ing in he excess o i and
monome s in he ob ained me as able p ecipi a es. PAH
seems o bind he examined ions mo e speci ically han PAA.
The eason o he obse ed di e ences in polyca ion and
polyanion ion-binding abili ies is s ill no ully unde s ood.
Howe e , he ac ha anions al e mo e s ongly he
o ma ion o bo h PAH/PAA and PAH/PSS complexes han
ca ions sugges s ha ion-speci ic e ec s a e mainly ela ed o
he ype o ion and no o such an ex en o he ype o
polyelec oly e. This s a emen is well suppo ed by a p e ious
s udy on PDADMAC/PSS mul ilaye s.
77
While anion e ec s
o his sys em become impo an abo e an ionic s eng h o
0.1 M, he ca ion e ec s become impo an a a much highe
ionic s eng h o 0.25 M. Ob iously, he g ea e speci ici y o
polyca ions owa d anions han ha o polyanions owa d
ca ions can he e o e be expec ed i espec i e o he polyion
ype.
4. CONCLUSIONS
To ob ain a be e insigh in o in e polyelec oly e neu aliza-
ion p ocesses be ween poly(allylammonium) ca ions and
poly(ac yla e) anions, a ela i ely no el HR-US echnique has
been applied. The ob ained esul s we e compa ed wi h he
p e ious s udy whe e mo e adi ional me hods (such as DLS
and calo ime y) we e used o examine PEC o ma ion.
9
The
esul s o HR-US measu emen s ha e con i med ha PAH/
PAA in e polyelec oly e neu aliza ion in he low sal egime
unde goes solid−liquid phase sepa a ion and p oceeds as
desc ibed in he pionee ing wo k o Fuoss and Sadek.
51
Fi s ,
posi i ely o nega i ely cha ged p ima y complexes a e o med
wi h he sign o su ace cha ge ha depends on he i a ion
di ec ion. Then, i he i a ion is pe o med s epwise,
opposi ely cha ged seconda y complexes a e gene a ed, and
hey eac wi h he p ima y complexes. This leads o
mac oscopic loccula ion ha is accompanied by he expulsion
o s i hyd a ion wa e om he su ace o PECs. Howe e , i
he i an is added o he suspension o p ima y complexes in
g ea excess, loccula ion will no occu , bu a he p ima y
PECs will o e cha ge. In addi ion, he esul s o HR-US
measu emen s made on PAH/PAA polyelec oly e complexes
in he di e en sal media ha e emphasized he impo an ole
o ions in PEC o ma ion. Coun e ions p esen in he solu ion
a e elec os a ically a ac ed by he cha ged polyelec oly e
uni s. Consequen ly, his cha ge sc eening causes asymme ic
polyme neu aliza ion (one o he polyelec oly es in he
o med complexes is p esen in conside able excess) and
agg ega ion o p ima y PECs.
7,9,12,16,78
In e es ingly, he
polyion mola a io a which loccula ion occu s ma kedly
depends on he ype o suppo ing sal , and ha e ec is mo e
p onounced in he case o anions.
In he end, i can be es ablished ha he esul s ob ained
he e and in he p e ious s udy
9
a e in good ag eemen and
mos ly in line wi h expec ed alues. Mo eo e , HR-US
complemen s he me hods no mally used o s udy PECs
wi h addi ional in o ma ion. Fo ins ance, HR-US is no
limi ed o he polyelec oly e mola a ios be o e he onse o
loccula ion like DLS so we can make u he conclusions
abou he p ocess beyond he loccula ion poin . Also, many
in e polyelec oly e neu aliza ions a e isoen halpic
5−12
o do
no include UV− is ac i e polyme s (like in his esea ch),
which limi s he applica ion o calo ime y and spec opho-
ome y. In such cases, HR-US can be used as an app op ia e
al e na i e. The e o e, i can be concluded ha HR-US is a
con enien and eliable new me hod ha can be used o
moni o ing in e polyelec oly e neu aliza ion. O e all, he
capabili ies o his echnique could make i a aluable ool
o u he in es iga ions o p ocesses ela ed o polyelec oly e
complexes such as p ecipi a e/coace a e/solu ion phase
ansi ions,
1,49,50
in e polyelec oly e exchanges,
79
o glass
ansi ions.
80,81
■ASSOCIATED CONTENT
*
sı Suppo ing In o ma ion
The Suppo ing In o ma ion is a ailable ee o cha ge a
h ps://pubs.acs.o g/doi/10.1021/acs.mac omol.2c02349.
Mac omolecules pubs.acs.o g/Mac omolecules A icle
h ps://doi.o g/10.1021/acs.mac omol.2c02349
Mac omolecules 2023, 56, 1434−1445
1442