Langmuir 1989,5, 1383-1387 than for their metal salt^.^^,^^ This is also true for the orientation of the long axes of the chromophoresin azobenzene-containing long-chain fatty acids and their barium salt.
Conclusion We measured the T-A isotherms and UV absorption spectra of spread monolayers of azobenzene-containing long-chain fatty acids and their barium salts on the water surface and found that the spread monolayers of 8A3H and 8A5H and their barium salts undergo the liquid crystalline-gel phase transition upon compression. The transition occurs stepwise for acids but gradually for their barium salts. On the contrary, the spread monolayers of 12A3H and 12A5H and their barium salts form the very high H-aggregate in the gel state without phase transition. In addition, UV absorption spectra of transferred LB monolayers were observed and compared with those of the spread monolayer prior to the transfer. It was found that the molecular aggregation of 8A3H and 8A5H mol(23) Rabolt, J. F.; Burns, F. C.; Schlotter, N. E.; Swalen, J. D. J. Chem. Phys. 1983, 78,946. (24) Allara, D. L.; Swalen, J. D. J. Phys. Chem. 1982,86,2700.
1383
ecules in the LB film is not as high as that in the spread monolayer on water. In the case of 8A3Ba and 8A5Ba, however, the molecular packing in the l-monolayer LB films is nearly the same as that in the spread monolayers. When the number of monolayers is increased, the packing is developed, finally forming a very high H-aggregate. For all compounds of m = 12, on the other hand, the molecules are in the same degree of aggregation in the spread monolayers and the LB films irrespective of the number of monolayers. Quantitative studies of the molecular orientation in the same LB films are in progress by using FT-IR reflectionabsorption and transmission spectroscopy.
Acknowledgment. We are indebted to Dr. K. Kina of Dojindo Laboratories for his kind supply of the samples. Thanks are also due to Otsuka Electronics Co., Ltd., for the use of a Model MCPD-100 spectrophotometer equipped with optical fibers and a multichannel photodiode array detector. This work was partly supported by the Grant-in-Aid for Special Project Research from The Ministry of Education, Science and Culture, Japan. Registry No. 8A3H, 112360-084;8A5H, 112360-09-5;12A3H, 121887-97-6;12A5H, 112360-10-8;8A3Ba, 121887-98-7;8A5Ba, 121918-59-0;12A3Ba, 121887-99-8;12A5Ba, 121888-00-4.
Thermotropic and Pressure-Induced Phase Behavior in a Dodecylammonium Bromide-Water System Shoji Kaneshinat College of General Education, Kyushu University, Ropponmatsu, Fukuoka 810, Japan Received December 12, 1988. I n Final Form: May 23, 1989 Differential scanning calorimetry (DSC) has been used to study the phase behavior of aqueous solutions of dodecylammonium bromide (DAB). Two kinds of endothermic transitions were observed by the heating scans of DSC. They are the transition (A) from the coagel phase (or so-called hydrated crystalline surfactant) to the micellar solution and (B) from the metastable gel phase to the supercooled micelle. The effect of pressure on both transition temperatures was determined in the pressure range up to 100 MPa. Regarding transition A, the transition temperature increased with an increase of pressure in such a manner as to be slightly convex upward. The temperature-pressure curve for this transition should become a critical solution line, representing the pressure dependence on the Krafft temperature. On the other hand, the temperature of transition B is linearly elevated by applying pressure in the range below 35 MPa. Above 35 MPa, a pressure-inducednew mesophase appeared. The metastable supercooled or supercompressed micelle is able to exist in the temperature and pressure ranges between phase boundaries for two transitions. The enthalpy, entropy, and volume changes associated with transitions were determined. The difference in the structure between coagel and gel phases is responsible for the differences in these thermodynamic quantities between two transitions.
Introduction With increasing temperature, the solubility of surfactants in water increases abruptly above a critical temperature called a Krafft point because of the dissolution as a micelle. Contrarily, with decreasing pressure the solubility increases suddenly below a critical pressure.' Loci
'Present address: Department of Biological Science and Tech-
nology, Faculty of Engineering, The University of Tokushima, Minamijosanjima, Tokushima 770, Japan.
of the critical temperature and pressure for the micellar dissolution become a critical solution line,'.' which represents the pressure dependence of the Krafft temperature. So far, few papers have been published on the pressure dependence of the Krafft temperat~re.'-~The crit(1) Tanaka, M.; Kaneshma, S.; Tomida, T.; Noda, K.; Aoki, K. J. Colloid Interface Sci. 1973,44, 525. (2) Tanaka, M.; Kaneshina, S.; Sugihara, G.;Nishikido, N.; Murata, Y. In Solution Behauior of Surfactants; Mittal, K. L., Fendler, E. J., Eds.; Plenum Press: New York, 1982; Vol. 1, p 71.
0 1989 American Chemical Society
1384 Langmuir, Vol. 5, No. 6, 1989 i d solution line, as well as the critical micelle concentration (cmc), is characteristic of solution behavior of surfactants. The present study proposes a novel method for the determination of the critical solution line of a surfactant. The occurrence of the supercooled or supercompressed micelle below the critical temperature or above the critical pressure for the micellar dissolution has been previously o b s e r ~ e d . ' ~ According ~-~ to the recent thermoanalytical study of surfactant solutions using differential scanning calorimetry (DSC),'*12 two kinds of phase transitions have been observed; one is the transition from coagel to micelle, the other is the transition from gel to micelle. The transition temperature from the coagel phase, where the hydrated crystalline surfactant is deposited,13 to the micellar solution coincides generally with the Krafft temperature. The existence of the coagel and gel phase below the Krafft temperature seems to be concerned with the appearance of the supercooled micelle. The knowledge of the pressure effect on the phase transitions and the thermodynamic characterization for these phase transitions will serve to elucidate the solution behavior of surfactants and to understand the states of phases. In the present study, we focused our attention on the phase behavior of aqueous solutions of a cationic surfactant, dodecylammonium bromide, whose solution properties such as the cmc, the solubility, and the pressure effect on them have already been known. The enthalpy and entropy changes associated with the transitions are determined by the DSC measurement. The volume changes associated with the transitions are estimated thermodynamically from the pressure dependence of transition temperature and are compared with the volume changes estimated from those for the dissolution and micellization processes of a surfactant.
Experimental Section Materials. A cationic surfactant, dodecylammonium bromide (DAB),was prepared by the reaction of a fractionally distilled dodecylamine with dried hydrogen bromide in acetone. The crude DAB was purified by recrystallization from acetone. The cmc determined by the conductimetry was 12.6 mmol kg-' at 35 "C, which is in good agreement with that in the 1iterat~re.l~ Water was purified by triple distillation,once from alkaline potassium permanganate solution. Differential ScanningCalorimetry (DSC). The phase transitions between surfactant assemblies under atmospheric pressure were determined by DSC using a Seiko calorimeter SSC560U. Several micellar solutions of DAB were prepared in the range of concentrationsup to 1.4 wt %. Weighed amounts of DAB solutions of 0.06 mL were sealed in DSC cells made of silver. Prior to the calorimetricscans, the temperature of the DSC cell was kept at 5 "C for a suitable period of time. The scanning rate was usually 0.8 "C min-' and occasionally 0.4 "C min-' in an ascending mode. The scanning rate did not affect the present results. Calibrationof the transition heat was car(3) Nishikido, N.; Kobayashi, H.; Tanaka, M. J.Phys. Chem. 1982,
86. --,3170. --
(4) Offen, H. W.; Turley, W. D. J. Colloid Interface Sci. 1982, 87, 442; 1983,92, 575. (5) Ikawa, Y.; Tsuru, S.; Murata, Y.; Okawauchi, M.; Shigematau, M.; Sugihara, G. J.Solution Chem. 1988, 17,125. (6) Franses, E. I.; Davis, H. T.; Miller, W. G.; Scriven, L. E. J.Phys. Chem. 1980,84, 2413. (7) Rosenblatt, C. Mol. Cryst. Liq. Cryst. 1986, 141,107. (8) Osugi, J.; Sam, M.; Ifuku, N. Reu. Phys. Chem. Jpn. 1965, 35, 32. (9) Hamann, S. D. Reu. Phys. Chem. Jpn. 1965,35,109. (IO) Kodama, M.; Seki, S. Hyomen 1984,22, 61. (11) Kodama, M.; Seki, S. J. Colloid Interface Sci. 1987, 117,485. (12) Andersson, B.; Olofsson, G. Colloid Polym. Sci. 1987,265,318. (13) Vincent, J. M.; Skoulios, A. Acta Crystallogr. 1966,20, 432. 1963,30,74. (14) Klevens, H. B. J. Am. Oil Chem. SOC.
Kaneshina
I
1
20
1
1
30
I
, 40
Temperature I OC Figure 1. DSC heating curves of DAB (1.023 w t %)-water system. Thermograms shown were recorded at a scanning rate of 0.8 "C min-'. Two endothermicpeaks refer to the transition (A) from the coagel phase to the micellar solution and (B) from the metastable gel phase to the supercooled micelle. A transition temperature was determined by extrapolating the linear slope of the endothermic peak to the base line, which is represented by dotted lines. ried out according to the manufacturer's instructions. Benzophenone was chosen for calibration (AH= 98.32 J g-l). Phase Transition under High Pressure. A high-pressure cell assembly with sapphire windows (Hikarihigh-pressureinstruments) was used for the determination of light transmittance of surfactant solution. Pressures were generated by a handoperated hydraulic pump (Hikari high-pressure instruments) and measured within an accuracy of f0.2 MPa by a Heise pressure gauge. The cell compartment assembly of a Hitachi 139UV-vis spectrophotometer was replaced with a high-pressure cell assembly. A glass tube (2-mm i.d.), sealed at one end, in which the DAB solution is filled, was placed in the optical high-pressure cell. The spectrophotometeroutput was recorded, together with the temperature signals, on an X-Y recorder (Riken Denshi Co. Ltd., Model F-35s). The temperature was monitored by a digital thermometer TR-2112A (Takeda Riken Co. Ltd.) with a C-C thermocouple probe inserted into the body of the cell block. The cell temperature was raised at a rate of 1 "C min-' by circulating water from a temperature-controlledwater bath through the jacket enclosing the pressure cell. The calibrationcurve of temperature difference between the sample solution in the pressure cell and the cell body was made under the same heating conditions used in the high-pressure experiments. The temperatures obtained were corrected for the sample solution by using the calibration curve. The sudden change of transmittanceaccompanyingthe phase transition was followed at 540 nm. The beginning of the increase in transmittance was taken as the phase transition temperature, which was in good agreement with the DSC results.
Results and Discussion DSC of Surfactant Solution. DSC thermograms for the DAB solution of 1.023 wt % are shown in Figure 1. There exist two kinds of endothermic peaks. An endothermic peak A was obtained from the heating scan of DAB solution after the solution was cooled at 5 "C for 18 h. The transition temperature for this endothermic peak A was 32.3 O C , and the hydrated solid of surfactant existed below this temperature. Therefore, the endothermic peak A was assigned to the transition from the coagel phase to the micellar solution. This transition temperature corresponds to the Krafft temperature of DAB surfactant. In the coagel phase, the alkyl chain takes the trans-zig-
Langmuir, Vol. 5, No. 6,1989 1385
Dodecylammonium Bromide Phase Behavior
Wt % of DAB 1.o
40L 0 I
+ 2ot
1
I
0.5 1.o Wt % of DAB
0
I
I
1.5
zag conformation, packed in parallel with each other, and the ionic head group is in a fixed state with the bound water.15 Thus, the melting of the hydrocarbon chain is responsible for this transition. Another endothermic peak B was observed from the heating scan after the solution was cooled to 5 "C in the calorimeter sink and kept at 5 "C for about 1 h. Since the sample shows the translucent gel phase at lower temperature than the endothermic peak observed, this endothermic peak was assigned to the transition from the metastable gel phase to the micellar solution. The existence of the metastable gel phase is already known in the system of octadecyltrimethylammonium bromide (ODAB) solutionlo Consequently, the metastable gel phase appears when the micellar solution is supercooled and is allowed to stand for a short time at 5 "C. On the other hand, the stable coagel phase appears when the metastable gel phase is allowed to stand for a sufficiently long time at 5 "C to be transformed completely into the coagel phase. The DSC thermograms were obtained for the micellar solutions with various concentrations of DAB. The effect of DAB concentration on the transition temperature for two kinds of transitions is shown in Figure 2. The transition temperature was evaluated by using the standard procedure,12 that is, by extrapolating the linear slope of the endothermic peak to the base line as indicated in Figure 1. As is seen from Figure 2, both transition temperatures are almost independent of the surfactant concentration up to 1.4 wt %. We can determine two transition temperatures from this figure. The temperature of transition A, namely, the T, transition, corresponds to the Krafft temperature. Above the transition temperature for both transitions, micellar aggregates exist in the solution. Therefore, the phase diagram of Figure 2 revealed the existence of the metastable supercooled micelle in the temperature range between A and B. The metastable supercooled micelle has been observed also by other
technique^.^" The heat of transition was obtained from the peak area of DSC thermograms. A linear relation between the heat of transition and the weight of DAB in the solution of 0.06 mL is shown in Figure 3. Two lines, A and B, are corresponding to two kinds of transitions shown in Figure 1,respectively. As is seen from Figure 3, all the transition heats are detectable above the cmc of DAB. From the slopes of these straight lines, the enthalpy changes ~
~
1.5
I
I
1
Figure 2. DAB-concentration dependence on the phase transtion temperatures. A and B correspond to the two transitions in Figure 1, respectively.
~~~~
0.5
Weight of DAB / mg
Figure 3. Heats of transition, which were determined from the area of DSC curves, as a function of the DAB concentration. A and B are the same as in Figure 1. Table I. Thermodynamic Characterization for Transitions in a Dodecylammonium Bromide-Water System
A 305.4 49.7 163 32.0 0.200 B 297.4 41.4 139 25.8 0.188 " A, the transition from the coagel to the micellar solution; B, the transition from the metastable gel to the micellar solution.
(AH,) associated with the transitions were determined. The entropy change (AS,) associated with the transition was calculated from the equation AS, = AH,f T
(1) where T is the transition temperature. The values of AH, and AS, thus obtained are given in Table I together with the transition temperature taken from Figure 2. With respect to the transition from the coagel phase to the micellar solution, the reported value of enthalpy change for an anionic surfactant, sodium dodecyl sulfate, which has the same alkyl chain, is 50.2 kJ This is comparable to AH, for DAB. The enthalpy and entropy changes for the transition B are smaller than those for the transition A. The difference in these thermodynamic quantities between two transitions may be attributable to water molecules interacting with surfactant assemblies in the states of coagel and gel phases, which will be described later. Pressure Effect on Transitions. An example of the phase transition measurements under various pressures up to 100 MPa is depicted in Figure 4. High-pressure experiments were carried out at a constant concentration of DAB (Le., 1.01 wt % ) because the phase transition temperature by the DSC method was almost independent of the DAB concentration, as shown in Figure 2. The upper two curves in Figure 4A show the transmittance vs temperature curve for the transition from the coagel phase to the micellar solution at a certain pressure. An abrupt increase in transmittance was observed at a transition temperature. The midpoint between the beginning of the increase in transmittance and the point where the transmittance reached its plateau was taken as the phase transition temperature. A lower group in Figure 4B illustrates the transition from the metastable
~
(15) Kawai, T.; Umemura, J.; Takenaka, T.; Kodama, M.; Seki, S. J . Colloid Interface Sci. 1985, 103, 56.
(16) Shinoda, K.; Hiruta, S.; Amaya,
1966, 21, 102.
K. J. Colloid Interface
Sci.
1386 Langmuir, Vol. 5,No. 6,1989
Kaneshina Regarding a similar phase behavior, it is known from the study on the effect of pressure on the phase transition in dipalmitoylphosphatidylcholine dispersed in excess water that a pressure-induced gel phase appears at about 93 MPa.17 The phase diagram shown in Figure 5 revealed that the metastable supercooled or supercompressed micelle is able to exist in the temperature and pressure ranges between two lines of A and B. The slopes of the temperature vs pressure curves, dT/ dP,for two kinds of transitions are listed in Table I.
A
f
-
a,
u
s
*
'E t
cz -
,
78.5
1 I I I J 30 40 50 Temperature I O C Figure 4. Transition measurementsunder high pressures. Transmittance scale is arbitrary. Numerical values are pressure in MPa. A and B are the same as in Figure 1.
I
1
1
20
monomer
50
where AV, and AV, are the volume change of dissolution and of micellization, respectively. Therefore, AVt can be expressed as
0 \
40
5
*
AV, = AV, + AV, (2) The volume change of dissolution (AV,) is thermodynamically related to the pressure dependence of the solubility below the Krafft temperature by the following equation, as has been described previously:'
za,
n 30
E
!-
20 0
Thermodynamic Characterization for Transitions. In order to estimate the volume change (AVJ associated with the transition from the coagel phase to the micellar solution, we consider the relation among three states of surfactant assemblies in water as follows:
20
60 80 Pressure I MPa 40
100
Figure 5. Effect of pressure on the transition temperature. A and B are the same as in Figure 1. Curve A should become the critical solution line of DAB and represents also the pressure dependence of the Krafft temperature. A new pressureinduced mesophase appears above 35 MPa.
gel phase to the supercooled or supercompressed micellar solution. More remarkable is the change in the transmittance above 35 MPa; two-step transitions were observed. The temperature-pressure diagram of the phase transition for a DAB-water system is shown in Figure 5. With respect to the transition from the coagel to the micellar solution (i.e., the curve A in Figure 5), the transition temperature increased with an increase of pressure in such a manner as to be slightly convex upward. Since the transition A corresponds to the Krafft point, the curve A in Figure 5 should be the critical solution line' of DAB, representing the pressure dependence on the Krafft temperature. In the temperature and pressure regions above this critical solution line, only a stable micelle can exist. Our previous results,' which were determined from the solubility measurement under high pressure, are completely euperimpoaable on the present curve A. The method presented here seems to be a novel method for an accurate and rapid determination of the critical solution line of a surfactant. On the other hand, the transition temperature from the metastable gel phase to the micellar solution is linearly elevated by applying pressure in the range below 35 MPa. A new pressure-induced mesophase appeared a t about 35 MPa. This pressure-induced mesophase is not elucidated yet with respect to its structure and properties but may be predictable from this temperaturepressure diagram to be a densely packed gel phase.
AV, = -2RT(a In X,/aP), (3) where X, is the surfactant solubility in mole fraction. On the other hand, the volume change of micellization (AV,) can be calculated from the pressure dependence of the cmc by the equationls AV, = (1+ P)RT(a In Xemc/aP)T (4) where X,,, is the cmc in mole fraction, P is a constant which indicates the ratio of the number of counterions to that of the surfactant ion in a micelle. P = 0.8 was used for the calculation of AV,. The recent theory of the phase separation model with the aid of the excess thermodynamic quantities adopts = The difference in AVm would be only 10% and probably not substantial. The pressure dependence on both the cmc and the solubility of DAB, which are taken from the previous data,' is depicted in Figure 6 at the temperature just above vaIues of AV, and AV, calculated from eq 3 and 4 by using the slopes in Figure 6 were 26.0 and 6.0 cm3 mol-', respectively. Therefore, the volume change of the transition from the coagel to the micelle was found to be 32.0 cm3 mol-' from eq 2. According to the Clapeyron-Clausius equation, the AV,lAS, ratio gives the temperaturepressure slope (dT/dP) for the transition. With respect to the transition from the coagel to the micelle, the AV,/ AS, ratio calculated is 0.199 X lo4 K Pa-l,which is in good agreement with the value obtained from the slope of the temperature-pressure diagram, namely, 0.200 X lo4 K Pa-'. This means the estimation of the transil.'9920
(17) Prasad, S: K.; Shashidhar, R.; Caber, B. P.; Chandrasekhar,S. C . Chem. Phys. Lcprds 1987,43,221. (18) Kaneshina, S.;Tanaka, M.; Tomida, T.;Matuura, R. J.Colloid Interface Sei. 1974.48, 450. (19) Yamanaka, M.; Aratono, M.; Motomura, K.; Matuura, R. Colloid Polym. Sci. 1984,262, 338. (20) Motomura, K.; Yamanaka, M.; Aratono, M. Colloid Polym. Sci. 1984,262, 948.
Langmuir 1989,5, 1387-1393
I 0
I
I
50
100
I
I
150
Pressure / MPa Figure 6. Effect of pressure on the solubility of DAB at 305.15 K and on the cmc of DAB at 308.15 K. Data for solubility and cmc were taken from ref 1. tion volume is valid and supports the idea21 that this transition is regarded as the melting point of the hydrated crystalline surfactant, although the concept of the Krafft phenomenon is divided into two different aspectsn a phase transition of solid surfactant and a solubility increase up to cmc for micellization. In the case of the transition from the metastable gel to the supercooled micelle, AVt was calculated from the Clapeyron-Clausius equation by using the values of AS, and dT/dP. Thermodynamic characterization for two transitions is summarized in Table I. The difference in the struc(21) Shinoda, K.; Hutchinson, E. J. Phys. Chem. 1962,66,577. (22) Moroi, Y.; Matuura, R. Bull. Chem. SOC.Jpn. 1988, 61,333.
1387
ture between the coagel and gel states seems to be responsible for the difference in these thermodynamic quantities between two kinds of transitions. Kawai and coworker~ have ~ ~ studied the structural features of the phase transitions in the system of octadecyltrimethylammonium chloride (0DAC)water by the method of Fourier transform infrared spectroscopy. The coagel phase, in which the methylene chains take the trans-zigzag conformation, intercalates water between the bilayers of the surfactant molecules as the bound water, coexisting with bulk free water. On the other hand, the gel phase, in which the rotational motion of methylene chain occurs around the chain axis, probably interposes the loosely bound water in the hexagonal lattice of surfactant assemblies. In the present DAB-water system, the coagel and gel states are considered to resemble the ODAB-water system in their structural features. Recently, with respect to the coagel and gel states of the related dioctadecyldimethylammonium chloride, Laughlin and M ~ n y o n ,have ~ reported from the viewpoint of kinetics that the physical state described as the coagel and gel by Kawai et al.24has been found to be in fact the stoichiometric crystal monohydrate and dihydrate, respectively.
Acknowledgment. This work was supported in part by Grant-in-Aid for General Scientific Research No. 5740008 from the Ministry of Education, Science and Culture of the Japanese Government. Registry No. DAB, 26204-55-7. (23) Laughlin, R. G.; Munyon, R. L. Abstracts of 6th International Conference on Surface and Colloid Science; Hakone, 1988; p 58. (24) Kawai, T.; Umemura, J.; Takenaka, T.; Kodama, M.; Ogawa, Y.; Seki, S. Langmuir 1986,2,739.
Interfacial Chemistry of MoS2 Films on Si P.A. Bertrand Chemistry and Physics Laboratory, The Aerospace Corporation, P.O. Box 92957, Los Angeles, California 90009 Received March 17, 1989. In Final Form: June 15, 1989 The interface between radio frequency sputter deposited MoS, films and single-crystal Si was studied by observing thin (
