Krypton adsorption on (0001) graphite pre-plated with cyclohexane

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Submitted on 1 Jan 1990

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Krypton adsorption on (0001) graphite pre-plated with

cyclohexane

A. Razafitianamaharavo, N. Dupont-Pavlovsky, A. Thomy

To cite this version:

Krypton

adsorption

on

(0001)

graphite

pre-plated

with

_cyclohexane

A. Razafitianamaharavo

_(1),

N.

_{Dupont-Pavlovsky}

₍₁₎

and A.

_Thomy

₍₂₎

(1)

CNRS, Laboratoire Maurice Letort, BP 104, 54600 Villers les

_Nancy,

France

(2)

Laboratoire Mixte CNRS-Saint Gobain, BP 28, 54703 Pont à Mousson, France

(Reçu

le 30

juin

1989,

accepté

sous

_forme

_définitive

le 20

septembre 1989)

Résumé. 2014 La

_{physisorption}

de

_krypton

sur une surface de

_{graphite préalablement}

recouverte de

cyclohexane

(C6H12)

également

physisorbé

a été _{étudiée par volumétrie} entre 71 et 83 K.

L’adsorption

de

_krypton,

fortement inhibée par la

présence

du film

_{préadsorbé,}

ne devient

importante qu’à

des

_pressions

nettement

_plus

_{élevées que celles observées} sur

_graphite

nu. Les résultats obtenus dans le domaine de la monocouche ont été

_{interprétés}

_par un

_déplacement

partiel

de

_C6H12

par le

krypton

et la formation de cristallites de

_C6H12

3D, ainsi

qu’une

miscibilité

partielle

des

_phases

2D. A

_plus

fort taux de recouvrement, le

krypton

s’adsorbe sur lui-même. Abstract.

_{2014 Krypton physisorption}

on

(0001)

_{graphite pre-plated}

with

_cyclohexane

(C6H12)

also

physisorbed

has been studied between 71 and 83 K

_by

means of classical volumetric methods.

Krypton adsorption

is

strongly

hindered

_{by preadsorbed}

_C6H12,

and becomes

_significant

at pressures

definitely higher

than on bare

_graphite.

In the first

_layer

_range, a

_{partial displacement}

of the

_preadsorbed

_C6H12

film

_{by krypton adsorption, according}

to a first order

_phase

transition process, was _assumed, as well as 3D

_C6H12

_crystallites

formation and

_{partial miscibility}

of the 2D

phases.

At

_higher

_coverages

_krypton

adsorbs on itself. Classification

Physics

Abstracts 68.45 - 64.70 - 68.60

1. Introduction.

The

_study

of

_composite

films adsorbed on well characterized surfaces have

_given

rise to

numerous

_{investigations}

within the last years, in order to determine two-dimensional

_(2D)

phase diagrams equivalent

to three-dimensional

_(3D)

solutions and

_alloy

_diagiams

_[1-11].

Classical volumetric methods can be

_applied

to the

_study

of

_binary

mixture

_{adsorption by}

means of the

_{pre-adsorbed layer technique,}

which has been introduced

_{by Halsey}

and

Singleton,

and extended later on

_[1-5].

This

_technique

is

_particularly

suitable when the

equilibrium

pressures of the two _{gases involved in the mixture} are _very

different,

as was the

case for the last

_system

studied

_{[5] :}

_krypton

_adsorption

isotherms on

(0001)

_graphite

pre-plated

with sulfur hexafluoride

_(SF6)

were measured between 70 and 80 K. At these

temperatures,

the

_krypton

_equilibrium

_{pressure is} more than 103 times

_higher

than that of

SF6,

and the presence of

SF6

in the

_gas-phase

can be

_{neglected. Krypton adsorption}

was

shown to be

_strongly

hindered

_{by pre-adsorbed}

_SF6,

_{becoming significant}

_{under pressures}

definitely higher

than on bare

_{graphite. Moreover,}

a

_displacement

of the

_SF6

film

_{by krypton}

adsorption

was

_shown,

_{occurring according}

to a _{first order transition process, and} followed

_by

adsorption

of

_krypton

on itself.

For

_comparison

with the

_krypton-SF6

_system,

we have undertaken a similar

_study

of the

krypton-cyclohexane

(C6H12 )

system

on the same substrate.

_C6H12

has been chosen because

of its very low

saturating

vapour pressure

(about

10-15

torr),

even

_compared

to that of

SF6

(about

10-7

_torr)

in the

_temperature

_{range of the}

_binary

mixture

_{investigation,}

which confers an utmost character to the

_{(krypton-C6H12)}

_system.

_C6H12

_pre-plating

of

_graphite

could then be

_expected

to lead to the formation of a film stable in presence of

_krypton,

which

would be

_equivalent

to a new substrate.

Experiments

were

_performed

on exfoliated

_graphite

_{(manufactured}

_by

« Carbone Lor-raine

_»)

which exhibits a _very

_homogeneous

surface of 45

_{m2. g-1}

_specific

area.

Before

_discussing

the

results,

the main 3D and 2D

_properties

of both

_adsorbates,

which are

needed for the

_composite

film

_study,

and the

_{experimental procedure}

are

_presented.

2. Adsorbates.

2.1 PURITY.

_Krypton

of 99.94 %

_purity

was

_{supplied by}

« L’Air

_Liquide

».

Cyclohexane

was a Merck

_product

of 99.7 %

_purity.

The adsorbates were

_purified

before each

experiment

by pumping

on the condensed

_phase

inside the introduction

_system

at 77 K for

_krypton,

and 193 K

_(dry

ice

_temperature)

for

_C6H12.

2.2 3D PROPERTIES. - The 3D

_{triple point}

and critical

_temperatures

are 115.8 and 209.4 K

for

_krypton

and 279.7 and 554 K for

_C6H12

_{respectively.}

The

_dependence

of

_saturating

_{vapour pressures} on

_temperature

are

_{given by}

the

_{equations :}

between 72 and 91 K for

_krypton

_[12],

and

between 203 and 248 K for

_{C6H12 [13].}

Equation

(2)

cannot be used in the

_temperature

_{range of the}

_composite

film

_{investigation}

(70-83 K)

because

_C6H12

_undergoes

a

_phase

transition from a cubic close

_packed

structure, stable between 279.8 and 186 K to a monoclinic structure stable at lower

_temperatures

_[14].

This is

_why

we have determined the

_dependence

of

_saturating

_{vapour pressure} on

temperatures

between 147 and 185

_K,

where the structure of 3D

_C6Hlz

is the same as that

observed at the

_temperature

of the

_composite

film

_{study :}

The value of the

_saturating

_{vapour pressure, calculated}

_by

means of

_equation

(3),

is

2 x

10-15

torr at 77

K,

which is

_{negligible, compared}

with the

_krypton

_{vapour pressure} at the same

_temperature

_(1.72

torr after

_Eq.

_{(1)). Consequently,}

_{the presence of}

_C6H12

_{in the}

gas-phase

is not taken into account in the

_temperature

_{range of}

_krypton

isotherms measurements

on

_C6H12

_{pre-plated graphite}

_{(70-83 K).}

2.3 2D PROPERTIES. - Pure

krypton

adsorption

properties

on

_graphite

have been

extensively

according

to a

_{complète wetting}

_process.

_Up

to five vertical

_steps

are observed on the 77 K

isotherm,

which are

_{représentative}

ofthe condensation ofthe first five monomolecular

layers,

according

to a first order

_phase

_{transition process. The}

_dependence

on

_température

of the first

three

_steps

_{pressure is}

_given

in table 1.

A

Table I.

_{- Ai and Bi coefficients of the}

relations

_{loglo Pi}

_(torr)

= Ai

+

_{Bi for the first}

three steps

of krypton adsorption

isotherms.

i : number of the step

(a)

: after

_[16]

(b)

: after

[17]

(c)

: our measurements.

A,*

and

_Bt

are estimated values

_assuming

a

1 /i 3

decrease of the

_adsorption

_potential

and

_{perpendicular}

entropy with

_increasing

number of

_layers.

Ao

and

_Bo

are A and B coefficients for the

_saturating

_{vapour pressure ; cf.}

_[17].

In the

_monolayer

_range,

_krypton

condensation leads to a commensurate

.J3

w5 .

R30° 2D solid. Near the

_monolayer

_completion,

this solid

_changes

into an incommensurate structure of

higher density.

The

_krypton

cross section at the

_monolayer

_completion

is 14.7

Â2

at 77

K,

as

determined from volumetric and LEED measurements

_[9,

_18].

Pure

_C6H12

_{adsorption properties}

have been studied

by

means of isotherm

measure-ments

_{[13, 19-21].}

In the last

_{investigation}

_[21],

which is also the most

_complete,

a set of isotherms has been measured in the 203-293 K

_temperature

_{range. The isotherms exhibit} two

steps,

representative

of the

_adsorption

of two

_layers

before 3D condensation. From the

temperature

dependence

of the second

_step

_{pressure, the second}

_layer

_completion

was

assumed to occur above 145

K,

only

one dense

monolayer

being

adsorbed before 3D condensation at lower

temperatures.

This

_assumption

was corroborated

_by

recent structural

investigations

[22].

Figure

1 shows a schematic

_diagram

of an

_adsorption

isotherm at 233 K. One of its characteristic features is the

_slope

of the

_plateau,

which

_corresponds

to a difference

of 20 % in the total adsorbed amount between the end of the first

_step,

and the

_beginning

of the second. In

_[21],

the

_completion

of the

_monolayer

was located at the

_beginning

of the

plateau

(M

point

of

_Fig.

_1),

the adsorbed molecule cross section

_being

then 36.75

Â2.

3.

Expérimental procedure.

The

_preadsorbed

_{layer technique}

is

_grounded

on the difference in condensation

properties

of

Fig.

1. - Schematic

drawing

of a

_C6H12

_adsorption

isotherm on

_graphite

at 233 K. 0 is the

_cyclohexane

fractional coverage.

temperature,

arbitrarily

chosen,

is

_{high enough}

to enable the

_{adsorption equilibrium}

control

_{[13, 21].}

The

_sample

is then

_slowly

cooled

_(0.5

_degree

_per

_minute),

in order to avoid any distillation on the cell

_walls,

down to the

_{krypton adsorption}

_temperature.

Two

_problems

had then to be solved before

_carrying

on this

_{investigation.}

First,

what is the adsorbed

C6H12

amount

_{corresponding}

to a

_{complete monolayer ?}

_Second,

at what

_temperature

should

krypton adsorption

be

_{performed ?}

A difference of 20 % in the

_C6H12

adsorbed amount is observed between the

_beginning

and the end of the

_plateau

of the

_adsorption

isotherm

_(points

M and M" in

_Fig.

_1).

In order to

Fig.

2. - Initial _stages of

_{krypton adsorption}

at 77 K on

_C6H12

_{pre-plated graphite.}

(1), (2)

and

(3)

correspond

to

_C6H12

_coverages at M, M’ and M"

_{points respectively}

on the isotherm of

figure

1.

define the location of the

_{monolayer completion}

the

_low-coverage

isotherm

_shape

of

_krypton

on

_{C6H12 pre-plated graphite}

at 77 K was examined for several

_preadsorbed

amounts. Results

are shown in

_figure

2. When

_graphite

is

_pre-plated

with

_C6H12

amounts

_{corresponding}

to

M’ and M"

_points

in

_figure

_1,

the linear

_Henry

law is observed

_during

the initial

_stages

of

krypton adsorption

(curves (2)

and

_(3),

_Fig.

_2).

_{For the lowest coverage considered}

_(M

point),

the

_Henry

law is not

_obeyed.

These observations can be

_{explained by assuming}

that the

_C6H12

_monolayer

is not

_complete

at

_point

M.

_{Consequently,}

the

_graphite

_surface

pre-plated

with such a

_C6H12

_{coverage is} not uniform with

_respect

to

_{krypton adsorption.}

The

monolayer

coverage was then set at

_point

_M’,

in the middle of the

_plateau

between the two

isotherm

_steps.

Curves shown in

_figure

3 are obtained

_by

_introducing

_krypton

at 77 K for isotherm

measurements on a

_graphite

surface

_pre-plated

at 233 K with a

_C6H12

_monolayer.

The

adsorption

isotherm exhibits three vertical

_steps

and an accident in the second

_plateau.

The

desorption

isotherm exhibits

_steps

at the same _{pressures, but the} two curves cannot be

superposed. Desorption plateaus

occur for

higher

adsorbed amounts, and the accident

observed in the second

_plateau

of the

_adsorption

isotherm does not _appear on the

_desorption

curve. The

_{irreversibility}

_{may be}

_assigned

to an

_{out-of-equilibrium}

state of the film

_during

krypton adsorption.

Such an

_assumption

is corroborated

_by

kinetic observations :

_quite

a

_long

time is _necessary to reach the

_equilibrium

_{pressure after admission of}

_krypton

amounts

corresponding

to _{the upper}

_part

of the isotherm first

_step

_(more

than one

_hour,

_{after each gas}

introduction,

instead of less than 15 min for pure

krypton

adsorption).

In order to increase

Fig.

3. -

Adsorption

and

_desorption

isotherms of

_krypton

at 77 K on

_{graphite pre-plated}

with a

C6H12

monolayer. Krypton

introduction at 77 K

_{(*) adsorption ;}

_{(A) desorption. Krypton}

introduction at 150 K :

_(0).

Dotted line :

_krypton

on bare

_graphite.

_(JKr:

: _{fractional coverage of}

_krypton

on bare

the adsorbate

_mobility,

the

_sample

was heated to 150 K before each

_krypton

introduction. The isotherm was measured at 77

K,

and results are shown in

_figure

3. The

_binary

mixture

adsorption

is then

_{reversible, except}

for the first isotherm

_step,

which will be discussed later.

Moreover,

the

_adsorption

isotherm coincides with the

_desorption

curve obtained after

introducing krypton

at 77 K. The second

_{experimental procedure}

was then

_adopted

and assumed to

_yield

an

_equilibrium

distribution of atoms on the surface. The accident observed in the isotherm second

_plateau

when

_{introducing krypton}

at 77 K was

_assigned

to a metastable

state of the film. Such a

_temperature

effect has

_already

been

observed,

and

_annealings

of the

sample

are

_frequently

carried out in order to reach the

_layer

_equilibrium

state in

binary

mixture

_adsorption

studies

_[6-9].

4. Results and discussion.

4.1 ADSORPTION OF KRYPTON AT 77.3 K ON GRAPHITE PRE-PLATED WITH A

_C6H12

MONO-LAYER. - The

_krypton

_adsorption

isotherm at 77.3 K obtained under

_equilibrium

conditions

(krypton

introduction at 150

_K)

on

_{graphite pre-plated}

with a

_C6H12

_monolayer,

exhibits three vertical

_steps.

Its

_comparison

with the

_adsorption

_{isotherm of pure}

_krypton

at the same

temperature

bears on two main

_{points :}

_step

_{pressures and}

_step

_heights

_{(Fig. 3).}

The first

_step

_{pressure is} two hundred times

_higher

than on bare

_graphite,

which indicates that

_{krypton adsorption}

is

_strongly

hindered

_{by pre-adsorbed}

_C6H12,

whereas the second and third

_step

_{pressure values} are _{very close} to those obtained for

_{krypton adsorption}

on bare

graphite.

Such a feature has

_already

been observed for the

_{(krypton-SF6)/graphite}

_system

_[5].

It was assumed to result from a

_SF6

film

_{displacement produced by krypton adsorption,}

according

to a _{first order transition process. The}

_similarity

of

_krypton

isotherms on bare and

SF6 pre-plated graphite

after the first vertical

_step

was

_assigned

to

_krypton

_growing

on itself. The same

_{interpretation}

can be

_given

to our results.

The

_{irreversibility}

shown

_by

the

_{krypton desorption}

curve in the first

_step

_{range may be}

explained by

the same

_{assumption :}

as the

_{C6H12 saturating}

_{vapour pressure is very low} at

77 K

_(about

10-15

_torr),

the

_exchanges

with the

gas-phase

are

_negligible.

At

_least,

_part

of the 3D

_C6H12

_crystallites

formed

_during

adsorbed

_C6H12

_{displacement by krypton adsorption}

cannot re-distillate to the 2D

_phase,

even when

_krypton

is desorbed. The

_shape

in the initial

part

of the isotherm would then

_represent

the

_{adsorptive properties}

of

_krypton

on a

graphite

surface

_{only partially pre-plated}

with

_C6H12.

But the

_similarity

between

_Kr-SF6

and Kr-

_C6H12

_couples

is not so close as far as we are

concerned about the

_step

_heights.

As a matter of

_fact,

the

_{krypton adsorption}

isotherm on

graphite pre-plated

with a

_{SF6 monolayer}

exhibits at 77.3 K

_steps

of

_{equal height,}

83 % that of pure

krypton

isotherm

_steps

_[5].

The authors

_assigned

this difference in

_height

to 3D

SF6

formed

_during

the film

_{displacement,}

and

_covering

a

_part

of the surface which is no

_longer

accessible to

_krypton.

This

_assumption

cannot be

_applied

to our case,

_owing

to the

_larger

difference observed. The first

_step

_height

of

_krypton

isotherm at 77.3 K on

_{C6H12 pre-plated}

graphite

is 70 % that on bare

_graphite,

and those of the second and third

_steps

_only

50 %. If the

_{non-accessibility}

to

_krypton

of 30 % of the total uniform surface area in the first

_layer

range were

_solely

attributed to

_displaced

_C6H12,

it would

_correspond

to a

_three-layer

thick

film.

_{Thermodynamical}

and structural _{studies of pure}

_C6H12

_{adsorptive properties}

have shown that

_only

one

_monolayer

can be adsorbed at 77.3 K before 3D condensation

_[13,

21,

22].

Moreover,

a

_three-layer

thickness is not

_{large enough}

to be

_{representative}

of a 3D

_phase.

This is

_why

our

_explanation

is that the

_C6H12

film

_{displacement by krypton}

is not

_complete,

_part

of the 2D

_preadsorbed

_C6H12

_remaining

on the surface. The most

_probable

reason for such a

the isotherm

_heights

of the second and third

_steps

with

_respect

to the first could be

_explained

by assuming

that 2D

_C6H12

in solution with

_krypton

in the first adsorbed

_{layer produces}

repulsive

sites for

_{krypton adsorption}

_{in the upper}

_{layers, resulting}

in a new decrease of the uniform surface accessible to

_krypton.

4.2 INFLUENCE OF

_C6H12

FRACTIONAL COVERAGE. -

_{Krypton adsorption}

isotherms at

temperatures

near to 77 K on

_{graphite pre-plated}

with

_{0.1, 0.48,}

0.7 and 0.77

_C6H12

monolayer

are shown in

_figure

4. The

_only

differences with the isotherm obtained on a

complete

C6H12

monolayer

lie in the first

_layer

_{range : the first}

_{step separates}

into two

sub-steps,

whose relative

_{heights depend}

on

_C6H12

_{coverage. The}

_higher

the

_C6H12

_{coverage, the} more

_developed

the second

_sub-step.

It is located at the same _pressure as that of the

_krypton

isotherm first

_step

on a

_complete

_C6H12

_monolayer

and is

_{representative}

of the same

phenomenon : displacement

of the

_C6H12

film

_{by krypton adsorption.}

Fig.

4.

_{- Krypton adsorption}

isotherms on

_C6H12

_pre-plated

_graphite

_{(1) :}

0.1

_C6H12

_monolayer

77.3 K ;

(2) :

0.48

_C6Hl2

_monolayer

77 _{K ;}

_{(3) :}

0.7

_C6HI2

_monolayer

77.24 K ;

(4) :

0.77

_C6H12

_monolayer

77.37 K.

_{(J Kr}

= fractional coverage of

krypton.

A detailed

_{representation}

of the initial

_parts

of the isotherms is

_given

in

_figure

5. For 0.1 and 0.48

_C6H12

_{fractional coverages,} a

_step

_clearly

_appears at the same _pressure as that of pure

krypton

isotherm first

_step,

and can be

_assigned

to

_{preliminary krypton adsorption}

on the

bare

_part

of the surface. The

_shape

of the curve obtained with 0.7

_monolayer

of

_pre-adsorbed

C6H12

is

_probably

_{the consequence of} an increase of the surface

_{heterogeneity}

towards

krypton

with

_increasing

_C6H12

_{coverage. In}

_{conclusion, krypton}

is first adsorbed on the bare

Fig.

5.

_{- Krypton adsorption}

isotherms on

_C6HI2

_{pre-plated graphite.}

Detailed

_{representation}

of the first

_sub-step.

₍₁₎

0.1

_C6Hl2

_monolayer

77.3 K.

₍₂₎

0.48

_C6H12

_monolayer

77 K.

₍₃₎

0.7

_C6H12

_monolayer

77.24 K. Dotted line :

_krypton

on bare

_graphite.

0 _{fractional coverage of}

_krypton

on bare

graphite.

However,

the commensurate-incommensurate transition

_occurring

near the

_completion

of a _pure

_{krypton monolayer}

_(B

_point

in

_Fig.

₅₎

does not _{appear for the}

_composite

_film,

even

when 0.1

_C6H12

_monolayer

is

_{pre-adsorbed.}

A similar

_study

of the

_{(krypton-xenon)/graphite}

system

has shown that for

_{krypton adsorption}

on 0.1 xenon

_{pre-adsorbed monolayer,}

the

commensurate-incommensurate _{transition still appears and is moved towards}

_higher

press-ures

_[4].

Thus we think that this transition

_actually

does not occur for the

(krypton-C6H12)/graphite

system.

Moreover,

as the

_krypton

isotherm first

_substep

_{pressure is that of}

pure

krypton

commensurate 2D solid

condensation,

pre-plated

C6H12

probably

results in a

stabilization of

_krypton

commensurate 2D solid. Such a stabilization of a commensurate

phase by larger

molecules has

_already

been observed

_{[4-8, 9]}

in

_adsorption

studies of

composite

films when the two

_components

of the film are miscible. This corroborates our

assumption concerning

the

_solubility

of

_C6H12

in

_krypton.

It should also be noted that in the first

_layer

_range,

_{krypton adsorption}

isotherm on 0.7

C6H12

layer pre-plated

graphite

is very similar in

shape

to

_krypton

_desorption

isotherm on one

C6H12

layer pre-plated

graphite,

in accordance with the

_assumption

of a

partially

irreversible

displacement

of

_C6H12

_{by krypton,}

as

_proposed

in

_{paragraph 4.1.}

4.3. DEPENDENCE ON TEMPERATURE. -

_{Krypton adsorption}

isotherms on

_graphite

pre-plated

with one

_C6H12

_monolayer,

measured at

_71,

77.5 and 83

_K,

are

_reported

in

_figure

6. The

_{general shapes}

of isotherms are _{very similar} at the three

temperatures

considered,

except

for the first

_step,

which is no

_longer

vertical at 83 K. The different

step

pressures are

_reported

Fig.

6. -

Krypton adsorption

isotherms on

_{graphite pre-plated}

with a

_C6H12

_monolayer.

₍₁₎

71 _K,

₍₂₎

77.5 K,

(3)

83 K.

8Kr

fractional coverage of

krypton

on bare

_graphite.

Table II.

_{- Step}

pressures

of krypton

isotherm _steps on bare and

_monolayer

_C6H12

_pre-plated

graphite. Pl, P2

and

_P3

are the pressures

of

the

first,

second and third

_steps

respectively.

with

_{Ai and Bi}

taken in table

_I,

where i is the number of the

step.

Ai

and

_B,

are taken from

[16],

A2, A3, B2, B3

from our measurements.

At the above

_{temperatures,}

the second and third

_step

_{pressure values} are _{very similar} on

bare and

_{pre-plated graphite,}

whereas the first

_step

_{pressure is much}

_higher

on

_pre-plated

Table III.

_{- Ai}

and

_{Bi coefficients of}

the relations

_loglo

_Pi

(torr) =- Ai

_{+ Bi}

_for

the

_first

T

three _steps

_{of krypton adsorption}

isotherms on

_{monolayer C6H12 pre-plated graphite.}

i is the number

of

the step.

with those for a _pure

_krypton

film

_{(Tab. I)}

corroborates the above observations : the

adsorptive properties

of

_krypton

on

_C6H12

_{pre-plated graphite}

differ from those on bare

graphite mainly

in the first

_layer

_range.

In so far as the substrate

(graphite

+

_C6H12)

is not invariant with

_respect

to

_krypton

adsorption,

and

_part

of

_C6H12

is

_probably

in solution with

_krypton,

it seems hazardous to

apply

to the

_composite

film the 2D

_{phase thermodynamical}

_{relations established for pure} adsorbed films. It should be noticed that the calculation of the isosteric heat of

_adsorption,

according

to the relation

leads to a lower value

(Qst

= 12.4 kJ.mole

1)

than that

corresponding

to _pure

krypton

adsorption

in the first

layer

range

(6st

= 16.5 kJ .

mole -1

), as

has

already

been observed for

the

_{(krypton-SF6)/graphite}

_system

_[5].

As for this last

_system,

the difference can be

_mainly

assigned

to the

_displacement

of the

_{preadsorbed layer.}

It would be worth

_checking

the

_validity

of these

_Qst

values

_by

means of calorimetric measurements.

_{Unfortunately,}

this is

_presently

impossible,

due to technical reasons.

5. Conclusion.

From these

_results,

a mechanism can be

_proposed

for the

_composite

film

organization,

when

krypton

is adsorbed on a

(0001)

_graphite

surface

_pre-plated

with a

_C6H12

_monolayer.,

A schematic

_drawing

of its successive

_stages

is

_presented

in

_figure

7. The initial

_stage

_(OA

_part

of the

_isotherm)

would be 2D

_krypton

_gas

_adsorption

on the

_C6H12

monolayer. Krypton

adsorption

on the

_graphite

surface itself would

_{only begin}

in the curved AB

part

of the

isotherm,

with a

_{possible compression}

of the

_C6H12

film.

_Displacement

of 2D

C6H12

would occur

_according

to a first order

_phase

transition _process,

represented by

the vertical BC

part

of the isotherm first

_step.

The two

_coexisting

2D solid

_phases

would be the

_compressed

C6H12

film,

and a

_C6H12-Kr

solution rich in

krypton,

whose extent would increase with

increasing krypton

coverage, while 2D

compressed C6H12

would transform into 3D

crystallites,

until

_reaching

the first

_{layer completion.}

The second and third isotherm

steps

would be

_{representative}

of

_{krypton adsorption}

on itself.

Concerning

the structure of the

_krypton

rich

_phase

in the first

_monolayer,

from the

Fig.

7. -

Schematic

drawing

on the

_composite

film

organisation.

(0)

krypton.

(O)

cyclohexane.

We intend to

_verify

all these

_{assumptions by}

a structural characterization of the

(Kr-C6H12 )/graphite

system,

scattering techniques beginning

to be

_applied

to the

_study

of

composite physisorbed

films

_[6-11].

The last

_point

to be noticed concerns the number of

_steps

exhibited

_{by krypton adsorption}

isotherms at 77 K on

_{pre-plated graphite. Only}

three

_steps

can be

_{distinguished}

for

krypton

adsorption

on

_{graphite pre-plated}

with a

_C6H12

_monolayer,

instead of five for

_krypton

on bare

graphite.

Such a decrease of the

_step

number can be

_assigned

to the surface

_{heterogeneity}

produced by C6H12 pre-plating,

in

_agreement

with Price and Venables

_[23].

These authors have shown that the number of rare _{gas adsorbed}

_layers

on the

_clivage

faces of

_graphite

is very

sensitive to the surface

_uniformity

and cleanliness. For

_C6H12

_{pre-coverages lower than about} 0.5

_monolayer,

four

_steps

are to be seen on the

_krypton

isoterm,

which would indicate that

surface

_{heterogeneity}

decreases with

_decreasing

_C6H12

_coverage.

In

_conclusion,

_despite

its

_saturating

_{vapour pressure, much lower than that of}

_SF6,

Acknowledgment.

The authors are

grateful

to Professor X. Duval for

_helpful

discussions.

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