Molecular surfactant head group

Water-in-oil concentrated emulsions have also been utilised in the preparation of polymer latexes, from hydrophilic, water-soluble monomers. Kim and Ruckenstein [178] reported the preparation of polyacrylamide particles from a HIPE of aqueous acrylamide solution in a non-polar organic solvent, such as decane, stabilised by sorbitan monooleate (Span 80). The stability of the emulsion decreased when the weight fraction of acrylamide in the aqueous phase exceeded 0.2, since acrylamide is more hydrophobic than water. Another point of note is that the molecular weights obtained were lower compared to solution polymerisation of acrylamide. This was probably due to a degree of termination by chain transfer from the tertiary hydroxyl groups on the surfactant head group. 

Arnold and co-workers attempted to prepare imprinted metal-coordinating polymers for proteins [25]. For this purpose, efforts were made to prepare metalcoordinating molecularly patterned surfaces in mixed monolayers spread at the air-water interface or liposomes. This approach was termed as molecular printing and is illustrated in Fig. 6.6. In this process, a protein template is introduced into the aqueous phase, which imposes a pattern of functional amphiphiles in the surfactant monolayer via strong interactions with metal-chelating surfactant head groups. The pattern is then fixed by polymerising the surfactant tails. The technique has also been employed for two dimensional crystallisation of proteins [26]. 

Hydration and steric-protmsion forces are repulsive forces that have been found to be present at rather short separations between hydrophilic siufaces such as surfactant head-groups. At least two molecular reasons for these forces have... 

Molecular interactions between two surfactants at an interface or in micelles are frequently described through the so-called parameters, which can be obtained from surface (or interfacial) tension or from critical micellar concentration data [13]. Attractive interactions are characterized by negative values of this parameter and, specifically, it has been found that it becomes less negative as the mole fraction of the co-surfactant increases. It has also been previously observed [14] that this tendency, for different mixed surfactant systems, can be explained by the role played by the interactions of the cationic surfactants head groups in the stability of the mixed micelles. Desai and Dixit [15] have found similar variations depending on the mixtures composition of cationic and polyoxyethylenic non-ionic surfactants. 

Ionic surfactants above the CMC form full coverage aggregates on metal and carbon electrodes [4, 22, 23]. Multilayers may form at extreme potentials of the opposite sign of the surfactant head group. Developing a definitive molecular picture of the aggregate structure based only on electrochemical results is difficult. Spectroscopy of the electrode-adsorbate solution interface has helped elucidate structural features [4,14]. 

The molecular structure of the surfactant. Adsorption profiles will be different depending on the surfactant head group (anionic, nonionic, cationic, zwitterionic) and on the surfactant hydrophobic chain (aliphatic or aromatic, linear or branched, etc.). 

Figure 14 Sketch of a molecular capacitor of thickness d at a spherical interface, which can be formed either by adsorbed zwitterionic surfactant or by charged surfactant head groups and their counterions the Gibbs dividing surface of radius a is chosen to be the boundary between the aqueous...

We review here results of computer simulations of monolayers, with an emphasis on those models that include significant molecular detail to the surfactant molecule. We start with a focus on hydrocarbon chains and simple head groups (typically a COOH group in either the neutral or the ionized state) and a historical focus. A less comprehensive review follows on simulations of surfactants of other types, either nonhydrocarbon chains or different head groups. More detailed descriptions of the general simulation techniques discussed here are available in a book dedicated to simulation techniques, for example, Allen and Tildesley [338] or Frenkel and Smit [339],... 

---