Internal phase behavior

In some of these models (see Sec. Ill) the surfactants are still treated as flexible chains [24]. This allows one to study the role of the chain length and chain conformations. For example, the chain degrees of freedom are responsible for the internal phase transitions in monolayers and bilayers, in particular the hquid/gel transition. The chain length and chain architecture determine the efficiency of an amphiphile and thus influence the phase behavior. Moreover, they affect the shapes and size distributions of micelles. Chain models are usually fairly universal, in the sense that they can be used to study many different phenomena. 

Viscosity is an important physical property of emulsions in terms of emulsion formation and stability (1, 4). Lissant (1 ) has described several stages of geometrical droplet rearrangement and viscosity changes as emulsions form. As the amount of internal phase introduced into an emulsion system increases, the more closely crowded the droplets become. This crowding of droplets reduces their motion and tendency to settle while imparting a "creamed" appearance to the system. The apparent viscosity continues to increase, and non-Newtonian behavior becomes more marked. Emulsions of high internal-phase ratio are actually in a "super-creamed" state. 

M. Kahlweit and R. Strey. Phase-behavior of ternary-systems of the type H20-oil-nonionic amphiphile (microemulsions). Angewandte Chemie. International edition in English, 24(8) 654—668,1985. 

Time - resolved spectra of a solid hydrocarbon layer on the surface of an internal reflection element, interacting with an aqueous solution of a nonionic surfactant, can be used to monitor the detergency process. Changes in the intensity and frequency of the CH2 stretching bands, and the appearance of defect bands due to gauche conformers indicate penetration of surfactant into the hydrocaibon layer. Perturbation of the hydrocarbon crystal structure, followed by displacement of solid hydrocaibon from the IRE surface, are important aspects of solid soil removal. Surfactant bath temperature influences detergency through its effects on both the phase behavior of the surfactant solution and its penetration rate into the hydrocaibon layer. 

Alany et al. [11,35] reported on the phase behavior of two pharmaceutical ME systems showing interesting viscosity changes. The viscosity of both systems increased with increasing volume fraction of the dispersed phase to 0.15 and flow was Newtonian. However, formation of LC in one of the two systems, namely the cosurfac-tant-free system, resulted in a dramatic increase in viscosity that was dependent on the volume fraction of the internal phase and a change to pseudoplastic flow. In contrast, the viscosity of the bicontinuous ME was independent of water volume fraction. The authors used two different mathematical models to explain the viscosity results and related those to the different colloidal microstructures described. 

Subbarao, D. Chester and lean-phase behavior, Powder Technology 46, 101-107 (1986). Wallis, G. B. One-Dimensional Two-Phase Flow, p. 182. McGraw-Hill, New York, 1969. Weinstein, H., Graff, R. A., Meller, M., and Shao, M. The influence of the imposed pressure drop across a fast fluidized bed, Proc. 4th Intern. Corf. Fluidization, Kashikojima, Japan, pp. 299-306 (1983). 

Small angle X-ray scattering (SAXS) (13,21) is used increasingly in biological sciences to determine dynamic structures of various molecular systems. The X-ray sources with high intensity allow the observation of weak scattering features that are associated with the internal structures of molecules studied. In the context of lipids, SAXS is often used to measure lipid system structures and their phase behavior. 

Chan, K.S., Shah, D.O., 1979. The effect of surfactant partitioning on the phase behavior and phase inversion of the middle phase microemulsions. Paper SPE 7869 presented at the SPE International Symposium on Oilfield and Geothermal Chemistry, Houston, 22-24 January. 

Hsieh, W.C., Shah, D.O., 1977. The effect of chain length of oil and alcohol as well as surfactant to alcohol ratio on the solubilization, phase behavior and interfacial tension of oil/brine/sur-factant/alcohol systems. Paper SPE 6594 presented at the SPE International Oilfield and Geothermal Chemistry Symposium, San Diego, 27-29 June. 

Martin, E.D., Oxley, J.C., 1985. Effect of various chemicals on phase behavior of surfactant/brine/ oil mixtures. Paper SPE 13575 presented at the International Symposium on Oilfield and Geothermal Chemistry, Phoenix, 9-11 April. 

Colina, C.M., Hall, C.K., and Gubbins, K.E., Phase behavior of PVAC-PTAN block copolymer in supercritical carbon dioxide using SAFT, presented at the 9th International Conference on Properties and Phase Equilibria for Product and Process Design, Kurashiki, Japan, May 20-25, 2001, 2001. 

Lv FF, Zheng LQ, and Tung CH. (2005). Phase behavior of the microemulsions and the stability of the chloramphenicol in the microemulsion-based ocular drug delivery system. International Journal of Pharmaceuticals, 301, 237-246. 

The physical properties that influence rheolc ical behavior are internal phase content size, shape, and panicle size distribution viscosity and rheological behavior of the continuous phase and temperature. For the case of emulsions two additional parameters, droplet deformabilily and viscosity of the dispersed phase, should also be considered. 

Deformable or soft particles and droplets show the same behavior as rigid particles. However, the high shear rate viscosity of deformable panicles is lower than that observed for rigid particles of the same internal phase content. 

The viscosity and rheological behavior of the continuous phase also modifies the behavior of emulsions and suspensions. In fact. Eqs. [50 -[53] establish that the larger the viscosity of the continuous phase (T) ), the larger the viscosity of the su.spension diJ. Furthermore, if the continuous phase is strongly viscoela,stic. die swelling and rod climbing may appear (26). Shear thickening has also been observed in concentrated emulsions of very viscous internal phase (27). 

In the previous discussion, it was stated that internal pha.se content is the most important parameter in terms of viscosity and rheological behavior. Anything that slightly modifies the effective internal phase fraction produces a steep change of viscosity (remember the exponential dependency of t), with [Pg.590]

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