Electrophoretic mobility of the polymer-like micelles of tetradecyldimethylamine oxide hemihydrochloride

Abstract

Effects of the surfactant concentration C_d and the NaCl concentration C_s on the electrophoretic mobilities U of the well-characterized polymer-like micelles have been investigated by the electrophoretic light scattering, using tetradecyldimethylamine oxide hemihydrochloride (C14DMAO·1/2HCl). At the high ionic strength of 0.1 mol kg⁻¹ NaCl, the electrophoretic mobilities were independent of C_d (5 mM < C_d < 100 mM), despite the concentration-dependent micelle growth of the polymer-like micelles. This suggests that the electrophoretic mobility of the polymer-like micelle at high ionic strengths is independent of the contour length (i.e., the molecular weight), as found on linear polyelectrolytes. Somewhat surprisingly, the entanglements of the polymer-like micelles gave small effect on the electrophoretic mobilities in the examined range of the surfactant concentration above an overlap concentration. The mobilities of the polymer-like micelle decreased with √C_s in a single exponential manner in the range of C_s from 0.02 to 0.3 mol kg⁻¹. It is suggested that the cylinder model can be applied to the electrophoretic mobilities of the polymer-like micelles at high ionic strengths (i.e. a free-draining behavior), since the persistence length of the polymer-like micelle (∼20 nm) is much larger than the Debye length at high ionic strength.

Introduction

Surfactant molecules can self-assemble reversibly into various organized structures (e.g. micelles and vesicles) in the aqueous media above a critical micelle concentration (cmc). The structures depend on the intensive variables (e.g. temperature, ionic strength and pH) as well as the surfactant geometry (e.g. length of hydrocarbon chains, size of polar headgroups, and counterion species). The aggregate structures are explained in terms of the surfactant packing argument [1] or surface-curvature argument [2]. Some ionic surfactants are known to undergo uniaxial growth (micelle growth) with increasing the surfactant concentration to form flexible polymer-like micelles [3]. The polymer-like micelles behave as equilibrium polyelectrolytes with continuous breaking and reformation of the micelles [3], [4]. The term “equilibrium polyelectrolyte” is used to distinguish the polymer-like micelles from ordinary linear polyelectrolytes [4], in which the neighboring monomers are connected by a covalent bond. The polymer-like micelles can entangle with each other above an overlap concentration $C_{\text{d}}^{*}$ [3]. Highly entangled polymer-like micellar solutions show pronounced viscoelastic behavior and interesting viscoelasticity described by single Maxwell model with only one set of relaxation time and plateau modulus [3], [4], [5], [6]. The experimental evidence for the formation of such polymer-like micelles has been obtained by other experimental techniques such as cryotransmission electron microscopy [7], [8] and small-angle neutron scattering (SANS) [9], [10].

Charged particles in a dilute dispersion move in response to an applied electric field. This phenomenon is well known as electrophoresis [11]. Linear polyelectrolytes show interesting electrophoretic behavior in free solutions. It has been reported that the electrophoretic mobility of the polyelectrolytes is independent of the molecular weight (M) at high ionic strength such as poly (sodium acrylate) in 0.1 M NaCl solution [10⁴ < M < 10⁶] [12], [13], [14]. These experimental facts have been interpreted as confirming the free-draining character of the polyelectrolyte motion in electrophoresis [15], [16], which is in contrast to the fact that the polyelectrolyte coil is essentially non-draining in viscous motion, particularly at high ionic strength. In the case of charged polymer-like micelles, however, little is known on the electrophoretic behavior. In the present study, we report the electrophoretic behavior of polymer-like micelles as a function of the surfactant concentration C_d and the ionic strength C_s. In particular, we are concerned about the following two points: (i) differences and similarities between polymer-like micelles and ordinary linear polyelectrolytes in the electrophoretic behavior, and (ii) how the electrophoretic mobility of polymer-like micelle depends on the molecular weight (i.e., the contour length of the polymer-like micelle) and the ionic strength of the solution.

The formation of very long polymer-like micelle is well known to be promoted remarkably in the case of cationic surfactant/hydrophobic aromatic salt systems [3], [4], [5], [6], [17], [18], [19], [20]. However, these systems are undesirable for our purpose because of the significant binding of the hydrophobic counterion to the micelles, resulting in the decrease in the electrophoretic mobilities of the micelles [21]. As a suitable micelle system to study the electrophoretic behavior of charged polymer-like micelles, we used tetradecyldimethylamine oxide hemihydrochloride (C14DMAO·1/2HCl), which forms very long polymer-like micelles at 0.1 M NaCl, in spite of no specific binding of counterion Cl⁻ [22]. The hydrogen bonding between the nonionic and the cationic head groups has been suggested as responsible for the formation of very long polymer-like micelle of C14DMAO·1/2HCl [22], [23], [24], [25], [26], [27]. Recently, the structural parameters of the polymer-like micelle have been reported by means of SANS such as molecular weight, contour length, the Kuhn length, semiaxes of the elliptical cross-section, and radius of gyration as a function of surfactant concentration [28]. These structural parameters give useful information on the theoretical calculation of the electrophoretic mobility of the polymer-like micelle.