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A Trimeric Surfactant: Surface Micelles, Hydration lubrication, and Formation of a Stable, Charged Hydrophobic Monolayer Nir Kampf, Chunxian Wu, Yilin Wang, and Jacob Klein Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02657 • Publication Date (Web): 19 Oct 2016 Downloaded from http://pubs.acs.org on October 24, 2016
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A Trimeric Surfactant: Surface Micelles, Hydration lubrication, and Formation of a Stable, Charged Hydrophobic Monolayer Nir Kampf*1, Chunxian Wu2, Yilin Wang*2 and Jacob Klein1 1
Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel. 2 Key Laboratory of Colloid and Interface Science，Beijing National Laboratory for Molecular Sciences (BNLMS)，Institute of Chemistry, Chinese Academy of Sciences Beijing 100190, China.
ABSTRACT:
The
surface
structure
of
the
trimeric
surfactant
tri(dodecyldimethylammonioacetoxy)-diethyltriamine trichloride (DTAD) on mica, and the interactions between two such DTAD-coated surfaces were determined using atomic force microscopy and a surface force balance. In an aqueous solution of 3 mM, five times the critical aggregation concentration (CAC), the surfaces are coated with worm-like micelles or hemi-micelles and larger (ca. 80 nm) bilayer vesicles. Repulsive normal interactions between the surfaces indicate a net surface charge and a solution concentration of ions close to that expected from the CAC. Moreover, this surface coating is strongly lubricating up to some tens of atmospheres, attributed to the hydration-lubrication mechanism acting at the exposed, highly hydrated surfactant headgroups. On replacing the DTAD solution by surfactant-free water, the surface structures have changed the DTAD monolayer, which then jump into adhesive contact on approach, both in water and following addition of 0.1 M NaNO3. This trimeric surfactant monolayer, which is highly hydrophobic, is found to be positively charged as evident from the attraction between the DTAD monolayer and negatively charged bare mica across water. These monolayers are stable over days even under salt solution. The stability is attributed to the several stabilization pathways available to DTAD on the mica surface.
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INTRODUCTION Surfactants are surface active materials that are widely used in industry and explored scientifically1, 2, frequently consisting of a hydrophilic polar headgroup and a hydrophobic alkyl tail. Due to this amphiphilic nature, surfactant aggregates in bulk solution as micelles, vesicles or other structures.3, 4 The morphological transitions are concentration dependent with a distinct critical micellization concentration (CMC) or more generally a critical aggregation concentration (CAC). Above the CAC, surfactants, depending on their detailed structures, often form micelles of different geometries including spheroids, thread-like or rodlike structures,5, 6 while certain surfactants may pack spontaneously into spherical bilayer vesicles similar to liposomes enclosing an aqueous cavity.7, 8
Recently it was found that star-shaped multimeric cationic surfactants in aqueous solution can transform to smaller micelles from large vesicles or network-like aggregates as a function of their concentration.9,
10
It was found that a cationic trimeric ammonium
surfactant, as used in the present study, forms bilayer vesicles, having a typical diameter of ca. 100 nm, just above their CAC, that transform into small micelles with an increase in concentration. Such multimeric surfactants are formed by covalently linking three or more monomeric surfactant units separated by a spacer.10 This covalent linkage between the surfactant units leads to major changes of the physical and chemical properties of the surfactants in solution, compared to those of monomeric surfactants.11,
12, 13, 14
The
influence of the spacer length and type on aggregate morphology has been directly related to changes in the specific area of the surfactant molecules.15, 16
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In addition to modifying solution properties via aggregate formation, surfactants can adsorb on and strongly modify surface properties, including wettability and surface energies,17 as well as friction and wear between two interacting surfaces.18 It was shown that there may be direct correlation between surfactant concentrations in solution and the patterns they form on a charged surface.19 Specifically, trimeric surfactants can arrange and rearrange on a surface depending on the surface type and surface charge density, or as a function of the surfactant concentration.19
Other factors like adsorption time,
counter-ion type and dewetting effects can also influence the adsorption of the surfactants onto surfaces.19, 20 Surface modification by surfactants can be probed by means of a wide variety of methods,18 including contact angle goniometry, X-ray photoelectron spectroscopy (XPS) and AFM measurements21, 22. Another widely-used method, as in this study, is to monitor directly the forces between surfactant-bearing surfaces.20, 23
Surfactant monolayers on charged surfaces in water can strongly reduce the friction between them,18 a theme of the present work. Surfactant aggregates on surfaces may also do this. For example, the friction coefficient (µ, the ratio between the friction force and the normal load) between mica surfaces bearing phosphatidylcholine phospholipids in the form of liposomes was found to be extremely low (around µ = 10-4-10-5). This was attributed to the efficient hydration lubrication mechanism between the close-packed, highly-hydrated phosphocholine groups exposed by the liposomes.24
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In this study we measured the normal and shear forces between two mica surfaces modified with a star-like cationic trimeric surfactant (Figure 1) by using a surface force balance (SFB), while AFM was used to obtain structural information on the surfactant aggregates on the mica surfaces immersed in the surfactant solution just above its CAC. Surface forces were measured at different loads and sliding velocities in surfactant solution; following rinsing in water to remove the surfactant solution; and also across pure water and across salt solution. VS
D C12 KS KN
A
B
Figure1. A - Chemical structure of tri(dodecyldimethylammonioacetoxy)diethyltriamine trichloride (DTAD). B - Schematic illustration of the surface force balance (not to scale) equivalent to the actual lens/spring assembly used.
EXPERIMENTAL SECTION Materials. This
work
used
a
cationic
trimeric
surfactant
(Figure
1-A),
tri(dodecyldimethylammonioacetoxy)-diethyltriamine trichloride (DTAD) with a star-like spacer. Its synthesis and characterization have been reported previously.10,
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DTAD
4
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consists of a tris(chloroacetyl) diethyltriamine moiety as a spacer, 3 ammonium headgroups, and 3 hydrocarbon chains each with 12 carbon atoms. It was shown that DTAD aggregates in aqueous solution can change between large vesicles and small micelles;10 and its CAC is 0.2 mM as determined by surface tension measurements.10 The 2 mM DTAD solution was prepared by dissolving the surfactant powder in purified water (so-called conductivity water, Barnstead NanoPure, TOC
