The solutions were stirred and left to stabilize overnight. NaCl (Sigma-Aldrich, > 98% pure) at different concentrations was subsequently added to the surfactant solutions to make the ternary CTAB/NaCl/water and Tween 80/NaCl/water systems. Stock solutions of CTAB (4 mmolal) and Tween 80 (0.1 % wt, or 0.15 mmolal) were prepared with deionized Millipore water and were later diluted to make smaller concentrations. The cationic surfactant CTAB (>99% pure Sigma-Aldrich) and the nonionic surfactant Tween 80 (polysorbate 80, Ameresco, proteomics grade) were chosen because of their exceptional stability in solution at high NaCl concentration as we have shown previously, (26) most surfactants are very sensitive to large amounts of salt and precipitate at high salt concentrations. Most of the results reported in the literature are limited to the small concentrations of salts (millimolar), (16в€’19) although in several applications, like in crystallization control processes, the salt concentration reaches to very high levels (several molar for NaCl) before salt precipitation. (10в€’15) However, the effect of salts on the DST has received much less attention. Salts can significantly affect the equilibrium surfactant behavior by changing the critical micellar concentrations (CMC), equilibrium surface tensions (Оі eq), aggregation number, or Krafft temperature. In many applications, salts are present or added as an extra additive to enhance the performance of surfactants. The dynamic surface tension (DST) is therefore controlled on the one hand by the nature of the surfactant, for example, chain length, size, and charge of the polar head, and on the other hand by the nature of the liquid and the presence of other additives. Subsequently, the adsorption of surfactant in time at the interface leads to the decrease of the surface tension of the liquid until the equilibrium value in the presence of surfactants is reached. In principle, immediately after the (instantaneous) creation of a new surface in a surfactant solution, the surface tension has the same value as that of the pure liquid. (7в€’9) In this regard, the understanding of time-dependent adsorption of different compounds at the interface and their interdependence is of crucial importance for atmospheric chemistry. In addition, adsorption at the liquid/air interface is of particular interest for studying the nucleation and growth of aerosol particles and other related atmospheric phenomena. (3в€’6) For example, in ink-jet printing and pesticide spraying in agriculture, the wetting of surfaces takes place on time scales that can be of the order of milliseconds consequently, the adsorption kinetics of surfactant molecules to the drop surface is of critical importance for the final result. (1,2) In addition to the equilibrium state of adsorbed surfactants at the interface, the kinetics at which the surfactants adsorb to the interface is also important in certain processes that involve fast wetting. The ability of surfactants to partition at the interface makes them ideal candidates for influencing the surface properties even in very small concentrations. Surface-active agents (surfactants) are one of the most commonly used compounds in both daily life as well as in diverse industrial processes.

This also correlates very well to the very low CMC of Tween 80.

For the nonionic surfactant, the kinetics of interfacial tension is seen to be already very slow, and the addition of salt does not influence it further. This change, however, is not able to account for the slowing down of adsorption dynamics we find that it is rather the decreases in the monomer concentration (CMC) in the presence of salt which has the major influence. Sum-frequency generation spectroscopy experiments and the equilibrium adsorption measurements show that the presence of NaCl in CTAB solution leads to the formation of ion pairs at the surface, thereby neutralizing the charge of the head group of CTAB. For the ionic surfactant, the dynamics agree with the usual diffusion-limited interfacial adsorption kinetics however, the kinetics become orders of magnitude slower when NaCl is present. Dynamic surface tension measurements allow comparing the surface tension as a function of time for pure surfactant solutions and in the presence of NaCl. Equilibrium surface tension measurements show that NaCl significantly reduces the critical micellar concentration (CMC) of CTAB but has no effect on the CMC of Tween 80. We study the influence of high NaCl concentrations on the equilibrium and dynamic surface tensions of ionic (CTAB) and nonionic (Tween 80) surfactant solutions.

Surface tension half life