Ordered monolayer

The stmcture of SAMs is affected by the si2e and chemical properties of surface functionahties. Indeed, the introduction of any surface functionaUty reduces monolayer order. The impetus toward disorder may result from stericaHy demanding terminal groups, eg, —O—Si(CH2)2(C(CH2)3) (245) and —C H N Ru(NH2)5 (345,346), or from very polar surface groups, eg, OH, COOH, etc. In both cases, the disorder introduced may be significant and not confined only to the surface. 

Mountain, R.D., Hubbard, J.B., Meuse, C.W., and Simmons, V., Molecular dynamics study of partial monolayer ordering of chain molecules, 7. Phys. Chem. B, 105, 9503, 2001. 

However, upon heating to 450 K the monolayer orders to form a (6 X 6) surface structure. Adsorbed naphthalene forms a disordered layer also on the Ag(l 11) crystal face at 300 K. However, below 200 K an ordered structure appears with a unit cell (-2 0 a s) sometimes another, less stable monolayer structure is also detectable. 

Fig. 8 Model for the insertion of a poly(phenylene ethynylene) oligomer into a lipid monolayer. Ordered domains (a) are broken down into small lipid patches as the oligomer penetrates the headgroup region and tilts the lipids (b) [85]...

Another experimental characteristic of polar mesogens is the intrinsic incommensurability of their structures. Nematic phases of polar compounds often exhibit diffuse X-ray scattering corresponding to a short range smectic order. Two sets of diffuse spots centered around incommensurate wavevectors i and 2 withqiassociated with the classical monolayer order is clearly of order 2 ydl where I is the length of a molecule in its most extended configuration. The wavevector q associated with the head to tail association of the polar molecules reveals the existence of another natu-... 

FIG. 22 Effects of surfactants, close to the cmc, on monolayer order for polystyrene spheres (diameter 2.6 /u,m) at the octane/aqueous solution interface. Monolayers (a) on water, (b) on 1 mM CTAB solution, and (c) on 10 mM aqueous SDS. 

There has been much activity in the study of monolayer phases via the new optical, microscopic, and diffraction techniques described in the previous section. These experimental methods have elucidated the unit cell structure, bond orientational order and tilt in monolayer phases. Many of the condensed phases have been classified as mesophases having long-range correlational order and short-range translational order. A useful analogy between monolayer mesophases and die smectic mesophases in bulk liquid crystals aids in their characterization (see [182]). 

McConnell et al. [196] and Andelman and co-workers have predicted [197,198] an ordered array of liquid domains in the gas-liquid coexistence regime caused by the dipole moment difference between the phases. These superstructures were observed in monolayers of dipalmitoyl phosphatidylcholine monolayers [170]. 

Grazing incidence excitation of a fluorescent probe in a phospholipid monolayer can also be used to indicate order. The collective tilt of the molecules in a domain inferred from such measurements is indicative of long-range orientational order [222]. 

A monolayer undergoes a first-order reaction to give products that also form monolayers. An equation that has been used under conditions of constant total area is (t - K°°)/(ifi - t") = exp(-)kr). Discuss what special circumstances are implied if this equation holds. 

Stigter and Dill [98] studied phospholipid monolayers at the n-heptane-water interface and were able to treat the second and third virial coefficients (see Eq. XV-1) in terms of electrostatic, including dipole, interactions. At higher film pressures, Pethica and co-workers [99] observed quasi-first-order phase transitions, that is, a much flatter plateau region than shown in Fig. XV-6. 

There is a fair amount of work reported with films at the mercury-air interface. Rice and co-workers [107] used grazing incidence x-ray diffraction to determine that a crystalline stearic acid monolayer induces order in the Hg substrate. Quinone derivatives spread at the mercury-n-hexane interface form crystalline structures governed primarily by hydrogen bonding interactions [108]. 

The effect is more than just a matter of pH. As shown in Fig. XV-14, phospholipid monolayers can be expanded at low pH values by the presence of phosphotungstate ions [123], which disrupt the stmctival order in the lipid film [124]. Uranyl ions, by contrast, contract the low-pH expanded phase presumably because of a type of counterion condensation [123]. These effects caution against using these ions as stains in electron microscopy. Clearly the nature of the counterion is very important. It is dramatically so with fatty acids that form an insoluble salt with the ion here quite low concentrations (10 M) of divalent ions lead to the formation of the metal salt unless the pH is quite low. Such films are much more condensed than the fatty-acid monolayers themselves [125-127]. 

Most LB-forming amphiphiles have hydrophobic tails, leaving a very hydrophobic surface. In order to introduce polarity to the final surface, one needs to incorporate bipolar components that would not normally form LB films on their own. Berg and co-workers have partly surmounted this problem with two- and three-component mixtures of fatty acids, amines, and bipolar alcohols [175, 176]. Interestingly, the type of deposition depends on the contact angle of the substrate, and, thus, when relatively polar monolayers are formed, they are deposited as Z-type multilayers. Phase-separated LB films of hydrocarbon-fluorocarbon mixtures provide selective adsorption sites for macromolecules, due to the formation of a step site at the domain boundary [177]. 

A fundamental approach by Steele [8] treats monolayer adsorption in terms of interatomic potential functions, and includes pair and higher order interactions. Young and Crowell [11] and Honig [20] give additional details on the general subject a recent treatment is by Rybolt [21]. 

If the desorption rate is second-order, as is often the case for hydrogen on a metal surface, so that appears in Eq. XVIII-1, an equation analogous to Eq. XVIII-3 can be derived by the Redhead procedure. Derive this equation. In a particular case, H2 on Cu3Pt(III) surface, A was taken to be 1 x 10 cm /atom, the maximum desorption rate was at 225 K, 6 at the maximum was 0.5. Monolayer coverage was 4.2 x 10 atoms/cm, and = 5.5 K/sec. Calculate the desorption enthalpy (from Ref. 110). 

In order to achieve a reasonable signal strength from the nonlinear response of approximately one atomic monolayer at an interface, a laser source with high peak power is generally required. Conuuon sources include Q-switched ( 10 ns pulsewidth) and mode-locked ( 100 ps) Nd YAG lasers, and mode-locked ( 10 fs-1 ps) Ti sapphire lasers. Broadly tunable sources have traditionally been based on dye lasers. More recently, optical parametric oscillator/amplifier (OPO/OPA) systems are coming into widespread use for tunable sources of both visible and infrared radiation. 

The e (2 X 2) unit eell ean also be written as (a/2 x V2)i 45°. Here, the original unit veetors of the (1 x i) stnieture have both been stretehed by faetors V2 and then rotated by 45°. Thus, sulfiir on Ni (100) fonus an ordered half-monolayer stnieture that ean be labelled as Ni (100) -i- e (2x 2) -S or, equivalently, Ni (100) -i-(V2 X V2) i 45°-S. The e (2 x 2) notation is elearly easier to write and also easier to eonvert into a geometrieal model of the stnieture, and henee is the favoured designation. 

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