What is the fine-grained topic of the following text: HDPy+ and HDTMA+ surfactants were retained by Zn-exchanged smectite, suggesting important surface properties (CEC = 80 meq/100 g clay, SBET = 116.5 m2/g) and acid-base properties (point of zero proton charge PZC, density charge σH). Only return the topic.
Retention properties of HDPy+ and HDTMA+ surfactants by Zn-exchanged smectite