J . Phys. Chem. 1992, 96, 314-383
314 0.0141
complex according to a report by Hayashi et al.,15J6but details of this research have yet to be presented.
7
'E
P cn
Conclusion
1
b
3 .-
.->
0.0140
s
L
?
8
0'm390
30
60
90
Time (sec.)
Figure 4. Results of simulation by eq 8' for response of conductivity.
A comparison of Figures 3 and 4 and Tables I and I1 shows no significant differences, meaning that we can view e q s 6' and 8' as virtually the same. We then took a look at parameter c, which represents transient response characteristics. The c values between 0.087 and 0.198 s-I were determined using a step-reaction method on similar samples8 and are close to the calculated k values. And calculated k values also agree with the k value obtained in eq 2. The results further corroborate the model submitted by the authors in ref 11 . There external magnetic field effects on the H2-02 reactions on the Sn02surface were caused by the increase in the frequency factor of the reaction between H2and 02-adsorbate on the Sn02 surface by the magnetic field. The model is further supported by the isotope effectI4 of magnetic effects of reactions reported earlier by us. Frequency factor variations are thought to be caused by magnetic field induced" fluctuations in the singlet-triplet conversion rate of the reaction's activated (14) Ohnishi, H.; Sasaki, H.; Ippommatsu, M. Surf. Sci. Lett., to be
submitted.
We studied the transient response characteristics of external reactions by magnetic field effects on S n 0 2 surface H& measuring fluctuations in the conductivity of Sn02thin films. The response was found to lag for the rather long period of several seconds. These results indicate that this phenomenon is solidly based on the change in reaction rate. Further corroboration is provided by results of studies on the steady state given in ref 11. For magnetic field effects, we tried to quantitatively explain transient response characteristics by adopting magnetic field term AB) in the form expressed by eq 9 to relational expression 1, which coherently expresses the relationship of flammable gas concentration to conductivity of Sn02 as determined in other studies. du/dt = (aPG b ) / u - cu (1)
+
du/dt = mB)aPG
+ b l / a - cu
(9) When we performed a curve fitting using least-squares optimization forflB) = (1 + 2kB2)andf(B) = (1 + kBz)2,we found agreement with experimental and calculated values for both cases. We also found agreement with our determined values when we used a pulse reaction technique with c, the parameter representing transient response characteristics. These results clearly support the idea that external magnetic field effects were caused by an increase in the frequency factor of the reaction between H2and 02-adsorbate on the Sn02surface was varied by magnetic fields. It is also clarified that magnetic field terms can be expressed as (1 2kB2) or with higher powers (magnitude of experimental error) in even numbers above 4 for B in that expression. Registry No. H2, 1333-74-0;SnO,, 18282-10-5.
+
(15) Hayashi, H.; Nagakura, S. Bull. Chem. SOC.Jpn. 1978, 51, 2862. (16) Sakaguchi, Y.; Hayashi, H. J. Phys. Chem. 1984, 88, 1437.
Physical Science of the Dioctadecyldimethylammonium Chloride-Water System. 3. Colloidal Aspects R. G. Laughlin,* R. L. Munyon, J. L. Burns, T. W. Coffindaffer, Miami Valley Laboratories, The Procter & Gamble Company, Cincinnati, Ohio 45239
and Y. Talmon Department of Chemical Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel (Received: June 13, 1991) Heating dilute aqueous mixtures of the monohydrate (X-W) crystal of dioctadecyldimethylammoniumchloride (DODMAC) to just below the Krafft discontinuity produces, without intervention of intense mechanical shear, a gel-like state having elaborate colloidal structure. This 'gel" is probably formed by cleavage of the X.W crystal by bulk water between the planes of ions within this crystal. It was found, using cryoelectron microscopy, that many of the particles formed display cusps or are lens-shaped. Unexpectedly, these angular structures are far more prevalent in samples quenched from above the Krafft discontinuity temperature than they are in samples quenched from below this discontinuity. Additional shear encloses many such structures within others, thereby reducing the average particle size and viscosity. It is suggested that both the structure and the composition of the membranes in these colloids are related to the structure and composition of the coexisting bulk phase. If so, DODMAC membranes below the Krafft discontinuity are relatively thin and include only 2 mol (6%) of water, while membranes above the Krafft discontinuity are about three times thicker and include 74 mol (68.5%) of water. The thermodynamic definition of an interface is reviewed, in the context of this issue. The collapse of colloidal structure requires nucleation by small crystals and produces single crystals with minimal excess surface and curvature energy. Once formed, these crystals undergo secondary aggregation into large composites. Nucleation crystals are probably formed at the air/fluid interface. Isothermal swelling of the lamellar (D) liquid crystal phase by water also produces colloidally structured mixtures of liquid and lamellar liquid crystal phases, which display myelin textures. Refractive index data suggest that the fraction of the liquid crystal phase in these mixtures is approximately 0.25. The equilibrium phase diagram of the dioctadecyldimethylammonium chloride (D0DMAC)-water system has recently been
determined' (Figure 1) and the kinetics of various phase reactions investigated.* Hydration (addition) reactions were found to be
0022-365419212096-374%03.00/0 0 1992 American Chemical Society
Dioctadecyldimethylammonium Chloride-Water System
0
L+D-
Xa
$ 4 0
20
Lo
-20
IC.
io
j,
40
klTZzq?+
+ x.2w
Percent Surfactant
X.N
Figure 1. Phase diagram of the DODMAC-water system. Symbols are defined in the Experimental Section. Reprinted with permission from ref 1.
highly variable kinetically and mechanistically simple. Dehydration (disproportionation) reactions were found to be also variable kinetically and mechanistically complex as well. New information regarding the colloid science of this system resulted from this study and is the focus of the present paper. It has been shown in earlier studies that colloidal dispersions of DODMAC can be prepared (at temperatures above the Krafft discontinuity) by the action of intense mechanical shear (sonication) on coarse di~persions.~*~ The sols which are produced contain either unilamellar vesicles or multilamellar liposomes. We found during the present work that colloidal structure can also be created in DODMAC-water mixtures without the intervention of mechanical shear.’ Such a structure may be produced simply by following selected isothermal (constant temperature, variable composition) or isoplethal (constant composition, variable temperature) process trajectories. Not all trajectories produce colloidal structure; those which do have been identified, the structure and stability of the resulting colloidal states have been characterized, and the effect of mild shear on some of these states has been determined. Structures and compositions within colloidal states have been analyzed in light of the thermodynamic and kinetic aspects of the phase chemistry of this system.
Experimental Section The phase symbolism used is the following: L = liquid; X = dry crystal; X-W = monohydrate crystal; X.2W = dihydrate crystal; D = lamellar (La or neat) liquid crystal; m = metastable. For the most part the DODMAC used was the same as that used for the phase study.I A larger sample of lesser purity, prepared by the same synthetic method, was used for the “gel” shearing experiments. DODMAC was assayed using pyrolysis gas chromatography with a Hewlett-Packard HP5880A gas chromatograph having an injector set at 290 OC. Dialkylmethylamines and methyl chloride are the major pyrolysis products. The peak areas of the tertiary amines were assumed to be proportional to the weight fraction of their parent ammonium salt. Each peak was identified as to molecular structure using gas chromatography-mass spectroscopy. The sample used for the phase study contained 99.83% DODMAC and 0.17% of the c2ocl8 homologue.’ The larger sample contained 0.29% C&16,0.73% c&7, 97.6% DODMAC, 0.41% CI9Cl8,0.71% c2&8, and 0.28% c22cl8 homologues. Both samples were recrystallized and equilibrated with air at ca. 35% relative humidity (RH)and were shown (using infrared data) to be the monohydrate crystal X.W.’ Isothermal DIT (Diffusive Interfacial Transport) studies provided unique insight into the colloid science of this system. During these studies an interface is created between DODMAC and water (1) Laughlin, R. G.; Munyon, R. L.; Fu, Y.-C.; Fehl, A. J. J. Phys. Chem. 1990, 94, 2546-2552. (2) Laughlin, R. G.; Munyon, R. L.; Fu, Y.-C. J . Phys. Chem. 1991, 95, 3852-3856. (3) Liposomes: From Physical Structure to Therapeutic Application; Knight, G. C.; Ed.; Elsevier-North Holland Biomedical: Cambridge, 1981. (4) Deguchi, K.;Mino, J. J . Colloid Interface Sci. 1978, 65, 155-161.
The Journal of Physical Chemistry, Vol. 96, NO. 1, 1992 375 within a geometrically and optically uniform capillary cell. The swelling phenomena which ensue, whether simple or complex, are displayed with unusual clarity during these studies. Details of the DIT method? and of its use during this study,’ have been described. Cryoelectron microscopy using ultrafast freezing methods: in combination with an environmental chamber to control temperature and relative humidity immediately prior to freezing,’ were utilized to characterize colloidal structure. The relative humidity was controlled at 95%. Most of the samples were examined in the vitreous state using a Gatan 626 cold stage. Others were freeze-fractured: and platinum/carbon replicas of the fracture surfaces were made using a Balzers 400T freeze-fracture unit. Vitrified specimens were examined in a Hitachi H-500TEM and the replicas in a Hitachi 12A TEM. Acceleration voltage was 100 kV; Kodak SO-163 negatives were developed in full strength Kodak D- 19 for 12 min. EM studies of colloidal structure were supplemented by qualitative observations made during DIT studies. It is possible to distinguish, by inspection, normal swelling (by diffusive transport) from anomalous swelling by nondiffusive processes. In the former case clean, sharp interfaces result, while in the latter case messy interfacialregions are formed within which dispersions of one phase in another exist. Figure 2A shows the clean D/crystal interface observed during the swelling of X.2W crystalsjust above the Krafft temperature, while Figure 2B shows the messy structure which develops in the vicinity of the L/X.W interface near the “gelation” temperature of X-W (below the Krafft temperature). The “gelation” temperature of bulk mixtures correlates well with the temperature at which anomalous swelling commences within the DIT cell.’ Shearing Experiments. Shear was applied to gelled mixtures in three ways. One process utilized passage through a continuous sonication chamber, another involved use of the piston mixing device,’ and the third involved passage through lengths of small diameter tubing. The continuous sonication process has been described el~ewhere.~The only modification necessary for this study was the application of pressure (14 psi) above the feed reservoir. Without this pressure the metering pump would not function. The piston mixing device has also been described.’ During its use in these studies ca. 2-g mixtures were forced back and forth through 0.7 mm diameter X 25 mm long orifices several hundred times. The third process utilized the feed pump of a Gaulin homogenizer, which is in essence a 2.5-L stainless steel syringe. The outlet of this pump was modified to eliminate dead space and to permit the attachment of lengths of small diameter stainless steel tubing. “Gels” were formed within the pump by heating mechanically stirred slurries of X-W with infrared lamps (3 X 250 W). After “gelation” the stirrer was removed, the piston inserted, and 100 psi air pressure was applied above the piston. The coil of tubing at the exit was heated with an infrared lamp. Exit temperatures and flow rates were monitored. The equation for the velocity gradient at the wall inside a tube during laminar flow (assuming Newtonian behavior) is dV,/dr = -4Q/ (aR3) where V, is the velocity in the direction of flow z, r is the radial distance from the centration axis, Q is the mass flow rate, and R is the radius of the tube.I0 These fluids are not Newtonian, but this calculation provides an order of magnitude estimate of the maximum shear rate to which they were exposed. The pressure required to maintain a particular flow rate also depends on the ( 5 ) Laughlin, R. G.; Munyon, R. L. J . Phys. Chem. 1987.91.3299-3305. (6) Talmon, Y. Colloids Surf. 1986, 19, 237-248. (7) Bellare, J. R.; Davis, H. T.; Scriven, L. E.; Talmon, Y. J. Electron Microsc. Tech. 1988, 10, 87-1 11. (8) Bums,J. L.; Talmon, Y. J . Electron Microsc. Tech. 1988, 10, 112-1 14. (9) Laughlin, R. G.; Munyon, R. L.; Ria, S. K.; Wert, V. F. Science 1983, 219, 1219-1221. (10) Bird, R. B.;Stewart, W. E.; Lightfoot, E. N. Transport Phenomena; John Wiley & Sons: New York, 1960; pp 42-44.
376 The Journal of Physical Chemistry, Vol. 96. No. I , I992 viscosity and the length of the tube, but the shear rate profile is independent of these variables. Use of the piston mixing device involved passage through IR orifices over a distance of roughly 1750 cm (700 0.35" passes X 2.5 cm/pass) at a maximum shear rate of ca. 400 s-!. The large "gel" sample prepared using the Gaulin feed pump was passed through 5 X 40 (200) cm lengths of either 0.49-mm or 0.38-mm IR tubing at an average maximum shear rate of ca. 53000 S-I and then through a 4 X 20 (80) cm length of the same tubing at an average maximum shear rate of ca. 90000 s-l. Particle size data on diluted dispersions were obtained using a Malvern System 3601 diffraction particle sizer.
Results Process Trajectories Which Do Not Result in Colloidal States. As colloidal states do not inevitably result from phase reactions within DODMAC-water mixtures, it is important to distinguish those reactions which do lead to colloidal structure from those which do not. For example, the following isothermal crystal hydration reactions do not introduce perceptible colloidal structure: X, + W (vapor) X-W (at room temperature) (1)
- + -
X-W + W (vapor or liquid) X.2W X, X-W, or X.2W
W
(at room temperature) (2) (at T > 47.5 "C) (3)
D
This conclusion follows in the case of reactions I and 2 from the observation that no visible alteration of crystal habit occurs. That colloidal structure is not formed during reaction 3 is clear from the observation that clean discontinuitiesexist at the D/crystal interface during DIT studies (Figure 2). Isoplethal crystal hydrate decomposition reactions at peritectoid phase discontinuities, although mechanistically complex, also do not produce colloidal structure. The two such reactions which occur in this system, and the paths by which they occur, are as follows:2
- - - + X-W
X.2W
fast
fast
[D-W,]
[D*2W,]
slow
slow
X +D,
X=W
D,
(at 72 "C)
(4)
(at 58 "C)
(5)
Phase Reactions Which Do Produce Colloidal States. Two phase reactions do produce colloidal structure without mechanical shear: (a) the isoplethal "gelation" of X-W with water at 39 "C along dilute (10:1 water:DODMAC weight ratio, or >300:1 molar ratio). If cleavage occurred at every polar plane, isolated platelets of single bilayers would initially result; these would be expected to close to form vesicles. It has been proposed that such platelets are also precursors to vesicles during sonication,'6 but in this process they are formed by intense mechanical shear. If small regions of X-2W phase are present, they might remain intact during this process and serve as nucleation sites for the subsequent growth of large crystals (see below). Effects of Shear on Colloidal Structure. The structure of the "gel" formed by heating X-W slurries is altered by additional shear. One pass through a continuous sonication chamber' yields a fluid, turbid dispersion. Extensive passage through small cylindrical orifices using the piston mixing device, at maximum shear rates of about 400 s-', yields a fluid, opaque dispersion.' Passage through short lengths of small bore tubing at higher maximum shear rates (50000 to 90000 s-I) produces only small changes in viscosity. The effects of shear on colloidal structure produced by use of the piston mixing device are evident from comparison of the structures seen in Figure 10 with those in Figures 3-7. The particles are much smaller and often show many levels of encapsulation (sometimes up to ten particles within particles). The outer membrane of the larger liposomes is smooth and without cusps, but many lens-like objects are encapsulated within them. These same objects are also present as individual particles. Encapsulation may occur via opening of the vesicles and liposomes. In some cases ruptured membranes are trapped within another enclosing membrane, without being able to close again (arrow in Figure IO). The reduction in size and volume fraction from many-level encapsulation causes a substantial reduction in the viscosity of the system. This decrease in viscosity allowed us to examine 2% DODMAC aqueous dispersions by direct-imaging cryo-TEM. Colloid Stability. The lifetime of the "gel" formed by heating 5% X-W slurries is greatly influenced by how it is subsequently treated. Samples removed from the source of heat before the Krafft discontinuity temperature is reached and kept at room temperature become white and opaque within a few hours. The whiteness is due to the formation of large crystals. If, however, the "gel" is formed in a sealed container with no void space, and (16) Finer, E. G.; Nook, A. G.; Hauser. H. Biochem. Biophys. Acra 1972, 260( 1 ), 49-58.
Stored at Room Temperature (ea. 26 OC)" mean median dav Darticle diameter Darticle diameter 5% 'Gel" after Modest Shear (Sample 2, Table I) 0 14 31
50 65
6.16 6.52 6.63 6.22 6.14
4.77 4.97 4.95 4.76 4.75
Sonicated 5% Dispersion (Fluid) (Sample 4, Table I) 8 36
11.38 3 1.30
7.65 9.49
"Mean and median particle diameters were determined using laser diffraction data (see Experimental Section). These data indicate that colloidal structure is more stable in the viscous sample than in the fluid sample.
is heated to above the Krafft discontinuity after preparation, it is incredibly long-lived at room temperature. Heated "gels" do not initially differ in appearance from unheated "gels", but one such "gel" prepared within a 50-mL gas-tight syringe, and then immersed in steam for 60 h, has shown no trace of instability (crystal growth) after more than 4.5 years' time. Stabilization of the "gels" hinges on both destroying nucleation sites and preventing their re-formation. Heating the system to above the Krafft temperature rapidly and quantitatively destroys existing sites, but to maintain stability new sites must also be prevented from re-forming. It is believed that new nucleation sites are formed at the interface of these "gels" with air. Ordinarily the air space above samples is not saturated with water, and evaporation will occur. The resulting increase in composition within the surface layer should collapse the "gel" structure. The products of collapse (small crystals) will facilitate further collapse of the "gel" if they are redispersed into the mixture. The extent of nuclei formation by surface evaporation should depend on the ratio of the air/fluid area to the fluid volume (Af/Vf), the ratio of the air space volume to the fluid volume (V,/Vr), and the relative humidity in the air phase. Since nuclei formed at the interface are likely to be redispersed by convection, the redispersion process should be influenced by the viscosity of the dispersion. It follows from these ideas that samples within which nuclei have been completely destroyed, and which lack an air interface altogether, should be indefinitely stable (as observed). In samples which do have an air interface, those having the smallest Ar/ Vf and V,/ Vf,and the highest viscosity, should be the most stable. Both qualitative stability observations (Table I) and quantitative measurements of particle size using laser diffraction (Table 11) are consistent with these ideas. "Gel-like" samples are more stable than are fluid dispersions, and large samples having small values of Af/ Vf and Val Vr are more stable than are small samples having large values of A r / V r and Va/Vr. Product of the Collapse of Colloidal Structure. Some of the particles formed within the small fluid sonicated sample, together with a small amount of the fluid, were isolated and characterized
380 The Journal of Physical Chemistry, Vol. 96, No. I , 1992
Laughlin et al.
+ Nucleation Crystal
Figure 11. Freeze-fracture replica of a vitrified sample of a large crystal that formed, during a span of several months, within a sonicated 5% DODMAC dispersion. The sample was quenched from 25 "C. Interlayer spacings, seen as terraces, are about 4 nm. No curvature of the crystal planes is perceptible. Bar = 100 nm.
using polarized light microscopy (at 1OOX) and freeze-fracture EM. Light microscopy indicated that the particles were large (0.4 to 1.5 mm) composites of smaller crystals. The individual crystals ranged in size from 4 to 120 pm and displayed sharp extinction each 90" of stage rotation. Freeze-fracture studies provided evidence that the crystals within the aggregates were well-formed and also that the liquid phase had been excluded (Figure 11). The layers within the crystals spanned several tenths of a micrometer and were flat and uniform. Interlayer spacings observed at terraces were about 4 nm-wmparable to the long d spacings of the DODMAC crystal hydrates (3.35 nm for XoW, 4.01 nm for X.2W'). These spacing data were not sufficiently precise to distinguish between these two crystals. Mechanism of Collapse. Two classes of mechanisms for the collapse of colloidal structure in this system can be envisioned: (1) ripening and (2) flocculation followed by stacking. It is impossible that these processes occur by ripening. The solubility of DODMAC is probably vanishingly small,' and it has been established that the rate of ripening is proportional to solubility." Flocculation, followed by 'stacking", remains a plausible pathway. Flocculation in this instance involves the collision and attachment of vesicles to the surface of nucleation crystals (Figure 12). The rate of this step should be governed by the product of the particle number concentrations of the crystals (PNC,) and of the vesicle particles (PNC,), respectively. When PNC, = 0, the rate of collapse should be zero, as was observed.
Stacking may be envisioned as the rearrangement of an adhering vesicle to form a new bimolecular layer at the crystal surface (Figure 12). During this process the interface to the crystal side of the vesicle membrane vanishes and becomes a structural plane, while that to the other side remains an interface (see Discussion). Both specific surface area and curvature are reduced and, as a result, the excess colloidal energy of the system is lowered. Once crystals of sufficiently low excess energy exist, the driving force for change becomes small in comparison to thermal energies and the rate of change slows. Our observations indicate that after the growth of crystals reaches a certain point, secondary flocculation to form composite aggregates occurs rather than continued crystal growth. Anomalolls Swelling of the Lamellar Liquid Crystal Phase. The other phase reaction which produces colloidal structure without
mechanical shear is isothermal swelling of the D phase by water. This conclusion is based on the observation that during DIT studies complex textures exist at the L/D interfacial region and on quantitative index data from these studies. Figure 13 depicts some of the textures observed. These textures are neither time-stable nor exactly reproducible. In comparing Figure 13A with Figure 13B, for example, the visible differences which exist had developed within a few minutes. Over a period of hours, the texture of this region was transformed into that shown in Figure 13C. These structures have macroscopic dimensions and are likely influenced by the flatness and geometry of the chamber. The coexisting phases which make up these structures above the Krafft discontinuity are the 0% liquid phase and the 31.5% D phase. The ratios of these two phases may be estimated from index elevation data,' which give an average value for indices within a 6-pm diameter (30 pm2) area.'* Statistical analysis of 42 data gave a mean index elevation to the D side of the L/D interface of 10.0 IEIOOO units and a standard deviation of 3.9 units. (IE1000 = (4- n,,,)IOOO, where n, is the index of the sample and n, is the index of water). This is an enormous variability in comparison with the intrinsic precision of these optical measurements. For replicate determinations of index elevation at the same position, standard deviations of 0.05 unit are typically observed." Index elevations at phase boundaries, which require the extrapolation of data at
( 1 7) Kabalnov, A. S.; Pertsov, A. V.; Shchukin, E. D. J . Colloid lnrerfuce Sci. 1987, 118, 590-597.
(18) Laughlin, R. G.; Marrer, A. M.; Marcott, C.; Munyon, R. L. J . Microsc. (Oxford) 198s. 139, 239-247.
crystal
+ vesicle
-
[crystal.vesicle]
rate = k(PNC,)(PNC,)
The Journal of Physical Chemistry, Vol. 96, No. I , I992 381
Dioctadecyldimethylammonium Chloride-Water System 1
b
I
Figure 13. Myelin textures seen a t the L/D interfacial region in the DIT cell a t 50 OC. The complex texture (A) formed within an hour after the interface was created. The clear region to the upper left is the liquid phase, and the untextured D phase band is to the right out of the picture. A few minutes later (B), perceptible changes had occurred (at arrow) in this texture. Five hours later, the texture had changed to C. The width of the light upper region is 280 @m.lR
six positions to the boundary position, display somewhat larger standard deviations. For example, at these same L/D interfaces the mean and standard deviation of the IElOOO values for the coexisting liquid phase were found to be 0.34 and 0.17 units, respectively. Assuming an IElOOO for the 3 1.5% D phase of 40,' it follows that the mean fraction of D phase within these textured mixtures is approximately 0.25, and the standard deviation of this phase fraction is about 0.10. These colloidal structuresare qualitatively similar to the myelin textures observed during the swelling of polar lipids and related compounds by water.'' The gross compositions, phase compositions, phase structures, and phase ratios of mixtures having myelin textures have not previously been characterized.
Discussion Irreversible colloidal structure within biphasic mixtures is defined by the manner in which the two phases are arranged in space. Such a structure is always metastable, to a degree governed by (19) Mishima, 149-153.
K.; Yoshiyama, K. Biochim. Biophys. Acta
1987, 904,
its excess surface and curvature energies. Interfaces vs Structural Planes. A fundamental issue with respect to the colloid science of DODMAC and related systems is "what defines an interface?". How interfaces are defined influences analysis of both the structure and the composition of the membranes within these DODMAC colloids. In the classical treatment of heterogeneous equilibria individual phases are regarded as being homogeneous (both structurally and thermodynamically), and an interface is the plane surface which separates coexisting phases.20 In the days when the only phases studied were gases, liquids, and crystals,2' there was little ambiguity as to the position of interfaces. More recently, the matter of deciding what is and what is not an interface has become less clear. The capacity to determine phase structure at the molecular level has been highly developed, and words such as "interface" and "surface" have been used to describe molecular aggregates, as in the pseudo-phase separation model of micelles22*23 and in describing the structures of microemulsion phase^.^^^^^ Classical Thermodynamic Definition of an Interface. During every DIT study a sequence of connected liquid, liquid crystal, or crystal phase bands develop which span the entire composition range. These bands are typically separated by, and coexist at, sharply defined interfaces. (The swelling behavior of X*W dzscribed in this work is very unusual and is not seen even in this system during the swelling of the equilibrium X-2W phase (Figure 2).) It is worth noting that phases within the bands which develop during DIT experiments display concentration gradients and are therefore not at equilibrium. Equilibrium phases exist during DIT studies only at the interface. Within the dilute liquid band of soluble surfactants discontinuities are not seen in passing from molecular to micellar solutions at the cmc. Since composition, as well as temperature and pressure, must be stated to explicitly define mixtures within the liquid phase band, this region clearly has three degrees of freedom (F). Since for binary systems the Phase Rule requires that P + F = 4, the number of phases (P)throughout the composition range of the liquid region is necessarily one.26 Discontinuities in composition exist at the interfaces between bands.5 If the upper composition limit of the liquid phase for a typical soluble surfactant is taken as 2596, the composition of the coexisting liquid crystal phase might be ca. 26%. For DODMAC, the composition of the liquid phase is almost exactly O%, while that of the D phase is 3 1.5%. Equilibrium phases do not exist within these gaps in miscibility (25-26%, or 0-31.5%), and mixtures having compositions within these gaps are not formed by swelling. The coexisting phases which do exist in mixtures within these gaps are invariant, at specified temperature and pressure. The concentrations of these phases and all of their properties are fully defined and are independent of gross composition. Since temperature and pressure suffice to fully define the phases, F = 2 and P = 2 as well. While molar free energies (and other density variables2') vary continuously within each band, discontinuities in the density variables exist at the interfaces. Chemical potentials (and other ~~~~~
~~~~
(20) Gibbs, J. W. In The Collecred Works of J. Willard Gibbs; Longley, W. R., Van Name, R. G., Eds.; Longmans: Green and Co., 1928; pp 55-353. (21) R m z e b m , H.W. B. Die Hererogene Gleichgewichrp, Braunschweig: Germany, 1904; Vol. 1 and 2, 1st Part. (22) Nagarajan, R.; Ruckenstein, E. J. Colloid Interface Sci. 1977, 60, 221-23 1. (23) Lindman, B.; Wennerstrom, H. Fortschr. Chem. Forsch. 1980,87, 1-83. In this review reference is made mostly to the micellar "surface", but occasionally (p 63) to the "interface" between the micelle and its aqueous
environment. (24) Bouml, M.;Schechter, R. S . Microemulsions and Related Systems; Marcel Dckker, Inc.: New York, 1988; p 15. The "C" region is described as the 'interfacial" region separating essentially bulk-phase water and bulkphase oil. (25) Langevin, D. Ado. Coll. Inrerface Sci. 1990, 34, 583-595. (26) Hall, D. G.;Pethica, B. A. In Nonionic Surfacrants, Surfacranr Science Series, Vol. 1; Schick, M. J., Ed.; Marcel Dekker, Inc.: New York, 1967; pp 516-557. (27) Griffiths, R. B.; Wheeler, J. C . fhys. Rev. A 1970, 2, 1047-1064.
382 The Journal of Physical Chemistry, Vol. 96, No. 1, 1992 field variables), in striking contrast, vary smoothly along the entire length of the sample-both within bands and across interfaces. Thus, the distinctive feature of the plane surfaces at the interfaces between coexisting phases is that spatial discontinuities in density variables exist at these particular surfaces. This is the classical thermodynamic definition of an interface. Defining the position and structure of interfaces in a microscopic sense, at molecular dimensions, is another matter. Studies of microstructure would reveal the existence of surfaces within many phase bands, but in samples a t equilibrium discontinuities in density variables do not exist at these internal surfaces. Local fluctuations may occur with time and with position along trajectories in space which span several structural planes, but neither unbalanced forces (tensions) nor discontinuities in any thermodynamic variable exist over a span of many molecular distances and during long times. It is in this sense that even structured phases remain “homogeneous”. A convenient term for surfaces within phases is “structural planes”.28 Structural planes are clearly evident within crystal and liquid crystal phases, within some liquid phases,2s at the surfaces of micellar aggregates, and within microemulsion phase^.^^^^^ Structural planes represent a nonuniform, but orderly, distribution of molecules within a macroscopically homogeneous phase. Actually, nonuniform molecular distributions, but not necessarily a high degree of order, exist within all phases. Even gas phases of low density display clusters, or *van der Waals” mole~~les,2~ and are nonuniform in density within sufficientlyshort spans of distance and time. Interfaces within Colloidal Structures. At the “chain-melting“ transition of vesicles, emphasis has historically been placed on the observable discontinuities in enthalpy and chain structure. If bulk phase chemistry is relevant, however, it follows that not only the structure and enthalpy of vesicle membranes but their composition as well should reflect these aspects of the coexisting bulk phases. Structures and compositions should change with temperature, both continuously and discontinuously, in a manner consistent with the phase diagram. Both structure and composition, however, may be perturbed relative to those of the bulk phase by the excess energy of the colloidal state. Applying these ideas to DODMAC suggests that the vesicle membranes at room temperature should (at equilibrium) resemble the dihydrate crystal X.2W. The molecular arrangement within the membranes should resemble that which exists within this crystal, and the composition of the membrane should be about 94% DODMAC and 6% water. Extrapolating information on bulk phase reaction kinetics2 to the vesicle state suggests that the equilibrium composition will be attained within minutes after cooling vesicle dispersions below the Krafft discontinuity, while the equilibrium structure may require up to 3 months to develop. If a DODMAC vesicle dispersion is heated, starting from room temperature, the structure and composition of its membranes should suddenly change at the Krafft discontinuity temperature from those of the dihydrate crystal to those of the coexisting liquid crystal (Figure 14). No mass transport is necessary for these changes to occur. The positions of the interfaces simply jump from near the surfactant bilayer to a position farther into the water region. At low temperatures the inner and outer interfaces will lie close to the DODMAC ions, encompassing between them (at equilibrium) two water molecules for each ammonium chloride ion pair. Above the Krafft discontinuity the interfaces will exist farther out, encompassing 74 water molecules per ion pair. The total thickness of the membrane will increase by a factor of about 3. The introduction of gauche chain conformations will cause the volume of the surfactant component to increase but its thickness to shrink. Some water transport through the membrane may be necessary to accommodate these dimensional changes. (28) Kilpatrick, P. M.; Davis, H. T.; Scriven, L. E.; Miller, W. G. J . Colloid Interface Sci. 1981, 118, 270-285. (29) Van der Waals Molecules, Faraday Discussion of the Chemical Society No. 73, Royal Society of Chemistry; London, 1982.
Laughlin et al.
L
L
L
\ ( , 1 ” ’ ~ ~ # ) I* 8’ 8 8
L
**
Figure 14. Cartoon depicting the changes in membrane structure and composition which m r in passing from below the Krafft discontinuity temperature to above it. The positions of interfaces are indicated by the dashes. Chloride ions are indicatedby minus signs (-). Water molecules, in the upper figure, are indicated by small carats ( A ) . Above the Krafft discontinuity(lower figure) about 74 water molecules per DODMAC ion pair exist between the interfaces, and the cationic molecules are less ordered conformationally.
Changes in membrane thickness and water content will not be visible using EM (or scattering) methods. EM detects electron density gradients-not thermodynamic discontinuities.30 EM can distinguish surfactant from water but most likely cannot distinguish water in membrane phases from water in liquid phases. A similar picture will apply at the surface of multilamellar structures (liposomes) as the Krafft discontinuity is crossed, but the internal situation is different. Transport of water-both across the interface and within the bulk phase-is required for equilibrium to be attained in these cases, and such transport will require time. Both colloidal structure and diffusion kinetics within the respective phases will influence the kinetics of equilibration of multilamellar particles. Changes in membrane composition at the Krafft discontinuity dictate changes in phase ratios, because in a closed system a change in the hydration of the membrane requires an equal and opposite change in the water content of the liquid phase. When the fraction of surfactant is high these changes may be globally important, because they will rob water from, or liberate water to, the coexisting liquid phase. This, in turn, will affect the chemical potentials of all the soluble components within the system. This particular consequence of heating or cooling a system past phase-transition temperatures needs to be considered, along with the changes in membrane structure which occur at these tem-
(30) Reimer, L. Transmission Electron Microscopy; Springer-Verlag: Berlin, 1985; pp 185-258.
J. Phys. Chem. 1992,96, 383-387 peratures.
Acknowledgment. The assistance of G. M. Bunke in synthesizing the materials used and in doing the shearing experiments, of Dr. M. H. Chestnut in analysis of the EM data, of Dr. J. D. Oliver, L. C. Strickland, and S.M. Thoman for the determination,
383
interpretation, and graphic displays of the crystal structure, of M. L. Crews in information science, and of E. Fletcher in doing the art work is gratefully Registry No. DODMAC, 107-64-2.
A Study of the Reaction of Ethylene Oxide on Coldly Deposited Silver Surfaces by SERS and EELS X.J. Gu, K.L.Akers, and M. Moskovits* Department of Chemistry and the Ontario Laser and Lightwave Research Centre, University of Toronto, Toronto, Ontario M5S l A l , Canada (Received: August 8, 1991)
Surface-enhancedRaman spectroscopy (SERS) and electron energy loss spectroscopy (EELS) were used to study the chemical transformation to products of ethylene oxide adsorbed on a coldly deposited silver surface heated to various temperatures. Spectra resulting from SERS indicate that, at temperatures below 110 K, ethylene oxide decomposes into an intermediate that is likely H2C=CH2bonded to a single adsorbed oxygen atom or to a metal site near an oxygen atom. At higher temperatures, ethylene oxide undergoes extensive decomposition to organic fragments and to CO. Reaction of ethylene oxide was also observed by EELS. However, the two spectroscopiesdid not seem to be equally sensitive to the various decomposition products observed. This leads to the conclusion that SERS and EELS probe different sites on the silver surface.
1. Introduction The study of chemical reactions occurring at metal crystal surfaces has proved invaluable in elucidating processes that are relevant to catalysis. Among the large number of techniques for studying the surface reactions, electron energy loss spectroscopy (EELS) has been widely used to obtain the vibrational spectra of adsorbed molecules, often leading to a conclusive identification of the adsorbed species. This is exemplified by the studies of adsorbed hydrocarbons on transition- and noblemetal Although EELS can detect even submonolayer quantities, it has two limitations: First, the resolution of EELS is about 50 cm-' and in most cases less. Second, EELS requires a well-ordered single crystal surface to achieve a good signal to noise ratio. Supported metal catalysts often involve surfaces with high surface to volume ratios and may possess radically different chemical properties from those of most low-index single-crystal planes. Surface-enhanced Raman spectroscopy (SERS) certainly overcomes the first limitation. Moreover, provided that one is studying reactions on the few metals that are strongly enhancing, it may reveal processes occurring at sites not unlike those found on supported metal catalysts. Interest in ethylene oxide (eo) adsorption and its surface reactivity stems mainly from the heterogeneous catalysis of the reaction of ethylene and oxygen to form ethylene oxide. Silver is known to be a unique catalyst for this reaction. Despite vigorous prior s t ~ d y , ~several -~ mechanistic issues remain unresolved. (1) Hatzikos, G. H.; Masel, R. I. Surf.Sci. 1987, 185, 479. (2) Keol, B. E.; Bent, B. E.; Somorjai, G. A. Surf. Sci. 1984, 146, 211. (3) Mate, C. M.; Kao, C. T.; Bent, B. E.; Somorjai, G. A. Surf.Sci. 1988, 197. 183. (4) van Santen, R. A.; Kuipers, H. P. C. E. Advances in Catalysis; Academic Press: New York, 1987; Vol. 35, p 265. (5) (a) Campbell, C. T.; Paffett, M. T. Appl. Surf. Sci. 1984, 19, 28. (b) Campbell, C. T.; Koel, B. E. J . Coral. 1985, 92, 272. (c) Campbell, C. T. J . Catal. 1986, 99,28. (d) Campbell, C. T. J . Phys. Chem. 1985,89, 5789. (e) Campbell, C. T. In Catalyst Characterization Science; Deviney, M. L., Gland, J. L.. Eds.; ACS Symposium Series 288; American Chemical Society: Washington, DC, 1985; p 210. (f) Grant, R. B.; Lambert, R. M. J . Catal. 1985, 93, 92. (g) Grant, R. B.; Lambert, R. M. Longmuir 1985, 1, 29. (6) (a) Campbell, C. T.; Paffett, M. T. Sur/. Sci. 1984, 139, 396. (b) Campbell, C. T. Vac.Sci. Technol. 1984, A2, 1024. (c) Campbell, C. T. J . Catal. 1985,94,436. (d) Grant, R. B.; Lambert, R. M. J. Chem. Soc. Chem. Commun. 1983, 662. (e) Grant, R. B.; Lambert, R. M. J. Catal. 1985, 92, 364.
Among them is the identification of the surface oxygen precursor that takes part in the formation of eo. On this point opinion falls into one of two global group: one favoring an O2precursor (either a surface superoxo or surface peroxo specie^)^,^ and the other a surface atomic oxygen precursor. Even in the latter case some propose or imply a simple 02species, while Carter and Goddard7 suggest that the important oxygen is a surface oxyradical (0-) whose presence on the silver surface is only expected at rather large values of the coverage. Van den Hoek et a1.,8 on the other hand, claim that while a single oxygen is indeed the species involved, efficacy in forming eo is only realized in the presence of subsurface oxygen atoms. The mechanism of the oxygen-ethylene interaction is also dictated by the form of the oxygen involved. The suggestion of Carter and Goddard implies strongly that the reaction is between adsorbed oxygen and gas-phase ethylene (Eley-Rideal mechanism). The other possibilities for the character of the surface oxygen, on the other hand, are less restrictive. Often they are equally consistent with either a reaction between coadsorbed species (Langmuir-Hinshelwd mechanism) or one between an adsorbed oxygen and a gaseous reaction partner. The role of chlorine and alkali promoters also remains a point of debate. Van den Hoek et al.,* for example, suggest that chlorine simply plays the same role as subsurface oxygen in the absence of the latter, while Campbell and c o - ~ o r k e r claim s ~ ~ that chlorine blocks the sites involved in the combustion reaction that competes with epoxidation. Carter and Goddard concur with this role for chlorine. Several quantum mechanical computations pertinent to epoxidation have appeared. Their results are not universally consistent. Carter and G ~ d d a r dfor , ~ example, find that ethylene forms a complex with their proposed surface oxyradical stabilized by 0.2 eV with respect to the reagents. Van den Hoek et a1.,* on the other hand, find that the interaction between ethylene and an adsorbed oxygen atom is repulsive in the absence of subsurface oxygen, in whose presence there is a stabilization in excess of 1 eV. One problem encountered in the study of ethylene oxidation under UHV conditions is the fact that ethylene desorbs in vacuum (7) Carter, E. A,; Goddard, W. A., 111. J . Catal. 1988, 112, 80. (8) van den Hoek, P. J.; Baerends, E. J.; van Santen, R. A. J . Phys. Chem. 1989, 93, 6469.
0022-365419212096-383$03.00/0 0 1992 American Chemical Society
