Plasma desorption mass spectrometry of Langmuir-Blodgett films: dependence of salt formation of arachidic acid monolayers on the subphase parameters

Thin Solid Films, 249 (1994) 215-218

215

Plasma desorption mass spectrometry of Langmuir-Blodgett films: dependence of salt formation of arachidic acid monolayers on the subphase parameters* D. Brandl, Ch. Schoppmann,

Ch. Tomaschko

and H. Voit

Physikalisches lnstitut der Universitiit Erlangen-Niirnberg, D-91058 Erlangen, Germany

(Received November 8, 1993; accepted March 9, 1994)

Abstract The conditions for complete salt formation of an arachidic acid monolayer spread onto an aqueous subphase were investigated by means of plasma desorption mass spectrometry in order to demonstrate that this method is very well suited to characterize Langmuir-Blodgett films. The combinations of subphase pH and metal ion concentration necessary for complete salt formation of arachidic acid monolayers were deduced from this analysis.

2. Experimental details

spread. For this purpose a small quantity of the subphase was taken from the trough. The subphase was maintained at a constant temperature of 25 °C by means of a control system. Arachidic acid (Sigma) dissolved in chloroform was spread onto the water surface. LB films consisting of 8 monolayers were deposited onto a gold surface (microscope slides covered with 500/~ gold) by means of the dipping technique with a dipping velocity of 3 - 8 mm m i n - k During the transfer the surface pressure (measured with a Wilhelmy-type sensor) was kept at the constant value rc = 30 m N m - i . The transfer ratio which was permanently recorded was 100%. Since the desorption is carried out in vacuum (10 -6 mbar) the vacuum stability of the LB films investigated is of some concern. In a first experiment it could be shown that evaporation of organic molecules of an 8 monolayer film consisting of pure arachidic acid data aquisition times used in the present PD analysis ( ~< 10 min).

2. I. L B film preparation

2.2. Plasma desorption mass spectrometry

For the LB film preparation a commercial trough (KSV 3000) positioned in a laminar flow box was used. The subphase was Milli-Q quality water containing dilute solutions of CdC12 with various concentrations (10 -5 to 10 -3 mol l-J). The subphase pH was adjusted by means of NaHCO3. It was measured with a p H electrode (Ingold 405-88TE-s7/9812) with an accuracy of 0.05 pH units just before the arachidic acid was

PD mass spectrometry makes use of the fact that MeV heavy ions are able to desorb fragile organic molecules as intact entities [2]. Desorption occurs from the surface and the first few layers beneath the surface of the sample. The escape depth for molecular ions increases with d E / d x , the electronic energy loss of the primary ion, and depends largely on the sample material. The escape depth for Cd arachidate samples was found to exceed 38 nm (14 monolayers) in the case where fission fragments from a 252Cf source with masses m ~ 100u and energies of ~ 1 0 0 M e V were used as primary ions [3].

1. Introduction Ultrathin organic films with a controlled molecular architecture are discussed for a number of applications [1]. These films can be fabricated by the L a n g m u i r Blodgett (LB) technique. Catchphrases concerning an application are wetting, biocompatibility, microlithography, lubrication, membranes and sensors. A successful application, however, demands a thorough characterization of these films. The aim of the present study is to demonstrate that plasma desorption (PD) mass spectrometry is very well suited for the characterization of Langmuir-Blodgett films. This is shown for a particular example, i.e. the investigation of the influence of subphase parameters (pH value, metal ion concentration) on the percentage of salt formation of an arachidic acid film.

*Dedicated to Professor R.D. Macfarlane on the occasion of his 60th birthday.

0040-6090/94/$7.00 SSDI 0040-6090(94)06127-7

© 1994 -- Elsevier Science S.A. All rights reserved

216

D. Bramfl et al. / PD mass spectromeoT o f L B films

The desorption yield Y, i.e. the number of desorbed molecular ions per incident primary ion increases with increasing electronic energy loss [4]. The yield for a particular ion desorbed from equally thick samples depends linearly on the number density of the corresponding molecule in the sample [5], Desorbed ions are usually mass analyzed in PD mass spectrometry by means of a time-of-flight (TOF) spectrometer. It has been shown in ref. 6 that ( A - H ) - , An(A - H ) - , (A + H) + and A,,(A + H) + ions are desorbed from a plain arachidic acid sample (A denotes the arachidic acid molecule and (A - H) - the deprotonated acid molecule). Negative ions emerging from a Cd arachidate sample (100% salt) are ( A - H ) and [(A - H ) 2 C d ] ( A - H ) - . The former ions are fragments of the ( A - H ) 2 C d molecule, the latter are either a fragment of the Cd arachidate dimer or originate during the desorption process via attachment of a ( A - H ) fragment to an evading salt molecule. Sample specific positive ions desorbed from a Cd arachidate sample are (A - H)Cd + and [(A - H)2Cd](A - H)Cd*. It should be mentioned that PD is a very sensitive analytic tool. Spectra containing the molecular ions mentioned above can be easily obtained from one monolayer films in reasonable data aquisition times. Besides this PDMS is a non-destructive method. Owing to the fact that only a rather small number of primary ions (10 5 to 106) is needed to generate a mass spectrum one can avoid the same spot at the sample being irradiated twice. This means that the irradiation has no influence on the information to be gained.

3. Results and discussion Figures l ( a ) - l ( d ) show PD mass spectra of negative ions desorbed from arachidic acid LB films (8 monolayers) by the same number of incident primary ions. The films were prepared with the same CdC12 concentration in the subphase (10-5 mol 1-~) and pH values varying from 5.73 to 6.65. All spectra, with the exception of spectrum (d), exhibit A , , ( A - H ) - ions (n = 1,2) indicating that the sample contains the arachidic acid molecule A. The number of A,(A - H) - ions desorbed from a particular sample decreases with increasing pH values used in preparing the sample. This means that the number of arachidic acid molecules in the sample decreases in the same way with increasing pH. Spectrum (d) of Fig. 1 does not exhibit A,(A - H ) ions. Instead the [ ( A - H ) 2 C d ] ( A - H ) - ion shows up. This means that the sample used to generate spectrum (d) is completely built from Cd arachidate molecules ( 100% salt). The intensity of the [ ( A - H ) 2 C d ] ( A - H ) - ion increases if the pH is raised from 5.73 to 6.65 (see Fig. 1),

A(A-H)

x

zE

2-

"E 1 O O

pH 5.73-

Az(A-H)

°i 3

pH = 6.40-

I

21 0

i

3 2!

200

i

,

pH = 6.55 I

0 ...... I 2~

•

~

pH 6.657

t

[(A'H)2Cd](A-H)- 1

450

700

950 m/z 1200

Fig. 1. PD mass spectra of negative ions desorbed from LB films (8 monolayers) of arachidic acid prepared with various pH values of the subphase. The subphase contained a dilute CdCI2 solution (1 x 10-5 mol I-1). Fission fragments from a 252Cfsource were used as primary ions PI. All samples were bombarded with the same number Np, of primary ions.

i.e. it behaves complementary to the A,(A - H) - intensity. A similar behaviour can be observed in the mass spectra for positive ions obtained from the same samples (see Fig. 2). Peak intensities of molecular ions containing the arachidic acid molecule decrease with increasing pH whereas an increase is observed for ions containing the Cd arachidate molecule (or a fragment of this molecule). This demonstrates clearly that a transition occurs in the film composition from films built up by acid and salt molecules (small pH values) to films containing only salt molecules (highest pH investigated). This transition can be visualized very easily by means of the PD mass spectrometry. Yields for ions which are indicative for the change of this composition are shown in Fig. 3. Actually the PD yield for [(A -- H)2Cd](A - H ) - ions obtained from a spectrum with vanishing A ( A - H ) - yield was arbitrarily set to 100%. The percentage of salt molecules obtained with this normalization from the sample prepared with the smallest pH was used to fix the percentage of acid molecules in this particular sample. It was assumed for this purpose that the latter is complementary to the salt percentage. Figure 3(a) contains

D. Bramtl et al. / P D mass spectrometry o f L B f i l m s

4- IA.xO~)+ x

3

~"

'

pH = 5.73

~A+H)+

A(! +H) +

2

Z

A2(A+H) +

co o

pH = 6.40 3 2 1 0 4

pH = 6,55

3 2 1

1

0 4.

pH 6.65

321.

[(A-H)2Cd](A-H)Cd +

(A-H)Cd +

0

200

460

7o0

9~0

1200 m/z

Fig. 2. PD mass spectra of positive ions desorbed from LB films (8 monolayers) of arachidic acid prepared with various subphase pH values. For further information see Fig. 1.

i CdCI

i

i

i

i

i

i

i

i

i

i

J

O}

2

100 __10-5 mo[.i-1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . "o o 50

j

100

,

~

Ik~

217

the data of Figs. 1 and 2. Figures 3(b) and 3(c) contain results obtained for additional measurements performed with different CdClz concentrations in the subphase (5 x 10 -5 and 1 × 10-4moi1-1). Obviously the percentage of salt ions in a LB film increases almost linearly with increasing subphase pH for a given CdC12 concentration until complete salt formation is reached. A further increase of the pH value leaves the number of salt ions constant (Fig. 3(c)). At the same time the percentage of ions containing the pure acid molecule decreases with increasing pH. It becomes zero for pH values representative of complete salt formation. In fact it behaves complimentary to the percentage of salt ions. This can be easily deduced from a comparison with the "complementary" lines (solid lines) shown in Fig. 3. It is obvious from Fig. 3 that complete salt formation depends both on the CdCI2 concentration K and the subphase pH. A small K value demands a large pH value and vice versa to obtain a pure salt film. Figure 3(c) demonstrates that a subphase pH equal to the pKA value of the fatty acid as measured at the subphase surface [7] (pKA ~ 5.6) indeed results in a film composition containing equal parts of acid and salt molecules provided a sufficient number of metal ions exists in the subphase. The smallest values of the two parameters (pH and CdCI2 concentration) which result in complete salt formation are given in Fig. 4. Obviously the CdC12 concentration has to be increased exponentially for a linear decrease of pH if complete salt formation is to be guaranteed. The solid line in the figure divides the (K, pH) plane in a region with incomplete (left side) and complete (right side) salt formation, respectively. Another series of LB films (8 monolayers) was prepared with a roughly constant subphase pH but various

----

-5.

i

10-3

,

i

,

J

i

,

i

i

i

i

Cd orochidote

50

m

E 0 100

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_

g

c o= 10-~ ~ou

50 . . . . . . . . . . . . . . . . . . . . . . . . . . .

i

5.6

i

5.8

i

i

6.0

~

Q

I

t

(..>

i

I

6.2

i

6.4 6'.6 subphose p H

Fig. 3. Percentage o f salt and acid molecules in 8 monolayer thick arachidic acid LB films prepared with various subphase pH values and CdCI 2 concentrations: A, [ ( A - H ) 2 C d ] ( A - H ) - ; O, A ( A - H ) - ; (3, ( A + H ) + ; I-7, A ( A + H ) +. The solid lines are complementary lines. For details see text.

5,6

5.8

6,0

6.2

6,4

6.6 6,8 subphese pH

Fig. 4. Subphase pH values and CdC]2 concentrations necessary for complete salt formation of arachidic acid rnonolayers spread on a water surface. The solid line is the border line between incomplete (left) and complete (right) salt formation.

218

D. Brandl et al. / PD mass spectrometry o f LB films [

i

I00~

i

0

i

~

4. Conclusion

T

/

~ =

"

Cd-arachidate /

80--

/

!,,

J4

/

i

®

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/

~.0

/ / / /

20

,~

/

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\

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5 CdCl z concentration

10

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Fig. 5. Percentage of salt/acid molecules in 8 monolayer thick arachidic acid LB films as a function of t h e CdCI2 concentration in t h e subphase: A, [ ( A - H ) 2 C d ] ( A - H ) - ; ~', ( A + H ) + ; F], A ( A + H ) + ; O, A ( A - H ) - . The pH value of the subphase was maintained between 5.55 and 5.75.

The present investigations show that plasma desorption mass spectrometry is well suited to characterizing Langmuir-Blodgett films consisting of only a few monolayers of a fatty acid. The method allows one to determine the composition of these films. This is demonstrated for the case of arachidic acid films consisting of various proportions of acid and salt molecules. The films were prepared with various subphase pH values and CdCI2 concentrations. The actual measurements show that the pH value which results in equal parts of acid and salt molecules in the film has to be increased with decreasing metal ion concentration K and that the percentage of salt (acid) molecules contained in the LB film increases (decreases) linearly with increasing pH for a given K. This is expected and is known already from other experiments [8-10]. The measurements were used to deduce the smallest CdCI2 concentration necessary for a given subphase pH (pH = 5.6-6.7) to obtain complete salt formation.

Acknowledgment CdC12 concentrations K. Actually the pH value varied between 5.55 and 5.75 depending on K (no buffer was added). Figure 5 shows the percentage of salt and acid molecules containing ions as deduced from the PD analysis. Again the same complementary behaviour as in Fig. 3 is found. The smallest K value which results in a complete salt formation fits nicely on the K versus pH curve of Fig. 4. The influence of the subphase pH on the salt formation of arachidic acid spread on a CdC12-containing subphase has been investigated already by various other methods. Kobayashi et al. [8] used X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) to analyse arachidic acid films prepared with different pH values and a 3 x 10-4 mol 1-~ CdC12 concentration. Petrov et al. [9] investigated samples prepared with 10-4 mol 1-I CdCIz by means of the neutron activation analysis. The pH value necessary to obtain an LB film consisting of equal parts of salt and acid was found to be 5.6 (for 10-4moll -l CdClz) in both investigations. This is in good agreement with the present result.

This work was supported by the Forschungsgemeinschaft, Bonn, Germany.

Deutsche

References 1 G. Roberts (ed.), Langmuir-Blodgett Films, Plenum, New York, 1990, p. 317. 2 K. Wien, Nucl. Instrum. Methods Phys. Res. B, 65 (1992) 149. 3 R. Schmidt, Ch. Schoppmann, D. Brandl, A. Ostrowski, H. Volt, D. Johannsmann and W. Knoll, Phys. Rev. B, 44 (1991) 560. 4 D. Brandl, Ch. Schoppmann, R. Schmidt, B. Nees, A. Ostrowski and H. Volt, Phys. Rev. B, 43 (1991) 5253. 5 R. Schmidt, Diploma Thesis, University of Erlangen-Nfirnberg, 1989. 6 R. Schmidt, D. Brandl, Ch. Schoppmann, H. Volt, T. Kr6hl, D. Johannsmann and W. Knoll, Int. J. Mass Spectrom. Ion Proc., 99 (1990) 223. 7 J. J. Betts and B. A. Pethica, Trans. Faraday Soc., 52(1956) 1581. 8 K. Kobayashi, K. Takaoka and S. Ochiai, Thin Solid Films, 159 (1988) 267. 9 J. G. Petrov, I. Kuleff and D. Platikanov, J. Colloid Interface Sci., 88 (1982) 29. 10 J. Bagg, M. B. Abramson, M. Fichman, M. D. Haber and H. P. Gregor, J. Am. Chem. Sot., 86 (1964) 2759.