Langmuir 1996,11, 1273-1276
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Langmuir-Blodgett Films of Docosylamine M. Bardosova, R. H. Tredgold,* and Z. Ali-Adib Department of Chemistry, University of Manchester, Manchester M13 9PL,
U.K.
Received May 31, 1994. In Final Form: January 9, 1995@ Films at the airlwater interface and Langmuir-Blodgett films of docosylamine have been formed over subphases having a wide range of pH values and containing a variety of different additives. These include polyacrylic acid, valeric acid, potassium arsenate, sodium arsenate, and chloroplatinic acid. Some of these systems have received some previous study but earlier workers have not all made use of X-ray diffraction to confirm the existence of a regular layer structure or to find the repeat distance associated with these layers. We agree with the conclusion arrived at by Gaines, namely that, in the case of films formed over subphases containing NaOH and valeric acid, carbon dioxide in the atmosphere combines with the amine to form a carbamate. These films are Y-type. The films formed over a subphase containingsodium arsenate incorporate the arsenate group and are also Y-type. On the other hand films formed over potassium arsenate are of inferior quality showing that the cation does more than simply act as a counterion in solution. Films formed over a subphase containing chloroplatinic acid exhibit a layer structure but are of poor quality and are Y-type.
Introduction Early work on the behavior of amines at the airlwater interface has been reviewed by Gained and subsequent work on the formation of Langmuir-Blodgett films formed from these materials has also been discussed by this author2 and by Petrov et aL3 Several other interesting papers have recently appeared dealing with various aspects of the formation of Langmuir-Blodgett films of these They have been studied in this context to a lesser extent than carboxylic acids as the latter materials can be obtained in a n un-ionized state at easily reached values of pH whereas amines require pH values of 11 or more before they cease to be ionized. Under these extreme conditions the majority of substrates are damaged and technical problems associated with dipping are made very difficult. On the other hand the incorporation of suitable anions in the subphase makes the formation of stable multilayers possible.2 In this paper we discuss the influence of various counterions on the structure of films making particular use of X-ray diffraction and infrared absorption spectroscopy as diagnostic techniques. Experimental Methods The docosylamine used throughout these experiments was kindly suppliedby Dr. J. G. Petrov and was used without further purification. Two different constant perimeter troughs were employed for forming the Langmuir-Blodgett films and for obtaining isotherms. One of these was partially automated and the other was fully automated. They were constructed in the Physics Department of the University of Lancaster and have been described el~ewhere.~ The material was spread at the air/ water interface as a solution in chloroform using a micropipet. The subphases were made up using doubly distilled deionized water. A Philips PW 1730 X-ray diffractometer was employed to obtain low angle diffraction patterns and an AT1 Mattson Genesis FTIR instrument was used to obtain the infrared spectra. Abstract published inAdvance ACSAbstructs, March 15,1995. (1)Gaines,G. L. Insoluble Monolayers at Liquid-Gas Interfaces;Wiley and Sons: New York, 1966. ( 2 ) Gaines, G. L. Nature 1982,298,544. (3) Petrov, J. G.;Kuhn, H.; Mabius, D. J . Colloid Interface Sci. 1992, 7.3. 213. . - ,-- . ( 4 )Angelova, A.; Petrov, J. G.;Dudev, T.;Galabov, B. Colloids Surf. 1991,60,351. (5) Angelova, A.; Petrov, J. G.; Kuleff, I. Langmuir 1992,8, 213. (6)Petrov, J. G.; Angelova, A. Langmuir 1992,8, 3109. (7) Ganguly,P.;Paranjape, D. V.; Sastry, M. J . Am. Chem. SOC.1993, @
-11.6 - - , 792 .- -.
(8) Ganguly, P.;Paranjape, D. V.; Sastry, M. Langmuir 1993,9,577. (9)Tredgold, R. H.Rep. Prog. Phys. 1987,50, 1609.
/
-7
/
I I I , I I I 0.1 0.2 0.3 0.4 0.5
I 0.6
0.7
[nm2 I Figure 1. Isotherms obtained with docosamine spread on a subphase containing NaOH adjusted to give values of pH between 12.8 and 5.4 shown as a three-dimensional graph. area per molecule
Throughout this work the X-ray results were interpreted as follows. The values of sin 6 were plotted against n,the order of the Bragg peak, and only results which gave sensibly straight lines were accepted.
Experimental Results In our initial experiments we studied the effect of pH on the isotherms obtained with this material (Figure 1) and on the formation of Langmuir-Blodgett films. The initial value of pH (below 7) was caused by the presence of COa in the atmosphere and progressively higher values were brought about by the addition of small quantities of NaOH. As will be seen the value of pH had only a minor effect on the isotherms. During the formation of Langmuir-Blodgett films the dipping speed was 4 mm min-l, the surface pressure was 30 mN m-l, and drying times required to obtain good deposition varied from 13 to 19 min as the pH was vaned from 12.6 to 9.5. At the highest value of pH employed the deposition ratio was 0.8 in both the upward and the downward directions. At pH = 9.5 the deposition in the upward direction was 0.8 and in the downward direction it was 0.65. Films of up to 100 layers were formed on silicon substrates and also on niobium
0743-746319512411-1273$09.0010 0 1995 American Chemical Society
Bardosova et al.
1274 Langmuir, Vol. 11, No. 4, 1995
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6
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2 0 (degrees,
2 0 (degrees)
Figure 2. X-ray diffraction pattern obtained from 100 layers of docosamine deposited on silicon. The subphase pH was 9.5. For further details see the text.
0
I
I
1
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' - - -* --.__ I
I
I
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i
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rea per molecule(nm2) Figure 3. Isotherms of docosamine on a subphase containing polyacrylic acid. Curve a corresponds to a concentration of 0.01 g L-l at a pH of 10. Curve b correspondsto a concentration of 0.1 g L-' at a pH of 9.2. Curve c corresponds to a concentration of 0.3 g L-' at a pH of 6.7.
deposited on glass. At the highest values of pH the niobium substrates tended to deteriorate, but below pH = 11this was not the case. In the pH range 7-12 the X-ray diffraction patterns remained constant (Figure 2) and a layer spacing of 5.75 nm was obtained which is a little over twice the molecular length of 2.76 nm. This length was obtained from a CPK model of the molecule, a process which usually gives results which agree well with other methods. In view of the long drying times needed and the fact that the multilayer structure is independent of pH over a wide range, we are inclined to the opinion expressed by Gaines? namely that the carbon dioxide in the atmosphere combines with the amine to form a carbamate. In an attempt to obtain Z deposition we tried the influence of dissolving polyacrylic acid in the subphase. The best results were obtained with a concentration of 4.2 mM. Here the concentration is expressed in terms of the number of repeat units present. Sodium hydroxide was added to adjust the pH to 6.5, a value which gave optimum results. Isotherms for various values of pH are given in Figure 3. Further details are given in the table. A n X-ray diffraction pattern obtained from a 100-layer specimen is shown in Figure 4. The linear plot referred to above yields a repeat distance of approximately 3.5 nm. A very small peak at an angle corresponding to about 5.8 nm probably arises from a small amount of a second phase. Dipping was Z type and the layer spacing of 3.5 nm is a plausible
Figure 4. X-ray diffraction pattern obtained from 100 layers of docosamine. The subphase contained polyacrylic acid at a concentration of 0.3 g L-l and the pH was 6.5. For further details see the text.
0
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Area per molecule(nmz> Figure 5. Isotherms for docosamine on subphases containing various additives: a, potassium arsenate; b, valeric acid; c, chloroplatinic acid; d, sodium arsenate.
value for a repeat unit consisting of one layer ofthe amine and one layer of the polymer. We thus conclude that we have a Z structure with a small admixture of a Y structure probably consisting of the amine alone. We have also studied the effect ofvaleric acid, potassium arsenate, sodium arsenate, and chloroplatinic acid in the subphase. The corresponding isotherms are shown in Figure 5. Angelova et aL4 and Vollhardt et aZ.I0 studied isotherms and other properties of docosylamine and octadecylamine, respectively, on a subphase containing phosphate ions and reported their results as a function of pH. Unlike these authors we have chosen values of pH which optimize deposition for each material and report our results only for this value. X-ray diffraction patterns for these materials are shown in Figure 6. With the exception of the results obtained from valeric acid and chloroplatinic acid the isotherms observed did not show characteristics of particular interest. Details are given in Table 1. It is interesting to note that when the pH is 7.7 o r less in the case of valeric acid, the isotherms associated with this material are extended in the lowpressure region. It is to be presumed that when valeric acid is not ionized, it becomes surface active. In this case the apparent area per molecule at high surface pressures is less than the known cross section of a hydrocarbon chain. The slight kink in the isotherm a t an area per molecule (10)Vollhardt, D.;Wittig, M.; Petrov, J. G.; Malewski, G. J.Colloid Interface Sci. 1985, 106, 28.
LB Films of Docosylamine
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Table 1. Synopsis of Roperties of Langmuir-Blodgett Films Formed from Docosylamine Using Various Subphase Additive@ dipping ratio subphase additive NaOH NaOH polyacrylic acid valeric acid* sodium arsenate chloroplatinic acid
concentration mM
PH
UP
down
drying time, min
layer spacing, nm
4.2 0.9 12 0.1
12.6 9.5 6.5 7.7 7.6 3.8
0.8 0.8 1.0 1.05 0.96 1.01
0.8 0.65 0 0.62 0.92 0.71
13 19 10-15
5.75 5.75 3.5 5.74 5.90 2.9
4 4 4
a The films formed over subphases containing NaOH only and polyacrylic acid plus NaOH were dipped at 4 mm min-l and the other materials at 6 mm min-l. Chloroplatinic acid films were formed at 10 mN m-l and the other materials at 30 mN m-l. These values were chosen to be below the collapse pressures of the materials in question. The asterisk indicates that the pH was adjusted by the addition of NaOH. The error in the layer spacing is estimated to be less than f0.05 nm except in the cases of polyacrylic acid and chloroplatinic acid where the error is less than 0.1 nm.
and the repeat distance is 5.74 nm. We conclude that, here too, carbamates are formed. Angelova et studied docosylammonium arsenate but did not carry out X-ray studies. We therefore examined the behavior of this amine on a subphase containing either potassium or sodium arsenate (Figure 5 ) . We also formed Langmuir-Blodgett multilayers of these materials. One might assume that the sodium or potassium merely acted as a counterion and would not influence the structure of the film. This however is not the case. Multilayers formed from a subphase containing potassium were always of very poor quality and we thus confined our attention to those formed from a subphase containing sodium. In this case good films were readily formed with a drying time of 4 min, but at a drying time of 2 min good films were still obtained. Y deposition occurred as was shown under similar circumstances by Angelova et aL4 (See the table for details.) The films formed from a subphase containing chloroplatinic acid were of an inferior quality to those formed in the other cases discussed here and appeared to consist of many small crystallites when examined under the polarizing microscope. It is reasonable to assume that films containing a heavy metal should lead to an X-ray diffraction pattern containing both even and odd order peaks. If we assume this to be the case, then the peaks that we observe are first, second, and third order and the repeat distance is 2.9 nm. This figure compares reasonably well with the results obtained by Ganguly et aL7 who studied octadecylamine under similar circumstances and found a repeat distance of 2.7 nm. They proposed a structure involving a large amount of interdigitation to account for this behavior. Figure 7 shows infrared absorption results obtained from the amine in chloroform and for LB films produced using subphases containing the various materials already discussed. In each case the pH employed was chosen to optimize dipping. Typical peaks for the free amine are seen at 3349 cm-', 1612 cm-1 (NH), at 2915 cm-l and 1612 cm-l (CH and CH2). For LB films deposited from all the subphases used, the peaks associated with CH and CH2 appear and the NH peak at 3349 cm-l does not appear. Peaks typical for carbamates appear in the region of 1560 cm-l for all cases except sodium arsenate. The infrared absorptionresults for the material dipped over a subphase containing sodium arsenate show peaks in the region where they would be expected for the arsenate group which are at 837 and 878 cm-l according to Nakamoto." These results confirm the supposition that the arsenate groups are actually incorporated in the multilayer. In the case
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Figure 6. X-ray diffraction patterns corresponding to the materials whose isotherms a r e shown in Figure 5 .
corresponding to a tightly packed layer could be connected with this phenomenon. It is interesting to note that Angelova et aL4also obtained isotherms which lead to an apparent area per molecule which is unreasonably low. In the case of a subphase containing chloroplatinic acid, it seems extremely likely that the curious shape of the isotherm is due to a collapse from a monolayer to a bilayer at a pressure of about 10mNm-l as postulated by Gunguly et al.7,sfor the case octadecylamine. Valeric acid is the longest straight chain carboxylic which is readily soluble in water and we hoped that we would experience a behavior similar to that seen with polyacrylic acid. In fact the valeric acid has very little influence on deposition. The deposition in the upward direction is 1.05 and in the downward direction it is 0.62
(11)Nakamoto, K. Infrared Spectra oflnorganic and Coordination Compounds; John Wiley and Sons: New York, 1965.
1276 Langmuir, Vol. 11, No. 4, 1995
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1000
Figure 7. FTIR spectra for films ofthe materials studied. Spectrum a corresponds to pure amine, b to film formed over a subphase containing valeric acid, c, over a subphase containing polyacrylic acid, d over a subphase containing sodium arsenate, and e over a subphase containing chloroplatinic acid.
of those films formed from a subphase containing chloroplatinic acid, a strong peak appeared at 3207 cm-'. Discussion We conclude that the films formed using subphases containing sodium hydroxide or valeric acid and sodium hydroxide are in the Y configuration. The layer spacings for these two materials are the same within the experimental error and it is thus likely that under both conditions the amine forms carbamates though the short drying time associated with the valeric acid is puzzling. Only odd orders of diffraction occur. As no heavy metal is present, difiaction must be largely due to variations in the density of the hydrocarbon material. It is to be noted that there is a significant difference of the deposition ratios in the downward and in the upward directions. If this fact corresponds to the monolayers formed on the upward stroke being more tightly packed than those formed on the downward stroke, then the electron density plotted in the direction of the normal to the film would approximate to a square wave. Simple Fourier analysis shows that such a structure will produce only odd order peaks, observed. The repeat distance is rather more than twice the length of the molecule in these cases. This implies that there is little or no tilt in the films formed in the Y configuration. The films formed using a substrate containing polyacrylic acid appear t o incorporate this material and deposit in the Z mode though with some slight imperfections. In view of the FTIR results it seems likely that the small amount of a second phase observed here arises from the
presence of carbamates. Several other examples of the technique of dipping over a subphase containing a polymer have been reported in the literature,12-14but we are not aware of an example using amines. The slightly larger repeat distance observed in the case of the multilayers containing arsenic is to be expected. As will be seen from the isotherms there is no room for the arsenate group to squeeze in between the amine molecules so that it must reside between the ends of the molecules as indeed would be expected. Again it is reasonable to infer that there is little tilt in this case. The films formed from a subphase containing chloroplatinic acid were all of rather poor quality but showed the interesting interdigitation observed by Ganguly et aL6 It appears from this study that Langmuir-Blodgett multilayers deposited from a subphase containing a suitable divalent anion or with a carbamate termination can readily be formed in a tilt-free form and in films which are largely defect free.
Acknowledgment. We wish to thank Dr. J. G .Petrov for providing the docosylamine and Professor P. Hodge
for providing facilities. We also wish to thank the Royal Society and the Engineering and Physical Science Research Council for supporting this research. LA940425X (12)Fujihira, M. Mater. Res. SOC.Symp. Proc. 1992, 277, 47. (13)Fujihira, M.; Gotoh,Y. Nanostructure Based MolecularMaterials; Goepel, W., Ziegler, C., Eds.; VCH: Weinheim, Germany, 1992. (14)Kajiyama, T.; Zhang, L.; Uchida, M.; Oishi, Y.; Takahara, A. Langmuir 1993, 9, 760.
