The role of competitive counterion adsorption on the electrolyte induced surface ordering in methyl ester sulfonate surfactants at the air-water interface

Hui Xu; Peixun Li; Kun Ma; Rebecca J L Welbourn; Jeffrey Penfold; Robert K Thomas; David W Roberts; Jordan T Petkov

doi:10.1016/j.jcis.2018.08.061

The role of competitive counterion adsorption on the electrolyte induced surface ordering in methyl ester sulfonate surfactants at the air-water interface

J Colloid Interface Sci. 2019 Jan 1:533:154-160. doi: 10.1016/j.jcis.2018.08.061. Epub 2018 Aug 22.

Authors

Hui Xu¹, Peixun Li², Kun Ma², Rebecca J L Welbourn², Jeffrey Penfold³, Robert K Thomas⁴, David W Roberts⁵, Jordan T Petkov⁶

Affiliations

¹ KLK Oleo, SDN BHD, Menara KLK, Muliara Damansara, 47810 Petaling Jaya, Selanger, Malaysia.
² ISIS Facility, Rutherford Appleton Laboratory, STFC, Chilton, Didcot, OXON OX11 0QX, UK.
³ ISIS Facility, Rutherford Appleton Laboratory, STFC, Chilton, Didcot, OXON OX11 0QX, UK; Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, UK. Electronic address: jeff.penfold@stfc.ac.uk.
⁴ Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, UK.
⁵ School of Pharmacy and Biomolecular Sciences, Liverpool John Moores University, Liverpool L3 3AF, UK.
⁶ Lonza UK, GB-Blackley, Manchester, Lancs M9 8ES, UK.

PMID: 30153592
DOI: 10.1016/j.jcis.2018.08.061

Abstract

The strong binding of Al³⁺ trivalent counterions to the anionic surfactants sodium polyethylene glycol monoalkyl ether sulfate and α-methyl ester sulfonate results in surface multilayer formation at the air-water interface. In contrast the divalent and monovalent counterions Ca²⁺ and Na⁺ result only in monolayer adsorption. Competitive counterion adsorption has been extensively studied in the context of surfactant precipitation and re-dissolution, but remains an important feature in understanding this surface ordering and how it can be manipulated. The α-methyl ester sulfonate surfactants are a promising class of anionic surfactants which have much potential for improved performance in many applications, greater tolerance to extreme solvent conditions such as water hardness, biocompatibility and sustainable production. Hence in this study we have used neutron reflectivity to extend previous studies on the surface ordering of the α-methyl ester sulfonate surfactant, sodium tetradecanoic 2-sulfo 1-methyl ester, in the presence of electrolyte to investigate the role of binary mixtures of electrolytes, AlCl₃/CaCl₂, and AlCl₃/MgCl_2. In the mixed electrolytes the evolution of the surface structure, from monolayer to multilayer with increasing AlCl₃ concentration, is observed. It is broadly similar to that reported for the addition of only AlCl₃. However with increasing CaCl₂ concentration the structural evolution is shifted progressively to higher AlCl₃ concentrations. Similar observations occur for the AlCl₃/MgCl₂ mixtures. However the presence of the MgCl₂ results in an additional phenomenon; the partial co-adsorption of a more compact lamellar structure which exists until the highest AlCl₃ concentrations. The results demonstrate the importance of the competitive adsorption of different counterions in driving and controlling the formation of surface multilayer structures with anionic surfactants. Furthermore it offers a facile route to the manipulation of these surface structures.

Keywords: Adsorption at air-water interface; Competitive counterion adsorption; Methyl ester sulfonate surfactants; Surface multilayers.