Su ace we abili y o lignin ma e ials om supe c i ical wa e hyd olysis
o wood
Vesna Leon ije ic
a
, Tijana Fech e
a
, Danilo Can e o
a
, Philip Jaege
b
, Ma ia Jos´
e Coce o
a,*
a
BioEcoU a, Ins i u e, P essTech G oup, Chemical Enginee ing &En i onmen al Technology Depa men Uni e si y o Valladolid, P ado de la Magdalena s/n, Valladolid
47011, Spain
b
Ins i u e o Subsu ace Ene gy Sys ems, Claus hal Uni e si y o Technology, Ag icola S . 10, Claus hal-Zelle eld 38678, Ge many
HIGHLIGHTS GRAPHICAL ABSTRACT
•SCWH enables con ol o cellulose con-
en allowing o ailo ed we abili y.
•Signi ican impac o ma e ial composi-
ion and p ocessing on we abili y and
su ace ene gy.
•Ma e ials wi h highe su ace ene gy
exhibi be e we abili y.
•Sligh inc ease in CA is obse ed wi h
inc easing p essu e.
ARTICLE INFO
Keywo ds:
Cellulose
Composi es
Su ace oughness
Su ace ene gy
P essu e
ABSTRACT
To mee he demands o he e ol ing ci cula economy, he e is a g owing need o enewable esou ces as base
ma e ials o inno a i e, easily ecyclable p oduc s. Lignin, he second mos abundan biopolyme , has eme ged
as a p omising sou ce o a oma ics and ein o cing agen in polyme composi es. Fo he success ul
manu ac u ing o homogeneous composi e ma e ials, good bonding be ween he coexis ing phases is essen ial o
p e en he o ma ion o oids and agglome a es. The e o e, unde s anding he su ace p ope ies o hese
ma e ials is c ucial o designing op imal composi e compounds. In his s udy, he we abili y o lignin-cellulose
composi es and lignin samples ob ained h ough supe c i ical wa e hyd olysis (SCWH) o bi ch wood is
in es iga ed. The con ac angle (CA) echnique, speci ically he sessile d op me hod, was employed o assess and
compa e he we abili y o SCWH lignin wi h comme cially a ailable lignin and aw bi ch wood. The esul s
p o ide insigh s in o hei su ace ene gy, adhesion, and hyd ophilic o hyd ophobic cha ac e is ics unde
p ocessing condi ions. All samples exhibi ed hyd ophilici y, wi h an ini ial CA app oxima ely 40 ◦, excep o aw
bi ch wood, which had a highe ini ial CA o ~ 64◦. No ably, when lignin is accompanied by signi ican amoun s
o cellulose, di e en ends in CA changes o e ime we e obse ed. The in luence o p essu e on he CA be-
ween wa e and hese polyme s was also analyzed, bu no signi ican impac was de ec ed. This esea ch ad-
ances he de elopmen o lignin-based ma e ials wi h ailo ed su ace p ope ies o a ious indus ial
applica ions.
* Co esponding au ho .
E-mail add ess: [email p o ec ed] (M.J. Coce o).
Con en s lis s a ailable a ScienceDi ec
The Jou nal o Supe c i ical Fluids
jou nal homepage: www.else ie .com/loca e/sup lu
h ps://doi.o g/10.1016/j.sup lu.2024.106458
Recei ed 5 Sep embe 2024; Recei ed in e ised o m 17 Oc obe 2024; Accep ed 11 No embe 2024
The Jou nal o Supe c i ical Fluids 217 (2025) 106458
A ailable online 13 No embe 2024
0896-8446/© 2024 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
As he ci cula economy con inues o e ol e, he demand o
enewable esou ces as ounda ional subs ances o inno a i e, ecy-
clable ma e ials is escala ing. Wi hin his amewo k, lignin is inc eas-
ingly ecognized as a aluable sou ce o a oma ics and a i al ein o cing
ma e ial o polyme composi es. Lignin, he second mos abundan
biopolyme , is he ocus o g owing esea ch aimed a ans o ming i
in o a key sou ce o a oma ics, which a e cu en ly de i ed om pe-
oleum [1]. New ma e ials based on polyme -lignin composi es, u i-
lizing lignin as a ein o cing agen , a e also eme ging [2]. Fu he , a
bene icial syne gis ic e ec o lignin and cellulose mix u es used in
di e en applica ions (wood adhesi es, ca bon ibe , nano ibe s, he -
moplas ics) has been demons a ed [3]. On he o he hand, o p oduce a
homogeneous composi e ma e ial, he polyme ma ix and any ein-
o cing ma e ials (such as ibe s o ille s) should be chemically and
physically compa ible o ensu e s ong in e acial bonding. A he same
ime, he ein o cemen ma e ials ha e o be uni o mly dispe sed wi hin
he polyme ma ix o a oid agglome a es, which can weaken he
composi e and c ea e s ess concen a ions [4,5]. To selec sui able
composi e compounds, hei su ace ene gies need o be aligned in o de
o each op imal composi e p ope ies [6,7]. The su ace ene gy o solid
ma e ials is ela ed o hei we ing beha io [8]. The we abili y e e s
o he deg ee o which a liquid d op will sp ead o e he solid su ace
su ounded by ano he luid (liquid o gas). In case o comple e we a-
bili y, he liquid will sp ead o e he en i e su ace spon aneously. While
we ing beha io is a uni e sal phenomenon, i can a y based on he
chemical na u e o bo h he solid and liquid phases [9]. When he
applied liquid is wa e , he commonly used e ms o explain he we ing
beha io a e hyd ophilici y and hyd ophobici y. Hyd ophilic and hy-
d ophobic ma e ials play c ucial oles in a wide ange o applica ions
due o hei dis inc i e in e ac ions wi h wa e . The unique p ope ies o
hyd ophilic and hyd ophobic ma e ials, enable inno a i e solu ions
ac oss di e se sec o s, enhancing unc ionali y and pe o mance. Hy-
d ophilic ma e ials a e essen ial in indus ies whe e a o able wa e
in e ac ion is equi ed, while hyd ophobic ma e ials play a c ucial ole
in c ea ing wa e - epellen su aces, o e ing ad an ages ac oss a ious
indus ial applica ions.
Fo he di ec measu emen o su ace we abili y, he con ac angle
(CA) is commonly de ec ed op ically. The e o e, he so-called sessile
d op me hod, a e sa ile and ela i ely simple echnique is equen ly
used [10]. The CA is he angle o med a he in e ace whe e a liquid
d ople mee s a solid su ace. I is measu ed h ough he liquid, be ween
he solid su ace and he angen line o he liquid d ople a he con ac
poin . The concep o CA and we abili y was in oduced in 1805 by
Thomas Young [11]. This echnique p o ides aluable insigh s in o
su ace ene gy, adhesion, and he hyd ophobic o hyd ophilic na u e o
ma e ials. The ma e ials ha a e a ge ed a in his wo k o igina e om
na u al was e ha unde goes a supe c i ical wa e ea men .
Supe c i ical wa e hyd olysis is a clean echnology ha o e s a
apid and e icien me hod o ea ing wood ma e ials in absence o any
addi i es o u he eac an s [12]. This p ocess acili a es he dissolu-
ion and hyd olysis o cellulose in less han a second, signi ican ly
educing p ocessing ime. Addi ionally, i enables he hyd olysis o he
lignin-cellulose complex ound in wood, aiding in he e ec i e ac-
iona ion o i s di e en polyme s. The use o a Sudden Expansion
Reac o in his echnology u he enhances i s e iciency by apidly
cooling he eac ing ma e ials, esul ing in he p ecipi a ion o unique
lignin and cellulose composi es [13]. Unlike con en ional me hods, his
app oach does no in ol e sol en dissolu ion o bleaching. Mo eo e ,
he na i e lignin s uc u e is p ese ed o a signi ican ex en , p oducing
dis inc p ecipi a es wi h he po en ial o inno a i e applica ions [13,
14].
The p ima y objec i e o his s udy is o e alua e he su ace
we abili y and he ela ed su ace ene gies o lignin-cellulose compos-
i es p oduced ia supe c i ical wa e hyd olysis (SCWH) and compa e
hei p ope ies o lignins ob ained h ough di e en ex ac ion
me hods, including sul ona ed k a lignin (SKL) and Indulin. By
examining he su ace p ope ies, pa icula ly h ough con ac angle
measu emen s, his s udy aims o p o ide c i ical insigh s in o he
in e ac ion be ween wa e and hese lignin-based ma e ials. Fu he -
mo e, con ac angle measu emen s o e a quan i a i e assessmen o he
su ace ene gy e ealing he unde lying mechanisms ha go e n he
hyd ophilic o hyd ophobic na u e o hese ma e ials. These insigh s a e
c ucial o op imizing he design o lignin-based composi es in e ms o
compa ibili y o he ma ix and he ille [6], and in gene al o unc-
ionalizing su aces o a ious indus ial applica ions. Fu he mo e, an
addi ional objec i e is o in es iga e he in luence o p essu e on he
con ac angle (CA) be ween wa e and polyme su aces. Since com-
posi e manu ac u ing o en in ol es ele a ed p essu es, his s udy seeks
o de e mine whe he p essu e a ec s he su ace we abili y and,
consequen ly, he pe o mance o he inal composi e ma e ials.
2. Ma e ials and me hods
2.1. Ma e ials
Fi e di e en lignin samples we e u ilized in his s udy. Sul ona ed
k a lignin (SKL) and comme cial k a lignin (Indulin) we e sou ced
om Sigma Ald ich (Da ms ad , Ge many) and Inge i y ( o me ly Mead
Wes aco, No h Cha les on, Sou h Ca olina, USA), espec i ely. The
hi d and ou h samples we e ob ained h ough SCWH o bi ch wood
and a e e e ed o as supe c i ical wa e lignin (SCWL). The dis inc ion
be ween he wo SCWL lies in hei cellulose con en . In addi ion, aw
bi ch wood was included as a i h sample. Bi ch wood was p o ided by
RISE Resea ch Ins i u es o Sweden (S ockholm, Sweden). Diiodo-
me hane, a nonpola compound used o es ima e he su ace ene gy (SE)
o he lignin- ich ma e ial, was pu chased om Fishe Scien i ic (Bleis-
wijk, Ne he lands). The ni ogen was pu chased om Linde (Valladolid,
Spain) a a pu i y o 5.0 (>99.999 % ).
2.2. Supe c i ical wa e hyd olysis o bi ch wood and SCWL
cha ac e iza ion
The SCW ea men o wood p ima ily hyd olyzes he ca bohyd a es
(cellulose and hemicellulose). The solid p oduc s a e SCW ea men
a e composed mainly o lignin and cellulose based ca bohyd a es.
De ailed expe imen al p ocedu es o supe c i ical wa e hyd olysis can
be ound in [13]. The 2 SCWL used in his s udy we e ob ained using
condi ions gi en in Table 1.
The chemical composi ion (cellulose, hemicellulose, and lignin) o
bi ch wood and he composi ion o SCWL we e analyzed ollowing
p ocedu es implemen ed by he Na ional Renewable Ene gy Labo a-
o ies [15].
2.3. P epa a ion and cha ac e iza ion o pelle s
P io o measu ing he con ac angle, he samples we e con e ed
in o pelle s using he ollowing echnique: App oxima ely 0.4 g o each
sample was placed in a mold wi h a 13 mm diame e and heigh o
19 mm and subjec ed o a hyd aulic p ess wi h an applied p essu e o
a ound 7.4 M . The esul ing pelle had a hickness o app oxima ely
4 mm.
Table 1
Supe c i ical wa e hyd olysis eac ions condi ions o wo supe c i ical wa e
lignin samples (SCWL).
Sample Reac ion ime Reac ion empe a u e Reac ion p essu e
SCWL1 0.34 s 377 ±10 ◦C 284 ±2 ba
SCWL2 1.16 s 390 ±2◦C 254 ±3 ba
V. Leon ije ic e al. The Jou nal o Supe c i ical Fluids 217 (2025) 106458
2
2.3.1. Nuclea magne ic esonance spec oscopy (NMR)
NMR analysis was conduc ed on ou di e en lignin samples o
u he cha ac e ize hem and gain a deepe unde s anding o hei
a ying we abili y beha io s o e ime. Sample p epa a ion o NMR
ollowed he p o ocol ou lined by Meng [16]. The analysis was pe -
o med using a B uke A ance Neo (500 MHz, 2 channels) spec ome e
(B emen, Ge many) equipped wi h an i-P obe, loca ed a he Labo a o y
o Ins umen al Techniques (LTI) Resea ch Facili ies a he Uni e si y o
Valladolid. The condi ions o 31 P[1 H] NMR analysis, using B uke ’s
zgig sequence (in e se-ga ed decoupling scheme), included a spec al
wid h o 20000 Hz, a ansmi e equency o se o 140 ppm, acquisi-
ion ime o 0.8 s, a elaxa ion ime be ween pulses o 11 s, a pulse wid h
o 90◦, and a o al numbe o 128 pulses. All measu emen s we e ca ied
ou a a empe a u e o 25 ◦C.
2.3.2. A omic o ce mic oscopy (AFM) – oughness measu emen
The su ace oughness o he p epa ed pelle s was measu ed by he
a omic o ce mic oscopy echnique using he Asylum Resea ch MFP3D
BIO (San a Ba ba a, Cali o nia, USA); AFM p obe 160AC, OPUS by
Mik oMasch (So ia, Bulga ia): AFM mode: AC ( apping mode). Fi e
ep esen a i e samples we e imaged in duplica e. In bo h eplicas, im-
ages, and da a a e aken a h ee di e en poin s a ound he cen al a ea
o he samples. The pelle s we e moun ed in s anda d mic oscopy slides
using epoxy glue. Images o wo sizes we e acqui ed a e e y poin :
10×10
μ
m
2
and 1×1
μ
m
2
.
2.3.3. The c ys allini y o cellulose in he samples
X- ay di ac ion was used o measu e and cha ac e ize he c ys al-
lini y o he cellulose in he samples on B uke D8 Disco e A25 de ice,
Gene a o 3Kw, Coppe ce amic ube 2.2Kw ype FFF, De ec o LynxEye
40K 30 Ma (B emen, Ge many).
2.4. De e mina ion o con ac angle
2.4.1. Measu emen o wa e con ac angle
The con ac angle measu emen s we e pe o med using a High-
P essu e View Cell om Eu o echnica GmbH (Ba g eheide, Ge many)
coupled wi h a high- esolu ion CCD came a [17]. The in e nal olume o
he cell is 30 ml and he maximum ope a ing condi ions a e 500 ◦C and
30 MPa. Measu emen s we e pe o med using he sessile d op me hod a
a empe a u e o 25 ◦C. Ni ogen was used o p essu ize he sys em in he
p essu e ange om a mosphe ic p essu e o 130 ba . The dynamic
con ac angle was also moni o ed o e app oxima ely 10 min. Mili-Q
wa e was used in all expe imen s. The con ac angle measu emen s
we e pe o med by i s placing app oxima ely en d ops o wa e a he
bo om o he cell o sa u a e he en i onmen (p essu ized gas) and
minimize he e ec o e apo a ion on he con ac angle. The sample was
hen posi ioned on he sample holde inside he cell. A single d op was
o med a he capilla y ip be o e p essu izing he sys em o a oid issues
wi h d ople o ma ion unde p essu e. Fo expe imen s conduc ed a
a mosphe ic p essu e, he d ople was placed on he sample immedia ely
a e i s in oduc ion in o he cell. Howe e , when he sys em was
p essu ized, he d ople was placed 10–15 minu es a e achie ing he
desi ed p essu e o ensu e ha he wa e was sa u a ed wi h ni ogen
and equilib ium be ween in e acial o ces was es ablished. Measu e-
men s o all condi ions we e epea ed a leas h ee imes, applying a
new pelle each ime. The ame a e o he ideo was 10 ames pe
second. A e wa d, indi idual ames we e ex ac ed a speci ied ime
in e als by adjus ing he eco ding a io o 10 (e e y en h image was
ex ac ed) in a ee, open - sou ce VLC media playe (VideoLAN, Pa is,
F ance). The so wa e Open D op, eleased unde he GNU GPL open –
sou ce license (F ee So wa e Founda ion (FSF), Bos on, Massachuse s,
USA), was used o measu e he con ac angle be ween he wa e and he
pelle s. All images ob ained om he ideo we e used in he analysis.
2.4.2. Measu emen o diiodome hane con ac angle
To de e mine he SE o all es ed ma e ials, addi ional CA measu e-
men s we e conduc ed using he nonpola compound diiodome hane,
ollowing he same sessile d op me hod unde ambien condi ions. A
10 µl d ople o diiodome hane was placed on he pelle using a mic o-
pipe e. The high- esolu ion CCD came a, as p e iously desc ibed, was
used o eco d he ideo. Each measu emen was epea ed wice o
accu acy. The Open D op so wa e was employed o calcula e he CA
be ween he diiodome hane and he pelle s.
Table 2
Chemical composi ion o he bi ch wood and wo supe c i ical wa e lignin
samples (SCWL).
Componen Amoun (% w/w d y basis)
Bi chwood SCWL1 SCWL2
Acid insoluble lignin 13.2 ±0.3 41.6 ±2.1 93.8 ±3.6
Acid soluble lignin 6.8 ±2.1 5.1 ±0.8 2.7 ±0.3
Cellulose 44.1 ±1.1 41.7 ±2.0 0.0
Hemicellulose 18.0 ±0.6 5.2 ±0.1 0.0
Ex ac i es 4.9 ±0.4 - -
Ash 0.5 ±0.1 - -
O he s 12.5 6.4 3.5
*S anda d de ia ion is based on analysis epea ed a leas h ee imes.
Fig. 1. Amoun o di e en OH g oups pe g am o lignin ma e ial ob ained by
NMR analysis.
Table 3
Su ace oughness and ini ial con ac angle o sul ona ed k a lignin (SKL), comme cial k a lignin (Indulin), wo supe c i ical wa e lignin samples (SCWL), and aw
bi ch wood.
Sample Roo Mean Squa e Heigh (Sq), nm A i hme ical Mean Heigh (Sa), nm Ini ial con ac angle
10£10 µm
2
1£1µm
2
10£10 µm
2
1£1µm
2
SKL 75.3 ±15.1 9.8 ±3.8 59.8 ±12.3 7.8 ±3.1 39.9 ◦±2.2 ◦
Indulin 79.6 ±19.6 12.2 ±6.6 59.9 ±14.5 9.7 ±5.7 40.9 ◦±2.1 ◦
Raw bi ch wood 186.5 ±54.6 6.9 ±0.7 140.3 ±40.9 5.4 ±0.7 63.7 ◦±5.4 ◦
SCWL 1 55.3 ±14.3 5.4 ±2.6 44.5 ±11.3 4.3 ±2.2 44.4 ◦±4.1 ◦
SCWL 2 30.9 ±17.7 5.5 ±1.5 22.5 ±13.0 4.2 ±0.8 43.0 ◦±2.6 ◦
V. Leon ije ic e al. The Jou nal o Supe c i ical Fluids 217 (2025) 106458
3
2.4.3. Calcula ion o su ace ene gy - heo y
When a d ople o a liquid is o med on a la solid su ace, he bal-
ance on he h ee-phase in e acial ene gies is exp essed by Young’s
equa ion
σ
s
=
σ
s,l
+cosθ
σ
l
(1)
whe e
σ
l
is he su ace ension o he liquid, θ he con ac angle be ween
he liquid-ai in e ace and he su ace,
σ
s,l
is in e acial ension o en-
e gy be ween he solid and he liquid and
σ
s
is he su ace ene gy o he
solid. To be able o calcula e he su ace ene gy om he con ac angle,
he unknown a iable
σ
s,l
mus be de e mined.
Fowkes [18] in oduced a concep o di iding he in e acial ension
(IFT) o ene gy in o pa s o pola and apola in e ac ions ha a e added
Fig. 2. AFM images o es ed samples.
Fig. 3. Images o d ople p o iles on a solid su ace in he i s and las minu e
o eco ding.
Fig. 4. Change o con ac angle wi h ime a a mosphe ic p essu e.
Table 4
Values o pola and dispe si e componen s and o al su ace ene gy o sul o-
na ed k a lignin (SKL), comme cial k a lignin (Indulin), and wo supe c i ical
wa e lignin samples (SCWL).
SKL Indulin SCWL1 SCWL2
SE solid pola , mN/m 27.2 ±1.0 27.6 ±2.0 21.1 ±2.4 28.2 ±2.5
SE solid dispe s, mN/m 46.1 ±0.3 45.4 ±2.3 46.6 ±1.6 39.5 ±2.3
SE solid, mN/m 73.3 73.0 67.7 67.7
*The s anda d de ia ion is de i ed om he s anda d de ia ions o he con ac
angle (CA) measu emen s, wi h h ee eplica es o wa e and wo eplica es o
diiodome hane.
Fig. 5. Dependence o ini ial CA and CA app oxima ely 10 min a e d ople
deposi ion on p essu e.
V. Leon ije ic e al. The Jou nal o Supe c i ical Fluids 217 (2025) 106458
4
Fig. 6. Change o con ac angle o e ime a a mosphe e p essu e (A), 10 ba (B), 20 ba (C), 30 ba (D), 40 ba (E), 50 ba (F), 100 ba (G), and 130 ba (H).
V. Leon ije ic e al. The Jou nal o Supe c i ical Fluids 217 (2025) 106458
5
o ob aining he in e acial ension as a mac oscopic measu e as shown
by Eq. (2)
σ
s
=
σ
d
s+
σ
p
s(2)
whe e d means dispe sion and p pola componen .
Acco ding o Fowkes [18], he in e acial ension o ene gy be ween
wo phases, in his case be ween liquid and solid (
σ
s,l
), is in e p e ed as
he geome ic mean o a pola pa and a dispe se pa
σ
s,l=
σ
s+
σ
l–2(
σ
d
s
σ
d
l
√+
σ
p
s
σ
p
l
√)(3)
Combining Eq. (1) and Eq. (3), he ollowing equa ion is ob ained
σ
l(1+cosθ) = 2(
σ
d
s
σ
d
l
√+
σ
p
s
σ
p
l
√)(4)
The su ace ene gy o he solid is de e mined om he con ac angle
da a in wo s eps: The dispe se pa is calcula ed i s wi h he help o a
pu ely nonpola liquid. The pola pa is hen de e mined wi h a
di e en liquid wi h pola pa s. The nonpola liquid is assumed o
in e ac only h ough dispe se in e ac ions wi h he solid, o which Eq.
(4) simpli ies o:
σ
d
s=
σ
diiodome hane (1+cosθ
2)2(5)
The dispe si e pa o he solid su ace ene gy (Eq. (5)) can be used o
calcula e he co esponding pola pa knowing he pola pa o he
su ace ension o a subsequen ly applied (pa ly) pola liquid such as
wa e .
σ
p
s=1
σ
p
w
(
σ
w(1+cosθ) − 2
σ
d
w
σ
d
s
√
2)
2
(6)
The su ace ension o wa e a di e en empe a u es is well-known
and abula ed [19]. Fu he , i can be assumed ha poly e a luo o-
e hylene (PTFE) is a comple ely non-pola solid su ace. Then, om Eq.
(2),
σ
d
PTFE =
σ
PTFE
. The su ace ene gy o PTFE is a e y well-known alue
in he li e a u e [18,20,21]. Consequen ly, applying hese assump ions
o Eq. (6) makes i possible o calcula e he dispe si e pa o wa e
su ace ension.
σ
d
w=
σ
2
w
σ
PTFE
(1+cosθ
2)2(7)
The pola componen o he su ace ene gy is calcula ed om Eq. (2)
as he di e ence be ween he su ace ension o wa e and i s dispe si e
componen . Using Eqs. 1 h ough 7, along wi h li e a u e da a o he
su ace ension o wa e and diiodome hane [21] unde ambien con-
di ions, and he su ace ene gy o PTFE, as well as con ac angle alues
ob ained om he p e iously explained expe imen s, i is possible o
es ima e he su ace ene gy o he lignin ma e ial. Fo he SE calcula-
ions, he CA alues we e aken a e he sys em eached mechanical
equilib ium.
3. Resul s and discussion
3.1. Composi ion o bi ch wood and SCWL
The chemical composi ions o he bi ch wood and wo SCWL p od-
uc s a e gi en in Table 2. I is e iden ha he amoun o ca bohyd a es,
p ima ily cellulose, emaining in he p oduc a ies based on he hy-
d olysis condi ions. In sample SCWL1, he cellulose con en is signi i-
can ; whe eas in SCWL2, i was unde ec able by HPLC. As expec ed, less
se e e hyd olysis condi ions esul s in a highe cellulose con en in he
solid p oduc .
3.2. Cha ac e iza ion o pelle s
3.2.1. Nuclea magne ic esonance spec oscopy (NMR)
NMR analysis (Fig. 1) p o ided insigh s in o he OH g oups con en
in lignin samples, ca ego ized as alipha ic, phenolic, and ca boxylic OH
g oups. SKL and Indulin showed simila amoun s o alipha ic and
phenolic OH g oups, while, due o i s high cellulose con en [22],
SCWL1 had a ound 3 imes mo e alipha ic OH g oups han Indulin and
SKL, and mo e han double compa ed o SCWL2. Fu he mo e, a
ema kably highe amoun o phenolic OH g oups can be obse ed in
SKL and Indulin in compa ison o he lignin sample ob ained in his
wo k. This phenomenon can be a ibu ed o he unique p ocess by
which lignin is ob ained using he desc ibed me hod, esul ing in a
s uc u e ha closely e ains i s na i e cha ac e is ics [13,14]. In his
nea -na i e lignin s uc u e, phenolic g oups engage in co alen
bonding, as e idenced by hei dis inc signa u es obse ed in NMR
analysis. Namely, i he OH g oup is in ol ed in co alen bonding, such
as es e i ica ion o e he i ica ion, he NMR signals o he OH p o ons
may change o e en disappea i he p o ons a e exchanged o i he OH
g oup is no longe ee.
3.2.2. A omic o ce mic oscopy (AFM) – oughness measu emen
Table 3 p esen s he su ace oughness alues o he p epa ed pelle s,
while Fig. 2 displays he co esponding AFM images. The su ace
oughness o aw bi ch wood pelle s (Sq =186.47 nm; Sa =140.26 nm)
is no ably highe compa ed o he hyd olyzed samples. Acco ding o
li e a u e, inc eased su ace oughness enhances we ing in wa e –we
sys ems, whe eas in non-we ed su aces, i ends o inc ease he con ac
angle, a phenomenon known as he lo us e ec [23]. Consis en wi h
his, he highe oughness o aw bi ch wood is associa ed wi h a sligh ly
highe ini ial CA be o e swelling occu s.
3.2.3. The c ys allini y o cellulose in he samples
X- ay di ac ion analysis o cellulose c ys allini y showed a sligh
dec ease om 61.5 % in aw bi ch wood o 59.0 % in SCWL1, while
SCWL2 showed no de ec able c ys allini y due o negligible ca bohy-
d a e con en , as con i med by HPLC. The ans o ma ion o igid c ys-
alline cellulose in o mo e lexible amo phous s uc u es in SCWH
p omo es easie hyd olysis, as wa e molecules a oid hyd ophobic zones
[12]. Lowe c ys allini y co ela es wi h highe su ace ene gy, in lu-
encing we abili y [24].
3.3. Con ac angle
3.3.1. Change o CA wi h ime
The p ope ies o solid ma e ials a e o en ca ego ized based on hei
con ac angle (CA). Solids wi h a CA below 90◦a e conside ed hyd o-
philic, while hose wi h a CA abo e 90◦a e classi ied as hyd ophobic
[11]. The expe imen al esul s demons a ed ha all samples we e hy-
d ophilic, wi h CA alues consis en wi h exis ing li e a u e [25].
Ini ially, he wa e d ople p o ile was moni o ed a a mosphe ic p es-
su e o e a 10 min pe iod, wi h he d ople p o ile shown in Fig. 3 a i s
and las minu e. Despi e simila ini ial CAs, a ia ions in he d ople
beha io we e obse ed (Fig. 4). SKL, Indulin, and SCWL2, cha ac e -
ized by a lignin-dominan s uc u e and negligible ca bohyd a es,
exhibi ed signi ican CA changes wi hin he i s 60 seconds. In con as ,
SCWL1, wi h a highe cellulose con en , displayed a mo e linea d ople
p o ile. These beha io s can be a ibu ed o di e en mechanisms such
as abso p ion, sp eading, swelling, and e apo a ion [10]. Abso p ion
and sp eading we e dominan in he i s minu e, while e apo a ion was
negligible due o he con olled en i onmen (22–25◦C). The swelling
was obse ed exclusi ely in he aw bi ch wood, which is p esumably
a ibu ed o i s high cellulose con en (44.1 %). This phenomenon has
been p e iously epo ed in cellulose c ys alli es [26,27]. In SCWL1, he
inc eased lignin con en (46.7 %) likely es ic ed swelling by educing
cellulose exposu e o wa e h ough hyd ogen bonding be ween
V. Leon ije ic e al. The Jou nal o Supe c i ical Fluids 217 (2025) 106458
6
cellulose and lignin. Then, he a ailabili y o he ee OH g oups o
in e ac wi h wa e is educed [28].
3.3.2. Su ace ene gy
Su ace ene gy es ima es o he es ed ma e ials (Table 4) indica ed
ha ma e ials wi h highe su ace ene gy exhibi be e we abili y [29].
SKL and Indulin, which had he same we abili y beha io (Fig. 4), also
had compa able su ace ene gies. Fu he mo e, hei pola and dispe -
si e componen s we e analogous. On he o he hand, he SE o SCWL1
and SCWL2 we e sligh ly lowe han SKL and Indulin. No ably, SCWL1
exhibi ed a dispe si e componen simila o SKL and Indulin, bu wi h a
lowe pola componen due o i s high cellulose con en , which con-
ibu es o he dispe si e SE. Despi e hei pola na u e, ca bohyd a es
also possess a dispe si e componen o su ace ene gy due o he p es-
ence o non-pola ca bon-hyd ogen bonds and ing s uc u es. As such, i
con ibu es o he dispe si e pa o he su ace ene gy o he SCWL1
sample. Addi ionally, i can be concluded ha a ela i ely high c ys-
allini y o cellulose p esen ed in his sample (59 %) con ibu es o he
lowe su ace ene gy, pa icula ly, i s pola componen . In con as , he
pola componen o SCWL2 is compa able o he pola pa s o SKL and
Indulin, while he dispe si e componen was signi ican ly lowe . I is
known ha he dispe si e componen o he su ace ene gy in lignin is
a ibu ed o an de Waals o ces a ising om he non-pola in-
e ac ions be ween he a oma ic ings and ca bon-hyd ogen bonds.
Al hough lignin has pola g oups, i s a oma ic na u e p omo es a sig-
ni ican dispe si e componen [18,30]. The me hod o ex ac ion
signi ican ly a ec s p ope ies like we abili y and SE [31], wi h he
molecula weigh o lignin in luencing dispe si e SE h ough chain
mobili y, su ace oughness, in e molecula in e ac ions, su ace
composi ion, solubili y, and he modynamic s abili y. Comme cial lig-
nins, like SKL and Indulin, ypically ha e lowe molecula weigh ,
esul ing in highe su ace ene gy due o inc eased molecula mobili y
and in e ac ion po en ial, while highe molecula weigh lignins end o
ha e lowe su ace ene gy due o educed mobili y and mo e ex ensi e
in e molecula in e ac ions [32]. In con as , SCWH allows o p ecise
con ol o condi ions, p oducing lignin wi h highe molecula weigh
and close o i s na i e s uc u e [13,14], which con ibu es o i s lowe
dispe si e SE.
Finally, his s udy highligh s he ad an age o using SCWH o ob ain
lignin and lignin-cellulose composi es. By ine- uning eac ion condi-
ions, SCWH allows selec i e ac iona ion o biomass componen s while
p ese ing lignin’s s uc u e and con olling he cellulose con en in he
inal p oduc . The biomass componen s can be selec i ely a ge ed and
e icien ly sepa a ed, op imizing he yield o desi ed p oduc s. In his
p ocess, when lignocellulose biomass is hyd olyzed o ob ain lignin, i is
possible no only o keep i s s uc u e close o na i e, bu also is easible
o con ol he amoun o cellulose emaining in he p oduc . As a esul ,
he we abili y o hese ma e ials can be ailo ed o op imal pe o -
mance in composi e manu ac u ing.
3.3.3. Dependence o CA on p essu e
Fig. 5 shows he a ia ion in ini ial CA and CA app oxima ely
10 minu es a e d ople deposi ion, as a unc ion o p essu e o h ee
es ed samples: SKL, Indulin, and SCWL1. In all cases, a sligh inc ease in
CA is obse ed wi h inc easing p essu e. Acco ding o Young´s ela ion
and conside ing he dec easing in e acial ension (IFT) wi h p essu e
[17], he su ace ene gy a he solid-gas in e ace dec eases as he
p essu e ises. These indings a e consis en wi h esul s epo ed by
Song and Fan [33]. Las ly, Fig. 6 shows ha he o e all dynamic
beha io o he SKL, Indulin, and SCWL1 su aces emains la gely un-
a ec ed by changes in p essu e.
4. Conclusions
In his wo k, he we abili y p ope ies o lignin-cellulose composi es
we e cha ac e ized and compa ed o lignins om di e en o igins and
ex ac ion me hods, such as SKL o Indulin. The hyd ophilici y o all
es ed samples was con i med, wi h con ac angles (CA) consis en wi h
li e a u e alues. Obse a ions o wa e d ople p o ile e olu ion o e
ime e ealed di e ences, p ima ily d i en by abso p ion and sp eading
du ing he ini ial s age a e d ople o ma ion. Raw bi ch wood, wi h i s
high ca bohyd a e con en , exhibi ed swelling. X- ay di ac ion analysis
showed a sligh educ ion in cellulose c ys allini y in SCWL1 a e he
p ocess o SCWH, whe eas he c ys allini y was unde ec ed in SCWL2
due o i s low ca bohyd a e con en . Ma e ials wi h highe su ace en-
e gy exhibi ed enhanced we abili y. SKL and Indulin displayed simila
su ace ene gy and we abili y beha io , while SCWL1 and SCWL2
p esen ed sligh ly lowe su ace ene gies and highe CAs. Di e ences in
he pola and dispe si e componen s we e a ibu ed o a ying cellu-
lose and lignin con en s. Nuclea magne ic esonance analysis e ealed
subs an ial di e ences in alipha ic and phenolic hyd oxyl g oup con en
among samples, wi h SCWH-ob ained lignin showing co alen bonding
ha in luenced NMR isibili y. These indings elucida e he signi ican
impac o ma e ial composi ion and p ocessing on we abili y and su -
ace ene gy. The e o e, when applied o lignocellulose biomass like
bi ch wood, SCWH no only p ese es he lignin’s s uc u e close o i s
na i e o m bu also enables con ol o cellulose con en in he inal
p oduc , allowing o ailo ed we abili y and op imal p ope ies o
composi e p epa a ion.
CRediT au ho ship con ibu ion s a emen
Philip Jaege : W i ing – e iew &edi ing, Valida ion, Fo mal
analysis, Da a cu a ion, Concep ualiza ion. Vesna Leon ije ic: W i ing
–o iginal d a , Me hodology, In es iga ion. Ma ia Jose Coce o:
W i ing – e iew &edi ing, Supe ision, Resou ces, P ojec adminis-
a ion, In es iga ion, Funding acquisi ion, Fo mal analysis, Concep u-
aliza ion. Danilo Can e o: W i ing – e iew &edi ing, Supe ision,
Fo mal analysis. Tijana Fech e : Me hodology, 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 inancial
in e es s o pe sonal ela ionships ha could ha e appea ed o in luence
he wo k epo ed in his pape .
Acknowledgmen s
This wo k was suppo ed by he Agencia Es a al de In es igaci´
on
(Gobie no de Espa˜
na) and FEDER Funds PID2022–140930NB-I00 and
TED2021–129837B-C42; and Jun a de Cas illa y Le´
on - Conseje ía de
Educaci´
on and FEDER Funds [CLU-2019–04]. V Leon ige ic hanks
JCyL o his p edoc o al con ac . D. Can e o is unded by he Spanish
Minis y o Science, Inno a ion and Uni e si ies (Bea iz Galindo
ellowship BEAGAL18/00247).
Da a A ailabili y
Da a will be made a ailable on eques .
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