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Langmuir 2004, 20, 8740-8753
External Reflection FTIR Spectroscopy of the Cationic Surfactant Hexadecyltrimethylammonium Bromide (CTAB) on an Overflowing Cylinder Richard A. Campbell, Stephen R. W. Parker, James P. R. Day, and Colin D. Bain* Department of Chemistry, University of Oxford, Chemistry Research Laboratory, Mansfield Road, Oxford, OX1 3TA, United Kingdom Received May 28, 2004. In Final Form: July 27, 2004 External reflection Fourier transform infrared spectroscopy (ER-FTIRS) has been used to study the adsorption of the cationic surfactant hexadecyltrimethylammonium bromide (CTAB) at the air-water interface under nonequilibrium conditions. An overflowing cylinder (OFC) was used to generate a continually expanding liquid surface with a surface age of 0.1-1 s. ER-FTIR spectra were acquired by a single bounce of p- or s-polarized radiation from the flowing surface of the OFC. The C-H stretching region of CTAB spectra was analyzed both by subtraction of a reference spectrum of pure water and by a chemometric technique known as target factor analysis (TFA). The TFA method is shown to give lower scatter in the weight of the component assignable to the adsorbed CTAB monolayer and to permit analysis of spectra at lower bulk surfactant concentrations. The surface sensitivity of ER-FTIRS is demonstrated both experimentally and by theoretical modeling. Modeling shows that surfactant adsorbed at the surface and dissolved in the bulk solution can be distinguished by reflection spectroscopy but also highlights potential errors that can arise from the neglect of the bulk surfactant contribution to the ER-FTIR spectra. Polarized spectra are consistent with an isotropic distribution of transition dipole moments of the hydrocarbon chains in CTAB. Component weights of the CTAB monolayer determined by TFA are compared with an independent determination of values of the dynamic surface excess, Γdyn, by neutron reflection and ellipsometry. The relationship between the component weights and Γdyn shows a small but significant deviation from linearity. Possible explanations for this deviation are discussed. The feasibility of using TFA to deconvolute ER-FTIR spectra of mixtures of hydrocarbon surfactants is demonstrated.
Introduction The adsorption of surfactants at the air-water interface controls the dynamic behavior of many important systems.1,2 In industry, surfactants influence the stability of turbulent foams, the droplet size in jets and sprays, the spreading of drops on solid surfaces, and the smooth coating of multiple layers in photographic films. In nature, pulmonary surfactants facilitate the expansion of alveoli in lungs by lowering the dynamic surface tension, σdyn. Surfactants affect dynamic interfacial properties in two distinct ways. First, by adsorbing at interfaces they lower the interfacial tension. Second, nonuniformity in the adsorption at the interface results in surface tension gradients, which generate a shear stress at the interface and alter the bulk hydrodynamic flow (Marangoni effects).2 While the equilibrium surface tension, σe, is uniform over a liquid surface at chemical and thermal equilibrium, σdyn may be either greater or less than the equilibrium value depending on whether the surface is expanding or contracting. In most practical examples, the rate of surface destruction or renewal is not constant across the interface, resulting in nonuniformity in the dynamic surface excess, Γdyn, and consequently in σdyn. The hydrodynamics together with intrinsic properties of the surfactant (such as monomer and micelle diffusion coefficients, critical micelle concentration (cmc), and surface equation of state, σ(Γ)) * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Edwards, D. A.; Brenner, H.; Wasan, D. T. Interface Transport Processes and Rheology; Butterworth-Heinemann: Boston, MA, 1991. Chang, C. H.; Franses, E. I. Colloids Surf., A 1995, 100, 1. (2) Dukhin, S. S.; Kretzschmar, G.; Miller, R. Dynamics of Adsorption at Liquid Interfaces; Elsevier: Amsterdam, 1995.
determines the mass transport of the surfactant to the surface and the resulting values of σdyn. Gradients in σdyn can fundamentally alter the hydrodynamics3 and even result in self-sustaining flow in a liquid.4 The coupling between mass transport and momentum transport greatly complicates the study of both the kinetics of adsorption at free surfaces and Marangoni flows. To study adsorption kinetics and Marangoni flows quantitatively under welldefined hydrodynamic conditions, we have developed two platforms for generating expanding liquid surfaces under steady-state conditions, the overflowing cylinder (OFC)3,5-12 and the liquid jet.13 The OFC is employed in the work described here. The objective of this paper is to show how external reflection Fourier transform infrared spectroscopy (ER(3) Manning-Benson, S.; Bain, C. D.; Darton, R. C. J. Colloid Interface Sci. 1997, 189, 109. (4) Bain, C. D.; Burnett-Hall, G. D.; Montgomerie, R. R. Nature 1994, 372, 414. (5) Piccardi, G.; Ferroni, E. Ann. Chim. (Rome) 1951, 41, 3. Piccardi, G.; Ferroni, E. Ann. Chim. (Rome) 1953, 43, 328. Padday, J. F. Proc. Int. Congr. Surf. Act., 2nd 1957, 1. (6) Bergink-Martens, D. J. M.; Bos, H. J.; Prins, A.; Schulte, B. C. J. Colloid Interface Sci. 1990, 138, 1. Bergink-Martens, D. J. M.; Bisperink, C. G. J.; Bos, H. J.; Prins, A.; Zuidberg, A. F. Colloids Surf. 1992, 65, 191. Bergink-Martens, D. J. M.; Bos, H. J.; Prins, A. J. Colloid Interface Sci. 1994, 165, 221. (7) Manning-Benson, S.; Bain, C. D.; Darton, R. C.; Sharpe, D.; Eastoe, J.; Reynolds, P. Langmuir 1997, 13, 5808. (8) Manning-Benson, S.; Parker, S. R. W.; Bain, C. D.; Penfold, J. Langmuir 1998, 14, 990. (9) Bain, C. D.; Manning-Benson, S.; Darton, R. C. J. Colloid Interface Sci. 2000, 229, 247. (10) Battal, T.; Shearman, G. C.; Valkovska, D.; Bain, C. D.; Darton, R. C.; Eastoe, J. Langmuir 2003, 19, 1244. (11) Valkovska, D.; Wilkinson, K. M.; Campbell, R. A.; Bain, C. D.; Wat, R.; Eastoe, J. Langmuir 2003, 19, 5960. (12) Campbell, R. A.; Bain, C. D. Vib. Spectrosc. 2004, 35, 205.
10.1021/la048680x CCC: $27.50 © 2004 American Chemical Society Published on Web 08/31/2004
Spectroscopy of CTAB on an Overflowing Cylinder
Figure 1. Schematic drawing of the OFC. A resistance plate (R) and a flow straightener (F) ensure that there is plug flow beneath the hydrodynamic boundary layer. The arrows indicate the direction of flow.
FTIRS) can be used to determine the amount of surfactant adsorbed at a flowing liquid surface. Using the cationic surfactant hexadecyltrimethylammonium bromide (CTAB) as a model system, we will establish the sensitivity of ER-FTIRS for submonolayers of adsorbed surfactant. We will use polarized spectra to provide information on structure in the adsorbed surfactant film and discuss the relationship between the intensities of surfactant peaks in ER-FTIR spectra and the amount of absorbed surfactant. Finally we will assess the prospects for extending ER-FTIRS to the dynamics of mixed surfactant systems. In an OFC, liquid flows vertically up an inner cylinder then pours over the horizontal rim to collect in an outer cylinder for recirculation. A schematic drawing of an OFC is shown in Figure 1. The free liquid surface accelerates radially outward from a stagnation point in the center of the cylinder. The surface undergoes a pure dilation with a surface expansion rate θ ) d ln A/dt (where A is the area of an element of the surface) that is almost constant across the surface of the liquid. In the presence of surfactant, Marangoni stresses can increase the radial surface velocity by an order of magnitude.9 An intriguing feature of the OFC is that in the presence of surfactants, the surface velocity is independent of both the dimensions of the cylinder and the bulk flow rate, within certain limits.3,6 The surfactant sets its own time scale, with θ typically in the range of 1-7 s-1. Thus the OFC allows the study of the dynamics of surfactant adsorption at the air-water interface on a time scale of θ-1 ) 0.1-1 s under steadystate conditions. For pure water in the OFC, the radial surface flow is driven by gravity and the free surface is slightly domed. In the presence of surfactants, the surface flow is driven by surface tension gradients and the surface is almost perfectly flat, except near the rim of the cylinder, making the OFC well-suited for spectroscopy, reflectometry, and scattering.3,7-12 Over the past decade, we have developed a range of noninvasive experimental probes of the surface properties of the OFC.3,7-12 Surface light scattering measures σdyn;7 laser Doppler velocimetry measures the vertical and radial velocity profiles, from which θ is inferred;3,9 neutron reflection (NR) provides a direct measure of the average value of Γdyn near the center of the cylinder;8,10,11 and ellipsometry, with a precision of
