Vol.:(0123456789)
Jou nal o The mal Analysis and Calo ime y
h ps://doi.o g/10.1007/s10973-024-13333-y
E ec o alkyl chain leng h on he he mal p ope ies and oxici y
o n‑alkyl‑ammonium ni a e ionic liquids (n = 2, 3, 4, 5, 6, 8) o ene gy
applica ions
M.Villanue a1 · P.Valle 1· T.Teijei a1· A.San iago‑Alonso1· A.Amigo2· E.Tojo3· L.M.Va ela1· J.J.Pa ajó1·
J.Salgado1
Recei ed: 2 No embe 2023 / Accep ed: 19 May 2024
© The Au ho (s) 2024
Abs ac
The mos cu en ly used ionic liquids (ILs) a e p o ic ionic liquids (PILs), subjec o ex ensi e in es iga ion ega ding hei
physical p ope ies. These compounds along wi h hei mix u es wi h o he subs ances such as sal s and sol en s, se e as
elec oly es in nex gene a ion elec ochemical sma de ices, and eme ge as iable candida es o eplace con en ional Hea
T ans e Fluids (HTFs) in a ious ene gy applica ions. Despi e he ex ensi e numbe o s udies, impo an in o ma ion abou
his kind o compounds is s ill unknown, such as he e ec o alkyl chain leng h on he mal and he mophysical p ope ies,
as well as oxici y. This wo k, ex ending p e ious s udies o ou g oup, summa izes he liquid ange, hea capaci y and acu e
oxici y le el o six ammonium ILs: speci ically, n-alkyl-ammonium ni a e ILs wi h inc easing alkyl chain leng h (n = 2, 3,
4, 5, 6, 8). Fo his s udy, he syn hesis o he h ee ILs wi h he longes alkyl chain was pe o med, along wi h DSC, TGA
and oxici y measu emen s. I was obse ed ha an inc ease in alkyl chain leng h esul ed in a dec ease in sho - e m he mal
s abili y and an inc ease in mel ing empe a u e, indica ing a educ ion in he liquid ange. A compensa ion e ec be ween
en halpy and en opy o mel ing was obse ed o he s udied chain leng hs. The isoba ic speci ic and mola hea capaci-
ies inc ease wi h empe a u e o all he compounds s udied he e, and good co ela ions we e ob ained be ween mola hea
capaci y and he numbe o ca bon a oms in he alkyl chain o e e y empe a u e. Finally, mos o he ILs a e non- oxic,
al hough oxici y inc eases wi h alkyl chain leng h.
Keywo ds Ionic liquids· The mal p ope ies· Eco oxici y· T anspo p ope ies
In oduc ion
Al hough he ield o ionic liquids (ILs) s a ed began mo e
han a cen u y ago, had long pe iods o insigni icance du ing
he wen ie h cen u y, bu in ecen decades has aken g ea
impo ance [1]. ILs a e widely de ined as ionic ma e ials, in
he sense ha ha e been o med o ca ions and anions wi h
low mel ing poin s, lowe han 100ºC, wi hou any physi-
cal o chemical sense a ached o his empe a u e. These
con o ma ional cha ac e is ics lead ha hese compounds
ha e e y in e es ing p ope ies, such as hei high chemical
s abili y and hei high miscibili y wi h o he sol en s [2].
Bu he mos iden i ying p ope ies a e i s low, almos ze o,
apou p essu e and he possibili y o modi ying i s p ope -
ies by in oducing unc ional g oups in o he mos apola
pa o i s s uc u e, which is known as he abili y o unning
hese compounds. Fo his eason, he applicabili y o he
ionic liquids has inc eased eno mously in he las decades,
* M. Villanue a
ma ia. illanue [email p o ec ed]
J. J. Pa ajó
[email p o ec ed]
1 G upo de Nanoma e iais, Fo ónica e Ma e ia B anda,
Depa amen os de Física Aplicada y Física de Pa ículas,
Uni e sidade de San iago de Compos ela, Campus Vida s/n,
15782San iagodeCompos ela, Spain
2 G upo de P opiedades Te mo ísicas e Supe iciais de
Líquidos, Depa amen o de Física Aplicada, Uni e sidade
de San iago de Compos ela, Campus Vida s/n,
15782San iagodeCompos ela, Spain
3 Depa men o O ganic Chemis y, Uni e sidade de Vigo,
Ma cosende,Vigo, Spain
M.Villanue a e al.
being now used in a ious ields, such as sol en s, ca alyse s,
elec oly es, hea ans e luids (HTFs), lub ican bases and
abso ben s in hea ing pumps [3–7]. Howe e , his unabili y
becomes a g ea challenge since he combina ions o ca ions
and anions ha can o m an IL a e almos unlimi ed [8].
The e o e, i is c ucial o unde s and, o example, how he
alkyl chain leng h and di e en unc ional g oups de e mine
he IL p ope ies.
Two subca ego ies o ILs can be ound, p o ic (PILs) and
ap o ic (APILs) ILs. The key p ope ies ha dis inguish PILs
om o he ILs a e he p o on ans e om he acid o he
base, esul ing in he p esence o p o on-dono and -accep o
si es. These si es can be used o build up a hyd ogen-bonded
ne wo k, leading o impo an p ope ies such as low is-
cosi y and high-wa e solubili y [9] and lowe oxici y [10].
The syn hesis o PILs equi es a simple equimola acid–base
eac ion ollowed by d ying, making PILs equen ly he
simples and mos economical o syn hesize. Alkylammo-
nium ni a es all wi hin his subca ego y.
Despi e he la ge numbe o pape s in es iga ing he p op-
e ies and applica ions o he ILs, signi ican gaps emain o
unde s anding hei beha iou specially o PILs [8] because
hey ha e, in gene al, be e anspo p ope ies and biodeg-
adabili y, as well as being simple and less expensi e o
syn hesize [11, 12]. Among he knowledge gaps o PILs, i
is c ucial o cha ac e ize he changes ha occu in di e en
he mophysical p ope ies based on he ca ion alkyl chain,
enabling he a ional design o PILs o speci ic applica ions.
Fu he mo e, aking in mind ha oxici y is also an
impo an pa ame e when selec ing a ma e ial o indus ial
applica ions. ILs ha e an inhe en epu a ion as en i onmen-
ally iendly sol en s due o hei negligible apou p es-
su e, al hough low ola ili y does no comple ely elimina e
he po en ial en i onmen al haza ds, and hei wa e solubil-
i y could be a eal conce n. I has also been obse ed ha an
inc ease in alkyl chain leng h, o lipophilici y, is associa ed
wi h highe deg ada ion a es and inc eased oxici y [13].
Mu alib and G ahem [14] in hei e iew, indica e ha alkyl
chain leng h appea s o be he dominan pa ame e con ol-
ling he oxici y o ILs owa ds di e en aqua ic o ganisms,
including Alii ib io Fische i (A. Fische i) and o he ophic
o ganisms. These au ho s ca ied ou a comp ehensi e s udy
o nume ous amilies o ILs wi h di e en anions and ca i-
ons, bu PILs as common as alkylammonium ni a es we e
no included in hei esea ch.
The aim o his wo k was o comple e he he mophysical
cha ac e iza ion o alkylammonium ni a e ILs, ini ia ed in a
p e ious wo k wi h e hyl-, n-p opyl and n-bu yl-ammonium
ni a e (EAN, PAN and BAN, espec i ely) [15]. Fo his
pu pose, he he mal beha iou and hea capaci y o n-pen-
yl, n-hexyl- and n-oc ylammonium ni a e, (PEAN, HEAN
and OAN), a e epo ed and compa ed wi h hose ob ained
o EAN, PAN and BAN, aiming o p ope ly es ablish he
e ec o he ca ion alkyl chain leng h on hese physicochem-
ical p ope ies.
Fu he mo e his s udy aims o also p o ide a comple e
cha ac e iza ion o he acu e oxici y, in e ms o biolumi-
nescence a ia ion o he ma ine bac e ium A. Fische i o
n-alkylammonium ni a e ILs (n = 2, 3, 4, 5, 6, 8).
Ma e ials andMe hods
Chemicals
N-amylamine (Ac os O ganics, ≥ 99%), N-hexylamine
(The mo Scien i ic Chemicals, 98%), N-oc ylamine (Sigma-
Ald ich, 99%), n-Hexane (≥ 99%, Ac os O ganics) and
Ni ic acid (65% solu ion in wa e , Ac os O ganics) we e
used as ecei ed wi hou any p e- ea men .
Syn hesis
1H and 13C NMR spec a we e eco ded on a BRUKER ARX
4CO spec ome e a 400.1621 (1H) and 100.6314 (13C)
MHz, espec i ely. CDCl3 (ACROS O ganics, 99.6 + a om %
D) was employed as deu e a ed sol en . Chemical shi s a e
quo ed in pa s pe million (ppm) ela i e o he signals co -
esponding o he esidual non-deu e a ed sol en (CDCl3:
δH = 7.26ppm, δC = 77.16ppm). ESI Mass spec a we e
eco ded on a BRUKER FTMS APEXIII. The glass ma e ial
employed in he syn he ic eac ions was d ied in an o en a
333K o 24h be o e use.
Syn hesis o n-Pen ylammonium ni a e (PEAN). N-amyl-
amine (45mL, 0.3834mol) and hexane (50mL) we e placed
in a ound bo om lask o e an ace one/d y ice ba h. Ni ic
acid (65% solu ion, 26.55mL) was added d op wise. The
solu ion was s i ed a oom empe a u e, and he o ma ion
o wo laye s was obse ed. Hexane was decan ed, sol en s
we e emo ed hea ing unde low p essu e and he esul ing
ionic liquid was d ied hea ing a 50 ºC unde high acuum
o 48h. Pen ylammonium ni a e (PEAN) was ob ained as
a colou less semisolid (56.46g, 98%). RMN-1H (CDCl3, δ,
400MHz): 7.44 (s, 3H, NH3), 3.04 (m, 2H, CH2NH3), 1.70
(m, 2H, CH2CH2NH3), 1.33 (m, 4H, CH2CH2CH2CH2NH3),
0.88 ( , 3H, J = 7.0Hz, CH3CH2CH2CH2CH2NH3). RMN-
13C (CDCl3, δ, 100.6MHz): 40.4, 28.4, 27.1, 22.1, 13.7.
HRMS (ESI) m/z (%): ound 88.1122 [A]+ (100), calcd o
[C5H14N]+: 88.1126; ound 238.2135 [A2B]+ (40), calcd o
[C10H28N3O3]+: 238.2131.
Syn hesis o n-Hexylammonium ni a e (HEAN). N-hex-
ylamine (89.40mL, 0.67mol) and hexane (100mL) we e
placed in a ound bo om lask o e an ace one/d y ice ba h.
Ni ic acid (65% solu ion, 46.40mL) was added d op wise.
The solu ion was s i ed a oom empe a u e, and he o -
ma ion o wo laye s was obse ed. Hexane was decan ed,
E ec o alkyl chain leng h on he he mal p ope ies and oxici y o n‑alkyl‑ammonium ni a e…
sol en s we e emo ed hea ing unde low p essu e and he
esul ing ionic liquid was d ied hea ing a 50 ºC unde ac-
uum o 48h. Hexylammonium ni a e was ob ained as a
colou less semisolid (109.68g, 99.7%). RMN-1H (CDCl3,
δ, 400MHz): 7.47 (s, 3H, NH3), 3.06 (m, 2H, CH2NH3),
1.73 (aquin , J = 7.6, 2H, CH2CH2NH3), 1.32 (m, 6H,
(CH2)3(CH2)2NH3), 0.89 ( , 3H, J = 6.8Hz, CH3(CH2)5NH3).
RMN-13C (CDCl3, δ, 100.6MHz): 40.4, 31.2, 27.5, 26.1,
22.4, 13.9. HRMS (ESI) m/z (%): ound 102.1278 [M]+ (40),
calcd o [C6H16N]+: 102.1283; ound 266.2440 [A2B]+
(100); calcd o [C12H32N3O3]+: 266.2444.
Syn hesis o n-Oc ylammonium ni a e (OAN). N-oc -
ylamine (96.45mL, 0.58mol) and hexane (100mL) we e
placed in a ound bo om lask o e an ace one/d y ice ba h.
Ni ic acid (65% solu ion, 40.0mL) was added d op wise.
The solu ion was s i ed a oom empe a u e, and he o -
ma ion o wo laye s was obse ed. Hexane was decan ed,
sol en s we e emo ed hea ing unde low p essu e and
he esul ing ionic liquid was d ied hea ing a 50ºC unde
acuum o 48h. Oc ylammonium ni a e was ob ained as
a colou less semisolid (110.7g, 99%). RMN-1H (CDCl3,
δ, 400MHz): 7.46 (s, 3H, NH3), 3.03 (m, 2H, CH2NH3),
1.70 (m, 2H, CH2CH2NH3), 1.26 (m, 10H, (CH2)5(CH2
)2NH3), 0.86 ( , 3H, J = 6.9Hz, CH3(CH2)7NH3). RMN-13C
(CDCl3, δ, 100.6MHz): 40.4, 31.8, 29.2, 29.1, 27.6, 26.5
(CH2(CH2)5NH3), 22.6, 14.0. HRMS (ESI) m/z (%): ound
130.1592 [A]+ (100), calcd o [C8H20N]+: 130.1596.
Phase ansi ion de e mina ion
A di e en ial scanning calo ime e DSC Q1000 om TA-
Ins umen s was used o de e mine he di e en phase ansi-
ions expe imen ed by he selec ed ILs du ing hea ing and
cooling cycles. Samples, wi hou u he pu i ica ion, we e
placed in a 40 μL he me ically sealed aluminium pan wi h
a pinhole a he op o he co e . Each sample (3–5mg) was
subjec ed o ou amps, wo in cooling and wo in hea ing
mode, wi h an iso he mal s ep be ween hem, ollowing he
same schedule han as ha used in p e ious wo ks [16, 17],
which is essen ial o enabling compa ison o he esul s
ob ained in he p esen wo k wi h hose o p e ious s ud-
ies. A p e ious iso he mal s ep a 120°C o 45min was
included o emo e impu i ies (mainly wa e abso bed du -
ing he sample p epa a ion) and e ase he he mal his o y o
he samples. T ansi ions empe a u es we e de e mined om
he DSC cu es, as he onse poin s o he di e en peaks,
du ing he ehea ing and e-cooling s eps. Tempe a u e and
hea o mel ing o indium we e used o calib a ion [6, 17].
The mog a ime ic analysis
To s udy he sho - e m he mal s abili y and de e mine
he highe empe a u e o he liquid ange window, he mal
s abili y o he selec ed ILs was pe o med. Expe imen s
we e ca ied ou using a DSC/TGA1 ins umen om
Me le Toledo in dynamic mode, wi h a hea ing a e o
10K min−1 and a pu ge gas low o 20 cm3 min−1, om
(50 o 800) ºC unde a N2 a mosphe e. Samples o 3–5mg
we e placed in an open pla inum pan [18, 19].
Hea capaci y
Speci ic hea capaci ies we e ob ained using he iso he mal
s ep me hod wi h a Mic o DSCIII di e en ial scanning
calo ime e om Se a am, F ance. Calib a ion was pe -
o med using a Joule e ec calib a ion essel (Se a am)
and checked using n-decane and squalane (Sigma-Ald ich
99%) as hea capaci y s anda ds [20]. The backg ound
noise o he Mic o DSCIII was less han 3µW. Dynamic
scans o samples o a ound 0.6g we e conduc ed a a hea -
ing a e o 0.25K min−1 wi hin he empe a u e ange o
(293.15–348.15) K. Squalene was used as he e e ence
luid o de e mine he hea capaci y alues due o i s simi-
la alues o hea low and he high p ecision o he a ail-
able li e a u e da a [20]. Expe imen s ollowed he expe i-
men al echnique and p ocedu e p e iously desc ibed [21,
22]. The s anda d unce ain y in Cp expe imen al alues is
es ima ed o be 0.002J·g−1·K−1.
Toxici y
The oxici y o he six ILs (CnNH3NO3; n = 3, 4, 5, 6 and
8) was assessed using he Mic o ox® Toxici y Tes ki
ollowing he same p ocedu e de ailed elsewhe e [23, 24].
The Mic o ox® Toxici y es e alua es he luminescence
inhibi ion o he G am‐nega i e ma ine bac e ia Alii i-
b io Fische i (A. Fische i) h ough a popula ion‐dependen
mechanism called quo um sensing sensi i e. I conside s
ha exposu e o a oxic subs ancedis up s he espi a ion
o he bac e ia, being he wi h luminescence p oduc ion
being di ec ly p opo ional o he me abolic ac i i y o
he bac e ial popula ion. The esul s he e ob ained we e
comple ed wi h hose p e iously ob ained o he IL wi h
he sho es alkyl chain leng h, EAN [24].
A se ies o dilu ed aqueous solu ions ( om 0 o 81.9%)
o each IL was p epa ed o he acu e oxici y es . Va ious
exposu e imes (5, 15 and 30min) o he IL we e analysed.
These da a we e used o es ima e he e ec i e concen a-
ions ha p omo e 50%, 20% and 10% (EC50, EC20 and
EC10, espec i ely) o luminescence inhibi ion, and he
co esponding 95% con idence in e als h ough a non-
linea eg ession, using he leas -squa es me hod o i he
da a o he logis ic equa ion[23].
M.Villanue a e al.
Resul s anddiscussion
Syn hesis
The syn hesis o he selec ed ILs n-pen yl-, n-hexyl- and
n-oc ylammonium ni a e (PEAN, HEAN, OAN) was ca ied
ou h ough a neu aliza ion eac ion o he p ecu so B Øn-
s ed acid/base pai using s oichiome ic a ios o he acid
(ni ic acid) and he base ( he co esponding n-alkylamine),
ollowing a p ocedu e p e iously desc ibed [25]. Thei
s uc u es we e de e mined by1H and 13C NMR, as well as
High Resolu ion Mass Spec a, and in he case o PEAN,
con i med by compa ison wi h he NMR da a p e iously
desc ibed [26]. Spec a a e p o ided in he ESI ( igu esS1-
S3). F om he NMR spec a a pu i y ≥ 99 mass% was es i-
ma ed o he h ee syn hesized ILs, de e mined using he
ollowing equa ion, whe e I ep esen s he ela i e a ea o
each signal: [27]
The ESI mass spec a o he syn hesized n-alkylam-
monium ni a e ILs showed he o ma ion o he ypical
ILs clus e ions o he ype [A2B]+, whe e A = ca ion and
B = anion. To ou knowledge, no NMR o Mass spec a ha e
been p e iously published o HEAN and OAN. The chemi-
cal s uc u es and pu i ies o he syn hesized ILs a e shown
in Table1.
Liquid ange
The liquid ange is he empe a u e in e al be ween mel ing
and he mal deg ada ion empe a u es.
DSC phase ansi ion analysis
In Fig.1, he DSC cu es ob ained o he syn hesized ILs,
PEAN (a), HEAN (b) and OAN (c), in hea ing and cooling
amps a e shown. Table2 summa izes he he mal ansi-
ions ob ained om DSC cu es o he h ee ILs in cooling
and hea ing amps. Resul s o EAN [6], PAN and BAN [15]
Pu i y
%=
�∑
Ip oduc
∑
I o al
�
⋅
100
Table 1 S uc u e, abb e ia ions, and pu i y o he selec ed ILs
Name Abb e ia ion Chemical s uc u e Molecula mass /
g·mol−1 Pu i y/
mass%
n-Pen hylammonium ni a e PEAN C5H14N2O3
+
H3NNO3−150.18 ≥ 99
n-Hexylammonium ni a e HEAN C6H16N2O3
+
H3NNO3−164.20 ≥ 99
n-Oc ylammonium ni a e OAN C8H20N2O3
+
H
3
NNO3−192.25 ≥ 99
5.0
0.0
0.0
– 4.0
– 8.0
2.5
1.5
0.5
– 0.5
– 1.5
– 2.5
– 3.5
– 4.5
4.0
– 5.0
– 10.0
– 15.0
– 20.0
– 40 – 20 0
Hea ing
Hea ing
Hea ing
Cooling
Cooling
Cooling
20 40 60
Tempe a u e/°C
– 40 – 20 020 40 60
Tempe a u e/°C
(c)
(b)
(a)
– 40 – 20 0204
06
0
Tempe a u e/°C
Hea low/W g–1
Hea low/W g–1 Exo
Exo Endo
Endo
Hea low/W g–1 ExoEndo
Fig. 1 DSC cu es on hea ing and cooling amps o  a PEAN,
bHEAN and cOAN ILs
E ec o alkyl chain leng h on he he mal p ope ies and oxici y o n‑alkyl‑ammonium ni a e…
ha e been p e iously epo ed bu , o a be e compa ison,
hey a e also included in Table2. As i can be seen, mel ing
(endo he mic peak in hea ing amp) and eezing (exo he -
mic peak in cooling amp) ansi ions we e ound o he
h ee ILs wi hin he s udied empe a u e ange. I mus be
ema ked ha eezing empe a u es a e lowe han mel ing,
e lec ing he cha ac e is ic supe cooling e ec ypical in
ILs [28, 29] Howe e , he supe cooled liquid egion wi h
inc easing ca ion alkyl chain leng h, and i ually disappea s
o he longes chains s udied. This supe cooling e ec ends
o diminish and p ac ically anish wi h inc easing ca bon
chain leng h.
F om Fig.1a, PEAN exhibi s se e al small peaks in hea -
ing and cooling amps, wi h a main endo he mic peak in
hea ing and an exo he mic peak in cooling. These could all
be associa ed wi h he mel ing o di e en c ys alline o ms,
wi h he p edominan o m ha ing an onse empe a u e o
33 ºC. As obse ed, he e is conco dance be ween he peaks
in he cooling and hea ing cu es, all o hem exhibi ing
he a o emen ioned supe cooling e ec . Ne e heless, in
he case o he peak obse ed a 40–50ºC du ing hea -
ing, i spli s in o wo peaks on he cooling scan. In o de
o check he dependence o he supe cooling e ec , alues
co esponding o he hys e esis o he six ILs a e p esen ed
in he ESI ile in TableS1. Fo HEAN (Fig.1b), a simila
beha iou is obse ed, wi h he peak onse a 36 ºC co e-
sponding o he main mel ing p ocess. PEAN and HEAN a e
polymo phic ILs [30], bu OAN (Fig.1c), wi h a single peak
upon hea ing and a single peak upon cooling, does no p e-
sen his beha iou . To he bes o ou knowledge, no simila
p e ious esul s o hese ILs ha e been epo ed. Figu e2a
shows he mel ing empe a u e o n-alkyl-ammonium ni a e
ILs in e ms o he numbe o ca bon a oms in he ca ion
alkyl chain leng h (n = 2, 3, 4, 5, 6, 8). PAN exhibi s he low-
es mel ing empe a u e, while BAN he highes , indica ing
ha he odd and e en numbe s o alkyl chain g oups p esen
di e en beha iou , as p e iously poin ed ou by Rod igues
& San os [31] who a ibu ed i o he con ibu ion o he
alkyl chain o he s abili y o c ys al packing. As seen in
Table2, he mel ing poin does no di ec ly co ela e wi h he
alkyl chain leng h, as p e iously obse ed o py olidinium-
based ILs [32]. G ea es & D ummond [33] no ed in hei
in e es ing e iew ha mos o he alkylammonium ni a e
compounds a e conside ed as high empe a u e mol en sal s,
gi en ha hei mel ing poin s exceed100 ºC. Howe e , all
six alkylammonium ni a e ILs s udied he e exhibi mel -
ing empe a u es below ha empe a u e h eshold. This
Table 2 Onse empe a u e, en halpy, and en opy o he main mel -
ing peak pe g am and pe mol o sample ( m, ΔmH and ΔmS), and
onse empe a u e and en halpy o he main eezing peak pe g am
o sample ( and Δ H) ob ained om DSC cu es o he ILs. Onse
empe a u e, peak empe a u e, and Woos e empe a u e ( onse , peak,
Woos e ) ob ained om dynamic TG and DTG cu es. Expe imen s
we e pe o med unde an a mosphe ic p essu e o (1008 ± 10) hPa
and a ela i e humidi y o (55 ± 10)%
Expanded unce ain ies a e U( ) = 6°C and U (ΔH) = 4% (0.95 le el o con idence (k = 2))
IL / °C m/ °C Δ H/ J g−1h ΔmH/ J g−1 ΔmH/ kJ mol−1 ΔmS/ J g−1 K−1 ΔmS/ J
mol−1 K−1 onse / °C peak/ °C Woos e / °C [37]
EAN [6] − 27 12 76 122 13.2 0.43 46 248 260 108
PAN [15] − 28 5 61 71 8.6 0.25 31 244 255 107
BAN [15] − 6 40 55 126 17.1 0.40 55 235 244 115
PEAN 24 33 45 42 6.3 0.14 21 208 218 109
HEAN 28 36 42 42 6.7 0.13 22 183 215 108
OAN 26 30 64 63 11.4 0.20 38 178 191 80
-5
5
15
25
35
45
55
100
140
180
220
260
123456789
Mel ing
empe a u e/ºC
Deg ada ion empe a u e
/°C
C numbe in alkyl chain/n
0
10
20
30
40
50
60
0
3
6
9
12
15
18
123456789
m
S/J K–1 mol–1
m
H/kJ mol–1
C numbe in alkyl chain/
n
(a)
(b)
Fig. 2 Ca ion chain leng h (n) dependence o a onse deg ada ion and
onse mel ing empe a u es ob ained om TGA and DSC expe i-
men s, espec i ely, and b en halpy and en opy o usion

M.Villanue a e al.
con adic ion could be explained by he ac ha G ea es
& D ummond analysed he mel ing empe a u e o wel e
ammonium ni a es wi h di e en alkyl g oups (a oma ic,
es e , and ami ied g oups), whe eas in he p esen wo k,
he ammonium compounds di e only in he leng h o he
ca ion alkyl chain. The use o di e en alkyl g oups esul s
in a ying ca ion–anion in e molecula o ces in he PIL,
which a ec he mel ing poin [34].
The mel ing poin o ILs is s ongly ela ed o he s eng h
o he c ys al la ice, which, in u n, is de e mined by in e -
molecula o ces, molecula symme y, and con o ma ional
deg ees o eedom, ha is by he packing e iciency. Bagno
e al. [35] ha e sugges ed ha in he case o ILs packing
ine iciency, low mel ing poin s a e obse ed [31]. In ou
case, al hough he mel ing poin s a e lowe han 100 ºC, only
EAN and PAN a e eally oom empe a u e ionic liquids
(RTILs). Then, he highes packing e iciency co esponds
o BAN, and inc easing he alkyl chain leng h om i esul s
in a dec ease in he packing e iciency and a dec ease in
he H-bond ac ion. Addi ionally, PEAN, HEAN and OAN
show lowe alues o mel ing en halpy and en opy com-
pa ed o EAN, PAN and BAN, despi e hei high molecula
mass. This, coupled wi h he nume ous addi ional ansi ions
obse ed in hei DSC p o iles, con i ms he di icul y o o
inding a s able c ys al o m and beha iou close o a plas ic
c ys al, as indica ed by Timme mans’ c i e ium [6, 36].
Figu e2b shows a simila end o mola en halpy and
o mola en opy wi h espec o he ca bon numbe o he
alkyl chain.
The mal s abili y
The he mal s abili y o he new h ee syn hesized ILs has
been analysed h ough he mog a ime y, in dynamic mode
a a hea ing a e o 10K min−1, and unde N2 a mosphe e.
The he mal s abili y o EAN [6], PAN and BAN [15] has
also been p e iously s udied and epo ed, and i was con-
side ed o be e compa ison. All ILs show a unique s ep in
he mog a ime ic cu e. Onse empe a u es, onse , pe cen-
ual emaining mass, Wonse , empe a u e a 90% o emain-
ing mass 10% and empe a u e o he minimum o DTG,
peak, we e de e mined ollowing he c i e ion indica ed in
p e ious wo ks [6, 16]. Table2 also shows he onse and
peak empe a u es, while he o he pa ame e s can be ound
in Supplemen a y ma e ial (TableS2). In he ESI ile a com-
pa ison o he TG cu es ob ained o he six ammonium ILs
has been included (Fig S6). A clea dependence on he alkyl
chain leng h is obse ed in he h ee empe a u es, onse (see
Fig.2a) and peak empe a u es, and empe a u e a 10% o
mass loss, all o which dec ease wi h he numbe o ca bon
a oms. Conside ing hese empe a u es, he ollowing end
can be s a ed ela ed o he mal s abili y:
EAN > PAN > BAN > PEAN > HEAN > OAN.
In he same way as o empe a u e o he he mal an-
si ions, o ou knowledge, no e e ences can be ound
in he li e a u e o hese pa ame e s o he ILs PEAN,
HEAN and OAN.
As i is well known, he c i e ion based on he onse
deg ada ion empe a u e is a use ul o compa a i e analy-
sis be ween di e en ma e ials, bu i ends o o e es ima e
long- e m he mal s abili y. Fo his eason, many au ho s
use he me hod p oposed by Woos e e al. [37] which
is mo e igo ous. They sugges ed ha he empe a u e a
which 1% deg ada ion occu s in 10h ( 0.01/10h) is a eliable
indica o o he mal s abili y. Fu he mo e, Woos e e al.
[37] es ablished a me hod o es ima e TWoos e , exp essed in
Kel in, om dynamic scans using he equa ion:
being
T(
dW
∕
d ≠0
)
he empe a u e in Kel in a which he i s
app eciable mass loss occu s. The esul s o his calcula ion
a e also shown in Table2. As expec ed, he alues ob ained
a e much lowe han he onse empe a u e. This seemingly
con adic o y di e ence needs o be cla i ied: Woos e c i e-
ion is ela ed o iso he mal s udy condi ions, while he onse
empe a u e is de e mined unde dynamic condi ions. This
implies ha , o long- e m exposu e, he Woos e c i e ion
is manda o y, whe eas o sho - e m exposu e, he s abili y
limi can be ex endedup o he onse empe a u e.
Finally, he liquid ange o hese ILs, calcula ed as he
di e ence be ween he onse deg ada ion empe a u e (i
he IL is exposed o high empe a u es o a sho ime)
o Woos e empe a u e ( o applica ions equi ing p o-
longed high empe a u es) and he main mel ing empe a-
u es, was de e mined and p esen ed in Table3. The liquid
ange dec eases wi h he alkyl chain leng h, al hough i
appea s o exhibi an asymp o ic endency o he highes
ca bon numbe s. This obse a ion is in ag eemen wi h
he assump ion p oposed by Se a e al. [38] which sug-
ges s he exis ence o a c i ical alkyl size (CAS). Beyond
his poin , u he inc ease in he alkyl chain leng h does
no signi ican ly change he in e ac ion wi hin he pola
TWoos e =0.82T(dW∕d
≠
0)
Table 3 Liquid ange o he selec ed PILs
IL Sho - e m Long e m
onse — mel ing /°C Woos e — mel ing /°C
EAN 236 96
PAN 242 102
BAN 195 75
PEAN 175 76
HEAN 147 72
OAN 148 50
E ec o alkyl chain leng h on he he mal p ope ies and oxici y o n‑alkyl‑ammonium ni a e…
ne wo k (anion-ca ion) o he s uc u al o ganiza ion o
he ILs in he c ys al.
Hea capaci y analysis
The speci ic (cp) and mola (Cp) hea capaci ies in he liq-
uid phase, be ween (310.15 and 348.15) K, o PEAN,
HEAN and OAN ILs a e a ailable as suppo ing in o ma-
ion (TableS3). Fo a comple e compa ison, he speci ic
hea capaci y o he six ammonium ILs (EAN, PAN, BAN,
PEAN, HEAN and OAN) is p esen ed in Fig. S4. Fo all six
ILs, cp inc eases wi h empe a u e as expec ed, and wi h he
alkyl chain leng h, al hough a clea endency wi h n canno
be ound.
The isoba ic mola hea capaci y alues, Cp, o hese
ILs a e plo ed agains empe a u e in hei liquid ange
(310.15- 348.15) K in Fig.3. Cp o en co ela es wi h mola
mass since he deg ees o eedom o he molecule s ongly
inc ease wi h i s size [39]. As s a ed F edlake e al.[40], he
g ea e he mola mass, he g ea e he mola hea capaci y.
A second-deg ee polynomial equa ion has been used o
i he mola hea capaci y agains empe a u e. The i ing
pa ame e s, along wi h he alues o he absolu e a e age
pe cen age de ia ion (AAD%), he maximum pe cen age
de ia ion (DMAX%), and he a e age pe cen age de ia ion
(Bias%) [41] a e displayed in Table4.
A good linea co ela ion be ween mola hea capaci y
and he ca bons numbe (n) o he alkyl chain o all he em-
pe a u es was ound, as can be obse ed in Fig.4 and Figu e
S5, which show he mola hea capaci y e sus n o wo
empe a u es (313.15K, and 343.15K) and o he es o
empe a u es s udied, espec i ely. The i ing pa ame e s a e
shown in Table5. This linea endency ag ees wi h he ind-
ings o Se a e al. [38], obse ed in a amily o [CnC1im]
[PF6] ILs and Rocha e al. [42] in he [CnC1im][NT 2] IL
se ies. Fo he ammonium ILs s udied in he p esen wo k,
a slope alue o (32.3±0.7)J mol-1K−1 was ob ained o he
linea ela ionship be ween mola hea capaci y and he num-
be o ca bons, a 313.15K. This alue a each empe a u e
ep esen s he con ibu ion o he CH2 g oup o Cp a his
empe a u e [43]. The ob ained alues ange be ween 32.2
and 33.7Jmol −1 K−1, show good conco dance wi h hose
ob ained by Se a e al. [38] o [CnC1im][PF6] a 298.15K,
wi h n = 2–10, 12, (32.1±0.5)Jmol−1 K−1, using a Se a am
μDSC III mic ocalo ime e .
Wi h he aim o compa e he slope o he i ing and he
CH2 con ibu ion each empe a u e, he g oup con ibu ion
me hod in oduced by Ga das & Cou inho [43] was used o
es ima e he ole o his unc ional g oup in he mola hea
capaci y, ob aining he ollowing equa ion:
430
380
330
280
230
180
290 300 310 320 330 340 350
Tempe a u e/K
Mola hea capaci y/J mol–1 K–1
Fig. 3 Compa isons o isoba ic mola hea capaci y o he six ILs:
●EAN, ▲PAN, ■BAN, ◌PEAN, △ HEAN and OAN. Expe imen s
we e pe o med a (978 ± 5) hPa o a mosphe ic p essu e. Values o
EAN, PAN and BAN a e ep oduced om e [15]
Table 4 Pa ame e s ob ained
om a i ing o expe imen al
mola hea capaci y alues o
a second-deg ee polynomial
equa ion. Cp = a + bT + cT2 and
AAD%, DMAX% and Bias%
IL a /J mol−1 K−1 b /J mol−1 K−2 c /J mol−1 K−3 AAD% DMAX % Bias %
EAN 368.13 − 1.16 0.002 0.3013 0.0104 0.0082
PAN 571.02 − 2.33 0.004 0.2471 0.0051 − 0.0023
BAN 906.72 − 4.40 0.008 0.6016 0.0291 − 0.3583
PEAN 450.01 − 1.15 0.002 0.3901 0.0061 0.0157
HEAN 959.15 − 4.12 0.007 0.3898 0.0068 0.0681
OAN 92.78 1.61 − 0.0020 0.3790 0.0083 − 0.0562
460
410
360
310
260
210
160
1 2 345 6 7 8 9
n
Mola hea capaci y/J mol–1 K–1
Fig. 4 Mola hea capaci y, Cp, o a unc ion o he numbe o ca bon
a oms in he alkyl side chain o he ca ion, n a 313K and 343K
M.Villanue a e al.
whe e R is he gas cons an and T he absolu e empe a u e.
Resul s o each empe a u e a e included in Table5,
showing a e y good ag eemen be ween hem and he
slopes ob ained.
As seen in Fig.4 and TableS3, he highes mola
hea capaci y alue o all he PILs analysed he e was
412J·mol−1·K−1, co esponds o he ammonium IL wi h he
longes ca ion alkyl chain (OAN). This alue is no pa icu-
la ly high compa ed o o he ILs, and i is in ag eemen wi h
he obse a ion o Liaqa e al. [44], who s a es ha ammo-
nium ILs a e hose wi h he lowes a e age hea capaci y
among all ca ion ypes p esen in he NIST ILThe mo da a
base [45], 375J·mol−1·K−1 e sus 315J·mol−1·K−1, calcu-
la ed he e as he a e age alue o he PILs analysed a he
highes empe a u e [36].
Toxici y
E ec i e concen a ion alue (EC50) and he espec i e 95%
con idence in e als a he h ee selec ed imes a e exposed
in Table6 o all he s udied compounds; in he case o
e hylammonium ni a e, p e iously published esul s we e
C
CH2
=R
[
−1.133 +2.443
(
T
100 )
−0.259
(
T
100 )2]
used [24]. EC20 and EC10 alues o he six ILs a e shown
in supplemen a y in o ma ion (TableS4 and TableS5,
espec i ely).
As p e iously epo ed [46], he oxici y o he IL
inc eases wi h leng h o he alkyl chain. This beha iou is
ela ed o he highe lipophilici y o long alkyl chain leng h,
p obably acili a ing he en ance o he IL h ough he cell
memb ane. I also can be seen ha he “cu -o ” e ec [47]
does no appea ed; his phenomenon sugges s ha oxici y
eaches a maximum le el a a ce ain chain leng h. In he
case o he s udied alkyl chain leng hs, his maximum ox-
ici y le el has no been eached. To isualize his endency
wi h espec o ca bon numbe , a ba g aphic o EC50, EC20
and EC10 a 5, 15 and 30min is shown in Fig.5.
Conside ing he EC and TU alues o he s udied com-
pounds, mos o he ILs s udied exhibi a lowe oxici y
compa ed o hose p e iously epo ed. The EC50 a 15min
o C4C1py TFSI is less han 1000mg L−1, C4C1C1im
TFSI is e y close o 100mg L−1 [48, 49], o e en cho-
line- and be aine-based ILs, usually associa ed wi h mo e
en i onmen ally iendly ILs, p esen alues be ween 10
and 1000mg L−1 [23, 50]. In he p esen s udy, he oxici y
alues a e highe han 1000mg L−1 (conside ed non- oxic)
om C2 o C5 alkyl chain leng h, and only he case o C8 can
be conside ed oxic.
The inc ease in oxici y wi h inc easing alkyl chain
leng h, as epo ed by o he au ho s [51, 52] can be seen
in Fig.5 [51, 52]. Al hough di e en s udies sugges ha
he anion is he main key pa ame e in oxici y [53, 54], ou
indings indica e ha he in luence o he ca ionic s uc u e
mus be aken in o conside a ion, as p e iously poin ed ou
o he au ho s [47, 55].
Conclusions
Th ee ni a e ILs de i ed om n-alkylammonium ca ions
o long alkyl chain leng h (n = 5, 6, 8) we e syn hesized and
hei he mophysical p ope ies we e compa ed o hose o
he h ee o he s ILs o he same amily wi h sho e alkyl
Table 5 Pa ame e s and co ela ion coe icien s ob ained om he lin-
ea i ing Cp = a + b·n, be ween mola hea capaci y and ca bon num-
be s a each empe a u e (Fig S5) and es ima ed alues o mola hea
capaci y co esponding o he CH2 g oup, CP, CH2, using he g oup
con ibu ion me hod o Ga das and Cou inho [43]
T /K a ± s(a)/ J
mol−1 K−1 b ± s(b)/J
mol−1 K−1 R2CP, CH2 /J
mol−1 K−1
313.15 139.6 ± 3.4 32.3 ± 0.7 0.997 33.07
318.15 139.5 ± 2.7 32.8 ± 0.5 0.998 33.40
323.15 142.3 ± 2.8 32.9 ± 0.5 0.999 33.73
328.15 144.2 ± 4.0 32.2 ± 0.8 0.997 34.04
333.15 142.6 ± 3.4 33.1 ± 0.7 0.998 34.35
338.15 141.5 ± 3.2 33.7 ± 0.6 0.998 34.64
343.15 146.9 ± 2.8 32.6 ± 0.6 0.998 34.92
Table 6 EC50 e ec i e concen a ion alues in mg·L−1 and he espec i e 95% con idence in e als, o he h ee exposu e imes o he ma ine
bac e ia A. Fische i
Compound EC50(5min)/mg L−1 EC50(15min)/mg L−1 EC50(30min)/mg L−1
EAN [24] 12,582.07 (8186.64; 16,977.50) 10,665.47 (6650.14; 14,680.80) 9711.63 (6561.46; 12,860.79)
PAN 8314.99 (7268.61; 9361.37) 5932.88 (5043.45; 6822.30) 5827.78 (4998.72; 6656.84)
BAN 1491.986 (636.69; 2347.04) 1066.71 (551.52; 1581.90) 1017.14 (478.49; 1555.78)
PEAN 1116.93 (945.07; 1222.79) 1073.63 (836.26; 1311.00) 1029.81 (792.52; 1267.11)
HEAN 85.69 (77.71; 93.68) 57.54 (52.98; 62.09) 50.11 (44.85; 55.39)
OAN 9.70 (6.37; 13.03) 7.33 (5.23; 9.43) 7.38 (5.51; 9.25)
E ec o alkyl chain leng h on he he mal p ope ies and oxici y o n‑alkyl‑ammonium ni a e…
chain leng h (n = 2, 3, 4) ha we e p e iously s udied. The
main conclusions a e as ollows:
• The akyl chain leng h dec eases he sho - e m he mal
s abili y, wi h his change being mo e signi ican o
longe chain ILs.
• The mel ing empe a u e changes wi h he alkyl chain
leng h, bu he e is no a clea endency wi h he ca bon
numbe . I can be in e ed ha he e a e wo di e en
ends o e en o odd ca bon a oms, as obse ed by o he
au ho s in o he ILs amilies.
• Because o he abo e esul s, he liquid ange o hese ILs
dec eases wi h he alkyl chain leng h.
• Isoba ic speci ic and mola hea capaci ies o he six PILs
inc ease wi h empe a u e as expec ed. Second-deg ee
polynomial equa ions we e ound o co ela e mola hea
capaci ies and empe a u e.
• Isoba ic speci ic and mola hea capaci ies inc ease wi h
he numbe o ca bon a oms in he ca ion alkyl chain.
Good co ela ions we e ob ained be ween he mola hea
capaci y and he numbe o ca bon a oms in he ca ion
alkyl chain o each empe a u e, wi h he slope o he
linea i ing ep esen ing he con ibu ion o CH2 g oup
o he Cp a each empe a u e.
• The gene al end indica es ha IL oxici y inc eases wi h
alkyl chain leng h, as epo ed by o he au ho s o a i-
ous amilies o ILs. This is likely due o he inc ease in
ca ion lipophilici y caused by longe alkyl chains, which
acili a es IL pene a ion h ough he cell memb ane.
Acknowledgemen s Au ho s acknowledge M. Gómez (RIAIDT-USC)
o he echnical suppo in DSC and TGA measu emen s. This wo k
was suppo ed by Spanish Minis y o Economy and Compe i i eness
and FEDER P og am h ough he p ojec s MAT2017-89239-C2-1-P,
PID2020-112846RB-C22 as well as by Xun a de Galicia h ough GRC
ED431C 2020/10 p ojec and he Galician Ne wo k o Ionic Liquids
(ReGaLIs) ED431D 2017/06. P. Valle hanks unding suppo o FPI
P og am om Spanish Minis y o Science, A. San iago hanks unding
o Dou o amen o indus ial p og am om GAIN-Xun a de Galicia and
J. J. Pa ajó (ED481D 2023/014) hanks he I2C pos doc o al P og am
o he Xun a de Galicia, o hei suppo in unding he s udy.The
au ho s a e also g a e ul o he esea ch suppo se ices o he Uni e -
sidade de Vigo (CACTI) o he NMR and MS di isions.
Funding Open Access unding p o ided hanks o he CRUE-CSIC
ag eemen wi h Sp inge Na u e.
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Re e ences
1. Plechko a NV, Seddon KR. Applica ions o ionic liquids in he
chemical indus y. Chem Soc Re . 2008;37:123–50. h ps:// doi.
o g/ 10. 1039/ B0066 77J.
2. Ren F, Wang J, Xie F, Zan K, Wang S, Wang S. Applica ions
o ionic liquids in s a ch chemis y: a e iew. G een Chem.
2020;22:2162–83. h ps:// doi. o g/ 10. 1039/ C9GC0 3738A.
3. Roge s RD, Seddon KR. Ionic Liquids - Sol en s o he Fu u e?
Science. 1979;2003(302):792–3.
Fig. 5 Ba g aphic o EC50,
EC20 and EC10 a e 5, 15 and
30min o exposu e in e ms o
ca bon numbe , n 14000
12000
10000
8000
6000
4000
2000
0
2
3
4
5
6
EC
50
5 min
EC
50
15 min
EC
50
30 min
EC
20
5 min
EC
20
15 min
EC
20
30 min
EC
10
5 min
EC
10
15 min
EC
10
30 min
8
n
EC/mg L–1