Excess adsorption of biopolymers and lactose sugar on soft surfaces. Adsorption of β-lactoglobulin on various types of soft surfaces

摘要

Excess adsorption (Γ2~1) of P-lactoglobulin (β-lg) from aqueous solution onto surfaces of served soft solid powder have been measured at pH 6.5 and ionic strength 0.05 and at 28°C. Γ2~1 in most cases increases with increase in bulk concentration C2 of β-lg until at a critical value of Cf at the state of monolayer saturation. The values of Γ2~m for the surfaces different soft surfaces stand in the order cellulose > tristearin > cholesterol > phosphatidyl choline > casein-fat mixture > stearic acid > lysolecithin. The affinities of P-lg for different soft surfaces in terms of standard free energy of adsorption -ΔG° in kg per kg of powdered solid can be calculated using integrated forms of the Gibbs adsorption equation. -ΔG° for monolayer adsorption for different soft powders are found to increase linearly with increasing values of Γ2~m and the constant value of ΔG°/Γ2~m is equal to ΔG_B° in kJ per mole P-lg adsorbed at the interfaces. In few systems, Γ2~m exceeds Γ2~m when C2 C2~m due to multilayer adsorption at the interface. Following thermodynamic approach, free energy of aggregation of P-lg at the interface forming multilayers have been calculated.Adsorption isotherms of p-lg on powdered stearic acid in the presence of different neutral salts (NaCl, KC1, CaCl2, Na2SO4 and KSCN) at pH 6.5 have been critically compared at pH 6.5. These isotherms have been also compared with the isotherms at β-lg at stearic acids at pH 4.0. The difference of results at pH 6.5 and 4.0 are due to the difference in surface charge of β-lg and stearic acids.