JOURNAT, OF POLYMJ<R SCIENCE: PAHT A-I VOT,. 9, 2503-2509 (1971)

Studies on Adsorption of Natural and Synthetic Hymatomelanic Acids at Air-Water and at

Oil-W a ter Interfaces

KUNAL GHOSH. B. C. CHATTERJEE, and S. I<. MUKHERJEE, Physical Chemistry Laboratories, University

of Science and Technology, Calcutta 9, India

synopsis By utilizing a static method, based on slight modification of the Wilhelmy plate

method, the interfacial properties of one natural and two synthetic hymatomelanic acids were studied at both the air-water and oil-water interfaces. On this basis their number- average molecular weights, flexibility and shape of the macromolecular chains at the inter- face, and the related thermodynamic parameters were evaluated.

INTRODUCTION

A good deal of literature on the study of surface films of proteins, poly- peptide polymers, and polyelectrolytes at air-water (A/W) and at oil-water (O/W) interfaces has been published.’-*

Following the work of Adam, Rideal and co-workers on the analogy be- tween surface film and three-dimensional states of matter, Guastalla5 sug- gested the use of unimolecular films for molecular weight determination. The technique involved the study of fairly expanded “gaseous” films exert- ing a small surface pressure, T. According to the kinetic theory, Adam6 showed that an ideal gaseous film of area A will have a surface pressure given by

s A = nRT (1) where n is the number of moles.

(W/M,J, where W is the weight of film spread of area A , so that For a substance of unknown number-average molecular weight an, n =

TA/W = RT/Mn (2)

In a real case, ideality is not attained6 and corresponding to the deviations from ideal behavior of gases, equations are employed analogous to that of van der Waals, i.e.,

(T - ?r0)(A - Ao) = nRT (3)

where A” is the limiting film area and AO is (s),,”. 2503

@ 1971 by John Wiley & Sons, Inc.

2504, GHOSH, CHATTEKJEE, AND MUKHERIEE

Bull' found that for large molecules the plots of TA against ?r at a fairly low pressure did rmt> show any intermolecular hydrocarbon attraction and, therefore, it had been assumed that, T O = 0, giving

lrA - 7rAu + nRl' (4)

ATn was calculated from the intercept on the ?rA axis of the plot of T A against T, whereas its slope determined the value of Ao. On the other hand, Adam6 emphasized the importance of ro, the cohesive or association force.

On the basis of the Flory-Huggins theory of polymer solutions, Singers proposed an equation for the evaluation of z, the coordination number of residues in the two-dimensional quasi-lattice at the surface, and also the de- gree of polymerization t. Later on Frisch and Simhae modified Singer's equation as follows

kT t (5)

where k is the Boltzmann constant and T is the absolute temperature. The intercept and slope on the TA axis of the plot of TA against A-' determined the values of t and z, respectively. The percentage of flexibility of the mac- romolecular chains at the interfaces 20 is defined as (w/2) X 1 0 0 ~ o , where w = z - 2. The interfacial molecular shapes can also be predicted on the ba- sis of the flexibility of the macromolecular chain.1°

The entropy change of the macromolecule at the surface AS is givenlo at large areas by

A S = [l - (2/t)]R In (z - 1) (6)

and hence for a rigid molecule (n = 2) the value of AS is zero. For mobile adsorption at the O/W interface, the total free energy AG

of adsorption or the adsorption energy per mole, referred t,o mole fraction unit, has the following form:"

On rewriting, eq. (7) becomes

where a is the bulk phase act,ivity of the molecules in solution, and c is their molar concentration. The slope of the linear plot of the left-hand side of eq. (8) against c passing through the origin determines the value of AG.

The value of the enthalpy change AH at the O/W interface can then easily be obtained from the well known thermodynamic relationship :

A G = A H - T A S (9)

ADSORPTION OF HYMATOMELANlC ACIDS 2505

EXPERIMENTAL

Preparation of the Samples

Natural hymatomelanic acid, the alkali- and alcohol-soluble but acid- insoluble fraction of humic acids, was obtained from a typical Indian soil. It was converted to its sodium salt and was designated DRDN correspond- ing to the place name of the soil.

Since the origin of humic substances, that is, whether they derive from aromatic or from carbohydrate materials, is still controversial; humus syn- theses were carried out with the persulfate oxidation and polycondensation of catechol as well as hydrochloric acid polycondensation of glucose in the presence of glycine as the source for nitrogen in both the cases. The cor- responding sodium hymatomelanates were designated as AN and CN, re- spectively.

Methods

As the samples of sodium hymatomelanate investigated were polyelectro- lyte in nature,I2 all the measurements were carried out at a sodium chloride concentration of 0.0254M, where they behaved as completely rigid mole- cules.

The n-heptane used was of E. Merck (Darmstadt) G. R. quality. With the aid of a static method based on slight modification of the Wil-

helmy plate method, the interfacial tension y was measured for samples ad- sorbed at the air-water (A/W) and at n-heptane-water (O/W) interfaces at a constant substrate concentration of 0.0254111 sodium chloride at 30 f 0.1"C.

The surface pressure x at each solute concentration has been calculated from the relationship, x = yo - y, where yo and y are the interfacial ten- sions of the solvent and of the solution, respectively.

The area available per sodium hymatomelanate residue at the interface A , was evaluated from the well known Gibbs equation by plotting y against

TABLE I

DRDN AN

A/W O/W A/W O/W

BTl 9745 9745 7721 7721 Ao, m Z h g 0.030 0.046 0.024 0.030 z 2 2 2 2 w, % 0 0 0 0 Interfacial shape of the

molecule in relation t,o tbe flexibili(.y of the macromolecnlzir chain Rigid Rigid Rigid Rigid

- AG, kcal/mole 12.12 11.98 AS, eu 0 0 0 0

- A H , kcal/mole 12.12 11.98

CN

A/W O/W

9042 9042 0.028 0.042 2 2 0 0

Rigid Rigid 0 0

12.06 12.06

2506 GHOSH, CHATTERJEE, AND MUKHERJEE

logarithm of concentration (not shown). pressure, T, was independent of time.

This area A for any given surface

The time allowed for the attainment of equilibrium was 15 min.

RESULTS Results are shown in Table I.

DISCUSSION It was noted in the y-log c plot (not shown) that for a given value of log c,

y is greater a t the A/W interface than at O/W interface; and hence in the T-A plot (Fig. l), for the same value of A , ?r is greater a t the O/W interface than a t the A/W interface. This is due to the presence of van der Waals attraction and cohesive forces a t the A/W interface. lo

The samples, both at A/W and O/W interfaces, afford linear plots of T A against ?r (Fig. 2) from a pressure ?r of 3 dyne/cm down to 0.5 dyne/cm. These observations differ from those of the previous workers,' where such linearity was observed only a t pressures 61 dyne/cm. The TA-T plot of any sample studied gives the same value of intercept on the ?rA axis at the A/W and O/W interfaces, whereas their respective slopes differ in magnitude. Thus, according to eq. (4), the molecular weight AT, remains unchanged a t the A/W and a t O/W interfaces, the values are in good agree- ment with those from osmotic pressure measurements,12 but the limiting

30t

6 A in m2x my-' -

Fig. 1. 7-A curves of hymatomelanic acid at air-water (A/w) and at n-heptane-water (O/w) interfaces at 30°C.

ADSOKPTION OF HYMATOMELANIC ACIDS -2507

0200- 1 2 3

R in dynes x cm-'

Fig. 2. TA-T curves of hymatomelanic acids at air-water (A/W) and at n-heptane- water (OjW) interfaces at 30°C.

areas available per sample molecule residue Ao at the two interfaces differ. The higher value of A0 at O/W interface can be attributed to the presence of electrostatic repulslon and the initiation of the dissolution of the polymer

The bulky configuration of the polymer molecules at the interface can be attributed to intramolecular interactions through van der Waals as well as electrostatic forces and the deviations from ideal state depends mainly on the intramolecular forces. At high-pressure regions, obviously, the differ- ent segments of the molecules are forced to close together, the interactions are increased, and the deviation from the ideal monomolecular layer is also increased. l3

From the relations between the surface potential of the polymer molecules and orientations of their functional groups a t the surface as discussed by Davies,14 we may conclude that the polar or ionized groups of the polymer molecules are immersed in the aqueous phase, whereas the nonpolar side chains have a tendency to enter into the oil phase and thus impose a strain on the molecule.

Both at A/W and at O/W interfaces, the value of (z = 2 ) , as calculated from eq. (5), indicates that the polymer phase is perfectly rigid. This conclusion is in agreement with the viscometric studiesI2 of the samples.

As, from statistical considerations, there is no change due to flexibility (z = 2) of the macromolecular chain both at A/W and at O/W interfaces, the values of AS, calculated from eq. (6), are zero.

fiims.*3

2508 GHOSH, CI-IATTERJEE, AND MUKHEHJEE

c x d i n moleej litre - Fig. 3. Adsorption isotherm for hymatomelanic acid at the n-heptane-water interface

at 30°C.

At O/W interface, the total free energy change of adsorption or adsorp tion energy per mole (AG) is calculated from the slope of the linear plot of the left-hand side of eq. (8) against the molar concentration (c ) . Such plots are satisfactorily linear at low concentrations (Fig. 3), whereas at higher con- centration deviations from linearity are observed (not shown), which proves that the adsorptions are completely mobile."

As the net entropy change Ah' is zero, the enthalpy change AH and the total free energy change of adsorption AG of any sample are equal in magni- tude. The values of total free energy change of adsorption at the O/W interface (- AG E 12 kcal/mole) are within the range of hydrogen-bond energy; thus, on adsorption some sort of bond is formed which is, as a whole, energetically intermediate between weak physical bonds and strong chemi- cal bonds.

Thanks are due to Prof. B. N. Ghosh, D.Sc. (Lond.), F.N.I., Department of Chemistry, Calcutta University, for his kind encouragement and helpful discussions.

References 1. 11. B. Bull, J . Amer. Chehem. Soc., 67,4 (1945). 2. J. T. Davies, Bwcheni. J . , 56,509 (1954). 3. C. W. N. Curnper arid A. E. Alexander, Trans. Furuduy SOL.., 46,235 (1950). 4. T. Isemura, K. Hamagucli, H. Tani, J. Noguchi, and H. Yuki, Nature, 168, 1G.5

5. J. Guastalla, C. R. Acud. Sci. (Paris), 206,993 (1938). (1946).

ADSORPTION OF HYMATOMELANIC ACIDS 2509

6. N. K. Adam, Physics and Chemistry of Surfaces, Oxford Univ. Press, Oxford, 1941. 7. H. B. Bull, Recent Advances in Protein Chemistry, Vol. 3, Academic Press, New

8. S. J. Singer, J . Chem. Phys., 16,872 (1948). 9. H. L. Frisch and R. Simha, J . Chem. Phys., 24,652 (1956).

York, 1947, p. 95.

10. J. T. Davies and E. K. Rideal, Interfacial Phenomena, Academic Press, New York,

11. F. Tokiwa and K. Ohki, J.Co2loid Interface Sci., 26.457 (1968). 12. K. Ghosh, Ph.D. thesis, University of Calcutta, 1970. 13. I. R. Miller, J . CoZloidSci., 9,579 (1954). 14. J. T. Davies, Trans. Faraday Soc., 49,949 (1953).

1963.

Received February 24, 1971