air interface

Effect

ofpara-Substituted

Phenols on the Surface Potential and on the Surface Tension at the Water/Air Interface

Electric surface potentials (by jet method) and surface tension (by maximum bubble pressure method) of aqueous solutions of phenol, p-methyl, p-methoxy, p-bromo, and p-nitrophenol at temperatures of 20, 30, and 4&C were investigated. The adsorption properties of the above-mentioned compounds (free energy of adsorption AG°, enthalpy AH°, entropy AS°, and surface dipole moment/2) at the free surface of water are discussed. INTRODUCTION The free surfaces of water solutions have been discussed in many papers (1-15). If a dissolved substance is adsorbed at the surface, it alters both surface tension and contact potential at the solution/air interface. A change in the contact potential due to the adsorption of the solute is usually called surface potential /XV of the solute (14-15). This paper presents the experimental results of surface potential and surface tension of aqueous solutions o f p a r a - s u b s t i t u t e d phenols with various substituted groups differing in their polarity and hydrophilicity. EXPERIMENTAL Phenol and p-nitrophenol (Odczynniki Chemiczne, Poland), p-bromopheuol (Reachim, USSR), p-methoxyphenol (BDH, England), and p-methylphenol, obtained from p-toluidine and sodium nitrate in the presence of sulfuric acid, were used in the course of the experiments. These compounds were further purified. The substances having been dissolved in water were heated in the presence of activated charcoal and filtered, and the compounds were extracted with petroleum ether from cold filtrate, and after that recrystallized from the petroleum ether. The surface potential was measured by the flowing jet method. The method has been described by Kenrick (13), Frumkin (1), and Kamiefiski (8), and therefore only a general outline of the method and some details directly concerning the apparatus used are given here. The solvent and solution were made to flow, the latter in a column down the center of a vertical tube, the former along the inside wall of the tube, such that there were constantly renewed surfaces of the solvent and the solution of a large area in fairly close proximity. The vessels with outflowing solutions were connected to Lindemann's quadrant electrometer by means of a 0.1 N calomel electrode in contact with the central jet, connected to the measuring instrument needle. The potential of the following system was measured:

Hg,Hg2CI2(0.1 N KCl)solution/air/solution(0.1 N KC1)Hg2C12,Hg. As long as both solutions were identical the potentials between them canceled each other out, yielding zero. When, however, a surface-active substance was added to one of the solutions, the adsorption of the solute dipoles on the free surface changed the potential drop, increasing or decreasing it to some particular extent. The method gives erroneous results for solutions of substances adsorbed too slowly for the surface to reach equilibrium with the interior very quickly, but it was reliable for solutions of investigated phenols. The above was checked by a radioactive method with a surface of several minutes life time. The entire assembly was placed in a Faraday cage. The sensitivity of the measurements was accurate to ---5 inV. The surface tension of the solutions was measured by the maximum bubble pressure method. Bubbles were blown at the rate of about one per 7-8 sec. The formation time of a single air bubble was empirically established. For a period of longer than 2 sec no changes in surface tension were observed. The method had an accuracy of 0.1 mN/m. The solutions were prepared in aqueous 0.1 M KC1 solution as solvent in order to eliminate the streaming potential which may arise by the flowingjet method. The surface potential and surface tension of p a r a substituted phenol solutions were measured at three different temperatures (i.e., 20, 30, and 40°C). The temperatures were held constant by submerging the solution in a water bath and by circulating water through the measuring apparatus. RESULTS AND DISCUSSION The results of surface tension and electric surface potential measurements of phenol, p-methylphenol, p-methoxyphenol, p-nitrophenol, and p-bromophenol in aqueous solutions are presented in Figs. 1 and 2. When the concentration of any of the investigated compounds in the solution was increased, changes in the electric potential and surface tension at the sohi-

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Journal of Colloid and Interface Science, Vol.73, No. 1, January 1980

NOTES

283

[ 7

0

~

I

3 50

L

0

i

0.02

0.04

I

I

0.06

0.08

2 c 3 I

0:1 c[mole/din3]

FIG. 1. Dependence of surface tension on concentration of aqueous solutions of phenol, curve a; pmethoxyphenol, curves b; p-methylphenol, curves c; p-nitrophenol, curves d; and p-bromophenol, curves e, at (1) 20°C, (2) 30°C, (3) 40°C. tion/air interface were observed. The greatest decrease in the water surface tension was caused by p-methylphenol, then by p-bromophenol, methoxyphenol, and, least of all, p-nitrophenol (Fig. 1). The surface poten-

tial was changed by p-bromophenol and by p-nitrophenol in an opposite way than by phenol, p-methyl, and p-methoxyphenol (Fig. 2). This fact can be easily explained by taking into consideration the assumption

Z200 -400

3° ~

~

e

""Ol

4~o~/~ ] FIG. 2. Dependence of changes in surface potential on concentration of aqueous solutions of phenol, curves a;p-methoxyphenol, curves b;p-methylphenol, curves c;p-nitrophenol, curves d; andp-bromophenol, curves e, at (1) 20°C, (2) 30°C, and (3) 40°C. Journal of Colloid and Interface Science,

Vol. 73, No. 1, January 1980

284

NOTES

that the potential drop of the free surface of water is due to the orientation of water molelcules. Molecules of an adsorbed substance change the natural surface potential of water by removing a number of oriented water molecules from the free surface and by charging the interface with their own fields. Thus the surface potential changes are strongly determined by the nature of the p-substituted group of phenol molecule. Figure 2 also shows that temperature increase affects only slightly the surface potential, reducing its values by about 1 mV per degree centigrade. Surface potential measurements are more difficult to interpret quantitatively in terms of molecules, but most authors (2, 4, 14, 15) express these results in terms of effective dipole moments /~, where AV is given by Helmholtz's formula:

parallel plate condenser if the dielectric constant of the adsorbed layer is taken to be unity. Figure 3 shows surface potential vs the number of psubstituted phenol molecules adsorbed on a surface of unit area. The number of molecules per square centimeter, n, was computed from the surface excess F, the latter being calculated from Gibb's adsorption equation. /2 was obtained from the slope of the linear portion of the curve presented in Fig. 3. Table I lists the values of /2 of the investigated phenols. As can be seen they are all of different order indicating that/~ is strongly determined by the nature of the p-substituted group. The linearity over a wide range of surface concentrations (Fig. 3) indicates that /2 changes only slightly as the film becomes more concentrated. The temperature rise causes a decrease in the dipole moment value. The adsorption process may be described quantitatively by one of the adsorption isotherms. For the investigatedp-substituted phenols, Volmer's isotherm is the best one corresponding to the experimental results. An integral Volmer's isotherm is represented by the following equation (16, 17):

AV = 4,rn#, n being the number of molecules per unit area of the film. This formula is based on an analogy between the electrical double layer existing at the surface and a

200'

100

J

-400 3 2 •

+0

i

r

100

200

-

-

i

300

i

_ _

400 ~1012

FIG. 3. Surface potential vs the number of molecules of phenol, curves a; p-methoxyphenol, curves b; p-methylphenol, curves c; p-nitrophenol, curves d; and p-bromophenol, curves e at (1) 20°C, (2) 30°C, and (3) 40°C. ' Journal o f ColloM a n d Interface Science, Vol. 73, No. 1, January 1980

285

NOTES TABLEI Standard Thermodynamic ParametersofAdsorptionofSomepara-SubsfitutedPhenols Temperature (°C)

Compound

~ (D)

AG° (kcallmole)

AH~ (kcaYmole)

AS° (caY(mote.K))

Phenol

20 30 40

0.034 0.031 0.029

--2.84 -2.85 -2.87

-2.45

1.33

p-Methylphenol

20 30 40

0.265 0.245 0.236

- 3.46 - 3.51 -3.55

- 2.14

4.51

p-Methoxyphenol

20 30 40

0.218 0.204 0.196

-2.88 -2.95 3.02

-0.88

6.83

p -Bromophenol

20 30 40

-0.603 - 0.590 -0.553

-3.36 - 3.44 -3.48

- 1.93

5.76

p- Nitrophenol

20 30 40

-0.408 - 0.379 -0.354

-2.51 - 2.52 -2.55

- 2.00

1.74

lna +K

=lnTr+b~r/kT

where a is the activity of the solute, K = --AG°/RT, ~r is the surface p r e s s u r e (Tr = tr - o-0, ~r and O-o are the surface tension of solution and water, respectively), and b is the limiting area of molecule in film. In Volm e r ' s isotherm equation we replaced the activity by concentration. This equaltion fitted the data to 1 - 2 % . T h e standard free energy change denoted by AG ° is the energy of transfer of 1 mole of solute f r o m the bulk solution in the standard state (c = 1) to the interracial film in the standard state (ideal two-dimensional gas, 7r° = 1 mN/m). T h e standard free energy of adsorption was obtained as follows: In C/Tr was plotted against ~- for various t e m p e r a t u r e s and the intersections of the lines obtained on the ordinate axis enabled us to estimate K values. The standard free energy w a s t h e n c o m p u t e d from the equation: - A G ° = RTK. T h e results are presented in c o l u m n IV o f Table I. In c o l u m n s V a n d VI of Table I the enthalpy and entropy o f adsorption are s h o w n . The enthalpy values were calculated from the formula:

A H o _ d(AG°/T) d(1/T) For this p u r p o s e AG°/T w a s plotted against 1/T and from the slope M-/° was found. K n o w i n g AG ° and M / ° , K~° was calculated. T h e enthalpy values f o r p - b r o m o , p-nitro, and p - m e t h y l p h e n o l are very similar to each other and a little lower t h a n t h o s e for phenol. A very small heat o f adsorption for p - m e t h o x y p h e n o l mole-

-

cules was obtained. T h e entropy c h a n g e s corresponding to the transfer of the molecule from the bulk to the surface of the solution were small for all o f the investigated c o m p o u n d s . This s h o w s that the molecules have about the s a m e mobility as s h o w n in the bulk solution. REFERENCES 1. F r u m k i n , A., Z. Phys. Chem. 109, 34 (1924); Frumkin, A., Donde, A., and K u l v a r s k a y a , R., Z. Phys. Chem. 123, 321 (1926). 2. Schulman, J. H . , and Riedel, E, K . , Proc. Roy. Soc. 130A, 229 (1931). 3. Sawai, I., Trans. Faraday Soc. 31, 765 (1935). 4. Powell, B. D., and Alexander, A. E., J. Colloid Sci. 7, 482,493 (1952). 5. Posner, A. M., A n d e r s o n , J. R., and Alexander, A. E., J. Colloid Sci. 7, 623 (1952), 6. Klevens, M. B., and Davies, J. T., " P r o c e e d . IInd Intern. C o n g r e s s Surf. A c t , " Vol. I, p. 31. Butterworths, L o n d o n , 1957. 7. Frumkin, A., Jofa, Z. A., and Gerowich, M. A., Zhur. Fiz. Khim. 30, 1455 (1956). 8. Kamiefiski, B., Bull. Intern. Acad. Polon. Sci., Ser. Sci. Math. Nat. 1935, 129. 9. Kamiefiski, B., "IIIrd Intern. C o n g r e s s on Surface Active S u b s t a n c e s , " Vol. II, p. 296. Cologne, 1960. 10. Kamiefiski, B.,Electrochim. Acta 1,272(1959); 10, 875 (1965); 12, 219 (1967).

Journal of Colloid and Interface Science, Vol. 73, No. 1, January 1980

286

NOTES

I1. Paluch, M.,Rocz. Chem. 45, 55 (1971). 12. Tokiwa, F., and Okki, K., Kolloid-Z. Z. Polym. 223, 135 (1%8). 13. Kenrick, B., Z. Phys. Chem. 19, 625 (1896). 14. Adam, N. K., "The Physics and Chemistry of Surfaces," p. 133. Oxford Univ. Press, London 1941. 15. Davies, J. T., and Rideal, E. K., "Interfacial Phenomena," p. 58. Academic Press, New York and London, 1963. 16. Parsons, K., "Trudy IV sovieshcheniya po elecktrokhimi," Moscow, 1959, p. 42.

Journal of Colloidand Interface Science, Vol. 73, No. 1. January 1980

17. Chen, E. S., Ind. Eng. Chem. 57, 40 (1965). MARIA PALUCH MARIA FILEK

Department of Physical Chemistry and Electrochemistry of Institute of Chemistry Jagiellonian University 3 Karasia 30-060 Krakow, Poland Received July 18, 1979; accepted July 25, 1979