The evolution of the microstructure and composition occurring in the aqueous solutions of di-alkyl chain cationic/nonionic surfactant mixtures has been studied in detail using small angle neutron scattering, SANS. For all the systems studied we observe an evolution from a predominantly lamellar phase, for solutions rich in di-alkyl chain cationic surfactant, to mixed cationic/nonionic micelles, for solutions rich in the nonionic surfactant. At intermediate solution compositions there is a region of coexistence of lamellar and micellar phases, where the relative amounts change with solution composition. A number of different di-alkyl chain cationic surfactants, DHDAB, 2HT, DHTAC, DHTA methyl sulfate, and DISDA methyl sulfate, and nonionic surfactants, C12E12 and C12E23, are investigated. For these systems the differences in phase behavior is discussed, and for the mixture DHDAB/C12E12 a direct comparison with theoretical predictions of phase behavior is made. It is shown that the phase separation that can occur in these mixed systems is induced by a depletion force arising from the micellar component, and that the size and volume fraction of the micelles are critical factors.

A new technology is tested for enzyme encapsulation. The capsules are small multilamellar vesicles of surfactant called spherulites which are produced by shearing a lamellar phase under well-controlled conditions. Encapsulation of alkaline phosphatase into spherulites is studied here as an example. Once encapsulated, the enzyme is shown to be unable to develop any enzymatic activity on its substrate, the p-nitrophenylphosphate. This is due to the absence of contact between the enzyme and the substrate. Interestingly, the whole enzymatic activity is recovered after destruction of the vesicles. Encapsulation efficiency ranges between 70% and 95% depending upon the enzyme over phospholipids ratio. Beyond the example of alkaline phosphatase, many applications of spherulites in the medical or in the biotechnology fields seem now at hand.

ABSTRACT:The system gelatin-poly(acrylic acid) (PAA) undergoes not only complex coacervation but also flocculation. The latter is incompatible with an encapsulation process. pH adjustment rate, ionic strength, temperature, and total macromolecular concentration have been studied to understand the origin of flocculation and to obtain a set of optimized parameters for coacervation using on-line turbidimetric titration. State diagrams were built, by varying gelatin/PAA mass ratio (R) and pH, for different PAA molar mass, which gave occurrence conditions of flocculation and coacervation. Flocculation can be avoided without significant yield decrease by pH adjustment. On the other hand, a modification of ratio (R) affects both coacervation yield and coacervate phase concentration. Spectrophotometric titration reveals a relative independence of the effective ratio within the coacervate and the initial mixing ratio before reaction. Conclusions are made concerning the use of this couple in an encapsulation process.

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