A Look at the Thermodynamics of the Association of Amphiphilic Polyelectrolytes in Aqueous Solutions:  Strengths and Limitations of Isothermal Titration Calorimetry

Abstract

The dissolution in water of the sodium salts of poly(2-acrylamido)-2-methylpropanesulfonic acid (PAMPS), N,N-di-n-octadecyl-endcapped-poly(2-acrylamido)-2-methylpropanesulfonic acid [(C18)2-PAMPS], cholesteryl-endcapped-poly(2-acrylamido)-2-methylpropanesulfonic acid (Chol-PAMPS), and a random copolymer of AMPS and N-dodecylmethacrylamide (PAMPS-Dod20) was studied by isothermal titration calorimetry (ITC). The endcapped polymers form multimolecular aggregates in aqueous solutions. The concentration range for aggregation and the enthalpy of micellization were determined as functions of the electrolyte concentration (0.01 M < [NaCl] < 0.3 M) and temperature (288 K < T < 308 K) for solutions of (C18)2-PAMPS. At 298 K and 0.2 M NaCl, aggregation of this amphiphilic polyelectrolyte occurs in solutions with concentrations ranging from about 1 to 14 mmol of AMPS L-1 (0.5−2.7 g L-1) with an enthalpy of micellization of ∼100 J (mol of AMPS)-1. The effect of the experimental conditions, such as the method of preparation of the polymer stock solution and its concentration, on the ITC results is described in the case of Chol-PAMPS. Titrations performed with poly(sodium-2-acrylamido-2-methylpropane sulfonate) (PAMPS) confirmed that PAMPS does not aggregate in aqueous NaCl, whereas PAMPS-Dod20 forms predominantly unimolecular micelles. The advantages and limitations of ITC for studying the micellization thermodynamics of amphiphilic polyelectrolytes are discussed.

What can isothermal titration microcalorimetry experiments tell us about the self‐organization of surfactants into micelles?

“…In many works, results were expressed in terms of enthalpy of demicellization (DH demic ) (Paula et al, 1995;Garidel et al, 2000;Raju et al, 2001), or in terms of enthalpy of micellization (DH mic ) (Bijma and Engberts, 1994;Majhi and Moulik, 1998;Dai and Tam, 2003;Bouchemal et al, 2009;Roques et al, 2009). Since results obtained from these two considerations are similar (DH mic ¼ ÀDH demic ), we suggest that it is thermodynamically more significant to comment this review in terms of ''the enthalpy of micellization'' which is the effective studied phenomena.…”

“…In commercial ITC apparatuses, the experimentally accessible range of temperature is comprised between 2 and 808C, but experimental temperatures investigated are usually varied from 48C (Roques et al, 2009) to 608C (Bijma and Engberts, 1994;Ropers et al, 2003;Beyer et al, 2006;Bouchemal et al, 2009). Water which serves as the dispersing medium for the micellar solution and water initially contained into the measurement cell can be replaced by phosphate buffer (Diab et al, 2007) or added with electrolytes such as NaCl (Raju et al, 2001;Roques et al, 2009), tyrode solution (Roques et al, 2009) or co-solvents such as propanediol-1,2 . Importantly, the surfactant concentration in the syringe has to be chosen in such a way that the increase in surfactant concentration in the measurement cell has to reach progressively the CMC during the experiment.…”

“…However, DH mic determined from ITC titration curve of ionic surfactant can take negative values (Bijma and Engberts, 1994;Diab et al, 2007;Bai et al, 2008;Poša et al, 2008), positive values (Raju et al, 2001;Bouchemal et al, 2009;Roques et al, 2009), or both negative and positive values depending on the temperature. For instance, it was reported for octylglucoside surfactants that DH mic was positive at high temperatures, decreased upon lowering the temperature, then, passed through zero, and became negative at low temperatures (Paula et al, 1995;Majhi and Moulik, 1998).…”

“…For instance, when no break in the enthalpy versus concentration curve is observed, the determination of DH mic and CMC becomes inaccurate or even impossible (Paula et al, 1995;Garidel et al, 2000;Rozycka-Roszak et al, 2008). In such circumstances, curve analysis is requested for determining the concentrations corresponding to the start and to the end of the micellization process (Klijn et al, 2000;Raju et al, 2001;Bouchemal et al, 2009;Roques et al, 2009). As explained in Figure 3, linear fits of the data sets in the lower and the upper concentration domains were performed.…”

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“…At present, PECs are used for such large-scale industrial applications as flocculants, coatings, and binders and for special purposes in biotechnology and medicine 45 . Promising fields include PEC-microencapsulation of drugs, enzymes, cells and microorganisms [46][47][48] , immobilization of proteins by complex formation 49,50 and polycation complexes with polynucleotides or oligonucleotides as vectors in gene therapy [51][52][53][54] . Depending on their composition, PECs can be either insoluble or soluble in water.…”