Surface force apparatus monolayer

Yamada S and Israelachvili J N 1998 Friction and adhesion hysteresis of fluorocarbon surfactant monolayer-coated surfaces measured with the surface forces apparatus J. Rhys. Chem. B 102 234-44... 

Section 4.1 briefly describes some of the commonly employed experimental tools and procedures. Chaudhury et al., Israelachvili et al. and Tirrell et al. employed contact mechanics based approach to estimate surface energies of different self-assembled monolayers and polymers. In these studies, the results of these measurements were compared to the results of contact angle measurements. These measurements are reviewed in Section 4.2. The JKR type measurements are discussed in Section 4.2.1, and the measurements done using the surface forces apparatus (SFA) are reviewed in Section 4.2.2. 

The LB monolayers of dimethyldioctyadecylammonium ions on molecularly smooth muscovite mica surfaces were investigated. Direct measurements of the interaction between such surfaces were carried out using the surface force apparatus. A long-range attractive force considerably stronger than the expected van der Waals force was measured. Studies on the electrolyte dependence of this force indicate that it does not have an electrostatic origin but that the water molecules were involved in this. 

Between hydrophobic surfaces a completely different interaction is observed. Hydrophobic surfaces attract each other [184], This attraction is called hydrophobic interaction. The first direct evidence that the interaction between solid hydrophobic surfaces is stronger than the van der Waals attraction was provided by Pashley and Israelachvili [185,186], With the surface force apparatus they observed an exponentially decaying attractive force between two mica surfaces with an adsorbed monolayer of the cationic surfactant cetyltrimethylammonium bromide (CTAB). Since then the hydrophobic force has been investigated by different groups and its existence is now generally accepted [189]. The origin of the hydrophobic force is, however, still under debate. 

The three major new atomic-scale experimental methods developed in the last decade are the quartz crystal microbalance (QCM) [2 4], atomic and friction force microscopes (AFM/FFM) [5,6], and the surface force apparatus (SEA) [7,7a,8]. These new tools reveal complementary information about tribology at the nanometer scale. The QCM measures dissipation as an adsorbed him of submonolayer to several monolayer thickness slides over a substrate. AFM and FFM explore the interactions between a surface and a tip whose radius of curvature is 10 100 nm [9]. The number of atoms in the contact ranges from a few to a few thousand. Larger radii of curvature and contacts have been examined by gluing spheres to an AFM cantilever [10,11]. SEA experiments measure shear forces in even larger-diameter ( 10 pm) contacts, but with angstrom-scale control of the thickness of lubricating hlms. 

It is important to note that the lamellar phase is thus stabilized by the balance of a negative interfacial tension (of the free oil/water interface covered by an amphiphilic monolayer), which tends to increase the internal area, and a repulsive interaction between interfaces. The result, Eq. (48), indicates that the scattering intensity in a lamellar phase, with wave vector q parallel to the membranes, should have a peak at nonzero q for d > d due to the negative coefficient of the q term in the spectrum of Eq. (40). just as in the microemulsion phase. This effect should be very small for strongly swollen lamellar phases (in coexistence with excess oil and excess water), as both very small [96]. Very similar behavior has been observed in smectic liquid crystals (Helfrich-Hurault effect) [122]. Experimentally, the lamellar phase under an external tension can be studied with the surface-force apparatus [123,124] simultaneous scattering experiments have to be performed to detect the undulation modes. 

After this stunning piece of work, Winterton joined the Nuclear Laboratories of the Electricity Generating Board at Berkeley, eventually becoming a lecturer at Birmingham University, while Tabor continued with a new PhD student, Jacob Israelachvili, who continued to develop the mica surface force apparatus as it was soon to become known. His immediate contribution was to push the jump distance from 5 nm down to 1.5 nm and then to measure the effect of stearic acid monolayers on the mica surfaces. " ... 

When the nanotube is withdrawn from the monolayer, the defects remain in the monolayer for some time after the load on the nanotube has returned to zero. Similar memory effects have been observed in surface force apparatus experiments (42). 

To measure the force between lipid layers, a lipid is chosen, which at a certain concentration and temperature range forms an L phase. The L phase is a regularly spaced stack of lamellar fluid bilayers separated by water. From a symmetry point of view, the L phase can be considered a smectic-A (SmA) liquid crystal. The mean repeat distance between lipid bilayers in the L phase is measured by X-ray diffraction versus an applied osmotic pressure. Direct experiments have been carried out with the surface force apparatus. Therefore, the bilayers are formed either by spontaneous vesicle fusion [1254-1256] or by depositing two subsequent monolayers with the Langmuir-Blodgett technique [1255, 1257]. Atomic force microscope experiments, which have been carried out between two bilayers formed by spontaneous vesicle fusion, confirmed earlier results [1258]. 

Although extraction of lipids from membranes can be induced in atomic force apparatus (Leckband et al., 1994) and biomembrane force probe (Evans et al., 1991) experiments, spontaneous dissociation of a lipid from a membrane occurs very rarely because it involves an energy barrier of about 20 kcal/mol (Cevc and Marsh, 1987). However, lipids are known to be extracted from membranes by various enzymes. One such enzyme is phospholipase A2 (PLA2), which complexes with membrane surfaces, destabilizes a phospholipid, extracts it from the membrane, and catalyzes the hydrolysis reaction of the srir2-acyl chain of the lipid, producing lysophospholipids and fatty acids (Slotboom et al., 1982 Dennis, 1983 Jain et al., 1995). SMD simulations were employed to investigate the extraction of a lipid molecule from a DLPE monolayer by human synovial PLA2 (see Eig. 6b), and to compare this process to the extraction of a lipid from a lipid monolayer into the aqueous phase (Stepaniants et al., 1997). 

The reasons behind the specific choice of apparatus geometry can best be shown by a brief review of prior work. The earliest canal type surface viscometer was introduced by Dervician and Joly (8). In this apparatus, an insoluble monolayer is floated on a substrate fluid in a straight channel. The film is forced to flow through the channel by movement of a floating barrier. This motion is resisted principally by surface viscosity. Thus, the small force required to propel the film at a given speed may be measured and used to determine the surface viscosity of the film. A relatively complete theoretical treatment has been provided by Harkins and Kirkwood (5) for insoluble films with Newtonian surface viscosity in deep channels. Actual measurements are typically made in shallow channels, however, which are formed by floating the channel boundaries on the liquid surface. This method is not applicable to soluble surface films, which tend to diffuse through the substrate fluid and pass behind the barrier. Nevertheless, the most accurate values of surface viscosity available have been produced by this approach. 

The second strand of recent evidence for slip at the wall is due to Zhu and Granick [7][8]. These authors employed a force balance apparatus to study the hydrodynamic squeeze force between a pair of crossed mica cylinders lubricated by both tetradecane and water. As with the alumina stufaces used by Pit et al. above, the mica surfaces were chemically modified, either by grafting on them a lyophobic monolayer of long chain octadecyl-triethoxysiloxane (OTE) or by adding a surfactant (1-hexadecylamine) to the tetradecane solvent. Contact angle measurements showed that these treatments converted the normally fully-wetted mica surface to one only partially wetted by both test liquids. 

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