Hydrophobic monolayers

Surfactants greatly improve the performance of trans-cinnamaldehyde as a corrosion inhibitor for steel in HCl [741,1590,1591]. They act by enhancing the adsorption at the surface. Increased solubility or dispersibility of the inhibitor is an incidental effect. N-dodecylpyridinium bromide is effective in this aspect far below its critical micelle concentration, probably as a result of electrostatic adsorption of the monomeric form of N-dodecylpyridinium bromide. This leads to the formation of a hydrophobic monolayer, which attracts the inhibitor. On the other hand, an ethoxylated nonylphenol, a nonionic surfactant, acts by incorporating the inhibitor into micelles, which themselves adsorb on the steel surface and facilitate the adsorption of trans-cinnamaldehyde. 

This chapter aims at reviewing the main development milestones of the SFA and look back at some of the most important experimental achievements in studying monolayers at interfaces with the SFA. It is structured as follows. In Section 2 the function of the basic SFA is described. This has been excellently done by several authors before [6-9] however, we feel that it is a necessary part of the review since it helps to appreciate the development of the SFA that is reviewed in Section 3. Further sections are devoted to an overview of experimental achievements obtained using the SFA. A particular emphasis is put on monolayer studies of surfactants and lipids (Chapter 4) where intriguing behaviour of hydrophobic monolayers and forces between hydrophilic monolayes, including specific interactions, are discussed. In Chapter 5 forces generated by... 

The most desirable liquid to use as a solvent would be water, from the standpoints of surface tension, cost, toxicity, and freedom from fire hazards. However, Compounds B, C, and D are not sufficiently water soluble to permit their isolation as hydrophobic monolayers from aqueous solution (11). Nevertheless, homologous acids having lower proportions of hydrophobic to hydrophilic substituents are available which do have appropriate water solubilities. 

Corrosion inhibitors The conditions that influence the onset of corrosion are the entrainment of atmospheric oxygen, moisture from the combustion of fuel, and stop-start running coupled with temperature cycling. In the marine diesel engine, the problem is exacerbated by contamination with fortuitous saline. Corrosion inhibitors are added specifically to cope with this electrochemical process. These additives operate by creating a physical barrier, in the form of a dense hydrophobic, monolayer of chemisorbed surfactant molecules, which prevent access of the water and oxygen to the metal surface. 

Dynamic control of wettability of the surface was described by Whitesides and coworkers. They formed a hydrophobic monolayer of a short-chain alkanethiol in a small region surrounded by a longer-chain thiol. Application of an electric potential results in a selective desorption of the short-chain thiol which renders the surface hydrophilic. At neutral potentials, the thiol readsorbs on the surface, making it hydrophobic293. 

To assemble the surface-bound membrane structure, we first form a hydrophobic monolayer by using alkylsilanization to covalently attach long-chained hydrocarbon chains to hydroxyl groups in the oxide layer on the electrode surface. Such a hydrophobic surface can be thought of as one leaflet of a membrane bilayer. Alkylsilane-modified surfaces have been widely used as substrates for lipid monolayers deposited by Langmuir-Blodgett techniques. The lipids in these monolayers have mobilities like those of lipids... 

Shafrin, E. G., Zisman, W. A., "Hydrophobic Monolayers and Their Adsorption from Aqueous Solutions," p. 129, in "Monomolecular Layers," Am. Assoc. Advance. Sci., Washington, D. C., 1954. 

The development of the photoresist was made by immersion in a solution of Su-8 developer for 6 min. The height of the photoresist was uniform at 50 pm. A hydrophobic monolayer of octadecyltrichlorosilane was finally deposited onto the mold. A 10 pL droplet was deposited on a glass wafer and placed with the mold in a desiccator for 1 h. The static contact angle with water droplet was about 110°. This last step produced a highly hydrophobic surface which was necessary for the PDMS pealing without damaging the Su-8 mold. 

For instance, we have recently shown that the monolayer-anchored ferrocene groups are not complexed by hosts such as the anionic calixarene 9 and p-CD[31]. Presumably, these two receptors are strongly solvated in aqueous media so that their approach to the hydrophobic monolayer is unfavorable, thus preventing any significant binding. In contrast, an... 

Shafrin, E.G. and Zisman, W.A., Hydrophobic monolayers adsorbed from aqueous solutions, J. Colloid Sci., 4, 571, 1949. 

Shafrin, E.G. and Zisman, W.A., Hydrophobic monolayers and their adsorption from aqueous solution, in Monomolecular Layers, H. Sobotka (ed.) American Association for the Advancement of Science, Washington, D.C., 1954 Somorjai, G.A., Chemistry in Two-Dimensions Surface, Cornell University Press, Ithaca, New York, 1981. 

For MEMS operating in a fluid environment, a survey of the literature shows that the choice of surface treatment depends on the operating fluid. For operation in aqueous solutions, a hydrophilic surface is preferable to avoid strong hydrophobic interactions. A hydrophobic monolayer decreases adhesion for actuation in nonpolar fluids such as iso-octane and hexadecane. 

Based on the multiple anodization approach and hydrophobic monolayer treatment of a nanotube layer, amphiphilic nanotubes consisting of bilayer nanotube arrays - one acting as hydrophobic and other as hydrophilic - can also be obtained (Fig. 8a(iv)) such arrays were proposed as a principle for drug delivery systems. ... 

Sudan 1 dye was extracted from chilli powder and detected using Au NPs in concentration as low as 48 ng in 1 g of powder (Cheung et al. 2010). Recently, Jahn and co-workers employed enzymatically generated Ag NPs prepared by a bottom-up and self-organizing procedure (see Sect. 3.4.2.1), in combination with a lipophilic sensor layer for the detection of water-insoluble azo-dye Sudan III in real food matrix, paprika products (Jahn et al. 2015). The surface modification consisted of self-assembled aliphatic hydrocarbons with a thiol moiety forming hydrophobic monolayer on the SERS substrate. The resulting layer repelled unwanted water-soluble analytes from the surface whereas water-insoluble ones were adsorbed and detected by SERS. The riboflavin was introduced as a water-soluble competitor, which is part of the relevant food matrices. Sudan 111 dye was detected down to 9 pmol 1 concentration from paprika powder extract in the methanol. Beside this, lipophilic sensor layer can also be applied for further analytical tasks for which it is beneficial to discriminate water-soluble from insoluble substances. 

Figure 9.18 Polyelecuolyce (a) in solution and (b) adsorbed on a charged surface. Adsorption of an amphiphilic ionic surfactant, forming (c) a hydrophobic monolayer and (d) a hydrophilic double layer...

Intercalation of amphiphilic molecules or polymers into the alkenyl chains of the stabilising ligands. This approach is based on hydrophobic forces that force the lipophylic (= hydrophobic) part(s) of the amphiphilic molecule or polymer into the hydrophobic monolayer on the nanoparticles. The hydrophilic parts then make these nanoparticles water dispersible. This method has been reported using PEG-oleate [25] and derivatised PM AO [poly (maleic anhydride-afi-octadecene)] [26-29]. 

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