Many active organic compounds and organic effect materials are poorly soluble in water, or even insoluble. Aqueous forms of application thus require special formulation techniques to utilize or optimize the physiological (pharmaceuticals, cosmetics, plant protection, nutrition) or technical (varnishes, printing inks, toners) action. The most interesting properties of nanodispersions of active organic compounds and effect materials include the impressive increase in solubility, the improvement in biological resorption, and the modification of optical, electrooptical, and other physical properties which are achievable only with particle sizes in the middle or lower nanometer range (50–500 nm). Hence in addition to economic and ecological constraints there are also technical demands which appear to urgently require the development of new processes for the production of organic nanoparticles as alternatives to the established mechanical milling processes. In this context attention is drawn to the recent increase in research activities which have as their objective the continuous, automatic preparation of nanodispersed systems by precipitation from molecular solution. In this review the current state of knowledge of the fundamentals of particle formation from homogeneous solution and the effect of solvent and polymer additives on the morphology and supramolecular structure of the nanoparticle will be discussed. The practical implementation of this new formulation technology will be explored in detail for the carotenoids, a class of compounds of both physiological and technical interest.

A combination of analytical methods and molecular modeling calculations has provided a detailed picture of the supramolecular and microscopic structure of precipitated lipophilic carotenoids. The nanoparticles have a core/shell structure (see schematic representation) in which the particle core (120 nm) consists of a variety of molecular aggregates of different sizes, and the shell (40 nm) consists of an adsorbed gelatin layer.

In this study, tapping mode scanning force microscopy was applied to characterize the distribution and adsorption structure of poly(ethylenimine) (PEI) macromolecules adsorbed onto negatively charged polystyrene latexes as colloidal model systems and onto molecularly flat mica surfaces as reference systems. On both surfaces, PEI macromolecules can be reproducibly mapped by tapping mode AFM, yielding molecular resolution without sample degradation despite the only relatively weak noncovalent coupling of the polyelectrolytes to the substrate surface. We are able to quantify their lateral dimensions as a function of the corresponding molecular weight. The lateral dimension of the adsorbed PEI macromolecules (60 down to 20 nm) on both types of substrates are in fair agreement with the diameters as measured by dynamic light scattering for the respective molecules in solution. Their adsorption structure is patch-like flat in the dried state under ambient air. However, mica and polystyrene surfaces result in a large difference in the height of the adsorbed macromolecules, which we interpret as being due to the grossly different surface charge densities of the substrates. Quasi-elastic light scattering (QELS) on PEI-covered polystyrene latexes in solution yields essentially the same heights of the adsorbed macromolecules as found by AFM in the dried state in ambient air. This indicates that there is no appreciable collapse upon drying at ambient conditions and further backs the notion of a dense patchlike adsorption structure in solution. These findings are discussed with respect to implications for the flocculation mechanism relevant for PEI.

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