The Langmuir adsorption isotherm is a model that describes the relationship between the concentration of a substance adsorbed on a surface adsorbate and the concentration of the substance in the bulk phase gas or liquid . The model assumes that adsorption occurs on a homogeneous surface, with a fixed number of adsorption sites, and that there is no interaction between adsorbed molecules.The Langmuir adsorption isotherm equation is given by: = K P / 1 + K P where: = fractional coverage of the adsorbent surface the ratio of occupied sites to the total number of sites K = Langmuir adsorption constant a measure of the affinity between the adsorbate and the adsorbent P = partial pressure of the adsorbate in the bulk phase for gases or concentration for liquids Now, let's consider the effect of a decrease in the surface area of the adsorbent. The total number of adsorption sites on the surface will decrease, but the Langmuir isotherm equation does not directly depend on the surface area. However, the decrease in surface area will affect the adsorption capacity of the adsorbent.The adsorption capacity q can be calculated using the following equation:q = N where:q = adsorption capacity amount of adsorbate adsorbed per unit mass of adsorbent N = total number of adsorption sites per unit mass of adsorbentWhen the surface area decreases, the total number of adsorption sites N will also decrease, leading to a decrease in the adsorption capacity q . This means that, for a given concentration of adsorbate in the bulk phase, the amount of adsorbate adsorbed on the surface will be lower when the surface area is smaller.In summary, the Langmuir adsorption isotherm equation itself does not change with a decrease in surface area of the adsorbent. However, the adsorption capacity of the adsorbent will decrease due to the reduced number of adsorption sites, leading to a lower amount of adsorbate adsorbed on the surface for a given concentration in the bulk phase.