Langmuir 1997, 13, 2527-2532
2527
Effect of Ionic Repulsion among Hydrophilic Groups on Aggregation Structure of Fatty Acid Monolayer on the Water Surface Yushi Oishi,‡ Yoshinari Takashima,† Kazuaki Suehiro,‡ and Tisato Kajiyama*,† Department of Chemical Science and Technology, Faculty of Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812, Japan, and Department of Applied Chemistry, Faculty of Science and Engineering, Saga University, 1 Honjo-machi, Saga 840, Japan Received March 8, 1996. In Final Form: December 27, 1996X The effect of ionic repulsion among hydrophilic groups of arachidic acid molecules on the aggregation structure of the arachidic acid monolayer on the water surface was investigated on the basis of surface pressure-surface area (π-A) isotherm measurements, Fourier transform infrared spectroscopic (FT-IR) measurements, and transmission electron microscopic (TEM) observations. π-A isotherm and FT-IR measurements revealed that the degree of ionic dissociation of hydrophilic groups in the monolayer drastically changed in a pH range of 10-12. The arachidic acid monolayer in a slightly dissociated state of hydrophilic groups (pH ) 5.8) did not exhibit the phase transition from an amorphous state to a crystalline one during compression. On the other hand, the monolayer in a highly dissociated state (pH ) 12.6) showed the phase transition from an amorphous state to a crystalline one by surface compression at the subphase temperature, Tsp, below the melting temperature, Tm, of the monolayer, and also, the monolayer was still in an amorphous state without the phase transition to a crystalline state at Tsp above Tm. The aggregation structure of the monolayers on the water surface was systematically classified into “the crystalline monolayer”, “the amorphous monolayer”, and “the compressing crystallized monolayer” with respect to the thermal and chemical (intermolecular repulsive) factors.
Introduction Langmuir-Blodgett (LB) films which are prepared by successive deposition of monolayers onto solid substrate provide the possibility of constructing supramolecular organizations exhibiting various functional properties, for example, electrically and optically characteristic properties.1 The ultimate functional properties of LB films can be attained by using defect-free or defect-diminished monolayers.2,3 Therefore, the construction of defect-free or defect-diminished monolayers can be realized with the systematic understanding of 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, Tsp, below and above the melting temperature, Tm, of the monolayer, respectively.4,5 It is also recognized that surface pressuresurface area (π-A) isotherm represents the aggregating process of isolated domains grown right after spreading a solution on the water surface.4-8 On the other hand, phosphatidylcholine9,10 and anionic amphiphile11 formed †
Kyushu University. Saga University. * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, April 1, 1997. ‡
(1) Roberts, G. G. Adv. Phys. 1985, 34, 475. (2) Yuda, E.; Uchida, M.; Oishi, Y.; Kajiyama, T. Rept. Prog. Polym. Phys. Jpn. 1989, 32, 151. (3) Kuri, T.; Honda, N.; Oishi, Y.; Kajiyama, T. Chem. Lett. 1994, 2223. (4) Kajiyama, T.; Oishi, Y.; Uchida, M.; Morotomi, N.; Ishikawa, J.; Tanimoto, Y. Bull. Chem. Soc. Jpn. 1992, 65, 864. (5) Kajiyama, T.; Oishi, Y.; Uchida, M.; Tanimoto, Y.; Kozuru, H. Langmuir 1992, 8, 1563. (6) Uyeda, N.; Takenaka, T.; Aoyama, K.; Matsumoto, M.; Fujiyoshi, Y. Nature 1987, 327, 319. (7) Ho¨nig, D.; Overbeck, G. A.; Mo¨bius, D. Adv. Mater. 1992, 4, 419. (8) Lu, Z.; Nakahara, H. Chem. Lett. 1995, 117. (9) Kajiyama, T.; Takashima, Y.; Kozuru, H.; Oishi, Y.; Suehiro, K. Supramol. Sci. 1995, 2, 107. (10) Albrecht, O.; Gruler, H.; Sackmann, E. J. Phys. 1978, 39, 301. (11) Kajiyama, T.; Zhang, L.; Uchida, M.; Oishi, Y.; Takahara, A. Langmuir 1993, 9, 760.
S0743-7463(96)00215-6 CCC: $14.00
a compressing crystallized monolayer, of which the phase transition from an amorphous state to a crystalline one was caused in a plateau region of π-A isotherm by compression at Tsp below Tm. In the case of the compressing crystallized monolayer, a fairly high surface pressure was required to crystallize the amphiphilic molecules with ionic hydrophilic groups owing to strong repulsion among ionic hydrophilic groups. Therefore, it is reasonable to consider that fatty acid molecules with ionic repulsion among hydrophilic groups in a dissociated state should form the compressing crystallized monolayer in a similar fashion to phosphatidylcholine and anionic amphiphile mentioned above. In this study, the effect of ionic repulsion among hydrophilic groups on the aggregation structure of arachidic acid monolayer on the water surface was investigated on the basis of π-A isotherm measurements, Fourier transform infrared spectroscopic (FT-IR) measurements, and transmission electron microscopic (TEM) observations. Experimental Section Monolayer Preparation. Arachidic acid12 (chromatographic reference quality) was used without further purification. Benzene with spectroscopic quality was used as a spreading solvent. A benzene solution of arachidic acid was prepared with a concen(12) It has been reported that the solubility of the ionized monolayer increases on alkaline subphases (for example: Adam, N. K. Proc. R. Soc. London 1922, A101, 516). Then, the authors have investigated the solubility of fatty acid monolayers on subphase at high pH. The surface pressure of palmitic acid (C16) monolayer at Tsp of 293 K a high pH of 12.6 did not increase over 0 mN m-1 even by compressing to surface area of about 0.1 nm2 molecule-1. In the case of stearic acid (C18) monolayer, the surface pressure increased to 20 mN m-1 at the most. On the other hand, π-A isotherm of arachidic acid (C20) monolayer was reproducible and a general one for monolayer, as shown in Figure 5. These results indicate that ionized monolayer is generally easy to soluble into subphase but that the solubility decrease with the alkyl chain length of amphiphile. This is because the solubility of monolayer depends on balance between a hydrophilic characteristic and a hydrophobic one of the amphiphile. From the above results, arachidic acid molecule was used as monolayer component in this study.
© 1997 American Chemical Society
2528 Langmuir, Vol. 13, No. 9, 1997
Oishi et al.
tration of 2.0 × 10-3 mol L-1. Subphase water was purified by the Milli-QII system (Millipore Co. Ltd.). Arachidic acid monolayers were prepared on the water subphase changing pH from 3.1 to 12.6 (adjusted by addition of NaOH (30 mM for pH 12.6)) at Tsp of 283-323 K. The trough dimensions were 502 × 150 × 5 mm. Every monolayer was compressed to a given surface pressure at an area change rate of 2 × 10-3 nm2 molecule-1 s-1. Modulus Measurement of Monolayer. π-A isotherms were obtained at various Tsp with a microprocessor-controlled film balance system (FSD-20, USI system Co. Ltd.). The static elasticity, KS, of the monolayer on the water surface was evaluated from the slope of π-A isotherm.4 The temperature dependence of the maximum of log KS, log KS(max), was adopted for the determination of the crystalline relaxation temperature, TRc, and the melting temperature, Tm, of the arachidic acid monolayer on the water surface. The log KS(max) corresponds to the rigidity of the monolayer for the case that arachidic acid molecules in the monolayer were packed most densely, and therefore, homogeneous compression force was transmitted through the monolayer. The crystalline relaxation phenomena correspond to a change from elastic to viscoelastic characteristics in a crystalline monolayer due to a remarkable increase in anharmonic thermal molecular vibration.4 Electron Microscopic Observation. The authors reported that the fatty acid monolayers on the water surface could be transferred onto hydrophilic substrate without any change of crystal structure.4 This is because the hydrophilic group of fatty acid molecule is in contact with the hydrophilic substrate surface, and the monolayer-substrate interfacial condition is similar to the monolayer-water interfacial one with respect to the magnitude of interfacial free energy between the hydrophilic group and substrate. The hydrophilic SiO substrate (water contact angle, θ ) 30°) was prepared by vapor-deposition of SiO onto a Formvar-covered electron microscope grid (200 mesh)13 for electron microscopic studies. The monolayer was transferred onto the hydrophilic SiO substrate by the upward drawing method4 at a transfer rate of 1 mm s-1 at various Tsp’s and pressures, except at the surface pressure of 0 mN m-1. The monolayer at 0 mN m-1 can be transferred only by the horizontal lifting method.14 Electron diffraction (ED) patterns were taken with a Hitachi H-7000 electron microscope, which was operated at an acceleration voltage of 75 kV and a beam current of 2.5 µA. The electron beam was 2 µm in diameter. Electron microscopic observations were carried out at a corresponding temperature to Tsp at which the monolayer was prepared. Infrared Measurement. The degree of ionic dissociation of hydrophilic groups was estimated from the absorbances of the stretching vibrations of carbonyl and carboxylate groups on the basis of FT-IR attenuated total reflection (ATR) measurements. According to Lambert-Beer’s rule, the absorbances A for CH2, COO-, and COOH groups, ACH2, ACOO-, and ACOOH, are obtained by eq 1,
ACH2 ) 1NCH2 ACOO- ) 2NCOO-
(1)
ACOOH ) 3NCOOH where 1, 2, 3, NCH2, NCOO-, and NCOOH are molar absorbance coefficients and molar numbers of CH2, COO-, and COOH, respectively. Also, the degree of ionic dissociation, R, for arachidic acid molecules in the monolayer is given by eq 2.
R ) NCOO-/(NCOO- + NCOOH)
(2)
Then, eq 3 can be obtained from eqs 1 and 2.
R ) (1/2)ACOO-/{(1/2)ACOO- + (1/3)ACOOH}
(3)
NCH2 is equals to NCOO- in the case that all carboxylic groups of arachidic acid molecule in the monolayer are in a dissociated (13) Fereshtehkhou, S.; Neuman, R. D.; Ovalle, R. J. Colloid Interface Sci. 1986, 109, 385. (14) Langmuir, I.; Schaefer, V. J. J. Am. Chem. Soc. 1938, 60, 1351. Fukuda, K.; Nakahara, H.; Kato, T. J. Colloid Interface Sci. 1976, 54, 430.
Figure 1. Subphase pH dependences of the degree of ionic dissociation for hydrophilic groups in an arachidic acid monolayer. state, and also NCH2 is equal to NCOOH in the case that all carboxylic groups are in an undissociated state. Based on the above relation, the values of 1/2 and 1/3 can be obtained from eq 4,
1/2 ) A′CH2/A′COO1/3 ) A′′CH2/A′′COOH
(4)
where A′CH2, A′COO-, A′′CH2, A′′COOH are the absorbances for CH2, COO- in a perfect dissociated state, CH2, and COOH in a perfect undissociated state. In this study, it was assumed that the arachidic acid molecules at pH 3.1 were in a perfect undissociated state because the absorbances of COO- at 1430 and 1550 cm-1 were hardly measured and that those at pH 12.6 were in a perfect dissociated state because the absorbance of the CdO stretching band at 1703 cm-1 was not measured at pH 12.6. On the basis of eq 4, the values of 1/2 and 1/3 for A′COO- (1550 cm-1) were evaluated to be 3.01 and 3.34, and also those for A′COO- (1430 cm-1) were evaluated to be 2.02 and 3.34, respectively. Thus, the degree of ionic dissociation was evaluated by substituting the values of 1/2, 1/3, and the measured absorbances into eq 3. In the above evaluation, the absorbance of the asymmetric stretching mode of CH2 (2917 cm-1) was used as a reference value. Infrared spectra were recorded on a Nicolet Model 510 FT spectrophotometer with a resolution of 2 cm-1. Coaddition of 100 scans was used. A Spectra-Tech internal reflection attachment was used for the ATR measurement. Seventy arachidic acid monolayers were transferred on a germanium ATR prism by the vertical dipping method. The monolayer transfer onto the prism was carried out at surface pressures of 25 or 28 mN m-1.
Results and Discussion Subphase pH Dependences of Ionic Dissociation and π-A isotherm. Figure 1 shows the subphase pH dependence of the degree of ionic dissociation of hydrophilic groups in the arachidic acid monolayer at Tsp of 303 K. The degrees of ionic dissociation of hydrophilic groups in the monolayer on the water subphase of pHs ranging from 3.1 to 10.0 were 0 in the case of the use of the absorbance of the COO- asymmetric stretching band at 1550 cm-1 and 0.03-0.06 in the case of the use of the absorbance of the COO- symmetric stretching band at 1430 cm-1. Also, the degrees of ionic dissociation at pH of 11.0 were 0.1 and 0.4 in the cases of the use of absorbances at 1430 and 1550 cm-1, respectively. The degree of ionic dissociation became unity at pH 12.0. It is, therefore, apparent from Figure 1 that the degree of ionic dissociation of hydrophilic groups in the arachidic acid monolayer strikingly increased at a pH range from 10 to 12, although the behavior in the case of 1430 cm-1 is slightly different from that in the case of 1550 cm-1. It is well-known that the pKa for fatty acid molecules in an aqueous solution state is ca. 6. Thus, the value of pKa for arachidic acid molecules in a monolayer state (pKa is ca.11 in this study) is markedly different from that in a solution state. It has been reported that the difference
Fatty Acid Monolayer on the Water Surface
Langmuir, Vol. 13, No. 9, 1997 2529
Figure 2. Subphase pH dependence of the π-A isotherm for an arachidic acid monolayer on the water surface at Tsp of 303 K.
of pKa between monolayer and solution originates from the ionic double layer.15 However, the π-A isotherm of the arachidic acid monolayer on the water subphase containing NaOH (30 mM) and NaCl (100 mM) was almost the same as that containing only NaOH. This indicates a slight effect of ionic double layer on the π-A isotherm and the aggregation structure. Moreover, the pKa difference, originating from CO2 adsorption from the atmosphere due to subphase without the buffer solution in this study, may be small. Hence, the large pKa difference cannot be explained by the effects of ionic double layer and/or CO2 adsorption. Recently, a quantum chemical calculation with the dielectric model revealed that the hydrophilic group of an amphiphile in monolayer was difficult to dissociate owing to a screening effect of twodimensional aggregation of amphiphiles at the air-water interface.16 The screening effect may be one reason for the large pKa difference. Anyway, the reason for the large pKa difference between a monolayer state and a solution one remains unclear at the present stage. Figure 2 shows the subphase pH dependence of the π-A isotherm for the arachidic acid monolayer at Tsp of 303 K. The π-A isotherms of the monolayers at pHs 3.3-9.6 were almost same; that is, the surface pressure abruptly increased at around 0.25 nm2 molecule-1 with a decrease in the surface area. On the other hand, in the case of pH above 11, the plateau region was observed on the π-A isotherms, and also the surface pressure at the plateau region increased with an increase in pH. It is apparent from Figures 1 and 2 that the plateau region on the π-A isotherm started to appear when the degree of ionic dissociation of hydrophilic groups in the monolayer increased remarkably. The drastic change of the π-A isotherm of the arachidic acid monolayer on the water surface at the pKa evaluated by FT-ATR-IR spectroscopic measurements of the transferred multilayered films on substrate indicates that the pH dependence of monolayer structure on the water surface is maintained on the substrate, which deserves special emphasis. Moreover, the increase of surface pressure at the plateau region corresponded to that of the degree of ionic dissociation of hydrophilic groups in the monolayer. These results indicate that the degree of ionic dissociation of hydrophilic groups in the arachidic acid monolayer, that is, the ionic repulsion among hydrophilic groups, causes the appearance of the plateau region on the π-A isotherm, resulting in a change in the aggregation structure of the monolayer on the water surface. Effect of Ionic Repulsion among Hydrophilic Groups on the Aggregation Structure of Arachidic Acid Monolayer. As mentioned in Figures 1 and 2, (15) Gaines, G. L. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966. (16) Sakurai, M.; Tamagawa, H.; Furuki, T.; Inoue, Y.; Ariga, K.; Kunitake, T. Chem. Lett. 1995, 1001.
Figure 3. π-A isotherms and ED patterns of arachidic acid monolayers at Tsp of 303 K on the water subphase of pH 5.8 (a) and pH 12.6 (b).
almost 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 3 show the π-A isotherms for the arachidic acid monolayers on the water subphase of pH 5.8 (pure water) and of pH 12.6 at Tsp of 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 in the monolayer on the water subphase of pH 5.8 (Figure 3a), the π-A isotherm showed a sharp rise of surface pressure with decreasing surface area without any appearance of plateau region. The ED patterns at surface pressures of 0 and 25 mN m-1 showed a crystalline arc and crystalline spot, respectively, indicating the formation of “the crystalline monolayer”.4,5,17 Kjaer et al.18 also reported from synchrotron X-ray diffraction studies that the arachidic acid monolayer on the pure water surface was in a crystalline phase at various surface pressures. The change of the ED pattern from the crystalline arc to the crystalline spot indicates that randomly oriented crystalline domains were fused or recrystallized on the interface of monolayer domains owing to sintering behavior caused by surface compression, resulting in the formation of larger twodimensional crystalline domains.4 In the case of a dissociated state of arachidic acid on the water subphase of pH 12.6 (Figure 3b), a plateau region of the π-A isotherm was observed in a surface area range of 0.3-0.5 (17) The aggregation state corresponding to the crystalline monolayer which was designated in this study has been classified in detail into S, L2, LS and so on: Kenn, R. M.; Bo¨hm, C.; Bibo, A. M.; Peterson, I. R.; Mo¨hwald, H.; Als-Nielsen, J.; Kjaer, K. J. Phys. Chem. 1991, 95, 2092. (18) Kjaer, K.; Als-Nielsen, J.; Helm, C. A.; Tippman-Krayer, P.; Mo¨hwald, H. J. Phys. Chem. 1989, 93, 3200.
2530 Langmuir, Vol. 13, No. 9, 1997
Oishi et al.
Figure 5. π-A isotherms of arachidic acid monolayers at different Tsp’s on the water subphase of pH 12.6.
Figure 4. Tsp dependences of log KS(max) and ED patterns of arachidic acid monolayers on the water subphase of pH 12.6.
nm2 molecule-1. The ED pattern at 5 mN m-1 showed an amorphous halo, whereas those at 12 and 28 mN m-1 exhibited crystalline arc and spot, respectively. Therefore, Figure 3b 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 monolayer” or “pressure-induced crystallized monolayer”.9,11 It is clearly concluded from Figures 1-3 that amphiphilic molecules are assembled in the crystalline monolayer and the compressing crystallized monolayer at Tsp below Tm in the cases of a neutral state (maybe the low degree of ionic dissociation) and a highly dissociated state of polar groups, respectively. Aggregation Structure of Arachidic Acid Monolayer in a Dissociated State. The magnitudes of TRc and Tm for fatty acid monolayers in a neutral state on the pure water surface were evaluated on the basis of the Tsp dependences of both monolayer modulus and the ED pattern of the monolayer.4 Tm of the monolayer was evaluated as the temperature at which both an apparent decrease in the Tsp vs modulus curve and the change of the ED pattern from a crystalline Debye ring to an amorphous halo were observed. Also, TRc was evaluated as the temperature at which both an apparent decrease in the Tsp vs modulus curve and the break in the thermal expansion coefficient of crystalline lattice constant were observed. The fatty acid monolayers on the pure water surface were systematically classified on the basis of Tm and TRc, that is, the fusing-oriented crystalline monolayer for Tsp < TRc < Tm, the randomly assembled crystalline one for TRc < Tsp < Tm, and the amorphous one for Tsp > Tm.4 Further, recent Brewster angle microscopic observations support the above evaluation method of TRc and Tm of fatty acid monolayers. In the same manner, the magnitudes of Tm and TRc for the arachidic acid monolayer in a dissociated state on the water subphase of pH 12.6 were evaluated, and also, the aggregation structure of the monolayer during a process of surface compression was investigated on the basis of TEM observations. Figure 4 shows the Tsp dependence of log KS(max) for the arachidic acid monolayer on the water subphase of pH 12.6 (obtained from the π-A isotherms of the arachidic acid monolayers at different Tsp’s, as shown in Figure 5) and the ED patterns of the monolayer transferred onto the hydrophilic SiO substrate at surface pressures of 28 or 32 mN m-1. The magnitude of log KS(max) started to decrease apparently at ca. 305 and 322 K. The ED patterns at 303 and 321 K were a crystalline spot and a crystalline
Figure 6. π-A isotherm and ED patterns of arachidic acid monolayer at Tsp of 323 K on the water subphase of pH 12.6.
Debye ring, respectively. Hence, it seems reasonable to conclude that the slight decrease of log KS(max) at around 305 K corresponds to the crystalline relaxation behavior of the monolayer as reported for the various fatty acid monolayers.4 Since the ED pattern at 323 K was an amorphous halo, a fairly remarkable decrease of log KS(max) at around 322 K might correspond to the melting behavior of the monolayer. Then, TRc and Tm of the arachidic acid monolayer on the water subphase of pH 12.6 were determined to be 305 and 322 K, respectively. As shown in Figure 3b, the ED pattern of the monolayer at Tsp of 303 K below TRc changed from an amorphous halo to a crystalline spot with an increase of surface pressure. This indicates that the large crystallized monolayer was formed, maybe due to sintering induced by compression. Whereas, in the case of Tsp above TRc and below Tm, the ED pattern changed from an amorphous halo to a crystalline Debye ring, indicating that the crystallized monolayer domains did not sinter at the domain interfacial region owing to an sufficient thermal molecular motion. Figure 6 shows the π-A isotherm for the arachidic acid monolayer on the water subphase of pH 12.6 at 323 K above Tm of the monolayer, as well as the ED patterns of the monolayers at two different surface pressures. The π-A isotherm exhibited a monotonous increase in surface pressure with a plateau zone during compression, and the ED patterns at different surface pressures were an amorphous halo. At Tsp above Tm, the monolayer was in an amorphous state even though in a higher surface pressure range owing to fairly active thermal molecular motion in the monolayer domain. Classification of Aggregation Structure of Fatty Acid Monolayer on the Water Surface. Figure 7 shows the newly proposed classification of fatty acid monolayers based on the molecular aggregation state on the water
Fatty Acid Monolayer on the Water Surface
Langmuir, Vol. 13, No. 9, 1997 2531
Figure 7. Classification for aggregation structure of fatty acid monolayer on the water surface.
surface with respect to thermal (Tsp, Tm) and chemical (the degree of ionic dissociation of hydrophilic group) factors. This figure is divided into the four quadrants by the two axes of Tsp and the repulsive force among ionic hydrophilic groups. In the case of amphiphiles with nonionic hydrophilic group (corresponding to the third and fourth quadrants of Figure 7), amphiphiles contact each other right after spreading a solution on the water surface owing to their self-assembling characteristics, resulting in the formation of isolated crystalline or amorphous domains.5 Especially in the case of Tsp < TRc, these domains were easily gathered to form a morphologically homogeneous monolayer by surface compression, as shown in Figure 3a. Then, at Tsp below Tm (the third quadrant), the monolayer is in a crystalline phase, that was designated as “the crystalline monolayer”. The crystalline monolayer was further classified into two types: the crystalline domains are assembled as a large homogeneous crystalline monolayer due to a surface compression-induced sintering at interfacial region among monolayer domains at Tsp below TRc, and also the crystalline domains are gathered without any special crystallographic orientation among domains at Tsp above TRc, due to sufficient thermal molecular motion at the domain interface. They were designated as a “fusingoriented crystalline monolayer” and a “randomly assembled crystalline monolayer”, respectively. At Tsp above Tm (the fourth quadrant), the monolayer is in an amorphous state that was designated as “the amorphous monolayer”. The amorphous monolayer cannot be crystallized by compression. This may be due to the monolayer collapse before reaching the surface pressure at which the monolayer can be crystallized at a given Tsp. In other words, the monolayer is not in a thermodynamically closed system because the monolayer collapse corresponds to a mass transfer outside the thermodynamic system. This is a reason why the aggregation structure of the monolayer during compression on the water surface cannot be speculated from the point of view of the compression process of a three-dimensional gas which is in a thermodynamically closed system. In the case of amphiphiles with ionic hydrophilic groups (the first and second quadrants of Figure 7), distinct monolayer domains are not formed at lower surface
pressure owing to an electrostatic repulsion among polar head groups. At Tsp below Tm (the second quadrant), though the monolayer cannot be crystallized at very low surface pressure owing to an electrostatic repulsion among polar head groups, the monolayer can be crystallized by a fairly strong compression because the mutual approach among amphiphilic molecules becomes sufficiently close to crystallize together, overcoming their electrostatic repulsive forces. This aggregation type of the monolayer was designated as “the compressing crystallized monolayer”. On the other hand, at Tsp above Tm (the first quadrant), the monolayer is not crystallized by compression owing to a fairly active thermal molecular motion even though at a fairly large surface pressure. The shapes of the π-A isotherms for the monolayers in an undissociated state at Tsp above Tm (the amorphous monolayer in the fourth quadrant) and in a dissociated state at Tsp below Tm (the compressing crystallized monolayer in the second quadrant) are similar to each other, to exhibit the plateau region, as shown in Figure 7. However, each molecular aggregation process in the monolayers during compression is different as follows. In the case of the amorphous monolayer, the fraction of trans conformation increased in the plateau region during the monolayer compression.5,19 On the other hand, in the case of the compressing crystallized monolayer, the phase transition from an amorphous state to a crystalline one occurs in the plateau region during the compression. An appearance of the plateau region on the π-A isotherm may be due to consumption of compression energy for the conformational regularization of alkyl chain (the amorphous monolayer) or the phase transition from an amorphous state to a crystalline one (the compressing crystallized monolayer). Then, it is reasonable to conclude that the aggregation structure of monolayers on the water surface during compression cannot be examined only by the shape of the π-A isotherm. (19) The authors have broadly classified the aggregation state of monolayer on the water surface into a crystalline state and an amorphous one. In the case of this broad classification, the phase transition from an amorphous state to a crystalline one did not occur during compression of “the amorphous monolayer (the fourth quadrant)”. However, a certain phase transition in an amorphous state, for example, liquid-liquid crystal transition may be occurred at the plateau region on the π-A isotherm for the amorphous monolayer (as described in ref 5).
2532 Langmuir, Vol. 13, No. 9, 1997
Conclusion The aggregation structure of the arachidic acid monolayer on the water surface was strongly dependent on the degree of ionic dissociation for the hydrophilic groups of arachidic acid molecules in the monolayer. The fatty acid monolayer was systematically classified into a “crystalline monolayer” and an “amorphous monolayer” in the case of an undissociated state and also “compressing crystallized
Oishi et al.
monolayer” and “amorphous monolayer”in the case of a dissociated state. The aggregation structure of the monolayer during compression on the water surface cannot be speculated by the shape of the π-A isotherm from the point of view of the compression process of a threedimensional gas because the monolayer on the water surface is not in a thermodynamically closed system. LA960215F
