CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. National Stage of PCT/JP2019/008188, filed Mar. 1, 2019, which claims priority to JP 2018-038769, filed Mar. 5, 2018.

TECHNICAL FIELD

The present invention relates to an ultrafine bubble generating agent for generating ultrafine bubbles in a liquid, a method for producing an ultrafine bubble-containing liquid, an ultrafine bubble-containing liquid produced by the producing method, and the like.

BACKGROUND ART

Microbubbles having diameters of 1000 nm or less in a solvent such as water under normal pressure are also called “ultrafine bubbles”. Such ultrafine bubbles, when compared with typical bubbles having diameters of 1 mm or more, have good qualities such as (1) having a notably large bubble interface surface area, (2) having a high bubble inner pressure, (3) having high gas dissolution efficiency, (4) having a slow bubble rise speed, and the like and for this reason they are considered useful for, for example, cleaning treatment of a semiconductor, water purification treatment or sterilization treatment, or farming oysters or shellfishes. For the method for producing such ultrafine bubbles, various methods have been proposed up to date and have been also practiced (Patent Documents 1, 2, 3). However, these producing methods require an ultrafine-bubble generator and thus have problems limiting usage environment of ultrafine bubbles and having an adverse effects for a consumer to handle easily, for example.

A substance called a CO2hydrate (carbon dioxide hydrate) is known as a type of ice having a high CO2-content rate. A CO2hydrate refers to a clathrate compound in which carbon dioxide molecules are trapped within cavities of the crystal structure of water molecules. Water molecules forming a crystal structure are called the “host molecules” and molecules trapped within cavities of the crystal structure of water molecules are called the “guest molecules” or “guest substances”. A CO2hydrate, when melted, breaks down to CO2(carbon dioxide) and water and thus generates CO2when melted. A CO2hydrate can be produced using CO2and water under the conditions of a low temperature and high CO2partial pressure and, for example, can be produced under the conditions of being at a certain temperature and CO2partial pressure being higher than the equilibrium pressure of a CO2hydrate at that temperature (hereinafter, also described as the “condition for producing a CO2hydrate”). The above conditions of “being at a certain temperature and CO2partial pressure being higher than the equilibrium pressure of a CO2hydrate at that temperature” are presented as, in the equilibrium pressure curve of a CO2hydrate (for example, the ordinate represents CO2pressure and the abscissa represents temperature) disclosed in FIG. 2 of Non-patent Document 1, and FIG. 7 and FIG. 15 of Non-patent Document 2, a combination condition of a temperature and CO2pressure within the region of the high pressure side of the curve (the upper part of the curve in the case where, for example, the ordinate represents CO2pressure and the abscissa represents temperature in the equilibrium pressure curve of the CO2hydrate). Alternatively, a CO2hydrate can also be produced by reacting micro ice, in place of water, with CO2under the conditions of a low temperature and low CO2partial pressure. The higher CO2pressure or the lower temperatures of CO2and water temperatures get during producing a CO2hydrate, a CO2-content rate of a CO2hydrate tends to be higher. A CO2-content rate of a CO2hydrate can be, depending on the producing method of the CO2hydrate, in the order of about 3 to 28 wt %, which is notably high compared with a CO2-content rate of carbonated water (in the order of about 0.5 wt %).

Further, Patent Document 4 discloses a method for producing a mass of consolidated carbon dioxide clathrate by compressing produced carbon dioxide gas clathrate (CO2hydrate) particles using a compression instrument such as a piston. It is described that the mass of carbon dioxide gas clathrate settle at a specified place of the deep-sea floor without being carried away by the deep-sea water. It is described that the technology can be utilized for reducing carbon dioxide gas in the atmosphere.

As other purposes of use of a CO2hydrate, it is known to add a CO2hydrate to a drink and mix them. For example, Patent Document 5 discloses the production of a carbonated drink by mixing a CO2hydrate with a drink to impart carbonic acid to the drink, and Patent Document 6 discloses that a carbonic acid supplementary medium formed by covering a CO2hydrate with ice is added to a drink for cooling the lukewarm drink and replenishing a carbon dioxide gas of the flat drink simultaneously.

Thus, it has been known that when ice having a CO2-content rate of 3 wt % or more (preferably a CO2hydrate) is added to a liquid, bubbles generate from the above-mentioned ice (preferably a CO2hydrate), however, it has not been known that ultrafine bubbles generate from the above-mentioned ice (preferably a CO2hydrate).

PRIOR ART DOCUMENTS

Patent Documents

Patent Document 1: Japanese unexamined Patent Application Publication No. 2008-149209Patent Document 2: Japanese unexamined Patent Application Publication No. 2004-330050Patent Document 3: Japanese unexamined Patent Application Publication No. 2007-275893Patent Document 4: Japanese unexamined Patent Application Publication No. 06-039243Patent Document 5: Japanese unexamined Patent Application Publication No. 2005-224146Patent Document 6: Japanese Patent No. 4969683

Non-Patent Documents

Non-patent Document 1: “Hydrates of Carbon Dioxide and Methane Mixtures”, J. Chem. Eng. Data (1991) 36, 68-71Non-patent Document 2: “Phase Equilibrium for Clathrate Hydrates Formed with Methane, Ethane, Propane, or Carbon Dioxide at Temperatures below the Freezing Point of Water”, J. Chem. Eng. Data (2008), 53, 2182-2188

SUMMARY OF THE INVENTION

Object to be Solved by the Invention

An object of the present invention is to provide an agent for generating ultrafine bubbles capable of easily generating ultrafine bubbles in a liquid without requiring an ultrafine-bubble generator, a method for producing an ultrafine bubble-containing liquid, an ultrafine bubble-containing liquid produced by the producing method, and the like.

Means to Solve the Object

While conducting extensive studies to solve the above object, the present inventors have found that when ice having a CO2-content rate of 3 wt % or more (preferably a CO2hydrate) is contacted with a liquid (preferably contained in a liquid), ultrafine bubbles can be generated in the liquid without requiring an ultrafine-bubble generator, whereby the present invention has been accomplished. Further, the present inventors have found that when a consolidated CO2hydrate was used as the ice having a CO2-content rate of 3 wt % or more, a density of the ultrafine bubbles (bubbles/mL) generated in a liquid can be notably increased, whereby the present invention was accomplished. Further, the present inventors have found that it is important to consolidate a CO2hydrate slurry even with low pressure and carry out sufficient dehydration treatment when preparing a CO2hydrate for obtaining ultrafine bubbles in a higher density by the CO2hydrate, whereby the present invention was accomplished. Further, the present inventors have found that when the ice having a CO2-content rate of 3 wt % or more (preferably a CO2hydrate) is directly melted, the melted liquid containing ultrafine bubbles in a much higher density can be obtained than the case that the ice is contained in a liquid such as water, whereby the present invention was accomplished.

In other words, the present invention relates to(1) an ultrafine bubble generating agent for generating ultrafine bubbles in a liquid, wherein the agent includes ice having a CO2-content rate of 3 wt % or more;(2) the ultrafine bubble generating agent according to the above (1), wherein the ice having a CO2-content rate of 3 wt % or more is a CO2hydrate;(3) the ultrafine bubble generating agent according to the above (1) or (2), wherein the ice having a CO2-content rate of 3 wt % or more is ice having a size with a maximum length of 3 mm or more and having a CO2-content rate of 3 wt % or more;(4) the ultrafine bubble generating agent according to any one of the above (1) to (3), wherein the ice having a CO2-content rate of 3 wt % or more is ice capable of generating ultrafine bubbles in water so that a density of the ultrafine bubbles is 5 million bubbles/mL or more when measured by the following Measurement Method P1;
(Measurement Method P1)

200 mg/mL of ice of −80 to 0° C. and having a CO2-content rate of 3 wt % or more is added to water of 25° C., allowed to stand for 1 hour under a condition of 25° C., and subsequently a density of the ultrafine bubbles in the water (bubbles/mL) is measured using Malvern NanoSight NS300 or SHIMADZU CORPORATION SALD-7500 nano;(5) the ultrafine bubble generating agent according to any one of the above (1) to (4), wherein the ice having a CO2-content rate of 3 wt % or more is a consolidated CO2hydrate;(6) a method for producing an ultrafine bubble-containing liquid, comprising a step of melting ice having a CO2-content rate of 3 wt % or more;(7) the method for producing an ultrafine bubble-containing liquid according to the above (6), wherein the step of melting ice having a CO2-content rate of 3 wt % or more is a step of melting ice having a CO2-content rate of 3 wt % or more by contacting the ice with another liquid, or a step of melting ice having a CO2-content rate of 3 wt % or more without contacting the ice with another liquid;(8) the method for producing an ultrafine bubble-containing liquid according to the above (6) or (7), wherein the ice having a CO2-content rate of 3 wt % or more is ice capable of generating ultrafine bubbles in water so that a density of the ultrafine bubbles is 5 million bubbles/mL or more when measured by the following Measurement Method P1;
(Measurement Method P1)

200 mg/mL of ice of −80 to 0° C. and having a CO2-content rate of 3 wt % or more is added to water of 25° C., allowed stand for 1 hour under a condition of 25° C., and subsequently a density of the ultrafine bubbles in the water (bubbles/mL) is measured using Malvern NanoSight NS300 or SHIMADZU CORPORATION SALD-7500 nano; and(9) the method for producing an ultrafine bubble-containing liquid according to any one of the above (6) to (8), wherein the ice having a CO2-content rate of 3 wt % or more is a consolidated CO2hydrate.

Effect of the Invention

According to the present invention, an agent for generating ultrafine bubbles capable of easily generating ultrafine bubbles without requiring an ultrafine-bubble generator, a method for producing an ultrafine bubble-containing liquid, an ultrafine bubble-containing liquid produced by the producing method, and the like can be provided.

