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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.

This paper is concerned with the interfacial behavior of surfactant solutions on short time scales. A gravity-driven laminar liquid jet is used to create a rapidly expanding liquid surface, which exposes the surfactant solution to highly nonequilibrium conditions. This expansion causes the surface tension to differ locally from its equilibrium value, generating a (Marangoni) shear stress that acts on the jet surface and retards the surface acceleration. A theory for the flow very near the nozzle shows that the cube-root dependence of the surface velocity on the distance traveled is altered through the adsorption of surfactant. In a boundary-layer treatment, both the surface velocity and the surface concentration increase linearly from the nozzle exit over a short distance, which we term the detachment region. The length of the detachment region is found to vary with the bulk concentration raised to the power ³/2. A numerical model of the surfactant adsorption process in the jet has been developed within the framework of the CFD code FIDAP. The numerical solution confirms the general features of the theory and shows that the maximum reduction in surface velocity occurs very close to the nozzle exit, except at high concentrations. A comparison with experiments on C16TAB at concentrations below the critical micelle concentration, which are described in part 1 of this series of papers, shows good agreement.

A quantitative model for the kinetics of adsorption of ionic surfactants to an expanding liquid surface is presented for surfactant concentrations below and above the critical micelle concentration (cmc). For surfactant concentrations below the cmc, the electrostatic double layer is accounted for explicitly in the adsorption isotherm. An overflowing cylinder (OFC) was used to create nonequilibrium liquid surfaces under steady-state conditions. Experimental measurements of the surface excess for solutions of cationic surfactants CH3(CH2)n-1N⁺(CH3)3 Br^- (CnTAB, n = 12, 14, 16) and the anionic fluorocarbon surfactant sodium bis(1H,1H-nonafluoropentyl)-2-sulfosuccinate (di-CF4) in the OFC are in excellent agreement with the theoretical predictions for diffusion-controlled adsorption for all concentrations studied below the cmc. For surfactant concentrations above cmc, the diffusion of micelles and monomers are handled separately under the assumption of fast micellar breakdown. This simplified model gives excellent agreement for the system C14TAB + 0.1 M NaBr above the cmc. Agreement between theory and experiment for C16TAB + 0.1 M NaBr is less good. A plausible explanation for the discrepancy is that micellar breakdown is no longer fast on the time scale of the OFC (ca. 0.1 s).

Dynamic adsorption and surface tension behavior of aqueous fluoro-surfactant solutions has been investigated using a range of experimental techniques, including neutron reflection (NR). Equilibrium tensions, γeq, have been measured by drop volume tensiometry (DVT), and dynamic surface tensions, γdyn, have been determined using the nonperturbative method, surface light scattering (SLS). Dynamic conditions for NR and SLS measurements were established using an overflowing cylinder (OFC). The OFC provides a dynamic surface on the 0.1−1 s time scale and offers a large (~50 cm²), near flat surface for interrogating interfacial properties. To exploit these techniques effectively, a fluorocarbon anionic surfactant, sodium bis(1H,1H-nonafluoropentyl)-2-sulfosuccinate (di-CF4) has been specifically selected. Molecular structure effects have been explored with the C6 analogue sodium bis(1H,1H,7H-dodecafluoro-n-heptyl) sulfosuccinate (di-HCF6). Using OFC−NR, dynamic surface excesses, Γdyn, have been measured directly, and these values have been compared to equilibrium coverages, Γeq, determined by DVT and NR. Close to the critical micelle concentration (cmc) of di-CF4 (1.58 mmol dm^-3), Γdyn and Γeq are very similar, and the ratio φ = Γdyn/Γeq is unity to within the precision of the experiment. For moderate differences in surface tension, up to Δγ = γdyn − γeq ≤ 15 mN m^-1, φ remains close to 1. At concentrations of 0.2−0.7 mmol dm^-3, the dynamic surface excess Γdyn is measurably different from the equilibrium value Γeq (φ < 1). This concentration range coincides with the largest differences in surface tension, Δγ. For both di-CF4 and di-HCF6, the maximum values of Δγ and ΔΓ occur around the same bulk concentration, ~0.7 mmol dm^-3, suggesting that dynamic surface behavior is determined mainly by mass transport (which is related to the bulk concentration), rather than surfactant properties such as cmc, for these surfactants.

The interfacial tensions and dihedral angles of hexadecane lenses on aqueous solutions of dodecyltrimethylammonium bromide (DTAB) were measured in air as a function of the molality of DTAB at 298.15 K and atmospheric pressure. The air/water interfacial tension of DTAB solution shows a break point that shifts to a lower concentration in the presence of a hexadecane lens. Ellipsometric measurements showed a continuous variation in ellipticity with concentration for pure DTAB solutions, but a discontinuous change in the ellipticity of the air/water interface in the presence of a hexadecane lens. This behavior can be viewed either as a first-order wetting transition driven by the adsorbed surfactant, or as a gaseous-expanded phase transition in the surfactant adsorbed film induced by the hexadecane. The formation of a microscopic oil film in equilibrium with a lens is rationalized by an attractive long-range potential across the thin film. From the measurement of dihedral angles, it was found that there was a concentration region where a lens exists stably even when complete wetting was expected from measured values of interfacial tensions.

Ellipsometry is a powerful experimental technique for studying the surfaces of surfactant solutions both at and away from equilibrium. From a theoretical perspective, there is no simple relationship between ellipsometric parameters at the air−water interface and the surface excess, Γ, of the surfactant. Here we report neutron reflection measurements of the dynamic surface excess, Γdyn, of the cationic surfactants CnTAB (n = 14, 18) in an overflowing cylinder (OFC) as a function of the bulk surfactant concentration, to complement earlier measurements on C16TAB. These results are compared with ellipsometric measurement of the coefficient of ellipticity, ρ, of the surfactant solutions in an OFC in order to generate a calibration curve ρ(Γ). For each surfactant, ρ was found to be a linear function of surface excess. For the shorter homologue, n = 12, surface tension measurements on equilibrium solutions have been used to determine the adsorption isotherm and the surface excess was then compared with the coefficient of ellipticity. Again, the relationship between ρ and Γ is linear. A protocol is described for determining surface excesses ellipsometrically in this class of surfactants, and the errors in the procedure are quantified.

Sum-frequency spectroscopy (SFS), ellipsometry, and surface tensiometry have been used to study the effect of halide counterion on monolayers of the cationic surfactant CTAX (CH3(CH2)15N⁺(CH3)3; X^-; X = F^-, Cl^-, Br^-, and I^-) at the air−water interface. The counterion was found to change the efficiency and effectiveness of the surfactant, both decreasing in order Br^- > Cl^- > F^-. Aqueous solutions of CTAI were well below the Krafft point at room temperature, and CTAI therefore functioned ineffectively as a surfactant under ambient conditions. Adding salt in the form of 0.1 M KX was found to reduce the cmc but appeared to have little effect on the limiting area per molecule attained at the cmc, which increased from 44 Ų for CTAB to 65 Ų for CTAC and ca. 94 Ų for CTAF. Neither SFS nor ellipsometry provided any firm evidence for specific effects of the halide ions on the structure of the surfactant monolayers. SFS was used to study monolayers of CTAB as a function of concentration. For surface coverages greater than 1 × 10^-6 mol m^-2, the mean tilt of the terminal methyl group from the surface normal was independent of the area per molecule. A decrease in the strength of the SF signal arising from the methylene groups at low surface coverages, which would be expected for molecules lying flat on the surface, was not observed. In the course of the analysis of the ellipsometric data, the contribution of the double layer to the coefficient of ellipticity was evaluated and was shown to be linear in the surface coverage and insensitive to the thickness of the double layer.

Total internal reflection sum-frequency spectroscopy (TIR−SFS) is shown to be capable of detecting molecules adsorbed on ultrathin gold films (≤10 nm) deposited on a sapphire prism. Octadecanethiol (ODT) and thiocyanate (SCN^-) were used as probe molecules in order to assess the usefulness of the approach. For ODT adsorbed on 5-nm Au films, SF signal enhancements of over an order of magnitude were observed with TIR−SFS compared to the standard external reflection geometry. While TIR−SF spectra were obtained for ODT on 5- and 10-nm Au films, no molecular signals were detected for 20-nm Au films. The C&tbd1;N stretch of SCN^- adsorbed on a 5-nm Au film was detected by TIR−SFS in the presence of either water or air. A theoretical model is presented to rationalize the different SF signal levels observed under various conditions. Future prospects of TIR−SFS for studying other oxide-supported metals are discussed, along with possible applications in the fields of heterogeneous catalysis and electrochemistry.

Low-density self-assembled monolayers (SAMs) on gold were generated by the adsorption of a series of specifically designed 2-monoalkylpropane-1,3-dithiol derivatives, CH3(CH2)nCH[CH2SH]2, where n = 11, 13, 14. The monolayers were characterized by optical ellipsometry, X-ray photoelectron spectroscopy, contact angle goniometry, polarization modulation infrared reflection absorption spectroscopy, and sum-frequency generation. Comparison of these data to those collected on SAMs generated from normal alkanethiols, CH3(CH2)n+2SH, and 2,2-dialkylpropane-1,3-dithiol derivatives, [CH3(CH2)n]2C[CH2SH]2, of similar chain length suggests that the new “monoalkanedithiol” SAMs are the least crystalline, exposing both methyl and methylene groups at the interface due to the low density of alkyl chains. Further comparison of these low-density SAMs to those obtained on branched and linear polyethylene films suggests that the exposure of interfacial methylene groups is greater for the polymer films.

A two-dimensional first-order phase transition has been studied by sum-frequency vibrational spectroscopy in mixed monolayers of dodecanol and alkyltrimethylammonium bromide surfactants (CnTABs) at the air/water interface, with n = 12, 14, and 16. At low temperatures (<19 °C), mixed monolayers adsorbed from solutions of 10 μM dodecanol and CnTAB at two-thirds of their critical micelle concentrations are densely packed and highly conformationally ordered. The density of the monolayer is comparable to that of a monolayer of dodecanol in its low-temperature crystalline phase. At a fixed temperature, which increases with CnTAB chain length, the monolayer melts to a phase that is less dense and more conformationally disordered. Specific deuteration experiments were carried out to determine the composition of the monolayer and the conformational order of the two components. The ‘solid' phase contains dodecanol and CnTAB in a ratio of approximately 3:2, and the ‘liquid' phase contains equal amounts of the two components. As the chain length of the CnTAB increases, the number of conformational defects in the dodecanol molecules, as measured by the line strength of the symmetric methylene stretch, decreases in both phases. The CnTAB in the mixed monolayer is more disordered than the dodecanol but much less disordered than in a pure CnTAB monolayer in the absence of dodecanol. Frequency shifts of the symmetric methyl stretch with changes in CnTAB chain length can be understood in terms of the changing environment of the dodecanol and CnTAB methyl groups.

Sum-frequency spectroscopy and ellipsometry have been used to study monolayers of the nonionic surfactants poly(ethylene glycol) monododecyl ethers (C12Em; m = 2−8) at the air−water interface. SF spectra were acquired for areas of 30−70 Ų per molecule, as determined from literature adsorption isotherms. These spectra show an increase in conformational disorder with increasing area per molecule and, surprisingly, an apparent decrease in the angle of tilt of the methyl group. There was no systematic variation in the SF spectra as a function of the length of the poly(ethylene glycol) chain at fixed areas of 45 and 62 Ų per molecule. For m ≥ 4, the ellipsometric data suggested that the density of the hydrocarbon region of the chain was independent of m at fixed coverage but that the density decreased for m = 2 and 3. This study suggests that the value of m does not, per se, affect the structure of a monolayer of C12Em at the air−water interface for m = 4−8.

Neutron reflection has been used to measure the dynamic surface concentration, Γdyn, of the cationic surfactant CTAB in an overflowing cylinder (OFC). This is the first direct measurement of Γdyn for a surfactant at an expanding liquid surface. Γdyn was measured for bulk concentrations between 1.8 and 0.125 mM, corresponding to areas per molecule of 42 to >300 Ų. Careful account had to be taken of the curvature of the surface in establishing calibration procedures. Γdyn was less than the equilibrium surface concentration Γe at all concentrations below the cmc but approached Γe at bulk concentrations well above the cmc, as expected for an expanding surface. Ellipsometry has been used previously to measure the coefficient of ellipticity of surfactant solutions in an OFC. The neutron reflection data were used to calibrate the ellipticity of the surface in terms of Γdyn, and ellipsometry was then used to map changes in Γdyn across the surface of the OFC with a precision of 2 × 10^-8 mol m^-2. The ellipticity can also be calibrated against Γe for static solutions, and this calibration curve can be used to calculate Γdyn from the dynamic ellipticities. The values of Γdyn determined by this indirect route were in excellent agreement with the direct measurements by neutron reflection.

Sum-frequency spectroscopy has been used to study coadsorption of dodecanol and sodium dodecyl sulfate (SDS) at the interface between aqueous solutions and a monolayer of octadecanethiol on gold. Selective deuteration was employed to distinguish between the two adsorbed surfactants. Mixed monolayers were studied over a range of SDS concentrations with dodecanol present at a level of 1 mol %. The results are compared with other data published previously and with literature studies of the air/water interface. A quantitative analysis of the monolayer adsorbed from 6 mM SDS + 10 μM dodecanol showed that the monolayer comprised 63% dodecanol and 37% SDS and had a packing density comparable to that of a monolayer of pure dodecanol. The composition of the monolayer was found to be isotope dependent. This dependence might arise from a highly surface active impurity in the SDS but can also be explained quantitatively by a lattice model of the monolayer within regular solution theory.

The optical dielectric constants, εe and εo, have been determined by ellipsometry for a monolayer of dodecanol on aqueous solutions of KBr. In the thin-film limit, ellipsometry yields only one independent parameter, η, which is insufficient to define uniquely the three unknowns εe, εo, and the thickness of the monolayer, d. For a monolayer of dodecanol on water, d is known independently from X-ray diffraction. Varying the concentration of KBr changes the dielectric constant of the subphase and allows the calculation of both εe and εo. The roughness of the interface and the depletion layer at the surface of the salt solution must be taken into account in the analysis of the ellipsometric data. We obtain values of εe = 2.23 and εo = 2.12 at an area per molecule of 20.9 Ų.

A two-dimensional phase transition has been studied in a mixed monolayer of sodium dodecyl sulfate (SDS) and dodecanol at the air/water interface by sum-frequency spectroscopy and ellipsometry. At low temperatures, a monolayer at the surface of a 6 mM solution containing 99.9% SDS and 0.1% dodecanol (w/w) is conformationally ordered and has a surface coverage comparable to that of a monolayer of pure dodecanol at the same temperature. At 16 °C there is a first-order transition to a phase that is less dense and more conformationally disordered. The high-temperature monolayer phase is more disordered than the corresponding liquid phase in pure dodecanol. The solid phase contains equimolar amounts of SDS and dodecanol while the liquid phase contains SDS and dodecanol in a ratio of approximately 3:2. The change in composition of the mixed monolayer can be understood qualitatively by a reduction in the interaction parameter in the monolayer at the phase transition.

Mixed monolayers of CTAB (hexadecyltrimethylammonium bromide) and tetradecane were formed at the air−water interface by addition of small amounts of tetradecane to the surface of a CTAB solution below the critical micelle concentration. Ellipsometry and sum-frequency spectroscopy were used to demonstrate that these mixed monolayers exhibit a first-order phase transition from a conformationally disordered to a conformationally ordered state as the temperature is lowered. The phase transition occurs ca. 11 °C above the bulk melting point of tetradecane.

Dynamic surface tensions have been measured for hexadecyltrimethylammonium bromide solutions in an overflowing cylinder by three independent methods. The surface tensions are derived by two noninvasive techniques, surface light scattering and ellipsometry, agreed well over the entire concentration range. Surface tensions determined by the classical Wilhelmy plate technique also agreed with the noninvasive measurements provided that the plate was completely wetted by the solution.