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Title: Formation and surface-stabilizing contributions to bare nanoemulsions created with negligible surface charge

Abstract

The stabilization of nanoemulsions, nanosized oil droplets dispersed in water, is commonly achieved through the addition of surfactants and polymers. However, nanoemulsions in the absence of emulsifiers have been observed to acquire a significant negative charge at their surface, which ultimately contributes to their stability. While the source of this negative charge is disputed to this day, its presence is taken as an inherent property of the aqueous–hydrophobic interface. This report provides a look at the molecular structure and bonding characteristics of bare aqueous–hydrophobic nanoemulsion interfaces. We report the creation of bare nanoemulsions with near zero surface charge, which are marginally stable for several days. The process of creating these low-charge nanoemulsions (LCNEs) required rigorous cleaning procedures and proper solvent storage conditions. Using vibrational sum-frequency scattering spectroscopy, we measure the structure and bonding of the interfacial aqueous and hydrophobic phases. The surfaces of these LCNE samples possess a measurable free OH vibration, not found in previous studies and indicative of a clean interface. Tuning the nanoemulsion charge through addition of anionic surfactants, modeling potential surface-active contaminants, we observe the free OH to disappear and a reorientation of the interfacial hydrophobic molecules at micromolar surfactant concentrations. Notably, the free OH vibrationmore » provides evidence for stronger dispersion interactions between water molecules and the hydrophobic phase at the LCNE surface compared with similar planar water–alkane interfaces. In conclusion, we propose the stronger bonding interactions, in addition to an ordered interfacial aqueous layer, contribute to the delayed droplet coalescence and subsequent phase separation.« less

Authors:
ORCiD logo; ; ;
Publication Date:
Research Org.:
Univ. of Oregon, Eugene, OR (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
1508690
Alternate Identifier(s):
OSTI ID: 1612132
Grant/Contract Number:  
SC0014278
Resource Type:
Published Article
Journal Name:
Proceedings of the National Academy of Sciences of the United States of America
Additional Journal Information:
Journal Name: Proceedings of the National Academy of Sciences of the United States of America Journal Volume: 116 Journal Issue: 19; Journal ID: ISSN 0027-8424
Publisher:
National Academy of Sciences
Country of Publication:
United States
Language:
English
Subject:
59 BASIC BIOLOGICAL SCIENCES; science & technology; other topics; nanoemulsions; surface spectroscopy; interfacial water; vibrational sum-frequency scattering spectroscopy; oil–water interfaces

Citation Formats

Carpenter, Andrew P., Tran, Emma, Altman, Rebecca M., and Richmond, Geraldine L. Formation and surface-stabilizing contributions to bare nanoemulsions created with negligible surface charge. United States: N. p., 2019. Web. doi:10.1073/pnas.1900802116.
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Carpenter, Andrew P., Tran, Emma, Altman, Rebecca M., & Richmond, Geraldine L. Formation and surface-stabilizing contributions to bare nanoemulsions created with negligible surface charge. United States. https://doi.org/10.1073/pnas.1900802116
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Carpenter, Andrew P., Tran, Emma, Altman, Rebecca M., and Richmond, Geraldine L. Wed . "Formation and surface-stabilizing contributions to bare nanoemulsions created with negligible surface charge". United States. https://doi.org/10.1073/pnas.1900802116.
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@article{osti_1508690,
title = {Formation and surface-stabilizing contributions to bare nanoemulsions created with negligible surface charge},
author = {Carpenter, Andrew P. and Tran, Emma and Altman, Rebecca M. and Richmond, Geraldine L.},
abstractNote = {The stabilization of nanoemulsions, nanosized oil droplets dispersed in water, is commonly achieved through the addition of surfactants and polymers. However, nanoemulsions in the absence of emulsifiers have been observed to acquire a significant negative charge at their surface, which ultimately contributes to their stability. While the source of this negative charge is disputed to this day, its presence is taken as an inherent property of the aqueous–hydrophobic interface. This report provides a look at the molecular structure and bonding characteristics of bare aqueous–hydrophobic nanoemulsion interfaces. We report the creation of bare nanoemulsions with near zero surface charge, which are marginally stable for several days. The process of creating these low-charge nanoemulsions (LCNEs) required rigorous cleaning procedures and proper solvent storage conditions. Using vibrational sum-frequency scattering spectroscopy, we measure the structure and bonding of the interfacial aqueous and hydrophobic phases. The surfaces of these LCNE samples possess a measurable free OH vibration, not found in previous studies and indicative of a clean interface. Tuning the nanoemulsion charge through addition of anionic surfactants, modeling potential surface-active contaminants, we observe the free OH to disappear and a reorientation of the interfacial hydrophobic molecules at micromolar surfactant concentrations. Notably, the free OH vibration provides evidence for stronger dispersion interactions between water molecules and the hydrophobic phase at the LCNE surface compared with similar planar water–alkane interfaces. In conclusion, we propose the stronger bonding interactions, in addition to an ordered interfacial aqueous layer, contribute to the delayed droplet coalescence and subsequent phase separation.},
doi = {10.1073/pnas.1900802116},
journal = {Proceedings of the National Academy of Sciences of the United States of America},
number = 19,
volume = 116,
place = {United States},
year = {Wed Apr 24 00:00:00 EDT 2019},
month = {Wed Apr 24 00:00:00 EDT 2019}
}
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https://doi.org/10.1073/pnas.1900802116
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