Ca bon nanoma e ials wi h Thymol + Men hol Type V na u al deep
eu ec ic sol en : F om su ace p ope ies o nano-Ven u i e ec h ough
nanopo es
Nu ia Aguila
a
, Rocío Ba os
b
, Juan An onio Tamayo-Ramos
b
, Sonia Ma el
b
, Al edo Bol
b,c
, Me A ilhan
d,
⇑
,
San iago Apa icio
a,b,
⇑
a
Depa men o Chemis y, Uni e si y o Bu gos, 09001 Bu gos, Spain
b
In e na ional Resea ch Cen e in C i ical Raw Ma e ials-ICCRAM, Uni e si y o Bu gos, Plaza Misael Banuelos s/n, 09001 Bu gos, Spain
c
Depa men o Physics, Uni e si y o Bu gos, 09001 Bu gos, Spain
d
Depa men o Chemical and Pape Enginee ing, Wes e n Michigan Uni e si y, Kalamazoo, MI 49008-5462, USA
a icle in o
A icle his o y:
Recei ed 30 Ma ch 2022
Re ised 15 Sep embe 2022
Accep ed 17 Oc obe 2022
A ailable online 21 Oc obe 2022
Keywo ds:
Na u al Deep Eu ec ic sol en s
Ca bon nanoma e ials
G aphene
Modelling
Adso p ion
Con inemen
abs ac
A heo e ical s udy using Densi y Func ional Theo y and classical Molecula Dynamics simula ions o he
s udy o ca bon nanoma e ials in a che ypical Men hol + Thymol Type V Na u al Deep Eu ec ic Sol en is
epo ed. The nanoscopic s uc u e o he ep esen a i e nano luid is analyzed conside ing con inemen ,
adso p ion and sol a ion e ec s, as well as consequences on di usion p ope ies h ough nano po es.
Di e en ypes o nanoma e ials we e conside ed such as ulle enes, nano ubes, g aphene and nanopo es.
The s udy o nanoscopic p ope ies allowed o analyze he esponse o he sol en o he p esence o he
nanoma e ials, aking in o accoun sol en ea angemen and con inemen in nanoca i ies and su aces.
This esponse shows liquid s uc u e and mobili y consequences, wi h a so o nano-Ven u i e ec
among hem. The epo ed esul s p o ide o he i s ime a cha ac e iza ion o his ype o na u al sol-
en s as a sus ainable pla o m o he de elopmen o ca bon – nanoma e ials-based echnologies.
Ó2022 The Au ho s. Published by Else ie B.V. This is an open access a icle unde he CC BY-NC-ND
license (h p://c ea i ecommons.o g/licenses/by-nc-nd/4.0/).
1. In oduc ion
Deep Eu ec ic Sol en s (DESs) [1] a e o med by he combina-
ion o wo o mo e compounds, which upon mixing a ce ain
mole a ios leads o a mel ing empe a u e ema kably lowe han
hose o he indi idual compounds (eu ec ic mix u e), hus
enabling liquid sys ems a close o ambien empe a u e condi-
ions. Depending on he na u e o he compounds mixed o DESs
o ma ion, hey may be classi ied om Type I o Type V [2,3,4]. Pa -
icula a en ion has been paid o Type III DESs o med by he com-
bina ion o an hyd ogen bond accep o (HBA, equen ly a
qua e na y ammonium sal ) and an Hyd ogen Bond Dono (HBD,
such as o ganic acids, alcohols, suga s, e c.), o which he o ma-
ion o s ong HBA – HBD hyd ogen bonding is he main eason
o he mel ing poin deple ion [5,6]. Fo he case o Type V DES,
non-ionic HBA – HBD mix u es a e conside ed [7]. Because o he
la ge numbe o HBA – HBD combina ions leading o DESs o ma-
ion, p ope ies can be ine- uned o p oduce DESs speci ically
designed o a la ge ange o possible applica ions [8,9].
Depending on he na u e o he compounds in ol ed in DESs
o ma ion, hey may be also classi ied as Na u al DESs (NADESs)
[10,11], which, pa icula ly o Types III and V, a e o med by HBAs
and HBDs wi h na u al o igin [12]. NADES a e pa icula ly ad an-
ageous conside ing hei low cos [13], biodeg adabili y, non-
oxici y [14], enewable o igin [15], as well as showing sui able
physicochemical p ope ies [16]. NADES ha e been p oposed o
applica ions in a wide ange o echnologies including hei use
as g een sol en s [17], in he ag i- ood sec o [18], bio echnology
[19], ex ac ion [20], o pha maceu ical applica ions [21].
Type V DESs a e especially con enien because he absence o
ionic species leads o mo e sui able physicochemical p ope ies
such as low iscosi y [22,23] o hyd ophobici y [24], allowing hei
scaling up o indus ial applica ions. The e o e, he design and use
o ype V – NADES may enable luids wi h he mos sui able p op-
e ies among he a ailable DESs being applicable o a la ge collec-
ion o echnologies. Among he possible applica ion o gene al
DES, hei use o nano echnologies [25] and nanoma e ials de el-
opmen a e o pa icula ele ance in a eas such as biosenso s [26],
me al nanoma e ials [27,28], g aphene p oduc ion by g aphi e
h ps://doi.o g/10.1016/j.molliq.2022.120637
0167-7322/Ó2022 The Au ho s. Published by Else ie B.V.
This is an open access a icle unde he CC BY-NC-ND license (h p://c ea i ecommons.o g/licenses/by-nc-nd/4.0/).
⇑
Co esponding au ho s a : Depa men o Chemis y, Uni e si y o Bu gos,
09001 Bu gos, Spain (S. Apa icio).
E-mail add esses: [email p o ec ed] (M. A ilhan), [email p o ec ed] (S.
Apa icio).
Jou nal o Molecula Liquids 368 (2022) 120637
Con en s lis s a ailable a ScienceDi ec
Jou nal o Molecula Liquids
jou nal homepage: www.else ie .com/loca e/molliq
liquid ex olia ion [29], 2D-ma e ials echnologies [30], nano ubes
unc ionaliza ion and dispe sion [31,32], nanoso ben s [33] and
nano luidics [34]. The pa icula case o ca bon-based nanoma e i-
als is o g ea in e es o DES / NADES applica ions, conside ing 0D
(e.g. ulle enes), 1D (e.g. nano ubes [35,36]) and 2D (e.g. g aphene
[37,38]) nanoma e ials, om he poin o iew o p oduc ion, s a-
biliza ion (dispe sion) and unc ionaliza ion. Taking in o accoun
he no el y and sui able physicochemical and echnoeconomic
p ope ies o Type V NADES, hei use o ca bon- ela ed nano ech-
nologies is a highly p omising a ea, sca cely explo ed.
The pu pose o his wo k is o p esen a s udy o he in e ac ion
o an a che ypical example o Type V NADES and ca bon nanoma-
e ials. As ep esen a i e o Type V NADES a mix u e o men hol
(MEN) and hymol (THY), Fig. 1, in 1: 1 mol a io has been selec ed.
The chosen ca bon nanoma e ials, Fig. 2, a e: i) ulle enes (C60 and
C540), ii) Single Walled Ca bon Nano ubes (SWNTs, wi h di e en
in e nal diame e s), iii) he e ojunc ions (HET, o med by he com-
bina ion o wo di e en SWNTs), i
) cylind ical and sli po es and
) g aphene shee s.
The gene al concep ual s udy scheme is as ollows: a de ailed
Densi y Func ional Theo y (DFT) s udy o he simples sys ems is
done, hen he s udy is epea ed wi h Molecula Dynamics (MD)
o alida ion pu poses o he esul s o his second echnique
and, inally, once he con idence on hese esul s is achie ed o
he p oposed sys ems i is applied o he mo e complex ones.
Hence, he equilib ium in e ac ion si ua ion p ope ies o hese
ma e ials in MEN: THY Type V NADES we e s udied using a heo-
e ical app oach combining Quan um Chemis y and Molecula
Dynamics me hods, as jus explained. MD has also been used o
s udy he di usion o MEN: THY 1: 1 liquid om one side o he
opposi e one o a g aphene bilaye connec ed wi h a SWNT,
Fig. 2 . Compu a ional s udies ha e been success ully used o he
nanoscopic cha ac e iza ion o DES / NADES conside ing bo h bulk
liquid phases [9,39] as well as hei in e ac ion wi h nanoma e ials
[30,40,41,42]. These s udies ha e p o ided ele an in o ma ion on
nano s uc u ing, dynamics, ene ge ics o ma e ials, which a e
equi ed o he cha ac e iza ion o he selec ed nano sys ems
and o MEN: THY + ca bon nanoma e ials conside ed in his wo k.
The epo ed esul s p obed o he i s ime he possibili y o
using Type V NADES o ca bon nanoma e ials echnologies, hus
p o iding a g een, sus ainable and low-cos pla o m o ma e ials
de elopmen .
2. Me hods
DFT s udies on single MEN: THY 1: 1 pai s adso bed on pe iodic
g aphene we e ca ied ou using he Quan um Esp esso so wa e
( 6.5) [43]. G aphene uni cell was ob ained om Ma e ials P ojec
Da abase [44] and used o build a single laye 10 10 1 supe cell
wi h dimensions a=b= 24.68 Å, c= 40 Å. The dimensions o his
supe cell allowed he in oduc ion o MEN: THY 1: 1 a he supe -
cell cen e as well as he la ge acuum laye on op, hinde ing he
in e ac ion o MEN: THY molecules wi h neighbo cells. The calcu-
la ions we e pe o med wi h Pe dew-Bu ke-E nze ho (PBE)
wi hin he gene alized g adien app oxima ion (GGA) o
exchange–co ela ion unc ional, wi h an de Waals in e ac ions
included wi h G imme’s D3 semiempi ical co ec ion [45], s an-
da d D3 co ec ion wi h ze o damping, i.e. DFT-D3 heo e ical le el
is used. The combina ion o s anda d DFT me hods (such as
PBE) + G imme Dispe sion co ec ion a e he bes comp omise
be ween quali y esul s and compu a ional ime. Ul aso pseupo-
en ials we e used o all he conside ed a oms [46]. Al hough
P ojec o -Augmen ed Wa e (PAW) pseudopo en ials a e e-
quen ly used in he li e a u e o he s udy o adso bed sys ems,
ul aso pseudopo en ials ha e been also success ully applied o
he s udy o adso p ion phenomena on g aphene su aces [47].
The B illouin zone sampling was ca ied ou using he Mon-
kho s –Pack me hod [48] wi h a 6 61 k-g id. Calcula ions
we e done wi h 40 Ry o he kine ic ene gy cu o . The h eshold
o he o ce minimiza ion was 1 10
–5
a.u. Full geome y op i-
miza ions we e comple ed, including a iable cell (wi h 0.5 kba
as h eshold), o he p e iously men ioned sys ems: i) pe iodic
g aphene supe cell, ii) MEN: THY 1: 1 dime s in acuum, conside -
ing THY as hyd ogen bond dono (P1), and MEN as hyd ogen bond
dono (P2), and iii) se e al con igu a ions o hese same dime s,
Fig. 3, on op o he g aphene laye wi h p e e ably MEN on he su -
ace (P1a and P2a) o p e e ably THY on he su ace (P1b and P2b).
Adso p ion ene gies, Eads, we e calcula ed acco ding o:
E
ads
¼E
GþM:T
E
G
þE
M:T
ðÞ ð1Þ
Whe e G + M:T s ands o he ene gy o he g aphene + MEN: THY 1:
1 sys em and Gand M:T o he ene gies o clean g aphene and iso-
la ed MEN: THY 1: 1, espec i ely.
Classical Molecula Dynamics simula ions (MD) we e ca ied
ou wi h MDynaMix .5.2 [49] so wa e o he di e en geome-
ies as epo ed in Fig. 2 o he o ce ield pa ame e iza ions
desc ibed in Table S1 (Supplemen a y In o ma ion). Ini ial simula-
ion boxes we e buil using Packmol [50] p og am, wi h he num-
be o MEN / THY molecules o each sys em adjus ed o he
expe imen al densi y o MEN: THY 1: 1 bulk liquid phase in
absence o ca bon nanoma e ials (
q
= 0.9332 g cm
-3
a 298.15 K,
unpublished expe imen al esul ob ained in ou labo a o y). Ten
di e en ini ial con igu a ions we e p epa ed o each conside ed
sys em, hus leading o en di e en ajec o ies which we e com-
bined o he analysis o imp o e s a is ical sampling and a oid
possible e ec s de i ed om ini ial con igu a ions. MD simula ions
we e done se ing pe iodic bounda y condi ions in he h ee space
di ec ions. MDs we e done in he NVT ensemble a 298 K o all he
sys ems, wi h he empe a u e being con olled using he Nosé-
Hoo e me hod [51], wi h 30 ps as empe a u e coupling ime.
The equa ions o mo ion we e sol ed using he Tucke man-Be ne
double ime s ep algo i hm, wi h long- and sho - ime s eps o 1
and 0.1 s. The Ewald me hod (1.5 nm cu -o adius) [52] was
applied o handling he elec os a ic in e ac ions whe eas
Lenna d-Jones po en ial was deal wi h 1.5 nm cu -o dis ance
and he Lo en z-Be helo mixing ules o c oss e ms. MD simu-
la ions o each sys em we e comple ed o 100 ns, o which he
i s 50 ns we e used o equilib a ion pu poses (assu ed h ough
he analysis o sys ems p ope ies) whe eas he las 50 ns we e
de o ed o p oduc ion pu poses and used o he analysis epo ed
along his wo k. MEN: THY 1: 1 is a low iscous NADES (
g
= 58.
6 mPa s a 298.15 K, unpublished expe imen al esul ob ained
in ou labo a o y), hus he simula ion ime is long enough o a
p ope sampling o he conside ed sys ems p ope ies. The isual-
iza ion, analysis and pos p ocessing o MD ajec o ies was ca ied
ou using VMD [53] and TRAVIS [54].
Fig. 1. Molecula s uc u es o men hol (MEN) and hymol (THY), and di ec ions
used o o ien a ional s udy (Fig. 6):
1
, is he di ec ion pe pendicula o g aphene
laye and
2
, is he di ec ions de ining main molecula axis o men hol and hymol.
N. Aguila , Rocío Ba os, J. An onio Tamayo-Ramos e al. Jou nal o Molecula Liquids 368 (2022) 120637
2
Fig. 2. Molecula s uc u es o NADES and ca bon-based nanos uc u es used in his wo k. A om labelling indica ed.
N. Aguila , Rocío Ba os, J. An onio Tamayo-Ramos e al. Jou nal o Molecula Liquids 368 (2022) 120637
3
3. Resul s and discussion
3.1. MEN: THY adso bed and con ined in g aphene
The adso p ion o MEN: THY 1: 1 isola ed dime s on g aphene
we e ini ially s udied using DFT – D3 o he o ien a ions and in e -
ac ions epo ed in Fig. 3, wi h he main esul s summa ized in
Fig. 4. The in e ac ion o MEN: THY dimme s wi h g aphene su ace
is cha ac e ized by a s ong adso p ion. Dimme s wi h THY ac ing
as hyd ogen bond dono (P1 sys ems) leads o la ge adso p ion
ene gies han hose wi h MEN ac ing as dono . Likewise, s uc u es
wi h THY placed on op o he g aphene su ace (P1b and P2b) lead
o s onge adso p ion, which can be jus i ied conside ing he
ma ching be ween he THY and g aphene a oma ic ings, as can
be iewed in Fig. 4. Ne e heless, adso p ion ene gies a e in he
0.69 o 0.89 eV, which con i ms s ong in e ac ions o all he pos-
sible o ien a ions. The adso p ion is cha ac e ized by NADES mole-
cules placed close o he su ace (1.2 o 2.0 Å ange), wi h globally
close dis ances o P1 o ien a ions, which would jus i y he la ge
adso p ion ene gies. A pa icula ly in e es ing ea u e o MEN: THY
adso p ion s ands on he dis up ion by expansion o he unde lying
g aphene su ace upon adso p ion, which is cha ac e ized by a
non-negligible ela i e cell expansion (one o de o magni ude la -
ge o P1 o ien a ions). Mo eo e , he adso p ion leads o losing
g aphene plana i y in he egion below he MEN: THY adso bed
molecules, quan i ied by a shi ing o ca bon a oms in he g aphene
shee up o 0.42 Å wi h espec o hose in egions beyond he
adso bed ones. The e o e, he adso p ion gene a es a p o usion
wi h an associa ed elec onic densi y deple ion in he g aphene
unpe u bed plane specially o P1 mechanisms o adso p ion.
Fig. S1a o S1d in Suppo ing In o ma ion (SI) con i ms g aphene
s uc u e buckling and i s consequences on elec onic densi y.
Mo eo e , he adso p ion o MEN: THY on he g aphene su ace
also dis up s he hyd ogen bonding be ween MEN and THY mole-
cules. The MEN: THY dime s adso bed on he g aphene su ace
we e ex ac ed and he ene gy calcula ed allowing he de e mina-
ion o he MEN – THY in e ac ion ene gy (i.e. hyd ogen bonding
s eng h). The MEN – THY in e ac ion ene gy calcula ed o op i-
Fig. 3. S uc u es conside ed o he adso p ion o MEN: THY 1: 1 dime on pe iodic g aphene shee using DFT: p e e ably MEN on op o he g aphene su ace (P1a and P2a)
p e e ably THY on op o he g aphene su ace (P1b and P2b). No e ha he supe cell used o he calcula ions is 40 Å high wi h g aphene laye loca ed a he middle.
Fig. 4. DFT esul s o he adso p ion o MEN: THY 1: 1 clus e on pe iodic g aphene su ace o he di e en con igu a ions and o ien a ions conside ed (Fig. 3).
D
Eindica es
adso p ion ene gy,
D
Vindica es cell olume expansion upon MEN: THY 1: 1 adso p ion,
D
z indica es maximum dis up ion o g aphene su ace measu ed o posi ion o
undis u bed a oms, hindica es molecule – g aphene dis ances conside ed a oms in molecules close o he su ace.
N. Aguila , Rocío Ba os, J. An onio Tamayo-Ramos e al. Jou nal o Molecula Liquids 368 (2022) 120637
4
Fig. 5. (le column) Numbe densi y p o iles in he di ec ion pe pendicula o he g aphene su ace and ( igh column) he co esponding in eg al o MEN: THY 1:1 con ined
be ween pe iodic g aphene shee s wi h din e shee dis ance (Fig. 2a). Dashed lines in panels on he le column indica e (black) he posi ion o he g aphene shee s and (g ay)
he cen e o he po e. Nume ic alues inside panels on he le indica e he dis ance o he g aphene shee s co esponding o ele an peaks. Numbe inside panels on he
igh indica e he numbe o molecules in he laye s in he i s adso bed laye s (in eg a ion o he i s numbe densi y peak).
N. Aguila , Rocío Ba os, J. An onio Tamayo-Ramos e al. Jou nal o Molecula Liquids 368 (2022) 120637
5
mized dime s in acuum (i.e. no adso bed on g aphene) is 0.49
and 0.31 eV, o P1 and P2 in e ac ing posi ions, espec i ely,
whe eas hese alues dec ease o 0.09 and 0.05 eV o MEN –
THY dime s adso bed on g aphene. The e o e, he adso p ion on
g aphene leads o a la ge dis up ion o MEN – THY hyd ogen bond-
ing al hough his in e ac ion is main ained on he su ace. I should
be ema ked ha he dispe sion con ibu ion (G imme – D3) is in
he 4–6 % ange (conside ing he di e en posi ions o adso p ion)
o he o al adso p ion ene gy, he e o e, no being he main con i-
bu ion o he mechanism o adso p ion on he g aphene su ace.
Fig. 6. O ien a ion o MEN o THY molecules placed in he i s adso bed laye on
g aphene measu ed by he de ined uangle o MEN: THY 1:1 (Fig. 1) con ined
be ween pe iodic g aphene shee s wi h din e shee dis ance (Fig. 2a).
Fig. 7. In e ac ion ene gy, E, be ween g aphene and DES componen (MEN o THY)
o MEN: THY 1:1 con ined be ween pe iodic g aphene shee s wi h din e shee
dis ance (Fig. 2a) as a unc ion o d.
Fig. 8. Veloci y, , dis ibu ion unc ions o he epo ed molecules in MEN: THY
1:1 con ined be ween pe iodic g aphene shee s wi h din e shee dis ance. Dashed
g ay lines show esul s o bulk liquid phase, i.e. in absence o con inemen be ween
g aphene shee s. Values inside each panel indica e he posi ion o he maxima.
Panel a and panel b show esul s o MEN and THY, espec i ely.
N. Aguila , Rocío Ba os, J. An onio Tamayo-Ramos e al. Jou nal o Molecula Liquids 368 (2022) 120637
6
The la ge adso p ion ene gies, he sho su ace o adso bed mole-
cule dis ance as well as he dis up ion o he s uc u e o he
unde laying g aphene su ace may indica e a ce ain chemical
so p ion beyond he nea physical so p ion mechanism.
The DFT con i med he la ge end o MEN: THY 1: 1 o be
adso bed on g aphene su ace, al hough conside ing a simpli ied
model o isola ed clus e s. The beha io o he ep esen a i e
NADES when con ined be ween g aphene shee s (i.e. sli like nano-
po es) is s udied using MD simula ions as a unc ion o he nano-
po e diame e in he 10 o 100 Å ange. Numbe densi y p o iles
in he di ec ion pe pendicula o g aphene shee s de ining he
nanopo es, Fig. 5, con i m he de elopmen o adso p ion laye s
in he icini y o he su aces. In he case o he na owe nanopo e
(d= 10 Å), Fig. 5a1, he densi y p o iles show a single peak nea he
su ace o each o he NADES componen s. The peaks co espond-
ing o THY a e sligh ly close o he su ace and a e na owe and
mo e in ense han hose o MEN, which ag ees wi h DFT esul s
in Fig. 4 showing he s onge adso p ion when THY a oma ic ings
a e placed on he g aphene su ace. Ne e heless, MEN adso p ion
is also con i med wi h he wide densi y peaks leading o a ele-
an numbe o adso bed molecules, Fig. 5a. The inc ease in nano-
po e size leads o he o ma ion o a second adso bed laye , which
is placed oughly 3.6 Å abo e he i s one. This second laye is spe-
cially de ined and mo e in ense o MEN molecules, whe eas THY
molecules seem o be concen a ed in he i s adso bed laye . This
laye ing leads o a i s adso bed laye iche in THY ( iche in he
20–30 % ange o he s udied nanopo e diame e ) and a second
laye iche in MEN, which ag ees wi h DFT esul s showing p e -
e ence o MEN: THY clus e s wi h THY close o he g aphene su -
ace. The i egula molecula dis ibu ion o adso bed molecules on
g aphene su ace is showed in Fig. S2, in SI. A pa e n o non-
o e lapping al e na ing spo s co esponding o MEN and THY
egions is obse ed. These spo s co espond o MEN: THY clus e s
wi h MEN o THY on he g aphene su ace and wi h he second
membe o he couple o he clus e placed abo e he shee co e-
sponding o he second laye in e ed om Fig. 5.
The o ien a ion o adso bed molecules on g aphene a e
epo ed in Fig. 6 wi h angle de ini ion o Fig. 1. A di e en beha -
io is obse ed o con inemen in small nanopo es (Fig. 6a, d = 1
0 Å) and he emaining wide po es (Fig. 6b o 6 ). In he case o
d = 10 Å nanopo e, MEN molecules show an angula dis ibu ion
spanning in he 60°120° ange, which con i ms ha molecules
a e no co-plana wi h espec o he su ace. Fo THY, he wo
peaks a 60 and 105°, indica es also a la ge p e e ence o non-
plana o ien a ions. As he nanopo e diame e inc eases he p e e -
ence o THY molecules o be a anged in a co-plana way wi h he
g aphene su ace is de eloped, as showed by he peaks cen e ed a
90°in Fig. 6b-6 . Complemen a y, o MEN wo peaks a oughly
60°and 105°indica e p e e ence o adop skewed non-pa allel o i-
en a ions, which ag ees wi h hose ob ained wi h DFT echniques,
Fig. 4. The e o e, he small a ailable space o d = 10 Å nanopo e
allows he de elopmen o a single adso bed laye on each g a-
phene su ace. This s e ic es ic ion leads o a dis up ion o he
end o THY molecules o be adso bed on he su ace wi h a co-
plana o ien a ion p omo ing MEN – THY hyd ogen bonding. Bu
as he nanopo e size inc eases he THY co-plana adso p ion is
accompanied wi h sui able hyd ogen bonding wi h MEN molecules
in he second adso bed laye on op o i , hus enabling e icien
THY – g aphene and THY – MEN in e ac ions. In he case o MEN
molecules, he weake in e ac ion wi h g aphene su ace gi es ise
o skewed o ien a ions upon adso p ion which allows e icien
MEN – THY hyd ogen bonding.
The s eng h o MEN: THY in e ac ions wi h he g aphene su -
ace upon adso p ion is epo ed in Fig. 7. The e olu ion o s eng h
Fig. 9. A e age numbe o hyd ogen bonds pe MEN molecule in MEN: THY 1:1
con ined be ween pe iodic g aphene shee s wi h din e shee dis ance (Fig. 2a).
Panel a shows esul s o molecules placed in he i s adso bed laye on g aphene;
panel b shows esul s o all MEN: THY molecules. Red dashed line in panel b shows
esul s o bulk MEN: THY 1: 1 liquid phase (i.e. in absence o g aphene shee s). (Fo
in e p e a ion o he e e ences o colou in his igu e legend, he eade is e e ed
o he web e sion o his a icle.)
Fig. 10. Cen e -o -mass sel -di usion coe icien in he xy plane (D
XY
), whe e xy
s ands o he plane con aining he g aphene shee s, in MEN: THY 1: 1 con ined
be ween pe iodic g aphene shee s wi h din e shee dis ance (Fig. 2a). Dashed lines
show he alues o bulk liquid phase.
N. Aguila , Rocío Ba os, J. An onio Tamayo-Ramos e al. Jou nal o Molecula Liquids 368 (2022) 120637
7
in e ac ion wi h nanopo e size shows a complex pa e n. Al hough
o mos o he nanopo es, THY – g aphene in e ac ion ene gies a e
la ge han MEN – g aphene ones, acco dingly o he end o THY
molecules o de elop plana a angemen s on he g aphene su -
ace, a in e media e nanopo e sizes (d= 40 Å) ene gies o MEN
a e su p isingly la ge han o THY. Ne e heless, esul s in
Fig. S3 indica e ha , as a ule, he la ge numbe o THY adso bed
molecules in compa ison wi h MEN ones co esponds o s onge
in e ac ions wi h he g aphene su ace.
The a angemen o MEN – THY molecules in he adso bed laye
o g aphene is analyzed by he si e – si e Radial Dis ibu ion Func-
ion (RDFs) be ween he oxygen a oms in MEN and THY (Fig. S4a)
as well as i s e olu ion wi h nanopo e size, i.e. g aphene in e laye
sepa a ion. RDF o MEN (O) – THY(O) is cha ac e ized in bulk liq-
uid phase by a s ong and na ow peak a 2.8 Å, which is main-
ained upon con inemen o he molecules adso bed on he i s
laye in di ec con ac wi h g aphene, Fig. S4a. The in ensi y o his
RDF peak ises wi h nanopo e size. This shows he inc ease, wi h
nanopo e g ow h, o he in e ac ion ough hyd ogen bonding
be ween MEN and THY molecules upon adso p ion. In eg a ion o
he RDFs, Fig. S4b, illus a es his a gumen and jus i ies he linea -
i y o he inc ease o he numbe o adso bed MEN: THY dime s
wi h he size o he nanopo e, Fig. S4c. The e o e, he e y e ec i e
adso p ion o MEN: THY on he g aphene su ace is also accompa-
nied by an e icien MEN – THY hyd ogen bonding wi h molecules
in he i s adso bed laye and hose in he icini y o molecules in
di ec con ac wi h he g aphene su ace.
The molecula dis ibu ion o molecules con ined in he nano-
po es is also analyzed by domain analysis [55] using Vo onoi –
based me hod, Fig. S5. Domain-coun numbe is one bo h o
MEN and THY, wi h he excep ion o he na owe nanopo e,
Fig. S5a. This is an e idence o an ex ended con inuous ne wo k
o hese species. Likewise, he domain olume and a ea, Fig. S5b
and S5c, inc ease linea ly wi h nanopo e size. This shows a cou-
pling be ween molecules in he i s adso bed laye and hose in
he egions no di ec ly in e ac ing wi h he su ace. I also ag ees
wi h he RDFs epo ed in Fig. S4 and con i ms ha he adso p ion
on g aphene is accompanied by e icien MEN – THY hyd ogen
bonding in he whole con ined luid. The calcula ed isope ime ic
quo ien , Fig. S5d, a measu e o he sphe ici y o he molecula
domains, dec eases wi h inc easing nanopo e size. The la ges al-
ues o his quo ien co espond o he na owe nanopo e, indica -
ing sphe ici y domain loss as he nanopo e size inc eases.
The dynamics o molecula mo emen was analyzed conside -
ing eloci y dis ibu ion unc ions, Fig. 8, ela ed wi h molecule
size as well as wi h he de eloped in e molecula in e ac ions.
The epo ed esul s show dis ibu ion cu es wi h he same max-
ima o MEN and THY, panels a and b o Fig. 8, in ag eemen wi h
he iew o MEN – THY sys em as a s ongly coupled one h ough
hyd ogen bonding and hus win displacemen o bo h ypes o
molecules. Likewise, he con inemen has a mino e ec on he
molecula mo emen . The eloci y dis ibu ion cu es show mo e
in ense peaks bu wi hou changing he posi ion o he maxima.
This is a mani es a ion o a sligh ein o cemen o he mo emen s
a lowe eloci ies because o con inemen . Al hough his mino
change p oduced by adso p ion and con inemen does no lead o
a la ge pe u ba ion o he global molecula mo emen because o
he ex ended hyd ogen bonding ne wo k. The almos negligible
pe u ba ion on he MEN – THY hyd ogen bonding upon con ine-
men is con i med by he epo ed an Howe co ela ion unc ions,
Fig. S6, which a e equal o he case o bulk luid and con ined
luid, wi h only a mino change o he main peak a 2.8 Å co e-
sponding o a sligh dec ease o molecula eloci ies as in e ed
om Fig. 8.
Fig. 11. (a,b,c) Snapsho s and (d,e, ) numbe densi y p o iles in he di ec ion pe pendicula o he SWNT long axis as a unc ion o he dis ance o he SWNT longi udinal axis,
, in MEN: THY 1: 1 + SWNTs.
N. Aguila , Rocío Ba os, J. An onio Tamayo-Ramos e al. Jou nal o Molecula Liquids 368 (2022) 120637
8
Al hough esul s in p e ious sec ions ha e p obed mino
changes in MEN – THY ela i e o ien a ion and hus hyd ogen
bonding should be mos ly main ained, he de elopmen o e ec-
i e hyd ogen bonding equi es pa icula in e molecula o ien a-
ion. We ha e de ined a geome ical c i e ia conside ing 3.5 Å
and 60° o dono – accep o hyd ogen bonding si es and quan i-
ied he numbe o H a oms ul illing his c i e ia. Resul s a e
shown in Fig. 9. The ex ension o MEN – THY o hose molecules
placed in he i s adso bed laye on g aphene, Fig. 9a, indica es
ha he numbe o hyd ogen bonds pe molecule dec eases in a
non-linea way wi h dec easing nanopo e size. Al hough RDF
esul s in Fig. S4 indica e ha ela i e MEN – THY dis ibu ion does
no change upon adso p ion, he ea angemen s induced by he
small nanopo es hinde s he hyd ogen bonding. This can be due
o s e ic cons ain s and only when emo ed, because o nanopo e
size inc ease, allow he numbe o hyd ogen bonds o app oach
hose o unpe u bed, bulk liquid, sys ems. The p esence o nano-
po e no only changes he hyd ogen bonding o he molecules in
he adso bed laye s bu he pe u ba i e e ec is ex ended beyond
his laye dis up ing he whole con ined luid, Fig. 9b. As i can be
obse ed in his Figu e only he la ge nanopo es (d > 50 Å) show
hyd ogen bonding ex ension simila o hose in bulk liquid phase.
These sub le changes in MEN – THY ela i e o ien a ion a e con-
i med by he epo ed Combined Dis ibu ion Func ions (CDFs)
o he dis ance and angle ele an o hyd ogen bonding, Fig. S7.
Al hough he ele an ( ed) spo s a 2.8 Å (in ag eemen wi h RDFs
in Fig. S4) a e main ained upon con inemen , CDF esul s indica e
ha hey a e weake o small nanopo e. This indica es ha he
geome ical condi ion ega ding he epo ed o ien a ion angle is
no ul illed, hinde ing hyd ogen bonding: MEN and THY molecules
a e a anged a sui able dis ances o hyd ogen bonding bu no a
sui able o ien a ion. This e ec can be unde s ood as he ea -
angemen ha imp o es MEN/THY – g aphene in e ac ions upon
adso p ion, and i diminishes as he nanopo e size inc eases
app oaching he CDFs bulk liquid phase. Hence, he ea angemen
o molecules on he adso bed laye s does no change he MEN –
THY ela i e dis ibu ion in e ms o sepa a ion o numbe o
neighbo molecules bu hinde s he ul illmen o he (angula o i-
en a ion) geome ical condi ions equi ed o de elop ue hyd o-
gen bonds.
I could be expec ed ha changes in hyd ogen bonding would
ha e a e lec on molecula dynamics. We ha e quan i ied he co -
esponding sel -di usion coe icien s, as epo ed in Fig. 10, and,
indeed, he dynamics o MEN and THY is coupled upon con ine-
men . This is in ag eemen wi h esul s o p e ious sec ions in spi e
o he weakening o hyd ogen bonding, as i was in he bulk liquid
phase. The dec ease o hyd ogen bonding upon con inemen leads
o an inc ease o molecula mobili y in he plane pa allel o g a-
phene walls (D
xy
) and only o he wide nanopo es hese alues
app oach hose o bulk liquid phase. The e is also a nanopo e size
egion in he d= 30 Å o 40 Å ange wi h la ge mobili ies, which
ag ees wi h he changes in in e ac ion ene gy epo ed in Fig. 7a:
mobili y echoes change in in e ac ion ene gy wi h g aphene, and
hus i poin s o a ansi ion om a la gely con ined luid o a luid
wi h p ope ies close o he bulk liquid phase, including he e o-
lu ion o hyd ogen bonding.
3.2. MEN: THY sol a ing and con ined in SWNTs / he e ojunc ions
The in e ac ion be ween MEN: THY 1: 1 and SWNTs leads o he
NADES being con ined inside he nano ube (i.e. esembling wi h
cylind ical symme y luid con inemen in sli nanopo es epo ed
in he p e ious sec ion) and also o he NADES being adso bed in
he ex e nal SWNT su ace (i.e mimicking on a con ex su ace
adso p ion ha on he plana one o g aphene). Snapsho s o he
NADES dis ibu ion inside and ou side he SWNTs a e epo ed in
Fig. 11a o 11c as a unc ion o nano ube in e nal diame e , con-
i ming MEN: THY con inemen and ex e nal adso p ion o all
he conside ed SWNTs. Fo he case o molecules inside he SWNTs,
he snapsho s and he densi y p o iles epo ed in Fig. 11d o 11
indica e he o ma ion o an in e nal adso bed laye on he conca e
in e nal wall o he nano ube. As he SWNT diame e inc eases a
egion o molecules placed a ound he cen e o he nano ube is
also de eloped. Fo he na owe SWNT, Fig. 11a and 11d, he
a ailable space does no allow o de elop a ully in e nal adso bed
laye as he absence o densi y peaks shows, and only o he wide
SWNT, Fig. 11c and 11 , he p ope ly de ined adso bed laye is
de eloped, wi h THY molecules close o he su ace and MEN
molecules wi h sligh excess in in e nal egions. The esul s o
SWNT(20,20) in e nal ca i y show analogous dis ibu ion o hose
o g aphene, Fig. 5: he inc easing a ailabili y o space and he
inc ease in nano ube diame e ha leads o a dec ease in he wall
Fig. 12. (a) SWNT – MEN /THY in e ac ion ene gy, E, (b) numbe densi y inside
SWNTs, and (c) cen e -o -mass sel -di usion coe icien o molecules inside he
SWNT ca i y, D. Dashed lines in panel c shows D alues in bulk liquid phase. All
esul s o MEN and THY molecules, o MEN: THY 1: 1 + SWNTs as a unc ion o
SWNTs in e nal adius,
SWNT
.
N. Aguila , Rocío Ba os, J. An onio Tamayo-Ramos e al. Jou nal o Molecula Liquids 368 (2022) 120637
9
