Adsorption of polystyrene to mica/cyclohexane interfaces at the θ-temperature

Abstract

The adsorption of polystyrene (M = 7.75 × 10⁵; M_w/M_n = 1.01) adsorbed from cyclohexane onto mica surfaces at a temperature of 34.5°C (the θ-temperature) and a solution concentration of 0.02 and 0.5 mg ml⁻¹ has been investigated by the combined use of two independent approaches based on direct measurements. First, the adsorption kinetics were studied. A commercial Mettler ultramicrobalance was used to determine the adsorbance (adsorbed mass per unit area) as a function of time to a precision of 0.1 mg m⁻². Secondly, the change in surface covering of mica surfaces with incubation time was investigated. The state of the surface covering can be seen by observation of the shape and size of polystyrene films which are floated off on a water surface. The polystyrene adsorption proceeds through two stages: initial rapid adsorption and subsequent slow adsorption. The rapid adsorption occurs in the initial stage of adsorption: the adsorbance determined at a dilute concentration of 0.02 mg ml⁻¹ rises to 2.7 mg m⁻² within 10 min and then becomes steady. It has been confirmed that, when the adsorbance reaches the steady level, the surface of the mica is completely covered by adsorbed polystyrene molecules. The steady level after the initial rapid adsorption is considered to be an equilibrium of polystyrene adsorption. Subsequent to the rapid adsorption, a slow adsorption proceeds. Although the adsorption is apparently steady after the initial rise in adsorbance, the adsorbance has been found to continue to increase at a much slower rate than the initial rise. After a long period of incubation, the adsorbance attains to about 8.5 mg m⁻² and levels off thereafter. This ultimate value of adsorbance has been determined by the adsorption from a comparatively concentrated solution of 0.5 mg ml⁻¹. This slow increase in adsorbance is due to the adsorption of polystyrene molecules onto the polystyrene layer already formed. Two points in relation to the adsorption process are discussed: (1) a reconsideration of the experimental method of determining adsorption equilibrium; (2) the occurrence of multi-layer adsorption at the θ-temperature.