Palmitic Acid Films Research Articles

Spontaneous collapse of palmitic acid films on an alkaline buffer containing calcium ions

Understanding the interaction of ions with fatty acids is important to identify their roles in various bioprocesses and to build novel biomimetic systems. In this study, the molecular organization of palmitic acid (PA) films on alkaline buffer solutions (pH 7.4) with and without divalent Ca2+ was measured at a constant surface area using Langmuir troughs coupled with microscopy and X-ray interfacial techniques. Without Ca2+, PA molecules remained a monolayer organization; however, with Ca2+, formation of the inverted bilayers of PA-Ca2+ superstructures caused a spontaneous 2D to 3D transformation under no compression due to the strong interaction between PA and the divalent cation. Self-assembly of this highly-organized inverted bilayer superstructure involved a two-step process of nucleation and nuclei growth. During nucleation, densely packed PA and Ca2+ monolayer firstly corrugated and some of PA and Ca2+ molecules ejected out from the monolayer; the ejected molecules then reorganized and formed the inverted bilayer nuclei. Nucleation was followed by nuclei growth, during which PA and Ca2+ in the monolayer kept integrating into the inverted bilayer structure through molecule migration and PA rotation around Ca2+.

The Adsorption of Synovene on ZDDP Wear Tracks: A Sum Frequency Generation (SFG) Vibrational Spectroscopy Study

The adsorption of the lubricant additive Synovene on steel and on ZDDP/steel wear tracks from base oil has been investigated by sum frequency generation (SFG) vibrational spectroscopy, an interface specific technique. SFG spectra (resonances) were investigated in the C–H stretching region and arise from the aliphatic chains of the palm oil constituent of Synovene. The observation of SFG spectra means that Synovene is adsorbed at the oil/metal and at the oil/ZDDP/metal interfaces and that the aliphatic chains of Synovene have a net polarisation order with respect to the surface. The intense spectrum observed when the film is first formed decreases in intensity with increasing temperature. It is proposed that this is due to a decrease in film thickness as the film tends towards monolayer thickness. A dependence of the intensity and shape of SFG resonances on film thickness due to a thickness-dependent interference effect has been observed in other thin film systems, most notably lipid films on gold. Supporting evidence for the film thickness hypothesis comes from examining the spectra of different thickness films of palmitic acid on steel, one of the constituents of Synovene. The spectra on the wear track are less intense and less reproducible than on the bare metal. After periods of several days at room temperature the spectra on both surfaces gain in intensity implying a return to thicker layers of Synovene under cold conditions.

Experimental study of the heterogeneous reactivity between atomic chlorine and palmitic acid films

The present study focuses on the heterogeneous reaction between gaseous atomic chlorine and solid palmitic acid films, used as a proxy of the fatty acids detected in atmospheric airborne particles. This reaction is investigated in a coated wall flow tube reactor coupled to a molecular beam mass spectrometer. The reactive surfaces were prepared by coating the inner surface of the reactor. The initial Cl˙ and Cl2 uptake coefficient measured for these heterogeneous reactions is found to be fast: γoCl = 0.07. The rapid formation of hydrogen chloride corresponding with the disappearance of atomic chlorine is highlighted. Furthermore, the formation of new chlorinated species on the solid substrate has been detected by TOF SIMS analysis leading to an ageing process of the surface. A heterogeneous recombination of Cl atoms to Cl2 molecules was observed for aged surfaces.

A Laser Imaging and Neutron Reflection Investigation Into the Monolayer Behaviour of Fatty Acids Used for Taste Masking Microspheres

Fatty acid microspheres have been used for taste masking purposes whereby the drug is preferentially released in the lower gastrointestinal tract, although the mechanisms involved are poorly understood. In this study, we use a combination of surface pressure measurements, Brewster angle microscopy (BAM) and neutron reflectivity measurements to study the phase miscibility and escaping tendency from mixed stearic and palmitic acid films with a view to relating this to drug dissolution behaviour. It was noted that mixed systems showed considerably greater film interaction and instability than those composed of the pure lipid, especially in alkaline media. BAM studies were able to identify a range of phase separated structures for both the pure and mixed systems. Neutron reflectivity studies indicated a marked selective dissolution of palmitic acid into the subphase as a function of time and allowed quantification of the rate of dissolution of this species. It is concluded that the fatty acids are interacting within the monolayer and in addition the palmitic acid is escaping the mixed monolayers and dissolving into the alkali subphase. These findings have strong relevance for understanding the mechanism of drug release from the associated microspheres.

Study of Calcium Carbonate Precipitation under a Series of Fatty Acid Langmuir Monolayers Using Brewster Angle Microscopy

Calcium carbonate was precipitated from a supersaturated calcium bicarbonate solution in association with a series of long-chain fatty acid monolayers, and the location of the growing crystals with respect to the monolayer domain structure was determined by in situ analysis using a Brewster angle microscope (BAM). Increase in the chain length from palmitic acid (C16) to triacontanoic acid (C30) strongly affected crystallization, resulting in changes in the crystal polymorphs and morphologies, in the homogeneity of morphology, and in the distribution of crystals over the monolayer surface. Induction times also increased with longer chain lengths, an effect consistent with the decreased rate of carbon dioxide diffusion through monolayers as a function of increasing thickness. While the palmitic acid films supported the growth of high proportions of aragonite, vaterite, and nonoriented calcite, almost exclusive precipitation of an extremely uniform distribution of oriented, regular triangular calcite crystal...

Palmitic acid and aquo-ruthenium complex monolayers at the liquid–air interface and electroactive Langmuir–Blodgett films

Liquid monolayers and Langmuir–Blodgett (LB) films of palmitic acid and Ru[L(4′-topty)(H2O)] (ClO4)2 (L=(C6H5)2PCH2CH2P(C6H5)2; 4-totpy= 4′-(4-tolyl-2,2′:6′,2″-terpyridine)) have been produced. Comparison of pressure–area and surface potential–area curves for pure palmitic acid monolayers and the mixed films of palmitic acid with ruthenium complex indicates that the minimum molecular area is determined only by the fatty acid. However, the metal complex bonding increases the monolayer compressibility, transforming the condensed state to expanded at higher area per molecule. The absence of any contribution of the metal complex to the minimum molecular area for the mixed films suggests that the complex may be ion paired with the hydrophilic head group of palmitic acid molecules at the condensed state. This observation is confirmed in the FTIR spectrum of the transferred films, by the absence of CO dimer stretching at 1700cm−1 suggesting the formation of the salt in the mixed films. The electronic spectra and cyclic voltammograms show the presence of the ruthenium complex in the films.

A Spectroscopic and Epifluorescence Microscopic Study of (Hexadecanoylamino)fluorescein Aggregates at the Air−Water Interface and in Langmuir−Blodgett Films

In this paper, we report the monolayer behavior of pure (5-n-hexadecanoylamino)fluorescein (HDFL) at the air−water interface. The spectroscopic characteristics of pure HDFL films at the air−water interface and the mixed Langmuir−Blodgett (LB) films of HDFL and palmitic acid (PA) deposited on quartz substrates have been studied and compared with the spectroscopic characteristics of HDFL in solution. The absorption and emission spectra of HDFL at the air−water interface were similar to those in polar, protic solvents but completely different in aprotic, apolar solvents. Detailed spectroscopic studies indicate that in protic solvents the fluorescein moieties exist as cations and zwitterions while in nonpolar environments the fluorescein chromophore exists as an inner lactone that is nonfluorescent. Spectroscopic studies of the mixed films of HDFL and PA transferred on quartz substrates provide evidence of aggregation and formation of at least two different aggregated species, one of them being fluorescent while the other is not. The large differences in the spectral characteristics between the monolayers at the air−water interface and the transferred layers indicate the existence of HDFL moieties in different molecular configurations that are microenvironment dependent. Surface pressure dependent steady-state in situ fluorescence studies confirm that fluorescence quenching of the dye with increasing surface pressure occurs as a result of efficient energy transfer from the fluorescent monomeric species to the aggregates that are likely nonfluorescent and decay therefrom by nonradiative processes. This study establishes that fluoresceinated dyes may be utilized as efficient molecular probes for ascertaining polarity in supramolecular assemblies.

On the stability of unimolecular films. Part II.— Dissolution in alkaline solutions

In the previous communications (Parts I and Parts II) it was shown that the conditions of equilibrium of a unimolecular film palmitic acid were influenced both by the temperature and the attraction of the polar head-group for the underlying solution, which in the case of the carboxyl group could be altered by changing the acidity or alkalinity of the surface layers of the solution. On alkaline surfaces this attraction increases to such an extent that not only does an expanded film tend to contract to form a condensed film, but actual dissolution may occur. Adam noticed that condensed films of palmitic acid showed a small slow decrease in area on a solution of P H 8∙5, while on a solution of P H 10∙0 this decrease in area look place so rapidly that he could not obtain force/area curves. An attempt has been made to examine the mechanism of the-dissolution process and to investigate the relationship between the rate of solution and the alkalinity of the underlying solution. The experiments were carried out on an apparatus designed so that the pressure of the film was maintained constant as the film dissolved. As the molecules left the film for the underlying solution the gaps in the film are closed by the pressure, and the area of the film gradually decreases. The factors influencing this rate of decrease in area at constant pressure have been studied.

The properties and molecular structure of thin films. Part II.—Condensed films

In this and a following paper, an account will be given of work done in continuation of the experiments on thin films of palmitic acid, described in Part I of this investigation (1), and, in particular, of the results obtained by extending the range of substances composing the films, and the range of temperature over which the experiments were made. As was there pointed out, the study of these films is a peculiarly attractive one, owing to the extremely simple manner in which the molecules are arranged; the films being of one molecule in thickness and the molecules in them being arranged not indiscriminately, but perpendicular to the surface and parallel to each other, as a rule. The experimental method (due in principle to Langmuir) has also a great advantage over those more commonly employed in capillary studies, that the forces tangential to the surface are measured directly. Exceptionally direct information is therefore obtainable as to some of the forces between the molecules composing the films, and by a study of the influence of chemical constitution on the properties of the films, much light should be shed upon the important problem of the relation between chemical and capillary forces. It will not be necessary to occupy space in proving that, for any of the substances here studied, the films are one molecule in thickness. Reasons for taking this view have been given by other workers, as well as on p. 344 of my preceding paper, and it is sufficient to say that the experiments described here afford evidence of the same character, which is equally conclusive for each one of the substances investigated.

The properties and molecular structure of thin films of palmitic acid on water. Part I

The experiments to be described in this paper have been undertaken in order to obtain confirmation of the very interesting views put forward by Langmuir (1) upon the arrangement of the molecules of various substances spread upon the surface of water. The study of these films has been carried on by a number of workers for many years, some of the principal publications being those of Miss Pockels (2), Rayleigh (3, 4), Hardy (5), Devaux (6), and recently Labrouste (7) : but in most of these papers the authors do not enter into much detail regarding the molecular structure of the films. So much information is now available however as to the dimensions of molecules and the forces about them, much of it being of an accurate quantitative nature, derived from structural organic chemistry, from the study of crystals, the kinetic theory of gases and the deviations from the simple gas laws, etc., that an attempt to deduce the arrangement of the constituent molecules from the properties of the films has become something more than a speculation and may be made with some certainty and definiteness.