Langmuir 1993,9, 1978-1979
1978
Novel Concept for the Aggregation Structure of Monolayers on the Water Surface Tisato Kajiyama,’ Yushi Oishi, Motoko Uchida, and Yoshinari Takashima Department of Chemical Science and Technology, Faculty of Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812, Japan Received September 8,1992. In Final Form: May 14,1993 The aggregation structure of arachidic acid monolayers on water subphases of different pHs was investigated by means of transmission electron microscopy. Arachidic acid monolayers exhibited the phase transition from an amorphousstate to a crystallineone by surface compressionin the case of a highly dissociated state of hydrophilic groups, whereas they did not show the phase transition in the case of a slightly dissociated state. The aggregationstructure of monolayers on the water surfacewas systematically classified into “the crystallinemonolayer”,“the amorphousmonolayer”,and ‘the compressingcrystallized monolayer” with respect to thermal and chemical (intermolecular repulsive) factors. Introduction Great attention has been paid to Langmuir-Blodgett films because of their potential applications as electronic or electro-optical devices and as models for biological membranes. The images of monolayers on the water surface have been proposed on the basis of recent morphologicaland structural studies.14 These images did not always agree with the general concepts which was accepted only from surface pressure-area (T-A) isotherms and also did not come to a universal understanding. We here present a novel and systematic classification for the aggregation structure of monolayers on the water surface. Fatty acid monolayers on the pure water surface have been classified into a crystalline monolayer and an amorphous one at the subphase temperature (Tap)below and above the melting temperature (T,)of the monolayer, respectively. These aggregation states are independent of the magnitude of the surface pressure.’I2 The T-A isotherm for the fatty acid monolayer represents the aggregating process of isolated domains grown right after spreading a fatty acid solution on the water surface.’-“ On the other hand, phosphatidylcholine: phosphatidic acid,’ and anionic amphiphilea formed a compressingcrystallized monolayer which was crystallized by compression of the monolayer on the water surface at Tapbelow T,. For these cases, a fairly high surface pressure was required to crystallize the amphiphilic molecules with ionic hydrophilic groups owing to strong repulsion among ionic charges of hydrophilic groups. In order to understand systematically the aggregation structure of monolayers, we investigate the effect of ionic repulsion among hydrophilic groups with respect to the aggregation structure of the monolayer on the water surface.
* To whom correspondence should be addressed.
(1)Kajiyama, T.; Oishi, Y.; Uchida, M.; Morotomi, N.; Ishikawa, J.; Tanimoto, Y. Bull. Chem. SOC.Jpn., 1992,65,864. (2)Kajiyama, T.;Oishi, Y.;Uchida, M.; Tanimoto, Y.;Kozuru, H. Langmuir 1992,8,1563. (3) Uyeda, N.; Takenaka, T.; Aoyama, K.; Mataumoto, M.; Fujiycehi, Y. Nature 1987,327,319. (4)Hbnig, D.; Overbeck, G. A.; Mbbiua, D. Adu. Mater. 1992,4,419. (5) Kjaer, K.; Als-Nielsen, J.; Helm, C. A.; Tippman-Krayer, P.; Mbhwald, H. J . Phys. Chem. 1989,93,3200. (6)Kozuru, H.; Oishi, Y.;Kajiyama, T. Polym. Prepr. Jpn. 1991,40, 2431. (7)M6hwald, H.Thin Solid F i l m 1988,159,1. (8) Kajiyama, T.; Zhang, L.; Uchida, M.; Oishi, Y.; Takahara, A. Langmuir 1993,9,760. (9)See for example: Gaines, G. L. Insoluble Monolayers at LiquidGas Interface; Interscience: New York, l w , Chapter 4.
Experimental Section Arachidic acid monolayers were prepared from a benzene solution on water subphases of pH 6.8 (pure water) and 12.6 (adjustedby addition of NaOH) at a T,of 303 K below T, (~328 K)’ of the monolayer. The ionic dlssociation state of the hydrophilic group was estimated on the basis of the stretching vibrations of carbonyl and carboxylate groups by Fourier transform infrared attenuated total reflection, FT-IR ATR, measurements. Seventy arachidic acid monolayers were transferred on a germanium ATR prism, resulting in the formation of the multilayered f i b . Transfer on the prism was carried out at surface pressures of 25or 28mN m-l. For transmission electron microscopic observations, the monolayer was transferred on hydrophilic Formvar or Si0 substrates, on which the monolayer could be transferred without a change of the aggregation state, by the horizontal lifting or upward drawing methods.’ Electron microscopic observations were carried out at a temperature corresponding to TSpat which the monolayer was prepared. Results and Discussion Infrared absorption measurements revealed that almost all the carboxylic groups of arachidic acid molecules did not dissociate on the water subphase of pH 5.8, whereas all carboxylic groups dissociated as carboxylate ions on the water subphase of pH 12.6. Parts a and b of Figure 1 show the F A isotherms for the arachidic acid monolayers on water surfaces of pH 5.8 (pure water) and pH 12.6 at a Tapof 303 K, respectively, as well as the ED patterns of the monolayers at several surface pressures. In the case of a neutral state of arachidic acid (pH 5.8), the r A isotherm showed a sharp rise of surface pressure with decreasing surface area without any appearance of a plateau region. The ED patterns a t surface pressures of 0 and 25 mN m-1 showed a crystalline arc and crystalline spot, respectively, indicating the formation of “the crystalline monolayer”.1.2 Kjaer et al.6 also reported from synchrotron X-ray diffraction studies that the arachidic acid monolayer on the pure water surface revealed the crystalline phase of the monolayer at various surface pressures. The change of the ED pattern from the crystalline arc to the crystalline spot suggests that crystalline domains were fused or recrystallized at the monolayer domain interface owing to sintering behavior caused by surface compression, resulting in the formation of larger two-dimensional crystalline domains.’Jo In the case of a dissociated state of arachidic acid on the water subphase of pH 12.6 at a Tapof 303 K, a plateau (10)Tanizaki,T.;Takahsra,A.;Kajiyama,T. J. Soc.Rheol.,Jpn. 1991, 19, 208.
0143-1~63/93/2409-~978$04.00/0 Q 1993 American Chemical Society
Letters
Langmuir, Vol. 9, No. 8,1993 1979 80
Ionic repulsion among hydrophilicgroups
Arachidic acid
70' (a)
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Compressing Crystallized Monolayer
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Figure 2. Classification of the aggregation structure of a monolayer on the water surface.
I 0.0
0.2 0.4 0.6 0.8AREA / nm' molecule
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Figure 1. F A isotherms and ED patterns of arachidic acid monolayers at a Tq of 303 K on the water subphase of pH 5.8 (a) and pH 12.6 (b).
region of the F A isotherm was observed in the range 0.30.5 nm2molecule-'. The ED pattern at 5 mN m-1 showed an amorphous halo, whereas those at 12 and 28 mN m-1 exhibited a crystalline arc or spot. Therefore, Figure l b indicates that the arachidic acid monolayer is crystallized by compression on the water surface of pH 12.6. This type of monolayer has been classified as "the compressing crystallized mono1ayer".6*8 It is clearly concluded from parts a and b of Figure 1 that amphiphile molecules form the crystalline monolayer and the compressing crystallized monolayer at Tapbelow T m in the cases of a neutral state (maybe the low degree of ionic dissociation) and a highly dissociated state of polar groups, respectively. Figure 2 shows the classification based on the aggregation structure of monolayers with respect to thermal (Tep,Tar,,T m )and chemical (the degreeof ionic dissociation of hydrophilic group) factors. This figure is divided into four quadrants by the two axes of Tapand the repulsive force among hydrophilicgroups. In the case of amphiphiles with a nonionic hydrophilic group (corresponding to the third and fourth quadrants), isolated domains grown right after spreadinga solution on the water surface are gathered to be a morphologically homogeneous monolayer by compression. Then, at Tapbelow T m (the third quadrant), the monolayer is in a crystalline phase which is designated "the crystalline monolayer". The crystalline monolayer is further classified into two types: crystalline domains
are assembled as a large homogeneous crystalline monolayer due to a surface compression-induced sintering at the interfacial region among monolayer domains at Tap below the crystalline relaxation temperature (Tar,), and also crystalline domains are gathered without any special The crystalline orientation among domains above TaC.l*lo relaxation phenomena correspond to a change from elastic to viscoelastic characteristics in a crystalline phase due to a remarkable increase from anharmonic thermal ~ibrati0n.ll-l~ At Tapabove T, (the fourth quadrant), the monolayer is in an amorphous (noncrystalline)phase which is designated "the amorphous monolayer". In the case of amphiphiles with an ionic hydrophilic group (the first and second quadrants), a distinct domain structure is not formed at lower surface pressure owing to an electrostatic repulsion among polar head groups. A t Tapbelow T m (the second quadrant), amphiphile molecules form a large homogeneous crystallized monolayer (Tap< Ta,) or an assembly of crystallized domains (Tat < Tap< Tm)owing to the contribution of the van der Waals force with an increase of surface pressure. On the other hand, at Tap above T m (the first quadrant), the monolayer is not crystallized by compression owing to fairly active thermal molecular motion.6
Acknowledgment. This research was supported in part by a grant from the Ministry of Education, Science, and Culture, Japan (Priority Area Research Program, "New Functionality Materials-Design, Preparation and Control"). (11) Takayanagi, M.; Matsuo, M. J. Macromol. Sci. Phys. 1967, BI, 407.
(12) Kajiyama, T.; Okada,T.;Sakoda,A.;Takayanagi,M. J.Macromol.
Sci. Phys. 1973, B7,583.
(13) Kajiyama,T.;Okada, T.; Takayanagi,M. J. Macromol. Sci. Phys.
1974, B9, 35.
(14) Kajiyama,T.;Takayanagi, M. J. Macromol. Sci. Phys. 1974, BlO, 131.
