Subscriber access provided by ORTA DOGU TEKNIK UNIVERSITESI KUTUPHANESI
Letter nd
From Discs to Ribbons Networks: The 2 Critical Micelle Concentration in Nonionic Sterol Solutions Dganit Danino, Ludmila L. Abezgauz, Irina Portnaya, and Nily R. Dan J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b00266 • Publication Date (Web): 31 Mar 2016 Downloaded from http://pubs.acs.org on April 1, 2016
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
The Journal of Physical Chemistry Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 17
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry Letters
From Discs to Ribbons Networks: The 2nd Critical Micelle Concentration in Nonionic Sterol Solutions
Dganit Danino†, Ludmila Abezgauz†, Irina Portnaya† and Nily Dan# †
Department of Biotechnology and Food Engineering, Technion- Israel Institute of Technology,
Haifa, Israel #
Department of Chemical and Biological Engineering, Drexel University, Philadelphia PA, USA
Corresponding Authors Dganit Danino
[email protected] Nily Dan
[email protected] ACS Paragon Plus Environment
1
The Journal of Physical Chemistry Letters
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 17
ABSTRACT At the Critical Micelle Concentration (CMC) amphiphiles self-assemble into spherical micelles, typically followed by a transition at the 2nd CMC to cylindrical micelles that are uniform in width but are polydispersed in length and have swollen ends. In this paper we report on a new structural path of self-assembly that is based on discoidal (coin-like), rather than spherical, geometry: The nonionic sterol ChEO10 is shown to form monodisperse equilibrium disc (disk) assemblies at the first CMC, transitioning at the 2nd CMC into flat ribbons, that (like the cylindrical micelles) have uniform width, polydispersed length, and swollen ends. Increase in ChEO10 concentration or the temperature lead to ribbons elongation, branching, and network formation. This self-assembly path reveals that: (1) surfactants can form equilibrium nonspherical assemblies at the CMC, and (2) aggregate progression around the 2nd CMC is similar for the disc and sphere geometries. TOC GRAPHICS
ACS Paragon Plus Environment
2
Page 3 of 17
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry Letters
The self-assembly of amphiphiles into distinct nanostructures such as spheres, cylindrical micelles and vesicles has been widely studied
1-7
. In this process, opposite non-covalent
hydrophobic and hydrophilic or electrostatic interactions act to minimize the free energy of the individual amphiphilic building blocks by forming core-shell structures. The process is cooperative, the assemblies form spontaneously, they are thermodynamically stable and dynamic, with characteristic life times as short as microseconds. It is well established now that amphiphile systems (e.g. surfactants or diblock copolymers) self-assemble into small, uniform spherical micelles at concentrations above a Critical Micelle Concentration (CMC) nd
second critical value (the 2
1,8,9
. Increasing the concentration of amphiphiles above a
CMC) usually triggers a distinct morphological transition to
cylindrical (also called threadlike or wormlike) micelles
6,10-13
. Around this concentration
spherical micelles coexist with short cylinders, displaying swollen micellar endcaps and significant size polydispersity. Similar changes occur with increase in temperature. The 2nd CMC is therefore a critical point that marks the onset of changes in the morphology of the already assembled amphiphile, as well as in the solution bulk properties, e.g., the viscosity. Identifying this point is immensely important for applications, for instance for adjusting the texture of personal care products or achieving stable encapsulation in nanomedicine. The 2nd CMC has been observed in single and multiple component systems of simple surfactants
14
and dimeric amphiphiles
15-18
, as well as block copolymers
3,19,20
. The sharply
defined 2nd CMC was linked to a high energetic penalty associated with the early stages of micellar elongation, that eliminates micelles shorter than a critical size
6,21,22
. As the
concentration is increased further, the length of the cylindrical micelles increases, and branching may occur, thereby leading to micellar networks 23,24. In this paper we present a similar structural progression in a single surfactant system that is based, however, on non-spherical geometry. The assemblies are formed by a sterol-derivative surfactant, ChEO10, that consists of a rigid, planar steroid backbone with hydrophobic and hydrophilic faces to which a short chain is attached, covalently linked to a flexible chain of 10 ethylene oxide (EO) units. This molecular architecture (Figure 1a), that differs from that of
ACS Paragon Plus Environment
3
The Journal of Physical Chemistry Letters
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 17
classic nonionic surfactants such as alkyl ethoxylates (CnEOm), affects the assembly characteristics. The self-assembly of sterol-based ethoxylate surfactants has been previously investigated, showing non-classical aggregation behavior
25-28
. In dilute solutions, several sterol ethoxylates
formed rod-like aggregates with varying aspect ratios, which did not change significantly with concentration thereby suggesting some type of an equilibrium width to length ratio
25-28
. In
particular and unexpectedly, ChEO10 was found to form ‘short rodlike’ micelles with an aspect ratio larger than 2 and length of order 15nm, which transitioned at high concentrations into an intermediate rectangular ribbon phase 40°C
28,29
28,29
. In dilute solutions, the 2nd CMC was detected at ~
, described as sphere-to-rod-to-planar structures, coupled with an increase in the
viscosity 29. In this work, the self-assembly of ChEO10 as a function of concentration and temperature was studied using cryo-transmission electron microscopy (cryo-TEM). Cryo-TEM is a noninvasive method that enables direct detection of soft nanostructures in solution at ~ 1nm resolution. The technique involves ultra-rapid cooling that captures the structures at their hydrated state
30-33
. Cryo-TEM uniquely allows determination of the detailed structure of
individual amphiphilic assemblies, information that cannot be obtained from scattering or rheology techniques that probe bulk properties 30. Thus, cryo-TEM is effective for determination of structural coexistence, e.g. the transition of spherical micelles into short rod-like assemblies at the 2nd CMC 14,34. The technique has also enabled resolution of fine structural details such as the swollen endcaps of cylindrical micelles
35,36
or their branching
37,38
: While theoretical work,
scattering and rheology suggested the existence of branching 23,24,39, cryo-TEM was the only tool that unambiguously proved their formation
38
, revealing different types of interconnected
micellar segments with 3-fold, 4-fold and even multiple connections 40. As shown in Figure 1, when dissolved in water at concentrations above the CMC ChEO10 spontaneously self-assembles into uniform disc elements. The disc diameter is of order 13 nm, and its thickness is ~ 4 nm, which is approximately twice the length of the hydrophobic moieties. Note that in the thin cryo-TEM specimen presented in Figure 1c the discs are randomly
ACS Paragon Plus Environment
4
Page 5 of 17
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry Letters
distributed and display different orientations ranging from circular, low contrast coin-like elements when imaged face-on, to dark rods when viewed edge-on.
a
Figure 1. Disc-like aggregates formed by of ChEO10. (a) Chemical formula. (b) A schematic of the disc-like aggregates. (c) A cryo-TEM micrograph of 1 wt% ChEO10 in aqueous solutions, at 30 °C, showing small discs at different orientations. Discs viewed from a top-like projection are circular, and—because of the aggregates small thickness—faint. Aggregates viewed from a sidewise projection seem like a line whose width is the disc width, and are much darker due to the larger diameter. The packing parameter (P) is 1/2
