The wetting properties of solid mineral samples (by contact angles) in original
surfactant-containing sea water (Gulf of Gdańsk,Ba ltic) 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),ca ptive bubble
and inclined plate methods. An experimental relation between the static contact angle θeq and stone density ρ was obtained in the form θeq = Bρ + C,wher e
B = 12.23 ± 0.92, C = −(19.17 ± 0.77),an d r2 = 0.92. The histogram of θeq distribution
for polished stone plates exhibited a multimodal feature indicating that
the most abundant solid materials (hydrophilic in nature) have contact angles
θeq = 7.2,10. 7,15. 7 and 19.2◦,whic h appear to be applicable to unspecified
field stones as well. The contact angle,a pH-dependent quantity,ap pears 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 γLV; it also enabled the advancing θA and receding θR contact
angles of this liquid to be calculated. Measurements of contact angle hysteresis
Δθ(= θA − θR) 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 ΔγS (−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,d emulgation,an ti-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.
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