Question: what are the nonpolar and polar phases in tlc

title: Thin-layer chromatography
text: Separation of compounds is based on the competition of the solute and the mobile phase for binding sites on the stationary phase . For instance , if normal - phase silica gel is used as the stationary phase , it can be considered polar . Given two compounds that differ in polarity , the more polar compound has a stronger interaction with the silica and is , therefore , better able to displace the mobile phase from the available binding sites . As a consequence , the less polar compound moves higher up the plate ( resulting in a higher R value ) . If the mobile phase is changed to a more polar solvent or mixture of solvents , it becomes better at binding to the polar plate and therefore displacing solutes from it , so all compounds on the TLC plate will move higher up the plate . It is commonly said that `` strong '' solvents ( eluents ) push the analyzed compounds up the plate , whereas `` weak '' eluents barely move them . The order of strength / weakness depends on the coating ( stationary phase ) of the TLC plate . For silica gel - coated TLC plates , the eluent strength increases in the following order : perfluoroalkane ( weakest ) , hexane , pentane , carbon tetrachloride , benzene / toluene , dichloromethane , diethyl ether , ethyl acetate , acetonitrile , acetone , 2 - propanol / n - butanol , water , methanol , triethylamine , acetic acid , formic acid ( strongest ) . For C18 - coated plates the order is reverse . In other words , when the stationary phase is polar and the mobile phase is non-polar , the method is normal - phase as opposed to reverse - phase . This means that if a mixture of ethyl acetate and hexane as the mobile phase is used , adding more ethyl acetate results in higher R values for all compounds on the TLC plate . Changing the polarity of the mobile phase will normally not result in reversed order of running of the compounds on the TLC plate . An eluotropic series can be used as a guide in selecting a mobile phase . If a reversed order of running of the compounds is desired , an apolar stationary phase should be used , such as C18 - functionalized silica . 

Answer: answer: If normal - phase silica gel is used as the stationary phase , it can be considered polar . When the stationary phase is polar and the mobile phase is non-polar , the method is normal - phase as opposed to reverse - phase .