Effect of Temperature and Alcohol on the Determination of Critical Micelle Concentration of Non- Ionic Surfactants in Magnetic Water

The determination of critical micelle concentration of selected non-ionic surfactants (Tween 20,40 and 80) have been investigated using magnetic water(MW)as an aqueous medium.Conductometry technique is used to determine critical micelle concentration.The effect of alcohol addition and temperature variation at the range(293.15 -303.15K) are also pursued. It is concluded that the process of micellization is spontaneous and endothermic because of the observed free energy of micellization (ΔG o m) , enthalpy change of micellization (ΔH o m), and entropy change of micellization (ΔS o m) for the system was also studied.The properties of the non-ionic surfactants were studied, both in absence and presence of alcohol. The results obtained were explained in light of hydrophobic-hydrophilic interactions and chain length of alcohol.The temperature change has slight effect on the micellization process. .

It has the ability of changing the physical properties of pure water (19) to give an aqueous solution having more acidity i.e. its new pH becomes 7.4-12% more than before which means causing more hydration and less aggregation leading to increasing the critical micelle concentration value. Also, it lowers the surface tension of the normal water by about 4.6-18% (20). Tween 20,40 and 80 (polyoxyethylenesorbitan fatty acid esters) have been chosen to be the suitable nonionic surfactants for this study.The interaction between (MW) molecules which is pure water exposed to a homogeneous magnetic field of 2000G over the region through which the water is allowed to stay in a calculated time , and the surfactants has been investigated via measuring the electrical conductance over a temperature range of 20 °C to 35 °C and were extensively studied in relative to its micellization using electrical conductivity (21). Micellization is affected by various factors including surfactant species (hydrophobic volume chain length, head group area), temperature, pressure, ionic strength, pH..etc.
Temperature change affects the micellization of ionic and amphoteric surfactants due to the relative change in the interaction of the hydrophobic and hydrophilic groups in the surfactant. critical micelle concentration is a distinguishing characteristic property of surfactants as well as the Krafft temperature which is defined as the minimum temperature at which micellization occurs (22).The aim of this work is to find out some new properties of surfactants that is created under the influence of magnetic water and the study of the effects of some alcohols on the micellization process of Tweens surfactant.
The chemical name, chemical formula and struxture of Tweens are listed in Table 1 and Fig.1

Materials and Methods:
Conductometic Apparatus: The conductometric measurements are taken with a WTW inolab cond 7110(made in Germany) conductometer using a cell with a cell constant of 0.476cm -1 with an accuracy measurement within 0.01µS.The surfactant conductance was measured after thorough mixing and temperature equilibration. The break point in the plot of specific conductivity (µS.cm -1 ) versus the total surfactant concentration is taken as the (critical micelle concentration) value.

Chemicals:
Polyoxyethylene (20,40 and 80) Sorbitan monolaurate, Sorbitanmono palmitate, Sorbitanmono oleate respectively (Tweens is the commercial name) were supplied by Oxford LABORATORY REAGENT -INDIA and it is used as received from the source. Solutions of individual surfactants were prepared using doubly distilled and deionized water. All the solutions were measured under the thermostated conditions at 293.15, 298.15, 303.15 and 308.15 K with an accuracy of (±0.1℃). Alcohol is used without any further purification.

Preparation of Solutions:
250 ml (0.1M) of Tween (20,40 and 80) were prepared as stock solution by dissolving 30.675,32.05,32.75 g/250ml respectively,by a minimum amount of distilled deionized water and then completed to the mark using volumetric flask.
A series of solutions of the desired concentrations of Tweens were prepared using a 25ml volumetric flasks.After maintaining the desired temperature by using the thermostating bath(the type HAKKE Nk22 was used to control the temperature of the solutions), conductometric measurements were carried out.

Distilled Deionized Magnetized Water
All conductance measurements were done using triple distilled deionized water with a conductance equal to (0.8-1.0 µS/cm). The water used was prepared using simple distillation method. A 1.5L of distilled water is placed in a round bottomed flask with1.0g of KMnO 4 . The process was repeated three times .The obtained water was left for 24 hrs with a stack permanent magnets to acquire a magnetic field more than 2000G.

Determination of Critical Micelle Concentration:
Surfactants are considered as surface active molecules containing hydrophilic head and hydrophobic tail in aqueous solution. It tends to aggregate to form what is called critical micelle concentration (23).This process depends on several factors such as the interaction between hydrophilic and hydrophobic groups of the alkane chain (24). This noteworthy unique structural feature of micelle makes it capable for interactions with hydrophobic and hydrophilic molecules).When the surfactant dissolves in solution it takes some sort of modification to minimize its volume occupied throughout the whole solution, which in turn minimize the interaction between hydrophobic tails of the non-polar groups, hence it shows a large value of critical micelle concentration at low temperature (25). The repulsion electrostatic forces between the non-polar groups lead to minimize the formation of critical micelle concentration. Temperature increments cause plugging out of water molecules from the ethoxylic surfactant molecule due to the low hydrogen-bond energy(29 Kcal/mol.)which leads to a decrease in the affinity 257 of the non-polar groups for hydration. (26). The solvent plays an important role in the formation of critical micelle concentration, which in turn depends on the physical properties of the applied solvent. The dielectric constant of the solvent have a prominent role in the formation of critical micelle concentration (27,28) Results and Discussion:

Surface Tension Measurements
The surface tension of the solvent plays an important role in determination of critical micelle concentration of the surfactant due to the interaction between the solvent and the surfactant molecules. The higher the surface tension, the higher interaction between water polar atoms and surfactant polar and non-polar groups.It facilitates the aggregation of surfactant species. Any change in surface tension up and down may cause an increase or a decrease in aggregation process respectively. In case of magnetic water, it shows a decrease in the surface tension by about(4.62-18% of the normal water) (20). Unfortunately, the results obtained using surface tension technique were not satisfactory enough to be discussed on the basis of the published literature (21,29). It is preferred to refer to it in this simple discussion because the study of surface tension of magnetized water requires high precision and sensitive equipment to be effective under special conditions (21).Surfactants are amphiphilic molecules. They are present in a dynamic equilibrium in magnetized water and associate at a low concentration solution to form a spherical or ellipsoidal shaped micelles. Accordingly, the micelle which is composed of a number of surfactant molecules has a duality property (polar-non polar) that start to dictate the geometry and size of micellization(30)

Conductance Measurement:
The electrical Conductance of Tweens solution continuously increases with surfactant concentration and suddenly changes the slope versus surfactant concentration at critical micelle concentration inflection point. The changed slope (from steep to smooth,) for electrical conductance indicates that the solution losses ionic charges. However, electrical Conductance of the solution still increases with increasing surfactant concentration after critical micelle concentration (micelle formation), because the additional surfactants are considered to form micelles (1), the increased electrical Conductance suggests that micelles are charged particles (aggregated). For nonionic surfactants, the activities of two forms of surfactants (monomer and micelle) should be the same at and above the critical micelle concentration and their activities should be unity, since the micelle form is essentially the reference state. A study has been done using the electric conductance technique to find out the interaction behavior and association of different molecules of surfactant in the system under study (31). Tables 3,4 and 5 indicates the values of critical micelle concentration for different Tweens in magnetized water solutions through a plot of concentration-Conductance diagram. The values of CMC are estimated from the break point of the linear relationship in the curve for each temperature. The table. shows a direct relationship between the conductance and concentration. It is obvious from the diagram that there are two regions with two linear segments. The above concentration line refers to the post-miceller region, while the other one is called pre-miceller region. The values of critical micelle concentration versus concentration are stated in Table 6. It shows a decrease in conductance for the sequence (Tween20 >Tween 40>Tween 80) at a given temperature, which is due to the increase in carbon atoms or hydrocarbon tail for surfactant molecule and the missing free ions responsible for Conductance. Effect of Temperature Many researches have been engaged in studying the effect of temperature changes on surfactant solutions (25,32). Non-ionic surfactants, and polyoxyethylenesorbitan fatty acid esters (polysorbate) are chosen to investigate the effect of temperature change (293.15 -308.15 K) on the critical micelle concentration determination using magnetic water,as shown in the results tabulated in Tables 3,4,5. To simulate the reality, temperatures similar to the living reality are chosen 20 to 35°C (293.15 -308.15K). Mesophiles, which comprise most of the species commonly found in waste water treatment processes, grow within the range of 10 to 45°C (283.15-318.15K), with an optimum of approximately 30 to 35°C (303.15-308.15K) . As the temperature changes in the waste water, one group of microorganisms will slow down until it dies. This diversity and adaptability in the number of microbes to the temperature of water continues at 35°C (308.15K). When the temperature is above 35°C, the combined capacity of many microorganisms decreases and optimum conditions are lost in the biological system (33).By increasing temperature up to 35 o C(303.15K) Tweens (20, 40 , 80 ) show a gradual decrease in values of critical micelle concentration. This may be due to i) Temperature elevation that may lower hydration of hydrophilic groups and favor micellization ii) Association and interaction of hydrophobic groups with each other which reduce critical micelle concentration values of Tweens (32,33)  Aggregation and critical micelle concentration values are dependent on the environmental conditions. Micellization, as well, is affected by various factors including surfactant nature (chain length, polar head group and hydrophobic volume), solvent, temperature, additives, pressure, ionic strength, pH,…etc (32).The results tabulated in Table 6 are compatible with the literature information (33)whereas the critical micelle concentration of each surfactant individually shows a slight increase in their values with respect to the corresponding temperature. In case of Tween 40, the critical micelle concentration shows very slight increase in values while it remains approximately constant in Tween 80 at temperatures used. The constancy in value may be attributed to two reasons Just as follows: first,the temperature increments causes plugging out of water molecules from the ethoxylic surfactant molecule due to the low hydrogen -bond energy(29 kcal/mol.)which leads to a decrease in the affinity of the non-polar groups for hydration (34), and the second reason, the small range of temperature used to minimize the effect of temperature on magnetic water to keep its ability for conversion, the increase in temperature favors the disorder of water phase .

Effect of Alcohol
Recently, the micellization of surfactants in the presence of different alcohols with different carbon atoms has been reviewed. (29,35).Surfactants that have low molecular weight tend to have true solutions with low concentration. The small molecules and ions are aggregated in groups to form micelles. The effect of the additives on the behavior of the surfactants has a great attention (36)(37)(38)(39)and it has been widely studied due to the alcohol ability to bind to the micelles in the surface region which penetrates between the ionic groups belong to the surfactant molecules and decreases the surface area per head group which finally increases the ionization, and its presence causes a decrease in the dielectric constant at the micellar interface because of replacement of water by alcohol molecules in the interface region, (30).The shape of the alcohol molecule has a good chance to interact with the surfactant molecules and penetrates between the molecules of the agent, causing the surface tension of the surfactant to be reduced to as much as 70%, as well as lowering dielectric constant of the Tweens, as well as the interference of the low hindrance factor that allows these molecules to intervene in favor of the interest interaction between the surfactant and the alcohol molecules (26).The micellization depends on the composition of the surfactant and the alcohol concentration in the medium concerned also. The difference in composition and nature of the alcohol and surfactant determines the micelle composition which gives an explanation about the aggregation number.

Addition of Methanol
The mutual influence of non-ionic surfactant (Tweens 20,40,80)and methanol, ethanol and 1propanol are considered on their behavior in the magnetic water. The consideration was based on the conductance measurements of the aqueous solutions of (Tweens) and alcohol mixture at 293.15K in the whole range of alcohol concentration and in the presence of the surfactant (Fig. 2). It is obvious from the Figure, that alcohols considerably affect the critical micelle concentration of (Tweens). The diagram shows the logical relationship between the Tween concentration and methanol (alcohol) as an additive. The graph shows that the addition of methanol to Tween-20 leads to an increase in the hydrophilic effect which increases the values of Conductance to values up to 170 (μS.cm -1 ), while during the addition of methanol to Tween-80, the hydrophopic effect shows a predominant property which favors micellization process and leads to a decrease in the conductance of about70(μS.cm -1 ). In the case of Tween-40, the process seems to be balanced in both cases and we got a straight line. consequently promotes the process of micellization and micelle formation by gathering and overlapping the oppositely charged surfactant molecules through the effect of hydrophobic and electrostatic attraction, and by increasing the concentration of alcohol(8), the steric effect will act as a new factor of disrupting the electrostatic attractive forces between polar groups of the surfactant and alcohol molecules. The temperature has a great role in the process of micellization, increasing temperature leads to increase the kinetic energy of the surfactant molecules which causes a spread of the molecules out of the zone of aggregation.In the presence of different alcohol concentration, Fig. (2, 3 and 4), the curves show up and down values giving more than one critical micelle concentration which explain disfavoring micellization specially at high temperature. Disturbance in the curves may be attributed to the former reasons explained before, the interaction is in the sequence (Propanol > Ethanol > Methanol) i.e. increasing hydrophobicity of the molecules added.

Addition of Propanol:
Propanol is an alcohol with three carbon atoms. It tends to increase the hydrophobicity of the surfactant molecules which in turn decreases the critical micelle concentration value and enhances aggregation. It is suggested that propanol molecules tend to penetrate between the Tweens head groups which lead to a decrease the surface area of the micelles per head group leading to an increase in the ionization (30). Consequently, the conductance shows a large increase in value. In the case of Tween 40, it seems that the compound suffers ionization more than others and have high conductance value as shown in Fig.4. Moreover, the shape and modulation of the molecule plays a role in the process away from giving any role to the type of alcohol.

Mechanistic Study
At low surfactant concentration the molecules adsorb at air water interface with the hydrophobic tail groups pointing away of the water surface, by increasing the surfactant concentration the molecules are tend to aggregate in a several dozens of simple molecules of surfactants to form a spherical collection of micelles named critical micelle concentration(CMC)with hydrophobic molecules toward the core of the sphere . Critical micelle concentration can be determined from the observed change in the plot of some physicochemical properties as conductance of the solution, against the surfactant concentration. Increasing surfactant concentration leads to formation of more micelles which results in a decrease in the average distance between the micelles and increase the inter-micellar repulsion,hence the spherical micelles may transform into worm like micelles which cause the molecules to spread of out of the zone of aggregation (13), thereby increasing the conductance of the species up to a maximum value as indicated in Figures (2,3,4). The temperature, also has a great role in increasing the electrical conductance of surfactant species by increasing the kinetic energy of the surfactant molecules.

Thermodynamics of Micelle Formation:
A plot of ln against temperature gives a good straight line Fig. 6. As shown in Table 6., the system temperature shows a decrease in the values of critical micelle concentration by increasing the temperature. Due to decreasing in hydrophilicity of the surfactant, molecules may cause a decrease in critical micelle concentration with temperature, which corresponds to reported review of non-ionic surfactant (33). The increase in temperature facilitates the reduction in hydration of the hydrophilic oxyethylene group, which favors 262 micellization process .It is widely known in surface chemistry that increasing temperature lead to an increase in the breakdown of the hydrogen bonds of water molecules surrounding the hydrophobic groups which disfavors micellization process ( 40). There are some aspects that govern the relation between surfactant molecules and alcohol in aqueous solution such as, hydrogen bonding, hydrophobic interaction and hydration which are reflected in the enthalpy and entropy contributions in the free enthalpy of solution (39). ΔG o m is the sum of the enthalpic (ΔH o m)and entropic (-TΔS o m) contributions.) contributions. By increasing the temperature the enthalpic contribution to the free energy increasing also and minimizing the entropic contribution (40).

Conclusion:
It can be concluded here that the micellar behavior of nonionic surfactant, polyoxyethylene (Tweens 20, 40 and 80), sorbitanmonolaurate in magnetized water in the presence of methanol, ethanol and propanol at a range of temperature 293.15-308.15K. The process was investigated with the help of c technique. The critical micelle concentration and of the micelles were determined from the conductance measurements at different temperature ranges. It has been observed that the temperature change does not mean much to us, perhaps because of the use of the magnetized water, which has changed many of the general properties of the substances under study. The presence of hydrophilic head and hydrophobic tail in the surfactant interact with any species present in solution taking advantage of the interference and adsorption properties of the surface active substances, which in turn affects in one form or another on the critical micelle concentration for alcohols. A systematic change observed in case of methanol and ethanol, but this is not true in the case of propanol which shows an unusual attitude. Effect of alcohol series in general was dependent on the hydrophobic character, hydrocarbon chain length, modification and shape of the alcohol molecule. The shape and modulation of the molecule play a role in the process away from giving any role to the type of alcohol. It seems that Tween 40 surfactant suffers ionization more than others and has high conductance value in presence if propanol.The study showed that there is a clear lack of practical results of surface tension where the study needs highly sensitive and more advanced equipments.
Conflicts of Interest: None.