Surface Tension and Adsorption Studies by Drop Profile Analysis Tensiometry

Abstract

Surface tension and dilational viscoelasticity of solutions of various surfactants measured with bubble and drop profile analysis tensiometry are discussed. The study also includes experiments on the co-adsorption of surfactant molecules from a solution drop and alkane molecules from saturated alkane vapor phase. Using experimental data for 12 surfactants with different surface activities, it is shown that depletion due to adsorption of surfactant from the drop bulk can be significant. An algorithm is proposed quantitatively to take into consideration the depletion effect which is required for a correct description of the co-adsorption of alkanes on the solution drop surface and the correct analysis of experimental dynamic surface tension data to determine the adsorption mechanism. Bubble and drop profile analysis tensiometry is also the method of choice for measuring the dilational viscoelasticity of the adsorbed interfacial layer. The same elasticity moduli are obtained with the bubble and drop method only when the equilibrium surface pressures are sufficiently small (Π < 15 mN m^-1). When the surface pressure for a surfactant solution is larger than this value, the viscoelasticity moduli determined from drop profile experiments become significantly larger than those obtained from bubble profile measurements.

Keywords: Bubble and drop profile analysis tensiometry; Surfactant adsorption layers; Surfactant depletion due to adsorption.

Fig. 1

Surface tension isotherms for solutions of SDS in pure water, in water with addition of 0.5 mol dm⁻³ NaCl, and for aqueous solutions of C₁₀OH; details are given in the text

Fig. 2

Surface tension isotherms for solutions of CTAB in phosphate buffer with a concentration of 10 mol m⁻³

Fig. 3

Surface tension isotherms for aqueous Inositol and Tween 20 solutions

Fig. 4

Surface tension isotherms for C₁₁DMPO and C₁₃DMPO, data from [39, 43, 85, 86, 90]

Fig. 5

Surface tension isotherms for C₁₀EO₈ and C₁₂EO₅, data from [, –89]

Fig. 6

Surface tension isotherms for Triton X45 and X100, data taken from [58, 83]

Fig. 7

Surface tension isotherms for C₁₄EO₈, data from [40, 48, 65]

Fig. 8

The dependencies of the ratio of initial concentration c ₀ to the subsurface equilibrium concentration c calculated via fitting the experimental data on the adsorption activity coefficient b for three constant surface tension values γ as labelled; dotted lines are guides for the eye

Fig. 9

The dependencies of the derivatives $-\text{d}\mathit{\gamma }/\text{d}ln{c}_0$ calculated from the isotherms shown in Figs. 4, 5, and 6, using the drop-based best fit values and plotted against the initial surfactant concentration c ₀ for the solutions of C₁₂EO₅, C₁₃DMPO, Tr-45, and Tr-100

Fig. 10

Dependence of the equilibrium adsorption of C₁₃DMPO on the equilibrium bulk concentration; squares experimental results obtained by the bubble and drop methods; triangles values calculated using the Gibbs’ equation; curve the values calculated using the Frumkin model with the parameters listed in Table 1

Fig. 11

Dynamic surface tension of aqueous solutions of ethoxylated surfactants with an initial concentration of 5 mmol m⁻³; symbols experimental data [, , , –89]; curves calculations using Eqs. (8)–(10)

Fig. 12

Dynamic surface tension of aqueous solutions of ethoxylated surfactants with an initial concentration of 10 mmol m⁻³; symbols experimental data [, , , –89]; curves calculations using Eqs. (8)–(10)

Fig. 13

Dynamic surface tension isotherms for aqueous surfactant solutions, experimental data from [39, 43, 58, 76] (symbols) and theoretical predictions (curves)

Fig. 14

The dynamics of surface tension decrease for a C₁₀EO₈ solution after the injection of heptane into the cell at different temperatures; dotted lines are eye guides

Fig. 15

Viscoelasticity modulus as a function of the surface pressure for C₁₂EO₅ solutions at two oscillation frequencies [0.1 Hz (filled triangle, unfilled triangle) and 0.01 Hz (filled diamond, unfilled diamond)], as measured by the drop method (filled triangle, filled diamond) and bubble method (unfilled diamond, unfilled triangle); red curves calculated with the diffusion model for a frequency of 0.01 Hz for the drop method (dashed line) and bubble method (solid line); the data were taken from [46]