Effect of oppositely charged hydrophobic additives (alkanoates) on the stability of C14TAB foam films

The impact of the hydrophobicity of oppositely charged additives on the stability of C14TAB foam films has been studied using a thin film pressure balance (TFPB). Disjoining pressure isotherms of 10−4 M C14TAB solutions containing negatively charged sodium alkanoates CnOONa with increasing chain length from n = 4 to n = 12 are measured. The results of disjoining pressure measurements are presented as maximum possible pressure Πmax applied to a stable film as a function of the alkanoate concentration. Furthermore, the surface properties of the mixed systems are described to gain a general understanding about the surface characteristics. Since C14TAB and alkanoates are oppositely charged, the question arises whether charge reversal at the film surfaces occurs and how it depends on the chain length of the alkanoate. The presented results indicate that C14TAB/alkanoate systems can be divided into three groups. Depending on the chain length, alkanoates act as organic salts (n = 4), co-surfactant (n = 6) or anionic surfactant (8 ≤ n ≤ 12). In case of alkanoates acting as anionic surfactant, the simulation of the isotherms by solving the non-linearized Poisson–Boltzmann equation under assumption of constant potential actually indicates that charge reversal occurs close to the nominal IEP. The film stabilities are markedly affected by the hydrophobicity and the amount of alkanoate.