The effect of certain hydrophilic polymers on suspension stability
The adsorption of nonionic surface active agents of polyoxyethylene
glycol monoethers of n hexadecanols on polystyrene latex and nonionic
cellulose polymers of hydroxyethyl cellulose, hydroxypropyl cellulose
and hydroxypropyl methylcellulose on polystyrene latex and ibuprofen
drug particles have been studied. The adsorbed layer thicknesses were
determined by means of microelectrophoretic and viscometric methods.
The conformation of the adsorbed molecules at the solid-liquid interface
was deduced from the molecular areas and the adsorbed layer thicknesses.
Comparison of the adsorption results obtained from polystyrene latex
and ibuprofen particles was made to explain the confonnation difference
between these two adsorbates.
Sedimentation volumes and redispersibility values were the main
criteria used to evaluate suspension stability. At low concentrations
of surface active agents hard caked suspensions were found, probably
due to the attraction between the uncoated areas or, the mutual
adsorption of the adsorbed molecules on the bare surface of the
particles in the sediment. At high concentrations of hydroxypropyl
cellulose and hydroxypropyl methylcellulose, heavily caked sediments
were attributed to network structure formation by the adsorbed
An attempt was made to relate the characteristics of the
suspensions to the potential energy of interaction curves. Generally,
the agreement between theory and experiment was good, but for
hydroxyethyl cellulose-ibuprofen systems discrepancies were found.
Experimental studies showed that hydroxyethyl cellulose flocculated
polystyrene latex over a rather wide range of concentrations I
similarly, hydroxyethyl cellulose-ibuprofen suspensions were also
flocculated. Therefore, it is suggested that a term to account for
flocculation energy of the polymer should be added to the total
energy of interaction. A rheometric method was employed to study
the flocculation energy of the polymer.