Spectroscopically Quantifying the Influence of Salts on Nonionic Surfactant Chemical Potentials and Micelle Formation

Influence of H+, OH- and salts on hydrophobic self-assembly

In spite of the ubiquity of acid/base ions and salts in biological systems, their influence on hydrophobic self-assembly remains an open question. Here we use a combined experimental and theoretical strategy to quantify the influence of H+ and OH-, as well as salts containing Li+, Na+, Cl- and Br-, on the hydrophobic self-assembly of micelles composed of neutral oily 1,2-hexanediol surfactants. The distributions of aggregate sizes, both below and above the critical micelle concentration (CMC), are determined using Raman multivariate curve resolution (Raman-MCR) spectroscopy to quantify the multi-aggregation chemical potential surface (MCPS) that drives self-assembly. The results reveal that ions have little influence on the formation of hydrophobic contact dimers but can significantly drive high-order self assembly. Moreover, the hydration-shells of oily solutes are found to expel the above salt ions and OH-, but to attract H+, with wide-ranging implications.

Hofmeister Effects of Group II Cations as Seen in the Unfolding of Ribonuclease A

This work studies the effects of alkaline-earth cation addition upon the unfolding free energy of a model protein, pancreatic Ribonuclease A (RNase A) by DSC analysis.  RNase A was chosen because it: a) does not specifically bind Mg 2+ , Ca 2+ and Sr 2+ cations and b) maintains its structural integrity throughout a large pH range. We have measured and compared the effects of NaCl, MgCl 2 , CaCl 2 and SrCl 2 addition on the melting point of RNase A.  Our results show that even though the addition of group II cations to aqueous solvent reduces the solubility of nonpolar residues (and enhances the hydrophobic effect), their interactions with the amide moieties are strong enough to "salt-them-in" the solvent, thereby causing an overall reduction in protein stability. We demonstrate that amide-cation interactions are a major contributor to the observed "Hofmeister Effects" of group II cations in protein folding. Our analysis suggests that protein folding "Hofmeister Effects" of group II cations, are mostly the aggregate sum of how cation addition simultaneously salts-out hydrophobic moieties through increasing the cavitation free energy, while promoting the salting-in of amide moieties through contact pair formation.

Cononsolvency of thermoresponsive polymers: where we are now and where we are going

Cononsolvency is an intriguing phenomenon where a polymer collapses in a mixture of good solvents. This cosolvent-induced modulation of the polymer solubility has been observed in solutions of several polymers and biomacromolecules, and finds application in areas such as hydrogel actuators, drug delivery, compound detection and catalysis. In the past decade, there has been a renewed interest in understanding the molecular mechanisms which drive cononsolvency with a predominant emphasis on its connection to the preferential adsorption of the cosolvent. Significant efforts have also been made to understand cononsolvency in complex systems such as micelles, block copolymers and thin films. In this review, we will discuss some of the recent developments from the experimental, simulation and theoretical fronts, and provide an outlook on the problems and challenges which are yet to be addressed.

Surfactant aggregate size distributions above and below the critical micelle concentration

Aggregate size distributions in an aqueous solution containing either charged or neutral surfactants are investigated using Raman multivariate curve resolution (Raman-MCR) spectroscopy and analyzed with the aid of a multi-aggregation chemical potential surface (MCPS) modeling strategy. Total least squares decompositions of the concentration-dependent Raman-MCR spectra are used to quantify the free and micelle surfactant populations, and the surfactant's C-H stretch frequency is used as a measure of its average aggregation state. MCPS predictions relate the experimental measurements to the underlying surfactant aggregate size distribution by fitting either the component concentrations or the average C-H frequency to MCPS predictions, and thus determine the critical micelle concentration (CMC) and estimate the corresponding micelle size and polydispersity. The Raman-MCR spectra of aqueous 1,2-hexanediol, sodium octanoate, and sodium dodecyl sulfate, measured both below and above CMC, provide critical tests of the assumed functional form of the MCPS and the presence of low-order premicellar aggregates. Our results indicate that the low-order aggregate population gradually emerges as the CMC is approached and then remains nearly concentration-independent after the appearance of micelles.

Theory of Gas Solubility and Hydrophobic Interaction in Aqueous Electrolyte Solutions

Ion-specific effects on the solubility of nonpolar solutes and on the solute-solute hydrophobic interaction in aqueous electrolyte solutions are studied on the basis of a continuum theory that incorporates the excluded volume of the molecules using the four-component (water, cations, anions, and solutes) Boublı́k-Mansoori-Carnahan-Starling-Leland model and ion hydration (electrostriction) using the Born model. We examine how the ordering of ions in the salt effect on the solubility as measured by the Sechenov coefficient K_S changes with varying sizes of ions and solutes. Our calculation reproduces the general trend of experimentally measured K_S and also provides insight into the irregular behavior of K_S for lithium ion. The correlation between K_S and the salt effect on the hydrophobic interaction that has been pointed out earlier is accounted for by an explicit connection between K_S and the salt-enhanced-association coefficient C_I in the expansion of the second osmotic virial coefficient B(n_s) = B(0) - C_In_s + ··· in powers of the salt density n_s at fixed pressure and temperature. The quadratic relation CI≈KS2/4 is derived for ions and solutes that are not very large.

Multistep Micellization of Standard Surfactants Evidenced by Second Harmonic Scattering

Processes involving in solution a reduced number of molecules are difficult to identify and characterize. Here, we show that micellization of standard surfactants, namely sodium dodecyl sulfate and trimethyl tetradecyl ammonium bromide, two nonefficient compounds for quadratic nonlinear optics, can be investigated by second harmonic scattering (SHS). In particular, the formation of aggregates at concentrations smaller than the critical micellar concentration is evidenced through a nonmonotonic behavior of the SHS intensity as a function of the surfactant concentration. A simple model based on chemical equilibria between monomers and micelles is proposed to account for the experimental observations. Signature of long-range molecular orientation correlation is revealed by polarization resolved experiments and is discussed regarding micellization and charge-induced effects.