Langmuir 1992,8, 1278-1284
1278
Perfluoropolyether Water/Oil Microemulsions: A ‘H NMR Self-Diffusion Study of Water Maura Monduzzi,**+ Alba Chittofrati,i and Mario Viscat Dipartimento Scienze Chimiche, Universitd di Cagliari, Via Ospedale 72, 09124 Cagliari, Italy, and Montefluos SPA, R&D Centre, Colloid Laboratory, 20021 Bollate, Milano, Italy Received November 20, 1991. I n Final Form: February 13, 1992 The water self-diffusioncoefficientswere determined by means of the Fourier transform pulsed gradient spin-echo NMR technique in three- and four-component W/O systems based upon perfluoropolyether (PFPE)oils and an anionicsurfactant with a PFPE hydrophobic chain. In order to investigatethe structural changes associated with microemulsion formation, the study has been performed by varying the water content within the monophasic isotropic region of the phase diagram, which is located at relatively low water content ( 10-15. The position of the maximum and its value and also the “nonconducting” boundary strongly depend on the oil chain length and on the presence of other components. Conductivity variations in a three-component PFPE system with oil and surfactant of comparable chain length suggested that upon addition of water, initially increasing hydration favors a partial surfactant dissociation and, thus, conductivity increases. At higher water content, static and dynamic light scattering measurements proved the occurrence of water droplets (diameter = 2-8 nm, depending on the composition), whose onset of formation is located at the composition for which the maximum of conductivity is observed. This composition corresponds to the geometrical limit due to the minimum surface per polar head.4p5J The gradual decrease of conductivity from values on the scale of percolative behavior699 to values typical of charge fluctuations among closed domains12has been associated with the occurrence of an intermediate region where strongly interacting hydrated species and water droplets coexist. The estimation of the relative fraction of water in droplets in this region indicated that only at W/S > 10-15 all water can be confined into droplets. (11) (a) Evans, D. F.; Mitchell, D. J.; Ninham, B. W.
J.Phys. Chem.
1986,90, 2817, and references therein. (b) Fontell, K.; Ceglie, A.; Lindman, B.; Ninham, B. W. Acta Chem. Scand., Ser. A 1986,40, 247. (c) Hyde, S.T.; Ninham, B. W.; Zemb, T. N. J.Phys. Chem. 1989,93,1464,
and references therein. (12) (a) Eicke, H. F.; Borkovec, M.; Das Gupta, B. J. Phys. Chem. 1989.93. 314. (b) Hall. D. G . J. Phvs. Chem. 1990.94.429. (c) Kallav. N.; Chittofrati,‘A. J. Phys. Chem. -1990, 94, 4755.’ (d) Hall;, B . Pr&: Colloid Polym. Sci. 1990, 82, 211.
0 1992 American Chemical Society
Perfluoropolyether Microemulsions
Although a great deal of research work has been made, a detailed characterization of microemulsion formation and possible structures is an unresolved problem in many important aspects. What can be generally stressed is a noticeable structuralvariability which is strongly dependent on the interplay of attractive and repulsive and also steric intermolecular forces.llJ3-17 Microstructure has been shown to be mainly set by packing parameter (ulal) along with geometric constraints dictated by the volume fractions of the In our systems, the chain length of the PFPE oil and surfactant produced significant effects both on the phase separation boundary and on the structural rearrangements within the isotropic area. For instance, a PFPE oil with a chain length largely exceeding that of the surfactant prevented the system from attaining the "nonconducting" state before evolving to a phase separation into a slightly birefringent liquid crystalline phase in equilibrium with a isotropic solution. Conversely, the shortest chain oil enlarged the range of composition where droplets exist, thus suggesting a higher degree of oil penetration. Hence, to deepen the knowledge on the structural features associated with microemulsion formation, a NMR self-diffusion study on these PFPE systems has been carried out. It has been largely proven that the NMR pulsed gradient spin-echo (PGSE) technique for selfdiffusion measurements can provide detailed information on the microstructure at a molecular level.l4-l6J7bJ&22 In these PFPE systems the determination of the selfdiffusion coefficients of water is particularly easy since only signals of water and ammonium protons are detected in the lH NMR spectra, and signals are always well resolved. Moreover water self-diffusion is extremely sensitive to the state of aggregation of the system. Indeed, the self-diffusion coefficient of pure water ( D o )is 2.3 X 10-9m2/s,23 but when water is confined into closed domains such as water droplets in oil, the self-diffusion coefficient decreases about 2 orders of magnitude.11b114-16a122 The PFPE surfactant (S) has average molecular weight of 723. Three analogous PFPE oils, namely D, L, and H with average molecular weights of 700, 800, and 1000, respectively, were used to investigate the effect of oil chain length. Besides the effect of the O/S ratio and ionic strength in the water phase, the effect of the addition of an alcohol such as a long chain PFPE alcohol (RfCHzOH, MW = 676) and ethanol was also examined. The two alcohols were chosen by considering that the long chain alcohol, soluble in oil but not in water, is expected to favor droplet formation and to enlarge the range of droplet (13)(a) Jada, A.; Lang, J.; Zana, R. J.Phys. Chem. 1990,94,381.(b) Jada, A.; Lang, J.; Zana, R.; Makhloufi, R.; Hirsch, E.; Candau, S. J. J. Phys. Chem. 1990,94,387. (14)Shinoda, K.;Lindman, B. Langmuir 1987,3,137,and references therein. (15)Guering, P.; Lindman, B. Langmuir 1987,1,464. (16)(a) Skurtveit, R.; Olsson, U. J. Phys. Chem. 1991,96,5353. (b) Soderman, 0.;Hansson, E.; Monduzzi, M. J.ColloidInterface Sci. 1991, 141,512. (17)(a) Israelachvili, J. N.; Wennerstrom, H. Langmuir 1990,6,873. (b) Anderson, D.M.; Wennerstrom, H. J.Phys. Chem. 1990,94,8683. (18)Stilbs,P.Prog.NMRSpectrosc. 1987,19,1,andreferencestherein. (19)Lindman, B.; Soderman, 0.;Wennerstrom, H. In Surfactant Solution. New Methods of Investigation; Zana, R., Ed.; Marcel Dekker: New York, 1986;p 263. (20)Lindman, B.;Stilbs, P.; Moseley, M. E. J. Colloid Interface Sci. 1981,83,569. (21)(a) Stilbs, P.J. Colloid Interface Sci. 1982,89,547. (b) Stilbs, P.;Rapacki, K.; Lindman, B. J . Colloid Interface Sci. 1983,95,583,and references therein. (22)(a) Jonsson, B.;Wennerstrom, H.; Nilsson, P. G.;Linse, P. Colloid Polym. Sci. 1986,264,77.(b) Faucompre, B.;Lindman,B. J . Phys. Chem. 19R7. 97. 383. - - - . . - - I
(23)Mills, R. J. Phys. Chem. 1973,77,685.
Langmuir, Vol. 8, No. 5,1992
1279
existence just like in hydrocarbon W/O system^.^^^^^^^ Conversely, ethanol is expected to cause a breakdown of the reverse structure as well as observed in aqueous micellar systems.21925t26Indeed, preliminary conductivity measurements agree with this mentioned beha~ior.~'
Experimental Section Materials. The perfluoropolyether (PFPE) oils, the anionic surfactant, and the fluorinated alcohol having the same PFPE hydrophobic tail have the following general molecular structure
where generally n > m and R and R' are perfluoroalkyl groups in the various oils (D, MW = 700, L, MW = 800; H, MW = 1O00), R' = CFzCOONHd (S, MW = 723) in the surfactant, and R' = -CHzOH (RfCHZOH, MW = 676) in the alcohol. The oils D, L, and H (Galden, manufactured by Montefluos, Milan, Italy) are completely insoluble in water. The oil L, which has been mainly used in this work, is a transparent liquid of density 1.8 g/cmS at 25 "C, dielectric constant 2.1, refractive index 1.282,andviscosity 6.84 CPat 25 "C. Oils D and H have similar characteristics but lower and higher viscosities,namely 3.2 and 13.0 cP, respectively. The surfactant (S)has a narrow molecular weight distribution (99% by gas chromatographic analysis). All values of water content in this work take into account the initial amount of water of the surfactant 1.1 w t % as determined by Karl Fisher titration. The PFPE alcohol (RfCHzOH) is insoluble in water but completely soluble in the oils. Ethyl alcohol (EtOH) is a RPEACS (95") reagent from Carlo Erba (Italy). Bidistilled water with a conductivity less than 2 X 10-4 S/m was used to prepare all samples. Three stock solutions containing the surfactant S and the oil L were prepared at the fixed mass ratio O/S of 3.00, 1.83, and 1.22. The mass ratio O/S = 1.83 was also used to prepare the samples to which a 0.1 M KNOs solution was added instead of pure water and to prepare the samples containing the alcohols. The two series with EtOH and RfCHzOH had an alcohol content of 0.45 and 1.62 wt %, respectively, which corresponds to the molar ratios EtOH/S = 0.20 and RfCHZOH/S = 0.05. The same mass ratio O/S= 1.83 was kept constant with the oils D and H. The samples for NMR measurements were prepared from the above stock solutions by adding a certain amount of water (or aO.l M KN03solution) to the S/Oor S/O/alcohol stock solutions within the range of composition for which a monophasic isotropic phase behavior has been ascertained. Samples were homogenized by moderate stirring and then were transferred into 5-mm NMR sample tubes and left to equilibrate at 25 "C for at least 48 h before measuring. These tubes were placed, carefully centered by means of a special Teflon device, into a 10-mm NMR tube containing CDC&for lock purpose. Care was taken in order to have always the level of both samples and CDCls at 5 cm from the bottom of the tubes. Methods. NMR Self-Diffusion Measurements. The selfdiffusion coefficients D were determined by using the pulsed field gradient spin-echo technique suggested by Stejskal and Tanners in the improved Fourier transform mode as suggested by Stilbs.lB To this purpose the spectrometer Varian FT80A was properly modified and equipped by a specially made pulsed magnetic field gradient unit (by Stelar, Italy) with a maximum gradient power of 0.25 T/m. The FT-PGSE measurements were performed, at the controlled temperature of 25 0.5 "C, by varying the length of the
*
(24)Friberg, S. E.; Venable, R. L. In Encyclopedia of Emulsion Technology; Becher, P., Ed.; Marcel Dekker: New York, 1983; Vol. 1, Chapter 4. (25)Leung, R.; Shah, D. 0. J. Colloid Interface Sei. 1987, 120,330. (26)Rao, I. V.; Ruckenstein, E. J. Colloid Interface Sci. 1986,3,375. (27)Chittofrati, A. Communicatin at 7th International Conferenceon Surface and Colloid Science, Compiegne, 7-13 Jul. 1991. (28)Stejskal, E. 0.; Tanner, J. E. J. Chem. Phys. 1965,42,288.
Monduzzi et al.
1280 Langmuir, Vol. 8, No. 5, 1992
Table I. Self-Diffusion Coefficients Obtained for the Different Series of Fluorinated Microemulsions at 25 f 0.5 'C
gradient pulse (6) and with a constant gradient pulse interval A (depending on the spin-spin relaxation time of the water protons of the samples, values A = 20-100 ms were used). The decay of the echo intensity (I)is given by the relation18*28
Z = Zo exp{-G2Dy262(6- A/3)]
series
(1)
where G is the gradient strength, y is the gyromagnetic ratio of theobservednucleus (lH,inthiscase),andZO is thesignalintensity in the absence of a gradient pulse. The gradient strength was calibrated with a sample of H2O for which D is known,i.e. Do = 2.299 m2/s a t 25 O C I z 3 by measuring in the same experimental conditions as the samples. The D values were calculated by means of a nonlinear twoparameter fitting of 14-20 experimental pointa. The occurrence of a single exponential decay of eq 1 was always ascertained. In the case of the two series containing the alcohols whose -OH groups might contribute to lH NMR signals of water, it has been verified that the molar fraction of the alcohol with respect to water is 0.03 in the less favorable case thus ita contribution to the measured self-diffusion coefficient was reasonably neglected.
Results and Discussion The self-diffusion coefficients of water, D,, obtained for the various systems with different water contents in the isotropic region are reported in Table I together with the corresponding water/surfactant (W/S) molar ratios and conductivities (from ref 5). The short spin-spin relaxation time of ammonium protons at 25 O C did not allow the simultaneous measurement of the self-diffusion coefficients of counterions. The water NMR signals always exhibited a Lorentzian bandshape thus indicating that a fast exchange on the NMR time scale occurs. In systems with W/S < 4 also water protons exhibited too short spin-spin relaxation times to give reliable and reproducible results in a FTPGSE sequence. Thus measurements could be performed in the range of water concentration between approximately the conductivity maximum and the phase separation boundary. Generally it can be observed that, with the exception of series F (oil H) and G (EtOH), the D, values decrease substantially with increasing water content, this behavior being paralleled by a marked decrease of the corresponding conductivity. As an example, Figure 1shows the trend of both conductivity and self-diffusion data as a function of W/S ratio for the system L/S = 1.22. It is worth mentioning the similarities in the phase behavior, conductivity, and water self-diffusion with several W/O microemulsions of the double-chain didodecyldimethylammonium bromide (DDAB)surfactant with various oils.11J6a In the proximity of the maxima of conductivity, D, values in the range (5-7) X 1O-IO m2/sare always observed. At high W/S ratio, D,values in the range (0.5-2) X lo-" m2/s, typical of water in droplets, are paralleled by low conductivities which have been described in terms of charge fluctuations among closed water domains. To better evidentiate the trend of self-diffusion coefficients as a function of water content, it is illustrative to plot the quantity DW/Do(where Do= 2.299 X 10-9 m2/s is the self-diffusion coefficient of bulk water a t 25 O C Z 3 ) vs WIS molar ratios as shown in Figures 2A, 3A, and 4A. In the case of the systems containing either the long-chain oil H (series F) or EtOH (series G), the results of Figures 3A and 4A clearly indicate that these components affect significantly the rearrangement related to droplet formation which seems to be largely inhibited. Hence, before analyzing the self-diffusion data in detail, from previous work4v5we can summarize for the threecomponent systems here examined the following points: (a) These systems show an onset of conductivity at W/S
w/sa x (XlO),b molar ratio S/m LIS = 1.83 + H20 6.17 7.35 8.50 11.01 13.10 15.40 18.28
2.240; 2.100 1.780 0.720 0.090 0.002 0.001
D, (XIOIO): m2/5 6.84f 0.440 3.82 f 0.300 3.36 f 0.290 2.42 f 0.100 1.13 k 0.080 0.32 f 0.031 0.16 f 0.009
L/S = 1.22 + H20 3.000 6.52 f 0.780 3.290; 5.68 f 0.680 2.800 2.76 f 0.100 1.500 1.54 f 0.050 0.280 0.74 f 0.042 0.015 0.42 f 0.009 0.001 0.23 f 0.006 L/S = 3.00 + HzO 5.80 1.282; 6.36 f 0.750 7.43 1.110 5.68f 0.280 9.38 0.577 3.90f 0.170 11.81 0.060 1.04 f 0.060 15.06 0.001 0.24 f 0.023 L/S = 1.83 + KN030.1 Md 6.17 2.170; 5.23 f 0.250 8.45 1.830 3.73 f 0.180 10.73 0.570 1.53 i 0.040 13.02 0.030 0.67 f 0.037 15.89 0.001 0.14f 0.009 D/S = 1.83 + HzO 5.28 1.500* 6.99 f 0.420 7.58 0.950 2.75 f 0.120 9.88 0.120 1.11 f 0.050 12.18 0.001 0.26 f 0.012 14.48 0.0005 0.12f 0.005 17.70 0.0001 0.05 f 0.003 H/S = 1.83 + H2O 5.62 2.800 5.48f 0.610 7.35 3.090; 4.89 f 0.500 8.50 3.040 4.26 f 0.260 9.65 2.800 3.71 f 0.180 L/S = 1.83 (+ EtOH 0.45wt % ) + HzOe 6.80 3.680 6.37 f 0.410 8.40 3.880; 4.90 f 0.280 10.85 3.600 3.34 f 0.110 13.16 3.000 2.50 f 0.080 L/S = 1.83 (+ RfCHzOH 1.62 wt %) + H2O' 4.76 0.997. 7.00 f 0.350 7.46 0.328 2.85 f 0.110 9.80 0.002 1.55f 0.090 14.71 0.0001 0.08f 0.007 In these calculations the effective amount of water is considered. Conductivities are from ref 5. The asterisk indicates the maximum of conductivity. The reported errors are the errors on the fitting of the experimental points to eq 1. The reproducibility of the measurements performed on different samples is better than 5%. d The self-diffusion coefficient obtained for a 0.1 M KN03 solution is D = (2.21f 0.04)X lo4 m2/s. This value was used to calculate Dw/Do values reported in Figure 2A. e Conductivities are from ref 27. 4.96 6.49 8.57 11.28 13.99 16.25 18.51
*
= 1.5.29 (b) Before the maximum conductivity is reached, which occurs at WIS = 5-7, no well-defined water cores are present in the systems. The W/S ratio corresponding to the conductivity maximum decreases with increasing the O/Sratio or with decreasing the oil chain length as _ _ _ _ ~
~
~~
~
~~~
(29) It should be mentioned that the pure surfactantcontainsaminimal amount of water correspondingto a W/S molar ratio of -0.45 and the
O/Smixtures, at 25 O C , are apparently stable isotropic systems which however become turbid within 2-3 h. No turbidity nor phase Separation is observed upon aging at 30 O C .
Langmuir, Vol. 8, No. 5, 1992 1281
Perfluoropolyether Microemulsions DwID" A
10
5
0
0
20'
15
5
10
15
1
WIS
WIS
Figure 1. Conductivity (u)and reducedself-diffusioncoefficients
Dw/Do(0) vs W/S molar ratio for the system L/S = 1.22.
A
4
WIS 0
5
10
20
15
WIS
fraction of water in droplets x& (from eq 4) vs W/S. Symbols are as in part A.
B
090 ' 0
5
10
15
Figure 3. Effect of the oil chain length (A) Dw/Dovalues vs W/Sfor O/S = 1.83, where oils were L ( O ) , D (A), and H (u);(B)
20
WIS
Figure 2. Effect of the L/Sratio and salinity: (A) Dw/Dovalues vs W/Smolar ratios for L/S= 1.83 (01, L/S = 1.22 (u),L/S= 3.00 ( + I , and LIS = 1.83 + KNO, (A),in this latter case W is the molar concentration of a 0.1 M KN03 solution and Do = 2.21 X lo+' m2/s;(B)fraction of water in droplets x b (from eq 4) vs W/S. Symbols and notations are as in part A.
shown in Table I, series A, B, C, and E. (c) The amount of water in droplets increases with increasing W/Suntil all water is confined. (d) Further increase of the water
content induces the droplets to grow in size until a phase separation occurs. With all these considerations in mind, it can be assumed that no "free" water is ever present in the oil phase. However it should be noted that in the highly conducting region, the microemulsion microstructure cannot be defined univocally. At low water content and below the maxima of conductivity, only hydration water is expected to occur. But the question is how does water uptake modify the initial organization, if any, of the surfactant molecules in the oil in order to permit the electric conductivity? Although a wide array of literature is available on this subject,618J0J1many questions are still unresolved. In our PFPE systems some trial self-diffusion measurements of counterions in the initial O/S mixtures, at 34 O C , gave D coefficients around (2-4)X 10-lom2/s(with errors of about 20% ) in all systems including thoee containing the alcohols. Although the mutual solubility of O/S mixtures as a function of temperature was not carefully determined (see ref 29), these data indicate a qualitatively constant behavior of counterion at W/S 0.45,independently of medium viscosity. It is presumed that these D values correspond to an average among the contributions of differently hydrated species in fast exchange on the NMR time scale. At present, however, no detailed structural information on this region of the phase diagram can be derived from our experiments.
-
1282 Langmuir, Vol. 8, No. 5, 1992
Monduzzi et al. Table 11. Parameters Used To Calculate Ddr from ComDosition in the Various PFPE Systems ~
~~~~
D& (X10-'2),*
system
W/S
(pw
cps
A
6.17 18.28 4.96 18.51 5.80 15.06 6.17 15.89 5.28 17.70 5.62 9.65 6.80 13.16 4.76 14.76
0.0890 0.2245 0.0910 0.2720 0.0610 0.1444 0.0890 0.2010 0.0772 0.2189 0.0817 0.1326 0.1055 0.1794 0.0690 0.1870
0.3219 0.2740 0.4095 0.3279 0.2347 0.2139 0.3219 0.2823 0.3261 0.2760 0.3245 0.3065 0.3161
B C
D E
m2/s
Rob
31.4 67.7 27.8 68.3 38.9 58.0 31.4 60.5 28.8 65.9 29.8 41.9 35.3 54.3 26.6 55.2
10.16 4.71 11.48 4.67 8.20 5.50 10.16 5.27 23.68 10.35 5.63 4.01 9.04 5.88 12.00 5.78
0.47 20.00 0.49 13.90
0.50 13.30 0.61 23.00 0.98 136.40 0.31 1.28
i F
0.0
&,'A
Gc
0
5
10
15
20
WIS
Hd
0.2900
0.3387 0.2958
0.50 1.28 0.46 40.00
From eq 2 introducing pI = 1.8 g/mL, = 30 A2, and I , = 13 A. (pw = (pw + (PA, where (PA is the volume fraction of EtOH. cps = cps + (PA, where VA is the volume fraction of RfCH20H. a
* From eq 3.
080
0
5
10
1
15
20
WIS
Figure 4. Effect of the alcohol chain length (A) D,/Do values vs WIS for LIS = 1.83 systems in the absence (a)and in the presence of the alcoholsEtOH (A)and RfCHzOH (m); (B)fraction of water in droplets .z& (from eq 4) vs W/S. Symbols are as in part A.
Turning attention to the conducting region at low W/S, below and up to the conductivity maxima, it is difficult to establish whether conductivity is produced by ionic transport within a water continuous network, shaped by a highly flexible surfactant layer. The occurrence of water droplets in this region can be ruled out on the basis of simple calculations. Table I1 shows the droplet radius and the water self-diffusion coefficients calculated from composition and assuming all water in monodisperse droplets, for the minimum and the maximum W/S of each system examined here by means of relations16a (2) = {39$~/98PflAZ + '81 where MEis the molecular weight of surfactant, NAis the Avogadro number, % and cps are the volume fractions of water and surfactant, respectively, while ps is the density of surfactant, Z is the area of the polar head group, and 1, is the surfactant chain length, and Rd
D, kT/6uqRd (3) where tl is the viscosity of the oil. Here the correction for hard sphere interactions [l - k(% + f i ) l l 3 ~ 2 2 was not introduced. The last column of Table IIshows the droplet radius obtained from the experiments by the use of the relation Rob = kT/6utlDw. It is evident from these data
that although a certain discrepancy between calculated (Rd) and observed (Rob) droplet radius is observed, a prevailing water-in-oil droplet structure can occur only at the highest W/Sratios with the exceptions of systems F (oil H) and G (EtOH). In fact, the discrepancy between Rd and Robs may be accounted for by considering that the presence of a very small amount (> Ddr, thus decreasing the calculated Rob. At the lowest W/S, besides light scattering results,5 the observed self-diffusion coefficients of water in the proximity of the conductivity maxima are typical of a water continuous p h a ~ e . 1In ~ particular it is worth noting that in the limiting case of self-diffusion confined to a cylindrical region of negligible diameter, the component of diffusion which determines the observed diffusion distance is in only one dimension; thus, in the case of water it can be predicted D, = D0/3 = 7.66 X 10-lom2/s at 25 "C. The D, values measured at WIS = 5-7 are slightly lower, probably due to the water molecules located in the first hydration shell of ionic groups for which a D, = D0/1016bcanbe hypothesized. Therefore we can treat the region in the proximity of the conductivity maxima as a water and oil continuous phase. Conversely, it is straightforward to assume that at high water content when systems become "nonconducting*the observed water self-diffusion coefficients are mainly due to water in droplets. In the region between the conductivity maxima and the "nonconducting"regime, the evolution of the microemulsion microstructure from a water continuous network to water-in-oil droplets, assuming spherical droplets, can be described as D, = (1- Xdr)Dc+ X d P d r [ l - 1 . 5 ( ~ ,+ CC,)] (4) where Xdr is the molar fraction of water enclosed into droplets while D, and Ddrare the self-diffusion coefficients of water in the continuous form and in droplets, respectively, and where D, is corrected for hard sphere interactionsz2using the lowest value reported in the literature for attractive interactions, i.e. k = 1.5.13bv22b In our systems assuming that hydration decreases the theoretical value D0/3 of about 10%, D = 7 X 10-10 m2/s would represent a good estimate of D,. This choice can be also supported by the relative constancy of the selfdiffusion coefficientsof water at compositions close to the conductivity maxima.
Langmuir, Vol. 8, No. 5, 1992 1283
Perfluoropolyether Microemulsions
.
'dr
Conductivity
--*
/'' t
t
f
1 . /.
Self-Diffusion
i
62
o,oo
5
,
10
.
,
.
15
I
1 20
WIS
Figure 5. zdr for system L/s = 1.22 vs w/s: (0)calculated in this work from eq 4; (B) obtained from conductivity data (ref 5).
The value of D& depends on composition but, assuming that only monodispersewater droplets are formed during the process, the values in Table I1calculated for the highest W/S of each system can be used to estimate the fraction of water in droplets from eq 4. The results of these calculations are reported in Figures 2B,3B,and 4B. It should be noted that the same D, was also used for the four-component systems(series G and H).In fact D,might be affected by alcohol hydration. However, either because of the constancy of D, in the proximity of conductivity maxima as mentioned above or because the molar fraction of alcohol with respect to water never exceeds 0.03,the possible contribution of alcohol hydration was neglected. From the propagation of errors the Xdr values are obtained with a maximum error of 10%. The system containing L/Swith O/S = 1.83was selected as the reference to investigate the effects of O/S ratios, salinity, oil, and alcohol chain length. However, in the following discussion it should not be forgotten that surfactant and oils, although the narrow distribution of MW, are polycomponent mixtures, thus observations concern a global effect rather than a specific action of a particular molecule, which is characterized by a defined conformation. Effect of O/S Ratio. From the analysis of the whole of the data concerning L/S systems and in particular the X& values reported in Figure 2B,only slight differences can be observed among the various systems with different O/S ratios. In particular X & values of series A-C are likely to have the same trend within the experimental errors. Although many aspects of the microemulsion formation have been necessarily neglected in the initial study of these new PFPE systems, it is worth stressing that different techniques allowed almost the same results. As an example Figure 5 reports the Xdr curves as a function of w / s , obtained from conductivity and self-diffusion data, for the system L/S = 1.22,which shows the highest water solubilization. The agreement is striking and further supports the assumptions made above and in particular the validity of eq 4 and the correctness of the D, and D d r values. Effect of Salinity. It has been shown that salinity generally causes dramatic effects on the microstructure of microemulsions.~~~~~~4-~~~ For instance, an increase of salinity can produce a water-in-oil droplet structure in microemulsions containing medium-chainedcosurfactant which, over certain ranges of composition, are of the bicontinuous type.
Water self-diffusion data as well as conductivity measurements, obtained in the L/S = 1.83 system by using a 0.1 M KN03 solution instead of pure water, indicated that the salt presence shifts the total confinement of water in droplets at lower W/Svalues while not affecting the onset of droplet formation (cf. series A, D and note d in Table I and Figure 2). Concerning the onset of droplet formation it should be considered that at W/S < 10the ionic strength of salt in water is rather small if compared to counterion concentration. In practice the increase of salinity promotes droplet formation by lowering head group area repulsion (ulal increases) in agreement with previous results obtained in hydrocarbon systems. Effect of Oil Chain Length. Figure 3A shows that the variation of D,/D" as a function of W/S strongly depends on the molecular weight of the oil. In particular, as shown in Figure 3B,the use of an oil with a lower MW (D, series E in Table I) causes water droplet formation at lower W/S ratio with respect to the oil L (series A). In the presence of the oil with the highest molecular weight (H, series F), either conductivities or self-diffusion Coefficients do not decrease significantly even at high water content, thus suggesting that oil H prevents the system from forming water droplets at a large extent before phase separation. It is clear that for the surfactant used here, the molecular weight of the oil L approaches the limiting value of the oil chain length in order to promote the appropriate packing at the interface and achieve the suitable spontaneous curvature for droplet formation. Moreover, the role of the shortest oil D in increasing the (ulal) ratio through higher penetration at the interface can be easily suggested from the Robs reported in Table 11,in the light of previous studies of alkanes.11130 It is worth noting that in the case of oil D, at high W/S, Robs largely exceeds Rd, thus indicating that surfactant hydrophobic tails are significantly swelled by the oil molecules. In the case of the oil H it should be noted that a waterin-oil droplet formation is even doubtful since the D, values obtained at any water content are typical self-diffusion coefficients of water in a continuous system. At present, however, no straightforward conclusions can be drawn about substantial differences occurring in the microstructure of the systems with different oils. Effect of Alcohol. It is well-known that alcohols, depending on their chain length, can cause dramatic modifications in the structure of microemulsions.6~20~21 In particular short-chain alcohols are often considered as structure-breaking constituents.21 Table I and Figure 4A show that in the L/S = 1.83system, in the presence of EtOH (EtOH/S = 0.21,neither conductivities nor D, values decrease significantlyup to water contenta close to phase separation. The water selfdiffusion coefficients decrease less than 1order of magnitude with respect to pure water, thus indicating that a prevailing water droplet structure is rather improbable even at the highest water content as also indicated by the data in Table 11. This agrees with previous findings.20v21 However it cannot be excluded that at W/S slightly higher than 8.4 (maximum of conductivity) a small amount of water is confined into closed domains, thus inducing a decrease of both conductivity and self-diffusion coefficients. As in the case of oil H, in the presence of EtOH the Xdr = 0.65 calculated at W/S = 13.16,with D, = 2.5 x 10-10 m2/s, seems to be overestimated. In this system (30)Ninham, B.W.; Chen, S.J.; Evans, F.D.J . Phys. Chem. 1984,88, 5855.
Monduzzi et al.
1284 Langmuir, Vol. 8,No. 5, 1992
the self-diffusion coefficient of EtOH,3' measured in some samples equal those measured for water within the experimental error. This means that, as expected, EtOH is almost completely solubilized in the water phase, thus determining an increase of the water volume fraction. Hence, from the whole of the data it can be stressed that the effect of EtOH is macroscopically the same for the long chain oil, thus suggesting a similar variation of (ulal). However, the molecular rearrangements involved in determining the phase separation of the microemulsion with EtOH are certainly rather different from those induced by the oil. As a matter of fact, a sample with EtOH at W/S a 17,upon a few hours of aging, separated into two well-defined phases. The water self-diffusion coefficient, measured in the lower and most abundant oil-rich phase, indicated the presence of a water-in-oil droplet microstructure (D, = 3.3 X 10-1' mz/s). All these findings give evidence of the actual cosolvent role of EtOH. A completelydifferent behavior is observedwhen a long chain alcohol (RfCHzOH) is used (seeTables I and I1series Hand Figure 4). In this system the occurrence of a waterin-oil droplet structure can be deduced from the marked decrease of both conductivityand D, values with increasing water content. The onset of droplet formation is shifted at the lowest value of W/S observed in our systems. We can also note that at high W/S, Rob approaches R d (Table 11)more closely than in the other systems, probably because of a more stable water-oil interface. Indeed long chain alcohols are well-known to favor droplet closure by inducing higher reverse c ~ r v a t u r e In . ~Figure ~ ~ ~ 4B ~ ~it~is evident that, on comparingwith the correspondingthreecomponent systems, droplet formation in the presence of RfCHzOH is favored at any water content, thus confiiing the substantial, and expected, cosurfactant action of this long chain PFPE alcohol.
Conclusions The analysis of the water self-diffusion coefficients of PFPE microemulsions allowed the clarification of some aspects of the structural rearrangements occurring in an isotropic region at low water content which was found to display a peculiar conductivity behavior. Generally water self-diffusion coefficients were in very good agreement with the interpretation of conductivity data (cf. Figure 5), thus confirming the validity of the assumptions used for the interpretation of the experimental data collected by both the techniques. In the _
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(31) D d values, although with large errors (about E%), were simultaneously measured in these systems from the proton NMR signal of -CHa group detectable up to W/S= 10.85.
examined systems the self-diffusion Coefficients of water measured at the maximum of conductivity indicated the occurrence of a water continuous network. The maximum of conductivity seems to be related to the largest extension of the water network allowed in each system before this network starta to disconnect upon further addition of water. Along a water dilution path most of the systems evolved to microemulsions with a well-defined water-in-oildroplet structure through a gradual process. Inverse structures can form in certain ranges of water content in agreement with suitable modifications of the parameters affecting (u/aO.
The effect of the oil chain length was quite remarkable. The use of an oil with the shortest chain length (D) favors droplet formation significantly by involving a high degree of oil penetration. Conversely, water droplet structures are certainly not promoted in the presence of an oil which, because of a longer chain length (HI, cannot penetrate the proximity of the surfactant hydrophobictails. The droplet formation seems to be strongly dependent on packing at the interface as expected on the basis of the (ulal)concept proposed by Ninham. In the four-component microemulsions the short chain alcohol EtOH, due to ita prevailing cosolvent role, inhibits the formation of a well-defined water-in-oil droplet structure, while the long chain alcohol RfCHzOH, acting as a real cosurfactant, favors droplet closure significantly. In droplet formation the effect of the shortest chain oil on packing is paralleled by the long-chain PFPE alcohol while the effect of the longest chain oil appears to be paralleled by EtOH, although completely different balancings among solvophobic and solvophilic interactions are expected to drive microstructure evolutions. What is worth stressing is that this PFPE surfactant, with NH4+ as counterion, displays features rather close to those observed for several double-chained ionic surfactants, particularly DDAB. The ability to obtain microemulsions without cosurfactants should be noted. Thus further work is in progress on PFPE systems in order to shed further light also on the more general problem of the structural features involved in microemulsion formation.
Acknowledgment. We wish to thank Dr. P. Gavezzotti for the preparation and purification of surfactant and Mr. P. Vanotti for his valuable technical assistance. A.C. is grateful to Professor N. Kallay for stimulating discussions; M.M. thanks Professor B. Lindman for useful suggestions and Montefluos (Mi) for financial support. Registry No. HzO, 7732-18-5.
