Peter R. Garrett

The effect of polytetrafluoroethylene particles on the foamability of aqueous surfactant solutions

Abstract It has been shown that the presence of PTFE particles may reduce the amount of foam which can be generated by vigorous shaking of surfactant solutions. PTFE is an inert and hydrophobic material. The experiments have therefore demonstrated that the formation of destabilizing surface tension gradients is not a necessary condition for antifoam behavior. Simple calculations have demonstrated that particles of hydrophobic material present in thin films may in principle spontaneously dewet to form a hole which would necessarily lead to film rupture. The critical condition for hole formation is shown to be dependent upon contact angle and particle shape. Particle size is an implicit consideration in that the smallest dimension of the particle must be approximately equal to the film thickness before hole formation can occur. This antifoam mechanism is shown to be consistent with all the experimental observations concerning the effect of PTFE particles on the foamability of surfactant solutions.

A reexamination of the measurement of dynamic surface tensions using the maximum bubble pressure method

Abstract The maximum bubble pressure method for measuring dynamic surface tension is reexamined, using high speed cinematography, particularly with respect to the process of bubble formation and the determination of the true surface age. A method for direct measurement of the time of bubble growth is suggested which readily permits routine determination of surface age. Sources of error are discussed and the evaluation of the method using submicellar surfactant solutions is described.

Mechanism for defoaming by oils and calcium soap in aqueous systems.

The effect of oils, hardness, and calcium soap on foam stability of aqueous solutions of commercial surfactants was investigated. For conditions where negligible calcium soap was formed, stability of foams made with 0.1 wt% solutions of a seven-EO alcohol ethoxylate containing dispersed drops of n-hexadecane, triolein, or mixtures of these oils with small amounts of oleic acid could be understood in terms of entry, spreading, and bridging coefficients, i.e., ESB analysis. However, foams made from solutions containing 0.01 wt% of three-EO alcohol ethoxysulfate sodium salt and the same dispersed oils were frequently more stable than expected based on ESB analysis, reflecting that repulsion due to overlap of electrical double layers in the asymmetric oil-water-air film made oil entry into the air-water interface more difficult than the theory predicts. When calcium soap was formed in situ by the reaction of fatty acids in the oil with calcium, solid soap particles were observed at the surfaces of the oil drops. The combination of oil and calcium soap produced a synergistic effect facilitating the well-known bridging instability of foam films or Plateau borders and producing a substantial defoaming effect. A possible mechanism of instability involving increases in disjoining pressure at locations where small soap particles approach the air-water interface is discussed. For both surfactants with the triolein-oleic acid mixtures, calculated entry and bridging coefficients for conditions when calcium soap formed were positive shortly after foam generation but negative at equilibrium. These results are consistent with the experimental observation that most defoaming action occurred shortly after foam generation rather than at later times.

Rates of solubilization of triolein into nonionic surfactant solutions

Abstract Videomicroscopy was used to measure the rates of solubilization of triolein drops injected into thin, rectangular glass cells containing solutions of nonionic surfactants and their mixtures with alcohols at various temperatures. In most cases it was observed that after an induction period where drop radius was nearly constant, the rate ( - d R d t ) of decrease in radius, which was proportional to the solubilization rate per unit area, was independent of time and of initial drop size. This behavior is not consistent with solubilization limited by diffusion in a stagnant surfactant solution with local equilibrium at the interface. Instead it suggests that processes occurring at the interface control the rate of solubilization. The rates of solubilization of triolein by the secondary alcohol ethoxylate Tergitol 15-S-7 at 30°C and 35°C, the latter temperature being just below the cloud point, were several times the corresponding rates for the pure linear alcohol ethoxylate C12E6 with enough n-dodecanol added to give it nearly the same cloud point. The difference in rates was much greater than could be explained by the relatively small differences in equilibrium solubilization of triolein and apparently stemmed from the smaller thickness and increased disorder in the hydrocarbon chain region of Tergitol 15-S-7 films which produced greater film flexibility. In contrast, the rates of solubilization of n-hexadecane by the two surfactant solutions were about the same and several times greater than the triolein solubilization rates. Addition of short-chain alcohols to the surfactant solutions increased the rates of solubilization of both triolein and n-hexadecane for both linear and secondary alcohol ethoxylates.

Thermodynamics of spreading from binary systems

Abstract An exact thermodynamic analysis of the equilibrium spreading behavior of binary systems of insoluble monolayer-forming substances has been developed. The analysis includes consideration of most types of possible phase behavior for such systems. The derived equations relate the equilibrium spreading pressures of binary systems to the equilibrium spreading pressures of the pure components and the surface pressure vs area isotherms for the pure and mixed monolayers. The extreme sensitivity of measurements of the spreading pressures of crystalline material to the presence of impurities is also discussed and is illustrated by an experimental example.

Dissolution of nonionic surfactant mixtures

Abstract Videomicroscopy was used to observe dissolution in water of drops of neat commercial nonionic surfactants and mixtures of pure nonionic surfactants. The time required for complete dissolution of such drops was greater at temperatures slightly below their cloud points than at lower temperatures, in contrast to behavior found for pure nonionic surfactants. Moreover, emulsification within and/or at the surfaces of surfactant drops was often observed at temperatures just below their cloud points. During the latter part of the dissolution process for these conditions, the rate of dissolution slowed, and drops became elongated and formed conical projections, apparently of the lamellar liquid crystalline phase. In some experiments, the projections emitted jets of small droplets or particles. While aspects of these intriguing phenomena are not completely understood, some interpretation is given, which makes use of relevant ternary phase diagrams.

Modeling oil droplets in plateau borders.

It is widely known that oil droplets can decrease the stability of aqueous films and foams. While less widely recognized, it has also been observed that oil droplets can, under certain circumstances, increase the stability of foams, especially if they are caught in the Plateau borders. In this paper, how the oil droplet deforms and is, in turn, deformed by the Plateau border is modeled using Surface Evolver. The two dimensionless parameters that affect these shapes are the size of the oil droplet relative to the Plateau border and the ratio of the oil-water interfacial tension to the air-water interfacial tension. The calculated pressures in all the phases were used to obtain the pressure exerted on the oil-water-air pseudoemulsion film, which allows the factors that influence the stability of these droplets in the Plateau border to be investigated. The final section of the paper demonstrates that the presence of an oil droplet in a Plateau border can have a major influence on the drainage of the aqueous phase along the Plateau border. This retardation of the flow would result in the oil droplets in the Plateau borders increasing the stability of foams in which they are found.

Solubilisation of triolein by microemulsions containing C12E4/hexadecane/water : Equilibrium and dynamics

Increasing triolein content of oil-in-water microemulsions in the pure C(12)E(4)/water/n-hexadecane/triolein system while maintaining a fixed surfactant concentration and volume fraction of drops raises the temperature of the solubilisation boundary, where excess oil separates, but has only a slight effect on the (higher) cloud point temperature, where excess water appears. Thus, the temperature range of the single-phase microemulsion shrinks and ultimately disappears. When such microemulsions are in equilibrium with excess oil, the hexadecane/triolein ratio is greater in the microemulsion, probably because the larger triolein molecules are unable to penetrate the hydrocarbon chain region of the surfactant films of the microemulsion droplets. Indeed, monolayer studies and calculations based on microemulsion and excess oil compositions indicate that the films have minimal triolein and similar ratios of hexadecane to surfactant. Triolein drops brought into contact with hexadecane-in-water microemulsions first swell as they incorporate hexadecane, then shrink owing to solubilisation. Interfacial tension decreases during this process until it becomes almost constant near 0.01 mN m(-1), suggesting that the drops in the final stages of solubilisation have high hexadecane contents. A microemulsion containing 10 wt% C(12)E(4) and 15 wt% hexadecane was able to remove over 50% of triolein from polyester fabric at 25 degrees C, more than twice that removed by an oil-free solution with the same surfactant concentration in similar experiments.