Polymer/surfactant interactions at the air/water interface

The development of neutron reflectometry has transformed the study and understanding of polymer/surfactant mixtures at the air/water interface. A critical assessment of the results from this technique is made by comparing them with the information available from other techniques used to investigate adsorption at this interface. In the last few years, detailed information about the structure and composition of adsorbed layers has been obtained for a wide range of polymer/surfactant mixtures, including neutral polymers and synthetic and naturally occurring polyelectrolytes, with single surfactants or mixtures of surfactants. The use of neutron reflectometry together with surface tensiometry, has allowed the surface behaviour of these mixtures to be related directly to the bulk phase behaviour. We review the broad range of systems that have been studied, from neutral polymers with ionic surfactants to oppositely charged polyelectrolyte/ionic surfactant mixtures. A particular emphasis is placed upon the rich pattern of adsorption behaviour that is seen in oppositely charged polyelectrolyte/surfactant mixtures, much of which had not been reported previously. The strong surface interactions resulting from the electrostatic attractions in these systems have a very pronounced effect on both the surface tension behaviour and on adsorbed layers consisting of polymer/surfactant complexes, often giving rise to significant surface ordering.     

Cationic Gemini surfactant at the air/water interface

The surface properties and structures of a cationic Gemini surfactant with a rigid spacer, p-xylyl-bis(dimethyloctadecylammonium bromide) ([C(18)H(37)(CH(3))(2)N(+)CH(2)C(6)H(4)CH(2)N(+)(CH(3))(2)C(18)H(37)],2Br(-), abbreviated as 18-Ar-18,2Br(-1)), at the air/water interface were investigated. It is found that the surface pressure-molecular area isotherms observed at different temperatures do not exhibit a plateau region but display an unusual "kink" before collapse. The range of the corresponding minimum compressibility and maximum compressibility modulus indicates that the monolayer is in the liquid-expanded state. The monolayers were transferred onto mica and quartz plates by the Langmuir-Blodgett (LB) technique. The structures of monolayers at various surface pressures were studied by atomic force microscopy (AFM) and UV-vis spectroscopy, respectively. AFM measurements show that at lower surface pressures, unlike the structures of complex or hybrid films formed by Gemini amphiphiles with DNA, dye, or inorganic materials or the Langmuir film formed by the nonionic Gemini surfactant, in this case network-like labyrinthine interconnected ridges are formed. The formation of the structures can be interpreted in terms of the spinodal decomposition mechanism. With the increase of the surface pressure up to 35 mN/m, surface micelles dispersed in the network-like ridges gradually appear which might be caused by both the spinodal decomposition and dewetting. The UV-vis adsorption shows that over the whole range of surface pressures, the molecules form a J-aggregate in LB films, which implies that the spacers construct a pi-pi aromatic stacking. This pi-pi interaction between spacers and the van der Waals interaction between hydrophobic chains lead to the formation of both networks and micelles. The labyrinthine interconnected ridges are formed first because of the rapid evaporation of solvent during the spreading processes; with increasing surface pressure, some of the alkyl chains reorient from tilting to vertical, forming surface micelles dispersed in the network-like ridges due to the strong interaction among film molecules.     

Cosurfactant and cosolvent effects on surfactant self-assembly in supercritical carbon dioxide

The impact of alcohol additives on the self-assembly of surfactants in supercritical carbon dioxide is investigated using lattice Monte Carlo simulations. We observe that all studied (model) alcohols reduce the critical micelle concentration. The reduction is stronger the longer the hydrocarbon chain of the alcohol, and the higher the alcohol concentration. Short-chain alcohols are found to concentrate in the surfactant layer of the aggregates, replacing surfactant molecules and leading to a strong decrease of the aggregation number and a large increase of the number of aggregates. On the other hand, only a small number of alcohol molecules with longer chain length are found in the aggregates, leading to a slight increase in the aggregation number. However, structural properties such as size and density profiles of aggregates at the same aggregation number are not influenced markedly. Consequently, short-chain alcohols act as cosurfactants, directly influencing the properties of the aggregates, while alcohols with longer hydrocarbon chains work as cosolvents, altering the properties of the solvent. However, the transition between both extremes is gradual.     

Polymerization of vinylidene fluoride with perfluoropolyether surfactants in supercritical carbon dioxide as a dispersing medium

The heterogeneous polymerization of vinylidene fluoride (VDF) was investigated at 50 °C with supercritical carbon dioxide (scCO₂) as a dispersing medium and diethylperoxidicarbonate as an initiator in the presence of different perfluoropolyether surfactants. When FLK 7004A ammonium carboxylate salts were used at a 5% (w/w) concentration with respect to VDF, with an initial pressure of 31–45 MPa and with an olefin concentration of about 5.5 mol/L, monomer conversions up to 63% were obtained, corresponding to a final solid content higher than 200 g/L, and the polymer was collected at the end of the process in the form of a white powder completely composed of microspheres. The effects of the density of the polymerization mixture, the monomer loading, and the surfactant concentration were studied. Collected experimental results suggest that Fluorolink ammonium perfluoropolyether carboxylic salts are the most effective surfactants yet tested in the dispersion polymerization of VDF in scCO₂. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2406–2418, 2006     

Specific counterion effect on the adsorbed film of cationic surfactant mixtures at the air/water interface

To investigate the counterion effects, we employed dodecyltrimethylammonium bromide (DTABr)-dodecyltrimethylammonium tetrafluoroborate (DTABF(4)) mixed aqueous solutions and measured their surface tensions, then analyzed these data in a thermodynamic treatment. The tensiometry showed that DTABF(4) was more effective in lowering the surface tension of water. The phase diagram of adsorption demonstrated that the surface was enriched with BF(4)(-) ions, but the composition of Br(-) ions in the adsorbed film was slightly enhanced compared to the ideal mixing criteria. These were explained in terms of the size and polarizability of counterions. Moreover, the distribution of counterions of the DTABr-DTABF(4) mixtures in the adsorbed film is greatly different from that of the 1-hexyl-3-methylimidazolium bromide (HMIMBr)-1-hexyl-3-methylimidazolium tetrafluoroborate (HMIMBF(4)) mixtures, where a stronger hydrogen-bonding exists between BF(4)(-) and HMIM(+) ions. These findings suggest that the adsorption of counterions in electric double layers is likely subject to two factors: the nature of counterion and their interactions with surfactant ions.     

Dispersion polymerization of styrene in supercritical carbon dioxide using monofunctional perfluoropolyether and silicone-containing fluoroacrylate stabilizers

The free radical dispersion polymerization of styrene was carried out in supercritical carbon dioxide (scCO₂) using two different stabilizers. The polymerizations are performed in the presence of poly(heptadecafluorodecyl acrylate-co-tris(trimethylsilyloxy)silyllpropyl methacrylate) p(HDFDA-co-SiMA) and a commercially available carboxylic acid-terminated perfluoropolyether (Krytox^® 157FSL) as polymerization stabilizers. Dry, fine powdered spherical polystyrene particles were produced under optimised conditions. The resulting high yield of spherical and relatively uniform micron-size polystyrene particles were formed utilizing various amounts of p(HDFDA-co-SiMA) random copolymer. However, it was observed that Krytox^® 157FSL was not a good stabilizer as p(HDFDA-co-SiMA) for the dispersion polymerization of styrene. The particle diameter was shown to be dependent on the type of the stabilizer and the weight percent of the stabilizer added to the system. The effect of varying the concentrations of stabilizers and initiator, reaction time and reaction pressure upon the polymerization yield, molar mass and morphology of polystyrene have been investigated.     

Adsorption of aromatic counterions at the surfactant/water interface: a neutron reflectivity study of hydroxybenzoate and chlorobenzoate counterions at the hexadecyl trimethylammonium surfactant/water interface

Specular neutron reflectivity has been used to investigate the adsorption of the aromatic counterions hydroxybenzoate and chlorobenzoate at the hexadecyl trimethylammonium bromide surfactant monolayer/water interface. The degree of counterion binding and the location of the counterions at the interface are shown to depend on the isomeric form of the counterion. For hydroxybenzoate, the para-substituted counterion is located within the headgroup region of the surfactant monolayer, and there is of order one counterion for every two surfactant ions. For the ortho-substituted counterion, the degree of counterion binding is higher. There is of order 0.85 counterions for each surfactant ion, and the counterion is located within the hydrophobic region of the monolayer, some 5 A from the center of the headgroup distribution. Similar results were found for the chlorobenzoate counterion, but in that case it was the para-substituted counterion that was more tightly bound and located within the hydrophobic region of the surfactant monolayer. The results for the ortho-substituted hydroxybenzoate and for the para-substituted chlorobenzoate are consistent with those previously reported for the para-tosylate. The results are discussed in the context of the ability of the specific aromatic counterion isomer to promote massive micellar growth, and the results shed light on that mechanism.     

Glass-liquid transition of carbon dioxide and its effect on water segregation

The interactions between CO(2) and D(2)O molecules have been investigated by using time-of-flight secondary ion mass spectrometry in the temperature rage 13-120 K. The monolayer of CO(2) tends to wet or intermix with the D(2)O film below 40 K and dewets the surface above 60 K. The water nanoclusters deposited on the CO(2) multilayers also start to segregate at 50-60 K and are finally incorporated in the bulk at 85-90 K, where the morphology of the film changes abruptly together with the desorption rate of the CO(2) molecules. The break at 85 K should be caused by the occurrence of the fluidized film whereas the glass-transition temperature of CO(2), as determined from the onset of translational molecular diffusion, is assigned to 50 K. This behavior may be related to the ultraviscous nature of the supercooled liquid, arising from the decoupling between the translational molecular diffusion and viscosity. The He(+) irradiation of the mixed CO(2)-D(2)O ice and the D(2)(+) irradiation of the CO(2) ice at 13 K do not yield any surface residues assignable to H(2)CO(3) and its precursors above 100 K. This result may be related to the segregation between the CO(2) and D(2)O molecules.     

Adsorption behavior of hydrophobin and hydrophobin/surfactant mixtures at the solid-solution interface

The adsorption of surface-active protein hydrophobin, HFBII, and HFBII/surfactant mixtures at the solid-solution interface has been studied by neutron reflectivity, NR. At the hydrophilic silicon surface, HFBII adsorbs reversibly in the form of a bilayer at the interface. HFBII adsorption dominates the coadsorption of HFBII with cationic and anionic surfactants hexadecyltrimethyl ammonium bromide, CTAB, and sodium dodecyl sulfate, SDS, at concentrations below the critical micellar concentration, cmc, of conventional cosurfactants. For surfactant concentrations above the cmc, HFBII/surfactant solution complex formation dominates and there is little HFBII adsorption. Above the cmc, CTAB replaces HFBII at the interface, but for SDS, there is no affinity for the anionic silicon surface hence there is no resultant adsorption. HFBII adsorbs onto a hydrophobic surface (established by an octadecyl trimethyl silane, OTS, layer on silicon) irreversibly as a monolayer, similar to what is observed at the air-water interface but with a different orientation at the interface. Below the cmc, SDS and CTAB have little impact upon the adsorbed layer of HFBII. For concentrations above the cmc, conventional surfactants (CTAB and SDS) displace most of the HFBII at the interface. For nonionic surfactant C(12)E(6), the pattern of adsorption is slightly different, and although some coadsorption at the interface takes place, C(12)E(6) has little impact on the HFBII adsorption.     

Adsorption behavior of hydrophobin and hydrophobin/surfactant mixtures at the air-water interface

The adsorption of the surface-active protein hydrophobin, HFBII, and the competitive adsorption of HFBII with the cationic, anionic, and nonionic surfactants hexadecyltrimethylammonium bromide, CTAB, sodium dodecyl sulfate, SDS, and hexaethylene monododecyl ether, C(12)E(6), has been studied using neutron reflectivity, NR. HFBII adsorbs strongly at the air-water interface to form a dense monolayer ～30 ? thick, with a mean area per molecule of ～400 ?(2) and a volume fraction of ～0.7, for concentrations greater than 0.01 g/L, and the adsorption is independent of the solution pH. In competition with the conventional surfactants CTAB, SDS, and C(12)E(6) at pH 7, the HFBII adsorption totally dominates the surface for surfactant concentrations less than the critical micellar concentration, cmc. Above the cmc of the conventional surfactants, HFBII is displaced by the surfactant (CTAB, SDS, or C(12)E(6)). For C(12)E(6) this displacement is only partial, and some HFBII remains at the surface for concentrations greater than the C(12)E(6) cmc. At low pH (pH 3) the patterns of adsorption for HFBII/SDS and HFBII/C(12)E(6) are different. At concentrations just below the surfactant cmc there is now mixed HFBII/surfactant adsorption for both SDS and C(12)E(6). For the HFBII/SDS mixture the structure of the adsorbed layer is more complex in the region immediately below the SDS cmc, resulting from the HFBII/SDS complex formation at the interface.     

Dynamics of surfactant sorption at the air/water interface: continuous-flow tensiometry

Dynamic interfacial tensiometry, gauged by axisymmetric drop shape analysis of static drops or bubbles, provides useful information on surfactant adsorption kinetics. However, the traditional pendant-drop methodology is not readily amenable to the study of desorption kinetics. Thus, the question of sorption reversibility is difficult to assess by this technique. We extend classical pendant/sessile drop dynamic tensiometry by immersing a sessile bubble in a continuously mixed optical cell. Ideal-mixed conditions are established by stirring and by constant flow through the cell. Aqueous surface-active-agent solutions are either supplied to the cell (loading) or removed from the cell by flushing with water (washout), thereby allowing study of both adsorption and desorption kinetics. Well-mixed conditions and elimination of any mass transfer resistance permit direct identification of sorption kinetic barriers to and from the external aqueous phase with time constants longer than the optical-cell residence time. The monodisperse nonionic surfactant ethoxy dodecyl alcohol (C(12)E(5)), along with cationic cetyltrimethyl ammonium bromide (CTAB) in the presence of added salt, adsorbs and desorbs instantaneously at the air/water interface. In these cases, the experimentally observed dynamic-tension curves follow the local-equilibrium model precisely for both loading and washout. Accordingly, these surfactants below their critical micelle concentrations (CMC) exhibit no detectable sorption-activation barriers on time scales of order a min. However, the sorption dynamics of dilute CTAB in the absence of electrolyte is markedly different from that in the presence of KBr. Here CTAB desorption occurs at local equilibrium, but the adsorption rate is kinetically limited, most likely due to an electrostatic barrier arising as the charged surfactant accumulates at the interface. The commercial, polydisperse nonionic surfactant ethoxy nonylphenol (NP9) loads in good agreement with local-equilibrium theory but shows deviation from the theoretical washout curve, presumably due to slow desorption of solubilized but otherwise water insoluble components. The polymeric nonionic triblock copolymer Pluronic exhibits almost complete irreversible adsorption at the air/water interface over a molecular-weight range from 3 to 14 kDa. Similar irreversible dynamic behavior is observed for adsorption/desorption of the protein bovine serum albumin (BSA) from dilute aqueous solutions at the air/water interface. The new continuous-flow tensiometer (CFT) is a simple, yet powerful, tool to investigate sorption dynamics at fluid/fluid interfaces, especially for larger molecular weight surface-active agents that exhibit significant hindrance to desorption.     

Tailoring porous silica films through supercritical carbon dioxide processing of fluorinated surfactant templates

The tailoring of porous silica thin films synthesized using perfluoroalkylpyridinium chloride surfactants as templating agents is achieved as a function of carbon dioxide processing conditions and surfactant tail length and branching. Well-ordered films with 2D hexagonal close-packed pore structure are obtained from sol-gel synthesis using the following cationic fluorinated surfactants as templates: 1-(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-octyl)pyridinium chloride (HFOPC), 1-(3,3,4,4,5,5,6,6,7,8,8,8-dodecafluoro-7-trifluoromethyl -octyl)pyridinium chloride (HFDoMePC), and 1-(3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluoro-decyl)pyridinium chloride (HFDePC). Processing the sol-gel film with CO2 (69-172 bar, 25 and 45 degrees C) immediately after coating results in significant increases in pore diameter relative to the unprocessed thin films (increasing from 20% to 80% depending on surfactant template and processing conditions). Pore expansion increases with CO2 processing pressure, surfactant tail length, and surfactant branching. The varying degree of CO2 induced expansion is attributed to the solvation of the "CO2-philic" fluorinated tail and is interpreted from interfacial behavior of HFOPC, HFDoMePC, and HFDePC at the CO2-water interface.     

The interaction between G-quadruplex-forming oligonucleotide and cationic surfactant monolayer at the air/water interface

We report on the interactions between a 21-mer quadruplex-forming oligonucleotide bearing human telomere sequence of dG(3)(T(2)AG(3))(3) (G4 DNA) and a positively charged dioctadecyldimethylammonium bromide (DODAB) monolayer at the air-aqueous interface, studied by surface film balance measurements. In the presence of G4 DNA, the π-A isotherm of the cationic Langmuir film shifted to lower molecular areas when compared with the reference isotherm recorded on the subphase containing only 50 mM triethylamine-acetate (TEAA) buffer. The presence of quadruplex-stabilizing metal cations (K(+) or Na(+)) further affected profiles of π-A isotherms. Further insight into processes related to the G4 DNA-monolayer interactions was provided by recording time profiles of the surface pressure of monolayer at a constant mean molecular area. In these experiments G4 DNA and/or metal ions were sequentially injected under the monolayer surface. Results indicated that multistranded assemblies of G4 DNA were formed at the monolayer interface even in the absence of metal ions, which suggested that the charged cationic surface of Langmuir monolayer induced aggregation of guanine-rich DNA strands. The presence of sodium and potassium ions inhibited formation of multi-stranded assemblies through the competitive G-quadruplex formation but to different extent that might be related to the differences in stability and topology of both quadruplexes.     

Dynamic adsorption of weakly interacting polymer/surfactant mixtures at the air/water interface

The dynamic adsorption of polymer/surfactant mixtures containing poly(ethylene oxide) (PEO) with either tetradecyltrimethylammonium bromide (C(14)TAB) or sodium dodecyl sulfate (SDS) has been studied at the expanding air/water interface created by an overflowing cylinder, which has a surface age of 0.1-1 s. The composition of the adsorption layer is obtained by a new approach that co-models data obtained from ellipsometry and only one isotopic contrast from neutron reflectometry (NR) without the need for any deuterated polymer. The precision and accuracy of the polymer surface excess obtained matches the levels achieved from NR measurements of different isotopic contrasts involving deuterated polymer, and requires much less neutron beamtime. The PEO concentration was fixed at 100 ppm and the electrolyte concentration at 0.1 M while the surfactant concentration was varied over three orders of magnitude. For both systems, at low bulk surfactant concentrations, adsorption of the polymer is diffusion-controlled while surfactant adsorption is under mixed kinetic/diffusion control. Adsorption of PEO is inhibited once the surfactant coverage exceeds 2 μmol m(-2). For PEO/C(14)TAB, polymer adsorption drops abruptly to zero over a narrow range of surfactant concentration. For PEO/SDS, inhibition of polymer adsorption is much more gradual, and a small amount remains adsorbed even at bulk surfactant concentrations above the cmc. The difference in behavior of the two mixtures is ascribed to favorable interactions between the PEO and SDS in the bulk solution and at the surface.     

Surface dilational rheology of mixed beta-lactoglobulin/surfactant layers at the air/water interface

The general theoretical model by Garrett and Joos proposed in 1976 for the estimation of the dilational elasticity of mixed surfactant solutions, and also the theoretical model proposed by Joos for the limiting elasticity of such mixtures, demonstrate quite satisfactory agreement with experimental results obtained from the oscillating bubble shape method for mixtures of a nonionic surfactant and a protein, that is, beta-lactoglobuline and decyl dimethyl phosphine oxide, C10DMPO.     

Adsorption behavior of oxyethylated surfactants at the air/water interface

The low-shear viscosity eta(0) of colloidal suspensions of acrylic latex or silica in aqueous gelatin has been measured at a temperature above the sol-gel transition. Measurements were made on dilution of a concentrated suspension with water or a gelatin solution. Thus, either the gelatin : colloid ratio was maintained or it was varied at constant aqueous gelatin concentration. Systems were studied with four lime-processed gelatins of different molecular weights at two concentrations of added salt. In addition, the latex particle size and the thickness of the adsorbed gelatin layer were measured by photon correlation spectroscopy (PCS) under dilute conditions. The dependence of the low-shear viscosity eta(0) on particle concentration was exponential and did not follow the well-established Krieger-Dougherty model for simple hard-sphere suspensions over the concentration range studied. A simple phenomenological model, eta(0)=eta(o)10(phi(e)/phi(s)), was found to predict the behavior well. Here, eta(o) is the viscosity of a gelatin solution of the corresponding solution concentration, phi(e) is proportional to the volume fraction of the particles, and phi(s) is a scaling factor, which was determined to have a value of 0.85. With this value of phi(s), the dimensions determined from PCS could be used to predict the viscosity values.     

New insight into the effect of interface supercapacitance on the performance of titanium dioxide/carbon nanowire array for photoelectrochemical water oxidation

The electrode/electrlyte interface is of great signifance to photoelectrochemical (PEC) water oxidation as the reaction mainly occur here. Herein, we focus on the effect of supercapactance of the electrode/electrlyte interface on the performance of PEC. It is discovered that the supercapacitor on the interface is crucial because it links the charge transport and solution ion adsorption on its two sides. In this study, we demonstrate an approach to promote the performance of TiO₂ nanowire array (TiO₂ NWs) photoanode in photoelectrochemical cells (PECs) by increasing its supercapacitance. A 2−5 nm carbon layer was coated and the interface supercapacitance increases by about 150 times. This enhances the separation rate of electron-hole pairs by collecting more holes. Meanwhile, it also promotes the water oxidation rate by adsorbing more OH⁻ on its surface. As a result, the photocurrent density of C-TiO₂ NWs was about 8 times higher than that of its carbon-free counterpart. This approach of increasing the supercapacitance of photoanodes would be attractive for enhancement of the efficiency of PECs and this work demonstrate the importance of supercapacitance of the interface for PECs.     

Dynamics of protein and mixed protein/surfactant adsorption layers at the water/fluid interface

The adsorption behaviour of proteins and systems mixed with surfactants of different nature is described. In the absence of surfactants the proteins mainly adsorb in a diffusion controlled manner. Due to lack of quantitative models the experimental results are discussed partly qualitatively. There are different types of interaction between proteins and surfactant molecules. These interactions lead to protein/surfactant complexes the surface activity and conformation of which are different from those of the pure protein. Complexes formed with ionic surfactants via electrostatic interaction have usually a higher surface activity, which becomes evident from the more than additive surface pressure increase. The presence of only small amounts of ionic surfactants can significantly modify the structure of adsorbed proteins. With increasing amounts of ionic surfactants, however, an opposite effect is reached as due to hydrophobic interaction and the complexes become less surface active and can be displaced from the interface due to competitive adsorption. In the presence of non-ionic surfactants the adsorption layer is mainly formed by competitive adsorption between the compounds and the only interaction is of hydrophobic nature. Such complexes are typically less surface active than the pure protein. From a certain surfactant concentration of the interface is covered almost exclusively by the non-ionic surfactant. Mixed layers of proteins and lipids formed by penetration at the water/air or by competitive adsorption at the water/chloroform interface are formed such that at a certain pressure the components start to separate. Using Brewster angle microscopy in penetration experiments of proteins into lipid monolayers this interfacial separation can be visualised. A brief comparison of the protein adsorption at the water/air and water/n-tetradecane shows that the adsorbed amount at the water/oil interface is much stronger and the change in interfacial tension much larger than at the water/air interface. Also some experimental data on the dilational elasticity of proteins at both interfaces measured by a transient relaxation technique are discussed on the basis of the derived thermodynamic model. As a fast developing field of application the use of surface tensiometry and rheometry of mixed protein/surfactant mixed layers is demonstrated as a new tool in the diagnostics of various diseases and for monitoring the progress of therapies.     

Dynamics of mixed protein–surfactant layers adsorbed at the water/air and water/oil interface

Drop and bubble shape tensiometry experiments are performed at the water/air and water/hexane interfaces in order to get more information about the differences in the adsorption layer structure of mixed protein/surfactant systems. For mixtures of β-lactoglobulin and sodium dodecyl sulphate the adsorption at the water/air interface is essentially a competitive process between protein/surfactant complexes and free surfactant molecules, while the water/oil interface is essentially covered by the complexes.     

Anomalous behavior of amine oxide surfactants at the air/water interface

A commonly stated requirement for the preparation of stable Langmuir monolayers of amphiphilic molecules at an air/water interface is that the surfactant must be insoluble in the subphase solution; however, a few prior studies have reported that some soluble surfactants can, under certain conditions, be compressed. The anomalous compression of soluble amphiphiles is extremely interesting and important, as it presents the possibility of greatly increasing the number of candidate compounds suitable for Langmuir monolayer studies and Langmuir-Blodgett deposition. The aim of this work was to obtain a better understanding of the factors that determine whether monolayers of a given water-soluble surfactant can be compressed. A series of amine oxide surfactants, including a novel gemini surfactant, were studied to explore the relationship between molecular structure and behavior at the air/water interface. Amine oxides are an especially interesting class of surfactants because their self-assembly in solution and at interfaces is pH-sensitive. Surface pressure-area isotherms show that the solubility of a surfactant in the subphase solution is not, in and of itself, a useful parameter in predicting whether the monolayer is compressible. Molecular modeling calculations suggest that the tendency of molecules to self-assemble plays a much more important role than solubility in this regard. The effect of pH was also investigated. We present a hypothesis that formation of dimers or small clusters of molecules at the interface inhibits the dissolution of these species into the subphase, and as a consequence the monolayer can be compressed.