Thermodynamic factors of foam stability

In one of the first attempts to explain foam stability in terms of thermodynamics it was assumed that the foam lifetime depends on the decrease in surface energy and increases when a definite value of A a is reached [e.g. 34], 

Another point of view concerning foam stability appeared in relation to the development of the general theory of stability of colloid systems (DLVO-theory). It has already been noted that this theory was verified for the first time with foam films [35]. This gave rise to the concept of foam stabilisation on the account of the electrostatic component of disjoining pressure [e.g. 24, 32, 36], 

it might be assumed that stabilisation of foam films will depend also on the action of other positive components of disjoining pressure. For example, equilibrium films are obtained from concentrated butyric acid solutions and, therefore, in this concentration range the foam lifetime also increases. On the basis of these concepts it should be expected that a foam consisting of films with equilibrium thicknesses at a constant capillary pressure pa = n, should be infinitely stable. In fact, a real foam decays both in bulk and as a disperse system, due to gas diffusion transfer and certain disturbances (shift of films and borders on structural rearrangement as a result of the collective effects , etc.) 

It should also be noted that in real foams as a result of reduced pressure in borders pa 1 kPa) films become relatively thin, while the electrostatic component (IX,/ - p ) strongly decreases. Furthermore, it has been shown [37] that with the increase in surfactant concentration while keeping the ionic strength constant, the transition from unstable to stable 

It follows from these data that the electrostatic component of disjoining pressure cannot alone provide the formation of a stable (long-living) foam. It is necessary to account for other positive components of n and the different conditions under which the films exist in the foam. 

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