Multivalent counterion induced multilayer adsorption at the air-water interface in dilute Aerosol-OT solutions

Zi Wang; Peixun Li; Kun Ma; Yao Chen; John R P Webster; Mario Campana; Zifeng Yan; Jeff Penfold; Robert K Thomas

doi:10.1016/j.jcis.2021.03.183

Multivalent counterion induced multilayer adsorption at the air-water interface in dilute Aerosol-OT solutions

J Colloid Interface Sci. 2021 Sep:597:223-232. doi: 10.1016/j.jcis.2021.03.183. Epub 2021 Apr 8.

Authors

Zi Wang¹, Peixun Li², Kun Ma², Yao Chen², John R P Webster², Mario Campana², Zifeng Yan³, Jeff Penfold⁴, Robert K Thomas⁵

Affiliations

¹ School of Science, State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, China; ISIS Facility, Rutherford Appleton Laboratory, STFC, Chilton, Didcot, Oxon OX11 0QX, UK.
² ISIS Facility, Rutherford Appleton Laboratory, STFC, Chilton, Didcot, Oxon OX11 0QX, UK.
³ School of Science, State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, China. Electronic address: zjgancat@upc.edu.cn.
⁴ 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.
⁵ Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, UK.

PMID: 33872879
DOI: 10.1016/j.jcis.2021.03.183

Abstract

The formation of surface multilayer structures, induced by the addition of multivalent counterions in dilute surfactant solutions, has been widely observed in a range of anionic surfactants. The phenomenon is associated with the ability to manipulate surface properties, especially in the promotion of enhanced surface wetting, and in the presence of an extensive near surface reservoir for rapid surface delivery of surfactant and other active components.

Hypothesis: In the single alkyl chain anionic surfactants, such as sodium dodecysulfate, SDS, sodium alkylethoxylsulfate, SAES, and alkylestersulfonate, AES, surface multilayer formation is promoted by trivalent counterions such as Al³⁺, and is generally not observed with divalent counterions, such as Ca²⁺ or with monovalent counterions. In the di-alkyl chain anionic surfactant, dodecylbenzenesulfonate, LAS, surface multilayer formation now occurs in the presence of divalent counterions. It is attributed to the closer proximity of a bulk lamellar phase, resulting in a greater tendency for surface multilayer formation, and hence should occur in other di-alkyl chain anionic surfactants.

Experiments: Aerosol-OT, AOT, is one of the most commonly used di-alkyl chain anionic surfactants, and is extensively used as an emulsifying, wetting and dispersing agent. This paper reports on predominantly neutron reflectivity, NR, measurements which explore the nature of surface multilayer formation of the sodium salt of AOT at the air-solution interface with the separate addition of Ca²⁺ and Al³⁺ counterions.

Findings: In the AOT concentration range 0.5 to 2.0 mM surface multilayer formation occurs at the air-solution interface with the addition of Ca²⁺ or Al³⁺ counterions. Although the evolution in the surface structure with surfactant and counterion concentration is broadly similar to those reported for SDS, SAES and AES, some notable differences occur. In particular the surfactant and counterion concentration thresholds for surface multilayer formation are higher for Ca²⁺ than for Al³⁺. The differences encountered reflect the greater affinity of the di-alkyl chain structure for lamellar formation, and how the surface packing is controlled in part by the headgroup structure and the associated counterion binding affinity.

Keywords: Aerosol-OT; Air–water interface; Anionic surfactants; Multivalent counterions; Surface multilayer formation.