A number of researches have been devoted to the developments of novel separation principles and methodology, because material separation is one of the most important fundamentals in scientific and technological disciplines. In particular, flow separation methods, such as chromatography, 1,2 electrophoresis, [3][4][5][6][7] and field flow fractionation (FFF), [8][9][10] have been extensively used in various fundamental and practical fields, and have facilitated the advancements of modern science. These methods cover separation of almost all classes of materials. A number of dissolved molecules, from simple ions to macromolecules, can be separated by chromatography or electrophoresis, FFF and electrophoresis being capable of resolving even particles. The dimensions of solutes to be separated by these methods thus range from 10 -10 m to 10 -5 m. This situation may suggest that separation science and technology have been completely matured. However, studying new separation principles is still important, because they can allow the separation based on material properties that have not been utilized for conventional methods, provide higher performance and more efficient separation than existing means, and facilitate the understanding of materials through separation processes.According to chromatographic theories, the intrinsic separation performance of liquid chromatography is restricted in comparison with that of gas chromatography because of much lower solute diffusion in liquid phases. 11However, a liquid flow obviously provides versatile ways for material separation because liquids have higher density and dissolution power than gases, and thus has made possible application to various types of solutes with wide ranges in molecular weights and sizes. In the usual flow separation methods, appropriate chemical (chromatography) or physical (FFF) separation fields are created, which retain solutes to different extents and separate them. A variety of stationary phases for chromatography 2,[12][13][14] and physical forces for FFF [15][16][17][18][19] have been exploited, and have provided different capabilities and selectivity from those performed by conventional separation. In contrast, a separation method that requires neither chemical interactions nor special external fields should also be useful because of its instrumental simplicity. Hydrodynamic chromatography (HDC) is such a case, which allows separation of particles or macromolecules according to their hydrodynamic sizes just by passing them through a narrow channel. [20][21][22][23][24] Particles with larger diameter for example cannot approach the channel wall, and thus flow through the channel faster than the smaller particles, when the laminar flow profile is maintained therein. Although the applicability of this method was very restricted, recent developments of chip technology have brought about a remarkable progress. 23,24 If a capillary of radius a is used for HDC, the time for the elution of a particle of radius rp is given bywhere t0 is the time required ...

When a solute passes through an open capillary, in which a laminar flow is established, different peak profiles can be obtained according to its diffusion property under a working condition, i.e., the radius and length of the capillary and the flow rate of the carrier solution. If a solute diffuses over the entire cross section of the capillary before it is eluted, a Gaussian-shaped diffusion peak appears, which has an apex at the travel time of the average flow. Insufficient solute diffusion, which is realized, e.g., by increasing flow rates or capillary radius, produces a new peak having an apex at the travel time of the maximum flow. This implies that two solutes can be resolved simply by passing through a capillary. However, our previous study indicated that the diffusion coefficients of two solutes should be at least one order different for their resolution based on this principle, suggesting that its applicability is highly restricted. In the present paper, this concept has been extended to the resolution between dissolved solutes that have similar intrinsic diffusion properties. The incorporation of molecular aggregates in the carrier makes a solute less diffusive according to the extent of their interaction and allows the resolution of a dissolved molecule from other ones differing in the affinity to the molecular aggregates. Several examples of peak resolution for phenols, aromatic hydrocarbons, and inorganic anions are shown and discussed on the basis of the modification of the diffusion natures due to their interactions with micelles or vesicles.

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