Capillaries dynamic surface modification

In capillaries, the EOF can be controlled by (1) operating at low pH, (2) chemical surface modification, (3) dynamic coating of the capillary walls for example with a polymer layer or (4) by using additives that change the viscosity, jj, and the zeta potential, C. 

The following so-called dynamic capillary method was developed by Van Hunsel Joos (1987b) and complements the area of application with respect to other methods. This method allows measurements from 50 ms up about 1 s, similar to the inclined plate and growing drop techniques described above, and can be used at liquid/liquid and liquid/gas interfaces without modification. The principle of the experiment is schematically given in Fig. 5.23. Two fluids are contained in a tube of diameter R. The interface (or surface in case of studies at the water/air interface) is located in such a way that its interfacial tension can be measured by the capillary rise of the lower liquid in a narrow capillary c, which connects the both fluids. The height of the capillary rise h is determined via a cathetometer Cat. 

Most approaches for controlling the electoosmotic velocity focus on altering the effective surface charge density of the capillary wall either by modification of the surface with a polymer layer (section 8.2.7.2), or by dynamic coating with additives... 

We have just the same situation in capillary gas-solid chromatography [26]. The replacement of a light carrier gas by a heavy one leads to dynamic modification of the sorbent surface, to the decrease of distribution coefficients in the solid-gas system, and, consequently, to the decrease of retention time. Table 3-3 contains data on capacity coefficient dependence on carrier gas nature using columns with an alumina layer (Chrompack, 50 m x... 

ILs have been used also in capillary electrophoresis (CE) for modification of running buffer and for dynamic coating or covalent attachment to the capillary walls. The main aim of this modification is reverse the electroosmotic flow (EOF) provoked by silanol groups on the inner surface of silica capillaries. 

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