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

The wetting properties of solid mineral samples (by contact angles) in original surfactaut-containing sea water (Gulf of Gdansk, 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 theta(eq) and stone density rho was obtained in the form theta(eq) = B rho + C, where B = 12.23 +/- 0.92, C = - (19.17 +/- 0.77); and r(2) = 0.92, The histogram of theta(eq) distribution for polished stone plates exhibited a multimodal feature indicating that the most abundant solid materials (hydrophilic in nature) have contact angles theta(eq) = 7.2, 10.7, 15.7 and 19.2 degrees, 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 gamma LV; it also enabled the advancing theta(A) and receding theta(R) contact angles of this liquid to be calculated. Measurements of contact angle hysteresis Delta theta(= theta(A) - theta(R)) with surfactant-containing sea water and distilled water (reference) on the same stone surfaces allowed the film pressure Delta Pi (1.22 to 8.80 mJ m(-2)), solid surface free energy Delta(gamma S) (-17.03 to -23.61 mJ m(-2)) and work done by spreading Delta W-S (-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.