J. Phys. Chem. C 2007, 111, 8757-8774
8757
Adsorption Kinetics in Binary Surfactant Mixtures Studied with External Reflection FTIR Spectroscopy† James P. R. Day, Richard A. Campbell, Oliver P. Russell, and Colin D. Bain*,‡ Department of Chemistry, UniVersity of Oxford, Mansfield Road, Oxford, OX1 3TA United Kingdom ReceiVed: October 27, 2006; In Final Form: NoVember 16, 2006
The adsorption kinetics of mixtures of soluble surfactants at the air-water interface was studied on an overflowing cylinder. Vibrational spectra of the adsorbed monolayers were acquired by external reflection Fourier transform infrared spectroscopy, and target factor analysis was used to determine the compositions of mixed monolayers. Laser Doppler velocimetry was employed to measure the surface expansion rates, and hence the effective surface age, which was in the range 0.1-1 s. Three surfactant mixtures with different interactions were investigated. Blends of the cationic hydrocarbon surfactant hexadecyl trimethylammonium bromide (CTAB) and the nonionic hydrocarbon surfactant octaethylene glycol monodecyl ether (C10E8) were found to mix ideally in the monolayer over a wide range of subsurface compositions. Combinations of C10E8 and the anionic fluorosurfactant ammonium perfluorononanoate (APFN) exhibited nonideal mixing that could be described with regular solution theory. APFN adsorbed to the interface at a rate limited by monomer diffusion but the adsorption of C10E8 appeared to be under kinetic control. The adsorption behavior of the cationic-anionic mixture of CTAB and APFN was dominated by the interaction of the oppositely charged headgroups. Either side of equimolar bulk composition only the species in excess was found to adsorb, which is rationalized by the presence of aggregates in the bulk that act as a sink for free monomer. Prior evidence in the literature suggests these aggregates may be vesicles. At equimolar compositions both species were found to coadsorb; this unexpected result may be explained by adsorption of vesicles to the uncharged interface. A semiquantitative model based on the interaction between the electrical double layers of the oppositely charged monolayer and vesicle explains the absence of adsorption of vesicles away from equimolar compositions. The combination of the overflowing cylinder, to generate a steadily expanding surface with well-defined hydrodynamics, and FTIR spectroscopy, to quantify the composition of the adsorbed monolayer, can be used to study a broad range of mixed monolayers at the air-water interface under nonequilibrium conditions.
Introduction Surfactants are used throughout industry, and within nature, to control the structure and properties of liquid interfaces.1 Invariably, practical applications of surfactants involve mixtures; in part because commercial drivers militate against expensive separations, in part because different surfactants may be present to perform different functions (such as dispersant and wetting agent in an ink), and in part because surfactant mixtures often deliver better performance than the individual components alone: a situation known as synergy.2,3 Natural surfactants are also mixtures: pulmonary surfactant (which facilitates the expansion and contraction of the mammalian lung) contains several lipids and four principal proteins.4 Owing to the ubiquitous exploitation of surfactant mixtures, there has been a long-standing effort to understand the equilibrium physical and thermodynamic properties of surfactant mixtures both in the bulk phase and, more recently, at interfaces. Although a knowledge of equilibrium properties is an essential step toward an understanding of the mode of action of surfactants, it is often not sufficient because surfactants are frequently used under †
Part of the special issue “Kenneth B. Eisenthal Festschrift”. * To whom correspondence should be addressed. E-mail: c.d.bain@ durham.ac.uk. ‡ Permanent address: Department of Chemistry, Durham University, South Road, Durham, DH1 3LE, U.K.
conditions that are far from equilibrium. Foaming, emulsification, wetting, coating, spraying, lubrication, and detergency are all examples of industrial processes that rely on the behavior of surfactants at dynamic interfaces.5 To model these processes, one needs not only equilibrium thermodynamic data but also kinetic data on bulk processes (diffusion, solubilization, demicellisation, and rheology) and interfacial processes (adsorption, interfacial rheology, and Marangoni flow). Although a significant corpus of knowledge is being developed on the dynamics of surfactant (and surfactant-polymer) mixtures in the bulk,2 our understanding of adsorption processes in surfactant mixtures is at an early stage. In this paper, we first show how external reflection Fourier transform infrared spectroscopy (ER-FTIRS), in conjunction with an overflowing cylinder (OFC) to generate a nonequilibrium surface, can be used to determine the composition of adsorbed monolayers at the air-water interface for surfaces ages in the range 0.1-1 s. We then apply this methodology to three binary mixtures of surfactants that show differing levels of nonideality. The Results and Discussion section is divided into two parts that can be read independently: the first part deals with the acquisition and analysis of the infrared spectra and the second part with the physical interpretation of the surfactant behavior. This structure allows the reader who is interested only in practical insights into dynamic surfactant behavior to skip the technical details of the experimental method. Conversely, readers interested in applying
10.1021/jp067051o CCC: $37.00 © 2007 American Chemical Society Published on Web 02/16/2007
8758 J. Phys. Chem. C, Vol. 111, No. 25, 2007 ER-FTIRS in other applications need not labor through the intricacies of nonideal surfactant behavior. However, first we will provide some general background to surfactant mixtures and set our work in context of other techniques. The simplest and most fundamental piece of information about an adsorbed film in a surfactant mixture is its composition. There are a few cases, such as phospholipids, where one can select components that are insoluble so that the surface composition is equal to the bulk composition used to spread the monolayer. Generally, the surfactants can exchange with the bulk and the surface composition and bulk composition are not the same. In extreme cases, such as sodium dodecyl sulfate containing a trace of dodecanol, the surface and bulk compositions may differ by several orders of magnitude.6,7 The most widely used experimental technique for determining the surface coverage in single component systems, tensiometry, can also be applied to mixtures provided that the chemical potentials of each component in the mixture can be varied independently. For dilute solutions well below the critical micelle concentration, the chemical potentials are simply related to the concentrations. When micelles are present, the variation in one chemical potential changes all of the others and the surface composition cannot be determined by tensiometry without first having a full understanding of the bulk thermodynamics. Direct measurements of surface composition are to be preferred. The most general approach is neutron reflectivity, in which selective deuteration of surfactants is used to distinguish between different chemical species.8 Vibrational spectroscopy also has chemical selectivity: attenuated total internal reflection infrared (ATR-IR) spectroscopy9 and Raman scattering10,11 have been used to study mixtures of surfactants at solid-liquid interfaces and sumfrequency spectroscopy (SFS) has been applied to mixtures of surfactants with dodecanol and alkanes at the air-water interface.12-15 Classical methods, such as radiotracers and McBain’s microtome, can also be used. For mixtures at dynamic air-water interfaces, the available information is much more sparse, and all work to date has focused on the determination of the dynamic surface tension. For a single surfactant, the surface tension, σ, yields the surface excess, Γ, provided that the equilibrium equation of state, σ(Γ), still holds under dynamic conditions. In a binary system, there is no unique relationship between surface tension and composition, and the dynamic surface tension can only be tested for consistency with an assumed model of the adsorption kinetics. Miller and co-workers have studied a number of binary mixtures by bubble pressure tensiometry.16,17 To our knowledge, there are no direct measurements of the dynamic surface composition of soluble surfactants at the air-water interface by any technique. The technique that we use to generate a continually expanding liquid interface, the overflowing cylinder (OFC), has its provenance at Kodak (Harrow, U.K.) in the 1940s.18-20 It was subsequently developed by Prins at Wageningen21-23 and Darton and Bain at Oxford in the 1990s.24-33 In an OFC, liquid flows vertically up an inner cylinder then pours over the horizontal rim to collect in an outer reservoir for recirculation. The free liquid surface accelerates radially outward from a central stagnation point with a surface expansion rate, θ, that is almost constant across the surface of the liquid. (θ is defined as d ln A/dt where A is the area of a small element of the surface.) In the presence of surfactant, Marangoni effects arising from surface tension gradients can increase the radial surface velocity, and hence the surface expansion rate, by up to an order of magnitude.28 The surface velocity is independent of the bulk flow rate and the dimensions of the cylinder, within certain
Day et al. limits; the expansion rate is determined autonomously by the surfactant and is typically in the range 0.5-7 s-1. Steady-state adsorption at the expanding surface of the OFC can be related approximately to time-dependent adsorption in the absence of convection if the mapping 2θ f t-1 is made:32 the characteristic surface age is 0.1-1 s. As in previous work, we use laser Doppler velocimetry (LDV) to measure the radial velocity profiles from which we calculate the surface expansion rates. The application of infrared spectroscopic reflectometry (often known as RAIRS or IRRAS, though we prefer the acronym ER-FTIRS since it is not really an absorption spectroscopy) to insoluble monolayers at the air-water interface was pioneered by Dluhy in 1985,34 and the first study of soluble surfactants was reported by Fina in 1993.35 Since then several other groups have used the technique to look at SDS monolayers,36,37 photochromic monolayers,38 polymer39,40 and protein41,42 films, and the interaction of pulmonary surfactant and lipid films43 but always under equilibrium conditions. We have recently extended ER-FTIRS to nonequilibrium surfaces in an OFC and demonstrated the benefits of using chemometric analysis over traditional spectral subtraction methods.30-32 The presence of bulk surfactant at mM concentrations can significantly distort the reflection signal from the surfactant monolayer. Target factor analysis (TFA) can be used to distinguish the signals from adsorbed and bulk surfactant species.31 To date, we have reported experimental data on pure solutions of the nonionic surfactant octaethylene glycol monodecyl ether (C10E8),30 the cationic surfactant hexadecyltrimethylammonium bromide (CTAB),32 and the anionic fluorosurfactant ammonium perfluorononanoate (APFN).31 By comparison with surface coverages determined independently by neutron reflection and ellipsometry, we have established protocols for determining the surface excess quantitatively from the infrared spectra. Here we extend the OFC/ER-FTIRS methodology to the three binary mixtures of the above surfactants at the air-water interface. We first set out the experimental methodology for acquiring spectra and surface expansion rates on the OFC. We then describe the general TFA protocol before discussing the specific application of TFA to each mixture. The second part of the Discussion section relates to adsorption kinetics. We use the surface compositions from ER-FTIRS and the surface expansion rates from LDV to investigate the adsorption kinetics and the molecular interactions within the monolayer for each binary mixture. We show that the behavior of CTAB-C10E8 is close to ideal. For C10E8-APFN, we use the framework of regular solution theory (RST) to analyze deviations from ideality and show that there is a synergistic interaction between the two surfactants that enhances the dynamic surface excess of APFN in the presence of C10E8. Finally, we address the highly nonideal catanionic mixture CTAB-APFN and show how the nature of bulk aggregates dictates the dynamic interfacial behavior. Experimental Section 1. Materials. CTAB (Fluka, 99%) was purified by recrystallization three times from acetone/methanol. C10E8 (Fluka, 98%) was used without further purification. APFN was prepared by neutralization of heptadecafluorononanoic acid (Fluorochem, 99%) with ammonium hydroxide solution (Aldrich, 28% NH3 in water, 99.99+%) followed by freeze-drying and recrystallization (3 times) from dichloromethane/isopropanol. Ultrahigh purity (UHP) water (Elga UHQ) was used throughout. Glassware and the OFC were cleaned in alkaline detergent (Decon 90) and rinsed thoroughly with UHP water. Stock solutions of
Adsorption Kinetics in Binary Surfactant Mixtures the surfactant mixtures were prepared at a number of different compositions; intermediate compositions were generated by successive dilutions of these stock solutions with a pure solution of one of the surfactants. This procedure does not introduce significant errors into the bulk concentrations.30 2. Overflowing Cylinder. The design of the OFC has been described in detail elsewhere.25 Separate stainless steel OFCs were used for LDV and ER-FTIRS measurements, with internal diameters of 80 and 50 mm, respectively. The smaller OFC was required for the IR experiments due to the geometrical constraints of the optical bench, whereas the larger OFC gives more accurate values of the surface expansion rate. Ellipsometric measurements have shown that that the surface excess is independent of the diameter of the cylinder, provided that the volume flow rate and the length of the free-falling films on the outside of the OFC are above the critical values where the surface hydrodynamics become independent of the bulk flow conditions: these conditions held in our experiments.25 All measurements were taken at 298 K. 3. Laser Doppler Velocimetry. A 10-mW HeNe laser beam (Uniphase) was split into two equal parts, focused, and recombined on the surface of the OFC in such a way that a set of interference fringes was formed perpendicular to the radial flow direction. The surfactant solution was seeded with a few mg of 2-µm TiO2 particles. When these particles pass through the interference fringes formed by the crossed laser beams they scatter light with an intensity that is modulated at a frequency, f, (in the range of 104 Hz) given by f ) [2u(r) sin φ/2]/[λ] where λ is the wavelength of the laser, φ is the angle between the crossed laser beams, and u(r) is the velocity of the particles. The maximum radial velocity, us(r), is obtained when the beams are crossed on the surface of the OFC. Measured profiles of us(r) on either side of the center of the cylinder were fitted to a function of the form us(r) ) a1(r - δ) + a3(r - δ)3, where δ represents the small (
