Neutron Scattering Study of the Structural Change Induced by Photopolymerization of AOT/D 2 O/Dodecyl Acrylate Inverse Microemulsions (original) (raw)
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Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2019
Microemulsion systems consisting of D 2 O, an alkane, an anionic internal olefin sulfonate surfactant, salt and secondary butyl alcohol (SBA) as co-solvent are studied in a systematic way. In four different sample sets, either the salt content, SBA content or alkane carbon number was varied in order to study the effects of the individual compounds on the structure sizes making up the microemulsion. Using complementary small-angle neutron scattering techniques SANS and Spin-Echo SANS, it was found that the microemulsion systems exhibit the largest structures in the optimum state (domain size of d/2 =144 nm in the model by Teubner and Strey), where the structure is considered bicontinuous. In comparison, at under-and over-optimum states where the structures consist of emulsified spherical droplets, the smallest measured diameter was 2R = 44 nm. Furthermore, the structure sizes in bicontinuous microemulsions decrease exponentially (down to d/ 2 =15 nm for pentadecane and 5 wt% SBA) as function of both SBA content and alkane carbon number. The observed trends in structure sizes combined with the trends observed in the area per surfactant molecule, are qualitatively explained with the extended Winsor R-ratio, the HLD-NAC model and surfactant film flexibility arguments.
The Journal of Physical Chemistry B, 2001
We present a preliminary examination of three isotopically substituted series of concentrated emulsions by small-angle neutron scattering (SANS). These have 90% internal phase water or salt solution droplets in continuous hexadecane. The surfactants have polyisobutylene oligomer tails with mainly acid-amide headgroups. The emulsion structure is well approximated by a polydisperse system of micrometer scale aqueous spheres surrounded by a continuous, surfactant/hexadecane phase L 2 microemulsion. Even though the aqueous volume fraction in the whole emulsion is ca. 90%, we see no evidence for nonsphericality of aqueous droplets, i.e., long-scale planarity of the aqueous-hexadecane boundary. The salt emulsion data fit well to a model in which there is 12-16% of the surfactant absorbed as a monolayer at a flat (0(3) Å) aqueous-oil interface, with the remainder as spherical 26-30 Å radius reverse micelles in the hexadecane continuous oil phase. The micelles contain 8-10% water and a large fraction of hexadecane as well as the surfactant. The water emulsion has less surfactant absorbed at a much rougher (62(1) Å) aqueous interface, and larger micelles containing more watersall reflecting less tightly held water in the aqueous as opposed to salt solution droplets. The structure is insensitive to heating from 20 to 70°C, but cooling to 5°C precipitates large surfactant aggregates, giving three phases. The three possible relative specific surface areas (aqueous/aggregate, aggregate/ hexadecane, and aqueous/hexadecane) show increasing intrusion of the surfactant aggregate into the aqueous droplets as the aggregate increases in size.
Langmuir, 1998
Microemulsions with entrapped nonaqueous polar solventssglycerol (Gly), ethylene glycol (EtGy), and formamide (Form)sand an oil (isooctane or n-heptane) continuous phase were studied by dynamic light scattering (DLS). The largest hydrodynamic radii evaluated for the two former systems, by the method of extrapolation to infinite dilution, were respectively 56.5 and 28 Å. The method was not applicable to the latter system (formamide), making the determination of the hydrodynamic radius impossible. It was found that, in these microemulsion systems, the interactions between the droplets are more attractive than the ones observed in water-in-oil microemulsions, being larger in formamide and smaller in glycerol. The droplet structure remains spherical, but evidence of clusters was obtained in all the systems studied. Their formation is attributed to strong attractive interactions.
The Journal of Chemical Physics, 1997
The small-angle neutron scattering ͑SANS͒ of bicontinuous microemulsions of 19 different watern-alkane-C i E j ͑n-alkylpolyglycolether͒ systems has been measured. All scattering curves exhibit a broad scattering peak which permits determining the characteristic length scale for bicontinuous structures at symmetric water and oil volume fractions, i.e., ϭ0.5. Various random models predict ϭ a␦(1 Ϫ)/ c. We find that is indeed inversely proportional to the surfactant volume fraction c. Approximating the effective surfactant chain length ␦ by ␦ ϭ c /a c , where a c and c are the area and the volume of the surfactant molecule, the numerical value for a is determined to be a ϭ 7.16, which is close to, but significantly different from those used in theoretical models. The head group area a c at the water-oil interface is obtained from the large q part of the scattering curves. It is found to be independent of i and k, the carbon numbers of the alkyl chain of the surfactant and of the alkane, respectively. However, it depends strongly, and nearly linearly, on the head group size j of the surfactant. Within experimental error it is described by a c ϭ29.3ϩ6.20j (Å 2).
Small-Angle Neutron Scattering of Ca(AOT) 2 /D 2 O/Decane Microemulsions
Langmuir, 1997
Small-angle neutron scattering (SANS) studies were performed in order to investigate the structural properties of the monophasic domain of the Ca(AOT)2/water/decane phase diagram, which shows a consistent reduction of the microemulsion region as compared to the parent sodium system. Microemulsions of Ca(AOT)2/D2O/decane and Ca(AOT)2/H2O/decane-d22 have been studied at 25°C, for an oil dilution line at constant [water]/[Ca(AOT)2] ) 25.1, as a function of decane concentration and of surfactant volume fraction, φS, in the range 0.0186 e φS < 0.0937 for the D2O system, or 0.0249 e φS e 0.0886 for the corresponding decane-d22 system. SANS spectra were analyzed according to a self-consistent method proposed by Sheu 1 and support, in analogy to the sodium AOT microemulsion system, the presence of polydisperse droplets in the diluted region studied, while the system shows a pecolative behavior for higher concentrations. These droplets have a 21.3 Å average radius with polydispersity around 22%. The mean area per polar head group and the principal geometrical parameters are also reported.
Journal of Dispersion Science and Technology, 2014
The structure of micelles formed by a four component water-in-oil nonionic microemulsion surfac-tant polyoxyethene (20) sorbitan monoleate (Tween 80), sorbitan monolaurate (Span 20) at ethyl oleate and deuterated water interface have been probed by small-angle neutron scattering (SANS). The total surfactant concentration in each of the samples studied (Tween 80: Span 20) is fixed at 3:2. The deuterated water content is variable at 5-60% w/w. The experimental SANS data from all the seven samples are fit well by spherical micelles interacting with hard sphere potential. Increased deuterated water leads to spherical to lamellar and rod-like micelle geometry featured in the SANS scattering data. The observed change in micelle geometry supports the characterization of phase transition between the self-assembled micelles of the nonionic microemulsion.
Small-angle light scattering studies of dense AOT-water-decane microemulsions
Il Nuovo Cimento D, 1996
We have performed extensive studies of a three-component microemulsion system composed of AOT-water-decane (AOT = sodium-bis-etbylhexylsulfosuccinate is an ionic surfactant) using small-angle light scattering (SALS). The small-angle scattering intensities are measured in the angular interval 0.001-0.1 radians, corresponding to a Bragg wave number range of 0.14~tm-1< Q < < 1.4 ~tm-1. The measurements were made by changing temperature and volume fraction ~ of the dispersed phase (water + AOT) in the range 0.05 < ~ < 0.75. All samples have a f'Lxed water-to-AOT molar ratio, w = [water]/[AOT] = 40.8, in order to keep the same average droplet size in the stable one-phase region. With the SALS technique, we have been able to observe all the phase boundaries of a very complex phase diagram with a percolation line and many structural organizations within it. We observe at the percolation transition threshold, a scaling behavior of the intensity data. This behavior is a consequence of a clustering among microemulsion droplets near the percolation threshold. In addition, we describe in detail a structural transition from a droplet microemulsion to a bicontinuous one as suggested by a recent small-angle neutron scattering experiment. The loci of this transition are located several degrees above the percolation temperatures and are coincident with the maxima previously observed in shear viscosity. From the data analysis, we show that both the percolation phenomenon and this novel structural transition are derived from a large-scale aggregation between microemulsion droplets. PACS 78.35 -Brillouin and Rayleigh scattering; other light scattering.
2005
The role of C4-alcohols (n-butanol, 2-butanol and tert-butanol) in four component microemulsions is studied using neutron scattering. These microemulsions are dispersions of nanoscale droplets consisting of a core of water surrounded by a surfactant-stabilized shell. The particles are suspended in an oil (octane) continuum. The addition of a co-surfactant, such as a short-chain alcohol, modifies the droplet size and structure, as well as the rigidity of the surrounding shell. In this work, we investigate water-in-oil microemulsions employing small-angle neutron scattering (SANS) experiments performed at the NIST Center for Neutron Research. Using this technique, it is possible to probe structure on the nanometer scale and extract information rele\ant to the structure of our system. The SANS data is analyzed with a model sensitive to the overall shape, size, polydispersity, and the internal structure of the dispersed droplets. In addition, the model also incorporates corrections for ...
Physical Review E, 2001
Microemulsions consisting of AOT water, and decane or iso-octane are studied in the region of the phase diagram where surfactant covered water droplets are formed. The polydispersity and shape fluctuations of the microemulsion droplets are determined and compared in the two different alkane types. Conductivity measurements show that there is a pronounced dependence of the temperature behavior of the microemulsion on the type of alkane used. In both cases the microemulsion droplets start to form larger aggregates when the temperature increases. But in the system with decane this aggregation temperature occurs at a temperature about 10°C lower than in a similar system with iso-octane. Aggregation phenomena are avoided and the two systems are at approximately the same reduced temperature with respect to the aggregation temperature when the temperature of the AOT/D 2 O/decane microemulsion is 10°C and the temperature of the AOT/D 2 O/iso-octane microemulsion is 20°C. Contrast variation small-angle neutron scattering measurements are performed at these temperatures on systems with volume fractions of 5% D 2 OϩAOT by varying the scattering length density of the alkane. The small-angle scattering for 11 different contrasts evenly distributed around the match points are studied for each sample. The scattering data for the different contrasts are analyzed using a molecular constrained model for ellipsoidal droplets of water covered by AOT, interacting as polydisperse hard spheres. All contrasts are fitted simultaneously by taking the different contrast factors into account. The analysis show that at the same reduced temperature with respect to the aggregation temperature the droplet size, polydispersity index, the size of the shape fluctuations are similar in the two systems. A polydispersity index (/R of the Gaussian size distribution͒ of 16% and an average axis ratio of the droplets of 1.56 is found in the AOT/D 2 O/decane microemulsion. In the AOT/D 2 O/iso-octane system the polydispersity index is also 16% while the axis ratio is 1.72. The bending elastic constant and the Gaussian bending elastic constant can be estimated from these numbers. For AOT/D 2 O/decane we find ϭ3.4k B T and ϭϪ5.9k B T and for AOT/D 2 O/iso-octane we find ϭ2.35k B T and ϭϪ3.8k B T, where k B is the Boltzmann constant and T is the absolute temperature.
Investigation of Pharmaceutical Oil/Water Microemulsions by Small-Angle Scattering
Pharmaceutical Research, 2002
Purpose. Stable oil/water (o/w) microemulsions are very effective vehicle systems for dermal administration of drugs having no or low skin penetration. These systems, consisting of oils, a blend of a high and a low HLB surfactant, and a hydrophilic phase (propylene glycol/water), were developed using pharmaceutically acceptable components. Methods. In this paper, the droplet size of these microemulsions was characterized by means of dynamic light scattering (DLS) and small-angle neutron scattering (SANS). Furthermore, different size parameters obtained by DLS and SANS experiments were compared and discussed. Results. Extremely small droplet radii of approximately 10 nm could be observed. A good agreement between the data of DLS and SANS experiments was found. The kind of oil only marginally influences the droplet size. Conclusions. Particle size determination via scattering techniques is a useful tool to characterize droplets in microemulsions for dermal drug delivery.