A Molecular Dynamics Study of Monolayers of Nonionic Poly(ethylene oxide) Based Surfactants Marco La Rosa, † Alfred Uhlherr,* ,‡ Carl H. Schiesser, † Keith Moody, § Robert Bohun, § and Calum J. Drummond †,‡,| School of Chemistry, The University of Melbourne, Victoria 3010, Australia, CSIRO Molecular Science, Bag 10, Clayton South, Victoria 3169, Australia, cap-XX Pty. Ltd., Units 9 and 10, 12 Mars Road, Lane Cove, New South Wales 2066, Australia, and Huntsman Corporation Australia Pty. Ltd., Newsom Street, Ascot Vale, Victoria 3032, Australia Received August 10, 2003. In Final Form: November 19, 2003 Molecular dynamics simulations of monolayers of nonionic, poly(ethylene oxide) based surfactants are reported. Specifically, alcohol ethoxylates and alkylphenol ethoxylates are compared in terms of the varying architecture of the molecules for the development of a structure-behavior relationship. Interfacial density profiles are used to assess the structure of the monolayers, the penetration of water and oil into the monolayers, and the solvation of the hydrophiles and hydrophobes. Chain conformational descriptors are used to examine the molecular structure of the surfactants.The simulations revealed that monolayers of alcohol ethoxylates are considerably more diffuse than their alkylphenol counterparts, with the packing being governed by the size of the hydrophile. With the exception of the branched alcohol ethoxylate, the intermixing of the bulk phases within monolayers of alcohol ethoxylates increases with increasing hydrophile length. By comparison, the packing of alkylphenol ethoxylates within the monolayer is governed by the aromatic nucleus in the molecule. No specific interaction is observed between the aromatic rings of neighboring molecules. Monolayers of alkylphenol ethoxylates are more compact than their alcohol counterparts, resulting in more effective separation of the bulk water and oil phases. Introduction Poly(ethylene oxide) based nonionic surfactants are typically found in a wide and diverse variety of applications including detergency, emulsification, lubrication, etc. 1 Of the various types of polyethoxylate surfactants available, alcohol and alkylphenol ethoxylates are the most common. Although both the alcohol and alkylphenols are found in similar applications, it is known that for the same mole ratio of ethylene oxide in the hydrophile, surfactants with alkylphenol hydrophobes are generally more effective than their counterparts with straight alkyl chain hydrophobes. 1 It is also known that alkylphenol surfactants have a long environmental persistence time while the process of biodegradation proceeds via potentially harmful aromatic intermediates. 2-7 By comparison, alcohol ethoxylates degrade considerably quicker and without aromatic byproducts. 2 Accordingly, one might ask whether alcohol ethoxylate surfactants with suitably refined chemical structures could form effective replacements for their alkylphenol coun- terparts. To answer this question, one needs to consider how the macroscopic phase behavior of a surfactant relates to the molecular architecture of the molecule. Given that the major structural difference between alcohols and alkylphenols is the presence of the aromatic ring in the alkylphenol ethoxylates, we consider what effect the aromatic ring has on the structural behavior of those surfactants. Specifically, we are interested in determining whether it is intermolecular interactions between the rings (π-stacking) that increases the efficacy of those surfac- tants or simply the size or hydrophobicity of the rings or other related factors. It follows that examination of the behavior of the molecules in terms of the individual components (hydrophiles and hydrophobes of varying architecture) will then allow the derivation of a structure- activity relationship for the two surfactant classes of interest. Recently, a number of experimental techniques have been used to partially address this issue. An X-ray diffraction 8 study examining the hydration and structure of a series of alcohol ethoxylate surfactant systems has been reported. 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