Transport of different substrates with calix[4]arene tetraphosphonic acid receptor 6 in a U-tube experiment. Source phase: MQ water (4mL), NBD-choline 11 (1 mm) and FITC-choline 9* (0.5 mm); organic phase: CH2Cl2 (10 mL), 6 (0.5 mm); receiving phase: MQ water; stirring speed = 400 rpm, room temperature, results are reported as the average of duplicate trials. For FITC-choline, the maximum deviation from the mean was ±0.3% (not shown), and for NBD-choline, it was ±0.8% (not shown). (* guest precipitation occurred at the interface when 1 mm solution was used.).