Thermodynamic molar properties of some aqueous micellar systems

167

Advances in Molecular Relaxation and Intemction Procesees, 19 (1981) 167-177 Elsevier Scientific Publishing Company, Amsterdam -Printed in Belgium

T1~10DYNAW.C

Mostafa

MOLAR PROPERTIES

11. Emara

Depariment Nasr

City

Nazik

A.

of

OF SONE AQUEOUS MICELLAR SYSTEES

*

Chemistry

, Cairo

, Faculty

of

Science

, Al-Azhar

University,

(Egypt)

Farid

Institute

of

( Received

Petroleum

Research

, Nasr

City

,

Cairo

( Egypt)

9 May 1980)

ABSTRACT

Densities partial at

and ultrasonic

molar

various

sulphate

,

detergents

volumes

the

sodium

and

are

cholatc

known

to

These

* Present

Abdulaziz

aggregates

model

03784487/81/0000-0000/$02.50

to

the

parameters

sodium

,Siven

These

put

of

Chemistry , Saudi

calculated dodecyl

three at

the

,

name

nonlinear with

forward

the

micelles

specific critical

behaviour

of

detergent’s

could

is

, Jeddah

of

called are

Due

then are

taurcholate.

conclusions

: Department

University

solutions

sodium

molar

quantitative

qualitaltive

address

aqueous

(cmc). partial

no

a reasonable

form

measured

compresoihilities

concentrations

concentration

thermodynamic

are

adiabatic of

concentration

King

and

concentrations

concentrations. micelle

velocities

be to

drawn

explain

, Faculty

. Bowever, the

of

results.

Science,

Arabia.

0 1981 Elsevier Scientific Publishing Company

168 INl!RODUCTION This study deals with an investigation of micellization of a very common detergent , sodium dodecyl sulphate , and two biological detergents , sodium cholate and sodium taurcholate , in pure aqueous solutions at 25'C. Simple densi*

measurements combined with sophisticated and

dalieafe ultrasonic velociiy measurements of these same systems have been carried out and should throw light on the micellization process itself. Also the partial molar volumes and adiabatic compressibilities are caloulated at various detergent concentrations at 25'C. As will be seen in the results and discussion sections several techniques have been applied to these systems to study the critical micelle concentration (cmo) and the various parameters affecting it, such as detergent chain length , ionic strength by addition of foreign electrolytes. However , the purpose of this study is to obtain some information on the thermodynamic apparent molar properties to enable some predictions to be made on the detergentand on the behaviour of water

detergent interaction

stabilization in the neighborhood of

the cmc range . In this connection , we consider this is somewhat different approach to understanding the micellization process in these important systems . This investigation

is , therefore valuable.

EXPERIMENTAL Chemicals Sodium dodecyl sulphate ; was purchased from Fisher Products and was used as received . Sodium cholate and sodium taurcholate rIere purchased from Sigma Company . Ilater was llcionized double distilled. Apparatus Density measurements were made with a 5 ml pycltnoneter which

169 was calibrated using deionized distilled water ( do of water = 0.99707 m-cm

-3

at 25OC) .Sound velocity measurements were performed using

the sound velocimeter ~~~-6100 in conjunction with a Beckman counter. 100 ml of sample was introduced into a thermostated water bath for 30 minutes before reading .This time was necessary for temperature equilibrium . This type of velocimeter has two transducers positioned in a probe which is immersed in the solution to be measured . A pulse is transmitted through the solution

by one transducer which

trigger the second one to send another pulse and so on . The measured quantie

is the pulse repetition frequency f that is

determined by (L/v) where L is the effective length of the bath between transducers . It is

determined also by Zthe

total electronic

delay time . Knowing both values v , sound velocity , can be obtained according to es.(l).

1 f

=

L v

+

2

The frequency readings were detected by the counter and automatically recorded on a printer assembled with the velocimeter . The temperature of the water bath was maintained constant at 25O

+, O.Oi°C during

the density measurements . Horrever , a Leeds Northrop oil bath maintained the temperature at 25'

_ + 0.002°C for measuring the

sound velocity .

RESUI,TS Various physico-chemical measurements of aqueous surfactant solutions , such as conductance freezing points

Cl-47,

1iC;ht scattering 653 ,

[F!] , viscosities [7,8] and surface tension [93

have been carried out since early 1940 . Ilo\.cver, during the last decade the study of micelle formation in these detergent solutions

170 has undergone almost explosive growth promoted by technological advances . An additional stimulus has been an upsurge of interest in the micellization phenomenon among chemists not primarily concerned with surfactant solutions for their own sake .This group consists principally of applied chemists and biochemists .Fortunatel.ymany of the newer contributions in micelle chemistry are cited in a recent review

CIO~ .Very recently new electrodes [11,12] are developed to

study the activity of surfactants in their aqueous solutions . It is the purpose of this communication to report densities and sound velocities for sodium dodecyl sulphate (SDS) at 25'C to enable the evaluation of thermodynamic apparent molar properties . The same kind of measurements along with viscosities are carried out for two biological detergents namely sodium cholate (SC) and sodium taurcholate (STC) . The latter are known to form small micelles while the SE% is known to form larger ones . The comparison , therefore , should be of great interest . Adiabatic compressibilities (p) volumes

(&)

, apparent molar

, and apparent molar compressibilities

( i,)

at

various concentrations of the above detergents are calculated as explained later. These thermodynamic parameters are considered as sensors for the water structure near the detercent molecules in general and at the critical micelle concentration in particular .

Calcilation of adiabatic compressibilities (p)

The adiabatic compressibilit;F

are obtained from comhini.nC

both density and ultrasonic velocities as shown from eq. (2)

B=-p- d

(2)

where v is the sound velocity and d is the density . These values are shown in Fig.

1 .

171

448

440 C (md

Adiabatic compressibility (p)

Fig. I.

concentration

I-‘)

against detergent

(e).

Calculation of apparent molar volumes (#r,,)

The apparent molar VOlUDIeS Of these 8qIIeOUS detergents 8t different concentrations are calculated according to eq. (3)

f,

=

+

-

1ooo(a - do) ---_-d do C

where M , d , do and C

are the molecular weight of detergent , densi*

of solution , density of water and concentration of detergent respectively Using the relation

The apparent molar volume at infinite dilution by plotting

tiV against

JC

, #t

, is determined

and extrapolating to zerc ccncentratdon.

Here S is constant for each detergent if linear relation ie olbtained. Th5s is shcm

in Fig.

2.

172

(4 I

I

I

0.04

I

I

I

I

0.24

0.44 6

Fig. 2.

Apparent molar volume ,flV , against

fi.

Calculation of apparent molar CompressSbilities (&

)

The precise measurements of the sound velocities of the above detergents at various concentrations allow calculation of apparent molar compressibklities at each concentration using eq. (5)

wkere

p

and PO

are the adiabatic compressibilities of solution at

concentration C and pure water respectively . Pig.

2

and Fig.

3

shav the variation of 8,

and

$h

with

concentration,respectively,for all detergents . No smooth. extrapolation of curve8 in both figures is possible to enable fl; and

Y;

the corresponding property at infinjte dilution to be

obtained .

173

A I

I

I

-0.000

I

0.02

I

I

I

I

0.10

C !O C(mol

Fig.

Apparent

3.

molar

compressi.bilities

I-‘)

,#g

,

against

(C).

DISCUSSION It

is

calculated simple the at

important

parameters

of

according

nv

=

to

plot

eq.

(6)

is

Similarly

the

behatiour. is

interesting

of

and

then

pure

water

the

concentration

with

of

of

each

sound

( v -

to

other

is

those

and

of

. For example

velocity

Vo)

measured

of

an electrolyte

generally

linear

=FC

the

behaviour

a function

behaviour

.

constant for

the

difference

C from

0

this

the

,

v-v

F is

compare

Y_13-151

concentration

where

as

electrolytes

plot

nv

to

not

for

each

detergents

electrolyte

or

investigated

solute

and

it

.

Figure

shows

that

k

shows

their

linear.

plot

The

(6)

of

v

slope to

notice

versus

for

SC is that

C

, Fig. larger

there

5

than is

, that

a change

shows of in

non

linear

STC and SDS . Tt the

slope

of

the

C(mol

Fig-

Av

k.

=

v-v

against

0

I-‘)

concentration(C).

STC

1494

I

0

10

I 20

I 30

I 40

I

I

I

50

60

70

I

I

I

80

90

100

Cm mol

Fig.

5.

I-‘)

Sound velocity (v) against Concentration (C).

curves in the three detergents at the corresponding cmc as reported by earlier investigatqax from other measurements

\2,113 . The cmc

increases in the order SC , STC and SDS .The adiabatic compressibility behav-iour with concentration is shown in fig.(T) that non linear behaviour

and it can be seen

is observed . Change in the slope is observed

175 at the corresponding cmc values . Bowever , a very important observation is worthnoting , mamely,the initial increase of /3 concentration (cmc ) , then a gradual other two cases , on the other hand, decrease in p

with

Vitlaes

up

to

0.01 m

decrease is obtained . The SC and STC there is a continuous

C in the whole range studied except at the cmc

(small increase ) . This behaviour of the two bile salts are somewhat similar to simple electrolytes while the SDS behaves like real micellar system . Figures %

2

and

3

show the behaviour of #V

and

with C,respectively . In both cases all the detergents go

through maxima at the cmc then through a minimum right after the cmc . This abnormal behaviour of v PBS

6

and $g with concentration

indicates a special interaction between solute - solute and solutesolvent for these classes of solutes in the concentration range investigated . The intermolecular forces and water structure effects responsible for these abnormalities have been the focus of much attention but are still not understood . One explanation is the tendancy of these detergents to exhibit aggregation in solutionho) i.e self-association and mutual association . llolecules containing large hydrophobic moities can show self- association at low concentrations. The slopes a&

/%I

and %?h/

aC

throw interisting light on

the solute - solute interactions . Before the cmc it is observed that a/d,/ 2C

for SDS and STC are almost the same while being di*ferent

from SC . Following Franks's

El61 model , that the larger a# / aC V

the larger the solute - solute interactions . "'his interaction reaches its maxima at the cmc . The slopes a#,

/ 8C

are also

revealing , the three detergents show very high values before cmc . If a strong solute - solute interaction for these detergents is accepfed , rapid increase in

fig

to positive values can be

176 understood

as

structural of

)

solute

-

assumed

cmc

can

be

free

energy

this

range

In

.

indicate

law

aqueous

say

a stabilizing

the detergent .. arrangement

are

the

are

of

or

stronger

after

coefficients

less

the

and

range

than

.

unity

apV

with

on water

In

(negative

forcing )

compressibility

surfa&ant structure.

measurements

way

surfactant

The larger

/ 9C

hydration

and

consistent

the .

and hence

(hydrophobic

appears

volumetric

influence

influence

occupies

the

concentration

and volumetric

/>C

much higher

increase

activity

this

that

distinction

2$JI

posses

to

.

detergents

stabilizing

due

a minima

in

)

contribution

that of

negative stabilization

the

basis

the

a structural

steeper

coefficients

compressibility

of

The

the

on

detergents

The fact

, we can

a possible

detergent

).

activity

having

their

positive

detergents

Rault’s

of

apparent

on

these

conclusion

molecules

exert

interaction

from

properties

of

water

explained

from

( pure

than

their

deviation

increasing

interaction

of

a superposition

( resulting

a rapidly

compressibility the

from

contribution

water

solute

resulting

the

more

water

the

further molecules

slopes

cavities into

Pesul

of of

the water

an ordered

.

ACKNOWLEDGEMI!?: T

for

the

The

authors

use

of

the

greatly sound

acknowledge velocimeter

Professor at

his

Gordon

laboratory

Atkinson .

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1

A.

Norman

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P.

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