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A new equation, a modified Langmuir-like equation (M-LLE), for describing adsorption from solution by activated carbons is proposed for the first time in this work. The M-LLE assumes that there are two types of interactions: (a) specific interactions which are typical, enthalpy-driven interactions and (b) nonspecific interactions driven by the loss of water structuring, upon adsorption (hydrophobic bonding), around the nonpolar parts of the drug. The proposed model was evaluated by studying the adsorption of three drugs: procaine, fluoxetine, and phenobarbital by four different activated carbons under different experimental conditions. As the hydrophobicity of the drug increased, the capacity constant representing the interactions driven by hydrophobic bonding (KHB, M-LLE equation) increased. Experimental conditions that decrease hydrophobic bonding, such as increased temperature and higher cosolvent concentration, resulted in a decrease in KHB. Salts that tend to increase water structuring and hydrophobic bonding caused an increase in KHB. All of these studies support the M-LLE, because they support the notion of hydrophobic-bonding-driven interactions.
Kamp et al. (Tue,) studied this question.