Article pubs.acs.org/Langmuir
Origins of Microstructural Transformations in Charged Vesicle Suspensions: The Crowding Hypothesis Mansi Seth,† Arun Ramachandran,‡ Bruce P. Murch,§ and L. Gary Leal*,† †
Department of Chemical Engineering, University of California, Santa Barbara, Santa Barbara, California 93106-5080, United States Department of Chemical Engineering and Applied Chemistry, University of Toronto, Ontario, Canada M5S 3E5 § The Procter and Gamble Company, Cincinnati, Ohio 45202, United States ‡
S Supporting Information *
ABSTRACT: It is observed that charged unilamellar vesicles in a suspension can spontaneously deflate and subsequently transition to form bilamellar vesicles, even in the absence of externally applied triggers such as salt or temperature gradients. We provide strong evidence that the driving force for this deflation-induced transition is the repulsive electrostatic pressure between charged vesicles in concentrated suspensions, above a critical effective volume fraction. We use volume fraction measurements and cryogenic transmission electron microscopy imaging to quantitatively follow both the macroscopic and microstructural time-evolution of cationic diC18:1 DEEDMAC vesicle suspensions at different surfactant and salt concentrations. A simple model is developed to estimate the extent of deflation of unilamellar vesicles caused by electrostatic interactions with neighboring vesicles. It is determined that when the effective volume fraction of the suspension exceeds a critical value, charged vesicles in a suspension can experience “crowding” due to overlap of their electrical double layers, which can result in deflation and subsequent microstructural transformations to reduce the effective volume fraction of the suspension. Ordinarily in polydisperse colloidal suspensions, particles interacting via a repulsive potential transform into a glassy state above a critical volume fraction. The behavior of charged vesicle suspensions reported in this paper thus represents a new mechanism for the relaxation of repulsive interactions in crowded situations.
1. INTRODUCTION Vesicle suspensions comprising charged surfactants form the base materials for a variety of household and personal-care products such as fabric enhancers and hair conditioners. Phase diagrams and thermal properties of common double-tailed charged surfactants such as didodecyldimethylammonium bromide1,2 (DDAB), dioctadecyldimethylammonium bromide3−5 (DODAB), and dioctadecyldimethylammonium chloride6,7 (DODAC) in water have thus been extensively studied in the past. These studies show that in the concentration range of 0.15−30 wt % in water and above the main phase transition temperature of the bilayer, such cationic surfactants selfassemble to form unilamellar and multilamellar or “onion” vesicles. In suspensions, these vesicles are believed to exist as a kinetically stabilized metastable phase rather than a thermodynamic equilibrium phase.8−10 Hence, over a period of time, they may undergo changes in properties such as their size, shape, and lamellarity among others, a process termed as “aging”. Aging of vesicle suspensions is generally undesired; for example, a change in the lamellarity and size of vesicles can modify their density and hence the existing balance of buoyancy forces in the suspension. This can lead either to creaming or sedimentation of the vesicles in the suspension, thereby destabilizing it. Furthermore, aging processes can significantly © 2014 American Chemical Society
alter the volume fraction and rheology of a vesicle suspension and in many cases the flow properties represent a crucial design parameter in vesicle-based products. Vesicles are also used as drug delivery vehicles,11,12 an application area wherein their size and lamellarity are important parameters influencing the efficiency of drug encapsulation and delivery.13−15 Studies of vesicle shape and lamellarity16−18 have shown that the shape of a vesicle is sensitive to a variety of parameters, with the most significant being the weight fraction of surfactant, the composition of the surfactant mixture and the temperature. Berndl et al.19 observed vesicle transformations such as budding and the discocyte−stomatocyte transition in giant unilamellar vesicles by making very modest changes in temperature. Vesicles can be prepared via a micelle−vesicle transition, or they can be broken down into micelles, by simply changing the composition of the mixture.20,21 Agarwal et al.22 used cryogenic transmission electron microscopy (cryo-TEM) imaging to reveal that the addition of phenolic organic dopants to aqueous CTAB solutions led to a series of microstructural evolutions from globular to worm-like micelles to unilamellar vesicles and Received: November 19, 2013 Revised: January 27, 2014 Published: January 27, 2014 10176
dx.doi.org/10.1021/la404434q | Langmuir 2014, 30, 10176−10187
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finally bilamellar vesicles. A transformation from unilamellar to bilamellar vesicles was also reported by Lee et al.23 upon the addition of an amphiphilic biopolymer to surfactant vesicles. Similarly, Hubert et al.24 observed via cryo-TEM imaging that the addition of a solute to a suspension of DODAB vesicles led to the formation of a large number of bilamellar vesicles (doublets) in a suspension that was initially composed primarily of unilamellar vesicles. They suggested that the addition of a solute imposes a hyper-osmotic stress which causes a unilamellar vesicle to deflate and eventually curl up and fuse at the tips to form a doublet via a route shown in Figure 1a.
repulsive (entropic/osmotic) pressure between neighboring vesicles in a suspension above a critical, effective volume fraction ϕeff. In the case of charged vesicles, the effective volume fraction includes the volume of the vesicle bilayer and the water enclosed within it, as well as the electrostatic double layer volume. Sources of the repulsive pressure include Brownian motion of colloidal vesicles,29 Helfrich undulations,30 and electrostatic repulsion;30 the latter being the dominant contribution in the case of charged vesicles.30 The more familiar response of a suspension of repulsive “particles” to crowded conditions is to form an ordered, crystalline phase (monodisperse particles) stabilized by an increase in the system’s entropy or a nonequilibrium, jammed or glassy state (polydisperse particles) above a critical volume fraction.31−33 A natural expectation might be that suspensions of vesicles should behave in a similar manner at high volume fractions. While highly monodisperse vesicle suspensions have been reported to form stable ordered phases at high volume fractions,34−36 vesicle suspensions as the ones examined in this study are unable to do so, on account of their high polydispersity of shape and size. However, vesicles have the unique ability of exchanging liquid with the surrounding medium, thereby allowing modification of their volumes. Thus, in polydisperse suspensions, vesicles can respond to “crowded” conditions by deflating and undergoing shape and size transformations in order to reduce their effective volume fraction. Indeed, if the ability of vesicles to modify their volume is diminished (for example by reducing the permeability of the membrane, or by cooling below the gel-fluid transition temperature), presumably polydisperse vesicle suspensions will also transform into glassy phases above a critical volume fraction. We demonstrate that the effective volume fraction of a charged vesicle suspension depends both on the amount of surfactant that is present and the salt concentration as well as the preparation procedure, and deflation is initiated once it exceeds a critical value. Furthermore, we provide a simple model to estimate parameter ranges for which this process is likely to occur. By tuning the surfactant and salt concentrations in a suspension, it is possible to either induce or inhibit vesicle deflation and subsequent transitions of the suspension microstructure, as desired. This understanding of the origins of microstructural transformations in a suspension is a first step toward rational design and control of charged vesicle suspensions. The ability to controllably transform a unilamellar vesicle into a doublet without the addition of undesired ionic species or the imposition of temperature gradients can also be potentially useful as a mechanism of encapsulating drugs or other desired molecules inside vesicles for applications such as drug delivery.
Figure 1. (a) Mechanism suggested by Hubert et al. (ref 24) and Regev and Khan (ref 26) for the transformation of unilamellar vesicles to bilamellar vesicles. (b) Unsaturated double-tailed cationic surfactant diC18:1 DEEDMAC used in this study.
Saveyn et al.25 investigated this phenomenon in greater detail for a suspension of DODAC and showed that for such a transition to occur, the bilayers must be above their chain melt temperature and the hyper-osmotic stress must be applied using an ionic species such as salt; presumably because an ionic species may lead to dehydration of the bilayer thereby aiding the fusion process at the curled tips.25 In the above-mentioned studies, the deflation of a unilamellar vesicle was initiated by the application of an external trigger such as a temperature gradient or a hyperosmotic salt gradient. Even without an imposed hyper-osmotic stress or temperature gradient, Regev and Khan26 used cryo-TEM imaging to show that DDAB vesicle suspensions had mostly unilamellar vesicles at low weight fractions (0.5 wt %), but predominantly bilamellar vesicles at a higher weight fraction (3 wt %). They also observed a variety of vesicle shapes such as deflated unilamellar vesicles and stomatocytes, and thereby suggested a mechanism similar to that in Figure 1a. for the transformation of a unilamellar vesicle into a doublet. However in this case it is unclear what triggers the initial deflation of a spherical vesicle (the first step toward forming a bilamellar vesicle). More recently, Tucker et al.27 observed that suspensions of cationic dihexadecyl dimethylammonium bromide (DHDAB) contained a large number of bilamellar vesicles but were unable to explain the reason behind their predominant presence. The occurrence of bilamellar vesicles has also been reported in other charged surfactant systems.28 To the best of our knowledge, neither the origins of any spontaneous deflation process nor the predominant presence of bilamellar vesicles in charged systems is yet understood. In this paper, we provide strong evidence that the driving force for spontaneous deflation of unilamellar vesicles is the
2. EXPERIMENTAL SECTION 2.1. Materials. The double-tailed cationic surfactant diethylesterdimethyl ammonium chloride (diC18:1 DEEDMAC, (MW = 697.5 g/ mol, Figure 1b) that we study is in the fluid state at room temperature. It was provided to us by Procter and Gamble Co. (Cincinnati, OH) and was used without further purification. 2.2. Suspension Preparation. Vesicle suspensions were prepared by dissolving cationic DEEDMAC surfactant in ethanol and subsequently hydrating it using a warm (40 °C) CaCl2 solution of concentration 0.1, 4.5, or 27 mM, resulting in the formation of vesicles. The suspension was then extruded eight times (unless noted otherwise) through 800 nm sized pores (EMD Millipore, Billerica, MA) in an extruder (Lipex Biomembranes Inc., Vancouver, BC) at a pressure of 200 psi. The pH of the suspension was maintained at 3.0 10177
dx.doi.org/10.1021/la404434q | Langmuir 2014, 30, 10176−10187
Langmuir
Article
Figure 2. (a) Classification of vesicle types observed in the cryo-TEM images. Vesicles are divided into three different size ranges of less than 100 nm, 100−200 nm, and greater than 200 nm in diameter. Unilamellar spherical vesicles that are smaller than 100 nm in diameter form the largest population group (27−35%) in the suspensions. (b) Cryo-TEM images of suspension A, a 35 mg/mL diC18:1 DEEDMAC suspension in 4.5 mM CaCl2 undergoing shape and lamellarity transformations. After preparation (t = 0), several unilamellar spherical, deflated, and stomatocyte are present and interacting with neighboring vesicles. c) After 3 days, an increase in the number of small doublets is observed. (d) Results of volume fraction measurements made using the dilution technique (ref 37); the suspension’s volume fraction decreases from 0.31 at t = 0 to 0.19 at t = 3 days. (e) Imaging summary showing relative number fractions: at t = 0, unilamellar deflated and stomatocyte vesicles form ∼19% of total vesicle population. (f) After 3 days, their combined population reduces to ∼9%. This is accompanied by an increase in the
