Materials Chemistrla and Ph_ysics, 21 (1989) 155-167 155

STUDY OF ADSORPTION OF LINEAR ALIPHATIC MONOHYDRIC

ALCOHOLS AND MONOCARBOXYLIC ACIDS FROM n-HEPTANE AND n-DECANE

SOLUTIONS ONTO a-IRON(II1) OXIDE IMMOBILIZED IN PHOTOSYNTHETIC

MEMBRANES*

I.R. BELLOBONO, E. SELL1 and L. RIGHETTO

Cattedra di Chimica, Dipartimento di Chimica Fisica ed Elettrochi-

mica, Universita di Milano, 20133 Milan (Italy)

F. MUFFATO

SORI S.p.A., 20070 Guardamiglio, Milan (Italy)

C. ERMONDI

Bosso Carte Speciali S.p.A., 10075 Mathi Canavese, Turin (Italy)

Received July 7, 1988; accepted August 12, 1988

ABSTRACT

Differential heats of adsorption at 298.2 K of a series of linear

aliphatic monohydric alcohols C n 2n+10H (n=2-14) and linear aliphatic H

monocarboxylic acids C H n 2n+,COOH (n=l-13), from n-heptane and n-decal

solutions (0.00005-0.03 molar fractions), onto cc-Fe,O, immobilized

by photosynthetic membranes, prepared by photochemically grafting

an epoxy-diacrylate copolymer onto cellulose, were measured. Langmuil

type treatment of data allowed us to calculate thermodynamic constanl

and adsorption heats for monolayer saturation, from which thermo-

dynamic functions (free energy, enthalpy, and entropy) (saturated

system as standard) were then computed. Mean areas occupied by

adsorbed molecules in the monolayer coincided with those obtained

for the unsupported sorbent, by allowing only about 20% of decrease

of surface area, as a consequence of immobilization. Behaviour of

immobilized sorbent is compared with that of 'free' cr-Fe,O,. The

sensitivity of adsorption measurements both to study configuration

of adsorbed molecules, solvent-solute and solute-solute interactions

* Part 10 of the series 'Photosynthetic Membranes'.

0254-0584/89/$3.50 0 Elsevier Sequoia/Printed in The Netherlands

156

for the system examined, as well as to obtain informations on the

specific surface area of immobilized sorbent, is discussed.

INTRODUCTION

In a previous paper [I] relations between the permeability of

composite membranes immobilizing sorbents and their structure, with

particular reference to the internal surface area of the porous

medium, have been investigated. For membranes prepared by photo-

chemical grafting of epoxy-diacrylate prepolymer onto cellulose,

satisfactorily linear plots of permeating volume of liquid per unit

time and unit apparent surface as a function of pressure drop have

always been observed in a certain range of membrane thickness. The

proportionality constant between these two quantities was represented

by the volume performance coefficient, that is the reciprocal of

overall hydraulic resistance. Linear portions in the graphs of

hydraulic resistance vs. - membrane thickness corresponded to constant

values of internal surface area. In this way, specific surface areas

of sorbents immobilized in the membrane structure could be evaluated

Cl]- These looked not so different from the values measured by the

classical B.E.T. method on pure sorbents at liquid nitrogen

temperature.

The investigation of adsorption properties of sorbents immobilized

in photosynthetic membranes was also aimed at, in order to obtain

further information on their active surface areas. Sorption studies

(adsorption and hydrogen ion exchange) of acetic acid from water by

photosynthetic membranes have already been carried out [2]. In the

present paper, a-iron(II1) oxide was chosen as model sorbent, both

in consideration of the fact that photosynthetic membranes immobiliz-

ing a-iron(II1) oxide had been subjected to flow dynamical character-

ization fl], and of the previous thermodynamic study of adsorption

of linear aliphatic monohydric alcohols and monocarboxylic acids

onto unsupported a-iron(II1) oxide from n-heptane and n-decane -

solutions [3]. The latter could consequently serve as a suitable

basis for comparison of behaviour between unsupported and immobilized

sorbent. This model has revealed quite useful knowledge of mean

157

areas of molecules adsorbed onto active sites under conditions of

monolayer saturation, and consequently of the configuration of these

molecules.

EXPERIMENTAL

Materials

a-Fe,O, was prepared and characterized as described previously

[II ( B.E.T. surface area 2.2kO.l m'g-I). All reagents and solvents

were of analytical grade purity. Solvents (n-heptane and n-decane)

were dehydrated by refluxing on sodium wires, followed by rectifica-

tion.

Membranes

Photochemical preparation of membranes immobilizing 30.0 wt.% of

cl-Fe,O, was carried out as in a preceding work [I].

Adsorption measurements

Differential heats of adsorption were measured microcalorimetri-

tally at 298 K, by employing the experimental device described in a

previous work [3] with some modifications, which will be reported

here. The flow microcalorimeter was fitted with an auxiliary chamber,

used as a pre-column, either filled with the sorbent membrane or

with the same membrane prepared without immobilized a-Fe,O,. This

chamber was identical to the cylindrical cell, 8 mm diameter and

height, interiorly coated by polytetrafluoroethylene, by which

adsorption heats were measured and in which an amount of membrane

material containing 48-51 mg of immobilized cc-Fe,O, was introduced.

A flux of 5.6 ml hr-1 was constantly maintained. This system allowed

us to determine simultaneously adsorbed quantities and the relative

adsorption heats. The method was based on determination of retention

times of solutes on the sorbent. When the pre-column contained the

'inert' membrane, while the calorimetric cell contained the'active'

membrane immobilizing a-Fe,O,, the solution emerged at time t,,

determined by the calorimetric peak. If, on the contrary, both pre-

column and calorimetric cell contained immobilized e-Fe203, the

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solution emerged at time t > t,. The difference tr= t - t, may be

defined as a retention time. The adsorbed amount of solute w (mol/

g of sorbent) may then be obtained by eqn. (I):

w=fct rjm (I)

where f denotes flow rate (1 s-i ), c concentration of solute (mol

l-l), m mass (g) of sorbent immobilized in the membrane and tr

retention time (s).

Adsorption heat was then determined as usual by peak area and

previous calibration. The maximum sensitivity was about 1 uJ with

temperature variations of 10 -"C. The standard state was considered

to be that of saturated sorbent. In the specific case this corre-

sponded to equilibration with about 0.2 M solutions of investigated

alcohols and acids (about 0.03 molar fraction). To reach the standard

state the differential method was followed [4] with at least 4 or 5

series of runs, in order to evaluate uncertainty.

Experimental data were found to satisfactorily fit the Langmuir

equation in the modified form [4]:

(x,/q,) = (l/q, Ki) + (xi/q,) (2)

where xi and q, represent the molar fraction of solute in a certain

solution 1 and its corresponding differential heat of adsorption

respectively, q, the adsorption heat for monolayer saturation, and

K, the equilibrium constant at concentration x1. Some examples of

these plots are given in Fig. 1. From K, at various xi values,

following the method of adsorption competition between solute and

solvent [5], thermodynamic equilibrium constants K could be finally

calculated, by successive approximations (the fourth approximation

usually gave satisfactorily constant values).

Thermodynamic functions of adsorption

From K values standard free energy of adsorption (saturated system

as standard) AGO was obtained (= -RT 1nK). From the ratio between

159

I I I

0.01 0.02 0.03

x, hdar fraction)

Fig. 1. Some examples of Langmuir isotherms, at 298.2 K, following

eqn. (2), relative to adsorption of linear aliphatic monohydric

alcohols and monocarboxylic acids onto cl-Fe,O,, immobilized by

photosynthetic membranes, from n-heptane solutions.

qo and the corresponding moles of adsorbed solute per unit mass of

sorbent, standard enthalpy of adsorption AH' was calculated. Finally,

standard free entropy AS" was calculated by the usual relation

AGo = AH' - TAS".

Mean area occupied by adsorbed molecules

From B.E.T. values of surface area, mean areas a, occupied by

adsorbed molecules in the monolayer were also calculated by some

approximations requiring on one side the full coverage of sorbent

surface area A, so that:

A = n,A,+ n,A, (3)

(where A, and A, are partial molar surface areas of solute 1 and

solvent 2 respectively per unit mass of sorbent, and n,, n2 the

corresponding moles per unit mass, provided solute and solvent have

160

similar molecular structures), and on the other side that n,A,<< nlAl,

an approximation which was fulfilled in the conditions of the present

work. As a consequence, eqn. (3) becomes:

A " n,A, (4)

from which a, (area occupied by a single molecule of adsorbed species)

was obtained:

al : A/n, N, (5)

with N, Avogadro's number. In eqns. (3)-(51, relative to experiments

carried out in the present work, sorbent surface area A was

considered equal to the B.E.T. surface area corrected by a factor

of 0.8 (see Results and Discussion) allowing for the decrease brought

about by immobilization in the photosynthetic membranes studied.

RESULTS AND DISCUSSION

Differential heats of adsorption at 298.2 K of a series of linear

aliphatic monohydric alcohols C H n 2n+,0H (n=2-14) and linear aliphatic

monocarboxylic acids C n 2n+,COOH H (n=l-13) onto u-Fe,O,, immobilized

by photosynthetic membranes prepared by photochemically grafting an

epoxy-diacrylate copolymer onto cellulose, from n-heptane and

n-decane solutions were measured at various concentrations

corresponding to 0.00005-0.03 molar fractions xi. Measurements have

been carried out microcalorimetrically by a technique using retention

times which allowed us to obtain adsorbed quantities and differential

heats simultaneously. Integral heats of adsorption relative to

'immobilized' sorbents resulted as 75-85% of those measured [3] in

a preceding work with 'free' a-Fe,O,.Differential heats q,, however,

were practically the same, within the limits of experimental un-

certainty, as the corresponding mass of adsorbed solute was lowered

accordingly. Ratios (x,/q,) as a function of xi satisfactorily

followed, within the limits of experimental error, the Langmuir

161

I I

6-

8 a-TAS’ IT-2982 KI

5 10

Number of carbon atoms

Fig. 2. Thermodynamic functions (J mol-lI, -AH', -AGO, and -TAS', at T = 298.2 K, for the adsorption of linear aliphatic monocarboxylic

acids from n-heptane solutions onto a-Fe,O,, as functions of the number of carbon atoms (carboxylic functions included). Data (present

work) relative to a-Fe,0 3 immobilized in the photosynthetic membrane

structure are denoted with full symbols, while open symbols refer to free, non immobilized sorbent [3].

I 1

5 10

Number of carbon atoms

Fig. 3. Thermodynamic functions (J molV1), -AH* and -AGO, at T =

298.2 K, for adsorption of linear aliphatic monocarboxylic acids

from n-decane solutions onto (X-Fe203, as functions of the number

of carbon atoms (carboxylic functions included). Data (present work)

relative to a-Fe,O, immobilized in the photosynthetic membrane

structure are denoted with full symbols, while open symbols refer

to free, non immobilized sorbent [3].

plot of eqn. (2). Adsorption heats corresponding to the monolayer

saturation and Langmuir constants K, could thus be obtained for

'free ’ [3] as well as for immobilized cr-Fe,O, (present work).

Finally, by the method of competing solute/solvent adsorption [5],

'thermodynamic ’ constants K were calculated and consequently thermo-

dynamic functions of adsorption at 298.2 K. In Fig.1 some examples

162

of linearized Langmuir plots following the form of eqn. (2) are

reported. In Figs. 2 and 3, thermodynamic functions AGO, AH', and

AS0 at 298.2 K for adsorption of linear monocarboxylic acids are

reported as functions of the number of carbon atoms (carboxylic

function included), in n-heptane (Fig. 2) and in g-decane (Fig. 3)

as solvents, respectively. In Figs. 4 and 5, thermodynamic functions

for adsorption of linear alkanols in n-heptane (Fig. 4) and in

p-decane (Fig. 5) as solvents are reported, respectively.

1 I

5 10 Number 01 carbon atans

Fig. 4. Thermodynamic functions (J mol-l), -AH", -AGO, and -TAS', at T = 298.2 K, for adsorption of linear alkanols from n-heptane solutions onto a-Fe,Og, as functions of the number of carbon atoms. Data (present work) relative to cc-Fe,03 immobilized in the photo- synthetic membrane structure are denoted with full symbols, while open symbols refer to free, non immobilized sorbent [3].

0 -AH'lwm nunbn of carbon atcmsl

6- O-AC’

I I

5 10 Number of carbon atoms

Fig. 5. Thermodynamic functions (J mol-I), -AH" and -AGO, at T = 298.2 K, for adsorption of linear alkanols from n-decane solutions - onto cr-Fe203, as functions of the number of carbon atoms. Data (present work) relative to cc-Fe20 3 immobilized. in the photosynthetic membrane structure are denoted with full symbols, while open symbols refer to free, non immobilized sorbent [3].

163

Fig. 6. Mean surface area a, (~2/molecule) of molecules adsorbed

onto cl-Fe,O, under conditions of monolayer saturation as a function

of the number of carbon atoms in the aliphatic chain (carboxylic

functions included). Data (present work) relative to cr-Fe,O, immobilized in the photosynthetic membrane structure are denoted

with full symbols, while open symbols refer to free, non immobilized

sorbent [3].

The surface area of a-Fe,O, being known, mean areas a, occupied

by single molecules saturating the monolayer were calculated.as

reported in Experimental. Their values for the various systems

examined are collected in Fig. 6.

In Figs, 2-6, results obtained for a-Fe,O, immobilized in the

membrane are compared with those relative to free, non immobilized

sorbent as measured previously.

It may be first observed that free energies of adsorption are

substantially independent of the number of carbon atoms in the

molecular chains and are nearly constant, ranging between (1.6-1.8)

x 104 J mol-i for both acids and alcohols. As a consequence, AH" and

AS' variations as a function of the number of carbon atoms follow

the same pattern.

164

The same order of magnitude of AH" values for acids and alcohols

(see Figs. 2,3 on one side and Figs. 4,5 on the other) apparently

contradicts the consideration that chemisorption of acids should

produce a greater enthalpic effect than the predominantly physical

adsorption of alcohols. As a matter of fact, however, one should

bear in mind that AH' values for acids are apparent values only,

incorporating both adsorption effects and effects of monomer 2 dimer

equilibrium, which stabilize solutions and destabilize adsorption.

The minima in AH' as well as in AS" values may be interpreted by

London-type interactions, attributable to oscillating dipole moments

even on apolar or slightly polar molecules, the relative importance

of which is magnified when both solute and solvent have comparable

size. as may be remarked from the shift of minima in Fig. 3

relative to Fig. 2 and in Fig. 5 relative to Fig. 4, when going from

n-heptane to c-decane as solvents. - This explains the minimum of AH'

for C, acid and C, alcohol, which have an almost equal molecular

'length', as well as the minimum of AH" for the C, acid in n-decane, -

which is shifted by one or two carbon atoms for the alcohols in the

same solvent. The parallel entropy effects denote clearly in these

cases a more ordered adsorption configuration on the solid surface.

By determination of mean areas of adsorbed molecules on active

sites under conditions of monolayer saturation (Fig. 6), this

hypothesis is corroborated and the configuration of adsorbed mol-

ecules may be further investigated. The values of Fig. 6 relative

to aliphatic monocarboxylic acids show a generally constant surface

area of about 22-25 A' per molecule, with some exceptions, e.g. for

C5 and C, acids on ;-heptane, for which surface areas are nearly

doubled. An array of adsorbed molecules perpendicular to the surface,

in the first kind of conformation, or rather relaxed parallel to it,

in the second kind, may be inferred. These observations are again a

clear evidence of London-type interactions with the solvent, which

also appears, though less markedly, in the alcohols.

From Figs. 4-6, a different behaviour for alcohols between even

and odd numbers of carbon atoms in the molecular chain may be

165

evidenced. It may be explained in terms of Debye-type (dipole-dipole

and dipole-induced dipole) interactions, which have a major

responsibility in physical adsorption, and which also appear in the

enthalpies of fusion [6]. A similar differentiation may also be

observed in the melting enthalpies of linear aliphatic acids, while

such a behaviour does not appear in the graphs of Figs. 2-3. This,

however, does not contradict the interpretation given for alcohols,

as for acids chemisorption is prevailing and conceals any effect

deriving from much weaker dipolar interactions. Furthermore, the

'chain matching' (which is the result of solvent-solute interactions

being important scientifically [7-81 and determinant technologically,

e.g. in extreme-pressure lubrication mechanisms [3]) is less marked

and more diffuse (peaks are less high) in alcohols than in acids.

This may mean that, due to the smaller steric hindrance of the

hydroxy group, London forces in alcohols have always a certain

importance, even if 'chain matching' is able to enhance their effect.

Finally, if behaviour of adsorption onto cr-Fe,O, is compared with

that onto a-Fe,O, immobilized by photosynthetic membranes, the

following facts deserve attention. When considering Langmuir iso-

therms (Fig. 1 and eqn. (2)), it was experimentally noted that

integral heats of adsorption at constant molar fraction of solute

per unit mass of immobilized sorbent were lower by 75-85% than the

values measured on the free one, but substantially similar q, and

K, values could be obtained (eqn. (2)). In other words, the effect

of immobilization is shown only as a certain, yet fairly limited,

decrease of surface area. Thermodynamic functions for immobilized

cr-Fe,O, calculated, as described in Experimental, from K, values

and from differential heats q0 at monolayer coverage allowing for

decrease of specific surface area of sorbent, substantially coincide

with those relative to free, non-immobilized sorbent, as may be seen

by comparison in Figs. 2-5. On the contrary, mean surface areas of

adsorbed molecules (see Fig. 6) onto immobilized u-Fe,O, give the

same values as for free cc-Fe,O,, only if the B.E.T. surface area in

eqn. (5) is corrected, by a factor of 0.8, as stated above in the

166

experimental section. Both these aspects point to the same conclusion

The lowering of specific surface area, which accompanies immobiliza-

tion during the preparation of photosynthetic composite membranes

containing sorbents, is scarce enough to require sophisticated

methods for its detection, such as that employed in the present

paper. This is not in disagreement with the statement that, when in

this kind of membrane surface area of sorbents is evaluated by

permeability measurements of liquids [1], this determination gives

values practically undistinguishable with respect to B.E.T. values.

This latter type of determination, in fact, being indirect in nature,

gives a very high uncertainty when specific surface area is low, as

for the case of a-Fe,O, [I] (about 50%), and which goes down anyhow

(but not less than 6-10%) with increasing values of surface area.

Methods based on adsorption measurements [2] are certainly more

sensitive, as it has been confirmed for the model system examined

in this work.

CONCLUSION

It may be concluded that immobilization of sorbents in a membrane

structure prepared photochemically by the technique employed is able

to leave almost entirely active the sorbent surface, a goal which

is not easy to achieve by other means.

ACKNOWLEDGEMENT

Technical assistance by Miss Cristina Crippa is gratefully

acknowledged.

REFERENCES

1 I.R. Bellobono, E. Selli, L. Righetto and F. Muffato, Mater. Chem.

Phys ., 19 (1988) 131.

2 I.R. Bellobono, M. Zeni and E. Selli, in B. Sedlacek and J.

Kahovec teds.), Synthetic Polymeric Membranes, W. de Gruyter,

Berlin, 1987, p. 113.

3 I.R. Bellobono, C. Di Fede and B. Bonura, Tribol. Lubrif., 17

(1982) 7.

4 T. Allen and R.M. Patel, J. Appl. Chem., 20 (1970) 165.

5 J.J. Kipling, Adsorption from solutions of non-electrolytes,

Academic Press, London and New York, 1965, pp. 248-250.

6 Landolt-BErnstein, Zahlenwerte und Funktionen, II Band, 4 Teil

Springer-Verlag, Berlin, 1961, pp. 306-332.

7 T.C. Askwith, A. Cameron and R.F. Crouch, Proc. Roy. Sot. A, 291

(1966) 500.

8 W.J.S. Grew and A. Cameron, Proc. Roy. Sot. A, 327 (1972) 47.