110

Figure 6 Effect of ionic strength /(Na2HPO4) on RF for three PS latexes of diameter 100 nm (triangles), 200 nm (squares) and 300 nm (circles) in the presence of 2.67 mmol L~1 surfactant. (Adapted with permission from results of Silebi CA and Mac Hugh J (1978) In: Becher P and Yudenfreund MN (eds) Emulsions, LatticesandDispersions, pp. 155-173. New York: Dekker.

Figure 6 Effect of ionic strength /(Na2HPO4) on RF for three PS latexes of diameter 100 nm (triangles), 200 nm (squares) and 300 nm (circles) in the presence of 2.67 mmol L~1 surfactant. (Adapted with permission from results of Silebi CA and Mac Hugh J (1978) In: Becher P and Yudenfreund MN (eds) Emulsions, LatticesandDispersions, pp. 155-173. New York: Dekker.

ionic ones, whereas stabilization by anionic surfactants is claimed to be considerably more effective than with nonionic ones. It is important to bear in mind that molecules of surfactant are in equilibrium with micelles above the critical micellar concentration. Depending on the amount and nature of surfactant in the sample, a new equilibrium between sample and eluent may explain the differences in observed results. It has been reported that SDS, tested from 0 to 4.8 gL"1 improves baseline, peak shape and reproducibility and increases RF in capillary columns.

Ionic strength Electrolytes may have four effects:

1. on the limiting RF value

2. on percentage recovery

3. on the size domain of interest

4. on sample chemical composition

Electrolytes greatly modify the critical micellar concentration of the surfactant and affect the repulsive electrostatic double layer around the particles. Basically, a low ionic strength I is necessary for screening charges. Recall that:

where ci = total concentration of species i, of valency Zi.

1. Increasing the ionic strength, I, leads to a decrease in Rf (Figure 6). In the initial work, even the ionic marker was affected: a 3% change in Vm was observed when increasing NaCl concentrations by a factor of 1000. Initial results obtained with packed columns have been confirmed by several authors, who observed a change in RF with ionic strength I, due to competition mainly between van der Waals (attractive) and double-layer (repulsive) forces. At high ionic strength, elution is opposite to the normal mode. The reason could be that very strong van der Waals interactions dominate over the hydrodynamic effect. Some authors have observed a normal decrease in RF when I increased, but no change in the slope of log Dp versus AV.

2. Percentage recovery is decreased for various colloids when I is increased on a 20 | m packing but is more constant for PS on large pore, large packings (63-125 |im Fractosil). A short column favours the recovery of large particles.

3. Change of I may make possible the separation or the elution of large particles (limit 850 nm instead of 500 nm), even if N or RF is decreased.

4. In addition, the chemical composition of the sample can affect the elution, so that a mixture of polystyrene and poly(methylmethacrylate) (PMMA) latexes of identical size (240 nm) can be separated in the presence of 0.4 mol L"1 NaCl, on a 1 m column packed with 20 |im PS beads. On the other hand, there is no elution difference between latexes of PS and PMMA of diameters from 58 to 207 nm, when NaCl is varied from 4.44 to 15.6 mmol L"1.

In capillary columns, no effect was observed for either NaNO3 (0-28.6 gL"1) or NaCl (0-48.7g L"1). Peak shape and elution volume of samples and marker were not modified - or only slightly - by variation in the salt concentration, but NaNO3 has some absorption and affects the baseline.

Viscosity additives Some authors have observed a change in RF mainly for larger particles, in the presence of ethylene glycol or sucrose. A small amount of ethylene glycol prevents aggregation. Theoretically, transverse motion must be decreased with decreased tailing and increase in RF, when the viscosity is increased. In this respect, polymers have been used with some success. Neither increase in RF nor shape improvement in peaks was observed in the range 0-6% ethylene glycol with capillary columns. The pressure is markedly increased, according to Darcy's law and a permeability coefficient, Ko, may be derived. For L = 60 m, a Ko value of 2.4 x 108m2 is far lower than that of filtration membranes (10~12 m2), which indicates the low separating power of a capillary column.

As a result of these observations, only the surfactant SDS at 3 g L"1 was used as an additive in capillary columns.

Flow rate, Q and nature of the eluent Flow rate may have two effects: one on N and one on the sample.

A high flow rate may induce polymer deformation or degradation. It was found that under 2.5 cm s"1 no deformation of soft polymer is expected from a calculation using a Deborah number.

Increase of resolution by a factor of 2 has been observed with a 10 times decrease of flow rate, but the effect is small if the packing is of very small particle size. It is known that, in general, low flow rates favour minimum equivalent plate height, He, according to the van Deemter equation:

where v is linear velocity. Here, this general expression holds with zero as c value, since it represents the mass transfer term. In capillary HDC diffusion has been found to be b = 7 cm4 min"1 and a = 10 cm for longitudinal and eddy terms, respectively. These values, which are far higher than in liquid chromatog-raphy, mean a high sinuosity coefficient and large equivalent bead sizes.

In capillary columns, N, RS and RF vary differently with eluent (water) flow rate Q, depending on sample size. Number of plates was found to increase with flow rate, but RF decreases for a 4 |im sample and the resolution is slightly affected. Some deformation of peaks was observed at low flow rates. For instance, the resolution between 0.84 |im particles and marker or 4 | m particles was unchanged, but was decreased between 4 | m and marker. The interpretation may be found in the tubular pinch effect, by taking account of the respective Reynolds numbers of the particles, Rep (see above).

Their values correspond to very different flow rates so that a change in mechanism occurs in the investigated region.

These different results for RS, N and RF show that operating conditions are not rigid and they must be chosen as a function of the particular analysis. Depending on the objective, it may be required to obtain either rapid results in 1.5 min with medium resolution or higher resolution in 36min. It is also interesting to note that the highest number of plates is obtained not only for the marker, but for the largest sample.

In methanol, the number of plates increased from a constant value for 0.84 and 4 | m particles and marker to a maximum for the 10 | m sample, when Q increased. The resolution, like RF, decreased when flow rate, Q, was increased.

Tetrahydrofuran has low viscosity and is a good solvent for polymers. These characteristics may allow higher flow rates and the study of polymers in solution. In fact, although excellent baselines, very high number of plates and good chromatograms of cross-linked PS (10 |im) were obtained, the upper limit of RF was decreased with this solvent. In tetrahydro-furan polybutadiene, polyisoprene and PS of nearly the same molecular weight (300 000) were eluted according to their respective dimensions, rg = 28.9, 26.6 and 25 nm on a packed column.

Water, being a more versatile eluent, and offering a good compromise between RF and N values, is the main solvent at a flow rate of about 1 mL min"1 with a capillary column. This flow rate also corresponds to a compromise between higher RF (Figure 1A) and lower resolution at the higher velocities. Table 1 summarizes the experimental conditions and results.

As an ancillary practical application, Poiseuille's law allows the determination of one of the parameters: flow rate Q, pressure P, length L, viscosity ¿u and capillary radius R:

Table 1 Experimental conditions with capillary columns

Length (m)

SG

aG

12G

12G

6G

12G

Radius (|im)

125

125

125

125

25G

125

Eluent

W

W

W

Meth

W

THF

Maximum RF

1.4

1.45

1.45

1.45

1.6

1.26

N(m)

5GG

9GG

19GG

7GGG

4GGa

5SGG

Maximum N (m)

16.7

15

15.B

5B.S

SB.S

25G

W, water: flow rate 1 mL min-1, except a5 mL min-1; Meth, Methanol; THF, tetrahydrofuran. Reproduced with permission from Revillon A and Boucher P (1989) Journal of Applied Polymer Science: Applied Polymer Symposium 43: 115-128.

W, water: flow rate 1 mL min-1, except a5 mL min-1; Meth, Methanol; THF, tetrahydrofuran. Reproduced with permission from Revillon A and Boucher P (1989) Journal of Applied Polymer Science: Applied Polymer Symposium 43: 115-128.

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