summary

J. DISPERSION SCIENCE AND TECHNOLOGY, 16(6),451-468 (t995)

SURFACE ACTIVITY AND ENERGETICS OF AGGREGATE FORMATION IN AQUEOUS SODIUM BENZOATE AND SALICYLATE SOLUTIONS B.A. Ali and M.B .Zughul Department of Chemistry-Faculty of Science, University of Jordan , Amman , Jordan

and

A.A.Badwan The Jordanian Pharmaceutical Manufacturing Co., P.O. Box 94. Naor, Jordan

SUMMARY Aqueous solutions of the two hydrotropic agents sodium benzoate and sodium salicylate exhibit moderate surface activity as surface tension measurements indicated. The surface cross sectional area of the salicylate ion is reasonably higher than that of benzoate. The area occupied per ion increases with the rise in temperature for both hydrotropes. The critical aggregate concentration (CAC) obtained from viscosity and conductivity measurements substantiales those obtained from surface tension measurements. Analysis of the temperature dependence of CAC showed thai aggregation of benzoate and salicylate ions in aqueous solution is driven by enthalpy and entropy faclors. The entropy contribution is larger especially for salicylate ion. This indicates that the entropy gain associated with freeing waler molecules structured around salicylate ions is the primary driving force for aggregation. The enthalpy contribution to aggregate formation of benzoate ion is relatively larger than that of salicylate, and this agrees well with more hydrophobic nature of the former species.

45' Copyr;,"! Cl I!WS by M.rcel Dekker . Inc

All, ZUGHUL, AND BADWAN

SURFACE ACTIVITY OF AGGREGATE FORMATION

452

'"

Any dramatic change in an otherwise smooth variation of the physical

INTRODUCTION The term hydrotropy was originally devised by Neuberg(1 ) to describe the increase in aqueous solubility of water insoluble organic substances, by the addition of a third component at a high concentration; usually an alkali metal salt of some organic acids such as sodium benzoate or sodium

salicylate. Following this definition, which was confined to some chemically related substances, the term was extensively used to describe

property with concentration is normally considered indicative of some kind of aggregalion(7.10.11). As yet there has not been any report in the literalure dealing with the surface activity of hydrotropic solutions. Moreover, the energetics involved in hydrotropic aggregate formation were not explored. This report is an attempt to answer some questions relevant to these areas of investigations.

the solubility increase caused by the presence of any organic substance that is not a surfactant(2).This usage was soj::lroad that a lot of confusion concerning the chemical nature of the solubilizers had appeared. Indeed, in the literature, a reference is found to cite urea, gelatin and caffeine as hydrotropic agents(3), thus adding to the confusion concerning the chemical nature of hydrotropes, and limiting a better understanding of this phenomena. In order to surmount this controversial aspect of recognizing and defining hydrotropy, attempts based on arbitrary parameters were made to classify them. For example, Higuchi and Kristiansen(4) classified some selected hydrotropes into two groups based on their binding tendency. A more general and detailed classification based on the aSSOCiation patterns of different organic molecules, induding Neuberg hydrotropes, in aqueouS .solutions was also provided by Mukerjee(5). A more restricted classification based on the original Neuberg definition of hydrotropy was also proposed by Saleh et al (61, who classified different hydrotropes into anionic, cationic and nonionic. This tatter classification is based on an earlier work where solubilization by these hydrotropes was proved to be a consequence of aggregate formation at reasonably higher concentrations(7). Such definition would cover a wide range of different chemical species and allow their classification according to their mechanism of solubilization. In different systematic investigation, the solubilization of water insoluble and structurally different compounds by aqueous sodium benzoate and salicylate was examined. The mechanism

EXPERIMENTAL MATERIALS Sodium benzoate and sodium salicylate were of 99% purity and supplied by Riedel·Oe Haen, FGR. Water used was deionised and double distilled to have a measured conductance of 2 Microsiemens and a surface tension of 71.9 mN/m.

EQUIPMENTS Surface tension was measured using a Kruss tensiometer equipped with a platinum ring (Kruss Instruments, FGR). Temperature control of solutions was maintained to within ±O.2°C, using a circulating water thermostatic bath which was connected to the surface tension cell compartment. SpeCific conductivity was measured using a Phillips (WP 9509) conductivity meter equipped with

an immersion type conductivity cell

(PW9510/60); its constant is 0.75 cm-1 (Phillips, Holland). The densities of

Solutions were measured using a density bottle (Technico 85733 IP196, Technico, U.SA). The density bolile was calibrated with mercury in the temperature range 20oC·550C. Viscosities were measured with a standard U tube Technico viscometer, size B, with an internal capillary tube of 12.00 em length and 0.015 em inner radius. The glassware was routinely cleaned with chromic acid followed by rinsing several times with double distilled water.

of their solubilization was attributed to factors including electron donor· acceptor complex formation, and salting in effect(2). It was found that at high concentrations of these solutions, the prevailing mechanism of solubilization

was

aggregate formation(81.

Consequently, different

physico..chemical techniques were utilized to examine the variation of n physical properties of such hydrotropic solutions with concentratio (91.

METHODS Preparation Of Sodium Benzoate And Salicylate Solutions: Stock solutions (1.0M) were prepared by dissolving accurately weighed qUantities of sodium benzoate and sodium salicylate in double distilled

ALi, ZUGHUl. AND BADWAN

454

water, and the volumes were made up to 500 mL with double distilled water. Different concentrations of sodium benzoate and salicylate ranging from 0.01 to 1.00 M were prepared by pipelting different volumes of the

freshly prepared stock solutions into 50 ml volumetric flasks and the


4>5

volume. The set of density measurements was repeated for all solutions at four different temperatures (15.00C. 20.0oC. 25.00C and 30. DOC). These values were used to sodium benzoate and salicylate solutions.

compute viscosi ties of

volumes made up to the mark with double distilled water.

Statistica' Analysis of Error

Surface Tension Measurements The surface tension of aqueous sodium benzoate and sodium salicylate solutions were measured over the molar concentration range-of 0.01 to 1.0 M. The measurements were repeated at three different temperatures

Error limits of all parameters obtained in this work were established from standard linear least square regreSSion. All confidence intervals were obtained from curve fitting of experimental data for a 95% confidence level, with errors assumed to follow the student-t probability distribution.

(20.0oC . 3D.OoC and 40.00C). In each case, 20 ml of the hydrotropic

RESULTS AND DISCUSSION .

agent solution was placed in the petri dish, and the work needed to detach the ring from the surface was recorded following thermal

The variation of surface tension of sodium benzoate and sodium salicylate

equilibrium.

solutions with concentraHon are presented in fig . (1) and (2), respectively.

Each measurement was

repealed

4 times using fresh

solutions and the average surface tension was calculated.

Both figures show that sodium benzoate and sodium salicylate solutions exhibit

SpecifiC Conductivity Measurements The specific conductivities of sodium benzoate and salicylate solutions

some moderate surface activity. AI lower concentrations, the

surface tension decreases smoothly with the logarithm of concentration till it reaches a plateau where no further reduction in the surface tension

covering the concentration range 0.01 to 1.0 M were measured at different

takes place. This pallern which occurs at different temperatures is similar

temperatures ranging form 20.00 C·to 50.0 0 C.

to the behavior of surface activity exhibited by surfactants(12). Thus the

Viscosity Measurements The isothermal relative viscosities of aqueous sodium benzoate and

explained in terms

moderate surface activity exhibited by these hydrotropic species may be

salicylate solutions were measured using an Ostwald viscometer, The concentrations covered were within the range 0.01 to 1,0 M for the temperatures : 15.0 o C, 20.00C and 30.00C. The flow time of these solutions through the capillary was measured for each solution and the corresponding viscosity was calculated accordingly. Density MeasurE!ments The density of each solution

was measured by filling the density

bottle with the solution of sodium benzoate or salicylate. The bottle was then placed in a thermostated water thermal and the

equilibrium was

attained,

of the

Hydrophile lipophile Balance(HlB); the

hydrophilic functional group being the carboxylate in the benzoate sail, while it is the carboxylate and the phenolic hydroxyl group in the salicylate saIl. The average cross-sectional area

of benzoate and salicylate ions

at the liquid surface were calculated from the surface excess (f in mole I m2l using the modified Gibbs adsorption equation for concentrated electrolyte solutions: adsorbed

r

= - (112) . (dy l dlnC), .... ..... .

.. .... ..... .... .... ..... (1)

bath for 30 min. After

the density

bottle was weighed

density of the solution was calculated from its weight and

Where y is the surface tension in mN/m, C is the molar concentration, R is the gas constant and T is the absolute temperature in Kelvin degrees.

ALI. ZUGHUL, AND BADWAN

456


457

An estimate of the surface area per molecule in Ao2 was obtained from the relation:

t

70

E , z

S

a

•

• 0.4-2 M

• ••

~

-,

I

much less than what is reported ifl the literature for benzene molecules adsorbed at the water surface 58 A02. It is generally understood that benzene molecules lie flat on the water surface. Yet the hydrophilic group in the hydrotropes used, being coplanar with the benzene ring, may exert a directional force on the benzene ring to stay perpendicular to the liquid surface, thus reducing the effective cross sectional area occupied by each ion. As the temperature increases, thermal agitation disrupts this highly ordered state of benzene rings, so they probably tilt at a given angle with respect to the liquid surface, thus increasing their effective cross sectional area . The tilt in angle is apparently more pronounced in the salicylate

o

-\

-2 In(C)

F'i9.(1). A plot of the surfac e tension 7 of aqueOUS sodium benzoate ogoinst In{C) at 3O"C

70

• ~ z S

6J

•

O.M to!

•

~

~o \

-3

•

•

••

... ... ...... .. ...... ...... .. ......... (2)

At 200C, both benzoate and salicylate ions seem to occupy comparable areas at the liquid surface (23 to 25 Ao2, ion ). However these values are

•••••• •••• 50

1Q20 /Nr .. ...

where N is the Avogadro's number. Table (1) lists the average surface cross sectional areas for sodium benzoate and salicylate at 20oC,30oC and 40oC.

•

60

=

I

......

I

1

1

-2

-1

0

In(C)

ions, where the presence of a hydroxyl group in an ortho position to the carboxylate group creates another six membered ring coplanar with benzene, and hence results in an increase in its effective cross sectional area. This is clear, as the average cross sectional area of salicylate at 40.00C is 75 Ao2 compared witt) 42 Ao2 for benzoate . The plots of specific conductivity against molar concentration fig. (3) and (4) show two distinct straight lines intersecting at a given concentration for each temperature. Attempls to plot the molar conductivity againslthe square rool of molar concentration also yielded curves resembling those of some bile salt solutions ' fig. (5)

, Fig.(2). A plot of the surface tension l' of aqueoUS sodium salicylate against In(C) at 3~

This may suggest a form of

aggregation other than micelle formation such as stack formation, which may be possible for these rigid , planar molecules. As with other organic molecules having similar structures, there are no geometrical restrictions 10 the height of the stack, other than those imposed by enlropic factors(13)

ALI. ZUGHUL. AND BADWAN

4S8


0.04

TABLE l1\ Average Cross Sectional Area per Ion for Sodium Salicylate and Sodium Benzoate Adsorbed at the Water Uquid Surface at Different Temperatures

Cross Sectional Area per Ion Ao

•

42

~

O.!!\6r.4

\.

0.02

0.01 0.00 I -

"" '4

"

30.0 40.0

q

-"-

2J

20.0

T T E

2

Sodium Salicylate

Sodium Benzoate

T{°C)

0.03

'"

,

I

I

0.2

0.6

1.0

Molo r eoncentnltion

Fig.( 4). A plot of the spec ific Conductonc It

II;

of aqueous sodium solicylate agoinst its molor

concentration at 4O"C. 0.04

..

0.451.1 O.OJ

\

T q

T

100

.'

.$

0.02

c;-

E

-"-

•

0.01

80

60

E

-"0.00