Thin Solid Films, 138 (1986) 151-156

GENERAL FILM BEHAVIOUR 151

G E N E R A T I O N AND TRANSMISSION OF A SURFACE PRESSURE IMPULSE IN MONOLAYERS*

MASAYOSHI SUZUKIt, DIETMAR MOBIUS AND RAMESH AHUJA Max-Planck-lnstitut fiir biophysikalische Chemie, Abteilung Molekularer Systemaufbau, D-3400 G~ttin- gen (F.R.G.)

(Received August 15, 1984; accepted March 26, 1985)

Surface pressure impulse propagation through a monolayer at the air-water interface has been investigated. The pressure impulse confined to the monolayer plane is produced by the photoisomerization of an amphiphilic spiropyran. Arachidic acid, dimyristoylphosphatidylcholine and monomethyloctadecanedioate have been used as transmitting layers. The pressure impulse is detected with a microphone-type sensor at various distances. The longitudinal pulse velocity is in the 50-260 cm s- 1 range. The results are interpreted in terms of a simple model in which a thin water layer under the monolayer moves along with it. The thickness of this layer is estimated to be about 100 lim, independent of the transmitting layer. It is also concluded that the dynamical compression modulus is up to four times larger than the stationary compression modulus.

1. INTRODUCTION

The organization of amphiphilic molecules in complex monolayers at the air-water interface is of great interest because of its implications in biological phenomena and the possibility of constructing microscopic functional units 1. A surface pressure impulse confined to the monolayer plane is an important and effective tool in the study of the viscoelastic properties and reaction kinetics in complex monolayers at the air-water interface. The surface pressure impulse is produced when a mixed monolayer of amphiphilic spiropyran (SP) and octadecanol (OD) (spread at the air-water interface and kept at constant area) is exposed to a flash illumination 2. Upon illumination with UV radiation, the SP molecules are photoisomerized to merocyanine (MC) leading to a jump in area per molecule and consequently in surface pressure. Using this technique, the kinetics of phase transitions such as J-aggregate formation in monolayers at the air-water interface may be investigated.

* Paper presented at the Sixth International Conference on Thin Films, Stockholm, Sweden, August 13-17, 1984. t Present address: Fundamental Research Laboratories, NEC Corporation, 1-1 Miyazaki 4-Chome, Miyamae-ku, Kawasaki, Kanagawa 213, Japan.

0040-6090/86/$3.50 © Elsevier Sequoia/Printed in The Netherlands

152 M. SUZUKI, D. Mt~BIUS, R. AHUJA

In this paper, we report the application of this technique to the invstigation of the surface pressure impulse propagat ion through monolayers at the a i r -water interface.

2. EXPERIMENTAL DETAILS

2.1. Materials The compounds used were as follows; SP, synthesized by J. Sondermann3; OD,

pro analyse Merck; arachidic acid (C20), biochemical grade Merck; dimyristoyl- phosphatidylcholine (DMPC), Sigma; monomethyloctadecanedioate (C18-HE), Larodan. These materials were used without further purification. Chloroform containing 1 vol.~o ethanol was used as a spreading solvent. The concentrations of the spreading solutions were (a) SP 5 x 1 0 4M plus O D 2 . 5 x 1 0 - 3 M , (b) C20 5 x 10 3 M, (c) D M P C 1 x 10 - 3 M and (d) C18-HE 1 × 10 3 M. Doubly distilled water was used as the subphase, the pH was 5.6 and the temperature was 22 °C.

2.2. Measurement technique The trough (50 cm x 10 cm x 0.4 cm) was constructed of glass plates which were

made hydrophobic by treatment with dimethyldichlorosilane in chloroform solution. It was equipped with a movable Teflon barrier and a Wilhelmy balance.

A microphone-type sensor was constructed to detect small amplitude surface pressure impulses. It was made of two parts, the sensing probe and the condenser. The sensing probe was a T-shaped glass fibre (0.5 mm in diameter, 8 cm wide and of mass 0.03 g) which was glued to one electrode of the condenser. This electrode (a glass plate 0.5 cm wide, 3.7 cm long and 0.17 m m thick coated with a layer of silver 100 nm thick and with a mass of 0.07 g) was attached to a rigid support carrying the second electrode by two thin tungsten wires (5 mm long and 0.05 mm in diameter). The distance between the two electrodes was 0.17 ram. The sensing probe was made hydrophobic by a Teflon spray.

The sensor was mounted on a micromanipulator and the sensing probe, which was positioned at 3cm from the trough edge, could be lowered to touch the monolayer on the water surface and was aligned perpendicular to the direction of pulse propagation.

The surface pressure pulse generating layer ( S P - O D mixed layer) and the adjacent transmitting layer (C20, D M P C or C18-HE) were separated by a thin floating Teflon tape 2 m m in width during the spreading and compression to a desired initial surface pressure. This Teflon tape was then removed and the monolayers were compressed again to the initial surface pressure. The pressure pulse was generated by illuminating the S P - O D layer with a photographic flash of duration 4 ms (Fig. l(a)). The magnitude of the pressure impulse depends on the initial pressure of the S P - O D monolayer which was varied in the 5 - 2 0 d y n c m - 1 range. At an initial pressure greater than 25dyncm -1, the photoisomerization reaction leads to no pressure impulse. The surface pressure impulse, on reaching the sensing probe, causes a change in the capacitance of the condenser. The resulting electric current was amplified and fed into a storage oscilloscope. The flash and the oscilloscope were triggered simultaneously by a function generator. The response of

SURFACE PRESSURE IMPULSE IN MONOLAYERS 153

the sensor was checked in the fol lowing way. The sensing p robe con tac ted the S P - O D mixed layer direct ly and SP molecules in the vicinity of the p robe were pho to i somer i zed by flash light f i l tered th rough a 366 nm filter. As shown in Fig. l(b) the signal rises within 3 ms even when the surface pressure change is 0.7 dyn c m - 1. F o r the veloci ty measurement s the t ime range was 3 0 - 2 0 0 m s and the surface pressure changes were in the 0 . 7 - 3 d y n c m -1 range except for C18-HE (at 5 d y n c m -1 init ial surface pressure) where the pressure change is l imited to 0.5 dyn c m - ~ because of the phase t ransi t ion. The de tec t ion system therefore has sufficient t ime reso lu t ion if there is no s t rong d ispers ion in the t ravel l ing wave. The pulse veloci ty was c o m p u t e d f rom d/t, where d is the d is tance between the b o u n d a r y of the two layers and the sensing probe , and t is the de lay t ime between the light flash and the signal onset ( indica ted by the a r row in Fig. l(c)). The value o f d was var ied by changing the a m o u n t of t r ansmi t t ing layer mater ia l .

250

• ~ 20C

E u

15C >,

o

100

g_ 5o

o o o

o

A ,

5 10 Distance (cm)

Fig. 1. Oscilloscope traces of the signal output for (a) light flash (500 Its per division), (b) sensor output without transmitting layer corresponding to a surface pressure change of 0.7 dyn cm-1 at 20 dyn cm- 1 initial pressure (5 ms per division) and (c) sensor output at a distance of 16.5 cm with C20 transmitting layer for a surface pressure change of 2 dyn cm - 1 at 20 dyn cm - 1 initial pressure (20 ms per division). Fig. 2. Pulse velocity for C18-HE at various initial pressures: O, 20dyn cm-~; O, 10dyncm-l; A, 5 dyn cm- 1.

154 M. S U Z U K I , D. MOBIUS, R. A H U J A

3. RESULTS A N D DISCUSSION

Since the photoisomerization is fast compared with the duration of the flash, we assume that the mechanical displacement of the molecule follows the time evolution of the light flash. The results for the longitudinal pressure impulse velocity in the case of the C18-HE monolayer are shown in Fig. 2. It is seen that the velocity increases with increasing stationary compression modulus of the monolayer, the value of which can be calculated from the surface pressure-area isotherm (Fig. 3). Figure 4 shows the results for the C20 monolayer. It is observed that the pulse velocity decreases at distances larger than 10 cm, which indicates dispersion. Similar trends were observed for DMPC.

The essential features of our results can be interpreted in terms of a simple model. As a force is applied to the monolayer, a thin water layer moves along with it.

40

35

30

~ 25

a_ 15

5

I J 1 ~ J

0 50 100 150

Area per Molecule (/&,2)

Fig. 3. Surface pressure-area isotherms for C20, D M P C and C18-HE.

E tJ

~o

300

250

200

150

100

Z . . . .

o

o o

o

o

o

o t o

I , , , , I . . . . I . . . . I

5 10 15 20

Distance (crn)

Fig. 4. Pulse velocity for C20 at various initial pressures: O , 20 dyn cm - 1 ; A , 5 dyn cm - 1.

SURFACE PRESSURE IMPULSE IN MONOLAYERS 155

The thickness a of this layer can be calculated by equalizing the inertial (pad 2 U/dt 2) and viscous (~l/a(dU/dt)) forces, which yields the relation a 2 = rlr/2p where r/is the viscosity, p the density of water, U the displacement of a material point from the equilibrium position and z/2 is the halfwidth of the pressure impulse. For r = 4 ms, we get a = 45 ~tm.

By considering the balance of forces acting on a small element, the wave equat ion can be written in the form

d 2 U _ e d2U ~/ dU (1) dt 2 ap dx 2 a2p dt

where e is the compress ion modulus of the monolayer . This equat ion is different f rom that of Lucassen et al. 4,s who neglect the inertial force term. The solution to this equat ion may be written as

U = U o e x p ( - f l x ) e x p ( i m ( t - x ) } (2)

where co is the angular frequency and the velocity c and damping coefficient fl are given by

- - ( 3 ) c = 21/2 I + l + a , - i ~ - ~ - ) ; L ° a /

~=2_1/2~f1+0 ~2 ~1/2)-x/2// 1 ~1/~ -¢- a,-2-#-~) ; ~o~3a3 ] (4) % %

Since the signal was easily detectable at a distance of 10 cm, the value of the damping coefficient fl should be smaller than 0.4 c m - 1. A Fourier analysis of the light flash showed that it included frequencies higher than 195 Hz. By putt ing the experimental values of c (in the non-dispersion range) into eqn. (3), assuming fl = 0.4 c m - ~ and neglecting r12/a4p20) 2 for ~o ~> 195 Hz we obtain the values ofe and a which are given in Table I. F r o m this table we see that the value of a (about 100 ~tm) is independent of

TABLE I P U L S E VELOCITY AND ESTIMATED VALUES FOR COMPRESSION M O D U L U S AND THICKNESS OF THE W A T E R

LAYER

Substance Initial surface Pulse Compression Water layer Modulus a pressure velocity c modulus e thickness a e' (dyn cm - 1) (cm s- a) (dyn cm - 1) (lam) (dyn cm- 1)

C20 20 260 600 90 130 10 230 500 70 130 5 200 400 100 110

DMPC 20 180 400 120 70 10 90 100 120 60 5 90 100 110 40

C18-HE 20 195 400 110 60 10 150 200 90 70 5 50 50 180 10

a Calculated from surface pressure-area isotherms.

156 M. SUZUKI, D. M()BIUS, R. AHUJA

the m o n o l a y e r mate r ia l and is larger by a factor of 2 than the es t imated value (45 lxm). It should be no ted tha t the es t imated values of the dynamica l e are larger than those of the s t a t iona ry e' (see Table I) by a factor of up to 4 and are in agreement with those ob ta ined from other me thods 6. The observed d ispers ion m a y be expla ined in terms of the f requency dependence of e and a.

ACKNOWLEDGMENTS

The au thors are grateful to Professor H. K u h n for many fruitful discussions and to H. Meyer for his assis tance in cons t ruc t ing the sensor. M. Suzuki par t i cu la r ly thanks Professor K u h n for the research o p p o r t u n i t y at the M a x - P l a n c k - I n s t i t u t and acknowledges the f inancial suppor t of the N E C Corpo ra t i on .

REFERENCES

1 H. Kuhn, Thin Solid Films, 99 (1983) 1. 2 D. M6bius and H. Grtiniger, in M. J, Allen and P. N. R. Usherwood (eds.), Charge and Field Effects in

Biosystems, Abacus, Tunbridge Wells, 1984, p. 265. 3 E.E. Polymeropoulos and D. M6bius, Ber. Bunsenges. Phys. Chem., 83 (1979) 1215. 4 J. Lucassen, Trans. Faraday Soc., 64 (1968) 2221. 5 J. Lucassen and M. Van Den Tempel, J. Colloid Interface Sci., 41 (1977) 491. 6 A.F.M. Snik, T. A. M. Beumer and J. A. Poulis, Biochim. Biophys. Acta, 689 (1983) 346.