Molecular relaxation behaviour of some simple liquids and their mixtures in dilute solutions

Journal of Molecular Liquids, 33 (1987) 203-211 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

MOLECULAR DILUTE

RELAXATION

BEHAVIOUR

OF SOM

SIMPLE

LIQUIDS

203

AND THEIR MIXTURES

IN

SOLUTIONS

M.P. MADAN Department Prince

of Physics,

Edward

(Received

University

Island, Canada

8 September

of Prince

Edward

Island, Charlottetown,

ClA 4P3

1986)

ABSTRACT

The dielectric mixtures region

absorption

has been examlned

over

are consistent

by us prevlously

other polar mixtures

In

The results

two proposed

acetone,

benzene

solutions

and their

In the microwave

parameters.

The relaxation

wlth those of some other rigid molecules

and also wlth the relaxatlon non-polar

solvents

are also compared

new relatlons

of two Debye-type

benzophenone

The data has been used to determine

times and the thermodynamic

times for mixtures

workers.

In dllute

a range of temperatures.

the relaxation

studled

of thlophene,

the relaxation

In a non-polar

of some

by several other

with the computed

to represent

polar components

reported

behavlour

values employing

process

of a system

solvent.

INTRODUCTION

Recently,

we (l-3) examined

molecules

and thelr mixtures

the relaxation In dllute

and over a range of temperatures. relaxation

of many

solvents,

solutions

the general

ses, In the present thlophene,

factors

was also mentioned

benzophenone

OXI-7322/87/$03.50

bearlng

paper we report

and the mixtures In benzene

at microwave

or thelr mixtures,

In order to gain more fnformatlon

more fully

of some rlgld polar

The lack of adequate

simple llqulds

state and In non-polar papers.

behavlour

at several

both In pure liquid and discussed

in these

In thls area and to appreciate

on molecular

the dlelectrlc

of thlophene

frequencies

data on molecular

reorientation relaxation

+ benzophenone

temperatures.

proces-

times of

and acetone

+

A few data on benzophenone

0 1987 Elsevier Science Publishers B.V.

and acetone previous

have been included

studies

data has been analyzed

to estimate

Flnally,

parameters. the relaxation

for comparison

(3,4) on these molecules

the varlous

two proposed

relations

times of polar mixtures

with the experimentally

and Interpretation

in dilute

determined

benzene

from our

solutions.

molar activation

energy

have been employed

in non-polar

solvents

The

to predict

for comparison

values.

EXPERIMNTAL

The apparatus

and procedures

used to determine

the dielectric

losses in the 3 cm microwave

region

have been described

(4-6).

errors

for the dielectric

The maximum

possible

were estimated

at ?0.5%

was determined

uslng the concentration

described Several

previously

solutions

and +5X, respectively.

The estimated

(5).

which did not, in general, dlpolar

mlxtures,

exceed

the concentration

kept the same, but the degree Relatively

was varled. thiophene

temperature

constancy

were obtained

RESULTS

a range For

estimate

were employed

of absorption

circulating

for

losses.

The chemicals

Scientific

purified

was

benzene

bath with the

of ?O.Ol"C.

and Fisher

They were further

solvent

Company

in

by fractional

The data was processed

with the

AN0 DISCUSSION

investigated

benzophenone included. sparse.

wlthln

were employed.

computer.

The values of the relaxation systems

solutlons

and uniformity Aldrich

and then used immediately.

aid of a VAX-II

in T is +10x.

in the non-polar

using a Neslab

of +O.O05'C

as pure a form as possible. distillation

an accurate

from BDH Chemicals,

time, T,

ratio of the two polar components

high concentration

was controlled

error

fraction

and loss

and has also been

concentrations

0.06 welght

of dilution

in order to obtain

The temperature

method

possible

and

by us previously constant

The relaxation

variation

for each system of varying

constants

and acetone Relaxation

Holland

time, T, in dilute

are presented

in Table 1.

which were measured

data on thiophene,

and Smyth

benzene

solutions

The relaxation

previously

in general,

(7) and Garg and Smyth

for the

times

for

(3,4) are also

seem to be rather

(8) determined

the

205 Table

1.

Relaxation

Times

and their

mixtures

temperatures

of

single in

(in

polar

benzene

components

at

different

psi)

Relaxation

Times

for

T =

20%

3o”c

4o”c

5o”c

60°C

18.6

16.0

14.1

12.1

10.6

2.6

2.4

2.2

2.1

1.9

3.1

2.9

2.7

Thiophene t benzophenone

15.8

13.6

11.9

10.4

9.1

Acetone + benzophenone

7.4

6.9

6.4

8enzophenonea Thiophene

2.8b 3.2b 2.1C Acetonea

a From our b Reference ’

previous 9,

Reference

Both

thiophene

by Crossley,

Hassell

at

solution

thiophene is

the

ring, It

firmed plotted

by Garg In

(TT)

tetrachloride

have

contributes

(9)

of

low

in

25’C

respectively.

for

and Kilp

reported

viscosities ring

(8)

for

l/T for

the

and

in

has

in

been

carbon

and Kilp

Table

and short compound

1 for

(10) dilute

acetone

in

cyclohexane liquid.

containing

times.

a sulfur

atom

polarfzability

to

a much smaller

(u = 2.9D). was found

pure

systems

but

relaxation

having

larger

volume

thlophene (10)

p-xylene

data

temperatures.

molar to

whereas

and by Hufnagel

a significantly

a,

and Smyth

state

The values of

larger

as compared

against

liquid

five-membered

has a slightly

by Hufnagel

pure

and Walker

20°C.

and acetone an aromatic which

in

a number

parameter,

zero

and carbon

temperature at

cover

moment u (u = 0.550) distribution

1 and 4.

cyclohexane.

a single

solutions

system.

from

p-xylene

of

Thiophene in

in

times

cyclohexane

benzene

25°C

in

tetrachloride in

at

References

10,

relaxation reported

study,

to which

the

dipole

The Cole-Cole be fndistinguishable was later

Furthermore, thfophene,

con-

when we a good

linear

206 relationship assumptlon

within

differing

loss peaks.

interpretation

assume

In this situatlon

(11,lZ).

that these molecules

relax following

It is seen from Table 1 that thiophene relaxation compare slightly

molecule,

having

the presence

benzene

solutions

moment

molecules

in dllute

molecules.

time of 3.2 ps in carbon cyclohexane definite

(lo), which

solute-solvent

of cyclohexane. ly, furan,

thiophene

tetrachloride

This behaviour

Is only

The dielectric

negligible

moment

slightly

data on polar mixtures

In benzene

was analyzed

The results

consistent

on some monosubstituted

(13) and some rigid polar molecules

them, were indicative

process

which

value for (7).

in a similar

on these mlxbenzenes

and

(l-3), along with those

(14-17) on some of the mixtures

of a molecular

structure,

polarizability

tures,

other workers

Interesting-

less than the corresponding smaller

of a

other than that

identical

solutions.

of several

T of 2.1 ps in

to that of acetone.

as for the single component with our results

of the sur-

is indicative

manner

their mixtures

dipole-dlpole

longer relaxation

in solutions

and an almost

of the comparatively

the

in T,

rearrangement

Inert solvent,

is similar

by

such that the solute

a significantly

interaction

this

affected

times and the small

(9) and a shorter

is a relatlvely molecular

(9,lO) because

barrier

significant

In contrast,

is hardly

The decrease

due to almcst

without

also with a low dipole

has a T value which

These values

which have approximately

viscosity.

The short relaxation

point to a low potential

are able to rotate solvent

has a

at 25°C (9), but are only

(=0.550),

molecules,

usually

roundlng

solution

in pure llquld of 2.8 and 2.3 ps at 20

and similar

is not notfceable.

to

a Debye behaviour.

a very small dipole moment

observed

wfth our

(8)., This shows that the dipole rotatlon.of

of non-polar

of

a significant

consistent

in dllute benzene

same order of molar volume

interaction,

measurements

of analysis

time, T, of 2.6 ps at 20°C and 2.2 ps at 4O'C.

less than the T value found

to

(l-3), it is reasonable

well with the value of 2.6 ps in p-xylene

and 40°C respectively

dipole

to our previous

Therefore,

processes

them into two

times does not provide

on some rigid molecules

the

it is difficult

or resolve

Is similar

relaxatlon

of the results

measurements

confirming

or relaxation

In such cases the method

(3,4).

the data into two separate

process

distribution

This behaviour

and acetone

was observed,

relaxation

them from a Cole-Cole

on benzophene

previous

errors

little from one another.

distlnguish dlstfnct

experimental

of a single molecular

investigated

gave the relaxation

by of

207 the mixture which

as the resultant

is, of course,

environments

of the relaxations

strongly

encountered

in rotation

the size, shape, viscosity, solvent

general,

polar

solvent,

workers

(20-22) have attempted

to analyze

the data on mixtures

C1 and C2,

experimental

of two partially by adjusting the

measurements.

an analysis

generally,

region

two or

into

many situations, (l-3,

13)

regions fit more, its

In

more

is

spite

the

the

times

ual

dispersion

regions

solutions,

that

in

dilute

a relaxing

unit

The observed time

with

This

is

t

In dilute

benzene

which is

falls

noticed

ation

at

behavfour

of

which

of

in

in

the

at

t

2O”C,

temperatures. is

of

It

true

presented

more

seems that

may it

seems

than

one

leads

to

region. relaxation

Table

of

1 for

the the

mixtures.

has to

dilute

decays

dispersion

of

fndfvid-

peaks

behaviour

in

benzophenone

by the

in

loss

type

terms

the

as an apparent

IS added

masked

if

Therefore,

the

retains

in

that

more.

the

best

relaxation

separate

of

the

component the

distinguish

benzophenone

ps when thiophene

thfophene

shape

< 6 Oebye

gives

likely

two Oebye

be termed

results

and acetone

quite

to of

the

then

out

100 or

difficult

overlap

can

is

amplitude,

as a characteristic

solution

other

is

change time

reflected

17.1

in

increased

benzophenone

to

broad,

is

interpretation

points

In

(11). Further-

polar

between

physical (18)

by a factor

little

Ta and considered

system.

their

when it

the

relaxation

thiophene

quite

differ

solutions

process,

of

TI/T

which

each

such

own merits.

be meaningful

that

of

charactertstic

resolution

differences

considerably

when T’S

relaxation

large

use

a dispersion

ratio

two

of polar

the

of

on its

the

seem to

behaviour

of

are

and differ only

attempted not

against

when the of

non-

some

regions of amplfthe best fit to the

analysis

be assessed

(18,19).Davidson

relaxation

appear

plausfble

sfgnfffcance

two

caution

example,

does

On the other hand,

Oebye obtain

each

overlap

the

absence

reduces

for

data

(23) that

must

significant

solutions,

of

al.

out

processes

experimental

individuality,

times

et

among others,

dilute

the

Hill

superimposed parameters to

and point

and there

in

to

(l-3, 15-171,

times.

terms

in

liquid state and In dilute

relaxation

in

observed

that

solutions,

60s (14) and others

in pure

solute-

assumption

on the dilute

single

compounds tudes

in nature.

bfnary polar mixtures

such as

and/or

with the customary

from measurements

tend to be molecular

investigating

molecules

of molecular

field and solute-solute

is in accord

time obtained

of the indlvidual

by the varlety

as well as by other factors

internal

This

interactions.

the relaxation

influenced

a T value

ft. in

of

A similar this

comparatively

mixture larger

18.6

ps

behavfour the

relax-

dipole

208

moment

and molar volume

molecule

of only slightly

comparable solution, mixture

dipole moment a considerably

the To values generally because

greater

freedom

volume

weights

consistent,

with the results

equation

entropy,

before,

variation

of the transition

(3).

of activation

the relaxing

units

Table 2.

rotational

In both cases, values

average

and are

values

and the respective however,

values

are

(l-3) as well as

(14-17).

of the relaxation

state theory,

*S+ , of activation

the usual manner

greater unit.

values,

with our previous

the free energy,

but of

The T for this

component

environment

The measured

of many other workers

reorientation,

a considerably

of the relaxing

in the molecular

times,

using the rate

the thermodynamic

AGf, the enthalpy,

for the relaxation

process,

parameters

for

AH', and the _

were determined

in

These are given in Table 2. It is seen that the free # differences, AG , between two equilibrium positions of

in dilute

solutions

Thermodynamic

of benzophenone

parameters

components

and mixtures

in benzene

solutions

and its mixtures

of polar components

AG+

AH+

(kcal/mol)a

(kcal/mol)

AS+ (cal mol-l

2.6

2.1

-2.2

AcetoneC

1.7

0.60

-3.6

Thioohene

1.7

0.66

-2.6

Thiophene + benzophenone

2.7

2.0

-2.4

Acetone + benzophenone

2.3

0.72

-5.4

b Reference

1.

are

for single polar

Benzophenoneb

a At 25'C.

a

is added to benzophenone

from the corresponding

of the molecules.

From the temperature

than that of thiophene

those of the individual

as expected,

as mentioned

when acetone,

drop in T is noticed.

of motion

lie between

different,

of the change

relaxing

energy

less molar

In contrast,

to that of benzophenone,

is 7.7 ps at 20°C and reflects

and translational

dipole

of benzophenone.

deg-I)

' Reference

4.

209 of

the

order

slightly

of

2 to

less.

temperature

3 kcal/mol

The values

dependence

indicate

of

the

AS+, are found

to

be negative,

tions

in

the

possible

activated

state

is

whereas

activated

state

that only state

out, of

it

is

desirable

mixtures.

to

behaviour

solvent.

studies

relations

barrier

and only of

are

to

of

Debye-type one has

a slight

activation, configura-

configurations

on mixtures have

predict

two

system,

fewer

are

the

state.

solutions

proposed

a system

a binary

a few

they

The entropies

these

normal

dilute

and acetone

there

the

in

we (2,24) of

For

or

develop

Recently,

relaxation polar

liquid

that and for

than

In view of the situation pure

energy time.

indicating

more ordered

thiophene

a small

relaxation

molecules

in

for

the

polar

dipolar carried

relaxation

relations

the

of been

to

componets

form

times

represent in

the

a non-

(2),

Cl1

where to

C = C:/C’ 2,

dipole

Another

Ci

moments

relation

are

u’i

parameters

and have

has

the

been

general

involving

effects

discussed

previously

form

other

than

those

due

(2).

(24),

121 nhere

Cl

and C2 are

solutions where can be taken to of

the

is

a function

degree

dipole

of

the

respective

the relaxation a first order

moments. of

the

overlap

of

f 12 is

the

statistical the

the

the

third

in

c21 corresponds

individual

f ~2 can,

'cl > T

, f

value

close

very

progressively

T

concentrations, to the square

magnitude

turn,

depends

The negative

> T , as

is

of

which

and the

the

upon

value case

for

of the

iy

j/T 2

shows

in

times. > T

- T 1

the

dilute

environments

which,

be r:pre&ed

= (T

1

departures

from

a zero

value

as the

12

- T ) increases. ti

regions

to

12

For

in

term,

molecular

relaxation

thus,

f

difference’(.r

of

dispersion

in

term

interference set

difference

mixtures.

which,

times are independent of the approximation as proportional

the

present

amplitudes

unity.

Furthermore,

L-&ten TV and ~~ differ

widely,

the

molecule

if

C

1

>> Cp,

f will

has

a

a relax-

atlon time T1 and amplitude

CL imposes

its relaxation

The ratio CI/CZ for thiophene-benzophenone

mixture

and, as such, the weight of the second dispersion yield more than a single relaxation suggestions

of Forest

fering moments benzophenone moments 0.17.

and Smyth

time.

mixture,

even though

the individual

for this mixture

value,

which would

> T

mixture.

is about 30 small

to

This is in accord with the with largely

Is undetectable.

little from each other,

Therefore,

from an average

wfth T

is negligfbli

(20) that in mixtures

a weak second dispersion

differing

on the whole

dlf-

For acetone-

components

have dipole

f,, is close to 0.83, TZ/TI

being

one would expect ~~~ to be different have resulted

if T, and TV were close

together.

Table 3.

Comparison

of observed

with those calculated

relaxation using

time ~~ (in ps),

Cl3 and C23.

Thlophene t benzophenone

Acetone t benzophone

T ("C)

Obs.

Calc.Cll

Calc.C23

Obs.

Calc.Cll

Calc.C21

20

15.8

17.1

17.1

7.4

7.7

8.3

30

13.6

14.8

14.7

6.9

7.1

7.2

40

11.9

13.1

13.0

6.4

6.5

6.4

50

10.4

11.3

11.1

60

9.1

9.9

9.7

The values

of T

dipole moments

have been compared

parameters,

tory for dilute

volumes

(25), are given

with the experimentally

agree well with each other the different

Cl1 and C21 and using the data for

, computed by employing (26) and molar

obtained

in Table 3 where

values.

and, in view of the uncertainties this agreement

can be considered

they

The values in estimating very satisfac-

solutions.

ACKNOW-EDGEMENTS

The author would assistance

like to express

in making

experimental

his sincere measurements

thanks

to Mr. Andrew

and to the Natural

Wtch

for

Sciences

211 and Engfneerfng Research

Research

Commfttee

Councfl

for their

of Canada

and the Unfversfty

Senate

support.

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