Dioctadecyldimethylammonium Bromide Monolayers Research Articles

Lipid exchanges between dioctadecyldimethylammonium bromide monolayer and vesicles in the subphase

AbstractStudies on the interaction of lipid monolayer with bilayer structures, such as vesicles, are relatively scarce in the literature. In order to ascertain whether these structures interact for cationic dioctadecyldimethylammonium bromide (DODAB) monolayers at the aqueous surfaces of 0, 0.05, and 0.5 mmol L−1 DODAB vesicle dispersions, differential scanning calorimetry (DSC) and surface film balance experiments were carried out. DSC results confirmed the presence of unilamellar vesicles in the subphase, while changes in the monolayer surface pressure–area per molecule (π–A) isotherm profile yielded by the presence of DODAB vesicle in the subphase revealed monolayer‐vesicle interactions as a result of monomer exchanges between the monolayer and the vesicles with stronger effects at the higher vesicle concentration investigated.

Resolving the structure of a model hydrophobic surface: DODAB monolayers on mica

The properties and interactions of hydrophobic surfaces in water are determining factors in a wide range of industrial applications, and represent a fundamental scientific problem that is far from solved. Langmuir–Blodgett (LB) lipid monolayers have often been used as model hydrophobic surfaces, but are only metastable, which compromises the interpretation of experiments. Using frequency-modulation atomic force microscopy (FM-AFM), we find that LB-deposited monolayers of dioctadecyldimethylammonium bromide (DODAB) on mica undergo two transitions upon immersion in water: (i) a rapid molecular rearrangement from a complete monolayer coverage to a more densely packed monolayer with holes exposing the mica substrate, followed by; (ii) a gradual flipping of lipids in the monolayer to form bilayers, at a timescale of many days, orders of magnitudes slower than previously reported. The (meta)stability of the monolayer shows little dependence on the deposition pressure (5–25 mN m−1), but strongly depends on the cleanliness of the preparation and, in AFM experiments, is reduced from days to minutes when the force applied by the AFM tip is not kept to well below 1 nN. When properly prepared and analysed, the DODAB/mica surface thus yields a well-defined structure of sufficient stability to study intersurface forces, albeit with a heterogeneity that gives rise to very distinct forces above the bare mica on one hand, and on the monolayer and bilayer areas on the other.

Structural organisation of cationic dioctadecyldimethylammonium bromide monolayers in presence of hyaluronic acid

Structural organisation of cationic dioctadecyldimethylammonium bromide monolayers in presence of hyaluronic acid

Investigation of the Protonation State of Novel Cationic Lipids Designed for Gene Transfection

In order to be used in versatile DNA delivery systems, novel cationic lipids were synthesized. The head groups of the new compounds represented by monoamines or oligoamines can be charged or uncharged depending on the environmental pH. Since their pK values are unknown, the protonation properties of these lipids have been studied in a wide pH range. In our experiments, the amphiphilic molecules were organized as a Langmuir monolayer at the air-water interface. Total reflection X-ray fluorescence (TRXF) was used to determine the 2D concentration of bromide counterions bound to a positively charged (protonated) Langmuir monolayer. The protonation rate of the novel cationic lipids was estimated by comparing the fluorescence intensity with that of dioctadecyldimethylammonium bromide monolayers as a reference. TRXF investigations were supplemented with results of film-balance measurements, grazing incidence X-ray diffraction, and X-ray reflectivity data. The results obtained display that the monolayers of all studied compounds are completely uncharged at pH values above 10. In the investigated pH region, the highest protonation rate of the monolayers is observed at pH 3. The influence of the monolayer packing density on the protonation properties is clearly shown.

Mesoscopic Organization of Two-Dimensional J-Aggregates of Thiacyanine in Langmuir−Schaefer Films

When dioctadecyl dimethylammonium bromide (DODAB) is compressed on a subphase containing 3,3'-disulfopropyl-5,5'-dichlorothiacyanine (THIAMS), adsorption of the dye to the DODAB monolayer results in the formation of J-aggregates which spontaneously organize into polygonal domains of micron size. The features of the domains depend on the surface pressure. The fluorescence of the individual domains is polarized. The shapes of the domains determined by fluorescence microscopy and atomic force microscopy (AFM) are identical. The domains differ however significantly from those observed after injection of a 3,3'-disulfopropyl-5,5'-dichloro-9-ethylthiacarbocyanine (THIATS) or THIAMS solution below a precompressed DODAB film, as well as from the domains observed upon compression of a DODAB monolayer on a subphase containing 10(-6) M THIATS.

Memory effects on the interfacial characteristics of dioctadecyldimethylammonium bromide monolayers at the air–water interface

The structural and dynamic characteristics of dioctadecyldimethylammonium bromide (DODAB) monolayers on a pure water subphase were investigated by surface film balance, Brewster angle microscopy, and relaxation in area and surface pressure at constant surface pressure and area, respectively. The first compression–expansion cycle of the monolayer is not reversible and the second π– A compression isotherm deviates to larger molecular areas relative to the first one. At a microscopic level this hysteresis may be assigned to an irreversible hydration of the ammonium groups of DODAB. The morphology and reflectivity of DODAB monolayers during compression and expansion on the monolayer depend on the monolayer history. Bright domains randomly dispersed were observed during compression before collapse. Surprisingly, this random distribution of domains changes into a fractal-like structure during the monolayer expansion in a narrow range of surface pressures. This morphology does not form when the monolayer is previously compressed above the collapse surface pressure. 2D foam-like structure is often observed when the film is expanded at maximum area. Relaxation phenomena in DODAB monolayers are attributed to monolayer reorganization and nucleation of liquid-condensed domains from the liquid-expanded phase. These time-dependent processes are irreversible.

Incorporation of β-lactoglobulin in monolayers of dioctadecyldimethylammonium bromide studied by Brewster angle microscopy

The deposition of pure bovine β-lactoglobulin (βLG) monolayer or the incorporation in a dioctadecyldimethylammonium bromide (DODAB) monolayer, were studied by π– A measurements at the air–water interface and by direct visualization of the interface by BAM. The co-spreading of the monolayer material dissolved in a mixed volatile solvent (chloroform+ethanol) was selected based on reproducible data and minimum consumption of protein. Conformational changes induced by the mixed solvent on the native structure of βLG, investigated by circular dicroism, disappear after the solvent evaporation at the interface. The π– A isotherms suggest and BAM images confirm, that, at low surface pressures, βLG is incorporated in the liquid expanded monolayer of DODAB, while at the plateau near 30 mN m −1 the protein is squeezed out into micro-domains partially immersed in the subphase and strongly adsorbed under the DODAB layer. The topography of DODAB/ yβLG mixed films changes with surface pressure and number of protein residues, y, incorporated per DODAB molecule. BAM observation shows that at low y DODAB molecules dominate the topography of monolayer at the interface.

Methods to monitor the polyelectrolyte adsorption employing monolayers of ionic amphiphiles as model surfaces

Abstract Monolayers of dioctadecyldimethylammonium bromide (DODA) at the air/water interface were used as a model for a charged surface to study the adsorption of a poly(p-phenylene sulfonate) (PPPS). Utilizing the Fromherz technique it is possible to spread the amphiphile on a pure water surface and subsequently transfer it onto a polyelectrolyte solution. The ensuing adsorption process was monitored by recording the change in the surface pressure, the change in the reflectivity at the Brewster angle, or by UV/Vis reflection spectroscopy. The first two methods give curves having induction periods, in which no change of the monolayer properties can be observed. However, when UV/Vis reflection spectroscopy is used, no induction period is detected. The induction period is explained to be a result of structural rearrangement processes at the interface due to the adsorption which are a prerequisite for the change in surface pressure and Brewster angle reflectivity.

Adsorption of anionic polyelectrolytes to dioctadecyldimethylammonium bromide monolayers

Monolayers of dioctadecyldimethylammonium bromide (DODA) at the air/water interface were used as model for charged surfaces to study the adsorption of anionic polyelectrolytes. After spreading on a pure water surface the monolayers were compressed and subsequently transferred onto a polyelectrolyte solution employing the Fromherz technique. The polyelectrolyte adsorption was monitored by recording the changes in surface pressure at constant area. For poly(styrene sulfonate) and carboxymethylcellulose the plot of the surface pressure as function of time gave curves which indicate a direct correlation between the adsorbed amount and surface pressure as well as a solely diffusion controlled process. In the case of rigid rod-like poly(p-phenylene sulfonate)s the situation is more complicated. Plotting the surface pressure as function of time results in a curve with sigmoidal shape, characterized by an induction period. The induction period can be explained by a domain formation, which can be treated like a crystallization process. Employing the Avrami expression developed for polymer crystallization, the change in the surface pressure upon adsorption of rigid rod-like poly(p-phenylene sulfonate)s can be described.

Using an effective surface charge to explain surface potentials of Langmuir monolayers from dialkyldimethylammonium halides with the Gouy–Chapman theory

The use of an effective surface charge density has allowed the Gouy–Chapman (GC) theory to explain surface potential isotherms of Langmuir monolayers of dioctadecyldimethylammonium bromide (DODAB). The effective surface charge density of DODAB monolayer increases with the electronegativity of the counterions in the subphase. The pressure–area isotherms indicate a very condensed monolayer for DODAB spread on an I −-containing subphase, which exhibits the lowest surface charge density, whereas the monolayer on a F −-containing subphase is extremely expanded owing to the high surface charge density or electrostatic repulsion between headgroups.

Adsorption of linear and star-shaped poly(acrylic acid) to model surfaces formed by amphiphiles at the air/water interface

Monolayers of dioctadecyldimethylammonium bromide (DODA) and an azobenzene containing ammonium amphiphile at the air/water interface have been used as model surfaces to study the polyelectrolyte adsorption. The influence of the polyelectrolyte architecture on the adsorption process was studied using linear and star-shaped poly(acrylic acid)s, which have been synthesized by atom transfer radical polymerization. The differences in the polyelectrolyte structure result in different structures of the polyelectrolyte-amphiphile-complex monolayer at the air/water interface. Compared to the amphiphiles on pure water the area per molecule is significantly enlarged for all polyelectrolyte complexes, the most for complex with the six-arm-star. Moreover, UV/Vis-spectroscopical measurements indicate that chromophore aggregation is supressed by the complexation with poly(acrylic acid)s. The differences in monolayer properties between the pure amphiphile monolayer and the polyelectrolyte complexes monolayer can be employed for the in situ investigation of the adsorption of the polyelectrolyte to an already compressed amphiphile monolayer. Preliminary experiments were carried out by monitoring the change of the surface pressure during adsorption over time, and significant differences between linear and star-shaped poly(acrylic acid) were found.

Effect of the spreading procedure on the formation of cationic–anionic mixed monolayers

Mixed monolayers of dioctadecyldimethylammonium bromide (DODAB) with sodium hexadecylsulfate (SHS) can be obtained by spreading both components at the interface, co-spread (1) or separately spread (2), or by incorporation of the water soluble surfactant SHS from the subphase (3). The most efficient procedure is co-spreading, when equimolar amounts are used. On the other hand, when the components are spread separately at the interface, or when SHS is adsorbed from the aqueous subphase, it is not possible to obtain an efficiency as high as that of co-spreading, even after 5 h. The equimolar composition at the interface can also be immediately obtained after solvent evaporation when SHS is added to the subphase in large excess to DODAB (9:1); alternatively, the same composition can be obtained from smaller excesses of SHS to DODAB, but only after larger time intervals. The composition of the mixed monolayers obtained by procedures 2 and 3 was estimated from the correlation compressibility–composition, obtained independently by co-spreading.

Ellipsometric model for two-dimensional phase transition in Langmuir monolayers

A microscopic ellipsometric model of dioctadecyldimethylammonium bromide monolayer compression has been proposed. The suggested model explains the dependence of the ellipsometric parameters on the surface density throughout the entire range of compression, comprising the liquid expanded (LE) and liquid condensed (LC) phases, as well as their coexistence region. The model uses unique microscopic parameters, such as molecular polarizabilities, Lorentz factors and geometric molecular dimensions. Molecules are treated as rods whose cross-section decreases linearly during the LE–LC phase transition. During compression the rods change their orientation from parallel to the interface to almost perpendicular. It was shown that the volume density of the LE phase is 10–15% lower than that of the LC phase.

Interaction of Hydrophobic Ions with a Langmuir Monolayer of Dioctadecyldimethylammonium Bromide

Interaction of a positively charged dioctadecyldimethylammonium bromide monolayer with three different single-charged hydrophobic anions, namely, hexafluorophosphate (HFP), picrate, and tetraphenylborate (TPB), was studied by surface pressure, surface potential isotherm, and ellipsometric techniques. Two models of monolayer−ion interaction were considered: adsorption to the head groups and incorporation into the monolayer. It was shown that the nature of ion−monolayer interaction depends on the hydrophobic properties of ions. The least hydrophobic HFP anion adsorbs to the head groups of the monolayer. This leads to a moderate decrease of the surface potential and more close packing of the monolayer. Ellipsometric data as well agree with the adsorption model. More hydrophobic picrate and strongly hydrophobic TPB anions incorporate into the monolayer. It reveals a drastic drop of the surface potential, which is due to the formation of an additional double electric layer inside the monolayer. Variations of ...

Counterion specific interactions in dioctadecyldimethylammonium bromide monolayers at the monolayer/subphase interface

We have investigated the influence of halogen counterions on the molecular organization characteristics of dioctadecyldimethylammonium bromide (DOMA) monolayers at the air/water interface. Surface pressure- and surface potential-area isotherms and Brewster angle microscopy were used to characterize the head group-counterion interactions. The counterions used were F −, Cl −, Br − and I −. The presence of halogen ions in the subphase leads to compression of the π-A and a decrease in the surface potential. The effects of counterions on the microscopic features of molecular organization are, however, the most dramatic. DOMA at the air/water interface shows a homogeneous and featureless morphology during the whole compression process. The presence of F − and Cl − ions in the subphase does not lead to any significant changes. However, the introduction of Br − ions results in the appearance of dendritic structures in the liquid/solid coexistence region. The I − ions have an even greater effect on the morphology, in that solid domains are formed immediately after spreading. The influence of counterions on DOMA isotherms and morphology may be rationalized in terms of the degree of hydration of the various ions.

Monolayer interfaces as templates for particle growth

Methodologies for the in situ generation of size-quantized semiconductor particulate films (SQSPFs) are described. Cadmium, zinc, lead and copper sulfides have been generated from their divalent metal ion precursors, electrostatically attached to well-compressed negatively charged monolayers by the infusion of hydrogen sulfide. Cadmium selenide has been generated at positively charged dioctadecyldimethylammonium bromide monolayers floating on an aqueous subphase which contained appropriate concentrations of sodium selenosulfate and cadmium nitrilotriacetate. SQSPFs have been characterized by absorption spectra, electron microscopy, scanning tunneling microscopy, X-ray diffraction and electrochemical measurements. Evidence is presented for the oriented growth and preparation-dependent rectification behavior of SQSPFs.

Size‐Quantized Semiconductor Particulate Films Generated in situ at Monolayers

AbstractMethodologies for the in situ generation of size‐quantized semiconductor particulate films (SQSPFs) are described. Cadmium, zinc, lead, and copper sulfides have been generated from their divalent metal ion precursors electrostatically attached to well‐compressed, negatively charged monolayers by the infusion of hydrogen sulfide. Cadmium selenide has been generated at positively charged dioctadecyldimethylammonium bromide monolayers floating on an aqueous subphase which contained appropriate concentrations of sodium selenosulfate and cadmium nitrilotriacetate. SQSPFs have been characterized by absorption spectra, electron microscopy, scanning tunneling microscopy, X‐ray diffraction, and electrochemical measurements. Evidence is presented for the oriented growth and preparation‐dependent rectification behavior of SQSPFs.

Effect of the membrane surface charge on the host-guest complex of valinomycin in a synthetic lipid monolayer at the air-water interface

The effect of the membrane surface charge on the host-quest complexation of valinomycin molecules with K[sup +] ions in synthetic lipid monolayers was studied by measuring surface pressure-molecular area isotherms. It was found that the formation of K[sup +]/valinomycin complexes took place when valinomycin was embedded in the monolayer of lipid with negatively charged head groups, such as didodecyl phosphate. On the contrary, no evidence for the complex formation was observed with valinomycin embedded into cationic lipid monolayers such as dioctadecyldimethylammonium bromide monolayer, or with a pure valinomycin monolayer without lipid. It was concluded that the use of negatively charged lipids was a prerequisite for the formation of the K[sup +]/valinomycin complex cation in the mixed monolayer assembly, thereby providing anionic sites for electroneutrality in the membrane environment. Furthermore, the result that two components, valinomycin and didodecyl phosphate, were found immiscible in the mixed monolayer assembly indicated that a possible location for the K[sup +]/valinomycin complex to be formed was at the boundary of the two immiscible micro-monolayer domains. 19 refs., 4 figs., 1 tab.

Scanning tunneling microscopic, optical, and scanning tunneling spectroscopic characterization of size-quantized cadmium selenide particulate films in situ generated at monolayer interfaces

Cadmium selenide particulate semiconductor films were in situ generated at positively charged, dioctadecyldimethylammonium bromide monolayers floating on aqueous subphase which contained appropriate concentrations of sodium selenosulfate and cadmium nitriloacetate.

Interaction of nitrophenols with lipids at the air/water interface

The uptake of ortho and para nitrophenol to charged and neutral lipid monolayers spread at the air/solution interface was studied by reflection spectroscopy. The adsorption characteristics of the two nitrophenols have been studied by measuring the surface pressure and surface potential as a function of molecular area of the different lipid monolayers in the presence of nitrophenols in the subphase. The results have been interpreted in terms of the electrostatic interaction between the negatively charged dissociated phenolate ions and the positively charged head group of dioctadecyldimethylammonium bromide monolayers.