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

The adsorption of polystyrene (M=7.75x10(5); M-w/M-n=1.01) adsorbed from cy clohexane onto mica surfaces at a temperature of 34.5 degreesC (the theta - temperature) and a solution concentration of 0.02 and 0.5 mg ml(-1) has bee n investigated by the combined use of two independent approaches based on d irect measurements. First, the adsorption kinetics were studied. A commerci al 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 t ime was investigated. The state of the surface covering can be seen by obse rvation of the shape and size of polystyrene films which are floated off on a water surface. The polystyrene adsorption proceeds through two stages: i nitial rapid adsorption and subsequent slow adsorption. The rapid adsorptio n 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 r eaches the steady level, the surface of the mica is completely covered by a dsorbed polystyrene molecules. The steady level after the initial rapid ads orption is considered to be an equilibrium of polystyrene adsorption. Subse quent to the rapid adsorption, a slow adsorption proceeds. Although the ads orption is apparently steady after the initial rise in adsorbance, the adso rbance has been found to continue to increase at a much slower rate than th e initial rise. After a long period of incubation, the adsorbance attains t o about 8.5 mg m(-2) and levels off thereafter. This ultimate value of adso rbance has been determined by the adsorption from a comparatively concentra ted solution of 0.5 mg ml(-1). This slow increase in adsorbance is due to t he adsorption of polystyrene molecules onto the polystyrene layer already f ormed. Two points in relation to the adsorption process are discussed: (1) a reconsideration of the experimental method of determining adsorption equi librium; (2) the occurrence of multi-layer adsorption at the theta -tempera ture.