Jou nal o Molecula Liquids 405 (2024) 124909
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Jou nal o Molecula Liquids
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On he molecula mechanisms o H2/N2up ake in confined ionic liquids: A
compu a ional s udy
Ma ín O e o-Lemaa,b,1, Raúl Lois-Cunsa,b,1, Pablo Ma ínez-C espoa,b,
Alejand o Ri e a-Pousaa,b, Had ián Mon es-Camposa,b,c,∗, T inidad Méndez-Mo alesa,b,∗,
Luis M. Va elaa,b
aG upo de Nanoma e iais, Fo ónica e Ma e ia B anda, Depa amen o de Física de Pa ículas, Uni e sidade de San iago de Compos ela, Campus Vida s/n, San iago de
Compos ela, E-15782, Galicia, Spain
bIns i u o de Ma e iais (iMATUS), Uni e sidade de San iago de Compos ela, A enida do Mes e Ma eo 25, San iago de Compos ela, E-15782, Galicia, Spain
cCIQUP, Ins i u e o Molecula Sciences (IMS)-Depa amen o de Química e Bioquímica, Faculdade de Ciências da Uni e sidade do Po o, Rua Campo Aleg e, Po o,
4169-007, Po ugal
A R T I C L E I N F O A B S T R A C T
Da ase link: h ps://
doi .o g /10 .5281 /zenodo .10495511
Keywo ds:
Ca bon nano ubes
Ionic liquids
Molecula modelling
Gas up ake
Hyd ogen
Ni ogen
Classical molecula dynamics and hyb id g and canonical Mon e Ca lo/molecula dynamics simula ions we e
combined o analyze he gas up ake mechanism o hyd ogen and ni ogen molecules inside ca bon nano ubes
filled wi h an ionic liquid. Se e al nano ube diame e s ( om 6 Å o 12.24 Å) and wo diffe en ionic liquids
(e hylammonium ni a e and 1-e hyl-3-me hylimidazolium e afluo obo a e) we e conside ed o s udy hei
effec on he gas cap u e capaci y and on he loca ion o gas molecules wi hin he nano ubes. The simula ions
showed ha ni ogen abso p ion abili y is, in gene al, g ea e han ha o hyd ogen, wi h he ap o ic ionic
liquid being mo e efficien o gas confinemen . In addi ion, gas cap u e was obse ed o inc ease om a sca ce
0.4% in bulk ionic liquids up o 8%-25% inside small nano ubes, and he maximum gas up ake was obse ed
o hose nano ubes ha allow o a g ea e deg ee o con o ma ional eedom o he ionic liquid. Howe e , ou
calcula ions show ha , whe eas hyd ogen s o age is mainly go e ned by he amoun o accessible ee olume,
o unde s anding ha o ni ogen sol a ion ene ge ics mus be also conside ed. In all cases, gas molecules a e
abso bed in he ionic liquid- ich- egion, bu he in e ac ions wi h he o he componen s o he sys em a ou
hei accommoda ion close o he ca bon wall han o he nano ube cen e. Finally, single-pa icle dynamics o
gas molecules was analyzed by means o he eloci y au oco ela ion unc ions and he ib a ional densi y o
s a es, which show a blue-shi ing when inc easing he adius o he nano ube.
1. In oduc ion
Wi h he g ow h o wo ld popula ion and he imp o emen o he
li ing s anda ds, he demand o ene gy is also inc easing and leading
o an u gen need o changing ou ene gy p oduc ion, dis ibu ion and
consump ion pa e ns o achie e g ea e sus ainabili y. In his con ex o
ene gy ansi ion, se e al s a egies ha e been ollowed o de elop e-
newable ene gy echnologies and achie e deep deca boniza ion a la ge
scale and in he sho es ime ame possible. Fo example, ca bon cap-
u e and s o age is expec ed o be he key o mi iga ing ca bon dioxide
(CO2) emissions and e e ing he end o he global clima e empe a-
u e ise [1]. G een hyd ogen (H2) has also been iden ified as a po en ial
* Co esponding au ho s.
E-mail add esses: [email p o ec ed] (H. Mon es-Campos),
[email p o ec ed] (T. Méndez-Mo ales).
1Con ibu ed equally o his wo k.
game change and he in e es in mechanisms ha enable i s s o age,
anspo a ion and use is eaching unp eceden ed le els. No in ain, i
can be used in uel cell elec ic ehicles, manu ac u ing p ocesses, elec-
ici y g id s abiliza ion and g een ammonia (NH3) p oduc ion [2]. The
la e is a much mo e p ac ical al e na i e o ossil uels, as i can be
p oduced and s o ed mo e easily han g een hyd ogen, bu i also cos s
mo e [3].
In any case, he global u gency o he implemen a ion o ossil uel
al e na i es has made he s o age o gases such as CO2, H2and ni-
ogen (N2) o be a majo challenge o ma e ials science. O e he las
decades, solid po ous ma e ials such as zeoli es [4,5], ca bon-based ma-
e ials [6], me al-o ganic amewo ks (MOFs) [7,8]o co alen o ganic
h ps://doi.o g/10.1016/j.molliq.2024.124909
Recei ed 2 Feb ua y 2024; Recei ed in e ised o m 19 Ap il 2024; Accep ed 2 May 2024
Jou nal o Molecula Liquids 405 (2024) 124909
2
M. O e o-Lema, R. Lois-Cuns, P. Ma ínez-C espo e al.
amewo ks (COFs) [9]ha e been epo ed as p omising candida es o
gas cap u e and s o age, mainly due o hei la ge accessible su ace
a eas and unc ionalizable po e en i onmen s. O he echnique ha
can also se e as a new co ne s one o suppo gas apping and ac-
cumula ion is he use o ionic liquids (ILs) [10–14], which a e “g een”
sol en s comple ely composed o ions wi h mel ing poin below 100
◦C
[15]. These compounds ha e ecei ed in ense a en ion due o hei
unique cha ac e is ics, which include non ola ily and nonflammabili y,
high he mal and chemical s abili y, biodeg adabili y and, mos impo -
an , he possibili y o sys ema ically une hei physical and chemical
p ope ies wi h se e al combina ions o ca ions and anions [16–19].
Ne e heless, he wo majo d awbacks o hei comme cial applica ion
a e hei high iscosi y, which leads o low adso p ion and deso p ion
a es, and ele a ed p oduc ion cos . The sea ch o new app oaches o
use ILs in a diffe en way and o e come hese issues has p oposed hei
inco po a ion in o po ous ma e ials, hus leading o he so-called sup-
po ed ionic liquids (SILs) [20]. This s a egy has po en ial ad an ages
such as enhanced mechanical s eng h, dec eased iscosi y and he use
o a lowe amoun o IL. In addi ion, i has been shown ha his IL
nanoconfinemen in o he po es o a solid suppo also inc eases gas
abso p ion. Fo example, Ha manli e al. [21]obse ed expe imen ally
a gian N2up ake, 10 imes highe han in nea IL, in an IL confined
in he po es o ca bon ma e ials. Many esea che s ha e in es iga ed
se e al op ions o be used as ideal encapsulan ma e ials o SILs, such
as memb anes [22–27], MOFs [28–35], zeoli e imidazola e amewo ks
(ZIFs) [36–39], ca bon-based ma e ials [21,40–43]and mesopo ous sil-
ica gels [44], h ough expe imen al and compu a ional means. In his
con ex , Wang e al. [23] in es iga ed CO2solubili y in ILs confined in
hyd ophobic and hyd ophilic memb anes, showing ha i was a li le
highe in he las case and also highe han ha in he bulk IL phase.
Also expe imen ally, Ib ahim and cowo ke s [29] highligh ed he im-
po ance o he choice o he IL ions, since CO2cap u e pe o mance
o 1-bu yl-3-me hylimidazolium ace a e suppo ed on MOFs exhibi ed
almos double capaci y han ha o he p is ine MOF, whe eas 1-p opyl-
3-me hylimidazolium bis( ifluo ome hylsul onyl)imide did no show
any up ake enhancemen .
All he a o emen ioned s udies ha e shown ha SILs can be ega ded
as po en ial candida es o gas cap u e and s o age. Howe e , mos o
hem ha e ocused on CO2, and weakly in e ac ing gases like H2o
N2 emained nea ly unno iced, despi e se e al wo ks ha ing iden i-
fied ILs as a ac i e elec oly es o he elec oca aly ic educ ion o
N2 o NH3[21,45,46]. Among he e y ew wo ks epo ed up o now,
Shi and Luebke [47]employed Mon e Ca lo (MC) and molecula dy-
namics (MD) simula ions o compa e he abso p ion and diffusi i y o
CO2, N2and H2in 1-n-hexyl-3-me hylimidazolium bis( ifluo ome hyl-
sul onyl)amide confined in silica sli po es, ob aining la ge alues han
in he bulk IL. Also by means o a omis ic simula ions, Shi and So escu
[48] analyzed he effec o confining diffe en amoun s in he same IL
inside ca bon nano ubes (CNTs) wi h a adius o 6.102 Å and 13.56 Å
upon CO2and H2so p ion. Fo bo h gases, he p is ine CNT and he
bulk IL exhibi ed he la ges and he smalles amoun o so p ion, e-
spec i ely. These s udies a e impo an o unde s and H2and N2up ake
in SILs, bu ele an aspec s such as he influence o po e size in a e y
s ong confinemen en i onmen and i s ela ion wi h he composi ion
o he IL a e ye o be add essed.
In an effo o shed some ligh on he cap u e and s o age o non-
pola gases wi h small quad upole momen s in confined ILs, we ha e
pe o med MC and MD simula ions o H2and N2abso p ion in wo di -
e en ILs confined in CNTs. The pe o mance o he CNTs con aining
ILs was compa ed wi h ha o he bulk ILs. Mo eo e , o analyze he
ole played by he IL, we examined a p o ic IL (PIL), e hylammonium
ni a e (EAN), and an ap o ic one (AIL), 1-e hyl-3-me hylimidazolium
e afluo obo a e ([EMIM][BF4]) (Fig. 1). Since he in e ac ions in he
ion pai s o PILs we e epo ed o be s onge han hose o AILs [49],
i is o undamen al impo ance o unde s and he impac ha hese in-
e ac ions can ha e on gas up ake. On he o he hand, CNTs wi h adii
Fig. 1. Molecula s uc u es o he ILs conside ed in his wo k: (a) 1-e hyl-3-
me hyl-imidazolium, (b) e afluo obo a e, (c) e hylammonium and (d) ni a e.
G ey, blue, whi e, g een, iole and ed co espond o ca bon, ni ogen, hyd o-
gen, fluo ine, bo on and oxygen.
Table 1
Co espondence be ween chi al indexes and CNT adii.
Chi al index
(12,0) (13,0) (14,0) (15,0) (16,0) (17,0) (18,0) (20,0)
Radius (Å)
3.00 3.39 3.78 4.18 4.56 4.96 5.35 6.12
be ween 3.00 Å and 6.12 Å we e chosen as a simple ca bon-based po e
model ha can cope wi h he effec o he po e wid h a ia ion on he
s uc u al ea u es o he ILs and he esul ing gas up ake. The la ges
nano ube employed in ou wo k co esponds wi h he smalles one an-
alyzed by Shi and So escu [48], which will allow us o analyze he
pe o mance o ILs in ul ahigh confinemen o gas s o age.
Thus, he aim o his wo k is o guide he design o p omising adso -
ben s o gas s o age analyzing he mos sui able po e size o a gi en
IL. Fo ha pu pose, fi s , we show he s o age capaci y o each o
he conside ed ILs unde confinemen and in he bulk phase. Then, he
s uc u al and ene ge ic cha ac e iza ion o he ILs confined in he ca -
bon s uc u e wi h diffe en po e sizes was ca ied ou o unde s and
gas up ake om an a omis ic poin o iew.
2. Simula ion de ails
CNTs we e gene a ed using Visual Molecula Dynamics [50], and
hey we e kep as igid s uc u es du ing he simula ions since he in-
fluence o he amewo k flexibili y on i s adso p ion p ope ies was
shown o be qui e small [51]. Simila ly, he helici y o he CNTs was
obse ed o ha e li le effec on he s uc u e o o he sol en s confined
inside [52], and on he adso p ion o CO2[53]. Thus, we employed
zigzag CNTs wi h chi al indexes anging om 12 o 20, whose co e-
spondence wi h he CNT in e nal adius (measu ed om he edge o he
ca bon a oms o he cen e o he nano ube) is shown in Table 1.
Fi s ly, he CNTs we e filled wi h he IL using he me hodology
o a nano-sy inge desc ibed in de ail in Re . 54. Once he CNTs we e
filled wi h he IL, he numbe o ionic pai s inside he ca bon s uc u e
anged om 100 o 115 depending on he CNT conside ed, and hei
leng hs anged om 25 Å o 127 Å. Then, hyb id g and canonical Mon e
Ca lo/molecula dynamics (GCMC/MD) simula ions we e ca ied ou o
e alua e he s o age capaci y o he ILs unde confinemen .
The OPLS-AA o ce field [55]was used o simula e he ILs, he
CNT and hei in e ac ion wi h he abso bed gas molecules. The ca -
bon a oms o he CNTs we e ep esen ed using he a om opls_147.
The pa ame e iza ion o [EMIM][BF4] was al eady used in Re . 54,
whe eas he one used o EAN was epo ed in Re . 56. Conce ning he
Jou nal o Molecula Liquids 405 (2024) 124909
3
M. O e o-Lema, R. Lois-Cuns, P. Ma ínez-C espo e al.
gas molecules, 3-si es models we e chosen in o de o ake in o accoun
hei quad upole momen . Ni ogen was modelled as wo cha ged eal
a oms ha ha e masses o 14.0067 u, cha ges o −0.482𝑒and Lenna d-
Jones pa ame e s o 𝜎=3.31 ⋅10−1 nm and 𝜀 =2.99156 ⋅10−1 kJ∕mol.
To main ain cha ge neu ali y, a poin cha ge o +0.964𝑒was placed
a he cen e o mass o he molecule [57]. On he o he hand, hyd o-
gen was pa ame e ized as wo cha ged eal a oms ha ha e masses o
1.008 uand cha ges o +0.4932𝑒, cons ained by a igid bond and a
cha ged i ual Lenna d-Jones si e whose pa ame e s a e 𝑞=−0.9864𝑒,
𝜎=2.958 ⋅10−1 nm and 𝜀 =3.051336 ⋅10−1 kJ∕mol [58].
2.1. Hyb id g and canonical Mon e Ca lo/molecula dynamics simula ions
Hyb id GCMC/MD simula ions o H2and N2up ake we e ca ied
ou using he LAMMPS package [59]in o de o es he loading capac-
i y o he confined ILs. These simula ions we e pe o med in he 𝜇𝑉 𝑇
ensemble a a empe a u e o 300 K, which was con olled by means
o he Nosé-Hoo e he mos a [60]wi h a elaxa ion ime o 10 s.
All he bonds in ol ing hyd ogen a oms we e kep cons ained using
he SHAKE algo i hm [61]. The gas ese oi p essu e, which is ela ed
o he chemical po en ial o he abso ba e, was fixed o 100 ba . The
MD simula ions we e ca ied ou o 500 ps wi h a ime s ep o 1 s.
The ial mo es in he GCMC p ocedu e we e inse ions and dele ions
(wi h equal p obabili y), and hey we e a emp ed an a e age o 100
imes e e y 10 s. The cen e o mass o he gas molecule is placed a
an a bi a y inse ion poin inside he CNT and i s o ien a ion is chosen
a andom by o a ing a ound his poin . The same p ocedu e was ol-
lowed wi h bulk ILs o he pu pose o compa ison, he only diffe ence
being ha hese bulk simula ions we e pe o med in he 𝜇𝑝𝑇 ensemble
(a 300 Kand 100 ba , as o ensu e an equilib ium wi h he ese oi ).
Then, a ame wi h he same gas occupancy as he a e age was se-
lec ed as he inpu configu a ion o con en ional MD simula ions o
each confined IL. Those CNTs in which he a e age gas up ake was
lowe han 5 gas molecules we e eplica ed 2-3 imes in o de o im-
p o e he s a is ics in he MD simula ions.
2.2. Molecula dynamics simula ions
Once he s o age capaci y o he ILs confined in each o he CNTs
was de e mined, MD simula ions we e pe o med in he NVT ensem-
ble using he GROMACS 2020.6 so wa e [62–64]. Fi s ly, all sys ems
we e elaxed using a conjuga e g adien s algo i hm. The maximum s ep
size and he ole ance we e se o 0.01 nm and 0.1 kJ∕(nm ⋅mol), e-
spec i ely. Second, a s abiliza ion un was done a 300 K o 5 ns. Nex ,
a p oduc ion un o da a collec ion was done o 20 ns a he same
empe a u e, and he ajec o y was eco ded wi h a ime in e al o
1 ps. Finally, a un o 500 ps in which he eloci ies we e sa ed a each
s ep was ca ied ou o he calcula ions o he Veloci y Au oco ela ion
Func ions (VACFs). The V- escale he mos a in oduced by Bussi e al.
[65]was employed o con ol he empe a u e wi h a damping cons an
o 0.1 ps. Pe iodic bounda y condi ions we e employed in he h ee di-
ec ions, using a box la ge enough as o ensu e ha diffe en eplicas o
he CNTs do no in e ac be ween hem. All bonds con aining hyd ogen
a oms we e limi ed by he LINCS algo i hm [66,67], wi h a ou h-o de
expansion o he cons ain coupling ma ix. Fo all he simula ions, he
equa ions o mo ion we e in eg a ed using a ime s ep o 1 s. The Ve le
cu off scheme wi h a buffe ole ance o 0.005 kJ∕mol ps was used o
long- ange in e ac ions in eal space, whe eas he Pa icle Mesh Ewald
(PME) [68]me hod wi h a g id spacing o 0.12 nm and cubic in e pola-
ion was used o deal wi h long- ange elec os a ic in e ac ions.
3. Resul s
3.1. Gas up ake
Fi s ly, we employed hyb id GCMC/MD calcula ions o s udy he N2
and H2cap u e abili y o ILs confined in ca bon-based s uc u es, wi h
Fig. 2. N2(g een) and H2(blue) up ake capaci y wi h confined [EMIM][BF4]
( op) and EAN (bo om) as a unc ion o he CNTs in e nal adius and hei
chi al index. Gas s o age capabili ies o bo h ILs in bulk phase a e included o
he pu pose o compa ison. E o ba s ep esen he s anda d de ia ion o he
a e age numbe o gas molecules adso bed.
he aim o es ing i a s ong confinemen egime is a sui able s a egy
o gas s o age. The s o age capaci y ob ained in he p esen GCMC/MD
simula ions is defined as he a e age numbe o gas molecules abso bed
(once he a io o inse ion/dele ion ials is s abilized) pe IL pai in-
side each CNT, and i is plo ed o bo h ILs in Fig. 2as a unc ion o
he in e nal adius and chi al index o he CNT excep o he EAN+H2
sys em, since o all CNTs no gas molecules we e ound o be abso bed
a e he Mon e Ca lo inse ions. Gas s o age capabili ies o bo h ILs
in bulk phase a e included o compa ison. I can be obse ed ha in
all cases he s o ed amoun o gas unde confinemen ends o ha in
he bulk IL as he CNT diame e inc eases. Simila ly, Shi and Luebke
[47] epo ed a dec easing CO2and H2solubili y in an IL confined
in a silica sli po e when dec easing he deg ee o confinemen . How-
e e , hey did no analyze he impac o po e wid h a ia ion on N2
abso p ion. On he o he hand, Shi and So escu [48]obse ed ha he
amoun o H2abso bed in an IL confined in a CNT(20,20), which has a
adius o 13.56 Å, was abou 1.1-1.4 imes la ge han ha in he bulk
IL a mode a e empe a u es. In ou case, gas s o age capaci y eaches
a maximum a he na ow adius o 3.78 Å and 3.39 Å o [EMIM][BF4]
and EAN, espec i ely. Gas cap u e is obse ed o inc ease om nea ly
0.4% up o 8%-25% when going om no confinemen o hose small
po e sizes, hus demons a ing ha confined ILs a e mo e beneficial o
cap u ing gases han bulk ILs. Fo bo h ILs, a second abso p ion peak is
ob ained o bigge CNTs, in pa icula , o hose wi h in e nal adius
o 4.56 Å and 4.96 Å o [EMIM][BF4]and EAN, espec i ely. On he
o he hand, N2abso p ion abili y is gene ally la ge han ha o H2,
p obably due o he in e ac ion be ween he highly pola ILs and he
la ge quad upole momen o ni ogen molecules. In addi ion, confined
[EMIM][BF4]shows o be mo e efficien o gas up ake han EAN.
The diffe en s o age capaci ies can be analyzed in e ms o he
emp y space a ailable wi hin he CNTs o dissol e he gas molecules.
This ee olume is s ongly dependen on he in e play be ween ionic
in e ac ions and po e size, as hey go e n he s uc u al con o ma ion
o he IL inside he CNTs. Thus, we ha e calcula ed he pe cen age o
accessible ee olume by inse ing ial molecules in andomly selec ed
posi ions and o ien a ions wi hin he CNTs. The ial molecules consis
o wo ha d sphe es sepa a ed by a igid bond, mimicking he s uc u e
o he gas molecules inse ed in he 𝜇VT simula ions. Thus, he inse ed
sphe es ha e adii o 1.2and 1.55 Å o H2and N2, espec i ely [69],
whe eas he sepa a ion be ween hem was se o 0.7414 and 1.0977
Jou nal o Molecula Liquids 405 (2024) 124909
4
M. O e o-Lema, R. Lois-Cuns, P. Ma ínez-C espo e al.
Fig. 3. Pe cen age o accessible ee olume o H2in [EMIM][BF4]( op), N2
in [EMIM][BF4](cen e) and H2in EAN (bo om). The colou s indica e he sys-
ems in which he inse ions we e pe o med. E o ba s ep esen he s anda d
de ia ion o he accessible ee olume.
Å, ollowing he MD pa ame e s o molecula hyd ogen and ni ogen,
espec i ely. The es ima ion was done o pu e IL confined inside he
CNTs, and also o he IL loaded wi h gas bu wi hou aking in o ac-
coun he olume occupied by he gas molecules. The pe cen age o ee
olume was defined as he a io be ween he numbe o accep ed inse -
ions and he numbe o ials, and i was compu ed ying 5 ⋅106 imes
pe ame. The esul s a e included in Fig. 3as a unc ion o CNT in e -
nal adius and he co esponding chi al index. I can be seen ha hose
CNTs o which we obse ed he maximum gas cap u e only co espond
wi h hose wi h he maximum amoun o ee olume o he sys em
whe e H2is abso bed. Fo N2, he e is no appa en ela ion be ween
a ailable ee olume and gas cap u e. This is qui e unexpec ed, since
he ex a ee olume in he es ic ed space o he CNTs due o some
us a ed a angemen o he ions compa ed o bulk IL has been iden-
ified as he basis o he ema kable enhancemen o gas up ake [21].
Howe e , ou esul s indica e ha he abso p ion o ni ogen molecules
is no only go e ned by he amoun o emp y accessible space wi hin
he sys em, bu also by he in e ac ions be ween he diffe en species,
unlike H2molecules. Hyd ogen is a molecule wi h a e y small size and
low in e molecula in e ac ions, due o which i always beha es nea ly
as an ideal gas. In ac , only dipole-quad upole in e ac ions, which a e
sligh ly s onge o ni ogen molecules, a e significan in hese sys ems.
In o de o gain g ea e molecula unde s anding o he nanocon-
finemen effec on he gas abso p ion mechanism, he obse ed s o age
capaci ies we e e alua ed bo h om he s uc u al and he ene ge ic
poin o iew. As i is well-known, he confinemen o ILs in nanoscale
geome ies leads o a diffe en physical and chemical beha iou , such
as changes in he we ing beha iou o he eezing-mel ing empe a-
u e [70]. Also, he way ILs eo ganize when encapsula ed inside CNTs
was p e iously epo ed by Mon es-Campos and cowo ke s [54], show-
ing ha i is es ic ed by he ca bon su ace. Consequen ly, hei s udy
showed ha , when he CNT wid h eaches he cha ac e is ic ion pai
size, he s uc u al con o ma ion o he ions changes om a highly o -
de ed one dimensional a ay o a mo e diso de ed s uc u e in which
he ions a e a anged o ming ings in he adial di ec ion.
Thus, in he p esen wo k he effec o CNT channel size was in es-
iga ed by calcula ing he numbe o IL laye s o med inside each o he
CNTs. The numbe o laye s is calcula ed by andomly inse ing planes
pe pendicula o he ubula axis in he simula ion box. Then, o each
molecule, he plane ha in e sec s he highes numbe o molecules is
selec ed (see Fig. S1 o he Suppo ing In o ma ion). The amoun o
ionic species cu h ough by his plane is he numbe o laye s associ-
a ed o ha molecule, and he global numbe o laye s is ob ained as
an a e age o e all molecules and ames o he simula ion. Planes ha
in e sec no molecules a e dis ega ded. The numbe o laye s as a unc-
ion o he CNTs in e nal adius and he co esponding chi al index a e
shown in Fig. 4, in which he colou g adien iden ifies he diffe en
numbe o laye s wi hin he CNTs.
Focusing on [EMIM][BF4], i can be obse ed ha , as he CNT diam-
e e inc eases, he accommoda ion o he ions changes om one laye
[in CNT(12,0), CNT(13,0) and CNT(14,0)] o wo laye s [in CNT(15,0)
and CNT(16,0)], and hen mo e complex s uc u es a e adop ed. In e -
es ingly, he wo p e iously men ioned peaks o he gas up ake capaci y
ob ained o CNT(14,0) and CNT(16,0) in Fig. 2co espond o he an-
si ion om one o wo laye s, and om wo o h ee laye s, espec i ely.
This indica es ha he maximum gas cap u e o a gi en s uc u e o
laye s akes place igh be o e eaching a nano ube size ha allows he
o ma ion o one mo e laye , which usually shows a lowe abso p ion
capaci y. This can be clea ly app ecia ed in hose CNTs ha le he o -
ma ion o 1 −2 laye s, bu o bigge CNTs he e a e p obably some
o he effec s playing a ole, since hey allow he appea ance o mo e
complex con o ma ions. The same beha iou is obse ed in he case
o EAN, whe e he gas up ake maxima we e ound o CNT(13,0) and
CNT(15,0), which in u n co espond o he la ges nano ubes capable
o hos ing one and wo laye s, espec i ely. The ac ha maximum gas
up ake alues a e egis e ed o hose CNTs in which he IL possesses
a g ea e deg ee o con o ma ional eedom is u he confi med by he
analysis o he dis ibu ion o ion o ien a ions. This is included in Fig.
S2 o he Suppo ing In o ma ion, and he mos ema kable ea u e is
ha in hose CNTs in which he numbe o laye s is abou o inc ease,
ha is, o he bigges CNT in each subplo , he ions a e accommoda ed
showing a wide a ie y o o ien a ions.
As we p e iously hypo hesized, he exis ence o ca i ies capable o
encapsula ing H2o N2molecules in e ms o size seems insufficien
o explain he gas up ake mechanism, specially in he case o N2. This
could be due o ele an in e ac ions be ween gas molecules and he IL
o he CNT, which could esul in ca i ies whe e hos ing a gas molecule
is no ene ge ically a ou able, e en hough i is big enough o accom-
moda e i . To pe o m an e alua ion o ee olume ha akes his effec
in o accoun , we calcula ed
𝜙=⟨min(1,𝑒−𝛽(𝐸−𝜇))⟩,(1)
whe e 𝐸is he ene gy o a gas molecule inse ed in a andom posi ion
inside a gi en CNT filled wi h pu e IL, 𝜇is he chemical po en ial o
he ese oi (which was modelled as an ideal gas), 𝛽=1∕𝑘𝐵𝑇, and
he b acke s indica e a e aging o e many IL configu a ions and gas
molecule inse ions. This exp ession is equi alen o he ac ion o ac-
cep ed inse ions du ing GCMC simula ions in a nano ube wi hou any
gas molecules abso bed. Gas molecules we e modelled using he same
pa ame e s as in he MD simula ions, and bo h Coulomb and LJ con-
ibu ions we e aken in o accoun o he calcula ion o 𝐸. A o al o
Jou nal o Molecula Liquids 405 (2024) 124909
5
M. O e o-Lema, R. Lois-Cuns, P. Ma ínez-C espo e al.
Fig. 4. Le : Numbe o ionic laye s o med unde confinemen by [EMIM][BF4]( op) and EAN (bo om) as a unc ion o he CNTs in e nal adii and hei chi al
index. The colou g adien is mean as a guide o he eye, o indica e he inc easing numbe o laye s. Righ : Snapsho s aken om CNTs wi h chi al indices ( om
bo om o op) 12, 15 and 17 and filled wi h [EMIM][BF4]depic ing he a angemen o ions in a ious laye s. Ca ions a e colou ed ed, while anions a e blue.
2 ⋅104configu a ions we e ex ac ed om p oduc ion uns in in e als
o 1 ps, and each one was sampled using 4 ⋅104inse ions.
The esul s a e shown in Fig. 5. I can be seen ha in gene al, a
much mo e simila beha iou o he gas up ake p ofiles o Fig. 2is
eco e ed. In ac , o all s udied sys ems, he fi s peak in 𝜙co e-
sponds o he CNT ha exhibi s he highes gas up ake, showing bo h
ha his a e age is indeed capable o p edic ing gas up ake maxima,
and ha gas abso p ion in he confined egime is highly dependen on
he in e ac ions be ween he hos molecules and he es o he sys em,
unlike bulk sys ems whe e he ele an me ic is jus he p obabili y
o ca i ies capable o hos ing gas molecules being o med inside he
IL [56]. As o he seconda y up ake peaks, i can be seen ha hey
a e co ec ly p edic ed in he case o H2in [EMIM][BF4]and N2in
EAN, whe eas o N2in [EMIM][BF4] he abso p ion peak is p edic ed
o CNT(17,0), while he abso p ion peak is ound in CNT(16,0). This
could be due o he complex s uc u ing o he IL gi ing aise o addi-
ional effec s, which is plausible since CNT(16,0) and CNT(17,0) exhibi
a wo and h ee laye s uc u e, espec i ely, as seen in Fig. 4. Finally,
i is in e es ing o no e ha o he same gas (N2), he alues o 𝜙a e
g ea e o [EMIM][BF4] han o EAN, which is consis en wi h he
ac ha he o me is capable o hos ing mo e gas molecules han he
la e .
In addi ion o he ene ge ically accessible ee olume, he in e -
ac ions be ween he gas molecules and he IL ions, as well as hose
be ween he gas molecules and he CNTs we e calcula ed using GRO-
MACS ools. The co esponding esul s a e plo ed in Fig. 6as a unc ion
o he CNTs in e nal adius and hei chi al index. The o al in e ac ion
ene gies a e much s onge (mo e nega i e) o hose CNTs in which
we obse ed a g ea abso p ion abili y [CNT(14,0) and CNT(16,0) o
[EMIM][BF4]and CNT(13,0) and CNT(15,0) o EAN] and hey become
weake as he adius o he CNT inc eases. Also, he in e ac ion ene -
gies o he ca ion and he anion a e simila , he la e being sligh ly
weake o mos cases. The esul s clea ly indica e ha gas in e ac ions
a e mainly d i en by he CNT con ibu ion, which is s onge o N2
han o H2in he p esence o he same IL, and i is also s onge o
N2abso bed in [EMIM][BF4] han in EAN. These findings a e consis-
en wi h he gas molecules being placed wi hin he olume o he IL
bu close o he CNT wall, as i will be confi med la e wi h he analysis
o he p obabili y densi y dis ibu ions o he componen s o he mix-
u e. In addi ion, gas-solid in e ac ions a e he mos significan , which
explains he ac ha gas up ake in ILs inc eases in a nanoconfined en-
i onmen compa ed o he bulk.
Fig. 5. Ene ge ic accessible ee olume o p obe adii abou he size o H2
( op) and N2(cen e) molecules in confined [EMIM][BF4], and N2molecules in
confined EAN (bo om) as a unc ion o he CNTs chi al index.
3.2. S uc u e
IL s uc u e confined in he CNTs is u he quan ified by analyzing
he dis ibu ion o he IL ions in he adial di ec ion o he CNTs be o e
and a e he gas cap u e. To ake in o accoun he possible o ma ion
o pola and apola domains, he adial densi y p ofiles o he ca ions
Jou nal o Molecula Liquids 405 (2024) 124909
6
M. O e o-Lema, R. Lois-Cuns, P. Ma ínez-C espo e al.
Fig. 6. In e ac ion ene gy be ween gas molecules and o he componen s o he
sys ems as a unc ion o he CNTs chi al index o H2confined in [EMIM][BF4]
( op), N2confined in [EMIM][BF4] (cen e) and N2confined in EAN (bo om).
we e calcula ed bo h o he pola head and he alkyl ail. Thus, o
[EMIM]+ he chosen a oms we e he ni ogen a om NA and he ca bon
a om CL o Fig. 1while o [BF4]– he bo on a om BF was used. On he
o he hand, he a oms aken in o accoun o analyze he PIL we e he
ni ogen a om N1 and he ca bon a om C8 o [EA]+, and he ni ogen
a om N15 o [NO3]–. The numbe densi y p ofiles co esponding o
he CNTs whe e gas up ake is maximized, along wi h he ones wi h
he g ea e chi al indices, a e shown in Fig. 7( he esul s o all he
s udied sys ems a e shown in Fig. S3 o he Suppo ing In o ma ion).
O e all, he p esence o sha p peaks indica es a s ong modifica ion o
he s uc u e o he liquid when i in e ac s wi h he CNT. Mo eo e ,
as he adii o he CNTs a e inc eased, new peaks appea , signalling he
exis ence o ex a IL laye s in addi ion o he one closes o he in e ace,
as al eady epo ed by Pensado e al. in Re . [71].
Fo he CNTs co esponding o gas up ake maxima ( op ow o
Fig. 7), i can be seen ha , while N2abso p ion does no esul in
changes o he densi y p ofiles, his is no he case o H2, which o ces
[EMIM][BF4] molecules o sligh ly modi y hei s uc u e o accommo-
da e i . This could in pa explain he ac ha H2is no abso bed in EAN
filled CNTs, since his IL is mo e igh ly packed han [EMIM][BF4], and
hus i would be mo e difficul o i o change i s s uc u e, e en i his
change is small. On he o he hand, o bigge CNTs (bo om ow o
Fig. 7) his effec disappea s, and nei he H2no N2cause a ea ange-
men o he IL. This is consis en wi h he ac ha in he bulk egime
H2molecules end o accommoda e in o he in e s i ial oids o he IL
ne wo k wi hou significan ly dis up ing i s s uc u e [56].
To ob ain a deepe unde s anding o he gas s o age mechanism, i is
also ele an o know in which egions he abso p ion o gas molecules
akes place. Tha is, whe he gas molecules a e abso bed wi hin he
olume o he IL—as obse ed by Ha manly e al. in a ca bon-based
ma e ial—o hey a e ins ead abso bed close o he po e walls [21].
This las si ua ion was epo ed by Shi and Luebke o silica sli po es
[47]. In o de o enligh en his poin , we de e mined he adial p oba-
bili y densi y dis ibu ion o bo h ILs and he gas molecules, which a e
shown in Fig. 8 o he CNTs wi h he g ea es gas abso p ion ( op ow)
and he ones wi h he la ges adii (bo om ow). All o hem can be
ound in Fig. S4 o he Suppo ing In o ma ion. Fo bo h IL sys ems, he
abso p ion o gas molecules wi hin he egion popula ed by he a oms
o he IL is clea ly app ecia ed ega dless o he size o he CNT. This
ac u he suppo s he ini ial hypo hesis o his wo k: ene ge ically
a ou able ee olume spo s o gas molecules appea unde a s ong
confinemen o he IL. Howe e , he ene ge ic con ibu ion o he CNT
o he gas molecules a ou s hem posi ioning close o he wall in mos
cases. As an excep ion o his, he e is a peak o H2molecules nea he
axis in he CNT(20,0) filled wi h [EMIM][BF4]. No ably, his is ela ed
o he change in ene gy con ibu ions in Fig. 6, wi h ha peak being
esponsible o he mo e a ou able ene gy con ibu ion by he ca ion.
Rega ding he N2dis ibu ions, he e is also some up ake nea he axis
in ha same CNT. Con a ily, he CNT(18,0) filled wi h EAN does no
eflec any abso p ion in ha egion due o he high packing densi y o
his IL, as men ioned abo e.
To u he confi m ha , despi e being weakly in e ac ing gases,
ni ogen and hyd ogen up ake and hei accommoda ion inside he nan-
o ubes is no only go e ned by molecula packing effec s, he p e ious
esul s can be compa ed wi h a oy model o he IL+CNT+gas sys em
in which bo h ions and gas molecules a e eplaced by cha geless ha d
sphe es. Mo e de ails on hese calcula ions can be ead in he Suppo -
ing In o ma ion. The gas up ake p ofile is depic ed in Fig. 9. The esul s
show a s iking diffe ence be ween he oy model and he simula ions.
The gas up ake p ofile o he model shows a single maximum, in con-
as wi h he mul iple ones obse ed o N2sys ems. Mo eo e , he
alue o he maximum is e y close o he bulk abso p ion alue, o
which la ge CNTs con e ge, in con as wi h he simula ed ones, whe e
gas up ake is significan ly highe in some CNTs han o he bulk egime.
This shows o a ce ain deg ee ha gas up ake is no de e mined solely
by he packing o he IL molecules, as e idenced by i s lack o co -
ela ion wi h ee olume. This conclusion is u he suppo ed by he
p obabili y dis ibu ions ob ained o he oy model, which a e included
in Fig. S5 o he Suppo ing In o ma ion and can be compa ed wi h
he ones ob ained om simula ions (Fig. S4). I can be seen ha in
he oy model, gas pa icles ha e a endency o lea e he IL- ich e-
gions and accommoda e in o in he emp y egion be ween he IL and
he CNT. Mo eo e , he gas popula ion wi hin he IL is low, and only
becomes no iceable o la ge CNTs. This is no obse ed in he simula-
ions, whe e gas molecules a e hos ed wi hin he liquid, and a e ne e
obse ed alone in he su ace o he CNT. The fi s effec can be a -
ibu ed o he ac ha molecules in he oy model a e s uc u eless,
and he e o e expec ed o lea e less in e s i ial oids, leading o he
lowe p esence o gas wi hin he IL. Howe e , he abundance o gas in
he egions whe e no IL molecules a e p esen is no clea ly co ela ed
wi h he de ails o he s uc u al pic u e. This sugges s ha in e ac ions
beyond simple ha d-co e epulsion, such as elec os a ic ene gies, mus
play an impo an ole in he abso p ion p ocess o hese gases, as p e-
iously discussed.
Finally, he analysis o he single-pa icle dynamics was accom-
plished h ough he calcula ion o he cen e-o -mass VACFs o he gas
molecules. The no malized VACF is compu ed as
𝐶(𝑡)= ⟨𝑣(𝑡)⋅𝑣(0)⟩
⟨𝑣(0) ⋅𝑣(0)⟩,(2)
whe e 𝑣(𝑡)is he eloci y o he cen e-o -mass o he molecule a ime 𝑡
and he b acke s indica e he ensemble a e age. The esul s a e included
in Fig. S6 o he Suppo ing In o ma ion. F om hese au oco ela ion
Jou nal o Molecula Liquids 405 (2024) 124909
7
M. O e o-Lema, R. Lois-Cuns, P. Ma ínez-C espo e al.
Fig. 7. Numbe densi y p ofiles in he CNT adial di ec ion. Solid, dashed and do ed lines co espond wi h he dis ibu ion o he ions be o e gas abso p ion, a e
he abso p ion o H2, and a e he abso p ion o N2, espec i ely.
unc ions he ib a ional Densi y o S a es ( DOS) can be ob ained in a
s aigh o wa d way as he Fou ie ans o m o he VACFs
𝐼(𝜔)=|
|
|
∞
∫
0
𝑒−𝑖𝜔𝑡𝐶(𝑡)𝑑𝑡|
|
|
2(3)
We conside he e ha 𝐼(𝜔)is no malized such ha
∞
∫
0
𝐼(𝜔)𝑑𝜔 =1 (4)
The esul s ob ained o all gas molecules abso bed in he diffe en CNTs
a e plo ed in Fig. 10. The e we can obse e ha he p ofiles a e clea ly
blue-shi ed as he size o he CNTs inc eases, which is consis en wi h
he gas molecules being mo e igh ly packed in egions in which he
ee olume is less ene ge ically a ou able, as i was shown in Fig. 5.
The e o e, hose DOS a low equencies can be a ibu ed o IL config-
u a ions wi h loose packing whe e gas molecules a e s o ed and ha e
la ge mean ee pa hs. As CNT adius inc eases, he liquid inside be-
comes mo e bulk-like, leading o a igh e packing, and hence, o he
blue shi p esen in he spec a. This is consis en wi h he DOS o N2
molecules abso bed in EAN being he leas displaced, since he packing
o his IL lea es he leas ee olume, and hus exhibi s smalle ca i ies.
4. Conclusions
In his wo k, he gas up ake mechanism o H2and N2in zigzag CNTs
wi h a ying chi al index was s udied by means o classical molecula
dynamic simula ions and hyb id g and canonical Mon e Ca lo/molecu-
la dynamics simula ions a 𝑇= 300 K. CNTs a e filled wi h one ap o ic
IL ([EMIM][BF4]) and one p o ic IL (EAN).
Fi s ly, he gas mola ac ion abso bed in each case was de e mined
by he g and canonical Mon e Ca lo/molecula dynamics simula ions
by inse ing gas molecules in o IL confined in CNTs. These simula ions
e ealed ha gas s o age capaci y eaches a maximum o CNTs wi h
adii o 3.78 Å and 3.39 Å filled wi h [EMIM][BF4]and EAN, espec-
i ely. An inc ease in he abso p ion o gas molecules up o 25% was
ound o hese CNTs, which is much g ea e han he 0.4% obse ed in
bulk IL. Seconda y abso p ion peaks we e also obse ed o bo h ILs in
hose CNTs wi h in e nal adii o 4.56 Å and 4.96 Å o [EMIM][BF4]
and EAN, espec i ely. In addi ion, N2abso p ion abili y appea s o be
la ge han ha o H2, and [EMIM][BF4]shows o be mo e efficien
o gas s o age han EAN ega dless o he channel size o he confining
CNTs. Finally, no H2molecules we e ound o be abso bed in EAN filled
CNTs, which is in ag eemen wi h he low ee olume in he liquid.
To unde s and he mechanism o gas cap u e, accessible ee olume
was analyzed as i is a key ac o in gas abso p ion by ILs. These esul s
show ha he sol a ion ene ge ics mus also be aken in o accoun o
a co ec unde s anding o N2up ake in confined ionic sol en s. This
is in con as wi h he case o H2abso p ion, which can be p ope ly
explained solely by ee olume calcula ions. Mo eo e , he g ea es gas
s o age capaci y was shown o be co ela ed wi h s uc u al changes in
he ILs, namely he numbe o ionic laye s wi hin he CNTs, which in
u n affec he a ailable olume o gas abso p ion.
Finally, he s uc u al analysis e eals he p e e ence o bo h gas
molecules o be placed in he IL- ich egion as ene gy- a ou ed s uc-
u al oids appea due o he confinemen o he ILs. Also he mo e
ele an ene ge ic con ibu ion o he gas molecules is he one o he
CNT, a ou ing hese molecules o accommoda e hemsel es close o
he ca bon wall. On he o he hand, he s udy o he single pa icle
dynamics shows ha he DOS is blue-shi ed wi h inc easing adii o
he CNTs, which is a ibu ed o he gas molecules being mo e densely
packed in a eas whe e he ee olume is less ene ge ically a ou able.
Jou nal o Molecula Liquids 405 (2024) 124909
8
M. O e o-Lema, R. Lois-Cuns, P. Ma ínez-C espo e al.
Fig. 8. P obabili y densi y dis ibu ion o finding an a om belonging o he IL ( ed), H2gas (blue) o N2gas (g een) a a ce ain dis ance om he adial axis o he
CNT. Fo each IL, he CNT wi h he g ea es gas abso p ion ( op ow) and he one wi h he la ges adius (bo om ow) a e depic ed. Fo he IL, solid lines co espond
o IL+N2sys ems, while dashed lines ep esen IL+H2.
Fig. 9. Gas up ake in he ha d sphe e model as a unc ion o CNT adius and
chi al index. E o ba s show he s anda d de ia ion.
In conclusion, his wo k confi ms ha a s ong confinemen o ILs
inside a po ous hos can g ea ly inc ease hei gas s o age capaci y,
allowing o he accommoda ion o la ge amoun s o gas wi hin ene -
ge ically a ou able s uc u al oids ha eme ge as a esul o modifi-
ca ions in he s uc u al con o ma ion o he ILs. The influence o po e
shape and geome y in gas cap u e and s o age in mo e complex po ous
ma e ials will be examined in a u u e wo k.
CRediT au ho ship con ibu ion s a emen
Ma ín O e o-Lema: W i ing – e iew & edi ing, W i ing – o iginal
d a , Visualiza ion, Valida ion, So wa e, Me hodology, In es iga ion,
Fo mal analysis. Raúl Lois-Cuns: W i ing – e iew & edi ing, W i -
ing – o iginal d a , Visualiza ion, Valida ion, So wa e, Me hodology,
In es iga ion, Fo mal analysis. Pablo Ma ínez-C espo: W i ing – e-
iew & edi ing, W i ing – o iginal d a , Visualiza ion, Valida ion, So -
wa e, Me hodology, In es iga ion, Fo mal analysis. Alejand o Ri e a-
Pousa: W i ing – e iew & edi ing, W i ing – o iginal d a , Visualiza-
ion, Valida ion, So wa e, Me hodology, In es iga ion, Fo mal analy-
sis. Had ián Mon es-Campos: W i ing – e iew & edi ing, W i ing –
o iginal d a , Visualiza ion, Valida ion, Supe ision, So wa e, P ojec
adminis a ion, Me hodology, In es iga ion, Fo mal analysis, Concep-
ualiza ion. T inidad Méndez-Mo ales: W i ing – e iew & edi ing,
W i ing – o iginal d a , Visualiza ion, Valida ion, Supe ision, So -
wa e, P ojec adminis a ion, Me hodology, In es iga ion, Funding ac-
quisi ion, Fo mal analysis, Concep ualiza ion. Luis M. Va ela: W i ing
– e iew & edi ing, Visualiza ion, Supe ision, P ojec adminis a ion,
Funding acquisi ion, Fo mal analysis, Concep ualiza ion.
Decla a ion o compe ing in e es
The au ho s decla e ha hey ha e no known compe ing financial
in e es s o pe sonal ela ionships ha could ha e appea ed o influence
he wo k epo ed in his pape .
Da a a ailabili y
Inpu files and analysis sc ip s a e a ailable a h ps://doi .o g /10 .
5281 /zenodo .10495511.
Jou nal o Molecula Liquids 405 (2024) 124909
9
M. O e o-Lema, R. Lois-Cuns, P. Ma ínez-C espo e al.
Fig. 10. Vib a ional densi y o s a es o ( op) H2molecules abso bed in
[EMIM][BF4], (cen e) N2molecules abso bed in [EMIM][BF4] and, (bo om)
N2molecules abso bed in EAN.
Acknowledgemen
The financial suppo o he Spanish Minis y o Science and In-
no a ion (PID2021-126148NA-I00 unded by MCIN/AEI/10.13039/
501100011033/FEDER, UE) is g a e ully acknowledged. Mo eo e , his
wo k was unded by he Xun a de Galicia (GRC ED431C 2020/10). A.
R. P. hanks he Spanish Minis y o Educa ion o his FPU18/01597
g an . M. O. L. and P. M. C. wish o hank he Xun a de Gali-
cia o hei “Axudas de apoio á e apa p edou o al” g an (ED481A
2022/236 and ED481A 2022/045). T. M. M. acknowledges he con-
ac unded by he pilo p og am o he USC o he ec ui men o
Dis inguished esea ch pe sonnel—call 2021 unde he ag eemen be-
ween he USC and he San ande Bank o 2021–2024. This publi-
ca ion and he con ac o T. M. M. a e pa o he g an RYC2022-
036679-I, unded by MCIN/AEI/10.13039/501100011033 and FSE+.
H. M. C. hanks he USC o his “Con oca o ia de Recualificación do
Sis ema Uni e si a io Español-Ma ga i a Salas” pos doc o al g an un-
de he “Plan de Recupe ación T ans o mación” p og am unded by
he Spanish Minis y o Uni e si ies wi h Eu opean Union’s Nex Gen-
e a ionEU unds. R. L. C. acknowledges his P edoc o al Con ac un-
de he amewo k o he p ojec PID2021-126148NA-I00 unded by
MCIN/AEI/10.13039/501100011033/FEDER, UE. This wo k was sup-
po ed by he Fundacão pa a a Ciência e Tecnologia (FCT) ( unded by
na ional unds h ough he FCT/MCTES (PIDDAC)) o CIQUP, Facul y o
Science, Uni e si y o Po o (P ojec UIDB/00081/2020), IMS-Ins i u e
o Molecula Sciences (LA/P/0056/2020).
Appendix A. Supplemen a y ma e ial
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a h ps://doi .o g /10 .1016 /j .molliq .2024 .124909.
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