Surface Rheology Of Monolayers.pdf

Thin Solid Films 284-285

( 1996) 361-364

Surface rheology of monolayers J.

KrSgel

‘, G. Kretzschmar b, J.B. Li b, G. Loglio ‘, R. Miller b,*, H. Miihwald b

a UniversitiitPotsdam, lnstitutfiir Festbrperphysik, Potsdam, Germany h Max-Planck-Institute fiirKolloid- und Gren.#ichenforschung, Rudower Chaussee 5, D-12489 Berlin, Germany ’ University of Florence, Florence, Italy

Abstract The surface rheological behaviour of spread monolayers of DPPC (L-cY-dipalmitoyl phosphatidyl chinoline) and DMPE (L-a-dimyristoyl phosphatidyl ethanolamine) on water has been investigated under shear and dilation/compression deformation. Harmonic area perturbations are performed using an oscillating barrier method which provides information on the monolayer dilational elasticity and the relaxation processes happening in the monolayers as a function of surface pressure. A surface shear rheometer has been used to measure the surface shear viscosity and elasticity of the same monolayers. The shear and dilational rheological properties can be explained by the morphology of the phospholipid monolayers as a function of surface pressure. Keywords: Phospholipid

monolayers;

Surface shear rheology;

Surface dilational rheology; Monolayer

1. Introduction It is well-known that some materials, for example polymeric materials and proteins, form very stiff or extremely viscous monolayers when they are spread on water [ l-6 I. Their behaviour is obviously characterised by surface rheology as they possess large shear viscosity and elasticity. This information is of great help in understanding the stability of such films during the transfer process to solid supports. Phospholipids are organic amphiphilic molecule with two aliphatic chains connected by a hydrophilic group. They can be anchored at an air-water interface to form a well-defined monolayer. Traditionally they are regarded as models for biological membranes [ 71. Numerous investigations on these monolayers or their mixture have been performed. Many characterisations, such as isotherms at various temperatures [ 81, surface nucleation in compression and expansion [ 93, growth or decay of a phase [lo], morphology of domain structure in the phase coexistence range [ 11,121, etc., have been carried out. Only very little work has been made so far in understanding the mechanical properties of such monolayers. These properties are particularly important from an application point of view. To know the surface viscoelastic behaviour of a film to be transferred to a solid support for the purpose of various optic and electronic devices would be advantageous.

relaxation

In this work we use recently developed techniques in order to characterise the mechanical behaviour of phospholipid monolayers. A torsion pendulum instrument allows one to determine the shear viscosity and elasticity of the monolayers at varying surface pressure. A modification of a conventional Langmuir trough equipped with a second barrier allows one to perform harmonic area oscillations of the monolayers. The response to such harmonic perturbation yields information about the dilational elasticity and viscosity of lipid monolayers [ 13-171.

2. Experimental

details

2.1. Sulfate shear rheometer The measuring principle of the surface shear rheometer ISRl (LAUDA, Germany) has been described elsewhere in detail [ 181 and was used for different applications [ 19-211. Briefly, it performs a simple pendulum experiment with a small deflection of 0.5-3”, resulting in a free oscillation of the ring which touches the interface under study. The curve of the damped oscillation is recorded automatically by the instrument and the important quantities, the damping coefficient and the circular frequency of the torsion oscillation, are calculated from r(r)=y,exp(-i)sin[$(r-b)]+c

* Corresponding

author.

0040~6090/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDIOO40-6090(95)08342-l

(1)

J. Krligel et al. /Thin Solid Films 284-285 (1996) 361-364

362

where a is the decay time, T is the period of oscillation, to is the phase shift and c is the offset. The parameters a and T contain the surface shear viscosity and elasticity [ 18,221. The shear rheology measurements were performed in the following way. After a small Langmuir trough had been filled with Millipore water, the monolayer was formed by spreading the phospholipids from a chloroform solution. After evaporation of the solvent the monolayer was compressed to a definite film pressure 7~by moving the barrier. The film pressure was measured by a Wilhelmy plate (,F 1 from LAUDA, Germany). The surface shear viscosity is measured in dependence on the film pressure. The Langmuir trough consists of two compartments. The film pressure measurements are carried out in the rectangular part while the surface shear rheological measurement are made in a concentric ring system. The shear stress acts in the slit between an outer fixed Teflon ring and the inner ring which is the measuring body of the surface shear rheometer. The fixed Teflon ring has two gaps to allow the compressed monolayer flowing into the concentric part of the trough. In that way the same film pressure is established in the rectangular and concentric part of the apparatus.

with 99% purity was purchased from MERCK. The water used in all experiments was prepared in a Milliporeapparatus.

3. Results The PA isotherms of DPPC and DMPE are shown in Fig. 1. The shape of the two isotherms show differences as discussed by other workers in the literature but both exhibit a.well-established coexistence region. The surface shear viscosity data for DPPC and DMPE are show in Fig. 2 as a function of surface pressure rr. The determined shear elasticities were independent of the surface pressure and close to zero and not shown here. At small surface pressure the shear viscosity is close to zero. With increasing r-values the viscosity increases first almost linearly and then abruptly to reach a plateau value at very high surface pressures.

40-

2.2. Langmuir trough with an oscillating barrier 30-

For measuring the dilational rheological parameters we used the oscillating barrier method. The set-up has been described in detail elsewhere [ 13,14,23,24]. The oscillating barrier method permits a direct measurement of the surface pressure oscillation and the phase angle between generation of the area oscillation and the resulting pressure oscillations. The complex elasticity modulus of the monolayer is given by -drr EC---_= d In A

Ed + iwr],

2. 2

E zoY IO-

o-

(2)

+,.,.,.,.,.,.

The dilational elasticity Ed is obtained from the surface pressure amplitude [ 171 and the surface dilational viscosity vd is given by [ 251

40

50

60

70

Area

Fig. 1. T-A isotherms 21 “C.

80

90

( AZ/Molec.)

of DPPC (solid line) and DMPE (dotted line) at

(3) Here o is the circular frequency of the barrier oscillation, and 0 is the phase shift between an extra peak produced by the excenter moving the oscillating barrier and the maximum in the surface pressure response. The phospholipids were spread OF the modified film balance as usual. 20 min after spreading the film was compressed to a definite film pressure IT. Via an electromagnet a second barrier was placed onto the monolayer at a definite distance to the force balance. The second barrier is moved periodically wilh an amplitude of 15 mm and a frequency of 0.1 Hz. 2.3. Material

g

2000

z t

0

5

10

15

20

25

30

3s

40

45

surface pressure [mN/m]

The phospholipid DPPC (99% + purity) purchased from Sigma was used without further purification. Chloroform

Fig. 2. Surface shear viscosity of DPPC (0) on 77.

and DMPE ( n ) in dependence

J. Kriigel et al. /Thin Solid Films 284-285 (1996) 361-364

140-

.

,

n

(

.

,

.

,

9

,

.

,

.

r700

J I

120-

- 600

-

a so0

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B 5. - 400 :: %. u" - 300 g z f

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- 2olJ -i!

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- 100

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.,.,.,.I.(

0

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.I.

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6 s pressure(mNlm)

10

12

14

Fig. 3. Dilational elasticity E and viscosity ~7~of a DMPE monolayer function of surface pressure, T=21 “C, L=7.5 cm, 0=0.628 S-I.

as a

363

not been reported so far the shear viscosity rises up sharply. Maybe the monolayer undergoes a phase transition which has not yet been observed. X-ray reflection in this range of surface pressure should be best suited to give evidence of such a process if it exists. The dilational elasticity and viscosity of both monolayers show a strong dependence on surface pressure with a minimum in the coexistence region. This behaviour can be explained as follows. As long as two phases coexists a monolayer compression does not result in a significant pressure increase but in a transfer of the liquid expanded (LE) into the LC phase. During expansion of the monolayer the opposite process hinders a surface pressure change. Thus almost no phase shift and no elasticity is observed. Only below and above the coexistence region surface pressure changes can be generated by area changes so that a measurable dilational elasticity and an increasing dilational viscosity are observed. Both parameters increase and level off at a surface pressure above the end of the coexistence region. From the present experiments it is not possible yet to specify the mechanism of the relaxation process in the monolayer which is responsible for the observed dilational viscosity (phase angle). Probably both a lateral relaxation along the monolayer and an orientational relaxation happen simultaneously. Further experiments with a different distance between the oscillating barrier and the pressure balance and variation in the oscillation frequency are necessary to enable us to distinguish between these two and possibly still more relaxation processes [ 261.

Acknowledgements

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The work was financially supported by a project of the European Community (HCM ERBCHRXT930322) and the Fonds der Chemischen Industrie (RM 400429).

IJO

14

pressure (mN/m)

Fig. 4. Dilational elasticity E and viscosity ~7~of a DPPC monolayer function of surface pressure, T= 20.5 “C. L = 7.5 cm, w = 0.628 s- ‘,

as a

References The dilational elasticity and the phase angle, expressed as dilational viscosity, of DMPE and DPPC are given in Fig. 3 and Fig. 4, respectively. All measurements were performed at 21 “C which allows a direct comparison of the results.

4. Discussion The shear viscosity is only insignificantly changed up to the end of the coexistence region. Beyond this point the shear viscosity increases linearly. For the DMPE an abrupt further increase to very high viscosity values is observed after the transition point from a liquid condensed (LC) to a solid-like state at a pressure of about rr= 21 mN m-i. For the DPPC a similar behaviour is observed. Although a transition from the LC phase to a solid-like film at about r= 32 mN m-’ has

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