Water, self-diffusion coefficient microemulsions

We have studied a variety of transport properties of several series of 0/W microemulsions containing the nonionic surfactant Tween 60 (ATLAS tradename) and n-pentanol as cosurfactant. Measurements include dielectric relaxation (from 1 MHz to 15.4 GHz), electrical conductivity in the presence of added electrolyte, thermal conductivity, and water self-diffusion coefficient (using pulsed NMR techniques). In addition, similar transport measurements have been performed on concentrated aqueous solutions of poly(ethylene oxide)... 

Figures 2-4 show the thermal and ionic conductivity, and water self-diffusion coefficient measured in these same systems. Also shown are the transport properties of PEO solutions of molecular weights ranging from 200 to 14,000 (12). The predictions of the Hanai and Maxwell relations are indicated, which were calculated on the assumption that the ionic conductivity or self-diffusion coefficient of the water or suspending electrolyte is equal to that of the pure liquid and that of the oil and emulsifier combined is zero. Also shown are similar results from the PEO solutions of various molecular weights. The thermal conductivity of the microemulsions and PEO solutions are shown in separate figures because the limiting thermal conductivity at zero water content is slightly different (0.27 times that of water for the microemulsion, vs. 0.31 for the PEO).

The striking observation is that the ionic conductivity and water self-diffusion coefficient, but not the thermal conductivity, deviate significantly from the predictions of the mixture theories. This could arise from structural effects, such as a gradual transition from 0/W to W/0 structure with decreasing water content. We argue instead that these deviations principally result from hydration effects, and not from structural properties of the microemulsions. This would be expected because of the similarity of the data from the microeraulsions and PEO, in which structure effects would be quite different. 

Effect of Microemulsion Structure on the Transport Properties. It appears from the discussion above that the reduction in the ionic conductivity and water self-diffusion coefficient is primarily attributable to hydration effects, not principally to changes in the structure of the microemulsion with higher phase volume. 

Figure 5. Water self-diffusion coefficients in a variety of ionic and nonionic microemulsions. The compositions of these microemulsions are given in Reference 2.

Thus, by measuring oil and water self-diffusion coefficients, it was quite easy to establish whether oil or water or none of them are confined to discrete domains, i.e. to droplets . In the first work on microemulsion structure by self-diffusion [46], using both tracer techniques and NMR spin-echo measurements, it was clearly shown that microemulsions can indeed be bicontinuous over wide ranges of composition, which is manifested by both... 

From the oil and water self-diffusion coefficients we can thus easily decide whether a given microemulsion is of the discrete oil-in-water (O/W), discrete water-in-oil (W/O), or bicontinuous type. Thus we have for the three cases (Z>o denoting the neat solvent value) ... 

Figure C2.3.8. Self-diffusion coefficients at 45°C for AOT ( ), water ( ) and decane ( ) in ternary AOT, brine (0.6% aqueous NaCl) and decane microemulsion system as a function of composition, a. This compositional parameter, a, is tire weight fraction of decane relative to decane and brine. Reproduced by pennission from figure 3 of [46].

FIG. 3 Self-diffusion coefficients of decane (A), water (B), and AOT ( ) in brine, decane, and AOT microemulsions at 45°C as a function of decane weight fraction, a (relative to decane and brine). Breakpoints in the self-diffusion data for both water and AOT are observed at a = 0.85 and at 0.7. (Reproduced by permission of the American Institute of Physics from Ref. 37.)... 

FIG. 9 Measured self-diffusion coefficients at 25°C for toluene (A), water ( ), acrylamide ( , and AOT ( ) in water, toluene, and AOT reverse microemulsions as a function of cosurfactant (acrylamide) concentration, f (wt%). The breakpoint at about 1.2% acrylamide approximately denotes, the onset of percolation in electrical conductivity. 

X 10 cm by measuring molecularly dispersed water in toluene and by correcting for local viscosity differences between toluene and these microemulsions [36]. Values for Dfnic were taken as the observed self-diffusion coefficient for AOT. The apparent mole fraction of water in the continuous toluene pseudophases was then calculated from Eq. (1) and the observed water proton self-diffusion data of Fig. 9. These apparent mole fractions are illustrated in Fig. 10 (top) as a function of... 

Further information on the dependence of structure of microemulsions formed on the alcohol chain length was obtained from measurement of self diffusion coefficient of all the constitutents using NMR techniques (29-34). For microemulsions consisting of water, hydrocarbon, an anionic surfactant and a short chain alcohol and C ) the self diffusion... 

In that case the self diffusion coefficient - concentration curve shows a behaviour distinctly different from the cosurfactant microemulsions. has a quite low value throughout the extension of the isotropic solution phase up to the highest water content. This implies that a model with closed droplets surrounded by surfactant emions in a hydrocarbon medium gives an adequate description of these solutions, found to be significantly higher them D, the conclusion that a non-negligible eimount of water must exist between the emulsion droplets. 

Figure 7. Schematic diagram comparing the behavior of self-diffusion coefficients of oil (D0), water (Dw), and surfactant (Ds) expected for the droplet inversion transition and the bicontinuous transition of microemulsion depicted in Fig. 6.

Physical Mechanisms. The simplest interpretation of these results is that the transport coefficients, other than the thermal conductivity, of the water are decreased by the hydration interaction. The changes in these transport properties are correlated the microemulsion with compositional phase volume 0.4 (i.e. 60% water) exhibits a mean dielectric relaxation frequency one-half that of the pure liquid water, and ionic conductivity and water selfdiffusion coefficient one half that of the bulk liquid. In bulk solutions, the dielectric relaxation frequency, ionic conductivity, and self-diffusion coefficient are all inversely proportional to the viscosity there is no such relation for the thermal conductivity. The transport properties of the microemulsions thus vary as expected from simple changes in "viscosity" of the aqueous phase. (This is quite different from the bulk viscosity of the microemulsion.)... 

The self-diffusion approach relies on the fact that molecular displacements over macroscopic distances are very sensitive to confinement and thus to microstructure. For example, we found that at the same composition (water, oil, surfactant), the ratio between water and oil self-diffusion coefficients could differ by a factor of 100 000. This also illustrates that the microstructure is primarily determined by the spontaneous curvature of the surfactant film and not by the oil-to-water ratio. Contributions to a better understanding of microemulsion structures with FT spin-echo NMR self-diffusion starting with Stilbs, included also Nilsson, Olsson, Soderman, Khan, Guering, Monduzzi, Ceglie, Das and many others in Lund. In this work [49-63], the access to suitable systems was very important. Here, the contacts with Friberg, Shinoda, Strey and Langevin played a central role. 

Figure 9 Relative self-diffusion coefficients of water and oil as a function of the oil volume fraction

Figure 10 Variations of self-diffusion coefficients of water, oil, surfactant, and cosurfactant in a salinity scan for a five-component microemulsion, SDS-butanol-water- NaCI-toluene. The experiments were performed with excess solvent(s) in the so-called Winsor sequence.The system...

Figure 11 Relative self-diffusion coefficients of ( ) water and (A) oil as a function of the oil content in a four-component microemulsion, AOT-water-NaCl-isooctane.The system is tuned by temperature at constant salinity. (Data taken from Ref. 45.)...

In many studies, much weaker variations are observed than those exemplified above. This refers, for example, to systems of short-chain surfactants and those of polar solvents other than water. Here the segregation of components between domains (oil, water, surfactant film) is weak, and distinct structures are not formed. This can be inferred from high values of the surfactant self-diffusion coefficient, which imply a considerable role of surfactant unimer translation. Such systems are intermediate between the organized microemulsions of strongly amphiphilic surfactants and simple molecular solutions. We note that, as expected, for nonassociating solvent mixtures, the self-diffusion coefficients vary little with composition, i.e., D/Do values throughout are not too different from unity. 

Figure 13 Relative self-diffusion coefficients of water and oil as a function of the surfactant mixing ratio in a five-component microemulsion consisting of Ri20CH2CH2S04Cai/2-/-R80CH2CH(0H)CH20H-water with 8 wt% CaCh and decane. Here R12 refers to a dodecyl (C12H25) chain and /-Rg to an isooctyl ((CHj)jC(CH2)4] chain. The system is tuned by the surfactant mixing ratio (Data taken from Ref 47.)...

Figure 14 Self-diffusion coefficients of surfactant, water, and oil in a three-component microemulsion (L2) phase with AOT, water, and /j-xylene. The samples are labeled from 1 to 18, and the compositions are indicated in the phase diagram. Note the very similar diffusion coefficients of water and AOT over the full concentration range, showing that the structure is made up of discrete reverse micellar aggregates. The fact that the diffusion coefficient of the oil is high everywhere confirms that the structure is oil-continuous. (Data taken from Ref. 94.)...

Figure 19 Double-oil diffusion experiment with nonionic surfactant, (a) Self-diffusion coefficients and (b) diffusion coefficient ratio A" as a function of temperature in a water-rich microemulsion with nonionic surfactant. A transition from oil-in-water droplets to a bicontinuous microstructure occurs with increasing temperature (decreasing spontaneous curvature of the C12E5 surfactant film). The maximum in K indicates that an attractive interaction between the micelles is operating prior to the formation of a bicontinuous structure. Kq = 1.69 is the diffusion coefficient ratio in the pure oil mixture and is indicated as a broken line in (b). Note that the initial decrease of the self-diffusion coefficients shows that the droplets grow in size before the bicontinuous transition. The phase boundary at 25.7 C is indicated as a vertical broken line. (Data from Ref 43.)...

Figure 20 Double-water experiment, the aqueous analogy of the double-oil experiment, performed on an AOT microemulsion as a function of temperature. The polar solvent is a 5% A-methyl formamide (NMF) solution in heavy water (D2O). The ratio of the water (here measured as trace impurities of HDO) and NMF diffusion coefficients is monitored as a function of temperature (c). Also shown as (a) the individual self-diffusion coefficients of water (O). NMF ( ), and AOT ( ) and (b) the relative diffusion coefficient of water. Kq = 1.73 is the diffusion coefficient ratio in the pure water-NMF mixture and is indicated as a broken line in (c). The phase boundary at 75"C is indicated as a vertical broken line. The behavior with increasing temperature is completely analogous to that of the nonionic system (Fig. 19) and illustrates a transition from reverse micelles to a bicontinuous structure via growing droplets that become attractive. (Data from Ref 49.)... 

Decreasing spontaneous curvature further to negative values (high salinity) leads to a progressive lowering of the surfactant self-diffusion coefficient to the value of water and to that of the droplets in the W/0 microemulsions. (This decrease is not pronounced for nonionic surfactants because of the high solubility in oil and the concomitant contribution from single-molecule diffusion.)... 

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