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Monolayers of double long-chain salts
Monolayers of Double Long-Chain Salts OSAMU SHIBATA, SHOJI KANESHINA, AND MAKOTO NAKAMURA College of General Education, Kyushu University-O1, Ropponmatsu, Fukuoka 810, Japan AND
RYOHEI MATUURA Department of Chemistry, Faculty of Science, Kyushu University-33, Fukuoka 812, Japan Received August 31, 1979; accepted December 8, 1979 The surface pressure of the monolayers formed by double long-chain salts, alkyltrimethylammonium alkylsulfonates, was measured at the air-water interface by the Langmuir method. When the alkyl chain lengths of cation (m) and anion (n) are short, i.e., m and n N 12, the film is soluble in the substrate, but when m and n > 12, the film becomes increasingly stable. The monolayer is of the expanded type first, and it becomes gradually condensed as the m or n length increases. The transition pressure from the expanded to the condensed state in the monolayer is lowered as the chain length of cation elongates under the condition (m + n) constant. In the case of the equal chain length of cation and anion, i.e., m = n, the condensing effect is pronounced. The apparent molar entropy, enthalpy, and energy of transfer from the expanded to the condensed phase were calculated from the temperature dependence of the transition pressure. The increasing chain length of cation and anion tends to increase the value of the apparent molar quantity change. The contributions of long-chain anion and cation to the phase transition of munolayer are not equivalent and the condensing effect of long-chain cation is larger than that of long-chain anion. INTRODUCTION
monolayers of these salts has been measured as a function of molecular area at various temperatures. The thermodynamic quantities on the phase transition from the expanded to the condensed state were determined. These results may be expected to give effective information on the surface of mixed micelles or the biomembrane.
Double long-chain salts, which are composed of a long-chain anion and a long-chain cation, are widely utilized as antistatic agents or finishing softeners in detergents. These salts, however, are only slightly soluble in water when the chain is long enough. Only a few studies have been made on surface chemical properties of these salts (1, 2). Mixed monolayers of long-chain anion and cation have been studied by Corkill et al. (3) and Hendrikx (4). However, the monolayer of double long-chain salts has not been studied yet. The double longchain salts with carbon number above 12 are capable of spreading on water or on aqueous electrolyte solutions. In the present study, alkyltrimethylammonium alkylsulfonates were prepared, and the surface pressure of
EXPERIMENTAL
The double long-chain salts, that is, alkyltrimethylammonium alkylsulfonates, were prepared by mixing equimolar solutions of alkyltrimethylammonium bromides and sodium alkylsulfonates (Tokyo Kasei Kogyo Co., Ltd.) and were purified by recrystallization from acetone. In this study the 11 salts shown in Table I were used.
182 0021-9797/80/090182-07502.00/0 Copyright© 1980by AcademicPress, Inc. All rightsof reproductionin any formreserved.
Journalof Colloidand Into:faceScience,Vol.77, No. 1, September1980
DOUBLE LONG-CHAIN TABLE I Materials Abbreviation
Double long-chain salt
C,.H2.,+1N+(CH3)3 "C,,H2~+~SO8 m = 12, 14, 16, 18 (cationic long-chain) n = 12, 14, 16, 18 (anionic long-chain)
-C14-C14 C16-C14 C18-C14
-C14-C16 C16-C16 C18-C16
rier is recorded on the X-axis of the X - Y recorder. The temperature of the substrate was controlled within __+0.I°K at the desired temperature by circulating thermostated water.
C . , - C,,
Salt used for monolayers
---C18-C12
183
SALT MONOLAYER
C12-Cl8 C14-C18 C16-Cl8 C18-Cl8
These salts are abbreviated as C , , - C , , in which m is the number of carbon atoms in the cationic long-chain and n is that in the anionic long-chain. The purity of these salts was checked by elemental analysis and differential scanning calorimetry, which gave correct results. Also powder X-ray diffraction patterns were obtained by a Rigaku Geiger-Flex 2001 diffractometer in order to confirm the crystallographic purity. Each double long-chain salt was spread from hexane:ethanol (24:1 - 21:4) mixture (both extrapure grade) at the a i r - w at er interface. Thrice-distilled water or 0.1 M sodium bromide solution was used as substrate. Here the sodium bromide was roasted at 973°K to remove any surfaceactive material. The surface pressure was measured by an automatic Langmuir film balance within an accuracy of _0.1 mN m -1 ( 5 - 7 ) . The block diagram of the apparatus is shown in Fig. 1. The most remarkable feature of this apparatus is that the float was connected with the electrobalance by means of the pivot. Changes of surface pressure were converted into changes of weight. The amplified signal was fed to the Y-axis input of an X - Y recorder. The motor driving barrier was coupled by a worm-gear arrangement with a ten-turn potentiometer which is equivalent to full-scale travel of the barrier. Feeding the constant dc-voltage to the potentiometer, the position of the bar-
RESULTS AND DISCUSSION
When the chain length of salts becomes shorter, the double long-chain salts are soluble in bulk water. Longer hydrocarbon chain compounds form stable monolayers on the electrolyte solutions, depending on the concentration of the electrolyte. Figure 2 shows the effect of NaBr concentration on the surface pressure-area (Tr-A) curves of C14-C14 monolayers. In this figure, (a) shows the ~--A curve on pure water, and (b) and (c) show the 7r-A curves on electrolyte solutions of various concentrations. Monolayers of C14-C14 spread on 0.04, 0.1, and 0.3 M NaBr solutions are in a more condensed state than those spread on 0.01 M NaBr solution and water. As is seen from this figure, the concentration above 0.04M NaBr does not affect the 7r-A curves, that is, the ~--A curves coincide with each other. Considering this result, the concentration of NaBr was held at 0.1 M. The 7r-A curves of C14-C16 and C14-C18 were also affected by the electrolyte and were measured on 0.1 M NaBr solution. In the case of the double long-chain salts having longer chains than these, the effect of sodium bromide concentration on the ~r-A curve was not recognized. Then the 7r-
4
l.ier forh--
II
~ioro
y I
l
Motor driving
FIG.
lec ro micro I
ba~ .... I*'-'--I h~1 ....
barrier
I. Block
I
pivot
Trough
diagram
of automatic
Langmuir
film balance. Journal of Colloid and Interface Science, Vol. 77, No. I, September 1980
184
SHIBATA ET AL. 40 C14-C14
30--
E e0
l0
I
0.5
1.0 A / nm 2
FIG. 2. The effect of NaBr concentration on the 7r-A curve of C14-C14 monolayers at 288.2°K. NaBr molar concentration of substrate: (a) water; (b) 0.01; (c) 0.04, 0.1, and 0.3, A curves of these longer chain salts (i.e., C16-C14, C16-C16, C16-C18, C18-C14, C 1 8 - C 1 6 , and C 1 8 - C 1 8 ) were measured only on pure water. The 7r-A curves of C 1 4 - C 1 4 , C 1 6 - C 1 6 , and C 1 8 - C 1 8 salts, which have equal numbers of carbon atoms in the long-chain cation and anion, are depicted in Fig. 3. The 7r-A curves of C 1 4 - C 1 4 exhibits the expanded (presumably partly soluble) film and that of C 1 8 - C 1 8 exhibits the condensed film, while that of C 1 6 - C 1 6 salt has a transition pressure from an expanded to a condensed state at 298.2°K. Comparing the m o n o l a y e r of a double long-chain salt with that of the corresponding single-chain fatty acid, we see that the 7r-A curve of tetradecanoic acid is of an expanded type, which has a transition f r o m the expanded to the condensed state at 298.2°K, but that of C 1 4 - C 1 4 does not have a transition pressure under this condition. The 7r-A curve of hexadecanoic acid and octadecanoic acid is of a condensed type, while that of C 1 6 C16 has a transition pressure at 298.2°K. When the alkyl chain of double long-chain salts is longer, i.e., m o r n > 14, the film has a transition pressure (Treq) f r o m the expanded to the condensed state and when Journal of Colloid and Interface Science, V o l . 7 7 , N o . 1, S e p t e m b e r 1980
m and n => 18, the m o n o l a y e r shows only a condensed-type film at 298.2°K. The transition pressure of these double long-chain salts is variable according to the combination of long-chain anion and cation. The t h e r m o d y n a m i c treatment of this transition pressure is considered to be useful for elucidating the properties of monolayers, which will be shown later. Figure 4 shows the 7r-A curves at 298.2°K for three double long-chain salts with a total carbon n u m b e r of long-chain cation and anion (n + m) equal to 32. It is seen that the C 1 6 - C 1 6 salt has the lowest transition pressure. It is n o t e w o r t h y that in the case of the equal-chain-length cation and anion, the condensing effect is most pronounced. This condensing effect seems to be due to the increase in molecular interaction between the alkyl chains. The 7r-A curves of C 1 6 C 16 and C 18- C 14 salts show the same limiting area of about 0.6 nm 2 m o l e c u l e - ' . This value is in good agreement with that obtained by Corkill et al. for mixed monolayers of long-chain anion and cation (3), and corresponds to the area of two hydrocarbon chains (8-13). These double longchain salts are probably aligned perpendicular to the water surface.
40
-
\
\1 30 r.,,. 'T
20
--
10
--
C18-CI I C16-C16~ 0.5
A/ nrn 2
1.0
FIG. 3. The rr-A curves of double long-chain salts on water at 298.2°K.
DOUBLE LONG-CHAIN SALT MONOLAYER
~-~ 30 18
20
~ 10
c
-C14
0-c16
0.5
1.0 A / nm
2
FIG. 4. The 7r-A curves in the case of total carbon number of long-chain cation and anion = 32, Substrate: water at 298.2°K. Figure 5 shows the 7r-A curves at 298.2°K for four double long-chain salts with a total carbon n u m b e r of long-chain cation and anion equal to 30. When the carbon n u m b e r difference between long-chain cation and anion is large as in the case of C 1 2 - C 18 and C 1 8 - C 1 2 , these salts form only the expanded film. On the contrary the salts with a small difference in chain length such as C 1 4 - C 1 6 and C 1 6 - C 1 4 give m o n o l a y e r s showing the transition from the expanded to the condensed film at this temperature. As is seen from the 7r-A curves of C 1 4 - C 1 6 and C 1 6 - C 1 4 , when the cation chain length is longer than that of the anion, the transition pressure is lower. F o r the case of total carbon n u m b e r of 34, the 7r-A curve of C 1 6 - C 1 8 salt has a transition pressure, while that of C l 8 - C 1 6 salt shows only a condensed film type at 298.2°K. We see that when the cation chain length is larger than that of the anion, the m o n o l a y e r is apt to be more condensed. The studies on the effect o f t e m p e r a t u r e on the m o n o l a y e r transition pressure are of m u c h interest, which will give us thermodynamic information about the state of the monolayers. Figure 6 shows the 7r-A isotherm of C 1 6 - C 1 6 at various temperatures.
185
All the curves have break points, showing the phase transition from the e x p a n d e d to the condensed state. As e x p e c t e d the transition pressure increases with an increase in temperature. The ~--A curves of other salts with transition pressure were also measured at various temperatures. In Fig. 7, the transition pressure (Treq) is shown as a function of t e m p e r a t u r e for various double long-chain salts. The curves are almost linear and their slopes can be related t h e r m o d y n a m i c a l l y to the e n t r o p y changes a c c o m p a n y i n g the transition. The open circle shows the transition pressure on N a B r solution, while the closed circle shows that on pure water. The transition pressures o f C 1 4 - C 1 6 and C 1 4 - C 1 8 monolayers were m e a s u r e d on both w a t e r and N a B r solution as a function of temperature. The difference in the slope between these two conditions is recognized and m a y be considered to be attributable to the partial dissolution of salts on pure water. But in the case of C 16C14 and C 1 6 - C 1 6 m o n o l a y e r s , the difference in slope between these two conditions is almost negligible. The slopes of these
~ 0.5
40
"T
1.0
C12
1
40
-- 30
30
20
20
I0
E
lO
Cl6-Cl4
~
i0
I 0.5
1.0 A /nm 2
FIG. 5. The 7r-A curves in the case of total carbon number of long-chain cation and anion = 30. Substrate: water at 293.7°K. Journal of Colloid and Interface Science,
Vol,77,No,1,September1980
186
SHIBATA ET AL. M o t o m u r a et al. (14-16, 19). This equation takes account of the contribution of water in monolayer. First, the apparent molar entropy change is evaluated by applying the relation
40
~- 30 ,E
As • = (a e - a e) 20
L\ O ' - ' ~ / p , x g ' e 10
which
J'
l.O A / nm2
FIG. 6. Temperature dependence of 7r-A curves of C16-C16 salt. Substrate: water; temperature: (a) 303.2°K; (b) 298.2°K; (c) 296.2°K; (d) 293.2°K. lines were determined in order to calculate the apparent molar entropy change on phase transition. The thermodynamic quantities on the transition from the expanded to the condensed film is given by the treatment of
t o E q . [29] o f R e f . ( ] 9 ) .
C14-C16 C14-C18
,o //
C16-C14
C16-C16 CI8-CI6
,o
o • 280
f
/
I
I
[]]
In this equation, As v is the apparent molar entropy change, a c and a e are the molecular area (nm 2) (superscripts c and e refer to the condensed and expanded state, respectively), ~-eq is the transition pressure from the expanded to the condensed state, and y0 is the surface tension of p u r e water. a ~ and a e are estimated as follows, a e is the area at the point where the film begins to change from the expanded to the condensed state, a t is determined in the following way: when the point (~-% a ~) is shifted parallel to the area axis, it comes into contact with the elongated line of the ~--A curve in the condensed state to the lower surface pressures.
I
0.5
corresponds
\'-or-Jp
Z
I
290
I 300
I
I 310
T / K
FIG. 7. Transition pressure (zreq) as a function of temperature. Substrate: (1) water; ((3) 0.1 M NaBr. Journal of Colloid and Interface Science, Vol. 77, No. 1, September 1980
DOUBLE LONG-CHAIN SALT MONOLAYER
187
TABLE II The Apparent Partial Molar Quantity Changes on the Phase Transition of Double Long-Chain Salts at 291.2°K Entropy change (_ 10-z AS~',j oK I mole-l)
Enthalpy change (-Ah ~, kJ mole -1)
Energy change (-Z~uv, k2r mole -~)
Anion
Cation~''~' ~
C14(-)
C16( )
C18(-)
C14(-)
Cl6( )
C18(-)
C14(-)
C16(-)
C18(-)
Cl4(+) C 16(+) C18(+)
-2.0 2.9
1.1 2.3 4.1 a
1.5 3.0 --
-59 85
32 69 125a
43 88 --
-66 99
36 83 138~
48 107 --
At 303.3°K.
This point of intersection gives the a e value. The right side of Eq. [1] is calculated numerically from the results given in Figs. 6 and 7. M o r e o v e r , the apparent molar enthalpy change is calculated by the relation Ah ~ = TAsL
[2]
N o w the apparent molar energy change 2~u• is related to ~s ~ by z~t ~ = -(Tr eq - y°)(aC - a e) + T A s ~,
[3]
which c o r r e s p o n d s to Eq. [32] of Ref. (19). Thus we can determine ~u r by use of the a b o v e experimental results. The apparent molar quantity changes on the p h a s e transition thus obtained are given in Table II. In Table II, the column represents the cation chain length and the row represents the anion chain length. The element of any column or row shows the value of the apparent molar quantity change on phase transition of the salt combined. F o r the apparent molar enthalpy changes, all the values are negative as expected. That is, the transition from the expanded to the c o n d e n s e d state is e x o t h e r m i c . T h e s e values c o m p a r e closely with two times the values for the corresponding fatty acid. F o r example, in the C14(+) system, the value is about twice that of tetradecanoic acid ( 1 7 19). The longer the chain length of cation and anion, the larger the apparent molar enthalpy change on the transition from the e x p a n d e d to the condensed state. Holding the total carbon n u m b e r of long-chain cation
and anion constant, increasing the length of cationic chain tends to increase the value of the enthalpy change. Apparent molar e n t r o p y change (AsD and apparent molar energy change (2~uv) follow similar trends. F r o m these results, for the apparent molar quantity changes, it m a y be concluded that the contribution of long-chain cation and anion to the phase transition of monolayer is not equivalent, the contribution of long-chain cation being slightly larger than that of long-chain anion. Corkill e t al. (2) studied solution properties of double long-chain salts (alkylt r i m e t h y l a m m o n i u m alkylsulfates). T h e y showed that C 8 - C 1 2 , C 1 0 - C I 0 , and a mixture of C12 and C8 (all total carbon n u m b e r 20) gave the same values of C M C or free energy of micelle formation, no difference being present b e t w e e n the contribution of long-chain cation and anion. It is evident that the results we obtained in the present study are characteristic of m o n o l a y e r properties. We expect that the data on the monolayers of this type (long double-chain c o m p o u n d s ) will also give useful information on the surface of mixed micelles or the b i o m e m b r a n e c o m p o s e d of anionic and cationic surfactants. ACKNOWLEDGMENTS We want to express our deep gratitude to Dr. K. Motomura of the Department of Chemistry, Kyushu University, for many valuable and most Journal of Colloid and Interface Science, Vol. 77, No. I, September 1980
188
SHIBATA ET AL.
stimulating discussions during this work. One of us (O.S.) extends his thanks to Drs. Y. Moroi and H. Matuo of the Department of Chemistry, Kyushu University, for many useful discussions. REFERENCES 1. Corkill, J. M., Goodman, J. F., Ogden, C. P., and Tate, J. R., Proc. Roy. Soc. London Ser. A 273, 84 (1963). 2. Corkill, J. M., Goodman, J. F., Harrold, S. P., and Tate, J. R., Trans. Faraday Soc. 62, 994 (1966). 3. Corkill, J. M., Goodman, J. F., Harrold, S. P., and Tare, J. R., Trans. Faraday Soc. 63, 247 (1967). 4. Hendrikx, Y., J. Colloid Interface Sci. 69, 493 (1979). 5. Gaines, G. L., Jr., "Insoluble Monolayers at Liquid-Gas Interface," p. 50. Interscience, New York, 1966. 6. Mann, J. A., Jr., and Hansen, R. S., Rev. Sci. lnstrurn. 31,961 (1960). 7. Mann, J. A., Jr., and Hansen, R. S., Rev. Sci. Instrurn. 34, 702 (1963).
Journal of Colloidand InterfaceScience, Vol. 77, No. 1, September1980
8. Pethica, B. A., and Few, A. V., Discuss. Faraday Soc. 18, 258 (1954). 9. Brady, A. P., J. Colloid Sci. 4, 417 (1949). 10. Davies, J. T., J. Colloid Sci. 11,377 (1956). 11. Betts, J. J., and Pethica, B. A., Trans. Faraday Soc. 52, 1581 (1956). 12. Matijevi6, E., and Pethica, B. A., Trans. Faraday Soc. 54, 1382 (1958). 13. Betts, J. J., and Pethica, B. A,, Trans. Faraday Soc. 56, 1515 (1960). 14. Motomura, K., J. Colloid Interface Sci. 23, 313 (1967). 15. Motomura, K., J. Colloid Interface Sci. 48, 307 (1974). 16. Motomura, K., Sekita, K., and Matuura, R., J. Colloid Interface Sci. 48, 319 (1974). 17. Kuramoto, N., Sekita, K., Motomura, K., Nakamura, M., and Matuura, R., Mem. Fac. Sci. Kyushu Univ. C 8, 67 (1972). 18. Sekita, K., Nakamura, M., Motomura, K., and Matuura, R., Mere. Fac. Sci. Kyushu Univ. C 10, 51 (1976). 19. Motomura, K., Yano, T., Ikematsu, M., Matuo, H., and Matuura, R., J. Colloid Interface Sci. 69, 209 (1979).
RYOHEI MATUURA Department of Chemistry, Faculty of Science, Kyushu University-33, Fukuoka 812, Japan Received August 31, 1979; accepted December 8, 1979 The surface pressure of the monolayers formed by double long-chain salts, alkyltrimethylammonium alkylsulfonates, was measured at the air-water interface by the Langmuir method. When the alkyl chain lengths of cation (m) and anion (n) are short, i.e., m and n N 12, the film is soluble in the substrate, but when m and n > 12, the film becomes increasingly stable. The monolayer is of the expanded type first, and it becomes gradually condensed as the m or n length increases. The transition pressure from the expanded to the condensed state in the monolayer is lowered as the chain length of cation elongates under the condition (m + n) constant. In the case of the equal chain length of cation and anion, i.e., m = n, the condensing effect is pronounced. The apparent molar entropy, enthalpy, and energy of transfer from the expanded to the condensed phase were calculated from the temperature dependence of the transition pressure. The increasing chain length of cation and anion tends to increase the value of the apparent molar quantity change. The contributions of long-chain anion and cation to the phase transition of munolayer are not equivalent and the condensing effect of long-chain cation is larger than that of long-chain anion. INTRODUCTION
monolayers of these salts has been measured as a function of molecular area at various temperatures. The thermodynamic quantities on the phase transition from the expanded to the condensed state were determined. These results may be expected to give effective information on the surface of mixed micelles or the biomembrane.
Double long-chain salts, which are composed of a long-chain anion and a long-chain cation, are widely utilized as antistatic agents or finishing softeners in detergents. These salts, however, are only slightly soluble in water when the chain is long enough. Only a few studies have been made on surface chemical properties of these salts (1, 2). Mixed monolayers of long-chain anion and cation have been studied by Corkill et al. (3) and Hendrikx (4). However, the monolayer of double long-chain salts has not been studied yet. The double longchain salts with carbon number above 12 are capable of spreading on water or on aqueous electrolyte solutions. In the present study, alkyltrimethylammonium alkylsulfonates were prepared, and the surface pressure of
EXPERIMENTAL
The double long-chain salts, that is, alkyltrimethylammonium alkylsulfonates, were prepared by mixing equimolar solutions of alkyltrimethylammonium bromides and sodium alkylsulfonates (Tokyo Kasei Kogyo Co., Ltd.) and were purified by recrystallization from acetone. In this study the 11 salts shown in Table I were used.
182 0021-9797/80/090182-07502.00/0 Copyright© 1980by AcademicPress, Inc. All rightsof reproductionin any formreserved.
Journalof Colloidand Into:faceScience,Vol.77, No. 1, September1980
DOUBLE LONG-CHAIN TABLE I Materials Abbreviation
Double long-chain salt
C,.H2.,+1N+(CH3)3 "C,,H2~+~SO8 m = 12, 14, 16, 18 (cationic long-chain) n = 12, 14, 16, 18 (anionic long-chain)
-C14-C14 C16-C14 C18-C14
-C14-C16 C16-C16 C18-C16
rier is recorded on the X-axis of the X - Y recorder. The temperature of the substrate was controlled within __+0.I°K at the desired temperature by circulating thermostated water.
C . , - C,,
Salt used for monolayers
---C18-C12
183
SALT MONOLAYER
C12-Cl8 C14-C18 C16-Cl8 C18-Cl8
These salts are abbreviated as C , , - C , , in which m is the number of carbon atoms in the cationic long-chain and n is that in the anionic long-chain. The purity of these salts was checked by elemental analysis and differential scanning calorimetry, which gave correct results. Also powder X-ray diffraction patterns were obtained by a Rigaku Geiger-Flex 2001 diffractometer in order to confirm the crystallographic purity. Each double long-chain salt was spread from hexane:ethanol (24:1 - 21:4) mixture (both extrapure grade) at the a i r - w at er interface. Thrice-distilled water or 0.1 M sodium bromide solution was used as substrate. Here the sodium bromide was roasted at 973°K to remove any surfaceactive material. The surface pressure was measured by an automatic Langmuir film balance within an accuracy of _0.1 mN m -1 ( 5 - 7 ) . The block diagram of the apparatus is shown in Fig. 1. The most remarkable feature of this apparatus is that the float was connected with the electrobalance by means of the pivot. Changes of surface pressure were converted into changes of weight. The amplified signal was fed to the Y-axis input of an X - Y recorder. The motor driving barrier was coupled by a worm-gear arrangement with a ten-turn potentiometer which is equivalent to full-scale travel of the barrier. Feeding the constant dc-voltage to the potentiometer, the position of the bar-
RESULTS AND DISCUSSION
When the chain length of salts becomes shorter, the double long-chain salts are soluble in bulk water. Longer hydrocarbon chain compounds form stable monolayers on the electrolyte solutions, depending on the concentration of the electrolyte. Figure 2 shows the effect of NaBr concentration on the surface pressure-area (Tr-A) curves of C14-C14 monolayers. In this figure, (a) shows the ~--A curve on pure water, and (b) and (c) show the 7r-A curves on electrolyte solutions of various concentrations. Monolayers of C14-C14 spread on 0.04, 0.1, and 0.3 M NaBr solutions are in a more condensed state than those spread on 0.01 M NaBr solution and water. As is seen from this figure, the concentration above 0.04M NaBr does not affect the 7r-A curves, that is, the ~--A curves coincide with each other. Considering this result, the concentration of NaBr was held at 0.1 M. The 7r-A curves of C14-C16 and C14-C18 were also affected by the electrolyte and were measured on 0.1 M NaBr solution. In the case of the double long-chain salts having longer chains than these, the effect of sodium bromide concentration on the ~r-A curve was not recognized. Then the 7r-
4
l.ier forh--
II
~ioro
y I
l
Motor driving
FIG.
lec ro micro I
ba~ .... I*'-'--I h~1 ....
barrier
I. Block
I
pivot
Trough
diagram
of automatic
Langmuir
film balance. Journal of Colloid and Interface Science, Vol. 77, No. I, September 1980
184
SHIBATA ET AL. 40 C14-C14
30--
E e0
l0
I
0.5
1.0 A / nm 2
FIG. 2. The effect of NaBr concentration on the 7r-A curve of C14-C14 monolayers at 288.2°K. NaBr molar concentration of substrate: (a) water; (b) 0.01; (c) 0.04, 0.1, and 0.3, A curves of these longer chain salts (i.e., C16-C14, C16-C16, C16-C18, C18-C14, C 1 8 - C 1 6 , and C 1 8 - C 1 8 ) were measured only on pure water. The 7r-A curves of C 1 4 - C 1 4 , C 1 6 - C 1 6 , and C 1 8 - C 1 8 salts, which have equal numbers of carbon atoms in the long-chain cation and anion, are depicted in Fig. 3. The 7r-A curves of C 1 4 - C 1 4 exhibits the expanded (presumably partly soluble) film and that of C 1 8 - C 1 8 exhibits the condensed film, while that of C 1 6 - C 1 6 salt has a transition pressure from an expanded to a condensed state at 298.2°K. Comparing the m o n o l a y e r of a double long-chain salt with that of the corresponding single-chain fatty acid, we see that the 7r-A curve of tetradecanoic acid is of an expanded type, which has a transition f r o m the expanded to the condensed state at 298.2°K, but that of C 1 4 - C 1 4 does not have a transition pressure under this condition. The 7r-A curve of hexadecanoic acid and octadecanoic acid is of a condensed type, while that of C 1 6 C16 has a transition pressure at 298.2°K. When the alkyl chain of double long-chain salts is longer, i.e., m o r n > 14, the film has a transition pressure (Treq) f r o m the expanded to the condensed state and when Journal of Colloid and Interface Science, V o l . 7 7 , N o . 1, S e p t e m b e r 1980
m and n => 18, the m o n o l a y e r shows only a condensed-type film at 298.2°K. The transition pressure of these double long-chain salts is variable according to the combination of long-chain anion and cation. The t h e r m o d y n a m i c treatment of this transition pressure is considered to be useful for elucidating the properties of monolayers, which will be shown later. Figure 4 shows the 7r-A curves at 298.2°K for three double long-chain salts with a total carbon n u m b e r of long-chain cation and anion (n + m) equal to 32. It is seen that the C 1 6 - C 1 6 salt has the lowest transition pressure. It is n o t e w o r t h y that in the case of the equal-chain-length cation and anion, the condensing effect is most pronounced. This condensing effect seems to be due to the increase in molecular interaction between the alkyl chains. The 7r-A curves of C 1 6 C 16 and C 18- C 14 salts show the same limiting area of about 0.6 nm 2 m o l e c u l e - ' . This value is in good agreement with that obtained by Corkill et al. for mixed monolayers of long-chain anion and cation (3), and corresponds to the area of two hydrocarbon chains (8-13). These double longchain salts are probably aligned perpendicular to the water surface.
40
-
\
\1 30 r.,,. 'T
20
--
10
--
C18-CI I C16-C16~ 0.5
A/ nrn 2
1.0
FIG. 3. The rr-A curves of double long-chain salts on water at 298.2°K.
DOUBLE LONG-CHAIN SALT MONOLAYER
~-~ 30 18
20
~ 10
c
-C14
0-c16
0.5
1.0 A / nm
2
FIG. 4. The 7r-A curves in the case of total carbon number of long-chain cation and anion = 32, Substrate: water at 298.2°K. Figure 5 shows the 7r-A curves at 298.2°K for four double long-chain salts with a total carbon n u m b e r of long-chain cation and anion equal to 30. When the carbon n u m b e r difference between long-chain cation and anion is large as in the case of C 1 2 - C 18 and C 1 8 - C 1 2 , these salts form only the expanded film. On the contrary the salts with a small difference in chain length such as C 1 4 - C 1 6 and C 1 6 - C 1 4 give m o n o l a y e r s showing the transition from the expanded to the condensed film at this temperature. As is seen from the 7r-A curves of C 1 4 - C 1 6 and C 1 6 - C 1 4 , when the cation chain length is longer than that of the anion, the transition pressure is lower. F o r the case of total carbon n u m b e r of 34, the 7r-A curve of C 1 6 - C 1 8 salt has a transition pressure, while that of C l 8 - C 1 6 salt shows only a condensed film type at 298.2°K. We see that when the cation chain length is larger than that of the anion, the m o n o l a y e r is apt to be more condensed. The studies on the effect o f t e m p e r a t u r e on the m o n o l a y e r transition pressure are of m u c h interest, which will give us thermodynamic information about the state of the monolayers. Figure 6 shows the 7r-A isotherm of C 1 6 - C 1 6 at various temperatures.
185
All the curves have break points, showing the phase transition from the e x p a n d e d to the condensed state. As e x p e c t e d the transition pressure increases with an increase in temperature. The ~--A curves of other salts with transition pressure were also measured at various temperatures. In Fig. 7, the transition pressure (Treq) is shown as a function of t e m p e r a t u r e for various double long-chain salts. The curves are almost linear and their slopes can be related t h e r m o d y n a m i c a l l y to the e n t r o p y changes a c c o m p a n y i n g the transition. The open circle shows the transition pressure on N a B r solution, while the closed circle shows that on pure water. The transition pressures o f C 1 4 - C 1 6 and C 1 4 - C 1 8 monolayers were m e a s u r e d on both w a t e r and N a B r solution as a function of temperature. The difference in the slope between these two conditions is recognized and m a y be considered to be attributable to the partial dissolution of salts on pure water. But in the case of C 16C14 and C 1 6 - C 1 6 m o n o l a y e r s , the difference in slope between these two conditions is almost negligible. The slopes of these
~ 0.5
40
"T
1.0
C12
1
40
-- 30
30
20
20
I0
E
lO
Cl6-Cl4
~
i0
I 0.5
1.0 A /nm 2
FIG. 5. The 7r-A curves in the case of total carbon number of long-chain cation and anion = 30. Substrate: water at 293.7°K. Journal of Colloid and Interface Science,
Vol,77,No,1,September1980
186
SHIBATA ET AL. M o t o m u r a et al. (14-16, 19). This equation takes account of the contribution of water in monolayer. First, the apparent molar entropy change is evaluated by applying the relation
40
~- 30 ,E
As • = (a e - a e) 20
L\ O ' - ' ~ / p , x g ' e 10
which
J'
l.O A / nm2
FIG. 6. Temperature dependence of 7r-A curves of C16-C16 salt. Substrate: water; temperature: (a) 303.2°K; (b) 298.2°K; (c) 296.2°K; (d) 293.2°K. lines were determined in order to calculate the apparent molar entropy change on phase transition. The thermodynamic quantities on the transition from the expanded to the condensed film is given by the treatment of
t o E q . [29] o f R e f . ( ] 9 ) .
C14-C16 C14-C18
,o //
C16-C14
C16-C16 CI8-CI6
,o
o • 280
f
/
I
I
[]]
In this equation, As v is the apparent molar entropy change, a c and a e are the molecular area (nm 2) (superscripts c and e refer to the condensed and expanded state, respectively), ~-eq is the transition pressure from the expanded to the condensed state, and y0 is the surface tension of p u r e water. a ~ and a e are estimated as follows, a e is the area at the point where the film begins to change from the expanded to the condensed state, a t is determined in the following way: when the point (~-% a ~) is shifted parallel to the area axis, it comes into contact with the elongated line of the ~--A curve in the condensed state to the lower surface pressures.
I
0.5
corresponds
\'-or-Jp
Z
I
290
I 300
I
I 310
T / K
FIG. 7. Transition pressure (zreq) as a function of temperature. Substrate: (1) water; ((3) 0.1 M NaBr. Journal of Colloid and Interface Science, Vol. 77, No. 1, September 1980
DOUBLE LONG-CHAIN SALT MONOLAYER
187
TABLE II The Apparent Partial Molar Quantity Changes on the Phase Transition of Double Long-Chain Salts at 291.2°K Entropy change (_ 10-z AS~',j oK I mole-l)
Enthalpy change (-Ah ~, kJ mole -1)
Energy change (-Z~uv, k2r mole -~)
Anion
Cation~''~' ~
C14(-)
C16( )
C18(-)
C14(-)
Cl6( )
C18(-)
C14(-)
C16(-)
C18(-)
Cl4(+) C 16(+) C18(+)
-2.0 2.9
1.1 2.3 4.1 a
1.5 3.0 --
-59 85
32 69 125a
43 88 --
-66 99
36 83 138~
48 107 --
At 303.3°K.
This point of intersection gives the a e value. The right side of Eq. [1] is calculated numerically from the results given in Figs. 6 and 7. M o r e o v e r , the apparent molar enthalpy change is calculated by the relation Ah ~ = TAsL
[2]
N o w the apparent molar energy change 2~u• is related to ~s ~ by z~t ~ = -(Tr eq - y°)(aC - a e) + T A s ~,
[3]
which c o r r e s p o n d s to Eq. [32] of Ref. (19). Thus we can determine ~u r by use of the a b o v e experimental results. The apparent molar quantity changes on the p h a s e transition thus obtained are given in Table II. In Table II, the column represents the cation chain length and the row represents the anion chain length. The element of any column or row shows the value of the apparent molar quantity change on phase transition of the salt combined. F o r the apparent molar enthalpy changes, all the values are negative as expected. That is, the transition from the expanded to the c o n d e n s e d state is e x o t h e r m i c . T h e s e values c o m p a r e closely with two times the values for the corresponding fatty acid. F o r example, in the C14(+) system, the value is about twice that of tetradecanoic acid ( 1 7 19). The longer the chain length of cation and anion, the larger the apparent molar enthalpy change on the transition from the e x p a n d e d to the condensed state. Holding the total carbon n u m b e r of long-chain cation
and anion constant, increasing the length of cationic chain tends to increase the value of the enthalpy change. Apparent molar e n t r o p y change (AsD and apparent molar energy change (2~uv) follow similar trends. F r o m these results, for the apparent molar quantity changes, it m a y be concluded that the contribution of long-chain cation and anion to the phase transition of monolayer is not equivalent, the contribution of long-chain cation being slightly larger than that of long-chain anion. Corkill e t al. (2) studied solution properties of double long-chain salts (alkylt r i m e t h y l a m m o n i u m alkylsulfates). T h e y showed that C 8 - C 1 2 , C 1 0 - C I 0 , and a mixture of C12 and C8 (all total carbon n u m b e r 20) gave the same values of C M C or free energy of micelle formation, no difference being present b e t w e e n the contribution of long-chain cation and anion. It is evident that the results we obtained in the present study are characteristic of m o n o l a y e r properties. We expect that the data on the monolayers of this type (long double-chain c o m p o u n d s ) will also give useful information on the surface of mixed micelles or the b i o m e m b r a n e c o m p o s e d of anionic and cationic surfactants. ACKNOWLEDGMENTS We want to express our deep gratitude to Dr. K. Motomura of the Department of Chemistry, Kyushu University, for many valuable and most Journal of Colloid and Interface Science, Vol. 77, No. I, September 1980
188
SHIBATA ET AL.
stimulating discussions during this work. One of us (O.S.) extends his thanks to Drs. Y. Moroi and H. Matuo of the Department of Chemistry, Kyushu University, for many useful discussions. REFERENCES 1. Corkill, J. M., Goodman, J. F., Ogden, C. P., and Tate, J. R., Proc. Roy. Soc. London Ser. A 273, 84 (1963). 2. Corkill, J. M., Goodman, J. F., Harrold, S. P., and Tate, J. R., Trans. Faraday Soc. 62, 994 (1966). 3. Corkill, J. M., Goodman, J. F., Harrold, S. P., and Tare, J. R., Trans. Faraday Soc. 63, 247 (1967). 4. Hendrikx, Y., J. Colloid Interface Sci. 69, 493 (1979). 5. Gaines, G. L., Jr., "Insoluble Monolayers at Liquid-Gas Interface," p. 50. Interscience, New York, 1966. 6. Mann, J. A., Jr., and Hansen, R. S., Rev. Sci. lnstrurn. 31,961 (1960). 7. Mann, J. A., Jr., and Hansen, R. S., Rev. Sci. Instrurn. 34, 702 (1963).
Journal of Colloidand InterfaceScience, Vol. 77, No. 1, September1980
8. Pethica, B. A., and Few, A. V., Discuss. Faraday Soc. 18, 258 (1954). 9. Brady, A. P., J. Colloid Sci. 4, 417 (1949). 10. Davies, J. T., J. Colloid Sci. 11,377 (1956). 11. Betts, J. J., and Pethica, B. A., Trans. Faraday Soc. 52, 1581 (1956). 12. Matijevi6, E., and Pethica, B. A., Trans. Faraday Soc. 54, 1382 (1958). 13. Betts, J. J., and Pethica, B. A,, Trans. Faraday Soc. 56, 1515 (1960). 14. Motomura, K., J. Colloid Interface Sci. 23, 313 (1967). 15. Motomura, K., J. Colloid Interface Sci. 48, 307 (1974). 16. Motomura, K., Sekita, K., and Matuura, R., J. Colloid Interface Sci. 48, 319 (1974). 17. Kuramoto, N., Sekita, K., Motomura, K., Nakamura, M., and Matuura, R., Mem. Fac. Sci. Kyushu Univ. C 8, 67 (1972). 18. Sekita, K., Nakamura, M., Motomura, K., and Matuura, R., Mere. Fac. Sci. Kyushu Univ. C 10, 51 (1976). 19. Motomura, K., Yano, T., Ikematsu, M., Matuo, H., and Matuura, R., J. Colloid Interface Sci. 69, 209 (1979).