The dispersive surface energy was determined by passing known
volumes of alkane probes through the column, with a helium carrier
gas, allowing sufficient time for the entire probe vapour to elute
through the column. The alkanes used were nonane, octane, heptane
and hexane, all HPLC grade (Sigma Aldrich). Both the Schultz and
Dorris–Gray Theories were used, with the Schultz Theory being
presented in the Results and discussion of this paper. The volumes of
probe added were equivalent to allow coverage of the mineral surfaces
ranging from 5% coverage, up to 38% coverage (0.05–0.38 fractional
coverage). This allows the heterogeneity of the surface energy to be
determined. Two polar probes were also passed through the column
in order to calculate the acid and base components of the surface
energy, and the work of adhesion to solvents. These probes were
dichloromethane (γ+ = 124.58 mJ m−2
; γ− = 0 mJ m−2
) and ethyl
acetate (γ+ = 0 mJ m−2
; γ− = 457.67 mJ m−2
), both HPLC grade
(Sigma Aldrich). These additions were also at volumes equivalent to a
surface coverage from 5% to 38%. The column temperature was kept
constant at 30 °C. At a fractional surface coverage the R2 value for the
dispersive component of the surface energy using both the Schultz
and the Dorris–Gray Methods was 0.997, and the difference in the dispersive
component of the surface energy is less than 0.40 mJ m−2
.
(Schultz = 46.49 mJ m−2
; Dorris–Gray = of 46.85 mJ m−2
.)