A humectant retards.. Surface evaporation

A humectant retards
a- Bacterial growth
b- Degradation
c- Surface evaporation***
d- Spreadability
e- All of the above.

Evaporation is a physical process that consists of a slow and gradual transition from a liquid state to a gaseous state, after having acquired enough energy to overcome the surface tension. Unlike boiling, evaporation can occur at any temperature, the faster it is. It produces water vapor and atmospheric vapor.
It is not necessary for the whole solution to reach the boiling point. When there is a free space above a liquid, some of its molecules is in gaseous form. When equilibrated, the amount of gaseous material defines the saturated vapor pressure, which does not depend on the volume, but varies depending on the nature of the liquid and its temperature. If the quantity of gas is lower than the saturated vapor pressure, a part of the molecules passes from the liquid phase to the gaseous phase: this is evaporation. When the vapor pressure is equal to the atmospheric pressure, a boiling occurs.
The evaporation, here of the water of a lake, causes a condensation in apparent droplets which ends up forming a mist or a fog.
- In hydrology, the evaporation of water is one of the important hydrological variables when establishing the water balance of a given watershed or a part of it. In this case, it is necessary to distinguish the evaporation of the free surfaces from the evaporation of the soil. The evaporation of water is important and indispensable in life, because water vapor, when condensed, turns into clouds and returns as rain, snow, fog or dew.
 - In thermodynamics, evaporation is an endothermic process, in that the heat is absorbed during evaporation.

Physical phenomenon:
The thermal movement of a liquid molecule must be sufficient to overcome the surface tension and evaporate, that is to say that its kinetic energy must exceed the cohesion work applied by the surface tension on the surface of the liquid. Therefore, evaporation occurs more rapidly at high temperatures, at high flow rates between liquid, vapor phases and in low surface tension liquids (ie with higher vapor pressure). .
With only a small proportion of molecules near the surface and moving in the right direction to escape the liquid at a given moment, the rate of evaporation is limited. In addition, as the higher energy molecules escape and those that remain have lower average kinetic energy, the temperature of the liquid is reduced. This phenomenon is also called evaporative cooling. An example of this phenomenon is perspiration (sweat).

Evaporative balance:
If evaporation occurs in a closed container, the molecules that escape from the liquid accumulate as vapor above the liquid. Many of these molecules return to the liquid state. When the exhaust and return process reaches equilibrium, the vapor is said to be saturated and no further change in vapor pressure or liquid temperature occurs.

Hygrometric deficit:
The factors that condition the evaporation rate (usually expressed in mm / day or in mm / month) are, on the one hand, those that characterize the state of the atmosphere in the vicinity of the evaporation surface, and on the other hand , the factors that characterize the nature and state of the surface being evaporated (open water, ice, bare soil, vegetation). As a correlation between evaporation and other meteorological factors affecting both media (water and air), Dalton (1802) proposes the following formulation: E = K x (ps-pv).
This equation expresses the evaporation rate E directly proportional to the difference between the saturated vapor pressure (ps) at the water temperature and the vapor pressure (pv) in the ambient air. The difference (ps-pv) is called hygrometric deficit. The vapor pressure pv, and thus the evaporation E, then depends on both the temperature of the water and the air.

determination (measurement) of evaporation:
Evaporation can be measured directly from small natural or artificial water surfaces (evaporation tanks) or with evaporimeters or lysimeters. The latter have a porous surface soaked with water and are located under conditions such that the measurement is conditioned by the meteorological characteristics of the atmosphere, such as humidity, temperature, insolation, wind, etc.
The evaporation rates thus observed can generally be considered as maxima and give a good approximation of the evaporative power of the atmosphere, with the probable evaporation index. By applying several reduction coefficients to said maximum values ​​and comparing the corrected results with those provided by the evaporation formulas, the most probable values ​​of the evaporation rates applicable to the area of ​​interest will be deduced.
The most used evaporimeter is the Piche type: it consists of a cylindrical glass tube 25 cm long and 1.5 cm in diameter. The tube is graduated and closed in its upper part, while its lower opening is closed by a circular sheet of standard filter paper 30 mm in diameter and 0.5 mm thick, fixed by capillarity and held by a spring. Filling the apparatus with distilled water, it gradually evaporates through the sheet of filter paper. The reduction of the water level in the tube makes it possible to calculate the evaporation rate (in mm per 24 hours, for example). The evaporation process is essentially related to the air humidity deficit; however, the device may not take sufficient account of the influence of insolation. This device, in hydrometeorological stations, is installed under shelter.
Evaporation tanks or tanks used in different countries have different shapes, sizes and characteristics, as specialists do not agree on the best type of use.

Evaporation from liquid surfaces:
Since the boundary conditions created have a significant influence, the results vary according to the evaporometer used for the determination.
Taking into account that the evaporation values ​​measured at the site of interest must be statistically valid for a period of at least 15 years, the difficulty is well understood. This has led many researchers to analyze empirical formulas, which allows us to achieve a result as close as possible.

Importance of the hydrological variable:
The water evaporates on the surface of the ocean, on the surface of the earth and also by the organisms, during the phenomenon of perspiration in the plants and transpiration in the animals. Living beings, especially plants, contribute 10% to the water incorporated into the atmosphere. This is the hydrological cycle or water cycle.
Water in the form of vapor rises and condenses to form the clouds, consisting of water in small drops. These are cooled by accelerating the condensation and joining other droplets of water to form larger drops that eventually precipitate on the surface of the Earth because of their greater weight. Precipitation can be solid (snow or hail) or liquid (rain). Water vapor can also condense in the form of fog or dew.
Some of the water that reaches the surface of the earth will be used by living beings. Sooner or later, all this water will return to the atmosphere, mainly because of evaporation.