4382
J. Phys. Chem. 1985,89, 4382-4386
Structure of AOT Reversed Mlcelles Determined by Small-Angle Neutron Scattering Michael Kotlarchyk, John S. Huang, Exxon Research and Engineering Company, Annandale, New Jersey 08801
and Sow-Hsin Chen* Nuclear Engineering Department, Massachusetts Institute of Technology, Cambridge, Massachusetts 021 39 (Received: December 18. 1984; In Final Form: May 16, 1985)
Small-angle neutron scattering measurements were carried out on AOT/decane reverse micellar solutions. The scattered intensity distributions were studied as a function of contrast by varying the scattering-length density of the solvent. Information was extracted on the size, shape, aggregation number, and internal structure of the reversed micelles. An additional study of dilute AOT/cyclohexane solutions shows that aggregates are only detectable above a concentration of about 0.225 mM.
Introduction TABLE I: Previous Investigations of the Cmc for AOT Reversed Micelle Formation in Hydrocarbon Solvents Over the past 40 years, there have been many investigations into the formation and solubilization of reversed micelles in solvent method cmc. m M ref nonpolar solvents.l4 Some of these studies address the question benzene light scat 2.9 11 of overall micellar size and shape both in the presence and in the light scat 2.8 12 absence of solubilized water. Experimentally, however, it is very vap press. depress. 2.3 12 difficult to measure internal structure within the micelle, especially dye absorption (TCNQ) 2.0 13 when only trace quantities of water are present. dye absorption (iodine) 13 0.9 spec conduc Because AOT (sodium di-2-ethylhexylsulfosuccinate; mol wt 0.5 14 2.1 positron annihilation 15 444.5) is soluble in many nonpolar solvents over a wide range of cyclohexane light scat 1.45 11 concentrations, a large number of studies addressing the structure dye absorption (iodine) 0.2 13 of reversed micelles have concentrated on examining hydrocarbon UV spectra 0.95-1.1 16 solutions of this surfactant. For example, combined ultracenisooctane positron annihilation 0.6-0.9 15 trifugation, light scattering, and viscometric measurements in decane X-ray scat 0.73 17 various solvents by Peri7 have suggested that the inverted AOT micelles are monodisperse, unsolvated aggregate that are only small-angle neutron scattering (SANS) technique. An external slightly aspherical in shape. In addition, a direct correlation was contrast variation e ~ p e r i m e n twas ' ~ performed on a dilute system observed between the micellar aggregation number and the solvent in order to obtain a direct measure of the structure of the AOT molal volume. Light and X-ray scattering studies by Ekwall et micelles. In this experiment, the intensity of scattered neutrons aL8 have yielded similar results for AOT/p-xylene solutions. These was measured as a function of the solvent scattering-length density, two studies indicate that the aggregation number is in the which was varied by mixing protonated and deuterated decane neighborhood of 20-30 monomers per micelle. in different proportions. Since a photon correlation experiment A question still remains as to whether a viable critical micelle by Zulauf and Eicke20 has demonstrated that the aggregation concentration (cmc) exists in these micellar s y s t e m ~ . ~For ~ ~ ~ ~ Jnumber ~ of AOT micelles remains constant over a wide range of example, Ekwall et a1.* failed to identify a marked critical conconcentrations, we felt it was necessary to only perform the centration below which no micelle formation occurs, while other contrast variation at a single, dilute concentration. At higher investigators have claimed to observe a ~ r n c ' ~ (Table - ' ~ I). The concentrations one might expect intermicellar interactions to start latter studies indicate that the precise cmc depends on both the playing a significant role, but the characteristic micellar size and polarity of the solvent13 and the amount of water present in the shape should not change as long as the solution remains below system.'* the close-pack volume fraction of approximately 74%.832' By Here, we have chosen to investigate the detailed characteristics combining the results of the contrast variation experiment with of AOT reversed micelles in n-decane solvent by applying the previous SANS results on AOT systems, we are able to construct a detailed picture of the AOT micelle. Furthermore, by per(1) Mathews, M. B.; Hirschhorn, E. J . Colloid Sci. 1953, 8 , 86. forming an additional series of SANS measurements on AOT/ (2) Singleterry, C. R. J . Am. Oil Chem. SOC.1955, 32, 446. cyclohexane solutions at various dilute concentrations, we are able (3) Winsor, P. A. Chem. Rev. 1968, 68, I . to identify a concentration below which no well-defined aggregate (4) Fendler, J. H. Acc. Chem. Res. 1976, 9, 153. can be detected. (5) Kertes, A. S.;Gutmann, H. In "Surface and Colloid Science"; Matijevic, E., Ed.; Wiley: New York, 1976; Vol. 8. Experimental Section (6) OConnor, C . J.; Lomax, T. D.; Ramage, R. E. Adu. Colloid Interface Sci. 1984, 20, 21. Samples. The surfactant, sodium di-2-ethylhexylsulfosuccinate (7) Peri, J. B. J . Colloid Interface Sci. 1969, 29, 6. (AOT), was supplied by Fluka. Our purification procedure has (8) Ekwall, P.; Mandell, L.; Fontell, K. J . Colloid Interface Sci. 1970, 33. 215. been described elsewhere.22 We employed Karl Fischer titration (9) Kertes, A. S.In "Micellization, Solubilization and Microemulsions"; and thermal gravimetric analysis to determine the water content Mittal, K. L., Plenum Press: New York, 1977; Vol. 1. of the purified AOT surfactant and found that the molar ratio (10) Muller, N. J . Colloid Interface Sci. 1978, 63, 383. X = [H20]/[AOT] was roughly 0.7 f 0.2. Study of the structure (1 1 ) Kitahara, A,; Kobayashi, T.; Tachibana, T. J . Phys. Chrm. 1962.66, of the AOT reversed micelles in n-decane was performed by means 363. (12) Kon-no, K.; Kitahara, A. Kogyo Kagaku Zasshi 1965, 68, 2058. of an external contrast variation experiment on solutions consisting (13) Muto, S.; Meguro, K. Bull. Chem. SOC.Jpn. 1973, 46, 1316. (14) Eicke, H. F.; Arnold, V. J . Colloid Interface Sci. 1974, 46, 101. (15) Jean, Y.-C.; Ache, H. J. J . Am. Chem. SOC.1978, 100, 6320. (16) Fendler, J. H.; Fendler, E. J. "Catalysis in Micellar and Macromolecular Systems"; Academic Press: New York, 1975. (17) Assih, T.; Larche, F.; Delord. P. J . Colloid Interface Sci. 1982, 89, 35. (18) Eicke, H. F.; Christen, €I. Helu. Chim. Acta 1978, 61, 2258.
(19) Jacrot, B. Rep. Prog. Phys. 1976, 39, 91 1. (20) Zulauf, M.; Eicke, H. F. J . Phys. G e m . 1979, 83, 480. (21) Huang, J. S.; Safran, S. A.; Kim, M. W.; Grest, G. S.; Kotlarchyk, M.; Quirke, N. Phys. Reu. Lert. 1984, 53, 592. (22) Kotlarchyk, M.; Chen, S. H.; Huang, J . S.; Kim, M . W. Phys. Rec. A 1984, 29, 2054.
0022-3654/85/2089-4382$01.50/00 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 20, 1985 4383
AOT Reversed Micelles of 3% (w/v) AOT in CIOH22/C1&mixtures. In order to identify a minimum AOT concentration for detectable aggregates, solutions of 0.225-6.75 mM AOT in deuterated cyclohexane (C6D12) were also measured by using the SANS technique. Cambridge Isotope Laboratories supplied both deuterated solvents at a purity of 98%. The protonated decane was a 99%+ gold label product from Aldrich Chemical Co. All samples were enclosed in cylindrical cells with high-quality 1-mm-thick quartz windows. For the contrast variation experiment, the path lengths of the samples containing the fully deuterated and fully protonated decane were 4 and 1 mm, respectively. All other samples, including those containing deuterated cyclohexane, were of 2-mm thickness. The samples were maintained at a temperature of 24 OC. SANS Measurements. The contrast variation measurements of AOT in decane were performed at the High Flux Beam Reactor of the Brookhaven National Laboratory (BNL). The samples in deuterated cyclohexane were measured at the 30-m SANS facility at the Oak Ridge National Center for Small Angle Scattering Research (NCSASR). Since the two facilities have been described e l s e ~ h e r e ,only ~ ~ ?the ~ ~neutron wavelength X and range of the wavevector transfer Q are quoted here for each spectrometer: X = 5.15 A and 0.02 < Q < 0.17 a t the BNL, X = 4.15 8, and 0.04 < Q < 0.21 at the NCSASR. The measured transmission of the cyclohexane solutions was near 87%, while that for the C10H22/C10D22 samples ranged between 40 and 68%. Most of the attenuation in the latter samples was due to incoherent scattering from the protonated solvent. The counting time for each sample was between 2 and 4 h. Following data collection, a pixel-by-pixel correction25 was applied to each spectrum to account for spatial nonuniformity of the two-dimensional detector sensitivity, background, and empty cell scattering. Each corrected array was then radially averaged to give values of the relative scattering intensity vs. Q. In the case of the external contrast variation measurements, the scattering from the pure solvent was independently measured and then subtracted from each spectrum. As described in the Appendix, the intensity spectra can be converted into an absolute scale in units of probability of scattering per unit path length per unit solid angle dZ/dCl (cm-I) by measuring the scattering spectrum from a pure protonated hydrocarbon solvent.
Data Analysis and Results In the external contrast variation experiment, the intensity dZ/dQ(Q) is measured as a function of the solvent scatteringlength density, ps, which is varied by mixing protonated and deuterated decane in different proportions. For a dilute solution of monodisperse rni~elles,’~
Here, np is the number density of micelles and p ( 3 is the local scattering-length density within the solvent-excluded micellar volume V,. In the limit of veryjmall Q, one may expand the If the center of gravity of exponential in eq 1 in powers of p(F) corresponds to the center of gravity of the micellar volume, then the odd powers vanish and one obtains, to the order of Q2 Q B i .
where the mean scattering-length density of the micelle is
(23) Koehler, W. C.; Hendricks, R. W.; Child, H. R.; King, S. P.; Lin, J. S.; Wignall, G . D. In “Scattering Techniques Applied to Supramolecular and Nonequilibrium Systems“;Chen, S . H., Nasal, R., a s . ; Plenum Press: New York, 1981. (24) Schneider, D. K.; Schoenborn, B. P. In “Neutrons in Biology”; Schoenborn, B. P., Ed.;Plenum Press: New York, 1984. (25) For example, see: “User Notes for the 30 Meter SANS Instrument”; National Center for Small-Angle Scattering Research, Septembex 1983; Schneider, D. K. ‘User Manual: H9B Biology Small Angle Spectrometer”: Brookhaven National Laboratory, June 1984.
and R , is the micellar radius of gyration defined by
One can now write eq 2 as the usual Guinier expression
where
The resulting Guinier plots of the 3% solutions of AOT in CIOD22/C10H22 mixtures are shown in Figure 1 as a function of the weight fraction W of deuterated decane.26 The intensities are displayed on a relative scale; however, the extrapolated zero-angle intensities were later converted to absolute cross sections by using the method described in the Appendix. An accurate calibration of the absolute intensity is essential for determining the micellar aggregation number, as shown below. Let nAOT be the number density of AOT micelles in the solution (nAOT = 3.96 x 1019 cm-3 for a 3% solution) and let iV be the aggregation number of AOT molecules in a micelle. Then one knows that
where uAOTis defined as the solvent-excluded volume of an AOT molecule and its associated bound water in the micellar state. One can now use eq 6 to write
i.e., a plot of [dZ/da(0)/nAoT]’/2 vs. ps should give a straight line, with the value of p given by the intercept of the line with the ps axis. From the value of p so obtained, one can calculate uAOT from the relation UAOT
= (~AoT+ x b H , o ) ~ ” ~
(10)
Here, bAm and bz0 are the scattering lengths of an AOT molecule and a water molecule, respectively. Since no D-H exchange between the micelle and the solvent is expected, the scattering ~ X 8, and bH20 lengths can be calculated to be b ~ =o4.155 = -0.168 X lo4 A. Furthermore, the slope of the line defined by eq 9 gives the quantity N ’ / 2 ~ A O T , thus providing a direct determination of the micellar aggregation number. Figure 2 shows a plot of [dZ/dn(0)/nAoT]’/2vs. ps for the 3% AOT solution. The scattering-length densities of the solvent were calculated from PS
= ~ P D D+ (1 - X)PPD
(1 1)
where the scattering-length densities of the protonated and deuterated decane are pPD = -0.49 X 10” A-2 and pDD = +6.36 X 10” k2, respectively. x, the molar fraction of deuterated decane, was computed from the weight fraction W by using the relation x =
W/ADD W / A D D+ (1 - W)/APD
(12)
where the molecular weights of protonated and deuterated decane are respectively ApD = 142.3 and A D D = 164.4. The results of the contrast variation experiment are summarized below:
.k2 = 612 A3
p = 0.66 UAOT
X
N = 22 (26) T h e values of Wwere corrected for the fact that the deuterated decane stock used in the sample preparation was 2% protonated.
4384
The Journal of Physical Chemistry, Vol. 89, No. 20, 1985
Kotlarchyk et al.
1000
W = 0.980
*
RG(A) = 11.7
0.783
11.6
0.686
11.2
0.591
11.8
0.490
10.5
0.408
11.0
0.000
7.3
CI
'I 100
10
' 0
2 x' '/2
0.03
0.02
0.01
@(A
- 2)
Figure 1. Relative intensity scattered at small Q by a solution of 3% AOT in various CIOH22/CIOD22mixtures (W= weight fraction of C10D22). The plots are the logarithm of the intensity against Q2 (Guinier plots) which, according to eq 5, are linear. RG is the radius of gyration, as calculated from the slope of the best fit line.
We believe that these values are accurate to within approximately 10%. The measured value of vAOT compares very well with the value of 649 A3 for the specific volume of an AOT molecule as measured in p-xylene by Ekwall et aL8 Our result for the aggregation number is about 20% less than the value of fl = 27 previously measured by Peri.' The spectra from AOT in deuterated cyclohexane are shown in Figure 3. Below 0.225 mM of AOT, the spectrum is indistinguishable from that of the pure solvent, and one is unable to detect a well-defined particle. At higher concentrations, however, the spectrum clearly rises above the base line, signaling the existence of surfactant aggregates. The value of R G between 0.675 and 6.75 mM lies between 9.5 and 10.2 A, which is less than the value of 11.7 A found in deuterated decane. Since the molal volume of cyclohexane (108 cm3) is less than that of decane (1 95 cm3), this measurement is in agreement with Peri's conclusion that the micellar size is an increasing function of solvent molal v01ume.~ Although the characteristic micelle size remains approximately constant, the extrapolated zero-angle scattering intensity dZldQ(0) increases considerably with the AOT concentration. This reflects the fact that the number density of micelles increases with concentration above the minimum value of about
0.225 mM. This concentration can be compared with the wide range of values reported by other investigators for the cmc of AOT in hydrocarbon solvents. Picture of the Reversed Micelle From the previous results, one can show that the reversed micelle is nearly spherical. The total solvent-excluded volume of the micelle is found from eq 8 to be 1.35 X lo4 A3, which corresponds to a radius of R = 14.8 A, assuming a spherical micelle with no oil penetration into the tails. This radius should be compared to the one extracted from the measured radii of gyration. To do this, one should note that by substituting p(7) = i j + Ap(7) into eq 4 R G is~ given by
Ro2 = RGM2+
1
(P - P s ) v M j V h s
where RGM2
[Ap(7)]r2dr'
(13)
-IvM$
= 1
di
VM
is the radius of gyration of a micelle assuming a uniform density. Here A p ( 3 represents fluctuations of the scattering-length density
AOT Reversed Micelles
The Journal of Physical Chemistry. Vol. 89, No. 20. 1985 4385
-
-2~0
in)
1.
psr1o-S
A-q
Fiyre 2. Variation of [d~/dll(0)/nAml'/'plotted against the mean scattering-length density of the solvent for solutions of 3% AOT in ndecane. The ordinates arc on an absolute scale. Following eq 9, this gives a linear plot.
\
. )
70
s
601
-E-E x
>
4"
L
40
-
30
-
""
2.250 1.580
7 10
0
.04
.08
0.12
0.16
0.20
Fiyrr 4. (a) Icosahedral packing of polar heads in an AOT reversed micelle. By slicing the micelle at the level of the dashed line. one wuld look down into the open aggregate. as shown in b. The polar core. including sulfonate head groups. bound water, and sodium wunterions, has a radius of 9.4 A. The very center of the core (4.4 A) is a region of bound w a w . The 8-A surfactant tails are penetrated by the oil solvent only to a depth of 2.4 A.
polar heads define the surface of the spherical wre. The rather large radius found for the polar core can be explained by realizing that the trace quantities of water present tend to remain bound to the AOT head groups. In fact, it has been that the presence of water molecules are necessary for the AOT aggregation process in nonpolar solvents. There is strong evidence that a hydrogen-bonded network develop between the sulfonate groups of the polar heads, the sodium wunterions, and a layer This of bound water having a thickness of about 4-5 A.'4.18.11." leaves a thickness of about 5 .&for the polar head layer, which is close to the expected diameter of a single sulfonate head group. The fact that the polar core consists of two distinct layers does not significantly alter our calculations which were originally based on a core of uniform density. For example, applying eq 13 and 1 4 to a 4.7-A water w r e coated by a 4.7-A layer of polar heads results in a radius of gyration of Rc = 7.6 A. This value differs from the measured one (7.3 A) by only 4%. One can also deduce from purely geometrical considerations that the head groups at the surface of the polar core are closely packed. For this to be the case, the area per head group, Le., aH = 51 A2,must approximately satisfy the relationship
qR.1 Figure 3. SANS spectra from dilute AOT/C6Dt2solutions. The curyes are labeled by the molar concentration of AOT. p ( 3 about the mean value p . From eq 13, it is seen that one measures & e RcM when the contrast p - p. is made very large.
Using the value of Rc extracted from the hi hest deuteration in Figure I , one sees that the value & = 11.7 is a good estimate for the radius of gyration of a micelle having a uniform density. Again, making the assumption of sphericity and no solvation, one or R = 15.1 A. This value obtains a radius of R = (5/3)1/2R~, wmpares very well with the one previously calculated, confirming the assumption that the micelle is indeed nearly spherical. Information on the size of the polar micellar w r e can now be found from the radius of gyration at 0%deuteration of the solvent, Le., at Rc = 7.3 A. This is because the scattering-length density of the tails is very close io that of the protonated solvent. If one assumes that the w r e is approximately spherical and the scattering-length density of the w r e is approximately uniform, then eq 13 shows that R = 9.4 A. If one divides the surface area of a wre having the above radius by the number of heads in the wre, N. one finds an area per head group of a" = 51 A2. This value wmpares well with the maximum area quoted by Zulauf and Eickem (55 A2) and the area measured by Assih et al." (52 Az). It is also in reasonable agreement with values obtained in AOT/water/oil system^^^'-^ (-63 A2). This suggests that the
1
(27) C a b , P. C.;Dclad. P. J. Appl. Crystallogr. IW, I Z , 502
where Rp is the radius of the polar core (9.4 A) and dH is the head-group diameter (5 A). Equation 15 is satisfied to within better than 4%. thus supporting the conclusion that the head groups are indeed packed closely together. Since N = 22 f 2, a reasonable scheme for configuring the polar heads is by close packing them on the faces of an icosahedron, Le., a twenty-sided regular polyhedron, as depicted in Figure 4a. By looking inside the polar core of such an aggregate, one would observe a void having a radius of approximately 4.4 A. This value is compatible with the thickness required to accommodate a region of bound water. Furthermore, it can be shown that the actual volume of the void is 6.18. If one computes this volume for the cased = 5 A and divides by ZOX, i.e., 14 water molecules per micelle, the resultant volume per water molecule is about 54 A'. This value is almost twice the volume of a water molecule in the bulk state ~~
___~
(28) Day. R. A : Robinson. B. H.. Clarke. J. H. R : Dohmy. J V. J Chrm. Soc., Fnroday 7 m n i . I 1979. 75. I32 ( 2 9 ) Kotlarchyk M ;Chcn. S H.. Huang. J S J . PhJr.C h m 19111.86. 3273. (30) Rouviere, J.; Count, J. M.; Lindheimcr, M.; Dejardin, J. L.: Marrony. R.J. Chim. Phys. 1979, 76.289. (31) Zundcl, 0. 'Hydration and Intermolecular Interactions"; Academic Prcss: New York. 1969. (32) Maitra, A. 1. Phys. Chcm. 1984, 88. 5122.
4386
J. Phys. Chem. 1985,89, 4386-4390
(-30 A3).However, for the case of water in the bound state, the larger value is probably a more reasonable one. On the basis of these rough calculations, it appears that icosahedral packing of the polar heads is indeed very plausible. SANS measurements have previously shown” that the thickness of the AOT tail layer in swollen reversed micelles is about 8 A. This value, taken along with the radius of the polar core, gives a total micellar radius of about 17.4 A, which is 2.4 A larger than the value of R that we calculated from RG. The apparent discrepancy can be explained if one relaxes the assumption that oil does not penetrate the surfactant tails. This is justified by some recent SANS experimentsz1on the interactions in AOT micelles and microemulsions which indicate that these aggregates attract one another by allowing the surfactant tails to interpenetrate over a distance of 2.4 A. Because our radius of gyration measurements only apply to the dry micellar volume, the very tips of the tails would not contribute significantly to RG if they were indeed fully penetrated by the decane solvent up to 2.4 A. Instead, most of the solvated region would be occupied by solvent, and the tips of the surfactant tails would appear virtually invisible to neutrons scattered from solutions as dilute as these.
Summary of Results On the basis of SANS measurements of AOT in deuterated cyclohexane, there appears to be a concentration below which no aggregates can be detected. The value we observe is lower than the cmc values generally found by other investigators. The structure of the AOT reversed micelle in decane is illustrated in Figure 4. It indicates that the micelle is approximately spherical in shape. The contrast variation measurement shows that the aggregation number is 22 f 2 monomers per micelle, with each monomer occupying about 612 A3.The micellar core consists of a monolayer of polar heads close packed on the faces of an icosahedron enclosing a region of bound water having a radius of about 4.4 A. The micelle is coated by an 8-A layer of surfactant tails. We estimate that only the last 2.4 A of the tails are penetrated by hydrocarbon solvent. The interpenetration distance is probably limited by the branched, double-chain structure of the AOT molecule.
the Chemgraf illustrations supplied by J. Newsam. This work was supported by the Exxon Research and Engineering Co. and the National Science Foundation. Appendix: Absolute Intensity Calibration Each SANS spectrum was converted to units of absolute cross section dZ/dn(Q) by independently measuring the isotropic incoherent scattering from a 1-mm-thick sample of protonated decane. Decane calibration spectra were obtained with the same scattering geometry as was used for the sample. Both the measured sample and calibration spectra were corrected for detector sensitivity, background, and empty cell scattering, as described in the Experimental Section. Following a radial average of the data at each wavevector transfer Q, the resultant spectrum, Is(Q), from a given sample is a measure of the number of neutrons per second that are detected within an annulus subtending a solid angle Ail. The spectrum is then normalized to the sample thickness, tS, and the sample transmission, Ts, allowing one to write
Here, 4 is the incident neutron flux, A is the irradiated crosssectional area of the sample, and t is the overall detector efficiency. If the decane scattering data is corrected in the same way, the resultant normalized decane spectrum, ID,can be expressed as
where tD and TD are the thickness and transmission, respectively, of the decane sample. By relating a normalized sample spectrum to a normalized calibration spectrum, one obtains the absolute cross section of the sample in terms of measured quantities, i.e.
where (dZ/dn), is the Q-independent scattering cross section for decane, given by
Acknowledgment. We acknowledge both the staffs of the BNL and the NCSASR for their generous allocations of spectrometer time and their assistance during the experiments. We are also grateful for experimental assistance provided by M. W. Kim and
Registry No. AOT, 577-11-7; CloH2,,124-18-5; C6HI2,110-82-7.
Interaction between Perfluoro Chemicals and Phosphatldylchollne Vesicles C. F. Kong: B. M. Fung,*t and E. A. O’Rearf Departments of Chemistry and Chemical Engineering, University of Oklahoma, Norman, Oklahoma 7301 9 (Received: January 18, 1985; In Final Form: May 13, 1985)
Interactions between three perfluoro chemicals (perfluorodecalin, PFD; perfluorotripropylamine, PFTPA; and perfluorotributylamine, PFTBA) with phosphatidylcholinevesicles were studied by F- 19 NMR, electron microscopy (EM), and differential scanning calorimetry (DSC). The NMR results show that PFD and PFTPA are partly soluble in the bilayer portion of the vesicles, while PFTBA is insoluble. The EM results indicate that PFD and PFTPA form droplets up to 1.5 pm in diameter in the emulsions, while PFTBA forms much larger droplets, up to 8 pm in diameter. The DSC data show that the transition temperature (T,) and enthalpy of transition ( A H ) of the lipid bilayers are not appreciably affected by the presence of PFD and PFTPA, while both T, and AH increase slightly in the presence of PFTBA. The increases may be due to artifacts caused by the inhomogeneity of the dispersions and the fusion of smaller vesicles into larger ones.
Introduction A number of perfluoro chemicals (PFC’s) is used to prepare emulsions that can be applied as blood substituta.1-3 The most commonly used PFC’s are perfluorodecalin (octadecafluoro‘Department of Chemistry. Department of Chemical Engineering
0022-3654/85/2089-4386%01.50/0
decahydronaphthalene; PFD), perfluorotripropylamine (henei~afluorotri-n-ProPYlamine;PflPA), and PrfluorotributYlamine (heptacosafluorotri-n-butylamine;P f l B A ) . Emulsions containing (1) Rim, J. G.; LeBlanc, M. Angew. Chem., Int. E d . Engl. 1978, 17,621. ( 2 ) Riess, J. G.;LeBlanc, M. Pure Appl. Chem. 1982, 12, 2383. ( 3 ) Geyer, R. P. Prog. Clin. Biol. Res. 1983, 122, 157.
0 1985 American Chemical Society
