Surface activity of amphiphiles in hydrogen fluoride ? water solutions

Hydrofluoric acid flow etching of low-loss subwavelength-diameter biconical fiber tapers

An etch method based on surface tension driven flows of hydrofluoric acid microdroplets for the fabrication of low-loss, subwavelength-diameter biconical fiber tapers is presented. Tapers with losses less than 0.1 dB/mm are demonstrated, corresponding to an order of magnitude increase in the optical transmission over previous acid-etch techniques. The etch method produces adiabatic taper transitions with minimal surface corrugations. A biconical fiber taper fabricated using this method is used to demonstrate an erbium doped silica microsphere laser.

Surface Activity of n-Alkyltrimethylammonium Bromides in Concentrated Orthophosphoric Acid

The aggregation and interface behavior of alkyltrimethylammonium bromides (CnH2n+1(CH3)3N+Br-: CnTMA with n = 14, 16, 18) have been investigated in concentrated orthophosphoric acid by means of surface tension measurements. Adsorption of these cationic surfactants at the interface between air and aqueous H3PO4 solutions has been determined at 298 K using the Wilehlmy plate method. Measurements in the most concentrated H3PO4-H2O mixture (95% by weight of acid or 17.5 mol/L) were done at the n-decane/H3PO4 interface using the ring method to avoid any contamination with atmospheric moisture. The critical micelle concentration (CMC), the maximum surface concentration, and surface pressure of C16TMA in water-H3PO4 mixtures and of the homologous series of surfactants, CnTMA, in 95%-H3PO4, were determined at 323 K. Benzene has been used as a test solute for solubilization measurements in 95%-H3PO4 containing C16 TMA micelles. From this experimental work, it can be concluded that micelles are forming even in the most concentrated solutions in acid and that the CMC displays a large depression between 4 and 10 mol/L of H3PO4. The maximum surface concentration increases by steps in the lowest (0 to 1 mol/L) and highest range of concentration (8 to 10 mol/L) in acid. The Klevens equation relating the CMC in relation to the chain length of the surfactants holds in 95%-H3PO4. Values obtained for the parameters of the Klevens equation and for the free energy of micellization in 95%-H3PO4 have been compared to other strongly acidic media like concentrated sulfuric or methanesulfonic acids. Copyright 1998 Academic Press.