Calorimetry is not color-blind. Molecular insights on association processes in surfactant-polymer mixtures derived from calorimetric experiments

Abstract

This comment will review and discuss recent results obtained with the use of calorimetry in assessing molecular information from complex phenomena such as association in surfactant/polymer mixtures. These examples were selected to support the view that it is possible to ally the great sensitivity of modern calorimeters with carefully planned experiments and, sometimes, ancillary techniques in order to derive detailed information on molecular interactions. calorimetry, polymer-surfatant interaction, self-assemblyCalorimetry is one of the earliest quantitative techniques employed in chemical and biological experiments, such as those developed by Lavoisier in the search for the origins of life [1], or in Watt's experiments on the relationship between heat and work. Nowadays, it is mostly used when high sensitivity is required, such as in the investigation of processes occurring in rather dilute solutions, as in the cases of biological interactions involving minute amounts of either biomolecules or drugs [2]. Recent developments in the design of calorimeters provide striking sensitivity, which allied to the general nature of heat exchange, produce a universal and very sensitive sensor to detect or monitor almost all chemical and biological processes. However, this detection is not selective in the sense that calorimeters are not capable of distinguishing the origin of exchanged heat, as opposed, for instance, to spectroscopic techniques which may follow the formation/breakdown of a specific chemical bond, release of a specific molecule fragment or chemical changes. This lack of selectivity leads to the current saying that heat bears no color, a conclusion that would encompass calorimetry as a technique incapable of providing information at the molecular level. We believe this is not true and we will discuss below a few examples that reveal the capacity of calorimetric experiments not only detecting features that may not be observable otherwise, but also providing insights at the molecular level.Calorimetry is now widespread in the investigation of other complex (chemical) systems, such as mixtures containing self-assembling molecules like surfactants and polymers. These studies are typically conducted in the titration mode, in which concentration of one of the components is increased, at constant temperature, named ITC (Isothermal Titration Calorimetry), producing information on the thermodynamics of these binding process--in this case, calorimetry is the only technique that allows the derivation of Gibbs energy, enthalpy and entropy changes from a unique experiment. Alternatively, experiments may be planned in the scanning mode (DSC-Differential Scanning Calorimetry) in which energy changes accompanying temperature driven processes are probed at constant composition.The use of calorimetry for investigation of association processes involving surfactants and/or polymers has been reviewed in recent articles [3][4][5], which in general indicate an increase of the use of these techniques, mostly due to the

Calorimetric and Light Scattering Investigations of the Transition from Spherical to Wormlike Micelles of C14TAB Triggered by Salicylate

“…A similar example was reported for the sphere-to-rod transition in mixed micelles of ethoxylated block copolymer and surfactant, also followed by ITC and confirmed by scattering techniques. Both cases support the use of calorimetric techniques as an invaluable tool to detect and to follow mixed surfactant interactions, aggregate formation, and even subtle shape transitions that they undergo …”

“…Both cases support the use of calorimetric techniques as an invaluable tool to detect and to follow mixed surfactant interactions, aggregate formation, and even subtle shape transitions that they undergo. 37 As indicated in Figure 3A, the interaction process involving 2-hydroxybenzoate and C 14 TAB can be divided into three regions. At the beginning of the titration of 2-hydroxybenzoate with C 14 TAB, the excess aromatic anion produced mixed micelles (region 1).…”

Thermodynamic Study of the Micellization of Zwitterionic Surfactants and Their Interaction with Polymers in Water by Isothermal Titration Calorimetry

“…It is widely known that ionic surfactants interact with ionic − and nonionic − polymers starting at concentrations below the cmc at a given surfactant concentration called the critical aggregation concentration, cac, forming aggregates with the polymer chains in a cooperative way. In calorimetric experiments, this is seen by an abrupt change in the enthalpy values at low surfactant concentration. ,,,,,,, This type of micellar aggregate formation increases with increases in the surfactant concentration, where a second critical concentration is achieved, as denoted by C 2 . This is where the polymer no longer influences the micellization and the concentration of free unimers in solution reaches the cmc and therefore unbound micelles start to form in solution.…”

From China to the world: Science China Chemistry celebrates the International Year of Chemistry

“…"The half-life time of the cis form of 2o was found to be 380 h, which is about 850 times longer than those of comparable 4m, 5m (4313 h) and 7p (7 h)" [27]. Watson Loh and Cesar Brinatti highlighted recent work in using calorimetry to gain molecular insights on association processes in surfactant-polymer mixtures [35]. Supramolecular chemistry is a relatively new research area in China.…”

“…"The substituted PAs have been found to show photoconductivity, optical nonlinearity, magnetic susceptibility, photo-and electroluminescence, liquid crystallinity, biocompatibility, chain helicity" [26]. Loh and Brinatti highlighted the modern applications of calorimetry in the investigation of other complex (chemical) systems, including the determination of thermodynamic parameters associated with surfactant/polymer interaction and providing important information on molecular features of these processes [35].…”