CO2-Responsive Surfactant-Free Microemulsion - Langmuir (ACS

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CO-Responsive Surfactant-Free Microemulsion Dongfang Liu, Zhiyu Huang, Yuxin Suo, Peiyao Zhu, Jiang Tan, and Hongsheng Lu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01518 • Publication Date (Web): 07 Jul 2018 Downloaded from http://pubs.acs.org on July 10, 2018

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CO2-Responsive Surfactant-Free Microemulsion Dongfang Liu, † Zhiyu Huang, *, † Yuxin Suo, † Peiyao Zhu, † Jiang Tan, † and Hongsheng Lu, *, †

†

College of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500, P. R. China

*Corresponding author: Email: Zhiyu Huang ([email protected]); Hongsheng Lu( [email protected]). Fax: +86-28-83037330; Tel: +86-28-83037330. College of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500, P. R. China.

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ABSTRACT A surfactant-free microemulsion (SFME) with CO2 stimuli responsive properties was prepared. The oil and the water phase were N, N-Dimethylcyclohexylamine (DMCHA) and deionized water, respectively, and N, N-dimethylethanolamine (DMEA) was used as an amphi-solvent. The single-phase and multi-phase zones were measured by the ternary phase diagram and the microstructure of SFME was determined by measuring the change trend of the electrical conductivity of the system with increasing DMCHA content. While using methyl orange (MO) as a probe, the microstructure of SFME was further confirmed by an UV-visible spectrometer. The microstructures of water-in-oil (SFME-I) and oil-in-water (SFME-II) microemulsions were obtained by changing the DMCHA content in the system. The SFME-I system has a significant phase separation after the action of CO2. However, with the continuous introduction of CO2, the upper phase of DMCHA is gradually protonated and dissolves in the aqueous phase, resulting in a gradual decrease in the volume of the upper phase, and eventually in an aqueous solution of ammonium bicarbonate. For SFME-II, CO2 does not directly cause phase separation, but eventually it becomes an aqueous solution of ammonium bicarbonate with the addition of CO2. Both the water-in-oil structure SFME-I and the oil-in-water structure SFME-II have excellent CO2 stimuli response performance.

Keywords: Surfactant-free, Microemulsion, CO2, Responsive, DMCHA, DMEA.

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INTRODUCTION Hoar and Schulman first reported microemulsions in 1943.1 Conventional microemulsions generally consist of surfactants, co-surfactants, oil and water phases,2 and require large amounts of surfactants. However, surfactants do not seem to be necessary for the formation of microemulsions. Barden reported in 19773 and 19794, 5 a surfactant-free microemulsion (SFME) consisting of hexane, 2-propanol and water. In general, the preparation of SFME requires an oil phase, an aqueous phase and an “amphi-solvent” (Completely or at least partially miscible with each of the two immiscible fluids).6 This discovery has greatly expanded the research and application prospects of microemulsion. Microemulsions are a thermodynamically stable system with excellent stability7. In surfactant-based microemulsion systems, we can prepare microemulsions with a stimulus-responsive property by stimulating response characteristics of surfactants, such as CO2-responsive,8,

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light-responsive10,

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and

magnetic-responsive12 and so on. Due to the absence of surfactants, SFME has more application scenarios. SFME with stimulus-response characteristics is undoubtedly more competitive. Is there any way to prepare SFME with stimulus-responsive properties? In 2013, Hou and co-workers prepared a nonaqueous ionic liquid SFME system consisting of the hydrophilic ionic liquid (IL) 1-butyl-3methylimidazolium tetrafluoroborate (bmimBF4), toluene and ethanol.13 Cyclic voltammetry, pulsed-field gradient spin-echo nuclear magnetic resonance and conductivity measurements showed that there were three microstructural transitions in microemulsion, 3


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IL-in-toluene (IL/O), bicontinuous (B.C.) and toluene-in-IL (O/IL). In the same year, Hou et al. Reported another SFME system prepared from oleic acid, n-propanol and H2O.14 Through the characterization of conductivity and FF-TEM and cryo-TEM, it was found that the microstructure of the SFME system completes the structural transition from oil-in-water (O/W) to bicontinuous (B.C.) and finally to water-in-oil (W/O) as the oil phase content increases. Hou15 and co-workers did a lot of very good work on the preparation and application of surfactant-free microemulsion. In 2014, Nora Ventosa reported a CO2-based SFME system16 and in 2017 reported a surfactant-free CO2-Based nanostructured fluids system consisting of water, organic-solvent and CO2 with pressure responses.17 The "water-rich" nano-regions in "water-deficient" matrices were observed to be characterized by Raman spectroscopy, molecular dynamics simulations, and small-angle neutron scattering. CO2 regulation emulsification and demulsification of SFME are highly desirable because CO2 is readily available as a greenhouse gas and the bicarbonate in the system

is

readily

decomposed

when

N2

is

present

or

when

heated.

Switchable-hydrophilicity solvent (SHS) first reported by Philip G. Jessop in 2010,18 The miscibility of this solvent with water changes with changing external conditions. In the air, this solvent is almost immiscible with water but appears hydrophilic and miscible with water in the presence of CO2. Through the study of the application of the switch hydrophilic solvent in the cleaning of oil-based drill cuttings19 and the heavy oil emulsion transportation,20 we found that the hydrophobic or hydrophilic properties of N,N-dimethylcyclohexylamine (DMCHA) can be changed under the 4


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control of CO2. This offers the possibility for us to prepare SFME with CO2 stimulation response properties. The application prospect of CO2 stimulation response SFME is very wide. For example, it can be used as a microreactor to synthesize materials15, 21 and can realize the separation of organic solvents under the action of CO2. In addition, we are conducting research on CO2 stimulation-responsive SFME for cleaning oil sands and oil-based drill cuttings. Therefore, it is very necessary to study the formation process of SFME and the mechanism of CO2 response. In combination with the characteristics of the SHS, we chose an excellent switchable hydrophilicity solvent-DMCHA as the oil phase to prepare a SFME with CO2 stimulation response. The ternary phase diagram was used to determine the single-phase region of the SFME composed of DMCHA, DMEA and water. The microstructural transition of SFME was investigated by conductivity and UV-vis spectrophotometer. The CO2 stimulation response behaviors of SFME was measured by conductivity and pH.

EXPERIMENTAL SECTION Chemicals and Materials N, N-Dimethylcyclohexylamine (DMCHA, 98%), N, N-dimethylethanolamine (DMEA, 98%) were all obtained from Aladdin Reagents of China. n-hexane (AR), Sodium chloride (NaCl, AR) and methyl orange (MO) were purchased from Chengdu Kelong Chemical Factory. CO2 (>99%) and N2 (>99%) were purchased from Chengdu Jinli gas Co., Ltd. The water used in all experiments was deionized water. 5


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The molecular structure of DMCHA, DMEA and MO can be found in Figure S1 of the supporting information. Ternary Phase Diagram of SFME DMEA, DMCHA and water occupy the three apexes of the ternary phase diagram as three components respectively. First, the mass ratio of DMEA to DMCHA (or water) was fixed to 1:9, 2:8, 3:7 ... 7:3, 8:2, 9:1 and then deionized water (or DMCHA) was added dropwise to the binary system with various DMCHA (or water) and DMEA ratios, at the same time record the quality of added water when the ternary system from clarification becomes turbid. Finally, ternary phase diagram of SFME was drawn by calculating the proportion of DMEA, DMCHA and water in the ternary system. The experiment was done at 25 oC. Electrical Conductivity and pH Measurements In order to study the effect of CO2 on SFME at 25℃, we measured the conductivity and pH of SFME with CO2 was continuous introduced. The instrument information for pH and electrical conductivity tests are as follows: PHS-3E pH meter (Shanghai INESA Scientific Instrument Co., Ltd) and DDS-307A conductivity meter (Shanghai Youke Instrument Co., Ltd). UV-visible Spectra In order to investigate the transition of the microstructure of SFME with increasing DMCHA concentration, methyl orange (MO) aqueous solution (0.015 g/L) was used as the water phase to prepare SFME. The mass ratio of DMEM to MO solution was 1: 1, then DMCHA was added to prepare SFME, and the change of the 6


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wavelength corresponding to the maximum absorption peak (λmax) of MO with the concentration of DMCHA was measured and recorded. UV-visible spectra were recorded using a computer-controlled UV-visible spectrometer (UV-1800, Shimadzu Co., Ltd., Japan). Dynamic light scattering (DLS) DLS measurement of the SFME-I and SFME-II was performed on a BI-200SM system (Brookhaven, United States) at 25 oC with a 90° back scattering angle and He– Ne laser (λ= 532 nm). In order to remove the effect of dust particles on the measurement results, all samples were filtered with a 0.45 µm filter before testing.

RESULTS AND DISCUSSION Mutual Solubility between DMCHA, DMEA and Water Before studying the formation of SFME and the response of CO2 stimulation, it is necessary to investigate the mutual solubility of DMCHA, DMEA and water. At room temperature, two of the three liquids (DMCHA, DMEA and water) were mixed together in the same volume into glass bottles. The picture of samples was shown in Figure 1 after standing for 24h.

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Figure 1. Mutual solubility between DMCHA, DMEA and water. (a) 3mL DMCHA+3mL H2O (Nile red as a detector); (b) 3mL DMEA+3mL H2O; (c) 3mL DMCHA+3mL DMEA; (d) 3mL DMCHA+3mL H2O+CO2. As depicted in Figure 1, DMCHA is almost immiscible with water, and we can see that a clear two-phase interface appears when two liquids of equal volume are mixed (Figure1 (a)). However, DMEA can form a single-phase system after mixing with an equal volume of DMCHA and water, respectively (Figure1 (b) and (c)). In DMCHA, DMEA and water ternary systems, DMCHA can be considered as an oil phase and DMEA as an amphi-solvent. Based on the properties of the liquids in the above ternary system, the preparation of SFME is made possible. Although DMCHA is not miscible with water in the air, we found after the introduction of CO2 that the original two-phase system became single phase (Figure1 (d)). As a kind of SHS18, 22, DMCHA has the characteristic that hydrophilicity alternates with the introduction of CO2 and N2. Due to the variable hydrophilicity of DMCHA, it has become a reality to successfully prepare a CO2 stimulus response SFME.

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Ternary Phase Diagram and Microstructure Analysis of SFME

Figure 2. Ternary phase diagram of SFME prepared by DMCHA, DMEA and water. The single-phase area was divided into three partial O/W, B.C and W/O zones. The red (I) and blue (II) balls are in the W/O and O/W regions and representing a composition of SFME respectively. Figure 2 presents the ternary phase diagram of DMCHA, DMEA and water at 25 ℃. The ternary phase diagram was divided into two parts, the single-phase zone and the multi-phase zone. The three-phase diagram tells us that it is not possible for the three components to form a microemulsion in any combination. The three components are capable of forming a microemulsion only in the proportions shown in the single-phase region and do not form a microemulsion in the multiphase region. In single-phase zone, the microstructure of SFME also changes with the proportion of three components. Single-phase zone is divided into W/O, B.C and O/W three areas. In order to study the CO2 stimulation response of SFME with different microstructures, we first fixed the ratio of DMEA to water and then gradually 9


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increased the DMCHA content to obtain SFME with different microstructures. The three components formed SFME with a microstructure of W/O and O/W under the composition shown by I (red ball) and II (blue ball) in Figure 2. The specific compositions of SFME-I and SFME-II are shown in Figure 3. In addition, the influence of temperature on the ternary phase diagram has already appeared in the supporting information (Figure S2).

Figure 3. The proportion of three components in SFME-I and SFME-II. The microstructure transition of a microemulsion can be determined by measuring the change in conductivity of the microemulsion as the content of a component (usually oil or water phase) changes.14,

23, 24

For surfactant-containing

microemulsions, the electrical conductivity of the microemulsions generally changes regularly as the water content increases.14 First, the conductivity of the microemulsion is nonlinearly increasing in the initial stage. Suggesting that the presence of percolation phenomenon25 may be due to droplet aggregation. Then, the conductivity increases linearly with the increase of water content, which is due to the gradual increase of water-in-oil droplets and collisions with each other. Thirdly, the conductivity shows a nonlinear curve increase, indicating that the microstructure of microemulsion has changed from W/O to B.C, and the conductivity appears 10


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maximum at this time. Finally, the B.C transitions to an O/W structure and the conductivity shows a linear downward trend with further increase of the water content, which are mainly attributed to the decrease of the O/W droplets concentration. To determine if there is a transition in the microstructure of SFME, as the water content increases, it behaves like a microemulsion containing surfactants. Water was added dropwise to the binary system consisting of DMEA and DMCHA and the conductivity was continuously measured as the water content increased. As shown in Figure S3, the conductivity increased with the increase of water content, and the trend did not appear that we expected. This shows that there is no change in the microstructure of SFME at this ratio. In order to enable the dilution line to cross different phase regions, DMCHA was added dropwise to a binary system consisting of DMEA and water and the conductivity was measured (Figure S4). This method is also widely used by peers.13, 23, 26 DMCHA was added dropwise to a binary system consisting of DMEA and water at a mass ratio of 1:1. Conductivity as DMCHA content increases trend shown in Figure 4.

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Figure 4. The trend of SFME conductivity changes with the content of DMCHA at 25 o C. Figure 4 presents the trend of the conductivity of SFME with DMCHA content changes. The conductivity curve of SFME showed an increasing trend in the initial stage, which may be due to the adsorption of OH- ions on the surface of O/W droplets.27 Immediately afterwards, the maximum value of conductivity appeared and started to decrease non-linearly, indicating the microstructure transition occurred from O/W to B.C. Finally, with the further increase of DMCHA content, the conductivity of SFME began to decrease rapidly and linearly, indicating that the microstructure of SFME was W/O. In addition, the microstructural transformation of SFME can also be analyzed using UV-Vis absorbance spectra.13,

14, 28

Methyl orange (MO), as a sensitive

solvatochromic probe, is sensitive to changes in microenvironment polarity and widely used in microenvironment detection of inverse microemulsions.23, 29 The water in SFME was changed to MO aqueous solution (0.015g/L), and then the mass ratio of 12


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DMEA to MO aqueous solution was still equal to 1. Different amounts of DMCHA (9.09wt% -50.00wt%) were added to the binary system consisting of DMEA and MO and the spectrum of the details shown in Figure S5.

Figure 5. The relationship between UV-vis absorption maximum (λmax) and DMCHA concentration corresponds to Figure 4. Figure S5 shows the UV-vis absorption spectra for MO, corresponding to different DMCHA concentrations and Figure 5 shows the relationship between DMCHA concentration and λmax. The λmax of MO is sensitive to the local microenvironment and the red shift phenomenon (λmax shifts to longer wavelengths) was observed when the polarity of the droplet domain increases.30, 31, 32 Conversely, when the polarity of the droplet domain decreases, λmax shifts to a shorter wavelengths. In Figure 5, the trend of λmax can be summarized in three phases as the increase of DMCHA concentration: first, a linear decrease, then a relatively steady, and finally a linear decrease. In the initial stage, λmax gradually decreases with the addition of 13


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DMCHA as the oil phase, indicating that the gradual formation of O/W droplets causes a decrease in the polarity of the system. Then, λmax almost did not change with the change of DMCHA concentrations. The reason why λmax does not change obviously in the bicontinuous phase is that the MO molecule is preferentially located in the DMCHA/H2O interface phase, and the interface phase is in a dynamic process of formation and destruction. The change of DMCHA concentration does not affect the DMCHA/H2O interface polarity.28 Therefore, λmax in the process did not change significantly. Finally, with the further increase of the concentration of DMCHA, W/O microemulsion droplets formed, the system polarity further reduced, so the λmax decreased linearly. The area of B.C that appears to have been determined by both conductivity and UV-vis may be slightly different due to experimental error, but the difference is insignificant and we do not consider the effect on the experimental results. Effect of CO2 on Phase Behavior, Conductivity and pH of SFME The proportions of the three components in SFME-I were shown in Figure 3, and the SFME at this ratio is just in the W/O area (Figure 4 and Figure 5). In SFME-I, the concentration of DMCHA was 37.5wt% higher than DMEA and water in three components. DMEA and DMCHA can be protonated in aqueous solution by CO2, and even can change the hydrophilic character of DMCHA itself. It is a common method to analyze the characteristics of CO2 stimulation response by monitoring the conductivity and pH of SFME-I, and the specific information of SFME-I after reacted with CO2 was shown in Figure 6. 14


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Figure 6. The conductivity and pH change of SFME-I with the continuous introduction of CO2. (a) Initial state of SFME-I, pH=12.48; (b) Photo of SFME-I at pH=10.38; (c) Photo of SFME-I at pH=9.69. Samples (b) and (c) were stained using Nile Red in order to clearly observe the phase interface. Figure 6 clearly shows that the conductivity and pH values of SFME-I show different trends with the introduction of CO2. The pH value gradually decreases with the increase of the time for the introduction of CO2, and the change in conductivity is obviously more complicated. After the introduction of CO2, some of the DMEA and DMCHA can be protonated and exist in the interior of the system in a cation state, so that the ion concentration increases rapidly and the electrical conductivity shows a rapid upward trend. During this process, the water-in-oil microemulsion remained stable without demulsification. Then, the conductivity of the system began to show a rapid decline. As a large number of DMCHA and DMEA are continuously protonated, the microemulsion breaks (Figure 6(b)). The system eventually becomes two layers. The upper layer is the oil phase DMCHA and the lower layer is the water phase, which leads to a rapid drop in electrical conductivity. Finally, the electrical 15


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conductivity starts to increase gradually. This is mainly due to the fact that the upper phase DMCHA is gradually protonated under the action of CO2, resulting in a further gradual increase of the ion concentration inside the system, and the electrical conductivity gradually increases. At the same time, the volume of the upper phase gradually decreases, and the volume of the lower phase gradually increases (Figure 6(c)). After a long enough period of CO2, the system can become a single-phase bicarbonate solution of DMCHA and DMEA. This is consistent with the hydrophilic variable properties before and after protonation of DMCHA.

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Figure 7. Molecule state distribution of DMCHA and DMEA with varying pH value. Both DMEA and DMCHA have good CO2 response performance and can achieve reversible transition between ion state and molecular state under the alternate regulation of CO2 and N2. This section was presented in the supporting information in Figure S6 and Figure S7.The pKa values for DMCHA and DMEA are 10.5034 and 9.2235, respectively. The species distribution of DMCHA and DMEA was shown in 16


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Figure 7. Both DMEA and DMCHA are molecular states when pH=12.48. With the continuous introduction of CO2, the pH value gradually decreased, and the amount of protonated DMCHA and DMEA gradually increased. When pH=10.38, more than 50% of the DMCHA has been protonated into a water soluble ammonium salt solution and some of the DMEA are also protonated as shown in Figure 7. The SFME-I system was gradually destroyed until it was divided into upper and lower layers when pH=10.38 as shown in Figure 6(b). The amount of DMEA and DMCHA in the molecular state is further reduced with further decrease in pH, and the amount of protonated state is gradually increased. As pH decreased to 9.69, we found that the amount of dyed upper layer gradually decreased (Figure 6(b) to Figure 6(c)). By analyzing the molecular state distribution of DMEA and DMCHA, we can clearly explain the CO2 response behavior of the SFME system. SFME-I has obvious CO2 response characteristics, so how about SFME-II? Due to the high water content of SFME-II, the introduction of CO2 will directly lead to the protonation of DMCHA and DMEA, eventually forming bicarbonates that are soluble in the aqueous phase. In this process, there is no obvious change in the appearance of the system and no obvious phase separation will occur. In order to confirm the effect on the system before and after the introduction of CO2, we prepared SFME-II by using different concentrations of NaCl aqueous solution (0wt%-10wt%) instead of pure water. CO2 was then introduced at a rate of 60 mL/min, and the changes of different samples were observed with the introduction of CO2.

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Figure 8. SFME-II prepared from different concentration NaCl aqueous solution (0wt%-10wt%), DMEA and DMCHA and its CO2-treated photos. It can be seen from Figure 8 that SFME-II can be formed by mixing DMEA, DMCHA and different concentrations of NaCl aqueous solution (0wt%-10wt%). However, with the introduction of CO2, there are different phenomena between samples. The samples containing 4% or more of NaCl all exhibited a very pronounced “salting out” phenomenon and a large amount of white matter precipitated after 8 min 18


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with the introduction of CO2. This shows that the introduction of CO2 leads to an increase in the internal ion concentration of the system (mainly the ammonium salt) so that salting out occurs in a system with a high NaCl concentration. The NaCl concentration of 2% or less is not enough to produce salting out phenomenon after 8 min of CO2 bubbling. Samples containing 2% NaCl concentration showed a white precipitate until after 30 min of continued CO2 infusion. We have found that the system containing NaCl can undergo salting out when CO2 was introduced into the system. The system that does not contain NaCl does not cause salting out even if CO2 was bubbled for 2 hours or longer. Although we did not observe the phenomenon similar to SFME-I after SFME- II (without NaCl) was introduced CO2, it can be seen from Figure 7 that the introduction of CO2 has a very significant effect on SFME-II. Unlike SFME-I, we did not observe the apparent demulsification of SFME-II (without NaCl) with the introduction of CO2. As can be seen from Figure 7, in the initial state, the system exists in the form of a microemulsion, and when CO2 was introduced, the system becomes a bicarbonate solution of DMEA and DMCHA.

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Figure 9. Distribution of hydrodynamic radius (Rh) of SFME initial system and after one CO2/N2 cycle. The reversibility of the phase transition was studied by bubbling CO2 and N2 sequentially into the SFME-I and SFME-II system. First, CO2 was introduced into SFME-I and SFME-II (20mL) at 25℃ for 5 minutes. The gas introduction rate is 0.06L/min. At this point, SFME-I has become cloudy, and SFME-II looks clear. Next, N2 was introduced into the above system at a rate of 0.3 L/min for 40 min at 45℃. After N2 action, both SFME systems became clear and transparent in appearance. The Rh before and after the cycle was measured by DLS at 25℃. The Rh of the initial system of SFME-I and SFME-II was 5.62 nm and 4.56 nm, respectively, as measured by DLS. After a CO2/N2 cycle, the Rh increased to 29.20 nm and 10.00 nm, respectively. The mean diameter and PDI data for SFMEs are presented in Table S1 in the supporting information. Although Rh slightly increased after one cycle, it remained at a very low level. This shows that the introduction of N2 can restore the 20


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microemulsion to its original state, and the SFME system has the reversibility of CO2 response.

CONCLUSION In general, a surfactant-free microemulsion with CO2 stimuli responsive properties can be prepared by mixing DMCHA, DMEA and water. The phase area of SFME can be clearly seen by drawing the ternary phase diagram of the microemulsion. After the CO2 was introduced into the SFME-I system, the microemulsion was destroyed and a distinct oil-water phase separation phenomenon appeared. At the same time, DMCHA was dissolved in water after protonation, the volume of the oil phase gradually decreases, and the volume of the water phase gradually increases as CO2 continues to be added. By comparing the changes in the SFME-II system with various NaCl concentrations after the introduction of CO2, it was found that the introduction of CO2 resulted in a significant increase in the internal ion concentration of the system, and the system eventually became the bicarbonate of the two tertiary amines. At the same time, we also confirmed the reversibility of the CO2 response of the microemulsion. SFME that has been demulsified by CO2 can be regenerated in the presence of N2, which makes the SFME CO2 response performance reversible. SFME as a very special microemulsion has a very broad application prospects and has been deeply studied. The SFME that we have proposed to have CO2 stimuli response characteristics will certainly receive extensive attention and research due to its unique smart response behavior. This also provides new opportunities for the development of the microemulsion industry. 21


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ASSOCIATED CONTENT Supporting Information 1.

Molecular structure of DMCHA, DMEA and methyl orange (MO).

2.

Influence of temperature on the area of phase region in ternary phase diagram of SFME system.

3.

The conductivity of the SFME system changes with increasing water content and the position of the dilution line in the ternary phase diagram.

4.

The UV-vis absorption curve of MO corresponds to different mass fractions of DMCHA in SFME-I.

5.

CO2/N2 Switching Performance of DMCHA and DMEA

AUTHOR INFORMATION Corresponding Author Zhiyu Huang College of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500, P. R. China. * Email: [email protected]; Fax: +86-28-83037330; Tel: +86-28-83037330. Hongsheng Lu College of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500, P. R. China. * Email: [email protected]; Fax: +86-28-83037330; Tel: +86-28-83037330. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (NSFC, NO. 21403173).

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CO2-Responsive Surfactant-Free Microemulsion - Langmuir (ACS

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