Microemulsions droplet

With 7ow close to zero, microemulsions will form spontaneously and are thermodynamically stable. The droplets of microemulsions tend to be monodispersed. A microemulsion may form as a separate phase in equilibrium with excess oil (O/W) or water (W/O) (i.e. it is saturated with respect to droplets). Microemulsions are usually of low viscosity. 

Figure 1. Schematic representation of the transition from a two-phase system with an oil-in-water droplet microemulsion to a three phase system with a bicontinuous microemulsion and to a two-phase system with a water-in-oil droplet microemulsion.

Calculations were carried out for a system consisting of the anionic surfactant sodium dodecyl sulfate, 1-pen-tanol (cosurfactant), cyclohexane, and water containing 0.3 M NaCl. As mentioned atthe very beginning, thechoice of this system was dictated by the possibility of identifying various types of phase behaviors for the same chemical components by merely changing the amount of added alcohol. In all calculations, we assumed the coexistence of an excess dispersed phase. This means that the droplet microemulsion phase is part of a two-phase system and that the amount of dispersed phase present in the droplet is the maximum achievable. 

Figure 5. Volume fractions of oil (squares), water (triangles), and alcohol (circles) in microemulsions containing O/W and W/O droplets as a function of the volume ratios of alcohol to surfactant in the microemulsions. Filled symbols refer to O/W droplet microemulsions, and open symbols, to W/O droplet microemulsions. All the system characteristics are identical to those described for Figure 3. In all cases, the volume fraction of the surfactant in the microemulsions is 0.01.

Figure 10. Volume fraction of oil in a bicontinuous microemulsion as a function of the volume ratio of alcohol to surfactant in the microemulsion. The values of Xaw and gAi/gsi are respectively 0.001 72 and 1.036 (filled squares) and 0.001 84 and 1.074 (open squares). In all cases, the volume fraction of surfactant is 0.01 and the volume fraction of water is found by subtraction from unity. The system consists of SDS, 1-pentanol, cyclohexane, water, and 0.3 M NaCl. Also shown are the corresponding results for O/W droplet microemulsions (filled circles) and for W/O droplet microemulsions (open circles).

A.l. A Water-in-Oil Droplet Microemulsion. (a) Description of the Phase. The microemulsion system is composed of Ngo droplets of various sizes g (each consisting of surfactant, alcohol, oil and water molecules) and outside the droplets,Noo oil molecules, Also surfactant molecules, Nao alcohol molecules, and ATW0 water molecules. The subscript g for a droplet denotes the total number of molecules of different kinds present in it (i.e., g =gs +8a +go +gw). The A ao alcohol molecules outside the droplets are present both as singly dispersed molecules and as aggregates. The number of alcohol aggregates containing j alcohol molecules is denoted by Njao-... 

A.2. An Oil-in-Water Droplet Microemulsion. Similarly, for the oil-in-water droplet-type microemulsion, the size and composition distribution of droplets is given by... 

The research on microemulsions currently concentrates on even more complex mixtures. By adding amphiphilic macromolecules the properties of microemulsions can be influenced quite significantly (see Chapter 4). If only small amounts of amphiphilic block copolymers are added to a bicontinuous microemulsion a dramatic enhancement of the solubilisation efficiency is found [27,28]. On the other hand, the addition of hydrophobically modified (HM) polymers to droplet microemulsions leads to a bridging of swollen micelles and an increase of the low shear viscosity by several orders of magnitude [29]. 

Scattering from droplet microemulsions 2.2.1 General outline... 

In this subsection, the theoretical background for SANS and neutron spin-echo measurements carried out with o/w- and w/o-droplet microemulsions will be presented. According to Milner, Safran and others, shape fluctuations in droplet microemulsions can be described in terms of spherical harmonics [42-44]. This offers the possibility to calculate a dynamic structure factor S(q,w) or its Fourier transform, i.e. the intermediate scattering function I(q,t) for the problem, which can be used to analyse dynamical measurements by neutron spin-echo spectroscopy [45]. For the scattering from thin shells I(q,t) was calculated [43]... 

In this equation, hs is the hard sphere volume fraction which is about 14% larger in o/w-droplet microemulsions of non-ionic surfactant than the dispersed volume fraction. This is caused by the water penetration in the surfactant layer [64]. S(q) approaches unity for q values smaller than the minimum of I(q). This behaviour occurs even for fairly high volume fractions in non-ionic surfactant systems (see for example Fig. 8 in Ref. [64]). Seeing that the value of the radius is fixed by the position of the minimum of I(q), the approximation of S(q) 1 in Eq. (2.12) does not lead to a significant error in the determination of Rq if the low q part of the experimental curve is not taken into... 

Figure 2.1 Comparison of SANS curves obtained for the system D20/n-octane-di8/C1oE4 on the (a) water-continuous (o/w-droplet microemulsion) and (b) the oil-continuous (w/o-droplet microemulsion) side, respectively. The solid lines in both plots are from factor curves according to Eq. (2.11). Usually, the polydispersity is slightly higher for w/o-droplet microemulsions. (Figures redrawn with data from Ref. [67].)...

As already pointed out the first work directly measuring the deformation dynamics in an o/w-droplet microemulsion using NSE was published by Huang et al. [45]. In this work, a microemulsion based on the surfactant AOT was studied and it was shown that the intermediate scattering functions contain information about the centre of mass diffusion and in addition also contributions from the deformation dynamics. The intermediate scattering functions obtained in this work are shown in Fig. 2.3. 

Figure 2.3 Intermediate scattering functions obtained for an AOT-based o/w-droplet microemulsion using NSE. The solid lines are fits with a single exponential function yielding an effective diffusion coefficient. Note that this was the first NSE study of a microemulsion showing the calculation of k on the basis of the intermediate scattering functions. (From Ref. [45], reprinted with permission of the American Physical Society.)...

After these first experiments it took 11 years until this problem was studied again exploiting the unique possibilities of NSE with respect to contrast variation and energy resolution [29]. The studied microemulsion was an o/w-droplet microemulsion in the system H2O/ -octane/C10E5. It turned out that the NSE data can be analysed using a double exponential fit according to Eq. (2.8), when the translational diffusion coefficient is already measured in advance using PCS. The same approach was also successfully applied to study another water-continuous microemulsion in the system H2 0/n-dodecane/Cio E-[49]. Since the approach works as well for oil-continuous systems an extended example for the approach will be discussed in the next subsection. 

Figure 2.5 Relaxation rates F of the intensity correlation functions as a function of q2 obtained via a photon correlation spectroscopy experiment. The sample was a w/o-droplet microemulsion made of D2 0/n-octane-di8/CioE4. On the oil-continuous side of the phase diagram the scattered light intensity is usually low leading to rather large errors of the individual data points. Nevertheless, from the slope of the linear fit the translational diffusion coefficient is obtained. (Figure redrawn with data from Ref. [67].)...

Figure 2.6 Measured intermediate scattering functions of a w/o-droplet microemulsion for the system D2 0/n-octane-d- 8/CioE4. The four curves were obtained at four different q values close to the minimum of the droplet form factor. The solid lines are double exponential fits with only two adjustable parameters. The translational diffusion coefficient was determined using PCS (see Fig. 2.5) and used as input for the analysis of the NSE data. (Figure redrawn with data taken from Ref. [67].)...

Hellweg, Th. and Langevin, D. (1998) Bending elasticity of the surfactant film in droplet microemulsions Determination by a combination of dynamic light scattering and neutron spin-echo spectroscopy. Phys. Rev. E, 57, 6825-6834. 

Gradzielski, M., Langevin, D., Sottmann, T. and Strey, R. (1997) Droplet microemulsions at the emulsification boundary The influence of the surfactant structure on the elastic constants of the amphiphilic film. /. Chem. Phys., 106, 8232-8238. 

Hellweg, T., Brfilet, A. and Sottmann, T. (2000) Dynamics in an oil-continuous droplet microemulsions as seen by quasielastic scattering techniques. Phys. Chem. Chem. Phys., 2, 5168-5174. 

There are basically two topics that need to be addressed regarding the effect of amphiphilic polymers on the physical behaviour of microemulsions. The first topic is related to phase behaviour and structure formation. Amphiphilic polymers can strongly influence phase behaviour because of their impact on the bending rigidity of the surfactant film. For both droplet micro emulsions and bicontinuous microemulsions such phenomena were studied. Especially in droplet microemulsions, amphiphilic polymers were used to interconnect microemulsion domains. This leads to ordering phenomena and can alter the phase behaviour. The second topic again is based on systems where microemulsion domains are connected via polymers. It covers dynamic phenomena with a focus on viscoelastic properties. Important in this area is the formation of transient or permanent networks. 

The difference between PEO being functionalised at one chain end with a hydrophobic sticker and its counterpart containing the hydrophobic units at both chain ends was examined with oil-in-water (o/w) droplet microemulsions [22-24]. Cetyl pyridiniumchlo-ride/octanol or alkylphenol ethoxylate were used as surfactant. The monofunctional polymers were only capable to decorate the oil droplets whereas the difunctional polymers could also bridge them. Experiments were carried out as a function of the droplet volume... 

Figure 4.5 Freeze fracture micrograph of an ordered w/o-droplet microemulsion containing PEO-PI-PEO triblock copolymer. (From Ref. [20], reprinted with permission of EDP Sciences.)...

The use of amphiphilic polymers to interconnect microemulsion domains does not only influence structural but also dynamic properties. This was investigated in droplet microemulsions of both the w/o and the o/w type. These systems allow to vary parameters... 

Figure 4.6 Phase behaviour of o/w-droplet microemulsions containing 0.2 M NaCI-decane-cetyl pyri-dinium chloride-octanol. The PEO additive is hydrophobically modified at one chain end (a) and at both chain ends (b), respectively, r represents the number of polymer chains per droplet. (From Ref. [23], reprinted with permission of the American Chemical Society.)...

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