Polyhedral oligomeric silsesquioxanes: a new class of amphiphiles at the air/water interface

Insoluble films of trisilanolisobutyl-POSS and octaisobutyl-POSS at the air/water interface are investigated by means of surface pressure - area per molecule isotherm (Pi - A) and Brewster angle microscopy (BAM). Analysis of the experimental results shows the partial cage molecule, trisilanolisobutyl-POSS, is a surface-active molecule that self-assembles into uniform monolayer upon compression; but the fully condensed cage molecule, octaisobutyl-POSS, is nonamphiphilic.     

Ions at the air/water interface

In a recent review of this topic [B.C. Garett, Science 303 (2004) 1146] the emphasis was on some recent experiments, in which it was found that some anions accumulate at the air/water interface and not in the bulk, as usually happens to the cations, and on some simulations which explained those positive surface adsorption excesses. Because a large number of these experiments could be explained on the basis of some simple physical models proposed by the authors for the interaction between the ions and the air/water interface [M. Manciu, E. Ruckenstein, Adv. Colloid Interface Sci. 105 (2003) 63; Adv. Colloid Interface Sci. 112 (2004) 109; Langmuir 21 (2005) 11312], those models are reviewed in the present note, the goal being to draw attention to them.     

Induced chirality of supramolecular assemblies of some amphiphiles with beta-cyclodextrin through the interaction at the air/water interface

4-(N-Stearoylamino)-2-amino-azobenzene (AzoNH2C18) and 4-(N-stearoylamino)-azobenzene (AzoC18) have been synthesized. The inclusion complex formation of AzoNH2C18 and beta-cyclodextrin (beta-CyD) at the air/water interface was investigated and compared to that of AzoC18. It has been found that both the amphiphiles can form stable monolayer films on water surface. When the amphiphiles were spread on the aqueous solution of beta-CyD, AzoNH2C18 can form inclusion complexes with the beta-CyD molecules at the interface while AzoC18 cannot. The inclusion complex formation was confirmed by the changes in the isotherms and the circular dichroism (CD) and Fourier transform infrared (FT-IR) spectra of the transferred LS films. Atomic force microscopy (AFM) observation found morphological changes in the course of complex formation. It was suggested that the additional amino group in the azobenzene ring plays an important role in forming the inclusion complex in situ at the air/water interface.     

Nanoaggregate shapes at the air/water interface

Chiral interfaces and molecular recognition phenomena are of special interest not only for the understanding of biological recognition processes but also for the potential application in material science. Langmuir monolayers at the air-water interface have successfully been used as simple models to mimic biological phenomena. Recent experimental studies revealed that both chirality and molecular recognition processes of amphiphiles are controlling the features of the nano-aggregates at the air/water interface. The objective of experimental studies has been to gain information about the properties of mesoscopic length scale aggregates obtained on the basis of chiral discrimation effects and the formation of supramolecular entities by molecular recognition of non-surface active species dissolved in the aqueous subphase. Differences in the two-dimensional morphology and lattice structures of the nano-aggregates cannot be explained by macroscopic theories and needed information about the detailed orientation and distance dependence of the intermolecular interaction within the aggregates. First new bottom-up studies have been directed toward understanding the driving forces for the aggregation processes of monolayers. Different types of interactions have been successfully considered using semi-empirical quantum chemical methods. The possibilities of Langmuir-Blodgett (LB) patterning to be an alternative paradigm for large-area patterning with mesostructured features are discussed.     

Electronic states at the water/air interface

Using combined path integral-molecular dynamics simulation techniques, we analyze electronic solvation at the water/air interface. Superficial electrons present a considerable extent of spatial confinement, somewhat less marked but still comparable to that found in bulk. The characteristics of the interfacial polarization promote an overall structure for the solvated electron-polymer which looks flatter along the direction perpendicular to the interface. Spatial and orientational responses of different slabs in the close vicinity of the interface were also investigated. Solvent configurations obtained from the simulations have been used to analyze electronic excited states and the optical absorption spectrum of superficial electrons. Compared to bulk results, the distribution of bound electronic states at the surface presents similar characteristics, that is, a ground s-state and three, quasi-degenerate, p-like excited states. The reduction of the energy gap between the ground state and the rest of excited states leads to a approximately 0.52 eV red-shift in the position of the absorption maximum.     

Protein-lipid interactions at the air/water interface

Surface pressure measurements and external reflection FTIR spectroscopy have been used to probe protein-lipid interactions at the air/water interface. Spread monomolecular layers of stearic acid and phosphocholine were prepared and held at different compressed phase states prior to the introduction of protein to the buffered subphase. Contrasting interfacial behaviour of the proteins, albumin and lysozyme, was observed and revealed the role of both electrostatic and hydrophobic interactions in protein adsorption. The rate of adsorption of lysozyme to the air/water interface increased dramatically in the presence of stearic acid, due to strong electrostatic interactions between the negatively charged stearic acid head group and lysozyme, whose net charge at pH 7 is positive. Introduction of albumin to the subphase resulted in solubilisation of the stearic acid via the formation of an albumin-stearic acid complex and subsequent adsorption of albumin. This observation held for both human and bovine serum albumin. Protein adsorption to a PC layer held at low surface pressure revealed adsorption rates similar to adsorption to the bare air/water interface and suggested very little interaction between the protein and the lipid. For PC layers in their compressed phase state some adsorption of protein occurred after long adsorption times. Structural changes of both lysozyme and albumin were observed during adsorption, but these were dramatically reduced in the presence of a lipid layer compared to that of adsorption to the pure air/water interface.     

Phase behavior and viscoelastic properties of trisilanolcyclohexyl-POSS at the air/water interface

A trisilanol polyhedral oligomeric silsesquioxane (POSS), trisilanolcyclohexyl-POSS (TCyP), has recently been reported to undergo a series of phase transitions from traditional Langmuir monolayers to unique rodlike hydrophobic aggregates in multilayer films that are different from "collapsed" morphologies seen in other systems at the air/water interface. This paper focuses on the phase transitions and morphology of films varying in average thickness from monolayers to trilayers and the corresponding viscoelastic properties of trisilanolcyclohexyl-POSS molecules at the air/water interface by means of surface pressure-area per molecule (Pi-A) isotherms, Brewster angle microscopy (BAM), and interfacial stress rheometry (ISR) measurements. The morphology studies by BAM reveal that the TCyP monolayer can collapse into different 3D structures by homogeneous or heterogeneous nucleation mechanisms. For homogeneous nucleation, analysis by Vollhardt et al.'s nucleation and growth model reveals that TCyP collapse is consistent with instantaneous nucleation with hemispherical edge growth at Pi = 3.7 mN.m(-1). Both surface storage (Gs') and loss (Gs") moduli obtained by ISR reveal three different non-Newtonian flow regimes that correlate with phase transitions in the Pi-A isotherms: (A) A viscous liquidlike "monolayer"; (B) a "biphasic regime"between a liquidlike viscous monolayer and a more rigid trilayer; and (C) an elastic solidlike "trilayer". These observations provide interesting insights into collapse mechanisms and structures in Langmuir films.     

Polyhedral oligomeric silsesquioxane amphiphiles: isotherm and brewster angle microscopy studies of trisilanolisobutyl-POSS at the air/water interface

A trisilanol derivative of polyhedral oligomeric silsesquioxanes (POSS), trisilanolisobutyl-POSS, has recently been reported to form stable monolayers at the air/water interface. Moreover, the trisilanolisobutyl-POSS monolayer undergoes a nonequilibrium structural transition (collapse) around a surface pressure of Rho approximately 18 mN.m(-1). This paper explores the mono- and multilayer properties of POSS molecules at the air/water interface by the Wilhelmy plate technique and Brewster angle microscopy. Surface concentrations are controlled by four mechanisms: (1) compression at a constant rate, (2) stepwise compression followed by surface pressure relaxation to an "equilibrium" value, (3) successive additions of spreading solution followed by relaxation to a stable surface pressure value, and (4) hysteresis loops to test the reversibility of the structural transitions. Results show that both an increasing compression rate and a decreasing temperature lead to an increase in the surface pressure of the structural transition, which is consistent with the formation of solidlike multilayer domains during the collapse process. For the case of compression at a constant rate, small domains initially form and later aggregate to form large solid masses. Cessation of compression allows these large solid masses to relax into equilibrium ringlike structures with a lower surface pressure, Rho approximately 13 mN.m(-1). In contrast, if the film is expanded rapidly, these large solidlike domains relax into "spaghetti" like networks with a residual surface pressure that depends on the initial amount of the solidlike collapsed phase. Finally, successive addition and stepwise compression isotherm experiments lead to different and time-dependent morphologies. Understanding these surface properties of POSS molecules affords an excellent opportunity to design and study POSS/polymer blends for coating applications where POSS molecules with rigid inorganic cores, soft organic coronae, and dimensions comparable to polymeric monolayers can serve as perfectly monodisperse nanofillers.     

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.     

Cyclodextrin-based self-assembled nanotubes at the water/air interface

Native alpha-cyclodextrin (alpha-CD) is found to spontaneously form films at aqueous solution/air interfaces. Shape-response measurements to volume perturbations on drops hanging from a capillary indicate that temperature and sodium dodecyl sulfate (SDS) concentration strongly modify the viscoelastic properties of such films. By using isothermal titration calorimetry (ITC), Brewster angle microscopy (BAM), atomic force microscopy (AFM), and molecular dynamics (MD) simulations, it is shown that the films consist of self-assembled nanotubes whose building blocks are cyclodextrin dimers (alpha-CD2) and alpha-CD2-SDS1 complexes.     

Surfactant headgroup orientation at the air/water interface

We have used vibrational sum-frequency spectroscopy to provide the first measurement of the spectrum and orientation of the polar headgroup of a charged alkyl surfactant at the air/water interface. Sum-frequency spectra of sodium dodecyl sulfate (SDS) are used to arrive at all participating elements of the second-order susceptibility tensor. We use these chi(2) elements, together with calculated values of the hyperpolarizability, to determine the tilt of the S-O bond attached to the alkyl chain and the twist of the S-O-C plane. Thus, a full characterization of the orientation of the surfactant headgroup has been achieved. This is the first demonstration of the feasibility of sum-frequency measurements of sulfate modes in the 1100 cm-1 region, opening possibilities for future investigations of surfactant behavior in this spectral region at aqueous and solid interfaces.     

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.     

On the adsorption properties of surface chemically pure CHAPS at the air/water interface

CHAPS, a surface-active derivative of the steroids' basic structure of the cholic acid [3-[(3-cholamidopropyl)dimethyl-ammonio]-1-propanesulfonate] has become a very important material in biological and pharmaceutical application. Investigations of the adsorption properties of aqueous, surface-chemically pure CHAPS solutions at the air/water interface were performed using surface tension and surface potential measurements. Unlike ordinary extended-chain surfactants, the amphiphilic structure of CHAPS is prone to adopt different concentration-dependent surface states of the adsorption layer. These are well reflected in the adsorption isotherm and in the electric surface properties. They are explained by changes in the adsorbate molecule's orientation and/or conformation as a result of the latter's different surface area demand. The versatile favorable application properties of the CHAPS molecule are obviously due to its complicated molecular structure, which enables it to comply with rather different interfacial and colloidal challenges.     

The properties of a binary mixture of nonionic surfactants in water at the water/air interface

The behavior of mixed nonionic/nonionic surfactant solutions, that is, p-(1,1,3,3-tetramethylbutyl)phenoxy poly(ethylene glycol)s Triton X-100 (TX100) and Triton X-165 (TX165) have been studied by surface tension and density measurements. The obtained results of the surface tension measurements were compared with those calculated from the relations derived by Joos, Miller, and co-workers. From the comparison, it appeared that by using these two approaches the adsorption behavior of TX100 and TX165 mixtures at different mole fractions can be predicted. The negative deviation from the linear relationship between the surface tension and composition of TX100 and TX165 mixtures in the concentration range corresponding to that of the saturated monolayer at the interface, the values of the parameters of molecular interaction, the activity coefficients, as well as the excess Gibbs energy of mixed monolayer formation calculated on the basis of Rosen and Motomura approaches proved that there is synergism in the reduction of the surface tension of aqueous solutions of TX100 and TX165 mixture when saturation of the monolayer is achieved. The negative parameters of intermolecular interaction in the mixed micelle and calculations based on MT theory of Blankschtein indicate that there is also synergism in the micelle formation for TX100 and TX165 mixture. It was also found that the values of the standard Gibbs energy of adsorption and micellization for the mixture of these two surfactants, which confirm the synergetic effect, can be predicted on the basis of the proposed equations, which include the values of the mole fraction of surfactant and excess Gibbs energy TX100 and TX165 in the monolayer and micelle.     

Ozonolysis of uric acid at the air/water interface

Uric acid (UA) epoxide, peroxide, and ozonide species produced in aqueous UA microdroplets exposed to O(3)(g) are detected by online mass spectrometry within approximately 1 ms. UA conversions are independent of its initial concentration below approximately 0.1 mM and are unaffected by addition of excess H(2)O(2) or t-butanol. UA reactivity increases approximately 380 times from pH 4 to 7, which is at variance with the pH-independent rates reported for the UA + O(3)(aq) reaction in bulk water. At pH approximately 7, UA and ascorbic acid (AH(2)) microdroplets react with O(3)(g) at similar rates, although UA is approximately 40 times more reactive than AH(2) toward O(3)(aq) in bulk water. Only the UA epoxide, plus traces of UA peroxide, are formed upon mixing UA(aq) and O(3)(aq) solutions. We infer that the gas-liquid ozonolysis of UA proceeds in an interfacial aqueous medium quite distinct from bulk water. Thus, UA, a component of the pulmonary epithelial lining fluid that scavenges atmospheric O(3)(g) into less deleterious species (similar to AH(2)), is rendered inactive below pH approximately 5. The potential implications of these findings on synergistic health effects between tropospheric ozone and acidic particulates are briefly analyzed.     

Adsorption of polyprotic acid at the water/air interface

A rigorous thermodynamic treatment appropriate for surface adsorption from mixed aqueous solution of alkali and polyprotic acid was derived. Those equations were applied to mixed aqueous solution/air systems of alkali metal hydroxide and Fe^III complex with ethylenediamine- N, N, N′,N′-tetraacetate (Fe-EDTA). Surface density of each species arising from Fe-EDTA was separately evaluated, and thus, surface activity of Fe-EDTA was studied, especially its dependence on pH and how it is influenced by the counter cations. Fe-EDTA was positively adsorbed at the water/air interface at very low pHs and negatively at high pHs. The pH range of positive adsorption of Fe-EDTA with potassium ion, as a counter ion, was wider than that with sodium ion. Thus, potassium ion, a structure breaker, tended to smooth surface adsorption of Fe-EDTA at the water/air interface, whereas sodium ion, a structure maker, tended to withdraw Fe-EDTA from the interfacial region.     

Adsorption of sugar surfactants at the air/water interface

The adsorption isotherms of n-decyl-β-D-glucoside (β-C(10)G(1)) as well as various n-alkyl-β-D-maltosides (β-C(n)G(2)) with n=8, 10, 12 and 14 were determined from surface tension measurements. Based on the analysis of the adsorption isotherms, the total free energy change of adsorption was determined and a novel method was proposed to determine the maximum adsorbed amount of surfactant. It can be concluded that the driving force for adsorption first increases with increasing adsorbed amount of the sugar surfactants and then levels off in a plateau. This peculiar behaviour is interpreted as formation of a thin liquid-like alkane film of overlapping alkyl chains at the air/water interface once a certain adsorbed amount is exceeded. The driving force of adsorption depends on the alkyl chain length only and is not affected by the type of the head group. The hydrophobic contribution to the standard free energy change of adsorption was compared with the values of sodium alkylsulfate and alkyltrimethylammonium bromide surfactants. This comparison reveals that the hydrophobic driving force of adsorption is the largest for the sodium alkylsulfates, whereas it is the same for the sugar surfactants and the alkyltrimethylammonium bromides.     

Insoluble monolayers of irisresorcinol at the air/water interface

Monolayer properties of irisresorcinol [5-(cis-10-heptadecenyl) resorcinol] were measured at the air/water interface. TheA-T isobars of the monolayers at 10 and 15 mN/m gave two-dimensional thermal expansivities of 1.4 × 10^–4/K and 1.3 × 10^–4/K at a temperature span from 7–40 C, respectively. The- A isotherms of the material showed only a little dependence on temperature from 5–35 C and onpH except at highpH, where monolayers expanded by ionization of resorcinol headgroups. Some types of saccharose in the subphase exhibited a characteristic interaction with irisresorcinol in monolayers, and there is a possibility that this material will be used for molecular recognition of some saccharoses.