air interfaces

NOTES Experimental Evidence for the Minimum of Surface Tension with Temperature at Aqueous Alcohol Solution/Air Interfaces The equilibrium surface tension of an aqueous solution of fatty alcohol/air goes through a minimum when the temperature is raised. As a consequence, the surface tension gradient originated by a given temperature difference between two regions of a free surface has to be reversed when the temperature increases. Simple devices are built-up to visualize this effect. A defined local region of the free surface is heated by a metallic plate near the interface. Talc particles on the surface show surface motion using a profile projector. The expected inversion of the direction of the talc particles with increasing temperature is often observed. 1. INTRODUCTION In the systems presenting a liquid-vapor interface, the gradient of surface tension which appears when the temperature is inhomogeneous at the surface can produce macroscopic movements. Such movements have an enhanced importance in reduced gravity (1-4). This may explain the attention paid to the systems in which the surface tension goes through an extremum. In such cases we can reverse the sense of the movement by modifying the temperature. Then we can define a small region where the Marangoni number is null. Vochten and P6tr6 (5) have proved the existence of a minimum of the surface tension at equilibrium (~r) with respect to temperature in aqueous solutions of fatty alcohols. Motomura et al. (6) have shown the presence of an analogous minimum in the case of an ionic surfactant solution (dodecylammonium chloride). Gannon and Farber (7) have indicated a maximum of ¢ in the case of a pure substance: the cyanobiphenil. The purpose of this work is to verify that it is possible to determine the sense of the surface movement for three aqueous fatty alcohol solutions by choosing the right temperature region. 2. MATERIALS AND METHODS The three following solutions prepared were: n-hexanol 40.4 × 10-3 m (minimum in ~r/T is about 50°C). n-heptanol 9 X 10-3 m (minimum in cr/T is about 40°C). n-nonanol 0.73 × 10-~ m (minimum in ¢/T is about 25°C). The order of magnitude of the gradient (&r/OT)N for these solutions ranges from -0.4 dyn/cm K at 30°C below the minimum, to +0.2 dyn/cm K 30°C above the minimum. The n-hexanol and the n-heptanol (quality puriss) were provided by Fluka. The n-nonanol was purified by distillation under vacuum by Professor R. Vochten.

The water used is bidistilled (the first distillation being done in presence of KMnO4). In order to create zones at the surface at different temperatures and to observe movements between them, we used the simple setup described in Fig. 1. A Pyrex container Ct (diameter 1 cm, depth 1 cm) contains the solution to be studied. The latter is maintained at the temperature T~ by circulating water in the container C2. A double-wall Pyrex lid filled with water at T3 (/'3 > Tt and T2) prevents air currents from inducing movements at the surface and also prevents condensation of the vapor emitted by the solution. Through the orifice E in the lid, talc powder can be blown on to the liquid surface with an adapted needle. Another circular orifice (diameter 1 cm) allows us to adjust the heating system (or the cooling). It consists of a copper disk (diameter 3 cm, thickness 1 mm) welded to a vertical inox tube (diameter 1 cm) in which water circulates at the temperature T2. The steady temperature is attained in 2 min, this has been checked by using a Pt resistor. A Teflon piece (D) (0.5 mm thick) insulates the bottom and the lateral parts of the copper disk. Small orifices (F) (diameter 1.5 mm) in the copper disk and the Teflon permit the liquid to flow. The whole setup represented in Fig. 1 is placed on the carriage of a profile projector (Nikon Model 6) under a telecentric objective giving a magnification of 10. The device described above can be lit either by the side or from below.

ii "

~-"- 2

FIG. 1. Schema of the apparatus, m, Stainless steel; [~, Teflon; [], copper; ~, rubber.

261 0021-9797/84 $3.00 Journal of Colloid and Interface Science, Vol. 98, No. 1, March 1984

Copyright © 1984 by Academic Press, Inc. All rights of reproduction in any form reserved.

262

NOTES TABLE I Surface Movement for Three Aqueous Fatty Alcohol Solutions with Respect to Temperature Aqueous alcohol solution Wate~

l

<0a___ a

I 1

Hot ~

-

: i i i )

Expected

20°C

II I I I l 1

60 . . . . . . . . .

20

54 . . . . . . . . .

46

C

I I I It

E_',__- 2

/

S7.S____E____52

H

C

1i I

34.5

3

I

54

40

II I I

43 . . . . . . . . .

40

46 . . . . . . . . .

40

i

58 . . . . . . . . .

H

Observed C

Water 40°C ......

H aq.n-bexanol 20 . . . .

tq.n-heptanol

H

aq.n-nonanol

C

{

~

*

Expected H

Observed C.

u

c

H

c

It ÷

37 C

19 . . . . . . . .

3

58 . . . . . . . .

50

14 . . . . . . . .

4

54 . . . . . . . .

37

27 . . . . . . . .

7

18 C°Id~c

H.

[

HotJ ~>0 Expected

Water

Observed

C

H

C

2 ........

21

2 ........

24

i H

I

I

i ii

Expected c

Observed H

' 43 . . . . . . . .

53.2

I ,,

4____E____33 3 ........

33

13 . . . . . . . .

33

I

C

H

I I

2 ........ E

20

i

c

H

C

H

aq.n-hexanol

23.5 . . . . . . .

40

29 ........

40

35 . . . . . . . . .

40

,)

I I

33

C

H

I

0 .......

37

I I

I I t

aq. n-nonanol

I I I I

The indication "E" means that it was verified that a steady state was reached.

Journal of Colloid and Interface Science,

V o l . 98, N o . 1, M a r c h

1984

_E___54.5

C

H

I

aq.n-heptanol

2

s2 ....

40

53

43 . . . . . . . . .

53.5

C

43 . . . . . . . . 40 . . . .

H

53.5 E--_53.5

NOTES The movements at the surface are made visible by test bodies. We used either talc powder or glass beads (Type Glaverbel Microcel Type M35, density 0.35, diameter 1/10 mm) the density of which is lower than that of the solution. 3. RESULTS Experiments were carried out with the three prepared solutions by imposing a temperature gradient in the region where the temperature is lower than the one for which the surface tension is minimum; a temperature gradient in the region above this minimum. Each gradient has been applied successively in both senses: for the same difference AT, T2 > TI, then TI > T2. The liquid height in the trough (CI, Fig. 1) was nearly 1 cm. The copper plate was placed at nearly 1 m m (or less) under the free liquid surface. The results concerning the alcohol solutions and pure water are gathered in Table I. An example of surface movement is represented on Fig. 2. In Table I, the two cases marked with stars have led in a reproducible way to the following observations. In the first case (*), the solution of n-nonanol is heated by the copper disk, with the whole solution being below the temperature of the minimum of ~r. Talc particles started by moving away from the disk as it was expected. Later the movement of the particles next to the disk reversed whereas the movement of further particles slowed down. Hence a clear zone without talc is created around the heated disk. In the second case (**) the solution of n-heptanol is cooled by the disk, the whole solution being under the minimum of a. The particles of talc started by moving away from the disk (contrarily to what was expected). Then further particles changed direction so that a concentric annulus of talc appeared around the cooling disk. COMMENTS AND DISCUSSION in order to obtain the results described in Table I, it is absolutely necessary that the regions of the surface where we create temperature gradients should be far enough

263

from the meniscus produced by the liquid in contact with the container walls. If that is not the case, the less dense liquid (at higher temperature) rises in the meniscus and this ascension becomes the phenomenon determining the observed hydrodynamic movements. In the region where the temperature at which crbecomes minimum, the density of the solution decreases continuously when T increases. This fact was observed in the n-pentanol and n-heptanol (8). Therefore, the inversions observed in the surface movements are not due to an inversion of a gradient of density, In Table I the column "expected" is based on the values of the surface tension at the equilibrium. It is worthwhile to notice that alcohol evaporates above the region with the highest temperature and condenses over the coldest region. This tends to increase the surface tension in the former region and to lower it in the latter one. This might explain the divergence between the expected sense and the observed one for some alcohol solutions and in some temperature regions as far as a sufficient shift from the equilibrium values is concerned. ACKNOWLEDGMENTS We are indebted to Professor R. Vochten for purification of n-nonanol. We thank Professor R. Vochten and Dr. J. C1. Legros for fruitful discussion and Professor J. Bernard and G. Thomaes for their aid. REFERENCES 1. Block, M. J., Nature (London) 120, 650 (1956). 2. Schwabe, D., and Scharmann, A., J. Cryst. Growth 46, 125 (1979). 3. Chun, C. H., J. Cryst. Growth 48, 600 (1980). 4. Guyon, E., and Pantaloni, J., C. R. Acad. Sci. Paris Ser. B T290, 301 (1980). 5. Vochten, R., and P~trr, G., J. Colloid Interface Sci. 42, 320 (1973). 6. Motomura, K., lwanaga, Sh-I, Hayami, Y., Uryu, S., and Matuura, R., J. Colloid Interface Sci. 80, 32 (198l). 7. Gannon, M. G. J., and Farber, T. E., Philos. Mag. 37, 117 (1978). 8. Vochten, R., Aggregaats thesis, p. 206. Ghent, Belgium, 1976. GEORGES PETRE

Chimie Physique Ecole Polytechnique Universit~ Libre de Bruxelles 1050 Bruxelles, Belgium MAHERZIA AZA AZOUNI FIG. 2. Schema of the movement in the surface of a 0.0484 m aqueous solution of n-hexanol, as observed on the screen of the profile projector. [], Shadow of the copper disk; ,-, visible movement of the talc particles.

Laboratoire d'A~rothermique 4 ter Route des Gardes 92190 Meudon, France Received March 16, 1983; accepted July 31, 1983 Journalof Colloidand InterfaceScience,Vol.98, No. l, March 1984