J. Phys. Chem. 1990, 94, 815-819
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Calorimetric Investigations of the Stability of Octadecyltrimethylammonlum Halide-Water Systems Michiko Kodama,* Department of Biological Chemistry, Faculty of Science, Okayama University of Science, 1 - 1 Ridai-cho, Okayama, 700 Japan
Kaoru Tsujii, Tokyo Research Laboratories, Kao Corporation, 1-3 Bunka 2-Chome. Sumida-ku, Tokyo 131 Japan
and Sylz6 Seki Department of Chemistry, Faculty of Science, Kwansei Gakuin University, Nishinomiya, 662 Japan (Received: March 28, 1989: In Final Form: June 12, 1989)
The thermodynamic stability of the gel phase of the binary systems of water and homologous series of octadecyltrimethylammonium halides of different counterions, Le., chloride (OTAC), bromide (OTAB), and iodide (OTAI), was systematically investigated. The gel phases in the present three systems were revealed to exist in the thermodynamic stable or metastable states; the stability of the gel phases relative to the coagel phase increased in the order OTAI < OTAB < OTAC, that is, in the order of an increase in the hydration interaction of these counterions. In accordance with this difference, the mode of phase transition also differed among the three systems. On the assumption that the OTAI- and OTAB-water systems exhibit a provisional phase transition of the coagel to gel, the present result was discussed from the enthalpy viewpoint on the basis of pseudolattice (lAH,I) and hydration (lAH,() enthalpies operating in the polar head groups of the surfactant molecules. The assumed enthalpy change associated with this transition was shown to increase in the order OTAC < OTAB OTAI, indicating that the hydration enthalpy of the halide counterions is a predominant factor in determining the thermodynamic stability of the gel phases in the three systems, which is related to the mode of phase transition.
Introduction In our previous paper,l the thermodynamic studies on the binary system of octadecyltrimethylammonium chloride (0TAC)-water revealed that the phase transitions of the coagel to gel ( TBeJand the following gel to micellar solution (or liquid crystal) (T,) appear in the heating direction. This fact indicated that the gel phase of this system exists in the thermodynamic stable state in the specified temperature region between the TgeIand T, transitions, contrary to the metastable state of the gel phase usually ac~ e p t e d . ~ -Furthermore, ~ we discovered that the gel phase interposed the structural interlamellar water, corresponding to the so-called intermediate water, between the bilayers of the OTAC molecules. This finding also suggested that the interaction between the polar head groups of surfactants and water molecules is an important factor in the appearance of the thermodynamic stable gel phase. In the present study, we deal with homologous series of octadecyhrimethylammonium halides of different counterions, bromide (OTAB) and iodide (OTAI) which have an affinity to water molecules less than that of chloride as the counterion of the OTAC, in order to systematically elucidate the nature of the thermodynamic stability of the gel phase. The stability of the gel phase of the OTAB- and OTAI-water systems, compared with that of the OTAC-water system, is discussed from the thermodynamic viewpoint, focusing on two kinds of binding force, hydration and electrostatic, operating between the positive ammonium ion and negative counterions of the polar head groups. Experimental Section The OTAB and OTAI were derived from the OTAC used in our previous study:' that is, the chloride ion of the OTAC was ( I ) Kodama, M.; Seki, S . J . Colloid Interface Sci. 1987, 117, 485. (2) Luzzati, V. In Biological Membranes: Chapman, D., Ed.: Academic Press: New York, 1968: p 7 1. (3) Winsor, P. A. Chem. Reu. 1968, 68, I . (4) Tiddy, G. J. T. Phys. Rep. 1980, 59, I . ( 5 ) Hauser, H. In Water; Franks, F., Ed.; Plenum: New York, 1974; Vol. 4, p 237.
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replaced with bromide and iodide ions, respectively, by using anion-exchange resins (Dowex). The OTAB and OTAI thus obtained were purified further by recrystallization three times from an acetone-methanol mixed solvent after they were completely dehydrated according to the procedure previously employed for the OTAC.' The traces of chloride contaminant for the OTAB and OTAI were checked by ion chromatographic method and were estimated to be below 0.01% for both compounds. Forty samples of the OTAB-water mixtures, containing from 0 to -95 g % water, were prepared by using a microsyringe to add increasing amounts of water to the completely dehydrated compound. The OTAI-water mixtures at different water content were also prepared in the same manner as that of the OTAB-water mixtures. All samples were preheated at the required temperature above the T, transition for about 2 h to ensure homogeneous mixing. Then, the samples were cooled to -20 "C and maintained at this temperature for about 12 h to complete the nucleation of the coagel phase, after which the differential scanning calorimetry (DSC) measurements were started, unless otherwise specified. DSC measurements were carried out with a Mettler DSC TA-2000 and a Seiko-Denshi DSC SSC/560 by placing the sample in a high-pressure crucible (pressure resistant to 100 kP/cm*) and heating it from -20 "C to temperatures above the T, transition at a heating rate of 0.2 "C/min unless otherwise specified.
Results Thermal Behavior of Phase Transitions of OTAB- Water System. Figure 1 shows a series of typical heating DSC curves of the OTAB-water system, which describes the variation of the T, transition with water content. The pronounced primary endothermic peak at 100.5 "C obtained for the completely dehydrated compound is followed by new second, third, and fourth endothermic peaks which appear at successively lower temperatures with increasing water content, similar to the thermal behavior of the T, transition for the OTAC-water system in our previous paper.' When a water content reaches about 30 g %, the fourth peak becomes a fixed, rather sharp peak of nearly constant size
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The Journal of Physical Chemistry, Vol. 94, No. 2, 1990
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I
1
I
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I
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1°C Figure 1. A series of typical DSC curves describing the T, transition of the OTAB-water systems with increasing water contents shown on the left-hand side of this figure. Endothermic peaks at around 0 "C show those due to a melting of ice frozen from the coexisting water.
and transition temperature and any peak other than the icemelting peak is no longer observed. This fixed peak due to the T, transition maintains with a further increase in water content up to 95 g % investigated in this study. However, the sample at a water content above 30 g % shows a dependence of the T, transition on the temperature to which the sample is cooled, as shown in the upper half of Figure 2A. In this figure, one cycle of the DSC curve and heating (+) in run I is compared with that on cooling (e) in run 11, based on the difference in the procedure whether or not the sample is cooled to a crystallization temperature of the coexisting water at around -20 'C. The endothermic peak (dotted area) of the heating DSC curve in run I shows a lower transition temperature and a smaller peak size, compared with that of the heating DSC curve in run 11. However, the size of the peak in run I is nearly the same as that of the exothermic peak (dotted area) of the cooling DSC curves in runs I and 11. This fact indicates that a phase which appears at a temperature below the T, transition depends on the limit of cooling temperature of the sample and, as a result, the phases at this temperature region in the heating direction differ between runs I and 11. Based on an observation by the naked eye, the phase of run I can be recognized as the gel phase, i.e., a homogeneous semitransparent state, while the phase of run I 1 can be recognized as the coagel phase, i.e., a hydrated crystalline state separated from the water phase.',6 The thermal behaviors shown in runs I and I1 of Figure 2A propose the following process of phase transition for the OTABwater system: (i) the gel phase appears on a cooling of the micellar solution to temperatures below the T , transition; (ii) the gel phase is allowed to exist in a supercooled state to the temperature where the coexisting water does not crystallize; (iii) the gel phase is converted into the coagel phase by a further cooling to the crystallization temperature of the coexisting water; and (IV) the coagel phase thus obtained is transformed directly into the micellar solution without passing through the gel phase on heating. Thermodynamic Stability of Gel and Coagel Phases, Related to T, and T,* Transitions in the OTAB-Water System. In order to clarify the thermodynamic relationship of the phases of the coagel, gel, and micellar solution in the OTAB-water system with a variation of temperature, the lower half of Figure 2A shows the schematic diagram of the Gibbs energy (G) versus temperature ( T ) curve, in comparison with the DSC curve in the upper half of this figure. The diagram of the G-T curve was contracted on the basis of the entropy changes and the transition temperatures ( 6 ) Winsor, P. A . In Liquid Crystals and Plastic Crystals; Gray, G . W., Wir.sor. P. A . Eds.: Halsted Press: Chichester, 1974; Vol. 1 . p 199
associated with the phase transitions shown in the DSC curves of this figure. Following the G-T curves in Figure 2A, the G T curve of the gel phase is shown to be always situated at positions higher than those of the coagel phase in all the temperature regions below the 7, transition. This fact indicates that the gel phase of the OTAB-water system exists in the thermodynamic metastable state in this temperature region and only the coagel phase exists in the thermodynamic stable state. Therefore, the phase transition at the point of intersection of the G T curves of the gel and micellar solutions corresponds to the metastable gel to metastable supercooled micellar solution, which we name the "Tc* transition", in contrast with the T, transition of the stable coagel to stable micellar solution. This result is different from that of the OTAC-water system, in which the gel phase exists in the thermodynamic stable state in the temperature region between the Tgeland T, transitions, as is evident in one set of the DSC curves and the G-T curves for this system shown in Figure 2C. In this figure, the G T curve of the gel phase is revealed to be located at the lowest positions in this specified temperature region. Figure 3 shows the phase diagram of the OTAB-water system which was constructed by all the DSC curves of about 40 samples of different water content. In this diagram, we can observe the T,* transition temperature curves characteristic of the OTABwater system, in place of the Tgeltransition temperature curves in the OTAC-water system,' reflecting the difference in the thermodynamic stability of the gel phase between both systems. Furthermore, we can notice the characteristic behavior of the T, transition curves which descend stepwise with increasing water content, particularly in the low-water-content region. The behavior of the T, curve obtained in this study is similar to that for all amphiphile-water systems previously investigated by u s ' ~ ~ - ' ~ a n d quite different from the smooth decreasing T, curve in many textbook^.**^,'^ Thermal Behavior of Phase Transition of the OTAI- Water System. The process of reaching a final, fixed peak of the T, transition in the OTAI-water system is nearly the same as that in the OTAB-water system shown in Figure 1; primary, second, third, and fourth endothermic peaks appear at successively lower temperatures with increasing water content, and the final fourth peak shows a limiting temperature of the T, transition and a nearly constant peak size at water contents above 20 g %. However, the thermal behavior of the T, transition is independent of the temperature to which the sample is cooled, as is obvious from a comparison of the DSC curves of runs I and I1 in the upper half of Figure 2B. In this figure, the endothermic peak in the heating direction is the same for runs I and I1 and the peak size is nearly the same as that of the exothermic peak in the cooling direction. This finding indicates that the micellar solution of this system is transformed directly into the coagel phase on cooling, and the coagel phase thus obtained is transformed reversibly to the micellar solution on heating. This mode of phase transition reveals a nonexistence of the gel phase, even in the metastable state. As a result, we can observe only the T, transition temperature curves in the phase diagram of this system as shown in Figure 4, and the schematic diagram of the GTcurves shown in the lower half of Figure 2B indicates a lack of the G-T curve of the gel phase. Dependence of Enthalpy and Entropy Changes (AH, AS) Associated with Phase Transitions on Water Content of OTABand OTAI-Water Systems. Figure 5 shows a variation of enthalpy changes (AH) associated with the phase transitions of the OTAB( 7 ) Kodama, M.; Kuwabara, M . ; Seki, S. Thermochim. Acfa 1981,50,81. (8) Kodama. M.; Kuwabara, M.; Seki, S. Biochim. Biophys. Acta 1982, 689, 567.
( 9 ) Kodama, M.; Kuwabara, M.; Seki, S . In Thermal Analysis; Proceedings ofthe 7th ICTA; Hayden: New York, 1982; Vol. 1; p 822. ( I O ) Kodama, M . ; Seki, S . Prog. Colloid Polym. Sci. 1983, 68, 158. (1 I ) Kodama, M.; Hashigami, H . ; Seki, S. Thermochim. Acra 1985, 88, 217. ( I 2 ) Kodama, M. Thermochim. Acra 1986, 109, 8 1. ( 1 3) Kodama, M . ; Seki, S. Ado. Colloid Interface Sci., in press. (14) McBain. J. W.; Sierichs, W. C. J . Am. Oil Chem. Sot. 1948, 25. 221.
The Journal of Physical Chemistry, Vol. 94, No. 2, 1990 817
Octadecyltrimethylammonium Halide-Water Systems
C ) OTAC
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Figure 3. Phase diagram of the OTAB-water system. T,* and T, represent temperatures of phase transitions of metastable gel to metastable micellar solution (or liquid crystal) and stable coagel to stable micellar solution (or liquid crystal), respectively.
0 0
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120\ 100 the entropy change (AS,) associated with the T, transition is nearly the same for both systems, while the T, transition enthalpy (AH,) is different for them.
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W nzo /%% Figure 4. Phase diagram of the OTAI-water system. T, represents temperatures of phase transition of stable coagel to stable micellar solution (or liquid crystal).
and OTAI-water systems with increasing water content. Entropy changes (AS) of the phase transitions were calculated according to the equation A S = A H / T t ( T t is the transition temperature) and are plotted against water content of the OTAB- and OTAI-water systems in Figure 6A and B, respectively. In these figures, we can notice the characteristic behavior of the enthalpy and entropy curves of the T, transition in both systems; the curves fall to minimum points at first and then rise with increasing water content, after which they are nearly parallel to the abscissa at water contents above at least 30 g %. In this water content region,
Discussion It is well-known that the so-called gel and coagel phases appear at a temperature below the T, transition in an amphiphile-water system. Many textbooks have described the gel phase as originating in the metastable However, Wilkinson and NagleIs elucidated the thermodynamic stability of the gel phase of the dimyristoylphosphatidylethanolamine-watersystem. Our previous and present studies also revealed that the gel phases of the octadecyltrimethylammonium halide-water systems exist in the thermodynamic stable or metastable states and their thermodynamic stabilities relative to the coagel phase increase in the order OTAI < OTAB < OTAC. Furthermore, based on the difference in the thermodynamic stability of the gel phase, the mode of phase transition also differs among the three systems, as is evident in Figure 2. This difference is more clearly recognized in Figure ( 1 5 ) Wilkinson, D. A,; Nagle, J. A. Biochemistry 1984, 23, 1538.
818
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TABLE I: Enthalpy and Entropy Changes Associated with Phase Transitions and Temperatures of the T , Transition at a Water Content of -80 g %
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OTACwater
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Figure 6. Variation with increasing water content of entropy changes (AS) associated with phase transitions of the (A) OTAB-water and (B) OTAI-water systems. ASc and hs,* represent entropy changes of phase transitions of stable coagel to stable micellar solution (or liquid crystal) and metastable gel to metastable micellar solution (or liquid crystal),
respectively.
micellar solution
gel phase coagel phase Figure 7. Schematic picture showing three different modes of phase transition among the phases of micellar solution, gel, and coagel in the OTAC-water (-) OTAB-water (-), and OTAI-water systems, respectively. The gel phase of this figure reveals a more loose packing and a more ionized state of the polar head group, compared with that (.-a)
of the coagel phase. Dotted marks surrounding the polar head groups of gel phase represent the existence of structural water, Le., intermediate water which we discovered.','
7 where a schematic picture showing the mode of phase transition in the three systems is presented. Following this picture, the outline of three different types of phase transition processes is given as follows: (i) in the OTAC-water system (solid line), the reversible phase transition of the coagel to gel to micelle occurs on cooling and heating, although the transformation of the supercooled gel to coagel phases needs the specified, adequate annealing treatment;' (ii) the OTAB-water system (dotted line) shows the irreversible phase transition of different processes in the heating and cooling directions, that is, the transformation of the micelle to coagel by way of the gel phase on cooling and direct transformation of the coagel to micelle on heating; and (iii) the OTAI-water system (dotted-and-dashed line) exhibits the direct,
OTABwater 63.5 205 (36.4)c (1 16)"
OTAIwater
51.2" 10.4 AH(coage1-micelle), k J / mol AS(coage1-micelle), J/(Kmol) 1996 204 AH(coage1-gel), kJ/mol 31.5 AS(coage1-gel), J/(Kmol) I IO AH&-micelle), kJ/mol 25.7 27.1 89 89 AS(gel-micelle), J/(K-mol) 16.0 31.7 71.5 T,, OC 'AH(coage1-micelle) = AH(coage1-gel) + AH(gel-micelle). AS(coagel-micelle) = AS(coage1-gel) + AS(gel-micelle). AH(coage1gel) = AH(coage1-micelle) - AH(gel-micelle). AS(coage1-gel) = S(coage1-micelle) - M(gel-micelle). reversible transition between the micelle and coagel without passing through the gel. To discuss the nature of the stability of the gel phases, related to the mode of phase transition in the three systems, from the thermodynamic viewpoint, Table I exhibits thermal data associated with the phase transitions. In this table, the three samples of different counterion are compared at nearly the same water content of approximately 80 g % where the enthalpy and entropy changes are independent of water content, as shown in Figures 5 and 6.' A comparison of the enthalpy data with the entropy data indicates that a total enthalpy change [AH(coagel micelle)] associated with the transformation from the coagel to micelle increases in the order OTAC < OTAB < OTAI, in contrast to nearly equal values of the corresponding entropy change [U(coagel micelle)] in the three systems. So, to proceed to a discussion from the enthalpy viewpoint, we assume that the OTAB- and OTAI-water systems exhibit provisionally the phase transition of the coagel to gel. According to this assumption, the assumed enthalpy change [M(coagelgel)] for the OTAB-water system will be estimated to be 36.4 kJ/mol, which is larger than the corresponding enthalpy change of the OTAC-water system as is evident in Table I. This is also the case for the enthalpy change of the phase transition of the gel to micelle as well as the coagel to micelle. Therefore, we could assume the largest enthalpy change to the coagel-to-gel transition in the OTAI-water system, in connection with the largest enthalpy change of the coagel-to-micelle transition of this system as shown in Table I. As a result, the enthalpy change associated with the coagel-to-gel transition in the three systems could be assumed to increase in the order OTAC < OTAB < OTAI. On the other hand, an outside appearance of the gel phase is apparently distinguished from that of the coagel phase, as mentioned above, and it has been generally accepted that the polar head group of the gel phase is in a partially ionized fused state.6 Judging from these facts, the polar head group of the gel phase is considered to have much more interaction with water molecules, compared with that of the coagel phase. This idea is additionally supported by the existence of the loosely bound water, corresponding to the intermediate water, other than the tightly bound water in the gel phase of the OTAC-water system which we dis~overed.'~'.~~-'* Accordingly, taking into account the cooperative interaction between the polar head groups and the water molecules at the coagel-to-gel transition, the enthalpy change associated with this phase transition may be approximated by the following equation: AH(coage1 gel) = AH, + lAH,I - lAHhI (1) +
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where AH, is the enthalpy change due to a conformational and/or packing change of hydrocarbon chain, AH, is the pseudolattice (16) Kawai, T.; Umemura, J.; Takenaka, T.; Kodama, M.; Seki, S. J . Colloid Interface Sci. 1985, 103, 56. (17) Kawai, T., Umemura, J.; Takenaka, T.; Kodama, M.; Seki, S. Langmuir 1986, 2, 739. (18) Fujiwara, T.; Kobayashi, Y.; Kyogoku, Y . ;Kuwabara, M.; Kodama, M.: Seki, S J. Colloid Interface Sci. 1989, 127, 26.
J . Phys. Chem. 1990, 94,819-828
-rP"
Figure 8. Relationship between pseudolattice (AH,) and hydration (AHh) enthalpiesassociated with the phase transition of coagel to gel. (A) and (B) correspond to the OTAI- and OTAC-water systems, respectively.
enthalpy due to a change of electrostatic interaction between positive trimethylammonium ions and negative counterions constituting the polar head groups, and ",is the hydration enthalpy due to a change of the interaction of the positive and negative ions of the polar head groups with water molecules. In eq 1, AH, may be nearly the same for the three systems, so that the following relation (2) may be obtained, on the assumption that the OTAB- and OTAI-water systems exhibit the provisional phase transition of the coagel to gel:
819
In order to clarify the relationship of AHl and Affh in eq 2, Figure 8 exhibits two types of representative enthalpy diagrams. Based on this figure, the following process of the phase transition of the coagel to gel in the heating direction is given: (i) a t the first stage, the polar head groups of the coagel phase are caused to separate into the positive and negative ions by taking up an energy corresponding to their lattice enthalpy (AHl) (endothermic phenomenon); (ii) at the following second stage, both ions have an interaction with water molecules by liberating an energy corresponding to their hydration enthalpy (A&) (exothermic phenomenon). As is well-known, the hydration enthalpy of halide ions increases in the order I- < Br- < C1-. Taking into account this effect, eq 2 provides the following idea that the value of IAHII- IAHhlin the three systems becomes larger in the direction to a negative value in the order OTAI < OTAB < OTAC and, as a result, the assumed enthalpy change of the coagel-to-gel transition given by eq 1 becomes smaller in the same order. This indicates that the hydration enthalpy of the halide counterions is a predominant factor in determining the thermodynamic stability of the gel phase in the three systems. Therefore, diagrams A and B of Figure 8 are assumed to correspond to the OTAI- and OTAC-water systems, respectively. On the basis of this figure, we want to summarize the present result as follows: (i) the gel phase in the OTAC-water system is thermodynamically stabilized by a large hydration enthalpy of the chloride ion, Le., IAZf,I < IAfZhl, and is allowed to exist as the thermodynamic stable state; (ii) the gel phase in the OTAI-water system is not allowed to exist, even in the metastable state, because of a small hydration enthalpy of the iodide ion, Le., I M h l < IAHII, in contrast to the metastable gel phase of the OTAB-water system in which IM,I is assumed to be slightly larger than I A f f h l . Registry NO.OTAC, 112-03-8;OTAB, 1120-02-1;OTAI, 4292-25-5; H,O, 7732-18-5.
X-ray Absorption Spectroscopy, X-ray Photoelectron Spectroscopy, and Analytical Electron Microscopy Studies of Cobalt Catalysts. 2. Hydrogen Reduction Properties David G. Castner,* Philip R. Watson: and Ignatius Y. Chan Chevron Research Company, 576 Standard Avenue, Richmond, California 94802 (Received: March 28, 1989)
XAS, XPS, AEM, and temperature-programmed reduction were used to characterize the hydrogen reduction of bulk Co30,
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and Co304supported on small (Co/Si02-62) and large (Co/Si02-923) pore size silica. The reduction process proceeded as Co304 COO Co, with the COOintermediate species spectroscopicallyidentified on all three samples. The first step of the reduction, Co304 COO,occurred near 300 OC for all three catalysts. The second step of the reduction, COO Co, showed significant differences among the samples, with the ease of reduction decreasing in the order Co304> Co/Si02-62 > Co/Si02-923. The ease of reduction appeared to be correlated to the ease of removing the H 2 0 produced during the COO Co step. The small pores (ca. 30 A) of the Co/Si02-923 sample made H 2 0 removal and reduction for that sample the most difficult. Upon reduction of the Co304supported particles to metallic Co the particle size decreased by 30-50% due to removal of oxygen from the Co304particles. For the reduced Co/Si02-62 sample, the Co particles were 90 A in size and grouped together in 0.1-1-pm aggregates. The crystallographic orientation of the individual particles in each aggre ate became radomized upon reduction and reoxidation. For the reduced Co/Si02-923 sample the Co particles were 35 in size.
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Introduction This study examines the hydrogen reduction properties of three different CO304 catalysts: one bulk Co304sample and two sili'Author to whom correspondence should be addressed at NESAC/BIO, Department of Chemical Engineering, BF-10,University of Washington, Seattle, WA
ca-supported c0@4 samples. A detailed characterization of these CO304 catalysts utilizing X-ray absorption sPectroscoPY (XAS), X-ray Photoelectron sPectroscoPY (XpS), analytical electron microscopy (AEM), and X-ray diffraction (XRD) has already been reported.' c o 3 0 4is the only c o species detected in the
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'Present address: Department of Chemistry, Oregon State University, Cowallis, OR 97331.
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(1) Castner, D. G.; Watson, P. R.; Chan, 1. Y . J . Phys. Chem. 1989, 93, 3188.
0 1990 American Chemical Society
