What Is the Cloud Point?

Cloud point, a characteristic constant of non-ionic surfactants, which is affected by the molecular structure of the surfactant and coexisting substances. The aqueous solution of surfactant will appear turbid as the temperature rises. The surfactant changes from being completely dissolved to being partially dissolved. The temperature at which the surfactant is transformed is the cloud point temperature. The cloud point (CP) is the temperature at which phase separation occurs in a nonionic surfactant (NS) homogeneous micellar solution and is a very important physical parameter. ^[1]

Cloud point, a characteristic constant of non-ionic surfactants, which is affected by the molecular structure of the surfactant and coexisting substances. The aqueous solution of surfactant will appear turbid as the temperature rises. The surfactant changes from being completely dissolved to being partially dissolved. The temperature at which the surfactant is transformed is the cloud point temperature. The cloud point (CP) is the temperature at which phase separation occurs in a nonionic surfactant (NS) homogeneous micellar solution and is a very important physical parameter. ^[1]

Chinese name

Cloud point

Foreign name

Cloud point

Nature

Physical Chemistry Concept

Belongs to

Oils, varnishes, etc .; non-ionic surfactants

Cloud point detailed definition

(1) Liquid samples such as oils and varnishes are cooled to a temperature at which turbidity begins to appear as the cloud point under standard conditions. Turbidity is due to the precipitation of moisture or solids from the sample. The lower the cloud point of fuel oil, lubricating oil, etc., the less water or solid paraffin it contains. Its estimation generally adopts the national standard GB / T 6986-2014 petroleum product cloud point determination method. ^[2]

(2) The solubility of non-ionic surfactants in aqueous solution decreases with increasing temperature. Turbidity appears when the temperature rises to a certain temperature. Two liquid phases can be obtained after being placed or centrifuged. This temperature is called the surface activity. Cloud point of the agent.

This type of surfactant is dissolved in water by combining the oxygen atom in its ether bond with the hydrogen atom in water in the form of hydrogen bond. The hydrogen bonding force is weaker, and it gradually breaks with increasing temperature, so that the solubility of the surfactant in water gradually decreases. When it reaches a certain temperature, it becomes insoluble and precipitates into a turbid liquid. The cloud point has a certain relationship with the mass ratio of hydrophilic groups and lipophilic groups in the surfactant molecule. ^[1]

The range of the cloud point has a certain relationship with the purity of the product. The cloud point of the product with good quality and high purity is obvious, and the quality of the product is not obvious.

Cloud point of commonly used nonionic surfactants: ^[3]

OP / NP series (alkyl phenol polyoxyethylene ether)	OP-9	OP-10	OP-15	OP-20	OP-40
Cloud point / ° C (1% solution)	60-65	68-78	94-99	> 100	> 100
O series (fatty alcohol polyoxyethylene ether)		O-10	O-15	O-20
Cloud point / ° C (1% solution)		72-76	81-85	88-91
EL series (hydrogenated castor oil polyoxyethylene ether)	EL-20	EL-30	EL-40	EL-60	EL-80
Cloud point / ° C (1% solution)	30	45	70-84	85-90	91

Influencing factors and mechanism of cloud point

The cloud point is not only determined by the molecular structure of NS, but also greatly affected by additives (such as inorganic electrolytes, polar organics, surfactants, polymers, etc.), and also related to the concentration of NS ^[3] .

1. Effect of NS concentration on cloud point

As the NS concentration increases, its cloud point decreases first and then increases. This is because before the cloud point reaches the lowest value, the increase in surfactant concentration only increases the number of micelles, which increases the probability of collisions between micelles and the possibility of aggregation, so it is easy to cause separation from the water phase To reduce the cloud point. When the cloud point reaches the minimum value, as the concentration of NS increases, the shape of the micelles changes from spherical to rod shape, which results in an increase in the radius of the micelle particles and the increase in the viscosity of the solution. Rise.

2. Effect of inorganic electrolyte on cloud point

Schott et al. Found that the change in cloud point and the concentration of the external electrolyte are approximately linear, especially in the low concentration region, and the change in cloud point caused by each ion has algebraic summation. Starting from NO _3- = 0, they calculated the cloud point changes of many anions and cations to some NS through experiments, and found that all tested cations except Na ⁺ , K ⁺ , Cs ⁺ , NH ₄ ⁺ , and Rb ⁺ Both have the effect of raising the cloud point. The reason is that many cations have complexation, that is, the lone pair of electrons provided by the oxygen in the ether can fill the empty orbit of the metal ion to form a complex, which greatly enhances the hydrophilicity of the polar group. However, Na ⁺ , K ⁺ , Cs ⁺ , Rb ^+, and NH4 ⁺ cannot form complexes with the ethoxy chain, but they can compete with the polar groups of NS for water molecules, so the cloud point is lowered.

The effect of anions on the cloud point of ethoxylated NS obeys the Hofmeister sensitive ion sequence. According to their influence on water structure, anions can be divided into structure-forming anions-which can promote the formation of aggregates of water molecules through hydrogen bonding, and structure-disrupting anions-which can promote the depolymerization of water molecule aggregates. The former has a small ionic radius and / or a multivalent negative charge, so the charge density is large, which can generate a strong electrostatic field, which can bind more water molecules, increase the viscosity and surface tension of water, and promote the formation of water molecule aggregates. Reduce the cloud point. Generally these ions have the characteristics of high electronegativity and low polarizability, and the number of sensitive ions is generally 8, such as F-, OH-, SO ₄ ^2- , PO ₄ ^3- and so on. The latter has the characteristics of low electronegativity and high polarizability. Due to the low charge density, only a weak electrostatic field can be generated. The water molecule aggregates in the body phase are easily deaggregated, and thus play a role in salt dissolution. The number is generally 11, such as I-, SCN- ^, etc. Therefore, these ions are also called chaotic anions. As the concentration of chaotic anions increases, when most or all of the water molecule aggregates become free water, a maximum value appears for the cloud point; as the concentration of inorganic salts increases, due to the salting out of the cations, Effect (because the sodium salt is generally used), it will reduce the cloud point. Goel pointed out that only when the electrolyte concentration is greater than a critical minimum concentration can they exhibit their effect on the cloud point. And for electrolytes with the same cation, the larger the anion-sensitive glue number, the larger the critical minimum concentration, and the smaller the cloud point change of NS. Therefore, for inorganic electrolytes with the same cation, the larger the number of anionic gels, the smaller the effect on the cloud point of NS.

3. The effect of polar organics on cloud point

The cloud point of NS is related to the length of the hydrocarbon chain, the type and number of polar groups of the polar organic matter, and it has a fairly good linear relationship with the concentration of each organic matter, especially in the low concentration area. Gu et al. Studied the effects of 21 polar organics on the cloud point of a 1 (wt)% TX-100 aqueous solution, and found that polar organics that are infinitely miscible with water increase the cloud point of TX-100, while in water Partially dissolved polar organics reduce the cloud point of TX-100. This is because the former type of organic matter reduces the polarity of the medium by changing the structure of the solvent water, and at the same time, part of the organic matter is adsorbed on the water interface of the micelle 2 and, through its solvation, the micellization of the NS is limited and the turbidity The point rises. As the latter type of organic molecules are solubilized in the micellar fence layer, the micellar volume expands, thereby reducing the cloud point of the NS. For a group of homologs, the degree to which they affect the cloud point of NS is related to their hydrophobic alkyl chain length. Generally, at a certain temperature, the longer the organic alkyl chain, the lower the organic concentration required for phase separation of the NS aqueous solution. It is therefore the relative solubility of NS and organic matter in water that determines the change in cloud point.

4. The effect of surfactants on the cloud point of NS

Adding an ionic surfactant (IS) to the NS solution can significantly increase its cloud point, and unlike the effect of the electrolyte on the cloud point, there is no critical minimum concentration for NS, and the cloudiness can be reduced at extremely low concentrations. Point makes an impact. This is because NS and IS form mixed micelles. When the concentration of IS is low, IS is inserted into the NS micelle interface membrane to form mixed micelles dominated by NS. The surface charge density of the micelles increases and the cloud point is significant. Increased; when the IS concentration gradually increased to form IS-based micelles, NS was inserted into the IS micelle interface membrane, which caused shielding between the IS polar heads, and the cloud point increased significantly again. The addition of amphoteric surfactants has almost no effect on the cloud point of NS, and adding equal amounts of cationic surfactants and anionic surfactants to the NS solution does not affect its cloud point.

5. Effect of Polymer on Cloud Point of NS

Qiao et al. Studied the effect of polyethylene glycol (PEG) of different molecular weights on the cloud point of NS, and found that small molecular weight PEG increased the cloud point of NS, while large molecular weight PEG played the opposite role. This is because the random strands of long-chain PEG are wrapped around the micelles, forming a special polymer-micelle complexthe coronal inner-chain micelles. Due to the exchange of NS monomers between adjacent micelles, the micelles are attracted to each other, so that the micelles are prone to collision. Therefore, the longer the polymer's chain, the more obvious the "bridge" role the polymer plays, and the more chances of micellar collision, leading to a decrease in cloud point. For PEG with small molecular weight, the ethoxylate chain is short and can only partially cover the micelle interface. Due to the steric effect and solvation of polymer chains, the chance of collision between micelles is reduced, resulting in an increase in cloud point. It should be mentioned that the molecular weight is relative to the size of the NS micelles.

Estimation of cloud point

For NS with a cloud point above 100 ° C, it cannot be measured directly at normal pressure. So the literature proposed two methods to estimate the cloud point above 100 ° C. One is the epitaxy method. An electrolyte that functions as a salt solution is selected. The cloud point values at different electrolyte concentrations are measured at normal pressure and plotted, and then epitaxialed to a concentration of zero. To reduce the arbitrariness of the epitaxy, it is advisable to measure several electrolytes and then take the average. Of course, this method has certain limitations because it is unknown whether the cloud point is linearly related to the electrolyte concentration under high pressure. But this has certain reference value for studying the phase behavior of NS under high temperature and high pressure. The second method is the comparison method. It is assumed that for similar NSs, the cloud point change mainly depends on the type of electrolyte. First select an NS that has a similar cloud structure to an unknown cloud point and has a cloud point below 100 ° C under normal pressure. Measure the cloud point change value caused by the addition of salting-out electrolyte, and then measure the unknown when the concentration of salting-out electrolyte is present The cloud point of the NS of the cloud point, and then add the change value to obtain the approximate value of the cloud point of the NS. ^[3]