An O/W-type emulsion of soybean oil (φ
v=0.05) was prepared in the presence of various concentrations of hydroxypropylcellulose (HPC) and sodium dodecylsulfate (SDS). The mean diameter (d) of secondary particles of the oil droplets decreased with concentrations of SDS and HPC after attaining a maximum at [SDS]=5 mmol/dm
3 and [HPC]=10
-3g/dl, while the relative viscosity (η
rel) of the emulsion increased with an SDS concentration after attaining a minimum around 5 mmol/dm
3 when the HPC concentration was kept constant. These facts were explained in terms of the formation of a surface complex by hydrophobic interaction between SDS and HPC adsorbed on the surface of the droplets. When the concentration of SDS is low, it bridges between the HPC segments intra-and intermolecularly on and between the particles, resulting in shrinking of the HPC surface layers as well as in bridging among the particles. Therefore, the total volume of the secondary particles effective on η
rel decreases while the d-value increases with the SDS concentration. When the concentration becomes high, the binding ratio of SDS to HPC also increases and the segments are repulsive each other owing to the negative charges given by the bound SDS. As a result, the adsorption layer swells and the secondary particles are redispersed. This fact causes an increase in η
rel and a decrease in the d-value with a concentration of added SDS.
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