We show that using mole fractions x s instead of activities a s in the Gibbs equation for analyzing surface tension curves for onset of amphiphilic association may be open for discussion. First, we determined the activity coefficients for the simple binary water-ethanol and water-n-propanol systems from vapor pressure measurements. The activity coefficients are neither unity nor are they constant in the range where the surface tensions σ of the binary systems decrease from the water value down to the value of the pure alcohols. Second, a break in the σ vs ln x s curves disappears when proper σ vs ln a s curves are constructed. The implication for determining critical micelle concentrations and headgroup areas of surfactants from surface tension curves is addressed.

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Chalcopyrite (CuFeS 2 ) is the principal mineral source of the ten million tons of copper produced annually worldwide. The main industrial extraction process involves crushing, grinding, and flotation, followed by pyrometallurgical treatment of the concentrate. However, falling mined copper ore grades and stricter environmental protection legislation have led to increased interest in more selective and efficient froth flotation and in hydrometallurgical processes to recover copper from its minerals. This has been the motivation behind many studies of copper sulfide and copper-iron sulfide oxidation and reduction mechanisms.Electrochemical techniques have been widely employed to investigate the redox behavior of chalcopyrite in aqueous solutions. Parker et al. 1,2 observed two distinct regions during the slow potential sweep of a chalcopyrite anode in acidic electrolytes. From potentials of 0.2 to 0.6 V vs. the saturated calomet electrode (SCE), the current density observed was less than 1 mA cm Ϫ2 , resulting in Fe 2ϩ , S, and Cu 2ϩ (Cu I in a chloride medium) as the main oxidation products. The current increased rapidly with increasing potential for >0.6 V vs. SCE. It was suggested that the passivating layer consists of a copper polysulfide which could be reactivated at 80ЊC. Biegler et al. 3,4 reported a prewave at applied potentials <0.8 V vs. SCE in acidic solutions due to a passivating layer, postulated to be CuS and S of about 3 nm thickness.Linge 5 found that iron was leached preferentially and that a metal-deficient protonated phase formed prior to sulfur film formation at higher potentials. Iron migration through this film was not found to limit the reaction rate. Warren et al. [6][7][8] interpreted initial anodic decomposition to form an intermediate product phase Cu 1Ϫx Fe 1Ϫy S 2Ϫz , mixed with sulfur, retarding further oxidation. Page 9 considered that the prewave process may involve the oxidation of chalcopyrite to covellite (CuS) through CuFe 0.2 S 0.8 and Cu 2 S as intermediates. Jones and Peters [10][11][12] proposed that covellite was the phase formed in the low-current region and that it was responsible for passivation during the anodic dissolution of chalcopyrite.Ammou-Chokroum et al. [13][14][15] proposed a mechanism involving the formation of CuS to explain the decrease in electrodissolution rate. McMillan et al. 16 interpreted anodic passivation as being caused by a solid electrolyte interphase, which slowed the rate of electron transfer. Holliday and Richmond 17 showed that as chal-copyrite was dissolved anodically in acidic solutions, the initial ratedetermining step was the production of adsorbed Cu 2ϩ ions and then Fe 2ϩ ions. The final ratio of aqueous Fe 2ϩ to Cu 2ϩ produced was determined as 5:1. Munoz et al. recently investigated the electrochemical behavior of chalcopyrite in the presence of silver and sulfolobus bacteria for bioleaching. 18 Less work has been carried out in alkaline than in acidic solutions. Gardner and Woods 19 suggested that the oxidation reaction of chalcopyri...

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