The Science of Defoaming

343

6

Deactivation of Mixed

Oil–Particle Antifoams

During Dispersal and

Foam Generation in

Aqueous Media

6.1 INTRODUCTION

It is well known that the effectiveness of hydrophobed silica–polydimethylsiloxane

antifoams in aqueous solutions deteriorates markedly during both use [1–7] and

dispersal as an emulsion [6]. Results of Marinova et al. [7] illustrating this effect

are reproduced in Figure 6.1. Here the time taken for foam collapse after repeated

shake–quiescent cycles of a solution of 0.01 M aerosol OT (sodium bis-octyl sulfo-

succinate) containing ~0.1 g dm

–3

of hydrophobed silica–polydimethylsiloxane anti-

foam is plotted against number of cycles. Antifoam was added neat to the solution

without predispersal. Marked deactivation of the antifoam is observed after 50–60

cycles.

There is some evidence that this deactivation is apparent with other hydrophobic

particle–hydrophobic oil mixed antifoams [8–10]. It also seems probable that the

prolongation of antifoam effectiveness accompanying increase in viscosity of the

oil [2, 3] concerns the effect of that increase on the process of antifoam deactivation

rather than on the intrinsic effectiveness of the antifoam. Moreover, partial deactiva-

tion of antifoams during dispersal to form emulsions appears to result from the same

cause as deactivation during foam generation [6]. This deactivation is commercially

important because antifoam products for certain applications are often necessarily

prepared as emulsions.

Deactivation is clearly a large effect and can involve complete loss of antifoam

effectiveness after prolonged exposure to foam generation. The evident commer-

cial signicance and apparent generality of this deactivation phenomenon therefore

clearly suggest that it merits detailed study. Indeed, its importance was recognized

by Ross [11] who wrote more than half a century ago that “not enough studies have

been made of the effect of the passage of relatively long periods of time on systems

containing antifoam agents.” It is, however, only in the past 20 years that signicant

attention has been given to this topic.

344 The Science of Defoaming: Theory, Experiment and Applications

In this chapter, we present a detailed review of deactivation of particle–oil anti-

foams, with particular emphasis on hydrophobed silica–polydimethylsiloxane anti-

foams about which most studies have been made. The available evidence suggests

that the phenomenon concerns disproportionation of antifoam drops during pro-

cesses of splitting and coalescence, which occur as the antifoam is either dispersed

or interacts with foam lms. We speculate here about the likely causes of this phe-

nomenon and describe theories that attempt to predict foam volume growth in the

presence of deactivating antifoam.

6.2 DEACTIVATION OF THE ANTIFOAM EFFECT OF

POLYDIMETHYLSILOXANE OILS WITHOUT PARTICLES

Hydrophobic oils such as polydimethylsiloxanes can exhibit signicant antifoam

effects even in the absence of hydrophobic particles. The antifoam efciency of such

oils is of course much enhanced by the presence of hydrophobic particles admixed

with the oil. However, the oils alone are known to lose antifoam effectiveness dur-

ing foam generation, albeit more rapidly than is the case with oil–particle mixtures

[3, 4]. It is argued by Basheva et al. [12] that this deactivation may be indirectly

attributed to the relatively high stability of the relevant polydimethylsiloxane–water–

air pseudoemulsion lms in the absence of hydrophobic particles. As we have seen

(see Chapter 4), stable pseudoemulsion lms mean that drops of oil do not have

time to emerge into the air–water surfaces of foam lms before they are swept out

by the draining ux present in those lms. If, however, such drops are sufciently

large (≫10 microns), they may subsequently accumulate in Plateau borders where

the capillary pressure may become high enough to overcome the disjoining forces

stabilizing the pseudoemulsion lms. Drops may then emerge into the air–water

surfaces of the Plateau borders, forming bridging congurations that are unstable [2]

and which lead to foam collapse as we discuss in Section 4.5.3. Continuous agitation

0

10

20

30

40

50

60

70

020406080

100

Time for foam collapse (s)

Number of cycles

FIGURE 6.1 Deactivation of hydrophobed silica–polydimethylsiloxane antifoam. Neat anti-

foam (~0.1 g dm

–3

added to 0.01 M AOT solution and subjected to repeated shake–quiescent

cycles). (Adapted with permission from Marinova, K.G., Tcholakova, S., Denkov, N., Roussev,

S., Deruelle, M. Langmuir 19, 3084. Copyright 2003 American Chemical Society.)