Langmuir 1992,8, 1806-1810
1806
Monolayer Properties of Octadecylammonium Perfluoroalkanoates Osamu Shibata,*l+Yoshikiyo Moroi,t Masahiko Saito,o and Ryohei Matuurat Laboratory of Chemistry, College of General Education, Kyushu University-01, Ropponmatsu, Fukuoka 810, Japan, Department of Chemistry, Faculty of Science, Kyushu University-33, Higashi-ku, Fukuoka 812, Japan, and Department of Chemical Science, Faculty of Education, Yamaguchi University, Yamaguchi 753, Japan Received December 10, 1991. In Final Form: April 7, 1992
Surface pressure (TI- and surface potential (AW-area (A) isotherms for monolayers of octadecylammonium perfluoroalkanoatesof six different perfluoroalkyl chains on water and on 4.4 M NaCl solutions were measured at various temperatures by the Langmuir method and the ionizing electrode method. Some T-A curves of these long-hydro-short-fluorosalts showed three phase transition points. Judging from the surfacepotentialand the apparent molar quantity together with our previous data on the transitions, the first was assigned to the transition from the expanded (E) phase to the condensed ((2-1) phase, the second from the C-I phase to another condensed (C-1’)phase, and the third from the C-I’ phase to another condensed ((2-11)phase. There exists a triple point on the phase diagram at which three phases (E, C-I, and C-1’) coexist. In this paper, negative inclination of transition pressure (C-I C-1’) against temperature was newly observed over a certain temperature range on the phase diagram. From the surface potential, the apparent molar entropy change, and phase diagram, the orientation of the long-hydroshort-fluoro-chainsalts in the monolayer state was also discussed.
-
Introduction The physicochemical properties of ionic surfactants which are composed of a surfactant ion and counterion are strongly influenced by the kind of surfactnt ion and counterion. Counterions, in particular, are more influential on their solid state and dissolved state, and their importance has been made Counterions of conventional surfactants so far investigated have mainly been inorganic alkali- or alkaline-earth-metal ions. In several years, anionic surfactants4 with nonmetallic cationic counterions of nonconcentrated, diffuse or separate, charges were found to have physicochemical properties much different from those of conventional surfactants. Moreover, ionic surfactants with a perfluorocarbon counterion5instead of a hydrocarbon counterion were also found to have physicochemical properties much different from those of ionic conventional hydrocarbon surfactants. These salts become hardly soluble in water when the chain of the surfactant ion is very long. However, the monolayer of the salts has not previously been studied. Many studies have been made on the adsorbed film at the air/water interfaces from the mixtures of cationic and anionic surfactant solution^.^^^^^ Mixed monolayers of + Kyushu University-01. t
Kyushu University-33.
i Yamaguchi University.
(1)Moroi, Y.; Ikeda, N.; Matuura R. J . Colloid Interface Sci. 1984, 101, 285. (2) Moroi, Y.; Sugii, R.; Akine, C.: Matuura. R. J . Colloid Interface Sci. 1985,108, 180. (3) Matuura, R.; Moroi. Y.: Ikeda, N. Proceedinas of the 5th International Congress on Surfactante in Solution, Bordiaux, France, 1984. (4) Moroi, Y.; Matuura, R.; Kuwamura, T.; Inokuma, S. J . Colloid Interface Sci. 1986, 113, 225. (5) Sugihara, G.; Nagadome, S.; Yamashita, T.; Kawachi, N.; Takagi,
H.; Moroi, Y. Colloids Surf., in press. (6) Corkill, J. M.; Goodman, J. F.; Ogden, C. P.; Tate, J. R. Proc. R. SOC.London, A 1963,273,84. (7) Corkill, J. M.; Goodman, J. F.; Harrold, S. P.; Tate, J. R. Trans. Faraday Soc. 1966,62,994. (8)Schwuger, M. J. Kolloid 2.2.Polym. 1971, 243, 129. (9) Lucassen-Reyndera, E. H. Kolloid 2.2.Polym. 1972, 250, 356. (10) Lucassen-Reynders, E. H.; Lucassen, J.; Giles, D. J. Colloid Interface Sci. 1981, 81, 150. (11) Lucassen-Reynders, E. H. J . Colloid Interface Sci. 1982,85,178.
long-chain anions and cations have been studied by Corkill et al., Hendrix, and others,18.22and mixed monolayers of fluorocarbon and hydrocarbon surfactants have been studied by Zhang et al.23 We have recently investigated surface chemical properties of the insoluble monolayers of double long-hydrocarbon-chain salts and triple longhydrocarbon chain salts on water and/or electrolyte s o l ~ t i o n s ~and - ~ properties ~ ~ ~ ~ of the planar bilayer membrane of triple long-hydrocarbon-chain salts.BJC’ Taking into account the above bulk properties of surfactants with a perfluorocarbon counterion, it is interesting to compare the three-dimensional bulk properties with the two-dimensional surface properties of such surfactants. For further investigation on the monolayer properties, it is necessary to compare them with several types of long-hydrocarbon-chain salts a t the same experimental conditions, i.e., on 4.4 M NaCl solution. (12) Goralczyk, D. J . Colloid Interfacr Sci. 1980, 77,68.
(13) Gbralczyk, D.; Waligora, B. J..Colloid Interface Sci. 1981,82,1. (14) Rodakiewicz-Nowak. J . Collord Interface Sci. 1982,85, 586. (15) Seplilveda, L.; PBrez-cotap, J. J . Colloid Interface Sci. 1986, 109, 21. (16) Gu, B.; Rosen, M. J. J . Colloid Interface Sci. 1989,129, 537. (17)Yu, Z.-J.; Zhao, G.-X. J . Colloid Interface Sci. 1989, 130, 414; 1989.130. 421.
(18)Corkill, J. M.; Goodman, J. F.; Harrold, S. P.; Tate, J. R. Trans. Faraday SOC.1967, 63, 247. (19) Hendrix, Y.; Ter-Minaeaian-Saraga, L. Monolayers; Goddard, E. D., Ed.; Advances in Chemistry Series 144; American Chemical Society: Washington, DC, 1975; pp 177-191. (20) McGregor, M. A.; Barnes, G. T. J . Colloid Interface Sci. 1977,62, 0.0
LLJ.
(21) Hendrix, Y. J . Colloid Interface Sci. 1979, 69, 493. (22) Hendrix, Y.; Mari, D. J . Colloid Interface Sci. 1980, 78, 74. (23) Zhang, L.-H.; Zhu, B.-Y.; Zhao,G.-X. J . Colloid Interface Sci. 1991, 144,483; 1991,144,491. (24) Shibata, 0.;Kaneshina, S.; Nakamura, M.; Matuura, R. J . Colloid Interface Sci. 1980, 77,182. (25) Shibata, 0.;Kaneshina,S.; Nakamura, M.; Matuura,R. Bull. Chem. SOC.Jpn. 1982,55,2243. (26) Shibata, 0.;Kaneshina, S.; Nakamura, M.; Matuura, R. J . Colloid Interface Sci. 1983, 95, 87. (27) Shibata, 0.J. Colloid Interface Sci. 1983, 96, 182. (28) Shibata, 0.;Kaneshina, S.;Matuura, R. Bull. Chem. SOC.Jpn. 1988,61,3077. (29) Shibata, 0.; Eicke, H.-F. Colloid Polym. Sci. 1990,268, 65. (30) Shibata, 0.; Moroi, Y.; Saito, M.; Matuura, R. J . Colloid Interface Sci. 1991, 142, 535.
0 1992 American Chemical Society
Alkylammonium Perfluoroalkanoate Monolayers
In the present study then, octadecylammonium perfluoroalkanoates (which are denoted as C18zn+1,whose n is the number of carbon atoms in the fluorinated counterion) were prepared, and the surface pressures and the surface potentials of their monolayers were measured as a function of molecular area at various temperatures. The sodium chloride concentration of the substrate was fixed at 4.4 M throughout the experiments. The phase types in the monolayer states could be judged from the surface potential, the phase diagrams, and the thermodynamic quantities of the phase transition in the previous s t ~ d i e s . ~ ~ ~ ~ ~ ~ ~
Langmuir, Vol. 8, No. 7, 1992 1807
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30 a t 298.21: o n pure water
.20
Experimental Section Starting materials of octadecylammoniumperfluoroalkanoates were octadecylamine and perfluoroalkanoic acid (CnFzn+lCOOH, n = 2-4,6-8). The former obtained from Tokyo Kasei Ltd. was purified by vacuum distillation. The purity was checked by gas chromatography showing a single peak. Perfluoroalkanoic acids were purchased from several companies (n = 2 and 7 from Tokyo Kasei Ltd., n = 3 from Nacalai Tesque, n = 6 from Aldrich Chemical Co., and n = 8 from Fluoro Chemicals Ltd.). Perfluoropentanoic acid (n = 4)was a kind gift from Japan Halon Co. Ltd. The octadecylammonium perfluoroalkanoates were prepared by mixing equimolaroctadecylamineand perfluoroalkanoic acid in ethanol solution and then purified by recrystallization three times from a water/ethanol mixture. The purities of these surfactanta were checked by elemental analysis; the observed and calculated values were in satisfactory agreement (*0.3% ). Each salt was spread at the air/aqueous interface from ita hexanel ethanol solution (9/1 v/v, the former solvent from Merck, Uvasol, and the latter from Nacalai Tesque). The substrate solution of 4.4 M sodium chloride (Nacalai Tesque) was prepared using triple-distilled water. Sodium chloridewas roasted at 973 K to remove any surface-activeorganic impurities. The surface pressure was measured by an automated Langmuir film balance, which was the same used in the previous studied.u-2a.90The monolayer was compressed at a speed of 2.00 X 10-1 nm2 molecule-' min-l, because no influence of the compression rate (at 6.6 X 10-2,1.00 X W, and 2.00 X 10-l nmz molecule-' mi+) could be detected within the limits of this experimental error. The surface potential was also recorded while the monolayer was compressed. They were monitored using an ionizing ulAm electrode at 1-2 mm above the interface and a reference electrode dipped in the subphase. The standard deviation of the surface pressure-area isotherm was approximately 1X 10-2 nm2,while that for the surface potential was f10 mv. Other experimental conditions were the same as described in a previous papersg0
Results and Discussion The shorter chain compounds with n < 6 did not make stable monolayers on pure water. The P A and AV-A curves obtained for C18zn+lsalts at 298.2 K on pure water are shown in Figure 1. Numerical values in Figure 1refer to the number of carbon atoms in the perfluorocarbon chain. As expected from the chemical structures, all the P A curves show the expanded type. As seen from Figure lA, the P A curve of C1813 ( n = 6) is shifted to a smaller area compared with those of the other two salts at high pressure. This means that the Cia13 salt seems soluble toward the subphase. But the extrapolated areas Of C1813 ( n = 61, CleF16 ( n = 71, and Cl81, ( n = 8) salts are approximatelysame. The collapse pressures of these salts are around 37 mN m-l. The change of AV of these salts is rather small over the measured area. The longer the perfluorocarbon chain of the counterion,the more negative the value of AV at constant mean area per molecule. It results from the properties of f l u o r ~ c a r b o n . ~ lIn - ~the ~
0.5
0
1.0 A
1.5
/ nm2
I 100
0
0. 7
-i 2
.1 00
I0 I
0
I
1.0
0.5
A
I
1.5
'
-200
nm2
Figure 1. (A) Surface pressure (*)-area (A) isotherms and surface potential (Am-area (A) isotherms of Cl&b' +l salts on pure water at 298.2 K 6, ClaFla; 7, CleF15; 8, CleF1,. (B)Surface dipole moment (rd-area (A) isotherms of the same system.
case of n = 6, there appears a peak in the AV-A curve (or pL-A curve), which is experimentally reproducible. This suggests that the conformational change takes place on the monolayer state at this combination of hydrocarbon chain and perfluorocarbon chain, although what kind of conformational change is occurring is uncertain. The pl-A behavior also shows the same trend. The concentration of NaCl in the substrate was held constant at 4.4 M throughout the experiments in order to avoid dissolution of the film-forming material toward the subphase and to compare the present phase diagram with the previous phase diagrams of the five kinds of salts: [CmN(CH3)-CnS03, CmNH3-CnS03, CmNH3-Cn1C00, BCnNCnS, and Cl4BP(Cn)21 (refer to refs 27,28, and 30 for abbreviations). Figure 2A show the P A curves of C18zn+lsalts at 298.2 K on 4.4 M NaC1. The shorter the chain length of the counterions, the smaller the value of the mean area per molecule at constant pressure, except for n = 8. Figure 2A might suggest that the P A isotherms can be classified into three types. The first is for n = 7 and 8, where both hydrocarbon chain and perfluorocarbon chain are perpendicular to the substrate surface. The second is for n = 2-4, where the perfluorocarbon chain and the hydrocarbon chain are aligned with the former dipping in the Substrate. The third is for n = 6 which is an intermediate type between the first and the second types. That is, at higher molecular area, both hydrophobic chains are perpendicular to the substrate like the first type, while the fluorocarbon chain moves down into the substrate by compression like the second type. (31)Fox, H.W.J. Phys. Chem. 1957,61, 1058. (32)Bernett, M.K.;Zieman, W.A. J. Phys. Chem. 1968,67, 1534. (33)Bernett, M.K.;Jarvie, N.L.;Ziman, W.A. J. Phys. Chem. 1964, 68,3520.
1808 Langmuir, Vol. 8,No.7, 1992
Shibata et al. 400
ii
a t 298.2 K
P 400
300
200
t
. >
100 a
0
50
n ‘ \
c
8
,100
0
300
-100
200
I
50
. 0
-
100 E
40
4 2
E
.
30
0
20
-100
L
\
10
C18NH3C1 I
4
0 0.2
1 .o
0.5 A
I
0.2
1.4
nm2
A
B 200
. 0
100
E
-I
2
0
-100
\ 1.o
0.5 A
I
I
J
I
-200
1.4
nm2
Figure 3. P A isotherms, AV-A isotherms,and pl-A isotherms of Cl,$’zn+1 (n = 4) salts and octadecylammoniumchloride (CISNH3Cl) on 4.4 M NaCl solution at 298.2 K: A; surface pressurearea, B; surface potential-area, C; surface dipole moment-area.
300
0.2
1.o
0.5
n-A
-200
1.4
nm2
Figure 2. (A) T-A isotherms and AV-A isotherms of Cl$h+l salts on 4.4 M NaCl solution at 298.2 K 2, C$6; 3, C1$,; 4, CI$B; 6, Cl$13; 7, ClgFl~;8, c&17. (B)Surface dipole moment ( p l ) isotherms of the same system. It can be seen from Figure 1 and 2A that the T-A isotherms of C1813 (n = 6) on 4.4 M NaCl electrolyte are shifted to a small area compared with that on pure water. So there comes the question whether the Cla2,,+1 compounds make a stable monolayer on 4.4M NaCl subphase or not. The hydrophilic anion of C&zn+l salt has a possibility to be replaced by the C1- in 4.4 M NaCl subphase solution. If this were the case, the *-A isotherm would be shifted to a smaller area. To make sure of the monolayer behavior of C182n+l salts, the monolayer test of octadecylammonium chloride were done on 4.4M NaCl subphase. Typical examples of the monolayer behavior of C182n+l (n= 4)and octadecylammoniumchloride (CiaNHsC1) are shown in Figure 3. As for the F A isotherm of C1$2,+1 (n = 4)and CleNH3C1, there is a big difference in the r A curve between them. For example, the number of transition pressures of the C$2n+1 (n= 4)salt is found
to be three, and on the contrary, that of ClaNHaCl is two. And ClaNH3Cl salt has a relatively high initial pressure at a higher area. There is also a big difference in surface potential and surface dipole moment behaviors between CIS9 (n = 4) and ClaNH3C1. The surface potential of Cl89 is found to be larger by about 100 mV than that of ClsNH3C1 a t the molecular area larger than 0.5 nm2. In addition, the AV-A curve of cl89 salt has three inflection points which correspond to the transition pressures on the T-A curve, while that of ClaNH3C1 is smooth. The surface dipole moment of C18NH3Cl steeply decreases by around 400 mD with increasing A , while that of C189 fluctuates within 100 mD. This big difference between them must originate from the counterion perfluoroalkanoate anion. Then, it is quite clear that the anionic part of the C182n+l salt is not replaced by the C1- ion of 4.4 M electrolyte subphase at the monolayer. As seen from Figure 2A,the T-A curves of n = 2-4 have two or three transition points at 298.2 K. Since the transition pressures remain constant irrespective of the compression rate, all the transition points on the r A curves can be regarded as equilibrium points between two phases on the monolayer. The phase transition pressures (e) change widely with varying the chain length of the (n counterions. Especially, the T-A curves of the C 1 8 ~ (n = 3 or 4)salts revealed two and = 2) salt and three distinct break points, respectively. These two or three points, however, have turned out to be different phase transitions in the condensed phase. The first break point a t a lower surface pressure of the T-A curve for the Cl$5 salt is due to the transition from the expanded (E) phase to the condensed film, while the second break at a higher surface pressure is the transition from a condensed film to another condensed film, as described in previous
Alkylammonium Perfluoroalkanoate Monolayers 50G,
I
I
,
I
C18Fg ( n
,
I
I
Langmuir, Vol. 8, No. 7, 1992 1809 40
4 )
on 4.4M NaCl
1 : 293.2 K
1
2 : 298.2 K 3: 303.2
. c
c - I ’
K
10
20
0 290
10
0
310 290
L 1 .o
0.5
A
/
“In2
Figure 4. Temperaturedependence of P A isotherms of C1@%+1 ( n = 4) salts. The substrate is 4.4 M NaCl solution.
p a p e r ~ . ~These ~ J ~ two condensed states are referred to the condensed phase I’ and the condensed phase 11, respectively, in the same manner as bef~re.~~aO On the other hand, judging from the previous phase diagrams, the break point at the lowest surface pressure of Cl87 (n = 3)and C189 (n= 4)salt can be assigned to the transition from the expanded (E) phase to the condensed phase I (C-I) which is different from the condensed phase I’ (C-1’) on the monolayer phase diagram. The second break point at a higher surface pressure is regarded as the transition from the condensed phase I (GI) to the condensed phase I’ (C-I’), which can also be assigned by the aid of thermodynamic quantities and the phase diagrams as mentioned later. The third transition at the highest surface pressure is from the condensed phase I’ ((2-1’) to the condensed phase I1 (C-11). On the other hand, the P A isotherms of Cl82n+l (n= 6-8) have no transition points, which is characteristic of the expanded type isotherms. As seen from Figure 2A,the surface potential measurement also clarifies the behavior of the above phase transitions. That is, a change in the inclination of AV vs the molecular area curve corresponds to the transition pressure in the *-A curve. In all cases where an inflection point appears in the T-A isotherms, a corresponding inflection point also appears in the AV-A isotherm. However, it is noteworthy that this is not the case for the pl-A isotherm (Figure 2B). Namely, a sudden change in the slope of the AV vs A curve is apparent for these salts, but the change of the corresponding pl-A isotherm is not so sharp as that of the AV-A isotherm. The molecular area at the inflection points of the AV-A isotherms for C 1 8 (n ~ = 2),( 2 1 8 7 (n= 3),and Cl8g (n= 4)salts coincides with the area at the inflection points of the u-A isotherm within less than 0.3 nm2. The temperature effects on the transition pressure in the monolayers are of much interest, since they provides us with thermodynamic information on a phase transition of monolayers. Figure 4 shows, for example, the u-A isothermsof Cl8g (n= 4)at threetemperatures. However, to avoid the overlapping with the P A isotherms at some other temperatures, they are not presented in Figure 4. Three break points are observed on the T-A isotherm at different temperatures. As was expected, the transition pressures increased with an increase in the temperature.
310 2 9 0
300 T
-
0.2
300
/
300
310
K
Figure 5. Transition pressure (+)-temperature diagrams for the monolayers of C136 ( n = 2), ClaF, (n = 3), and C 1 8 0 ( n = 4) salts on 4.4 M NaCl solution.
Table I. Apparent Molar Quantity Changes on the Phase Transition of Alkylammonium Perfluoroalkanoatee at 298.2 IC (on 4.4 M NaCl) -10-2ASt/(J K-1 mol-’) Clfizn+l transition n =2 n=3 n=4 E C-I 1.04 (A0.03) 1.4 (i0.05) E C-I‘ 0.7 (AO.1) c-I c-I’ -0.075 (A0.008) -0.09 (A0.005) C-I’ C-I1 0.03 (A0.002) 0.07 (AO.01) 0.41 (10.05)
--.
4 4
Cl$%+l transition
E C-I E C-I’ c-I c-I’ c-I’ c-I1
--
n =2
+
-Aur/(kJ mol-’) n=3
n=4
41 ( i l )
51 (A2)
0.4 (a0.04) 3.5 (A0.5)
1.7 (A0.07) 15 (il)
+
30 (i4) 3.5 ( i 0 . 3 )
However, the middle transition points were observed to decrease with temperature, which is opposite of the other two. In Figure 5 are illustrated the two types of phase diagrams for C182,+1 (n = 2-4) salts from the transition pressure (uw)-temperature relation, where the slope of +q against temperature is a key point to assigning the phases. The thermodynamic quantities on the phase transition of monolayers were calculated by the same method as described in our previous s t u d y . 2 4 - ~The ~ ~apparent molar entropy change ( M y ) on the phase transition was evaluated by employing eq 29 of ref 34:
AF = (aC- ae)[(6uW/6T),- (6y0/6~,1
(1) In this equation, ASY is the apparent molar entropy change, uc and ae are the molecular area (in square nanometers; superscripts c and e refer to the condensed and expanded states, respectively), +q is the transition pressure from the expanded to the condensed state, and yois the surface tension of the substrate. The apparent molar entropy changes on the various types of phase transitions are given in Table I for C&zn+l salts. As seen in Table I, the first transition includes two kinds of transitions: one from the expanded phase (E) to the condensed phase I ((2-1) and the other from the expanded phase (E) to the condensed phase I’ (C-1’). The phase transition for the C182n+l ( n = 2) salt at 298.2 K can be classifiedas the transition from the expanded phase (E)to the condensed phase I’ (C-1’) by the aid of the phase diagram (Figure 5): the slope of vs Tcurves and surface potential behavior. In this case, surfacepotential behavior (34) Motomura, K.;
Yano,T.; Ikematau, M.; Matuo, H.; Matuura, R.
J. Colloid Interface Sci. 1979, 69, 209.
1810 Langmuir, Vol. 8, No. 7, 1992
Shibata et al.
is also the key point to classifying the type of the phase (Figure 2). The phase transition for the C182,,+1 (n = 3 and 4) salta a t 298.2 K can be classified as the transition from the expanded phase (E) to the condensed phase I (C-I) by the aid of the phase diagram (Figure 5 ) and the slope of f l vs T curves. The longer the perfluorocarbon chain length of the salt, the larger the apparent molar entropy on the transition from their expanded to their condensed states. The apparent molar entropy and the apparent molar energy changes on the different types of the phase transition are also given in Table I. Equation used for the energy change is AuY = - ( T ~- ro)(ac- a”) + TGS (2) which corresponds to eq 32 of ref 34. The second transition includes two kinds: C-I C-I’ and C-I’ (2-11. The apparent molar entropy change of the second phase transition decreased in magnitude by about 1 order, compared with that of the first phase transitions (E C-I and E C-1’). The columns of C#zn+1 (n = 3 and 4) salts have three numerical values for the corresponding phase transitions. Especially in these cases, the assignment of the type of phase transition is difficult only from the apparent molar entropy change.2s@ The values of the apparent molar entropy changes are too small to compare with those of other phase transitions. So, the phase transitions for these salts were classified from the slopes of ?reg vs T curves in Figure 5 and from the surface potential behavior in Figure 2. But in this case, the negative value of the apparent molar entropy change reflects the difference in the type of the phase transition between the C-I C-I’ and C-I’ C-11. As seen from Table I, the longer the perfluoro chain of the counterions, the larger in negative magnitude the apparent molar entropy change on the phase transition at the second transition (C-I C-1’). The larger entropy change on the first transition (Le., E C-I and E C-1’) suggests the considerable change of the molecular orientation in the monolayers, whereas the small entropy change on the second transition (i.e., C-I- C-I’ and C-1’- C-11)suggests the slight change in the monolayer configuration. In these salt systems, the phase transition from the C-I phase to the C-I1 phase could not be observed. Especially, the r e q vs T curve for the transition from the C-I phase to the C-I’ phase has a negative slope. This might suggest that the phase transition is accompanied by a positive entropy change, which leads us to the conclusion that the monolayer becomes less ordered as a whole. As for the values
-
-
-
-
-
- -
-
-
of the apparent molar energy change (Auy) in Table I, it can be said that the second entropy term of eq 2 has a much larger contribution to AUT than the first area term for the large change of molecular surface area on the phase transition (Le., E C-I and E C-I’), while the first and the second terms become comparable for the smaller change of the surface area (C-I C-I’ and C-I’ C-11). It might be quite fruitful to compare the properties of hydrocarbon compounds with those of hydrocarbon-perfluorocarbon compounds for the same Cl&JH3+. Let us look at the column n = 4 in Table I. The apparent molar entropy change on the transition from the E phase to the C-I phase is -1.4 X J K-’mol-’. On the contrary, by our previous work,26 that of CIFJNH~-C~~COO is -2.8 X J K-’mol-’. The former value is just half of the latter, which can be expected from a hydrocarbon with a carbon number of 6. From bulk properties of many perfluorocarbon compounds, an effect of the chain length of perfluorocarbon is equal to 1.5-foldof that of the chain length of the hydrocarbon. So our experimental value is in good agreement with the above result, if it can be assumed that the chain length of the perfluorocarbon in CuF9 (n = 4) salts is equivalent to the hydrocarbon chain length (n =
-
-
-
6).
There still remains questions such as whether the C-I and the C-I’ phases can be distinguishable from the phase diagram. This distinction may be made clear by other techniques like an ellipsometric measurement, which will be reported in a separate paper. In conclusion, the monolayers of octadecylammonium perfluoroalkanoates on concentrated NaCl subphase solution were found to have two or three kinds of phase transitions from their r-A isotherms. From surface potentials, thermodynamic quantities, and phase transition pressures, the phase diagrams were drawn and the phase types were classified. Especially, the f l vs Tcurve for the C-I C-I’ transition has a negative slope. This might suggest that the phase transition is accompanied by a positive entropy change, which leads us to the conclusion that the molecular orientation in the monolayer becomes less ordered as a whole. The four kinds of phases were found to appear especiallyfor the amphiphiles having both hydrocarbon and perfluorocarbon chains.
-
Acknowledgment. This work was supported by a Grant-in-Aid for Scientific Research No. 03453013 from the Ministry of Education, Science, and Culture (Japan), which is greatly acknowledged. This work has also been financiallysupported from Riken Denshi Co., Ltd. (Japan).
