Phase Transitions in Mixed Monolayers of Sodium Dodecyl Sulfate and Dodecanol at the Air/Water Interface

Impact of Fluorocarbon Gaseous Environments on the Permeability of Foam Films to Air

A foam film, free to move and stabilized with tetradecyltrimethylammonium bromide or sodium dodecylsulfate surfactants, is deposited inside of a cylindrical tube. It separates the tube into two distinct gaseous compartments. The first compartment is filled with air, while the second one contains a mixture of air and perfluorohexane vapor (C₆F₁₄), which is a barely water-soluble fluorinated compound. This foam film thus acts as a liquid semipermeable membrane for gases equivalent to the solid semipermeable membranes conventionally used in fluid separation processes. To infer the rate of air transfer through the membrane, we measure the displacement of the mobile foam film. From this, we deduce the instantaneous permeability of the membrane. In contrast to the permeability of solid membranes, which inexorably decreases over time because they become clogged, an anticlogging effect is observed with a permeability that systematically increases over time. Because the thickness of the film is constant over time, we attribute this to the possibility of adsorbing or desorbing fluorinated gas molecules on the liquid membrane. Indeed, because the partial pressure of the fluorinated gas is high at the beginning of the experiment, the density of the adsorbed molecules is also high, which leads to a low permeability to air transfer. On the contrary, at the end of the experiment, the partial pressure in fluorinated gas and thus the density of the adsorbed molecules are low. This leads to a higher permeability and a less clogged membrane.

Thinning and thickening transitions of foam film induced by 2D liquid-solid phase transitions in surfactant-alkane mixed adsorbed films

Mixed adsorbed film of cationic surfactant and linear alkane at the air-water interface shows two-dimensional phase transition from surface liquid to surface frozen states upon cooling. This surface phase transition is accompanying with the compression of electrical double layer due to the enhancement of counterion adsorption onto the adsorbed surfactant cation and therefore induces the thinning of the foam film at fixed disjoining pressures. However, by increasing the disjoining pressure, surfactant ions desorb from the surface to reduce the electric repulsion between the adsorbed films on the both sides of the foam film. As a result, the foam film stabilized by the surfactant-alkane mixed adsorbed films showed unique thickening transition on the disjoining pressure isotherm due to the back reaction to the surface liquid films. In this review, we will summarize all these features based on the previously published papers and newly obtained results.

Coming to Order: Adsorption and Structure of Nonionic Polymer at the Oil/Water Interface as Influenced by Cationic and Anionic Surfactants

Polymer-surfactant mixtures are versatile chemical systems because of their ability to form a variety of complexes both in bulk solution and at surfaces. The adsorption and structure of polymer-surfactant complexes at the oil/water interface define their use surface chemistry applications. Previous studies have investigated the interactions between charged polyelectrolytes and surfactants; however, a similar level of insight into the interfacial behavior of nonionic polymers in mixed systems is lacking. The study herein uses vibrational sum frequency (VSF) spectroscopy to elucidate the molecular details of nonionic polyacrylamide (PAM) adsorption to the oil/water interface in the presence of surfactant. The polymer's adsorption and conformational structure at the interface is investigated as it interacts with cationic and anionic surfactants. Where the polymer will not adsorb to the interface on its own in solution, the presence of either cationic or anionic surfactant causes favorable adsorption of the polymer to the oil/water interface. VSF spectra indicate that the cationic surfactant interacts with PAM at the interface through charge-dipole interactions to induce conformational ordering of the polymer backbone. However, conformational ordering of polymer is not induced at the interface when anionic surfactant is present.

Specific Ion Effects of Dodecyl Sulfate Surfactants with Alkali Ions at the Air-Water Interface

The influence of Li⁺, Na⁺ and Cs⁺ cations on the surface excess and structure of dodecyl sulfate (DS⁻) anions at the air–water interface was investigated with the vibrational sum-frequency generation (SFG) and surface tensiometry. Particularly, we have addressed the change in amplitude and frequency of the symmetric S-O stretching vibrations as a function of electrolyte and DS⁻ concentration in the presence of Li⁺, Na⁺ and Cs⁺ cations. For the Li⁺ and Na⁺ ions, we show that the resonance frequency is shifted noticeably from 1055 cm⁻¹ to 1063 cm⁻¹ as a function of the surfactants’ surfaces excess, which we attribute to the vibrational Stark effect within the static electric field at the air–water interface. For Cs⁺ ions the resonance frequency is independent of the surfactant concentration with the S-O stretching band centered at 1063 cm⁻¹. This frequency is identical to the frequency at the maximum surface excess when Li⁺ and Na⁺ ions are present and points to the ion pair formation between the sulfate headgroup and Cs⁺ counterions, which reduces the local electric field. In addition, SFG experiments of the O-H stretching bands of interfacial H₂O molecules are used in order to calculate the apparent double layer potential and the degree of dissociation between the surfactant head group and the investigated cations. The latter was found to be 12.0%, 10.4% and 7.7% for lithium dodecyl sulfate (LiDS), sodium dodecyl sulfate (SDS) and cesium dodecyl sulfate (CsDS) surfactants, which is in agreement with Collins ‘rule of matching water affinities’.

New Evidence of Head-to-Tail Complex Formation of SDS-DOH Mixtures Adsorbed at the Air-Water Interface as Revealed by Vibrational Sum Frequency Generation Spectroscopy and Isotope Labelling

Details about the molecular structures of surfactant mixtures adsorbed at the air-water interface have been controversial. Using sum frequency generation vibrational spectroscopy (SFG) and isotope labeling, we show here for the first time that mixtures of dodecanol (DOH) and sodium dodecyl sulfate (SDS) adsorb at the air-water interface with the formation of a head-to-tail complex. We observed this complex formation to occur first in the aqueous subphase, followed by complex adsorption onto the interface. This new piece of evidence for the head-to-tail complex conformation contradicts the conjectured tail-to-tail adsorption of the surfactant mixtures. The SFG data also show the dominating adsorption of the SDS-DOH complex over the single molecules of SDS and DOH at the air-water interface. The interfacial DOH-to-SDS molecular ratio of approximately 2.2:1 at a DOH-to-SDS bulk concentration ratio of 10 μM/2 mM was determined by isotope labeling of the surfactants. In addition to a smaller number of gauche defects, the DOH-SDS complex was found to adopt a higher level of orderliness than the adsorbed single surfactants. These findings provide important insights into the descriptions and interpretation of DOH-SDS adsorption at the air-water interface and its properties.

Sodium dodecyl sulfate adsorption onto positively charged surfaces: monolayer formation with opposing headgroup orientations

The adsorption and structure of sodium dodecyl sulfate (SDS) layers onto positively charged films have been monitored in situ with vibrational sum-frequency-generation (SFG) spectroscopy and surface plasmon resonance (SPR) sensing. Substrates with different charge densities and polarities used in these studies include CaF2 at different pH values as well as allylamine and heptylamine films deposited onto CaF2 and Au substrates by radio frequency glow discharge deposition. The SDS films were adsorbed from aqueous solutions ranging in concentration from 0.067 to 20 mM. In general the SFG spectra exhibited well resolved CH and OH peaks. However, at SDS concentrations between 1 and 8 mM the SFG CH and OH intensities decreased close to background levels. Combined data sets from molecular conformation, orientation, and order sensitive SFG with mass sensitive SPR suggest that the observed changes in SFG intensities above 0.2 mM are related to structural arrangements in the SDS layer. A model is proposed where the SFG intensity minimum between 1 and 8 mM is associated with a monolayer containing two headgroup orientations, one pointing toward the substrate and one pointing toward the solution phase. The SFG peaks observed at concentrations below 0.2 mM are dominated by the presence of adsorbed contaminants such as fatty alcohols (e.g., dodecanol), which are more surface active than SDS. As SDS solution concentration is increased above 1 mM SDS molecules are incorporated in the surface layer, with dodecanol continuing to be present in the surface layer for solution concentrations up to at least the critical micelle concentration.

Infrared studies of the potential controlled adsorption of sodium dodecyl sulfate at the Au(111) electrode surface

Quantitative subtractively normalized interfacial Fourier transform infrared reflection spectroscopy (SNIFTIRS) was used to determine the conformation and orientation of sodium dodecyl sulfate (SDS) molecules adsorbed at the single crystal Au(111) surface. The SDS molecules form a hemimicellar/hemicylindrical (phase I) structure for the range of potentials between -200 ≤ E < 450 mV and condensed (phase II) film for electrode potentials ≥500 mV vs Ag/AgCl. The SNIFTIRS measurements indicate that the alkyl chains within the two adsorbed states of SDS film are in the liquid-crystalline state rather than the gel state. However, the sulfate headgroup is in an oriented state in phase I and is disordered in phase II. The newly acquired SNIFTIR spectroscopy measurements were coupled with previous electrochemical, atomic force microscopy, and neutron reflectivity data to improve the current existing models of the SDS film adsorbed on the Au(111) surface. The IR data support the existence of a hemicylindrical film for SDS molecules adsorbed at the Au(111) surface in phase I and suggest that the structure of the condensed film in phase II can be more accurately modeled by a disordered bilayer.

Freezing transitions in mixed surfactant/alkane monolayers at the air-solution interface

Mixed monolayers at the air-water interface of the cationic surfactant, hexadecyltrimethylammonium bromide (CTAB), with alkanes show a first-order freezing transition as a function of temperature. Sum-frequency spectra and ellipsometric measurements are consistent with a structure in which the high-temperature phase is liquid-like and the low-temperature phase has all-, upright chains. There are strong structural similarities between the low-temperature phase in the mixed monolayers and frozen monolayers at the alkane-air and alkane-CTAB solution interfaces. The difference between the surface freezing point and the freezing point of the bulk alkane ranges from 1 °C for alkane chain length = 17, to 28 °C for = 11. Surface freezing is more favourable in mixed monolayers at the air-water interface than at the bulk alkane-water interface for the same surfactant concentration. Long-chain alkanes do not wet water, but it is postulated that if they did, they would also show surface freezing analogously to alkanes on silica. The surfactant plays the dual role of enhancing wetting and surface freezing of the alkane on water.

Computer Studies on the Effects of Long Chain Alcohols on Sodium Dodecyl Sulfate (SDS) Molecules in SDS/Dodecanol and SDS/Hexadecanol Monolayers at the Air/Water Interface

Molecular dynamics simulations of sodium dodecyl sulfate (SDS)/dodecanol and SDS/hexadecanol monolayers at the air/water interface were investigated where the monolayer mixtures were prepared by two different configurations. In the first configuration, all of the dodecanol (or hexadecanol) molecules were placed together and also the SDS molecules were placed together in the surface area. In the second configuration, the dodecanol (or hexadecanol) molecules were uniformly distributed with the SDS molecules, forming a homogeneous mixture. The results showed that the alcohol tails are more ordered and thicker than the SDS tails in monolayers where the alcohol molecules are close to each other and separated from the SDS. However, the reverse trend is observed in monolayers where the SDS and alcohol molecules are well mixed; that is, the alcohol tails seem to have less order. Studies of how the SDS tails are affected by the presence of long chain alcohols are also discussed. Basically, by increasing the alcohol chain length, the order and the thickness of the SDS tails increased when those molecules were placed all together in a region of the surface area. When both surfactants were well mixed, the order and thickness of the SDS chains decreased as the alcohol chain length increased. Comparisons of the present results with actual experiments of similar systems were performed, and they showed similar tendencies.

Adsorption and phase transition of alkanol and fluoroalkanol at electrified mercury/aqueous solution interface

The adsorption behavior and the phase transition of alkanol and fluoroalkanol at the electrified mercury/aqueous solution interface were investigated by the interfacial tension measurements and the thermodynamic analysis. In the alkanol system, it is found that the phase transitions in low interfacial densities occur: the ones from the zero adsorption to the gaseous or the expanded state and the gaseous to the expanded state at the electrified interface depending on the electrostatic nature as well as the concentration in the bulk phase. These phase transitions were verified by the thermodynamic equations derived by the assumption of coexistence of two phases at the electrified interface. Furthermore the distribution of ionic species in the interfacial region is discussed on the basis of dependence of the interfacial charge density of solution phase on an applied potential. Fluoroalkanol, on the other hand, was practically not adsorbed at the electrified interface within this experimental condition. The zero adsorption of fluoroalkanol molecules suggests the driving force of the adsorption may be the interaction hydrophobic group of alcohol molecule and mercury.

Hydration State of Nonionic Surfactant Monolayers at the Liquid/Vapor Interface: Structure Determination by Vibrational Sum Frequency Spectroscopy

The OH stretching region of water molecules in the vicinity of nonionic surfactant monolayers has been investigated using vibrational sum frequency spectroscopy (VSFS) under the polarization combinations ssp, ppp, and sps. The surface sensitivity of the VSFS technique has allowed targeting the few water molecules present at the surface with a net orientation and, in particular, the hydration shell around alcohol, sugar, and poly(ethylene oxide) headgroups. Dramatic differences in the hydration shell of the uncharged headgroups were observed, both in comparison to each another and in comparison to the pure water surface. The water molecules around the rigid glucoside and maltoside sugar rings were found to form strong hydrogen bonds, similar to those observed in tetrahedrally coordinated water in ice. In the case of the poly(ethylene oxide) surfactant monolayer a significant ordering of both strongly and weakly hydrogen bonded water was observed. Moreover, a band common to all the surfactants studied, clearly detected at relatively high frequencies in the polarization combinations ppp and sps, was assigned to water species located in proximity to the surfactant hydrocarbon tail phase, with both hydrogen atoms free from hydrogen bonds. An orientational analysis provided additional information on the water species responsible for this band.

Adsorption and micellar properties of a mixed system of nonionic–nonionic surfactants

We study the surface adsorption and bulk micellization of a mixed system of two nonionic surfactants, namely, ethylene glycol mono-n-dodecyl ether (C12E1) and tetraethylene glycol mono-n-tetradecyl ether (C14E4), at different mixing ratios at 15 degrees C. The pure C14E4 monolayer cannot show any indicative features of phase transition because of both hydration-induced and dipolar repulsive interactions between the bulky head groups. On the other hand, the monolayers of pure C12E1 and its mixture with C14E4 undergo a first-order phase transition, showing a variety of surface patterns in the coexistence region between the liquid expanded (LE) and liquid condensed (LC) phases under the same experimental conditions. For pure C12E1, the domains are of a fingering pattern while those for the C12E1/C14E4 mixed system are found to be compact circular and small irregular structures at 2:1 and 1:1 molar ratios, respectively. The critical micelle concentration (cmc) values of both the pure and the mixed systems were measured to understand the micellar behavior of the surfactants in the mixture. The cmc values of the mixed system were also calculated assuming ideal behavior of the surfactants in the mixture. The experimental and calculated values are found to be very close to each other, suggesting an almost ideal nature of mixing. The interaction parameters for mixed monolayer and micelle formation were calculated to understand the mutual behavior of the surfactants in the mixture. It is observed that the interaction parameters for mixed monolayer formation are more negative than those of micelle formation, indicating a stronger interaction between the surfactants during monolayer formation. It is concluded that since both the surfactants bear EO units in their head groups, structural parity and hydrogen bonding between the surfactants allow them to be closely packed during monolayer and micelle formation.

Mixtures of sodium dodecyl sulfate/dodecanol at the air/water interface by computer simulations

Molecular dynamics simulations of monolayers of surfactant mixtures at the air/water interface were performed where the binary mixture was composed of sodium dodecyl sulfate (SDS) and dodecanol molecules. At the same ratio of SDS and dodecanol molecules, two monolayer mixtures were prepared. In the first monolayer, all the dodecanol molecules were placed together in the center of the simulation box, whereas in the second monolayer, those molecules were uniformly distributed in the surface area in such a way that they were far from each other. Simulations of both systems indicate that the dodecanol tails in the first monolayer are straighter and more ordered than those in the second monolayer. From the present results, we observed new insights of how the different molecules should array or distribute at the interface in real systems. Finally, studies of the interfacial water around the different surfactants were also analyzed, showing that they are closer to the polar headgroups of dodecanol than to the SDS headgroups.

Study of the adsorption of sodium dodecyl sulfate (SDS) at the air/water interface: targeting the sulfate headgroup using vibrational sum frequency spectroscopy

The surface sensitive technique vibrational sum frequency spectroscopy (VSFS), has been used to study the adsorption behaviour of SDS to the liquid/vapour interface of aqueous solutions, specifically targeting the sulfate headgroup stretches. In the spectral region extending from 980 to 1850 cm(-1), only the vibrations due to the SO(3) group were detectable. The fitted amplitudes for the symmetric SO(3) stretch observed at 1070 cm(-1) for the polarization combinations ssp and ppp, were seen to follow the adsorption isotherm calculated from surface tension measurements. The orientation of the sulfate headgroup in the concentration range spanning from 1.0 mM to above the critical micellar concentration (c.m.c.) was observed to remain constant within experimental error, with the pseudo-C(3) axis close to the surface normal. Furthermore, the effect of increasing amounts of sodium chloride at SDS concentrations above c.m.c. was also studied, showing an increase of approximately 12% in the fitted amplitude for the symmetric SO(3) stretch when increasing the ionic strength from 0 to 300 mM NaCl. Interestingly, the orientation of the SDS headgroup was also observed to remain constant within this concentration range and identical to the case without NaCl.

Observation of a Liquid−Gas Phase Transition in Monolayers of Alkyltrimethylammonium Alkyl Sulfates Adsorbed at the Air/Water Interface

The equilibrium adsorption layers of symmetric chain alkyltrimethylammonium alkyl sulfates (Cn+.Cn- for n = 8, 12) were investigated at the air/water interface by sum-frequency vibrational spectroscopy in the function of the bulk surfactant concentration. To ensure the surface purity of the solutions investigated, an improved version of the foam fractionation method was used for the purification of the constituent ionic surfactants and the surface purity of the solutions was also checked. In the monolayer of the C12+.C12- surfactant, a two-dimensional first-order gas/liquid phase transition was observed. At surfactant bulk concentrations just exceeding the concentration corresponding to the phase transition, the monolayer is conformationally disordered, liquidlike, but with increasing bulk surfactant concentration the conformational order of the monolayer increases. The SFG spectra of the C8+.C8- monolayer did not indicate the occurrence of phase transition at room temperature.

External reflection FTIR spectroscopy of the cationic surfactant hexadecyltrimethylammonium bromide (CTAB) on an overflowing cylinder

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, Gammadyn, by neutron reflection and ellipsometry. The relationship between the component weights and Gammadyn 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.

Fluorescence Evidence That a Phase Transition Causes the Induction Time in the Reduction in Dynamic Tension during Surfactant Adsorption to a Clean Air/Water Interface and a Kinetic–Diffusive Transport Model for the Phase-Induced Induction

An aqueous soluble surfactant adsorbing from solution onto an initially clean air/water interface often exhibits an induction period in the surface tension relaxation in which, as the adsorption begins, the tension remains near the clean interface value for an extended period of time before decreasing rapidly to the equilibrium value. In this study, using a model nonionic soluble surfactant, C14E6(CH3(CH2)13-(OCH2CH2)6-OH), we present direct fluorescence evidence that this induction is due to a first-order phase transition from a gaseous (G) to a liquid expanded (LE) phase that the assembling monolayer undergoes at constant surface pressure. An open channel flow cell is initially filled with water, and onto its air/water interface is spread an insoluble amphiphilic dye that fluoresces upon irradiation in the LE phase and whose fluorescence is quenched in the G phase. An aqueous solution of C14E(6) is then allowed to flow through the channel. We observe the immediate appearance of bright islands of the LE phase growing in a dark (G) background, confirming the presence of the G/LE phase transition. These islands eventually occupy the entire surface, after which the interface remains uniformly bright. We correlate this phase transition to the induction period by simultaneously measuring the tension of the interface of the open channel, and verifying that as the islands grow the tension remains at the clean value until the bright LE phase occupies the entire surface, whereupon the tension rapidly decreases. We further develop a phase transition surfactant transport model for the induction period in which surfactant diffuses toward and kinetically adsorbs onto the surface, and then rapidly equilibrates between the G and LE phases. For our model surfactant C14E6, we independently measure the surface concentration of the nucleating LE phase, the LE phase surfactant equation of state, the kinetic rate constants for adsorption into the LE phase, and the bulk diffusion coefficient. Using these measurements, we predict induction times for adsorption onto a clean surface without convection. We also measure these induction times in tension relaxation for adsorption onto a pendant bubble using axisymmetric shape analysis, and demonstrate agreement with the simulations with no adjustable constants.

Patterning of small particles by a surfactant-enhanced Marangoni-Bénard instability

Evaporating drops provide a means of organizing particles suspended within them. Here, the manner in which surfactants alter these patterns is studied as a function of the surface state of an insoluble monolayer at the drop interface. The surface state is visualized throughout the drop evolution using fluorescence microscopy. A regime of surfactant coverage is identified that creates conditions that enhance the Marangoni-Bénard instability. This result was not anticipated in prior studies, in which surfactants are predicted to prevent this instability. These data demonstrate that, by tuning the liquid-gas boundary condition, the patterns formed from an evaporating drop can be controlled.

Modern physicochemical research on Langmuir monolayers

Recent developments in characterising Langmuir monolayers of a variety of film-forming materials and employing several physicochemical techniques are reviewed. The extension of the LB method to non-amphiphilic substances, especially macromolecular systems, has increased the need of a thorough understanding of Langmuir film properties, which requires characterising techniques that provide complementary information. Since there is vast literature in the subject, only selected examples are given of results that illustrate the potential of the techniques discussed.

Structure and bonding of molecules at aqueous surfaces

Significant advances toward understanding the structure of aqueous surfaces on a molecular level have been made in recent years. This review focuses on the recent contributions of surface vibrational sum frequency spectroscopy (VSFS) to this field of study. An overview of recent VSFS studies of the molecular structure and orientation of molecules at the vapor-water interface and the interface between water and an immiscible organic liquid is presented, with particular emphasis on studies that compare the molecular properties and adsorbate behavior at these two different but related interfaces. This discussion is preceded by a general introduction to VSFS studies at aqueous surfaces and a description of the fundamental principles underlying the technique.

Surfactant layers at the air/water interface: structure and composition

The use of neutron reflectometry to study the structure and composition of surfactant layers adsorbed at the air/water interface is reviewed. A critical assessment of the results from this new technique is made by comparing them with the information available from all other techniques capable of investigating this interface.