J. Phys. Chem. C 2010, 114, 11133–11139
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Inversion of Hofmeister Series by Changing the Surface of Colloidal Particles from Hydrophobic to Hydrophilic Jose´ Manuel Peula-Garcı´a,† Juan Luis Ortega-Vinuesa,‡ and Delfi Bastos-Gonza´lez*,‡ Department of Applied Physics II, UniVersity of Ma´laga, 29071 Ma´laga, Spain, and Biocolloid and Fluid Physics Group, Department of Applied Physics, UniVersity of Granada, AVenida FuentenueVa S/N, 18071 Granada, Spain ReceiVed: December 21, 2009; ReVised Manuscript ReceiVed: April 9, 2010
In this paper, we demonstrate that in addition to the nature of the ions the nature of the surface is also of vital importance in order to elucidate the origin of Hofmeister effects. Specifically, we show that for the solid/ water interface when the surface turns from hydrophobic to hydrophilic, an inversion in the Hofmeister series occurs. Results were recorded from colloidal-stability experiments performed with seven different anions located at different positions in the Hofmeister series and working with four colloidal systems that varied in their surface-charge sign and hydrophobic/philic degree. A mechanism based on the structural modifications that ions and surfaces induced in water is proposed to explain all these results. The existence of hydration forces on hydrophilic systems enables us to explain the data and to reinforce our arguments concerning the relevance of considering the water structure around both the ions and the interfaces on Hofmeister effects. Introduction It is well-known that many charged hydrophilic colloids undergo anomalous stability at moderate or high ionic strengths. At certain salt concentrations, these hydrophilic colloids aggregate but at higher amounts of salt the colloidal particles stabilize regardless of the ionic strength. These phenomena are usually known as restabilization processes and the origin of this behavior at microscopic level is ascribed to so-called hydration forces. The repulsion of the colloidal particles under such conditions is usually associated with the structure of water molecules near the hydrophilic surfaces and the ions located in the proximities of the surfaces.1 In fact, this type of restabilization has never been observed in highly hydrophobic particles. Therefore, the way in which water is structured at the hydrophilic interface seems to be essential for short-range repulsive forces.2 With respect to the ions, only those positively charged, strongly hydrated and acting as counterions were thought to be able to cause restabilization. However, recent results have shown that not only cations but also anions can also trigger restabilization.3 These latter results were found in connection with the generally known ion-specificity or Hofmeister effects.4 Hofmeister effects refer to the ability of different anions and cations to alter diverse properties of many phenomena that take place at interfaces from surface tensions5 to polyelectrolyte multilayers.6 In addition, with very few exceptions cations and anions consistently order with the same sequence, called the Hofmeister series, regardless of the property studied.7,8 Explaining Hofmeister effects is a significant challenge in contemporary Colloid Science as they are present in many biophysical and biochemical systems. However, the large diversity of interactions that must be taken into account among surfaces, ions, and water has made it very difficult to find a general theory capable of explaining all the disparate results. Nonetheless, theoretical * To whom correspondence should be addressed. E-mail: dbastos@ ugr.es. Phone: +34 958 240016. Fax: +32 958 243214. † University of Ma´laga. ‡ University of Granada.
researchers are making great strides, and a satisfactory theory describing Hofmeister effects appears to be within reach. Accumulation/exclusion mechanisms of anions or cations near surfaces seem to be the key underlying Hofmeister effects. The still debatable matter is whether these mechanisms are mediated mainly by alterations of the structure of water at the interface9 and/or by the specific polarizability of the ions (accounted for by means of dispersion forces).10,11 Experimentally, however, most research agrees that Hofmeister effects share some common characteristics, namely, (i) they usually appear when electrostatic interactions are screened; (ii) the effects are greater and usually dominated by anions; and (iii) independently of the property studied positive surfaces usually order ions in an inverse sequence that negative ones. In previous works, we have extensively studied the effect of ionic specificity over a broad gamut of several colloidal particles with different surface characteristics: hydrophobic/hydrophilic degree, sign (positive or negative), and surface charge density. Our investigations have been focused mainly on the stability and electrokinetic behavior of the different colloidal systems.3,12,13 The main conclusion of that research is that the mechanisms governing Hofmeister effects depend on the water structure, not only of the ions but also of the surface. Experimentally, we found that when the sign of the surface charge is kept constant, the sequence in which the ions are ordered according to colloidal stability with hydrophilic surfaces is reversed in comparison to hydrophobic surfaces. Moreover, in intermediate situations of hydrophilicity, partial reversions were observed.3 Restabilization processes observed in hydrophilic surfaces helped us to shed light on the origin of Hofmeister effects. However, in the previous works, we used many different surfaces and relatively few Hofmeister ions, most of them with chaotropic character. For this reason, the aim of the present work was to test the above conclusion with more ions belonging to Hofmeister series and to delve into the mechanisms responsible for such observations. Highly hydrophobic and hydrophilic, positive and negative, systems have been used to demonstrate that the stability sequence of different ions can be reversed when surfaces change
10.1021/jp912035v 2010 American Chemical Society Published on Web 06/03/2010
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J. Phys. Chem. C, Vol. 114, No. 25, 2010
Peula-Garcı´a et al.
from hydrophobic to hydrophilic. In addition, the hydrophilic nature of the systems has been exploited to analyze restabilization processes and their relation to Hofmeister effects. Seven different sodium salts, NaIO3, NaF, NaCl, NaBr, NaI, NaNO3, and NaSCN, enable us to compare the specific effects of the anions used. Experimental Section Materials. All the salts were of analytical grade and purchased from different firms, Merck-Sigma and Scharlau. Deionized Milli-Q water was used throughout. When used, nonbuffered solutions were prepared by adding HCl or NaOH to water to reach the desired pH. The bovine serum albumin (BSA) was Pentex Fraction V fatty acid free (Milles Inc., reference 82-002). The goat immuno-γ-globuline-G from serum (IgG) was supplied by Sigma-Aldrich (ref I5256). Nanoparticle Preparation. Four different types of particles were used in this work. Two polystyrene latexes prepared by the emulsion polymerization method in absence of surfactants were used as hydrophobic particles. One of them carried a negative charge on the surface from sulfonate groups,12 and the other one had positive charge coming from amine groups.14 The hydrophobic character of sulfonate latex is guaranteed, as the ratio between the monomer styrene and the comonomer sodium styrene sulfonate used in the synthesis was 116:1, and latex has a low surface charge density of 0.09 C/m2.12 However, in the results section the hydrophobic character of these latex will further discussed. On the other hand, as hydrophilic particles, protein-coated polystyrene particles were used as negative systems,15 while chitosan nanocapsules were synthesized to have positive ones.3 All the details concerning particle synthesis and characterization can be found in the corresponding references. Briefly, protein was adsorbed onto negative polystyrene particles as described below; dialyzed IgG (or BSA) was added to an aqueous solution at the desired pH containing latex particles with a total polystyrene area of 0.3 m2. The IgG (or BSA) protein concentration used was high enough to guarantee a maximum coat. Incubation was carried out at 25 °C for 21 h. Afterward, samples were centrifuged at 25 000×g for 30 min, and the pellets were redispersed and stored at the desired pH. More details can be found elsewhere.15 Colloidal Stability. Seven different sodium salts containing the Hofmeister anions were used, independently, as aggregating agents. According to the classical Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory, increased salinity triggers the coagulation of lyophobic colloidal systems. During aggregation, the turbidity of the system increases when the average size of the scattering particles enlarges. Therefore, a simple spectrophotometer working with a visible wavelength is able to detect and analyze the aggregation kinetics of many colloidal systems. In our case, we used a Beckman DU 7400 spectrophotometer and the wavelength was set at 570 nm, and the absorbance was recorded for 120 s in each experiment. The scattering cell was rectangular with a 1 cm path length. Equal volumes (0.3 mL) of salt and particle solutions were mixed and injected into the cell. Figure 1 shows a typical aggregation experiment of a colloidal system partially hydrophilic. As can be seen, an increment of the salt concentration speeds up the aggregation kinetics, as predicted by the DLVO theory. However, in hydrophilic systems, the kinetics may slow down if the salinity is increased even more. This “anomalous” behavior is not explained by the DLVO theory, and the origin of this phenomenon must be sought in the repulsive hydration forces. Information on the kinetics-aggregation constant “k” of dimer formation
Figure 1. Variation of the optical absorbance (λ ) 570 nm) with time for a hydrophilic colloidal system at different NaCl concentrations: 0.003 M (\), 0.005 M (right-pointing triangle), 0.008 M ( PO4H2- > IO3- > F- > CH3COO- > Cl- > Br-> -
-
-
-
I > NO3 >ClO4 > SCN
Anions on the left side are more hydrated and they are usually known as kosmotropes or structure makers. On the contrary, anions on the right side are referred to as chaotropes or structure breakers and they are weakly hydrated. Cl- is usually considered to be an indifferent ion or as reference point in the Hofmeister series. In this study, we chose seven anions ranging from kosmotropic to chaotropic, although only monovalent anions were analyzed to simplify the data analysis. First, the results for hydrophobic systems will be presented. In Figure 3a,b, W is plotted as a function of the concentration of the different salts. The results clearly reflect that the stability patterns depend on the nature of the salt. These curves gave the CCC data displayed in Table 1. According to these CCC values the anions can be arranged for both latexes in the following sequences
Figure 3. (a) W versus salt concentration for the positive hydrophobic interface and different sodium salts: (right-pointing triangle) SCN-; (b) I-; (2) NO3-; ([) Br-; (1) Cl-; (9) F-; (f) IO3-. Solid lines serve to guide the eye in order to determine CCC values. (b) W versus salt concentration for a negative hydrophobic interface and different sodium salts: (right-pointing triangle) SCN-; (b) I-; (2) NO3-; ([) Br-; (1) Cl-; (9) F-; (f) IO3-. Solid lines serve to guide the eye in order to determine CCC values.
TABLE 1: CCC Values (in mM Units) Obtained for the Positive and Negative Hydrophobic Interfaces Using Different Sodium Salts As Aggregating Agents CCC
IO3-
F-
Cl-
Br-
NO3-
I-
SCN-
positive surface negative surface
85 380
80 385
70 430
45 428
36 465
18 510
12 535
Positive latex: Negative latex:
IO3->F->Cl- > Br->NO3 >I >SCN
IO3- e F-
