Surface properties of aqueous cetylpyridinium bromide solutions

Surface Properties of-Agueous Cetylpyridinium Bromide Solutions In diluted aqueous solutions simple inorganic cations and anions do not possess surface activity (1, 2). However, when the solutions contain cationic or anionic surfactant, they change the surface activity of those substances (3-5). In the present paper the results of the influence of various amounts of potassium bromide on the surface tension and the electric surface potential of aqueous solution of cetylpyridinium bromide (CPB), as well as the equilibrium thickness of the three thin films obtained from these solutions, are presented.

Surface tension of the solutions studied has been measured by the drop weight method with a 1 min drop life time. The time of formation of the drop was empirically established. The radius of the stalagmometer tip was 0.243 cm. The surface tension measurements were performed at 25°C. The Harkins and Brown's corrections have been used for determination of the surface tension. The accuracy of the measurements was +_0.1 dyn/cm. The thickness of vertical thin free films was measured by a setup similar to the one used by Lyklema et al. (7). The apparatus consisted of the vessel which was used for preparing the film and an electronicoptical part. The film was produced on the rectangular frame (1.5 × 2 cm) by immersing the frame and then by partially emerging it from the investigated solution. The film thickness was determinated by measuring the intensity of light reflected by the film. A detailed description of the setup used can be found elsewhere (8).

EXPERIMENTAL METHODS Cetylpyridinium bromide (Fluka AG, Switzerland) used in the experiments was twice recrystallized from anhydrous acetone. A detailed examination of the surface properties of the CPB solutions used in the present work did not reveal any minimum in the surface tension against log concentration curve above the critical micelle concentration, and this was taken as an indication that the sample was not contaminated by other surface active compounds. Potassium bromide (POCh, Poland) was pro analysi grade. Before preparation of the solutions the salt was heated at a temperature above 350°C for 12 hr to eliminate any organic impurities. The solutions were prepared in water which had been distilled three times. Prior to its use, it was boiled down to two thirds of its volume. The electric surface potentials have been measured by the radioactive method. 6 The air over the investigated solutions was ionized by plutonium 239Pu in the oxide form (activity: 0.02 mCi). The radioactive probe consisted of two parts: a slab with the radioactive material and a gilt brass spiral below. The spiral was insulated from the slab and was placed several millimeters from the radioactive electrode. The investigated solution was connected with a 0.1 N calomel electrode to Lindemann's quadrant electrometer which was used as a measuring apparatus. The spiral was always connected with a needle of the electrometer while the calomel electrode was grounded. The difference between the potential jump on the surface of pure solvent (that is, water with the addition of inorganic electrolyte) and the potential jump measured on the surface of the CPB solution was considered to be the surface potential AV. The accuracy of the measurements was _+5 mV. All measureme~ats were carried out at 20-22°C.

RESULTS AND DISCUSSION The results of measurements of surface tension, electric surface potentials and equilibrium thicknesses of the free thin films of aqueous solutions of cetylpyridinium bromide in the presence of various concentrations of potassium bromide are presented in Table I and Fig. 1. The addition of a small amount of CPB reduces significantly surface tension of water up to critical micelle concentration (0.00066 M). This value of CMC is a little lower than the value 0.0008 M obtained by Hartley et al. (9) from measurements of transport numbers at the temperature 30°C. The differences may be due to the fact that various methods for determination of CMC were used and TABLE I The Influence of Addition of Potassium Bromide on the Surface Properties of Aqueous CPB Solutions Concn of KBr in water solutions of CPB (N)

CMC × 104 (moles/liter)

AV (mV)

he (/~)

0 0.001 0.01 0.1

6.6 4.5 1.05 1.0

565 490 435 380

105 95 90 --

582 0021-9797/78/0663-0582502.00/0 Copyright © •978 by Academic Press, Inc. All rights of reproduction in any form reserved.

Journal of Colloid and Interface Science, Vol. 66, No. 3, October 1, 1978

NOTES I

r

i

i

--

i o

00

583

CPB (5 × 10-4 M) and appropriate amounts of potassium bromide. The hydrostatic pressure was ~'h = 500 dyn/cm~. From the CPB solutions of the above given concentration, very thin and stable black films were formed. Their thickness was the lowest value, while the higher one was the potassium bromide concentration in the solution.

REFERENCES

-7

-6

-5

-Z tg c[M]

-3

FIG. 1. Dependence of electric surface potential on CPB concentration in solutions: pure water (a), 0.001 N KBr (b), 0.01 N KBr (c), 0.1 N KBr (d). the temperatures at which measurements were carried out were different. Such magnitude of deviations of CMC values appears in literature data also for dodecylpyridinium halides (4, 10-14). Above the CMC value the changes in surface tension were small. The constant amount of potassium bromide added to CPB solutions decreased the surface tension and shifted the value of CMC to a lower concentration. The CMC values for various concentrations of potassium bromide are presented in Table I. When the concentration of potassium bromide in the solution was increased, the CMC of CPB was decreased. The addition of potassium bromide considerably influenced the AV value only above CMC. The smaller changes in AV were observed when the coiltent of potassium bromide in the solutions was greater. The values of the greatest changes in AV are listed in Table I. In solutions where the CMC had not been yet reached a sharp increase of AV was observed with increasing concentration of CPB and in these cases an addition of potassium bromide did not considerably influence AV value. The equilibrium thickness of macroscopic vertical free thin films is given in the last column of Table I. The films were obtained from the solution containing

1. Frumkin, A., Z. Phys. Chem. 109, 34 (1924). 2. Kamiefiski, B., Bull. Int. Acad. Polon. Sci., Cracovie, ClassellI, Ser. A 309 (1935); 255 (1936); 434 (1937). 3. Shinoda, K., J. Phys. Chem. 59, 432 (1953). 4. Shinoda, K., Nakagawa, T., Tamamuski, B., and Isemura, T., "Colloidal Surfactants," p. 58, Academic Press, New York, 1963. 5. Pytasz, G., and Szeglowski, Z., Zesz. Nauk. Univ. Jagiellon. Pr. Chem. 11, 199 (1966). 6. Kamiefiski, B., Kulawik, I., Kulawik, J., Mikulski, J., and Pawetek, L, Bull. Acad. Polon. Sci., Ser. Sci. Chim. 15, 249 (1967); 15, 253 (1967). 7. Lyklema, J., Scholten, P. G., and Mysels, K. J., J. Phys. Chem. 69, 116 (1965). 8. Paluch, M., Zesz. Nauk. Univ. Jagiellon. Pr. Chem. 20, 145 (1975). 9. Hartley, G. S., Callie, B., and Samis, C. S., Trans. Faraday Soc. 32, 795 (1936); see Ref. (4), p. 54. 10. Harkins, W. D., Krizek, N., and Corrin, M. W., J. Colloid Sci. 6, 576 (1951). 11. Anderson, J. E., and Taylor, H., J. Colloid Sci. 19, 495 (1964). 12. Ford, W. P., Ottewill, R. H., and Parreira, H. C., J. Colloid Interface Sci. 21, 522 (1966). 13. Meguro, K., and Kodo, T., Nippon Kagaku Zasshi 80, 818 (1959). 14. Mukerjee, F., and Ray, A.,J. Phys. Chem. 70, 2t50 (1966). MARIA PALUCH

Department o f Physical Chemistry and Elektrochemistry o f lnstitute o f Chemistry Jagiellonian University 4l Krupnicza 30-060 Krak6w, Poland. Received August 25, 1977; accepted March 10, 1978

Journal of Colloidand Interface Science, Vol.66, No. 3, October1, 1978