The c ys alline s uc u es o he e en alkanes hexane, oc ane, decane,
dodecane and e adecane monolaye s adso bed on g aphi e a
submonolaye co e ages and om he liquidy
Thomas A nold,
aa
Robe K. Thomas,
aa
Miguel A. Cas o,
bb
S ua M. Cla ke,*
cc
Loic Messe
cc
and Aki a Inaba
dd
a
Physical and T heo e ical Chemis y L abo a o y, Uni e si y o Ox o d, Sou h Pa ks Road,
Ox o d, UK
b
Ins i u o de Ciencia de Ma e iales de Se illa, A da. Ame ico Vespucio, Se illa, Spain
c
BP Ins i u e and Depa men o Chemis y, Uni e si y o Camb idge, Madingley Rise,
Madingley Road, Camb idge, UK CB3 0HE
d
Depa men o Chemis y, G adua e School o Science, Osaka Uni e si y, Toyonaka,
Osaka 560, Japan
Recei ed 10 h Sep embe 2001, Accep ed 1s No embe 2001
Fi s published as an Ad ance A icle on he web 4 h Janua y 2002
Neu on and X- ay diff ac ion ha e been used o s uc u ally cha ac e ise he c ys alline monolaye s uc u es o
hexane, oc ane, decane, dodecane and e adecane adso bed on g aphi e a sub-monolaye co e ages and when
coexis ing wi h liquid alkane. The s uc u es o all he molecules in es iga ed a bo h co e ages and low
empe a u es a e isomo phous wi h ec angula uni cells o plane g oup pgg con aining wo molecules pe cell. In
bo h high- and low-co e age s uc u es he molecules ha e hei ex ended axes pa allel o he su ace. The plane o
he ca bon skele on is ound o be pa allel o he g aphi e su ace. The monolaye s a sub-monolaye co e ages
a e in e p e ed as uniaxially commensu a e while hose monolaye s coexis ing wi h he liquid a e ully
commensu a e. Dodecane and e adecane a e excep ional: dodecane o ms addi ional phases a high
empe a u es jus p io o mel ing, a bo h low and high co e ages. In hese s uc u es he molecules in he uni cell
a e pa allel o each o he wi h plane g oup cmm. Te adecane only o ms a single phase a high co e ages in which
he molecules appea o be pa allel and up igh , simila o he dodecane high empe a u e, high co e age phase.
In oduc ion
The s uc u e, composi ion and beha iou o alkanes, ca -
boxylic acids and alcohols, adso bed om he apou , liquid
o bina y solu ions o solid su aces is he esul o he delica e
balance o adso ba e and su ace in e ac ions which o en
leads o a complex phase beha iou on he su ace, qui e di -
e en om ha o he bulk adso ba es. Fo example, many
sys ems a e ound o adso b solid, o de ed monolaye s, e en
whe e he s able phase in he bulk sys em is a liquid o solu-
ion.
1–6
I is impo an o iden i y and cha ac e ise such solid
monolaye s in de ail, no only o unde s and he beha iou o
pu e adso ba es bu also o unde s and he beha iou o
mix u es whe e he ela i e s uc u es o he pu e ma e ials can
be a key ac o .
5–7
Physiso bed monolaye s ha e been ex ensi ely s udied by a
a ie y o echniques including STM,
3
calo ime y
8,9
and iso-
he m measu emen s.
10–12
Mo e ecen ly hese echniques ha e
been complemen ed by he applica ion o incohe en elas ic
neu on sca e ing (IQNS) and X- ay and neu on diff ac ion
(ND).
1,8,9,13
Incohe en elas ic neu on sca e ing is able o
indica e he p esence o solid monolaye s coexis ing wi h liquid
adso ba e and o dis inguish he beha iou o each componen
o mul icomponen sys ems. The neu on diff ac ion echnique
is able o p o ide de ailed c ys allog aphic in o ma ion abou
he solid monolaye s.
In his wo k we p esen de ailed s uc u al analysis o he
solid monolaye s o med by he e en membe ed alkanes hex-
ane (C6), oc ane (C8), decane (C10), dodecane (C12) and
e adecane (C14) a bo h submonolaye and mul ilaye co -
e ages using a combina ion o neu on diff ac ion and X- ay
diff ac ion da a.
Expe imen al
Only a b ie ou line o he diff ac ion echniques is p esen ed
he e (see e . 14 o a ull desc ip ion). The appa a us and
p ocedu es o such expe imen s ha e been desc ibed else-
whe e.
15
The ins umen s used o he neu on diff ac ion we e
D20 and D1B a he Ins i u Laue-Lange in, G enoble
15
wi h
inciden wa eleng hs o 0.242 and 0.252 nm espec i ely. The
X- ay expe imen s we e pe o med a he Depa men o
Chemis y, Osaka Uni e si y. The equipmen he e is o he
symme ical ansmission ype wi h Cu-Ka adia ion, as
desc ibed p e iously.
16,17
Sca e ing om c ys alline wo-
dimensional adso bed laye s gi es ise o diff ac ion peaks
18,19
wi h a cha ac e is ic saw- oo h line shape. The solu ion o such
pa e ns can be used o de e mine he s uc u e o he laye
in a manne analogous o diff ac ion om h ee-dimensional
c ys als. Solu ion o such pa e ns can be assis ed by con-
side a ion o close packing a gumen s and he possible plane
g oups a ailable o molecules wi h pa icula poin g oups.
17
The diff ac ion pa e n om an adso bed monolaye is
ob ained by sub ac ion o he sca e ing om he subs a e
alone om ha o he subs a e and adso bed ma e ial oge-
yElec onic Supplemen a y In o ma ion a ailable. See h p:==
www. sc.o g=suppda a=cp=b1=b108190j=
DOI: 10.1039/b108190j Phys. Chem. Chem. Phys., 2002, 4, 345–351 345
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/ Jou nal Homepage
/ Table o Con en s o his issue
he . A highe co e ages whe e he e is a solid monolaye
coexis ing wi h bulk fluid phase he e y b oad peak cha -
ac e is ic o a fluid phase can be dis inguished and emo ed in a
s aigh o wa d ashion. Fo X- ay measu emen s p o ona ed
ma e ials we e used. Fo he neu on expe imen s deu e a ed
alkanes we e used o minimise he incohe en sca e ing
backg ound ha would ha e a isen om p o ona ed samples.
In he de e mina ion o he s uc u e o an adso bed
monolaye i is impo an o combine bo h X- ay and neu on
da a, pa icula ly when he molecules con ain significan
numbe o hyd ogen a oms such as o he alkanes desc ibed
he e. The sca e ing by X- ays is domina ed by he hea ie
elemen s while he neu on pa e ns a e sensi i e o all he
elemen s. In se e al cases he s uc u al solu ion based only
on X- ays has been e oneous as he symme y o he hyd o-
gen-con aining pa s o a s uc u e ha e a lowe symme y
han ha o he hea y elemen s.
20
The adso ben used was ecomp essed ex olia ed ecom-
p essed g aphi e Papyex (Le Ca bone Lo aine). Se e al g a-
phi e samples we e used, each cha ac e ised by adso p ion o
ni ogen and ound o ha e specific su ace a eas o 14–31.6 m
2
g
1
. The deu e a ed alkanes we e ob ained om Camb idge
Iso ope Labo a o ies wi h quo ed deu e a ion le els o
>99%. The g aphi e subs a es we e ou gassed unde acuum
in an o en a 350 C be o e known quan i ies o alkane we e
added as liquid by mic o-sy inge unde an ine a mosphe e
o helium. When dosing he g aphi e i is con enien o know
he app oxima e numbe o equi alen monolaye s adso bed.
This can be es ima ed om he a eas pe molecule, aken om
he wo k o G oszek,
21
and he specific su ace a ea o he
g aphi e.
Resul s
1. Low-co e age s uc u es
Fig. 1 p esen s he expe imen al (a) X- ay and (b) neu on
sca e ing pa e ns om 0.8 monolaye s o hexane, oc ane,
decane, dodecane and e adecane adso bed on g aphi e.
The neu on da a we e collec ed a 50 K and he X- ay da a
a 10 K. The g aphi e backg ound has been sub ac ed
in p epa ing hese figu es. The ea u es a 2y¼42and
2y¼26.6in he neu on and X- ay pa e ns espec i ely a ise
om impe ec sub ac ion o he g aphi e backg ound. The
neu on pa e ns ha e also been co ec ed o ins abili y o
Fig. 1 (a) Expe imen al X- ay di ac ion pa e ns om 0.75 monolaye s o hexane (bo om), oc ane, decane, dodecane and e adecane ( op)
adso bed on g aphi e a low empe a u e. The sca e ing om he g aphi e subs a e in he absence o any adso ba e has been sub ac ed in
p epa ing his figu e. (b) Expe imen al neu on di ac ion pa e ns om 0.8 monolaye s o hexane (bo om), oc ane, decane, dodecane and
e adecane ( op) adso bed on g aphi e a low empe a u e. The ea u es a 2y¼42a e egions o impe ec g aphi e sub ac ion. Calcula ed
(c) neu on and (d) X- ay di ac ion pa e ns om he solid monolaye s o hexane (bo om), oc ane, decane, dodecane and e adecane ( op) on
g aphi e.
346 Phys. Chem. Chem. Phys., 2002, 4, 345–351
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he mul ide ec o on one o he diff ac ome e s (D20). The
peaks in hese figu es a e hen he diff ac ion om he adso -
bed monolaye s and ha e he cha ac e is ic asymme ic line
shape expec ed om wo-dimensional laye s.
18,19
In Fig. 1(b)
he x-axis, ‘ wo he a’ e e s o D20 da a wi h a wa eleng h
0.242 nm. D1B da a has been included on his figu e by cal-
cula ing he effec i e wo he a om he a io o wa eleng hs
o he wo ins umen s.
The mel ing poin s o he laye s, iden ified by he eplace-
men o hese sha p peaks by b oad fluid-like eflec ions, a e
in excellen ag eemen wi h calo ime y da a.
12
2. Coexis ing liquids and solid monolaye s
Fig. 2 p esen s he expe imen al neu on sca e ing pa e ns
om monolaye s o hexane, oc ane, decane, dodecane and
e adecane adso bed on g aphi e a highe co e ages. The
co e ages and empe a u es we e 2 monolaye s and 190 K (C6),
3 monolaye s and 220 K (C8), 2 monolaye s and 250 K (C10),
3 monolaye s and 270 K (C12) and 4 monolaye s and 295 K
(C14). Fo all he ma e ials in es iga ed he pa e ns om he
monolaye s we e unchanged a co e ages highe han 2 laye s,
indica ing ha hese a e he s able monolaye phases in he
p esence o bulk liquid, he empe a u es o hese pa e ns
being abo e he bulk mel ing poin s (178 K (C6), 217 K (C8),
243 K (C10), 264 K (C12) and 279 K (C14)). In p epa ing hese
figu es bo h he g aphi e backg ound and he bulk fluid sca -
e ing ha e been sub ac ed as desc ibed p e iously.
13
Fea u es
a ising om impe ec sub ac ion and de ec o ins abili y ha
a e p esen in Fig. 1(b) a e also p esen in his figu e.
In ma ked con as o he sub-monolaye co e ages, he
mel ing poin o he solid monolaye s which coexis wi h he
liquid a e highe han he bulk mel ing poin s, as desc ibed
p e iously.
13
3. Dodecane
Recen calo ime y and incohe en neu on sca e ing expe i-
men s ha e clea ly indica ed ha dodecane shows an addi-
ional phase change wi h inc easing empe a u e and wi h
inc easing co e age.
12,22
Thus, he e a e ou s uc u es pos-
sible o adso bed dodecane. The low- empe a u e pa e ns
ha e been p esen ed abo e. The neu on diff ac ion pa e n
om 0.75 monolaye s o deu e a ed dodecane a a empe a u e
o 220 K is shown in Fig. 3(a) and he pa e n a 3 monolaye s
a 290 K is shown in Fig. 3(b). The p incipal diffe ence been
Fig. 2 (a) Expe imen al neu on di ac ion pa e ns om he solid
monolaye s o hexane (bo om), oc ane, decane, dodecane and e a-
decane ( op) adso bed on g aphi e a mul ilaye co e ages coexis ing
wi h he liquid. The sca e ing om he ba e g aphi e and ha o he
bulk fluid alkane ha e been sub ac ed in p epa ing his figu e. (b)
Calcula ed neu on di ac ion pa e ns om he solid monolaye s o
hexane (bo om), oc ane, decane, dodecane and e adecane ( op) on
g aphi e coexis ing wi h he liquid.
Fig. 3 (a) Expe imen al neu on di ac ion pa e ns om he solid
monolaye o dodecane adso bed on g aphi e a high empe a u es and
submonolaye co e ages. (b) Expe imen al neu on di ac ion pa -
e ns om he solid monolaye o dodecane adso bed on g aphi e a
high empe a u es a mul ilaye co e ages. The calcula ed neu on
di ac ion pa e ns a e also gi en in he figu es.
Phys. Chem. Chem. Phys., 2002, 4, 345–351 347
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he low- and high- empe a u e pa e ns is he loss o a numbe
o peaks and a shi in he posi ions o o he s. On cycling he
empe a u e i was ound ha he high- empe a u e o low-
empe a u e phase ansi ion a high co e age exhibi ed hys-
e esis, indica ing a fi s o de ansi ion. In keeping wi h
adi ional nomencla u e he low- and high- empe a u e pha-
ses a e e e ed o as phase II and phase I espec i ely.
Fig. 4 gi es he X- ay diff ac ion pa e ns o dodecane a low
empe a u e wi h inc easing co e ages o 0.6, 0.75, 0.84, 0.90
and 1.05 ( op) monolaye s. This figu e clea ly shows ha he e
is coexis ence o wo phases o e a significan co e age ange
be ween he sub-monolaye and high co e age monolaye s,
i.e. be ween 0.84 and 1.05 monolaye s.
S uc u al analysis
Unlike con en ional 3D c ys allog aphy, he e a e usually
many less eflec ions in he expe imen ally de e mined di -
ac ion pa e ns om adso bed monolaye s, and only a lim-
i ed numbe o s uc u al pa ame e s can ealis ically be
ex ac ed. When fi ing he expe imen al diff ac ion pa e ns
we ha e he e o e aken he molecula bond leng hs and angles
o be he same as in he bulk c ys alline phases o he
alkanes.
23
Addi ionally, he alkanes we e assumed o be
adso bed in he all- ans configu a ion. The diff ac ion pa e n
o each ial s uc u e was calcula ed as desc ibed p e-
iously.
14,17,24
In his analysis we ha e adop ed he usual
con en ion o sol ing he pa e ns wi h he mos highly con-
s ained pa ame e se possible. In his app oach we conside
high symme y ec angula uni cells fi s be o e mo ing o less
symme ic and hen oblique uni cells i solu ions canno be
ound. I is always easie o fi an expe imen al pa e n by
adding mo e a iables.
In he ollowing discussion we ha e g ouped he s uc u es
acco ding o hei simila i y. The low- empe a u e, low-co -
e age phases o hexane, oc ane, decane, dodecane and e a-
decane, wi h he low- empe a u e, high-co e age phases o
hexane, oc ane and decane, o m a single isomo phic g oup.
The o he s uc u es o dodecane and e adecane a e discussed
sepa a ely.
He ingbone s uc u es
I was ound ha a good fi o he expe imen ally de e mined
peak posi ions o Fig. 1(a) and (b) o all he e en alkanes
and o Fig. 2(a) o hexane, oc ane and decane we e
ob ained wi h he isomo phic s uc u es illus a ed schema-
ically in Fig. 5. The expe imen ally de e mined cell pa a-
me e s a e gi en in Table 1. Close packing conside a ions and
he indexing o he expe imen al eflec ions can be used o
de e mine he plane g oup o he adso bed laye s.
17
The
highes symme y possible o he space g oup o he uni
cell, cen ed, can be elimina ed due o he p esence o a
s ong (2,1) eflec ion which is o bidden unde cen ed
symme y ( eflec ions (h,k)hþk¼odd a e absen ) o all he
low- empe a u e phases. Fu he mo e, he sys ema ic absen-
ces o eflec ions (h,0) wi h h¼odd and (0,k) wi h k¼odd
indica e a pgg space g oup. The symme y ela ed posi ions
unde pgg a e gi en in Table 2. Wi h his space g oup and he
assump ion o a fixed molecula con o ma ion, he only
adjus able pa ame e s a e he angles o o ien a ion o he
molecule in he cell. Fo any gi en molecule hese wo angles
de e mine he in ensi ies o he en i e se o B agg peaks. The
calcula ed diff ac ion pa e ns a e gi en in Fig. 1(c) and (d)
and 2(b). The ac ional coo dina es o he epea ing mo i s
in he uni cells a e gi en as Elec onic Supplemen a y
In o ma ion in Tables S1–S12.yThe fi s o all he pa e ns
based on his simple model a e excellen and gi e s uc u es
isomo phic o ha illus a ed in Fig. 5(a), whe e he deu e -
ium a oms o one molecule fi in he hollows o he adjacen
chain. We discuss he accu acy o he de e mina ion o hese
s uc u es below.
Fo hexane, oc ane, decane and dodecane he high-co e age,
low- empe a u e phases a e isomo phic wi h he low-co e age
phases wi h he only diffe ence being ha he uni cell is
comp essed along he b-di ec ion.
In he he ingbone s uc u es he angle he molecules make
wi h espec o he a-axis o he uni cell (‘HB’) is app oxi-
Fig. 4 X- ay di ac ion pa e ns o dodecane a low empe a u e a
inc easing co e ages o 0.6, 0.75, 0.84, 0.90 and 1.05 ( op) monolaye s.
Fig. 5 Schema ic ep esen a ions o he monolaye c ys al s uc u es
o (a) hexane, (b) dodecane I and (c) e adecane adso bed on g a-
phi e.
348 Phys. Chem. Chem. Phys., 2002, 4, 345–351
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ma ely 30in all cases (see Table 1). The e a e small a ia ions
be ween he diffe en s uc u es bu hese diffe ences a e close
o he expe imen al e o (±4).
I is in e es ing ha he a-axis pa ame e s o he uni cells o
all he low empe a u e monolaye s a e close o in ege mul-
iples o p3a
g
(0.426 nm), whe e a
g
is he la ice pa ame e o
he g aphi e (0.246 nm). Howe e only a high co e age does
he b-axis pa ame e appea o be ela ed o he g aphi e la ice
pa ame e (2a
g
). This esul indica es ha he monolaye s a e
uniaxially commensu a e a low co e ages and ully com-
mensu a e a high co e ages. Clea ly he e is a uniaxial com-
p ession in he b-di ec ion on inc easing he co e age. Table 1
also indica es small diffe ences in he leng h o he uni cell in
he b-di ec ion be ween X- ay and neu on measu emen s a
low co e ages o samples no ionally made up a he same
co e age, al hough he apa ame e is he same. This indica es
ha he b-axis is s ongly co e age dependen a co e ages
close o a monolaye . Such a co e age dependence sugges s
ha he ansi ion is a con inuous one.
In he ideal commensu a e s uc u e CH
2
g oups si in he
g aphi e hexagons and he molecula axis makes an angle o
exac ly 30wi h he a-axis. The expe imen ally de e mined
molecula o ien a ion is he e o e also consis en wi h a com-
mensu a e laye . The molecula o ien a ion abou he long axis
o he molecules has been he subjec o some deba e. Fo all
he alkanes in es iga ed he e he pa e ns om he low em-
pe a u e phases indica e ha he molecules lie on he su ace
wi h he plane o he ca bon backbone pa allel o he plane o
he g aphi e. Al hough he calcula ed in ensi ies o he
obse ed peaks in he pa e ns a e no e y sensi i e o his
o ien a ion, hey define i o 20. Fig. 6 p esen s examples o
calcula ed sca e ing pa e ns o diffe en molecula o ien a-
ions demons a ing his sensi i i y o he o a ional posi ion
and clea ly showing ha he fla o ien a ion o he molecules
ep esen s a significan ly be e fi o he expe imen al da a
han an up igh o ien a ion.
Scanning unnelling mic oscopy (STM) has also been used
o in es iga e physiso bed monolaye s o simple alkyl species
on g aphi e. Howe e , i is clea ha such da a should be
ea ed wi h ex eme cau ion bo h because in e p e a ion o he
expe imen al da a is e y complex and because i is now well
es ablished ha he sweeping o he STM p obe can pe u b
he delica e s uc u es ha a e being imaged. Recen STM o
much longe alkanes han s udied he e adso bed on g aphi e
ha e shown he appea ance o c ys alline monolaye s uc u es
wi h oblique uni cells. Fo such long molecules he posi ions
o he peaks in he diff ac ion pa e ns a e domina ed by he
la ge a-pa ame e , esul ing in li le diffe ence in he sca e ing
pa e ns be ween ec angula and oblique cells. Howe e , o
he much sho e molecules in es iga ed, using he non-in a-
Table 1 Expe imen ally de e mined cell pa ame e s o he adso bed monolaye s o he e en alkanes and he closes commensu a e s uc u es.
All cell pa ame e s a e in nm. The leng h p3a
g
is 4.26 A. The unce ain ies in pa ame e s aand ba e es ima ed o be 0.02 and 0.01 nm espec i ely.
HB is he angle be ween he molecula axis and he a-axis o he uni cell. Ax is he o a ion angle o he molecule abou i s own axis
Low co e age
Alkane X- ay Neu on High co e age
Hexane 0.54 1.69 0.53 1.69 0.49 1.69
0.54 4p3a
g
0.53 4p3a
g
2a
g
4p3a
g
Ax ¼0, HB ¼25 Ax ¼0, HB ¼25 Ax ¼0, HB ¼25
Oc ane 0.544 2.12 0.53 2.12 0.49 2.12
0.544 5p3a
g
0.53 5p3a
g
2a
g
5p3a
g
Ax ¼0, HB ¼28 Ax ¼20, HB ¼26.5 Ax ¼30, HB ¼25
Decane 0.547 2.55 0.540 2.55 0.51 2.55
5.47 6p3a
g
0.540 6p3a
g
0.51 6p3a
g
Ax ¼0, HB ¼29 Ax ¼10, HB ¼28 Ax ¼0, HB ¼29
Dodecane (Phase II) 0.546 2.98 0.546 2.98 0.426 3.50
0.546 7p3a
g
0.546 7p3a
g
Ax ¼0, HB ¼30 Ax ¼0, HB ¼28 Ax ¼0, HB ¼0
Te adecane 0.555 3.40 Composi e pa e n 0.445 3.90
0.555 8p3a
g
Axial diso de ed,
Ax ¼0, HB ¼33 HB ¼0
Dodecane (Phase I) N=A 0.474 3.52 0.450 3.46
0.474 14a
g
0.450 14a
g
Axial diso de ed, Axial diso de ed,
HB ¼0HB¼0
Table 2 Symme y ope a ions o he pgg space g oup
x,yx,y
1=2þx,1=2y1=2x,1=2þy
Fig. 6 Calcula ed neu on sca e ing pa e ns o di e en molecula
o ien a ions o adso bed molecules in adso bed monolaye s. See ex
o discussion.
Phys. Chem. Chem. Phys., 2002, 4, 345–351 349
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si e echnique o neu on sca e ing, he esul s clea ly suppo
he o ma ion o ec angula cells and he dis inc ion be ween
ec angula and oblique uni cells is unambiguous.
Phases o dodecane and e adecane
The low-co e age, low- empe a u e monolaye s uc u e o
dodecane is isomo phic wi h he o he e en alkanes discussed
abo e. Howe e , on inc easing he empe a u e he expe i-
men al pa e ns change o gi e he new pa e n o Fig. 3(a).
The eflec ions o Fig. 3(a) can be indexed on a ec angula
uni cell as gi en in Table 1. The e y limi ed numbe o
eflec ions clea ly indica es a highly symme ic uni cell wi h
appa en plane g oup cmm. The symme y ela ed a omic
posi ions unde cmm symme y a e gi en in Table 3. Fo e en
alkanes his highly symme ic s uc u e can only a ise in ce -
ain molecula o ien a ions, in pa icula i he molecules ha e
he plane o he ca bon backbone up igh . Al e na i ely, i he
molecules a e o a ing abou hei long axis hen he ime
a e aged s uc u e will appea o be highly symme ical. We
a e no able o dis inguish hese wo al e na i es om he
in ensi ies o he peaks.
Significan ly he leng h o he uni cell a-axes is essen ially
double he leng h o he molecules. Hence we can conclude
ha he molecules do no lie in he he ingbone a angemen
ound a low empe a u es bu a e a anged pa allel o each
o he in he s uc u e shown schema ically in Fig. 5(b).
O en, he appea ance o a highe empe a u e solid phase is
associa ed wi h an inc ease in symme y ollowing he in o-
duc ion o some diso de . Significan ly he in ensi ies o he
(h,0) peaks o his high- empe a u e s uc u e a e s onge
ela i e o he (h,k) han would be sugges ed by simple s a ic
models. This sugges s significan diso de in he laye . Apa
om he possibili y o o a ional diso de al eady men ioned
abo e, he e is he possibili y o diso de associa ed wi h
ansla ional mo ion. Recen STM s udies o longe alkanes
adso bed on g aphi e ha e also epo ed ha he alkane
molecules line up pa allel o each o he o ming ‘lamellae’.
25
In
he STM wo k he e we e indica ions o a sliding o he
lamellae. I his we e he case han he pe iodici y cha ac e is ic
o he lamellae would be expec ed o emain s ong, i.e. he (h,0)
eflec ions would emain s ong. Howe e , he gene al eflec-
ions o he ype (h,k) could be b oadened o e en disappea
al oge he . In he absence o o he expe imen al in o ma ion
abou he dynamics and spa ial ex en o his mo ion i is no
possible o quan i y i om he diff ac ion pa e n.
The co e age-dependen X- ay pa e ns o Fig. 4 show he
e olu ion o he high-co e age low- empe a u e s uc u e o
dodecane. Again, he a-pa ame e o he uni cell clea ly
indica es pa allel molecules jus as o he high- empe a u e,
low-co e age phase. Howe e , he b-pa ame e is sligh ly
sho e a highe co e ages, showing he comp ession e iden
o he he ingbone s uc u e o he o he e en alkanes. Fo
his s uc u e he symme y is no as high as he low-co e age,
high- empe a u e phase, indica ing ha he molecules a e no
diso de ed and in ensi y calcula ions sugges ha he plane o
he ca bon backbone is wi hin 20o being pa allel o he
su ace, bu we canno de e mine he angle wi h any accu acy.
The high-co e age, high- empe a u e phase o dodecane
appea s o ha e he molecules all pa allel, ei he wi h he plane
o he backbone up igh o o a ionally diso de ed, simila o
he low-co e age high- empe a u e s uc u e. The diso de is
e iden in a b-axis alue la ge han he commensu a e alue
and in he loss o highe o de eflec ions. Howe e he high-
empe a u e s uc u es a low and high co e age diffe sligh ly
in ha he high-co e age phase is sligh ly comp essed.
De ailed in es iga ion o he high-co e age phase indica es
ha i expands con inuously wi h inc easing empe a u e,
p io o mel ing, and con ac s on inc easing he co e age.
The low-co e age phase o e adecane is isomo phic wi h
all he o he e en alkanes a his co e age. Howe e , he high
co e age phase o e adecane is ound o adop a simila
s uc u e o high-co e age, high- empe a u e dodecane wi h a
pa allel a angemen o he molecules. The neu on pa e n o
Fig. 1(b) is a a co e age whe e bo h phases coexis , simila o
he co e age dependence seen wi h dodecane, Fig. 4. This high-
co e age phase has a b-pa ame e la ge han commensu a e
and no high o de peaks. Howe e , as desc ibed abo e o
dodecane, we canno eliably dis inguish up igh backbones
om o a ional diso de .
Some o he cha ac e is ics o he s uc u es o high-co e -
age dodecane and e adecane, a e eminiscen o he o a o
phases obse ed in bulk alkanes, al hough bulk o a o phases
in e en alkanes do no occu un il C20.
26–29
As men ioned
abo e, specific calcula ions o dis inguish he s a ic cmm
s uc u e om a o a o phase we e gene ally inconclusi e.
Discussion and conclusions
The low- empe a u e monolaye s uc u es o he e en alkanes,
oc ane o e adecane, a e ound o be isomo phic, diffe ing only
in an expansion by an in eg al alue o 0.426 nm o each addi-
ional CH
2
CH
2
and a uniaxial comp ession on inc easing he
co e age abo e a monolaye . This s uc u e is also isomo phic
wi h hose o adso bed bu ane and hexane monolaye s ha ha e
been p e iously epo ed based on neu on da a only.
4,30
The molecules in nea ly all he s uc u es appea o ha e he
planes o hei ca bon backbones pa allel o he g aphi e.
Howe e , he expe imen ally de e mined pa e ns a e no always
pa icula ly sensi i e o he o ien a ion o he alkanes abou hei
long axis on he su ace. This is pa icula ly he case o dode-
cane whe e i appea s ha he ca bon plane is pa allel o he
g aphi e su ace a low empe a u e bu may be up igh o
o a ionally diso de ed in he high- empe a u e phase. The high-
co e age phases o dodecane and e adecane also diffe om he
majo i y o he s uc u es in ha he molecules a e in a pa allel
a angemen . A ecen STM s udy
3
o he C27 alkane has
iden ified lamella s uc u es wi h ex ended ca bon chains
o ien ed pa allel o he g aphi e basal plane, in good ag eemen
wi h ou esul s. In he STM s udy he alkane chains a e almos
pe pendicula o he lamellae, simila o he high- empe a u e
phases o dodecane and e adecane. In he homogolous se ies o
linea alkanes i appea s ha odd alkanes and e en alkanes o
C12 o longe may adop his pa allel a angemen o molecules.
The c ys allog aphic s uc u e o all monolaye s ha e pgg
symme y in ag eemen wi h he close-packing p edic ions o
Ki aigo odskii.
17
Howe e , he de ailed compa ison wi h wha
is o is no he ue close-packed s uc u e is complica ed by
he effec s o he commensu abili y wi h he unde lying g a-
phi e. The complexi y o hese effec s is well illus a ed by he
ac ha he laye s a e commensu a e along one di ec ion and
con inuously a iable up o commensu abili y in he o he .
Acknowledgemen s
The au ho s hank UK EPSRC (TA), The Spanish DGICYT,
The Le e hulme T us (LM) and The Yamada Science
Founda ion (AI) o financial suppo and he S aff and sci-
en is s a he ILL and ISIS o beam ime and echnical
assis ance.
Table 3 Symme y ope a ions o he cmm space g oup
x,yx,y
1=2þx,1=2þy1=2x,1=2y
x,yx,y
1=2þx,1=2y1=2x,1=2þy
350 Phys. Chem. Chem. Phys., 2002, 4, 345–351
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