Article pubs.acs.org/jced
Effect of Salt on the Micellization of Partially Fluorinated Octylesters and Hydrogenated Dodecylesters in Water Elif B. Olutaş* and M. Acımış Department of Chemistry, Faculty of Arts and Sciences, Abant Iż zet Baysal University, 14280 Bolu, Turkey ABSTRACT: The effect of added salt, NaCl, on micellization and structural properties of four enantiomeric/racemic surfactant pairs obtained from partially fluorinated octylesters (PFOEs) and hydrogenated dodecylesters (DDEs) of alanine and serine were investigated by electrical conductivity, surface tension, dynamic light scattering (DLS), and transmission electron microscopy (TEM). The Krafft temperatures, the critical micelle concentrations, the degree of counter−ion ionization, and the Gibbs free energy of micellization were determined. The Gibbs free energy of adsorption of the esters at the air/solution interface was greater than that of micellization. The hydrodynamic radius (RH) increased with salt concentration, and the polydispersity index exhibited a shift from monodisperse to polydisperse aggregates. The PFOEs of serine could incorporate only a minimum amount of salt, but it exhibited the largest RH (37.9 nm). DLS and TEM results indicated that the esters formed micelles of spherical shapes. Neither of the methods could detect an appreciable difference in the micelle sizes of the L- and DL- amphiphiles. The DDEs exhibited greater antibacterial activity against some gram−positive bacteria than the reference antibiotic, tetracycline, but the PFOEs did not show any activity.
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INTRODUCTION Surfactants, (amphiphilic molecules) owing to the delicate balance of their hydrophilic and hydrophobic moieties, selfassociate in water to form aggregates called micelles. The micelle formation first occurs at a certain minimum concentration called critical micelle concentration (cmc). This is of paramount importance because the application of surfactants in technology and science is based on the formation of a micelle structure that is able to solubilize a large number of ionic, polar, or neutral materials. The structure of micelles of chiral amphiphilic molecules, because their properties related to chiral recognition may be of importance, then the most biologically active compounds vital to life are chiral. Therefore, in the foregoing paper we studied the micellization process in aqueous solution of a new class of enantiomeric and racemic amphiphilic esters (surfactants) obtained from esterification of the amino acids, alanine (A), and serine (S) with partially fluorinated n-octyl alcohol and hydrogenated dodecyl alcohol, respectively.1 As a result, we found that the cmc values, the thermodynamic functions (ΔG°mic, ΔH°mic, ΔS°mic), the adsorption parameters at the air/ water interface (the surface excess concentration, Γmax, the minimum area per headgroup of the molecule, Amin, and the surface tension at the cmc, γcmc) and the volumetric parameters (the changes in the molal volumes, ΔVmic φ ) of the partially fluorinated esters were strikingly different from those of the hydrogenated ones. Among the adsorption parameters, the quantities, Γmax and Amin, for the partially fluorinated octylesters PFOEs of serine were particularly different from all of the studied systems. For comparison and space saving purposes only the minimum area per headgroup is depicted from ref 1: Amin = 1.14 nm2 for LSPFOE versus (0.61, 0.70 and 0.75) nm2 for L-APFOE, L© 2014 American Chemical Society
ADDE and L-SDDE, respectively. On the bases of the adsorption results, it was then suggested that the SPFOE system would be in part as dimer at the air/water interface. On the other hand, the structures of the micelles in aqueous solution at and above the cmc of these systems were not investigated. Moreover, we recently have shown that the partial fluorination in L-APFOE and L-SPFOE systems had an enormous effect on chirality in the lyotropic liquid crystalline state.2 The amphiphilic esters, L-APFOE, and L-SPFOE, did not require any salt for the formation of the liquid crystalline state in contrast to the hydrogenated amphiphilic esters, ADDE and SDDE. It is known that the addition of salt in micellar aqueous solutions and lyotropic liquid crystals reduces the repulsion forces between head groups.3−7 Thus, for completion of these studies, it was of interest to investigate the salt effect on these surfactants and compare them with those in aqueous solution. As mentioned above, due to chiral recognition properties, the structure of chiral micelles may have a potential for a drug delivery systems. Therefore, the main objectives of this study are to (i) elucidate the structures and structural changes of these enantiomeric and racemic micelles above the cmc in water and NaCl solutions and (ii) focus on the adsorption and antimicrobial properties of these surfactants. The micellization and adsorption processes were investigated using the electrical conductivity and surface tension methods, whereas the structures and structural changes were monitored by dynamic light scattering (DLS) and transmission electron microscopy (TEM). Received: November 29, 2013 Accepted: January 30, 2014 Published: February 6, 2014 869
dx.doi.org/10.1021/je4010382 | J. Chem. Eng. Data 2014, 59, 869−879
Journal of Chemical & Engineering Data
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Article
MATERIALS AND METHODS Chemicals. Sodium chloride, DL-alanine, L-alanine, DLserine, L-serine, n-dodecanol, and 3,3,4,4,5,5,6,6,7,7,8,8,8tridecafluoro-1-octanol were used as received. Their purities are listed in Table 1.
and DL-SPFOE occurred in 24 h to 48 h, in contrast the systems, L/DL-APFOE, DL-SDDE, and L-SPFOE did not form any precipitate in this time period. Therefore, the latter systems were first kept at 0 °C, but then if no precipitate had formed after 24 h, they were frozen and then left to melt slowly until the precipitation appeared. The precipitated solutions were placed into the conductivity cell. Then, the temperature of the precipitated solutions was slowly increased using a thermostatted bath with an accuracy ± 0.1 °C. The conductance of these solutions was measured under constant stirring until it reached a steady value at each temperature. To determine the cmc, the average degree of counter ion ionization (α), and Gibbs free energy of micellization (ΔGmic ° ), an appropriate amount of aqueous 0.05 mol·kg−1 NaCl solution was placed into the cell, which was in a thermostatted water bath at 25 °C (± 0.1 °C), and then, a known volume of concentrated solution of ester prepared in 0.05 mol·kg−1 NaCl was progressively added to the cell with a micropipet. The conductance was measured after each addition followed by thorough mixing and temperature equilibration. The measurements were repeated at least twice to check reproducibility. Surface Tension. Surface tension measurements were performed using a du Noüy ring digital tensiometer (KSV Sigma 702, Finland) equipped with a thermostatted vessel holder (± 0.01 °C). The tensiometer, with a precision of ± 0.01 mN·m−1, was calibrated and checked by measuring the surface tension of the distilled water. The solution placed in the vessel holder was gently stirred by magnetic stirrer and allowed to reach thermal equilibrium at each temperature. The du Noüy ring was thoroughly cleaned by washing with distilled water, ethanol, and acetone, and dried before each measurement. Sets of measurements were obtained by adding appropriate volumes of the concentrated solution to the aqueous NaCl solution in the vessel.
Table 1. Sources and Purities of the Chemicals Used chemical name sodium chloride L/DL-alanine L/DL-serine
n-dodecanol 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-1octanol
source
mole fraction purity
Merck Merck Merck Merck Alfa Aesar
0.99 0.99 0.99 0.99 0.97
The amphiphilic enantiomers and their racemates as esters of alanine and serine were synthesized using the partially fluorinated octanol and dodecanol as described before.2 The structures, IUPAC, and common names, and abbreviations of the esters are listed in Table 2. All solutions were prepared in double distilled water at room temperature. The solutions were homogenized by using a centrifuge or magnetic stirrer. Each solution was left at room temperature at least 2 h before each measurement. Electrical Conductivity. Conductivity measurements were done with a Cyberscan PC 510 digital conductivity meter (Oakton Instruments, USA) using a conductivity cell having a cell constant of 1.03 cm−1. To determine the Krafft temperatures (TKs), clear solutions of the esters with a mass fraction 0.01 were prepared in the presence of 0.05 mol·kg−1 NaCl and placed in a refrigerator at about 5 °C until precipitates were formed. Although the precipitation of L/DL-ADDE, L-SDDE
Table 2. The Structures, IUPAC Nomenclatures, Common Names2, and Abbreviations of the Esters
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dx.doi.org/10.1021/je4010382 | J. Chem. Eng. Data 2014, 59, 869−879
Journal of Chemical & Engineering Data
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Table 3. Experimental Data for Temperature T, and Conductivity κ, for the Systems L-ADDE, L-SDDE, DLSPFOE, and DL-ADDE in Solutions of 0.05 mol·kg−1 NaCl + Watera
Dynamic Light Scattering. DLS measurements were performed with an ALV/CGS-3 compact goniometer system (Malvern, UK) using a scattering angle of 90°. A He−Ne laser operating at λ = 632.8 nm was used as a light source (output power of 22 mW). To avoid dust particles, all solutions were filtered through 0.20 μm pore size NY filter (Sartorius, Germany) prior to measurements. The temperature of the scattering cell was controlled at 25 °C (± 0.2 °C). Transmission Electron Microscopy. TEM measurements were done with a FEI Tecnai G2 (120 kV) transmission electron microscope. Micelles were observed by using the negative-staining method. A drop of the solution was placed onto a 300 mesh copper grid coated with Formvar (polyvinyl formal) carbon film. In this way prepared samples were allowed to dry at room temperature. Then, they were painted with uranyl acetate and air dried for TEM detections. Antimicrobial Activity. The disc diffusion method8 was used to measure antimicrobial activity of the studied esters. This is one of the more commonly used methods for antimicrobial susceptibility testing. In this test, a standard amount of antibiotic impregnated with the sample is placed onto an agar plate to which bacteria have been swabbed. The plates were incubated overnight, and at the end of the incubation time, the diameter of the clear zone of inhibition surrounding the sample was measured in millimeters using a ruler or with a sliding caliper. This zone was taken as a measure of the inhibitory power of the sample against the tested bacteria.
T/°C
κ/mS·cm−1
T/°C
L-ADDE
4.0 5.1 6.0 7.0 7.6 8.0 8.3 8.6 9.0 10.0 11.0 12.0
L-SDDE
5.97 6.00 5.98 6.00 6.11 6.51 7.09 7.25 7.25 7.21 7.21 7.21
6.0 7.0 8.1 9.1 10.1 11.1 11.7 12.0 12.2 12.5 13.0 14.0 15.0
5.51 5.53 5.54 5.60 5.64 5.74 5.94 6.13 6.30 6.59 6.67 6.66 6.66
17.0 18.0 19.0 20.0 21.0 21.5 22.0 22.5 23.0 24.0 25.0 26.0 27.0
DL-SPFOE
14.0 15.0 16.0 17.0 18.1 18.5 19.0 19.3 19.6 20.0 21.0 22.1 23.0
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RESULTS AND DISCUSSION Electrical Conductivity Measurements. It is well-known that the TK is a useful property for ionic surfactants because it represents the temperature that is required for dissolving the hydrated surfactant crystals completely where their solubility is equal to the cmc.9−11 In other words, surfactant molecules begin to precipitate from aqueous solution at the higher concentrations below the TK, whereas they form micelles above the TK. This behavior arises from the fact that an unassociated surfactant molecule has a limited solubility, whereas the micelles, aggregates of surfactants, are highly soluble. It has been previously shown that the TK of ionic surfactants increases with hydrocarbon chain length and decreases with branching or unsaturation in that chain in a homologous series.11 It also depends on the headgroup and nature of the counterion.11,12 Generally, salt addition raises the TK, while many other cosolutes decrease it.12,13 As described before, the TKs of the esters in 0.05 mol·kg−1 NaCl solution were determined via the conductivity measurements (Table 3 and Figure 1) to choose a convenient temperature range suitable for investigation of their micellar properties. The TK, indicated by the arrow in Figure 1, is evaluated from the intercept of the two straight lines drawn through the data above and below the TK. Table 4 summarizes the results, and as seen, the systems, L-APFOE, L-SPFOE, DLAPFOE, and DL-SDDE, exhibited TK values below 0 °C, indicating their rather high solubility in NaCl solution. However, the TKs of the systems, L-ADDE, L-SDDE, DLADDE, and DL-SPFOE, in NaCl were approximately 3−6 °C higher than the TKs of those in pure water, indicating a reduced solubility of these systems in NaCl solution. The increase in TK values can be explained by the surfactant solubility reduction due to the common ion effect. However, the rest of the esters, L-APFOE, L-SPFOE, DL-APFOE, and DL-SDDE, had TK values
a
κ/mS·cm−1 5.66 5.66 5.66 5.66 5.82 6.11 6.49 6.83 7.13 7.14 7.13 7.11 7.09 DL-ADDE
5.85 5.85 5.89 5.85 5.88 6.14 6.39 6.66 6.93 6.95 6.93 6.95 6.95
Standard uncertainties u are u(T) = 0.05 °C, u(κ) = 0.01 mS·cm−1.
Figure 1. Conductivity (κ) versus temperature (T) behavior of the studied esters in 0.05 mol·kg−1 NaCl solution: ○, L-ADDE; ●, LSDDE; ◇, DL-SPFOE; and ◆, DL-ADDE, respectively. The Krafft temperature (TK) shown by the arrow is the point of complete clarification of the solutions. The solid lines show the method to obtain the TK values.
below 0 °C even in NaCl, showing their good solubility both in water and NaCl solution. The effect of fluorination on ester chains led to some electronegativity change on the fluorinated esters compared to nonfluorinated esters. The conductivity is a convenient method to determine the cmc of surfactants over a range of concentrations.10,11 As 871
dx.doi.org/10.1021/je4010382 | J. Chem. Eng. Data 2014, 59, 869−879
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Table 4. The TK Values of the Enantiomeric/Racemic Surfactants in Solutions of 0.05 mol·kg−1 NaCl + Watera
Table 5. Experimental Data for Molality m, and Conductivity κ, for the Enantiomeric/Racemic Pairs of APFOE, SPFOE, ADDE, and SDDE in Solutions of 0.05 mol·kg−1 NaCl + Water at Pressure p = 0.1 MPa and Temperature T = 25 °Ca
TK/°C L-APFOE L-SPFOE L-ADDE L-SDDE
