Drugs and the pharmaceutical sciences

Concerning Mechanisms of Particle Removal by Filters 95
The hydrophilic portion of the surfactant molecule extends into the aqueous
solution where its repetitively spaced etheryl oxygens, (–O–), the seat of its polarity,
feature the electronegativity of their unshared electrons that form hydrogen bonds with
the water molecules.
The surfactant molecules mediate between the particle’s hydrophobic, non-polar
surface and the water, the hydrophilic medium. In effect, the arrangement is a waterwettable
surface adsorptively grafted onto the hydrophobic particle which, as a result, is
now amenable to the aqueous medium.
Surfactant Particle Size Enlargement
The findings of Bowman et al. (1967) wherein penicillinase occupied the adsorptive sites
of a filter that would otherwise have served to retain B. diminuta have been discussed.
Surfactants can likewise preempt the adsorptive sites of filters to deny latex particles their
access. In studies involving the retention of latex particles serving as surrogates for
organisms, it was found that retention efficiencies decreased in the presence of
surfactants. An alternate explanation is possible, namely, that surfactant deposited on the
latex particles increases the energy barrier to their coming together. The resulting steric
stabilization, also called entropic stabilization, enlarges the distance of their separation to
such an extent that the weak forces of attraction cannot overcome it.
Pall et al. (1980), in reporting that the presence of surfactant diminished the latex
bead retention, noted that different surfactants did so to different extents. Emory et al.
(1993) corroborated that not all surfactants have the same effect on a given membrane.
Confirmation of Pall et al.’s findings were made by Wrasidlo et al. (1983), in respect to
both pH, and surfactant (Tables 4 and 5). Tolliver and Schroeder (1983) compared the
retention of 0.198 mm latex beads suspended in water, with those suspended in an aqueous
solution of 0.05% Triton X-100. The comparisons were made using various commercially
available 0.2-mm-rated membranes. Table 6 shows differences in results between the two
vehicles. The dissimilarity is greatest for the nylon 66 membrane. The polyamide
polymers are known to exhibit adsorptive interactions with surfactants that interfere
with, for example, the uptake of proteins. The action of surfactant in differentiating
among the extents of latex particle retentions in otherwise similar situations is taken as a
confirmation of the adsorptive sequestration mechanism.
Significance of Mechanism
Size exclusion is so dominantly the mechanism of particle retention that it is erroneously
still regarded by some to be the exclusive mode of organism (particle) retentions. This is
TABLE 4 0.198-mm Latex Percent Retention for Various 0.2 mm-Rated Membranes as a
Function of pH
Filter type Bubble point pH 4 pH 6 pH 8 pH 9
Asymmetric polysulfone 51 100 100 100 100
Polycarbonate (track-etched) 63 100 100 100 100
Polyvinylidene difluoride 55 86.8 74.8 79.5 67.3
Cellulose esters 58 36.3 89.4 23.0 31.3
Nylon 66 45 99.9 82.1 23.7 28.4
Source: From Wrasidlo et al., 1984.