SURFACE THERMODYNAMIC PROPERTIES OF CADMIUM-SULFIDE

Measurements of the contact angle for water, formamide and diiodomethane on pellets of cadmium sulfide were made. Also, the thin-layer wicking technique was used with n-decane, water and formamide as the penetrating liquids. The measurements were used to determine apolar, Lifshitz-van der Waals (gamma(LW)) and polar, acid-base (Lewis) (gamma(AB)) surface free energy components of monosized, spherical CdS colloidal particles. The results were compared with those obtained on a pure, commercial sample of cadmium sulfide. They both are monopolar materials, although synthetic CdS has a slightly higher electron-donor character (Lewis base) (gamma(s)- = 61.4 mJ/m2) than commercial cadmium sulfide (gamma(s)- = 56.0 mJ/m2). The gamma(LW) component has a similar value for both samples, in the range of 44-47 mJ/m2. DELTAG121(TOT), the free energy of interaction between particles in aqueous medium is for both synthetic and commercial CdS around 34.0 mJ/m2. In spite of the (small) attractive contribution due to the Lifshitz-van der Waals interaction (DELTAG121(LW)), the large acid-base repulsion would ensure stability of CdS dispersions. Surface characteristics of the synthesized cadmium sulfide are similar to those of polydispersed commercial particles concerning their wetting properties. In consequence, the stability of dispersions of either kind of particles is expected to be the same. However, the use of perfectly spherical particles of this material is still valuable for testing model theories concerning interactions between particles in suspension.