J. Phys. Chem. 1992,96,9422-9424
9422
Thermodynamlc Study on the Interface Formation of Water-Lortg-Chaln Alcohol Systems Makoto Aratono,* Takanori Takiue, Norihiro Ikeda, M r a Nakamura, and Kinsi Motomm Department of Chemistry, Faculty of Science, Kyushu University 33, Hakozaki, Higashiku, Fukuoka 812, Japan (Received: May 4, 1992)
The interfacial tensions of the water-decyl alcohol (DEA), -undecyl alcohol (UNA), and -dodecyl alcohol (DOA)systems were measured as a function of temperature and pnssure. The thermodynamic quantitie of interface formation were evaluated from the experimental results;the entropy, volume, and energy were found to have negative values and change discontinuwly at the break point on the interfacial tension vs temperature and pressure curves. These results were discussed in terms of the orientation of alcohol molecules at the interface caused by the interaction between alcohol and water molecules. Further, it was suggested that the phase transition takes place between the expanded and condensed states at the interface.
Introduction The adsorption of alcohols at oil-water interfaces has been studied with continuous interest.'-" We have investigated the adsorbed films of long-chain alcohols at oil-water interfaces by measuring the interfacial tension as a function of temperature, pressure, and concentration and by analyzing the results to obtain the thermodynamic quantities such as the interfacial densities of the alcohols and the entropy, volume, and energy changes associated with a d s ~ r p t i o n ' ~based - ~ ~ on the thermodynamics of interface~;'~ the state of their adsorbed films is found to be appreciably dependent on the thermodynamic variables, the hydrocarbon chain length of the alcohols, and the kinds of oils. These results and our previous works on the interface formation of water-organic compound systemsIb18 have aroused much interest to study the interface formation of water-long-chain alcohol systems in the absence of oil. In this study, we choose decyl alcohol (DEA), undecyl alcohol (UNA), and dodecyl alcohol (DOA) because they are liquid at room temperature and their structures are simple. The interfacial tension between the water-rich and alcohol-rich phases was measured as a function of temperature for the DEA, UNA, and DOA systems and also of pressure for the DEA and UNA systems.
Experimental Section Materials. DEA, UNA, and DOA were guaranteed reagents purchased from Tokyo &sei Kogyo (Japan). They were purified by fractional distillation under reduced pressure, and their purities were estimated to be more than 99.9% by gas-liquid chromatography. Water was distilled three times from alkaline permanganate solution. The purity of water was checked by measuring the interfacial tension at the water-air and waterhexane interfaces. Density Measurement. The density value ( p ) at atmospheric pressure was measured by a digital density meter (Anton Paar 601602). First, the water and alcohol mixture was stirred by a magnetic rotor for 1 h and then allowed to stand for several hours to separate into two transparent phases. Then a small amount of the phase was introduced individually into the glass vibration tube of the density meter. The temperature was kept constant within 0.001 K by circulating thermostated water around the tube. The error of the density values was within 5.0 X le2kg m-3. The density value at high pressure (p ) was calculated from the values of p at atmospheric pressure anBthe compression (k)through the equation Pp = P ( 1 - k) Here k is defined by
k = ( V - V&/V where V and V, are respectively the volumes of a given mass at atmospheric pressure and high pressure and measured as a function of pressure with a precision of 2 X lo4 by using a pyezometer.
Inteafacial Teasion M e " e n t . The interfacial tension (y) of the preequilibrated two-phase system was measured by the pendant drop meth0dl9as a function of temperature and also of pressure for the DEA and UNA systems. The accuracy was 0.05 mN m-l. Results and Discuapion Let us first show the thermodynamic equations describing y between the water-rich and alcohol-rich phases. The interfacial tension is written as a function of temperature (T), pressure (p), and the chemical potentials of water ( k )and alcohol (I,) as dy = -s" d T + U" dp - rwu d k - ro' d k (1) where f,vu, and rf are respectively the interfacial excess entropy, the volume, and the number of moles of component i per unit interfacial area defied with respect to the two arbitrary dividing plane^.'^^^ Since the degree of freedom of this system is 2, however, we cannot determine the values of the thermodynamic quantities indicated by the superscript u from experiments. It is natural to choose temperature and pressure as the independent variables. Now we define the excess thermodynamic quantities of interface with respect to the two dividing planes, making the excess number of moles of water and alcohol zero:Is rwH
=o
(2)
and
=o (3) where the superscript H is used to stress the way of choosing the dividing planes. The change of interfacial tension is then described by the equation roH
dy = -sH d T + VH dp (4) By using the quasithermodynamicsof the interface, we have shown that, for the twoamponent systems, P is equivalent to the entropy of interface formation (h):
-
h=
rwIhw + r:bo
(5)
where I!' and hi are the number of moles inherent in the interfce and the entropy change of adsorption per unit area of component i, respecti~ely.'~Similarly, UH is equal to the volume of interface formation (Av): ~v =
r$bw+ r:Avo
(6) Therefore, we can estimate the values of As and Av from the temperature dependence of the interfacial tension at constant pressure and the pressure one at constant temperature through the relations ( w m , = -h (7) and (aY/aP)T = Av
0022-3654/92/2096-9422S03.00/0Q 1992 American Chemical Society
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The Journal of Physical Chemistry, Vol. 96, No. 23, 1992 9423
Interface Formation of Water-Alcohol Systems
0
c
I I
N
I I I
I
I
-0.2Y 7
E
-0.3-
I
I
I
I
I
I
I
I
I
I
v)
Q
-0.4-
51
290
280
300
31 0
320
T I K
Figure 1. Interfacial tension vs temperature curves under atmospheric pressure: (1) decyl alcohol, (2) undecyl alcohol, (3) dodecyl alcohol.
-5.0-
280
290
300
310
320
T I K
Figure 3. Entropy of interface formation vs temperature curve8 under atmosphericpressure: (1) decyl alcohol, (2) undecyl alcohol, (3) dodecyl alcohol. '
o
l ........... .... ... ........... ... ... ....... ..........................................
0
-0.01
5 ' 0
'
kF=F-=-=
I 20
40
60
2
80
p / MPa
Figure 2. Interfacial tension vs pressure curves at constant temperature: (1) decyl alcohol at 298.15 K, (2) undecyl alcohol at 293.15 K, (3) undecyl alcohol at 298.15 K, (4) undecyl alcohol at 303.15 K.
respectively. In Figure 1 are shown the interfacial tension values measured as a function of temperature under atmospheric pressure. The y values are considerably low because of the relatively high mutual solubilities between alcohol and water?1J2and they increase with increasing temperature; this behavior is in contrast to that of the water-hydrocarbon systems.I6 It should be noted that the y vs T curve of the DEA system is smooth, while those of the UNA and DOA systems break sharply at a certain temperature. The interfacial tension values under elevated pressures were measured at 298.15 K for the DEA system and at three temperatures for the UNA system. The results are given in the form of the y vs p curve at constant.temperature in Figure 2. It is seen that the y value decreases with increasing pressure, which is contrary to the results of the waterhydrocarbon system^^^^'^ and is similar to the results reported by Lm et al?3 It should be noted that the curve of the DEL4 system at 298.15 K and that of the LTNA system at 293.15 K are almost linear with negative slopes, while those of the UNA system at 298.15 and 303.15 K consist of the two almost linear portions with different negative slopes. Now we evaluate the thermodynamic quantities of interface formation. By applying eq 7 to Figure 1, we obtained the entropy of interface formation and plotted it against temperature in Figure 3. All the As values are negative, and those of the UNA and DOA systems change discontinuously at the temperatures corresponding to the break points on the y vs T curves given in Figure 1. The negative value may be mainly attributable to the restricted orientation of alcohol molecules at the interface. The discontinuous change in As from the more negative value to the less negative value indicates that the structure of the interface is changed abruptly at this temperature from a more rigid one to a more loose one as temperature increases. This will be proved to be the phase transition from the expanded film to the condensed film of alcohol at the interface in our next paper.24 It is noted that the As value hardly depends on temperature in the temperature range below the discontinuous point, while it does appreciably depend on
-401
-140
280
290
300
310
320
T I K
Figure 5. Energy of interface formation vs temperature curve8 under atmosphericpwure: (1) decyl alcohol, (2) undecyl alcohol, (3) dodecyl alcohol.
temperature in the range above the point. The volume of interface formation was obtained by applying eq 8 to Figure 2 and plotted against pressure in Figure 4. The Av values are negative and almost independent of pressure except for the discontinuous point. It should be noted, however, that the value depends strongly on the hydrocarbon chain length of alcohol and temperature; as the chain length increases and temperature decreases, it becomes more negative, and therefore, the structure of the interface becomes more condensed. Further, it is noted that the discontinuous change from the less negative Av value to the more negative one corresponds to the phase transition mentioned above. These findings are in harmony with the results of the entropy of interface formation. The energy of interface formation (Au) at 298.15 K under atmospheric pressure was estimated by substituting the values of
J. Phys. Chem. 1992,96, 9424-9431
9424 y, &, and Av into the equation
Au=y+T&-pAv
(9)
The values are piotted against temperature in Figure 5 , where the Au values of the DOA system were calculated on the assumption that the value of p h , is negligibly small compared with those of the other two terms. It is found that the value of the water-alcohol system is very low compared with those of the waterhydrocarbon systems;I6the energy of the interfacial region is lowered by an attractive intermolecular interaction between alcohol and water such as the hydrogen bond. Further, the phase transition from the expanded film to the condensed film accompanies a large decrease in energy. In our next we will demonstrate the y vs p curves at various temperatures and prove thermodynamicallythe break given in Figures 1 and 2 to be due to the phase transition at the alcohol-water interface, which is analogous to the one at the oil solution of long-chain alcohol-water interfaces.13g14.25-28 Registry No. Decyl alcohol, 112-30-1; undccyl alcohol, 112-42-5; dodecyl alcohol, 112-53-8.
References and Notes (1) Hutchinaon, E. J. Colloid Sci. 1948, 3, 219. (2) Hutchinson, E.; Randall, D. J . Colloid Sei. 1952, 7 , 151. (3) Franks, F.; Ives, D. J. G . J . Chem. Soc. 1960, 741. (4) Jasper, J. J.; Van Dell, R. D. 1. Phys. Chem. 196569, 481. ( 5 ) Lutton, E. S.; Stauffer, C. E.; Martin, J. B.; Fehl, A. J. J . Colloid Interface Sei. 1969, 30, 283.
(6) Aveyard, R.; B h , B. J. J. Chem. Soc., Faraday Trans. I 1972,68, 478. (7) Sagert, N. H.; Quinn, M. K. J . Colloid Interface Sci. 1985, 105, 58. (8) Van Hunscl, J.; JW, P. Longmuir 1987, 3, 1069. (9) Villers, D.; Platten, J. K. J . Phys. Chem. 1988, 92,4023. (IO) Turkevich, L. A.; Mann, J. A. Longmuir 1990, 6, 445. (1 1) Coninck, J. D.; Villers, D.; Platten, J. J. Phys. Chem.1990,94,5057. (12) Motomura, K.; Matubayasi, N.; Aratono, M.; Matuura, R. J . Colloid Interface Sci. 1978, 64, 356. (13) Matubayasi, N.; Motomura, K.; Aratono, M.; Matuura. R. Bull. Chem. Soc. Jpn. IW8,51,2800. (14) Ikenap, T.; Matubayasi, N.; Aratono, M.; Motomura, K.; Matuura, R. Bull. Chem. Soc. Jpn. 1980,53, 653. (15) Motomura, K. J . Colloid Interface Sci. 1978,64, 348. (16) Motomura, K.; Iyota, H.; Aratono, M.; Yamanaka, M.; Matuura, R. J . Colloid Interface Sci. 1983, 69, 264. (17) Aratono, M.; Motomura, K. Bull. Chem. Soc. Jpn. 1985,58,3205. (18) Aratono, M.; Motomura, K. J. Colloid Interface Sci. 1987, I1 7, 159. (19) Matubayasi, N.; Motomura, K.; Kancahina, S.; Nakamura, M.; Matuura, R. Bull. Chem. Soc. Jpn. 1977.50, 523. (20) Motomura, K.; Aratono, M. Lungmuir 1987, 3, 304. (21) Stephenson, R.; Stuart, J. J. Chem. Eng. Dofa 1986, 31, 56. (22) Donahue, D. J.; Bartell, F. E. J . Phys. Chem. 1952, 56, 480. (23) Lin, M.; Firpo, J. L.; Mansoura, P.; Baret, J. F. J. Chem. Phys. 1979, 71, 2202. (24) Aratono, M.; Takiue, T.; Ikcda, N.; Motomura, K. To be submitted. (25) Matubayasi, N.; Dobzono, M.; Aratono, M.; Motomura, K.; Matuura, R. Bull. Chem. Soc. Jpn. 1979,52, 1597. (26) Aratono, M.; Yamanaka, M.; Motomura, K.; Matuura, R. Colloid Polym. Sei. 1982, 260, 632. (27) Motomura, K.; Iwanaga, S.; Hayami, Y.; Uryu, S.;Matuura, R. J . Colloid Interface Sei. 1981, 80, 32. (28) Aratono, M.; Uryu, S.; Hayami, Y.; Motomura, K.; Matuura, R. J . Colloid Interface Sci. 1984, 98, 33.
Vibrational Spectra and Thermal Decomposltlon of Methylamine and Ethylamhe on Ni( 111) Denis E.Cardia and Cabor A. Somorjai* Centerfor Advanced Materials, Materials Sciences Division, Lawrence Berkeley Laboratory. 1 Cyclotron Road, Berkeley, California 94720, and Department of Chemistry, University of California, Berkeley, California 94720 (Received: May 13, 1992; In Final Form: August 17, 1992)
The bonding and geometry of methylamine (CH3NH2)and ethylamine (CH3CH2NH2)on Ni(l11) have been investigated with high-resolution electron energy loss vibrational spectroscopy (HREELS). Both amines adsorb molecularly at 150 K through the nitrogen lone pair. Significant metal-hydrogen interactions in the alkyl chain were indicated by ‘softened” C-H stretching modes with frequencies shifted to 2660-2680 cm-’. Temperatureprogrammed desorption (TPD)and HREELS were used to monitor their desorption and thermal decomposition on the Ni( 111) surface. Both CH3NH2 and CH&H2NH2 are dehydrogenatedin the temperature range 300-400 K. CH3NH2is dehydrogenated to HCN at about 330 K,which further d a m p a m above 360 K. CH3CH2NH2is dehydrogenated to CH3CN. initially by a-C-H bond Scission, leading to dearorption of that molecule at 350 K. On the basis of our spectra,we propose a mechanism for the dehydrogenation proc*rses of CH3NHz and CH3CH2NHzon Ni( 111).
1. Iatroduction
The adsorption of amines on metal surfaces is of considerable importance in catalysis and surface coatings chemistry. These molecules have the ability to adsorb molecularly through the nitrogen lone pair on transition-metal surfaces under ultrahighvacuum (UHV) conditions, at a temperature high enough for activating C-H, N-H, C-N, or C-C bonds. When heating the surface, the investigation of their surface reactivity by various surface science techniques is possible. The high-mlution electron energy loss vibrational s p e c b w q y (HREELS) study of CH3NH2on Ni(100). Ni(l1 l), Cr(100), and Cr(ll1) by Baca et al.’ confirmed that methylamine adsorbs molecularly at 300 K like ammonia through the nitrogen lone pair. The adsorption of CH3NH2was also investigated on Ni( 111),2 Ni(100),3R(111),4R(100),Mo(100),6 5 W(100),’Rh(lll)?and R U ( ~ W ) .Methylamine ~ was found to dehydrogenate on all 0022-3654/92/2096-9424503.oo/o
surfaces. No C-N bond scission was observed on Pt( 111)5 (desorption of HCN and C2N2was detected), while some was found On Ni( 111)2and Ni( methylon Rh( 111)9and Pt( amine was totally decomposed, leaving atomic carbon and nitrogen on the surface. Ethylamine thermal decomposition was studied on W(lOO), W(lO0)-(5Xl)-C, and W(100)-(2X1)-0.’0 On W(lOO), ethylamine undergoes C-N and C-C bond scission, leading mainly to methane and ammonia desorption. In contrast, on W(100)(5Xl)-C, neither C-C nor C-N bond scission was observed after ethylamine adsorption. Acetonitrile (CH3CN) was the major product detected. An initial selective a-C-H bond activation was proposed to account for the product distribution. The W(100)-(2Xl)-O surface was inert with respect to C-H, N-H, or C-N bond scission, resulting primarily in molecular ethylamine desorption. 0 1992 American Chemical Society
