Effect of hydroxyl group position and system parameters on the features of hydroxystearic Acid monolayers

The characteristic features of hydroxystearic acid monolayers OH-substituted in the mid position of the alkyl chain deviate considerably from those of the usual nonsubstituted stearic acid. The phase behavior, domain morphology, and two-dimensional lattice structure of 9-, 11-, and 12-hydroxystearic acids are studied, using pi-A isotherms, Brewster angle microscopy (BAM), and grazing incidence X-ray diffraction (GIXD), to obtain detailed information on the effect of the exact position of the OH-substitution. The pi-A isotherms of all three hydroxyoctadecanoic acids have an extended flat plateau region, the extension of which only slightly decreases with the increase of temperature. At the same temperature, the extension of the plateau region increases and the plateau pressure decreases from 9-hydroxyoctadecanoic acid to 12-hydroxyoctadecanoic acid. The absolute -DeltaH and -DeltaS values for the phase transition increase slightly from 9-hydroxyoctadecanoic acid to 12- hydroxyoctadecanoic acid and indicate differences in the ordering of the condensed phase under consideration of the special reorientation mechanism of these bipolar amphiphiles at the fluid/condensed phase transition. The morphology of the condensed phase domains formed in the fluid/condensed coexistence region is specific for the position of the OH-substitution of the alkyl chain, just as the lattice structures of the condensed monolayer phase. 11-hydroxyoctadecanoic acid monolayers form centered rectangular lattices with the chain tilt toward the NNN (next nearest neighbor) direction, and 12-hydroxyoctadecanoic acid monolayers have an oblique lattice over the entire pressure range. A special feature of 9-hydroxystearic acid monolayers is the phase transition between two condensed phases observed in the pi-A isotherm of 5 degrees C at approximately 18 mN/m, where the centered rectangular lattice shows a NNN/NN transition. The morphology of the condensed phase domains formed in the fluid/condensed coexistence region, just as the lattice structures of the condensed monolayer phase, reveal the high specifity of the monolayer feature of the bipolar hydroxystearic acids OH-substituted in the mid position.     

Effect of interfacial molecular recognition of non-surface-active species on the main characteristics of monolayers

Recent progress in studies of the main characteristics of supramolecular assemblies formed by interfacial molecular recognition between an amphiphilic monolayer and a non-surface-active species, which is dissolved in the aqueous subphase, by complementary hydrogen bonding and/or electrostatic interaction at the air-water interface is reviewed. Systems consisting of an amphiphilic melamine-type monolayer and an pyrimidine derivative dissolved in the aqueous subphase are representative model systems for molecular recognition on the basis of complementary hydrogen bonding. Most of the studies have been performed with 2,4-di(n-undecylamino)-6-amino-1,3,5-triazine (2C11H23-melamine) monolayers as host component and thymine, uracil or barbituric acid as dissolved non-surface-active pyrimidine derivatives. The combination of surface pressure studies with Brewster angle microscopy (BAM) imaging and Grazing incidence X-ray diffraction (GIXD) measurements is optimal for the characterization of the change in structure and phase behavior at the interfacial recognition process. The molecular recognition of all pyrimidine derivatives dissolved in the aqueous subphase changes drastically and in a specific way the characteristic features (pi-A isotherms, morphology of the condensed phase domains) of the 2C11H23-melamine monolayer. The small condensed phase domains of the pure 2C11H23-melamine monolayer are compact without an inner texture. The monolayers of the supramolecular 2C11H23-melamine entities with thymine or uracil form specifically well-shaped condensed phase domains with an inner alkyl chain texture essentially oriented parallel to the periphery. The completely different morphology of the 2C11H23-melamine-barbituric acid monolayers is characterized by the formation of large homogeneous areas of condensed phase that transfer at smaller areas per molecule to a homogeneous condensed monolayer. The striking differences in the main characteristics between the supramolecular entities are related to their different chemical structures: complementary hydrogen bonding of two thymine or uracil molecules by one 2C11H23-melamine molecule and a linearly extended hydrogen bonding network between 2C11H23-melamine and barbituric acid. The high values of hydrogen bonding energy obtained by quantum chemical calculations on the basis of the semi-empirical PM3 method state the high stability of the supramolecular entities. The GIXD results reveal that the formation of hydrogen-bond based superstructures between the polar head groups of the amphiphilic 2C11H23-melamine monolayer and the non-surface-active pyrimidine derivatives gives rise only to quantitative changes in the two-dimensional lattice structure of the alkyl chains. The alternative possibility to construct interfacial molecular recognition systems on the basis of acid-base interaction is demonstrated by the experimental results obtained by molecular recognition of the heptadecyl-benzamidinium chloride monolayers with dissolved non-surface-active phenylacetate ions. The formation of supramolecular assemblies causes also drastical changes of the surface features in these systems. Here, the development of a substructure in the condensed phase domains consisting of long filigree strings and the favoured formation of bilayers overgrowing the strings indicates a linearly extended amidinium-carboxylate interfacial structure of the base and acid component in alternating sequence.     

Phase behavior and morphology of equimolar mixed cationic-anionic surfactant monolayers at the air/water interface: isotherm and Brewster angle microscopy analysis

The phase behavior and morphological characteristics of monolayers composed of equimolar mixed cationic-anionic surfactants at the air/water interface were investigated by measurements of surface pressure-area per alkyl chain (pi-A) and surface potential-area per alkyl chain (DeltaV-A) isotherms with Brewster angle microscope (BAM) observations. Cationic single-alkyl ammonium bromides and anionic sodium single-alkyl sulfates with alkyl chain length ranging from C(12) to C(16) were used to form mixed surfactant monolayers on the water subphase at 21 degrees C by a co-spreading approach. The results demonstrated that when the monolayers were at states with larger areas per alkyl chain during the monolayer compression process, the DeltaV-A isotherms were generally more sensitive than the pi-A isotherms to the molecular orientation variations. For the mixed monolayer components with longer alkyl chains, a close-packed monolayer with condensed monolayer characteristics resulted apparently due to the stronger dispersion interaction between the molecules. BAM images also revealed that with the increase in the alkyl chain length of the surfactants in the mixed monolayers, the condensed/collapse phase formation of the monolayers during the interface compression stage became pronounced. In addition, the variations in the condensed monolayer morphology of the equimolar mixed cationic-anionic surfactants were closely related to the alkyl chain lengths of the components.     

Monolayer characteristics of a long-chain N,O-diacyl substituted ethanolamine at the air/water interface

The N- and/or O-acylation of amphiphilic ethanolamine attracts particular attention because of its interesting biological, pharmaceutical, and medicinal properties. Tetradecanoic acid-2-[(1-oxotetradecyl)amino]ethyl ester (TAOAE) as the selected N,O-diacyl derivative of ethanolamine has been synthesized in order to obtain first information about its main interfacial characteristics, such as the surface pressure-area (π-A) isotherms, the morphology of the condensed phase domains, the lattice structure of the condensed phase, and information about the existence of interfacial hydrogen bonds (-NH···O═C-). The π-A isotherms of TAOAE, similar to those of the most usual monolayers of amphiphiles, show a sharp break point (A(c)) indicating the first-order phase transition from the fluid (liquid-expanded (LE), gaseous (G)) to the condensed (liquid-condensed (LC)) phase. On the mesoscopic scale, the dendritic domains homogeneously reflecting suggest an orientation of the alkyl chains perpendicular to the aqueous surface. The grazing incidence X-ray diffraction (GIXD) studies reveal hexagonal packing of the TAOAE molecules oriented perpendicular to the surface in an LS phase. The existence of a hydrogen-bonding network in the monolayer is supported by infrared reflection absorption spectroscopy (IRRAS) experiments.     

The role of nonsurface-active species at interfacial molecular recognition by melamine-type monolayers

The main characteristics of Langmuir monolayers are radically changed by molecular recognition of hydrogen bond nonsurface-active species. The change in the thermodynamic, phase, and structural features by molecular recognition of dissolved uracil or barbituric acid by 2,4-di(n-undecylamino)-6-amino-1,3,5-triazine (2C11H23-melamine) monolayers is characterized by combination of surface pressure studies with Brewster angle microscopy (BAM) imaging and Grazing incidence X-ray diffraction (GIXD) measurements. Phase behavior of the 2C11H23-melamine monolayer and morphology of the condensed phase domains are changed drastically, but in a specific way, by molecular recognition of uracil or barbituric acid. The main characteristics of the interfacial system can be essentially affected by the kinetics of the recognition process. Pure 2C11H23-melamine monolayers show only small compact, but nontextured domains. The monolayers of 2C11H23-melamine-uracil assemblies develop well-shaped circular condensed-phase domains having an inner texture with alkyl chains essentially oriented parallel to the periphery and having a striking tendency to two-dimensional (2D) Ostwald ripening. The 2C11H23-melamine-barbituric acid monolayers form large homogeneous areas of condensed phase that transfer at smaller areas per molecule to a homogeneous condensed monolayer. BAM imaging of corresponding assemblies with ((CH3(CH2)11O(CH2)3)2-melamine having modified alkyl chains demonstrates the specific effect of the monolayer component. GIXD results reveal that molecular recognition of pyrimidine derivatives gives rise only to quantitative changes in the two-dimensional lattice structure. The striking differences in the main characteristics between the supramolecular species are related to their different chemical structures. Quantum chemical calculations using the semiempirical PM3 method provide information about the different nature of the hydrogen-bonding-based supramolecular structures.     

Influence of temperature and alkyl chain length on phase behavior in Langmuir monolayers of some oxyethylenated nonionic surfactants

We study the surface phase behavior in Langmuir monolayers of a series of nonionic surfactants of the general formula CnE1 with n=14, 16, and 18 by film balance and Brewster angle microscopy (BAM) over a wide range of temperatures. A cusp point followed by a pronounced plateau region in the pressure-area (pi-A) isotherms indicates a first-order phase transition in the coexisting state between a lower density liquid expanded (LE) phase and a higher density liquid condensed (LC) phase at the air-water interface. The formation of bright two-dimensional (2D) LC domains in a dark background visualized by BAM further confirms this observation. In addition to the cusp point at the onset of the LE-LC coexistence state, another cusp point followed by a small plateau is observed for the C14E1 and C18E1 monolayers, indicating a second phase transition between two condensed phases of different compressibility and tilt orientation of the molecules. This unusual two-step phase transition is explained by the Ostwald step rule. The C16E1 and C18E1 monolayers show a kink in their respective isotherms, after which the surface pressure increases steeply with only a little decrease in the molecular area, suggesting that the molecules undergo a transition from a tilted to an almost vertical orientation with respect to the water surface. The thermodynamic parameters for the condensation of the molecules in the LE-LC coexistence state were calculated by employing the 2D Clapeyron equation. The temperature coefficient of the critical surface pressure dpi(c)/dT values shows a decreasing trend from C14E1 to C18E1, suggesting that the condensation process becomes less and less prone to thermal perturbation as the chain length increases. For all the amphiphiles, the DeltaH values are found to be negative, suggesting an exothermic nature of condensation. The negative DeltaS values obtained from the relation DeltaH/T probably come from the restriction on the rotational and translational motion of the molecules constrained in a confined area in the LE-LC transition region.     

Monolayer formation of alkyl chain-containing phosphoric acid amphiphiles at the air/water (pH 5.6) interface: influence of temperature and cations

In the four studied monoalkyl phosphoric acids (n-C(12)H(25)OPO(OH)(2), MDP; n-C(14)H(29)OPO(OH)(2), MTP; n-C(16)H(33)OPO(OH)(2), MHP; and n-C(18)H(37)OPO(OH)(2)MOP), only MOP can form an insoluble monolayer at the air/water interface (pH 5.6), suggesting that the longer alkyl chain (> or =C(18)) is essential for the formation of insoluble monolayers. On the contrary, all four corresponding dialkyl phosphoric acids ((n-C(12)H(25)O)(2)PO(OH), DDP; (n-C(14)H(29)O)(2)PO(OH), DTP; (n-C(16)H(33)O)(2)PO(OH), DHP; and (n-C(18)H(37)O)(2)PO(OH) DOP) can form insoluble monolayers, with only the pi-A isotherm of DDP showing a phase transition plateau at 25 degrees C. The enhancement of the subphase temperature not only increases the plateau pressure of the DDP monolayer, but also induces the emergence of a plateau for the DTP monolayer. In contrast to the weak influence of Na(+) and K(+) (1 x 10(-4) M in the subphases, pH approximately 5.6) on the pi-A isotherm of DDP, Ca(2+), Sr(2+), and Ba(2+) (1 x 10(-4) M in the subphases, pH approximately 5.6) have an evident impact on the isotherms of DDP, and the different isotherm results indicate that DDP can recognize the three divalent cations at the air/water interface. In addition, the gaseous portion and phase transition plateaus of the isotherms of some DAPs on pure water and on subphases containing Ca(2+), Sr(2+), or Ba(2+) were well simulated by Volmer's equation of state and Vollhardt's equation, except for a small difference for gas phases around critical points. The relationship between the plateau and the net molecule area is also discussed.     

Influence of an amide group in methyl octadecanoates on the monolayer stability

The influence of a hydrogen bond donor and acceptor in the hydrophobic part of an amphiphile on the monolayer stability at the air/water interface is investigated. For that purpose, the amide group is integrated into the alkyl chain. Eight methyl octadecanoates have been synthesized with the amide group in two orientations and in different positions of the alkyl chain, namely, CH3O2C(CH2)m NHCO(CH2)n CH3 (n + m = 14): 1 (m = 1), 3 (m = 2), 5 (m = 3), 7 (m = 14); and CH3O2C(CH2)m CONH(CH2)n CH3: 2 (m = 1), 4 (m = 2), 6 (m = 3), 8 (m = 14). The monolayers have been characterized by their pi/A isotherms, their temperature dependence and Brewster angle microscopy (BAM). Amphiphile 1 with the amide group close to the ester group (m = 1) behaves like an unsubstituted fatty acid ester, while 3, 5, and 7, with the amide group in an intermediate and terminal position, exhibit a two-phase region. The amphiphiles 2, 4, 6, and 8, with a reversed orientation of the amide group, all exhibit a two-phase region with higher plateau pressures and lower collapse pressures than those of 1, 3, 5, and 7. For 7 and 8, domains of the liquid condensed (LC) phase are visualized by BAM in the two-phase region. The liquid expanded (LE)/LC-phase transitions are all exothermic with enthalpies deltaH ranging from -31 to -12 kJ/mol. Comparison with other bipolar amphiphiles indicates that the LC phase is better stabilized by the hydroxy and dihydroxy groups than by the amide group. For model compounds of 1-4, optimized conformers in the LE and LC phases have been determined by density functional theory (DFT) calculations.     

Langmuir and Langmuir-Blodgett films of N-(4-octadecyloxy-2-hydroxybenzylidene) derivatives of amino acids

Langmuir and Langmuir-Blodgett monolayers of N-(4-octadecyloxy-2-hydroxybenzylidene) derivatives of glycine, tyrosine, and phenylalanine were studied using pi-A isotherms and photoelastic modulated FTIR (PEM-FTIR). Based on compression modulus and interaction parameters, mixed monolayers of these compounds with stearylamine (SAM) showed well-organized monolayers compared to mixed systems with stearic acid (SA) and stearyl alcohol (SAL). The pure amphiphiles exhibited fairly well-ordered packing in the films, and in the mixtures, the ordering increased and showed a triclinic packing arrangement. For the phenylalanine amphiphile the packing showed slight disorder compared to the other two compounds. Surface properties of the LB films of these compounds on solid substrates were analyzed using static and dynamic contact angles of a series of liquids. The surface tension of coated substrates reflected clearly the highly acidic character. Fluidlike monolayers having a molecularly rough surface indicated high wettability for n-alkanes. In contrast, the monolayer containing well-ordered, well-packed alkyl chains indicated low wettability and small hysteresis.     

Chiral discrimination of a gemini-type surfactant with rigid spacer at the air-water interface

Spontaneous separation of chiral phases was observed in the monolayers of a racemate of gemini-type twin-tailed, twin-chiral amphiphiles, (2R,3R)-(+)-bis(decyloxy)succinic acid and (2S,3S)-(-)-bis(decyloxy)succinic acid. The pressure-area isotherms of the interfacial monolayers formed at the liquid-air interface, and the 2D lattice structures studied through surface probe measurements revealed that the racemate exhibits a homochiral discrimination of the enantiomers in two dimensions. An enantiomeric excess (e,e) of 20% was sufficient to break the chiral symmetry at the air-water interface for a homochiral interaction. Langmuir monolayers on ZnCl2 and CaCl2 subphases manifested chiral discrimination with Zn2+ evidencing homochiral interaction with a chelate-type complex, whereas Ca2+ resulted in a heterochiral interaction forming an ionic-type complex. For the chiral asymmetric units, oblique and rectangular unit cells of the racemic monolayer had exclusive requirements of homo- and heterochiral recognitions for Zn2+ and Ca2+ ions, respectively. Monolayers transferred from the condensed phase at 25 mN/m onto hydrophilic Si(100) and quartz substrates revealed the formation of bilayers through transfer-induced monolayer buckling. The emergence of homochiral discrimination was explained using the effective-pair-potential (EPP) approach.     

Interactions of L-arginine with Langmuir monolayers of di-n-dodecyl hydrogen phosphate at the air-water interface

The surface phase behavior of di-n-dodecyl hydrogen phosphate (DDP) in Langmuir monolayer and its interactions with L-arginine (L-arg) have been investigated by measuring pi-A isotherms with a film balance and observing monolayer morphology with a Brewster angle microscopy (BAM). The DDP monolayers on pure water show a first-order liquid expanded-liquid condensed (LE-LC) phase transition and form fingering LC domains having uniform brightness at different temperatures. At 15 degrees C, the pi-A isotherms on pure water and on different concentration solutions of L-arg show a limiting molecular area at approximately 0.50 nm(2)/molecule. With increasing the subphase concentration of L-arg up to 4.0 x 10(-4)M, the LE and the LE-LC coexistence regions shift to larger molecular areas and higher surface pressures, respectively. With a further increase in the concentration of L-arg beyond this critical concentration, these isotherms show little or no more expansion. These results have been explained by considering the fact that the L-arg undergoes complexation with the DDP to form L-arg-DDP that remains in equilibrium with the components at the air-water interface. As the concentration of L-arg in the subphase increases, the equilibrium shifts towards the complex. At a concentration of L-arg > or =4.0 x 10(-4)M, the DDP monolayers get saturated and show the characteristics of the new amphiphile, L-arg-DDP. BAM is applied to confirm the above results. When the concentration of the L-arg is <4.0 x 10(-4)M, domains always start forming at an area of approximately 0.64 nm(2)/molecule, which is the critical molecular area for the phase transition in the DDP monolayers on pure water. In contrast, when the monolayers are formed on a solution containing > or =4.0 x 10(-4)M L-arg, comparatively smaller size domains are formed after the appearance of a new cusp point at approximately 0.55 nm(2)/molecule. With an increase in the concentration of L-arg in the subphase, the size of the domains decreases indicating that the fraction of the DDP gradually decreases, whereas the fraction of the complex gradually increases. In addition, a very simple procedure for determination of the stability constant, which is 2.6 x 10(4)M(-1) at 15 degrees C, has been suggested.     

Progress in characterization of Langmuir monolayers by consideration of compressibility

Over decades, information about the rheological properties of the condensed monolayer phases has been obtained by introduction of a two-dimensional compressibility which is defined on the basis of the surface pressure-molecular area (Pi-A) features of the monolayer. Since the last decade, fundamental progress was attained in the experimental determination of the main characteristics of Langmuir monolayers in microscopic and molecular scale. Already smallest changes in the molecular structure of the amphiphile can result in changes in the molecular arrangement in the monolayer and thus, in changes of the main characteristics of the monolayer such as, the surface pressure-area per molecule (Pi-A) isotherms, the shape and texture of the condensed phase domains and the two-dimensional lattice structure. As the classical equations of state allowed only characterisation of the fluid (gaseous, liquid-expanded) state, thermodynamically based equations of state, which consider also the aggregation of the monolayer material to the condensed phase, have been developed. The present review focuses particularly to amphiphilic monolayers, the Pi-A isotherms of which indicate the existence of two condensed phases. For this case, the experimental results of the differences in the structure features and phase properties are discussed. The generalisation of the equation of state for Langmuir monolayers developed for the case that one, two or more phase transitions in the monolayer take place, is in agreement with the experimental results that the two-dimensional compressibility of the condensed phases undergoes a jump at the phase transition, whereas the compressibility is proportional to the surface pressure within one of the condensed phases. An example is presented which explains the procedure of the theoretical analysis of Pi-A isotherms indicating the existence of two condensed phases. An element of the procedure is the application of the general principle that the behaviour of any thermodynamic system is determined by the stability condition. An interesting anisotropy of the compressibility is revealed by GIXD studies of the S-phase of octadecanol monolayers. However, similar studies performed close to the LS-S-phase transition would result in a thermodynamically impossible negative compressibility. Close to this phase transition, the compressibility cannot be determined from the positions of the maxima because the monolayer is in a disordered state attributed to elastic distortions by fluctuations with the structure of the new phase in the surrounding matrix without destroying the quasi-long-range positional order.     

Spider-web amphiphiles as artificial lipid clusters: design, synthesis, and accommodation of lipid components at the air-water interface

As a novel category of two-dimensional lipid clusters, dendrimers having an amphiphilic structure in every unit were synthesized and labeled "spider-web amphiphiles". Amphiphilic units based on a Lys-Lys-Glu tripeptide with hydrophobic tails at the C-terminal and a polar head at the N-terminal are dendrically connected through stepwise peptide coupling. This structural design allowed us to separately introduce the polar head and hydrophobic tails. Accordingly, we demonstrated the synthesis of the spider-web amphiphile series in three combinations: acetyl head/C16 chain, acetyl head/C18 chain, and ammonium head/C16 chain. All the spider-web amphiphiles were synthesized in satisfactory yields, and characterized by 1H NMR, MALDI-TOFMS, GPC, and elemental analyses. Surface pressure (pi)-molecular area (A) isotherms showed the formation of expanded monolayers except for the C18-chain amphiphile at 10 degrees C, for which the molecular area in the condensed phase is consistent with the cross-sectional area assigned for all the alkyl chains. In all the spider-web amphiphiles, the molecular areas at a given pressure in the expanded phase increased in proportion to the number of units, indicating that alkyl chains freely fill the inner space of the dendritic core. The mixing of octadecanoic acid with the spider-web amphiphiles at the air-water interface induced condensation of the molecular area. From the molecular area analysis, the inclusion of the octadecanoic acid bears a stoichiometric characteristic; i.e., the number of captured octadecanoic acids in the spider-web amphiphile roughly agrees with the number of branching points in the spider-web amphiphile.     

Phase behavior of 2,3-disubstituted methyl octadecanoate monolayers at the air-water interface

The phase behavior of 2,3-disubstituted methyl octadecanoate monolayers at the air-water interface is studied by film balance and a Brewster angle microscope (BAM). The comparison of the surface pressure-molecular area (pi-A) isotherms with the corresponding BAM images provides information on the phase behavior of the monolayers. Variations in the phase behavior of different 2,3-disubstituted methyl octadecanoate monolayers can be correlated with the size of the headgroups, the interactions between the polar molecular moieties and the subphase, and the intermolecular interactions. The enlarging of the headgroups makes forming a condensed monolayer difficult for the molecules, even after introduction of substituents giving rise to the formation of hydrogen bonds between the molecules, which may balance the steric repulsion and stabilize the monolayers. Model calculations of the two-dimensional lattice structure of the 2,3-disubstituted methyl octadecanoates on basis of the pg and p1 space group are performed and correspond well with the experimental results.     

Faceted structures in Langmuir monolayers of diethylene glycol mono-n-octadecyl ether at the air--water interface

We have concurrently studied the surface pressure (pi) versus area (A) isotherms and microscopic surface morphological features of Langmuir monolayers of diethylene glycol mono-n-octadecyl ether (C18E2) by film balance and Brewster angle microscopy (BAM) over a wide range of temperature. At temperatures < or =10 degrees C, the monolayers exist in the form of condensed phase even just after the evaporation of the spreading solvent, suggesting that the melting point of the condensed phase is above this temperature. At > or =15 degrees C, the monolayers can exist as gas (G), liquid expanded (LE), and liquid condensed (LC) phases and undergo a pressure-induced first-order phase transition between LE and LC phases showing a sharp cusp point followed by a plateau region in the pi-A isotherms. A variety of 2-D structures, depending on the subphase temperature, are observed by BAM just after the appearance of the cusp point. It is interesting to note here that the domains attain increasingly large and compact shape as the subphase temperature increases and finally give faceted structures with sharp edges and corners at > or =30 degrees C. The BAM observations were coupled with polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) to gain better understanding regarding the conformational order and subcell packing of the molecules. The constancy of the methylene stretching modes over the studied temperature range suggests that the hydrocarbon chains do not undergo any conformational changes upon compression of the monolayer. However, the full width at half-maximum (fwhm) values of the asymmetric methylene stretching mode (nu(as)(CH(2))) are found to respond differently with changes in temperature. It is concluded that even though the trans/gauche ratio of the hydrocarbon chains remains virtually constant, the LE-LC phase transition upon compression of the monolayer is accompanied by a loss of the rotational freedom of the molecules.     

Impact of the Spacer on the Condensing Effect of Fluorinated Chains Investigated by Grazing Incidence X-ray Diffraction on Ultrathin Langmuir Monolayers

Langmuir monolayers of double perfluoroalkyl(alkyl) chain amphiphiles fitted with a monomorpholinophosphate polar head, [C(n)F(2n+1)(CH(2))(m)O](2)P(O)[N(CH(2)CH(2))(2)O] (di(FnHm)MP with n = 6, 8, or 9; and m = 1 or 2), were investigated by surface pressure (π)/molecular area (A(0)) compression isotherms for temperatures ranging from 15 to 50 °C, and by grazing-incidence X-ray diffraction (GIXD) at 25 °C. Ultrathin monolayers were obtained for these short surfactants. Though the hydrocarbon spacer is short, it has a remarkable impact on the monolayer's organization. At 25 °C, whereas di(F8H2)MP monolayer presents a liquid expanded (LE)/liquid condensed (LC) transition, simply replacing one CH(2) by a CF(2) in the latter compound's structure at constant chain length, i.e. shortening the spacer from 2 to 1 CH(2) (as in di(F9H1)MP), suppresses the LE phase. At 25°, GIXD established that for both di(F8H2)MP and di(F9H1)MP, the chains form an hexagonal lattice in the LC phase. The collective tilt of the two compounds is close to zero. The lattice of the dense phase can be compressed, as assessed by the continuous linear decrease of the d spacing with increasing pressure. This indicates that the azimuthal distribution of the molecular tilts is progressively reduced upon compression. The d value for di(F9H1)MP is significantly lower than that of di(F8H2)MP, providing evidence for strong condensing effect of the fluorinated chains. Molecular areas were determined directly from the compression curves and also from the X-ray data, the latter allowing reconstruction of the compression isotherms. The calculated lattice compressibilities are ～30% and 50% of the macroscopic compressibilities for di(F9H1)MP and di(F8H2)MP, respectively. Comparison with the experimentally determined isotherms shows that the monolayer of di(F9H1)MP is more stable than that of di(F8H2)MP. The enthalpies and entropies determined for di(F9H1)MP and di(F8H2)MP, derived from the Clausius-Clapeyron equation, confirm that the observed transitions are both of the LE/LC type, although the triple point temperatures are strikingly different (27 °C vs -18 °C); this large difference further illustrates the stabilizing effect of the fluorinated chains. Disorder is hindered by the fluorinated chains and facilitated by a hydrocarbon spacer when larger than 1 CH(2).     

Brewster angle microscopy: A preferential method for mesoscopic characterization of monolayers at the air/water interface

Knowledge of the mesoscopic morphology of condensed phase domains formed after the main phase transition in the two-phase coexistence region of Langmuir monolayers progressed rapidly with the development of the highly-sensitive imaging techniques, particularly by Brewster angle microscopy (BAM). Latest developments of commercial BAM instruments have been developed to a high technical level and allow upgrading to imaging ellipsometers which combine optical microscopy and ellipsometry and make the assessment of small layered structures or patterned thin films possible. A large variety of condensed phase domains different in mesoscopic sizes and shapes as well as their textural features has been observed which depend sensitively on the chemical structure of the amphiphilic monolayer and the system conditions, such as surface pressure and temperature. This unsuspected morphological variety of condensed phase domains has been proven not only in Langmuir monolayers but also in adsorbed monolayers (Gibbs monolayers), in Langmuir monolayers penetrated by dissolved surfactants or in adequate molecular recognition systems. The inner textures of domains can be explained on the basis of their geometry and the two-dimensional lattice in dependence of the tilt angle of the alkyl chains and gave rise to the development of a geometric concept on the basis of the molecular packing. New knowledge has been gained about non-equilibrium structures and their transition kinetics into the equilibrium state. Combined results obtained recently by BAM have enhanced the understanding of molecular organization in phase diagrams and binary mixtures. Recent advances in model studies about chiral discrimination effects and of the highly specific structural changes of host-monolayers by recognition of non-surface active guest-components have made progress. Semi-empirical quantum chemical methods have been used to gain insight into the role of different types of interactions involved in the main characteristics of mesoscopic length scale aggregates of mimetic systems.     

Interfacial molecular recognition of dissolved thymine by medium chain dialkyl melamine-type monolayers

Systems consisting of an amphiphilic melamine-type monolayer and a pyrimidine derivative dissolved in the aqueous subphase are good candidates for the formation of interfacial supramolecular assemblies by molecular recognition of hydrogen-bond nonsurface-active species. In the present work, the change in the thermodynamic, phase, and structural properties as a result of molecular recognition of dissolved thymine by 2,4-di(n-undecylamino)-6-amino-1,3,5-triazine (2 C11H23-melamine) monolayers is studied. The combination of surface pressure studies with Brewster angle microscopy (BAM) imaging and grazing incidence X-ray diffraction (GIXD) measurements is optimal for the characterization of the change in structure and phase behavior at the interfacial recognition process. The molecular recognition of the nonsurface-active thymine dissolved in aqueous subphase changes drastically the characteristic features (surface pressure-area isotherms, morphology of the condensed phase domains) of the 2 C11H23-melamine monolayer. It is demonstrated that the kinetics of the recognition process affect largely the main characteristics (phase behavior, morphology of the condensed phase domains) of the interfacial system. The monolayers of 2 C11H23-melamine-thymine assemblies form dumbbell-shaped condensed phase domains not yet observed in other Langmuir monolayers so far. GIXD results show that the molecular recognition of thymine causes only quantitative changes in the two-dimensional lattice structure. Complementary hydrogen bonding of two thymine molecules by one 2 C11H23-melamine molecule is concluded from the chemical structure of both components. Additional information about the nature of the hydrogen bonding on the basis of supramolecular assemblies is obtained by using the quantum chemical PM3 approximation. Energy and lengths of the hydrogen bonds of the optimized thymine-2 C11H23-melamine-thymine structure are calculated.