Surfactant based microstructures

In many ways, bound water is the most interesting type of water related to surfactant-based microstructures. Differences in the structure of interfacial water... 

Schulz, P. C., Soltero, J. F. A., and Puig, J. E. 2001. DSC analysis of surfactant-based microstructures. In Thermal Behaviour of Dispersed Systems. Marcel Dekker, New York, Chapter 4, pp. 121-182. 

Simmons BA, Irvin GC, Agarwal V, Bose A, John VT, McPherson GL, Balsara NP (2002) Small Angle Neutron Scattering Study of Microstructural Transitions in a Surfactant-Based Gel Mesophase. Langmuir 18 624-632... 

A basic question that might be raised here concerns whether objective criteria for establishing this quite detailed picture can be ascertained. We can seldom reach this level of description with regard to microstructure of surfactant-based systems when we rely on only calorimetric data. The salient features of such... 

Differential scanning calorimetry (DSC) is widely used for studying binary and multicomponent systems containing surfactants. Transition temperatures and enthalpies are often determined and used to draw the limits of existence of the different phases of surfactant-based systems [1-6], The state of the surfactant molecules in these phases is studied by means of thermal analysis [7,8], DSC is also a useful technique to obtain information about the phase diagrams of surfactant-based systems and the various microstructures formed in these systems. The properties that can be obtained from DSC for binary systems are the following [6] ... 

The gel-liquid crystal transition in surfactant-based systems is the result of the cooperative melting of the hydrocarbon chains [58]. The melted state in a surfactant-based system is less disordered than that of liquid hydrocarbons due to the anchoring of one end of the molecule to the microstructure surface via its polar headgroup. This transition can be sharp in pure synthetic phospholipids and surfactants, but it is broad and ill-defined in natural phospholipids and surfactants, which are usually mixtures with a variety of hydrocarbon chain lengths. The transition temperature, T, depends on the nature of the polar headgroup and the... 

Nature uses membrane-based microstructures which organize redox enzymes and reactants to provide living organisms with highly efficient redox processes. Micellar solutions and microemulsions contain surfactant aggregates which can be considered crude models for biological membranes. 

The procedure consists of three steps. The first step is to identify all the desired product quality factors or attributes for the new product. Then what follows is the selection of the appropriate product form and microstructure, a stable surfactant system with the right performance based on phase behavior, and the appropriate active ingredients in order to realize those quality factors previously identified. Finally the process flowsheet will be created with the equipment units and process operating conditions determined. 

Two main microemulsion microstructures have been identified droplet and biconti-nuous microemulsions (54-58). In the droplet type, the microemulsion phase consists of solubilized micelles reverse micelles for w/o systems and normal micelles for the o/w counterparts. In w/o microemulsions, spherical water drops are coated by a monomolecular film of surfactant, while in w/o microemulsions, the dispersed phase is oil. In contrast, bicontinuous microemulsions occur as a continuous network of aqueous domains enmeshed in a continuous network of oil, with the surfactant molecules occupying the oil/water boundaries. Microemulsion-based materials synthesis relies on the availability of surfactant/oil/aqueous phase formulations that give stable microemulsions (54-58). As can be seen from Table 2.2.1, a variety of surfactants have been used, as further detailed in Table 2.2.2 (16). Also, various oils have been utilized, including straight-chain alkanes (e.g., n-decane, /(-hexane),... 

The ideas underlying elemental structures models are to establish microstructures experimentally, to compute free energies and chemical potentials from models based on these structures, and to use the chemical potentials to construct phase diagrams. Jonsson and Wennerstrom have used this approach to predict the phase diagrams of water, hydrocarbon, and ionic surfactant mixtures [18]. In their model, they assume the surfactant resides in sheetlike structures with heads on one side and tails on the other side of the sheet. They consider five structures spheres, inverted (reversed) spheres, cylinders, inverted cylinders, and layers (lamellar). These structures are indicated in Fig. 12. Nonpolar regions (tails and oil) are cross-hatched. For these elemental structures, Jonsson and Wennerstrom include in the free energy contributions from the electrical double layer on the water... 

The polymeric material based on nonionic surfactant (Neodol 91-5) has diferent pore morphology (Figure5(f)) as compared to the anionic system. Even the polymeric material based on the anionic system with a different cosolvent, butyl cellosolve, shows a different morphology (Rgure5(g)). Figure5(h) illustrates the structure of polymers obtained from a microemulsion using a different nonionic surfactant (Emsorb 6916). Thus, the pore morphology depends on the initial microstructure of the microemulsion as determined by the type of surfactant and cosolvent in addition to composition and polymerization conditions. 

DSO via molybdate-catalyzed disproportionation of HjOj provides a readily scalable alternative to photooxidation. It can be carried out in commonly available stirred-tank reactors. However, the reaction does not work at low temperatures and organic media are limitedto alcoholic polar solvents (methanol or the safer ethylene glycol) or to microstructured media such as one-, two-, or three-phase microemulsion systems. The latter based on balanced catalytic surfactants advantageously combine low surfactant concentration with easy product isolation and catalyst recycling via simple phase separation. Safe processing may be further enhanced by microreactors, which minimize peroxide hold-up. 

M. He, Z. Lin, H.T. Davis, L.E. Scriven and R.M. Hill, Phase behavior and microstructure of nonionic heptamethyltrisiloxy-based surfactants in water, AIChE Annual meeting, Miami Beach, FA, 1992 and submitted o Langmuir. 

Ternary surfactant-solute (oil)-solvent (water) systems based on a model along the lines described in the previous section have been studied extensively by Larson and coworkers [34-38]. These studies focus mostly on the phase behavior of micelles and microemulsions, with an emphasis on the microstructures formed by the surfactant aggregates and on the microstructural transitions at high surfactant concentrations (10-80% by volume). Larson has also reported results on micellization at relatively low surfactant concentrations in the absence of solutes [37], which is of particular interest here. In addition, variations of Larson s model have been presented by others [39-41]. 

The procedure of evaluating self-diffusion data in terms of microstructure is to calculate the reduced or normalized diffusion coefficient, D/Dq, for the two solvents. Do being the neat solvent value under the appropriate conditions. Here we also have to account for reductions in D resulting from factors other than microstructure, mainly solvation effects. As discussed above, solvation will lead to a reduction of solvent diffusion that is proportional to the surfactant concentration. Normally the correction has been empirical and based on diffusion studies for cases of established structure, notably micellar solutions. We need to distinguish between corrections due to polar head-water and alkyl chain-oil interactions. The latter have often been considered insignificant, but a closer analysis (either experimental or theoretical) is lacking. However, it is probably reasonable to assume, for example, that the resistance to translation is not very different in the lipophilic part of the surfactant film and in an alkane solution. (This is supported by observations of molecular mobilities of surfactant allQ l chains on the same order of magnitude as for a neat hydrocarbon.)... 

The data obtained in cohesion characteristics studies of pol5mr-ethane systems based on PPG and ODA mixtures are well correlated with the results of specific interactions studies. As in the case of polyurethane systems based on compatible oligomer pairs, growth in the number of self-associations, formation of more ordered microstructure, and increase of polyiu-ethane cohesion strength are observed for the same surfactant contents. 

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