On How Surfactant Depletion during Foam Generation Influences Foam Properties

Abstract

Although it is known that foaming a surfactant solution results in a depletion of the surfactant in the bulk phase, this effect is often overlooked and has never been quantified. Therefore, the influence of surfactant depletion on foam properties using solutions of the two nonionic surfactants, n-dodecyl-β-D-maltoside (β-C(12)G(2)) and hexaethyleneglycol monododecyl ether (C(12)E(6)), were investigated. These investigations were conducted in two steps. First, different foam volumes were generated with the same surfactant solution at a concentration of c = 2 cmc. It was found that the higher the foam volume, the larger the surfactant depletion. Second, two different bulk concentrations (c = 2 and 1.33 cmc) were used for the generation of 50 and 110 mL of foam, respectively. For a foam volume of 50 mL, no differences were observed, whereas generating 110 mL led to different results. The surfactant loss in the bulk solution was measured via surface tension measurements and then compared to the results of purely geometric considerations that take into account the amount of interface created in the foam. Both results were in very good agreement, which means that surfactant depletion can be calculated in the way suggested here. Under conditions where depletion plays a role, our approach can also be used to estimate the bubble size of a foam of known volume by measuring the surfactant concentration in the bulk solution after foaming.

Surfactant selection for a liquid foam‐bed photobioreactor

“…The surfactant molecules, due to their amphiphilic nature, partition to the foam phase when gas is introduced to the solution, thus the amount of surfactant molecules is reduced in the bulk liquid remaining underneath the foam. The depletion effect is especially relevant when large foam volumes are formed, or the initial surfactant concentration is low . Furthermore, the rate of diffusion of the surfactant molecules to the interface also influences the foam stability and thereby might result in further foam stability increase above CMC.…”

Effect of surfactant concentration on foam: From coreflood experiments to implicit-texture foam-model parameters

“…It is suggested that the slow increase in foam apparent viscosity above the CMC can be partly due to surfactant depletion in the base solution in each case. In order to estimate this depletion, we consider how much of the surfactant is adsorbed to the gas-water interface as compared to the quantity in the solution [41]. This can be approximated by considering the ratio of the surface area of the bubbles formed with the solution volume in each case (Appendix 3).…”

“…The implication of this higher depletion is that the concentration in the solution can be reduced to levels where there is an influence on foam stability. Boos et al [41] observed that even if the initial solution was above the CMC, the concentration in a solution can fall below the CMC as surfactant is adsorbed to the water-gas interfaces as foam is produced. The weaker foam behavior of solutions above the CMC in the coreflood could then be attributed to a greater surfactant depletion of the solution in the core.…”

“…The estimation of the relative surfactant depletion in the aqueous phase for the bulk foam and coreflood tests is based on the work of Boos et al [41]. They considered the change in surfactant concentration, c, and showed that:…”

Investigation on the Properties of Aqueous Foams Stabilized by Cetyltrimethylammonium Bromide in Terms of Free Drainage and Bubble Size

“…Many foam properties such as drainage, foam column stability, and depletion are closely interrelated to foam bubble dynamics (Boos et al, ; Magrabi et al, ; Saint‐Jalmes, ). Figure presents bubble images during foam destabilization at various CTAB concentrations and different times.…”