Contributions to the theory of crystal growth

Contains fulltext : mmubn000001_025263803.pdf (publisher's version ) (Open Access)Promotor : P. Bennemavi, 185 p

A Biologically Plausible Mechanism for Neuronal Coding Organized by the Phase of Alpha Oscillations

Contains fulltext : 161819.pdf (publisher's version ) (Open Access)The visual system receives a wealth of sensory information of which only little is relevant for behavior. We present a mechanism in which alpha oscillations serve to prioritize different components of visual information. By way of simulated neuronal networks we show that inhibitory modulation in the alpha range (~10 Hz) can serve to temporally segment the visual information to prevent information overload. Coupled excitatory and inhibitory neurons generate a gamma rhythm in which information is segmented and sorted according to excitability in each alpha cycle. Further details are coded by distributed neuronal firing patterns within each gamma cycle. The network model produces coupling between alpha phase and gamma (40-100 Hz) amplitude in the simulated local field potential similar to that observed experimentally in human and animal recordings. This article is protected by copyright

Phase behaviour in binary mixed Langmuir-Blodgett monolayers of triglycerides

Binary mixed monolayers of the triglycerides (TAGs)-tripalmitin (PPP), tristearin (SSS) and triarachidin (AAA) at the air–water interface are investigated with the Langmuir method. Langmuir–Blodgett (LB) layers obtained by deposition on mica are investigated by Atomic Force Microscopy. Combining Langmuir and AFM results the relation between the phase behaviour of binary mixed TAGs and their chain length is established. TAG mixtures form monolayers with molecules in trident conformation at the air–water interface, like pure TAGs. The area Acond = 63 Å 2 and the pressure πcond = 8–10mN/m that separate ‘‘gas’’ and ‘‘condensed’’ film structures are the same for all mixtures and pure systems. In the π-A isotherms the sharpness of the transition from ‘‘gas’’ to ‘‘condensed’’ phase decreases with the average chain length for all systems. Using AFM data the monolayer thicknesses for mixtures and pure systems is found to be linearly dependent on the average chain length of the TAG molecules. A linear relation between film thickness and applied AFM force is established. The corresponding coefficient K is higher for mixed monolayers ( K = 0:08 ± 0:01nN-¹) than for pure systems ( K = 0:07 ± 0:01nN-¹). AFM images show phase separation in the systems PPP–SSS and PPP–AAA. The solubility of the shorter PPP molecules in the ‘‘long’’ (SSS- and AAA-rich) phase is significant. For the mixture SSS–AAA, phase separation is not observed. In that mixture the monolayer thickness varies linearly with composition, supporting the conclusion that SSS and AAA mix almost ideally. The main driving force for phase separation is the difference in the alkyl chain length. Indeed PPP–AAA (length difference 4 C atoms) shows the most clear phase separation. The relatively weak phase separation in PPP–SSS and the absence of phase separation in SSS–AAA show that the influence of chain length difference decreases with increasing average chain length. In air PPP–SSS and PPP–AAA mixed monolayers are unstable and crystals with a- and b-like structure are formed on top of the monolayer as in pure PPP and SSS systems

Structure and stability of triglyceride monolayers on water and mica surfaces

The structure and the stability of tripalmitin (PPP), tristearin (SSS), and triarachidin (AAA) monolayers at the air-water interface are investigated with the Langmuir method. The Langmuir-Blodgett (LB) layers obtained by deposition on mica were investigated with atomic force microscopy (AFM). Our experiments show that the three triglycerides can form monolayers with molecules in trident conformation at the air-water interface. We determined the equilibrium spreading pressure πeq below which such monolayers are thermodynamically stable. Under isobaric conditions, a slow compression was sometimes observed for PPP and SSS, corresponding to crystal formation with molecules in tuning fork conformation on top of the monolayer. This isobaric compression takes place at pressures significantly larger than πeq but still smaller than the collapse pressure. The isobaric compression rate was highest for PPP and almost zero for AAA. Through the use of AFM, the thickness of the trident monolayers was measured. It is 1.49 for PPP, 1.75 for SSS, and 2.2 nm for AAA, corresponding to tilt angles of the molecules of about 46, 49, and 59°, respectively. The LB monolayers of PPP and SSS are thermodynamically unstable in air. Small crystals form on top of the monolayer, presumably in β-phase for SSS. Domains with R-like and β-like structure coexist in the LB film of PPP. The nucleation rate increases with increasing surface pressure π and with decreasing chain length of the triglyceride. For AAA, no well-defined crystals were found on top of the LB monolayer during the periods of days. The trident monolayer is the less mobile, and the crystal phase is the more stable the longer the alkyl chains are