Polydispersity microemulsions

Arriagada FJ, Osseoasare K (1994) Silica Nanoparticles Produced in Aerosol Ot Reverse Microemulsions - Effect of Benzyl Alcohol on Particle-Size and Polydispersity. J Dispers Sci Technol 15 59-71... 

Kotlarchyk M, Stephens RB, Huang JS (1988) Study of Schultz Distribution to Model Polydispersity of Microemulsion Droplets. J Phys Chem 92 1533-1538... 

It can now be said that the microemulsion-mediated silicon alkoxide sol-gel process has come of age. The ability to form monodisperse spherical silica particles (20-39) and monolithic gels (40-53) by this method has been amply demonstrated. Recipes are available to prepare materials with predetermined characteristics, especially particle size and polydispersity. Potential applications of these microemulsion-derived... 

Small-angle neutron scattering (SANS) can be applied to food systems to obtain information on intra- and inter-particle structure, on a length scale of typically 10-1000 A. The systems studied are usually disordered, and so only a limited number of parameters can be determined. Some model systems (e.g., certain microemulsions) are characterized by only a limited number of parameters, and so SANS can describe them fully without complementary techniques. Food systems, however, are often disordered, polydisperse and complex. For these systems, SANS is rarely used alone. Instead, it is used to study systems that have already been well characterized by other methods, viz., light scattering, electron microscopy, NMR, fluorescence, etc. SANS data can then be used to test alternative models, or to derive quantitative parameters for an existing qualitative model. 

Shioi, A., Harada, M., and Tanabe, M. (1995), Static light scattering from oil-rich microemulsions containing polydispersed cylindrical aggregates in sodium bis(2-ethylhexyl) phosphate system,/. Phys. Chem., 99,4750 1756. 

Since the PS reference sample is almost monodisperse, a cumulant analysis of that material would yield a very small Q, say Q < 0.03. That is, all the correction terms are negligible and Eqs. (17) collapse to Eqs. (12). But cumulant analysis is a useful way to handle practical samples such as pigments, inks, microemulsions, swollen micelles, globular proteins, and spherical virus particles, where there is a size distribution but one that is not very broad (say Q < 0.3). This analysis should be made for the milk data using a non-linem teast-squares fitting of Eq. (17a), neglecting /1.3 and all higher order terms. Report the F, D, and R values as well as the second cumulant /t2 aiid the polydispersity index Q. 

Fig. 9 (A) SANS from EC/Ci2AO/water microemulsion (15 25) 1.4% v/v. Lines are simultaneous fit using polydisperse oil core with some mixing of oil into surfactant shell. Dashed line is interparticle S Q). Dotted lines are best simultaneous fit with no oil mixing into the C12AO tails. (B) SANS data and simultaneous fit to 1.8%v C12AO, 0.7% v EC oil, 0.7%v drug, microemulsion circles—drop xlO xxx— /i-shell x5 squares—d-shell dot-dash—charged sphere S(Q). (Reproduced from Ref. l)...

The low temperature properties of a dodecane-hexanol-K.oleate w/o microemulsion from 20°C to -190°C were studied vs. increasing water content (C,mass fraction) in the interval 0.021+-0.1+, by Differential Scanning Calorimetry and dielectric analysis (5 Hz-100 MHz). A differentiation between w/o dispersions is obtained depending on whether they possess a "free water" fraction. Polydispersity is evidenced by means of dielectric loss analysis. Hydration processes occurring, at constant surface tension, on the hydrophilic groups of the amphiphiles, at the expenses of the free water fraction of the droplets, are shown to develop "on ageing" of samples exhibiting a time dependent behavior. 

The conclusions drawn on the basis of the dielectric loss analysis of liquid samples, support the interpretation that a very gradual confluence of the different types of dispersions takes place.Such an interpretation could explain the instauration of polydispersed samples in terms of the coexistence, at equilibrium, first, of micellar aggregates with w/o microemulsion droplets and, successively, of a microemulsion with l-I O-per hydrophilic group monolayer, in equilibrium with a hydrated type of microemulsion (U-water molecule per polar head of the surfactant hydrophilic groups monolayer). The latter interpretation is in accordance with Steinbach and Sucker findings that the two types of structures ( 1-HpO and U-HgO molecule), may coexist at equilibrium (23.). 

The theoretical description in terms of spherical harmonics also yields a relation between the size polydispersity index p of the microemulsion droplets and the bending elastic constants [43]. The quantity p is accessible by SANS [51, 52, 59-61]. For polydisperse shells as obtained by using deuterated oil and heavy water for the preparation of the microemulsion (contrast variation), one can account for the droplet polydispersity by applying an appropriate form factor, e.g. containing a Gaussian function to model the size distribution [52, 59, 62]. A possible often-used choice is the following form factor... 

In this relation, N is the number density of the scattering microemulsion droplets and S(q) is the static structure factor. Equation (2.12) is only strictly valid for the case of monodisperse spheres. However, for the case of low polydispersities the occurring error is small [63, 64]. S(q) describes the interactions between and the spatial correlations of the droplets. These are in general well approximated by hard sphere interactions in microemulsion systems [65], The influence of inter-particle interactions as described by S(q) canbe estimated at least for S(0) using the Carnahan-Starling expression [52,64,66]... 

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].)...

The analysis of the light scattering data using CONTIN also allows for a determination of the size polydispersity of the microemulsion droplets, because all the moments u = / pm G(r)rndr which describe the distribution function G(T) are computed (for details see Ref. [98]). The polydispersity index is obtained from... 

Gradzielski, M., Langevin, D., Magid, L. and Strey, R. (1995) Small-angle neutron scattering from diffuse interfaces. 2. Polydisperse shells in water n-alkane-C10E4 microemulsions. /. Phys. Chem., 99, 13232-13238. 

Arleth, L. and Pedersen, J.S. (2001) Droplet polydispersity and shape fluctuations in AOT [bis(2-ethylhexyl)sulfosuccinate sodium salt] microemulsions studied by contrast variation small-angle neutron scattering. Phys. Rev. E, 63, 61406-61423. 

In Ref. [42], PEO was embedded in a w/o-droplet microemulsion and studied by small-angle neutron scattering. The authors state that this polymer does not adsorb considerably at the SDS monolayer. The important statement is that both the size polydispersity and the shape fluctuations are increased compared to the reference system without polymer. Larger shape fluctuations are also found for gelatine embedded in w/o-droplet microemulsions (see Fig. 4.10 in [43]). Here, by strong confinement, the elongated shapes... 

The formation of nanoparticles from microemulsions need not essentially follow the template shape. Pileni [32] (as quoted by Ganguli and Ganguli) has shown that with water/isooctane/Cu( AOT)2 shapes like sphere to cylinder to mixed spherulites and cylinders to other polydisperse shapes were possible with increasing to. According to Pileni [33], the presence of salt anions can control the shape while chloride ions favour formation of nanorods, nitrate ions hinder it. The surfactant content also can have a say on the shape of nanoparticles. The infrequently observed morphologies of nanoparticles, viz. wires, trigons, hexagons, cubes etc. have so far no specific and reliable reasons for formation in micro emulsion templates. 

Microemulsion polymerisation has shown a great advantage over conventional polymerisation strategies such as emulsion polymerisation with respect to the end particle size, polydispersity and reproducibility of the product characteristics. Although we still face severe problems regarding the polymerisation of microemulsions (see Section 11.2 in Chapter 11), it has been employed for the synthesis of polymeric nanoparticles of pharmaceutical interest. Microemulsion polymerisation involves free-radical polymerisation in a large number of monomer-swollen microemulsion droplets and represents a thermodynamically stable, transparent one-phase reaction system. Generally, the microemulsion droplet is considered as initiation locus for the polymerisation. The type of microemulsion used for the polymerisation depends on the monomer properties [148]. 

Akkara, J.A., Kaplan, D.L., and Ayyagari, M. (2000) Process to control the molecular weight and polydispersity of substituted polyphenols and polyaromatic amines by enzymatic synthesis in organic solvents, microemulsions, andbiphasic systems, United States Dept, of the Army, USA US 6096859 US 20010003774 US 6362314. 

Scattering techniques provide the most definite proof of micellar aggregation. Zielinski et aL (34) employed SANS to study the droplet structures in these systems. Conductivity measurements (35) and SANS (36) were also used to study droplet interactions at high volume fraction in w/c microemulsions formed with a PFPE-COO NH4 surfactant (MW = 672). Scattering data were successfully fitted by Schultz distribution of polydisperse spheres (see footnote 37). A range of PFPE-COO NH/ surfactants were also shown to form w/c emulsions consisting of equal amount of CO2 and brine (38-40). 

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