Adso p ion beha iou o he bina y mix u es o oc ane and nonane a
sub-monolaye co e age on g aphi e
Miguel Cas o,aS ua M. Cla ke,*bAki a Inaba,cRobe K. Thomasdand Thomas A noldd
aIns i u o de Ciencia de Ma e iales de Se illa,A da.Ame ico V espucio,Se illa,Spain
bDepa men o Chemis y and BP Ins i u e,Madingley Rise,Madingley Road,Camb idge,UK
CB3 0ZE
cDepa men o Chemis y,G adua e School o Science,Osaka Uni e si y,Osaka,Japan
dPhysical and T heo e ical Chemis y L abo a o y,Sou h Pa ks Road,Ox o d,UK OX1 4BH
Recei ed 5 h Ma ch 2001,Accep ed 12 h June 2001
Fi s published as an Ad ance A icle on he web 18 h July 2001
The beha iou o bina y mix u es o oc ane and nonane adso bed on g aphi e has been in es iga ed wi h a
combina ion o adiaba ic calo ime y and neu on di† ac ion. In con as o he eu ec ic beha iou o
monolaye s o hese ma e ials a highe co e ages, his combina ion o alkanes is ound o o m mixed c ys als
on he su ace o e an ex ensi e composi ion ange a he sub-monolaye co e ages in es iga ed he e. In he
mixed phase he sho e oc ane molecules a e inco po a ed in o he la ice o he longe nonane molecules.
This inco po a ion occu s despi e pu e oc ane and nonane monolaye s ha ing di†e en symme ies and is
a ibu ed o he expanded na u e o he submonolaye phases, ela i e o highe co e age monolaye s,
p o iding oom o mixed c ys al o ma ion.
In oduc ion
Adso p ion om liquids and solu ions o solid su aces is
cen al o many academic and indus ial p oblems, including
lub ica ion, ca alysis and de e gency. Almos all indus ial
si ua ions in ol e mul icomponen mix u es, ei he because
mix u es a e cheape han pu e componen s o because he
simul aneous adso p ion o mo e han one componen can
lead o an enhanced beha iou ela i e o he pu e com-
ponen s alone. Howe e , ob aining de ailed s uc u al in o -
ma ion o e en de e mining he phase beha iou a he
solid/solu ion in e ace is pa icula ly di icul and, as a esul ,
ou unde s anding o hese impo an adso bed laye s is s ill
e y limi ed.
P e ious wo k in his a ea has ocussed on adso bed bina y
mix u es whe e one componen is signiÐcan ly longe han he
o he .1h6In such cases i is gene ally epo ed ha he longe
componen is p e e en ially adso bed o he comple e exclu-
sion o he o he a any p ac ical solu ion composi ion.
Howe e , ecen compu e simula ion esul s ha e epo ed
he opposi e beha iou ,7wi h he sho e alkane being p e e -
en ially adso bed. Adso p ion om solu ion has been epo ed
o gi e ise o monolaye s uc u es1 ha a e di†e en om
hose o med by adso p ion om he apou . Howe e , he
s uc u al changes epo edÈuniaxial comp ession in he b-
axisÈcan also a ise when he o al co e age is inc eased.
To in es iga e he mixing beha iou o adso bed bina y
solu ions a he su ace i is impo an o ca e ully con ol he
alkyl chain leng hs o he componen s and he composi ion o
he bulk solu ion such ha bo h componen s a e co-
adso bed.4,8 We ecen ly epo ed he adso p ion beha iou
o oc ane and nonane mix u es adso bed on g aphi e a high
(g ea e han monolaye ) co e ages.8This wo k, combining
calo ime y, incohe en and cohe en neu on sca e ing,
clea ly indica ed phase sepa a ion o he wo componen s,
ha we e simul aneously adso bed in a solid monolaye ha
coexis s wi h bulk solu ion. This phase sepa a ion a ises
because he wo componen s ha e monolaye s wi h di†e en
space g oups.
He e we desc ibe da a om monolaye s o oc ane and
nonane a sub-monolaye (0.8) co e age using sensi i e adia-
ba ic calo ime y and cohe en neu on di† ac ion ha indi-
ca es ha he e is mixed solid monolaye o ma ion o e a
wide composi ion ange.
Expe imen al
Sample p epa a ion
The g aphi e subs a e used in hese expe imen s was Papyex,
a comme cial ma e ial desc ibed p e iously,9and he speciÐc
su ace a ea was de e mined by adso p ion o ni ogen. Fo
he adiaba ic calo ime y measu emen s a s ack o small discs
o app oxima ely 20 mm in diame e we e used. Fo he
neu on di† ac ion measu emen s a s ack o g aphi e ings o
ex e nal diame e 20 mm and in e nal diame e 8 mm wi h a
o al mass o app oxima ely 12 g we e used. Rings o g aphi e
we e used in he neu on expe imen s o minimise ansmis-
sion p oblems bu maximise he di† ac ed signal. The samples
we e cleaned a 350È400 ¡C unde acuum be o e dosing, in
si u, wi h he equi ed amoun o liquid adso ba e. The
amoun equi ed o a pa icula co e age was es ima ed om
he a ea pe molecule (using he G oszek model3) and he
speciÐc su ace a ea o he Papyex. In he calo ime y mea-
su emen s he o al co e age was main ained a 0.8 o a
monolaye . The di† ac ion measu emen s we e made a o al
co e ages o 0.8 and 5 monolaye s. In all cases he samples
we e annealed a ele a ed empe a u e.
Adiaba ic calo ime y
The me hod and expe imen al de ails o adiaba ic calo ime y
ha e been p esen ed in de ail elsewhe e.10 Essen ially, he hea
capaci y o a sample o subs a e (g aphi e) dosed wi h a
known amoun o adso ba e is di ec ly de e mined as a unc-
ion o empe a u e by adding iny amoun s o elec ical
hea ing ene gy o he sample and measu ing he empe a u e
ise. T ansi ions a e indica ed as peaks in he hea capaci y.
3774 Phys.Chem.Chem.Phys., 2001, 3, 3774È3777 DOI: 10.1039/b102054o
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Neu on di† ac ion
The use o neu on di† ac ion o s udy adso bed bina y mix-
u es has been desc ibed elsewhe e.8,11,12 The expe imen s
we e conduc ed on OSIRIS a he Ru he o d Apple on
Labo a o y, Ox o dshi e, UK. This ins umen has a a iable
inciden wa eleng h and a single Ðxed de ec o bank such ha
almos all he coun ed neu ons a e back-sca e ed. Used in
his geome y OSIRIS has a high esolu ion ha is equi ed
o hese expe imen s.
Resul s
Adiaba ic calo ime y
Fig. 1(a) p esen s he adiaba ic calo ime y da a om oc aneÈ
nonane mix u es adso bed on g aphi e a a o al co e age o
0.8 monolaye a composi ions o (mole ac ion o oc ane,
0, 0.217, 0.427, 0.626, 0.817 and 1.0. The equi alen a eaXoc )
ac ions, a e 0, 0.2, 0.4, 0.6, 0.8 and 1.0 espec i ely. TheAoc ,
posi ions o he peaks in Fig. 1(a) a e plo ed as a unc ion o
monolaye composi ion in Fig. 1(b).
Fig. 1(a) shows a single sha p peak a a empe a u e o 183
K co esponding o mel ing o he adso bed laye o pu e
nonane. This sha p peak mo es o lowe empe a u e (175 K)
and b oadens on addi ion o a small quan i y o oc ane. Such
a dep ession in mel ing poin has been obse ed p e iously in
adso bed laye s and in bulk alkane mix u es.4,8 We a ibu e
his b oadening o he sepa a ion o he solidus and liquidus
a his composi ion. The b oadening in he peak emains on
Fig. 1 (a) Adiaba ic calo ime y om mix u es o oc ane and nonane
adso bed on g aphi e a a o al co e age o 0.8 monolaye s wi h
oc ane mole ac ions o 0, 0.217, 0.427, 0.626, 0.817 and 1.0. (b) Posi-
ions o he peaks in Fig. 1(a) as a unc ion o monolaye composi ion.
u he addi ion o oc ane un il a composi ion o 0.6ÈXoc
0.8 when he peaks na ow again, emaining na ow un il he
adso bed laye is pu e oc ane.
The colo ime y da a a con ains a sha p peak a Xoc 0.8
166 K bu , signiÐcan ly, also exhibi s a shoulde a T 168
K. We assign he lowe empe a u e ea u e o a eu ec ic
in a ian .
I is in e es ing o de e mine he en opy inc eases h ough
he mel ing ansi ions o he monolaye s and o compa e
en opy changes wi h monolaye composi ion. These ela i e
en opy changes can be calcula ed om C/T(C, hea capac-
i y; T, absolu e empe a u e) in eg a ed wi h inc easing em-
pe a u e om some common e e ence empe a u e (125 K in
his wo k) well below he ansi ion. These en opy calcu-
la ions a e gi en in Fig. 2(a). By compa ing he en opies a a
empe a u e well abo e he monolaye ansi ions we can
iden i y ela i e changes in en opy on monolaye mel ing
wi h composi ion, gi en in Fig. 2(b).
Fig. 2(b) can be used o indica e he mixing beha iou o he
monolaye s i he liquid mix u es a e assumed o be ully
mixed. Mel ing o a mixed solid laye will ha e only an
en opy o mel ing. This may a y sligh ly wi h composi ion
bu can be es ima ed as a simple linea sum o ha o he wo
pu e componen s (indica ed by he diagonal dashed line in he
Ðgu e). This end does indeed appea o be ollowed o com-
posi ions sugges ing ha o e his composi ionXoc 0È0.5
Fig. 2 (a) Rela i e en opy inc emen om 125 K ( aken as ze o) o
he di†e en monolaye composi ions o Fig. 1(a). (b) The en opy
inc emen s a 200 K ex ac ed om Fig. 2(a) o he di†e en mono-
laye composi ions. The diagonal dashed line indica es he en opy
expec ed i mix u es a e simple linea sums o he wo pu e com-
ponen s.
Phys.Chem.Chem.Phys., 2001, 3, 3774È3777 3775
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Fig. 3 Schema ic phase diag am o submonolaye oc ane and
nonane adso bed on g aphi e. Composi ion o is a solidXoc 0.55
compound. Di†e en egions o he phase diag am ha e been labelled
wi h he app op ia e phase(s).
ange, on he nonane ich side o he phase diag am, he e is a
mixed solid monolaye .
Howe e , i he solid s a e is no mixed hen mel ing will
ha e no only an en opy o mel ing bu also an addi ional
en opy o mixing. The e o e he en opy will exceed ha o
he simple linea sum, as illus a ed on Fig. 2(b). This beha -
iou is obse ed o composi ions This addi-Xoc 0.5È1.0.
ional excess en opy can be es ima ed depending upon
composi ion, Xoc :
*Smis [RMx1ln x1]x2ln x2N
Whe e and Fo a composi ion o x1 Xoc x2 1[Xoc .
he excess en opy would be 5.5 J K~1 mol~1. TheXoc 0.6
expe imen ally de e mined de ia ion om he ully mixed line
is ound o be 3.1 J K~1 mol~1.
We now eÐne his es ima e o he en opy o include he
ac ha he nonane ich side is in ac no pu e nonane bu a
mixed c ys al. The composi ion o his mixed c ys al is es i-
ma ed as ollows: Many long alkanes o m adso bed laye s
wi h molecules ha a e pa allel o each o he 13 in lines. We
Fig. 4 OSIRIS neu on di† ac ion da a om ( op) a phase sepa a ed
mix u e o oc ane and nonane a mul ilaye co e ages and (bo om)
sub-monolaye co e age mix u e o oc ane and nonane wi h mole
ac ion The a ows indica e he posi ions o peaks omXoc 0.5.
he pu e oc ane and pu e nonane monolaye s. The symme ical peaks
a d-spacings o 3.82, 3.88 and 4.28 in all pa e ns a e om heA
aluminium sample can.
assume a simple model o he mixed c ys al con aining lines
o oc ane and lines o nonane molecules side by side. A line o
nine oc ane molecules will ha e he same leng h as a line o
eigh nonane molecules; he e o e he oc ane mole ac ion in
he mixed laye will be Xoc
mix 9/17 0.53.
In a comple e monolaye a a composi ion o 0.4Xoc 0.6,
monolaye s will be nonane. In a mixed c ys al, his amoun o
nonane can include 0.45 monolaye s o oc ane. 0.15 mono-
laye s o pu e oc ane canno be included. The alues o andx1
o go in o he calcula ion o he excess en opy a e he e-x2
o e and gi ing an excess en opy o 3.5 Jx1 0.15 x2 0.85
K~1 mol~1. This is in e y good ag eemen wi h he expe i-
men al alue o 3.1 J K~1 mol~1 gi en he simplici y o he
model.
Thus we conclude, based on he calo ime y da a, ha
oc ane and nonane will o m a mixed solid monolaye o e a
wide ange o composi ion, and o m a solidXoc 0.0È0.53,
molecula compound wi h a composi ion o app oxima ely
Abo e his composi ion he e is phase sepa a ionXoc 0.53.
o he ma e ial o essen ially pu e oc ane and he oc ane/
nonane mixed c ys al. This phase sepa a ion is e iden om
he excess en opy and he eu ec ic in a ian e iden in Fig.
1(a) a a empe a u e o 166.5 K. A schema ic phase diag am
ep esen ing hese ea u es is gi en in Fig. 3.
Neu on di† ac ion
Fig. 4 illus a es he sca e ing pa e n om ( op) a phase
sepa a ed oc aneÈnonane mix u e a mul ilaye co e ages,
showing he peaks om pu e oc ane and pu e nonane mono-
laye s and hose om he aluminium sample can (labelled in
he Ðgu e). Fig. 4 (bo om) shows he sca e ing pa e n om
an oc aneÈnonane mix u e a sub-monolaye co e age and a
su ace mole ac ion o I is clea om he lowe Xoc 0.5.
ace ha he e is no e idence o he pu e oc ane peaks. This
indica es ha he e is a mixed phase a his sub-monolaye
co e age and mole ac ion, in ag eemen wi h he adiaba ic
calo ime y esul s abo e. In he mixed phase he e is only a
single peak obse ed a app oxima ely he posi ion o he pu e
nonane monolaye peak. This again suppo s he conclusion
ha oc ane molecules a e inco po a ed in o he la ice o he
longe nonane molecule as indica ed by he adiaba ic calo im-
e y da a. The limi ed accessible ange o d-spacing on
OSIRIS means ha a ull s uc u al cha ac e isa ion o he
mixed phase is no cu en ly possible.
Discussion and conclusions
In his wo k we ha e p o ided a combina ion o calo ime y
and di† ac ion da a o demons a e he mixing beha iou o
solid monolaye s o oc ane and nonane adso bed on g aphi e
a sub-monolaye co e age. O e a signiÐcan composi ion
ange i is ound ha he wo componen s do mix on he
su ace, in ma ked con as o he beha iou ound a highe
co e ages.8The e is a solid molecula compound o med wi h
a composi ion o app oxima ely wi h he sho e Xoc 0.53,
oc ane molecules inco po a ed in o he la ice o he longe
alkane nonane. We p opose ha a highe mole ac ions han
phase sepa a ion occu s because he nonaneXoc 0.53
la ice is “ ullÏ. This beha iou is iden iÐed by he calo ime y
da a and simple s uc u al a gumen s.
The mixing beha iou in bulk alkanes has been
in es iga ed14 and can be summa ised in simple e ms as
ollows: whe e he c ys al symme y o he pu e componen s
a e di†e en mixing is no p e e ed. Fo example, bina y mix-
u es o he sho e alkanes C11ÈC12 ,C
12ÈC13 ,C
13ÈC14 ,
and show a p onounced minimum in heC14ÈC15 C15ÈC16
phase diag ams indica ing non-ideal beha iou 15 a ising om
he di†e en space g oups o he pu e compounds (e en
membe s a e usually iclinic and odd membe s o ho-
3776 Phys.Chem.Chem.Phys., 2001, 3, 3774È3777
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hombic). E en when he c ys al symme ies o he wo pu e
compounds a e he same, mixing can s ill be p e en ed i he
uni cells o he wo la ices a e signiÐcan ly di†e en in size.
The bulk beha iou o oc ane and nonane eÑec s his beha -
iou as, al hough hey a e bo h iclinic, hey s ill show a p o-
nounced minimium in he mel ing cu e. P e iously we ha e
demons a ed ha , a highe , mul ilaye co e ages, oc ane and
nonane adso bed laye s ollow hese simple ules and phase
sepa a e because he symme ies o he uni cells a e di†e en .8
The phase beha iou o adso bed alkanes a co e ages
a ound a monolaye is complex. In pa icula , many a e ound
o be expanded a low, sub-monolaye co e ages ela i e o
he high co e age s uc u es.16 Such an expansion sugges s
ha close packing c i e ia a e elaxed in he s uc u es and
he e may be addi ional space in he laye o accommoda e
molecules o a di†e en kind.
The di† ac ion da a p esen ed he e sugges ha i is he
sho e oc ane molecules ha Ð in o he la ice o he longe
nonane molecules, as migh be expec ed. I he e we e a “one-
o -oneÏ eplacemen o nonane molecules by oc ane his
would c ea e gaps in he la ice co esponding oughly o a
uni pe oc ane molecule. Howe e , in ce ain a ange-CH2
men s he oc ane can be inco po a ed wi hou gapsÈ o
example i he e we e “linesÏ o oc ane molecules pa allel o
“linesÏ o nonane molecules. In con as , eplacing oc ane mol-
ecules by nonane would equi e dis o ion o he whole la ice
o accommoda e hem as he space a ailable is oo small.
Acknowledgemen s
The au ho s a e g a e ul o he s a† and scien is s o ISIS a
he Ru he o d Apple on Labo a o y, UK o he neu on
beam ime alloca ions and assis ance wi h he expe imen s.
We also hank he Le e hulme T us (SMC) and EPSRC o
unding (TA).
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Phys.Chem.Chem.Phys., 2001, 3, 3774È3777 3777
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