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Adsorption of natural surfactants present in sea waters at surfaces of minerals: contact angle measurements

Mazurek A., Pogorzelski S.J., Boniewicz-Szmyt K.

Oceanologia

2009

No. 51 (3)

377-403

The wetting properties of solid mineral samples (by contact angles) in original surfactant-containing sea water (Gulf of Gdańsk, Baltic) were characterised under laboratory conditions on a large set (31 samples) of well-classified stones of diverse hydrophobicity using the sessile drop (ADSA-P approach), captive bubble and inclined plate methods. An experimental relation between the static contact angle θ_eq and stone density ρ was obtained in the form θ_eq = B-ρ+ C, where B = 12.23 š 0.92, C = - (19.17 š 0.77), and r2 = 0.92. The histogram of ?eq distribution for polished stone plates exhibited a multimodal feature indicating that the most abundant solid materials (hydrophilic in nature) have contact angles θ_eq = 7.2, 10.7, 15.7 and 19.2^(o), which appear to be applicable to unspecified field stones as well. The contact angle, a pH-dependent quantity, appears to be a sensitive measure of stone grain size, e.g. granite. The captive bubble method gives reproducible results in studies of porous and highly hydrophilic surfaces such as stones and wood. The authors consider the adsorption of natural sea water surfactants on stone surfaces to be the process responsible for contact angle hysteresis. In the model, an equation was derived for determining the solid surface free energy from the liquid's surface tension γ_LV it also enabled the advancing θ_A and receding θ_R contact angles of this liquid to be calculated. Measurements of contact angle hysteresis Δθ(=θ_A - θ_R) with surfactant-containing sea water and distilled water (reference) on the same stone surfaces allowed the film pressure ΔΠ (1.22 to 8.80 mJ m-2), solid surface free energy ??S (-17.03 to -23.61 mJ m-2) and work done by spreading ΔWS (-1.23 to -11.52 mJ m-2) to be determined. The variability in these parameters is attributed to autophobing, an effect operative on a solid surface covered with an adsorptive layer of surfactants. The wetting behaviour of solid particles is of great importance in numerous technological processes including froth flotation, demulgation, anti-foaming procedures and the coal industries. It is believed that the approach presented here and the examples of its application to common sea water/solid mineral systems could be successfully adapted to optimise several surfactant-mediated adsorption processes (see below) of practical value in natural water ecology.