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Langmuir 1994,10, 1670-1672
Exchange of Interlamellar Cations with Its Aqueous Environment in Charged Layered Systems: Similarity between Clays and Langmuir-Blodgett Films of Metal Salts of Fatty Acids P. Ganguly,* D. V. Paranjape, S. Pal, and Murali Sastry Physical Chemistry Division, National Chemical Laboratory, Pashan Road, Pune 411 008, India Received December 20, 1993. I n Final Form: March 14, 1994@ The general principle involving the exchange of interlamellar cations in clays with those in its aqueous environment has been examined in the case of Langmuir-Blodgett films of metal salts of fatty acids. X-ray diffraction as well as X-ray photoelectron studies on Langmuir-Blodgett films made up by sequential deposition from two troughs containing various metal ions with the same fatty acid condensed Langmuir monolayer have been studied. Evidence is presented for the first time of the exchange of metal ions between the interlamellar spaces of the negatively charged fatty acid bilayers with those in its aqueous environment. Entropic factors associated with equalization of chemical potential are believed to be responsible for the swelling of smectite clays and the consequent exchange between cations in the interlamellar spaces of negatively charged sheets of these clays and those in the bulk aqueous envir0nment.l Since any thermodynamic prinicple should be of general application, these same considerations must apply to other charged lamellar systems such as the Langmuir-Blodgett (LB)films of metal salts of fatty acids2 or anionic complexsalts of long-chain amine^.^ This simple aspect has not been considered so far in the more than 60 years history of Langmuir-Blodgett films. Such an exchange has far-reaching consequences for devices4from LB films which seek to incorporate different charged species in different layers, for example. However, it is the generality of the thermodynamic principle that could prove to be useful in understandingseveral phenomena involving, say lipid bilayers in biological systems or as a model for clays. In this communication, we give evidence for such an exchange in LB films of metal salts of fatty acids. Y type c-axis oriented LB films were formed by dipping a conventionally cleaned Si(ll1) substrate at oleic acid piston pressure (-30 dynlcm) into two different troughs containing various metal ions M (=Pb, Cd, Y, etc.) covered with a condensed solid phase arachidic acid monolayer. The films were dried for half an hour after the first dip and for 10 min for every subsequent dip. Measurement of the piston displacement along with ellipsometricstudies showed that the transfer ratios for the films were not affected by alternating the dipping sequence, the films being Y type as expected. We shall use the notation (Ah: (Pb),(Cd),Jq to refer to a film grown by p alternating cycles of m dips in PbClz solution (lo4 M) followed by n dips in CdClz solutions (lo4 M). Abstract published in Advance ACS Abstracts, June 1, 1994. (1)(a) Langmuir, I. J.Chem. Phys. 1938,6,873.(b) See, for example, Deville, A. Langmuir 1991,7,547, and references therein. (c) See articles in Clay Minerals: Their Structure Behaviour and Use. Philos. Trans. R. SOC.London A 1984,311,219-432. (2)Langmuir Blodgett Films,Roberts, G .G., Ed.; Plenum Press: New York, 1990. (3)Ganguly, P.;Paranjape, D. V.; Sastry, Murali J. Am. Chem. SOC. 1993,115,793. (4)See for example Martin, A. S.; Sambles, J. R.; Ashwell, G. J. Phys. Reu. Lett. 1993,70,218.Burrows,P.E.;Donovan,K. J.;Scott,K.;Sudivala, R. V.; Wilson, E. G. Synth. Met. 1991,41-43,1523. @
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20 ("i Figure 1. (a)X-ray diffraction pattern for (Ah(Pb)l(Cd)l)sLB films on Si(ll1)substrate. Filled circles correspond to positions of reflections calculated for the doubled unit unit cell. The backgroundat low anglesis from the instrument. (b)Calculated X-ray diffraction pattern for the ideal doubled unit cell and (c) for different partial occupancy of Pb sites (100%)and Cd (50%)
sites.
The X-ray diffraction pattern (Figure la) from (Ah (Pb)l(Cd)l)g (pH = 5.5; dipping rate 2 mm/min) clearly does not show evidence for the expected superlattice reflections (Figure Ib) due to doublingof the c-parameter5 even when account is taken (Figure IC)of the reported variation of the transfer ratios of lead and cadmium6 for 0 1994 American Chemical Society
Langmuir, Vol. 10, No.6, 1994 1671
Letters Table 1. Observed and Calculated X-ray Photoelectron
Intensities from Various Langmuir-Blodgett Films {Ah (M)r(M')mjp (M' Layer is Outermost) m
M
n
M'
p
1 1 1 1 15 15
Cd Cd Pb Pb Pbb Cdb
1 1 1 1 0 0
Pb Pb Cd Cd
5 5 4 4
acid Ah Ah Ah Ah Ah Ah
0
56 26 56 26 66 66
IPdICd IdIPb obsd calcdo obsd calcd" 60 60 6.7 7.7 66 61 6.3 25.7 5.0 0.5 61 240 7.7 0.16 68 780 3.3b 54 4.3b 54
0 The calculations of the intensity ratios were carried out using the parameters for attenuation length, h (=50 A), and bilayer thickness, d( -55 A), as discwed in ref 7. The intensity of the XPS signal as a function of the take-off angle, e, is given by I ( @ = [exp(d/X sin @(1- exp(-nd/h sin 8))]/[1 - exp(-d/h sin e)], where n is the total number of bilayers. The lead (or cadmium) arachidate film was immersed fully in a 1 W M solution of CdCla (or PbC12) for 15 min. The cadmium (or lead) k incorporated into the film during the time the f i i was immersed in the solution. The thickness per monolayer over the whole f i b as measured by ellipsometry did not change appreciably (
